  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.
