Tailoring electrolyte salt concentration to achieve reversible Li storage in pure-Ni layered oxide

Abstract

Abstract Modulating electrolyte formulation is an efficient strategy to mitigate side reactions at the electrode/electrolyte interface and thereby enhance the cycling stability of lithium batteries. Herein, the effects of electrolyte salt concentration on the solution structure and interfacial reactivity toward a pure-Ni layered oxide positive electrode are systematically studied using LiN(SO 2 F) 2 (LiFSA)/1,2-dimethoxyethane (DME) model electrolytes. Raman spectroscopy and X-ray total scattering analyses reveal a pronounced structural evolution of the electrolyte solution at high salt concentrations, wherein free DME molecules are nearly absent, and extensive anion coordination to Li⁺ ions becomes prominent beyond 3.5 M. In this regime, coordination by ether oxygen atoms of DME alone becomes insufficient to satisfy the preferred Li⁺ coordination environment, triggering anion participation and the formation of contact ion pairs and ionic aggregates. Electrochemical measurements demonstrate that increasing salt concentration markedly suppresses capacity fading of the pure-Ni layered oxide electrode in the 4.2 V plateau region, with degradation becoming negligible at salt concentrations above 3.5 M. Online electrochemical mass spectrometry further confirms that gas evolution arising from parasitic reactions between the charged pure-Ni layered oxide electrode and ether solvents is effectively suppressed in highly concentrated electrolytes. While the intrinsically high viscosity of concentrated electrolytes remains a challenge for achieving practical rate performance, the present findings elucidate the critical role of electrolyte solution structure in governing electrolyte–electrode reactivity and provide important design principles for electrolytes compatible with pure-Ni layered oxide positive electrodes in advanced lithium batteries.