Study On Preparation And Electrochemical Properties Of High Voltage Electrode Material LiNi_0.5Mn_1.5O₄

High voltage electrode material LiNi0.5Mn1.5O4due to its operating potential ofabout4.7V, has15-30%higher energy density than commercial cathode materials,become one of the most promising power battery cathode materials. But due to theformation of impurity phase in synthesis process, the decomposition of electrolyteunder high potential and manganese can be dissolved under high temperature, thebattery’s rate performance and cycling performance are not as decent as expected, sothe application of LiNi0.5Mn1.5O4was limited. Spinel LiNi0.5Mn1.5O4and porousmicrospheres LiNi0.5Mn1.5O4had been prepared in this paper. The structure,morphology and electrochemical performance were characterized by X-ray diffraction(XRD), scanning electron microscopy (SEM) and charge-discharge techniques,respectively. Cyclic voltammetry was used to study the reaction mechanism of thebattery. The diffusion coefficient of lithium ion in spinel structure at roomtemperature can be calculated according to CV result. Electrochemical impedancespectroscopy was introduced to study the effects of temperature on theintercalation-deintercalation process of lithium ion in LiNi0.5Mn1.5O4. The specificresearch information can be concluded as follows:(1) Spinel LiNi0.5Mn1.5O4electrode material was synthesized by sol-gel method.The CV results indicated that there were two pairs of redox peaks about Mn3+/Mn4+and Ni2+/Ni3+, Ni3+/Ni4+appeared at the voltage of4.0V and4.7V. Two platforms canbe seen in4.0V and4.7V in charge-discharge curves, which is consistent with theCV results. At the rate of0.1C,0.2C and1C, the discharge capacity ofLiNi0.5Mn1.5O4electrode was90.9,69.8and41.9mAh/g, respectively. Cyclingperformance showed that during the activation process in the initial few cycles, thedischarge capacity of LiNi0.5Mn1.5O4electrode was rising slowly, at the rate of0.1C,0.2C and1C, the discharge capacity was maintained at120、108and73mAh/g. EISresults indicated that at the voltage of4.0V and4.7V, impedance spectroscopy wascomposed of two semicircle and a slash, represented by HFA, MFA and LF,respectively. The SEI film resistance at the voltage of4.7V was larger than that of atthe voltage of4.0V, which indicated that the side effects of electrolyte decompositionmade the SEI film thick. The diffusion coefficient calculated by CV method were0.9×10-9,1.05×10-9,1.02×10-9and1.01×10-9cm2/s.Furthermore, EIS method was used to study charges in impedance spectroscopy at different temperature in1mol/L LiPF6in EC/EMC (3:7). The results showed thatthe EIS should be composed of HFA, MHFA, MFA and LF four parts. With theincrease of temperature, HFA and MHFA began to overlap, at the temperature of-5℃, they merged to a large semicircle at high frequency area. An appropriateequivalent circuit was selected to fit the EIS spectroscopy, the results shown thatlnRSEIand T-1, lnReand T-1, lnRctand T-1were all showed a good linear relationship.The activation energy of the ion jump, the thermal activation energy of the electricalconductivity as well as the activation energy of the delithiation/lithiation reaction ofLiNi0.5Mn1.5O4electrode can be also calculated from21.61,24.36and52.24KJ/mol.(2) Then porous microspheres LiNi0.5Mn1.5O4was synthesized by templatemethod. The charge-discharge results showed that at the rate of0.5C and1C, theelectrode material had excellent cycle performance, the discharge capacity showed noattenuation after cycling. At the rate of5C, discharge capacity retention was about85.6%after200cycles, at the temperature of55℃and rate of1C, dischargecapacity retention was about92%after100cycles, indicated that the synthesizedmaterial still had good structure stability in the large current and high temperature.EIS method was used to study charges in impedance spectroscopy. With the increaseof temperature, the resistance of lithium ion deintercalation in the electrode materialhad become smaller. At the same time, HFA and MHFA were fusioned, impedancespectroscopy eventually was composed of four parts into three parts. After fitting theEIS spectroscopy, found that lnRSEIand T-1, lnReand T-1, lnRctand T-1were all showeda good linear relationship. The activation energy of the ion jump, the thermalactivation energy of the electrical conductivity as well as the activation energy of thedelithiation/lithiation reaction of LiNi0.5Mn1.5O4electrode can be also calculated from16.89,33.57and59.75KJ/mol.