# Capacity-expanding O/Cl-bridged catholyte boosts energy density in zero-pressure all-solid-state lithium batteries

## TL;DR

A new catholyte design improves zero-pressure all-solid-state lithium batteries by boosting energy density and performance.

## Contribution

A novel O/Cl-bridged catholyte with polymer-like viscoelasticity and high ionic conductivity is introduced for zero-pressure ASSLBs.

## Key findings

- The catholyte achieves 86.6% capacity retention after 100 cycles, a 35.7% improvement over rigid catholytes.
- It delivers a capacity of 97.7 mAh g−1 and increases energy density by 21.1% when paired with LFP cathodes.

## Abstract

The advancement of all-solid-state lithium batteries (ASSLBs) requires innovative breakthroughs in catholyte design to eliminate the need for external pressure and mitigate the adverse effects of inactive catholytes on energy density. Here, we present a capacity-expanding O/Cl-bridged catholyte (1.2LiOH–FeCl3) featuring an abundant, freely rotating FexOyClz framework, endowing it with polymer-like viscoelasticity and an impressive ionic conductivity (6.1 mS cm−1 at 25°C). The polymer-like viscoelasticity creates a soft interface that alleviates volume changes during cycling, enabling zero-pressure ASSLBs to deliver a high capacity retention of 86.6% after 100 cycles, which is a 35.7% improvement compared to the rigid Li2ZrCl6 catholyte (50.9%). Moreover, the fast Li+ transport capability and variable-valence iron coordination center endow 1.2LiOH–FeCl3 catholyte delivering a capacity of 97.7 mAh g−1. When used as a catholyte alongside an LiFePO4 (LFP) cathode material, it increases capacity by 31.3% (196.4 vs. 149.6 mAh g−1LFP) and boosts energy density by 21.1% (609.4 vs. 503.4 Wh kg−1LFP) compared to Li2ZrCl6 catholyte. Beyond these properties, the 1.2LiOH–FeCl3 catholyte offers significant cost advantages, priced at just $2.6 kg−1 (16% of the cost of Li2ZrCl6), and supports scalable production at 60°C, making kilogram- to ton-level manufacturing feasible.

A capacity-expanding O/Cl-bridged catholyte with polymer-like viscoelasticity and high ionic conductivity enables zero-pressure all-solid-state lithium batteries, achieving superior capacity retention and enhanced energy density.