Some Fluorinated Carbonates as Electrolyte Additives for Li(Ni 0.4 Mn 0.4 Co 0.2 )O 2 /Graphite Pouch Cells

The effects of four ﬂuorinated carbonates including ﬂuoroethylene carbonate, diﬂuoroethylene carbonate, bis(2,2,2-triﬂuoroethyl) carbonate and 2,2,3,4,4,4-hexaﬂuorobutyl methyl carbonate as electrolyte additives were studied in Li(Ni 0.4 Mn 0.4 Co 0.2 )O 2 /graphite pouch cells using ultra-high precision coulometry, in situ measurements of gas evolution, gas chromatography, electrochemical impedance spectroscopy, and long-term cycling experiments. The differential capacity vs. voltage curves during formation showed that ﬂuoroethylene carbonate and diﬂuoroethylene carbonate are solid electrolyte interphase (SEI) forming additives, while bis(2,2,2- triﬂuoroethyl) carbonate and 2,2,3,4,4,4-hexaﬂuorobutyl methyl carbonate do not alter the SEI on the negative electrode during formation. Cells containing diﬂuoroethylene carbonate have the highest coulombic efﬁciency and lowest charge end point capacity slippage rate at both 4.2 and 4.4 V. However the performance was not as good as that of cells containing a state of the art additive blend. Long-term cycle-hold-rest tests at 55 ◦ C showed that at 4.4 V all cells with ﬂuorinated additives that gave promising capacity retention generated unacceptable quantities of gas. Only diﬂuoroethylene carbonate during cycling tests to 4.2 V at 55 ◦ C provided promising capacity retention and moderate gas generation. These results suggest that the use of only these ﬂuorinated additives in Li(Ni 0.4 Mn 0.4 Co 0.2 )O 2 /graphite pouch cells with ethylene carbonate-based electrolytes is not competitive to alternative approaches.

One way to increase the energy density of the Li-ion cells is to utilize high voltage positive electrode materials. The discharge capacity of Li(Ni 0.4 Mn 0.4 Co 0.2 )O 2 (NMC442) increases almost linearly with voltage as the upper cutoff is increased from 4.1 V to 4.7 V. NMC442 can deliver a reversible capacity of ∼155 mAh/g to 4.3 V and ∼207 mAh/g to 4.7 V vs. Li/Li + , without incurring structural changes. [1][2][3][4][5] Therefore NMC422 is a suitable material to use to test electrolytes proposed for high voltage NMC Li-ion cells.
The cycling stability of NMC442/graphite cells is quite poor when charged to cutoff voltages higher than 4.4 V. 6 As an example, Nelson et al. 7 have shown that NMC442/graphite cells can be cycled aggressively at 4.4 V using state-of-the-art electrolyte additives in conventional solvents, but the same cells fail rapidly when the upper cutoff is increased to 4.5 V. Nelson et al. 7 used an electrolyte consisting of 1 M LiPF 6 in ethylene carbonate (EC): ethyl methyl carbonate (EMC) 3:7 (by wt) with 2% prop-1-ene,1,3,sultone (PES), 2% 1,3,2-dioxathiolane-2,2-dioxide (DTD, also called ethylene sulfate) and 2% tris-trimethylsilyl phosphite (TTSPi). This electrolyte, called PES222, was very successful at 4.4 V, even though fluorinated solvents and additives were not used. Based on work by Xia et al., 8 fluorinated solvents appear to extend the voltage range of NMC442/graphite cells. The purpose of this paper is to compare the effectiveness of a variety of fluorinated additives in EC:EMC electrolyte and also to compare those to PES222 and to a related additive blend PES211 which has 2% PES + 1% DTD + 1% TTSPi.
In this work, four fluorinated carbonates-FEC, DiFEC, TFEC and 2,2,3,4,4,4-hexafluorobutyl methyl carbonate (HFBMC), shown in Figure 1-were studied as electrolyte additives in various concentrations in 1 M LiPF 6 EC:EMC 3:7 electrolyte. The electrolyte was added to NMC442/graphite Li-ion pouch cells which were then tested to different voltages. Experiments were made using Ultra-High Precision Coulometry (UHPC), 26 an in-situ gas evolution apparatus, 27 and electrochemical impedance spectroscopy (EIS). 28 Long-term Figure 1. Differential capacity (dQ/dV) versus potential (V) during formation step 1 for the 180 mAh NMC442/graphite pouch cells with different concentrations of a) FEC and DiFEC, b) TFEC and HFBMC. The insertions in Figure 1 show the chemical structure of the four fluorinated additives. UHPC barn-charge protocols.-The "barn-charge" cycling procedure was designed so that the cells were exposed to higher potentials for significant fractions of their testing time, thereby highlighting the effects of electrolyte oxidation at high voltage. This was carried out using the Ultra-High Precision Charger (UHPC) at Dalhousie University. 26 Xia et al. 33  /graphite Li-ion cells which are charged and then exposed to high potential (≥4.4 V) for extended periods of time using barn-charge protocols have much lower CE and shorter life time compared to those that are continuously cycled as has been recently reported in long-term test results. The "barn charge" protocol consisted of a C/15 charge to 0.2 V below the desired upper cutoff voltage, followed by a slower, C/40 charge for the remaining 0.2 V. The cells were then discharged, using a slow, C/40 current to 0.2 V below the upper voltage limit, followed by a C/15 discharge to 2.800 V. This process was repeated on the UHPC for 15 cycles where comparisons were made.
In-situ and ex-situ gas volume measurements .-Both in-situ (dynamic) and ex-situ (static) gas measurements were used to measure gas evolution during formation and during cycling. 34 Both measurements were made using Archimedes' principle, with cells suspended from a balance while submerged in liquid. The changes in the weight of the cell suspended in fluid, before, during, and after testing are directly related to the volume changes by the change in the buoyant force. The change in submerged weight (the scale reading of the balance as mass, m) of a cell, m, suspended in a fluid of density, ρ, is related to the change in cell volume, v, by v = − m/ρ [1] Ex-situ measurements were made by suspending pouch cells from a fine wire hook attached under a Shimadzu analytical balance (AUW200D). The pouch cells were immersed in a beaker of deionized "nanopure" water (18 M ) that was at 20. ± 1 • C for measurement. Before weighing, all cells were equilibrated to the same state of charge (3.80 V). In-situ measurements were made using the apparatus and procedure described in Ref. 34. During the in-situ measurements, the cells were suspended in silicone vacuum pump oil and their submerged weight was measured using sensitive strain gauges (or load cells) as they were cycled. All in-situ gas volume measurements were performed in a temperature controlled box, at 40. ± 0.1 • C. During these measurements, cells were charged and discharged, without degassing, using a current of 9 mA (C/20).

Gas chromatography and thermal conductivity detection (GC-TCD).
-After the in-situ gas measurement, gas chromatography coupled with a thermal conductivity detector (GC-TCD) was used to analyze the different gases produced during formation step 1. The analysis of the gas formed in the pouch cells followed the technique described by Petibon et al. 35 This technique uses a sealed brass chamber into which the pouch cells are fitted. The chamber is then evacuated through a Swagelok quick-connect to 100 mTorr for 20 min. Once the vacuum line is removed (the chamber is still at 100 mTorr), a sealed shaft fitted through the top of the chamber is lowered, thus puncturing the cell. The low pressure in the chamber allows the gas to be extracted from the pouch cell. The chamber is then back-filled with ultra-pure Ar to a gauge pressure of 10 kPa. Some of the gas inside the chamber is then extracted through a rubber GC-septum (Bruker) using a gastight syringe and injected into the GC equipped with a thermal conductivity detector (TCD).
The GC-TCD used consisted of a Bruker 436-GC equipped with a split/splitless injector (270 • C) and a thermal conductivity detector (Bruker) equipped with a custom-made capillary column. The column consisted of 5A molecular sieve column (Bruker, 10 m, 0.32 mm ID, 30 μm coating), in parallel with a Q-PLOT column (Bruker, 50 m, 0.53 mm ID, 20 μm coating). This custom column allows for permanent gases (H 2 , O 2 , N 2 , CO) and light hydrocarbons (CH 4 , C 2 H 6 , C 2 H 4 , etc.) as well as CO 2 to be well-separated in a single injection. Argon was used as the carrier gas at a flow rate of 9 mL min −1 . In order to maximize the sensitivity of the detector, the reference cell flow rate of the TCD was set to 30 mL.min −1 and the make-up flow rate of the analytical cell was set to 5 mL.min −1 . The TCD temperature was set to 230 • C while the filament temperature was set to 370 • C.

Electrochemical impedance spectroscopy (EIS).-Electrochemi-
cal impedance spectroscopy (EIS) measurements were conducted on NMC442/graphite pouch cells after formation and also after cycling on the UHPC. 28 Cells were charged or discharged to 3.80 V before they were moved to a 10. ± 0.1 • C temperature controlled box. Alternating current (AC) impedance spectra were collected with ten points per decade from 100 kHz to 10 mHz with a signal amplitude of 10 mV at 10. ± 0.1 • C. A Biologic VMP-3 was used to collect these data. Figure 1 shows the differential capacity (dQ/dV) vs. V curves of NMC442/graphite pouch cells with different concentrations of FEC, DiFEC, TFEC and HFBMC, during formation step 1. From the dQ/dV vs. V curves, one can determine at which potential the additives initially react with the graphite electrode. The control cells showed a pronounced peak at 2.85 V, which corresponds to a potential of ∼0.75 V vs. Li/Li + . This peak is therefore associated to the reduction of EC on graphite electrode. 36 When 2% TAP was added to the control electrolyte, two peaks were observed near cell terminal potentials of 2.65 V and 2.85 V, which correspond to approximately 0.95 V and 0.75 V vs. Li/Li + , respectively, the former due to TAP and the latter due to EC. 31 Figure 1a shows that cells containing 2% FEC have three peaks at cell potentials of 2.45 V, 2.65 V and 2.85 V, which correspond to potentials of ∼1.15 V, 0.95 V and 0.75 V vs. Li/Li + , where the first two are presumably due to FEC and the last is due to EC. When the FEC content was increased to 5%, the peaks at 2.65 V and 2.85 V decreased and while the peak at 2.45 V increased. This may suggest that the graphite can be passivated completely in a one electron reduction of FEC when there is sufficient FEC present as 5% FEC virtually eliminates the 2.65 V peak due to FEC and does eliminate the 2.85 V due to EC. Figure 1a shows that cells containing 2% DiFEC have four peaks near cell terminal voltages of 2.20 V, 2.45 V, 2.75 V and 2.85 V, which correspond to potentials of approximately 1.40 V, 1.15 V, 0.85 V and 0.75 V vs. Li/Li + , respectively. The first two peaks are clearly caused by DiFEC. When the DiFEC content was increased to 5%, the peak at 2.45 V was virtually eliminated and the peak at 2.20 V became much larger. This behavior is similar to that observed for FEC. In addition, the peak due to EC is eliminated or significantly depressed and shifted when 2% or 5% DiFEC is used. Figure 1b shows that cells containing TFEC or HFBMC have similar reduction peaks as control cells. It is therefore believed that the fluorinated cyclic carbonates (FEC and DiFEC) are SEI forming additives and react on the negative electrode while the fluorinated linear carbonates (TFEC and HFBMC) do not have an obvious impact on the negative electrode SEI during formation. Figure 2 shows the cycling data collected using the UHPC on NMC442/graphite pouch cells cycled to 4.4 V during barn-charge cycling experiments (the protocol is shown in Figure S1b). Five electrolyte solutions, including control, 2% TAP, 2% FEC, 2% DiFEC and PES211 were selected for comparison. From top to bottom, the 4 panels in Figure 2 show: a) the difference between the average cell voltage during charge and the average cell voltage during discharge, V (V), b) the charge end point capacity, Ch. End Cap (mAh), c) the discharge capacity, Q d (mAh), and d) the coulombic efficiency (CE), all plotted versus cycle number. Differences in V are caused by differences in cell polarization during cycling and smaller values of V generally indicate lower DC resistance. 13 Figure 2a shows that cells containing TAP have higher V (high impedance) than control, while cells containing FEC or DiFEC have smaller V than control cells.

Results and Discussion
In addition, Figure 2a shows that V increases with cycle number (bad) for cells with control and 2% TAP, while V is stable for cells with FEC, DiFEC and PES 211. Figure 2b shows that all cells have smaller charge end point capacity slippage (less electrolyte oxidation) than cells with control electrolyte, and cells containing 2% DiFEC and PES 211 have the smallest charge end point capacity slippage among the cells shown here. Figure 2c shows all that cells have a slow capacity fade during UHPC cycling to 4.4 V, except PES 211, which is stable during the first 15 cycles. Figure 2d shows that the CE of cells decreases from cells with PES 211 (best) > 2% DiFEC > 2% FEC > 2% TAP > control (worst). Figure 3a summarizes the coulombic inefficiency (CIE = 1 -CE) for all the NMC442/graphite cells with electrolyte additives, cycled on the UHPC at 40. ± 0.1 • C, using barn-charge protocols to 4.2 V (protocol in Figure S1a) and 4.4 V (protocol in Figure S1b). The detailed CE vs cycle number data are given in Figures S2d, S3d, S4d and S5d in the supporting information. The CIE was calculated from an average of the final three data points (cycles 13-15) collected on the UHPC (see Figure 2d). Each data point in Figure 3a represents the  Figure 3b summarizes the discharge capacity fade rate for the same cells shown in Figure 3a. Detailed discharge capacity vs cycle number data are given in Figures S2c, S3c, S4c and S5c in the supporting information. The fade rate was calculated from the slope of a best fit line to the final five points (cycles 11-15) of the discharge capacity versus cycle number curves (see Figure 2c). The discharge capacity fade is caused by depletion of the lithium inventory, due to SEI growth at the negative electrode and was shown to be relatively independent of voltage in earlier publications. 33,38 Figure 3b shows that the fade rate at 4.2 V is higher than at 4.4 V which suggests a possibly beneficial electrode/electrode interaction above 4.2 V. Figure 3b shows the fade rate increases with higher concentration of additives. Figure 3b shows  Figure 3c summarizes the charge end point capacity slippage rate for the same cells as Figure 3a. Detailed charge end point capacity vs cycle number data are given in Figures S2b, S3b, S4b and S5b in the supporting information. The charge end point capacity slippage rate was calculated from the slope of a best fit line to the final five points (cycles 11-15) of the charge end point capacity versus cycle number curves (see Figure 2c). Charge end point capacity slippage is caused by undesired reactions such as electrolyte oxidation or transition metal dissolution at the positive electrode. 38 Figure 3c shows that cells containing FEC or DiFEC have smaller charge end point capacity slippage rates than the cells containing control or the two linear fluorinated carbonates. Cells with PES211 have the smallest charge end point capacity slippage rate at 4.2 V but cells containing DiFEC have the smallest charge end point capacity slippage rate at 4.4 V. This suggests there may, perhaps, be some benefit in combining some DiFEC with PES211. Figure 3d summarizes the average rate of increase in chargedischarge polarization ( V/cycle) for the same cells as Figure 3a. Detailed V vs cycle number data are given in Figures S2a, S3a, S4a and S5a in the supporting information. The rate of increase in V was calculated from the slope of a best fit line to the final five points (cycles 11-15) of the V vs. cycle number curves. The increase in V/cycle is caused by an increase in cell polarization during cycling and smaller values of V/cycle generally indicate lower impedance growth during cycling. 38 Figure 3d shows cells cycled at 4.4 V have much larger values of V/cycle than those of cells tested at 4.2 V, indicating more impedance growth at high voltages, which agrees well with previous results. 7,8 Figure 3d shows that cells containing all these fluorinated additives have smaller V/cycle than cells containing control or 2% TAP, at both 4.2 V and 4.4 V. However, cells with PES211 have the smallest impedance growth (the impedance actually gets slightly smaller) at 4.2 V and have competitive impedance growth at 4.4 V. Figure 4a shows the volume of gas produced in NMC442/graphite pouch cells with some selected electrolyte additive blends, a) during formation step 1 (charged to 3.5 V) and b) formation step 2 (charged to 4.5 V). Figure 4a shows that cells containing FEC or DiFEC produced much less gas than cells containing control or 2% TAP, while cells containing TFEC or HFBMC produced more gas than cells containing control or 2% TAP during formation step 1. Figure 4a shows cells containing PES211 produced a very small amount of gas during both formation steps and had the smallest total amount of gas produced during formation of all the cell tested. Figure 4a shows that increasing the amount of FEC or DiFEC decreases the amount of gas produced during formation step 1 and increases the amount of gas produced during formation step 2. It is important to decrease the gas evolution during formation since too much gas during formation can cause the deformation of the jelly roll and can lead to inhomogeneous current distributions and Li plating. 39 Cells containing control, TAP, TFEC and HFBMC produced too much gas during formation step 1 to be considered practical in 2.2 mL pouch cell. Figure 4b shows the volume of gas produced during UHPC cycling to 4.2 V and during UHPC cycling to 4.4 V. A volume change certainly less than 10% (0.22 mL) is desired during cycling. Figure 4b shows that all cells satisfy this requirement during UHPC cycling, except cells which contained 5% DiFEC cycled to 4.4 V, which produced about 0.5 mL gas. Figure 5a shows the cell voltage versus time at 40. ± 0.1 • C for the selected electrolyte additives in NMC442/graphite cells measured during in-situ gas measurements in formation step 1. Figure 5b shows the in-situ gas volume versus time during the first charge to 3.5 V for the same cells as shown in Figure 5a. Figure 5b shows that all cells have an initial gas evolution peak, mainly between 2.4 V and 3.4 V associated with the passivation of the graphite electrode. Figure 5b shows that the gas volume produced during in-situ gas measurement agrees well with that measured during the ex-situ gas measurements shown in Figure 4a. Cells containing FEC or DiFEC produce a small amount of gas, while cells containing control, TAP, TFEC or HFBMC produce a large amount of gas.
After the in-situ gas measurement, the gaseous products were extracted and analyzed by GC-TCD. 35 The main gaseous products during formation included CH 2 =CH 2 , H 2 , CO and small amounts of CO 2 , CH 4 and CH 3 CH 3 , as is shown in Figure 5c. In the TAP containing cell, C 3 H 8 was present, which most likely comes from the reduction of TAP at the graphite surface following: [2] The formation of C 2 H 4 comes from the reduction of EC as shown by many researchers. [40][41][42] Carbon monoxide is produced following the 2-electron reduction of EC and EMC as proposed by Onuki et al., 43 and some of the H 2 results from the reduction of residual moisture. [44][45][46] Finally, CH 4 and C 2 H 6 come from the reduction of EMC. 47,48 For cells containing FEC or DiFEC, the amount of C 2 H 4 evolution is much less than that of cells containing control, TAP, TFEC or HFBMC. This is likely due to the early reduction of FEC and DiFEC at the graphite surface as seen in the dQ/dV vs. V curves shown in Figure 1. The reduction of FEC and DiFEC partially passivates the negative electrode surface before the graphite electrode reaches a potential below the reduction potential of EC. This then greatly lowers the amount of EC getting reduced and the associated C 2 H 4 produced. The formation of polymeric or oligomeric species as well as LiF and Li 2 CO 3 by FEC instead of (CH 2 OCO 2 Li) 2 by EC on the graphite electrode provide a different passivation and prevents excessive gassing during formation. 49 Given the similarity between the structure of FEC and DiFEC, it is likely that DiFEC acts in a similar manner. Cells containing FEC and DiFEC also showed smaller H 2 generation. This most likely comes from the early passivation of the graphite surface by FEC and DiFEC which prevents further water and HF reduction. For instance, Bernhard et al. 50 showed that the addition of VC lowered the amount of H 2 generated in graphite/Li half cells containing an electrolyte to which water was intentionally added. It is then likely that FEC and DiFEC act in a similar manner. Figure 6 shows a summary of EIS data after formation, after UHPC cycling to 4.2 V and after UHPC cycling to 4.4 V. Detailed EIS spectra for all of the cells tested before and after UHPC cycling are given in Figure S6. The EIS measurements were made at 3.80 V and 10. ± 0.1 • C. Figure 6 shows that the diameter of the semicircle in the Nyquist plot, called R ct here, of all the cells, except those for PES211 tested to 4.4 V, increases during UHPC cycling tests compared to cells after formation. Figure 6 shows adding TAP, PES211 or 5% DiFEC to the cells increases R ct after formation while adding FEC, TFEC, HFBMC or 2% DiFEC does not obviously increase R ct . A high concentration of DiFEC in these cells is not recommended. Figures 7a and 7b show the capacity versus cycle number for the NMC442/graphite pouch cells containing different additives during the grid cycling protocol to 4.2 V at 55. ± 0.5 • C (protocol is shown in Figure S7a). The grid cycling protocol contains a C/4 (45 mA) charge, an eight hour hold at top of charge, a C/4 discharge and then an eight hour rest at the bottom of discharge. Therefore, one cycle takes 24 hours. During grid cycling, all cells were clamped to ensure firm pressure. A low rate, C/20 cycle was included every 50 cycles, to gauge what fraction of the capacity loss was due to impedance growth during the high rate cycling, but the capacity of the cells at C/4 and at C/20 were almost exactly the same at 55 • C so this was ineffective. Figures 7e and 7f show the difference between average charge and discharge voltage ( V) vs cycle number for the same cells shown in Figures 7a and 7b, respectively. Figures 7a, 7b, 7e and 7f show that cells containing control and 2% TAP have rapid capacity loss and impedance growth during cycling at 4.2 V, while cells containing fluorinated additives have better capacity retention than cells containing control or 2% TAP. Figures 7a and 7b show cells containing FEC or DiFEC have better capacity retention and less capacity loss than cells containing TFEC or HFBMC. Figures 7a and  7e show that cells containing 5% DiFEC have low discharge capacity, due to its high impedance. Figures 7e and 7f show cells containing control and 2% TAP have increased impedance growth rates compared to those with fluorinated additives. Figures 7c and 7d show the capacity versus cycle number for the NMC442/graphite pouch cells containing different additives during the grid cycling protocol to 4.4 V at 55. ± 0.5 • C (protocol is shown in Figure S7b). Figures 7g and 7h show V vs cycle number for the same cells shown in Figures 7c and 7d  performance to control cells which is much worse than cells containing 2% TAP. Figures 8a and 8b show the summary of EIS and the volume change data collected after the long-term grid cycling experiments of the same cells shown in Figure 7. Detailed EIS spectra are given in Figure S8. Figures 8a and S8 show the impedance after cycling to 4.4 V is much larger than for cells cycled to 4.2 V. Figure S8 shows that the impedance spectra of cells cycled to 4.2 V do not change shape after grid cycling, while the spectra of the cells cycled at 4.4 V show large impedance growth. Figure 8a summarizes the EIS spectra. Figure 8b shows the gas produced during grid cycling for the same cells. Figure 8b shows that cells cycled to 4.4 V generally produced more gas than cells cycled at 4.2 V. Figure 8b shows cells containing control, TAP, FEC, DiFEC and HFBMC all produce a large amount of gas during cycling, especially at 4.4 V. In fact, these electrolytes in NMC442/graphite cells produce too much gas to be useful. Figure 8b shows cells containing TFEC have much less gas production at both 4.2 V and 4.4 V. However, cells with TFEC do not display acceptable properties.
There is a large amount of data in this paper which may be difficult for the reader to assimilate. Table I is an attempt to summarize all the data in one place. Table I lists the outcomes of all the tests in this paper. For example, in Table I, a " √ " indicates that the electrolyte has possibly suitable properties in that test, while an "X" indicates that the properties are unacceptable. When the word "BEST" appears in Table I, it designates that the particular electrolyte led to the best properties. Table I shows that none of the fluorinated electrolyte additives, on their own, create NMC442/graphite cells which are acceptable in all tests. However, 2% DiFEC appears to be the most preferred of the fluorinated additives showing only a problem with gassing during 4.4 V 55 • C cycling. Cells with 2% DiFEC outperformed cells with 2% of all of the other fluorinated additives. Although data for PES211 is not available in all tests, previous work 6,48 has shown that NMC111/graphite cells show excellent charge-discharge capacity retention at 55 • C with small amounts of Table I. Summary of the results. X -completely unsuitable for pouch-type Li-ion; √ -possibly suitable; BEST -best data; N/A -head to head comparative data not available. The columns list from the left: electrolyte type; does the negative SEI differ from the control SEI?; amount of gas during formation step 1; amount of gas during formation step 2; the coulombic inefficiency during 4.2 V UHPC barn cycling at 40 generated gas, although the protocol was different than that used here.

Conclusions
Four fluorinated carbonates were studied as electrolyte additives in NMC442/graphite pouch type Li-ion cells. The results showed that fluorinated cyclic carbonates such as FEC and DiFEC are negative electrode SEI forming additives while fluorinated linear carbonates such as TFEC and HFBMC are not. The results showed that cells containing 2% DiFEC had higher CE, lower charge end point capacity slippage rate, lower impedance as well as lower gas evolution during cycling tests on UHPC at both 4.2 V and 4.4 V, compared to cells containing FEC, TFEC, HFBMC or TAP. High concentrations of DiFEC (5%) cause high impedance as well as gas evolution during UHPC cycling. Grid cycling experiments showed that cells containing DiFEC have better capacity retention and less impedance growth than cells containing FEC or TAP. These results suggested that DiFEC is a useful additive and could replace FEC in high voltage applications.
Comparisons were made between the NMC442/graphite cells with fluorinated additives with cells containing 2% prop-1-ene,1,3,sultone + 1% ethylene sulfate + 1% tris(trimethylsilyl) phosphite (PES211) and no fluorinated additives. Cells with PES211 outperformed cells with single fluorinated additives in virtually all tests. This suggests that single fluorinated additives are not competitive to PES211 in NMC442/graphite cells destined for high potential applications. Further work should be done to find co-additives that can be used together with DiFEC to improve cell performance.