Trace filling strategy of amphoteric molecules for large-capacity and long-lasting Li-Fe-F conversion all-solid-state batteries

Abstract

Polyethylene oxide (PEO)-based solid-state polymer electrolytes (SPE) face the challenges of insufficient ionic conductivity and uncontrollable Li dendrite growth. The filler strategy can reinforce anode interface stability, but at the cost of a large filler content (usually more than 10 wt%). This would increase the granular sensation, gravitational separation risk, and electrolyte membrane roughness with the creation of inhomogeneous Li⁺ transport channels between filler and polymer. Herein, we propose a trace filling strategy to address the above problems by introducing an amphoteric molecule L-Cysteine (LCy) as an eco-friendly and low-cost electrolyte additive. Only trace amount of LCy is required and integrated into PEO to form a homogenous, granule-less SPE with enhanced ionic conductivity and dendrite suppression capability. The ionic conductivity increases to 0.54 mS cm⁻¹ at 60 ℃ after introducing only 1 wt% LCy. The amphotericity of LCy with basic –NH₂ and acidic –COOH groups can promote the dissociation of Li salt and release more free Li ions through Lewis acid-base synergy, as well as the formation of multiple hydrogen bonds between PEO and LCy. The trace LCy additive swiftly leads to the formation of more ionic conductive interphases at both the anode and cathode sides. The composite SPE enables the stable cycling of Li metal for over 1400 h at 0.2 mA cm⁻² and sustains a maximum current density up to 1.4 mA cm⁻² in Li‖Li symmetric cells. The corresponding all-solid-state Li‖FeF₃ full cells exhibit a high specific capacity up to 567 mA h g⁻¹ at 0.2 C and stable cycling performance for at least 700 cycles at 0.5 C with a high capacity retention. The excellent interface compatibility also guarantees the achievement of high-capacity Li-Fe-F conversion reaction even under the thin electrolyte membrane thickness and larger-scale pouch cell configuration.