Improved positive electrode materials for lithium-ion batteries: Exploring the high specific capacity of lithium cobalt oxide and the high rate capability of lithium iron phosphate.

摘要

During the past decade, the search for better electrode materials for Li-ion batteries has been of a great commercial interest, especially since Li-ion technology has become a major rechargeable battery technology with a market value of {dollar}3 billion US dollars per year. This thesis focuses on improving two positive electrode materials: one is a traditional positive electrode material—LiCoO₂; the other is a new positive electrode material—LiFePO ₄.; Cho et al. reported that coating LiCoO₂ with oxides can improve the capacity retention of LiCoO₂ cycled to 4.4 V. The study of coatings in this thesis confirms this effect and shows that further improvement (30% higher energy density than that used in a commercial cell with excellent capacity retention) can be obtained. An in-situ XRD study proves that the mechanism of the improvement in capacity retention by coating proposed by Cho et al. is incorrect. Further experiments identify the suppression of impedance growth in the cell as the key reason for the improvement caused by coating. Based on this, other methods to improve the energy density of LiCoO₂, without sacrificing capacity retention, are also developed.; Using an XRD study, the structure of the phase between the O3-phase Li _1−xCoO₂ (x > 0.5) and the O1 phase CoO₂ was measured experimentally for the first time. XRD results confirmed the prediction of an H1-3 phase by Ceder's group. Apparently, because of the structural changes between the O3 phase and the H1-3 phase, good capacity retention cannot be attained for cycling LiCoO₂ to 4.6 V with respect to Li metal.; An effort was also made to reduce the carbon content in a LiFePO ₄/C composite without sacrificing its rate capability. It was found that about 3% carbon by weight maintains both a good rate capability and a high pellet density for the composite.