Sodium Storage Performance and Mechanism of MnO₂ with Different Phase Structures (α, β, γ, δ) as Anode Materials for Sodium-Ion Batteries

An in-depth understanding of the links between the phase structure and electrochemical property is crucial for the advancement of high-performance anode materials. Herein, the sodium storage performance and mechanisms of MnO₂ with four distinct phase structures (α, β, γ, and δ) as anodes are systematically investigated. Among the four materials, the layered δ-MnO₂ nanoflowers exhibit the best sodium storage performances, characterized by a specific capacity of 303.6 mA h g^-1 after 100 cycles at 200 mA g^-1, cyclability of 247.3 mA h g^-1 after 500 cycles at 1000 mA g^-1, and high-rate performance of 184.5 mA h g^-1 at 3000 mA g^-1. Furthermore, δ-MnO₂ shows the most pronounced pseudocapacitance behavior during discharge/charge processes among the four materials. <i>Ex situ</i> XRD and TEM analyses reveal that the sodium storage reactions of α-, β-, and γ-MnO₂ proceed via a conversion reaction mechanism, while the sodium storage reaction of δ-MnO₂ is controlled by an insertion/deinsertion mechanism. The findings presented in this study may offer insights into the structure regulation and performance promotion of MnO₂-based anode materials for SIBs.