| In this paper, we utilize solid-state reaction method to synthesize a series of Li3-xNaxV2(PO4)3/C (0≤x≤3) as energy storage materials for lithium ion batteries and sodium ion batteries, and then study the phase structure, electrochemical performances and lithium/sodium storage mechanism. The detailed contents are as follows:Firstly, Combined the XRD and electrochemical performances, it is obvious seen that the Li3V2(PO4)3/C formed at 800℃ exhibits the highest initial discharge capacity and best capacity retention among all samples. So the optimal sintering temperature is 800℃. From the XRD results, it can be found that Li3-xNaxV2(PO4)3/C is composed of one monoclinic phase with 0<x<0.5, one monoclinic phase and two rhombohedral phases with 0.5≤x<1,two rhombohedral phases with 1≤x≤2.5 and one rhombohedral phases with 2.5<x≤3.Secondly, the CV results show that the Li+(Na+) extraction/insertion plateaus of Li3-xNaxV2(PO4)3/C gradually evolve with the increase of Na content in the formula, which is induced by different phases formed in the as-prepared samples. Li3-xNaxV2(PO4)3/C (0≤x≤3) deliver initial discharge capacities of 115.5,111.7,108.9,108.6,101.4,95.9,96.6 mAh g-1 at 35 mA g-1 in the potential range of 3-4.3 V used as cathode material for lithium ion batteries, corresponding the coulombic efficiencies of 99.5,88.0,86.2,87.9,87.9,84.8,87.2%, respectively. And the initial discharge capacities can achieve the theoretical capacities of 86.8,86.6,86.4,87.6,83.1,80.0, 81.2%, respectively. The capacity retentions are 96.8,91.2,81.5,77.7,70.3,74.3,82.9% after 45 cycles, respectively. Therefore, Li3V2(PO4)3/C, Li2.5Na0.5V2(P04)3/C and Li2NaV2(PO4)3/C has better initial charge/discharge capacity and cycle performance.It is demonstrated that excessive Na+ may occupy other crystal positions outside Li+ with the increase of the content of Na so that the monoclinic structure transforms into rhombohedral phase structure. This phenomenon can reduce reversible lithium storage capacity and then affect the long-term cycling performances. Used as cathode material for sodium ion batteries, Li3-xNaxV2(PO4)3/C (0≤x≤3) can deliver initial discharge capacities of 94.3,103.6,115.1,112.7,99.6,94,91.9 mAh g-1 at 20 mA g-1 in the potential range of 2.8-4.1 V, corresponding the coulombic efficiencies of 84.0,94.7,92.1,92.9,94.2,95.1,86.3%, respectively. And the initial discharge capacities can achieve the theoretical capacities of 70.9,80.3, 91.3,90.1,81.6,78.3,77.9%, respectively. The capacity retentions are 94.3,85.1,70.0,88.4,87.6, 77.3,78.7% after 45 cycles, respectively.Thirdly, the CV analysis indicates that the Li+ diffusion coefficients of Li3-xNaxV2(PO4)3/C (0<x<3) are about 10-12-10-14 cm2 s-1, mainly concentrated in a magnitude of 10-12 cm2 s-1, which is higher than that of LiFePO4 (10-14-10-18 cm2 s-1), demonstrating that the structure is more favorable for the diffusion of Li+. It should be noticed that the Li+ diffusion coefficient of Li2.5Na0.5V2(P04)3/C is much higher than those of the other five rhombohedral samples, showing that the coexistence of monoclinic phase and rhombohedral phase can improve the diffusion of Li+Lastly, in-situ XRD, ex-situ TEM and ex-situ XPS observations reveal that the lithium storage mechanism in Li2NaV2(PO4)3 and LiNa2V2(PO4)3 can be related to the reversible formation of rhombohedral LiV2(PO4)3 phase and rhombohedral NaV2(PO4)3 phase. In-situ XRD data shows that the structural changes of Li3V2(PO4)3/C used as cathode materials for sodium ion battery during the charge process is a two-phase reaction between Li3V2(PO4)3 and LiV2(PO4)3 with volume change of only 2.82%. The structural changes between charge/discharge of Li2NaV2(PO4)3, LiNa2V2(PO4)3 and Na3V2(PO4)3 used as cathode and anode material for lithium ion batteries and sodium ion batteries is highly reversible, respectively. |
PDF Full Text Request