Hierarchically Structured Potassium-Vanadium-Phosphate/C Composites As Possible High-Voltage Cathodic Materials for Potassium-Ion-Batteries

Abstract

Potassium-ion-batteries (PIBs) share technological similarities with Lithium- and Sodium-ion batteries (LIBs/SIBs) and are based on the inter- and deintercalation of K ions between two host materials.[1] The higher atomic weight and larger ionic radius of potassium ions results in lower gravimetric and volumetric capacities compared to LIBs.[2] But they offer similar advantages as SIBs (i.e. use of aluminium as current collector on both sides, evenly distributed potassium reserves) as well as graphite could be used as anode.[2,3] Also, they are expected to offer higher cell voltages due to the low standard potential of K/K+ vs. SHE of -2.93V.[3] To unlock the full potential of PIBs, cathode materials with high average potentials above 4 V need to be developed, which would significantly improve the energy density.[4] Polyanionic compounds, especially different Vanadium-Phosphates, are prepared and investigated for this purpose. Their thermal stability and diverse structural chemistry are advantageous for high energy PIBs. [2,5] One of the often-used synthesis methods for polyanionic compounds is the solid-state reaction. [4] In my work this well-known synthesis was coupled with a spray-drying process and a subsequently sintering process for K3V2(PO4)3/C composites. This created spherical secondary particles with a size in the micrometer range and an open micro-porosity. (see attached image)Through the addition of different amounts of organic sugars as a carbon source during the synthesis or sintering step an additional carbon coating of the primary and secondary particles was achieved, as well as the morphology of the secondary particles could be influenced. The influences of the different carbon contents during synthesis and sintering were investigated by thoroughly characterizing the morphology and crystal structure of the secondary particle with the help of SEM, BET, particle size distribution, XRD and porosity measurements. First electrochemical evaluation of the synthesized material was done by galvanostatic cycling the KVP/C composites against potassium.