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

• Discharge of γ-MnO2 was tracked within sealed batteries as a function of position.
• Spinel formation immediately followed phase transition to α-MnOOH.
• Well-formed α-MnOOH occurred after insertion of 0.79 H+ per Mn atom.
• Insertion of 0.79 H+ correlated to 104% of the 2 × 1 ramsdellite tunnel capacity.

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.

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