Pomegranate-Like KVPO 4 F@C Microspheres as High-Volumetric-Energy-Density Cathode for Potassium-Ion Batteries.

KVPO 4 F 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 KVPO 4 F microspheres with a high volumetric energy density are designed in this work. The nano-sized primary particles with carbon sheets in KVPO 4 F 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 KVPO 4 F@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 KVPO 4 F in the first three cycles is also revealed. The presented KVPO 4 F@C microspheres cathode provides a new sight for developing KIBs with large volumetric energy density.