The Modification Of Spinel LiMn₂O₄by Doping And Coating Used For Lithium Ion Batteries

Lithium-ion batteries are considered to be the most efficient energy storage systems forelectric vehicles （EVs）, hybrid electric vehicles （HEVs） and plug-in electric vehicles（PHEVs）. Spinel LiMn₂O₄is one of the most competitive cathode active materials for the4Vrechargeable lithium-ion batteries because of its low cost, abundant in nature, environmentalamity and high safety properties. However, the cycleability of the material has not beensufficient enough to be used as a commercial cathode.The spinel LiMn₂O₄has been plagued by three problems that contribute to a capacity fade oncycling:（1） the Mn³⁺ion concentration is high enough for a cooperative Jahn-Tellerdistortion in Li-rich regions;（2） an ordering of Li at x=0.5creates regions of x where twocubic phases coexist; and （3） surface Mn³⁺ions undergo the disproportionation reaction2Mn³⁺=Mn²⁺+Mn⁴⁺followed by dissolution of Mn²⁺into the liquid electrolyte.Tetravalent Ti-ion doped spinel LiMn₂O₄has been prepared by solid-state reaction. Thedoped samples have well-defined spinel structure with tetravalent Ti ion substituting Mn⁴⁺ion in the octahedral （16d）. Due to the stronger bond of Ti-O than Mn-O, Ti doping couldenhance the stability of the spinel structure. Moreover, due to the larger radius of Ti⁴⁺（0.061nm） ions than Mn⁴⁺（0.053nm） ions, the lattice size of the doped spinel increases, which isfavorable to lithium ion migration. The doped sample LiMn_1.97Ti_0.03O₄exhibits significantlyimproved capacity retention and much higher capacities even at high rates, which isattributed to the more stable lattice structure, higher ion diffusion coefficient and lowercharge-transfer resistance. The results of electrochemical test demonstrate that appropriatetetravalent Ti doping could greatly improve cycling performance and rate capability of thespinel LiMn₂O₄.Multi-cations （Cu, Al, Ti） doped spinel LiMn₂O₄has been synthesized by lactic acid assistedPechini method. The X-ray diffraction analysis indicates the doped sample has well-definedsingle-phase in the cubic spinel with doping-ions occuping in the octahedral sites; and all thedoping metal ions are homogeneously distributed in the spinel which confirmed by scanningelectron microscopy mapping. The doped spinel delivers initial capacity of134mAh g^-1andexhibits97%and91%capacity retention after400cycles at25℃and50cycles at55℃ respectively. The multi-cations doped spinel still exhibits relatively high rate capacity; and itshows111mAh g^-1even at12C. The doped spinel exhibits much less Mn dissolution contentcompared to the undoped spinel. The electrochemical impedance analysis indicates that themulti-cations doping could effectively enhance the ion diffusion ability in the spinel.Surface-modified spinel is prepared by heat-treating aluminum nitrate coatedLiMn_1.97Ti_0.03O₄at700℃for3h. The X-ray diffraction and Fourier transform infraredspectroscopy analysis reveal that Al ion diffuses into the spinel structure to formLiAl_xMn_1.97-xTi_0.03O₄solid solution. Transmission electron microscopy analysis shows thatAl₂O₃coated on the surface of the spinel particles. The Al₂O₃layer compactly coats on theparticle surface and exhibits well developed crystalline. The modified spinel exhibitssignificantly improved capacity retention, especially at55℃. The improved cyclingperformance is ascribed to the double layers, i.e., LiAl_xMn_1.97-xTi_0.03O₄solid solution andAl₂O₃metal oxides, coated on the surface. The compact metal oxides thin layer couldeffectively suppress the HF damage by scavenging of HF in electrolyte and block the directcontact between the electrolyte and spinel particles, which is confirmed by Mn dissolutionanalysis. Additionally, the thinner Al₂O₃layer reduces the lithium ion diffusion distance ininactive layer. The LiAl_xMn_1.97-xTi_0.03O₄solid solution layer could enhance the stability ofthe spinel structure, and such stability-enhanced structure is benefit to improve theelectrochemical performance during cycling.The coating and doping co-modified spinel LiMn₂O₄is synthesized by a simply one-stepsolid state reaction. The thickness of Li₂MnO₃coating layer increases with the increase ofLiF content in the raw materials. The XRD and FTIR analysis indicate that the spinelstructure is doped by other ions. The Li₂MnO₃coated Li(Li_yMn_2-y)O_4-zF_zmaterials exhibitsexcellent cycling stability and good rate capability. At1C charge-discharge current, themodified sample delivers98.8%capacity retention after1000cycles. The greatly improvedelectrochemical performance of the prepared sample indicates practical application as acathode in lithium ion batteries used for energy storage.