Ultrafine Manganese Oxide As Sodium-Ion Battery Anode Involving Charge-Transfer Between Mn(III) and Mn(II) Oxides

Our investigation on manganese dioxide (MnO₂) as an anode material for sodium (Na)-ion batteries (NIBs) reveals a remarkable size effect within the nanometer range on the electrochemical sodiation activity of the oxide. 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 392 mAh g⁻¹ compared with the negligible activity shown by the aggregates of larger (14 nm) MnO₂ nanocrystallites. To our knowledge, the present study is the first to show that MnO₂ is electrochemically active as an NIB anode. The UF-MnO₂ anode exhibits exceptional rate and cycle performance, including a reversible sodiation capacity of >100 mAh g⁻¹ at a current density of 7500 mA g⁻¹ and >70% capacity retention after 500 cycles at 150 mA g⁻¹. The enhanced cycle stability may be attributable to the solid-embedding architecture that enables the reduction of the dimensional variations in the active material. In operando synchrotron X-ray absorption near-edge structure analysis reveals combined charge–storage mechanisms involving a unique two-oxide conversion reaction between Mn(III) and Mn(II) oxides, Mn(III)–O_1.5 + Na⁺ + e^- « 1/2 Na₂O + Mn(II)–O, and non-Mn-centered redox reactions.