Mg/F Doped Lithium Magnate Spinels Cathode For Lithium-ion Batteries

Normal spinel LiMn2O4, due to its advantages such as low cost, low toxicity, and relatively high energy density, is considered as a promising cathode material for lithium-ion batteries in place of LiCoO2, which is expensive and toxic. However, it suffers from gradual capacity loss during cycling, which prevents it from commercial use. Ion-doping can obtain improvement in cycleability for spinel cathode. The recent developments in normal spinel LiMn2O4 were reviewed on its crystal structure and properties. The Mg/F-doped lithium magnate spinels, used as the cathode for power polymer Li-ion batteries (PPLIB), were synthesized from Li2CO3 and HMD by solid state reaction-and were characterized by XRD, SEM, as well as electrochemical performance testing. The structure, electrochemical performance and the correlations between them were all discussed in detail for these doped spinels.The research found that the isotropically cubic-structured lithium magnate spinels, prepared at 750癈 for 20h in air, were packed with large and small particles, which were bridged by acetylene black.The results of electrochemical testing showed that Mg-doped LiMn2O4 had excellent cycle performance. LiMn1.92Mg0.08O4 (vs. Li) showed better capacity retention than LiMn2O4, exhibiting no capacity fade over 300 cycles, even though the first discharge capacity (90mAh/g) of LiMn1.92Mg0.08O4 was lower than that (102 mAh/g) of LiMn2O4, at 0.5C charge and discharge rate. PPLIB based on Li0.9gMn1.94Mg0.06O4/CMS also had good electrochemical performance. The discharge capacity over 300 cycles was about 75% that of the first cycle 90mAh/g (vs.Li: 106 mAh/g) at 0.5C rate for the cathode. Mg-doping could improve the cycle performance of the lithium magnate spinels due to facilitation of the crystal growth of the spinels and inhibition of Jahn-Teller distort in LiMn2O4, thus enhancing the atomic cohesive force in the cathode hosts to strength their stability. It was also found that the active particles of the cathode surface would gradually break up upon cycle, which caused adequate penetration of the electrolyte into the cathode, increasing the contact between the cathode and the electrolyte. As a result, the capacity loss, due to the inevitable structural degeneration in lithium magnate spinels, could be compensated.F-doping could increase first specific discharge capacity and meliorate cycle performance of LiMn2O4. Significant improvement in capacity and cycleability was achieved using LiMn2O3.96F0.04 (vs. Li), whose discharge capacity in the 1st and 150th were 113mAh/g and 90mAh/g, respectively. The discharge capacity after 300 cycles was about 70% that of the first cycle. All those were achieved through increasing the content of Mn3+ in lithium magnate spinels and building up theatomic cohesive force in the cathode hosts by F-doping.The first discharge capacity and longevity of Mg and F co-doped LiMn2O4, which had a rather high discharge capacity, were affected by small change of lithium content. PPLIB based on Lio.98Mn1.94Mg0.06O4/CMS also had good cyclability and a large increase of capacity, and the discharge capacity in the 1st and 200th cycles were 98 mAh/g (vs.Li:115 mAh/g) and 73 mAh/g, respectively, for the cathode. A great increase in capacity was achieved using LiMn196Mg0.04O3 96Fo.o4, which exhibited the first discharge capacity of l06mAh/g (vs.Li: 120 mAh/g) for the cathodei but it showed poor cyclability. More work should be undertaken to investigate the relations between the structure and electrochemical performance of Mg/F co-doped lithium magnate spinels.