The rational design of calcium metal anode batteries requires connecting the speciation of an electrolyte to its ability to plate and strip calcium metal. While Ca(BH4)2 in THF can reversibly plate and strip calcium with coulombic efficiencies greater than 97 percent, the concentration of the active CaBH4+ contact ion pair (CIP) remains below 0.1 M even at an overall Ca(BH4)2 concentration of 1.5 M. We hypothesize that the concentration of the active CaBH4+ species can be boosted through addition of a weakly coordinating calcium salt to a Ca(BH4)2 electrolyte; this change in speciation should enable Ca metal cycling at higher current densities. Using Ca(CB11H12)2 as a weakly coordinating salt, we observe a significant increase in ionicity as Ca(CB11H12)2 is added to a Ca(BH4)2 electrolyte. However, the combination of dielectric relaxation spectroscopy and 43Ca nuclear magnetic resonance spectroscopy unexpectedly indicates that the addition of Ca(CB11H12)2 to Ca(BH4)2 electrolytes produces a significant population of Ca(CB11H12)+ solvent-separated ion pairs (SSIPs) in addition to boosting the population of CaBH4+ CIPs. The Ca(CB11H12)+ SSIP induces distinct electrochemical behavior in the mixed salt electrolyte. In the base Ca(BH4)2 electrolyte, calcium deposition is kinetically limited while, in the mixed salt electrolyte containing both the Ca(CB11H12)+ SSIP and the CaBH4+ CIP, calcium deposition becomes mass transport limited. The accelerated kinetics for Ca2+ reduction in the SSIP suggests that removal of the strongly bound anion from the first coordination shell of Ca2+ can facilitate electron transfer. Furthermore, the SSIP may provide adequate electrostatic screening to lower the desolvation barrier of the Ca2+ cation. This work provides insight into how coordination of the calcium delivery species affects its ability to plate calcium metal. Creating this link is key to the rational design of multivalent electrolytes.

Manipulating the speciation of Ca-ion electrolytes through the addition of a weakly coordinating calcium salt to enhance calcium metal anode performance

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