Abstract
Calcium-ion batteries represent a promising alternative to the current lithium-ion batteries. Nevertheless, calcium-ion intercalating materials in nonaqueous electrolytes are scarce, probably due to the difficulties in finding suitable host materials. Considering that research into calcium-ion batteries is in its infancy, discovering and characterizing new host materials would be critical to further development. Here, we demonstrate FeV3O9∙1.2H2O as a high-performance calcium-ion battery cathode material that delivers a reversible discharge capacity of 303 mAh g−1 with a good cycling stability and an average discharge voltage of ~2.6 V (vs. Ca/Ca2+). The material was synthesized via a facile co-precipitation method. Its reversible capacity is the highest among calcium-ion battery materials, and it is the first example of a material with a capacity much larger than that of conventional lithium-ion battery cathode materials. Bulk intercalation of calcium into the host lattice contributed predominantly to the total capacity at a lower rate, but became comparable to that due to surface adsorption at a higher rate. This stimulating discovery will lead to the development of new strategies for obtaining high energy density calcium-ion batteries.
Highlights
Lithium-ion batteries (LIBs) have been an essential energy storage device for mobile applications in the last three decades and have received particular attention for their use in recently commercialized electric vehicles, primarily because of their outstanding energy densities and power [1]
In the Fourier transform infrared (FTIR) transmittance spectrum (Figure 1b), the bond at 1004 cm−1 corresponded to V=O stretching, while the absorption bands at 1621, 3191, and 3417 cm−1 indicated the existence of adsorbed water and crystal water in the host structure
Considering that natural kazakhstanite (FeV3O9·2.6H2O or Fe5V15O39(OH)9·9H2O) [30] has more crystal water than we obtained in the lab, the amount of crystal water can be varied depending on the synthetic conditions
Summary
Lithium-ion batteries (LIBs) have been an essential energy storage device for mobile applications in the last three decades and have received particular attention for their use in recently commercialized electric vehicles, primarily because of their outstanding energy densities and power [1]. The calcium-insertion materials in nonaqueous electrolytes reported so far include Prussian-blue analogues [10,11,12,13,14], CaCo2O4 [15], NaFePO4F [16], NH4V4O10 [17], α-MoO3 [18], CaxMoO3 [19], TiS2 [20], α-V2O5 [21], CaMnO3 [22], Mg0.25V2O5·H2O [23], VOPO4·2H2O [24], NaV2(PO4)3 [25,26], and Ca0.13MoO3·(H2O)0.41 [27] Most of these materials exhibit low Coulombic efficiency, capacity, or cyclability in dried nonaqueous electrolytes. We report that layered iron vanadate FeV3O9·1.2H2O is a potential cathode material for nonaqueous CIBs. Calcium ions could be reversibly intercalated into the material with a high Coulombic efficiency at room temperature. The trace amount of water in the electrolyte was analyzed by the Karl Fischer titrator (831 KF coulometer, Metrohm, Herisau, Switzerland)
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