A new high-voltage calcium intercalation host for ultra-stable and high-power calcium rechargeable batteries

Abstract

Rechargeable calcium batteries have attracted increasing attention as promising multivalent ion battery systems due to the high abundance of calcium. However, the development has been hampered by the lack of suitable cathodes to accommodate the large and divalent Ca2+ ions at a high redox potential with sufficiently fast ionic conduction. Herein, we report a new intercalation host which presents 500 cycles with a capacity retention of 90% and a remarkable power capability at ~3.2 V (vs. Ca/Ca2+) in a calcium battery. The cathode material derived from Na0.5VPO4.8F0.7 is demonstrated to reversibly accommodate a large amount of Ca2+ ions, forming a series of CaxNa0.5VPO4.8F0.7 (0 < x < 0.5) phases without any noticeable structural degradation. The robust framework enables one of the smallest volume changes (1.4%) and the lowest diffusion barriers for Ca2+ among the cathodes reported to date, offering the basis for the outstanding cycle life and power capability.

Sodium‐rich NASICON‐structured cathodes for boosting the energy density and lifespan of sodium‐free‐anode sodium metal batteries

“…The lattice parameter c representing the interlayer distance of Na3VP decreases, whereas the lattice parameter a corresponding to the basal dimension increases upon sodiation to 1.0 V (Figure S13B). The reduced c parameter can be attributed to the decreased electrostatic repulsion between O 2− layers as the Na + layer is continuously filled, while the increased a parameter is associated with the expansion of vanadium octahedra because of the electrochemical reduction of vanadium ions 54 . The volume expansion from pristine Na3VP to Na4VP is ~3.0%, while a volume shrinkage of 9.2% takes place upon the transition from pristine Na3VP to Na1VP, consistent with the DFT result.…”

“…The reduced c parameter can be attributed to the decreased electrostatic repulsion between O 2À layers as the Na + layer is continuously filled, while the increased a parameter is associated with the expansion of vanadium octahedra because of the electrochemical reduction of vanadium ions. 54 The volume expansion from pristine Na3VP to Na4VP is ~3.0%, while a volume shrinkage of 9.2% takes place upon the transition from pristine Na3VP to Na1VP, consistent with the DFT result. The small volume change during Na insertion and extraction can explain the excellent cyclic performance (see the discussion below).…”

Calcium-tin alloys as anodes for rechargeable non-aqueous calcium-ion batteries at room temperature

“…Concurrently, identifying a suitable cathode is still fundamentally restricted by high kinetic barriers for ion transport at the Ca anode-electrolyte interfaces and insufficient electrochemical stability of electrolytes 12 , 15 , 17 . Some potential cathode materials could only be examined with specially designed cell setups (for example, using activated carbon cloth as the counter electrode) and veritable full-cell tests with these cathodes have not been validated 18 – 20 . Very recently, calcium tetrakis(hexafluoroisopropyloxy)borate (Ca[B(hfip) 4 ] 2 ) based electrolyte has been demonstrated, which exhibits electrochemical properties in terms of high efficiency (~80%) for reversible Ca deposition, high oxidative stability (4 V), high ionic conductivity (~8 mS cm −1 ) at room-temperature (25 °C), and good chemical stability under ambient conditions 21 .…”

Searching Ternary Oxides and Chalcogenides as Positive Electrodes for Calcium Batteries

“…While a Ca-based chemistry is promising in theory, a number of unresolved challenges stand out. First of all, the lack of practical electrolytes that can simultaneously tolerate the reducing environment of a Ca-metal anode and the oxidizing nature of a high-voltage (and high-capacity) Ca-intercalation cathode. − Another significant challenge is the identification of fast Ca 2+ conductors that can act as reversible, high-voltage, and high-capacity intercalation cathodes, which is the focus of this work. − …”

“…These experimental results on olivine-FePO 4 are in qualitative agreement with Rong et al, who predicted a moderate Ca migration barrier (∼580 meV) in the charged state of olivine-FePO 4 . Recently, the group of Kang could reversibly extract Ca 2+ ions from Na 1.5 VPO 4.8 F 0.7 and reported a capacity of 87 mAh/g and 90% capacity retention over 500 cycles …”