Investigations on the improved cycling stability of Kazakhstanite phase Fe-V-O layered oxide by using superconcentrated electrolytes: Generalized solubility limit approach (Part I)

Abstract

In this article, we address the issue of vanadium dissolution pertinent in the layered Fe5V15O39(OH)9.9H2O using the solubility limit approach. This layered oxide is prepared via a low-cost solution phase synthesis route and crystallizes in the Kazakhstanite phase (Space Group: C2/m), confirmed using selected area electron diffraction and x-ray diffraction.The layered oxide exhibits the 2 electron redox reaction of vanadium (V5+ to V3+) along with the 1 electron redox reaction of iron within the voltage window of 1.5-3.8V. This results in a high specific capacity of ~350mAhg-1 which can be extracted from this material. However, the transition from V4+ to V3+ is identified to initiate a dissolution process at ~2.5V, resulting in a loss of active material and poor cycling stability. The vanadium dissolution is found to be arrested by switching to a superconcentrated electrolyte, wherein the amount of 'free' solvent is low. An electrolyte, consisting of seven molar lithium bis(trifluoromethanesulfonyl)imide in 1,3-Dioxolane: 1,2-Dimethoxyethane = 1:1 (v:v), is found to be suitable in providing the best cycling stability amongst the other compositions tested. The electrochemical characteristics of the passivation layers formed over lithium foil are mathematically modeled to indicate the preference of superconcentrated electrolytes over relatively dilute ones.