Li-Ion Intercalation Kinetics at LiMn2O4 Thin Film Electrode/Localized High-Concentration Electrolyte Interface

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The charge transfer resistances (R ct) of Li+ intercalation/deintercalation at the electrodes in lithium-ion batteries (LIBs) are affected by several factors. Decreasing R ct is essential to improve the power density of LIBs. Desolvation of Li+ is the rate-determining step of the interfacial charge transfer reaction in electrolytes containing 1 mol dm− 3 of Li salts.1 Furthermore, decreasing viscosity (η) of electrolytes decreases R ct.2 The high-concentration electrolytes (HCEs) exhibit high η and the different liquid structure from that of 1 mol dm− 3. In this work, we investigated the charge transfer kinetics at the intercalation electrode/HCE interface. Here, we use lithium bis(fluorosulfonyl)amide (LiFSA)/monoglyme (G1)/1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) ternary mixtures as electrolytes. The molar ratio of LiFSA and G1 in the ternary electrolytes was set to be 1:2 and the amount of HFE was altered. Adding HFE into HCE of LiFSA/G1 = 1:2 can decrease η while HFE is not involved in the coordination of Li+ due to its weekly coordinating ability.3 Such electrolytes are called the localized high-concentration electrolytes (LHCEs). We evaluated R ct at LiMn2O4 thin-film electrode (LMO)/LHCE interface to reveal the effects of liquid structure and η on the charge transfer kinetics. If the Sumi-Marcus theory is valid for Li+ intercalation reaction, R ct is proportional to dielectric longitudinal relaxation time of solvent (τ L).4 τ L is roughly proportional to η, therefore, R ct is assumed to be proportional to η. However, the salt concentration dependence of R ct in the LHCE showed the minimum value at around 2.5 mol dm− 3 while η continuously decreases with increasing the salt concentration. Therefore, the electrode/LHCE interfacial charge transfer kinetics is not solely dominated by η. To estimate the electrochemically reactive area of the LHCE/electrode interface, we evaluated electric double layer capacitance (C dl) by electrochemical impedance spectroscopy. C dl decreased gradually at salt concentration lower than 2.5 mol dm− 3, indicating that the electrochemically reactive area decreases at lower than 2.5 mol dm− 3. As aforementioned, HFE is not involved in the coordination of Li+, and the liquid structure of LHCE consists of high-concentration electrolyte domain and HFE domain.5 This domain structure is assumed to be dynamic, however, the volume fraction of HFE domain becomes larger in LHCE with increasing the HFE (decreasing the salt concentration). In summary, η of the electrolyte decreases with increasing HFE concentration, which enhance the interfacial kinetics, however, the electrochemically reactive area also decreases with high volume fraction of HFE, resulting in the minimum value of R ct at 2.5 mol dm− 3. Acknowledgements This study was partially supported by JSPS KAKENHI (Grant Numbers 22H00340 and 23K17370) from the Japan Society for the Promotion of Science (JSPS). References (1) T. Abe et al., J. Electrochem. Soc. 151, A1120 (2004).(2) Y. Uchimoto et al., Solid State Ionics, 176, 2377 (2005).(3) K. Dokko et al., J. Electrochem. Soc. 160, A1304 (2013).(4) H. Sumi and R. A. Marcus, J. Chem. Phys., 84, 4894–4914 (1986).(5) S. Lin et al., ACS Appl. Mater. Interfaces., 12, 33710–33718 (2020).