Evaluating electrolyte additives for lithium-ion cells: A new Figure of Merit approach

Journal of Power Sources 365 (2017) 201e209

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Evaluating electrolyte additives for lithium-ion cells: A new Figure of Merit approach Adam Tornheim, Cameron Peebles, James A. Gilbert, Ritu Sahore, Juan C. Garcia, ~ o, Hakim Iddir, Chen Liao, Daniel P. Abraham* Javier Baren Argonne National Laboratory, Argonne, IL 60439, USA

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Figure of merit approach developed to rank electrolyte formulations.
Separate figures of merit defined based on cell energy and power retention.
Fifteen electrolyte additive combinations evaluated relative to a baseline electrolyte.
Cells containing a 0.25 wt% tVCBO þ 1 wt% TMSPi additive mix outperformed the baseline.
Mechanisms that govern energy and power retention are likely independent of each other.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 August 2017 Received in revised form 20 August 2017 Accepted 23 August 2017

Electrolyte additives are known to improve the performance of lithium-ion cells. In this work we examine the performance of Li1.03Ni0.5Mn0.3Co0.3O2-graphite (NMC532/Gr) cells containing combinations of lithium bis(oxalate)borate (LiBOB), vinylene carbonate (VC), trivinylcyclotriboroxane (tVCBO), prop-1ene-1,3-sultone (PES), phenyl boronic acid ethylene glycol ester (PBE), tris(trimethylsilyl) phosphite (TMSPi), triethyl phosphite (TEPi), and lithium difluoro(oxalate)borate (LiDFOB) added to our baseline (1.2 M LiPF6 in EC:EMC, 3:7 w/w) electrolyte. In order to rank performance of the various electrolytes, we developed two separate figures of merit (FOM), which are based on the energy retention and power retention of the cells. Using these two metrics in conjunction, we show that only one of the fifteen electrolyte formulations tested significantly outperforms the baseline electrolyte: this electrolyte contains the 0.25 wt% tVCBO þ 1 wt% TMSPi additive mix. Little correlation was observed between the FOMs for energy retention and power retention, which indicates that the mechanisms that govern these performance parameters are likely independent of each other. Our FOM approach has general applicability and can be used to develop electrolyte and electrode formulations that prolong the life of lithiumion batteries. © 2017 Elsevier B.V. All rights reserved.

Keywords: Capacity fade Impedance rise Energy density Power density Lithium-ion Full cells

1. Introduction

* Corresponding author. E-mail address: [email protected] (D.P. Abraham). http://dx.doi.org/10.1016/j.jpowsour.2017.08.093 0378-7753/© 2017 Elsevier B.V. All rights reserved.

Lithium-ion batteries (LIBs) have seen widespread deployment in the transportation sector due to their superior power and energy densities compared to other battery chemistries [1]. Among the

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positive electrode (cathode) types used in LIBs, layered oxides of the formula LiMO2 (M ¼ Mn, Co, Ni) are common because of their high theoretical gravimetric capacity (>270 mAhg1) [2] and relatively high discharge voltages [3]. These LiMO2-based cathodes are used in conjunction with graphite (Gr)-based negative electrodes (anodes) and carbonate-based electrolytes in LIB cells. However, conventional organic carbonate electrolytes are not electrochemically stable at the elevated voltages (>4.4 V) required to achieve high cell capacities [3e5]. One strategy to maintain high voltage performance is through the use of additives; these components make up a small percentage (<10%) of the electrolyte and are designed to change its effective properties. An effective electrolyte may comprise a combination of additives to address the various degradation mechanisms in the cell. The electrochemical performance of LIB cells is characterized in many ways, with one of the most prevalent being the measurement of discharge capacity (and capacity retention) as a function of cycle number. Impedance rise, which would decrease both power delivery during vehicle acceleration and efficiency of regenerative braking, is also an important metric. Long term cycling tests of LIB cells have shown that capacity fade results from a continual growth of the solid-electrolyte interphase (SEI) layer at the graphite (negative) electrode through electrolyte reduction reactions that immobilize Liþ ions. Impedance rise is often attributed to processes at the LiMO2 positive electrode [6,7], where oxygen can be released from the oxide surface at high voltage [5,8], and transition metalion dissolution becomes more prevalent [9]. Cross talk between the electrodes has also been reported [10,11]. For example, transition metal (TM) ion dissolution from the cathode can lead to both impedance rise as well as capacity fade through additional electrolyte decomposition at the anode [12e14]; also, electrolyte decomposition species that form at one electrode can migrate and react at the other electrode [15]. These degradation processes, which lower both energy and power densities, are exacerbated when the cells are cycled at high voltages [5]. The degradation can be mitigated by using electrolytes that are more stable at both electrodes than the conventional carbonate electrolytes. Finding stable electrolytes is crucial to the long term deployment of LIBs for transportation applications. A combinatorial approach to electrolyte-additive formulations has been pursued in previous work, with Li(Ni0.4Mn0.4Co0.2)O2/Gr pouch cells with a 4.4 and 4.5 V upper cutoff voltage (UCV) [16], Li(Ni1/3Mn1/3Co1/3)O2/Gr pouch cells with a 4.2 V UCV [17], and Li1.2(Ni0.15Mn0.55Co0.1)O2/Gr coin cells with a 4.6 V UCV [7], as examples. In this work, we investigate fifteen electrolyte-additive combinations using Li1.03(Ni0.5Mn0.3Co0.2)0.97O2 (NMC532)/Gr full cells. The electrochemical data from these cells were evaluated using multiple metrics: we developed a Figure of Merit (FOM) approach to rank the performance of the various electrolyte formulations. The additive compounds we tested have been reported to improve performance through beneficial reactions at the positive electrode and/or the negative electrode. Triethyl phosphite (TEPi) [18], tris(trimethylsilyl) phosphite (TMSPi) [19e21], and lithium difluoro(oxalate)borate (LiDFOB) [22,23] were selected because these compounds are believed to minimize degradation processes at the cathode. Lithium bis(oxalate)borate (LiBOB) [24,25], vinylene carbonate (VC) [16,26,27], trivinylcyclotriboroxane (tVCBO) [28], prop-1-ene-1,3-sultone (PES) [29e31], and phenyl boronic acid ethylene glycol ester (PBE) [7] were selected because these compounds are believed to enhance SEI stability on the graphite anode. For many of these additive compounds, details of the “electrode protection” mechanism are not fully known; some additives are hypothesized to work on both electrodes, and are considered bifunctional in LIB cells [22,23,27,29]. The supporting tables and figures for this article are placed in

the Supporting Information (SI). When referenced in the text, these materials have the designator “S”, as in Fig. S1. 2. Experimental 2.1. Electrode and electrolyte details Our electrodes are from Argonne's Cell Analysis, Modeling and Prototyping (CAMP) facility [6]. The positive electrode contained a coating of 90 wt% Li1.03(Ni0.5Mn0.3Co0.2)0.97O2 (Toda), 5 wt% C45 (Timcal), and 5 wt% PVdF (Solvay 5130), with a loading of 9.17 mgcm2, on a 20 mm Al current collector: the electrode was calendared to 33.5% porosity and a coating thickness of 34 mm. The negative electrode contained a coating of 91.8 wt% graphite (ConocoPhillips), 2 wt% C45 (Timcal), 0.17 wt% oxalic acid, and 6 wt% KF9300 PVdF binder (Kureha), with a loading of 5.88 mgcm2, on a 10 mm Cu current collector: the electrode was calendared to 38.4% porosity and a coating thickness of 44 mm. In our study, the baseline electrolyte was 1.2 M LiPF6 in 3:7 w/w ethylene carbonate (EC):ethyl methyl carbonate (EMC). The structure, molecular weight, and concentrations (by wt%) of the electrolyte additives are shown in Fig. 1. The additive concentrations (2% VC, 0.25% tVCBO, 2% PES, 0.25% PBE, 1% LiBOB, 1% TEPi, 1% TMSPi, and 2% LiDFOB) were based on prior work at Argonne and from information in the research literature. The selected concentrations are reported to yield performance benefits when used

Fig. 1. Structure, molecular weight (MW) and concentration (wt%) of compounds used in our electrolyte formulations. These compounds were added to our baseline (1.2 M LiPF6 in EC:EMC, 3:7 w/w) electrolyte.

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individually in LIB cells. For this work, we did not optimize the concentrations for best performance in our NMC532/Gr cells. The 2,4,6-trivinylcyclotriboroxane (tVCBO) (ACS Scientific, Inc., 97%), prop-1-ene-1,3-sultone (PES) (TCI America, >98%), tris(trimethylsilyl) phosphite (TMSPi) (Sigma Aldrich, >95%), and triethyl phosphite (TEPi) (Sigma Aldrich, 98%) were all purchased and used without further purification. Vinylene carbonate (VC) (TCI America, >97%) was distilled under vacuum to remove trace impurities and any residual water. Phenyl boronic acid ethylene glycol ester (PBE) was synthesized and purified according to a previously published procedure [32]. The lithium difluoro(oxalato)borate (LiDFOB) and lithium bisoxalatoborate (LiBOB) are from Argonne's Materials Engineering Research Facility (MERF); the salts were purified via recrystallization according to a previously reported procedure [33]. 2.2. Cell assembly and electrochemical testing The 2032-format coin cell assembly and testing followed standard procedures that are detailed in a recent article by Long and coworkers [34], which evaluated the effect of cathode/anode electrode diameters and electrolyte contents [34]. Before cell assembly, the electrodes were dried at 110  C, and the separator at 70  C, in vacuum ovens. Each cell, containing a 14 mm dia. cathode, 15 mm dia. anode, 16 mm dia. Celgard 2325 separator, and ~25 mL electrolyte, was assembled in an argon-atmosphere glove box. The coin cells were contained inside a 30  C constant temperature chamber and all the 3e4.4 V cycling tests were conducted with a Series 4000 Test Unit battery cycler (MACCOR). The electrochemical cycling protocol was described earlier in the work by Long and coworkers [34]; details of the protocol are provided in Table S1 of the Supplementary. In essence, after the four initial (formation) C/10 cycles, the protocol includes repetitions of the following: (i) a C/10 cycle to obtain capacity data at low-rates; (ii) a C/1 cycle; (iii) a modified hybrid pulse power characterization (HPPC) protocol, using 2C 10s discharge pulses and a 1.5C charge pulses, to determine area-specific impedance (ASI); (iv) 20 C/3 (aging) cycles with a 3 h hold at top of charge (4.4 V) to accelerate aging, and a voltage hold during the 20th discharge cycle until the current decreases to a < C/20 value, to better estimate the “true” cell capacity. A graphical representation of the cycling protocol is provided in Fig. S1 of the Supplementary. 3. Results and discussion 3.1. Methodology to determine the Figures of Merit As explained later, the energy and power FOMs are expressed relative to performance of the NMC532/graphite cells with the baseline electrolyte; data from these baseline cells are displayed in Fig. 2. Fig. 2a shows that the discharge capacity decreases with cycle number; the average initial discharge capacity 1 is 191 mAhgoxide and the (C/10) capacity retention after 119 cycles is 86%. The energy density (ED) values for the various cycles can be obtained by integrating the area in the capacityvoltage plots: we obtained these ED values directly from the cycler data output. Fig. 2b shows that the ASI increases with cycle number; as seen in the data, this increase is not uniform over the voltage range. The average initial ASI at the 2nd HPPC pulse (~4.08 V) is 24 Ucm2 (see curve 1 in Fig. 2b) and the ASI increase (difference between curve 5 and curve 1) at this pulse location is 20.9 Ucm2. To obtain power density (PD) values we used the equation described by Bloom et al. [35], which is as follows:

Discharge pulse power ¼ Vmin

203

OCV  Vmin Discharge resistance

(1)

In this formula, Vmin is the lower cutoff voltage (3.0 V for our protocol) and OCV is the open circuit potential at which the discharge resistance (ASI) is measured. To determine the FOM for energy density (FOME) we assumed that a baseline LIB cell would be unsuitable for transportation ap1 plications once the energy (mWhgoxide ) decreases to 80% of its “initial” value. Note that this 80% cutoff value is simply selected as an example to explain how the FOME is obtained; our approach is applicable, no matter what cutoff value is selected. For a baseline 1 cell the average (initial) ED is 714 mWhgoxide (C/10 rate). Because this C/10 value decreases linearly with cycle number (as shown previously) [36] we can determine by extrapolation the number of 1 cycles at which the ED decreases to 570 mWhgoxide (80% of initial). As shown in Fig. 3a, the FOME for baseline cells is 170 cycles. By extending our cycling protocol beyond the initial 119 cycles we have confirmed that the baseline cell ED indeed decreases to 570 mWh/goxide in ~170 cycles. For cells containing the electrolyte additives we defined the FOME as the number of cycles at which the ED (C/10 rate) de1 creases to 570 mWhgoxide (80% of the initial baseline value). Through this definition we can obtain a direct comparison of the other electrolytes with that of the baseline. Cells that have a higher (lower) FOME would be superior (inferior) to the baseline in terms of long-term energy performance. An advantage of this definition is that the FOME takes into account the initial ED value of the additive-bearing cells. This feature is important because some electrolyte formulations show a lower initial ED (because of capacity loss during SEI formation), whereas others shows a higher initial ED (because of electrolyte oxidation), than the baseline cells. In a similar manner, we determine the FOM for power density (FOMP) by assuming that a baseline cell would be unsuitable once the power density (mWcm2) decreases to 80% of its “initial” value. For a baseline cell the average initial PD is 135 mWcmm2 at the 2nd HPPC pulse (~4.08 V). By fitting the cell power versus cycle number data we can determine the number of cycles at which the PD decreases to 108 mWcmm2 (80% of initial). As shown in Fig. 3b, FOMP for the baseline cells is 23 cycles. For cells with the additives, FOMP is the number of cycles for which the PD (at the 2nd HPPC pulse) decreases to 108 mWcmm2 (80% of initial baseline value). Cells that have a higher (lower) FOMP would be superior (inferior) to the baseline in terms of long-term power performance. As is evident from the definition, electrolyte formulations that have lower initial impedance, and display lower impedance rise, would have a higher FOMP than the baseline cells. 3.2. Effect of electrolyte additives seen in the first charge cycle For comparison with the baseline we tested the performance of NMC532/Gr cells containing 15 different additive combinations, which are as follows: (1) 1% LiBOB þ 2% LiDFOB, (2) 1% LiBOB þ 1% TEPi, (3) 1% LiBOB þ 1% TMSPi, (4) 0.25% PBE þ 2% LiDFOB, (5) 0.25% PBE þ 1% TEPi, (6) 0.25% PBE þ 1% TMSPi, (7) 2% PES þ 2% LiDFOB, (8) 2% PES þ 1% TEPi, (9) 2% PES þ 1% TMSPi, (10) 0.25% tVCBO þ 2% LiDFOB, (11) 0.25% tVCBO þ 1% TEPi, (12) 0.25% tVCBO þ 1% TMSPi, (13) 2% VC þ 2% LiDFOB, (14) 2% VC þ 1% TEPi, (15) 2% VC þ 1% TMSPi (all in wt%), where one additive of each pair is from the “Negative Electrode” category and the other from the “Positive Electrode” category as listed in Fig. 1. Multiple cells containing identical chemistries were tested using the protocol described in Table S1 of the Supplementary. Cells with identical chemistries

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Fig. 2. (a) Discharge capacity vs. cycle number for the baseline NMC532/Graphite cells cycled in the 3e4.4 V range at 30  C. Each data point is an average value from 3 cells. The values include data obtained at C/10, C/3 and C/1 rates. Locations of the HPPC pulse are also indicated. (b) Area-specific impedance vs. voltage obtained with a discharge pulse at the locations indicated in (a). Each curve represents data from a single HPPC cycle; the data show that cell impedance increases with cycle number.

1 Fig. 3. FOME (a) and FOMP (b) derivation schematic for the baseline electrolyte cell. For FOME, the top line indicates the Energy Density (ED) at cycle 5 (713 mWhgoxide , C/10), and 1 the bottom line indicates 80% of that value (570 mWhgoxide ). The FOME is the cycle number at which the extrapolated C/10 ED reaches that 80% threshold, which occurs at 170 cycles for the baseline. For FOMP, the top line indicates the Power Density (PD) calculated for the baseline cell at 4.08 V for the first HPPC test. The cell power is calculated from the ASI and fit to a logarithmic decay. The FOMP is the cycle number at which the fit of cell power crosses 80% of the initial baseline power, which occurs at 23 cycles for the baseline.

showed similar performance. Therefore, only average data from multiple measurements, or representative plots, are shown in this article to highlight differences between the various electrolyte formulations. The effect of electrolyte additives is seen during the early portion of the first charge cycle. A voltage vs. time plot of this region is shown in Fig. S2 of the Supplementary. Differential capacity (dQdV1) values obtained from those data are plotted versus cell voltage in Fig. 4; these plots mainly yield information on the amount of charge that is consumed during SEI formation reactions. For comparison, data from the baseline cells is shown in grey in all plots. From an examination of the plots in Fig. 4 we make the following observations: e For the baseline electrolyte, dQdV1 intensities are observed starting around 2.4 V; the main peak, with an apex at ~3.0 V, can be attributed to EC reduction at the graphite electrode. e The peak apex for all LiDFOB cells is at ~2.1 V. The addition of 0.25 wt% PBE or 0.25 wt% tVCBO to a LiDFOB-bearing electrolyte barely alters the dQdV1 plots, indicating that these compounds are either (i) not reduced at the anode, or (ii) may undergo reactions that do not consume charge, such as polymerization reactions.

e On the other hand, LiBOB reduction with a peak apex at ~1.96 V, does consume charge. Note that the LiBOB reduction reduces the peak area (or amount of charge consumed) during the subsequent LiDFOB and EC reduction reactions. The shape and position of the EC peaks is also altered, which suggests that the additives may react with the baseline components (EC, EMC, LiPF6) and form species that have other reduction potentials. e The apex of the PES reduction peak is around 2.5 V, and its reduction suppresses the intensity of the EC reduction peak. In a mixture with LiDFOB, which reduces earlier, both PES peak area and location are altered but the reduction is not suppressed. e The TMSPi compound shows a reduction peak at around 2.67 V. Calculations of redox potentials have suggested that TMSPi would not be reduced at the anode [20]. Our mechanistic studies, which will be detailed in upcoming articles, indicate that TMSPi interacts with the baseline electrolyte, as was also observed by Martin et al. in a recent study [37]. The presence (or absence) of the reduction peak depends on “electrolyte age”: the peak is seen for electrolyte solutions that are used within hours after preparation, but not seen for electrolyte solutions that are used several days after preparation. e The profile of the 0.25% PBE þ 1% TEPi cell resembles that of the baseline cell, suggesting that TEPi is not reduced at the graphite anode.

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Fig. 4. 1st charge cycle differential capacity (dQdV1) vs. voltage plots, in the 1.8e3.3 V range, for NMC532/Gr cells containing various additive combinations. The main peaks are marked by the voltage values at the apex. The baseline data is shown in grey in all plots; the peak at ~3.0 V arises from EC reduction at the negative electrode.

e VC reduction, with an apex around 2.9 V, appears to decrease the quantity of charge consumed during the subsequent EC reduction. The LiDFOB reduction, which occurs earlier, barely changes the VC reduction profile.

3.3. Capacity retention and FOME for the various electrolytes The complete 1st cycle capacities (charge and discharge) and capacity loss for the various electrolytes are given in Table S2 of the Supplementary. Plots showing the discharge capacity versus cycle number, for the duration of the test protocol, are shown in Fig. 5; for comparison, data from the baseline cells are shown in grey. From an examination of Fig. 5 and Table S2 we can make the following observations: d The initial discharge capacity of all cells containing the LiDFOB additive is lower than the initial capacity of the baseline cell. This lower capacity indicates that additional Liþ ions are immobilized during LiDFOB reduction, which accompanies the SEI formation reactions as shown in Fig. 4. d Most of the TEPi cells display a higher initial discharge capacity than the baseline cell. In addition, Table S2 shows that

205

Fig. 5. Discharge capacity versus cycle number plots of NMC532/graphite full cells 1 containing the various electrolyte formulations. The initial capacity (in mAgoxide ) and capacity retention at the end of cycling (in %) are indicated for each plot. For comparison, the baseline data is shown in grey in all plots; for these cells the initial capacity and capacity retention are 191 mAg1 oxide and 86%, respectively.

the 1st cycle charge capacity of the TEPi cells is almost always higher than that of the baseline. The additional capacity results from the oxidation of TEPi at the positive electrode; capacity gain from electrolyte oxidation has been reported previously [38]. For cells that contain VC and PES (in addition to TEPi), this capacity gain is counteracted by the capacity loss that results from VC and PES reduction, resulting in a slightly lower initial capacity. d Despite the higher initial capacity, the capacity loss on cycling of most TEPi cells is higher than that of the baseline. In a recently published study we suggest that enhanced HF generation, and increased TM dissolution, could explain the higher capacity fade of TEPi cells [39]. Some of this capacity loss is mitigated by VC addition, presumably by forming a better SEI. d The highest discharge capacity at the end of cycling is shown by cells containing the 0.25% tVCBO þ 1% TMSPi combination. The initial energy density, and energy loss at the end of cycling, for the various electrolytes are shown in Table 1. The FOME values derived from these data are included in Table 2. As would be

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Table 1 1 Initial energy density (mWhgoxide ), with loss after cycling (%) in parenthesis. These 1 values for the baseline electrolyte are 714 mWhgoxide and 11.9%, respectively.

LiDFOB TEPi TMSPi

LiBOB

PBE

PES

tVCBO

VC

666 (11.2%) 718 (16.8%) 694 (15.1%)

684 (18.3%) 720 (23.3%) 721 (12.2%)

693 (12.8%) 697 (24.8%) 703 (12.6%)

693 (15.3%) 728 (21.4%) 724 (10.8%)

664 (12.4%) 707 (12.5%) 682 (8.8%)

Table 2 FOME values for the various electrolytes. For the baseline, FOME ¼ 170 cycles.

LiDFOB TEPi TMSPi

LiBOB

PBE

PES

tVCBO

VC

123 116 115

89 84 169

128 78 154

111 97 204

110 147 200

expected, there are strong correlations between the energy density trends and FOME values. The TEPi cells show some of the highest initial energy densities, but also the highest energy loss. The FOME value for these cells is significantly lower than that of the baseline, because of this high loss. The LiDFOB cells, on the other hand, show lower initial energy densities, but also show lower loss. The FOME value for these cells is also significantly lower than that of the baseline because of the low initial ED values. The highest FOME value is obtained for cells with the 0.25% tVCBO þ 1% TMSPi electrolyte, which also shows the best combination of high initial energy density and low loss during cycling. The FOME value for the 2% VC þ 1% TMSPi cell is also higher than that of the baseline, whereas the value for the 0.25% PBE þ 1% TMSPi is comparable to that of the baseline. These latter conclusions are not immediately evident from the ED values (in Table 1), which highlights the importance of the FOME values.

3.4. Impedance rise and FOMP for the various electrolytes Plots showing the ASI changes as a function of cycle number for the 15 electrolytes are shown in Fig. 6; the ASI were calculated from the 2nd 2C discharge pulse (around 4.08 V) during the cell discharge cycle. For comparison, data from the baseline cells are also included in the plots. The initial ASI, final ASI, and ASI change values for the various electrolytes are given in Table S3 of the Supplementary. From an examination of Fig. 6 and Table S3, we make the following observations: d The initial ASI of the baseline cell (24.0 Ucm2) is lower than the initial ASI of all the additive-containing cells. However, the ASI increase of the baseline (20.9 Ucm2) is larger than the increase for all the other cells. d Cells containing the PES compound show higher initial ASI than most of the other cells. The highest initial ASI (32.4 Ucm2) is observed for the 2 wt% PESþ 1 wt% TEPi cell; however, the ASI increase of this cell (7.0 Ucm2) is relatively small. A similar comment can be made about the 1 wt% LiBOB þ 2 wt% LiDFOB cell, which shows higher initial impedance (29.1 Ucm2) but a relatively small impedance rise (4.9 Ucm2). d Cells that show among the lowest ASI increase are those with 0.25% PBE þ 2% LiDFOB, (2.6 Ucm2), 1% LiBOB þ 1% TMSPi (3.5 Ucm2), 0.25% tVCBO þ 2% LiDFOB (4.0 Ucm2), 0.25% PBE þ 1% TMSPi (4.7 Ucm2). The positive electrode is known to be the main contributor to impedance rise in these NMC532/Gr cells [6,40]; these additive combinations apparently act on

Fig. 6. Discharge ASI as a function of cell voltage for the NMC532/graphite cells containing various electrolyte formulations. The ASI values were obtained from the HPPC test conducted periodically during cell aging: the curves in increasing order of aging are red (1), orange (2), yellow (3), green (4), then blue (5). The initial ASI (in Ucm2) and ASI rise (in D Ucm2) for the 2nd discharge pulse (~4.08 V) are indicated for each plot. For comparison, the baseline data is shown in grey in all plots; for these cells the initial ASI and ASI rise are 24 and 20.9 Ucm2, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

and stabilize the positive electrode surfaces, and thereby reduce the impedance rise. The initial power density, and power loss at the end of cycling, for the various electrolytes are shown in Table 3. The initial power density of the baseline cell (135 mWcm2) is higher than those of all the other cells; however, the power loss at the end of cycling (47.1%) is also the highest for this cell. Among the additive cells, some show higher initial power but also higher loss, whereas

Table 3 Initial power density (mWcm2), with loss after cycling (%) in parenthesis. These values for the baseline electrolyte are 135 mWcm2 and 47.1%, respectively.

LiDFOB TEPi TMSPi

LiBOB

PBE

PES

tVCBO

VC

112 (16.4%) 127 (24.0%) 127 (16.7%)

124 (12.5%) 119 (33.3%) 131 (17.5%)

114 (19.1%) 101 (29.5%) 116 (18.9%)

120 (15.2%) 127 (39.0%) 134 (18.9%)

134 (35.2%) 128 (39.2%) 127 (30.9%)

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others show lower initial power but also lower loss; hence, it is difficult to rank the relative performance of the cells. A clearer picture emerges from the FOMP values (shown in Table 4) which accounts both for the initial power and power loss during cycling. Cells with 0.25% tVCBO þ 1% TMSPi (133 cycles), 2% PES þ 2% LiDFOB (116 cycles), and 0.25% PBE þ 1% TMSPi (113 cycles) have a much larger FOMP than the baseline cell (23 cycles); cells with these additives would hence show the best long-term power performance. In contrast, cells with 2% PES þ 1% TEPi (5 cycles) would be expected to perform poorly, whereas cells with 0.25% tVCBO þ 1% TEPi (23 cycles) and 2% VC þ 1% TEPi (23 cycles) would match the long-term performance of the baseline.

3.5. FOME and FOMP e comparison and correlations From the above results, it is evident that some electrolyte formulations would improve the long-term energy performance relative to that of the baseline, whereas others would improve the long-term power performance. In order to obtain a more holistic view of cell performance, which includes both the energy and power criteria, we plot the FOME and FOMP values together for the various electrolytes in Fig. 7. In Fig. 7a, the electrolytes are shown in descending order of FOME and the corresponding FOMP values are also indicated. The plot indicates no obvious correlations between the FOME and FOMP values. Electrolyte formulations that have a relatively high FOME could either have a low FOMP (VC and TMSPi, for example) or a relatively high FOMP (PBE and TMSPi, for example). This observation suggests that the processes and mechanisms that govern the long-term energy performance are likely independent from the processes that govern the long-term power performance of the cells. Fig. 7b contains a plot of FOME versus FOMP values for the various electrolytes. From this figure we can get a direct assessment of cell performance relative to the baseline. Electrolytes that have values in the top right portion of the plot would yield the best overall long-term performance, whereas those with values in the bottom left corner would yield the worst performance. An examination of the data shows that only cells with the 0.25% tVCBO þ 1% TMSPi combination would have significantly better long-term energy and power retention than the baseline. Most of the electrolyte formulations tested would yield long-term performance that is inferior to the baseline. Some formulations, such as 0.25% PBE þ 2% LiDFOB or 1% LiBOB þ 1% TMSPi would have better power performance but poor energy performance, while others such as 2% VC þ 1% TMSPi would have better energy performance and slightly better power performance than the baseline. Fig. 7b can also be used to examine the performance trends for a particular additive. For example, all the VC-bearing cells have similar FOMP values (23e31) but very different FOME values, which are lowest in combination with LiDFOB and highest in combination with TMSPi (VC þ LiDFOB: 110, VC þ TEPi: 147; VC þ TMSPi: 200). As evident, the addition of the 2nd component (LiDFOB, TEPi, TMSPi) has a strong effect on FOME and hence on the long-term energy performance: the apparent synergy between VC

Table 4 FOMP values for the various electrolytes. For the baseline, FOMP ¼ 23 cycles.

LiDFOB TEPi TMSPi

Fig. 7. (a) FOME and FOMP values (X-axis) for the various electrolytes (Y-axis). The filled circles represent the FOME cycles and the unfilled circles represent the FOMP cycles. (b) FOME (X-axis) vs. FOMP (Y-axis) plots for the various electrolytes, compared to those of the baseline (black triangle). Values in the top right corner have the highest values for FOME and FOMP and would be expected to yield the best long-term performance. In both (a) and (b), additives intended to improve anode performance are indicated by the symbol color and additives intended to improve cathode performance are indicated by symbol geometry. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

and TMSPi, is especially intriguing. Another example relates to the LiBOB-bearing cells, which have similar FOME values (115e123) but very different FOMP values. These values are lowest in combination with LiDFOB and highest in combination with TMSPi (LiBOB þ LiDFOB: 10; LiBOB þ TEPi: 54; LiBOB þ TMSPi: 99). That is, the addition of the 2nd component (LiDFOB, TEPi, TMSPi) has a strong effect on FOMP and hence on the long-term power performance: the high values for the LiBOB þ TMSPi mix are particularly noteworthy. The mechanistic details associated with these effects on FOME and FOMP values are beyond the scope of this article. However, it is evident that plots, such as Fig. 7b, can be used to compare and rank overall cell performance and would thus be useful in developing new electrolyte and electrode formulations that show better longterm performance than our baseline chemistry. Furthermore, the FOME and FOMP values provide preliminary insights into processes within the cells, and can be used in conjunction with additional experiments, to determine the various mechanisms that influence cell lifespans. 4. Summary and conclusions

LiBOB

PBE

PES

tVCBO

VC

10 54 99

116 16 113

18 5 22

30 23 133

29 23 31

We have developed a new methodology to examine long-term performance of novel cell chemistries relative to that of a baseline chemistry. Our methodology uses conventional equipment for the cycling tests and complements other methods, such as the

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coulombic efficiency measurements with high-precision cyclers, which have been discussed in the research literature [38,41]. Our approach uses tests conducted over a roughly 6 week period to make long-term predictions about cell performance and is suitable for identifying electrolyte and electrode formulations that could prolong the life of lithium-ion cells. In this work we prepared 15 different electrolyte formulations by adding various additive compounds to a baseline electrolyte, and then compared the cycling behavior of all cells. We developed two separate figures of merit (FOM), one related to cell energy (FOME) and the other related to cell power (FOMP), and used them in conjunction to rank electrolyte performance. Just like the other short-term methods (coulombic efficiency measurements, for example), our approach assumes that the mechanisms that govern performance during the duration of our tests are still dominant during longterm cycling. The FOMs are defined by the cycle number at which the cell performance (energy, power) drops below 80% of the initial value of a cell containing the baseline electrolyte. For the baseline cell, the energy density (calculated from the discharge capacity vs. cycle number data) decreases to 80% of the initial value at 170 cycles; the power density (calculated from the impedance data) decreases to 80% of the initial value at 23 cycles. Only two electrolyte formulations (0.25% tVCBO þ 1% TMSPi, 2% VC þ 1% TMSPi) had higher FOMEs than the baseline, whereas several formulations had higher FOMPs than the baseline. When the two FOMs are included in the same plot, it becomes evident that only the cell with 0.25% tVCBO þ 1% TMSPi would have long-term energy and power performance that is significantly superior to that of the baseline cell. We are currently using the FOM methodology to optimize additive concentrations and discover new formulations that far exceed the performance of our baseline cells. Furthermore, we are conducting detailed mechanistic studies to determine synergies between electrolyte components in the best-performing cells and interactions between components in the worst-performing cells. Data from these various studies will be reported in upcoming publications. Acknowledgments The lead authors A.T., C.P., and J.A.G. contributed equally to this article. We gratefully acknowledge support from the U.S. Department of Energy's Vehicle Technologies Program (DOE-VTP), specifically from Peter Faguy and Dave Howell. The electrodes in this article were fabricated at Argonne's Cell Analysis, Modeling and Prototyping (CAMP) Facility, which is supported within the core funding of the Applied Battery Research (ABR) for Transportation Program. We are grateful to the HEHV team members for their suggestions, especially to S. Trask, B. Polzin, A. Jansen, J. Croy, Z. Zhang, I. Bloom, and D. Dees. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.08.093.

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