Operando identification of the point of [Mn 2 ]O 4 spinel formation during γ-MnO 2 discharge within batteries

Abstract The rechargeability of γ-MnO 2 cathodes in alkaline batteries is limited by the formation of the [Mn 2 ]O 4 spinels ZnMn 2 O 4 (hetaerolite) and Mn 3 O 4 (hausmannite). However, the time and formation mechanisms of these spinels are not well understood. Here we directly observe γ-MnO 2 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-MnO 2 the α-MnOOH phase forms rapidly. α-MnOOH is the precursor to spinel, which closely follows. ZnMn 2 O 4 and Mn 3 O 4 form at the same discharge depth, by the same mechanism. The results show the final discharge product, Mn 3 O 4 or Mn(OH) 2 , is not an intrinsic property of γ-MnO 2 . While several studies have identified Mn(OH) 2 as the final γ-MnO 2 discharge product, we observe direct conversion to Mn 3 O 4 with no Mn(OH) 2 .