Phosphate Based Cathodes and Reduced Graphene Oxide Composite Anodes for Energy Storage Applications

This chapter highlights the importance and principle of Lithium ion batteries (LIBs) along with a concise literature survey highlighting the research trend on the different components of LIBs namely, cathode, anode and electrolyte. The aims of the present study and the thesis outline are given at the end of the chapter.

The different experimental procedures used in this study for the synthesis of various electrode materials and their principle involved are discussed in this chapter. In addition, methodology and principles of different techniques used in the physicochemical and electrochemical characterization of the materials are also discussed in this chapter.

This chapter deals with the synthesis and ab initio structure determination of a novel lithium containing Metal Organophosphate Open Framework (MOPOF) material, [Li₂(VO)₂(HPO₄)₂(C₂O₄)]·6H₂O. It was synthesized by hydrothermal method at 120 °C and the crystal structure was solved and refined from its powder X-ray diffraction data. The anhydrous phase, [Li₂(VO)₂(HPO₄)₂(C₂O₄)] obtained by dehydration of the hydrated phase at 200 °C was studied as a novel cathode material for Lithium ion batteries. The presence of extractable Li⁺ ions in the inter-layer space together with the feasibility of V⁴⁺/V⁵⁺ redox couple and a good theoretical capacity of 125 mAh g⁻¹ make this compound suitable as a cathode material. Electrochemical properties of the material was investigated using cyclic voltammetry, galvanostatic charge-discharge cycling, electrochemical impedance spectroscopy (EIS) and ex situ XRD studies. The material exhibits reversible lithium insertion at ~4 V with a reversible capacity of 80 mAh g⁻¹ at 0.1 C current rate.

This chapter deals with the synthesis of a MOPOF material, [K₂(VO)₂(HPO₄)₂(C₂O₄)] and its rGO composites for application as cathode materials for Lithium ion batteries. Synthesis of a hydrated phase, [K₂(VO)₂(HPO₄)₂(C₂O₄)]⋅4.5H₂O was achieved at room temperature by a simple magnetic stirring. The rGO composites of the material was prepared by carrying out the reaction in the presence of graphene oxide. During the synthesis, tartaric acid was used as the organic ligand which was found to undergo in situ oxidation to oxalate resulting in formation of oxalatophosphate framework. The anhydrous phase, [K₂(VO)₂(HPO₄)₂(C₂O₄)] and the composite rGO/[K₂(VO)₂(HPO₄)₂(C₂O₄)] were obtained by dehydration of the respective hydrated phases at 120 ℃. These phases were investigated as 4 V cathode for Lithium ion batteries. The pristine compound undergoes highly reversible lithium storage with good capacity. However, there was slight capacity fading. The rGO composites (4 and 8 wt% of rGO) exhibit enhanced lithium cycling with excellent capacity retention.

This chapter deals with the synthesis of monoclinic Li₃V₂(PO₄)₃ using the MOPOF material, [Li₂(VO)₂(HPO₄)₂(C₂O₄)]·6H₂O as a single source precursor. Thermal decomposition of the oxalatophosphate precursor at 800 °C in argon atmosphere resulted in a Li₃V₂(PO₄)₃–V₂O₃ composite. Carbon coating of the particles were carried out by addition of sucrose to the precursor prior to thermal decomposition, to achieve better electronic conductivity. The carbon coated sample exhibits a large BET surface area of 75 m² g⁻¹ while the pristine sample has a lesser surface area of 5.4 m² g⁻¹. In the voltage range of 2.5–4.3 V, carbon coated sample shows a reversible capacity of 132 mAh g⁻¹ at 0.1 C current rate with good capacity retention. In addition, it shows good rate capability with 56 mAh g⁻¹ obtained at a high current rate of 20 C. In comparison, the pristine sample shows inferior electrochemical performance with capacity fading. The carbon coated sample was also investigated as an anode material. It exhibits a good reversible capacity of 125 mAh g⁻¹ when cycled in the voltage range of 1–3 V at a current density of 50 mA g⁻¹ (0.4 C).

In this chapter, synthesis of a metastable phosphate material, α _I-LiVOPO₄ via dehydration of a hydrothermally prepared precursor, LiVOPO₄·2H₂O have been presented. Single crystal X-ray diffraction analysis ascertained the formation of LiVOPO₄·2H₂O in the orthorhombic space group, Cmca with the lattice parameters; a = 8.9454(7) Å, b = 9.0406(7) Å and c = 12.7373(10) Å. We have also investigated the crystal structure transformation of orthorhombic LiVOPO₄·2H₂O to tetragonal α _I-LiVOPO₄ accompanying the solid-state dehydration. A probable mechanism for the phase transformation was proposed with the help of crystal structures of the two phases. Electrochemical studies on this rarely studied tetragonal phase was carried out using cyclic voltammetry and galvanostatic cycling studies which revealed its excellent lithium cycling at 4 V and a capacity of 103 mAh g⁻¹ was obtained at a current rate of 0.1 C.

In this chapter, the synthesis of Sb₂S₃ and rGO/Sb₂S₃ composites for Lithium ion battery applications have been reported. The stibnite phase was obtained employing single source precursor approach. A metal complex, Sb(SCOPh)₃ was prepared at room temperature by the reaction of sodium thiobenzoate with SbCl₃. Annealing of the precursor at 400 ℃ in Ar leads to the stibnite phase of Sb₂S₃. To improve the battery performance, rGO composite of Sb₂S₃ were also prepared by annealing the rGO/Sb(SCOPh)₃ mixtures obtained by different techniques. Electrochemical performance of the prepared composites were investigated using galvanostatic cycling, cyclic voltammetry and ex situ XRD studies. The pristine sample shows very poor lithium cycling while the rGO composites demonstrate better lithium storage. The rGO/Sb₂S₃ obtained by microwave irradiation of rGO/Sb(SCOPh)₃ followed by annealing at 400 ℃ shows good capacity retention.

This chapter deals with the investigation of lithium storage in rGO/Fe₃O₄ nanocomposites prepared by a simple precipitation method followed by annealing at different temperatures and environments such as 80 °C in air, 600 °C in Ar, 700 °C in Ar and 700 °C in Ar–H₂. The sample obtained at 80 °C exhibit a high surface area of 30 m² g⁻¹. Electrochemcial properties of the different rGO wrapped magnetite nanoparticles were investigated by cyclic voltammetry and galvanostatic cycling and EIS studies. They exhibit stable and high capacity and minimal capacity fading. In addition, they exhibit good rate capability. The rGO/Fe₃O₄ composite obtained at 700 °C in Ar–H₂ exhibits the best rate capability with a high reversible capacity of 480 mAh g⁻¹ at a high current density of 3000 mA g⁻¹.