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, alpha-layered double hydroxides (alpha-TM(OH)(2), TM = transition metal) are promising due to their high specific capacities. Yet, irreversible alpha-to-beta phase transitions of alpha-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 alpha-TM(OH)(2) by stabilizing the octahedral frameworks of alpha-TM(OH)(2). In particular, an alpha-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 alpha-TM(OH)(2). This study indicates that proper mixing of TMs is a prolific approach in enhancing the vital properties of alpha-TM(OH)(2), a promising family of pseudocapacitor materials.