Stable interfaces in lithium metal batteries constructed via in-situ electrolyte reformulation

Abstract

Li2CO3 was introduced into LiPF6-based electrolytes and the electrolytes were stored at 40 °C. Nuclear magnetic analysis of electrolytes and X-ray diffraction characterization of reaction residues demonstrate the formation of LiPO2F2 and LiF during storage. This reformulated electrolyte boosts lifespan and Coulombic efficiency (CE) in Li||Li and Li||Cu cells, with Li||Li cells stably cycling for >800 h and 300 h at 0.5 mA/cm2 and 1.0 mA/cm2, respectively. Moreover, with the optimal content of Li2CO3, the CE of the reformulated electrolyte (91.56%) is greatly improved compared to that of the standard electrolyte (81.99%). The compatibility and enhanced rate performance of the reformulated electrolyte are also exhibited in Li||NCM full cells with a moderately high mass loading of 9.6 mg/cm2.

1. Introduction

Lithium-ion batteries (LIBs) have been employed globally to power the majority of portable electronic devices since their initial commercialization in 1991 [1]. However, the further applications in emerging technologies such as electric vehicles and the smart grid require energy storage systems with higher energy densities [2,3]. From this perspective, lithium metal, which has an ultra-high capacity (3860 mA∙h/g) and a low redox potential (−3.04 V versus the standard hydrogen electrode), has been regarded as a desired anode material for the next generation of energy storage systems [4−7]. But the commercial applications of Li metal batteries (LMBs) have been significantly impeded by the low Coulombic efficiency (CE) observed during repeated Li plating and stripping processes, as well as the formation of Li dendrites, which are an unfavourable growth phenomenon. As lithium metal is a hostless anode that necessitates deposition and stripping reactions for the storage and release of lithium, the interface between the lithium anode and electrolyte undergoes dynamic alteration throughout the charge−discharge process. Moreover, the instability of the interface leads to the continuous occurrence of parasitic side reactions, which affect the lithium deposition morphology and the electrochemical properties, including the CE, resistance, and cycling stability. As a result, these effects have been observed in numerous studies [8−11].

It is widely acknowledged that interfacial control represents a crucial aspect in the enhancement of electrochemical performance [7,10]. Considerable research has been conducted to elucidate the underlying mechanisms associated with dendrite formation by controlling the interface [7,12−14]. One of the attractive methods to form a stable solid electrolyte interphase (SEI) layer is to incorporate a small amount of additives into the electrolytes [15−19]. A plethora of electrolyte additives have been employed to enhance the electrolyte−electrode interface in LMBs, including FEC [20], VC, LiNO3 [21,22], RbF [23], CsPF6 [24], MgCl2 [25], borate lithium salts [26−29] and C60 [30]. The modified SEI layers are demonstrated to be dense and uniform, thus enabling the effective suppression of Li dendrites and the attainment of high Li CE. Among the various types of additives investigated, additives that generate LiF-rich SEI films are favored due to their beneficial properties such as the good electronic insulation and low barriers for lithium-ion diffusion, which result in improvements in the cycling performance of Li metal anodes, as evidenced by studies [31−35]. For example, LiPO2F2 has recently been introduced into the electrolyte with the intention of suppressing the dendritic growth of lithium [36−38]. However, the beneficial role of the additive in lithium metal battery has been mainly studied