The Achilles' Heel of Lithium-Ion Batteries: Protonic Species.

Lithium-ion batteries (LIBs) are central to the global energy transition, dominating the markets for electric vehicles and grid-scale renewable energy storage. However, as LIBs are pushed toward their theoretical energy density limits to meet ever-escalating performance demands, they are confronted with challenges to their intrinsic operational safety and long-term cycling stability. In recent years, protonic species within LIBs have been increasingly recognized as critical drivers of accelerated cell degradation and premature failure. In this contribution, we first recapitulate the diverse sources and generation pathways of protonic species, with a particular emphasis on the chemical and electrochemical oxidation of state-of-the-art carbonate-based electrolytes. Subsequently, we elucidate the proton-mediated failure network and associated self-amplifying proton-regeneration cycle, including electrolyte degradation and proton-driven parasitic interfacial side reactions at both the anode- and cathode-electrolyte interphases. Furthermore, we highlight critical advancements in in situ/operando characterization techniques for elucidating the sources and degradation mechanisms of protonic species, and propose a mitigation framework centered on proton source suppression and proton scavenging strategies. Finally, we pinpoint unresolved challenges and outline prospective research directions in this field. This contribution establishes critical connections between fundamental proton chemistry and failure modes, which will provide key strategic guidelines toward durable and safe high-energy-density LIBs.