Porous amorphous CMC/MnO2 with abundant oxygen vacancies for calcium ion storage

Manganese dioxide (MnO 2 ) materials have recently garnered attention as promising cathode materials for aqueous calcium-ion batteries, owing to their low cost, environmental friendliness, and high-capacity characteristics. However, the low electrical conductivity and slow diffusion kinetics of MnO 2 severely limit its application. Herein, a carboxymethylcellulose sodium (CMC)/MnO 2 (CMO) were synthesized through a simply hydrothermal method for Ca 2+ storage for the first time. CMC was employed to induce the structure of MnO 2 from crystal to amorphous transition, which was investigated by morphological evolution characterization. Notably, the CMO composite has an amorphous structure with numerous mesopores and oxygen vacancies, and this greatly facilitates the migration of Ca 2+ and results in high pseudocapacitive behavior. As a result, the CMO cathode achieved a specific capacity of 83 mAh g −1 at 1 A g −1 and retained 73 % after 2000 cycles. Importantly, used active carbon as anode, the full coin battery CMO//AC achieved up to 4000 cycles with an energy density of 126 Wh kg −1 and power density of 434.8 W kg −1 . This work provides a potential alternative and development for aqueous calcium-ion batteries. The CMO with abundant oxygen vacancies, mesoporous structure, and amorphous nature was synthesized via a facile hydrothermal method, achieving superior Ca 2+ storage performance. • 5-CMO possesses an amorphous, mesoporous structure along with abundant oxygen vacancies • Unveiling a CMC-induced amorphization evolution mechanism in MnO 2 • The 5-CMO electrode demonstrated rapid ion diffusion and a dominant pseudocapacitive charge storage behavior • The CMO//AC full cell delivered a remarkable capacity retention of 54 % after 4000 cycles at 1 A g −1