# Ionic Liquid Electrolyte Suppresses Deep Sodiation in Nb4P2S21/Mo2CT x Enabling Transition from Mixed-Voltage to Pure High-Voltage Operation for Sodium-Ion Battery Cathodes

## TL;DR

This study shows that using an ionic liquid electrolyte improves the performance of a sodium-ion battery cathode material by suppressing unwanted low-voltage reactions.

## Contribution

The use of an ionic liquid electrolyte enables high-voltage operation in a sulfur-rich cathode material for sodium-ion batteries.

## Key findings

- The material retains 96.3% of its initial discharge capacity above 0.8 V in ionic liquid electrolyte.
- The ionic liquid suppresses deep sodiation and low-voltage redox activity.
- The material shows structural and compositional stability after 100 cycles.

## Abstract

Elemental sulfur
has garnered significant attention due
to its
low cost and high theoretical capacity; however, its reliance on ether
electrolytes leads to the formation of soluble polysulfides, thereby
limiting its application. Sulfur-rich transition metal polysulfides
demonstrate potential as sulfur-equivalent cathodes to replace conventional
sulfur in alkali metal–sulfur batteries; however, adequate
research in this area remains unrevealed. In this study, we investigate
the Nb4P2S21 in carbonate, ether,
and ionic liquid electrolytes for sodium-ion battery testing. The
material exhibits a high discharge capacity exceeding 1000 mAh/g and
a prolonged discharge plateau at low potentials in both ether and
carbonate electrolytes, same with other high-capacity phosphorus sulfide
anodes via conversion reactions. When switching to the NaTFSI/[Emim]­TFSI
ionic liquid electrolyte, 96.3% of the initial discharge capacity
in the 0–3 V range is retained above 0.8 V, with the suppression
of low-voltage redox activity. This shift is attributed to the cointercalation
of Na+ and Emim+ ions, preventing the materials
from deep sodiation at lower voltage range. The incorporation of Mo2CT
x
MXene into the material further
reduces electrochemical polarization and enhances cycle stability.
During 100 cycles, a self-activation phenomenon occurs, resulting
in a maximum capacity of 384 mAh/g, while the median voltage remains
above 1.5 V, predominantly governed by a pair of reversible redox
peaks. X-ray photoelectron spectroscopy (XPS) and high-resolution
transmission electron microscopy (HRTEM) analyses of postcycled material
confirm the structural and compositional stability of the material
during cycling. This study advances the understanding of sulfur-rich
materials in sodium-ion batteries across various electrolytes, particularly
ionic liquids.