Oxide Cathodes: Functions, Instabilities, Self Healing, and Degradation Mitigations

Recent progress in high-energy-density oxide cathodes for lithium-ion batteries has pushed the limits of lithium usage and accessible redox couples. It often invokes hybrid anion- and cation-redox (HACR), with exotic valence states such as oxidized oxygen ions under high voltages. Electrochemical cycling under such extreme conditions over an extended period can trigger various forms of chemical, electrochemical, mechanical, and microstructural degradations, which shorten the battery life and cause safety issues. Mitigation strategies require an in-depth understanding of the underlying mechanisms. Here we offer a systematic overview of the functions, instabilities, and peculiar materials behaviors of the oxide cathodes. We note unusual anion and cation mobilities caused by high-voltage charging and exotic valences. It explains the extensive lattice reconstructions at room temperature in both good (plasticity and self-healing) and bad (phase change, corrosion, and damage) senses, with intriguing electrochemomechanical coupling. The insights are critical to the understanding of the unusual self-healing phenomena in ceramics (e.g., grain boundary sliding and lattice microcrack healing) and to novel cathode designs and degradation mitigations (e.g., suppressing stress-corrosion cracking and constructing reactively wetted cathode coating). Such mixed ionic-electronic conducting, electrochemically active oxides can be thought of as almost “metalized” if at voltages far from the open-circuit voltage, thus differing significantly from the highly insulating ionic materials in electronic transport and mechanical behaviors. These characteristics should be better understood and exploited for high-performance energy storage, electrocatalysis, and other emerging applications

Pt₃Co/Co Composite Catalysts on Porous N‑Doped Carbon Support Derived from ZIF-67 with Enhanced HER and ORR Activities

The primary challenge for efficient H2 evolution and hydrogen energy conversion is to develop highly active and stable catalysts with simple and reliable preparation processes. In this regard, we have designed and synthesized a porous carbon-supported low-Pt alloy catalyst (Pt3Co/Co@C composite) using ZIF-67 as a template. It showed uniformly dispersed Pt3Co/Co on the porous carbon layer due to the confinement effect of the porous carbon layer. Pt3Co/Co@C demonstrated excellent activity for the hydrogen evolution reaction in the full pH range, with an overpotential of 187 mV in 0.5 M H2SO4 to attain 100 mA/cm2 as well as long-term stability. It also displayed superior mass activity for the oxygen reduction reaction (ORR) at 0.85 V (vs RHE) compared to the commercial Pt/C. Furthermore, the Pt3Co/Co@C catalyst exclusively enabled a four-electron reaction process under ORR conditions without the competitive pathway to H2O2. The current work provides guidance for the design and facile synthesis of Pt-based catalysts with enhanced performance