Balancing Polysulfide Distribution in "Anode-Free" Lithium-Sulfide Batteries.

Abstract

Lithium-sulfide positive electrodes represent a promising alternative to established transition metal-based positive electrodes due to enhanced specific capacity and sustainability. While positive electrodes containing elemental sulfur require a lithiated negative electrode, lithium-sulfide can serve as the lithium reservoir and thus be paired with bare copper electrodes in "anode-free" or "zero-excess" cell concepts. This boosts energy density and avoids handling of thin lithium metal electrodes. While promising electrochemical performance of "anode-free" lithium-sulfide batteries has already been demonstrated, many reported cell configurations rely on nickel- instead of copper-based negative electrodes, undermining the enhanced sustainability bestowed by lithium-sulfide positive electrodes. Demonstrating a continuous reaction between copper electrodes and soluble polysulfide species, two approaches are evaluated that restrict the migration of polysulfide species. While both, in situ polymerization of an electrolyte additive as well as electrospinning of a polymer layer at negative electrodes, enable reversible operation of copper-based "anode-free" lithium-sulfide batteries, the former approach offers notably enhanced capacity retention. Counterintuitively, the quantification of polysulfide distribution throughout the individual battery components reveals less confinement within the positive electrode as beneficial for the overall reversibility. This demonstrates that a balance between positive and negative electrode reversibility is required to advance "anode-free" lithium-sulfide batteries.