Towards the development of calcium ion batteries

Abstract

A novel system for the study of calcium-ion electroactive materials has been developed, characterized, and utilized to screen a number of candidate calcium intercalation compounds. The system is comprised of a dried, pre-electrolyzed calcium perchlorate salt in acetonitrile solvent electrolyte combined with solid-liquid metal slush counter and reference electrodes utilizing inert molybdenum, borosilicate glass, and polytetrafluoroethylene cell components. The counter and reference electrodes consist of saturated calcium amalgam and a calcium mercury intermetallic phase, denoted as CaHglI-Ca(Hg) sal , with a nominal calcium concentration of 5 mole percent. Reference electrodes were found to be stable for many weeks with no drift and high precision (+/- 2 mV), and lie at a potential value of approximately -2.043 V versus the standard hydrogen electrode or 0.825 V versus the Ca/Ca2 couple. Several transition metal oxide and other chalcogen-based structures were explored as calcium cathode materials. Vanadium oxide (V2O5), iron sulfide (FeS2) and molybdenum selendide (Mo3Se4) could be reversibly cycled. The behavior of Mo3Se4 was studied in greater detail, and its electrochemical performance suggested sluggish calcium transport resulted in rate limited capacity. Microscale (-2.5 pm particle diameter) powders demonstrated a reversible capacity less than 3 percent of theoretical for the host compound. Nanoscaling, higher temperature cycling, and chemo-structural alteration of Mo3Se4 increased capacity utilization fourfold. Calcium content of electroactive samples was confirmed by energy dispersive x-ray spectroscopy. X-ray photoelectron spectroscopy and x-ray diffractometry studies provided supporting evidence of calcium intercalation into the Mo3Se4 Chevrel phase structure. Furthermore, preliminary results are presented involving beryllium and aluminum electrochemistry in the Mo3Se4 Chevrel phase.