Correlation Between Solvation Structure and Ion-Conductive Properties of Highly-Concentrated Poly(ethylene carbonate)-Based Electrolytes

Abstract

Ion-conductive solid polymer electrolytes (SPEs) are of great interest as key materials to realizing safe and flexible rechargeable batteries, particularly Li batteries. Since the discovery of ionic conductions in poly(ethylene oxide) (PEO)/metal salt complexes, numerous efforts have been made to improve their electrochemical properties. However, it is difficult to increase Li-ion conductivity that is the product of overall conductivity and Li transference number (t +). This drawback mainly arises from excessively tight complex structures between polar polymers and Li ion. To overcome the drawback and create all-solid-state batteries based on SPEs, we specifically focus on poly(alkylene carbonate)s derived from CO2/epoxide copolymerization as SPE polymer hosts [1,2]. Consequently, we have found an unusual ion-conductive behavior of poly(ethylene carbonate) (PEC)-based electrolyte, including high ionic conductivity of highly-concentrated electrolytes and extremely high t +, which in some cases is above 0.8 [3,4]. In this PEC-based electrolyte system, ionic conductivity continues to increase with increasing salt concentration, which is a behavior scarcely shown in conventional electrolyte systems. In addition, we have already confirmed operations of prototype Li/LiFePO4 cells based on PEC-based electrolytes combined with plasticizing and reinforcing additives [5,6]. In this context, we considered that elucidation of salt solvation structure is a key to better understanding the origin of the particular ion-conductive behavior, and improving the performance further. In the present study, we adopted spectroscopic studies including FT-IR, Raman spectroscopy, and solid-state 7Li magic angle spinning (MAS) NMR for PEC/lithium bis(fluorosulfonyl)imide (LiFSI) electrolytes with varying concentration. The FT-IR and Raman spectroscopy indicate that most FSI ions interact with more than one Li ion at high concentration region, which is referred to as aggregates, while C=O groups dissociates the salt. The 7Li MAS NMR suggested that a relatively weak contact between Li ions and FSI ions withdraws Li ions from a strong interaction with C=O groups. We conclude that this moderate complex structure, where C=O groups allows the highly concentrated ions to remain amorphous and mobile, leads to reasonable conductivity with extremely high t +. We believe that this finding will provide a new insight for developing SPEs having both high conductivity and high t + simultaneously.