| As polyanion cathode materials for lithium ion batteries, monoclinic lithium vanadium phosphate Li3V2(PO4)3 is attractive for high structural stability, reliability, low cost and abundant resources, which has high electron and ionic conductivity,high theoretical charge and discharge capacity, high charge and discharge voltage plateau. It is considered to be one of great market potential applications for lithium ion battery cathode material since Li Fe PO4. In this paper, Li3V2(PO4)3 is prepared by rheological phase method reaction. Then XRD, SEM is used to analyze its crystal structure or morphology, and their electrochemical performances are investigated by galvanostatic current charge-discharge, cyclic voltammetry, electrochemical impedances spectroscopy. The work is to find out the effects of doping different contents of cations on the physical properties and electrochemical performances of active materials.The cathode material Li3V2(PO4)3 is prepared by rheological phase method reaction. Choosing different temperature to roast the materials which doping or not, and the synthesis conditions are optimized. We conform the technical parameters, which is listed as follows: carbon source(sucrose, reducing agent amount), lithium source(Li2CO3), the initial heating is up to 350°C for 3h and then press into pellets before final heating performed at 750°C for 6 h. The pure and doped products show the best performance: in the voltage range of 3.0-4.3V and 3.0-4.8V, the discharge capacity of 110.7 and 149.8m Ah/g(pure Li3V2(PO4)3), 111.5 and 144 m Ah/g(doped Li3V2(PO4)3)can be reached at 0.2C, respectively, which owning the highest charge discharge efficiency of 91.7 and 87.3%(pure Li3V2(PO4)3), 93.3 and 87.2%(doped Li3V2(PO4)3), respectively.After Nd3+ ions doped Li3V2(PO4)3 with different contents of modified, we find that when doping amount is 0.08, the sample shows the best the discharge capacity of 115.8 m Ah/g(3.0-4.3V), 146.6m Ah/g(3.0-4.8V) in 0.2 C ratio, and without doping samples delivers 110.7 m Ah/g(3.0-4.3V), 149.8 m Ah/g(3.0-4.8V) discharge capacity. Meanwhile, Li3V2-x Ndx(PO4)3(x=0.08) acts good ratio performance obviously, which keeps the discharge capacity retention at 78.2% in 3.0-4.3V voltage range and 2C ratio, after 50 weekly cycle. By contrast, the undoped sample maintains only 41.1% of capacity. From the cyclic behavior, it can be seen that when x = 0.05, Li3V2-x Ndx(PO4) 3 sample has the best cycle performance. In the 3.0-4.3V voltage range, it can still reach 92.4% capacity retention after 80 weekly cycles in 0.2C ratio, and not doped samples is only 80.8%.Gd3+ ion doping modification samples of Li3V2-x Gdx(PO4)3(x=0.02, 0.05, 0.02, 0.1) are synthesized. The electrochemical test demonstrates that compared with Nd3+ ions, when doped content is 0.02, the sample gets the highest discharge capacity in 0.2 C ratio. The special capacities are 117.4m Ah/g(3.0-4.3V), 161.1m Ah/g(3.0-4.8V). In the potential range 3.0-4.3 V, the Li3V2-x Gdx(PO4)3(x = 0.02) doped sample, which charge and discharge at 2C ratio after 50 cycles has a special capacity of 90 m Ah/g and retention ratio reaches 80.8%, apparently higher than the undoped Li3V2(PO4)3 sample, which is 44.9m Ah/g. Moreover, Li3V2-x Gdx(PO4)3(x=0.05) sample keeps upon a high discharge capacity of 102 m Ah/g(94.5% capacity retention) at 0.2C after 80 cycles, and the pure one is 85.1 m Ah/g. In the 3.0-4.8V voltage range, ratio performance also is strengthened certainly, but the improvement of cycle stability is little, all samples after 80 weekly cycles at 0.2C ratio only reach around 80% of capacity retention.In addition, we choose Gd3+ and Cl- as the candidate ions for co-doping. The results show that the highest initial discharge capacities of co-doping samples are 118.7, 169 m Ah/g(3.0-4.3V, 3.0-4.8V) and the capacities of 50.2, 72.8 m Ah/g(3.0-4.3V, 3.0-4.8V) can be obtained at 10 C rate. By contrast, the capacities of Gd3+ doped sample are 120.5, 170.7 m Ah/g(3.0-4.3V, 3.0-4.8V) and 68.2, 100.3 m Ah/g(3.0-4.3V, 3.0-4.8V) at 0.1C and 10 C, respectively. It indicates that the effect of compound doping modification is poor for the samples we synthesized. Key words: Li-ion battery; Cathode material; Lithium vanadium phosphate; Rheological... |
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