The generalized solubility limit approach for vanadium based cathode materials for lithium-ion batteries

Abstract

In this article, we address the issue of vanadium dissolution pertinent in several vanadium containing cathode materials using the solubility limit approach. This article is divided into three major sections. In the first section, we introduce and demonstrate this concept on a layered Fe–V–O Kazakhstanite phase, which undergoes active material dissolution. The vanadium dissolution observed in this material is found to be arrested by switching to a superconcentrated electrolyte, wherein the amount of “free” solvent is low. An electrolyte, consisting of 7 M 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 in the material amongst the compositions tested. The second section is focused towards providing a robust explanation for the observed experimental results in the previous section. Two major avenues, namely the local structure within the superconcentrated electrolyte and the electrochemical characteristics of the passivation layers over the lithium counter electrode, are identified to be the cause of the improved cycling stability of the Kazakhstanite phase. The local structure within the superconcentrated electrolyte is studied through molecular dynamics simulations. A high anion content in the first solvation shell of vanadium cations is observed for superconcentrated electrolytes through ion-clustering calculations. Ion-dynamics of the clusters reveal that vanadium cation transport occurs against its concentration gradient due to strong coulombic interactions with the anions in superconcentrated electrolytes. On the other hand, the passivation layers are modeled with a bi-layer diffusion mechanism, and validated experimentally using electrochemical impedance spectroscopy. The experimental results indicate a preference for superconcentrated electrolytes over relatively dilute ones. The third section of the article is focused on extending the concept to several vanadium containing cathode materials, thereby providing concrete proof for the experimental and theoretical framework described in this article.