(Invited) Cation-Disordered Li3VO4-Based Materials As a Pseudocapacitive Negative Electrode

Abstract

The growing demand for fast charge-discharge electrical energy storage (EES) devices with long cycles lifetimes has led to the need for alternatives to current battery systems, which store energy via slow faradic reactions. Among different EES technologies, electric double layer capacitors (EDLCs) are considered as promising devices due to their high-power, safe and long-lived characteristics. One of the approaches to further enhance the cell voltage and energy density of EDLCs while maintaining their high power is to replace the activated carbon with ultrafast lithium ion battery materials as hybrid supercapacitors.The orthorhombic β-Li3VO4 (β-LVO) has been identified as a promising negative electrode material for hybrid supercapacitors, with theoretical specific capacity of 394 mAh g-1 (2 lithium accommodation)[1]. The electron-donor effect of Li, exactly opposite to the inductive effect of polyanions, lowers the redox potentials of V5+/V4+ and V4+/V3+ to a safe but still low potential range between 1.3 and 0.4 V vs. Li/Li+, compared to the 3.0 V usually observed for V2O5. Recent our studies [1-2] unveiled that both the structure and electrochemical signature change of the β-LVO during the initial cycling, resulting in what we termed as electrochemical activation process. This electrochemical activation process induces the transformation of β-LVO from the pristine cation-ordered structure into an activated LVO, which has a Li+/V5+ cation-disordered structure. Charge discharge curves also change from “battery-like” plateau to “pseudocapacitive” slope via such cation-disordering of LVO, along with its reaction mechanism change from two-phase to solid-solution reaction[2]. Li+ diffusion coefficient (DLi ) evaluation by galvanostatic intermittent titration technique (GITT) reveals that the DLi of cation-disordered LVO is two orders of magnitude higher than the one of pristine β-LVO. Yet, the preparation method of cation-disordered LVO has been a major issue as the electrochemical activation process of β-LVO is impractical from the industrial point of view. Such electrochemical process requires a precycling of β-LVO for 10-20 cycles down to low potential below 0.4 V vs. Li/Li+, which is difficult to control and inevitably produces undesirable irreversible capacity for a full cell assembling.In this talk, we introduce an alternative to the electrochemical process, i.e., the direct synthesis of fully cation-disordered LVO via simple mechanical milling of the pristine β-LVO powder (Fig.1). The mechanochemical process brings about the disordering of LVO cation sublattice and thus enables to obtain thermodynamically metastable cation-disordered LVO phase without any precycling. Comparison of X-ray and neutron powder diffraction patterns confirmed the successful direct synthesis of cation-disordered LVO. The mechanochemically cation-disordered LVO shows superior rate performances to those of β-LVO and even electrochemically cation-disordered LVO. Another strategy to synthesize pseudocapacitive LVO-based material is a partial substitution of V5+ with Si4+ in β-LVO, which results in a transformation into γ-Li3+xV1-xSixO4. The γ-Li3+xV1-xSixO4 , with a partially V5+/Si4+ cation-disordered structure shows similar psuedocapacitive behavior as a fully cation-disordered LVO with enhanced DLi and rate capability compared to the β-LVO, thanks to the randomly distribution of V5+. In the presentation, we introduce those two approaches, “fully” or “partially” cation-disordering, as new paths for transformation of battery into pseudocapacitive materials with improved electrochemical performances by simple cation mixing.