(40d) A Freestanding Calcium Hydroxide Interlayer As a Selective Separator for Rechargeable Alkaline Zn/MnO2 Batteries

      The
alkaline Zn/MnO₂ battery has dominated the primary battery market
since its invention because of its low cost and safety characteristics, as well
as its high theoretical energy density that arises from the high gravimetric
capacity of MnO₂. Currently, a long-term rechargeable Zn/MnO₂
battery is achievable at a reduced depth of discharge (DOD) (~10%-20% of the
first electron of MnO₂) with a curtailment of energy density [1].
Its energy density higher than that of lead-acid batteries and the cost lower
than $100/kWh make it a disruptive technology. Accessing higher DOD results in
higher energy densities, but leads to detrimental characteristics like phase
transformation of MnO₂ and the zinc redistribution [2][3]. Recently,
a class of Bi-birnessite (a layered MnO₂ structure with bismuth
oxide) material intercalated with Cu²⁺ has been reported as being
able to stabilize the MnO₂ structure and deliver near-full
two-electron capacity for more than 6,000 cycles in the absence of zinc [4].
However, when a zinc anode is employed, the most detrimental effect arises from
zincate ions ([Zn(OH)₄]^2-) formed during discharge that
traverse to the cathode side and react to form a spinel phase ZnMn₂O₄,
a highly resistive and electrochemically inactive compound, leading to rapid
energy density loss and battery failure [5][6]. Therefore, inhibiting ZnMn₂O₄
formation remains a major challenge for the long-term cycling of an energy dense
secondary Zn/MnO₂ battery.

    In
this presentation, we report the use of an interlayer fabricated of calcium
hydroxide for zincate sequestration, which successfully prevents ZnMn₂O₄
formation. Ca(OH)₂ is known to reversibly react with zincate ions
and form an insoluble complex, calcium zincate (Ca(OH)₂•2Zn(OH)₂•2H₂O).
Its role as an anode additive in Ni/Zn batteries is well known; however, its
use has been limited due to its high resistivity and low density. We find that
Ca(OH)₂ as an interlayer successfully avoids these drawbacks. It
shows good properties such as a porous structure, good electrolyte retention
ability, and a high ionic conductivity (Fig. 1a). It stabilizes the zincate
concentration at a lower number in the bulk electrolyte by forming calcium
zincate with excess zincate ions during discharge, and working as a zincate
source during charge through decomposition (Fig. 1b). It is effective in
localizing and trapping the zincate ions, while not affecting the transport of
hydroxyl ions. More importantly, it inhibits the formation of zinc manganese
spinel phase at high DOD as observed through X-ray diffraction (Fig. 2a) and
energy dispersive X-ray spectroscopy analyses, which results in improved
capacity retention and energy density in a Zn/MnO₂ cell (Fig. 2b).

References

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[5] J.
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Fig. 1 (a)
Nyquist plots showing ionic
conductivities of different separators measured by a symmetric cell; (b) Zincate
ion concentrations in the cathode and anode chambers of a cycling cell at the
end of each discharge half-cycle, with different separators applied.

Fig. 2
(a) XRD patterns of different EMD electrodes i) uncycled, ii) cycled vs. a
Hg/HgO reference electrode, iii) cycled in a cell with a Ca(OH)₂
interlayer and iv) cycled in a Ca(OH)₂-free cell; (b) Curves of
specific discharge capacity with cycle number.