Preparation And Modification Of LiNiO₂ Cathode Material For Lithium-ion Batteries

Lithium-ion batteries with many advantages of high energy density, long cycling life and low pollution are widely applied in many regions, such as potable electric devices and electric vehicles, etc. LiNiO₂ has been intensely investigated because of its low cost and large theoretical capacity (275mAh·g^-1). Study on preparation and modification of LiNiO₂ has theoretical and practicable significance.Cathode material LiNiO₂ was synthesized by solid-state reaction method at high temperature with a little excess lithium, adding fusing agent and increasing certain pressure in closed oxygen atmosphere. The charge-discharge specific capacity and cycling performance were researched by electrochemical tests. The influence of preparation conditions, such as partial pressure of oxygen and temperature during the second sintering process, mole ratio of LiOH/LiNO₃ and the mole ratio of Li/Ni, on the electrochemical performances of samples were discussed. The optimum conditions were obtained by Lstin orthogonal experiments of L₉(3⁴).LiNiO₂ prepared by optimal synthetic technology was characterized through XRD, SEM and particle size analysis. Results showed it had a-NaFeO₂ layered structure, normal particle size distribution and fine crystalline. Electrochemical tests showed that the initial charging and discharging capacities of the battery using LiNiO₂ as the cathode materials were 224.9 mAh·g^-1 and 182.2 mAh·g^-1 respectively and the cycling performance was good.Ni_1-yCo_y(OH)₂ and Ni_1-x-yMg_xCo_y(OH)₂ precursors were prepared by co-deposition method. Precursors were used as resources to synthesize cathode materials LiNi_1-yCo_yO₂ and LiNi_1-x-yMg_xCo_yO₂. The optimum doping contents were got by experiments. The results of XRD analysis showed that the sample powder crystallized perfectly. There were notmiscellaneous phases after doping. On the other hand, the layered structure of LiNii-yCoyO2 was more regular with the increment of Co coping content. Electrochemical tests showed that the initial charging and discharging capacities of the batteries decreased with increasing of the Co coping content, but the coulomb efficiency of the first charge-discharge and cycling performance were improved. The comprehensive performance of LiNio.8Coo.2O2 was the best. As for Co and Mg co-doping, electrochemical performance of LiNio.75Mgo.1Coo.15O2 was excellent. Its synthetic property was increased by synergetic effect of Mg and Co.The sample LiNio.sCoo.2O2 was modified by surface treatment. XPS, SEM and XRD tests were carried out. The results showed that initial charging and discharging capacities of coated LiNio.8Coo.2O2 were 193 mAh-g'1 and 163.7 mAh-g"1. The capacities decreased after coating but the cycling performance was improved. It was found that the surface modification can avoid harmful interactions between cathode materials and electrolyte and the cycling performance of cathode materials were improved effectively.LiNiO2 was synthesized by a sol-gel method using lithium nitrate and nickel nitrate as materials, and citric acid as a chelating agent. Through orthogonal experiments, the best technology conditions were obtained: the mole ratio of Li/Ni was 1.05 ' 1, the mole ratio of citric acid and metal ions was 0.6 '. 1, pH value of sol forming was 3.25, temperature of sol forming was 85°C, drying temperature and time were 115°C and 2h respectively.LiNiO2 prepared by sol-gel method under the optimum conditions had large discharge capacity and good cycling performance. The initial charge and discharge capacities were 186.9 mAh-g'1 and 157.2 mAh-g"1 respectively. The study on Co doping was also studied. The best doping content was 20%. The first charge and discharge capacities of LiNio.8Coo.2O2 were 173 mAh-g'1 and 149.8 mAh-g"1 respectively. The discharge capacity at the 20th cycle was 143.8 mAh-g-1. Discharge capacity decreased after Co-doping comparing with LiNiO2, but the cycling performance was obviously improved.