ngfang Li

Synthesis of Li3V2(PO4)3/C for use as the cathode material in lithium ion batteries using polyvinylidene fluoride as the source of carbon

We synthesized two samples of the cathode material Li3V2(PO4)3/C via a solid state method and a hybrid sol–gel method using polyvinylidene fluoride (PVDF) as the source of carbon. Electrochemical testing and XRD, SEM, TEM and EIS were used to characterize their electrochemical performance and micro-morphology. The pyrolitic carbon of the PVDF polymer behaved as a folded film and formed a conductive net that efficiently enhanced the electrochemical performance of the Li3V2(PO4)3. It is easier to coat this pyrolitic carbon on the Li3V2(PO4)3 particles than other types of organic carbon. Compared with the solid state method, the sol–gel method combined with the surfactant cetyl trimethyl ammonium bromide is better at controlling the particle size of the cathode material at the nano-level. The sample prepared by this method showed outstanding rate and cycle performances, with a capacity >90 mA h g−1 at 15 C and a capacity retention of almost 100% after 50 cycles at rate of 5 C at 3.0–4.3 V (theoretical capacity 130 mA h g−1 at 3.0–4.3 V).

Influences of the molecular structure of carbon sources on the structure, morphology and performances of the Li3V2(PO4)3–C cathode for lithium ion batteries

The influences of carbon sources in the synthesis of Li3V2(PO4)3–C (LVP) by carbothermal reduction method have been investigated. All the Li3V2(PO4)3–C samples possess the typical monoclinic structure. Of all the samples, LVP-PR (phenolic resin) possesses the largest discharge capacities of 128.4 mA h g−1, 125.7 mA h g−1 and 103.9 mA h g−1 at 0.2 C, 2 C and 5 C respectively. Its charge transfer resistance has the lowest value of 59.04 Ω. These excellent electrochemical performances result from its maximum conductivity of 1.93 × 10−2 S cm−1. The intensity ratio of peaks 1350 cm−1 to 1580 cm−1 in Raman spectra of LVP-PR has the lowest value of 1.162. The appearance of needle-shaped crystallites in LVP-PR and LVP-ER (epoxy resin) shows that phenolic resin and epoxy resin possess a strong reduction ability. This is also verified by the size of the crystallite. All these are because of the structural differences of carbon sources. The certain amount of benzene rings in the phenolic resin enables LVP-PR to possess high conductivity. The space steric effect of the methyl in epoxy resin and the lack of conjugated π bonds in glucose decrease the conductivity of LVP-ER and LVP-GL.

Structure, conductive mechanism and electrochemical performances of LiFePO4/C doped with Mg2+, Cr3+ and Ti4+ by a carbothermal reduction method

Mg2+, Cr3+ and Ti4+ with various valences at the ratio of 0.02 were used to dope, in order to improve the electrochemical performances of LiFePO4/C. LiFe0.98Mg0.02PO4/C (LFMPC), LiFe0.97Cr0.02PO4/C (LFCPC) and LiFe0.96Ti0.02PO4/C (LFTPC) were successfully synthesized by a carbothermal reduction method using FePO4·2H2O as the iron source and phenol-formaldehyde resin as the reducing agent and carbon source. The reaction mechanism is put forward. A LiFePO4 crystallite develops very well, and the lattice constants decrease after doping. LFTPC possesses the largest conductivity of 8.01 × 10−4 S cm−1, compared to LFMPC and LFCPC. The capacities of LFMPC, LFCPC and LFTPC at 0.1 C are 126.2 mA h g−1, 132.3 mA h g−1 and 134.7 mA h g−1 respectively, which are much larger than the 122.4 mA h g−1 of LiFePO4/C. LFTPC possesses the maximum capacity of 83.1 mA h g−1 at 3 C and a stable potential platform of 3.3 V. The energy gap of LFTPC is 0.61 eV, which is smaller than the 0.63 eV and 0.65 eV of Cr3+ and Mg2+ doped LiFePO4/C, respectively. The vacancy content of LFTPC is much more than for the other samples. This improves the electronic conductivity of doped LiFePO4/C. It is found that Ti4+ plays a significant role in improving the electronic conductivity and performances of LiFePO4/C.