Molten Salt Synthesis Of Cathode Material For Li-ion Batteries: Parameters Of Synthesis And Material Properties

Lithium-ion battery as a clean and efficient energy storage device has been widely used in cameras, mobile phones, notebook computers and other portable mobile devices. Lithium-ion battery also gradually finds application in electric vehicle, but its high price considerably restricts its viability as vehicule power source. The key methods to reduce the cost of lithium-ion battery rely on the new low cost materials and low cost preparation processes. On one hand, development of new materials often requires decades to reach commercial product and research on a selected material has often prove to be very specific. On the other hand, optimization of preparation technique has more impact as the same preparation route can be readily applied to synthesize a variety of materials.Molten salt synthesis method is a simple process for preparing multiple complex oxides. Despite those key advantages, molten salt technique has not been studied in a complete and systematic way. This thesis uses molten salt method to synthesize layered structure material Li(Ni0.5Mn0.5)O2, Li (Ni0.2Mn0.2Co0.6)O2, and spinel structure LiMn2O4. The effects of the molten salts and reaction conditions on the structure, morphology, and electrochemical properties have been systematically investigated. One of the aims of the present thesis is to outline general rules when choosing molten salts in synthesis of layered and spinel structure mateirals.Li(Ni0.5Mn0.5)O2 materials can not be synthesized from LiCl molten salt, since this synthesis route could easily lead to lack of lithium in the final product. Alternative salts to synthesize Li(Ni0.5Mn0.5)O2 layered materials include LiNO3 and Li2CO3. The material made by LiN03 molten salt shows good cycling stability and its discharge capacity is greater than material made by Li2CO3 molten salt. However, the material prepared with Li2CO3 molten salt shows an increasing discharge capacity with cycling. To tentatively use advantages of both materials, combination of LiNO3 and Li2CO3 in different proportions also can be used as molten salt, which have different effect on the morphology of Li(Ni0.5Mn0.5)O2. When 0.9LiNO3-0.05Li2CO3 is used as molten salt, the Li(Ni0.5Mn0.5)O2 has a homogeneous particle size distribution. The discharge capacity is 150 mAh g-1 in CC mode, and 200 mAh g-1 in CCCV mode.0.9LiNO3-0.05Li2CO3 and 0.38LiOH-0.62LiNO3 molten salt combination both can be used to synthesize Li(Ni0.2Mn0.2Co0.6)O2 materials. Because of the different melting point of the molten salts, the materials have different morphology. Both materials have low discharge capacity. Additionnal high temperature heat treatment can improve the discharge capacity of the material synthesized using 0.38LiOH-0.62LiNO3 as molten salt. The discharge capacity of Li(Ni0.2Mn0.2Co0.6)O2 can reach 150 mAh g-1 after two heat treatments. Other salts such as chloride and sulfate have proved not to be suitable for preparation of layered Li(Ni0.2Mn0.2Co0.6)O2 materials.Chloride is good at preparation of spinel materials.0.5NaCl-0.5KCl and 0.6LiCl-0.4KCl both can be used to synthesize LiMn2O4 materials. Morphology of LiMn2O4 prepared using 0.5NaCl-0.5KCl as molten salt has aggregated spherical particles in the nanosized range. The resulting discharge capacity is 124 mAh g-1 with excellent cycling performance.Finally, on the basis of the above-mentioned materials, the thesis summarizes molten salt selection rules and the preparation parameters for the molten salt synthesis of lithium-ion battery cathode material. Except for chloride and sulfate, other lithium containing salts or combinations can be used for the preparation of layered materials. Chloride molten salt is useful to prepare spinel structure material. Molten salt synthesis at around 800℃for 5-6 hours is enough to obtain battery materials with good morphology and electrochemical properties. Paremeters of salts such as melting point affects the morphology and thereby affects the electrochemical performance. Therefore choice of the salts combination as well as the synthesis parameters has proved to be critical to the cycling performance.