Carboxymethylcellulose Induced the Formation of Amorphous MnO2 Nanosheets With Abundant Oxygen Vacancies for Fast Ion Diffusion in Aqueous Zinc-Ion Batteries

Abstract

Manganese dioxide with high theoretical capacity has drawn a great deal of attention for aqueous zinc-ion batteries (ZIBs). Nevertheless, their sluggish diffusion kinetics, low electrical conductivity and limited active sites are still hindering the potential application in batteries. Herein, amorphous MnO₂ nanosheets with abundant oxygen vacancies are facilely prepared by using carboxymethylcellulose sodium (CMC) as a capping agent. During the growth process, CMC can preferentially attach to the (003) facet of MnO₂ to guide its crystal growth and morphology. The small nanosheets can expose abundant edge sites, along with the surface oxygen vacancies, to facilitate the insertion/extraction of H⁺ and Zn²⁺ in ZIBs. Moreover, the presence of CMC can stabilize the Mn³⁺ to inhibit the Jahn–Teller effect in the preparation process. As a result, CMC-MnO₂ based ZIB can provide a specific capacity of 324 mAh g⁻¹ at 0.5 A g⁻¹, and achieve 86.2% retention after a long cycle test of 1000 cycles. Furthermore, the energy storage mechanism may be attributed to the insertion/extraction of H⁺/Zn²⁺, dissolution/deposition of MnO₂ and Zn₄SO₄(OH)₆·nH₂O, and irreversible transformation of vermiculite during long cycles. This work may open new perspectives for the development of MnO₂-based cathodes in ZIBs.