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Abstract The Bi-modified MnO2 cathode is an enabling technology for rechargeable alkaline Zn batteries, which are under intense study for low-cost, sustainable, non-flammable grid-scale energy storage. Although the effect of Bi on MnO2 rechargeability has been known for decades, its mechanism has remained a longstanding challenge in the field. Here, we detail an oxidative electrochemical coupling between Bi and Mn in electrodes of commercially relevant areal capacity (12 mAh/cm2). Specifically, operando X-ray absorption spectroscopy and differential capacity analysis reveal that electrochemical capacity associated with Bi oxidation is shifted up into a voltage range where only Mn oxidation is expected. This oxidative coupling occurs precisely when failure by Mn3O4 formation is known to occur in the absence of Bi, and coincides with the potential range at which the β′-MnOOH intermediate exists during charge. The stabilization of β′-MnOOH by Bi-Mn coupling reveals its critical role in rechargeability, as this intermediate is the primary material source of Mn3O4 in unmodified electrodes. Only a small fraction of the total Bi capacity, corresponding to approximately 2 wt % Bi2O3, participates in this coupling, and it is demonstrated that the overall extent of rechargeability is controlled by the diffusion of dissolved Bi ions through the microporous structure of the MnO2 active material. Consistent with this transport-limited scenario, structurally incorporating Bi3+ directly into the MnO2 lattice is shown to be counterproductive, as it pillars the structure and impedes the electrochemical conversion it was intended to assist. 

Abstract The alkaline battery MnO₂discharge mechanism progresses from tunneled γ-MnO₂to tunneled α-MnOOH to layered Mn(OH)₂as the reduction proceeds from charged Mn(IV) to discharged Mn(II). However, the existence of a disordered intermediate has recently been suggested in the “second electron” region between Mn(III) and Mn(II). The authors useoperandoextended X-ray absorption fine structure (EXAFS) to observe the structural evolution. A disordered γ-MnOOH was identified, which was the majority material for a substantial period before the sudden appearance of Mn(OH)₂in the final moments of discharge. In rechargeable Bi-modified electrodes, the disordered γ-MnOOH was more efficiently converted to Mn(OH)₂. Graphical abstract 

Alkaline Zn anodes are fundamental to commercial Zn-MnO₂batteries as well as emerging rechargeable designs. In these electrodes, Zn particles are both the active material and source of electronic conduction. However, there are known cases in which electronic connectivity between Zn particles or the current collecting pin is lost even though the battery continues to function. Here we use X-ray computed tomography (CT) of AA batteries to demonstrate several examples of Zn particle-to-particle connectivity breakdown, which is observed even in cases at relatively high discharge rate. This indicates maintenance of the electronic network through the less-conductive ZnO discharge product. We introduce a new equation for electronic conductivity maintained through bridges of ZnO formed between percolating Zn particles. This necessitates discarding the Bruggeman correlation and instead redefining effective electronic conductivity using percolation theory. We demonstrate that such a model for conduction enables prediction of an inverted reaction zone, which is an experimentally observed case in which significant Zn dissolution and ZnO formation occurs heavily near the current collecting pin. Current computational Zn-MnO₂models never predict an inverted reaction zone, and thus the updated conductivity enables models to accurately explain a wider range of experimental conditions. 

Sulfide electrolyte all-solid-state lithium batteries (ASLBs) with uncoated Li-Ni_xMn_yCo_1−x−yO₂(NMC) cathodes suffer from a large capacity loss during initial cycling and an increase in cell impedance. Decomposition reactions are known to occur at the Li₆PS₅Cl-NMC111 interface due to incompatibility between the two materials. If a stabilizing coating is applied to the NMC, it delivers full capacity during initial charge. However, the loss in capacity during discharge still occurs. The interface was studied byμXANES and through EIS analysis. A chemically-formed interphase was detected byμXANES, evident from reduction of Co at an uncoated NMC particle surface. This interphase was produced by decomposition at rest. To study the effect of the interphase on electrochemically active surface area, piecewise in situ EIS was performed and the data was modeled using a transmission line model (TLM). The charge transfer resistance R_CTwas used to estimate the volume specific active surface area (a_act). The median value for a_actwas 296 cm⁻¹, a factor of 7.5 lower than the theoretical value of 2216 cm⁻¹. This provided evidence of a lower electrochemically active surface area in the ASLB.