Jiafeng Zhang

Comparative investigation of phosphate-based composite cathode materials for lithium-ion batteries.

Li3V2(PO4)3-LiVPO4F, LiFePO4-Li3V2(PO4)3, and LiFePO4-Li3V2(PO4)3-LiVPO4F composite cathode materials are synthesized through mechanically activated chemical reduction followed by annealing. X-ray diffraction (XRD) results reveal that the obtained products are pure phase, and the molar ratio of each phase in the composites is consistent with that in raw material. Transmission electron microscopy (TEM) images show that each phase coexists in the composites. The LiFePO4-Li3V2(PO4)3-LiVPO4F composites exhibit the best electrochemical performance. These composites can deliver a capacity of 164 mAh g(-1) at 0.1 C and possess favorable capacities at rates of 0.5, 1, and 5 C. The excellent electrochemical performance is attributed to the mutual modification and the synergistic effects.

Electrochemical properties of VPO4/C nanosheets and microspheres as anode materials for lithium-ion batteries.

VPO4/C nanosheets and microspheres are successfully synthesized via a hydrothermal method followed by calcinations. The XRD results reveal that the obtained products both have an orthorhombic VPO4 phase. The SEM and TEM images demonstrate that nanosheets and spherical morphology can be obtained by controlling the synthesis conditions. The samples are both uniformly coated by amorphous carbon. The electrochemical test results show that the sample with a nanosheet structure has a better electrochemical performance than the microsphere samples. The VPO4/C nanosheets can deliver an initial discharge capacity of 788.7 mAh g(-1) at 0.05 C and possessed a favorable capacity at the rates of 1, 2, and 4 C. The nanosheet structure can effectively improve the electrochemical performances of VPO4/C anode materials.

Novel synthesis of LiMnPO4·Li3V2(PO4)3/C composite cathode material

A carbon-coated LiMnPO4·Li3V2(PO4)3 composite cathode material is synthesized from a rod-like MnV2O6·4H2O precursor prepared via aqueous precipitation for the first time, followed by chemical reduction and lithiation with oxalic acid as the reducing agent and glucose as the carbon source. XRD results indicate that orthorhombic LiMnPO4 and monoclinic Li3V2(PO4)3 co-exist. SEM results reveal that the thickness of the rod-like MnV2O6·4H2O precursor is about 80 nm and that the LiMnPO4·Li3V2(PO4)3/C composite possesses a micro/nano sphere-like morphology. HRTEM results indicate that the sample is a core–shell structure, where the external shell is amorphous carbon and at the core of the sample are LiMnPO4, Li3V2(PO4)3 and LiMnPO4·Li3V2(PO4)3 unit cells. The initial discharge capacity of the LiMnPO4·Li3V2(PO4)3/C composite is 110 mA h g−1, 104.4 mA h g−1, 100.6 mA h g−1 and 80.4 mA h g−1 at the rate of 0.1 C, 1 C, 3 C and 10 C, respectively. The cell shows excellent cycling stability and good rate capability as a cathode for lithium-ion batteries.

Comparative investigation of microporous and nanosheet LiVOPO4 as cathode materials for lithium-ion batteries

LiVOPO4 cathode materials are synthesized by freeze drying and spray drying methods. X-ray diffraction results reveal that the products obtained using the two methods are both in the β-LiVOPO4 phase. SEM images demonstrate that the stacked nanosheets LiVOPO4 were synthesized by freeze drying, whereas the microporous ones were synthesized by spray drying. Upon comparing the two methods, results indicate that the stacked nanosheets LiVOPO4 synthesized by freeze drying exhibit much better electrochemical performance than microporous LiVOPO4 synthesized by spray drying. The stacked nanosheets can deliver a capacity of 128.4 mA h g−1 at 0.1 C, and possess favorable capacity at rates of 1 C and 2 C.

CNT-Decorated Na3V2(PO4)3 Microspheres as a High-Rate and Cycle-Stable Cathode Material for Sodium Ion Batteries

A novel cathode material, carbon nanotube (CNT)-decorated Na3V2(PO4)3 (NVP) microspheres, was designed and synthesized via spray-drying and carbothermal reduction methods. The microspheres were covered and embedded by CNTs, the surfaces of which were also covered by amorphous carbon layers. Thus, a carbon network composed of CNTs and amorphous carbon layers formed in the materials. The polarization of a 10 wt % CNT-decorated NVP (NVP/C10) electrode was much less compared with that of the electrode with pristine NVP without CNTs. The capacity of the NVP/C10 electrode only decreased from 103.2 to 76.2 mAh g-1 when the current rates increased from 0.2 to 60 C. Even when cycled at a rate of 20 C, the initial discharge capacity of the NVP/C10 electrode was as high as 91.2 mAh g-1, and the discharge capacity was 76.9 mAh g-1 after 150 cycles. The charge-transfer resistance and ohmic resistance became smaller because of CNT decorating. Meanwhile, the addition of CNTs can tune the size of the NVP particles and increase the contact area between NVP and the electrolyte. Consequently, the resulted NVP had a larger sodium ion diffusion coefficient than that of the pristine NVP.

Composite cathode material β-LiVOPO4/LaPO4 with enhanced electrochemical properties for lithium ion batteries

A composite cathode material, β-LiVOPO4/LaPO4, is synthesized by a sol–gel method. The synthesized samples are characterized by XRD, SEM, TEM, EDS, XPS, and electrochemical tests. Results indicate that LiVOPO4 has an orthorhombic structure with a Pnma space group and that LaPO4 has a monazite structure with a P21/n space group. EDS and TEM results illustrate that LaPO4 with typical sizes of 10–40 nm is homogeneously distributed on the surface of primary LiVOPO4 particles. The synthesized β-LiVOPO4/LaPO4 exhibits much better electrochemical performance than bare β-LiVOPO4. The β-LiVOPO4/LaPO4 samples delivered an initial discharge capacity of about 127.0 mA h g−1 at 0.1 C and possessed favorable capacities at rates of 0.5 and 1 C. Therefore, surface modification of crystalline LaPO4 is an effective way to improve the electrochemical performance of β-LiVOPO4.

3D-porous β-LiVOPO4/C microspheres as a cathode material with enhanced performance for Li-ion batteries

3D porous β-LiVOPO4/C microspheres were synthesized through a solvothermal method followed by a post-heat treatment. TG-DSC and FTIR results illustrate crystal structure transformation from α→β-LiVOPO4. XRD results reveal pristine and synthesized powders that were crystallized in the triclinic α-LiVOPO4 and orthorhombic β-LiVOPO4 phase, respectively. Scanning electron microscopy (SEM) and pore distribution results reveal that β-LiVOPO4/C spheres were built from small nanoplates and pores with a wide diameter distribution. HRTEM results indicate encapsulation of β-LiVOPO4/C particles with amorphous carbon shells. A porous β-LiVOPO4/C cathode delivered 134 mA h g−1 and 74 mA h g−1 initial discharge capacities at 0.1 C and 1 C, respectively. The cell presented superior capacity retention attributed to the contributions of surface coating, high specific surface area, and porous architecture that serve as facile electrical conduits for ion/electron transport.

Effect of Nb and F Co-doping on Li1.2Mn0.54Ni0.13Co0.13O2 Cathode Material for High-Performance Lithium-Ion Batteries

The Li1.2Mn0.54−xNbxCo0.13Ni0.13O2−6xF6x (x = 0, 0.01, 0.03, 0.05) is prepared by traditional solid-phase method, and the Nb and F ions are successfully doped into Mn and O sites of layered materials Li1.2Mn0.54Co0.13Ni0.13O2, respectively. The incorporating Nb ion in Mn site can effectively restrain the migration of transition metal ions during long-term cycling, and keep the stability of the crystal structure. The Li1.2Mn0.54−xNbxCo0.13Ni0.13O2−6xF6x shows suppressed voltage fade and higher capacity retention of 98.1% after 200 cycles at rate of 1 C. The replacement of O2− by the strongly electronegative F− is beneficial for suppressed the structure change of Li2MnO3 from the eliminating of oxygen in initial charge process. Therefore, the initial coulombic efficiency of doped Li1.2Mn0.54−xNbxCo0.13Ni0.13O2−6xF6x gets improved, which is higher than that of pure Li1.2Mn0.54Co0.13Ni0.13O2. In addition, the Nb and F co-doping can effectively enhance the transfer of lithium-ion and electrons, and thus improving rate performance.

High-rate electrode material 2LiFePO4·Li3V2(PO4)3@carbon/graphene using the in situ grown Fe4(VO4)4·15H2O precursor on the surface of graphite oxide

2LiFePO4·Li3V2(PO4)3@carbon/graphene (2LFP·LVP@C/G) as a cathode material, based on an in situ grown Fe4(VO4)4·15H2O precursor on the surface of graphene oxide, was synthesized by a solid-state process. The X-ray diffraction Rietveld refinement and Raman spectroscopy results indicated that multi-phase structural 2LiFePO4·Li3V2(PO4)3 with a carbon/graphene coating was obtained. The morphology is characterized by HRTEM tests, which reveal well crystallized 2LFP·LVP@C/G with bridging graphene nanosheets, forming an effective three-dimensional conducting network. Compared with 2LiFePO4·Li3V2(PO4)3@carbon (2LFP·LVP@C), the electrochemical results demonstrate that the 2LFP·LVP@C/G electrode measured at 0.1 C and 10 C can deliver a high specific discharge capacity of 151.2 mA h g−1 and 125.4 mA h g−1, respectively, and has a discharge capacity of 100.2 mA h g−1 at −30 °C at 0.1 C, indicating better rate capability and thermal properties.

Preparation and electrochemical performance of 2LiFe1–xCoxPO4–Li3V2(PO4)3/C cathode material for lithium-ion batteries

Abstract 2LiFe 1– x Co x PO 4 –Li 3 V 2 (PO 4 ) 3 /C was synthesized using Fe 1–2 x Co 2 x VO 4 as precursor which was prepared by a simple co-precipitation method. 2LiFe 1– x Co x PO 4 –Li 3 V 2 (PO 4 ) 3 /C samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical measurements. All 2LiFe 1– x Co x PO 4 –Li 3 V 2 (PO 4 ) 3 /C composites are of the similar crystal structure. The XRD analysis and SEM images show that 2LiFe 0.96 Co 0.04 PO 4 –Li 3 V 2 (PO 4 ) 3 /C sample has the best-ordered structure and the smallest particle size. The charge–discharge tests demonstrate that these powders have the best electrochemical properties with an initial discharge capacity of 144.1 mA·h/g and capacity retention of 95.6% after 100 cycles when cycled at a current density of 0.1 C between 2.5 and 4.5 V.