Controllable Synthesis Of Lithium Iron Phosphate As Cathode Materials For Lithium-ion Batteries

The lithium-ion battery is a kind of green secondary battery with many advantages such as low self-discharge, high specific energy, no memory, wide operating temperature range, high output voltage, safety and stability, rapid charge-discharge ability and so on, which has been extensively and intensively used in the aerospace, military, electronics, electric vehicles (EVs) and other fields. Among the various components of the lithium-ion battery, the cathode material is essential. Olivine lithium iron phosphate (LiFePO4) has attracted wide attention because of its safety performance, high specific capacity, good cycle performance and low price and toxicity, which is considered to be one of the most promising cathode materials.However, the rate performance of the original LiFePO4was significantly restricted by the low conductivity and sluggish kinetics of lithium-ion transport. To solve the problem, strategies have mainly focused on the synthesis of LiFePO4/carbon, which not only promises good conductivity, but it also acts as a rigid nano-confinement support that controls the particle size. In this paper, a series of LiFePO4/carbon composites with different morphologies and electrochemical performance were prepared through carbothermal reduction method, solvothermal method and carbon template impregnation method. Technical means including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2adsorption, thermogravimetry (TG) analysis were used to characterize the morphology and structure of the synthesized materials. Furthermore, cyclic voltammetry, galvanostatic charge/discharge cycling, AC impedance and long cycling and rate performance tests were used to study their electrochemical performance. This paper mainly contains three sections:reviews, experiments and conclutions. The main research contents and results has been summarized as follows:(1) High tap density LiFePO4/C composites were synthesized using iron oxide precursor with different morphology and size through a carbothermal reduction method. LiFePO4/C composite by using rhombic iron oxide precursor prepared with lysine, glucose as carbon source shows a nano-porous morphology with small LiFePO4nanoparticles uniformly embedded in3D conductive network. The discharge capacity of the composite is132mA h g-1with the carbon content less than5%, and the capacity retention is almost100%after100cycles. (2) LiFeP04was prepared by a solvothermal method in a water-polyethylene glycol (PEG400) binary solvent using ascorbic acid (VC) as reducing agent. The results showed that the addition of VC can not only prevent the oxidation of divalent iron, and more importantly, the crystal shape and discharge capacity varied with VC content. With the coaction of VC and PEG400, the pure LiFePO4could deliver a discharge capacity of143.2mA hg-1at0.1C. Over20cycles, the electrode could maintain74.4%of its initial capacity. After heat treatment, the residual PEG400on the surface of LiFePO4was converted into carbon, which avoids the structural defects and non-uniform of carbon coating layer caused by the process of milling and made a21.2%increase in the charge-discharge capacity. Over30cycles, the electrode showed97%capacity retention compared to an initial capacity and the coulombic efficiency stayed at about100%.(3) Mesoporous LiFePO4/C composites containing80%and85.4%of highly dispersed LiFePO4nanoparticles were fabricated by using bimodal mesoporous carbon (BMC) and3D interconnected carbon with large pore volume and thin walls as continuous conductive networks, respectively. The unique pore structure of BMC not only promises good particle connectivity for LiFePO4, but it also acts as a rigid nano-confinement support that controls the particle size. A correlation between the electrochemical performances and their structural features has been discussed systematically through different capacity calculation method. Based on the weight of LiFePO4, the cathode with80%of LiFePO4loading exhibits a sttible reversible capacity of132.9mA h g-1at0.1C. the electrode with47%of LiFePO4loading exhibits101.5mA h g-1at10C and75.4mA h g-1at20C. The electrochemical performance shows that high LiFePO4loading benefits large tap density and to further energy storage at low rates, while the electrode with low content of LiFePO4displays superior high rates performance greatly due to the small particle size, developed porosity of carbon support and high utilization of the active material which lead to a fast ion and electron diffusion. Especially the cathode with85.4%of LiFePO4loading exhibits a stable reversible capacity of155.8,116and85.3mA h g-1at0.1,10and20C. The cell retains96.7%of its initial capacity at10C over200cycles and the coulombic efficiency is almost100%.