Synthetic Mechanisms And Electrochemical Properties Of Lithium Manganese Oxygen Solid Solutions With Different Structures

Li-Mn-O solid solutions with spinel, orthorhombic and hexagonal structure are synthesized by different method such as solid-state method and liquid method, respectively. Process and vauious factors affecting the product structure and properties are studied in detail. Effects of adulteration on solid solution formation are analysed. The reaction mechanicsms for different synthesis methods are studied. Effects of excessive Li source, different environment atmosphere, and oxygen deficiency on the structure and electrochemical property of Li-Mn-O solid solution are discussed. New type doped Li-Mn-O materials are determined, and synthesized successfully according to defect theory.The structure model of spinel LiMn₂O₄-doped compounds is developed. The mechanism of the solid solution reaction is analysized. Effects of structure defect of spinel LiMn₂O₄ on conductivity are discussed on the bases of defect theory. The oxygen deficiency inside the spinel Li-Mn-0 solid solution deteriorate the electrochemical properties and can be removed by adding of small amount of LiOH·H₂O or O₂ flowing.The phase changes of manganese source during processes of solvothermal reaction are studied. A new solvo-thermal method is proposed to synthesize orthorhombic-type lithium manganese oxide directly from low cost starting materials such as MnO₂, LiOH·H₂O-NaOH and C₂H₅OH. The optimized synthesis conditions are obtained, such as temperature 170℃, raw ratio n(MnO₂):n(LiOH·H₂O):n(NaOH)= 1:4:8, time beyond 24 h. The solvothermal reaction mechanicsim is discussed based on defect theory.Li-Mn-O solid solutions doped Cr, Ni, Ni/Co are successfully synthsied by sol-gel method combined high temperature solid-state reaction in the air. Li(Cr_xLi_1/3-x/3Mn_2/3-2x/3)O₂ (x=0.1,0.13,0.16,0.22) solid solution exhibits the highest discharge capacity of 221 mAh·g^-1 in the range of 2.5-4.5 V at a specific current of 14 mA·g^-1 at 25℃, and the discharge capacity is 180 mAh·g^-1 after the cell being charged/discharged for 10 times.Li_1+yNi_0.5Al_xMn_0.5-xO₂ doped with 5 mol% Al exhibit a discharge capacity of 192 mAh·g^-1 in the range of 2.5-4.7 V, and 4 % capacity loss after 30 cycles. An addition of 8 mol% Li⁺ shows a smaller voltage drop and the capacity retention is improved. XPS studies indicate that the oxidation states of manganese, nickel and cobalt in the Li[Mn_1/3Co_1/3Ni_1/3]O₂ surface region are 4+, 2+ and 3+ respectively with small content of Ni³⁺ ion. From the voltage profile and cyclic voltammetry, the redox processes occurring at -3.7 and -4.4 V are assigned to the Ni²⁺/Ni⁴⁺ and Co³⁺/Co⁴⁺ couples, respectively. Initial discharge capacity of 195 mAh·g^-1 at 55℃is obtained and 170 mAh·g^-1 is retained after 10 times cycle in the range 2.5-4.6V and at a specific current of 28 mA·g^-1.The results show that the prepursors obtained at 400, 600℃have a hexagonal layered structure. Solid solution formation is hypothesized to be a three-step: Firstly, small particle Li-Ni-Co-Mn-O solid solution with hexagonal layered structure as main phase is formed at lower temperature; secondly, residual lithium carbonate is decomposed; thirdly, lithium ion diffuse from exterior into the bulk of Li-Ni-Co-Mn-O solid solution. The TG-DSC curves are measured at different heating rates in quiescent air atmosphere for decomposition of the Li-Ni-Co-Mn-O precursor, and confirmed the solid solution formation mechanicsim. The kinetics model and parameters are obtained by differential method, integral method, and Kissinger method. For the first stage, E=126.45 kJ·mol^-1, lnA=10.03, the coefficients of reaction order n=3. The control steps are formation of nuclei randomly and growth of nuclei subsequently.