NMR monitoring of electrode/electrolyte interphase in the case of air-exposed and carbon coated LiFePO₄

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• 作者：M. Cuisinier ; N. Dupr¨¦ ; P. Moreau ; D. Guyomard
• 关键词：Lithium batteries ; Positive electrode ; Interface ; MAS NMR
• 刊名：Journal of Power Sources
• 出版年：2013
• 出版时间：1 December, 2013
• 年：2013
• 卷：243
• 期：Complete
• 页码：682-690
• 全文大小：2059 K

文摘

Degradation of the electrochemical performance of LiFePO₄ upon air exposure, assigned to a corrosion-type aging mechanism, implies the incorporation of hydroxyl groups and the formation of an amorphous tavorite-like phase at the surface. Using a carbon coating provides an efficient protection from this detrimental process but also modifies the surface in contact with the electrolyte. The formation and evolution of electrode/electrolyte interphases forming on both air-aged and carbon coated LiFePO₄ are discussed based on combined quantitative ⁷Li, ¹⁹F MAS NMR, EIS and EELS measurements. Concerning the air-aged LiFePO₄, the electrode/electrolyte interactions are dominated by the dissolution of the active material and an exacerbated reaction of incorporated hydroxyl groups with the electrolyte salt, resulting in a LiF rich interphase. This dissolution of the outer part of active material particles is accompanied by the departure of the previously formed interphase and a new interphase is then formed on a newly exposed surface. The resistive LiF rich interphase passivates the active material particles during cycling, forming a resistive film, hindering both Li ion transfer and material corrosion. Cellulose acetate based carbon coating prevents air-aging but yields to an accumulation of organic lithiated species, allowing Li transfer and maintaining good electrochemical performance.