Synthesis,Modification And Electrochemical Characterization Of LiCo_1-xNi_xO₂ As Cathode Materials For Lithium Ion Batteries

A series of compounds in the LiCo1-xNixO2 system (0≤x≤1), which are considered as next-generation methods for lithium-ion batteries, have been synthesized by citrate sol-gel method. In the whole composition range (0≤x≤1), a pure phase LiCo1-xNixO2 has been obtained by this method. The evolution of the structure and electrochemical properties of the synthesized LiCo1-xNixO2 (0≤x≤1) with nickel content x was studied. The a parameter increases continually with x as a result of the difference in size between the Co3+ and Ni3+. In the same way, the c parameter increases slowly when x increases. But c/a decreases while Ni3+ substituted for Co3+. LiCo0.3Ni0.7O2 exhibited better electrochemical performance in the LiCo1-xNixO2 system (0≤x≤1). The synthesis process of LiCoO2 prepared by citrate sol-gel method was studied by using MS, FTIR, TG/DTA, XRD and elemental analysis. The results show that lithium and cobalt metal ions were trapped homogeneously on an atomic scale throughout the precursor. Lithium carbonate, Co3O4 and CoO were intermediate products during heat treatment of the precursor and Li2CO3 undergoes direct reactions with corresponding reactants to form LiCoO2. Moreover, the kinetics of formation of LiCoO2 by the citrate sol-gel method is faster than the case of the conventional solid-state reaction between lithium carbonate and corresponding reactants. Phase pure HT-LiCoO2 was obtained at 750℃. In comparison with the solid-state reaction, the sol-gel method significantly shortens the required reaction time for synthesizing LiCoO2, and also reduces the particle size. In the electrochemical test, it is found that the specific discharge/charge capacity as well as the coulomb efficiency substantially increases with increasing the calcination temperature. It is considered that the formation of a pure LiCoO2 with a good-layered structure facilitates the insertion and de-insertion of lithium ions. As a result, the combination of the sol-gel method with proper calcination processes is highly successful in producing LiCoO2 powders with large specific capacity and good cycle performance. The synthesis process of LiCo0.3Ni0.7O2 was also investigated by FT-IR, mass spectroscopy, elemental analysis, SEM, BET, TG/DTA and XRD in this paper. The results revealed that lithium and transition metal ions were trapped homogeneously on an atomic scale throughout the precursor. Li2CO3, NiO and CoO were the intermediate products obtained after decomposition of the precursor and Li2CO3 undergoes direct reactions with NiO and CoO to form LiCo0.3Ni0.7O2. Moreover, the kinetics of formation of LiCo0.3Ni0.7O2 by citrate sol-gel method is faster than the case of the conventional solid-state reaction between lithium carbonate and corresponding reactants. The single phase of LiCo0.3Ni0.7O2 was synthesized at temperature as low as 550℃. The discharge capacity of LiCo0.3Ni0.7O2 increases from 67 to 165mAh/g as the calcinations temperatures increasing from 550 to 750℃. An electrochemical study showed that the LiCo0.3Ni0.7O2 powders had high capacity and good cycling behavior for lithium ion batteries. The effects of varying the ratio of citric acid to transition metal ions in the precursor on the structure, thermal and electrochemical properties of the synthesized compound were investigated. The compound synthesized with the citric acid to transition metal ions R=1 yielded better electrochemical performance.The influence of calcination time on the physic-chemistry properties of LiCo0.3Ni0.7O2 was also studied. Increasing calcination time leaded to the volatilization of lithium and resulted in decreased capacity of the material. However, the material also, which not being calcined for enough time, did not show good electrochemical performance. In the present study, the optimum time was 8h. The effects of the solvents of the precursor solution and the partial pressure of O2 on the product characteristics were also discussed. The material synthesized at water system synthesized exhibited better electrochemical performance than that synthesized at ethanol system. The optimum partial pressure of O2 was 40.5KPa. A series of polymer have been studied as chelating agents for the synthesis of LiCo0.3Ni0.7O2. The decomposition temperature of the polymer precursors was found to increase with their chelating ability. The product obtained from the PVA-precursor gave the promising capacity while that from the PAAm-precursor exhibited the worst property. The ICP-AES method was used to determine Li content in cathode materials of lithium ion battery. Nitric acid was chosen as medium of determination and the influence was investigated on coexisted ions which contained Co, Ni, Mn, Fe, PO 34 ? and so on. It was swift, accurate and selective. The RSD of lithium is ≤1.02% and the range of the recovery is 99.2%～101.3%. Cobalt and Nickel in the cathodes LiCo1-xNixO2 for rechargeable lithium secondary batteries can be determined simultaneously by dual-wavelength equal absorption spectrophotometry combining with standard addition method. The results were compared with that of ICP. The experimental results show that the method is satisfied. The linear ranges are 3μg~30μg/25mL for Co2+ and 0μg~25μg/25mL for Ni2+. The recoveries of Co2+ and Ni2+ in synthetic samples are between 97.7%~103.7% and between 97.2%~103.3%, respectively. The relative standard deviations of analytical results in LiCo1-xNixO2 samples are less than 2.2% for Co2+ and less than 1.2% for Ni2+, and relative errors are less than ±2.2% for Co2+ and less than ±1.9% for Ni2+. The method is simple operation, easy master, economic,utilitarian, accurate and reliable. The specific application of one-step oxidation method to measure the average oxidation state of (Co,Ni) in the cathodes LiCo1-xNixO2.The results showed that the dissolution of the samples was the key step. The effects of the factors such as the volume of acid and temperature on the analytical results have been studied. The method was proved to simple, accurate and reliable. The recoveries of the sample was between 99.77%~100.3%. The standard deviation was less than 0.25% and the relative standard deviation was under 0.30%. Meanwhile, a novel route used preoxidized nano-size powders Ni3O2(OH)4 as nickel sources to prepare LiCo0.3Ni0.7O2 as cathode materials for rechargeable lithium ion batteries is reported. The synthesized LiCo0.3Ni0.7O2 was characterized by FTIR, ICP, XRD, SEM, BET, valence analysis and electrochemical performance. The result shows that the novel route reported in this paper is a successful route to prepare LiCo0.3Ni0.7O2 as cathode materials of lithium-ion batteries with good electrochemical performance. The initial discharge capacity of LiCo0.3Ni0.7O2 synthesized at 800℃for 8h is 154mAh/g . There is no decrease in capacity over 10 cycles. LiCo0.3Ni0.7O2 has also been prepared at 700℃for 8h in air by using β-Co0.3Ni0.7OOH and LiOH as starting materials. The sample was characterized by XRD, ICP, SEM, XPS, valence analysis and electrochemical performance. The results show that the sample has a good layered structure and the average oxidation value of (Co,Ni) in the sample is +3. Moreover, the sample has a good electrochemical performance with an initial discharge capacity of 145mAh/g and there is no decrease in capacity over 10 cycles. XRD, XPS and ICP were used to study the hydroscopic property of Li1-xCoO2 (0