Reversible electrochemical potassium deintercalation from >4 V positive electrode material K6(VO)2(V2O3)2(PO4)4(P2O7)

Abstract

Reversible electrochemical deintercalation of potassium from the K6(VO)2(V2O3)2(PO4)4(P2O7) phosphate-pyrophosphate-oxide of potassium and vanadium (IV,V) obtained through a ceramic route has been demonstrated. The material has been tested as a high voltage positive electrode (cathode) for K-ion batteries, demonstrating 53% of the theoretical capacity of 88 mAh/g at the current density C/50 in the 1.8–4.4 V vs K/K+ potential range. The crystal structures of K6(VO)2(V2O3)2(PO4)4(P2O7) in the pristine, charged and discharged states were refined by Rietveld method from powder X-ray diffraction data using a structure model with stochastic orientation of the pyrophosphate groups (space group Pnma, a = 6.9891(1)Å, b = 13.3735(2)Å, c = 14.2495(2)Å in the pristine state). In spite of deintercalation of bulky K+ cations, the cell volume change upon charge amounts to 0.9% rendering K6(VO)2(V2O3)2(PO4)4(P2O7) a “low strain” cathode. The potassium (de)intercalation occurs equally from two distinct crystallographic sites through a single ~4.2 V sloped plateau featuring solid-solution-like behavior, which was demonstrated by operando powder X-ray diffraction. The evaluation of diffusion barriers from the first principles has been conducted by the nudged elastic band method for two ordered arrangements of the pyrophosphate groups derived from the refined disordered structure. The diffusion barriers along crystallographic direction b for both investigated ordered variants were found to be below 0.3 eV.