Electrolyte Interphases to Enable Wide

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Constructing Robust Electrode/Electrolyte Interphases to Enable Wide-Temperature Applications of Lithium Ion Batteries Bin Liu, Qiuyan Li, Mark H. Engelhard, Yang He, Xianhui Zhang, Donghai Mei, Chongmin Wang, Ji-Guang Zhang, and Wu Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019

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ACS Applied Materials & Interfaces

Constructing Robust Electrode/Electrolyte Interphases to Enable Wide-Temperature Applications of Lithium Ion Batteries Bin Liu,† Qiuyan Li,† Mark H. Engelhard,‡ Yang He,‡ Xianhui Zhang,†,|| Donghai Mei,§ Chongmin Wang,‡ Ji-Guang Zhang,† and Wu Xu*,† † Energy

and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA

99354, United States ‡

Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory,

Richland, WA 99354, United States §

Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory,

Richland, WA 99354, United States || Ningbo

Institute of Materials Technology and Engineering, Chinese Academy of Sciences,

Ningbo, Zhejiang 315201, China * To whom correspondence may be addressed. E-mail: [email protected]

KEYWORDS: Lithium ion battery, electrolyte, additive, electrode/electrolyte interphase, wide temperature

ACS Paragon Plus Environment

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Abstract The electrolyte generally dictates the working temperature range of lithium ion batteries (LIBs), thus developing new class of electrolytes (primarily functional additives) in LIBs for wide temperature applications will be quite essential for further development of LIBs for electric vehicle market. In this study, we develop new functional electrolytes containing multiple additives to enable the LIBs to perform well in a wide temperature range from −40°C to 60°C. Importantly, those cells based on the above optimized electrolytes have been proven to behave significantly enhanced discharging performance at −40°C, the long-term cycling stability at 25°C (more than 85% of capacity retention after 1000 cycles at 1C/1C rates in 1 Ah pouch cells), as well as the obviously improved cycling stability at 60°C. These remarkable cell performances originate from the highly conductive, uniform and compact passivating films formed on both anode and cathode surfaces by the synergistic effects of the multiple additives. Our finding on the synergistic benefits originating from certain additive combination in the optimized electrolytes would favorably widen the working temperature range of future high-performance LIBs.

1. Introduction Lithium (Li) ion batteries (LIBs) have been successfully commercialized and widely applied in portable consumer electronics, electric vehicles and stationary energy storage devices.1–6 Generally, the state-of-the-art LIB systems mainly include a graphite (Gr) anode, an aprotic organic electrolyte, and a lithium transition metal oxide cathode (e.g., LiCoO2, LiNixMnyCozO2 (NMC), LiNi0.80Co0.15Al0.05O2 (NCA), and LiMn2O4, etc.) or a lithium transition metal phosphate cathode (LiFePO4). The conventional organic electrolytes are often composed of lithium hexafluorophosphate (LiPF6) and organic carbonate solvent mixtures, e.g., ethylene carbonate


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(EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), propylene carbonate (PC), and diethyl carbonate (DEC).7–9 EC is an indispensable component in the conventional electrolytes and it helps generate a robust protective layer that is also called solid electrolyte interphase (SEI) on the Gr anodes, which plays a key role in preventing the further intercalation of solvent molecules into Gr layered structure and the continuous decompositions of electrolyte components.3 However, the high melting point of EC (36.4 °C) inevitably brings a relatively high solidification temperature of the electrolytes containing it at about 20% or more, thus leading to a narrow temperature range for most of current and future LIBs’ operations. In particular, the use of large amount of EC solvent in electrolytes is detrimental for LIB performances at low temperatures. Although the temperature range of LIBs can be widened by partially or completely replacing EC with PC due to a much lower melting point (−48.8 °C) of PC, the addition of large amount of PC results in irreversible capacity loss because of its inability to form a stable SEI on Gr anode, which causes the continuous solvent co-intercalation into Gr and subsequent exfoliation of Gr structure. In our previous studies, cesium hexafluorophosphate (CsPF6) as an additive in EC-PC-containing electrolytes can help form ultrathin, compact and uniform SEI layers on Gr electrodes, which promote the widetemperature performance enhancement of LIBs.3,10,11 However, there is still room to further widen the operation temperature range (i.e., −40 °C to 60 °C) of electrolytes that directly dominate the working temperature range of LIBs. It is well known that different functional additives in electrolytes can tune the compositions and properties of the electrode/electrolyte interface layers on both anode and cathode, then directly affecting the LIB performances.12–17 Therefore, to meet the ever-increasing requirements for high performance LIBs under wide-temperature operation in various applications particularly electric vehicles, it’s highly expected to explore cost-effective strategies to develop novel electrolytes for widening working temperature range of LIBs.


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In this work, we present a facile strategy to greatly enhance LIB performances in a wide temperature range of −40 ~ 60 °C through combining previously optimized electrolytes with multiple additives. CsPF6-containing LiPF6-EC/PC/EMC (1:1:8 by weight) electrolyte was employed as a benchmark electrolyte in this study since it has been proven as the optimized widetemperature electrolyte in our earlier report,11 where the contents of PC and EC in the above baseline electrolyte have been optimized to not only ensure their beneficial effects but also minimize negative effects on the wide-temperature battery performances. This baseline electrolyte has shown nearly the same cycling performances as the conventional electrolyte of 1.0 M LiPF6 in EC-EMC (3:7 by volume) at room temperature and elevated temperatures but exhibited much superior low-temperature discharge behavior to the conventional electrolyte down to −40 °C.11 More importantly, the new electrolytes containing multiple additives have been developed as well, and they are able to make LIBs behave greatly enhanced discharging performance at −40°C, improved capacity retention (more than 85 %) after 1000 cycles at 25 °C as well as strong cycling endurance at 60 °C. These significant improvements are attributed to the formation of highly conductive, dense, and robust SEI film on the anode surface as well as ultrathin cathode electrolyte interface (CEI) film on the cathode surface during cycling processes of the Li ion cells. The above results reveal that the feasibility of widening temperature operation range of high performance energy storage systems relies upon the synergistic effects from the additive combinations in the aforementioned optimized electrolytes.

2. Results and Discussions 2.1. Screening of Electrolyte Additives


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The screening of five electrolyte additives, fluoroethylene carbonate (FEC), 1,3-propane sultone (PS), tris(trimethylsily) phosphite (TTMSPi), N-(trimethylsilyl)diethylamine (TMSDEA), and vinylene carbonate (VC), was first conducted to evaluate the low-temperature discharge performance based on the baseline electrolyte E1 (0.05 M CsPF6-containing 1 M LiPF6/EC-PCEMC (1:1:8 by weight)) with different additives in Gr||LiNi0.80Co0.15Al0.05O2 (i.e. Gr||NCA) coin cells. Molecular structures of these additives are provided in Figures S1-S5 and their major functions in the electrolytes are listed in Table 1. The corresponding electrolyte formulations for screening and studying in this work have been provided in Table S1. As seen from Figures 1a−1e, the coin cells with E1+0.5 % FEC, E1+0.5 % PS, and E1+0.5 % TTMSPi indicated superior discharging performance at −40 °C to cells with other additives. It reveals that both type and concentration of additives dictate the low-temperature performance of Li ion cells. More interestingly, the coin cells with E2 (E1 + 0.5 % FEC + 0.5 % TTMSPi) and E3 (E1 + 0.5 % FEC + 0.5 % TTMSPi + 0.5 % PS) demonstrated further enhanced low-temperature performance, especially using E2 electrolyte (Figure 1f). Moreover, their corresponding discharging performances of Li ion cells with the above various electrolytes at 25°C behave very similar, except for those with TMSDEA-based electrolytes (Figure S6). However, the cells with E2 and E3 electrolytes are able to show enhanced discharging performance at room temperature (Figure S6f). As a comparison, their related charging voltage profiles are given in Figure S7.

Table 1. Functions of electrolyte additives in this research Additive CsPF6 FEC

Functions Contribute to formation of utrathin SEI on graphite anodes Contribute to formation of robust and dense SEI on anodes and improvement in low-temperature cell performance


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TTMSPi

Contribute to formation of robust and ultrathin CEI on cathodes and consume HF produced in LiPF6-based electrolytes during cycling

PS

Suppress gassing of cells (especially on cathodes) during long-term cycling and at elevated temperatures

TMSDEA VC

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Consume H2O and HF produced during cycling Contribute to formation of robust SEI on anodes for long cycling stability

Figure 1. Voltage profiles of Gr||NCA coin cells with different electrolyte systems (E1 and E1 with various additives at different contents) at a discharge rate of C/5 (where 1C = 1.50 mA cm-2) at −40°C. (a) FEC additive, (b) PS additive,( c) TTMSPi additive, (d) TMSDEA additive, (e) VC additive, and f) multiple additives.

2.2. Wide-Temperature Electrochemical Performance of Optimized Electrolytes Further, we characterized the discharge performance of 1 Ah pouch cells of Gr|| LiNi1/3Mn1/3Co1/3O2 (or Gr||NMC) system (with the NMC electrode loading of 1.5 mAh cm-2) in


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E1, E2, and E3 electrolytes at a current rate of 1C (i.e. a current of 1 A) under different temperatures. The discharge curves of the pouch cells with the three electrolytes were very similar at 25°C (Figure S8), while the pouch cells with E2 and E3 showed slightly better discharge performance than the E1-containing pouch cell at −18 °C (Figure 2a). More obviously, the pouch cells with E2 and E3 electrolytes, especially E2, indicated greatly improved low-temperature discharge performance at −40 °C, compared to the baseline E1 electrolyte (Figure 2b). The longterm cycling performance of the 1 Ah pouch cells with the three different electrolytes were evaluated at discharge/charge rates of 1C/1C in a cutoff voltage window between 3.0 V and 4.2 V at 25°C. Although the pouch cell with E1 showed a good cycling endurance with capacity retention of 79% after 1000 cycles, the capacity retentions of pouch cells with E2 and E3 were surprisingly as high as 87% and 85%, respectively, even after 1000 cycles (Figure 2c). Correspondingly, as shown in Figure 2d about the evolution of the direct current resistances of the 1 Ah pouch cells with cycling, the electrolyte E1 led to rapid increase in direct current resistance of the cell, while the cells with E2 and E3 electrolytes demonstrated not only much lower direct current resistances but also relatively slight resistance changes during the 1000 cycles, and E2 resulted in the lowest cell resistance. It reveals that the electrode/electrolyte interface layers formed on cathodes and anodes represent higher and more stable ionic conductivity in E2 and E3 electrolytes, compared to those films on both electrodes in baseline electrolyte E1. After cycling, the thickness change of the pouch cells was measured, as shown in Figures 2e, S9, and S10. The thickness values of the cycled pouch cells with E1, E2, E3 electrolytes were 5.247 mm, 4.933 mm, and 4.928 mm, respectively, whereas the original thickness of these pouch cells after two initial formation cycles was 4.878 mm. Thus, the corresponding thickness change ratios of cycled pouch cells with E1, E2, E3 electrolytes are 7.6%, 1.1 %, and 1.0%, respectively. These results indicate that both E2 and


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E3 electrolytes, especially E3 that contains PS additive, exhibit largely improved electrochemical stabilities on both cathode and anode, leading to less electrolyte decomposition and passivation layer accumulation on electrodes, then enhanced long-term cycling performance, and the addition of PS into E2 electrolyte to form E3 electrolyte can indeed further suppress the gas generation during long cycling of the pouch cells.


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Figure 2. (a-c) Voltage profiles of 1 Ah Gr||NMC pouch cells at 1C discharge rate to the cutoff voltage of 3 V at different temperatures: (a) −18 °C, and (b) −40 °C. (c) Cycling performance of 1 Ah pouch cells of Gr||NMC with E1, E2, and E3 electrolytes at discharge/charge rates of 1C/1C rate in a voltage window between 3.0 V and 4.2 V for 1000 cycles at 25 °C. (d) The corresponding evolution curves of direct current resistances during above cycles. (e) Gas generation characterizations of the pouch cells after 1000 cycles with E1, E2, and E3.

Besides the evaluations of the performances of the above three electrolytes at low and room temperatures, we further measured their high-temperature cycling performance at 60 °C. As shown in Figure 3, the cycling stability of the Gr||NCA coin cells with E2 and E3 electrolytes completely outperformed that of E1 electrolyte-based cells during 300 cycles at a current rate of 1C, and E3 also behaved slightly better than E2. Based on our previous findings,11 EC content in electrolytes can largely influence the cell performances at high temperatures (e.g. 60°C), more specifically, the cycling stability of Li ion cells can be gradually enhanced as the amount of EC in the solvent mixture increases from 10% to 50%. However, we used only 10% EC in the electrolyte solvent mixture for all of the electrolytes in this work, thus the above high-temperature cycling performances directly prove that the combination of the multiple efficient additives in the baseline electrolytes plays a critical role in contributing to cell performance enhancement at elevated temperatures. Compared to cell cycling performances of E2 and E3 electrolytes, PS additive does slightly improve the cycling stability of the batteries at elevated temperatures, which is in accordance with literature reports.18


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Figure 3. Capacity retention of Gr||NCA coin cells in different electrolyte cycled at 1C rate at 60°C.

2.3. Mechanism Analysis of Enhanced Battery Performance Relying on the Multiple Additives 2.3.1. Morphological Characterization of CEI on NCA Cathode and SEI on Gr Anode The underlying mechanisms for the origin of enhanced battery performances relying on the multiple additives in the electrolytes in the wide temperature range have been explored. Usually, the CEI on the cathode surface and the SEI on the anode surface are gradually generated during initial few cycles (i.e. formation cycles) of Li ion cells. In this work, after two initial formation cycles (charge/discharge rate at C/20) of Li ion cells, there formed a layer of CEI with the thickness of ~7 nm coated on the NCA cathode surface in E1 baseline electrolyte (Figure S11a), whereas the thickness of the SEI layer on Gr anode surface is 2 nm (Figure S11d). Surprisingly, the thickness of the CEI layers formed on the cathode surfaces in the two electrolytes with multiple additives was dramatically decreased to about 2 nm (E2 in Figure S11b) and 1.5 nm (E3 in Figure S11c). This should be mainly attributed to the passivation by the oxidation products of TTMSPi additive. Considering the very high value of the highest occupied molecular orbital (HOMO) energy of


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TTMSPi additive (-6.24 eV, shown in Table S2 along with the calculated results for other additives, salts and solvents), TTMSPi itself could be firstly oxidized to quickly form a very stable and ultrathin inorganic passivation film with high ionic conductivity on the cathode surface during the formation cycles,[6] which directly suppressed continuous decomposition of electrolytes and other side reactions. This is why a uniform and ultrathin CEI layer can be observed on the cathode surfaces in E2 and E3 electrolytes containing TTMSPi. Without the help of TTMSPi, E1 baseline electrolyte induced relatively thicker CEI on the cathode because the lack of effective passivation film at the initial cycles of cells led to the excess decomposition of electrolytes. The thin SEI film (about 2 ~ 4 nm) on Gr anode in E1 electrolyte (Figure S11d) is well consistent with the previous observations by our research group due to the first reduction decomposition of Cs+-(EC)m solvates which have lower values of the lowest unoccupied molecular orbital (LUMO) energies to form ultrathin SEI layers on graphite anodes.3,10,11 The thickness of SEI on Gr anodes in E2 and E3 electrolytes was found to be about 14 nm (Figure S11e) and 13 nm (Figure S11f), respectively. Obviously, the SEI layers formed in E2 and E3 electrolytes are thicker than the one on the Gr anode surface in E1 electrolyte. This is due to decomposition of multiple additives in E2 and E3 occurred at the anode surface to build up a relatively thicker but robust and dense SEI layer. The corresponding compositions of the SEI layers will be discussed in the following section about X-ray photoelectron spectroscopy (XPS) analysis. Although E2 and E3 electrolytes induced a bit thicker SEI layer on Gr anodes than E1, they generated thinner CEI on cathode surfaces, and overall the cells with E2 and E3 possessed higher ionic conductive capability for Li-ion transport through the electrode/electrolyte interfaces, which have been demonstrated by the much lower cell resistance and change (Figure 2d).


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Additionally, the CEI and SEI films on both electrodes recovered from cells after 100 cycles at a current density of 1C and high temperature (60 C) have been characterized as well. As shown in Figure 4, although the thickness of CEI on NCA cathodes and the thickness of SEI on graphite anodes with different electrolytes have increased after 100 cycles at such a more rigorous testing temperature (60 C), the CEI and the SEI layers with E2 and E3 are still relatively less changed when compared to significantly thicker CEI and SEI layers with E1 electrolyte. These observations are consistent with their electrochemical cycling performances shown in Figure 3. These results reveal that the mixed additives in E2 and E3, especially E3 preferably guide the formation of high conductive, dense and uniform passivation films on the interphases of cathode/electrolyte and anode/electrolyte during the formation cycles and the follow-on cycling.

Figure 4. TEM images of NCA cathodes and graphite anodes in different electrolytes recovered from cells after 100 cycles at 60 C.


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2.3.2. Linear Sweep Voltammetry and Cyclic Voltammetry of Cells with Different Electrolytes In addition, linear sweep voltammetry (LSV) results of the Li|Li|Pt three-electrode cells with different electrolytes at a scan rate of 0.5 mV s-1 are provided in Figure 5. The oxidation voltage (~5 V) of E1 baseline electrolyte (Figure 5a) is very similar to those of FEC-added E1 electrolyte (Figure 5d) and PS-added E1 electrolyte (Figure 5f), while the LSV curves of the three-electrode cells based on both E2 (Figure 5b) and E3 (Figure 5c) containing TTMSPi additive are almost same as that of the cell with E1 containing only TTMSPi additive (Figure 5e). It means that the oxidation potential of TTMSPi is lower than that of E1 baseline electrolyte. The oxidation potentials are well consistent with the calculated values of the HOMO energies of salts, solvent, additives, as shown in Table S2. TTMSPi is more easily oxidized among the studied additives. The cyclic voltammetry (CV) measurements of Li||Gr cells containing various electrolytes for three cycles at a scan rate of 0.2 mV s-1 were conducted as well and the results are shown in Figure 6. Clearly, there are two remarkable peaks in anodic scan and cathodic scan, corresponded to lithiation and de-lithiation processes of the Gr anode, respectively, but the poor reversibility of lithiation/de-lithiation for Gr anode with E1 baseline electrolyte was observed due to the lack of robust and thin SEI that can mitigate the intercalation of solvent molecules into the Gr layered structure (Figure 6a). Moreover, much better reversibility of lithiation/de-lithiation for Gr anodes in E2 (Figure 6b) and E3 (Figure 6c) should be attributed to the formation of thin and uniform protective SEI layers on Gr surface, as shown in Figures 4e and 4f. Interestingly, single additive component (i.e., FEC in Figure 6d, TTMSPi in Figure 6e, and PS in Figure 6f, each having amount of 0.5 wt %) added into E1 baseline electrolyte is not able to completely address poor lithiation/delithiation reversibility issue of Gr anodes, although the addition of TTMSPi indeed enhances


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electrochemical reaction activity (Figure 6e). Also, the computational calculation data demonstrate that CsPF6 has a LUMO energy value lower than LiPF6, which means that the reduction of CsPF6 occurs easier, compared to LiPF6. As reported in our previous work,19 the decomposition of CsPF6 in the LiPF6/PC electrolyte occurred at about 2.0 V, which could not be shown up in this test since the CV scans started at 2.0 V. There are reduction peaks with different intensities in various electrolytes located at about 0.7 V during the first cathodic scan, which are corresponding to the decomposition of EC and PC molecules.10 The critical synergetic benefits induced by the combination of multiple additives in the electrolytes (e.g., E2 and E3) should be regarded as a dominant factor for the formation of robust and ultrathin SEI on Gr anodes.

Figure 5. LSV curves of Li|Li|Pt three-electrode cells with different electrolytes at a scan rate of 0.5 mV s-1 between 3.0 V and 5.5 V. (a) E1, (b) E2, (c) E3, (d) E1 + 0.5% FEC, (e) E1 + 0.5% TTMSPi, and (f) E1 + 0.5% PS.


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Figure 6. CV curves of Li||Gr cells containing various electrolytes at a scan rate of 0.2 mV s-1 between 0.02 V and 2.0 V. (a) E1, (b) E2, (c) E3, (d) E1 + 0.5% FEC, (e) E1 + 0.5% TTMSPi, and (f) E1 + 0.5% PS.

2.3.3. Composition Analysis of CEI and SEI Produced in Different Electrolytes The compositions of CEI and SEI layers formed in various electrolytes were characterized by XPS. Figure 7 shows the XPS quantification analysis on the thin film compositions on cathode and anode surfaces after two initial formation cycles. The CEI on the NCA cathode in E2 electrolyte contains some Si-based species, whereas the CEI formed in E3 electrolyte contains Si- and S-based compounds, as shown in Figure 7b. This is attributed to the decomposition of additives TTMSPi and PS during the formation cycles. In addition, Cs+ species should be considered as one of key components in CEI layers. According to the calculation results in Table S2, the HOMO energy values of three additives of FEC, TTMSPi, and PS are -8.44 eV, -6.24 eV, and -7.97 eV, respectively, thus both TTMSPi and PS are more easily oxidized when compared to FEC.


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However, compared to the lower carbon (C) amount in the CEI film from the E1 baseline electrolyte, the increase in the C amount in the CEI film from the E1 + FEC electrolyte is indicative of participation of FEC in the CEI formation on the cathode (Figure 7a). For the element analysis for SEI films on the Gr anode surfaces, the reductive decomposition of TTMSPi and PS additives on Gr can also be confirmed in Figure 7d. Rapidly increased C content with E1 containing only FEC additive illustrates the formation of side-reaction products (i.e., carbonates) induced by the reductive decomposition of FEC (Figure 7c).20–22 Although the LUMO energy value of CsPF6 is as low as -1.96 eV which is even lower than that of LiPF6 salt, there is no obvious signal of Cs as component of SEI on anodes with the following electrolytes: E1, E2, E3, E1 + FEC, and E1 + PS (Figure 7d). This is consistent with our previous report about using CsPF6 as additive to suppress Li dendrite growth.23 This probably originates from the dissolution of Cs-based species into the electrolytes after their formation in SEI layers, or their contents in the SEI layers are lower than the detection limit of the XPS instrument. Interestingly, a signal of Cs was found on the Gr anode surface with E1 + TTMSPi electrolyte (Figure 7d), but the reason is unclear. From Figures 7b and 7d, it can be seen that there are much more Si-based species on cathode surface to form CEI due to the decomposition of TTMSPi, while there are more S-based species from the decomposition of PS contributing to the formation of SEI on the anode surface. These results certify that the compositions of SEI contain some decomposition products from multiple additives in E2 and E3 electrolytes, which can be well explained by the differences in thickness and morphology of CEI and SEI in the TEM images shown in Figure 4.


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Figure 7. XPS quantification analysis on CEI compositions on NCA cathode surfaces (a, b) and SEI compositions on Gr anode surfaces (c, d) after two initial formation cycles.


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Figure 8. XPS P 2p spectra of NCA cathode surfaces (a-f) and Gr anode surfaces (g-l) with different electrolytes after two initial formation cycles. (a, g) E1, (b, h) E2, (c, i) E3, (d, j) E1 + 0.5% FEC, (e, k) E1 + 0.5% TTMSPi, and (f, l) E1 + 0.5% PS.

The narrow scan XPS P 2p spectra with the above different electrolytes are given in Figure 8. The salt decomposition by-products (LixPFy and LixPOyFz) are clearly detectable in all CEI layers on the cathode surfaces in all studied electrolytes.24,25 LixPFy is far more than LixPOyFz on the cathode surface in E1 baseline electrolyte (Figure 8a). Similarly, LixPFy is more than LixPOyFz in the CEI layers with E1 + FEC electrolyte (Figure 8d) and E1 + PS electrolyte (Figure 8f). Instead, the significantly enhanced intensity of LixPOyFz peaks is much stronger than that of LixPFy in the CEI layers with E2, E3, and E1 + TTMSPi electrolytes. As previously reported,26


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LixPOyFz has been considered as one of the effective SEI components to improve surface stability, thus the higher content of LixPOyFz in more robust and thin films on cathode surfaces with E2 electrolyte (Figure 4b) and E3 electrolyte (Figure 4c) leads to greatly improved battery performances. Importantly, some P-O-Si peaks are attributed to the oxidation of TTMSPi additive in those TTMSPi-containing electrolytes (i.e., E2 in Figure 8b, E3 in Figure 8c, and E1 + TTMSPi in Figure 8e) because of its very high HOMO value of –6.24 eV (Table S2), which has been also proven by XPS quantification analysis (Figure 7b). Similarly, its reductive decomposition also occurred on the Gr anode side to produce SEI layer in the above TTMSPi-based electrolytes, as shown in Figures 8h, 8i, and 8k, respectively. Some possible pathways and mechanisms of TTMSPi upon oxidation on the cathode and reduction on the anode were discussed by other researchers.26–28 Moreover, the additional merit for use of TTMSPi is to consume hydrogen fluoride (HF) produced in PF6-based electrolytes during cell cycling.29 Besides reduced TTMSPi species existed in the SEI on Gr anode, LixPOyFz should be one of key dominant components in SEI layers with both E2 electrolyte (Figure 8h) and E3 electrolyte (Figure 8i), however, the SEI on the anode surface with E1 baseline electrolyte contains much more P-F anions from decomposition of LiPF6 and/or CsPF6 (Figure 8g). It demonstrates that the use of multiple additives facilitates the formation of LixPOyFz with improved surface stability for SEI with high ionic conductivity,26 which is well consistent with the above cell resistance change based on E2 and E3 electrolytes during long-term cycles (Figure 2d).


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Figure 9. XPS F 1s spectra of NCA cathode surfaces (a-f) and Gr anode surfaces (g-l) with different electrolytes after two initial formation cycles. (a, g) E1, (b, h) E2, (c, i) E3, (d, j) E1 + 0.5% FEC, (e, k) E1 + 0.5% TTMSPi, and (f, l) E1 + 0.5% PS.

Accordingly, XPS F 1s spectra of NCA cathode and Gr anode surfaces with the above electrolytes after two initial formation cycles are shown in Figure 9. In E1 electrolyte, after formation cycles of Li ion cells, the CEI on cathode surface and the SEI on anode surface include much more by-products (LixPFy and especially LiF) from Li salt decomposition, compared to those components in E2 electrolyte (Figures 9b and 9h) and E3 electrolyte (Figures 9c and 9i). In contrast, relative to less decomposed compounds, obviously stronger peaks of PVDF on cathode surface with E2 and E3 electrolytes are indicative of ultrathin CEI on cathode surface (Figures 9b


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and 9c). In addition to the above narrow scan XPS characterizations, the wide scan XPS spectra of cycled cathodes and anodes with different electrolytes, and non-cycled pristine electrodes shown in Figures S12-S25, clearly reveal that the formation of CEI on cathodes and SEI anodes is induced by multiple additives (Figures S12-S23), when compared to limited peaks (C, F, O) on the pristine electrode surfaces in Figures S24 and S25.

3. Conclusion The above results demonstrate that the optimized combination of multiple functional additives, CsPF6, FEC, TTMSPi and PS, in the baseline electrolyte E1 to make E2 and E3 preferably favor building highly conductive, uniform and robust CEI and SEI on cathode and anode surfaces, respectively after initial formation cycles. These additives indeed participated in the formation of the CEI on cathode and the SEI on Gr anode surface. In the case of E2 and E3, the CEI layers on cathodes contain P-O-Si, F-enriched, or S-based (only in E3), and Cs-containing species, as well as less LixPFy and LiF, and much LixPOyFz as one of effective components to improve surface stability and ionic conductivity. On the anode side, the SEI layers have most compounds similar to those in the CEI layers, such as P-O-Si, F-enriched, and S-based (only in E3), as well as numerous LixPOyFz along with less LixPFy and LiF, but there is nearly no Cs-containing species. According to their calculated molecular energy values, the four additives studied in this work have the oxidation potential order from low to high as TTMSPi < PS < FEC < CsPF6, while the reduction potential order from high to low follows the order of CsPF6 > FEC > PS > TTMSPi. Relying on the aforementioned synergetic effects of multiple additives, the E2 and E3 electrolytes generate highly conductive and dense electrode/electrolyte interphase films on both electrodes which in turn enable significantly improved cycling reversibility of Gr anode and NCA cathode, and wide-


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temperature application range of LIBs. This work represents some inspiring progresses in the development of electrolytes and additives for wide-temperature application range of LIBs and the considerable understandings and findings for guiding future development of rechargeable battery electrolytes.

4. Experimental Section Materials. LiNi0.80Co0.15Al0.05O2 (NCA) cathode (1.5 mAh cm-2) and graphite (Gr) anode (1.53 mAh cm-2) obtained from Argonne National Laboratory (ANL) were used in coin cells. LiNi1/3Mn1/3Co1/3O2 (NMC) cathode sheets (1.5 mAh cm-2) and Gr anode sheets (1.65 mAh cm-2) were made in the Advanced Battery Facility (ABF) at Pacific Northwest National Laboratory (PNNL) and used in pouch cells. Celgar 2500 (polypropylene) was used as separator in coin cells and pouch cells. LiPF6, EC, PC, EMC, VC, and FEC of battery grade were ordered from BASF Battery Materials and were used as received. CsPF6 (≥99.0%) was purchased from SynQuest Laboratories (Alachua, FL) and dried at 60 C for 24 h under vacuum prior to use. PS (98%), TTMSPi (95%) and TMSDEA (≥98%) were ordered from Sigma-Aldrich and dried with molecular sieves for 1 week before use. The electrolytes were prepared inside an argon-filled glovebox (MBraun), where both O2 and H2O levels were below 1 ppm. Electrochemical Measurements. The electrochemical performances of Gr||NCA 2032-type coin cells filled with 100 all of various electrolytes were evaluated on Land battery testers, where 1C rate corresponded to a current density of 1.5 mA cm−2. 1 Ah Gr∥NMC pouch cells with stacked electrodes were made at the ABF of PNNL and evaluated on an Arbin BT2000 battery tester, in which 1C corresponded to a current of 1.0 A. All cells were tested under galvanostatic charge−discharge cycles at different temperatures inside TestEquity temperature chambers or


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Tenney JR environmental chambers as required. The above coin cells and pouch cells were first conducted two formation cycles at a current density of C/20 at room temperature, and then followed by selected testing protocols. The cutoff voltages of the Gr||NCA coin cells were set at 2.5 V for discharge process and 4.3 V for charge process, while those of the Gr||NMC pouch cells were in the range of 3.0 V and 4.2 V. In case of low-temperature discharge tests, the coin cells and pouch cells were galvanostatically charged to the cutoff voltage at C/5 and then held potentiostatically at that voltage to C/10 at room temperature, kept in the temperature chamber at the specified testing temperature for 6 h to reach thermal equilibrium, and then discharged at C/5 at the selected temperature. For room temperature cycling tests of Gr∥NMC pouch cells, the cells were cycled at 1C/1C for charge/discharge with a voltage window of 3.0 V and 4.2 V at 25 °C. After certain cycles, direct current resistance (DCR) tests were conducted for the pouch cells at 25 °C. Briefly, the pouch cells were first charged to 4.2 V at C/2 and maintained at 4.2 V until the current decreased to C/15. After that, the cells were discharged to 3.7 V at C/2 rate followed by 15 min rest, and then fast discharged at 3C rate for 10 s. The DCR value was calculated by dividing the voltage change value during the above 10 s process by the constant current of 3C. For high temperature cycling evaluation of Gr||NCA coin cells, the cells were cycled at 1C/1C for charge/discharge with a voltage window of 2.5 V and 4.3 V at 60 °C. LSV tests of threeelectrode cells composed of Pt as working electrode, and Li as counter electrode and reference electrode were conducted at a scan rate of 0.5 mV s-1 between 3.0 V and 5.5 V in 3 mL of various electrolytes (E1, E2, E3, E1 + 0.5% FEC, E1 + 0.5% TTMSPi, and E1 + 0.5% PS). Besides, CV curves of Li||Gr coin cells were tested at a scan rate of 0.2 mV s-1 with 100 µL of different electrolytes (E1, E2, E3, E1 + 0.5% FEC, E1 + 0.5% TTMSPi, and E1 + 0.5% PS). The Gr and


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NCA electrodes after two formation cycles were recovered from the Gr∥NCA full cells, soaked in anhydrous EMC solvent for 30 min, rinsed with fresh EMC three times, and dried under vacuum inside the antechamber of the glovebox. Characterizations. TEM was carried out to observe the SEI layers of Gr anodes and the CEI layers on NCA cathodes on an FEI Tian 80−300 microscope at 300 kV. XPS measurements of the precharged electrodes were performed with a Physical Electronics Quantera scanning X-ray microprobe with a focused monochromatic Al Ka X-ray (1486.7 eV) source for excitation and a spherical section analyzer. The samples were sealed on standard sample holders inside a glove box filled with argon gas prior to characterization. Computational Calculations. The molecular orbital energies of the selected salts, solvents and additives were calculated by density functional theory using the generalized gradient approximation (GGA) as implemented in the Gaussian 09 suite of programs.[30] The B3LYP functional combined with the 6–311++G (d, p) basis set was used in geometry optimization calculations.[31,32]

ASSOCIATED CONTENT Supporting Information. Major functions of electrolyte additives; more details of electrolyte formulations; molecular structures of fluoroethylene carbonate (FEC), tris(trimethylsily) phosphite (TTMSPi), 1,3-propane sultone (PS), N,N-diethyltrimethylsilylamine (TMSDEA), and vinylene carbonate (VC) additives; voltage profiles of coin cells of NCA cathodes and Gr anodes with different electrolyte systems at 25°C; voltage profiles of 1 Ah Gr||NMC pouch cells at 1C discharge rate at 25 °C; photo of thickness measurement of cycled pouch cell by using an electronic


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micrometer; optical images of cycled pouch cells with different electrolytes; HOMO and LUMO values of salts, solvents, and additives; the wide-scan XPS spectrum of NCA cathodes and Gr anodes with different electrolytes after two initial formation cycles.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Technology Transfer Office of the U.S. Department of Energy (DOE) through the Technology Commercialization Fund under the contract number DE-AC06-76LO1830. The microscopic and spectroscopic characterizations were conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the DOE under the Contract DE-AC05-76RLO1830.

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