Deciphering the Formation and Accumulation of Solid-Electrolyte Interphases in Na and K Carbonate-Based Batteries

The continuous solid-electrolyte interphase (SEI) accumulation has been blamed for the rapid capacity loss of carbon anodes in Na and K ethylene carbonate (EC)/diethyl carbonate (DEC) electrolytes, but the understanding of the SEI composition and its formation chemistry remains incomplete. Here, we explain this SEI accumulation as the continuous production of organic species in solution-phase reactions. By comparing the NMR spectra of SEIs and model compounds we synthesized, alkali metal ethyl carbonate (MEC, M = Na or K), long-chain alkali metal ethylene carbonate (LCMEC, M = Na or K), and poly(ethylene oxide) (PEO) oligomers with ethyl carbonate ending groups are identified in Na and K SEIs. These components can be continuously generated in a series of solution-phase nucleophilic reactions triggered by ethoxides. Compared with the Li SEI formation chemistry, the enhancement of the nucleophilicity of an intermediate should be the cause of continuous nucleophilic reactions in the Na and K cases

Molecular Sieve Induced Solution Growth of Li₂O₂ in the Li–O₂ Battery with Largely Enhanced Discharge Capacity

The formation of the insulated film-like discharge products (Li₂O₂) on the surface of the carbon cathode gradually hinders the oxygen reduction reaction (ORR) process, which usually leads to the premature death of the Li–O₂ battery. In this work, by introducing the molecular sieve powder into the ether electrolyte, the Li–O₂ battery exhibits a largely improved discharge capacity (63 times) compared with the one in the absence of this inorganic oxide additive. Meanwhile, XRD and SEM results qualitatively demonstrate the generation of the toroid Li₂O₂ as the dominated discharge products, and the chemical titration quantifies a higher yield of the Li₂O₂ with the presence of the molecular sieve additive. The addition of the molecular sieve controls the amount of the free water in the electrolyte, which distinguishes the effect of the molecular sieve and the free water on the discharge process. Hence, a possible mechanism has been proposed that the adsorption of the molecular sieves toward the soluble lithium superoxides improves the disproportionation of the lithium superoxides and consequently enhances the solution-growth of the lithium peroxides in the low donor number ether electrolyte. In general, the application of the molecular sieve triggers further studies concerning the improvement of the discharge performance in the Li–O₂ battery by adding the inorganic additives