| The high-energy lithium-ion battery is one of the important technical directions for newenergy development in the future. At present, many breakthroughs have been achieved in anodedevelopment, especially in silicon and alloy materials. The reversible capacity of anode hasreached as high as4200mAh g-1. A wide range of layered intercalation compounds such asLiCoO2, LiMn2O4, LiNiO2and LiFePO4, etc. have been developed as cathode materials forcurrent commercial LIBs. However, these compounds exchange only one electron per a3Dmetal, corresponding to a limited capacity. Ror example, the theory capacities of LiCoO2,LiMn2O4, LiNiO2and LiFePO4are274,184,148and170mAh g-1, respectively. However, theactual capacities are only150mAh g-1. Thus, it is necessary to explore new redox mechanismsand electrode materials to meet the requirements.With higher working voltage and energy density obtained through reversible conversionreaction, transition metal fluorides are regarded as potential cathode material for futurelithium-ion batteries. Despite the above advantages, there are also several problems to be solvedfor transition metal fluoride. First, fluoride generally has stronger polarity, wider band gap andlower electronic conductivity, which would lead to serious electrode polarization problems.Secondly, the conversion reactions of fluoride are accompanied by obvious volume changes,which will affect the cyclic performance of batteries. In order to search for cathod materials ofhigh density, different kind of conductive agents are mixed with fluoride in this paper. Theelectrochemical impedance spectroscopy (EIS) techniques are used to explore the electrodekinetic processes and the electrode interface performance.The main research content and results are as follows:(1) The commercial NiF2/C composites were prepared by milling with conductive agent(graphite and carbon black) in a ratio of5:3:1(w/w) for3h in a high-energy milling machine at500rpm.And its discharge capacity in the first cycle reached up to1100mAh g-1. However,after tenth cycles, the capacity is200mAh g-1. The main Nyquist characteristic of NiF2/Celectrode recorded by EIS showed a semicircle in both high and middle frequencyregion(HFS,MFS), and an arc or a line in the low-frequency region(LFS/L). An equivalentcircuit was proposed according to the different impedance response time of each part inside theelectrode, and the fitting results are satisfactory. This study reveals that, the impedance of eachpart is related to voltage changes, particularly HFS is commonly attributed to the process oflithium ions migrating through SEI film, while LFS/L is ascribed to charge transfer process onthe electrolyte-electrode interface. MFS reflects the Schottky Contact resistance formed byfluoride and conductive agent inside the electrode, which doesn’t exist in lithium intercalation compound electrodes, indicating that the choice of conductive agent has significant influenceon the resistance of the metal fluoride electrode. And the NiF2/C electrode reaction model wasproposed.(2) The commercial FeF3composites were prepared by milling FeF3with differentconductive agents (carbon black, TiO2, MoS2and V2O5) in a ratio of5:3(w/w) for3h in ahigh-energy milling machine at500rpm.The charge/discharge tests showed that the initial specific discharge capacity of FeF3/Celectrode was close to712mAh g-1at the current density of10mA g-1during4.5~1.5V. After100cycles, the discharge capacity was266mAh g-1with the capacity retention of37%; the firstdischarge capacity of FeF3/TiO2/C electrode is close to340mAh g-1, and reduced to244mAhg-1after20cycles, with the capacity retention of71.7%; as for FeF3/MoS2/C compositeelectrode, the first discharge capacity was423mAh g-1, after100circles, only45mAh g-1ofthe capacity remained and the capacity retention is10.6%; The first circle discharge capacity ofFeF3/V2O5/C electrode was490mAh g-1at the current density of20mA g-1, and34mA g-1after100circles, with a capacity retention of only7%. According to the above results, FeF3/Celectrode has the highest capacity and the best cyclic performance, FeF3/TiO2/C electrode isnext and FeF3/V2O5/C electrode is the worst. The EIS results of the above composite electrodesare similar to that of NiF2/C electrode, and all are composed of three parts: HFS, MFS andLFS/L. However, there is difference in the correspon ding Schottky contactimpedance.Schottky contact impedance of FeF3/MoS2/C composite electrode materials is theminimal. Schottky resistance is one of the important factors which affect electrochemicalperformance of the electrode. According to comprehensive analysis of the experimental results,FeF3/C electrode reaction model was proposed.(3) FeF3/C and FeF3/TiO2of different morphologies were synthesized respectively throughtwo liquid phase methods. Then FeF3/C and FeF3/TiO2/C compounds were prepared byhigh-energy ball milling. The FeF3/C electrode is charge/discharged at a current density of71.2mA g-1between4.5and1.5V. The first circle discharge capacity of the electrode is293mAhg-1,34mAh g-1after30circles, and the capacity retention is58.0%. The FeF3/TiO2/C electrodeis charge-discharged at10mA g-1between4.5and1.5V with initial discharge capacity of504mAh g-1,124mAh g-1after30circles, and the capacity retention of24.6%. When cycled underthe current density of20mA g-1, the capacity became a little lower.The EIS results show that, the Nyquist plots of FeF3/C electrode and FeF3/TiO2/Ccompound electrode are both consisted of three parts, namely HFS, MFS and LFS/L. This EISresult is similar to that of the commercial material. According to the fitting results, in thedischarge process the SEI resistance of the synthesized FeF3/C electrode ranges from8.97to 21.98, and its Schottky contact resistance is between13.42to36.01. As to the FeF3/TiO2/Ccompound electrode, the SEI resistance ranges from14.76to33.20, and Schottky contactresistance from70.52to846.30. It is seen that the interfacial impedance is larger for thiscompound material, similar to the commercial material, indicating TiO2leads to larger Schottkycontact resistance than carbon black, therefore resulting in decline in the electrochemicalperformance of electrode.(4) Furthermore, the electrochemical performance of CuF2as cathode material forlithium-ion battery was discussed in this paper. The CuF2/MoO3/C and CuF2/C compoundmaterials were prepared by high-energy ball milling. And then Galvanostatic discharge/chargemeasurements were carried out in the Neware battery test system at0.01C (7.12mA g-1)between1.5V and4.5V versus Li+/Li at room temperature.The results indicate that thecombination of conduct agents (MoO3and carbon black) has better improvement on theelectrochemical activity of CuF2than single carbon black. The EIS of CuF2/MoO3/C compoundelectrode remains typical three parts, namely HFS, MFS and LFS/L. MFS is related tocontacting resistance among CuF2, MoO3and C. LFS/L reflects the large charge transferresistance during the reaction process.Although the introduction of MoO3and carbon black canimprove the conductivity of CuF2to a certain extent and reduce the polarization of electrodes,there is little improvement in relieving the electrochemical inactivation of reaction products inconclusion.In this paper, there are113figures,18tables and169reference articles. |
