Accelerated Kinetics Revealing Metastable Pathways of Magnesiation-Induced Transformations in MnO2 Polymorphs
Accelerated Kinetics Revealing Metastable Pathways of Magnesiation-Induced Transformations in MnO2 Polymorphs
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DOI：
10.1021/acs.chemmater.1c02011
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发表时间：
2021-08-17
影响因子：
8.6
通讯作者：
Ichitsubo, Tetsu
中科院分区：
文献类型：
--
作者：
Hatakeyama, Takuya;Li, Hongyi;Ichitsubo, Tetsu
二氧化锰的内在电位被认为是可充电镁电池的高容量阴极，在Mg迁移动力学得到充分增强的条件下，二氧化锰的内在电位被清楚地暴露出来。迄今为止已有报道，由于MnO2在室温下的动力学缓慢，镁的插入基本上局限于MnO2颗粒的表面，从而导致局部过镁化条件导致转化反应等。为了揭示MnO2多晶（α, β, γ， δ和λ）在镁化过程中的遍历或亚稳态相变途径，本研究使用耐热离子液体电解质进行了中温电化学实验（150℃）。无论其最初的多晶型结构如何，每个MnO2多晶型都被发现在镁化作用下转变为含镁尖晶石，然后转变为岩盐状相。考虑到这种转变趋势，具有尖晶石/岩盐结构的相干框架的缺陷尖晶石λ - mno2相可能遵循拓扑转变途径，但热不稳定。在电解液中镁化前，MnO2自发还原为Mn3O4。相反，α -MnO2被发现在MnO2多晶中足够健壮，在150℃的极限容量条件下表现出可逆的镁嵌入。这一结果表明，对于不可逆转化为尖晶石和岩盐结构的结构，可逆镁嵌入氧化物阴极是可行的。
The intrinsic potential of manganese dioxides, considered high-capacity cathodes of rechargeable magnesium batteries, was clearly exposed under conditions where the Mg migration kinetics are sufficiently enhanced. It has been reported to date that magnesium insertion into MnO2 is substantially confined to the surfaces of MnO2 particles due to its sluggish kinetics at room temperature, which leads to local overmagnesiation conditions causing conversion reactions etc. To unveil its ergodic or metastable phase-transformation pathways of MnO2 polymorphs (alpha,beta,gamma,delta, and lambda) during magnesiation, this study employed intermediate-temperature electrochemical experiments (at 150 degrees C) using heat-tolerant ionic liquid electrolytes. Regardless of its original polymorphic structure, each MnO2 polymorph was found to transform into a Mg-including spinel and then to a rocksalt-like phase by magnesiation. Given this tendency of transformation, the defect spinel lambda-MnO2 phase possessing the coherent framework of spinel/rocksalt structures is expected to follow a topotactic transformation pathway, but thermally unstable.MnO2 underwent spontaneous reduction into Mn3O4 before magnesiation in an electrolyte. Instead, alpha-MnO2 was found to be robust enough among MnO2 polymorphs to exhibit reversible magnesium intercalation at 150 degrees C under limiting capacity conditions. This result highlights that reversible magnesium intercalation in oxide cathodes is feasible for structures that are kinetically resistant to irreversible transformation pathways to spinel and rocksalt structures.