The redox chemistry of sulfur in cathode materials for rechargeable Li-ion batteries

Thesis

There is an urgent need for rechargeable batteries with higher energy density. Lithium-rich cathodes represent one of the few routes available to increase the energy density of Li-ion batteries by invoking redox chemistry of both the transition metal (TM) and anionic redox in the lattice. However, in Li-rich oxide cathodes, the oxidation of O2– is accompanied by the formation of O2 trapped in nano-sized pores within the cathode and irreversible O2 loss from the particle surfaces, leading to voltage hysteresis, voltage fade and low first-cycle coulombic efficiency. Moving from Li-rich oxides to Li-rich sulfides using S-redox offers a potential way to mitigate most of these limitations. In this thesis, model Li-rich sulfides have been studied to understand the fundamental process of S-redox.

Firstly, Li1.2Ti0.8S2, with S-redox, is investigated as a direct analogue of layered Li-rich oxides. The formation of persulfide S22− species during charge is identified. Dimerization of oxidized S is accompanied by rearrangement of the TM layer to form nanovoids, as in Li-rich oxides, accounting for the presence of voltage hysteresis. In Li-rich sulfides, the S22− dimers remain coordinated to TM atoms, offering insights into the differing electrochemical properties of O-redox and S-redox cathodes.

Secondly, the electrochemical performance of Li1.2Ti0.8S2 was examined in all-solid-state batteries (ASSBs). At room temperature, ASSBs using Li1.2Ti0.8S2 deliver an areal capacity of ~ 2.8 mAh cm−2 with > 99% capacity retention over 200 cycles under a low stack pressure of 2 MPa. This excellent performance highlights the promise of Li-rich sulfides as cathodes for ASSBs, beyond conventional high-voltage oxides.

Finally, S-redox materials Li1.2Mn0.2Ti0.6S2 and Li1.2Ti0.8S2 are studied in parallel, and the critical role of the TM-S covalency is revealed. Stronger Mn 3d-S 3p hybridization significantly enhances electronic conductivity during S oxidation in Li1.2Mn0.2Ti0.6S2, while it does not suppress S22− dimer formation.

Authors: Qi, R

• DOI: 10.5287/ora-dp9ymqxmj
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: s-redox; Li-rich cathodes; anionic redox; persulfide dimers
• Subjects: Oxidation-reduction reaction; Solid state chemistry; Chemistry, Inorganic; Lithium ion batteries; Solid state batteries