Electrochemical And Structural Study On Spinel LiMn₂O₄ Cathode Materials By Doping And Surface Modification

Lithium ion batteries are undergoing a rapid and innovative development due to increasing market demand for EVs and HEVs. The layered oxides, LiCoO₂, LiNiO₂ and spinel LiMn₂O₄ have been widely studied as cathode materials for lithium ion batteries. Among them, spinel LiMn₂O₄ is considered to be one of the most promising candidates due to its low cost, nontoxicity and easy preparation. However, spinel LiMn₂O₄ suffered from severe capacity fading during cycling, especially at elevated temperatures (higher than 50°C), which hindered its commercialization. Doping or suface modification was adopted to overcome this problem in previous studies. In this dissertation, spinel LiMn₂O₄ with Fd3m group was synthesized by high temperature solid state reaction, and the effect of doping or surface modification on the structure and electrochemical performances at both room temperature (RT) and 55℃ have been investigated. The improved performances of these samples at room and elevated temperatures ascribed to structural and chemical changes resulting from doping or surface modification.Sample LiMn_2-2xLi_xNi_xO₄(x=0.03, 0.05, 0.075, 0.1) were synthesized with well-defined octahedral configuration and lattice parameters of them decreased linearly with the increase of x. XRD, FTIR and XAS ananlyses show that Li/Ni co-doping stabilizes the structure of spinel LiMn_2-2xLi_xNi_xO₄ by increasing the valence of Mn and decreasing the distortion of Mn³⁺O₆. Ex Situ XRD shows that Li/Ni co-doping can suppress the two-phase co-existence in higher voltage plateau and reduce the lattice and volume change during electrochemical cycling. Compared with pure LiMn₂O₄, Samples LiMn_2-2xLi_xNi_xO₄(x=0.03, 0.05, 0.075, 0.1) show better performance at both RT and 55℃. The cyclability data become better with the increase of x.1%, 2.5% and 4% (atom ratio) La-modified spinel LiMn₂O₄ were prepared and Rietveld refinement shows that ca. 1% can be doped into \6d sites of the lattice. After the doped amount exceeded 1%, the impurity LaMnO₃ was formed. La doping, which suppresses the oxygen losses and loss of crystallinity during the charge-discharge process, inhibits the structural destruction as well as the increase of the electrode impedance during prolonged cycling, and thus good performances can be achieved. Compared with spinel LiMn₂O₄, sample LiLao.01Mn1.99O4 show excellent capacity retention with only 9.5% capacity loss after 300 cycles at RT.Cationic substitution improved the room-temperature performance significantly, but the improvement was achieved at the expense of the reversible capacity, oxides coating such as ZnO, MgO etc are effective in improving performances at 55℃, but the coating procedures adopted in above studies were based on the sol-gel or micro-emulsion methods, which are beset with problems such as the high cost of alkoxide oxide precursors and questionable industry-scale production. In this dissertation, the surface of as-prepared LiMn₂O₄ was modified successfully by a melting impregnation method, which can improve the cycleability greatly at both RT and 55℃. Among all samples, Sample LiMn₂O₄/ZnO exhibits the lowest capacity fading rate (0.064% per cycle over 100 cycles) at 55°C, and samples LiMn₂O₄/LiAlO₂ and LiMn₂O₄/Al₂O₃ Show the best capacity retention ability (87.5% after 300 cycles and 96.3% after 100 cycles, respectively). Comparing with LiMn₂O₄ and LiAl_0.04Mn_1.96O₄, TEM and XAS analyses demonstrate that not only nano-Al₂O ₃ is coating on the surface of LiMn₂ O₄/Al₂O₃, but also a substitutional shell has formed. The improvement of surface modified samples by a melting impregnation method is due to suppression of the surface Jahn-Teller distortion and a slow-down of manganese dissolution by the existence of substitutional shell and oxide coating on the surface, hence keeping good structural stability and good electric contact.At last, as a combination of doping and surface modification, sample LiAl_0.04Mn_1.96O₄ was modified by LiCoO₂ or ZnO with a melting impregnation method. XRD and TEM show that Li/Co atoms have diffused into the bulk after surface modification with 3%, 5% and 8% LiCoO₂. The chemical diffusion coefficiences of surface modified samples are about 3.2×10¹⁹¹.3×l0^-11 cm² s^-1, which is one order magnitude higher than sample LiAl_0.04Mn_1.96O₄, and thus the surface modified sample show better rate characteristics. 5% ZnO-modified sample, LA/ZnO, shows a capacity of 140 mA h g^-1 between 2.6 and 4.35 V without remarkable capacity loss over 50 cycles despite the Jahn-Teller distortion at 3 V. High temperature aging experiment shows that the ZnO on the surface can suppress the dissolution of Mn and keep the structure intact. Sample LA/ZnO exhibits better performance at both room temperature and ℃ with a capacity fading of 0.036% at RT and 0.041% at 55℃.