Effect of Sub-nanoparticle Architecture on Cycling Performance of MnO₂ Battery Cathodes through Thermal Tuning of Polymorph Composition

Abstract

Reversible cycling of MnO2 cathodes remains a challenge in alkaline rechargeable battery development. In prior work, we have developed new platelet shaped MnO2 nanoparticles with stable performance in LiOH electrolyte for over 50 cycles. In this study, the effects of sub-nanoparticle organization of MnO2 polymorphs on cycling performance, phase activation, and charge/​discharge mechanisms in LiOH electrolyte are investigated by ex situ X-ray powder diffraction (XRD). Different phase compositions with the same particle morphology are achieved by annealing as-synthesized nanoplatelets composed of 50% ramsdellite (R-MnO2) and 50% akhtenskite (ε-MnO2) polymorphs. Thermal treatments result in a gradual change of polymorph composition, due to reorganization of the 1 × 2 channels in ramsdellite and akhtenskite phases to 1 × 1 channels with initial appearance of the gamma phase (γ-MnO2), followed by complete conversion of ramsdellite to pyrolusite (β-MnO2) at 400 °C. Electrochemical activity of thermally treated nanoparticles is correlated to the phase compositions before and after cycling. Rietveld refinement of the XRD patterns suggests material activation in the initial cycles through intercalation of Li+ and H+ ions into 1 × 2 channels, resulting in lattice expansion in both akhtenskite and ramsdellite, while intercalation of ions into structural faults present in akhtenskite and gamma phases results in amorphization of the active material. The detailed mechanism of the polymorph conversion during annealing as well as phase activation and reversible redox activity are discussed. For the first time, it is clearly demonstrated that the electrochemical activity of MnO2 material strongly depends not only on the lattice structure of individual polymorphs but also on the sub-nanoparticle architecture, including surrounding polymorphs and interphases.

Opportunities of Aqueous Manganese‐Based Batteries with Deposition and Stripping Chemistry

“…[ 92,93 ] The general failure mechanism of MnO 2 cathode in the alkaline electrolyte is ascribed to the partial irreversible conversion reactions between MnO 2 and the reduced Mn 2 O 3 and Mn(OH) 2 , or the gradual dissolution of MnO 2 into the strong alkaline electrolytes (Figure 2). [ 94–98 ] In order to improve the MnO 2 cathode instability issue, researchers have developed various strategies such as utilizing mixed KOH and LiOH instead of pure KOH as the electrolyte, [ 99–101 ] or different metals doping of MnO 2 . [ 102–105 ] It was found that the battery capacity retention rate and the number of cycles can be largely increased.…”

Oxygen Defect Engineering of β‐MnO2 Catalysts via Phase Transformation for Selective Catalytic Reduction of NO

“…The calcination treatment could lead to the collapse of the 1 × 2 channels to smaller 1 × 1 channels, thus leading the transformation of γ-MnO 2 to β-MnO 2 . [32] Compared to the hydrothermal condition, the phase transformation induced by post-calcination forms more abundant oxygen defects, benefitting from the intrinsic incomplete lattice rearrangement. We find that the oxygen defects favor the reactants adsorption and *NH 2 formation.…”