Upcycling Regeneration of Spent Layered Oxide Cathodes with Gradient Elements‐Distribution and Deep Vacancies Toward Ultra‐Fast Energy‐Storage Properties

Abstract Sodium‐ion batteries (SIBs) are regarded as next‐generation energy storage system with considerable commercialization achievements, especially mass production of commercial metal‐oxides cathodes. Similar to lithium‐ion batteries (LIBs), they will be also restricted by the limited lifespan. Considering their low value, the elements‐extracting recycling manners hardly display effective profits. Thus, from the experience of LIBs recycling, direct regeneration maybe suitable approach. Herein, assisted by water‐exfoliation, spent cathodes are successfully collected from retired electrode pieces, but with the serious loss of Na‐elements, poor phase transformation, and microcracks. Through the introduction of Na‐resources and thermal energy, spent cathodes are successfully regenerated. And, the as‐regenerated sample shows smooth surface and O3‐type phase. Surprisingly, the gradient sodium concentration distribution is noted, along with deep Na‐vacancies in bulk phase. As SIBs cathode, the optimized sample shows considerable cycling stabilities with the capacity retention of 89.0% at 0.2 C. And, even at 5.0 C, the relative capacity can be remained ≈79.3% after 400 cycles. Assisted by kinetic analysis, the existing of deep Na‐vacancies serves as the fast ion‐diffusion “accessboard”, and Na‐rich surface/subsurface successfully prevent interfacial side reactions. Given this, the work is anticipated to provide direct regeneration method and clear upcycling mechanism for spent SIBs cathodes.