Electrochemical Quantification of Lithium Plating: Challenges and Considerations

Abstract

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.

Full text

Journal of The Electrochemical Society,166 (12) A2689-A2696 (2019) A2689
Electrochemical Quantification of Lithium Plating: Challenges and
Considerations
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 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−1signal 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, 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: 10.1149/2.1581912jes]
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.7–9
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 plating10–12 which is known
to deleteriously impact cycle life.12–14 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.10–12,15–17
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.
zE-mail: Eric.Dufek@inl.gov
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

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 (mAhg−1)∗OLBOL(%) OLabs (mAhg−1)
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.
Experimental
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 cm−2[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 mAhg−1was used to calculate the absolute percentage of OL,
OLabs.
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 mAhg−1) 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 mAhg−1with 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.dV−1plots 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 mAhg−1.

Journal of The Electrochemical Society,166 (12) A2689-A2696 (2019) A2691
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.dV−1plots (iR corrected) during
lithiation, (c) Voltage vs delithiated capacity: BOL and cycling at C/20, and (d) dQ.dV−1(iR corrected) plots during delithiation.
show the corresponding dQ.dV−1peaks 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.dV−1profiles 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.dV−1plots 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.21,33–34 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.dV−1peak 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 I´in 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
ratesusedinSuetal.’sstudy.
Figures 3a and 3c show that increasing the extent of OL results
in a stronger dQ.dV−1peak 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.dV−1
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.dV−1peaks 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

A2692 Journal of The Electrochemical Society,166 (12) A2689-A2696 (2019)
Figure 3. dQ.dV−1plots 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 (I´in Fig. 2c).
Such analysis enables quantification of capacity from graphite and Li
stripping. Coulombic efficiency is CE =100 ×QD/QL,whereQ
D=
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 g−1)
lower than the theoretical capacity. Similar underutilization behav-
ior of graphite is reported elsewhere with capacities ranging from
229–360 mAhg−1for 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 mAhg−1or 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 g−1of 59 mAhg−1) 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).
PriortoOLattheBOL,Q
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

Journal of The Electrochemical Society,166 (12) A2689-A2696 (2019) A2693
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.

A2694 Journal of The Electrochemical Society,166 (12) A2689-A2696 (2019)
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.dV−1peaks related to Li stripping at
the beginning of delithiation (<0.05V) after specific OL cycles with
and without rest. The dQ.dV−1peaks 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 g−1) 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
18,and
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 mAhg−1) 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
lithiation.
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.dV−1signature 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.dV−1signature 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.dV−1meth-
ods. Such behavior indicates that dQ.dV−1may 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

Journal of The Electrochemical Society,166 (12) A2689-A2696 (2019) A2695
Figure 7. (a), (c) Delithiated capacity balance with and without rest time, respectively, for 5.5% OLabs cycling condition, (b) dQ.dV−1peaks related to Li stripping
at the beginning of delithiation (<0.05V) with rest (d) dQ.dV−1peaks 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.
Conclusions
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.
Acknowledgment
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

A2696 Journal of The Electrochemical Society,166 (12) A2689-A2696 (2019)
SEM scanning electron microscopy
XRD X-ray diffraction
ORCID
Tanvir R . Tanim https://orcid.org/0000-0002-1864-6868
Eric J. Dufek https://orcid.org/0000-0003-4802-1997
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