The use of calcium
(Ca) metal anodes in batteries is currently
challenged by the development of a suitable solid electrolyte interface
(SEI) that enables effective Ca2+ ion transport. Native
calcium electrolytes produce a passivation layer on the surface of
the calcium electrodes during cycling, causing a decrease in capacity
during cycling and the need for large overpotentials. The use of a
hybrid SEI is a strategy to mitigate the uncontrolled production of
a passivation layer and reduce the overpotentials needed for the plating
and stripping of calcium. Here, we report the development of a hybrid
potassium (K)/Ca SEI layer investigated in symmetric Ca//Ca cell configurations.
Using KPF6 salt in a ternary mixture of carbonate solvent
(EC/EMC/DMC), Ca//Ca cells can be cycled up to 200 h at a capacity
of 0.15 mAh/cm2 with a current density of 0.025 mA/cm2. The symmetrical cells consistently cycle at overpotentials
of 1.8 V. Ex-situ X-ray diffraction (XRD) of cycled electrodes reveals
plating and stripping of both calcium and potassium. Energy dispersive
X-ray (EDX) maps confirm the plating of calcium and potassium during
galvanostatic cycling. Scanning electron microscopy (SEM) cross-sectional
views of the calcium electrodes reveal a continuous SEI layer formed
over the calcium metal. XRD analysis reveals the SEI layer consists
of K-based inorganics along with the identification of permanent and
transient phases. FTIR outlines the parallel plating of both calcium
and potassium at both regions of redox activity. Raman spectroscopy
of the electrolyte reveals compositional changes over the course of
cycling that promote increased plating and stripping. The results
indicate that potassium electrolytes are a possible route for tuning
the SEI to enable reversible calcium electrochemical cycling.
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