Metastable and nanosize cation-disordered rocksalt-type oxides: revisit of stoichiometric LiMnO₂ and NaMnO₂

Abstract

Stoichiometric LiMnO2 and NaMnO2 with a cation-disordered rocksalt-type structure as metastable polymorphs were successfully prepared by mechanical milling.

Material Design Concept of Lithium‐Excess Electrode Materials with Rocksalt‐Related Structures for Rechargeable Non‐Aqueous Batteries

“…Metastable phases prepared by mechanical milling consists of nanosize particles with low crystallinity, and therefore anionic redox is destabilized compared with submicrometer‐sized particles. Indeed, reversibility as electrode materials is significantly improved for nanosize NaMnO 2 with the cation‐disordered rocksalt structure prepared by mechanical milling …”

Nanosize Cation‐Disordered Rocksalt Oxides: Na2TiO3–NaMnO2 Binary System

“…Despite a large size gap between Na and Ti/Mn ions, all cations are randomly distributed in the same site of the rocksalt structure, similar to Na 1.3 Mn 0.4 Nb 0.3 O 2 . A lattice parameter of Na 1.14 Mn 0.57 Ti 0.29 O 2 ( x = 0.34) is calculated to be 4.457 Å, which is longer than 4.443 Å for NaMnO 2 with the rocksalt structure obtained by mechanical milling . This fact also suggests the incorporation of larger Na ions into the rocksalt structure and the formation of Na‐excess phases.…”

“…Charge/discharge curves of rocksalt Na 1.14 Mn 0.57 Ti 0.29 O 2 and NaMnO 2 prepared by mechanical milling are also compared in Figure a. Nanosize NaMnO 2 with the rocksalt structure delivers a reversible capacity of ≈200 mA h g −1 with pure cationic redox . In contrast to NaMnO 2 , presence of the voltage plateau at 4 V is clearly noted for the Na‐excess rocksalt oxide, which is also indicative of the use of anionic redox as electrode materials.…”

“…Synthesis of the sample at 1000 °C results in crystallization of an unknown phase, but phase segregation of Mn and Ti ions is clearly evidenced by scanning electron microscopy (SEM) with energy‐dispersive X‐ray (EDX) analysis in Figure S2 in the Supporting Information. Synthesis of this binary system by calcination was unsuccessful, and therefore, mechanical milling approach, which allows to synthesize thermodynamically metastable phases by using mechanical energy, including friction and shear stress, was applied. Mechanical impact and friction during milling reduce particle sizes of materials into nanoscale, and many structural defects are induced.…”

“…The rate capability of Na 1.14 Mn 0.57 Ti 0.29 O 2 is also tested in a Na cell, and a result is shown in Figure c. The result is competitive with that of nanosize NaMnO 2 with the rocksalt structure . The sample delivers large reversible capacity even though it is expected that higher concentration of grain boundaries in particles would be sacrifice facile Na migration as electrode materials.…”