Research on preparation and property of the spinel LiMn2O4 material by method of co-doping anti-electricity ions
...4;体晶体场稳定能 (Kcal/mol) Li +1 0.68 340.6/577.0 — Mn +3 0.62 402.0/423.4 35.9 Al +3 0.50 512.1/663.6 — Cr +3 0.64 427.0/437.2 60.0 Co +2 0.63 368.0/435.0 45.0 Mg +2 0.65 393.7/461.9 — O -2 1.40 498.35/222.0 — F -1 1.36 — — 2.2 Preparation of the LiMn2O4 material co-doped with anti-electricity ions The LiMn2O4 material co-doped with anti-electricity ions was synthesized by the method of conventional solid-state fusion. The author utilized electrolyzed MnO2 (EMD) and Li2CO3 as the raw materials, and LiF and MgF2 as the reagents offering F, and Al(OH)3、Co3O4、Cr2O3、Mg(OH)2 and MgF2 as the reagents offering Al3+、Co3+、Cr3+ and Mg2+ separately, to synthesize LiMn1-xMxO4-yFy (x=0.05;y=0.05; M= Al、Co、Cr、Mg). 3. Results and discussion 3.1 Charge–discharge studies The first charge–discharge capacity, the first coulomb efficiency, the data of reversible capacity and the efficiency of capacity retention of LiMn1-xMxO4-yFy（x=0.05;y=0.05; M= Al、Co、Cr、Mg）co-doped with anti-electricity ions during prior 20 times charge-discharge process were listed in Table 2, where the data of the first cycle process showed that the first reversible capacity was relatively changed. That the first reversible capacity of both LiMn0.95Al0.05O3.95F0.05 and LiMn0.95Cr0.05O3.95F0.05 were higher than that of pure-phase LiMn2O4 and lower than LiMnO3.95F0.05, while such capacity of LiMn0.95Co0.05O3.95F0.05 and LiMn0.95Mg0.05O3.95F0.05 were both lower than pure-phase LiMn2O4 and LiMnO3.95F0.05 indicated that this result may be related to 价态 of the co-doped ions. In addition, the first coulomb efficiency of the materials co-doped was also changed. The first coulomb efficiency of materials co-doped with Al、Cr、Mg and F were considerably enhanced, especially that of LiMn1-xMgxO1-yFy up to 97.2%,from which the author presumed that its efficiency of capacity retention may increase after more cycle times. Table.2 The capacities and the cycle efficiency of LiMn1-xMxO4-yFy (x=0.05;y=0.05; M= Al、Co、Cr、Mg） First Tenth Twentieth Charge Discharge Efficiency Discharge Efficiency Discharge Efficiency LiMn2O4 133.51 122.04 91.41 115.80 94.89 112.27 91.99 LiMnO4-yFy 141.05 127.63 90.49 109.74 85.98 — — LiMn1-xAlxO4-yFy 131.99 124.59 94.40 117.31 94.16 118.07 94.77 LiMn1-xCoxO4-yFy 123.45 111.18 90.10 105.54 94.93 102.89 92.54 LiMn1-xCrxO4-yFy 132.01 124.34 94.20 116.12 93.41 106.87 85.97 LiMn1-xMgxO4-yFy 122.11 118.71 97.22 118.05 99.44 117.57 99.04 Fig.1 depicted the cycle performance curves of such materials during prior 20 times charge-discharge process. It could be seen, from Table.2 and Fig.1, that reversible capacities and the efficiency of capacity retention of LiMn0.95Al0.05O3.95F0.05 and LiMn0.95Mg0.05O3.95F0.05 during the prior 20 times charge-discharge process were superior to those of other materials. Compared with LiMn0.95Mg0.05O3.95F0.05, the first capacity of LiMn0.95Al0.05O3.95F0.05 was 5.88mAh/g higher, and yet such data (118.07mAh/g和117.57mAh/g) of these two materials were closed after 20 times charge-discharge. Moreover, the efficiency of capacity retention of LiMn0.95Al0.05O3.95F0.05 and LiMn0.95Mg0.05O3.95F0.05 during the twentieth charge-discharge process was respectively 94.77% and 99.04%, and the latter one had more promising trend of capacity retention. Fig.1 cycle longevities curves of LiMn1-xMxO4-yFy（x=0.05;y=0.05; M= Al、Co、Cr、Mg） Thus, the author though that the performances of LiMn1-xMxO4-yFy（x=0.05;y=0.05; M= Al、Mg）materials obviously preceded the pure LiMn2O4 material. So it was necessary to make further research on these two kinds of materials mentioned above from the angles of different synthesizing temperatures and reagents offering metal ions, which would be described in the following contents. 3.2 Effects of different synthesizing temperature systems on the performances of LiMn1-xAlxO4-yFy The cycle performances of LiMn1-xAlxO4-yFy（x=0.05;y=0.05）during 20 times charge-discharge process under different temperatures were displayed in Fig.2. We could find that the electrochemical performances varied under different temperatures. At the temperature of 850℃, the performances of the material were the worst with the capacity of the first charge-discharge not reaching 110mAh/g and rapid fading, because spinel LiMn2O4, at such high temperature, suffered from severe loss of oxygen, which gave rise to impurity phases which, and then, affected electrochemical performances of the material. Nevertheless, at the temperature of 750OC and 800OC, their performances were similar: the first capacity at 750OC was slightly higher; the cycle longevity curve of this material at 800OC was almost a beeline parallel to the X-axes and showed favorable cycle stability; after 20 times cycle process, the capacities of the material at 750OC and 800OC corresponded to each other. Fig.2 The cycle longevity curves of LiMn0.95Al0.05O3.95F0.05 at different temperatures The XRD patterns of LiMn0.95Al0.05O3.95F0.05 at the synthesizing temperature of 800℃ was showed in Fig.3 which showed that the diffraction maximum of the material was considerably acute and went all the way with the criterion patterns. From this result, we could estimate that LiMn0.95Al0.05O3.95F0.05 was the typical spinel material, whose position and intensity of characteristic absorption peak, compared with the XRD patterns of pure-phase LiMn2O4, were not evidently changed and whose diffraction maximum and 半高峰 were more narrow. This demonstrated that LiMn0.95Al0.05O3.95F0.05 had better crystallization capacity. Fig.3 The XRD patterns of LiMn0.95Al0.05O3.95F0.05 at the synthesizing temperature of 800℃ Fig.4 was the SEM patterns of LiMn0.95Al0.05O3.95F0.05 at the synthesizing temperature of 800℃. From these patterns, we could find that LiMn0.95Al0.05O3.95F0.05 maintained the favorable spinel octahedral structure with fine and uniform granularity around 0.8μm and thus possessed relatively large active areas. Fig.4 The SEM patterns of LiMn0.95Al0.05O3.95F0.05 at the synthesizing temperature of 800℃. (the left for 3,000 times, the right for 10,000 times) 3.3 Effects of different reagents offering Mg ions on performances of LiMn1-xMgxO4-yFy When utilizing the method of solid-state fusion, the author had two choices for the raw materials: one reagent was MgF2 which could offer both Mg and F to synthesize the cathode material of LiMn1-xMgxO4-yFy(x=0.05; y=0.05); the other were Mg(OH)2 and LiF which respectively could offer Mg and F to synthesize the cathode material of LiMn1-xMgxO4-yFy(x=0.1; y=0.05). The curves of cycle performances of LiMn1-xMgxO4-yFy were depicted in Fig.5, where Li-Mn-Mg-O-F(1) and Li-Mn-Mg-O-F(2) respectively represented LiMn1.95Mg0.05O3.95F0.05 and LiMn1.90Mg0.10O3.95F0.05 which were synthesized with Mg(OH)2 and LiF as reagents, while Li-Mn-Mg-O-F(3) and Li-Mn-Mg-O-F(4) respectively represented LiMn1.95Mg0.05O3.95F0.05 and LiMn1.90Mg0.10O3.95F0.05 synthesized with only MgF2 as reagent. We could find, from Fig.5, that Li-Mn-Mg-O -F(1) possessed not only higher capacity, but better cycle stability. Therefore, LiMn1.95Mg0.05O3.95F0.05, synthesized with Mg(OH)2 and LiF as reagents, was best of all the materials of LiMn1-xMgxO4-yFy(x=0.1;y=0.05). Fig.5 The curves of cycle performances of LiMn1-xMgxO4-yFy doped with Mg offered by different reagents Fig.6 was the SEM patterns of the material of LiMn1.95Mg0.05O3.95F0.05. From this, the integrate crystal shape, which presented polyhedron appearance with trenchant edges and corners and uniform granularity between 0.5μm and 2μm, was found. Fig.6 The SEM patterns of the material of LiMn1.95Mg0.05O3.95F0.05 (the left for 3,000 times, the right for 20,000 times) 3.4 Cyclic voltammetry tests of LiMn2O4 material co-doped with anti-electricity ions The fir...