# Highly Stable Quasi-Solid-State Sodium Batteries via Facile Grain Boundary Engineering

## TL;DR

This paper introduces a new method to improve the stability and performance of sodium batteries by modifying grain boundaries with a zinc oxide coating.

## Contribution

A novel grain boundary engineering approach using a ZnO coating to enhance ionic conductivity and stability in solid-state sodium batteries.

## Key findings

- Symmetric cells showed ultrastable cycling for over 12,000 hours with suppressed dendrite formation.
- Quasi-solid-state batteries achieved 93.2% capacity retention after 1300 cycles at 0.5 C.
- Batteries maintained 95.8% of initial capacity after 1200 cycles at 2 C without external pressure.

## Abstract

The development of ceramic solid-state electrolytes such
as sodium
superionic conductors (NASICON) is critical in the advancement of
all-solid-state sodium batteries. However, the key fundamental issue
lies in the large interfacial impedance and instability due to the
mismatched mechanical properties across different battery components.
Herein, we propose a novel interfacial engineering approach to tackle
the challenge at the interface, where a Na+ conducting
grain boundary complexion phase can be formed by cosintering NASICON
with a thin layer of zinc oxide (ZnO) coating. The grain boundary
complexion envelopes the NASICON grains and forms an ion-conducting
network that enhances the ionic conductivity at the grain boundaries.
Ultrastable symmetric cell cycling over 12,000 h was demonstrated,
showing the efficacy in suppressing dendrite formation. Electrochemical
impedance spectroscopy (EIS) measurements revealed a minimal increase
in internal resistance over cycling. In addition, quasi-solid-state
batteries using Na3V2(PO4)3 as the cathode, sodium metal as the anode, and cosintered NASICON
as the electrolyte were assembled and tested at room temperature with
no externally applied stack pressure. When cycled at 0.5 C, a high
initial capacity of 116.7 mAh g–1 was obtained,
and 93.2% capacity retention was achieved at the 1300th cycle with
an average Coulombic efficiency of 99.98%. Even when cycled at 2 C,
the battery maintained 95.8% of the initial capacity after 1200 cycles.
Overall, this work provides insights into facile and strategic approaches
to interfacial modification of solid-state batteries and shows the
great potential of grain boundary engineering.