A Study On Thermal Analysis Kinetics Of Raw Materials Used For Preparing Cathode Materials For Lithium Ion Batteries

The lithium ion battery is of considerable importance in many applications, account of its excellent performance in rate capability, cell voltage and cycling life. It is widely used in communications, traffic, military affairs, electrical equipment and so on. The performance of the lithium ion battery largely depends on the performance of anode and cathode materials. As anode materials exhibit much better performance than cathode materials, more attention is fixed on cathode materials. The commercial cathode materials contain LiCOO2with the layed structure, LiFePO4with the olivine structure, LiMn2O4with the spinel structure, ternary materials and so on.Co3O4, CoC2O4·2H2O, FePO4·2H2O and MnO2used for preparing cathode materials for lithium ion batteries have attracted much more attention. In this research, the thermal analysis kinetics of these materials have been investigated by thermogravimetric (TG), derivative thermogravimetric (DTG), Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) techniques, including the thermal decompositions of Co3O4in nitrogen and MnO2in air, dehydrations of CoC2O4·2H2O and FePO4·2H2O in air, oxidation of CoO while cooling in oxygen. The kinetic triplets of each process were obtained by TG and DTG data.The calculation of non-isothermal kinetic parameters contains three steps.First, the values of the activation energy (Ea) are obtained by the iso-conversional method, which contains integral and differential methods respectively. And then we gain the average value. This can avoid the error of assuming the reaction mechanism functions.Second, substitute the values of Ea and proper P(x) into the function of the average square of the deviation and then confirm the reaction mechanism by the minimum deviation approach.Third, the value of the pre-exponential factor (A) is obtainde by above data.As is different from the calculation of non-isothermal kinetic parameters, the calculation of isothermal kinetic parameters is much simpler. The reaction mechanism is firstly determined and then the values of Ea and A are evaluated by Arrhenius equation.The results are as below:1. The thermal decomposition of Co3O4in nitrogen is a single step. It can be described as: Co3O4(s)�'3CoO(s)+(1/2)O2(g)The non-isothermal kinetic parameters are as following. The values of Ea and A are308.82kJ·mol-1and4.92～5.51－1013s-1. The process is controlled by the random nucleation and nuclei growth model (A2). The integral form and differential form are G(α)=[-ln(1-α)]1/2and f(α)=n(1-α)[-ln(1-α)]1/2.The oxidation of CoO while cooling in oxygen can be characterized by:3CoO(s)+(1/2)O2(g)�'Co3O4(s)The non-isothermal kinetic parameters are as follows. The values of Ea and A are350.19kJ·mol-1and3.17×1017s-1. The process is controlled by the random nucleation and nuclei growth model (A3). The integral form and differential form are G(α)=[-ln(1-α)]1/3and f(α)=n(1-α)[-ln(1-α)]2/3.2. The dehydration of CoC2O4-2H2O in air is a single step. The process can be expressed as: CoC2O4·2H2O(s)�'CoC2O4(s)+2H2O(g)The non-isothermal kinetic parameters are as following. The values of Ea and A are84.45kJ·mol-1and1.64～1.84×109s-1. The process is controlled by the random nucleation and nuclei growth model (A1.5). The integral form and differential form are G(α)=[-ln(1-α)]2/3and f(α)=n(1-α)[-ln(1-α)]1/3.The isothermal kinetic parameters are as follows. The values of Ea and A are80.56kJ·mol-1and5.67×106～5.54×107s-1。The process is controlled by the random nucleation and nuclei growth model (An). When the temperatures are135and140℃, the integral form and differential form are G(α)=[-ln(1-α)]1/n and f(α)=n(1-α)[-ln(1-α)]1-1/n, where n are1.3and1.4. While the temperatures are145、150and160, the integral form and differential form are G(α)=[-ln(1-α)]1/n and f(α)=n(1-α)[-ln(1-α)]1-1/n, where n are1.5,1.6and1.6, respectively.3. The dehydration of FePO4·2H2O in air is also a single step and can be represented as: FePO4·2H2O(s)�'FePO4(s)+2H2O(g)The non-isothermal kinetic parameters are as following. The values of Ea and A are84.38kJ·mol-1and2.43～2.69×109s-1. The process is controlled by the n-order model (Fn). The integral form and differential form are G(α)=[1-(1-α)1-n]/(1-n)�'�f(α)=(1-α)n, where n is0.6.The isothermal kinetic parameters are as follows. The values of Ea and A are75.56kJ·mol-1and1.69×106～1.08×108s-1The process is controlled by the n-order model (Fn). The integral form and differential form are G(α)=[1-(1-α)1-n]/(1-n) and f(α)=(1-α)n, where n is0.5.4. The thermal decompositions of MnO2in air undergo two stages: MnO2(s)�'(1/2)Mn2O3(s)+(1/2)O2(g)(1/2)Mn2O3(s)�'(1/3)Mn3O4(s)+(1/6)O2(g) The non-isothermal kinetic parameters of the first stage are as following. The values of Ea and A are339.43kJ·mol-1and3.73～3.86×1019s-1. The process is controlled by the random nucleation and nuclei growth model (A2). The integral form and differential form are G(α)=[-ln(1-α)]1/2and f(α)=2(1-α)[-ln(1-α)]1/2.The non-isothermal kinetic parameters of the second stage are as follows. The values of Ea and A are1164.22kJ·mol-1and1.74～1.90×1049s-1. The process is controlled by the random nucleation and nuclei growth model (A1.2). The integral form and differential form are G(α)=[-ln(1-α)]1/n and f(α)=n(1-α)[-ln(1-α)]1-1/n, where n is1.2.According to above results, the isothermal processes have been predicted, which are the thermal decompositions of Co3O4in nitrogen and MnO2in air, dehydrations of CoC2O4·2H2O and FePO4·2H2O in air. We hope that the predicted results will do some help in determining the calcination temperature and holding time.