Research On The Cathode Material For Lithium Ion Secondary Battery

Following the extensive usage of LiCoO₂ with layer structure, served as cathode material in lithium ion battery, which have small capacity and used with low multiple ratio discharge current, several new cathode materials have being proposed. The layer structure LiNiO₂, was one kind of compounds among them, which have the feature of plenty raw material source, relatively low price and higher specific capacity. It was believed to be a hopeful cathode material for lithium ion batter}'. However, in practice it is difficulty to synthesis pure phase layer structure LiNiO₂, and the instability of its layer structure during cycling lead to the decline of its specific capacity, all these shortcomings hinder it from practical applying. In order to modify the crystal structure stability of LiNiO₂ and improve its electrochemical property, one useful method commonly used in the field of researching LiNiO₂ based cathode material was partially substituting the Ni atom in LiNiO₂ by variety metal elements to form solid solution of multi-components.In this thesis, several additive compounds of LiNi_0.7Co_0.3-xM_xO₂(M= Ga³⁺, Mg²⁺, Al³⁺, Ti⁴⁺, Sr³⁺) belong to the family of LiNi_0.7Co_0.3O₂ were synthesized for lithium ion battery cathode material The method used here was tow steps sintering process which contains high temperature solid reactions in two different temperature ranges. The raw material used here were several nickel containing compounds. The additive metal elements were that with the valance of two, three and four. The effects of technology parameters upon the feature of yields, have been studied comparatively, using the technologies of powder X-ray diffraction (XRD), thermo-gravimetric analysis (TG), scanning electric microscope (SEM), cyclic voltammegram(CV), and charge-discharge feature probe by simulated cell, from the views of crystal structure, thermal stability, particle morphology and electrochemical property. The technology parameters been taken into account are, the raw materials of nickel containing compounds, the technology for producing precursors, the temperature used in former and later sintering stage, the sustaining time and the atmosphere for sintering. The results are showed as follow. 1. Research on the material based on LiNi_0.7Co_0.3O₂In oxygen atmosphere, LiNi_0.7Co_0.3O₂ was produced using different nickel contained compounds, the effects of sintering temperature and continuing sintering time upon the crystal structure of target products have been studied systematically.1.1 In the oxygen atmosphere, all the products (LiNi_0.7Co_0.3O₂) with well-ordered hexagonal layer structure are synthesized by the two-step calcinations process, with the 1st step sintering temperature range from 600℃to 650℃, and the 2nd step sintering temperature range from 700℃to 800℃, when using different nickel contain compounds as raw material, such asγ-NiOOH,β-NiOOH,Ni(OH)₂,Ni₂O₃ and so on . But the product fromγ-NiOOH does not have electrochemical activity. The optimum sintering temperature and sustaining sintering time in second calcinations process for synthesizing LiNi_0.7Co_0.3O₂ fromβ-NiOOH, Ni₂O₃, Ni(OH)₂ raw materials, are sintering at 700℃for 20 hours, sintering at 700℃for 30 hours and sintering at 750℃for 20～25 hours respectively. The products of LiNi_0.7Co_0.3O₂ made from Ni(OH)₂ andβ-NiOOH are much more suitable to be used as cathode material for lithium ion battery than other products made from other nickel containing compounds.1.2 When Ni(OH)₂ were used to produce LiNi_0.7Co_0.3O₂, with increasing the temperature of second calcinations process, the crystal structural (degree of crystal) of the product was enhanced and the splitting of fine peak in XRD spectra was obviously. When the temperature of second calcinations process was maintained at 800℃, the electrochemical performance of the products declined, because of the escape of lithium lead deficiency and impurity phase may be produced during calcinations at relatively high temperature. The same result was occurred, when the sustaining sintering time is lengthened under the same calcinations temperature.2. Research of doped compound LiNi_0.7Co_0.3-xM_xO₂ (M= Ga³⁺, Mg³⁺, Al³⁺, Ti⁴⁺, Sr³⁺)LiNi_0.7Co_0.3-xM_xO₂ (M= Ga³⁺, Mg²⁺, Al³⁺, Ti⁴⁺,Sr³⁺) was synthesized by the high temperature solid state reaction . The performance feature of the target products were compared, from the views of crystal structure, thermal stability and electrochemical property, by means of X-ray diffraction (XRD), thermal gravimetric analysis (TGA), charge-discharge of simulated cell and cyclic voltammogram (CV). The results are showed as followed:2.1 In the oxygen atmosphere , the structural performance of LiNi_0.7Co_0.3-xGa_xO₂ (x=0.01,0.03,0.05) synthesized by the high temperature solid state reaction waseffected by the preparation condition of the precursor . The precursor was prepared by two different processes: (i) the raw material were compressed to be pellet before the first calcinations step, (ii) the raw material were mixed only without compress before the first calcinations step. The structural performance of LiNi_0.7Co_0.3-xGa_xO₂ (x=0.01,0.03,0.05) from the first processes was better .2.2 The influence of doped Gallium to the crystal structure and the thermal stability of LiNi_0.7Co_0.3-xGa_xO₂ (x=0.01,0.03,0.05) . With increase the quantity of doped Gallium in LiNi_0.7Co_0.3-xGa_xO₂, its crystal performance (degree of crystal) was enhanced. The results of thermo-gravimetric analysis (TG) shown that, if the temperature of second calcinations process was lower than 800℃, the additive of Gallium was conducive to increase the thermal stability of LiNi_0.7Co_0.3O₂ cathode material.2.3. The tests of charge-discharge of simulated cell and cyclic voltammogram (CV) revealed that when Ga doped, the capacity of LiNi_0.7Co_0.3-xGa_xO₂ (x=0.01,0.03,0.05) increased and structural stability of the cathode was improved during the charge-discharge. The gallium doping stabilized the crystal structure of the LiNi_0.7Co_0.3-xGa_xO₂ (x-0.01,0.03,0.05) during charging, suppression of the phase transitions.2.4 The results of preliminary study about LiNi_0.7Co_0.2M_0.1O₂ (M= Mg²⁺, Al³⁺, Ti⁴⁺, Sr³⁺) shown that: when simultaneously multiple additive Al³⁺ and a few Ti⁴⁺ or Sr³⁺ to LiNi_0.7Co_0.2M_0.1O₂, the unit cell volume of LiNi_0.7Co_0.2M_0.1O₂ was decreased, and the ratio value of I₍₀₀₃₎ /I₍₁₀₄₎ was increased, these revealed that the inter layer binding force were enhanced. If Mg²⁺ was doped, the value of a does not change ,the value of c increase slightly, the reason for that were the radius of Mg²⁺is approaches to the radius of Li+, and it is bigger than the radius of other positive ions (Mg2+ :0.072nm; Li+:0.076nm;: 0.051nm; Co3+:0.053nm; Ni3+:0.056nm).