Abstract
Ca-ion batteries (CIBs) have the potential to provide inexpensive energy storage, but their realization is impeded by the lack of suitable electrolytes. Motivated by recent experimental progress, we perform ab initio molecular dynamics simulations to investigate early decomposition reactions at the anode-electrolyte interface. By examining different combinations of solvent—tetrahydrofuran (THF) or ethylene carbonate (EC)—and salt—Ca(BH 4 ) 2 , Ca(BF 4 ) 2 , Ca(BCl 4 ) 2 , and Ca(ClO 4 ) 2 —we identify a variety of behavioral trends between electrolyte solutions. Next, we perform a separate trajectory with pure THF and gradually increased negative charge; despite an addition of -32 e , no THF decomposition is detected. Charge analysis reveals that in a reductive environment, THF distributes excess charge evenly across its hydrocarbon backbone, while EC concentrates charge on its ester oxygens and carbonyl carbon, resulting in decomposition. Graphs of charge vs. time for both solvents reveal that EC decomposition products can be reduced by up to five electrons, while those of THF are limited to a single electron. Ultimately, we find Ca(BH 4 ) 2 and THF to be the most stable solution investigated herein, corroborating experimental evidence of its suitability as a CIB electrolyte.
Highlights
Li-ion batteries (LIBs) are ubiquitous in current technology, powering a variety of devices ranging from portable electronics to electric vehicles
We aim to address this outstanding issue by means of ab initio molecular dynamics (AIMD), allowing us to observe preliminary interfacial decomposition products under a variety of conditions
In order to define a baseline for solvent behavior, we first investigated two 20 ps AIMD trajectories of either pure ethylene carbonate (EC) or pure THF with a Ca anode
Summary
Li-ion batteries (LIBs) are ubiquitous in current technology, powering a variety of devices ranging from portable electronics to electric vehicles. Further concerns—including dendritic plating behavior, the lower melting point of Li metal, safety issues, and an approaching performance ceiling—motivate the development of an alternative technology [3, 4]. Assuming the development of an affordable cathode, Ca-ion batteries (CIBs) could significantly improve upon the cost of current battery technology due to the much lower price of Ca metal relative to Li metal [3]. CIBs could provide performance benefits relative to LIBs. Due to its divalent nature, Ca2+ can provide higher volumetric capacity than Li+. Ca2+ has a very low redox potential relative to SHE—nearly that of Li+—a feature that allows theoretical CIBs to have higher volumetric (and, with certain cathodes, gravimetric) energy density than LIBs [5]. Multivalent ions can suffer from high diffusion barriers due to their increased charge density, the large ionic radius of Ca offsets these issues [4, 6]
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