Cyclic ethers are promising solvents for low-temperature electrolytes, but they still suffer from intrinsic poor antioxidant abilities. Until now, ether-based electrolytes have been rarely reported for high-voltage sodium-ion batteries (SIBs) operated under a low-temperature range. Herein, a novel ether-based electrolyte consisting of tetrahydrofuran as the main solvent is proposed and it could be utilized for a high-voltage Na2/3Mn2/3Ni1/3O2 (MN) cathode in a wide-temperature range from −40 to 25 °C. Meanwhile, a thin and robust inorganic component-rich cathode electrolyte interface layer is elaborately introduced on the MN cathode by this tailored electrolyte, resulting in excellent cycle life of MN cathode. Specifically, a capacity retention of 97.2% after 140 cycles could be delivered by MN at 0.3 C at room temperature (RT). Especially at an ultra-low temperature of −40 °C, the initial discharge capacity of MN could still approach 89.3% of that at RT, and the capacity retention is 94.1% at 0.2 C after 100 cycles. This work provides a new insight into the rational design of ether-based electrolytes for high-voltage and stable SIBs operated in a wide-temperature range.
KVPO4F is one of the most competitive cathode candidates for potassium‐ion batteries (KIBs) because of its high output voltage and energy density. Although the gravimetric energy density of KIBs is intensively discussed in literature, little attention is paid to the volumetric energy density. In view of this, pomegranate‐like carbon‐coated KVPO4F microspheres with a high volumetric energy density are designed in this work. The nano‐sized primary particles with carbon sheets in KVPO4F microspheres enable promis rate capability by enhancing the K+ diffusion kinetics, while the micro‐sized spheres guarantee the improvement of cycling stability. Owing to the dense hierarchical microspheres, the volumetric energy density of cells is greatly improved compared to bulk materials. This cathode delivers a reversible capacity of 101.5 mA h g−1 at 0.3 C with an average output voltage of 4.0 V and a capacity retention of 85.1% after 200 cycles. The KVPO4F@C microspheres have a compact density of 2.45 g cm−3 and further offer a high volumetric energy density up to 891.3 Wh L−1. The overcharge behavior of KVPO4F in the first three cycles is also revealed. The presented KVPO4F@C microspheres cathode provides a new sight for developing KIBs with large volumetric energy density.
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