Development of Electrolytes for Single Crystal NMC532/Artiﬁcial Graphite Cells with Long Lifetime

NMC532/artiﬁcial graphite cells using single crystal NMC532 active material can have excellent long term lifetime at 4.4 V and elevated temperature if appropriate electrolytes are used. However, electrolytes developed earlier for these cells and reported in the literature cannot support even C/2 rates during charging without unwanted lithium plating at room temperature. This work is thus focused on the development of new electrolytes for single crystal NMC532/artiﬁcial graphite cells that can yield long lifetime and support higher charging rates. Ex-situ and in-situ gas measurements, ultra-high precision coulometry, isothermal microcalorimetry, lithium plating tests and long term cycling tests were used for the screening of electrolytes. Electrolytes with 2% vinylene carbonate (VC) + 1% ethylene sulfate (DTD) additives or 2% ﬂuoroethylene carbonate (FEC) + 1% DTD additives yield single-crystal NMC532/graphite cells with long lifetime that can support C-rate charging at 20 ◦ C. unrestricted in any

Capacity loss in lithium ion cells can be caused by the loss of lithium inventory to the solid electrolyte interphase (SEI). 1-3 Active material loss due to structural degradation and due to electrical disconnection at the particle/electrode level can also lead to capacity loss. Internal impedance or polarization increase is another major contributor to capacity loss under high rate discharge conditions. Moreover, unwanted lithium plating, which can occur during high rate or low temperature charging, can also result in severe capacity fade. At high potentials, accelerated unwanted reactions in the electrolyte such as electrolyte oxidation occur and can hasten capacity loss by causing reconstruction of the positive electrode surface which can lead to impedance growth. [1][2][3][4][5] In addition the oxidized by-products can migrate to the negative electrode surface and be reduced there. 6,7 Such reactions can lead to the consumption of lithium ions from the electrolyte (to maintain charge neutrality in the electrolyte), a reduction in lithium inventory, as well as a thickening of the negative electrode SEI which together ultimately cause impedance growth and capacity loss. 8,9 These processes are usually accelerated by higher charging potentials and higher temperatures.
Liu et al. found that additives and electrolytes that increase the negative electrode area-specific resistance lead to a decreased onset current for unwanted lithium plating. 23 Such additives are often those that also increase lifetime of cells under moderate rate conditions. PES211 electrolyte usually leads to large negative electrode charge transfer resistance and therefore is not suitable for applications requiring high rates during charging. Liu et al. showed that NMC111/graphite cells with PES211 electrolyte could not be charged at a C/2 rate at room temperature without unwanted lithium plating (detailed electrode information can be found in Reference 23). 23 If single crystal NMC532 A627 where A was VC, FEC or PES. Once cells were filled with electrolyte, they were sealed with a compact vacuum sealer (MSK-115 V, MTI Corp.) to 94% of full vacuum (-95.2 kPa gauge pressure or 6.1 kPa absolute pressure) with a 4 second sealing time at 165 • C.
All cells were placed in a temperature-controlled box at 40. ± 0.1 • C and held at 1.5 V for 24 hours to ensure complete wetting. For the formation process, cells were clamped with rubber blocks in cell holding "boats" and charged at C/20 using a Maccor series 4000 automated test system (Maccor Inc). The cells were charged to the upper cutoff voltage and discharged to 3.8 V at 40 • C. The cells were then transferred back to an argon-filled glove box and cut open under the previous seal to release any gas that was produced. The cells were then vacuum sealed again as previously described, clamped in boats and then were ready for electrochemical impedance spectroscopy (EIS) measurements.
Electrochemical impedance spectroscopy (EIS).-All EIS spectra were measured at a temperature of 10. ± 0.1 • C using a Biologic VMP3 electrochemical test station. Data were collected with ten points per decade from 100 kHz to 10 mHz with a signal amplitude of 10 mV. The experimental setup did not allow for reproducible solution resistance measurements due to cable and connector impedance. Therefore, all impedance spectra were manually shifted to zero on the real axis at the highest frequency measured. The area specific resistance reported here is extracted from a Nyquist plot as the total "diameter" of the "semicircle" which is the sum of charge (both ion and electron) transfer resistance (Rct) (both electrodes) from the full cell.

Ultra-high precision coulometry (UHPC).-
The coulombic efficiency of cells was measured using the ultra-high precision chargers at Dalhousie University described in Reference 24. The cells after formation were tested at C/20 for 16 cycles between 3 and 4.3 V. The cells were placed at 40. ± 0.1 • C in temperature-controlled boxes during the tests. 20 and 40 • C long term cycling.-The cells were tested for long term cycling with a C/3 rate between 3 and 4.2 or 4.3 V respectively. A cycle at a C/20 rate was included every 50 cycles. The cells were held at the top of charge until the current reached C/20 during each cycle.
The tests were made at 20 or 40. ± 0.1 • C in temperature-controlled boxes. Neware (Shenzhen, China) chargers were used for these tests. 20 • C Plating tests.-The NMC532/graphite cells were charged with a current (C-rates) of 210 mA (1C) for 30 cycles, 315 mA (1.5 C) for 30 cycles and 420 mA (2 C) for 30 cycles between 3 and 4.3 V using a Neware charger system at 20. ± 0.1 • C. The cells were discharged with a current of 105 mA (0.5 C). In order to determine the active lithium loss during cycling, cells were cycled at C/5 three times before and after each high charge rate segment.
Ex-situ gas measurement.-The gas produced in cells due to electrolyte decomposition during cycling was measured using Archimedes's principle. 25 The pouch cells after cycling were first discharged to 3.8 V. Ex-situ gas measurements were carried out by suspending pouch cells from a fine wire "hook" attached under a Shimadzu balance (AUW200D) and then immersed in a beaker of de-ionized "nanopure" water (18 M ) that was at 20. ± 1 • C for measurement. The change in the weight of the cell suspended in fluid, before and after testing is directly related to the volume change by the change in the buoyant force. The change in weight of a cell, w, suspended in a fluid of density, ρ, is related to the change in cell volume, v, in mL by: where the weight is measured in milliNewtons, the density in g/mL and g is the acceleration due to gravity in m/sec 2 . It is important to realize that w/g is what a balance reports as the mass in grams.
In-situ gas measurement.-The gas produced in cells due to electrolyte decomposition during testing was measured using Archimedes' principle. 25 A strain gauge load cell was used to measure changes in the buoyant force of cells submerged in mechanical pump oil kept at a steady temperature as the pouch cells were charged and discharged. A detailed account of this method was described by Aiken et al. 25 One batch of cells filled with electrolyte underwent formation during simultaneous in-situ gas measurements between 3 and 4.7 V using currents corresponding to C/20. A new batch of cells after formation in the usual way were first tested using the in-situ gas equipment between 3-4.3 V for one cycle and then were charged and held at 4.4, 4.5 and subsequently 4.6 V for 100 h each. The current used during testing was ∼C/20 calculated based on the capacity measured between 3-4.3 V. The tests were made at 40 • C. The changes in the volume of the cells during testing were tracked as a function of time.
Isothermal microcalorimetry.-Cells used for isothermal microcalorimetry measurements underwent formation, then were transferred into a TAM III Microcalorimeter (TA Instruments: stability ± 0.0001 • C, accuracy ± 1 μW, precision ± 1 nW) at 40.0 • C and connected to a Maccor 4000 series cycler. The baseline drift over the course of the experiments did not exceed ± 0.5 μW. All specifications and information regarding microcalorimetry calibration, cell connections and operation procedures can be found in previous literature. 26 Cells were cycled four times at a C/20 rate between 2.8 V and 4.2 V to ensure a well formed, stable negative electrode SEI and were then cycled between 4.0 V and different upper cutoff limits: 4.2 V, 4.3 V (twice), 4.4 V (twice) and again to 4.2 V (twice) at 1 mA to investigate the performance and the parasitic heat flow in different voltage ranges. A description of the analysis techniques is provided in the Results and discussion section.

Results and Discussion
The electrolytes were designed to contain a primary additive of VC, FEC or PES, which are known to form stable passivation films on the graphite negative electrode and a secondary additive of MMDS or DTD which were used to improve electrolyte stability at high voltages based on the previous studies. 27 Figures 1a and 1b show a summary of the area specific charge transfer resistance (R ct ) of cells after formation and the volume of gas generated during formation, respectively, for cells with each electrolyte. The red, gray and green bars show the results for cells with PES, FEC and VC as the primary additive, respectively. Figure 1a shows that cells containing PES had significantly larger R ct than cells without PES, which was between 180-350 * cm. 2 This result is consistent with previous reports that cells with PES usually have large R ct . 12,28 For cells containing FEC, the addition of the secondary additive MMDS or DTD slightly increased R ct compared to cells with 2FEC only. R ct of cells containing FEC was around 90-100 * cm. 2 In cells with VC as the primary additive, R ct increased with increasing VC content from 1% to 2% when the same secondary additive was used. In general, cells with MMDS showed slightly larger R ct than cells with DTD when the same primary additive was used.
Large amounts of gas produced during formation can lead to difficulties maintaining electrode stack integrity due to: a) deformation of the jelly roll due to gas bubbles and b) gas pushing electrolyte out of the electrode stack. So it is important to identify systems that simply produce too much gas and eliminate them from further consideration. Figures 1b shows that the replacement of 1% A, where A is a primary additive of PES, FEC or VC, with 1% DTD or 1% MMDS increased the amount of gas generated during formation compared to cells with 2% A. For instance, cells with 2PES and 2FEC showed 0.22(3) and 0.22(2) mL of gas, respectively, while cells with 1PES+1DTD, 1FEC+1DTD and 1VC+1DTD showed 0.30(2), 0.54(1) and 0.77(2) mL of gas, respectively. Additionally, cells with 2A+1DTD and 2A+1MMDS produced less gas than cells with 1A+1DTD and 1A+1MMDS, respectively. For instance, cells with 2PES+1DTD, 2FEC+1DTD and 2VC+1DTD a b Figure 1. The summary of the area specific charge transfer resistance (Rct) of cells after formation (a) and the volume of gas generated during formation (b) for cells with each electrolyte. The red, gray and green bars show the results for cells with PES, FEC and VC as primary additive, respectively. produced 0.23(2), 0.38(3), and 0.60(2) mL of gas, respectively, while cells with 1PES+1DTD, 1FEC+1DTD and 1VC+1DTD produced 0.30(2), 0.54(1) and 0.77(2) mL of gas, respectively. When FEC and VC were used as primary additives, cells containing DTD produced more gas than cells with MMDS while the same primary additive was used. For instance, cells with 2FEC+1DTD and 2VC+1DTD showed 0.38(3) and 0.60(2) mL of gas while cells with 2FEC+1MMDS and 2VC+1MMDS produced 0.26(2) and 0.23(1) mL of gas.
To generally evaluate the performance of the designed electrolytes, the cells with 2A, 1A+1DTD, 2A+1DTD, 1A+1MMDS and 2A+1MMDS, where A is a primary additive of FEC, VC or PES, were first tested with long term cycling at 40 • C between 3.0 and 4.3 V. A current corresponding to C/3, assuming a capacity of 230 mAh, was used. Every 50 cycles, one C/20 cycle was performed. During every charge, the cells were held at the top of charge until the current reached C/20. Figures 2a1, 2b1 and 2c1 show the discharge capacity, normalized discharge capacity and V (difference between the average charge and discharge voltage), respectively, as a function of cycle number for cells with FEC as the primary additive.  Figure 2c1 shows that cells with MMDS exhibited the highest V growth of ∼0.025(5) V after 100 cycles, while cells containing DTD showed the lowest V growth of ∼0.007(1) V after 850 cycles.
When VC was used as the primary additive, Figure 2b2 shows that cells with MMDS also exhibited the worst capacity retention of ∼97% after 100 cycles. The cells with 2VC + 1DTD exhibited better capacity retention, above ∼96% after 850 cycles, than cells with 2VC only. Figure 2c2 shows that cells with MMDS exhibited the largest V growth of ∼0.015(1) V after 100 cycles, while cells with DTD showed the lowest V growth of ∼0.008(1) V after 850 cycles.
When PES was used as the primary additive, Figure 2b3 shows that cells with PES all lost about ∼2-3% capacity during the first 100 cycles. Additionally, Figure 2b3 shows that cells with 2PES + 1DTD exhibited the best capacity retention. Figures 2c3 shows that cells with PES all exhibited fast V growth of ∼0.005(1) V in 100 cycles, which is much worse than cells with FEC or VC with DTD. Overall, Figure 2 shows that combinations of FEC or VC with DTD can significantly improve the cell capacity retention and suppress the V growth comparing to FEC or VC only. The following work, therefore, focuses on combinations of FEC or VC with DTD.
To test high rate charging capability of cells with the designed electrolytes, cells were charged with currents (C-rates) of 210 mA (1C) for 30 cycles, 315 mA (1.5 C) for 30 cycles and 420 mA (2 C) for 30 cycles between 2.8 and 4.3 V at 20. ± 0.1 • C. The electrode loadings of the cells used in this work are listed in the Experimental section. The cells were discharged with a current of 105 mA (0.5 C).
To determine the capacity loss due to unwanted lithium plating during high rate charging tests, cells were cycled at C/5 for three times before and after each high rate segment (30 cycles). Figures S2, S3 and S4 show the discharge capacity as a function of cycle number for cells with FEC, VC and PES as primary additive, respectively. Figure  3 shows a summary of the accumulated capacity losses after each high rate segment. The capacity loss was calculated by subtracting the discharge capacities of the third C/5 cycle after 1 C (cycle 37), 1.5 C (cycle 71) and 2 C (cycle 107) charging tests from the third C/5 cycle before any high rate tests (cycle 3), respectively. The gray, green and red bars show the capacity losses of cells after the 1 C, 1.5 C and 2 C charging tests, respectively. Figure 3 shows that cells with PES as the primary additive exhibited large capacity losses of ∼50-90 mAh after testing with 1 C charging for 30 cycles, which is primarily due to unwanted lithium plating. 23 Cells with FEC or VC or/and DTD showed negligible capacity loss during the 1 C charging test, indicating the absence of unwanted lithium plating. This result agrees well with Figure 1a which shows that cells with PES showed significantly higher R ct than cells with FEC or VC. 23 Figure 3 also shows that cells containing VC had higher capacity losses than cells containing FEC after the 1.5 C charging test. The capacity losses for cells with 1FEC + 1DTD, 2FEC + 1DTD, 1VC + 1DTD and 2VC + 1DTD after the 1.5 C test were about 3.7(3), 2.5(3), 6(2) and 17(3) mAh, respectively. Cells with 2FEC + 1DTD could be charged at 1.5 C with little lithium plating, however, cells with 2VC + 1DTD experienced severe unwanted lithium plating. Further developments in the electrolyte system are required if 2VC + 1DTD is selected. This result agrees well with Figure 1a that cells with 2VC + DTD showed higher R ct than cells with 2FEC+1DTD. 23 Additionally, Figure 3 shows that all cells exhibited significant capacity losses of more than 20 mAh after the 2 C charging test, indicating severe unwanted lithium plating at this rate regardless of the electrolyte additives selected. Nevertheless, Figure 3 shows that cells with 2FEC + 1 DTD or VC + 1DTD can be charged at 1 C without lithium plating at 20 • C, which is a significant improvement over the PES211 electrolyte which cannot support C/2 rate charging at room temperature without unwanted lithium plating. 20 To further investigate the gas generation during formation, cells after filling with electrolytes were charged to 4.7 V and then discharged to 3.8 V at 40 • C with currents corresponding to C/20, while the volumes of gas produced in the cells due to electrolyte decomposition were measured in-situ. Figure 4a shows the cell voltage as a function of time while Figure 4b shows the corresponding volume changes of Figure 2. The discharge capacity (a1), normalized discharge capacity (b1) and V (difference between the average charge and discharge voltage) (c1), respectively, as a function of cycle number for cells with FEC as the primary additive. The discharge capacity (a2), normalized discharge capacity (b2) and V (c2), respectively, as a function of cycle number for cells with VC as the primary additive. The discharge capacity (a3), normalized discharge capacity (b3) and V (c3), respectively, as a function of cycle number for cells with PES as the primary additive. The blue squares, black triangles, red crosses, green diamonds and purple crosses show the results for cells with 2A, 1A+1DTD, 2A+1DTD, 1A+1MMDS and 2A+1MMDS, where A is the primary additive of FEC, VC or PES, respectively.   Figure 4b shows the first region of gas generation caused by electrolyte reduction when the cell voltage was below 3.5 V. Figure 4b shows that the peak gas volumes of cells with 2VC, 2VC + 1DTD, 2FEC and 2FEC + 1DTD in this region were about 0.36(2), 0.87(2), 0.34(2) and 0.73(2) mL, respectively. This shows that the addition of DTD leads to significant additional gas generation. Between 3.5 and 4.5 V (3-25.5 h), the cell volumes decreased due to gas consumption in the cells. Cells with DTD showed faster gas consumption rates than cells without DTD. The decrease in volumes of cells with 2VC, 2VC + DTD, 2FEC and 2FEC + DTD were 0.09(2), 0.26(2), 0.15(2) and 0.38(2) mL, respectively. Figure  4b shows the second region of gas generation above 4.5 V, which is primarily due to electrolyte oxidation. The volumes of gas generated in cells with 2VC, 2VC + 1DTD, 2FEC and 2FEC + 1DTD in this region were 0.4(2), 0.61(2), 0.22 (2) and 0.20(2) mL, respectively. Cells with FEC showed less gas generation than cells with VC when the cell voltage was above 4.5 V, which indicates that cells with FEC may have better electrolyte stability than cells with VC at voltages above 4.5 V. This is consistent with previous work in the literature. 29 To further study the electrolyte stability at the positive electrode surface, the cells after formation were charged and held at 4.4, 4.5 and 4.6 V for 100 h while the volumes of gas produced in the cells due to electrolyte decomposition at high voltages were measured in-situ. The temperature of the experiment was 40.0 • C. Figure 4c shows the cell voltage as a function of time while Figure 4d shows the corresponding volume changes of the cells as a function of time. The black, blue, red and green solid lines show the results for cells with 2VC, 2VC + 1DTD, 2FEC and 2FEC + 1DTD, respectively. Figure 4d shows that cells with 2FEC and 2FEC + 1DTD produced a minimal amount of gas during the high voltage holds. The volumes of gas generated in cells with 2FEC were 0.1(1), 0.04(1) and 0.11(2) mL after the holds at 4.4, 4.5 and 4.6 V for 100 h, respectively, while the corresponding volumes of gas generated in cells with 2FEC + 1DTD were 0.06(1), 0.06(1) and 0.12(2) mL, respectively. These are extremely small volumes of gas considering that the initial cell volume was about 2.5 mL. Cells with 2VC generated slightly more gas at voltages higher than 4.4 V. The volumes of gas generated after the holds at 4.4, 4.5 and 4.6 V for 100 h were 0.03(1), 0.15(3) and 0.17(2) mL, respectively. Cells with 2VC + 1DTD produced relatively more gas when the voltages were at and above 4.5 V, the corresponding volumes of gas generated were 0.11(1), 0.12(1) and 0.45(2) mL, respectively. This indicates that cells with 2VC + 1DTD may have worse electrolyte stability than cells with 2FEC + 1DTD at voltages above 4.4 V. Figure 5 shows the results from UHPC measurements of cells tested between 3 and 4.3 V using currents corresponding to C/20 at 40 • C. Each panel includes a plot of coulombic efficiency (CE), charge end point capacity slippage (Ch. End. Pt.), normalized discharge capacity (Q d ), and V (the difference between the average charge voltage and the average discharge voltage) as a function of cycle number. Figure 5a shows the UHPC results for cells with 1DTD (red crosses), 2VC (blue crosses) and 2VC + 1DTD (green diamonds). Figure 5a shows that the CEs of cells with 1DTD, 2VC and 2VC + 1DTD at the 16 th cycles were ∼0.99836(1), 0.99862(3) and 0.99876(6), respectively. The corresponding charge end point capacity slippage at the 16 th cycle were 6.1(1), 4.7(1) and 4.2(2) mAh, respectively, while the corresponding normalized capacities at the 16 th cycle were 0.9895(1), 0.984(1) and 0.9873 (9), respectively. Figure 5a shows that cells with 2VC + 1DTD exhibited higher CE and smaller charge end point capacity slippage than cells with 2VC only and 1DTD only. This suggests that that cells with 2VC+1DTD will have longer lifetime than cells with 2VC only and 1DTD only in low rate testing at 40 • C. Figure 5b shows 2FEC (black crosses) and 2FEC + 1DTD (red diamonds). Figure 5b shows that the CEs of cells with 1DTD, 2FEC and 2FEC + 1DTD at the 16 th cycle were ∼0.99836(1), 0.99844(1) and 0.99864(1), respectively. The corresponding charge end point capacity slippage at the 16 th cycle were 6.1(1), 5.2(1) and 4.61(4) mAh, respectively, while the corresponding normalized capacities at the 16 th cycle were 0.9895(1), 0.9829(7) and 0.9885 (8) respectably. Figure 5b shows that cells with 2FEC + 1DTD showed higher CE and smaller charge end point slippage capacity than cells with 2FEC only and 1DTD only. This suggests that cells with 2FEC + 1DTD will have longer lifetime than cells with 2FEC only and 1DTD only in low rate testing at 40 • C.
where t is the time of each charge-discharge cycle and the results from the 16 th cycle were plotted.
The cells with 2FEC + 1DTD and 2VC + 1DTD electrolytes were also studied with isothermal microcalorimetry to probe the magnitude of the parasitic reactions by separating the parasitic heat flow from the total measured heat flow from the cells. When current is applied to a cell, the measured heat flow comes from the voltage polarization, entropy changes in the electrode materials, and the parasitic reactions occurring in the cell. 26,31 Since the entropic heat flow is reversible between charge and discharge, the average parasitic heat flow at each voltage point can be found by taking the average of charge and discharge heat flow and subtracting the average overpotential heat flow. The details of this method were introduced by S. Glazier et al. 32 Figure 7a shows that cells with 2VC+1DTD had much less parasitic heat flow than cells with 2FEC + 1DTD at the same voltage. Figure 7b shows a summary of the results by plotting the mean parasitic heat flow over each cycle (the average value in Figure 7a) to the various voltages, where the red and green circles show the mean parasitic heat flow for cells with 2FEC + 1DTD and 2VC + 1DTD pair cells, respectively. Figure 7b shows that cells with 2VC + 1DTD have a lower mean parasitic heat flow than cells with 2FEC + 1DTD in all voltage ranges, which suggests that cells with 2VC + 1DTD should have longer lifetime than cells with 2FEC +1DTD during low rate cycling at 40 • C. This result agrees well with the UHPC results ( Figure 6).
Cells with 1DTD, 2FEC, 2FEC + 1DTD, 2VC, and 2VC + 1DTD were tested with long term cycling at 40 • C using currents corresponding to C/3, assuming a capacity of 230 mAh, between 3 and 4.   Figure  8b shows the cells with 2FEC + 1DTD had ∼93% capacity retention after 1500 cycles at 4.2 V. Figure 8B shows similar trends at 4.3 V. Cells with 2VC + 1DTD and 2FEC + 1DTD exhibited better capacity retention than cells with 1DTD only, 2VC only and 2FEC only. The normalized capacities of cells with 2FEC + 1DTD and 2VC + 1DTD were 0.917(3) and 0.931, respectively, after 1650 cycles when charged to 4.3 V. Cells with 2FEC only and 2VC only showed the worst capacity retention with a normalized capacity of ∼0.955 at cycle 550. Cells with 1DTD only showed a small capacity fade during early cycles as predicted from UHPC measurements. Figure 8C shows that cells with 1DTD only had faster V growth rate than cells with 2FEC + 1DTD and 2VC + 1DTD. The V growth rate for cells with 1DTD only was ∼1.64(2) mV per 100 cycles while it was ∼0.1.16(3) mV per 100 cycles for cells with 2FEC + 1DTD and 2VC + 1DTD in the first 800 cycles, however, the V growth rate slowed down significantly to about 0.36(3) mV per 100 cycles for cells with 2FEC + 1DTD and 2VC + 1DTD after 800 cycles. This suggests that cells with 1DTD only would have shorter lifetime than cells with 2FEC + 1DTD and 2VC + 1DTD in the long term. Figure 9 shows long term cycling results of cells tested at 20 • C. The cycling protocol was the same as that for cells cycled at 40 • C ( Figures  8). Figures 9a, 9b and 9c Figure  9b shows that the capacity losses of all cells cycled at 4.2 V were small, less than 4% in 1400 cycles, however, the cells with 2FEC + 1DTD and 2VC + 1DTD showed better capacity retention than cells with 2FEC, 2VC and 1DTD. Additionally, Figure 9c shows that cells with 2FEC or 2VC had rapid V growth rate while cells with 2FEC + 1DTD and 2VC + 1DTD showed the least V growth in 1400 cycles. Similar trends were observed at 4.3 V. Figure 9B shows that cells with 2FEC + 1DTD and 2VC + 1DTD had less than 2% capacity fade in 1400 cycles, while cells with 2FEC showed ∼10% capacity fade. Figure 9C shows that cells with 2FEC + 1DTD and 2VC + 1DTD showed the least V growth in 1400 cycles while cells with 2FEC showed the largest V increase.  Figures 10a and 10b show the normalized discharge capacity and V, respectively, as a function of cycle number. The blue and red triangles show data for cells with 2FEC + 1DTD electrolyte charged to 4.3 and 4.4 V, respectively, with a C/3 rate. The green diamonds show data for cells with 2VC + 1DTD charged to 4.3 V with a C/3 rate. The black circles and black crosses show the historical cells with PES211 and Q5FEC (5% FEC in 1 M LiPF 6 in EMC) electrolytes, respectively, charged to 4.4 V with a C/2 rate. Figure 10a shows that the historical cells with PES211 and Q5FEC electrolytes exhibited capacity retentions of more than 80% after 3000 cycles which took more than 18 months of testing at 40 • C. Figure 10a also shows that the capacity retention of cells with 2FEC + 1DTD and 2VC + 1DTD were comparable to the historical cells (following the same track). The 2FEC + 1DTD and 2VC + 1DTD electrolytes can provide excellent lifetime like PES211 electrolyte at elevated temperatures and can also support 1C charging rates at room temperature, suggesting they could be potentially useful simple electrolytes for single crystal NMC532/graphite cells.

Conclusions
Single crystal NMC532/artificial graphite cells can have excellent long term lifetime with PES211 electrolyte at 4.4 V and elevated temperature. 22 More than 80% capacity was maintained after testing for 18 months (∼3000 cycles with C/2 rate, CCCV) at 40 • C. However, the use of PES211 electrolyte usually leads to large negative electrode charge transfer resistance and therefore is not suitable for applications requiring high rates during charging. This work focused on the development of new electrolytes for single crystal NMC532/artificial graphite cells that can support long lifetime as well as 1C charging rates at room temperature.
Electrolytes with FEC, VC or PES as the primary additive and with MMDS or DTD as the secondary additive were first designed. Cells containing VC and FEC showed much smaller charge transfer resistance than cells containing PES. It was found that cells with DTD as the secondary additive had much better capacity retention than cells with MMDS during 40 • C cycling tests. High rate charging tests at 20 • C showed that cells with 2VC + 1DTD or 2FEC + 1DTD can support a 1C charging rate without unwanted lithium plating. UHPC results showed that cells with 2VC + 1DTD or 2FEC + 1DTD exhibited lower CIE/h and lower Fract. Slip./h than cells with 1DTD, 2VC or 2FEC only. Long term cycling results confirmed the predictions from UHPC testing and from microcalorimetry that cells with 2VC + 1DTD and 2FEC +1 DTD had better capacity retention than cells with 1DTD, 2VC or 2FEC only at 40 • C. Cells with 2VC + 1DTD or 2FEC + 1DTD exhibited more than 92% capacity retention after 1400 cycles to an upper cutoff of 4.3 V at 40 • C. The cells with 2FEC + 1DTD or 2VC + 1DTD showed less than 2% capacity fade and had a stable V for over 1400 cycles at room temperature. Cells with 2VC + 1DTD or 2FEC + 1DTD had comparable capacity retentions to the historical cells with PES211 electrolyte at 40 • C. Electrolytes containing 2VC + 1DTD or 2FEC + 1DTD as additives are excellent candidates for single-crystal NMC532/graphite cells which have long lifetime and can support C-rate charge at room temperature.