Operando identification of the point of [Mn₂]O₄ spinel formation during γ-MnO₂ discharge within batteries

The rechargeability of γ-MnO₂ cathodes in alkaline batteries is limited by the formation of the [Mn₂]O₄ spinels ZnMn₂O₄ (hetaerolite) and Mn₃O₄ (hausmannite). However, the time and formation mechanisms of these spinels are not well understood. Here we directly observe γ-MnO₂ discharge at a range of reaction extents distributed across a thick porous electrode. Coupled with a battery model, this reveals that spinel formation occurs at a precise and predictable point in the reaction, regardless of reaction rate. Observation is accomplished by energy dispersive X-ray diffraction (EDXRD) using photons of high energy and high flux, which penetrate the cell and provide diffraction data as a function of location and time. After insertion of 0.79 protons per γ-MnO₂ the α-MnOOH phase forms rapidly. α-MnOOH is the precursor to spinel, which closely follows. ZnMn₂O₄ and Mn₃O₄ form at the same discharge depth, by the same mechanism. The results show the final discharge product, Mn₃O₄ or Mn(OH)₂, is not an intrinsic property of γ-MnO₂. While several studies have identified Mn(OH)₂ as the final γ-MnO₂ discharge product, we observe direct conversion to Mn₃O₄ with no Mn(OH)₂.