The development of a rechargeable battery technology using light electropositive metal anodes would bring in a breakthrough in energy density. For divalent charge carriers (M2+), the number of ions that must react to achieve a certain electrochemical capacity is diminished by two when compared to Li+.
Amongst divalent electropositive metals, calcium is especially attractive as it is the fifth most abundant element on earth crust and its standard reduction potential is only 170 mV above that of lithium, enabling significantly larger cell potential than that achievable with magnesium. Moreover, Ca2+ would hold promise for faster reaction kinetics than Mg2+ (and thus better power performance) due to its lower polarizing character. Despite the electrodeposition of calcium being thought to be impossible, the feasibility of calcium plating was demonstrated in conventional organic electrolytes, provided moderate temperatures (100 °C) are used to mitigate the degree of ion pairing and enhance Ca2+ mobility within the electrolyte. The electrolytes used are similar to those used in the Li-ion battery technology and thus exhibit very wide operation potential window. The reversibility of the process upon cycling was ascertained and thus these findings are the first proof of the viability of calcium metal anodes.
The study was carried out by researchers at ICMAB with the support of results obtained at ALBA Synchrotron in the framework of a project funded by Motor Europe (TME). The results achieved open the way to exploratory screening and testing of potential cathode materials which would reversibly insert and deinsert calcium in the electrolytes used. Efforts are currently underway to achieve proof-of-concept of a new high energy density rechargeable battery technology using calcium anodes.
1Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Spain
2Toyota Motor Europe