Year

2022

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

With increasing demand for more functionalized, sophisticated and durable electronic devices and electric vehicles, rechargeable batteries are required to be more powerful, have longer life, more safety, less cost and be environment friendly. Although Li-ion batteries (LIBs) have been very successful as rechargeable batteries over the past decades, the theoretical energy density of LIBs is limited and the price of components (e.g. Co, Ni) of LIBs’ cathodes is rapidly increasing. Rechargeable Lithium-Sulphur batteries (LSBs) are believed to be one of the potential candidates to substitute LIBs as the next generation of commercial rechargeable batteries due to their exceptionally high energy density and low cost of sulphur. Both Li anodes and S cathodes have a high theoretical specific capacity (3860 and 1672 mAh g-1, respectively), and coupling these two electrodes can deliver an average cell voltage of 2.1 V. This battery system, thus, can theoretically produce a remarkable gravimetric and volumetric energy density (nearly 2600 Wh kg-1 and 2200 Wh L-1, respectively). However, there are some challenges in this system, including the insulating nature of the S and Li2S, the dissolution and shuttle effect of lithium polysulphides (LPSs) and dendritic growth of Li metal, all of which hindered the commercialization of LSBs. To address problems at the Li metal anode side, Li2S cathodes have been introduced to simultaneously supply Li and S, paired with Li-metal-free anodes (such as Graphite, Silicon, and Tin), which exhibit advantages in terms of safety, cost, and facile cell assembly procedure compared with earlier LSBs. However, the problem of the insulating nature of the S and Li2S and the shuttling of lithium polysulphides still need to be overcome.

Polyvinylidene difluoride (PVDF) binder is commonly used in LIBs and LSBs. The influence of PVDF on the operation of sulphur-based cathodes has carefully investigated in this thesis. In this thesis, Li2S cathodes with/without polyvinylidene difluoride (PVDF) binder were prepared by using traditional electrode preparation method and an in-situ high-temperature formation method. The influence of the chemical interaction between LPSs/Li2S and PVDF on both Li2S1−2 deposition and electrochemical performance was found by electron microscopy techniques, spectroscopy techniques, operando X-ray diffraction techniques, and theoretical calculations. The results show that, by avoiding PVDF binder, uniform deposition of Li2S1−2 across the whole cathode can be achieved and the rate performance of the Li2S electrode is greatly improved. The strong interaction between the binder and LPSs/Li2S in PVDF-containing LPSs/Li2S was found to be responsible for the different electrochemical performances. Moreover, PVDF weakens not only the Li dissociation during the activation of Li2S, but also the Li bonding capability of LPSs during discharge. Therefore, the Li2S cathodes without PVDF binder have fast reaction kinetics. The Li2S electrode without PVDF in a full cell can deliver an areal capacity of 12.7 mAh/cm2 and a specific capacity of 154 mAh/g with 16.2 mgLi2S/cm2 mass loading and 2.0 μL/mgLi2S electrolyte usage. The gravimetric and volumetric energy density of the full cell could reach 331.0 Wh/kg and 281.5 Wh/L, respectively. This work may shed light on practical high-energy S and Li2S cathode design.

FoR codes (2008)

0306 PHYSICAL CHEMISTRY (INCL. STRUCTURAL), 0904 CHEMICAL ENGINEERING, 0912 MATERIALS ENGINEERING, 0915 INTERDISCIPLINARY ENGINEERING

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.