Multivalent-ion batteries are gaining attention as energy storage devices. The major limitation in multivalent metal anodes is the reduction of aprotic-based electrolytes, resulting in the formation of passivating layers that inhibit plating and stripping of the metal. In Ca-ion battery systems using propylene carbonate, butyrolactone and acetonitrile based electrolytes, Ca(OH)2, CaCO3 and calcium alkoxides are formed during electrochemical reduction while the decomposition products passivate the electrode surface and inhibit further plating of Ca. Plating and stripping of Ca has been shown to be feasible in electrolyte solutions containing Ca(BF4)2 in ethylene carbonate and propylene carbonate mixtures only at elevated temperatures of the order of 75 to 100 °C, with small capacities of the order of 0.165 mAh/cm2, accompanied by the formation of CaF2. Since Ca(BH4)2 is readily soluble in THF at room temperature, researchers at Departments of Materials and Chemistry, University of Oxford, UK have investigated plating and stripping of Ca in this electrolyte to obtain capacities of 1 mAh/cm2 at a rate of 1 mA/cm2, with low polarization (~100 mV) and in excess of 50 cycles.
Electrochemistry of plating/stripping of Ca in 1.5 M Ca(BH4)2 in THF, evaluated by cyclic voltammetry (CV) using a three-electrode assembly indicates plating of Ca during reduction and its stripping on subsequent oxidation with a Coulombic efficiency of 94.8% (Fig. 1). The X-ray diffraction pattern and FT-IR spectrum confirms that the dominant product after deposition is Ca metal, along with a small amount of CaH2. The total amount of Ca deposited (sum of Ca as metal as well as CaH2) corresponds to 98% of the charge passed (1.828 µmol for 0.1 mAh of charge). For a charge/mass ratio of 2.04 e–/Ca, majority of charge passed is utilized for the deposition of Ca metal while only a small amount of the deposited Ca metal (5.0 mole%) reacts with the electrolyte to form CaH2.
The morphology acquired at the cross section during plating and stripping of Ca at 1st, 5th and 10th cycles at 1 mAh/cm2 is shown in Fig. 2. Deposition of a thick film of Ca is evident after the first plating (Fig. 2(a)), which upon stripping leaves some CaH2 as residue on the surface of the electrode (Fig. 2(b)). The morphology acquired after 5th and 10th cycles of plating and stripping of Ca indicates the presence of CaH2 at the end of each stripping process (Figs. 2(c)-2(f)). Time-of-flight secondary ion mass spectroscopy and gas chromatography mass spectrometry results confirm the formation of CaH2 following the reaction of freshly deposited Ca with THF, which is distributed throughout the film. Allowing the Ca deposit in contact with the electrolyte to rest at its open circuit potential enables the growth of CaH2 until a protective film of CaH2 with sufficient thickness to suppress further reaction between Ca and the electrolyte.
Fig. 1 (a) CV of plating/stripping of Ca in 1.5 M Ca(BH4)2/THF using Au, Ca and Pt as the working, reference and counter electrodes, respectively at a scan rate 25 mV/s. (Inset: charge passed on plating/stripping)
Fig. 2 Cross-sectional morphology at the end of (a, c) 1st, (b, d) 5th and (c, e) 10th cycle of (a, c, e) plating and (b, d, f) stripping of Ca in 1.5 M Ca(BH4)2/THF on Au electrode.
The cycling efficiency of a metal anode for rechargeable batteries needs to be as high as 99.98% per cycle. To achieve this level of efficiency in Ca-ion battery system, the SEI layer should be electronically insulating while the Ca2+ ions are conducting. The formation of CaH2 acts as a passivating layer at open circuit and mitigates further reaction of Ca with the electrolyte. However, the SEI layer fails to acquire the required properties, thus limiting the cycling efficiency to 96%, which is not sufficient for practical applications. The proposed Ca(BH4)2/THF electrolyte for the plating and stripping of Ca though not solves all the problems of the Ca anode for rechargeable batteries, it opens up an avenue to make further progress towards achieving Ca-ion batteries with better cycling efficiency.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Da Wang et al., Plating and stripping calcium in an organic electrolyte, Nature Materials, doi:10.1038/nmat5036
Recent Advancements in Science 