Investigations on the Improved Electrochemical Performance of Layered Fe-V-O Kazakhstanite Phase By Using Superconcentrated Electrolytes
Authors: Mitra Arijit , Majumder Subhasish Basu , Das Siddhartha 
Published: July 2021

In order to develop high energy-density lithium-ion batteries, multi-electron redox reaction based cathode materials are being actively researched across the globe, such as those observed in vanadium and molybdenum based compounds. The vanadium based cathode materials provide significantly higher specific capacity in comparison to the other state of the art materials such as over-lithiated layered oxides (OLO), NMC, NCA etc. However, their drawback of possessing a lower nominal voltage of operation brings their energy density values at the same level as those observed for OLO.

In this presentation, we investigate the electrochemical performance of one of the vanadium containing compounds, namely the Kazakhstanite phase (Fe5V15O39(OH)9.9H2O), as a cathode material for lithium-ion batteries. Kazakhstanite is a naturally occuring mineral found in Kazakhstan and United States. Preliminary structural characterization is performed to identify the microstructural, compositional and structural features of the as-synthesized material. Electrochemical investigation with the conventional electrolyte (1M LiPF6 in EC:DMC = 1:1 v:v, Code named: PL) reveals that the material delivers a specific capacity of ~350mAhg-1 between 1.5-3.8V (Li+/Li). However, the cycleability of the material is observed to be extremely poor with the conventional electrolyte. The cause is identified to be elemental dissolution of Fe and V from the active material with the help of EDX analysis of the cycled separators. It is observed that the active material dissolution is arrested to a great extent by changing the electrolyte system to a solvent-in-salt based one, without any additional engineering done on the material. By reducing the amount of solvent in the electrolyte, a solubility limit can be attained which prevents dissolution of active material. The electrochemical performance of the Kazakhstanite phase is evaluated with a series of electrolytes containing LiTFSI as the salt and DOL:DME = 1:1 v:v as the solvent, with varying concentration of the LiTFSI salt ranging from 1M to 7M. The cycleability of the Kazakhstanite phase with 7M LiTFSI in DOL:DME=1:1 v:v (Code named: OL) is found to be the best amongst all the other electrolyte compositions tested. Electrochemical impedance spectroscopy reveals that there should be a bilayer diffusion mechanism operational over the Lithium counter electrode. This indicates that the dissolved products form a passivation layer over the Lithium electrode apart from the SEI. The lithium-ion diffusion across the bilayer of the SEI and the passivation layer is observed to follow equation (1).

Zmt ∝ Δc1(0,s)/ΔJ1(0,s) = [Λtanh(λu)+tanh(u)]/[mu(Λtanh(u)tanh(λu)+1)] (1)

where:

D1, L1 : Diffusion coefficient of Li-ion through the passivation layer and thickness of the passivation layer

D2, L2 : Diffusion coefficient of Li-ion through the SEI layer and thickness of the SEI layer

λ = √(D1/D2) (L2/L1)

Λ = √(D1/D2)

m = -D1/L1

u = √(s/D1) L1 with s being the complex variable jω

The proposed bilayer diffusion model, along with the experimental results, explains the superiority of superconcentrated electrolytes over conventional organic electrolyte in tuning the electrochemical characteristics of the SEI and the passivation layer.

Molecular Dynamics Simulations are carried out to understand the co-ordination environment of the dissolved vanadium ions in the superconcentrated and conventional electrolytes. The simulated physico-chemical properties such as density, diffusion coefficient of the ions, and ionic conductivity match well with the values reported in the literature. For the superconcentrated electrolyte OL, it is observed from the simulations that the fraction of TFSI- is higher in the first solvation shell of vanadium ions in comparison to the one observed for PL case. The magnitude of the solvation energy of the vanadium ions, calculated using Free Energy Perturbation (FEP) and Finite-Difference Thermodynamic Integration (FDTI) methods, is found to be lower in superconcentrated electrolytes. This indicates that the dissolution of the active material in superconcentrated electrolyte is unfavorable in comparison to the conventional organic electrolyte. Additional EMD simulations are carried out to correlate the motion of the vanadium ions with respect to Li+, TFSI- and solvent molecules.

Figure Caption: (a) Co-ordination environment of Vanadium ions (0.16M) inside PL electrolyte. (b) Co-ordination environment of Vanadium ions (0.16M) inside OL electrolyte

Figure 1