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

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 Fe_xO_yCl_z 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^-1 _LFP) and boosts energy density by 21.1% (609.4 vs. 503.4 Wh kg^-1 _LFP) 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.