(644b) Operando Studies to Enumerate the Electrochemical Phase Transformations of MnO2

This talk will concern the electrochemical phase
transformations that occur during cycling of electrolytic manganese dioxide
(EMD) when used as a cathode material in alkaline batteries. While previous
studies have focused on the initial and final states of the material, we focus
on the pathway as observed during cycling. The goal is identification of the materials
that prevent battery recharging once they have formed, and the conditions that
lead to these terminal materials. MnO₂ is inexpensive and has a high
theoretical capacity (617 mAh/g-MnO₂), making it an attractive
option for large-scale battery storage. Figure 1 shows the discharge pathway of
EMD. Two strategies have been employed to use EMD for rechargeable batteries.
The first is limitation of the discharge capacity to prevent phase changes,
i.e. shallow cycling.[1-5] The second is
modification of the material to allow deep discharge along a favorable pathway,
i.e. modified deep cycling.[6-17]

EMD is a commercially-relevant polymorph of MnO₂
used extensively in primary alkaline batteries. Its structure is typically
described as an intergrowth of 1x1 tunnel MnO₂ (pyrolusite or β-MnO₂) and 2x1
tunnel MnO₂ (ramsdellite), and is variously classified as either γ-MnO₂ or ε-MnO₂ in the
literature.[18-20] Its diffraction
pattern can be matched to standards of the MnO₂-based minerals
akhtenskite and nsutite. However there can be variation in the observed
diffraction peaks depending on the material source. As a cathode in alkaline
electrolyte, its discharge proceeds via the insertion of protons into the
crystal lattice following the reaction:

MnO₂ + xH₂O
+ xe^-⇌ MnO_2-x(OH)_x
+ xOH^-

Initially the discharged material maintains the same crystal
structure, and the unit cell expands anisotropically as x increases.[21] We report a series of experiments
performed on thick electrodes that identify the crystalline materials formed in
the later stages of discharge, beginning with a solid state phase
transformation to α-MnOOH
that occurred at x = 0.79 regardless of rate or location in the electrode.[22] From this, a series of other materials was
formed. First Mn₃O₄ and ZnMn₂O₄
spinels rapidly followed α-MnOOH,
which was the precursor material for spinel formation. At x = 1.92, layered
Mn(OH)₂ appeared as the discharge end member. yes">[23] We show evidence that the spinels displayed some
rechargeability, which was limited by their low conductivity, effectively rendering
them self-isolating. These experiments were performed using high energy, high
flux energy dispersive X-ray diffraction (EDXRD) from a synchrotron source.[24-26]

Standard electrodes were limited to one discharge due to Mn₃O₄
and ZnMn₂O₄. However, if Bi³⁺ ions were
present Mn(OH)₂ was preferentially formed in greater amount, and
this was recharged to the layered polymorph Arial">δ-MnO₂ or birnessite. The birnessite discharge
occurred at a lower potential than that of EMD. The insertion of cations into
the birnessite interlayer or van der Waals gap caused contraction of the
birnessite unit cell during discharge.[23]
Operando XANES data indicated the presence of Arial">β-MnOOH as a discharge intermediate, although this material
was poorly crystalline and was not observed by X-ray diffraction. The
recognition of non-crystalline intermediate species within the discharge
pathway aids in expanding understanding of the mechanisms underlying alkaline
MnO₂ cycling.

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