Manipulating Interfacial Stability via Preferential Absorption for Highly Stable and Safe 4.6 V LiCoO2 Cathode

Abstract

Elevating the upper cutoff voltage to 4.6 V could effectively increase the reversible capacity of LiCoO2 (LCO) cathode, whereas the irreversible structural transition, unstable electrode/electrolyte interface and potentially induced safety hazards severely hinder its industrial application. Building a robust cathode/electrolyte interface film by electrolyte engineering is one of the efficient approaches to boost the performance of high-voltage LCO (HV-LCO); however, the elusive interfacial chemistry poses substantial challenges to the rational design of highly compatible electrolytes. Herein, we propose a novel electrolyte design strategy and screen proper solvents based on two factors: highest occupied molecular orbital energy level and LCO absorption energy. Tris (2, 2, 2-trifluoroethyl) phosphate is determined as the optimal solvent, whose low defluorination energy barrier significantly promotes the construction of LiF-rich cathode/electrolyte interface layer on the surface of LCO, thereby eventually suppresses the phase transition and enhances Li+ diffusion kinetics. The rationally designed electrolyte endows graphite||HV-LCO pouch cells with long cycle life (85.3% capacity retention after 700 cycles), wide-temperature adaptability (−60–80 °C) and high safety (pass nail penetration). This work provides new insights into the electrolyte screening and rational design to constructing stable interface for high-energy lithium-ion batteries.

1. Introduction

Lithium cobalt oxide (LiCoO2), as one of the most popular cathode materials, has overwhelmingly dominated the lithium-ion battery (LIB) markets of consumer electronics due to its high theoretical capacity, favorable operating voltage and volumetric energy density [1–4]. To cope with the energy density demands of the ever-expanding information and communication revolutions, researchers have attempted to elevate the energy density of LIBs by charging the LiCoO2 to a higher voltage above 4.5 V [5, 6]. However, increasing the charging cutoff voltage induces the detrimental phase transition of LiCoO2 from O3 to H1-3 phase [7, 8]. Worse still, traditional carbonate solvents (such as ethylene carbonate) suffer from oxidation and dehydrogenation at higher voltage triggered by the catalytic action of high oxidation state of Co4+ ions, forming an instable cathode/electrolyte interface (CEI) [9–11]. The dehydrogenation of the solvent generates highly corrosive HF, which accelerates the dissolution of Co and the collapse of LiCoO2 structure, and eventually leads to severe capacity decay [12, 13] (Fig. 1a). The above interface instability issue is more prominent when LiCoO2-based batteries are operated or stored at high temperature, which will bring more serious safety hazards.

Extensive efforts have been devoted to easing these issues of high-voltage LiCoO2 (HV-LCO). The most explored methods are surface coating [14–17] and elemental doping [18–23], which could improve the stability of Co–O bond, prevent the surface degradation and inhibit the irreversible phase transition, but the structural modification (especially coating) also increases the complexity of industrial production [17] and is unable to affect the intrinsic safety of the battery system. Compared to the modification of LCO electrode, electrolyte engineering is a more straightforward and efficient approach to improve the LCO/electrolyte interface property. Highly concentrated electrolytes and all-fluorinated solvents aid the formation of LiF-rich high oxidation stability CEI interfaces [24–26], but the high viscosity and high cost would heavily restrain their practical applications.