The recent discovery of reversible plating and alloying of calcium
has invoked considerable interest in calcium-based rechargeable batteries
toward overcoming the limitations of conventional Li-ion batteries.
However, only a few cathode materials have been tested thus far, and
these exhibit low energy-storage capability and poor cyclability.
Herein, the highly reversible Ca-intercalation capability of NASICON-type
NaV2(PO4)3 makes it a potential cathode
material for nonaqueous Ca-ion batteries, with high capacity and voltage
and good cyclability (90 mA h g–1 and ∼3.4
V at 11.7 mA g–1 and 75 °C; 70 mA h g–1 and ∼3.2 V at 5.85 mA g–1 and 25 °C).
Although this work shows only the capability of the cathode, not a
full-cell performance, it does demonstrate experimentally that a poly-oxyanionic
material can provide an outstanding host structure for Ca diffusion
at room temperature with high energy-storage capability.
VOPO4⋅2 H2O is demonstrated as a cathode material for potassium‐ion batteries in 0.6 m KPF6 in ethylene carbonate/diethyl carbonate, and its distinct exchange reaction mechanism between potassium and crystal water is reported. In an anhydrous electrolyte, the cathode shows an initial capacity of approximately 90 mAh g−1, with poor capacity retention (32 % after 50 cycles). In contrast, the capacity retention dramatically improved (86 % after 100 cycles) in a wet electrolyte containing 0.1 m of additive water. VOPO4⋅2 H2O contains two types of water (structural and crystal). Upon discharge, potassium ions are intercalated whereas the crystal water is simultaneously de‐intercalated from the structure. Upon charging, a completely reverse reaction takes place in the wet electrolyte, resulting in high stability of the host structure and excellent cyclability. However, in the anhydrous electrolyte, some portion of the extracted crystal water molecules cannot be reinserted into the host structure because they are distributed over the anhydrous electrolyte. Keeping some concentration of water in the electrolyte turns out to be was the key to achieving such high reversibility. The potassium ions (90 %) and proton or hydronium ions (10 %) seem to be co‐intercalated in the wet electrolyte. This work provides a general insight into the intercalation mechanism of crystal‐water‐containing host materials.
Rhombohedral
potassium–zinc hexacyanoferrate K1.88Zn2.88[Fe(CN)6]2(H2O)5 (KZnHCF)
synthesized using a precipitation method is demonstrated as a high-voltage
cathode material for potassium-ion batteries (PIBs), exhibiting an
initial discharge capacity of 55.6 mAh g–1 with
a discharge voltage of 3.9 V versus K/K+ and a capacity
retention of ∼95% after 100 cycles in a nonaqueous electrolyte.
All K ions are extracted from the structure upon the initial charge
process. However, only 1.61 out of 1.88 K ions per formula unit are
inserted back into the structure upon discharge, and it becomes the
reversible ion of the second cycle onward. Despite the large ionic
size of K, the material exhibits a lattice-volume change (∼3%)
during a cycle, which is exceptionally small among the cathode materials
for PIBs. The distinct feature of the material seems to come from
the unique porous framework structure built by ZnN4 and
FeC6 polyhedra linked via the CN bond and a Zn/Fe
atomic ratio of 3/2, resulting in high structural stability and cycle
performance.
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