Li3V2(PO4)3/C Research Articles

A simple diethylene glycol-assisted synthesis and high rate performance of Li3V2(PO4)3/C composites as cathode material for Li-ion batteries

Spherical Li3V2(PO4)3/C(LVP/C) composites were synthesized by sol–gel method using diethylene glycol as the spheroidizing medium and glucose as the carbon source. The crystal structure, morphology, the lithium diffusion behavior and high rates capacities were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and electrochemical methods. Results indicated that the sphere-like LVP/C sample prepared with 10wt% glucose has a uniform carbon layer about 10nm on the surfaces, and presented a high discharge capacity of 131.8, 126.5, 102.4, 82.8mAhg−1 at 0.1, 2, 10, 20C between 3.0 and 4.3V with no obvious capacity fading during 200 cycles at the rate of 2C. While in the voltage region of 3.0–4.8V, it owned the largest reversible capacity of 169.4, 139.8, 121.7mAhg−1 at 0.5, 1, 5C, respectively. Its capacity retained 79.9% after 200 cycles at 2C, and the apparent Li-ion diffusion coefficient was calculated to be 3.37×10−9cm2s−1.

Evolution of electrochemical performance in Li3V2(PO4)3/C composites caused by cation incorporation

Li3V2(PO4)3/C (LVP/C) composites incorporated by a series of electrochemically active cations (Fe, Co, Ni, Mn) have been successfully prepared by a conventional solid-state reaction. M-incorporation (M=Fe, Co, Ni, Mn) in Li3V2(PO4)3 does not change the monoclinic structure. Analyzed with X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and high-resolution transmission electron microscopy, we find that the valence is between +2.67 and +3 for Fe, and is +2 for Co, Ni and Mn. M-doped LVP and LiMPO4 phases coexist in the incorporated LVP/C composites. Compared with pristine LVP/C, Fe-incorporated LVP/C shows the best electrochemical performance with the highest initial discharge capacity of 131.4mAhg−1 at 0.1C between 2.5 and 4.3V. The Fe-incorporated LVP/C sample also exhibits excellent rate capability with an average capacity of 122.4mAhg−1 at 1C and 93.5mAhg−1 at 5C, resulting from the reduced particle size, the improved electronic conductivity, the high Li-ion diffusion coefficient, and the contribution of LiFePO4 to the capacity.

A comparative structural and electrochemical study of monoclinic Li3V2(PO4)3/C and rhombohedral Li2.5Na0.5V(2−2x/3)Nix(PO4)3/C

In order to synthesize pure derivative of rhombohedral Li3V2(PO4)3 (LVP), lithium-ion batteries materials Li2.5Na0.5V(2−2x/3)Nix(PO4)3/C (x=0.03, 0.06, 0.09) and its control, monoclinic Li3V2(PO4)3/C (LVP/C), were prepared by sol–gel method. The samples were investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) spectroscopy, scanning electron microscopy (SEM), Raman spectroscopy, and electrochemical methods. The XRD patterns of Li2.5Na0.5V(2−2x/3)Nix(PO4)3/C are in good agreement with that of rhombohedral LVP, which indicates that the Na+–Ni2+ composite doping can change the structure of monoclinic LVP. All the composite doping samples displayed a single flat plateau at 3.7V in the charge/discharge voltage profile, which is caused by transformation of multi-phase mechanism to single-phase mechanism. For Li2.5Na0.5V1.98Ni0.03(PO4)3/C, a specific discharge capacity of 108mAhg−1 was achieved at a 0.5C charge rate and a 1C discharge rate, and a 99.0% retention rate of the initial capacity was obtained after 50 cycles.

Novel synthesis of low carbon-coated Li3V2(PO4)3 cathode material for lithium-ion batteries

A novel refluxing-assisted solid-state reaction (RE-SSR) has been developed to prepare Li3V2(PO4)3/C (LVP/C) composite; hydrothermal-assisted solid-state reaction (HT-SSR) method is employed for comparison. The as-obtained composites have been characterized with XRD, Raman spectrum, SEM, and electrochemical measurements. XRD results indicate that the samples are well indexed as monoclinic LVP. Low carbon-coated LVP prepared by RE-SSR (LVP/C(RE)) shows better electronic conductivity and lower charge-transfer resistance. For LVP/C(RE), SEM image shows uniform morphology and small particle size. It delivers a high initial discharge capacity of 162.3mAhg−1 at 0.1C and an average capacity of 138.5mAhg−1 at 0.5C. Our experiments demonstrate that the low carbon-coated LVP cathode material prepared by RE-SSR exhibits better electrochemical performance than that prepared by HT-SSR.

Improvement of electrochemical performance for Li3V2(PO4)3/C electrode using LiCoO2 as an additive

A Li3V2(PO4)3/C (LVP/C) electrode and a new Li3V2(PO4)3/C electrode using LiCoO2 as an additive (LVP/LCO/C) have been successfully fabricated and investigated via physical and electrochemical methods. Compared with the LVP/C electrode, the LVP/LCO/C electrode presents better rate capability and cyclic performance in the range 3.0–4.3V. For the LVP/LCO/C electrode, the largest capacities of 99.3 and 68.0mAhg−1 are delivered at 1 and 15C charge rates and 51C discharge rate, respectively. 80.4% of the largest value is kept at 1C charge and 51C discharge rates in the 800th cycle. The excellent rate capability and cyclic performance are attributed to the small crystallite size of LVP particles, the small particles, narrow particle size distribution in the LVP/LCO/C electrode, large specific surface area and pore size, low electrical resistance for the LVP/LCO/C electrode, and the existence of the other electroactive LCO.

Controllable synthesis of spherical Li3V2(PO4)3/C cathode material and its electrochemical performance

Spherical Li3V2(PO4)3/C (LVP/C) composites are successfully synthesized via two different spray-drying processes. XRD results reveal that some impurities, such as Na3V2(PO4)3, Li2NaV2(PO4)3 and Li4(P2O7), appear when sodium salt of carboxy methyl cellulose (CMC) acts as carbon source. SEM images show two different morphologies: solid spherical particles for LVP/C(A) prepared by adding CMC after pre-sintering, and hollow for LVP/C(B) prepared by adding CMC before pre-sintering. Compared to LVP/C(B), LVP(A) presents higher tap density and better electrochemical performance at any C-rate. Remarkably, another charge/discharge plateaus or anodic/cathodic peaks around 3.85V are detected for both LVP/C(A) and LVP/C(B) electrodes, which is attributed to the formation of Li3−xNaxV2(PO4)3 or some other electrochemically active composites containing Na+. CMC addition sequence has a significant influence on morphology of Li3V2(PO4)3. Furthermore, acting as carbon source and Na dopant, CMC shows a notable effect on electrochemical behavior of Li3V2(PO4)3 cathode material.

A carbon-coated Li3V2(PO4)3 cathode material with an enhanced high-rate capability and long lifespan for lithium-ion batteries

A facile sol–gel approach combined with a carbon-coating technique via high-temperature thermally decomposing C2H2 has been developed for the synthesis of a Li3V2(PO4)3/C (LVP/C) cathode material employing the biomass of phytic acid as an eco-friendly phosphorus source. The effects of the carbon-coating on the structural, morphological and electrochemical properties of LVP have been investigated. Compared with pristine LVP, the LVP/C composite presents a higher discharge capacity of 127 mA h g−1 at 0.1 C, better rate capability and long-term cyclability in the voltage range of 3.0–4.3 V. Even at a high charge–discharge rate of 5 C, it can still deliver a reversible capacity of 107 mA h g−1 over 400 cycles without obvious fading, demonstrating great potential as a superior cathode material for lithium-ion batteries.

Significantly Improved Electrochemical Performance in Li3V2(PO4)3/C Promoted by SiO2 Coating for Lithium-Ion Batteries

Li3V2(PO4)3/C (LVP/C) coated with various amounts of SiO2 has been synthesized, and the effect of surface modification by SiO2 on the performance of LVP has been systematically investigated with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), magnetic susceptibility, Raman spectroscopy, and electrochemical measurements. Based on the analysis of XRD, XPS, and TEM, it is confirmed that SiO2 coating on the surface of LVP particles does not change the monoclinic structure of LVP. The XAS, XPS, and magnetic susceptibility results indicate that the valence of V in both the pristine and SiO2-coated LVP are close to +3. Furthermore, our results reveal that the electrochemical performance of LVP/C can be significantly improved by the SiO2 coating, which is due to the enhanced structural stability and reduced charge-transfer resistance.

Preparation and electrochemical performance of Na-doped Li3V2(PO4)3/C cathode material

Na-doped Li3V2(PO4)3/C (LVP/C) cathode materials are prepared by a sol–gel method. X-ray diffraction results show that the Na ion has been well doped into the crystal structure of LVP/C and does not disturb the extraction–insertion behavior of lithium ion seriously. The initial discharge capacity of the Na-doped LVP/C is 112.2 mA h g−1 at 5 C, and the capacity retention reaches 98.3 % over 80 cycles. Cyclic voltammetry and electrochemical impedance spectra indicate that the reversibility of electrochemical redox reaction and the charge-transfer resistance of LVP/C cathode material have been significantly improved by Na doping. The improved performances can be attributed to the more convenient route for lithium ion diffusion and the lower activation energy of the extraction–insertion of lithium ion due to the weakness of Li-O bond.

Synthesis and electrochemical performance of Li3V2(PO4)3−xClx/C cathode materials for lithium-ion batteries

The Cl-doping Li3V2(PO4)3/C (LVP/C) cathode materials were synthesized by a sol–gel method. The effects of Cl-doping amount on electrochemical properties, structure and morphology of Li3V2(PO4)3/C were studied by electrochemical impedance spectroscopy, cyclic voltammetry, X-ray diffraction, FTIR spectrum and scanning electron microscopy. The results indicate that the Li3V2(PO4)2.88Cl0.12/C composite presents an excellent discharge capacity as high as 106.95mAhg−1 after 80 cycles at 8C. The same monoclinic structure of Li3V2(PO4)2.88Cl0.12/C sample can be obtained from XRD analysis while the particle size is smaller than that of pristine sample (x=0). The FTIR results further confirm the substitution of Cl− for PO43−. The reasons for the relatively enhanced capability are attributed to the decreased polarization and the reduced charge transfer resistance of electrode, with the suitable Cl-doping amount of x=0.12.