Direct Evaluation of the Electrode/Electrolyte Interface with Additives by Single-Particle Electrochemical Measurement

To achieve high-performance lithium-ion batteries (LIBs), controlling interfacial reactions at the electrode/electrolyte interface is intensely studied by introducing chemical additives into the electrolyte solution. These additives preferentially decompose over other electrolyte components, forming a stable interphase film at the electrode/electrolyte interface, which protects against capacity degradation and overcharging. However, the composite nature of conventional LIB electrodes makes it challenging to directly observe the electrochemical properties and formation process of the passivation film on the active material alone. To address this challenge, we used single-particle electrochemical measurement (SPEM), which uses an open-type measurement cell, enabling the direct observation of resistance component changes within a single particle during the in-situ introduction of additives. In this study, SPEM was applied to a LiCoO₂ single particle (LCO-SP) to evaluate changes in electrochemical and resistance properties with the in-situ introduction of an additive solution under a charged state. The electrolyte solution and additive used were 1.0 mol kg^-1 ethylene carbonate-LiN(SO₂F)₂, with LiPO₂F₂ as the additive avoiding concentration changes of LiN(SO₂F)₂. In the additive-free system, SPEM and AC impedance measurements revealed a single asymmetric semicircular arc, indicating resistance components related to the internal LCO SP, charge transfer, and the interphase layer at the electrode/electrolyte interface. In the additive-containing system (1.0 wt %), the semicircular arc from AC impedance measurements exhibited a decrease in time constant and slight noise, suggesting changes in the charge transfer process. Upon in-situ introduction of the additive under a charged state, the impedance spectra exhibited two semicircular arcs and an increasing trend in the resistance of their lower frequency component, while maintaining potential, attributed to the growth of the interphase layer at the LCO SP/electrolyte interface. Therefore, SPEM enables direct and precise observation of resistance behavior at the electrode/electrolyte interface on a single particle scale during additive introduction.