Mechanism of Gases Generation during Lithium-Ion Batteries Cycling

Abstract

This paper studied the gases release of a graphite//NMC111(LiNi1/3Mn1/3Co1/3O2) cell during cycle in the voltage ranges of 2.6-4.2V and 2.6-4.8V and the temperatures of at 25°C and 60°C. It was proved that the CO2, CO, and H2 gases are released as a result of electrolyte decomposition. And it shows that the CO and H2 gases evolution is a direct consequence of the electrochemical reaction of electrolyte decomposition, while the CO2 generation is a consequence of the additional chemical reaction of interaction between the O2 released from the cathode atomic lattice oxygen and CO released from the same place on the cathode (appearing because of the electrolyte decomposition). That is why at the same electrochemical reaction of electrolyte decomposition, the ratio CO2/CO varies in the wide range from 0.82 to 2.42 depending on cycling conditions (temperature and cutoff voltage). It was proved that a potential-independent H2 evolution is a consequence of its adsorption in pores of powdered graphite on anode. There was proposed the mechanism of the electrolyte decomposition and the gases evolution in lithium-ion cells at their cycling, which corresponds quantitatively to all obtained experimental results.

Full text

Journal of The Electrochemical Society,166 (6) A897-A908 (2019) A897
Mechanism of Gases Generation during Lithium-Ion Batteries
Cycling
N. Е. Galushkin, ∗,zN. N. Yazvinskaya, and D. N. Galushkin
Laboratory of Electrochemical and Hydrogen Energy, Don State Technical University, Town of Shakhty, Rostov Region
346500, Russia
This paper studied the gases release of a graphite//NMC111(LiNi1/3Mn1/3 Co1/3O2) cell during cycle in the voltage ranges of 2.6-4.2V
and 2.6-4.8V and the temperatures of at 25°C and 60°C. It was proved that the CO2,CO,andH
2gases are released as a result of
electrolyte decomposition. And it shows that the CO and H2gases evolution is a direct consequence of the electrochemical reaction
of electrolyte decomposition, while the CO2generation is a consequence of the additional chemical reaction of interaction between
the O2released from the cathode atomic lattice oxygen and CO released from the same place on the cathode (appearing because
of the electrolyte decomposition). That is why at the same electrochemical reaction of electrolyte decomposition, the ratio CO2/CO
varies in the wide range from 0.82 to 2.42 depending on cycling conditions (temperature and cutoff voltage). It was proved that a
potential-independent H2evolution is a consequence of its adsorption in pores of powdered graphite on anode. There was proposed
the mechanism of the electrolyte decomposition and the gases evolution in lithium-ion cells at their cycling, which corresponds
quantitatively to all obtained experimental results.
© The Author(s) 2019. Published by ECS. This is an open access article distributed under the terms of the Creative Commons
Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any
medium, provided the original work is properly cited. [DOI: 10.1149/2.0041906jes]
Manuscript submitted November 1, 2018; revised manuscript received March 5, 2019. Published March 15, 2019.
Now the lithium-ion batteries get more and more widespread use.
First of all, this is connected with their high specific capacity and en-
ergy as well as their long enough service life.1–3Now the lithium-ion
batteries prevail in the segment of batteries of small-format. They are
used in smartphones, notebooks, hover boards, etc. Recently, more per-
vasive use is typical for lithium-ion batteries of large-formatin connec-
tion with their application in electric vehicles and airplanes.4The elec-
trochemical processes in these batteries running during their charge
& discharge have been studied quite well.5However, for lithium-ion
batteries, processes of aging and gases evolution during their cycling
are studied insufficiently.6
For energy density increase, a high voltage is necessary of the
cathode material.7–9At present time, a number of potentially promis-
ing cathode materials exists.7One of such promising materials is the
lithium nickel manganese cobalt oxide (LiNixMnyCozO2,NMC).For
these batteries capacity and energy density increase, it is necessary
to use the high cutoff voltage.10–12 The high cutoff voltage increases
lithium efficiency rate.12,13 However at the same time, the high cutoff
voltage results in a considerable gases evolution growth and a cells’
coulombic efficiency decrease.14–19 That is why nowfor batteries of the
kind, the cutoff voltage is used around 4.3V. Nevertheless, even under
this cutoff voltage on both cathode and anode, gases are released.20,21
In general, the gases evolution in the lithium-ion batteries is a seri-
ous problem; it is especially so in the case of their work under high
voltages and temperatures.22,23
In the paper20 in order to find out, what kind of gases are released
on cathodes and anodes at cycling of lithium-ion batteries, a two-
chamber cell was used, in which its cathode and anode were separated
by Li+-ion conducting glass.
The released gases were analyzed with aid of OEMS (on-line elec-
trochemical mass spectrometry). The experimental studies showed
that at cycling of lithium-ion batteries on their cathodes, the gases
CO2and CO are released, while on their anodes the gases C2H4,CO
and H2do. The majority of researchers believe that the hydrogen is
released due to reduction of residual moisture on an anode in line with
the formula H2O+e−→OH−+1/2 H2. The residual moisture can
appear as a result of electrolyte contamination by water or incorrect
drying of electrodes and other battery components. However the ex-
perimental studies20,21 showed that the released hydrogen amount is
much bigger than its content in a residual moisture. Hence, the hy-
drogen is released also due to the electrochemical decomposition of
∗Electrochemical Society Member.
zE-mail: galushkinne@mail.ru
the carbonate electrolyte. It should be observed that the hydrogen is
released also when lithium-ion batteries are stored in charged state.24,25
Generally accepted is the consideration that gases CO2and CO are
generated on a cathode only as a result of the carbonate electrolyte
decomposition.20 However in the paper,21 it was proved experimen-
tally that in this process, the essential role is played by the atomic
oxygen, which is released from the cathode in the course of batteries
charge at high voltages. In the papers,20,21 the probable mechanisms
were proposed of the gases generation at lithium-ion batteries cycling.
At the present time, studying of all electrochemical reactions lead-
ing to generation of gases and other side products is one of the most
important scientific problems connected with lithium-ion batteries as
these processes result in batteries degradation and aging.14–23
Therefore, the purpose of this study is to establish the mechanism
of electrolyte decomposition and the generation of gases and other
side products during the cycling of lithium-ion cells.
Experimental
Electrochemical cell.—The lithium-ion cell in the Fig. 1was used
for these experiments. The cell (Fig. 1) consists of an upper part 1 and
a lower one 12 made of stainless steel (316L). The electric isola-
tion of the upper and lower parts is fulfilled by Kel-F O-rings 5. The
cell gastight is provided by a virgin PTFE O-ring 6 (2.62 mm cord
1
2
3
4
56
7
8
910
11
12
5
Figure 1. Schematic cross-section of sealed электрохимической cell: 1- up-
per part of cell made of stainless steel, 2 - inlet, 3 - outlet, 4 - connection to
OEMS, 5 - Kel-F insulation rings, 6 - virgin PTFE O-ring, 7 - spring made of
stainless steel, 8 - mesh current collector made of stainless steel, 9 - positive
electrode, 10 - separator, 11 - negative electrode, 12 - lower part of a cell made
of stainless steel.

A898 Journal of The Electrochemical Society,166 (6) A897-A908 (2019)
diameter, 30 mm inner diameter, Angst+Pfister, Switzerland). An
electric contact between a cell upper part with a working electrode
is established via a spring 7 made of stainless steel (13 mm diam-
eter). The spring 7 makes pressure on the mesh current collector 8
located over the working electrode 9. The mesh current collector is
made of stainless steel; its diameter makes 21 mm; wire diameter
makes 0.22 mm; openings size is 1 mm. Between the positive and
negative electrodes, a separator is located (this is glass microfiber fil-
ter, 691, VWR, Germany, 28 mm diameter). The cell inner volume
makes 8.5 mL. The electrolyte amount makes 120 μL. Assembling of
a cell was executed in an argon-filled glove box (MBraun, Germany,
O2and H2O<0.1 ppm). Before assembling in the glove box, all cell
parts were dried in a vacuum furnace at the temperature 70°C dur-
ing 12 hours. In whole, the cell construction was very similar to the
cell construction in the classical papers20,21 on studying of electrolyte
decomposition in lithium-ion cells, which facilitates a comparison of
obtained results.
Electrodes and electrolyte.—The active mass of the graphite
electrode was obtained from the following mixture: SLP30 graphite
powder (TIMCAL, Switzerland, BET surface area of 7 m2g−1);
polyvinylidene fluoride binder (PVDF, Kynar HSV 900, Arkema,
France) with weight ratio 90:10 and N-methyl-2-pyrrolidone (NMP,
anhydrous, chemical purity 99.5%, Sigma-Aldrich, Germany; solid
content 30 wt%). The active masses were mixed in a planetary orbital
mixer (Thinky, USA) during 10 minutes at 2000 rpm and 50 mbar.
The obtained active mass was blade-coated onto a porous separator
(C480 Celgard USA, thickness 20 μm, porosity 45%), with use of
an automatic coater (RK PrintCoat Instruments, UK). The thickness
of the wet-film made 250 μm. After drying at the temperature 55°C
during 12 hours, from the obtained sheets, negative electrodes 15 mm
in diameter were punched out. Then additionally, the electrodes were
dried at the temperature 95°C in a glass furnace (Buchi, Switzerland)
in conditions of dynamic vacuum. Now the ready-to-use electrodes
were brought into the argon-filled glove box (MBraun, Germany, O2
иH2O<0.1 ppm,). The obtained electrodes had the following pa-
rameters: 6.8 ±0.4 mgSLP30 cm−2average graphite loading, 80 μm
electrode thickness, 50% porosity.
In this study as positive electrodes, the electrodes
LiNi1/3Mn1/3 Co1/3O2(NMC111) were used. These electrodes
were produced from the following mixture: NMC111 powder (HED
NMC-111, BASF SE, Germany, BET surface area of 0.29 m2g−1);
Super C65 carbon black (Imerys Graphite & Carbon, Switzerland);
binder PVdF at the weight ratio 96:2:2 and N-methyl-2-pyrrolidone
(solid content 65 wt%). The active masses were mixed in the planetary
orbital mixer (Thinky, USA) during 16 minutes (2000 rpm) at atmo-
spheric pressure and other 2 minutes at 50 mbar. The obtained active
mass was blade-coated onto a porous Celgard separator (C480 Celgard
USA, thickness 20 μm, porosity 45%). The wet-film thickness made
150 μm. The positive electrodes and separator drying-up procedure
was the same as for the negative electrodes. From the obtained
sheets, the positive electrodes 15 mm in diameter were punched out.
Such construction of the electrodes gave a free electrolyte access
from both sides of the electrodes; besides, it ensured a free gases
diffusion into the cell head space, which was a necessary condition for
measurements with aid of the mass spectrometer.26,27 The obtained
electrodes had their average loading on the level 11±1mg
NMC cm−2.
In this study, the standard electrolyte LP57 (1 M LiPF6in EC:EMC,
3:7 by weight) was replaced with the electrolyte 1.5 M LiPF6in ethy-
lene carbonate (EC) as the same was done in the investigation.21 The
mixture of EC with LiPF6is a liquid at room temperature due to the
melting point depression caused by the addition of LiPF6. Firstly, this
replacement simplifies the electrolyte composition and hence facili-
tates an interpretation of gases obtained during the cells cycling as
in this case any cross-impact of EC and EMC is excluded. Secondly,
this electrolyte excludes signal fluctuations (on the oxygen channel
m/z =32) occurring from the transesterification of the linear carbon-
ate EMC.28–31 Thirdly, in virtue of the low pressure of the ЕC steams,
the background signals from the electrolyte are decreased by two or-
ders. All this results in improvement of the signal/noise ratio in the
mass spectrometer.32
It should be noted that linear carbonates decompose into gases
5 times less efficiently than EC.33 In our experiments, the amount
of gases generated using the standard electrolyte LP57 was about the
same as when using the electrolyte EC. This experimental result proves
that, in a standard electrolyte (LP57), mainly decomposes EC during
gases generation.
The Karl-Fischer titration (Titroline KF, Schott Instruments, Ger-
many) showed that the prepared electrolyte contained around 20 ppm
water.
On-line electrochemical mass spectrometry (OEMS).—After the
connection of the cell to the mass spectrometer system (QMA 410,
Pfeiffer Vacuum, Germany), the cell was purged by argon during
3 minutes (0.05 L min−1). This allowed avoiding contaminations from
the atmosphere of the glove box, inside of which the cell was as-
sembled. Before measurement conducting, the cell was kept at the
open circuit voltage (OCV) during 4 hours. This ensured a signal
equilibration.
A transformation of the measured ion currents (obtained with aid of
OEMS) into the concentrations was conducted using two calibrating
gases. Gas I was Ar with 2000 ppm H2,CO,O
2,andCO
2. Gas II was Ar
with 2000 ppm H2,C
2H4,O
2,andCO
2. By using the calibrating gases,
it is possible to calculated the gases concentrations quantitatively: H2
(m/z =2), C2H4(m/z =26), CO (m/z =28), O2(m/z =32) and CO2
(m/z =44) in the cell free space. All the currents of the mass signal
(IZ) were normalized to the ion current of the isotope 36Ar (I36 ), i.e.
(IZ/I36). This allowed avoiding signal fluctuations because of minor
changes of pressure/temperature.
Unlike all the rest gases studied in this investigation, CO has no
unique channel m/z. On the main signal CO (m/z =28) is superim-
posed by signals from C2H4(m/z =26) and CO2(m/z =44). That
is why in order to find out a signal matching only to CO (i.e. I28(CO)),
one needs subtracting contributions C2H4(m/z =26) and CO2(m/z =
44) from the signal of I28. These contributions were found out by the
way of pure gases C2H4and CO2passing through the OEMS cell and
registration of the resultant signals.28 The measurements showed that
the mass signal for CO (I28(CO)) could be calculated on the following
formula: I28(CO) =I28 - (1/0.63)·I26 - 0.14·I44.
Cells cycling.—The cell cycling was fulfilled on the following
scheme. The cells charge was executed in the mode CC/CV (constant
current/constant voltage) in the voltages range 2.6-4.2V (first group
of experiments) and in the voltages range 2.6-4.8V (second and third
groups of experiments). Charging was performed by the current 0.2C
with due account of the following theoretical capacities of the NMC
electrodes: 150 mAh/gNMC for the 4.2V cell and 190 mAh/gNMC for
the 4.8V cell.14 The step (application of the constant voltage (CV) at
the upper cutoff potential) was performed until reaching the limiting
current 0.05C. Discharging was performed in the mode CC (constant
current 0.2C was applied). It should be observed that the graphite
electrodes capacity is much higher than that of the NMC electrodes
(theoretical capacity of graphite electrodes makes 370 mAh/gSLP30
(Ref. 20)).
Results of Cells Cycling in the Voltage Range 2.6−4.2V at 25°C
In the first group of experiments, the evolution of gases when cells
were charged in the standard mode was studied. For this purpose,
four charge/discharge cycles of the experimental cell (Fig. 1) were
performed in the voltages range 2.6−4.2V at the temperature 25°C.
The cycling results are represented in the Fig. 2.
From the Fig. 2, it is seen that most intensively, the gases C2H4,
CO, H2are evolved during the first hour of charging. This isconnected
with sharp growth of the cell voltage from 2.6 V to 3.6V.
In the interval from the beginning of the charge first cycle to 4.0V,
a sharp growth of ethylene concentration is observed approximately
up to 10 μmol m−2SLP30, which corresponds quite well to the data of

Journal of The Electrochemical Society,166 (6) A897-A908 (2019) A899
2.5
3
3.5
4
4.5
Voltage (V)
010 20 30 40
0
2
4
6
8
10
12
Time (h)
Concentration( )
μmol m-2
SLP30
CH
24
H2
CO
CO2
(a)
(b)
Figure 2. (a) Voltages change during cycling of a cell SLP30//NMC111 at
25°C in the voltage range from 2.6 to 4.2V for a cell containing 120μLof
1.5 M LiPF6 in ethylene carbonate (EC). (b) Gases concentration in the cell
head space, normalized to the SLP30 BET surface area. The OEMS data are
smoothed and converted into units of [μmol m−2SLP30].
other experimental researches.34–36 The ethylene concentration growth
is a consequence of EC reduction during the process of the SEI layer
formation on the graphite electrode:20,35,37,38
2Li +C3H4O3(EC)→Li2CO3+C2H4[1]
and33,39,40
2Li +2C3H4O3(EC)→(CH2OCO2Li)2+C2H4[2]
or a consequence of SEI decomposition41
2Li +(CH2OCO2Li)2→2Li2CO3+C2H4.[3]
Upon reaching the voltage 4.0V, the sharp growth of the C2H4con-
centration stops, which is usually connected (in researchers’ opinion)
with the SEI layer formation end.20,21 However, as it is seen from the
Fig. 2, slowly, the C2H4concentration grows even farther at the cell
cycling. Notably it is going on independently on a cycling voltage,
i.e. the ethylene release is potential-independent. Usually the ethylene
concentration growth during the cell cycling is subjected to no analysis
at all as it is thought that this concentration growth lies in limits of an
experimental error.20,21 Nevertheless from the obtained experimental
data (Fig. 2), for C2H4concentration growth speed, we have the value
0.075 μmol m−2SLP30 h−1. This process will be discussed by us below
in this section.
Now let us consider hydrogen evolution on the Fig. 2. The con-
sideration is widely accepted20–25,34,44 that the hydrogen evolution in a
first cycle of charging till the voltage 4.0V reaching is connected with
a residual moisture removal from the electrolyte. Indeed according
to our experimental data (Fig. 2) in the first cycle, while the voltage
grows up to 4.0V, the hydrogen is evolved in amount of approximately
2.4 μmol m−2SLP30. With due consideration of the fact that at full de-
composition of one water molecule (on any mechanism), one molecule
of hydrogen is formed, i.e.
2H2O→2H2+O2,[4]
so for the water concentration in the electrolyte, we obtain the value
20.1 ppm. This value (in limits of the experimental error) matches
well to direct water concentration measurements in our electrolyte
(see the section Electrodes and electrolyte). In the calculations, the
following data were used: anode surface area on BET equal to 0.084m,2
electrolyte density 1.5 g mL−1,Ref. 45 electrolyte amount in the cell
120μL.
Later on during cycling, the hydrogen concentration grows and
moreover independently on a cell voltage, i.e. the hydrogen evolu-
tion is potential-independent. In the papers,20,21 it was experimentally
proved that this hydrogen evolution is connected with the electrolyte
decomposition. This problem will be studied by us in detail below
(see the section Mechanism of electrolyte decomposition and gases
evolution at lithium-ion cells cycling).
Let us give consideration to carbon dioxide evolution shown in
the Fig. 2. In the beginning of the charge first cycle, we observe CO2
evolution and then its consumption. This process will be discussed by
us below in this section.
An essential CO2evolution starts at cell charge voltage higher
than 4.0 V. In the papers,20,42,43,46–48 it was proved that at voltages
higher than 4.0 V on the cathode, there runs oxidation of Li2CO3with
CO2evolution. The authors of those papers note that the oxidation of
Li2CO3runs according to the following electrochemical reaction
2Li2CO3→2CO2+O2+4e−+4Li+.[5]
However with use of the reaction (Eq. 5), the authors did not come
to a quantitative correspondence with the experimental data on the
CO2release. Besides, both in their experiments43 and in our ones
(Fig. 2), there was not observed an oxygen evolution at these voltages
in compliance with the reaction (Eq. 5).
Below (see the section Mechanism of electrolyte decomposition
and gases evolution at lithium-ion cells cycling) it will be shown that
the evolving-on-cathode (because of Li2CO3decomposition (Eq. 5))
very reactive atomic oxygen interacts in situ with CO evolving-on-
cathode (because of electrolyte decomposition20), which results in
CO2formation, i.e.
CO +Oad →CO2[6]
Thus, with due account of the reactions (Eqs. 5and 6), the total
reaction of oxidation of Li2CO3on a cathode will look like as follows
Li2CO3+CO →2CO2+2e−+2Li+.[7]
It should be mentioned that in the paper,21 it was experimen-
tally proved that the adsorbed-on-cathode atomic oxygen is an ac-
tive participant of generation of gases CO2and CO at electrolyte
decomposition.
Based on the Equation 7, let us evaluate an amount of the re-
leased CO2. Earlier it was found that at the course of the cell cy-
cling after reaching the voltage 4.0V (in the first cycle), the amount
of C2H4grows slowly with the rate 0.075 μmol m−2SLP30 h−1. Hence,
during a total cycle of cell charge/discharge (10h) in the electrolyte,
will be dissolved Li2CO3in amount of 0.75 μmol m−2SLP30 (Eqs. 1–3).
For the same time, the total amount of Li2CO3additionally dissolved
in the electrolyte must get oxidized on the cathode according to the
Equation 7and CO2formation must take place. Hence, for the total
cycle, 1.50 μmol m−2SLP30 carbon dioxide must be evolved. This value
in the limits of the experimental error matches very well to the CO2
amount evolved within second, third and fourth cycles (Fig. 2). Thus,
the amount of CO2evolved on the second and next cycles has a strict
quantitative explanation based on the electrochemical reaction (Eq. 7).
However in the first cycle, the amount of evolved CO2is much
higher, approximately 2.4 μmol m−2SLP30 (Fig. 2). As it has been
shown earlier, on expense of the electrochemical reaction (Eq. 7)in
each cycle, the carbon dioxide is formed in the amount of 1.5 μmol
m−2SLP30. Hence, in the first cycle, the amount of additionally released
CO2is 2.4-1.5 =0.9 μmol m−2SLP30.
The first cycle differs from all the next cycles only with the fact
that within this cycle, residual moisture is removed from the elec-
trolyte. Therefore, on the cathode because of electrochemical decom-
position of the residual moisture, the atomic oxygen must be ad-
sorbed and additionally form CO2in accordance with the reaction
(Eq. 6). For a formation of the deficient amount of the carbon dioxide
(0.9 μmol m−2SLP30 based on the Equation 6), the oxygen is needed

A900 Journal of The Electrochemical Society,166 (6) A897-A908 (2019)
only in the amount of 0.45 μmol m−2SLP30. Thus, only 0.45 μmol
m−2SLP30 oxygen and not oxygen total amount (1.2 μmol m−2SLP30)
obtained from residual moisture decomposition takes part in the for-
mation of the deficient amount of CO2. The remaining oxygen amount
must participate in other chemical or electrochemical reactions.
An investigation in-detail of these chemical and electrochemical
reactions likewise of all the processes running in the beginning of the
first charge cycle is beyond the scope of this research. Nevertheless
in order to give a complete picture, we would like to speak out our
assumptions about the mechanism of these reactions.
In general at the present time, there is no full understanding of gases
evolution processes running in lithium-ion cells in the beginning of
the their first charge cycle. These processes were under investigation
in the papers.32,34 The authors put forward a hypothesis that in the
cells charge beginning, the evolution of CO2runs through electrolyte
decomposition on the anode (Fig. 2). Then CO2consumption goes
on expense of its reduction to the lithium oxalate.46,47 However, the
performed electrolyte study described in the paper49 showed that cells
cycling result in formation of neither lithium oxalate nor another prod-
uct of electrolyte decomposition except for the following gases: CO,
CO2,H
2and C2H4.
That is why taking our experimental studies experience as a guide,
we incline to the opinion that more probable is the other mechanism
of CO2evolution and consumption in the cell charge first cycle. In the
beginning of the charge first cycle, a chemical reaction must take place
between residual moisture and CO evolving on the anode20 (because
of electrolyte decomposition (Fig. 2)), which results in CO2formation,
i.e. in the water-gas shift reaction (WGS)Ref. 50
CO +H2O→CO2+H2.[8]
The role of catalysts for the reaction WGS can be played by carbonates
of alkali metals, in particular, Li2CO3(Refs. 51–54). The reaction runs
in a basic environment, which also must be formed on the anode in
presence of residual moisture55
2Li +2H2O→H2+2LiOH.[9]
Thus, the reaction (Eq. 8) must run on the anode SEI layer during the
cell charge first cycle as long as the electrolyte contains any residual
moisture. For the common reaction WGS in virtue of the low solubility
of CO in water, a high operating pressures are required for reaching of
at least moderately active process on the alkali metals carbonates.51–54
Meanwhile for the reaction (Eq. 8), such problem does not exist as CO
is formed on the anode just in the place of reaction running as a result of
the electrolyte decomposition.20 Our preliminary experimental studies
showed that in the first cycle beginning of the lithium-ion cell charge,
indeed on the anode, the reactions (Eqs. 8and 9) run. However, study
in detail of these reactions is beyond the scope of this paper and will
be a goal for our further investigations.
In sequel, with charge progress and lithium sufficient amount ac-
cumulation in the anode, the reaction must run of CO2reduction in
the interaction with the intercalated lithium56–58
2CO2+2Li++2e−→Li2CO3+CO.[10]
The reaction (Eq. 10) will consume already evolved CO2(in compli-
ance with the Fig. 2) and – in addition – form the SEI layer on the
anode.
Besides, the formed lithium hydroxide (Eq. 9) is a very effective
CO2absorbent, in connection with what it is used in systems of breath-
ing gas purification.59
2LiOH +CO2→Li2CO3+H2O [11]
Thus, the reactions (Eqs. 8,10,11) explain evolution (Eq. 8)and
consumption (Eqs. 10 and 11)CO
2in the very beginning of the cell
charge first cycle (Fig. 2).
The proposed mechanism explains all the experimental data given
in the Fig. 2on the CO2evolution in the beginning of the first cycle
of the cell charge/discharge.
Undoubtedly, the proposed mechanism requires additional re-
searches both experimental and theoretical. That is why in this paper,
2.5
3
3.5
4
4.5
5
Voltage (V)
010 20 30 40
0
5
10
15
20
25
Time (h)
Concentration ()
μmol m-2
SLP30
(b)
CO2
H2
CH
24
CO
Oх10
2
(a) 4.57V
Figure 3. (a) Voltage change during cycling of a cell SLP30//NMC111 at 25°C
in the voltage range from 2.6 to 4.8V for a cell containing 120μLof1.5M
LiPF6 in ethylene carbonate (EC). (b) Gases concentration in the cell head
space, normalized to the SLP30 BET surface area (O210-fold magnified). The
OEMS data are smoothed and converted into units of [μmol m−2SLP30].
the proposed mechanism of gases evolution (Eqs. 8–11) in the begin-
ning of the cell charge first cycle is to be considered as one of possible
hypotheses.
Let us consider the carbon monoxide evolution on the Fig. 2.In
a quite a number of papers,20,21,33 it was proved experimentally that
CO evolution at a cell charge is connected with the electrolyte decom-
position. This question will be studied by us in detail below (see the
section Mechanism of electrolyte decomposition and gases evolution
at lithium-ion cells cycling).
In the paper21 with use of 13C-labeled carbon, it was proved that
CO and CO2evolution is not connected with an oxidation of the car-
bon additive into the cathode. That is why the observed CO and CO2
evolution can be connected only with the electrolyte decomposition.
In conclusion, we would like highlighting the fact that in this group
of experiments, the concentrations ratio is CO2/CO =0.82. This eval-
uation will be of importance for the discussion of the electrolyte de-
composition mechanism.
Results of Cells Cycling in Voltages Range 2.6−4.8 V at 25°C
In the second group of experiments, there was studied, how the up-
per cutoff voltage influences gases evolution velocity.For this purpose,
four cell charge-discharge cycles were performed in the voltages range
2.6−4.8V at the temperature 25°C. The cycling results are represented
in the Fig. 3.
From the beginning of the first charge cycle and up to the voltage
4.0 V (Fig. 3), a sharp growth is observed of the ethylene concentration
up to 10.1 μmol m−2SLP30. This value differs not much from results
obtained in the first group of experiments and the results obtained in
the other researches.20,21,34–36 This is nothing to be surprised about
as the ethylene evolution is determined by the SEI layer formation
and must not depend heavily on the upper cutoff voltage. In sequel at
cell cycling, the C2H4concentration grows – potential-independently
– with the approximate speed 0.075 μmol m−2SLP30 h−1. Within the
discussion of the first group of experiments, it was shown that this C2H4
concentration growth was connected with the dissolution of Li2CO3
from the SEI layer and further restoration of this layer with C2H4
evolution according to the reactions (Eqs. 1–3).

Journal of The Electrochemical Society,166 (6) A897-A908 (2019) A901
Then the dissolved Li2CO3is oxidized on the cathode with parallel
CO2evolution according to the reaction (Eq. 7). It should be noted that
the C2H4concentration growth speeds in the first and second groups
of experiments are the same in limits of the experimental error. This is
just what should be expected as the C2H4concentration growth speed
must depend only on Li2CO3dissolution speed from the SEI layer,
which is determined only by properties of Li2CO3and the electrolyte
at the same temperature. Hence, it must not depend on a value of
the upper cutoff voltage, which indeed was shown by the experiment
represented in the Fig. 3.
Now let us give consideration to hydrogen evolution on the Fig. 3.
The hydrogen evolution from the beginning of the first charge cycle
up to reaching the voltage around 4.0 V is connected with the removal
of the residual moisture from the electrolyte.20–25,34,44 According to
our experimental data (Fig. 3), from the beginning of the first charge
cycle and up to reaching the voltage 4.0 V, the hydrogen is released
in amount of approximately 2.5 μmol m−2SLP30, which – in limits
of experimental error – coincides with the similar value in the first
group of experiments. Also this result should be expected as in both
groups of experiments, the same electrolyte was used with the same
residual moisture. But in this group of experiments as a result of the
four charge/discharge cycles, the hydrogen was evolved in the amount
of 26.2 μmol m−2SLP30, which is 2.5 times more than in the first group
of experiments.
Let us give look to the CO2evolution in the Fig. 3. From the Fig. 3,
it is seen that within the voltages range from 4.0 V to 4.57 V, the
CO2concentration grows slowly and almost in linear fashion. How-
ever starting with the voltage 4.57 V and up to the end of charging,
the speed of the concentration growth sharply increases. This fact is
an indication that the CO2evolution in these two voltages ranges is
connected with two different electrochemical processes. Here and now
we would like to attract readers’ attention that the CO2evolution in
this group of experiments at charge up to the voltage 4.57V is the
same as in the first group of experiments. Indeed, within the voltages
range from 4.0 V to 4.57 V, as it is known, the Li2CO3oxidation
runs on the cathode.20,42,43,48 In view of the fact that in these exper-
iments, the C2H4concentration growth speed is the same as that in
the first group of experiments, it follows that according to the reaction
(Eq. 7), the same amount of CO2will be evolved, i.e. approximately
1.50 μmol m−2SLP30. This value corresponds very well to the experi-
mental data (Fig. 3) at charge within the voltages range from 4.0V to
4.57V for second, third and fourth cycles of charge/discharge. In the
first cycle of charge in the voltages range from 4.0 V to 4.57 V, the car-
bon dioxide is released in amount approximately 2.4 μmol m−2SLP30
just as in the first group of experiments. As it was shown earlier, the
additional CO2evolution is connected with the residual moisture re-
moval according to the reactions (Eqs. 6and 8).
The coincidence of the evolved amounts of CO2in the first and
second groups of experiments (within voltage range from 4.0 V to
4.57 V) follows immediately from the two facts given below. Firstly,
in this voltages range, only one reaction takes place and this is the
reaction of Li2CO3oxidation on the cathode (Eq. 7)(Refs.42,43,48).
The reaction (Eq. 7) speed is determined only by Li2CO3dissolution
speed in the electrolyte and it does not depend on the cutoff voltage
at the same temperature. Secondly, the amount of residual moisture in
the electrolyte in both experimental groups is the same. However it is
exactly the amount of the residual moisture in the electrolyte, which
determines the amount of additionally evolved CO2in the first cycle
of charge according to the reactions (Eqs. 6and 8).
The fundamental difference in CO2evolution on the Fig. 2and
Fig. 3starts (at cell charge) from the voltage 4.57 V and to the charge
process end. From the Fig. 3, it is seen that after reaching the voltage
4.57 V, CO2evolution speed grows sharply. It is indication that from
this voltage value, a new electrochemical process starts, a result of
which is the CO2vigorous evolution. It should be observed that also
after reaching the voltage 4.57 V, the oxygen evolution starts (Fig. 3).
In the paper,21 the series of experimental proves were given, which
establish the following facts. Firstly, after reaching the voltage 4.57 V
on the cathode surface, the highly reactive atomic lattice oxygen starts
releasing from the cathode body. Secondly, the oxygen release is the
reason of the electrolyte decomposition and the gases CO2and CO
evolution.
Thus, the CO2evolution during cell charge in the voltages range,
from 4.57 V and up to the charging process end is connected with
the electrolyte decomposition, an active participant of which is the
atomic oxygen, releasing from cathode under these voltages. This pro-
cess will be studied in-detail below in the frame of the discussion of
the electrolyte decomposition mechanism (see the section Mechanism
of electrolyte decomposition and gases evolution at lithium-ion cells
cycling).
Let us give consideration to the O2evolution in the Fig. 3.Fromthe
Fig. 3, it is seen that the oxygen is released stepwise and besides, its
concentration is decreased continuously. In our opinion, the mecha-
nism of consumption of already released oxygen looks like as follows.
After the atomic oxygen release from the cathode, two ways appear
for the oxygen further reactions. Firstly, it can take part in the reaction
of the electrolyte decomposition. Secondly, it can recombine and exit
into the cell head space:
2Oad →O2.[12]
The oxygen obtained according to the reaction (Eq. 12) is measured
with aid of OEMS inside of the cell head space (Fig. 3). After charging,
the oxygen release from the cathode stops. Then the oxygen located in
the cell head space can return on the cathode (Fig. 1) and be adsorbed
on it (the reverse reaction (Eq. 12)). Then the adsorbed oxygen will
take part in the chemical reaction (Eq. 6), which consumes the oxygen
and decreases its concentration in the cell head space. Besides, in
each charge cycle, the amount of the released oxygen (i.e. oxygen
concentration top peaks on the Fig. 3) also falls. According to the
researches,21 this is connected with the oxygen exit from the cathode
superficial layer and its structure change (from the layered structure
to the spinel structure or rock-salt structure). Such structure contains
much less oxygen. That is why in the subsequent cycles, it gets much
harder for the oxygen to leave the cathode with increase of the oxygen-
poor superficial layer.
In conclusion, we would like to highlight several features of the
composition of the released gases mixture in this group of experi-
ments. Firstly, the concentrations ratio is CO2/CO =2.4. Secondly,
the amount of the released gases CO2+CO in this group of experiments
is 2.6 times more than in the first group of experiments. Hence, the
intensity of the electrolyte decomposition sharply grows at increase of
the upper cutoff voltage, above the voltage at which the atomic oxygen
starts releasing from the cathode.
Results of Cells Cycling in Voltages Range 2.6−4.8 V at 60°C
In the third group of experiments, the temperature impact was stud-
ied on the gases evolution. For the clarification of this question, four
cell charge/discharge cycles were performed in the voltages range
2.6−4.8 V at the temperature 60°C. The cycling results are repre-
sented in the Fig. 4.
From the beginning of the first charge cycle and up to the voltage
3.65 V, a sharp growth is observed of the ethylene concentration – up to
23.5 μmol m−2SLP30, which is 2.3 times more than it be in the second
group of experiments during the formation of the SEI layer. Hence
at the temperature 60°C, the formed SEI layer is 2.3 times thicker
than at the temperature 25°C. Afterwards at further cell cycling, the
concentration C2H4grows (potential-independently) with the speed
0.61 μmol m−2SLP30 h−1, which 8.2 times more than at the temperature
25°C. As it is known,60–62 the standard electrolyte LP57 is not able to
form the stable SEI layer at high temperatures without use of additives
(such as LiBOB). This is exactly, what this experiment shows.
Now let us consider hydrogen release in the Fig. 4.Fromthebe-
ginning of the first charge cycle and up to reaching the voltage 4.0V, a
sharp growth of hydrogen concentration is observed – approximately
up to 5.5 μmol m−2SLP30, which is 2.2 times more than in the second
group of experiments. In all the experiments, the same electrolyte was
used by us. Hence in this experiment already on the stage of the sharp

A902 Journal of The Electrochemical Society,166 (6) A897-A908 (2019)
2.5
3
3.5
4
4.5
5
Voltage (V)
0 9 18 27 36
0
5
10
15
20
25
30
35
40
Time (h)
Concentration( )μmol m-2
SLP30
CH
24
H2
CO2
CO
Oх10
2
4.57V
(a)
(b)
Figure 4. (a) Voltage change during cycling of a cell SLP30//NMC111 at 60°C
in the voltage range from 2.6 to 4.8V for a cell containing 120μLof1.5M
LiPF6 in ethylene carbonate (EC). (b) Gases concentration in the cell head
space, normalized to the SLP30 BET surface area (O210-fold magnified). The
OEMS data are smoothed and converted into units of [μmol m−2SLP30].
hydrogen concentration growth, the processes take place of not only re-
moval of residual moisture from the electrolyte but also the decomposi-
tion of the electrolyte itself. Then at cell cycling, the H2concentration
grows potential-independent. Total in the four charge/discharge cy-
cles, the hydrogen was released in the amount of 35.0 μmol m−2SLP30,
which is 1.3 times more than in the second group of experiments. This
fact is an indication of an intensification of the electrolyte decompo-
sition processes with the temperature increase up to 60°C.
Let us give consideration to the CO2evolution in the Fig. 4.The
amount of the evolved CO2in the voltagesrange of the electrochemical
reaction (Eq. 7) (from 4.0 V to 4.57 V (Fig. 4)) is approximately
the same that we see in the second group of experiments (Fig. 3).
The velocity of the electrochemical reaction (Eq. 7) is determined by
Li2CO3solubility in the electrolyte. Hence the Li2CO3solubility in
the electrolyte is not too much change in the temperatures range from
25°C to 60°C.
In the voltages range corresponding to the electrochemical reaction
of electrolyte decomposition (running at voltages from 4.57 V up to
charging session end with aid of the released-from-cathode reactive
atomic oxygen), the amount of the released gas CO2is approximately
the same as in the experiments given in the Fig. 3. Hence also the
general amount of the released gas CO2in this group of experiments
(29.1 μmol m−2SLP30 (рис. 4)) is approximately the same as in the
second group of experiments (Fig. 3). This is connected with the fact
that in the second and third groups of experiments, the amount of the
released-from-cathode oxygen is also approximately the same (Figs. 3
and 4). Below (see the section Mechanism of electrolyte decomposi-
tion and gases evolution at lithium-ion cells cycling) it will be shown
that the reactive atomic oxygen is the reason of the CO2evolution.
That is why the amount of the released gas CO2is determined in full
by the amount of the released-from-cathode atomic oxygen.21 Hence
the velocity of the atomic oxygen diffusion from the cathode does not
change much at a cell temperature growth – from 25°C to 60°C.
In conclusion, we would like to highlight several features of gases
evolution in this group of experiments. Firstly, the concentrations ratio
is CO2/CO =1.2. Secondly, the amount of the released gas mixture
CO2+CO (53.9 μmol m−2SLP30) in this group of experiments exceeds
1.3 times the same in the second group of experiments. Hence the
intensity of the electrolyte decomposition grows with the cell temper-
ature increase.
Reasons of Potential-Independence of Gases Evolution at
Lithium-Ion Cells Cycling
An electrochemical decomposition of any electrolyte can take place
only within a period of cell charge. Hence, if in the course of the elec-
trolyte decomposition, gases are released, they are supposed to be
released only over the period of the cell charge. However in our ex-
periments (Figs. 2–4) all through the cycling, some gases (H2,C
2H4)
were released during both cell charge and discharge, i.e. in the po-
tential independent manner. Hence, in the experiments (Figs. 2–4)
onto the classical process of the electrolyte decomposition, an addi-
tional process is overlapped, which changes the pattern of the gases
evolution.
The purpose of this group of experiments is to study this unusual
phenomenon.
This phenomenon was also experimentally established in the
papers.20,21,28,34 In the paper,20 the authors take the view that the ap-
parent potential-independence of the H2evolutioniscausedbythe
rate-limiting reduction of protic electrolyte decomposition species on
anode. However the rate-limiting reduction of protic species on anode
can result only in the fact that on the classical steplike curve of the
gases evolution at cell cycling, the steps will become lower and less-
pitched. This limitation cannot lead to the hydrogen evolution on the
stage of a cell discharge as protic species are reduced on anode with
the H2evolution only on the stage of cell charge. However the experi-
ments given in the Figs. 2–4show that hydrogen is released on stages
of cell charge and discharge, i.e. completely potential-independently.
Prior to a discussion of the potential-independence of the gases H2
and C2H4evolution in the Figs. 2,3, let us note that CO2is released
only on cathode20 during cell cycling. Notably, the CO2evolution
on cathode has the classical stepwise pattern, while the gases H2and
C2H4, which are released only on anode,20 have potential-independent
nature of their evolution. The carbon monoxide is released on both an-
ode and cathode.20 Along with it, its evolution has partially potential-
independent and partially stepwise pattern (Figs. 2and 3). From this
fact, the conclusion can be drawn that all the gases released on cath-
ode are featured with the stepwise pattern of their evolution, while
for the gases released on anode the potential-independent pattern of
their evolution is typical. Hence, the potential-independent pattern of
gases evolution is connected not with properties of the gases evolution
process (as the authors of the paper20 think) but instead with anode
properties.
An active mass of anode, on which gases are released in potential-
independent manner, is represented by a finely dispersed powder of
graphite. As it is known, the finely dispersed powder of graphite is
able to adsorb gases in abundance.63 That is why we take the view that
the ability of the finely dispersed powder of graphite to absorb gases
ensures the potential-independent pattern of their evolution. Indeed,
at a cell charge, when electrolyte decomposition and gases evolution
take place, a part of the gases released on anode will be adsorbed in
pores of the graphite powder, while the other part of the gases will
exit into the cell head space and be measured by OEMS. While a
cell is discharging (electrolyte does not decompose and gases are not
generated), the accumulated in graphite pores gases will exit into the
cell head space due to the diffusion processes from the porous graphite.
Hence in the case of gases adsorption on anode, their concentration
must rise in the cell head space on both stages of charge and discharge,
i.e. potential-independently.
In the paper,64 it was shown that in the case of pure carbon-based
materials, it is impossible to reach a high gravimetric capacity of hy-
drogen accumulation. An increase of the gravimetric capacity of hy-
drogen accumulation in carbon-based materials is possible only either
by way of their structure change (in particular by way of reducing the
pore size65,66) or by way of doping of carbon-based materials.67

Journal of The Electrochemical Society,166 (6) A897-A908 (2019) A903
In the case of graphite, a considerable growth of gravimetric capac-
ity, the hydrogen accumulation takes place in the case of use of finely
dispersed powders, which can be obtained by milling in ball mills.68–70
Any imperfections of crystalline structure (particularly dislocations)
are traps for hydrogen, as they decrease the energy of hydrogen atom
as compared to their location in normal interstice. Besides they are the
centers of hydrogen absorption, and also contribute to hydrogen pen-
etration into the graphite depth. Hence, imperfections of the graphite
crystalline structure cause sharp rise of hydrogen adsorption on it. It
should be noted that for manufacturing of anodes of lithium-ion cells,
exactly finely dispersed powders of graphite are used.
In quite a number of papers, it was shown that doping of
carbon-based materials by alkaline or transitional metals results in
a considerable growth of their gravimetric capacity of hydrogen
accumulation.67,68,71 This is connected with a general energy rise of hy-
drogen binding with the doped graphite powder.71 It should be noted
that on anode, the graphite powder is doped by lithium. To the full
extent, the said has relation also to other gases released on anode.
However, the hydrogen cannot only adsorb on the graphite powder
surface but also intercalate inside of the graphite and be accumulated
there in abundance.72–74 The ability of the hydrogen to be accumulated
in electrodes is proved also by the studies,24,25 in which it was shown
that the hydrogen was released at lithium-ion batteries storage in their
charged state.
If our assumptions are correct, at both cell discharge and charge
before reaching of voltage value of electrolyte decomposition (4.57 V),
hydrogen is not supposed to be generated. Hence, it can get in the
cell head space (and augment the H2concentration there) only due
to its diffusion from the graphite powder pores. Hence, if (after cell
charge completion) to interrupt the cycling and let hydrogen leave the
graphite powder, then at subsequent discharge&charge to the voltage
value 4.57 V, the hydrogen concentration is not supposed to rise in
the cell head space. With this purpose in mind, there were executed
four cell charge/discharge cycles in the range of voltages 2.6-4.8 V at
the temperature 25°C. Notably, after second and third charge cycles,
a pause was done for 10 h (equal to the cycle duration). This pause
is enough so that the hydrogen diffusion process from the graphite
powder into the cell head space had time to be completed. The results
of the cycling are represented in Fig. 5.
The conducted experimental studies prove categorically that the
potential-independence of evolution of the gases (H2,C
2H4and CO)
on anode is connected with the gases adsorption in the pores of anode
graphite powder. It should be noted that in our previous paper75 by
direct experiments, it was proved that during lithium-ion cell cycling,
the hydrogen is accumulated in anode graphite powder.
Mechanism of Electrolyte Decomposition and Gases Evolution at
Lithium-Ion Cells Cycling
Before establishing the mechanism of the electrolyte decomposi-
tion and gases evolution at lithium-ion cells cycling, let us (based on
the obtained-to-the-date experimental data) put in words criteria for
which this mechanism must meet.
1. In the experiments conducted in the frame of the study,20 the Li+-
ion conductive glass ceramic was used, which was impermeable
for gases generated in cathode and anode departments of a cell.
This allowed – during cell cycling – analyzing generated gases
separately in cathode and anode departments. As a result, it was
proved that at cycling of lithium-ion cells and electrolyte decom-
position, the following gases are released: on cathode CO2and
CO, while on anode H2and CO. Besides, on anode, ethylene is
released, too, but in the paper,20 it was proved that the C2H4evolu-
tion is connected with formation of SEI layer and has no relation
to the electrolyte electrochemical decomposition.
2. Besides, the use of the Li+-ion conductive glass ceramic allowed
proving20 that in general, the electrolyte decomposition is deter-
mined by the interaction between the processes on cathode and
anode, which often is referred to as “crosstalk”.76,77 It means that
2.5
3
3.5
4
4.5
5
Voltage (V)
010 20 30 40
0
5
10
15
20
25
Time (h)
Concentration ( )
μmol m-2
SLP30
CO2
H2
CH
24
CO
break (10 h)
break (10 h)
(a)
(b)
Figure 5. (a) Voltage change during cycling of a cell SLP30//NMC111 at 25°C
in the voltage range from 2.6 to 4.8V (with 10h-long pause after the charge
process completion in the second and third cycles) for a cell containing 120μL
of 1.5 M LiPF6 in ethylene carbonate (EC). (b) Gases concentration in the cell
head space, normalized to the SLP30 BET surface area. The OEMS data are
smoothed and converted into units of [μmol m−2SLP30].
the electrolyte is oxidized on cathode and afterwards the oxy-
genated ions of the electrolyte move to anode and are reduced on
it. Upon that, the gases evolution rises sharply on both anode and
cathode as compared to cells, where the anode and cathode de-
partments are separated by the Li+-ion conductive glass ceramic.
3. At electrolyte oxidation on cathode, also the acidity grows of
the surrounding electrolyte.20,78 In the paper,78 it was shown ex-
perimentally that at the electrolyte oxidation on cathode, a high
concentration of protons is generated on the cathode surface and
the protons promote a corrosion of aluminum current collectors.
Along with it the protons motion to anode together with the Li+
ions reduces the coulombic efficiency at cycling.76,78
4. In the paper,21 a number of experimental proves was given that
the release from cathode of highly reactive atomic oxygen is the
main reason of the electrolyte decomposition and the CO2and CO
evolution. Firstly, in all the experiments,21 the CO2and CO evolu-
tion occurs only after that the oxygen evolution can be observed.
Secondly, on these or that cathodes (of different composition), the
oxygen is released at less or higher voltage values than 4.57 V;
then at the same voltages, the electrolyte is decomposed and the
gases CO2and CO are released.21,32 More to it, if the atomic oxy-
gen is released in a less amount, then equally less will be the
evolution of CO2+CO (Ref. 21).
Here it should be noted that the chemical reaction between oxygen
and ЕC (ethylene carbonate) at room temperature is possible only in
the case that the oxygen is in its highly reactive form, for example, in
the form of the atomic oxygen21 as ЕC does not decompose in dry air
at working temperature of the lithium-ion cells.
From this, it follows that the electrolyte decomposition is a com-
plicated electrochemical and chemical process, a very important stage
of which is one of the chemical oxidation of the electrolyte by very
reactive atomic oxygen.
Also in the paper,21 it was proved that due to the purely electro-
chemical process of the electrolyte decomposition (without the stage
of the chemical oxidation of the electrolyte by the atomic oxygen), not

A904 Journal of The Electrochemical Society,166 (6) A897-A908 (2019)
more than 10% of the entire gas CO2+CO obtained in the experiments
can be generated.
5. In the paper49 with aid of the method GC-MS (Gas
chromatography–mass spectrometry), the electrolyte of lithium-
ion cells was examined after a long cycling. It was found out
that in the electrolyte, no new components occur connected with
the electrolyte decomposition except for the gases H2,CO
2,CO.
Hence, as a result of cells cycling, the electrolyte decomposes in
full onto the gases H2,CO
2and CO. To the same conclusion, we
came, too. We obtained it with the method GC-MS, while study-
ing various electrolytes used in lithium-ion cells after them long
cycling.
6. The ratio CO2/CO can vary (at the electrolyte decomposition as a
result of cells cycling in the wide range from 0.82 to 2.4 (Figs. 2–4
and (Refs. 20,21)) depending on temperature and the upper cutoff
voltage.
Let us start our discussion of the electrolyte decomposition mecha-
nism at cells cycling from consideration of the last criterion. Only two
ways are possible, in which the ratio CO2/CO would vary considerably
in dependence with a temperature and an upper cutoff voltage.
Firstly, it is possible to make an assumption that two groups of
concurrent electrochemical reactions exist. In the first group of the
reactions, mainly CO2is generated, while in the second group, mainly
CO is generated. Thus one can make an assumption that in dependence
with a temperature and an upper cutoff voltage, one of the reactions
groups receives an advantage and the ratio CO2/CO changes much.
However if to look at the formula of our electrolyte – C3H4O3(EC), –
it is seen that it is impossible to write two different overall reactions for
two electrochemical processes of the electrolyte (EC) decomposition
so that as a result of one of them, mainly gases CO2and H2would be
generated, while as a result of the second one, mainly gases CO and
H2would be generated. It should be noted that at the electrolyte (EC)
decomposition, no other components are supposed to be generated
except for the gases CO, CO2and H2(the criterion 5). Thus, this way
of the electrolyte decomposition is impossible.
Secondly, it is possible to make an assumption that there exists
only one electrochemical reaction of the electrolyte decomposition
and along with it, the CO2formation is a consequence of an additional
chemical reaction running between the releasing from cathode of the
reactive atomic oxygen (criterion 4) and the releasing – at the same
place on cathode – of the gas CO (because of the electrolyte decom-
position) (Eq. 6). In this case, it is unambiguously possible to write
the overall reaction for the electrochemical process of the electrolyte
(EC) decomposition meeting all the mentioned above criteria and this
can be done as follows:
C3H4O3→3CO +2H2.[13]
As it was highlighted earlier based on the studies specified in the cri-
terion 4, the process of the electrolyte decomposition is a complicated
multistep process running on cathode and maybe also on anode. Never-
theless based on the overall reaction (Eqs. 13) and on compliance with
all the criteria, for sure, it is possible to write the total electrochemical
reactions running separately on cathode and on anode.
2CH
2
O
CH
2
C
O
O2CH
2
O
C
+
O+ 2CO + 4H
+
+ 6e
-
(cathode)
[14]
2CH2
O
C
+
O+ 2e
-
CH2O
CH2C
O
O+ CO
4 H+ + 4 e-2 H2
(anode)
[15]
The electrochemical reactions (Eqs. 14 and 15) (with due account
of the reaction (Eq. 6)) meet the criteria 1–3,5,6. As a result of these
electrochemical reactions, the electrolyte decomposes and only the
gases H2,COandCO
2are formed (with due account of the reaction
(Eq. 6)) (criterion 5). On anode, the gases H2and CO are released,
while on cathode the gases CO and CO2(with due account of the
reaction (Eqs. 6)) (criterion 1). As a result of the electrolyte decompo-
sition, its acidity rises (criterion 3). In general, the processes running
on cathode and anode are interdependent (criterion 2).
However, as it was shown in the paper,21 a direct electrochemi-
cal decomposition of electrolyte (Eqs. 14 and 15) is unlikely and it
is able to make only a small contribution into the general process of
the electrolyte decomposition. Much more intensively, the process of
the electrolyte decomposition runs, when prior to the electrochemical
stage (Eqs. 14 and 15), the chemical process takes place of the elec-
trolyte oxidation by the very reactive atomic oxygen released from
cathode.21 In this case, the overall reaction (Eqs. 13) must be re-write
in the form:
C3H4O3+2Oad →3CO +2H2O,[16]
as the very reactive atomic oxygen released from cathode can be
reduced only by the hydrogen coming from the electrolyte, which
promotes the electrolyte (EC) decomposition. In this case, the electro-
chemical reactions (Eqs. 14 and 15) will accept the following form:
2CH
2
O
C
+
O+ 2CO + 2H
2
O + 2e
-
2CH
2
O
CH
2
C
O
O + 2O
ad
,(cathode
[17]
2CH
2
O
C
+
O + 2e
-
CH
2
O
CH
2
C
O
O+ CO
.(anode
[18]
The resulted from the electrolyte decomposition water will be decom-
posed due to the additional electrochemical reaction (see below).
As because of the electrolyte decomposition its acidity must grow
(criterion 3), the water is supposed to be decomposed on the acidic-
type mechanism:
2CO +2H2O→4H++2CO2↑+4e−(cathode)[19]
4H++4e−→2H2↑(anode)[20]
Upon that the H+ions motion to anode together with the Li+ions
reduces the coulombic efficiency at cycling according to the experi-
mental results represented in the paper.76
From a comparison between the electrochemical reactions (Eqs. 17
and 18) and (Eqs. 14 and 15), it becomes evident that running of
the electrochemical reactions (Eqs. 17 and 18) is more energetically
profitable than running of the electrochemical reactions (Eqs. 14 and
15). Thus in the case of the atomic oxygen release from the cathode,
in general, oxygen-type mechanism of the electrolyte decomposition
must take place, i.e. the reactions (Eqs. 17–20). These reactions meet
all the described above criteria 1–6. However in the case of atomic
oxygen absence on cathode, the direct mechanism of the electrolyte
decomposition is possible, too (criterion 4), i.e. the reactions (Eqs. 14
and 15). Also these reactions meet all the criteria 1–6. With the aid of
the reactions (Eqs. 13–20), it is possible to explain all the experimental
results obtained in this paper and other papers.20,21
Discussion of the First Group of Experiments (Fig. 2)
The main purpose of this section is to explain the results of the first
group experiments (Fig. 2) quantitatively based on the established
mechanism of electrolyte decomposition (Eqs. 7,13–20).
First of all, let us evaluate the ratio of the released gases
(CO2+CO)/H2based on the experimental data given in the Fig. 2.
According to both the direct mechanism of electrolyte decomposition

Journal of The Electrochemical Society,166 (6) A897-A908 (2019) A905
(Eqs. 13–15) and the oxygen-type of the same (Eqs. 16–20), this ra-
tio should be equal to 1.5. While estimating the ratio (CO2+CO)/H2,
one should take into consideration only the gases released as a result
of the electrolyte decomposition. Besides, as a part of the released
gases are adsorbed on anode (Fig. 5), an estimation of the ratio must
be performed only when the adsorbed gases would leave the anode.
Let us perform this estimation at the charge voltage 4.0 V in the last
cycle. According to the investigations described in Figure 5, from this
voltage level, the electrolyte decomposition starts and on anode the
least possible amount of the adsorbed gases stays.
As for hydrogen, from the total amount of the hydrogen re-
leased before reaching the charge voltage 4.0 V in the fourth cycle
(9.0 μmol m−2SLP30.seeFig.2), it is needed to subtract the hydrogen
amount released because of the decomposition of a residual moisture
(2.4 μmol m−2SLP30,seeFig.2). Hence, only due to the electrolyte
decomposition, there was released hydrogen in amount of 9.0-2.4 =
6.6 μmol m−2SLP30.
Now let us evaluate the amount of the gases CO2+CO released
only due to the electrolyte decomposition. For the reaction (Eq. 7), a
half of CO2amount is generated due to the electrochemical decom-
position of Li2CO3(Eq. 5), while the second half of CO2amount is
generated due to the electrolyte decomposition with CO subsequent
oxidation on cathode under action of the atomic oxygen from the re-
action (Eqs. 5), i.e. it is generated on the oxygen-type mechanism of
the electrolyte decomposition (Eqs. 17–20). Hence, from the CO2to-
tal volume released within the cycle (1.5 μmol m−2SLP30,seeFig.2),
only a half is released due to the electrolyte decomposition. That is
why from the total amount of the gases CO2+CO released before
reaching the charge voltage 4.0 V in the fourth cycle (13.0 μmol
m−2SLP30,seeFig.2), it is needed to subtract the CO2volume obtained
due to the Li2CO3decomposition in the three previous cycles, i.e.
1.5·3/2 =2.25 μmol m−2SLP30. Besides, the additional carbon dioxide
in the amount of 0.9 μmol m−2SLP30 is obtained because of the resid-
ual moisture removal (see the section Results of cells cycling in the
voltage range 2.6-4.2V at 25°C). Also this carbon dioxide amount is to
be subtracted from the total amount of CO2. Hence, only the released
due-to-the-electrolyte-decomposition amount of the gases CO2+CO
is equal to 13.0 -2.25-0.9 =9.85 μmol m−2SLP30.
As a result for this ratio, we obtain the value (CO2+CO)/H2=
1.492. This value coincides with the theoretical value 1.5 in the limits
of relative experimental error (less than 1%). Thus the proposed elec-
trolyte decomposition mechanism (Eqs. 7,13–20) gives a very good
correspondence between the experimental data and the theoretical
calculations.
Now let us give consideration to the experimental ratio
CO2/CO≈0.82 for the released gases CO2and CO within all the four
cycles. According to the data given in the Fig. 2during all four cy-
cles of cell charge/discharge, the gas CO2was released in amount
of 6.9 μmol m−2SLP30, while CO of 8.4 μmol m−2SLP30 . According
to the studies, due to the electrochemical decomposition of Li2CO3
(Eq. 5) over the period of four cycles, the carbon dioxide was re-
leased in amount of 1.5·4/2 =3μmol m−2SLP30 (see the section
Results of cells cycling in the voltage range 2.6-4.2V at 25°C). The
same amount of CO2(3 μmol m−2SLP30) was released on the oxygen-
type mechanism (Eqs. 17–20) due to the electrolyte decomposition
with the subsequent CO oxidation (on cathode) by the atomic oxygen
from the reaction (Eq. 5). Besides, it was shown that the additional
0.9 μmol m−2SLP30 carbon dioxide is released due to the residual mois-
ture removal according to the reaction (Eq. 19) and possibly to the
reaction (Eq. 8) (see the section Results of cells cycling in the voltage
range 2.6-4.2V at 25°C). Therefore, in accordance with the calcula-
tions, the total amount of released gas CO2should be 3+3+0.9 =
6.9 μmol m−2SLP30, which is fully consistent with the experimental
data.
Simultaneously with the release of CO2on cathode, on anode on
the oxygen-type mechanism, the CO amount must be released twice
less (Eqs. 17–20) than that of CO2i.e. 1.5 μmol m−2SLP30. Hence, the
remaining carbon monoxide in amount of 8.4-1.5 =6.9 μmol m−2SLP30
is released due to the direct mechanism of electrochemical electrolyte
decomposition (Eqs. 14 and 15). Indeed, at cells cycling until the upper
cutoff voltage (4.2V at 25°C), the atomic oxygen from cathode is not
released (Fig. 2and (Ref. 21)). Hence, an electrolyte decomposition
on the oxygen-type mechanism (Eqs. 17–20) is impossible.
Thus, in the first group of experiments on the oxygen-type
mechanism (Eqs. 17–20), only a small amount of gases CO2
(3 μmol m−2SLP30) and CO (1.5 μmol m−2SLP30 ) is released using
oxygen from the reaction (Eqs. 5). The main amount of CO (6.9 μmol
m−2SLP30) is released due to the direct mechanism of electrochemi-
cal electrolyte decomposition (Eqs. 14 and 15). That is why the ratio
CO2/CO≈0.82 (Fig. 2) has a rather small value.
Discussion of Second Group of Experiments (Fig. 3)
The main purpose of this section is to explain the results of the sec-
ond group experiments (Fig. 3) quantitatively based on the established
mechanism of electrolyte decomposition (Eqs. 7,13–20).
Now we’ll give consideration to cells cycling up to the upper limit
voltage of 4.8 V at 25оC(Fig.3). Prior to the charge voltage 4.57 V
reaching (in the first cycle), the gases evolution is the same as in the
experiments of the first group (Fig. 2). After the voltage value 4.57 V
is reached, the release starts of the atomic oxygen from cathode (Fig. 3
andRef. 21). Hence, after the voltage 4.57 V, the electrolyte decompo-
sition is supposed to run on the oxygen-type mechanism (Eqs. 17–20).
As a result, the portion of CO2rises sharply in the total balance of the
gases CO and CO2; so the ratio CO2/CO becomes equal to 2.4 (Fig. 3).
Let us estimate several ratios for the gases released due to the
electrolyte decomposition. First of all, let us evaluate the ratio of the
released gases (CO2+CO)/H2based on the experimental data given
in the Fig. 3. According to Fig. 3(prior to the charge voltage 4.0 V
reaching in the fourth cycle), the hydrogen was released in the amount
of 22.58 μmol m−2SLP30, while CO+CO2in amount of 33.3 μmol
m−2SLP30. It should be noted that the amount of hydrogen released due
to residual moisture decomposition in this group of experiments is the
same as in the first group of experiments, i.e. 2.4 μmol m−2SLP30.This
is because the same electrolyte was used (see the section Results of
cells cycling in voltages range 2.6-4.8 V at 25°C). Hence, only due to
the electrolyte decomposition, hydrogen was released in the amount
of 22.58-2.4 =20.18 μmol m−2SLP30.
Now let us estimate the amount of gases CO+CO2(prior to the
charge voltage 4.0V reaching in the fourth cycle) released only due
to the electrolyte decomposition. In this group of experiments, the
amount of CO2released due to the electrochemical decomposition of
Li2CO3(Eq. 5) (in the first three cycles) is the same as in the first
group of experiments (i.e. 1.5·3/2 =2.25 μmol m−2SLP30)asitis
determined by the solubility of Li2CO3(which in the experiments of
the first and second groups is the same) and it does not depend on
an upper cutoff voltage (Fig. 3). Besides, the amount of CO2released
due to the residual moisture removal is also approximately the same
(0.9 μmol m−2SLP30) because the same electrolyte was used (see the
section Results of cells cycling in voltages range 2.6-4.8 V at 25°C).
Hence, only due to the electrolyte decomposition, the gases CO2+CO
were released in the amount of 33.3-2.25-0.9 =30.15 μmol m−2SLP30.
As a result for this ratio, we obtain the value (CO2+CO)/H2
=1.494 which corresponds very well to the theoretical value 1.5
(Eqs. 16–20) (the relative experimental error is less than 1%). Hence,
also in this group of experiments, the proposed mechanism of the elec-
trolyte decomposition (Eqs. 7,17–20) shows a very good correspon-
dence between the experimental data and the theoretical calculations.
Second, let us estimate ratios for the gases released due to the
electrolyte decomposition in the first and second groups of experi-
ments. With due account of the obtained data, the ratio between the
amount of CO+CO2released only due to the electrolyte decompo-
sition in the second and first groups of experiments (prior to reach-
ing the charge voltage 4.0 V in the fourth cycle) will be equal to
(CO+CO2)2/(CO+CO2)1=3.061, while the ratio for the amount of
releasing hydrogen will be equal to (H2)2/(H2)1=3.058. From com-
paring these ratios, it is possible to draw a number of conclusions.
Firstly, at growth of an upper cutoff voltage up to 4.8 V, the rate of

A906 Journal of The Electrochemical Society,166 (6) A897-A908 (2019)
the electrolyte decomposition grew three times. Secondly, the amount
of the gases CO+CO2and H2grew by the same value (in the limits
of experimental error). This is possible only in the case that in the
experiments of the first and second groups, the same chemical com-
pound was decomposed onto the same gases CO and H2. Meanwhile
the considerable difference of the ratio CO2/CO in the experiments
of the first and second groups is the result of the additional chemical
reaction (Eq. 19).
Exactly this mechanism of the electrolyte decomposition at
lithium-ion cells cycling is proposed by us based on the electrochem-
ical and chemical reactions (Eqs. 7,13–20). Thus, this experimental
result is one more confirmation of the proposed mechanism of the
electrolyte decomposition during cells cycling.
Third, let us consider more in detail the experimental ratio CO2/CO
=2.4. It is possible to make an assumption that in this group of exper-
iments, the electrolyte decomposition runs on the oxygen-type mech-
anism (Eqs. 17–20). Therefore, according to the oxygen-type mecha-
nism of the electrolyte decomposition (Eqs. 17–20), the ratio CO2/CO
must be equal to two.
As a result of the experiments (Fig. 3) for the four cycles of cell
charge/discharge, the carbon dioxide was released in the amount of
28.6 μmol m−2SLP30, while the сarbon monoxide of 12 μmol m−2SLP30 .
The сarbon monoxide is released only due to the electrolyte decom-
position (Eqs. 13–20). However, the gas CO2is released not only due
to the electrolyte decomposition but instead also due to the decompo-
sition of Li2CO3. (It is released in the amount of 1.5·4/2 =3μmol
m−2SLP30 over the period of four cycles). Besides, a part of the released
on cathode CO is transformed to CO2due to the residual moisture
removal from the electrolyte according to the formula (Eq. 19)and
maybe (Eq. 8). In the sections (see description of the first and second
group of experiments (Figs. 2–3)), it was shown that thus additionally,
the carbon dioxide is generated in the amount of 0.9 μmol m−2SLP30.
Hence, only due to the electrolyte decomposition, there was released
the carbon dioxide in the amount of 28.6-3-0.9 =24.7 μmol m−2SLP30.
Thus only due to the electrolyte decomposition, we obtain the fol-
lowing value for the ratioя:CO
2/CO≈2.058. This value coincides with
the theoretical value 2.0 in the limits of experimental relative error 3%.
Hence, in this group of experiments, the electrolyte decomposition
runs only on the oxygen-type mechanism (Eqs. 17–20). To the same
conclusion also, it is possible to come via analysis of the results of
the experiments given in the Fig. 3. After reaching the voltage 4.57V,
the release starts of atomic oxygen from cathode; and in this case, the
oxygen-type mechanism of the electrolyte decomposition is realized
(Eqs. 17–20). Besides, also the released oxygen (according to Fig. 3)
enters into the cell head space (Fig. 1). And within the following cycle
from beginning of cell charge and up to the voltage 4.57 V, the oxygen
is consumed from the cell head space (Fig. 3). Hence, again the oxygen
is adsorbed on cathode (Fig. 1). Then it dissociates on atoms and takes
part in the electrolyte decomposition on the oxygen-type mechanism
(Eqs. 17–20). Thus, over the period of the entire cycle of cell charge
(Fig. 1), the oxygen-type mechanism of electrolyte decomposition
takes place.
Discussion of Third Group of Experiments (Fig. 4)
The main purpose of this section is to explain the results of the
third group experiments (Fig. 4) quantitatively based on the established
mechanism of electrolyte decomposition (Eqs. 7,13–20).
Let us give consideration to cell cycling up to the upper limit voltage
of 4.8 V at 60°C (Fig. 4). The amount of the released gases CO2+CO
and H2for four cycles grew 1.3 times as compared to the second group
of experiments (Figs. 3and 4). Hence, also the electrolyte decompo-
sition rate grew 1.3 times. However, as it was shown (see the section
Results of cells cycling in voltages range 2.6-4.8 V at 60°C), at tem-
perature rise up to 60°C, the diffusion rate of oxygen from cathode
at charge stays approximately the same as in the second group of ex-
periments. As a result, the share of CO2sharply falls in the general
balance of the gases CO and CO2, so the ratio CO2/CO becomes equal
to 1.2.
From knowing the mechanism of the electrolyte decomposition
(Eqs. 13–20), it is possible to calculate this ratio purely theoretically.
According to Figs. 3and 4, the general ratio of the released gases
(CO+CO2) in the third and second groups of experiments is equal to
(CO+CO2)3/(CO+CO2)2≈1.3. The amount of the released gas CO2
in these groups of experiments is the same (as the diffusion rate of
oxygen from cathode at charge is the same (see description of the
third group of experiments (Figs. 4)), i.e. (CO2)3=(CO2)2.From
theses two equations, we obtain the ratio as follows: (CO2/CO)3=
1/(0.3+1.3/((CO2/CO)2))≈1.2 as (CO2/CO)2=2.4 (see the section
Results of cells cycling in voltages range 2.6-4.8 V at 25°C). Thus, the
obtained calculated value for the ratio (CO2/CO)3coincides with the
direct experimental value 1.2 (see the section Results of cells cycling
in voltages range 2.6-4.8 V at 60°C).
Now let us give consideration to the dilemma, on what mechanism
the electrolyte is decomposed at the temperature 60°C. The amount
of the released gas CO2in the experiments of the second and third
groups is approximately the same (Figs. 3and 4) as at temperature
rise up to 60°C, the diffusion rate of oxygen from cathode at charge
stays approximately the same as at the temperature 25°C (see the sec-
tion Results of cells cycling in voltages range 2.6-4.8 V at 60°C).
However, the amount of the released gas CO grows sharply (Figs. 3
and 4). This is possible only due to the direct mechanism of the elec-
trolyte decomposition (Eqs. 14 and 15). According to the Figs. 3and
4, at the temperature rise up to 60°C, the amount of the released gas
CO rises approximately twice. In the second group of experiments, the
gas CO was generated only due to the oxygen-type mechanism of the
electrolyte decomposition. In the third group of experiments, approx-
imately the same amount of CO is generated due to direct mechanism
of the electrolyte decomposition. Hence at the cell temperature 60°C,
contributions of both mechanisms into the process of the electrolyte de-
composition are approximately the same. The conducted studies show
that with cell temperature rise, the probability of the direct mechanism
of electrolyte decomposition (Eqs. 14 and 15) increases drastically.
Additionally let us estimate other ratios for the gases released due
to the electrolyte decomposition. First of all, let us evaluate the ratio
of the released gases (CO2+CO)/H2based on the experimental data
given in the Fig. 4.
According to the Fig. 4(up to the charge voltage 4.0 V in
the fourth cycle), the hydrogen was released in the amount of
30.1 μmol m−2SLP30, while the gases mixture CO+CO2was released
in the amount of 44.8 μmol m−2SLP30. Besides, the amount of the hy-
drogen released due to the decomposition of residual moisture in this
group of experiments is the same as in the first and second groups of
experiments (2.4 μmol m−2SLP30) because the same electrolyte was
used. Hence, only due to the electrolyte decomposition, the hydrogen
was released in the amount of 30.1-2.4 =27.7 μmol m−2SLP30.
Now let us estimate the amount of the released gases CO+CO2
(up to the charge voltage 4.0 V in the fourth cycle) only due to the
electrolyte decomposition. As it was shown, at the temperature rise up
to 60°C, the Li2CO3solubility does not change much (see the section
Results of cells cycling in voltages range 2.6-4.8 V at 60°C). Hence,
the amount of the gas CO2(over the period of first three cycles) re-
leased due to the Li2CO3electrochemical decomposition (Eq. 5)is
supposed to be approximately the same as in the first and second
groups of experiments, i.e. 1.5·3/2 =2.25 μmol m−2SLP30. Besides,
the amount of CO2released due to the residual moisture removal is
also approximately the same (0.9 μmol m−2SLP30) because the same
electrolyte was used. Hence, only due to the electrolyte decompo-
sition, the gases mixture CO2+CO was released in the amount of
44.8-2.25-0.9 =41.65 μmol m−2SLP30.
As a result, for this ratio, we obtain the experimental value
(CO2+CO)/H2=1.504, which matches very well to the theoretical
value 1.5 (The relative error is less than 1%). Hence, also in this group
of experiments, the proposed mechanism of the electrolyte decompo-
sition (Eqs. 7,13–20) shows a very good correspondence between the
experimental data and the theoretical calculations.
Second, let us estimate ratios for the gases released due to the elec-
trolyte decomposition in the third and second groups of experiments.

Journal of The Electrochemical Society,166 (6) A897-A908 (2019) A907
With due account of said above, in the third and second groups of
experiments only due to the electrolyte decomposition, the ratio for
the mixture of CO+CO2will be equal to (CO+CO2)3/(CO+CO2)2=
1.38, while the ratio for the amount of releasing hydrogen will be equal
to (H2)3/(H2)2=1.37. The synchronous rise of the gases amounts of
CO2+CO and H2is a strong indication that in the experiments of the
second group (Fig. 3) and the third group (Fig. 4), the decomposition
runs of the same chemical compound and onto the same gases CO
and H2. Meanwhile the considerable difference of the ratio CO2/CO
in the experiments of the second and third group is the result of the
additional chemical reaction (Eq. 19).
From the discussion of experimental results in the last three sec-
tions, we can draw the following conclusions about the overall mech-
anism of gases generation under different cycling conditions.
1. From a comparison between the electrochemical reactions
(Eqs. 17 and 18) and (Eqs. 14 and 15), it becomes evident that
running of the electrochemical reactions (Eqs. 17 and 18) is more
energetically profitable than running of the electrochemical re-
actions (Eqs. 14 and 15). Thus in the case of the atomic oxygen
release from the cathode (at charge voltage over 4.57 V), oxygen-
type mechanism of the electrolyte decomposition must take place,
i.e. the reactions (Eqs. 17–20). This mechanism of electrolyte
decomposition and gases generation was in the second group
of experiments (see the section Discussion of second group of
experiments (Fig. 3)).
2. The direct mechanism of electrolyte decomposition (Eqs. 14
and 15) take place when there is not enough atomic oxygen
at the cathode. For example, in the first group of experiments
when cell cycling in the voltage range 2.6-4.2V, atomic oxy-
gen is not released from the cathode. In this case, the direct
mechanism of electrolyte decomposition makes a large contri-
bution to the total amount of released gases (see the section
Discussion of the first group of experiments (Fig. 2)).
3. In the last section, it was shown that with increasing cell tempera-
ture, the probability of the direct mechanism of electrolyte decom-
position (Eqs. 14 and 15) increases. At the same time, the velocity
of the atomic oxygen diffusion from the cathode does not change
much at a cell temperature growth – from 25°C to 60°C. Thus,
when the cell temperature rises above 25°C, both the oxygen-type
mechanism of the electrolyte decomposition (Eqs. 17–20) and the
direct mechanism of electrolyte decomposition (Eqs. 14 and 15)
begin to work. Moreover, at the cell temperature 60°C, contri-
butions of both mechanisms into the process of the electrolyte
decomposition are approximately the same (see the section Dis-
cussion of third group of experiments (Fig. 4)).
4. In the papers,20,42,43,46–48 it was proved that at voltages higher than
4.0 V on the cathode, there runs oxidation of Li2CO3with CO2
evolution. Moreover, a half of CO2amount is generated due to
the electrochemical decomposition of Li2CO3(Eq. 5), while the
second half of CO2amount is generated due to the electrolyte
decomposition with CO subsequent oxidation on cathode under
action of the atomic oxygen from the reaction (Eqs. 5), i.e. it
is generated on the oxygen-type mechanism of the electrolyte
decomposition (Eqs. 17–20). Simplified, this mechanism of de-
composition of Li2CO3and CO2generation is represented by the
reaction (Eq. 7). This CO2generation mechanism is present in all
the experiments performed. It should be noted that in this mech-
anism only half of the CO2is generated due to the electrolyte
decomposition.
Conclusions
At present, there is no generally accepted mechanism that could
quantitatively explain experiments (Figs. 2–5and (Ref. 20,21,28)) in
which gases are generated during the cycling of lithium-ion cells.
In a number of papers,20,21 various mechanisms of electrolyte de-
composition and various qualitative schemes for the generation of
gases were proposed. However, these schemes could not quantitatively
explain the resulting gases and the ratio between them.
The mechanism of electrolyte decomposition during cells cycling,
established in this paper, has several advantages.
First, electrochemical reactions (Eqs. 7,13–20) underlying the es-
tablished mechanism of electrolyte decomposition, for the first time
allowed to quantitatively explain the resulting gases and the ratio be-
tween them. These studies are done in the last three sections. There-
fore, the established mechanism of electrolyte decomposition during
cells cycling is of great theoretical importance for understanding the
electrochemical processes inside the cells during their operation.
Secondly, all electrochemical reactions leading to generation of
gases and other side products in the process of cells cycling lead to the
degradation and aging of cells.14–21 In addition, the gases evolution
in the lithium-ion batteries is a serious problem; it is especially so
in the case of their work under high voltages and temperatures.22,23
The established mechanism of electrolyte decomposition for the first
time provides a quantitatively correct explanation of these undesirable
phenomena. Only the knowledge of the electrochemical mechanism
of any process allows them to manage optimally. Consequently, the
established mechanism of electrolyte decomposition (Eqs. 7,13−20)
can be the basis for research to reduce the processes of degradation
and increase the service life of cells.
Third, in paper,75 it was experimentally proved that when cells cy-
cling, hydrogen accumulates in the anode, and therefore the probability
of thermal runaway in lithium-ion cells sharply increases. The estab-
lished mechanism of electrolyte decomposition (Eqs. 14–20) explains
the reason for the evolution of hydrogen at the anode and its accu-
mulation in the anode (Fig. 5). Knowing the causes and mechanism
of hydrogen accumulation, it is possible to effectively control such
a dangerous phenomenon as thermal runaway and therefore improve
the safety of lithium-ion cells operation.
Fourth, in a number of papers,20,21,28,34 it was experimentally es-
tablished that some gases (H2,C
2H4) are released during both cell
charge and discharge, i.e. in the potential independent manner. Within
the framework of the established mechanism of electrolyte decompo-
sition, it was experimentally proved for the first time that a potential-
independent H2evolution is a consequence of its adsorption in pores
of powdered graphite on anode (Fig. 5).
Undoubtedly, the proposed mechanism of the electrolyte decom-
position (Eqs. 7,13–20) at cycling of lithium-ion cells requires further
investigations both experimental and theoretical. Notwithstanding, it
allows explaining quantitatively all the experimental data available to
the date.
ORCID
N. Е. Galushkin https://orcid.org/0000-0002-1613-8659
D. N. Galushkin https://orcid.org/0000-0001-8261-6527
References
1. G. E. Blomgren, J. Electrochem. Soc.,164, A5019 (2017).
2. T. M. Bandhauer, S. Garimella, and T. F. Fuller, J. Electrochem. Soc.,158, R1 (2011).
3. N. Nitta, F. Wu, J. T. Lee, and G. Yushin, Materials Today,18, 252 (2015).
4. O. Groger, H. A. Gasteiger, and J.-P. Suchsland, J. Electrochem. Soc.,162, A2605
(2015).
5. P. Kurzweil and K. Brandt, in Encyclopedia of Electrochemical Power Sources, Vol.
5, J. Garche, Editor, p. 1, Elsevier, Amsterdam (2009).
6. T. Waldmann, A. Iturrondobeitia, M. Kasper, N. Ghanbari, F. Aguesse,
E. Bekaert, L. Daniel, S. Genies, I. J. Gordon, M. W. Loble, E. De Vito, and
M. Wohlfahrt-Mehrens, J. Electrochem. Soc.,163, A2149 (2016).
7. D. Andre, S.-J. Kim, P. Lamp, S. F. Lux, F. Maglia, O. Paschos, and B. Stiasznya, J.
Mater. Chem. A,3, 6709 (2015).
8. K. G. Gallagher, S. Goebel, T. Greszler, M. Mathias, W. Oelerich, D. Eroglu, and
V. Srinivasan, Energy Environ. Sci.,7, 1555 (2014).
9. J. Choi and A. Manthiram, J. Electrochem. Soc.,152, A1714 (2005).
10. N. Yabuuchi, Y. Makimura, and T. Ohzuku, J. Electrochem. Soc.,154, A314 (2007).
11. M. G. Kim, H. J. Shin, J.-H. Kim, S.-H. Park, and Y.-K. Sun, J. Electrochem. Soc.,
152, A1320 (2005).
12. S.-K. Jung, H. Gwon, J. Hong, K.-Y. Park, D.-H. Seo, H. Kim, J. Hyun, W. Yang, and
K. Kang, Adv. Energy Mater.,4, 1300787 (2014).

A908 Journal of The Electrochemical Society,166 (6) A897-A908 (2019)
13. H. Gabrisch, T. Yi, and R. Yazami, Electrochem. Solid-State Lett.,11, A119 (2008).
14. I. Buchberger, S. Seidlmayer, A. Pokharel, M. Piana, J. Hattendorff, P. Kudejova,
R. Gilles, and H. A. Gasteiger, J. Electrochem. Soc.,162, A2737 (2015).
15. D. R. Gallus, R. Schmitz, R. Wagner, B. Hoffmann, S. Nowak, I. Cekic-Laskovic,
R. W. Schmitz, and M. Winter, Electrochim. Acta,134, 393 (2014).
16. H. Zheng, Q. Sun, G. Liu, X. Song, and V. S. Battaglia, J. Power Sources,207, 134
(2012).
17. Y.-C. Lu, A. N. Mansour, N. Yabuuchi, and Y. Shao-Horn, Chem. Mater.,21, 4408
(2009).
18. A. M. Andersson, D. P. Abraham, R. Haasch, S. MacLaren, J. Liu, and K. Amine, J.
Electrochem. Soc.,149, A1358 (2002).
19. J. Wandt, A. Freiberg, R. Thomas, Y. Gorlin, A. Siebel, R. Jung, H. A. Gasteiger, and
M. Tromp, J. Mater. Chem. A,4, 18300 (2016).
20. M. Metzger, B. Strehle, S. Solchenbach, and H. A. Gasteiger, J. Electrochem. Soc.,
163, A798 (2016).
21. R. Jung, M. Metzger, F. Maglia, C. Stinner, and H. A. Gasteiger, J. Electrochem. Soc.,
164 A1361 (2017).
22. I. Belharouak, G. M. Koenig, and K. Amine, J. Power Sources,196, 10344 (2011).
23. I. Belharouak, G. M. Koenig, T. Tan, H. Yumoto, N. Ota, and K. Amine, J. Elec-
trochem. Soc.,159, A1165 (2012).
24. K. Wu, J. Yang, Y. Zhang, C. Wang, and D. Wang, J. Appl. Electrochem.,42, 989
(2012).
25. K. Wu, J. Yang, Y. Liu, Y. Zhang, C. Wang, J. Xu, F. Ning, and D. Wang, J. Power
Sources,237, 285 (2013).
26. N. Tsiouvaras, S. Meini, I. Buchberger, and H. A. Gasteiger, J. Electrochem. Soc.,
160, A471 (2013).
27. M. Metzger, J. Sicklinger, D. Haering, C. Kavakli, C. Stinner, C. Marino, and
H. A. Gasteiger, J. Electrochem. Soc.,162, A1227 (2015).
28. B. Strehle, S. Solchenbach, M. Metzger, K. U. Schwenke, and H. A. Gasteiger, J.
Electrochem. Soc.,164, A2513 (2017).
29. E. S. Takeuchi, H. Gan, M. Palazzo, R. A. Leising, and S. M. Davis, J. Electrochem.
Soc.,144, 1944 (1997).
30. H. Yoshida, T. Fukunaga, T. Hazama, M. Terasaki, M. Mizutani, and M. Yamachi, J.
Power Sources,68, 311 (1997).
31. R. Petibon, L. Rotermund, K. J. Nelson, A. S. Gozdz, J. Xia, and J. R. Dahn, J.
Electrochem. Soc.,161, A1167 (2014).
32. M.Metzger, C. Marino, J. Sicklinger, D. Haering, and H. A. Gasteiger, J. Electrochem.
Soc.,162, A1123 (2015).
33. M. Onuki, S. Kinoshita, Y. Sakata, M. Yanagidate, Y. Otake, M. Ue, and M. Deguchi,
J. Electrochem. Soc.,155, A794 (2008).
34. R. Bernhard, M. Metzger, and H. A. Gastegier, J. Electrochem. Soc.,162, A1984
(2015).
35. B. Zhang, M. Metzger, S. Solchenbach, M. Payne, S. Meini, H. A. Gasteiger,
A. Garsuch, and B. L. Lucht, J. Phys. Chem. C,119, 11337 (2015).
36. M. Armand and J.-M. Tarascon, Nature,451, 652 (2008).
37. M. Nie, D. Chalasani, D. P. Abraham, Y. Chen, A. Bose, and B. L. Lucht, J. Phys.
Chem. C,117, 1257 (2013).
38. D. Aurbach, Y. Gofer, M. Ben-Zion, and P. Aped, J. Electroanal. Chem.,339, 451
(1992).
39. G. Gachot, S. Grugeon, G. G. Eshetu, D. Mathiron, P. Ribiere, M. Armand, and
S. Laruelle, Electrochim. Acta,83, 402 (2012).
40. D. Aurbach, A. Zaban, Y. Ein-Eli, I. Weissman, O. Chusid, B. Markovsky, M. Levi,
E. Levi, A. Schechter, and E. Granot, J. Power Sources,68, 91 (1997).
41. M. N. Richard and J. Dahn, J. Electrochem. Soc.,146, 2068 (1999).
42. B. D. McCloskey, D. S. Bethune, R. M. Shelby, G. Girishkumar, and A. C. Luntz, J.
Phys. Chem. Lett.,2, 1161 (2011).
43. S. Meini, N. Tsiouvaras, K. U. Schwenke, M. Piana, H. Beyer, L. Lange, and
H. A. Gasteiger, Phys. Chem. Chem. Phys.,15, 11478 (2013).
44. R. Bernhard, S. Meini, and H. A. Gasteiger, J. Electrochem. Soc.,161, A497 (2014).
45. M. Metzger, B. Strehle, S. Solchenbach, and H. A. Gasteiger, J. Electrochem. Soc.,
163, A1219 (2016).
46. K. Xu and A. von Cresce, J. Mater. Chem.,21, 9849 (2011).
47. S. E. Sloop, J. B. Kerr, and K. Kinoshita, J. Power Sources,119–121, 330 (2003).
48. S. A. Freunberger, Y.Chen, Z. Peng, J. M. Griffin, L. J. Hardwick, F.Bard´e, P. Novak,
and P. G. Bruce, J. Am. Chem. Soc.,133, 8040 (2011).
49. A. Friesen, F. Horsthemke, X. Monnighoff, G. Brunklaus, R. Krafft, M. Borner,
T. Risthaus, M. Winter, and F. M. Schappacher, J. Power Sources,334, 1 (2016).
50. G. Jacobs and B. H. Davis, Catalysis,20, 122 (2007).
51. D. Elliott and L. Sealock Jr., Ind. Eng. Chem. Prod. Res. Dev.,22(3), 426 (1983).
52. D. Elliott, R. Hallen, and L. Sealock Jr„ Ind. Eng. Chem. Prod. Res. Dev.,22(3), 431
(1983).
53. D. Elliott, L. Sealock Jr, and R. Butner, Ind. Eng. Chem. Prod. Res. Dev.,25(4), 541
(1986).
54. U. Schuchardt and M. Sousa, Fuel,65, 669 (1986).
55. U. Wietelmann and M. Steinbild, Lithium and Lithium Compounds. Ullmann’s En-
cyclopedia of Industrial Chemistry, B. Elvers, Editor, 7th ed., J. Wiley & Sons, New
York (2016).
56. D. Aurbach, A. Zaban, Y. Gofer, Y. E. Ely, I. Weissman, O. Chusid, and O. Abramson,
J. Power Sources,54, 76 (1995).
57. A. D. Pasquier, F. Disma, T. Bowmer, A. S. Gozdz, G. Amatucci, and J.-M. Tarascon,
J. Electrochem. Soc.,145, 472 (1998).
58. L. D. Ellis, J. P. Allen, L. M. Thompson, J. E. Harlow, W. J. Stone, I. G. Hill, and
J. R. Dahn, J. Electrochem. Soc.,164, A3518 (2017).
59. J. R. Jaunsen, The behavior and capabilities of lithium hydroxide carbon dioxide
scrubbers in a deep sea environment. US Naval Academy Technical Report. USNA-
TSPR-157 (1989).
60. K. Xu, S. Zhang, T. R. Jow, W. Xu, and C. A. Angell, Electrochem. Solid-State Lett.,
5, A26 (2002).
61. M. Herstedt, D. P.Abraham, J. B. Kerr, and K. Edstrom, Electrochim. Acta.,49, 5097
(2004).
62. D. M. Seo, D. Chalasani, B. S. Parimalam, R. Kadam, M. Nie, and B. L. Lucht, ECS
Electrochem. Lett.,3, A91 (2014).
63. D. P. Broom, Hydrogen storage materials, Springer, London (2011).
64. M. Fichtner, Adv. Eng. Mater,7, 443 (2005).
65. I. Cabria, M. J. Lopez, and J. A. Alonso, Int. J. Hydrogen Energy,36, 10748 (2011).
66. Y. Gogotsi, C. Portet, S. Osswald, J. M. Simmons, T. Yildirim, G. Laudisio, and
J. E. Fischer, Int. J. Hydrogen Energy,34, 6314 (2009).
67. R. T. Yang, Carbon,38, 623 (2000).
68. Y. Yurum, A. Taralp, and T. N. Veziroglu, Int. J. Hydrogen Energy,34, 3784 (2009).
69. H. Imamura, I. Kitazawa, Y. Tanabe, and Y. Sakata, Int. J. Hydrogen Energy,32, 2408
(2007).
70. Y. Kojima, Y. Kawai, and T. Haga, J. Alloys Compd.,424, 294 (2006).
71. V. J. Surya, K. Iyakutti, N. Venkataramanan, H. Mizuseki, and Y. Kawazoe, Int. J.
Hydrogen Energy,35, 2368 (2010).
72. N. E. Galushkin, N. N. Yazvinskaya, and D. N. Galushkin, J. Electrochem.Soc.,164,
A2555 (2017).
73. N. N. Yazvinskaya, N. E. Galushkin, D. N. Galushkin, and I. A. Galushkina, Int. J.
Electrochem. Sci., 12, 2791 (2017).
74. N. N. Yazvinskaya, N. E. Galushkin, D. N. Galushkin, and I. A. Galushkina, Int. J.
Electrochem. Sci., 11, 5850 (2016).
75. N. E. Galushkin, N. N. Yazvinskaya, and D. N. Galushkin, J. Electrochem.Soc.,165,
A1303 (2018).
76. J. C. Burns, A. Kassam, N. N. Sinha, L. E. Downie, L. Solnickova, B. M. Way, and
J. Dahn, J. Electrochem. Soc.,160, A1451 (2013).
77. R. Dedryvere, D. Foix, S. Franger, S. Patoux, L. Daniel, and D. Gonbeau, J. Phys.
Chem. C,114, 10999 (2010).
78. T. Ma, G.-L. Xu, Y. Li, L. Wang, X. He, J. Zheng, J. Liu, M. H. Engelhard, P. Zapol,
L. A. Curtiss, J. Jorne, K. Amine, and Z. Chen, J. Phys. Chem. Lett.,8, 1072 (2017).