| Positive electrodes for rechargeable Li cells have been under intense scrutiny since theadvent of Li-ion batteries in1991. In this paper, we mainly focus on the preparation,substitution, and performance of two novel Li-rich vanadium-based phosphates, Li3V2(PO4)3and Li9V3(P2O7)3(PO4)2, as cathode materials for lithium ion batteries.Co-doped Li3V2-xCox(PO4)3/C (0≤x≤0.15) and Na-doped Li3-xNaxV2(PO4)3/C (0≤x≤0.05)compounds were prepared via solid-state reaction and sol-gel method, respectively. The initialspecific capacity decreased as the Co-doping content increased, increasing monotonicallywith Co content for x>0.10. For the Li3V1.85Co0.15(PO4)3/C, the initial capacity was163.3mAh/g (109.4%of the initial capacity of the undoped one) and73.4%capacity retention wasobserved after50cycles at a0.1C rate. The Co2+doping is favorable for the structuralstability, and thus resulting in the improvement of cell cycling ability. On the other hand,among three Na-doped samples and the undoped one, Li2.97Na0.03V2(PO4)3/C sample has thehighest electronic conductivity of6.74×103S/cm, and presents the highest initial capacity of118.9mAh/g and12%capacity loss after80cycles at2C rate. The partial substitution of Liwith Na (x=0.03) is favorable for electrochemical rate and cyclic ability due to theenlargement of unit cells, optimizing the particle size and morphology, as well as resulting ina higher electronic conductivity.Moreover, carbon-coated Li3V2(PO4)3was firstly synthesized at850oC via two-stepreaction method combined sol-gel and conventional solid-state synthesis by usingVPO4/carbon as an intermediate. Two different carbon sources, citric acid and glucose ascarbon additives in sequence, ultimately deduced double carbon-coated Li3V2(PO4)3as ahigh-rate cathode material. The Li3V2(PO4)3/carbon with4.39%residual carbon has asplendid electronic conductivity of4.76×10-2S/cm. Even in the voltage window of2.54.8V,the Li3V2(PO4)3/carbon cathode can retain outstanding rate ability (170.4mAh/g at1.2C,101.9mAh/g at17C), and no degradation is found after120C current rate.What is more important, an unexplored compound Li9V3(P2O7)3(PO4)2is synthesized at750C via solid-state reaction method for the first time. The Rietveld refinement resultsshow that the trigonal system (space group: P3一c1) with the lattice parameters a=9.724, c=1.3596are obtained. Its intrinsic electronic conductivity of1.43×108S/cm is higher thanthat of LiFePO4, and the electronic conductivity comes to2.07×10-3S/cm after carboncoating. Li-ion diffusion coefficient (4.19×10-10cm2/s) for Li9V3(P2O7)3(PO4)2is close to that of LiCoO2and much higher than that of LiFePO4, and it exhibits a paramagnetic behaviorin5300K. In the range of2.04.6V, two discharge plateaus (4.46V and3.74V) can beobserved and110mAh g-1of discharge capacity is achieved. The XRD refinement result atthe end of discharge after the first cycle suggests that the structural reversibility can beretained during electrochemical reactions. In2.04.8V, almost six lithium ions are extractedand the trigonal structure is still recovered after30cycles. Furthermore, we first correlate thestructural characters with the electrochemical process by using a combined experimental andcomputational method. The electrochemical recrystallization of Li9V3(P2O7)3(PO4)2isaccomplished along with a metastable superstructure phase in different but related spacegroup. Nevertheless, the structure as well as oxidation state can be easily recovered onreduction-oxidation, and the volume change is minimal.Finally, the Li9V3xAlx(P2O7)3(PO4)2solid solutions are obtained via solid-state reaction at750C in high-purity N2. The cell parameters of Li9V3xAlx(P2O7)3(PO4)2decrease in a linearway with Al-doping content. The electronic conductivity of Al-doped samples reaches107S/cm, which is more than one order of magnitude higher than that of the undoped one. After30cycles, Li9V2.75Al0.25(P2O7)3(PO4)2can keep92.4%,87.7%, and85.5%of the initialcapacity under the current rates of0.2C,0.5C and1.0C, respectively. The favorable voltagepolarization, enhanced electronic conductivity, and dramatically contractible particle size afterAl doping play significant contributions of comparatively good electrochemical performanceof Li9V2.75Al0.25(P2O7)3(PO4)2sample. |
