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Electrochemical Quantification of Lithium Plating: Challenges and Considerations


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Low-level overlithiation (1.5 to 5.5% based on the theoretical capacity of graphite, 372 mAh-1) behavior of graphite electrode is reported to illustrate the challenges of detecting Li plating via electrochemical methods such as voltage, incremental capacity (dQ.dV-1), and coulombic efficiency. Low-level overlithiation is closely tied to multiple in-vehicle mild-abuse and aging-related conditions such as overcharge and fast-charge where early detection of plating is highly desirable. Analysis following overlithiation includes the interplay between reversibly and irreversibly plated Li with and without rest after overlithiation, impacts on the electrode, and the feasibility of using reversible Li stripping as a detection method. Following overlithiation the capacity for graphite readily approaches theoretical values. Inspection of the lithiation voltage profile suggests that a portion of this capacity is associated with the chemical lithiation of graphite and effectively suppresses the dQ.dV-1 signal related to Li stripping. Analysis of coulombic efficiency and the lithiation and delithiation profiles are used to quantify the capacity of graphite, irreversibly and reversibly plated lithium. It has been found that the extent of reversibility of lithium plating is distinctly impacted by rest with periods as short as one hour reducing the extent of reversibility and hence electrochemical detection.
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Journal of The Electrochemical Society,166 (12) A2689-A2696 (2019) A2689
Electrochemical Quantification of Lithium Plating: Challenges and
Tanv i r R . Ta n i m , Eric J. Dufek, ,zCharles C. Dickerson, and Sean M. Wood
Idaho National Laboratory, Idaho Falls, Idaho 83415, USA
Low-level overlithiation (1.5 to 5.5% based on the theoretical capacity of graphite, 372 mAh1) behavior of graphite electrode
is reported to illustrate the challenges of detecting Li plating via electrochemical methods such as voltage, incremental capacity
(dQ.dV1), and coulombic efficiency. Low-level overlithiation is closely tied to multiple in-vehicle mild-abuse and aging-related
conditions such as overcharge and fast-charge where early detection of plating is highly desirable. Analysis following overlithiation
includes the interplay between reversibly and irreversibly plated Li with and without rest after overlithiation, impacts on the electrode,
and the feasibility of using reversible Li stripping as a detection method. Following overlithiation the capacity for graphite readily
approaches theoretical values. Inspection of the lithiation voltage profile suggests that a portion of this capacity is associated with the
chemical lithiation of graphite and effectively suppresses the dQ.dV1signal related to Li stripping. Analysis of coulombic efficiency
and the lithiation and delithiation profiles are used to quantify the capacity of graphite, irreversibly and reversibly plated lithium.
It has been found that the extent of reversibility of lithium plating is distinctly impacted by rest with periods as short as one hour
reducing the extent of reversibility and hence electrochemical detection.
© 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,, which permits unrestricted reuse of the work in any
medium, provided the original work is properly cited. [DOI: 10.1149/2.1581912jes]
Manuscript submitted June 27, 2019; revised manuscript received July 25, 2019. Published August 6, 2019.
Graphite is a widely used anode material for lithium-ion batter-
ies (LIBs). As performance for LIBs has improved, the applications
that these batteries are used in has ballooned spanning consumer elec-
tronics to battery electric vehicles (BEVs).1,2Existing, commercial
graphite-based LIB packs in the BEV domain typically can sustain
charging up to a nominal 2C rate which ultimately leads to a charge
duration of 0.5 to 1h.3,4Reducing the time necessary to recharge BEV
batteries to 10–15 minutes, at rates up to 6C, would diminish the gap
between refueling and recharging time which exists between internal
combustion engine vehicles and BEVs. This reduction in charging
time is seen as appealing to consumers and a key driver to enhance
adoption of BEVs.3,5,6Efforts to understand key scientific limitations
associated with high rate charging have begun globally.79
The electrochemical lithiation of graphite occurs in multiple dis-
tinct stages, which when combined, leads to a reversible potential for
Li intercalation and deintercalation near 0.1 V vs. Li/Li+.Thislow
potential provides a broad voltage window for maximizing cell en-
ergy when coupled with an appropriate cathode. The low reversible
potential of graphite is, however, very close to the thermodynamic po-
tential for Li plating (0 V) and allows a very narrow usable polarization
window during charging. As a result, moderate and high rate charg-
ing increases the likelihood of lithium plating1012 which is known
to deleteriously impact cycle life.1214 Balancing the ability to charge
at moderate and high rates without plating Li in cells that have high
enough specific energy to enable cost effective BEVs is a challenging
and active research area.3
Understanding Li plating and the scenarios in which it is likely to
occur is crucial to advancing fast charge-capable vehicle adoption. Li
plating has been reported to become favorable in the early stages of
LIB life due to improper cell design and aggressive usage.1012,1517
Even in non-aggressive cases, Li plating may become thermodynam-
ically favorable as a cell ages and the loss of active anode material
outpaces the loss of Li inventory.3,13,18,19 Increased charging rates are
typically associated with higher polarization arising from transport
and kinetic overpotentials and could make plating favorable locally
much earlier than thermodynamically favorable conditions. To aid in
the development of technological means to advance fast charging, in-
operando detection methods based on electrochemical signals would
provide benefit to ensure safe and reliable operation of LIB throughout
the service life.
Multiple reports have noted that plated Li is partially
reversible.10,12,13,20,21 This reversible behavior offers opportunities for
Electrochemical Society Member.
the electrochemical detection of plating. For example, a distinct volt-
age signature associated with reversible cycling of plated Li has previ-
ously been used as an indication of Li plating.13,15,18,20,22,23 However,
recent analysis has shown that this plateau cannot solely be used to
quantitatively identify Li plating.24 Irreversibly plated Li can broadly
be broken down into two main types of Li loss: i) excessive solid elec-
trolyte interphase (SEI) growth due to reaction with metallic Li25 and
ii) electronically isolated Li metal.3While reversible Li plating can
have a direct electrochemical signal, irreversibly plated Li requires
indirect observation such as reduced coulombic efficiency, which has
been tracked using high precision measurements as another indication
of the occurrence of Li plating.26
Both standard and high rate charging pose the concern of full or
localized overlithiation of graphite electrode in LIB’s early and/or
later stage of life. Thus, mitigation of Li plating in conjunction with
developing reliable detection methods requires a comprehensive un-
derstanding of the overlithiation (OL) behavior of the graphite elec-
trode. Many studies have investigated the plating or OL behavior in
full cell settings.17,18,20,22,23,27 Few researchers, however, have dedicat-
edly investigated the plating or overlithiation behavior of the graphite
electrode and tried to understand as how processes at the graphite an-
ode impact the ability to realistically use electrochemistry to detect Li
plating. Ohzuku et al. first performed OL tests on graphite electrode.28
In their limited cycling study with 50% OL, they observed the pres-
ence of non-dendritic Li deposits, but the graphite structure remained
unharmed. Lu et al. studied 10% OL based on theoretical capacity and
observed tube-like hollow dendrites at the surface of graphite particles
though the bulk properties of the graphite particles remained intact.21
Honbo et al. studied 13 to 15% OL based on theoretical capacity and
concluded that surface modification of graphite particles was able to
change the lithium morphology from needle-like to granular.29 Su et
al. studied slow rate OL behavior of graphite and observed the pre-
dominance of SEI formation at the graphite surface during OL.25
A comprehensive understanding of the impact of OL of graphite
electrodes during both normal and fast charging conditions is vital
to advance reliable and safe operation and early detection of plating.
In particular, understanding the interplay between reversibly and ir-
reversibly plated Li, how it impacts electrode behavior, and whether
the electrochemistry of reversible Li stripping could viably be used
as a potential detection method is crucial in the endeavor of develop-
ing advanced diagnostic, prognostic and detection methods. This work
systematically overlithiates graphite electrodes to investigate different
aspects of reversible and irreversible Li and the impacts that both have
on electrochemical detection and quantitation. At the same time the
challenges associated with Li plating detection via electrochemical
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A2690 Journal of The Electrochemical Society,166 (12) A2689-A2696 (2019)
Table I. BOL specific capacity and the extent of overlithiation.
Group Cell number BOL Sp. cap. at C/20 (mAhg1)OLBOL(%) OLabs (mAhg1)
A 1 326 15.7 377.2 (1.5%)
2 295.3 27.7
3 301.2 25.2
B 4 312.9 22.5 383 (3%)
5 313.1 21.3
C 6 352 11.5 392.4 (5.5%)
7 320.3 22.5
8 282.8 38.7
Capacities are normalized by active mass of graphite for each electrode.
methods are identified and discussed as an aid for the development of
diagnostic and prognostics for large format cells and batteries.
Graphite (Conoco Philips, A12) electrode laminate received from
the Cell Analysis, Modeling, Prototyping (CAMP) Facility at Argonne
National Laboratory was used for this study. The electrode laminate
with 44 μm coating thickness and 38.4% porosity was cut into 1.27 cm
diameter disks with loading of 5.4 mg cm2[6.88 mg ±0.05 mg ac-
tive graphite]. The graphite electrode contains 91.8 wt% Phillips 66
CPreme A12 active material, 2 wt% TimcalC45 carbon, 6 wt% Kureha
9300 PVDF binder and 0.17wt% oxalic acid. Overlithiation and cy-
cling performance experiments were performed using the graphite
discs paired with a 0.6 mm thick Li metal counter electrode of 1.43 cm
diameter and a 1.59 cm diameter Celgard 2500 separator in CR-2032-
coin cells. Cells used a total of 28 μL of 1.2M LiPF6in ethylene
carbonate/ ethyl methyl carbonate (3:7 wt/wt) electrolyte with 14 μL
applied to the Li electrode prior to the placement of the separator and
another 14 μL after placing the separator. Following assembly, cells
were placed in a thermal chamber and held at 25 ±1°C and allowed
to soak for 2.5 h before undergoing three formation cycles between
1.2V and 0.002V at a C/10 rate (0.25mA) with one-hour rest between
the charge and discharge steps. After the three C/10 cycles each cell
underwent a single C/20 charge and discharge at the current defined
by the last C/10 discharge capacity.
After formation, the cells were split into three sets (Table I)and
were cycled to varying degrees of OL. Within the sets, the OL based on
the beginning of life (BOL) capacity at C/20, OLBOL, varies with the
BOL capacity variability, but the OL based on the theoretical capacity
of graphite (OLabs) remains the same for each set. For cycling, the
cells were first lithiated to a specified capacity (Table I) at C/3 without
imposing any constraint on lower voltage limits. This closely mirrors
full cells which either have seen a loss of negative active material
and have an electrode negative: positive (N:P) ratio less than 130 at an
aged state or cells which have undergone fast charge where Li can be
readily plated.12,14 Delithiation was performed using a staged process
where a C/20 rate (0.125 mA) was used until the voltage reached
0.3V. Above 0.3V a C/3 rate was used until the Vmax of 1.2V. Table I
includes the specific capacities (scaled by active material mass for
each cell) applied during OL cycling. Graphite’s theoretical capacity
of 372 mAhg1was used to calculate the absolute percentage of OL,
Scanning electron micrographs were obtained using a JEOL Neo-
Scope JCM-5000 at 10 kV. Samples were transferred out of the glove
box in a sealed vial to prevent air exposure during transport to the
SEM. Samples were briefly (<10 s) exposed to air while loading
into the SEM chamber. To perform ex situ X-ray Diffraction (XRD),
another set of cells that went through the same cycling protocols as
mentioned in Table Iwere first brought to the desired state of charge
or discharge. Electrodes were extracted from each cell in an Ar-filled
glove box and rinsed lightly in dimethyl carbonate to remove resid-
ual LiPF6salts. After drying under vacuum, electrodes were sealed
between two pieces of Kapton tape (still under an Ar-atmosphere) in
order to prevent air exposure upon transfer to the XRD system. XRD
spectra were measured using a Rigaku SmartLab diffractometer with
aCuKαradiation source at 40 kV and 44 mA with λ=1.54059 Å.
Results and Discussion
Utilization behavior of graphite at plating conditions.—Plotted in
Fig. 1are the delithiated specific capacities of graphite between 0.05V
to 1.2V before and during 10 OL cycles as a function of OLBOL.The
extent of OLBOL varies from 11.5% up to 38.7% due to capacity vari-
ability at the BOL as shown in Fig. 1. The variability in BOL capacity
is not uncommon21,31,32 and correlates with the slightly variable cell
polarization overvoltage at the beginning of delithiation (see supple-
mental Fig. S1) where cells with higher overvoltage displayed lower
BOL capacity. As shown in Fig. 1, delithiation of the graphite elec-
trodes following OL resulted in a significant increase in the specific
capacity with the average (366 mAhg1) readily approaching the the-
oretical specific capacity of graphite. Over 10 OL cycles, the average
delithiation specific capacity remained nearly unchanged between 357
to 365 mAhg1with a variability between 0.5% to 1.5% (1σ). In all
cases, the delithiation capacity with OL exceeded the delithiation ca-
pacity at BOL when no OL was applied.
Electrochemical behavior of graphite at plating conditions and
its implications to electrochemical detection signals.—The voltage
evolution and the corresponding dQ.dV1plots are shown in Fig. 2
for both the lithiation and delithiation of cell 1 (15.7% OLBOL or 1.5%
OLabs) as an example case. Similar behavior was observed for other
cells (see supplemental Fig. S2). Following formation, the primary
electrode processes are the intercalation and deintercalation of Li in
graphite which occurs in three main stages at voltages <0.25V.1,28
Figures 2a and 2c distinctly show the expected plateaus (I, II and IV,
stages IIIa and IIIbare not distinguishable here)28 and Figs. 2b and 2d
Figure 1. Delithiated specific capacity of graphite electrode between 0.05V–
1.2V before (at BOL with no OL) and during OL cycling. The error bars
represent 1σstandard deviation for the 10 OL cycles. The dashed line indicates
the theoretical capacity for graphite at 372 mAhg1.
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Figure 2. Cell 1 with 15.7% OLBOL or 1.5% OLabs: (a) Voltage vs lithiated capacity: BOL at C/20 and cycling at C/3, (b) dQ.dV1plots (iR corrected) during
lithiation, (c) Voltage vs delithiated capacity: BOL and cycling at C/20, and (d) dQ.dV1(iR corrected) plots during delithiation.
show the corresponding dQ.dV1peaks at the BOL with no indication
of Li plating. During the 1st OL cycle without any lower voltage limit,
the voltage continued to decrease below 0V, where simultaneous Li
intercalation into not fully lithiated graphite and electrodeposition of
Li occurs.10,21 After the 1st OL cycle, measurable changes in the volt-
age and dQ.dV1profiles can be observed. To more clearly visualize
the capacity variations in the different stages, the profiles in Fig. 2c
have been aligned such that the final delithiation capacity is uniform
across the cycle sets. The result of this alignment is that the BOL
profile and the profiles after 5 and 10 OL cycles are all shifted to the
right. Note that, the lithiation C-rate during OL cycling is C/3, higher
than the BOL rate of C/20. This rate difference shows up as reduced
peak height, peak broadening and some shift in onset voltage for the
incremental capacity signatures in Fig. 2b. For the delithiation cycles,
the C-rates remain the same in the voltage of interest (<0.25V) and
rate-based variations are not seen in the dQ.dV1plots presented in
Fig. 2d.
As the cells underwent multiple OL cycles, the extent of overvolt-
age and the voltage profiles change. In the 1st OL cycle, a distinct
peak (near 0.1V) followed by a plateau (near 0.05V) are observed
in Fig. 2a. These signatures can be attributed to the overpotential and
voltage relaxation during the electrodeposition of Li.21,3334 Upon con-
tinued cycling, the 0V crossing point transitions to higher capacities
with lower overall overpotential during OL and an enhanced utiliza-
tion of stage I of the graphite is observed (Fig. 2c). The other stages
of graphite did not see any increase in utilization as shown in Fig. 2c.
The shift in the zero-crossing point likely arises from the enhanced
utilization of graphite, which effectively reduced the effective C-rate
along with possible slight increase in electrode kinetics from Li depo-
sition also noted by Schindler et al.23 In-line with the plateau related to
the electrodeposition of Li, a distinct dQ.dV1peak is observed near
0.05V during the 1st OL cycle (Fig. 2b) which gradually diminishes
with repeated OL.
The delithiation voltage profile following the 1st OL cycle shifts
asshowninFig.2c. As noted above, the bulk of the change can be
attributed to the enhanced utilization of stage I of the graphite. The
extended capacity which resulted in near theoretical capacity for the
graphite was observable through the 10 cycles, though aging and loss
of active material did become visible as cycling advanced as shown in
Figs. 2c and 2d, e.g., a shrinkage in voltage vs. capacity plot in Fig. 2c
and decrease in peak intensity in Fig. 2d. Only a small portion of the
capacity gain can be attributed by the stripping of reversibly plated Li
at voltages below 0.05V [plateau Iin Fig 2c]. A previous report by Su
et al.,25 indicated a more distinct increase in capacity for Li stripping
using the same graphite material. The distinct differences between this
study and the prior work are the different extent of OL, rest time and
Figures 3a and 3c show that increasing the extent of OL results
in a stronger dQ.dV1peak near 0.05V associated with more Li
deposition. However, in all cases, the peak diminishes in the later cy-
cles due to the increased graphite utilization. The discharge dQ.dV1
plots in Figs. 3b and 3d follow a similar, but to a lower extent for the
reversible stripping of Li. Upon continued cycling, the Li stripping
dQ.dV1peaks gradually become more prominent especially for the
5.5% OLabs case due to the apparent loss of active material that ef-
fectively increases the percent OL. The incongruity between lithiation
and delithiation incremental capacity plots underscores the fact that
an OL-based shift in graphite utilization impacts and complicates the
electrochemical detection of Li.
To address some of the complications in electrochemical detection,
the balance between the reversible and irreversible capacities during
delithiation were estimated. In doing so, the 0.05V was used as the
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Figure 3. dQ.dV1plots during cycling: (a) lithiation at 1.5% OLabs and (b) delithiation after 1.5% OLabs, (c) lithiation at 5.5% OLabs and (b) delithiation after
5.5% OLabs. 1.5% and 5.5% OLabs correspond to 15.7% OLBOL (cell 1) and 38.7% OLBOL (Cell 8), respectively.
cutoff between the reversible Li and graphite capacity21 (Iin Fig. 2c).
Such analysis enables quantification of capacity from graphite and Li
stripping. Coulombic efficiency is CE =100 ×QD/QL,whereQ
delithiated capacity and QL=lithiated capacity. QDand QLare both
measured values. The reversible stripping efficiency is SE =100 ×
Qrev/(Qrev+Qirr ), where Qirr =QL-QDand Qrev is the reversible capac-
ity below 0.05V during delithiation (see Fig. 2c). The graphite capacity
is essentially the delithiated capacity above 0.05V and calculated from
Qgr =QD-Qrev formula.
Figures 4a and 4c show the delithiated capacity balance for cells
1 and 8 after selected OL cycles. Figure 4b includes the evolution of
the delithiation capacities at the primary stages of graphite during cy-
cling for cell 1 as an example case. At BOL, prior to OL, the graphite
remained significantly underutilized, e.g., about 16% (59 mAh g1)
lower than the theoretical capacity. Similar underutilization behav-
ior of graphite is reported elsewhere with capacities ranging from
229–360 mAhg1for rates between C/2 and C/20.21,31,32 Following
the 1st OL, graphite capacity across the samples increases to near
theoretical levels (366 ±2.7 mAhg1or 98.5% of the theoretical
capacity with less than 1% variability) irrespective of their BOL ca-
pacity. Additional utilization of stage I (Figs. 2and 4b) primarily
dominates the increase in graphite capacity, e.g., of the total increase
in capacity about 90% (53 mAh g1of 59 mAhg1) came from the
additional utilization of stage I. The stripped Li (Qrev) during delithi-
ation made up the remainder 10% of increased capacity (Figs. 4a
and 4c).
irr remained insignificant as shown in
Figs. 4a and 4c. However, after multiple OL cycles loss of active
graphite shifted the percent OL to higher values. This additional Li
ended up either in further SEI growth, i.e., irreversible loss or in in-
creased Qrev. Over the course of the OL cycles the CE (Fig. 4d)re-
mained within 1.5% of the BOL value of 99.3% and improved as Qrev
increased. The SE followed a similar, but more pronounced trend from
60% during the first OL cycle to 80% by the 10th OL cycle.
Structural behavior of graphite due to overlithiation.—After
completion of cycling, post-test investigation included SEM and XRD
analysis for samples in both the lithiated and delithiated states (Figs. 5
and 6). As expected from the electrochemical data the images suggest
that Li metal is present on the graphite electrode after OL based on the
presence of rod-like structures in Figs. 5b and 5c. After delithiation
some evidence of the Li remains suggesting that some of the Li is
electrically isolated and that the full extent of Qirr is not solely limited
to additional SEI formation.
Post-cycling XRD characterization was performed on samples in
both lithiated and delithiated states which had undergone similar OL
protocols. The XRD analysis indicates little change to the graphite
structure due to OL as evidenced by lack of shift in the location of the
diffraction peaks (Figs. 6a and 6b) and the calculated d-spacings which
were calculated using the 2θvalue and Bragg’s Law. Assignment of
the diffraction peaks was accomplished by comparing with the work
of Konar et al.35
For the samples that were lithiated and OL, a distinct dynamic
emerges as shown in Fig. 6b. For the sample which wasnot OL (0.002V
at C/20 without cycling at BOL), peaks associated with graphite, LiC6
and LiC12+LiC18 +LiC24 phases are observed. This observation aligns
with the data in Fig. 2where incomplete utilization of stage I (LiC6)
occurred for these electrodes even when cycling to 0.002V at very low
C-rates. Similar inhomogeneity has also been observed using in-situ
neutron spectroscopy following charging of graphite.36 In the present
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Figure 4. (a), (c) Delithiated capacity balance during OL cycles, (b) evolution of graphite capacities at different stages with cycling, where the average delithiated
capacity after 10 OL cycles is used for scaling, (d) Average coulombic and Li stripping efficiency as a function of cycle number.
case some of the inhomogeneity for the non-OL samples is likely due
to the overpotential for the cell (Fig. S1). As OL occurs the combined
presence of Li metal and the lower voltage facilitates the conversion
to LiC6and the reduction in intensity of the peaks associated with
graphite and LiC12+LiC18 +LiC24.
Effect of relaxation on the detection signature of plating.—The
data in Figs. 2and 3strongly suggest that following the deposition
of Li during OL, a portion of the deposited Li intercalates into the
graphite. The extent of this process should thus be dictated by the rest
time between the lithiation and delithiation steps. Thus, we cycled a
Figure 5. SEM image of graphite samples: (a) pristine graphite, (b) cell 1 from group A at OL state (1.5% OLabs, lithiated) after 10 OL cycles, (c) cell 6 from
group C at OL state (5.5% OLabs, lithiated) after 10 OL cycles, (d) Cell 2 from group A (1.5% OLabs , delithiated) after 10 OL cycles, and (e) Cell 8 from group C
(5.5% OLabs, delithiated) after 10 OL cycles.
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Figure 6. (a) XRD reflections at completely delithiated state at 1.2V, (b) XRD reflections at lithiated and OL states, (c) d-spacings at lithiated and OL states. Here,
intensity is normalized by the pristine graphite’s peak intensity.
subset of cells without providing any rest after the OL cycles. Fig-
ures 7a and 7c show the comparison between different fractions of
delithiated capacities for the 5.5% OLabs cycling condition with and
without rest. While the cell with no rest still see an increase in graphite
capacity following OL, the extent in increase is less when a 1 h rest was
used. While the extent of irreversible capacity loss remained roughly
the same, significant difference in reversible stripped capacity can be
noticed in the sample with no rest after overlithiation. Figures 7b and
7d show the corresponding dQ.dV1peaks related to Li stripping at
the beginning of delithiation (<0.05V) after specific OL cycles with
and without rest. The dQ.dV1peaks with no rest after OL are well
defined and prominent from the very beginning of OL cycling unlike
the condition with rest.
Combined the incremental capacity curves during and after OL
suggest that complex electrode behavior occurs during cycling of
graphite electrodes which could impact the electrochemical detec-
tion of Li plating. The present experimental results align with other
published reports showing the underutilization of graphite by up to
16% (59 mAh g1) after formation at the BOL (Figs. 2and 4a
4c). The XRD data in Fig. 6highlights that this is in part due
to the incomplete conversion of graphite and the LiC12,LiC
LiC24 phases to LiC6. Upon OL any limitations due to overpoten-
tial are overcome and the plating of Li becomes favorable. Combined
the two conditions result in near complete lithiation during the lithi-
ation step of the cycling. While the specific capacity associated with
graphite delithiation readily approaches theoretical values (98.5% of
372 mAhg1) after OL it is evident from Figs. 2and 7that a dis-
tinct portion of the Li removed from the graphite during delithia-
tion is coming from deposited Li that subsequently chemically in-
serted into the graphite after the completion of the electrochemical
The chemical lithiation following OL has implications for the elec-
trochemical detection of Li plating when only minor amounts of plat-
ing have occurred. Based on the present data any underutilized space in
the graphite is available for subsequent end-of-charge chemical lithi-
ation if given sufficient time. Underutilized conditions are likely to
occur during overcharge, during fast charge scenarios and for cells
which display a low N:P ratio due to aging. One specific complication
is that the chemical lithiation which occurs during a rest (Figs. 3,4and
7) distinctly decreases the intensity of the dQ.dV1signature which
arises from the electrochemical stripping of plated Li. In part this is
likely at least partially the reason some reports on extreme fast charg-
ing in the literature fail to clearly observe a dQ.dV1signature for Li
stripping even when there is other evidence of Li plating.12 From Fig. 3
it is also evident that the reversibility (or reversible Li stripping) of the
Li is strongly dictated by the extent of OL, and a significant amount of
lithium must be plated to be able to detect plating via dQ.dV1meth-
ods. Such behavior indicates that dQ.dV1may be a lagging indicator
for Li plating detection.
Another prime method to characterize the presence of Li plating
is the use of CE. While Li plating broadly has lower CE in most
carbonate-based electrolytes than a graphite anode, complications in
use of only CE arise when only small quantities of Li are plated un-
less high precision equipment is used. For cells which undergo a 1h
rest between lithiation and delithiation a lower CE is observed but the
values still remain near 98.5% and the efficiency recovers on subse-
quent cycles. When no rest is included the CE shifts to 96% with a
similar but slightly lower recovery (see supplemental Fig. S3). The
relatively higher CE with rest could be attributed to the additional Li,
which chemically lithiated into the graphite, and becomes reversible
afterward, subsequently reducing the fraction of isolated dead Li as
compared to the no rest condition. This highlights that the rest period
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Figure 7. (a), (c) Delithiated capacity balance with and without rest time, respectively, for 5.5% OLabs cycling condition, (b) dQ.dV1peaks related to Li stripping
at the beginning of delithiation (<0.05V) with rest (d) dQ.dV1peaks related to Li stripping at the beginning of delithiation (<0.05V) without rest.
has a profound impact on the ability to use CE for detection purpose.
The recovery and still relatively high efficiency are in large part due to
the maintained SE, which exceeds 80% after 10 OL cycles. Combined
the efficiency measurements showcase that for CE to be relevant, very
tight experimental control is needed along with instrumentation with
appropriate measurement accuracy, precision and resolution for both
lithiation and delithiation.
Following overlithiation, it has been observed that the overall
delithiation capacity attributed to graphite increases. Based on elec-
trochemical and XRD analysis this has been linked with the chemical
lithation of graphite and the distinct shift in Li staging to a LiC6com-
position. The process is reversible with no short-term impact to the
graphite structure though repeated OL cycles do result in enhanced
loss of active material. The distinct complication from the chemical
lithiation is that it exacerbates complications in the early detection of
low-level Li plating during charging using methods such differential
capacity analysis. The present work also identifies that it is possible to
clearly quantify the stripping efficiency of low-level Li plating while
also highlighting efficiency trends may improve even after the ini-
tial OL cycles. This implies that the coulombic efficiency-based Li
plating detection methods, where monotonous decrease in coulom-
bic efficiency is considered as an indication of Li plating may not
be reliable for plating detection in full cell over extended cycling.
The possible complications for both differential capacity and electro-
chemical efficiency measurements highlight the need to further refine
electrochemical methods to identify and quantify the early stages of
Li plating in cells which are expected to maintain high cycle life while
undergoing aggressive use cases such as extreme fast charging.
Funding was provided from the Vehicle Technologies Office of the
U.S. Department of Energy’s Office of Energy Efficiency and Renew-
able Energy under the guidance of the Advanced Battery Cell Research
Program (eXtreme Fast Charge Cell Evaluation of Lithium-ion Bat-
teries, XCEL). This manuscript has been authored by Battelle Energy
Alliance, LLC under Contract No. AC07-05ID14517 with the U.S.
Department of Energy. The authors thank Bryant Polzin at Argonne
and Peter Faguy in the Vehicle Technologies Office of DOE-EERE
for access to electrode laminate samples. The authors thank Samuel
Gillard from the U.S. Department of Energy for supporting this project.
The United States Government retains and the publisher, by accept-
ing the article for publication, acknowledges that the United States
Government retains a non-exclusive, paid-up, irrevocable, worldwide
license to publish or reproduce the published form of this manuscript,
or allow others to do so, for United States Government purposes.The
XRD measurements were performed at the Center for Advanced En-
ergy Studies (CAES). The authors would like to thank Jatuporn Burns
for her assistance in collecting the XRD spectra.
List of Symbols
BEV battery electric vehicles
BOL beginning-of-life
CC constant current
CV constant voltage
CE coulombic efficiency
Gr graphite
LIB lithium-ion battery
OL overlithiated
SEI solid electrolyte interphase
) unless CC License in place (see abstract). address. Redistribution subject to ECS terms of use (see on 2019-08-06 to IP
A2696 Journal of The Electrochemical Society,166 (12) A2689-A2696 (2019)
SEM scanning electron microscopy
XRD X-ray diffraction
Tanvir R . Tanim
Eric J. Dufek
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... We believe that the varying anode morphology is a result of the formation of the plated metallic Li from extended XFC cycling at 9 C-rate for 450 cycles, which we have previously confirmed with XRD. [41][42][43] In the present study, the plated, metallic Li was exposed to moisture during the pouch disassembly and/or subsequent sample handling and converted into lithium hydroxide (LiOH) as per Eq. 1 and confirmed by our XRD measurement ( Figure S2). ...
... For example, the lines labeled c and d on the 3D visualization and 2D segmented slices inFigure 5B(left) correspond to the thick yellow lines labeled c and d on the optical image inFigure 5B(right). We know from our previous works 36,[41][42][43] on mm-scale spatial XRD maps of the cycled anode that the white deposits in the optical image correspond to irreversibly plated Li containing trapped lithiated graphite (LiC 6 ...
Full-text available
Extreme fast charging (XFC) of commercial lithium-ion batteries (LIBs) in ≤10-15 minutes will significantly advance the deployment of electric vehicles globally. However, XFC leads to considerable capacity fade, mainly due to graphite anode degradation. Non-destructive three-dimensional (3D) investigation of XFC-cycled anodes is crucial to connect degradation with capacity loss. Here, we demonstrate the viability of simultaneous neutron and X-ray tomography (NeXT) for ex-situ 3D visualization of graphite anode degradation. NeXT is advantageous because of the sensitivity of neutrons to Li and H and X-rays to Cu. We combine the neutron and X-ray tomography with micron resolution for material identification and segmentation on one pristine and one XFC-cycled graphite anode, thereby underscoring the benefits of the simultaneous nature of NeXT. Our ex-situ results pave the way for the design of NeXT-friendly LIB geometries that will allow operando and/or in-situ 3D visualization of graphite anode degradation during XFC.
... 5,[20][21][22][23][24][25] Up to now, no reliable electrochemical operando or in situ method exists to meet the demands in terms of specificity and sensitivity for an early Li plating detection in a given 2-electrode full cell system. [26][27][28][29][30][31][32][33][34] Generally, Coulombic efficiency or differential analysis of voltage and/or capacity are used for early Li plating detection, however, none of these techniques are able to prevent Li plating in the first place. [26][27][28]30,31,33,[35][36][37][38][39] The easiest and safest method to prohibit occurrence of Li plating at the NE is by staying well above its onset conditions. ...
... [26][27][28][29][30][31][32][33][34] Generally, Coulombic efficiency or differential analysis of voltage and/or capacity are used for early Li plating detection, however, none of these techniques are able to prevent Li plating in the first place. [26][27][28]30,31,33,[35][36][37][38][39] The easiest and safest method to prohibit occurrence of Li plating at the NE is by staying well above its onset conditions. Doing so, both energy density as well as charging power in LIB cells are sacrificed. ...
Full-text available
A method to determine threshold voltage conditions for Li plating in lithium ion battery cells is presented. Transferring open-circuit values determined in a 3-electrode electrochemical measurement onto a 2-electrode cell setup, the boundary conditions for Li plating can be assessed. In multi-layer pouch cells, these boundary conditions agree perfectly with the exact onset of Li plating as proven by post mortem analysis. By knowledge of the Li plating threshold voltage conditions, plating-free fast-charging procedures can be exercised leading to an increase in charging rate by 84% and 79% for two different cell systems, respectively. Cycling above or below the Li plating threshold voltage, Li plating occurrence can be deliberately controlled. Comparing plating and plating-free conditions, the applied charging voltage properties differ hardly. Hence, the applied analysis of overvoltage proves a more sensitive and specific operando method to predict Li plating.
... The estimation of plating reversibility on graphite at standard SOC (below 100%) and ambient temperatures is challenging due to the rapid dissolution of reversible Li deposits, which supports Li + re-intercalation into the graphite 27,29,30 . A workaround to this is to study plating during graphite overcharge (above 100% SOC) 34,35 , which has also emerged in the context of hybrid graphite/lithium anodes [36][37][38] , but reversibility estimates have only been reported at low current rates (<0.5C) and/or are deduced from qualitative voltage plateau transitions. ...
Full-text available
Fast charging of most commercial lithium-ion batteries is limited due to fear of lithium plating on the graphite anode, which is difficult to detect and poses considerable safety risk. Here we demonstrate the power of simple, accessible and high-throughput cycling techniques to quantify irreversible Li plating spanning data from over 200 cells. We first observe the effects of energy density, charge rate, temperature and state of charge on lithium plating, use the results to refine a mature physics-based electrochemical model and provide an interpretable empirical equation for predicting the plating onset state of charge. We then explore the reversibility of lithium plating and its connection to electrolyte design for preventing irreversible Li accumulation. Finally, we design a method to quantify in situ Li plating for commercially relevant graphite|LiNi0.5Mn0.3Co0.2O2 (NMC) cells and compare with results from the experimentally convenient Li|graphite configuration. The hypotheses and abundant data herein were generated primarily with equipment universal to the battery researcher, encouraging further development of innovative testing methods and data processing that enable rapid battery engineering.
... However, their detection time is up to several hours, 21 and they can only detect lithium plating after the charging process instead of in real-time. Incremental capacity analysis (ICA) 22,23 can only detect lithium plating after long-term cycling and is difficult to distinguish between lithium plating and other aging mechanisms. Coulombic efficiency (CE) 24 and electrochemical impedance spectroscopy (EIS) 25 can detect lithium plating by manually finding a unique feature in their evolutions during cycling, but the requirements to fully charge and discharge the battery, the long detection time, and the requirement for high precision instruments limit their applicability. ...
Full-text available
Lithium plating seriously threatens the life of lithium‐ion batteries at low temperatures charging conditions, but the onboard detection and quantification of lithium plating are severely hampered by the limited available signals and volatile operating conditions in real scenarios. Herein, we propose a detection method to predict the occurrence and quantification of lithium plating under uncertain conditions by only using constant‐current curves during charging based on deep learning. A deep neural network (DNN) is developed to extract data‐driven features induced by lithium plating from the charge curves, avoiding the challenge of manual feature selection. Only using the most common voltage and current signals as inputs, the network exhibits superior adaptability and accuracy. The detection accuracy of the proposed method is 98.64%, while the quantity of the lithium plating can be accurately predicted with a root‐mean‐square error <4.1712 mg. Moreover, the generalization ability of the proposed method is verified by its reliable detection accuracy under conditions that are not used in the training dataset. The detection accuracy is 92.39% for brand new charging conditions and 95.53% for brand new aging states. This method shortens the detection time that currently takes more than several hours (the widely used differential curve analysis) to milliseconds and eliminates the need for a rigorous testing environment, showing great potential for onboard application in future battery management systems. A data‐driven detection method is proposed to predict the occurrence and quantification of lithium plating under uncertain conditions by only using constant‐current curves during charging based on deep learning. Moreover, the generalization ability of the proposed method is verified by its reliable detection accuracy under conditions that are not used in the training dataset.
... We note that lithium plating can also be a partially reversible process, where stripping efficiency can be improved by changing electrolyte properties, the rest period and the rate at which the charging event occurs [58]. When stripping efficiency is high, the total capacity loss due to lithium plating is minimal during early cycles, but the combined effect of even minor loss due to irreversible lithium plating and additional SEI formation does eventually lead to increased capacity loss [59]. Due to the loss of lithium and the complexity of achieving high reversibility for plated lithium, lithium plating has been identified as a significant inhibitor to reliable fast charging and XFC cells and has been a subject of intense recent research [1]. ...
Extreme fast charging (XFC) has become a focal research point in the lithium-battery community over the last several years. As adoption of electric vehicles increases, fast charging has become a key driver in enhancing consumer recharge experience. Recently, the research community has made significant improvements in developing charge protocols to support XFC. New charge protocol designs derived using a combination of advanced, physically derived models, and electrochemical and secondary characterization methods, increase charge acceptance and decrease aging. By coordinating these methods and modifying protocols to account for different material constraints, including lithium plating and cathode particle degradation, novel charge protocols have increased the energy accepted during charging by over 25% in 10 min and increased the charge acceptance prior to a constant-voltage step by approximately 3x. Here, we review several charge-protocol advances, aging factors which are enhanced by XFC and advances which will enable adoption of XFC capable vehicles. These advances include implementing machine learning and other detection algorithms to reduce and classify lithium plating, which is known to significantly degrade cell performance and reduce cell life. The review concludes by discussing full-system fast charge requirements, including electric vehicle service equipment needs for implementing XFC protocols.
Increasing the charging rate and reducing the charging time for Li-ion batteries are crucial to realize the mainstream of electric vehicles. However, it is formidable to avoid the Li plating on graphite anode upon fast charging. Despite the tremendous progress in Li detection techniques, the fundamental mechanism of Li plating and its chemical/electrochemical responses upon cycling still remains elusive. Herein, we present a comprehensive electrochemical method to investigate the fast charging behavior of graphite electrode. A detailed analysis is directed toward understanding the changes in phase, composition, and morphology of the fast-charged graphite. By applying a resting process, we scrutinize the further reactions of the plated Li, which readily transforms into irreversible (dead) Li. We further develop a modified graphite electrode with a thin Ag coating as the Li reservoir. The plated Li can be "absorbed" by the Ag layer to form the Li-Ag solid solution that suppresses the formation of dead Li and provides structural stability, thus promoting the further lithiation of graphite and enhancing the reversibility. This work not only provides additional insights into the fast charging behavior of graphite electrode but also demonstrates a potential strategy to improve the fast charging performance of graphite anode.
Advanced battery characterization using neutron and X-ray-based imaging modalities is crucial to reveal fundamental degradation modes of lithium-ion batteries (LIBs). Taking advantage of the sensitivity of neutrons to some low-Z (Li) and X-rays to high-Z materials (Cu), here we demonstrate the viability of simultaneous neutron- and X-ray-based tomography (NeXT) as a non-destructive imaging platform for ex situ 3D visualization of graphite electrode degradation following extreme fast charging (XFC). In addition, we underscore the benefits of the simultaneous nature of NeXT by combining the neutron and X-ray data from the same sample location for material identification and segmentation of one pristine and two XFC-cycled graphite electrodes (9C charge for 450 cycles). Our ex situ results and methodology development pave the way for the design of NeXT-friendly LIB geometries that will allow operando and/or in situ three-dimensional (3D) visualization of electrode degradation during XFC.
Lithium plating on the negative electrode of Li-ion batteries remainsa great concern for durability, reliability, and safety in operation under low temperatures and fast charging conditions. High-accuracy detection of Li-plating is critically needed for field operations. To detect the lithium plating is to track its multiphysics footprint since Li plating often is a localized event while the driving force from chemical, electrical, thermal, and mechanical origins could vary with time and locality which makes the detection and characterization challenging. Here, we summarize the multiphysical footprints of lithium plating and the corresponding state-of-the-art detection methods. By assessing and comparing these methods, the combination of capacity/voltage differential, R-Q mapping and Arrhenius outlier tracking could be promising and effective for battery diagnosis, prognosis, and management. We analyze the origins of quantitative error in sample preparation, overly simplified assumption, and dynamic evolution of the plated Li, and recommend the in-situ and quantitative chemical analysis methods, such as in-situ nuclear magnetron resonance, electron paramagnetic resonance, X-ray, and neutron. In addition, we propose four conjectures on the capacity plunge, Li plating, pore clogging, electrolyte drainage, and rapid SEI growth, can be aligned and unified to one scenario basically triggered by Li plating.
A key degradation mechanism in lithium-ion batteries (LIBs) is the irreversible loss of cyclable lithium during cycling. At the graphite negative electrode, this loss occurs through the deposition of lithium-containing compounds in the solid-electrolyte interphase (SEI) and through plating of metallic lithium, resulting in so-called dead lithium. The separate quantification of SEI and dead lithium has so far been a challenge in post mortem analysis of commercial LIBs. Here we report a simple and fast ⁷Li nuclear magnetic resonance spectroscopy (NMR) protocol applied to solid-state samples derived from lab-built batteries to independently quantify these and other lithium species in graphite electrodes without the need for specialized cell design nor knowledge of prior charging history. The metallic lithium content is corroborated by electrochemical calculations; the total amount of lithium is also determined from ⁷Li liquid-state NMR and inductively coupled plasma optical emission spectroscopy (ICP-OES) in suitably digested samples. Factors influencing accuracy like the sample handling process, the radiofrequency skin effect, and re-intercalation losses are investigated. Measurements on samples from commercial cells aged under realistic conditions demonstrate quantification of dead lithium and remaining ionic species (SEI), and further reveal lithium dendrites entrained in the separator following cell disassembly. The method uses conventional and widely available NMR instrumentation and is applicable to samples from lab-scale test cells or commercial batteries, thereby presenting a vast improvement over prior post mortem methods.
Excellent fast‐charging performance is a key requirement for Li‐ion batteries intended for automotive applications. Rational particle design for active materials within electrodes represents a strategic approach to minimize kinetic limitations ‐ especially for the anode, where the lithium intercalation rate affects the overall cell charging capacity at elevated current densities. Typically, for practical applications, natural graphite flakes are shaped into rounded particles via a mechanical spheroidization process. In this work, we show that both surface and bulk particle properties correlate strongly with the applied spheroidization conditions, and directly affect the electrochemical performance, particularly in terms of lithium intercalation rate. We demonstrate that graphite particles with a surface rich in prismatic planes, structural defects, and oxygen‐rich groups are favorable for fast lithium uptake. The influence of the graphite particle characteristics on the lithium intercalation rate plays a key role at the electrode and cell level, affecting the overall cell performance. We provide new insights into particle optimization during spheroidization as an effective strategy for developing fast‐charging Li‐ion batteries.
Full-text available
Enabling a 10 min fast charge for electric vehicles is a possible route to reduce range anxiety and increase the utility of electric vehicles. While lithium plating during fast charging is a known challenge, the full suite of limitations which occur in full cells during a 10 min fast charge are unknown. In the present work the central constraints of extreme fast charging are explored through extensive experiments and analysis in single layer graphite/NMC532 pouch cells. Methods of developing fast-charging protocols considering the impedance and transport limitations are presented and the relative benefits of altering the charging rate, profile, relaxation, etc., are investigated. Analysis during and at the end of cycling identified both known and unexpected aging pathways. The most distinct outcomes from the work are a significant increase in cell-to-cell variability as the number of fast charge cycles increase [up to 11% (1σ)] and the identification of distinct aging of the NMC532 positive electrode including cracking of the secondary particles and a trend towards under-lithiation of the positive electrode. While significant aging of the positive electrode was observed, only a few conditions had discernible Li plating and no distinct reversible Li signature was seen during periodic reference performance tests.
Full-text available
Fast charging of batteries is currently limited, particularly at low temperatures, due to difficulties in understanding lithium plating. Accurate, online quantification of lithium plating increases safety, enables charging at speeds closer to the electrochemical limit and accelerates charge profile development. This work uses different cell cooling strategies to expose how voltage plateaus arising from cell self-heating and concentration gradients during fast charging can falsely indicate plating, contrary to prevalent current assumptions. A solution is provided using Differential Voltage (DV) analysis, which confirms that lithium stripping is observable. However, scanning electron microscopy and energy-dispersive X-ray analysis are used to demonstrate the inability of the plateau technique to detect plating under certain conditions. The work highlights error in conventional plating quantification that leads to the dangerous underestimation of plated amounts. A novel method of using voltage plateau end-point gradients is proposed to extend the sensitivity of the technique, enabling measurement of lower levels of lithium stripping and plating. The results are especially relevant to automotive OEMs and engineers wishing to expand their online and offline tools for fast charging algorithm development, charge management and state-of-health diagnostics.
Full-text available
This study investigates the effect of 50-kW (about 2C) direct current fast charging on a full-size battery electric vehicle's battery pack in comparison to a pack exclusively charged at 3.3 kW, which is the common alternating current Level 2 charging power level. Comparable scaled charging protocols are also independently applied to individual cells at three different temperatures, 20 °C, 30 °C, and 40 °C, to perform a comparative analysis with the packs. Dominant cell-level aging modes were identified through incremental capacity analysis and compared with full packs to gain a clear understanding of additional key factors that affect pack aging. While the cell-level study showed a minor impact on performance due to direct current fast charging, the packs showed a significantly higher rate of capacity fade under similar charging protocols. This indicates that pack-level aging cannot be directly extrapolated from cell evaluation. Delayed fast charging, completing shortly before discharge, was found to have less of an impact on battery degradation than conventional alternating current Level 2 charging
Technical Report
Full-text available
In this report, researchers at Idaho National Laboratory teamed with Argonne National Laboratory and the National Renewable Energy Laboratory to identify technical gaps to implementing an extreme fast charging network in the United States. This report highlights technical gaps at the battery, vehicle, and infrastructure levels. For full text, go to:
Full-text available
Lithium ion batteries as a power source are dominating in portable electronics, penetrating the electric vehicle market, and on the verge of entering the utility market for grid-energy storage. Depending on the application, trade-offs among the various performance parameters—energy, power, cycle life, cost, safety, and environmental impact—are often needed, which are linked to severe materials chemistry challenges. The current lithium ion battery technology is based on insertion-reaction electrodes and organic liquid electrolytes. With an aim to increase the energy density or optimize the other performance parameters, new electrode materials based on both insertion reaction and dominantly conversion reaction along with solid electrolytes and lithium metal anode are being intensively pursued. This article presents an outlook on lithium ion technology by providing first the current status and then the progress and challenges with the ongoing approaches. In light of the formidable challenges with some of the approaches, the article finally points out practically viable near-term strategies.
Most lithium-ion batteries still rely on intercalation-type graphite materials for anodes, so it is important to consider their role in full cells for applications in electric vehicles. Here, we systematically evaluate the chemical and physical properties of six commercially-available natural and synthetic graphites to establish which factors have the greatest impact on the cycling stability of full cells with nickel-rich LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes. Electrochemical data and post-mortem characterization explain the origin of capacity fade. The NMC811 cathode shows large irreversible capacity loss and impedance growth, accounting for much of full cell degradation. However, six graphite anodes demonstrate significant differences with respect to structural change, surface area, impedance growth, and SEI chemistry, which impact overall capacity retention. We found long cycle life correlated most strongly with stable graphite crystallite size. In addition, graphites with lower surface area generally had higher coulombic efficiencies during formation cycles, which led to more stable long-term cycling. The best graphite screened here enables a capacity retention around 90% in full pouch cells over extensive long-term cycling compared to only 82% for cells with the lowest performing graphite. The results show that optimal graphite selection improves cycling stability of high energy lithium-ion cells.
The battery technology literature is reviewed, with an emphasis on key elements that limit extreme fast charging. Key gaps in existing elements of the technology are presented as well as developmental needs. Among these needs are advanced models and methods to detect and prevent lithium plating; new positive-electrode materials which are less prone to stress-induced failure; better electrode designs to accommodate very rapid diffusion in and out of the electrode; measure temperature distributions during fast charge to enable/validate models; and develop thermal management and pack designs to accommodate the higher operating voltage. Find the full text in the appendix of the XFC report at:
Using information gained from parallel investigations on lithiated/over-lithiated graphite and hard carbon, we propose two different processes for lithium nucleation and dendrite growth in the carbon allotropes based on their different crystal structures. In the case of graphite, lithium nucleation and initial growth of lithium dendrites occur inhomgenerously on graphite‘s surface during the over-lithiation process, which exposes lithium nucleation to the electrolyte causing formation of a large amount of Li2CO3, LiRC2 and knobby dendrites covered with a thick lithium electrolyte interphase (LEI). However, in the case of hard carbon, lithium nucleation and initial growth occur inside the hard carbon nanopores, which limits side reactions, provides a high capacity (~550 mAh/g vs ~370 mAh/g for graphite), and generates dendrites with smooth clean surfaces during the over-lithiation process. These findings could profoundly influence the design enhancement, the improvement of performance, and the inherent safety of carbon based electrodes for lithium ion batteries.
In this study, lithium gradients forming in the graphite anode of a commercial 18650-type lithium-ion battery during discharge and the associated relaxation processes after discharge were monitored by neutron diffraction. The experiments reveal the coexistence of several Li1-xC6 phases with different lithium contents during discharge, which can be explained by the formation of an inhomogeneity or lithium gradient in the graphite anode. The observed inhomogeneity is more pronounced at higher discharging rates, but at low temperatures it appears at a rate as low as C/10. After discharge, the inhomogeneity gradually disappears and the coexisting phases diminish in favor of one or several Li1-xC6 phases with close to mean lithium content. At room temperature these relaxation processes take 20–40 min with the main changes in the first 10 min. In contrast, at −20 °C changes are still observed after 11 h. The observed phenomena can be explained by a faster delithiation of the graphite particles than the equilibration of the resulting lithium gradient by lithium diffusion in the solid phase during discharge.
A physics-based Li-ion battery (LIB) aging model accounting for both lithium plating and solid electrolyte interphase (SEI) growth is presented, and is applied to study the aging behavior of a cell undergoing prolonged cycling at moderate operating conditions. Cell aging is found to be linear in the early stage of cycling but highly nonlinear in the end with rapid capacity drop and resistance rise. The linear aging stage is found to be dominated by SEI growth, while the transition from linear to nonlinear aging is attributed to the sharp rise of lithium plating rate. Lithium plating starts to occur in a narrow portion of the anode near the separator after a certain number of cycles. The onset of lithium plating is attributed to the drop of anode porosity associated with SEI growth, which aggravates the local electrolyte potential gradient in the anode. The presence of lithium metal accelerates the porosity reduction, further promoting lithium plating. This positive feedback leads to exponential increase of lithium plating rate in the late stage of cycling, as well as local pore clogging near the anode/separator interface which in turn leads to a sharp resistance rise.