The Influence of MXene Chemistry and Structure on Electrochemical Energy Storage Applications

  • Francis Moissinac

Student thesis: Phd

Abstract

The global decarbonisation strategy has stimulated an imminent synchronous shift to renewable energy, and electrification of the transport industry. This has created a surge in demand for electrochemical energy storage devices such as advanced battery systems, fuel cells and supercapacitors. Understandably, novel energy materials and batteries are being rapidly developed to address the growing need for these devices to possess greater energy densities, power densities and cycling stability. The research project presented will focus on the development of a field of two-dimensional materials called MXenes, and their applications for lithium-ion battery anodes, lithium-sulphur battery cathodes, and symmetric supercapacitor devices. Synthesis and characterization of MXenes beyond Ti3C2Tx specifically Mo2TiC2Tx and V2CTx (T = O, OH, F and/or Cl; 0 < x V2CTx > Mo2TiC2Tx, oxidative stability follows the general trend of Mo2TiC2Tx > Ti3C2Tx > V2CTx. As lithium-ion battery anode materials, the charge storage of Ti3C2Tx MXenes are based on pseudo redox mechanisms of intercalation and surface mediated reactions. On the other hand, Mo2TiC2Tx and V2CTx MXenes exhibits a conversion reaction as the predominant charge storage mechanism. Incidentally discovered V2C-VOx MXene heterostructures in lithium half-cells demonstrated notable specific capacitance of 532.4 F g-1 at 2 mV s-1 and capacity of 493.3 mA h g-1 at 50 mA g-1. In lithium-sulphur (Li-S) cells, composite V2C-S electrodes were capable of 1178.5 mA h g-1 at 0.05 C with 45.9% retained up to 1.0 C and negligible capacity decay across 200 cycles. Mo2TiC2Tx-S electrodes achieved 1145.8 mA h g-1 at 0.05 C, with only 19.7% retained at 1.0 C. Ti3C2Tx-S exhibited the best rate capability with 62.7% of the initial capacity retained up to 1.0 C. Further electrochemical impedance spectroscopy studies and post-testing SEM analysis provided greater insights on the MXene Li-S electrode dynamics. As porous ice-templated supercapacitor electrodes, the MXenes were evaluated in various aqueous electrolyte systems (3M H2SO4, 6M KOH, 1M LiCl). The highest specific capacitance was observed with the acidic electrolyte albeit with abnormally poor cycling stability. For the first time, these electrodes were also evaluated in water-in-salt electrolytes (10m and 21m LiTFSI) which allowed for an electrochemical stability window of 1.4 V and 1.6 V respectively. This resulted in a symmetric supercapacitor device with templated Ti3C2Tx electrodes to achieve state-of-the-art energy density of 18.5 W h kg-1 at a power density of 372.7 W kg-1 in 21m LiTFSI. Good cycling stability was also observed with 86.1% of initial specific capacitance being retained after 10,000 cycles at 5 A g-1. Cumulatively, the experimental results provided greater insight on how as-synthesized MXene structures and transition metal chemistry influenced the electrochemical performance in various application scopes.
Date of Award1 Aug 2024
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorPing Xiao (Supervisor), Rob Lindsay (Supervisor) & David Lewis (Supervisor)

Keywords

  • V2CTx
  • MXene synthesis
  • Mo2TiC2Tx
  • Energy materials
  • Nanomaterials
  • Ti3C2Tx
  • Functional materials
  • Supercapacitor
  • Li-ion Battery
  • Li-S Battery
  • MXene
  • Electrochemical Energy Storage
  • 2D Materials

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