# Understanding the Role of Triple Phase Boundaries on Coating-Free Solid-State Cathodes

## TL;DR

This paper explores how triple phase boundaries affect decomposition in uncoated solid-state cathodes, showing ways to improve performance without coatings.

## Contribution

The study reveals the critical role of triple phase boundaries in oxidative decomposition and demonstrates a coating-free cathode with high performance.

## Key findings

- Triple phase boundaries significantly increase oxidative decomposition in uncoated solid-state cathodes.
- Regulating electronic pathways at triple phase boundaries enables high areal capacity and good cycle retention in thick electrodes.
- Uncoated cathodes achieved ~4.6 mAh cm–2 initial areal capacity and 85% retention after 500 cycles.

## Abstract

Sulfide
solid electrolytes have high ionic conductivities necessary
to achieve high-rate solid-state cathodes at room temperature and
low pressure. Cathode active materials generally require coatings
to avoid deleterious oxidative decomposition reactions with the electrolyte.
Coatings add cost and complexity to the manufacture. Here we decouple
the effect of double and triple phase boundaries on the decomposition
in the thick (i.e., ∼110 μm) uncoated solid state cathode.
We show that more severe oxidative decomposition of solid electrolytes
occurs when the cathode active materials, carbon, and the solid electrolyte
coexist, highlighting the importance of the triple phase boundary
concerning the decomposition. By regulating the electronic pathways
at the triple phase boundary, a thick uncoated electrode at 1 mA cm–2 and 2 MPa stack pressure, delivers an initial areal
capacity of ∼4.6 mAh cm–2 at 30 °C and
∼85% capacity retention after 500 cycles.