Stabilized Octahedral Frameworks in Layered Double Hydroxides by Solid‐Solution Mixing of Transition Metals

Pseudocapacitors have received considerable attention, as they possess advantages of both rechargeable batteries and electric double layer capacitors. Among various active materials for pseudocapacitors, α‐layered double hydroxides (α‐TM(OH)₂, TM = transition metal) are promising due to their high specific capacities. Yet, irreversible α‐to‐β phase transitions of α‐TM(OH)₂ hinder their long‐term cyclability, particularly when the TM is nickel. Here, it is reported that binary TM ion mixing can overcome the limited cycle lives of α‐TM(OH)₂ by stabilizing the octahedral frameworks of α‐TM(OH)₂. In particular, an α‐TM(OH)₂ with equal amounts of nickel and cobalt exhibits long‐term capacity retention (89.0% after 2000 cycles) and specific capacity (206 mA h g⁻¹), which are better than those of individual TM counterparts. A series of analyses reveals that the improved performances originate from the synergistic effects between the TM ions; the preferred trivalent state of cobalt ions stabilizes the octahedral framework by accommodating the detrimental Jahn–Teller distortion of Ni³⁺. The stabilized framework also widens the redox swing range of the nickel up to 4+, thus, increasing the specific capacity of the corresponding α‐TM(OH)₂. This study indicates that proper mixing of TMs is a prolific approach in enhancing the vital properties of α‐TM(OH)₂, a promising family of pseudocapacitor materials.