Aaron M. Melemed and Betar M. Gallantz

Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States of America

Electrochemical characterization.—All steps occurred within a vacuum glove box (Xiamen Tmaxcn Inc.). As-received THF (anhydrous, 99.9%, Sigma Aldrich) and EC (99%, Acros Organics):PC (99.7%, Sigma Aldrich) (1:1 v/v) were dried over 4 Å molecular sieves for over 72 h; water content was determined to be <10 ppm through KF titration.Ca(BH4)2·2THF (Sigma Aldrich) was added to THF and the solution was rested overnight until translucent. Ca(TFSI)2 (99.5%, Solvionic) was vacuum dried at 120 °C overnight in a Buchi apparatus.Ca(BF4)2 (95%, Apollo Scientific) was vacuumed dried at 120 °C overnight before mixing with EC:PC and then further drying the solution with 4 Å molecular sieves for several days. A glass threeelectrode cell (Pine Research, Low Volume Cell) was used for electrochemical measurements. The Au working electrode was 1.6 mm in diameter and surrounded by a PCTFE shroud (Pine Research, LowProfile). Au foil (0.025 mm thick, 99.99%, Sigma Aldrich) was cut into thin strips or 12 mm disks, soaked in sulfuric acid for 20 min, and rinsed with deionized water before vacuum drying overnight. Freshly-prepared Ca or Au foil working electrodes were affixed in stainless steel holders and submerged in the electrolyte; the submerged area was measured manually for each electrode after testing. Ca reference electrodes utilized the same setup. A Pt wire coil (Pine Research, LowProfile) was used as the counter electrode. 2032-type coin cells were assembled using a coin cell crimper (Xiamen Tmaxcn Inc.) inside the Ar glovebox with a crimping pressure of 0.81 tons. Whatman glass fiber separators were used for coin cell measurements. A Bio-Logic potentiostat was used for cyclic voltammetry and galvanostatic experiments. Cyclic voltammetry (CV) sweep rates were 25 mV s−1, beginning at the open circuit potential and scanning to −1 and 2 V vs the Ca reference, unless indicated otherwise.