Study on High-rate Charge/Discharge of LiFePO4 Cathode Material for Lithium-Ion Batteries
Date Issued
2009
Date
2009
Author(s)
Chang, Hao-Hsun
Abstract
Lithium iron phosphate (LiFePO4) is a new-generation cathode material for lithium ion batteries. It has the characteristics of environmental benignity, high safety, and low cost, and is proven to have the ability for high-power applications. In this dissertation, the analysis based on a non-isothermal methodology was conducted to establish a model for the synthesis of LiFePO4 via solid-state reaction, thus a rapid synthetic route was acquired which was able to produce a batch of powders at 600oC in merely two hours. The product has the capacity of ~140 mAh/g and the tap density of 1.33 g/cc. Compared with the commercial powders, the volumetric energy density of our product is at least 20% higher.oreover, the structural transformation of LiFePO4 during charge/discharge was monitored by using synchrotron X-ray diffraction. The results revealed serious delay of structural change, especially at 1C at 55oC, the phase changed abruptly when current stopped. The structural transformation became “normal” while no rest period was between every charge and discharge process, indicating the effect of charge/discharge protocols on the structural change. In the prolonged cycle test, the cell experienced abruptly structural change every cycle showed higher fading rate after a certain cycle. The reason was the abruptly structural change inducing the defects which, after accumulation along cycling, caused structural collapse. These results impact the applications of LiFePO4 at very high rates. vital problem of LiFePO4 cycling at high temperatures is the dissolution of Fe ions which is induced by HF in the electrolyte. The dissolution not only influences the cycle life of positive electrode itself, but also the carbon negative electrode due to the migration of Fe ions through electrolyte and consequently depositing on the surface of carbon particles. The deposited Fe would catalyze the decomposition reaction of electrolyte and cause a thick passivated layer on the carbon electrode, thus reduce the cycle performance of LiFePO4/C full cells. Two methods in this dissertation were proposed to solve this problem. TiO2 coating on LiFePO4 particles was tried to prevent the direct contact between LiFePO4 particles and HF. The results showed that the cycle performance was indeed enhanced in the case of TiO2-LiFePO4/Li half cell, but not of TiO2-LiFePO4/C full cell. It was revealed that the TiO2 layer was alternatively corroded and the dissolved Ti ions also deposited on the carbon electrode. The canalysis ability of Ti was found even stronger than Fe. This problem has never revealed in the literatures trying to coat metal oxides on LiMn2O4 or LiCoO2 for the same reason in that they didn’t look into the performance of full cells. In the other modification, a metal layer was sputtered on the carbon electrode. It was discovered that the Fe ions which diffused from the positive electrode were almost all collected on the top of metal “sieving” layer. The catalytic reaction of the decomposition of electrolyte was prevented because Fe ions were not directly deposited on the surface of carbon particles.
Subjects
Li-ion battery
LiFePO4
Kinetics
Structural transformation
Coating
SDGs
Type
thesis
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