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Synthesis, Modification and Storage Research of LiNi0.8-Co0.15Al0.05O2Cathode Materials for Lithium-ion Batteries

Author: LiuWanMin
Tutor: HuGuoRong
School: Central South University
Course: Metallurgical Engineering
Keywords: lithium ion battery nickel-rich LiNiO2-based cathodematerial co-oxidation-controlled crystallization molten salt method storage performance
CLC: TM912
Type: PhD thesis
Year: 2012
Downloads: 337
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Abstract


Among the nickel-rich LiNiO2-based cathode materials for lithium-ion batteries, LiNi0.8Co0.15Al0.05O2has been considered as the substitution for LiCoO2in that it owns the advantages of LiNiO2, LiCoO2and LiAlO2, such as high capacity, good thermal stability, low cost and low toxicity. However, the loss of capacity during cycling, insufficient rate capability and poor storage performance have seriously hindered its application in large scale. In this dissertation, in order to improve its electrochemical properties and storage performance, the LiNi0.8Co0.15Al0.05O2cathode material was prepared and modified using different methods.The LiNi0.8Co0.15Al0.05O2cathode material was synthesized under the elevated oxygen pressure from the spherical Ni0.8Co0.15Al0.05(OH)2.05precursor prepared by a controlled crystallization method. The effects of molar ratio of Li to Ni+Co+Al, oxygen pressure, temperature and time on the structure and electrochemical properties of LiNi0.8Co0.15Al0.05O2were examined. The spherical LiNi0.8Co0.15Al0.05O2cathode material with tap density of2.57g·cm-3was prepared under the optimum condition. The initial discharge specific capacity of these powders was190mAh·g-1at0.2C in the voltage range of2.8-4.3V, and91%of the initial discharge capacity was maintained after30cycles.The spherical Ni0.8Co0.15Al0.05O2OOH precursor was synthesized by a co-oxidation-controlled crystallization method. The effects of (NH4)2S2O8, pH value, temperature and time on the precursor’s tap density and nickel ions’ average oxidation state, as well as the precursor’s forming mechanism were studied systematically. In the initial stage, the [Ni0.8Co0.15Al0.05(H2O)x-y(NH3)y]2.05+ions were precipitated and later oxidized. In the middle and final stages, the [Ni0.8Co0.15Al0.05(H2O)x-y(NH3)y]2.05+ions were oxidized and later precipitated. Eventually, Ni0.8Co0.15Al0.05OOH with tap density of1.88g·cm-3and nickel ions’ average oxidation state of three was obtained. The LiNi0.8Co0.15Al0.05O2cathode material prepared from this Ni0.8Co0.15Al0.05OOH precursor had the best-ordered hexagonal layer structure and least cation mixing. The charge-discharge tests demonstrated that these powders exhibited excellent electrochemical properties, with an initial discharge capacity of196.8mAh·g-1and capacity retention of96.1%after50cycles when cycled at a current density of0.2C between2.8and4.3V.The LiCoO2-coated LiNi0.8Co0.15Al0.05O2cathode material was prepared successfully via a molten salt method. The results showed that a uniform LiCoO2layer with a thickness of～50nm was coated on the surface of LiNi0.8Co0.15Al0.05O2under the optimum condition. The3.0wt.%LiCoO2-coated LiNi0.8Co0.15Al0.05O2showed an initial discharge capacity of196.2mAh·g-1and capacity retention of98.7%after50cycles. While the corresponding values of the bare LiNi0.8Co0.15Al0.05O2were196.8mAh·g-1and96.1%, respectively. Moreover, the coated sample exhibited better rate capability and high-temperature performance than the bare sample. The electrochemical impedance spectroscopy analyses indicated that the LiCoO2layer coated on the LiNi0.8Co0.15Al0.05O2hindered effectively the direct contact between Ni3+and electrolyte, and decrease of NiO amount on the coated material reduced impedance during charge and discharge, which improved the electrochemical performance of LiNi0.8Co0.15Al0.05O2. On the other hand, the LiCoO2-coated LiNi0.8Co0.15Al0.05O2cathode material prepared by a precipitation method showed lower discharge capacity than the pristine LiNi0.8Co0.15Al0.05O2and the same cycle performance as the bare sample, which was attributed to the formation of the mixture of LiCoO2and LiNi0.8Co0.15Al0.05O2, with a few LiCoO2coated on LiNi0.8Co0.15Al0.05O2.The effects of washing LiNi0.8Co0.15Al0.05O2with deionized water and ethanol, and surface coating with LiCoO2on its storage performance were systematically studied. The results showed that washing with low-temperature deionized water could effectively eliminate the lithium impurity (LiOH and Li2CO3) on the surface of LiNi0.8Co0.15Al0.05O2powders, and improve slightly its storage capability without changing structure and discharge capacity. After storage in air for three months, the initial discharge specific capacity of the fresh LiNi0.8Co0.15Al0.05O2material was127.5mAh·g-1, and the capacity retention was66.5%after30cycles. While the corresponding values of the washed sample were160mAh·g-1and82%, respectively. The heat-treatment temperature after washing had obvious influence on the structure and storage capability of washed LiNi0.8Co0.15Al0.05O2. After heat treatment under450-600℃, the storage performance of the material was enhanced due to the formation of NiO on the material surface, but the discharge capacity was decreased. However, the cathode material treated under higher than600℃or lower than450℃showed the similar storage performance with the fresh sample. In addition, the air humidity during storage had remarkable effects on the fresh LiNi0.8Co0.15Al0.05O2material. The higher the humidity was, the more the discharge capacity and capacity retention decreased. However, the moisture had little influence on the storage capability of the material treated with washing and heat treatment under450℃. More meaningfully, washing twice the fresh LiNi0.8Co0.15Al0.05O2material with ethonal showed the same effect as the said washing with low-temperature water.The storage performance of the3.0wt.%LiCoO2-coated LiNi0.8Co0.15Al0.05O2cathode material prepared by the molten salt method was substantially enhanced. After storage in air with80%relative humidity for three months, the bare LiNi0.8Co0.15Al0.05O2material delivered an initial discharge capacity of127.5mAh·g-1and the capacity retention was66.5%after30cycles. Contrastively, the initial discharge capacity of the LiCoO2-coated LiNi0.8Co0.15Al0.05O2material was186.6mAh·g-1, and the capacity retention was95.3%after30cycles,90%after50cycles, respectively. XPS analyses demonstrated that Co3+in the LiCoO2coating layer has excellent chemical stability and can resist the erosion of CO2and H2O in the air. Thus, the reduction of Ni3+to Ni2+on the surface of LiNi0.8Co0.15Al0.05O2was effectively suppressed by the LiCoO2coating, and the formation amounts of NiO and Li2CO3were decreased greatly, which was responsible for the improvement of storage property of LiNi0.8Co0.15Al0.05O2.

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