Calcium battery

Calcium (ion) batteries are energy storage and delivery technologies (i.e., electro-chemical energy storage) that employ calcium ions (cations), Ca2+, as the active charge carrier in solution as well as in the electrodes (anode and cathode). Calcium (ion) batteries remain an active area of research, with work persisting in the discovery and development of electrodes and electrolytes that enable stable, long-term battery operation.
Benefits and Advantages.
Calcium batteries are considered a next-generation, or post-Li-ion energy storage system, namely one of the many candidates that may potentially replace lithium-ion technology. Key advantages sought in such post-Li-ion systems is lower cost, earth abundance (41,500 ppm), higher energy density, and higher power output. There are several benefits to the development and use of calcium batteries. Calcium is the fifth most abundant mineral in the Earth's crust, and the most abundant alkaline earth metal, and the third most abundant metal after aluminum (Al) and iron (Fe). A calcium metal anode offers a higher volumetric capacity and gravimetric capacities (2072 mAh.mL-1 and 1337 mAh.g–1, respectively) than current commercial graphite anodes in Li-ion batteries (300–430 mAh mL–1 and 372 mAh g–1). Calcium metal anodes have a 2+ oxidation state which would provide a greater energy density over monovalent systems (i.e., Li+ and Na+), and it has a standard reduction potential of 2.9 V, which is 0.17 V greater than that of Lithium. Compared to divalent systems, calcium batteries can have a higher cell voltage than magnesium because of the 0.5 V lower standard reduction potential of the latter. Ca2+ ions also have the potential for faster reaction kinetics as compared to magnesium (Mg2+) owing to its lower polarizing properties and charge density both in the electrode as well as in an intercalation cathode.
The United States is the largest producer (by annual production) of calcium sources (primarily lime), promising domestic supply and manufacture.
Battery Components.
A calcium (ion) battery has not yet been commercially realized, but remains in the realm of research and development. Efforts concentrate on developing effective anode and cathode materials, as well as stable electrolytes.
Anodes.
Examples of anode materials include: vanadium oxide, copper-calcium alloying, MgV2O5, graphite, metallic calcium, and silicon anodes. Recent work on plating/stripping calcium was done in ethylene carbonate/propylene carbonate (EC/PC) solutions at elevated temperatures. It was also shown at room temperature in different electrolytes such as tetrahydrofuran and a binary mixture of ethylene carbonate and propylene carbonate. Aqueous batteries have used calcium vanadate.
Cathodes.
Cathodes examined recently include calcium manganese oxide, calcium cobalt oxide and titanium disuphide, as well as hexacyanoferrates, or dual carrier batteries, as well as for aqueous calcium ion batteries. Theoretical work has been performed to ascertain the potential of cathodes from different crystal structures such as perovskite, spinel, other naturally occurring calcium compounds, metal selenides, as well as other calcium lanthanide oxide phases.
Investigated Battery Cells.
Several calcium metal batteries with different cathodes have thus far been examined: Ca//V2O5, Ca//Ca4Fe9O17, Ca//LiTiO2, Ca/Carbon-Fiber, Ca//TiS2, Ca//FePO4, Ca//Ca3Co2O6, Ca//PAQ, and Ca//S. C-rates range from 0.2 to >5 C.
Other battery architectures.
A primary Ca-S battery was examined. Ca-S batteries have also been examined using Li as a mediator to make it reversible. In addition, a calcium-air (Ca-O2) battery has also been examined.
Electrolytes.
Salts.
Salts explored thus far in liquid electrolytes include: calcium nitrate, calcium tetrafluoroborate, calcium borohydride, calcium bis(trifluoromethanesulfonimide), and calcium perchlorate. New salts based on Calcium hexafluoroisopropoxide (Ca(Ohfip)2·"x"THF) and tris-hexafluoroisopropoxy borate (B(Ohfip)3) have also been examined.
Liquid Electrolytes.
Several different electrolyte systems have been examined for calcium (ion) batteries. Electrolytes are still an area of investigation, where previous work has shown that many show low electrochemical stability. Redox reactions on calcium metal in several organic electrolytes was initially examined by Aurbach et al. Water as the electrolyte has been examined in a calcium ion battery. Alkyl carbonate electrolyte have also been examined. Theoretical studies have also been conducted on aprotic solvents showing they have favorable solvation/de-solvation properties. This has also been followed by experimental observations of salt solvation by different solvents. Ionic liquids have also been examined.
Polymer Electrolytes.
Polymer electrolytes have also been examined. One of the first samples of a polymer electrolyte was PVA/PVP complexed with CaCl2. Subsequent studies demonstrated polymer electrolytes made from polyethylene glycol diacrylate (PEDGA) and polytetrahydrofuran (PTHF) both with calcium nitrate (Ca(NO3)2), polyethylene oxide, and PEDGA based gel polymer electrolytes. 
Challenges.
Calcium batteries current show capacity fading and relatively lower energy densities that Li-metal batteries. The solid electrolyte interface (SEI) also shows slow migration of Ca2+ ions. Ca metal also undergoes dendritic growth at high current rates.