Study On Layered Lithium Nickel-based Oxides As Cathode Materials For The Application Of Lithium-ion Batteries

Lithium-ion battery is an excellent, "green", rechargeable battery after the nickel-cadmium and nickel-hydrogen ones with the advantages of high cell voltage, high specific energy, low rate of self-discharge, no-memory effect and etc. Since its first emerging, it has been widely used in small portable electronic products: such as cellar phone and laptop computers. And now it anticipates promising prospects in the fields of electric vehicle, aviation and aerospace.The cathode materials have a strong influence over the properties of the lithium-ion battery. Compounds such as LiCoO₂, LiMn₂O₄ and LiNiO₂ are attractive cathode materials for lithium-ion battery. Now the commercialized lithium-ion battery mostly uses LiCoO₂ as cathode material, but cobalt suffers the disadvantage of limited resource, high price and heavy pollution to environment. Though LiMn₂O₄ enjoys the advantage of low-price, easy preparation and non-toxicity to environment, yet its cycleability not good and specific capacity low, especially capacity fading under high temperature. While LiNiO₂ is the most promising cathode materials: plenty in resource, cheapness in price, high specific capacity and non-toxicity. But it is difficult to be synthesized, and its unstable structure results in rapid capacity fading and bad thermal stability. This thesis focuses on the study of the layered lithium nickel-based oxides as cathode materials of lithium-ion batteries. The main aspects follow: 1 Study on the synthesis and electrochemical properties of LiNiO₂By the solid-state method usually happens the non-uniformity of raw materials in mixing. In order to prepare LiNiO₂ with good performance, the preheating treatment was adopted according to TG-DTA. We have meticulously studied the synthesizing conditions: the Li/Ni molar ratio of raw materials, calcining temperature, calcining time and atmosphere, and their influence over the crystal structure and electrochemical properties of LiNiO₂. Based on the work above, the route and conditions for preparing LiNiO₂ have been optimized. We have systematically studied the influence of charging/discharging conditions on the specific capacity and cycling performance of LiNiO₂. Experimental results showed that, synthesized in the best conditions , the first discharge capacity of LiNiO₂ is 193.2mAh/g at the current of 5mA/g and voltage between 3.00 and 4.30V; at 55℃, its first discharge capacity up to214.4mAh/g.2 Study on the synthesis and properties of LiNiCh doped with single kind of elements.In order to improve the crystal structure and cycling performance of LiNiO2, we doped LiNiC>2 with Co, Al, Mg, Ti and Sr respectively, and studied the influence of the dopants and their amount on the crystal structure and electrochemical properties of LiNiO2. Except for Sr2+ whose ionic radius is larger than other's, a suitable amount of the dopants of Co, Al, Mg or Ti in LiNiO^ all can improve the crystal structure and cycling performance of LiNiO2 .We have also systematically studied the influence on the specific capacity of LiNio.85Coo.15O2 under different charging /discharging conditions and analyzed the cause of capacity fading during the process of storage. When voltage in the range of 3.00^<sup>4.30V and current at 5mA/g, the first discharge capacity of LiNio.g5Coo.15O2 is 203.6mAh/g; at 18mA/g, its first discharge capacity is 192.6mAh/g, after 10 cycles, its capacity 186.8mAh/g; when the voltage between 3.00 and 4.50V , the current at 18mA/g, its first discharge capacity is 217.4mAh/g, after 10 cycles, its discharge capacity 192.4mAh/g; at 55r, 36mA/g^ 3.00—4.30V, its first discharge capacity is 216.7mAh/g. These results show that it has good electrochemical properties and promises a hopeful cathode material for lithium-ion battery.3 Study on the synthesis and properties of L1N1O2 doped with multi-elementsOn the basis of LiNii.xCoxO2, other elements M (M=A1, or Ti+Mg) were further doped in LiNij.xCoxO2. The ions of multi-element dopants exert different effects on the crystal structure of LiNiO2 and, thus, improve the performance of LiNiO2. The dopants of Co and M (=A1, or Ti+Mg) in LiNiO2can improve its cycling performance and thermal stability, and, nevertheless, reduce its capacity. The best amount of multi-element dopants in LiNiO2 is proposed.4 Study on the Synthesis and properties of LiNii-xCoxO2 by particle sol-gel (PSG) Compared with solid-state method, PSG enjoys the advantage of lower calciningtemperature and small uniform particle size of products. The results showed that LiNii.xCoxO2 synthesized via PSG have an improved electrochemical performance.5 The calculation of the cathode material of the lithium-ion battery by the first principle calculations.According to density functional theory, we used the first principle calculations tocalculate the cathode material of lithium-ion battery. The calculating results showed that the average voltage of LiNiC>2 is 3.21V and that of LiCoO2 is 3.87V. These values are close to the experimental ones. Therefore, such calculations provide a theoretical basis for the synthesis of materials.