Evolution of the LiFePO4 positive electrode interface along cycling monitored by MAS NMR
Résumé
Decreasing particle size ensures a good accessibility of LiFePO4 to lithium ions and electrons and allows reaching the theoretical capacity and achieving high cycling rates. It nevertheless leads to an increase of the surface area and thus a subsequent amplification of the parasitic reactions at the interface between active material surface and electrolyte. The formation and evolution of the interphase on the surface of LiFePO4 may affect strongly its electrochemical performance. This work aims at monitoring the interphase forming on LiFePO4 upon its operation in a lithium battery, and correlating its evolution with the electrochemical behaviour of the active material. Combined XPS and multinuclear quantitative 7Li, 1H and 19F NMR indicate a trend for the interphase structure that can be described as a mostly stable inner interphase composed of fluorinated inorganic products while outer lithiated organic species undergo a reversible formation at high potential and dissolution at low potential. Although an irreversible accumulation of interphase occurs, spin-spin relaxation time analysis indicates that interphasial species tend to stack on the top of each other rather than cover the whole active material surface. Moreover, the dissolution/precipitation process seems to prevent a blocking of the electrode surface by resistive species such as LiF.