Study on LiCoPO4 Cathode Materials for Lithium-ion Batteries
Date Issued
2014
Date
2014
Author(s)
Kuo, Ching-Liang
Abstract
Olivine orthophosphates LiMPO4 (M=Fe, Co, Ni, Mn) is a new-generation cathode materials for Li-ion batteries due to remarkable electrochemical properties, high theoretical capacity, and thermal stability as well as increased safety. Among them, Lithium cobalt phosphate (LiCoPO4) bares highest redox potential at 4.8V versus Li/Li+, which ensures the highest energy density so it receives a lot of attention. In this research, the main purpose is to develop a high capacity cathode material with good cycling stability, consequently the inherent weaknesses of LiCoPO4 like low electronic conductivity, low Li-ion diffusivity and some irreversible structural transformation during cycling have to be conquered. Two synthetic method included spray drying method and sol-gel method was used. To achieve our purpose, a series of modifications like carbon coating, Fe doping and different stoichiometric ratio were adopted to improve the electrochemical performance.
At first, the influence of calcination atmosphere was investigated, and it was found that oxidizing atmosphere Air was helpful for creating porous secondary particle which can boost the diffusivity of lithium ion. Besides, adding H2O2 in the precursor solution also could create the porous structure by reacting with the carboxyl group of the citric acid, and also the Fenton-like reaction is helpful for decomposing the organic compound. With porous secondary particle, the electrochemical performance was improved.
To increase the low electronic conductivity, carbon coating method was adopted. Besides, large capacity fading of LiCoPO4 might result from the decomposition of the structure during charge and discharge process, hence Fe doping was attempted to stabilize the structure. Different heat-treatment conditions like calcination temperature and holding time were tried to find out the most suitable condition. Also, the ratio between Li and transition metal Co and Fe was adjusted. By these modifications, discharge capacity and cycling stability were improved gradually.
As the electrochemical performance could be improved by coating a carbon layer on the particle and by Fe doping, Li1.1Co0.8Fe0.2PO4, which means the sample with Fe doping, was coated by carbon to observe the influence. From charge-discharge curves, a plateau of LiFePO4 at around 3.5V could be observed. By X-ray absorption near edge structure (XANES) tests, it can be confirmed that with carbon coating the reactivity of LiFePO4 was activated. Consequently, except for the capacity devoted by LiCoPO4, LiFePO4 also contributed to some discharge capacity. The experimental results show that carbon coated Li1.1Co0.8Fe0.2PO4 calcined at 650°C for 12 hours in Air delivered highest discharge capacity and good cycling stability.
Subjects
鋰離子電池
磷酸鋰鈷
噴霧乾燥
溶膠凝膠法
碳披覆
鐵摻雜
SDGs
Type
thesis
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