Sodium insertion/extraction mechanisms in Chevrel phase cathodes for sodium ion batteries

Wang, Yejun (2026) Sodium insertion/extraction mechanisms in Chevrel phase cathodes for sodium ion batteries. PhD thesis, University of Glasgow.

Abstract

As issues related to the resource availability and cost of lithium-ion batteries become increasingly prominent, the development of emerging energy-storage systems based on elements with high crustal abundance and low cost has become a current research hotspot. Sodium-ion batteries are regarded as one of the most promising next-generation rechargeable battery systems owing to the abundance and low cost of sodium resources, as well as their similar working mechanism to lithium-ion batteries. However, developing sodium-ion battery cathode materials that combine high reversibility and long cycle life still faces significant challenges.

Chevrel phase (CPs) materials possess unique three-dimensional open ion-diffusion channels, favourable structural stability, and good electronic conductivity, and have demonstrated reversible ion-storage behaviour in several multivalent-ion systems, such as magnesium- and zinc-ion batteries. However, research on their application in sodium-ion batteries remains relatively limited, particularly with respect to the sodium-storage mechanism and the irreversible processes occurring during the initial cycle. Therefore, this thesis focuses on the binary Chevrel phase derivative Mo₆S₈ and its solid solution Mo₆S₈-xSex (x = 2-8), systematically investigating their sodium storage behaviour and structural evolution, and specifically exploring the underlying causes of irreversible capacity loss and ion trapping in the first cycle.

The preliminary study focused on Mo₆S₈ cathode materials, prepared via a high-temperature solid-state route followed by oxidative extraction. By optimising the electrode coating formulation and electrolyte system, a stable testing platform for Mo₆S₈-based sodium-ion batteries was established. Based on this, the sodium storage behaviour of Mo₆S₈ was investigated using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and in operando X-ray diffraction (in operando XRD). The results demonstrate that, owing to ion trapping within the crystal structure, this material undergoes an irreversible electrochemical cycling process, accompanied by substantial capacity loss.

To improve the diffusion kinetics of Na⁺ and reduce ion trapping behaviour, this thesis further employs Se to partially or completely substitute S in Mo₆S₈, successfully constructing the Mo₆S₈-ₓSeₓ solid solution system. In addition, this thesis explores the direct synthesis of pre-sodiated Chevrel phase materials, namely NaMo₆S₈ and NaMo6₆Se₈, with the aim of bypassing the initial Na+ insertion process and reducing irreversible capacity loss. Synthesis was performed using solid-phase ion exchange and Na(benzophenone)/THF chemical insertion methods, respectively; neither method successfully yielded a stable pre-sodiated phase.

Furthermore, this thesis investigates the electrochemical behaviour of Cu-based ternary Chevrel phase precursors in sodium-ion batteries, with the aim of elucidating the influence of structural Cu⁺ on the Na⁺ storage mechanism. The results indicate that the introduction of Cu does not improve the sodium storage performance of the materials. Instead, the cycling stability further decreased due to Cu⁺’s occupation of diffusion channels and possible structural reconstruction during cycling.

This thesis demonstrates that the initial irreversible capacity loss of Mo₆S₈ in sodium-ion batteries mainly originates from the initial insertion of Na⁺ and its subsequent retention within the structure. Se substitution cannot completely eliminate the Na⁺-trapping phenomenon, while direct pre-sodiation strategies are similarly difficult to realise. Therefore, Na⁺ trapping is more likely due to the inherent cation occupancy characteristics in the Chevrel phase structure, rather than being solely determined by lattice size or ion diffusion kinetics.

Future research could further focus on elucidating the atomic-scale Na⁺ trapping sites and migration mechanisms within Chevrel phases. Solid-state NMR, X-ray absorption spectroscopy, and DFT calculations could be combined to investigate Na⁺ site occupancy, local structural evolution, and diffusion behaviour. Simultaneously, strategies such as anion substitution and pre-sodiation could be explored to improve the reversibility of Na⁺ insertion/extraction, thereby further establishing the structure-property relationships linking structure, ion migration, and electrochemical performance.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Sodium-ion battery, Chevrel phase, Mo₆S₈, ion trapping, solid solution.
• Subjects: Q Science > QD Chemistry
• Colleges/Schools: College of Science and Engineering > School of Chemistry
• Supervisor's Name: Ganin, Dr. Alexey
• Date of Award: 2026
• Depositing User: Theses Team
• Unique ID: glathesis:2026-86201
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 27 Aug 2026 14:52
• Last Modified: 27 Aug 2026 14:53
• Thesis DOI: 10.5525/gla.thesis.86201
• URI: https://theses.gla.ac.uk/id/eprint/86201

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