Calcium Cobalt Hexacyanoferrate Cathodes for Rechargeable Divalent Ion Batteries

Abstract

Calcium cobalt hexacyanoferrate (CaCoHCF) was synthesized and tested as a cathode material for rechargeable batteries, using divalent cations (Mg2+, Ca2+, Ba2+). CaCoHCF demonstrated reversible specific capacity and coulombic efficiency (in parentheses) of 45.49 mAh/g (99.18%) for Mg2+, 55.04 mAh/g (99.2%) for Ca2+, and 44.09 mAh/g (99.42%) for Ba2+, at a current density of 25 mA/g. Of the three ions, Ca2+ resulted in the highest absolute specific capacity as well as high specific capacity utilization. The cathodes were also subjected to rate capability measurements using current densities of 50 mA/g (30 cycles) and 0.1 A/g (100 cycles). Upon addition of 2 mL water to the non-aqueous electrolyte, the fraction of theoretical specific capacity increased to 0.55 for Mg2+, 94.8% for Ca2+, and 95.53% forBa2+. This increase has been interpreted as the ability of the cathode material to intercalate and de-intercalate more ions due to the electrostatic shielding provided by water molecules between the host lattice and the guest cations. An empirical relationship between the cation size and specific capacity utilization is presented.Calcium cobalt hexacyanoferrate (CaCoHCF) was synthesized and tested as a cathode material for rechargeable batteries, using divalent cations (Mg2+, Ca2+, Ba2+). CaCoHCF demonstrated reversible specific capacity and coulombic efficiency (in parentheses) of 45.49 mAh/g (99.18%) for Mg2+, 55.04 mAh/g (99.2%) for Ca2+, and 44.09 mAh/g (99.42%) for Ba2+, at a current density of 25 mA/g. Of the three ions, Ca2+ resulted in highest absolute specific capacity as well as high specific capacity utilization. The cathodes were also subjected to rate capability measurements using current densities of 50 mA/g (30 cycles) and 0.1 A/g (100 cycles). Upon addition of 2 mL water to the non-aqueous electrolyte, the fraction of theoretical specific capacity increased to 0.55 for Mg2+, 94.8% for Ca2+, and 95.53% forBa2+. This increase has been interpreted as the ability of the cathode material to intercalate and de-intercalate more ions due to the electrostatic shielding provided by water molecules between the host lattice and the guest cations. An empirical relationship between the cation size and specific capacity utilization is presented.

A Ca‐Ion Electrochromic Battery via a Water‐in‐Salt Electrolyte

“…In addition, under a high current density of 0.4 mA cm −2 , discharge capacities of 74.7 and 90.2 mAh m −2 are achieved in the 1 and the 5 M electrolytes, respectively (Figure 5d). Notably, the average discharge potential of the InHCF film in the 5 M electrolyte is 0.63 V at 0.05 mA cm −2 , which is much higher than the values obtained in the reported hexacyanoferrates [15,[32][33][34][35]37,38] and other reported cathodes [38,39] (Figure 5e). Furthermore, the Fe 2+ /Fe 3+ redox couple in the InHCF film also gives rise to electrochromism accompanying with the energy storage process.…”

“…[ 21 ] Typically, the large interstitial sites and open channels of transition metal hexacyanoferrates can accommodate Ca 2+ insertion/extraction. [ 31 ] So far, NiHCF, [ 32 ] CoHCF, [ 33 ] CuHCF, [ 34 ] and BaHCF [ 35 ] have been studied as cathode candidates in aqueous Ca 2+ electrolytes in three‐electrode systems. However, their low average discharge potentials limit the energy densities of their assembled cells.…”

Understanding Ca Electrodeposition and Speciation Processes in Nonaqueous Electrolytes for Next-Generation Ca-Ion Batteries

“…Aurbach and co-workers tested the electrochemical behaviors of Ca electrodes in typical solvents such as acetonitrile (MeCN), tetrahydrofuran (THF), γ-butyrolactone, and propylene carbonate (PC) with common salts including Ca­(ClO 4 ) 2 and Ca­(BF 4 ) 2 and demonstrated that Ca deposition was impossible because of the formation of a passivation layer, thus blocking the Ca 2+ transport and eventually ceasing the deposition process . Other efforts regarding research on Ca batteries focus either on primary cells , and on aqueous electrolytes as well as Prussian blue analogues, ,− fluorinated sodium iron phosphate, and titanium sulfide as potential cathode materials in nonaqueous electrolytes. Ponrouch et al reported reversible Ca electrodeposition/stripping in a solution containing Ca­(BF) 4 in ethylene/PC at 100 °C.…”

Plating and stripping calcium in an organic electrolyte

“…Also, whereas Mg has a potential of 0.67 V vs. Li, Ca is much closer to Li at 0.17 V vs. Li, and Ca 2+ mobility in cathodes may be higher than Mg 2+ due to the lower charge density of Ca 2+ . As a result, interest in Ca based batteries is rising rapidly, although still at the research stage and with significant research challenges to be met 7,[20][21][22][23][24][25][26][27] . The problem generally encountered with multivalent metal anodes in aprotic-based electrolytes is their tendency to reduce the solvent producing passivating layers that inhibit plating and stripping of the metal 1-4, 7, 12, 13 .Early work on Ca metal anodes demonstrated that although calcium can be stripped it requires a relatively high overpotential.…”