Chen, Guanxu; (2023) Design of transition metal oxide anode for fast-charging energy storage devices. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

As the demand for electrical energy storage (EES) has increased worldwide, rechargeable lithium-ion batteries (LIBs) have been extensively investigated and have in many respects dominated the energy storage market in the past few decades. However, the shortage and uneven distribution of lithium and other key LIB materials is widely considered to present a bottleneck to future LIB development. Sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) have been considered as promising alternatives to LIBs in recent years because they utilise similar mechanisms to LIBs and because of the extensive global sodium and potassium resources. Nevertheless, major challenges still must be overcome to enable the commercialisation of SIBs and PIBs, including inferior cycling stability, low energy density and low power density. To enhance the electrochemical performance of energy storage devices for the post-LIB era, research into the introduction of new, and optimisation of existing, electrode materials has become a major international focus. Nb-based transition metal oxides (TMOs) have been reported as promising anode materials for fast-charging energy storage devices that rely on lithium-ions, hence to improve the rate performance of next-generation technologies I studied the application of Nb-based TMOs as anode materials for SIBs and PIBs. In the first results chapter, a new composite anode consisting of pseudohexagonal Nb2O5 (TT-Nb2O5) nanoparticles strongly anchored to carbon nanotubes (CNT) via a glucose-derived carbon framework (Nb2O5/g-CNT) is synthesised and assessed for use as the anode in SIBs. The composite is shown to offer high specific capacity (135 mAh g-1 at 0.2 A g-1 after 300 cycles), good rate capability (~53 mAh g-1 at 5 A g-1) high-capacity retention rate (97% capacity retention after cycling at high rates). This is attributed to high conductivity and flexibility offered by the designed carbon framework, the superior capacity of the TT-Nb2O5 and the linkage between the two provided by the bonding glucose derived carbon. This work therefore both represents the first application of TT-Nb2O5 in SIBs and offers a scalable route for the application of metal oxides in future high-performance SIB systems. In the second results chapter, it is demonstrated that pseudohexagonal Nb2O5 (TT-Nb2O5) can also offer high specific capacities, good lifetimes and excellent rate performance in PIBs, when it is composited with a highly-conductive carbon framework; this is the first reported investigation of TT-Nb2O5 for PIBs. When strongly tethered to multi-walled carbon nanotubes, via a one-step hydrothermal method, the Nb2O5 forms a highly conductive and porous needle-like composite. This material is shown to exhibit a maximum specific capacity of 179 mAh g-1 at 0.2 A g-1 (~ 1C) and 72 mAh g-1 at high rate of 5 A g-1 (~25C). This work therefore offers a route for the scalable production of a viable PIB anode material and hence improves the feasibility of fast-charging PIBs for future applications. In the final results chapter, MnNb2O6 was anchored on reduced graphene oxide (MNO/rGO) in the first study of Mn-Nb-O compounds as an anode for SIBs. The MNO in this work exhibited a high surface area and a six-pointed star morphology, which facilitated the specific capacity and cyclability of the composite. The MNO/rGO composites showed good rate capability (reversible capacity retention rate of 97%) and outstanding stability during long charge/discharge cycles, offering a specific capacity of ~150 mAh g-1 at current density of 0.2 A g-1 after 450 cycles. Hence, although the overall capacity presented was not as high as some others presented in literature, this work provides a model for onward research regarding high stability anodes for SIBs. In summary, the novel anode materials presented in this thesis provide excellent electrochemical performance, particularly in terms of high-rate performance and long-cycle stability. This thesis will provide guidance for future research on the investigation of high-performance SIB and PIB anodes in the field of materials science and engineering.

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