The gap between long lifespan Li-S coin and pouch cells: The importance of lithium metal anode protection

Lithium-sulfur (Li-S) batteries have been strongly regarded as next-generation energy storage devices for their very high theoretical energy density (2600 W h kg⁻¹), low cost, and non-toxicity. Li polysulfide (LiPS) shuttle in the cathode and Li dendrite growth in the anode are among the toughest issues on the practical applications of Li-S batteries. By efficient cathode and membrane design, LiPS can be effectively trapped in the cathode side and the coin cells deliver a superior cycling performance of 2000 cycles. When the coin cell strategy is transplanted to a pouch cell, the possibility to achieve a long-lifespan Li-S pouch cell should be further explored. Herein, the gaps between the coin and pouch cell were probed and the failure mechanism of a Li-S pouch cell was explored. Compared with LiPS shuttle issue, Li metal powdering and the induced polarization are more responsible for pouch cell failure. Electrochemical cycling performance and ex-situ morphology characterization indicated that the continuously striping/plating of Li ions results in Li dendrite growth during cycling test, which exposes more fresh Li to electrolyte and induces the formation of dead Li powder. Therefore, the Li ion diffusion resistance is increased and the Coulombic efficiency is reduced gradually. When the cycled Li anode was updated by a fresh Li metal, the renaissance cell with cycled cathode vs fresh anode exhibited an improved discharge capacity from 314 to 1030 mA h g⁻¹ at 0.1 C. Relatively to a neglected role in coin cells in most cases, Li metal anode tremendously affects long-term cycling performance of Li-S pouch cells. More attentions should be concentrated on efficiently protecting Li metal anode to achieve large capacity and safe Li-S cells with high energy density.