Discharge Reactions of γ-MnO2 and Mo6S8 Tracked in the Electrode Bulk of Sealed Devices By Energy Dispersive X-Ray Diffraction (EDXRD)
Authors: Gallaway Joshua W. , Kim Matthew A , Jadhav Ankur , Messinger Robert J. , Okasinski John 
Published: February 2020

Energy dispersive X-ray diffraction (EDXRD) from a high energy source allows the evolution of materials to be tracked from deep within large specimens, due to (a) the use of highly penetrating X-rays and (b) the ability to define a well-controlled diffraction gauge volume in space.1,2 This non-destructive characterization tool lends itself to the study of batteries, whose electrodes are generally composites of compressed particles, with electrolyte filling the pore space, and a sealed containment surrounding the cell designed to be as compact as possible. The fundamental electrochemistry of the active material can be known from laboratory experiments, but reaction inhomogeneity that is a consequence of the system itself is best observed in an in situ fashion without disassembly, or in an operando fashion within the battery as it cycles. EDXRD allows tomographic-like observation of the bulk interior of electrodes and critical locations such as the electrode-separator interfaces.3,4 This talk will illustrate the technique for understanding complex electrochemical reactions in three diverse systems: (1) proton insertion into γ-MnO2 in alkaline electrolyte, (2) Al3+ intercalation into Mo6S8 chevrel in chloroaluminate electrolyte, and (3) Zn2+ intercalation into Mo6S8 in aqueous ZnSO4.

Deeply cycling MnO2 (617 mAh/g-MnO2) has the potential to provide battery storage of high safety, high energy density, and low cost, for applications such as intermittent renewable generation backup at the scale of the power grid.5 Figure 1a shows the γ-MnO2 discharge curve, in which reduction to one electron equivalent per Mn (x = 1) proceeds through proton insertion. Reaction beyond this point involves new phase formation at relatively constant potential. Operando EDXRD data in Figure 1b reveals that the nominal discharge product Mn(OH)2 was found only in a relatively small region clustered at the Ni current collector, while a propagating front of Mn3O4 traversed the electrode (panel iii). Figure 1c correlates the spatial locations of the various crystalline phases with the potential data in 1a. This shows that the final potential plateau at x = 1.8 corresponded to sudden Mn(OH)2 formation throughout the electrode, a phenomenon not previously reported.6 Dopant strategies to provoke earlier Mn(OH)2 formation and suppress Mn3O4 hold the key to MnO2 rechargeability.7

EDXRD is also useful in situations when electrode discharge products are prone to damage or alteration by cell disassembly, casting doubt on material structures observed by ex situ analysis. The chevrel phase Mo6S8 has been shown to intercalate divalent and trivalent ions, making it an important material of study. It is known that two second order phase transitions are expected during intercalation of a guest ion A into the host chevrel: Mo6S8 → AMo6S8 → A2Mo6S8.8 However, a recent ex situ study of Zn2+ intercalation suggested that conversion to Zn2Mo6S8 may be incomplete.9 We demonstrate this was a consequence of material oxidation during cell disassembly, and that when observed by EDXRD conversion to Zn2Mo6S8 is complete. The corresponding Al3+ intercalation will be compared to show there is only one phase transition in this case.

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Figure 1