Phosphate-based cathode materials for rechargeable batteries

Publication Type:
Thesis
Issue Date:
2018
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The design of electrode materials depends critically on understanding the underlying electrochemical processes. Material composition, morphology, structure, and preparation method affect and can alter electrochemical performance of electrochemically active materials. In this research project, a series of phosphate-based polyanionic electrode materials have been fabricated and their electrochemical properties for the use in lithium-ion and sodium-ion batteries are evaluated. We successfully prepared carbon-coated LiFePO₄ cathode materials by industrial ball milling and a solid-state reaction with Li₂CO₃, NH₄H₂PO₄, and FeC₂O₄·2H₂O as starting materials. Soluble starch as the primary carbon source was investigated for its capability of generating a highly graphitic carbon coating, whilst sufficiently controlling the crystal growth of LiFePO₄. XRD analysis, Raman spectroscopy, and electrochemical testing revealed the significant impact of the amount of starch added to the pre-sintered precursor on phase purity, carbon quality, and electrochemical performance of the final LiFePO₄/C composite. The optimum soluble starch content to achieve a highly sp²-coordinated carbon coating is 10 wt%, which enabled our LiFePO₄/C composite to achieve competitive reversible capacities as well as improved rate performance The spray method is well-trusted in practical applications, such as food manufacturing, fertilizers, oxide ceramics, and pharmaceuticals. The ability to produce uniformly spherical particle clusters ranging from nano- to micrometre in size is one of the main advantages of this method, which is immensely important for large scale production of electrochemically active materials for the energy storage market. In this report, we systematically evaluated spray-drying conditions and equipment settings in regards to electrochemical performance of carbon coated LiFePO₄ cathode materials. In an optimisation trial, the most suitable process conditions for the precursor materials and spray-dryer model used to prepare pure and practical LiFePO₄ cathode materials were identified. The impact of different organic additives on the resulting particle morphology of the final product was also investigated. It was found that the addition of polyvinyl alcohol (PVA) generates particle clusters that provide a high tap density product without sacrificing electrochemical performance. The LiFePO₄ cathode material prepared with the addition of PVA achieved remarkable rate performance results and could maintain a capacity of 113.95 mA h g⁻¹ at 10C. Lithium-ion batteries (LIBs) are widely implemented to power portable electronic devices and are increasingly in demand for large-scale applications. One of the major obstacles for this technology is still the low cost-efficiency of its electrochemical active materials and production processes. In this work, we present a novel impregnation–carbothermal reduction method to generate a LiFePO₄–carbon paper hybrid electrode, which does not require a metallic current collector, polymeric binder or conducting additives to function as a cathode material in a LIB system. A shell of LiFePO₄ crystals was grown 𝘪𝘯 𝘴𝘪𝘵𝘶 in situ on carbon fibres during the carbonization of microcrystalline cellulose. The LiFePO₄–carbon paper electrode achieved an initial reversible areal capacity of 197 μA h cm⁻² increasing to 222 μA h cm⁻² after 500 cycles at a current density of 0.1 mA cm⁻². The hybrid electrode also demonstrated a superior cycling performance for up to 1000 cycles. The free-standing electrode could be potentially applied for flexible lithium-ion batteries. Sodium-ion batteries (NIBs) are an emerging technology, which can meet increasing demands for large-scale energy storage. One of the most promising cathode material candidates for sodium-ion batteries is Na₃V₂(PO₄)₃ due to its high capacity, thermal stability, and sodium (Na) superionic conductor 3D (NASICON)-type framework. In this work, the authors have significantly improved electrochemical performance and cycling stability of Na₃V₂(PO₄)₃ by introducing a 3D interconnected conductive network in the form of carbon fibre derived from ordinary paper towel. The free-standing Na₃V₂(PO₄)₃-carbon paper (Na₃V₂(PO₄)₃@CP) hybrid electrodes do not require a metallic current collector, polymeric binder, or conducting additives to function as a cathode material in an NIB system. The Na₃V₂(PO₄)₃@CP cathode demonstrates extraordinary long-term cycling stability for 30 000 deep charge– discharge cycles at a current density of 2.5 mA cm⁻². Such outstanding cycling stability can meet the stringent requirements for renewable energy storage.
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