An unexpected large capacity of ultrafine manganese oxide as a sodium-ion battery anode

Abstract

MnO₂ is shown for the first time to be electrochemically active as a conversion anode for Na-ion batteries (NIBs). Space-confined ultrafine (UF)-MnO₂, with an average crystal size of 4 nm, synthesized using a porous silicon dioxide templated hydrothermal process exhibits a high reversible sodiation capacity of 567 mA h g⁻¹, in contrast to the negligible activity shown by the aggregates of larger (14 nm) MnO₂ nanocrystallites. The remarkably enhanced sodiation activity of the UF-MnO₂ is attributable to its greatly reduced crystal size, which facilitates diffusion of Na ions, along with high surface energy arising from extensive heterogeneous interfacial bonding with the SiO₂ surrounding. The UF-MnO₂ anode exhibits an exceptional rate and cycle performance, exhibiting >70% capacity retention after 500 cycles. In operando synchrotron X-ray absorption near-edge structural analysis reveals combined charge-storage mechanisms involving conversion reaction between Mn(III) and Mn(II) oxides, Mn(III)–O_1.5 + Na⁺ + e⁻- ↔ 1/2Na₂O + Mn(II)–O, and non-Mn-centered redox reactions. The finding suggests a new strategy for “activating” the potential electrochemical electrode materials that appear inactive in the bulk form.