1. Introduction
Acetonitrile (AcN) is a promising organic solvent for lithium-ion battery (LIB) electrolytes from the viewpoint of fast-charging and extremely low temperature performance due to its low viscosity and high relative dielectric constant. Asahi Kasei succeeded in developing an AcN-containing electrolyte for LIBs by deriving the appropriate combination of additives that suppress the reduction of AcN as shown in Fig.1.1) Nevertheless, the potential formation of cyanide as a result of AcN decomposition remains a significant concern.2) In this study, we analyzed the solid electrolyte interphase (SEI) of a 1-Ah pouch type LIB cell with an AcN-containing electrolyte by TOF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) to consider SEI formation mechanism in AcN-containing electrolyte.

2. Experiment
1-Ah pouch type LIB cells were assembled in a dry-air atmosphere. Lithium iron phosphate (LFP) was used as a cathode active material, and graphite as an anode active material. Mixture of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) was used as lithium salt, and electrolytes with and without AcN were used for the electrolyte solution respectively. After the formation of the LIB cells, C-rate performance at 25 °C, fast-charge performance at 25 °C, charge / discharge cycles in a rangeof 2.0 V and 3.6 V at 50°C, and a 60°C storage test at 3.6 V were performed. In addition to the LIB cells used for the above electrochemical evaluations, other LIB cells for analysis of SEI were also assembled. These LIB cells were disassembled in an argon-filled glove box and the negative electrodes were washed with diethyl carbonate (DEC). Then they were dried and analyzed by TOF-SIMS in an inert atmosphere.

3. Results and Discussion
The changes in discharge capacitywith different electrolytesat different rates in the range of 2.0 –3.6 V are compared in Fig.2. The rate capability is enhanced with AcN-containing electrolyte, suggesting that C-rate performance highly correlates with their ionic conductivity.
The capacity retention of LIB cells using different electrolytes for more than 1,000 cycles of charge / discharge at 50°C is shown in Fig.3. The AcN-containing electrolyte shows good capacity retention, suggesting that high durability with AcN-containing electrolyte can be achieved by forming an appropriate SEI on the anode surface although AcN is easily reduced electrochemically.
In comparison between AcN-containing electrolyte and conventional electrolyte, the analysis results of the negative electrode by TOF-SIMS suggested the absence of cyanide, which could have been generated by the decomposition of AcN and included in the SEI.
In this presentation, further analysis data will be provided. This deeper exploration will dispel the concern about the safety of LIBs using AcN-containing electrolyte.

References
1) N. Matsuoka, H. Kamine, Y. Natsume, A. Yoshino, ChemElectroChem, 2021, 8, 3095–3104.
2) S. Pons, S. B. Khoo. Electrochim. Acta, 1982, 27, 1161–1169.