The massive demand for batteries required by electromobility and stationary storage is pushing the research activity to evaluate electrochemical storage chemistries based on highly abundant elements, as alternatives to state-of-the-art Li-ion ones.[1] The use of more abundant and sustainable elements is expected to reduce the cost and the environmental impact of the energy storage devices. In particular, calcium, the fifth most abundant element in the Earth’s crust also offering non-toxicity, low standard reduction potential (-2.87 vs. SHE), is an appealing candidate for the realization of electrochemical storage devices.[1] However, the conditions to achieve efficient Ca stripping and deposition have not been found yet, due to the formation of passivation products at the Ca/electrolyte interphase, blocking any electrochemical reaction.[2] Although a limited number of electrolytes enabling the stripping deposition have been found, [3–6] they are characterized by a rather limited efficiency, thus making the Ca metal unsuitable for counter and reference electrodes. The resulting lack of a reliable cell configuration limits the possibility to test candidate electrode materials suitable for Ca²⁺-based electrochemical storage devices, impeding the progress and research on the topic. Here, we propose an alternative cell configuration, employing activated carbon (AC) as counter electrode and quasi reference electrode (QRE), as a reliable system for the electrochemical characterization of electrode materials for Ca-ion batteries.[7] As a prove-of-the-concept, a few electrode materials, i.e., CaV₆O₁₆ cathode and graphite anode exploiting co-intercalation mechanism, have been demonstrated electrochemically active in Ca²⁺-based electrolytes.

References:

[1] M.E. Arroyo-De Dompablo, A. Ponrouch, P. Johansson, M.R. Palacín, Achievements, Challenges, and Prospects of Calcium Batteries, Chem. Rev. 120 (2020) 6331–6357.

[2] D. Aurbach, R. Skaletsky, Y. Gofer, The Electrochemical Behavior of Calcium Electrodes in a Few Organic Electrolytes, J. Electrochem. Soc. 138 (1991) 3536–3545.

[3] A. Ponrouch, C. Frontera, F. Bardé, M.R. Palacín, Towards a calcium-based rechargeable battery, Nat. Mater. 15 (2016) 169–172.

[4] D. Wang, X. Gao, Y. Chen, L. Jin, C. Kuss, P.G. Bruce, Plating and stripping calcium in an organic electrolyte, Nat. Mater. 17 (2018) 16–20.

[5] A. Shyamsunder, L.E. Blanc, A. Assoud, L.F. Nazar, Reversible Calcium Plating and Stripping at Room Temperature Using a Borate Salt, ACS Energy Lett. 4 (2019) 2271–2276.

[6] Z. Li, O. Fuhr, M. Fichtner, Z. Zhao-Karger, Towards stable and efficient electrolytes for room-temperature rechargeable calcium batteries, Energy Environ. Sci. 12 (2019) 3496–3501.

[7] X. Liu, G.A. Elia, S. Passerini, Evaluation of counter and reference electrodes for the investigation of Ca battery materials, J. Power Sources Adv. 2 (2020) 100008.