Investigations into perovskite oxides as potential positive electrode materials for potassium-ion batteries

Abstract

Today, nations worldwide are transitioning from fossil fuels to renewable energy sources to achieve net-zero carbon emissions. Grid-scale energy storage technologies, such as potassium-ion batteries, have gained attention as cost-effective and sustainable alternatives to lithium-ion batteries. However, their commercial adoption remains limited due to the lack of suitable positive electrode materials.

This thesis explores the synthesis, crystal structure, morphology and structural stability of four double perovskite materials, KLaMnWO₆, KLaFeNbO₆ (Ar), KLaFeNbO₆ (reduced), and KLaFeMoO₆-K₂MoO₄, as positive electrode materials for KIB. Their electrochemical performance, including redox reaction, specific capacity, and cycling stability, were also investigated.

KLaMnWO₆ was first investigated due to its previously reported synthesis method by G. King. Structural analysis revealed a potassium deficiency in the as-synthesised KLaMnWO₆, which could not be fully avoided after optimising synthesis conditions. Electrochemical testing demonstrated that KLaMnWO₆ had a low reversible specific capacity of 20 mA h/g after 30 cycles. In-situ/operando X-ray absorption spectroscopy confirmed the Mn²⁺/Mn³⁺ served as the redox couple.

KLaFeNbO₆ was synthesised for the first time in this study under either a pure Ar atmosphere or a reducing atmosphere of 5% H₂ in Ar. The crystal structure of KLaFeNbO₆, with a space group of Pnma, was determined by conducting Rietveld refinements against the neutron powder diffraction data. After particle size reduction and carbon-coating, KLaFeNbO₆ (Ar) presented a reversible specific capacity of 24 mA h/g after 30 cycles, while KLaFeNbO₆ (reduced) showed inferior electrochemical performance.

KLaFeMoO₆-K₂MoO₄ demonstrated the best electrochemical performance among all studied materials. The crystallographic analysis reveals that the double perovskite phase adopts a monoclinic P2₁/n structure, thereby refuting the reported cubic Fm-3m structure by
G. Huo, et al. Upon optimisation through particle size reduction and carbon coating, the KLaFeMoO₆-K₂MoO₄ composite showed a reversible specific capacity of 46.76 mA h/g over 30 cycles, closely approaching its theoretical capacity.

Date of Award	3 Jul 2025
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Julia Payne (Supervisor)