Enabling high mass loadings for polymer based all solid-state batteries with temperature assisted solvent-free vacuum melt infiltration

Abstract

This study introduces a solvent-free cathode infiltration method for polymer electrolytes and catholytes, leveraging melt infiltration under vacuum conditions. The present work focuses on polycaprolactone-based polymer electrolytes, utilizing non-crosslinked polymers for catholyte infiltration and crosslinked variants for better mechanical stability, which are operated in solid-state batteries. Notably, the method boosts accessible cathode mass loadings and electro-chemical performance by optimizing contacts upon infiltration of melted solid polymers into porous cathodes at elevated temperatures under vacuum (∼0.5 mbar). The employed polymers included grafted polycaprolactones (GCD-PCL and BT-PCL) and PEO reference, each mixed with lithium bis(trifluoro sulfonyl)imide (LiTFSI) in a 5:1 and 10:1 mol-ratio respectively. The infiltrated cathodes (LFP, NMC and LCO) with mass loadings of 6 up to 13 mg cm⁻², which corresponds to areal capacities of 1 and 2 mAh cm⁻², were cycled and characterized in lithium metal-based cells, utilizing impedance spectroscopy and DRT analysis. Discharge capacities of 106 mAh g⁻¹ were achieved at a current density of 0.25 mA cm⁻² (representing C/4 at mass loadings of 6 mg cm⁻² NMC₆₂₂) in fully dry batteries, except for addition of small amounts of propylene carbonate as catholyte, whereby the discharge capacity could be boosted to 158 mAh g⁻¹. In summary, the introduced infiltration strategy holds promise for further advancing achievable electrochemical performance of polymer-based solid-state batteries.