Sodium Storage Performance\nand Mechanism of MnO<sub>2</sub> with Different Phase Structures (α,\nβ, γ,\nδ) as Anode Materials for Sodium-Ion Batteries

An in-depth understanding of the links between the phase\nstructure\nand electrochemical property is crucial for the advancement of high-performance\nanode materials. Herein, the sodium storage performance and mechanisms\nof MnO<sub>2</sub> with four distinct phase structures (α, β,\nγ, and δ) as anodes are systematically investigated. Among\nthe four materials, the layered δ-MnO<sub>2</sub> nanoflowers\nexhibit the best sodium storage performances, characterized by a specific\ncapacity of 303.6 mA h g<sup>–1</sup> after 100 cycles at 200\nmA g<sup>–1</sup>, cyclability of 247.3 mA h g<sup>–1</sup> after 500 cycles at 1000 mA g<sup>–1</sup>, and high-rate\nperformance of 184.5 mA h g<sup>–1</sup> at 3000 mA g<sup>–1</sup>. Furthermore, δ-MnO<sub>2</sub> shows the most pronounced\npseudocapacitance behavior during discharge/charge processes among\nthe four materials. <i>Ex situ</i> XRD and TEM analyses\nreveal that the sodium storage reactions of α-, β-, and\nγ-MnO<sub>2</sub> proceed via a conversion reaction mechanism,\nwhile the sodium storage reaction of δ-MnO<sub>2</sub> is controlled\nby an insertion/deinsertion mechanism. The findings presented in this\nstudy may offer insights into the structure regulation and performance\npromotion of MnO<sub>2</sub>-based anode materials for SIBs.