Shaochang Han

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.

Improving the initial Coulombic efficiency of hard carbon-based anode for rechargeable batteries with high energy density

Recently, hard carbons have been extensively studied as anode materials for high-energy rechargeable batteries owing to their low costs, potential high capacities and talented rate capability. Nevertheless, they suffer a low initial Coulombic efficiency (ICE) problem which prohibits their broad practical application. Here we develop a facile prelithiation scheme for hard carbon/graphene (HCG) anodes based on a spontaneous electrochemical reaction with lithium metal foils. The ICE can reach a desirable level by easily tuning the prelithiation time. Importantly, the accurate amount of lithium preloaded into HCG is determined by an atomic adsorption spectrum method. Besides, a similar presodiation process is employed to demonstrate the versatility of this strategy. The surface characterization of prelithiated and presodiated HCG confirms that generated solid electrolyte interface layers have almost identical compositions as those formed during the conventional electrochemical charge–discharge cycles. Moreover, the prelithiated HCG paring with a commercial high-capacity cathode, LiNi0.5Co0.2Mn0.3O2 (NMC), enables the full-cell a comparable galvanostatic capacity and rate capability to NMC half-cell (vs. Li) and a superior cycling performance. These encouraging results indicate an accessible solution to solve problems related to low ICEs of hard carbons.

Chemical reaction characteristics, structural transformation and electrochemical performances of new cathode LiVPO4F/C synthesized by a novel one-step method for lithium ion batteries

A new cathode LiVPO4F/C with a high working voltage of around 4.2 V was synthesized by a novel one-step method. The color of the solution turns green, which implies that V2O5 is successfully reduced to V3+. The reaction thermodynamics indicates that LiVPO4F/C is formed when the sintering temperature is higher than 650 °C, while the accompanying impurity phase Li3V2(PO4)3/C is also generated. The reaction kinetics proves that the reaction is third order and the activated energy is 208.9 kJ mol−1. X-ray photoelectron spectra imply that the components of LiVPO4F/C prepared at 800 °C (LVPF800) are in their appropriate valence. LVPF800 is composed of micron secondary particles aggregating from nano subglobose. The structural transformation shows that the V : P : F ratio in LVPF800 is close to 1 : 1 : 1. The reason behind generation of impurity Li3V2(PO4)3 at a high temperature of 850 °C is demonstrated directly, which is mainly due to the volatilization of VF3. The electrochemical performances of the cathode are related to the crystallite content of LiVPO4F/C and Li3V2(PO4)3/C. The specific capacities at 0.2 and 5C of LVPF800 are as high as 139.3 and 116.5 mA h g−1. Electrochemical analysis reveals that LVPF800 possesses an excellent reversibility in the extraction and insertion process and minimum charge transfer resistance.

Effect of environmental temperature on the content of impurity Li3V2(PO4)3/C in LiVPO4F/C cathode for lithium ion batteries

Previous studies have shown that the impurity Li3V2(PO4)3 in LiVPO4F will adversely affect its electrochemical performance. In this work, we show that the crystalline composition of LiVPO4F/C is mainly influenced by the environmental temperature. The content of Li3V2(PO4)3 formed in LiVPO4F/C is 0, 11.84 and 18.75% at environmental temperatures of 10, 20, and 30°C, respectively. For the sample LVPF-30C, the SEM pattern shows a kind of alveolate microstructure and the result of selected area electron diffraction shows two sets of patterns. The LiVPO4F/C cathode without impurity phase Li3V2(PO4)3 was prepared at 10°C. The selected area electron diffraction result proves that the lattice pattern of LiVPO4F is a regular parallelogram. Electrochemical tests show that only one flat plateau around 4.2 V appears in the charge/discharge curve, and the reversible capacity is 140.4 mAh·g−1 at 0.1 C, and 116.3 mAh·g−1 at 5 C. From these analyses, it is reasonable to speculate that synthesizing LiVPO4F/C at a low environmental temperature is a practical strategy to obtain pure crystalline phase and good electrochemical performance.