Preparation And Performance Study Of LiFePO₄ As Cathode Material For Lithium Ion Battery

With high voltage, high capacity, high energy and good cycle performance, lithium ion batteries developed rapidly since 1990s. And high power lithium ion battery becomes a new kind of power source with the emergency of energy crisis. Cathode material is very important part of the whole battery, and the research of new cathode material with high electrochemical performance and low cost is necessary. LiCoO₂, LiNiO₂ and LiMn₂O₄ are studied widely by now, but they all can not fit for requirement of high power lithium ion batteries.In 1997 a new cathode material LiFePO₄ with olivine structure was reported. It has high theoretical capacity, good cycle performance, low cost, safety and low harm. But its shortcomings are low electronic conductivity and low ion diffusion kinetics, which restrict LiFePO₄ commercialized.In this thesis, several synthesis methods were used to prepare LiFePO₄. Carbon mixing, carbon coating and doping with different elements were used to improve its electrochemical properties and advanced results were obtained. XRD, SEM, TEM, FTIR, Raman , CV and EIS were used to study properties of products and reaction mechanism.(1) Carbon mixed, carbon coated, doped, and carbon mixed-doped LiFePO₄ were prepared by solid state reaction. In this part, more than 20 kinds of unreported dopants were used to prepare doped LiFePO₄; combining carbon mixing or carbon coating with doping method to improve electrochemical properties of LiFePO4, and bi-element doping and carbon mixing was firstly used to study for improving electrochemical behavior of LiFePO₄. The results showed the materials doped with Ti-, Al-, Pr-, Zn-, Sn- and Ni- exhibited good electrochemical characteristic.(a) Iron oxalate, lithium hydroxide and ammonium dihydrogen phosphate and acetylene black were used as main raw materials for preparing LiFePO₄ mixed with carbon. Carbon mixed LiFePO₄ was obtained by two-step heat treating. By XRD analysis, pure phase LiFePO₄ could be synthesized when calcined temperature above 650°C. Product synthesized at 750°C showed the best electrochemical performance. It could deliver 131mAh/g initial discharge capacity at 0.1C rate, and discharge capacity remained 98.8% of initial capacity after 35 cycles.(b) Carbon coated LiFePO₄ was prepared using citric acid, PVA, and sucrose as carbon source respectively. In TEM images, a layer carbon film with 3-4nm thickness surrounding material particles could be observed. From Raman spectra, the ratio ofI_d/I_g was calculated to estimate the degree of graphitic carbon. Carbon coated materials showed better properties than that of carbon mixed materials by charge discharge results. Among these carbon sources, sucrose was the best one. The LiFePO₄/C composite prepared with sucrose presented the discharge capacity of 148mAh/g at discharge rate of 0.1C. Its initial discharge capacity was 125mAh/g at discharge rate of 0.5C, and it could reach 130mAh/g at the 75^th cycle.(c) More than 25 kinds of dopants were used to synthesize Li_0.99M_0.01FePO₄. The conductivities of these materials were measured by AC impedance method. It was found that their conductivities were higher than that of pure LiFePO₄, but most of them had bad electrochemical performance. To improve the electrochemical properties of samples, doping method was combined with carbon mixing. Cathode materials prepared by this method showed higher discharge capacity and longer cycle life compared with carbon mixed materials or doped materials. Good results could be got when Ti, Al, Sn, Pr, Zn were used as dopants. Among them, C-Li_0.99Ti_0.01FePO₄ exhibited the best electrochemical behavior. Its initial discharge capacity was 154.5mAh/g at discharge rate of 0.2C. When discharged at the rate of 0.5C, its initial discharge capacity was 140mAh/g and after 80 cycles its discharge capacity remained 122mAh/g. Even discharged at the rate of 1.0C, its initial discharge capacity could keep as 130mAh/g. The apparent lithium ion diffusion coefficient in C-Li_0.99Ti_0.01FePO₄ was calculated by CV experimental. It was 2.34×10¹⁰cm²/s, which is higher than that of C-LiFePO₄ (3.66×10^-11cm²/s). The apparent lithium ion diffusion coefficient calculated from AC impedance results for C-Li_0.99Ti_0.01FePO₄ was 3.47×10¹⁰cm²/s, which was corresponded well with that obtained from CV results. Moreover, citric acid was used to substitute acetylene black as carbon source for preparing carbon coated Li_0.99Ti_0.01FePO₄, Li_0.98Ti_0.02FePO₄ and Li_0.97Ti_0.03FePO₄ materials. From charge/discharge results, the conclusion could be drawn as follows: low dopant content material showed better electrochemical performance at low discharge rate, and high dopant content material showed better electrochemical performance at high discharge rate.(2) Using very cheap raw material Fe₂O₃ as iron source to synthesize LiFePO₄/C composites by thermal reduction method.(a) Using sucrose as reduction agent, according to following supposed reaction to prepare LiFePO₄/C composite:3Fe₂O₃ + 6LiOH·H₂O + 6NH₄H₂PO₄ + C₁₂H₂₂O₁₁→ 6LiFePO₄ + 3CO + 9C + 29H₂O + 6NH₃By studying the influence of calcined temperature, it was found that the material synthesized at 700°C showed the best electrochemical properties. Its initial discharge capacity was 144.5mAh/g and after 190 cycles the discharge capacity reached 149.2mAh/g at the rate of 0.1C. At the rate of 0.2C, its initial discharge capacity was 135mAh/g and the discharge capacity reached 141.3mAh/g at the 248^th cycle.(b) In first time using Fe₂O₃ and Fe powder to prepare LiFePO4/C composite according to following supposed reaction:Fe₂O₃ +Fe + 3NH₄H₂PO₄ + 3LiOH·H₂O→3LiFePO₄ + 3NH₃ + 9H₂OThe sample prepared at 700°C showed good electrochemical behavior. Its initial discharge capacities were 138.3mAh/g and 129.5mAh/g for charge/discharge at 0.1C and 0.2C respectively. The discharge capacity was 142.2mAh/g at the 201^st cycle for 0.1C and 126.2mAh/g at the 170^th cycle for 0.2C discharge rate.(3) Using very cheap raw material FePO₄ as iron source to synthesize LiFePO₄/C composites.(a) According to following supposed reaction, sucrose was used as reduction agent:6FePO₄ + C₁₂H₂₂O₁₁ + 6LiOH ? H₂O → 6LiFePO₄ + 3CO + 20H₂O + 9CThe LiFePO₄/C composite was synthesized at 700°C. When charged and discharged at 0.1C, and 0.2C, its initial discharge capacities were 142.1mAh/g and 137.1mAh/g respectively. The discharge capacity was 141.2mAh/g at the 200^th cycle at 0.1C. It remained 94.7% of its initial discharge capacity at the 200th cycle for 0.2C discharge rate.(d) According to following suggested reaction, Fe powder was used as reduction agent:2FePO₄ +Fe + 3LiOH ? H₂O + NH₄H₂PO₄→3LiFePO₄ + NH₃ + 6H₂OThe LiFePO₄/C composite was synthesized at 700°C. At the rate of 0.2C, the first cycle discharge capacity was 152.3mAh/g and the discharge capacity kept 151.5mAh/g at the 196^th cycle. At the rate of 1.0C, the first cycle discharge capacity was 134.3mAh/g and the discharge capacity remained 92.3% of its initial capacity after 40 cycles. The apparent diffusion coefficient of lithium ion in LiFePO₄/C was 1.17×10^-9cm²/s from CV results and was 9.63×10¹⁰cm²/s from AC impedance results.LiFePO₄/C composites were synthesized successfully by thermal reduction method using Fe₂O₃ and FePO₄ as raw materials, which reduced the cost and was more suitable for the requirement of industrialization.(4) It is the first time that complex agent was introduced to precipitation method to prepare LiFePO₄/C composites. In this method Fe(NO₃)₃·9H₂O, LiOH·H₂O, NH₄H₂PO₄ and sucrose were used as raw materials, citric acid was used as complex agent. The results showed that when the ratio between complex agent and metal ions was less than 2.5, precipitation could be obtained. The complex agent could decrease the precipitation rate of metal ions and relieve unevenness of each composition. The electrochemical properties of products prepared with complex agent were better than that of those without complex agent. Composite prepared at 750°C exhibited the best performance. When charged and discharged at 0.2C, its initial discharge capacities were 134.9mAh/g and the discharge capacity was 136.2mAh/g at the 180^th cycle.(5) Based on the complex property and reduction property, oxalic acid was firstly used as complex agent to prepare LiFePO₄/C by sol-gel method. Discharge capacity of LiFePO₄/C composite prepared with oxalic acid under the ratio between complex agent and metal ions being 3.5 and pH being 4.0 could reach 123mAh/g and its discharge capacity was 121.6mAh/g at the 30^th cycle. But this initial research study need go further to improve the electrochemical behavior of LiFePO₄/C composites.