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

Abstract

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)–O1.5 + Na+ + e - « 1/2 Na2O + Mn(II)–O, and non-Mn-centered redox reactions. Figure 1