Reversible calcium alloying enables a practical room-temperature rechargeable calcium-ion battery with a high discharge voltage

Abstract

Calcium-ion batteries (CIBs) are attractive candidates for energy storage because Ca has low polarization and a reduction potential (-2.87 V versus standard hydrogen electrode, SHE) close to that of Li (-3.04 V versus SHE), promising a wide voltage window for a full battery. However, their development is limited by difficulties such as the lack of proper cathode/anode materials for reversible Ca intercalation/de-intercalation, low working voltages (<2 V), low cycling stability, and especially poor room-temperature performance. Here, we report a CIB that can work stably at room temperature in a new cell configuration using graphite as the cathode and tin foils as the anode as well as the current collector. This CIB operates on a highly reversible electrochemical reaction that combines hexafluorophosphate intercalation/de-intercalation at the cathode and a Ca-involved alloying/de-alloying reaction at the anode. An optimized CIB exhibits a working voltage of up to 4.45 V with capacity retention of 95% after 350 cycles.

Discovery of Calcium‐Metal Alloy Anodes for Reversible Ca‐Ion Batteries

“…[33] The calciation voltages corresponding to these phases declines from 0.99, 0.72, 0.59 to 0.53 V relative to Ca/Ca 2+ ( Figure 1B) while the volume expansion of the system per Ca accommodated increases from 7.4, 27.5, 27.6, 28.8 to 31.1 Å 3 ( Figure 1C). For instance, the experimental calciation of Sn ends at Ca 7 Sn 6 , [14] and the calculated calciation voltage of the Sn-Ca system ( Figure 1B) reaches a value of 0.53 V (relative to Ca/Ca 2+ ) for Ca 7 Sn 6 . For instance, the experimental calciation of Sn ends at Ca 7 Sn 6 , [14] and the calculated calciation voltage of the Sn-Ca system ( Figure 1B) reaches a value of 0.53 V (relative to Ca/Ca 2+ ) for Ca 7 Sn 6 .…”

“…[17] Alloying-type anodes, widely studied for Li-ion batteries, show great promise for reversible CIBs. Using a Sn anode and graphite cathode, Wang et al [14] recently reported a highvoltage (4.45 V) CIB cell with a reasonable capacity of 85 mAh g and a remarkable cyclability (95% capacity retention in 350 cycles). Besides Sn, several metals and metalloids including Zn, Al, Si, Li, and Na have also been investigated for the use of alloying-type CIBs anodes with largely disparate capacities achieved.…”

“…Using a Sn anode and graphite cathode, Wang et al [14] recently reported a highvoltage (4.45 V) CIB cell with a reasonable capacity of 85 mAh g and a remarkable cyclability (95% capacity retention in 350 cycles). [13,14,30,31] All of them except Na have been reported to mix with Ca in wide composition ranges, forming various intermetallic compounds. [13,14,30,31] All of them except Na have been reported to mix with Ca in wide composition ranges, forming various intermetallic compounds.…”

“…[13,14,30,31] All of them except Na have been reported to mix with Ca in wide composition ranges, forming various intermetallic compounds. [14] It is therefore important to examine the metal-calcium (M-Ca) reaction mechanisms during the electrochemical calciation and understand the variation of the calciation driving force as a function of Ca-ions accommodated. However, in experimental full-cell operations, the calciation of Sn ends at Ca 7 Sn 6 with a theoretical capacity of 527 mAh g −1 while the calciation of Zn and Li is even more truncated with very limited capacity observed.…”

“…[7,8] (2) Ca is the fifth most abundant element in the earth's crust with an extensive global resource distribution, in contrast to lithium. Material systems including Prussian blue compounds, [10,13,14] Chevrel phases, [15,16] spinels, [17][18][19] perovskites, [20] layered transition metal (TM) sulfides, [21] and iron phosphate, [22] were suggested to be effective Ca-ion electrodes with the spinels and perovskites attracting extra attention because of their predicted high voltage (>3.5 V) and large theoretical capacities (>240 mAh g −1 ) during discharge at room temperature. [9][10][11] The development of CIBs was originally pioneered by the study of Ca-ion electrochemical intercalations into layered transition metal oxides and sulfides.…”