A Tale of Two Additives: Effects of Glutaric and Citraconic Anhydrides on Lithium-Ion Cell Performance

The use of electrolyte additives is an important method to improve lithium-ion cell lifetime and performance without signiﬁcantly affecting costs. This work evaluates two organic anhydrides, glutaric anhydride (GA) and citraconic anhydride (CA), as additives in Li(Ni 0.6 Mn 0.2 Co 0.2 )O 2 (NMC622)/graphite and Li(Ni 0.5 Mn 0.3 Co 0.2 )O 2 (NMC532)/graphite pouch cells, using ultrahigh precision coulometry and high-temperature storage. The additives were tested singly and in binary blends. GA-based additive blends give high coulombic efﬁciencies (CEs) and good storage performance. However, GA leads to substantial impedance during formation. Most notably, GA is extremely effective at suppressing gas during cell formation and storage. Whereas CA-containing blends yield good CEs, they show rapid voltage drop during storage. Both additives may provide speciﬁc beneﬁts for target applications. Long-term cycling data indicates that GA is a negative electrode SEI-forming additive that is useful for capacity retention and limiting cell impedance growth when used as a binary blend with vinylene carbonate or lithium diﬂuorophosphate. These results are also intended to facilitate comparison between chemically related additives in order to better understand the underlying chemistry behind their function in lithium-ion cells. © The The continued adoption of electric vehicles and grid energy stor- age technologies has the potential for signiﬁcant societal and environ-mental beneﬁts. It is therefore desirable to develop lithium-ion cell chemistries that optimize long cycling and calendar lifetimes, high energy density, and low cost. The introduction of electrolyte additives is a practical and increasingly ubiquitous approach to improve cell lifetimes at higher cell voltage without signiﬁcantly affecting manu- facturing costs. these additives function by leading to the formation of passivating solid-electrolyte interphase electrode-electrolyte

The continued adoption of electric vehicles and grid energy storage technologies has the potential for significant societal and environmental benefits. It is therefore desirable to develop lithium-ion cell chemistries that optimize long cycling and calendar lifetimes, high energy density, and low cost. The introduction of electrolyte additives is a practical and increasingly ubiquitous approach to improve cell lifetimes at higher cell voltage without significantly affecting manufacturing costs. 1 Many of these additives function by leading to the formation of passivating solid-electrolyte interphase (SEI) layers at one or both of the electrode-electrolyte interfaces. Well-known examples include vinylene carbonate (VC), 2-6 fluoroethylene carbonate (FEC), [5][6][7][8][9] prop-1-ene-1,3-sultone (PES), 10-13 1,3,2-dioxathiolane-2,2dioxide (DTD), [14][15][16] methylene methane disulfonate (MMDS), 10,11,17 pyridine boron trifluoride (PBF), [18][19][20][21] and lithium difluorophosphate (LFO). [22][23][24] Succinic anhydride (SA, Figure 1a) and maleic anhydride (MA, Figure 1b) are organic anhydride compounds that differ only by a carbon-carbon double bond and that have been used previously as electrolyte additives in lithium-ion cells. [25][26][27][28] This work considers whether compounds that are chemically similar to SA and MA may also be used as additives. The testing of new compounds that are chemically similar to known electrolyte additives is one approach to additive discovery. 29,30 The substitution of a hydrogen atom (H) with a methyl group (CH 3 ) in SA and MA yields methylsuccinic anhydride (MSA, Figure 1c) and citraconic anhydride (CA, Figure 1d), respectively. The expansion of the five-membered ring of SA to a six-membered ring, via the insertion of an additional CH 2 moiety, gives the structure of glutaric anhydride (GA, Figure 1e). These three structures were therefore considered potentially interesting to test as additives in lithium-ion cells. The commercial list price of MSA is significantly more expensive than either CA or GA, making it less favorable for study and development. Therefore, this study was limited to comparing CA and GA additives. GA has shown promise in LiNi 0.4 Mn 1.6 O 4 /Li 4 Ti 5 O 12 cells, 31 whereas the focus of the present work is LiNi 1-x-y Mn x Co y O 2 (NMC)/graphite cells. In addition to their use as single additives, bi-nary combinations of the anhydrides with well-known blend components VC, DTD, MMDS, and LFO were tested. 11,32,33 Experimental Lithium-ion cells.-Dry (no electrolyte), vacuum-sealed LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532)/graphite (gr) and LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622)/gr pouch cells, with capacity of ∼220 mAh and ∼230 mAh, respectively, were received from LiFun Technology (Tianyuan District, Zhuzhou, Hunan, China). The NMC532 material was 'single crystal' (sc), as described previously, 34,35 and the particles were uncoated (NMC532(sc)u) or had a proprietary inorganic surface coating (NMC532(sc)c). The NMC622 active materials were coated with either an Al 2 O 3 coating or and Al and F-based coating (denoted NMC622a and NMC622b, respectively). Except as noted, the NMC622 was conventional polycrystalline material. For selected prolonged cycling tests, single crystal NMC622 with a proprietary inorganic surface coating of unknown composition was used, denoted as "NMC622(sc)c". The negative electrodes were made of an artificial graphite (AG), except as noted; select prolonged cycling tests used cells with natural graphite (NG) negative electrode material. The cells were cut below the heat seal in an argon-atmosphere glove box, dried under vacuum at 80°C for 14 h, and then returned to the glove box for filling. All solutions in this work used 1.2 mol L -1 LiPF 6 (BASF, ≥ 99.9%) in a 3:7 solvent blend, by mass, of ethylene carbonate (EC) and ethylmethyl carbonate (EMC), as received from BASF (< 20 ppm H 2 O). The additives, GA (Sigma, 95%), CA (Aldrich, 98%), VC (BASF, ≥ 99.8%), DTD (Guangzhou Tinci Materials Tech. Co. Ltd., ≥ 98%), and LiPO 2 F 2 (called LFO in this work, Shenzhen CapChem Tech. Co. Ltd.) were added singly or as binary blends to this electrolyte solution in the indicated mass percentages. Cells were filled with 1.0 ± 0.1 g of solution, sealed at −90 kPa gauge pressure using a compact vacuum sealer (MSK-115A, MTI Corp.), and immediately held at 1.5 V at room temperature (21 -25°C) to prevent corrosion of the copper current collector during the ∼24 h wetting period that followed. Cells were then loaded into temperature-controlled boxes (40.0 ± 0.1°C) and connected to a Maccor 4000 Series automated test system (Maccor Inc.). Because gas formation is frequently observed during formation, storage, and cycling, the pouch cells were clamped using soft rubber (at about 25 kPa gauge pressure) during all electrochemical testing, which has previously been observed to significantly improve the experimental precision.
Electrochemical testing.-SEI formation was performed by charging cells at C/20 to 4.3 V (at 40°C), holding at 4.3 V for 1 h, discharging at C/20 to 3.8 V, and then holding cells at 3.8 V for 1 h. Cells were weighed under water before and after formation, allowing the change in displacement volume to be determined using the Archimedes principle. Cells were then degassed by cutting the pouch open in an argon-atmosphere glove box, and resealed using the compact vacuum sealer. Cells were weighed again and electrochemical impedance spectroscopy (EIS) was then measured at 10.0 ± 0.1°C using a BioLogic VMP3 instrument (100 kHz -10 mHz, ± 10 mV sinusoidal amplitude). Following formation, cells were either taken for ultrahigh precision coulometry (UHPC) cycling or high temperature storage. In this work, R ct represents the width of the depressed semi-circle in the Nyquist plot.
For UHPC cycling, cells were maintained at 40.0 ± 0.1°C and cycled at C/20 between 3.0 -4.3 V. Measurements were performed with the custom-built Dalhousie UHPC system, described previously, 36,37 or with a Novonix UHPC charging system. For storage, cells were maintained at 40.0 ± 0.1°C while they were cycled at C/10 between 2.8 -4.3 V twice and then held at 4.3 V for 24 h. Cells were then moved to a storage box with fixed temperature of 60.0 ± 0.1°C and the cell voltage was recorded at open circuit for 500 h. Following storage, cells were charge-discharge cycled at C/10 between 2.8 -4.3 V twice again and then charged to 3.8 V. Cells were then taken for EIS testing at 10°C, as described above, and then weighed under water again to measure gas evolution during storage.
For long-term cycling, cells were maintained at 40.0 ± 0.1°C and cycled at C/3 between 3.0 -4.3 V using a Neware testing system. A slow cycle was performed every 50 cycles at C/20 to evaluate impedance growth.

Results and Discussion
Cells were prepared with GA-and CA-containing electrolyte solutions. Since these are previously untested additives, multiple concentrations were tested. When used individually, GA and CA were added in 1% or 2%, by weight, which was based on the amounts used in SA and MA studies. [25][26][27][28] When used in combination with DTD, MMDS, or LFO, the anhydrides were tested at 2% concentration. This is based on the prediction that GA and CA may act as the 'primary' filmforming additive at the negative electrode surface. This hypothesis was based on previous studies that combined DTD, MMDS, or LFO with negative electrode SEI-forming additives such as PES, VC, or FEC. 10,11,32,33,38,39 Finally, GA and CA were tested at lower concentra-tions, i.e., 0.5% and 1%, when used in combination with VC. This is because co-additives tend to behave as the 'secondary' additive when used in combination with VC. Figure 2 shows the differential capacity (dQ/dV vs E cell ) of SEI formation for cells containing GA-and CA-based additive blends in NMC622b/gr pouch cells. At these relatively low cell voltages, the electrode potential at the positive is approximately constant at E pos ∼ 3.6 V vs Li/Li + , whereas the potential at the negative electrode decreases relatively quickly. 38,40 Therefore, the features in Figure 2 correspond to reduction processes at the negative electrode surface. The peak positions may be used to estimate reduction potentials relative to the lithium electrode according to 1: Control cells prepared without any additives exhibit a peak that onsets at 2.90 -2.95 V, corresponding to the expected reduction of EC on the graphite surface at 0.65 -0.70 V vs Li/Li + . Cells that contained 2%VC show a reduction feature that onsets at 2.75 -2.80 V, corresponding to 0.80 -0.85 V vs Li/Li + . The addition of VC suppresses the EC reduction peak, indicating the graphite surface has been effectively passivated. This is consistent with the expectation that VC reduction forms a passive SEI on graphite negative electrodes. 2 Cells prepared with GA-based additives exhibit a new reduction feature at 2.40 -2.45 V (Figure 2a), corresponding to E red ∼ 1.15 -1.20 V vs Li/Li + . The addition of 1% GA greatly suppresses EC reduction and all cells prepared with 2%GA (including binary blends) suppress EC reduction completely. These results indicate that GA reduction forms a passivating SEI on the graphite electrode surface. Moreover, the GA reduction feature occurs at lower cell voltage than the feature corresponding to VC reduction at ∼2.8 V. As a result, in cells prepared with a blend of 2%VC and 0.5%GA, the magnitude of the VC reduction feature is greatly decreased. This suggests that 0.5%GA partially passivates the negative electrode. The VC reduction feature is completely absent in cells that contained 2%VC and 1%GA. Therefore, at 1%GA content, the negative electrode is completely passivated by the time the cell charges to the potentials at which VC reduction normally occur.
In contrast, cells prepared with CA do not show any new reduction features (Figure 2b). The position of EC reduction is shifted by ∼0.05 V, which suggests some chemical interaction occurs, but that the graphite surface is not passivated by CA. The cells prepared with CA/VC blends similarly exhibit a slightly shifted VC reduction peak. Finally, the 2%CA/1%DTD blend shows a peak at ∼2.80 -2.85 V, corresponding to E neg ∼ 0.75 -0.80 V vs. Li/Li + . This is a poor match for the reduction potential of DTD, E 0 red = 1.3 -1.4 V vs Li/Li + . The volume change due to gas evolution during the first cell chargedischarge cycle (i.e., the formation cycle) is shown in Figure 3. In all cell types, the introduction of GA significantly decreased the volume of gas evolved, relative to additive-free control cells. The suppression of formation gas is observed whether GA is used as a single or binary additive. The magnitude of gas suppression is, on average, comparable to that observed for VC-containing cells. In contrast, the addition of CA decreases the volume of gas produced during formation by only a minor amount. That is, cells that contained CA produced a similar volume of gas as the control cells. The exception to this was that cells prepared with both CA and VC displayed significant gas suppression. This decrease is attributed to the VC, rather than the presence of CA. Therefore, GA and VC are effective at suppressing gas during formation, whereas CA is not. This is consistent with the hypothesis that GA and VC form SEIs that passivate the negative electrode, whereas CA does not.
The suppression of gas in GA-containing cells may suggest that this additive forms an effective SEI. Although the lack of gas suppression in CA-containing cells could be interpreted as a counter-indication of electrode passivation, some electrolyte additives that lead to passive SEIs, such as DTD, nonetheless generate gaseous species as by products during the course of SEI formation. 12,38,41 The R ct of the cells following formation was measured to further explore the effects of GA-and CA-based additive blends, as shown in Figure 4. Whereas the magnitudes of the effects vary by positive electrode material and coating composition, the general trends are the same for the four cell types tested here. As GA is introduced into a cell, the R ct following formation is greatly increased, up to nearly an order of magnitude greater in 2% GA cells, relative to additive-free control cells. Given that one dominant feature is visible in the Nyquist plots of GA-containing cells following formation (NMC622b/AG cells are shown for example in Figure 5), that GA prevents EC reduction ( Figure 2) and that GA limits gas evolution during formation, it is considered most likely that the impedance growth is primarily attributable to the reduction product, or products, of GA on the graphite surface. When VC is used as a co-additive with GA, the R ct is similar to the equivalent cell with GA by itself. That is, 2%VC/1%GA cells have a similar post-formation R ct as cells containing 1%GA only. Together with the volumetric data in Figure 3, it is concluded that GA produces a high impedance negative electrode SEI that limits the formation of gas from electrolyte decomposition during the formation cycle.
The introduction of CA also increases the post-formation R ct , however the extent of the increase is considerably smaller than is observed for GA. As with GA, the general trends are the same for all four positive electrode types studied in this work. It is similarly observed that for CA/VC binary blends, the R ct is dominated by the presence of the organic anhydride, rather than the carbonate. That is, the R ct of 2%VC/1%CA cells is comparable to the R ct of cells that contained 1%CA only. The results suggest that CA is reacting in such a way that the impedance of the cell is increased. However, the lack of suppression of formation gas precludes any conclusions, at this point, on whether CA leads to the formation of a passivating SEI.
The coulombic efficiency (CE) from UHPC measurements is strongly correlated with the long-term cycling performance and stability of lithium-ion cells. 36,37,42,43 UHPC was therefore chosen as an efficient method to rapidly screen the suitability of GA-and CAcontaining electrolytes in NMC/graphite cells. The results for GAcontaining cells, shown in Figure 6, illustrate considerable variation in the results depending on the combination of positive electrode material and coating. For example, the CE of the NMC532(sc)c/gr cells was > 99.75% after 16 cycles for all electrolyte compositions tested in this work. In contrast, the NMC532(sc)u/gr cells had a visibly greater distribution in the measured CE values. It is emphasized that both cells utilized the same positive electrode material -only the presence or absence of an inorganic surface coating was different. For the NMC622a/gr cells, the greatest CE after 16 cycles is observed for the 2%VC/1%GA blend. In contrast, the largest CE in NMC622b/gr cells is observed for 2%GA/1%LFO. Again, only the surface coatings on the positive electrodes varied between these cells. This highlights a significant challenge faced by battery scientists and engineers; a complex interplay exists between the electrolyte composition, electrode materials, and many other aspects of cell design. Yet despite these subtleties, some broad observations can be made. The cells prepared with binary combinations of GA generally outperformed those that contained GA as a singular additive. In particular, 2%GA/1%LFO and 2%VC/0.5%GA offer potentially promising balance between high CE and low impedance growth (based on V, the difference in average voltage between charge and discharge). Figure 7 shows the UHPC cycling data for cells prepared with CA and CA-based additive blends. Relative to GA-containing cells, CA-based electrolytes lead to a smaller distribution of CE values and lower V in NMC622a/gr, NMC622b/gr, and NMC532(sc)u/gr cells. This result is especially marked in the last of these cell types, which performed especially poorly with GA-containing solutions. Yet the CE of cells prepared with the uncoated NMC532(sc)u material is nevertheless the worst of the four cell types tested here. Moreover, the greatest CE observed in this work was for binary GA-based blends in NMC532(sc)c/gr (Figure 6).
High temperature storage is another practical method to rapidly evaluate the stability of new lithium-ion cell chemistries. The cell  voltage drop over time for cells initially at 4.3 V and stored at 60°C is shown in Figure 8 for NMC622a/gr and NMC622b/gr cells. For both cell chemistries, GA clearly outperforms CA at high temperature storage. In both cases, the best storage behavior is observed for the VC/GA blends, although in NMC622a/gr, this electrolyte actually underperforms 2%VC alone. The volume of gas produced during storage (Figure 9) supports the conclusion that GA-containing electrolytes perform better at high-temperature storage than those prepared with CA. However, in the 622a/graphite cells, the use of VC as a co-additive eliminated the gas suppression quality of GA. Given that the VC is not fully reduced in cells that also contain GA (as concluded from Figure 2), there is some of the additive left over in the electrolyte fol-lowing formation. While the exact mechanism is not known, it is likely that VC oxidation leads to 'cross-talk'. This occurs when oxidation products generated at the positive electrode cross over to affect the composition and stability of the SEI at the negative electrode. [44][45][46] Due to limited availability of materials and resources and the poor storage behavior of CA in both NMC622 cells, only the GA-containing additives were used for high-temperature storage in NMC532(sc)c/gr and NMC532(sc)u/gr cells. In the cells with the coated positive electrode material, 2%VC/0.5%GA and 2%VC/1%GA binary blends retain their voltage better than cells prepared with VC alone (Figure 10a). In contrast, 2%VC had the least cell voltage drop in NMC532(sc)u/gr cells (Figure 10b). It is again observed that GA and VC generally, but not always, decrease the volume of gas produced during high temperature storage ( Figure 11). The exception is the combination of 2%GA/1%LFO in an uncoated NMC532(sc)u/graphite cell, which produced slightly more gas than the additive-free control cells. However, when used in the coated NMC532(sc)c/graphite cells, GA-containing solutions are observed to produce very little or even no gas after 500 h storage. The results support that GA may be a useful additive for applications where it is important to produce very little gas. The development of higher energy density batteries for portable electronics is one example where cell volume expansion can be catastrophic for device lifetime.
The R ct following storage is compared for the GA-containing solutions in all four cell types ( Figure 12). It is generally observed that the improvements of gas suppression and decreased voltage drop come with a trade-off of greater impedance growth over time. However, the R ct following storage is nonetheless significantly less than observed immediately following cell formation.
The results highlight the complex interplay between the electrolyte composition and the electrode compositions and morphologies. As expected, the single crystal positive electrode materials display greater capacity retention (Figures 13c-13d) and slower cell impedance growth, as evaluated by the slope of V vs cycle number (Figures 13g-13h, V is the difference between average charge and average discharge voltages), relative to cells prepared with polycrystalline electrode materials (Figures 13a-13b and Figures 13e-13f, respectively). Generally, the results indicate that GA can be beneficial for long-term capacity retention and for limiting cell impedance growth. Especially promising is the combination of 2%VC and 0.5%GA, which shows equal or greater performance in all cell types, relative to 2%VC by itself. The advantages of combining LFO and GA (rather than using LFO by itself) are less obvious, based on the first ∼100 cycles where inferior capacity retention is observed. However, the addition of GA to LFO-containing cells is observed to limit cell impedance growth, which may lead to greater long-term cycling behavior. This is already observed for NMC622b/NG and NMC532(sc)c/AG pouch cells, where the capacity retention for 1%LFO with 0.5% GA is greater than the other blends in this work (Figure 13b).

Conclusions
Two relatively inexpensive organic anhydrides, glutaric anhydride (GA) and citraconic anhydride (CA) were tested as electrolyte additives for lithium-ion cells. The additives were tested singly and as binary additive blend components. Two positive electrode compositions (NMC622 and NMC532), each with two surface coatings, were tested, for a total of four cell material chemistries. The results exemplify the complex interplay that exists between electrolyte composition, electrode materials, and electrode surface coatings. Cells prepared with GA show very little gas production after formation or after high-temperature storage. GA-based additive blends lead to large CE and low V growth during UHPC cycling at 40°C, and small voltage drop during storage at 60°C. However, GA leads to very large impedance during formation. Whereas CA-containing blends perform well in UHPC testing, they performed relatively poorly in     high-temperature storage. Both additives may provide specific benefits for some applications, for example GA is very promising as a gas suppressant. Prolonged cycling testing demonstrates that GA is effective at suppressing long-term cell impedance growth in a variety of cell types, when used in combination with VC or LFO. GA/VC and GA/LFO cells also display good long-term capacity retention. It is further hoped that this work will contribute to larger, ongoing efforts to compare chemically similar additives in order to better understand the underlying chemistry behind their function in lithium-ion cells.