Constructing the best symmetric full K-ion battery with the NASICON-type K3V2(PO4)3

Symmetric full-cells, which employ two identical electrodes as both the cathode and anode, attract great research attention, because it has high safety, facial fabrication and lower costs. Unfortunately, the practical utilization of full symmetric energy storage systems, especially the symmetric potassium ion batteries (KIBs), is hindered by the limited choice of the available electrode materials. In this work, a novel NASICON-type K3V2(PO4)3 is prepared and first employed for the symmetric KIBs. Through in-situ measurement, a highly lattice reversibility is found during the K+ insertion/extraction process. KV2(PO4)3 and K5V2(PO4)3 was generated after the depotassiation and potassiation process at about 4.0 V and below 1.0 V, respectively. The reversible capacity of the full symmetric KIBs is about 90 mAh g−1 between 0.01 and 3.0 V at 25 mA g−1, corresponding to an initial coulombic efficiency of 91.7% which is the highest one among all the previous reported symmetric energy storage systems (including the symmetric lithium/sodium ion batteries). 88.6% reversible capacity was maintained even after 500 cycling test. More importantly, a largest working potential at about 2.3 V was obtained in this work, benefiting the output energy of this symmetric energy storage system. The outstanding cycling stability, large working potential and the highest initial coulombic efficiency endow this work with promising advantages for the future development of the novel energy storage system.