Research On Design, Preparation And Modification Of Lithium Transition Metal Phosphate Cathodes For Lithium-Ion Batteries

Lithium ion battery has been extensively used in mini-type portable electronic devices and has been undergoing its applications in electric vehicles due to its large energy density, low self-discharge performance, long cycle life and environmental friendliness. The main aim is improving electrochemical properties and decreasing costs of batteries. Because the cathode materials occupy lots of costs in lithium ion battery, it is important to modify and investigate the cathode materials.In this dissertation, the design, preparation and modification of lithium transition metal phosphate cathodes for lithium-ion batteries have been researched on. The following aspects were mainly included:1. A hydrothermal reaction has been adopted to synthesize pure LiFePO4first, which was then modified with carbon coating and cupric ion (Cu2+) doping simultaneously through a post-heat treatment. X-ray diffraction patterns, transmission electron microscopy and scanning electron microscopy images along with energy dispersive spectroscopy mappings have verified the homogeneous existence of coated carbon and doped Cu2+in LiFePO4particles with phospho-olivine structure and an average size of400nm. The electrochemical performances of the material have been studied by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge-discharge measurements. The carbon-coated and Cu2+-doped LiFePO4sample (LFCu5/C) exhibited an enhanced electronic conductivity of2.05×10-3S cm-1, a specific discharge capacity of158mAh g-1at50mA g-1, a capacity retention of96.4%after50cycles, a decreased charge transfer resistance of79.4Ω and superior electrode reaction reversibility. The present synthesis route is promising in making the hydrothermal method more practical for preparation of the LiFePO4material and enhancement of electrochemical properties.2. A simple sodium dodecyl benzene sulfonate (SDBS) mediated hydrothermal method has been described in this paper to prepare morphology-controlled LiFePO4, cathode material such as nanoparticles, nanorods and nanoplates with controllable b-axis thickness. When used in lithium-ion batteries, the LiFePO4/C nanoparticles (200nm in size) and nanorods (90nm in diameter along b-axis and200nm-1μm in length) display initial discharge capacities of145.3and149.0mAh g-1at0.1C rate,33.9and61.3mAh g-1at10C rate, respectively. The LiFePO4/C nanoplates (20nm thickness along b-axis and50nm width) deliver a discharge capacity of162.9mAh g-1at0.1C rate and107.9mAh g-1at10C rate. The Li-ion diffusion coefficients of the LiFePO4/C nanoparticles, nanorods and nanoplates are calculated to be1.66×10-12,2.99×10-12and1.64×10-11cm2s-1, respectively. In general, the discharge capacity and rate performance have been found to increase with the decreasing thickness of the b-axis. The experimental results demonstrate that decreasing the crystallite size in the b-axis and increasing the surface area of (010) plane can shorten Li-ion diffusion path and increase the electrode reaction, which significantly improve electrochemical performance of the LiFePO4/C nanocomposites.3. Li3V2(PO4)3/carbon nanocomposites have been synthesized by a solvothermal method and polymerization reaction, followed by post-heat treatment. Polyaniline (PANI) was coated on the outer surface of Li3V2(PO4)3precursor formed by a solvothermal method through a polymerization reaction, and subsequent heat treatment led to the formation of Li3V2(P04)3/carbon nannocomposites. The polymer shell is transformed into in-situ carbon shell that restricts the crystallite growth of Li3V2(PO4)3. The Li3V2(PO4)3/carbon nanocomposies have a size of about250nm and a carbon shell with a thickness of about5nm. The Li3V2(PO4)3/carbon nanocomposite cathodes display discharge capacities of, respectively,174.3and139.1mAh g-1at0.1C rate and5C rate between3.0and4.8V. The cathode material displays capacity retention of93.2%after40cycles at0.1C rate between3.0and4.8V. The good electrochemical performance was mainly ascribed to the uniform carbon shell and the small Li3V2(PO4)3particle size.4. Li3V2(PO4)3/reduced graphene oxide (designated as Li3V2(PO4)3/rGO) and Li3V2(PO4)3/reduced modified graphene oxide (designated as Li3V2(PO4)3/rmGO) nanocomposites have been synthesized by a solvothermal method, followed by post-heat treatment at800℃, and explored as cathodes in lithium-ion cells. Lamellar GO sheets were modified with cetyltrimethylammonium bromide (CTAB) to form mGO with good dispersibility. The Li3V2(PO4)3/rGO (～350nm particles) and Li3V2(PO4)3/rmGO (～200nm particles) nanocomposite cathodes display discharge capacities of, respectively,169.8and186.3mAh g-1at0.1C rate and117.7and134.9mAh g-1at10C rate between3.0and4.8V. The higher discharge capacity and rate capability of Li3V2(PO4)3/rmGO compared to Li3V2(PO4)3/rGO are ascribed mainly to the smaller particle size of Li3V2(PO4)3and the tight contact between the Li3V2(PO4)3nanoparticles and the rmGO sheets. The tight contact enables fast electron transport through the underlying rmGO sheets to Li3V2(PO4)3nanoparticles.