Expansion counteraction effect assisted vanadate with rich oxygen vacancies as a high cycling stability cathode for aqueous zinc-ion batteries†

In this study, a novel tunnel structure vanadate NaVO (Na_0.465V₂O₅) cathode for aqueous zinc ion batteries (AZIBs) is facilely fabricated by thermal decomposition of polyoxovanadate containing NH₄⁺ ions. The NaVO cathode is characterized by abundant oxygen vacancies and nanometer dimensions. These attributes can offer extra reaction sites and suppress structural collapse during circulation. In the charge–discharge process, a unique phenomenon occurs where NaVO undergoes opposite expansion (positive vs. negative expansion) along its different crystal planes. This opposite expansion produces an “expansion counteraction effect”, which effectively buffers the volume change of NaVO. Additionally, the irreversibly inserted Zn²⁺ ions as “pillars” are maintained in the framework after the first discharge, further improving the structural stability of NaVO. Consequently, the NaVO cathode exhibits superior cycling stability. The capacity retention rate can reach 87.3% after 350 cycles at 0.1 A g⁻¹. With a high current density of 2 A g⁻¹, the specific capacity can be maintained at 206.3 mA h g⁻¹ with a capacity retention of 95.5% after 2100 cycles. This study not only provides a novel approach for synthesizing nanoscale vanadate cathodes with rich oxygen vacancies, but also proposes the “expansion counteraction effect” theory, offering innovative insights into the design of high cycling stability cathodes for AZIBs.