The current electric vehicle market primarily uses lithium-ion batteries (LIBs) for energy storage. However, for long-range vehicles that require high energy densities (> 350 Wh/kg), LIBs are unsuitable.1,2 Multivalent ion batteries such as magnesium, aluminum, and calcium ion batteries
(CIBs) could fill this gap. Among the multivalent ions, Ca is non-toxic and highly electropositive (E0Ca2+/Ca = -2.87 V vs. SHE). It is also the fifth most abundant element in the earth’s crust. Moreover, a Ca anode could deliver high cell voltages since its reduction potential is just 170mV lower than a Li anode. Also, since it can hold/deliver more than one electron per ion, it exhibits high specific charge capacities (Ah/kg). Together with high specific capacities (Ah/kg) and voltages (V), CIBs are expected to deliver high energy densities (Wh/kg). Despite these
advantages, due to the high reduction potential of Ca, electrolytes decompose spontaneously at the Ca surface and form a solid-electrolyte interphase (SEI), which prohibits the reversible deposition of Ca.3,4 As such, the major impediment to the growth of CIBs is the sluggish kinetics
of calcium ions through the SEI. However, recent experiments showed that the Ca ions can be reversibly shuttled through the SEIs at high temperatures, and furthermore, the extent of Ca shuttling can be controlled by tuning the nature of SEI.5,6 As such, these results demand further examination of the nature of SEI to improve the percolation of Ca2+ ions through it. In my thesis, we aim to investigate the formation and evolution of SEI on the Ca(001) surface when it is in contact with 0.45 M Ca(TFSI)2 salt or Ca(BF4)2 salt in a 1:1 wt% ethylene carbonate (EC) and propylene carbonate (PC) solvent mixture. Our preliminary results using ab initio molecular dynamics suggest that the SEI could be formed either due to the decomposition of the salt or solvent or both at the Ca surface. Moreover, the generated SEI contains both inorganic and
organic parts, where the inorganic part percolates into the Ca matrix, and the organic part stays away from the Ca surface. Further, we find that the stability of a salt highly depends on the presence of the prepassivated layer on the Ca surface. All these results are in good agreement with the experimental observations. Encouraged by these findings, we would like to use the SEIs obtained from our simulations to identify various Ca-ion migration pathways through these SEIs and the corresponding migration energy barriers. Moreover, by using appropriate salts, solvents, and additives, we would like to tune the nature of SEI to further improve the performance of CIBs.

References
(1) Banerjee, S.; Ghosh, K.; Reddy, S. K.; Yamijala, S. S. R. K. C. Cobalt Anti-MXenes as Promising Anode Materials for Sodium-Ion Batteries. J. Phys. Chem. C 2022, 126 (25), 10298–10308.
(2) Banerjee, S.; Narwal, A.; Reddy, S. K.; Yamijala, S. S. R. K. C. Promising Anode Materials for Alkali Metal Ion Batteries: A Case Study on Cobalt Anti-MXenes. Phys. Chem. Chem. Phys. 2023, 25 (16), 11789–11804.
(3) Yamijala, S. S. R. K. C.; Kwon, H.; Guo, J.; Wong, B. M. Stability of Calcium Ion Battery Electrolytes: Predictions from Ab Initio Molecular Dynamics Simulations. ACS Appl. Mater. Interfaces 2021, 13 (11), 13114–13122.
(4) Aurbach, D.; Skaletsky, R.; Gofer, Y. The Electrochemical Behavior of Calcium Electrodes in a Few Organic Electrolytes. J. Electrochem. Soc. 1991, 138 (12), 3536–3545.
(5) Ponrouch, A.; Frontera, C.; Barde, F.; Palacin, M. R. Towards a Calcium-Based Rechargeable Battery. Nat. Mater. 2016, 15 (2), 169–172.
(6) Forero-Saboya, J.; Davoisne, C.; Dedryvere, R.; Yousef, I.; Canepa, P.; Ponrouch, A. Understanding the Nature of the Passivation Layer Enabling Reversible Calcium Plating. Energy Environ. Sci. 2020, 13, 3423-3431.