Degradation diagnosis of aged Li4Ti5O12/LiFePO4 batteries

• Li4Ti5O12/LiFePO4 batteries exhibit huge capacity fade after 4–5 months of aging.
• Degradation explained using electrochemistry, spectroscopy and electron microscopy.
• Both positive and negative electrodes show no significant changes after aging.
• Capacity fade result from internal imbalance likely caused by parasitic reactions.

Li4Ti5O12/LiFePO4 cells are cycled under 4 different conditions of discharge profile (galvanostatic or driving-based) and cycling rates (C/8 or 1C) during 4–5 months. All the cells exhibit capacity fade whose extent is not correlated with the aging condition. In order to understand aging phenomena, cells are disassembled at the end of cycle life and the recovered electrodes are analyzed using electrochemistry, electron microscopy, XRD and MAS-NMR. Positive and negative electrodes show no loss in active material and no change in electrochemical activity, active material structure and composite electrode structure. This rules out any irreversible electrode degradation. Lithium stoichiometry estimated by both XRD and electrochemistry is unexpectedly low in the positive electrode when the aging is stopped at full discharge. That indicates a loss of cyclable lithium or electrons leading to cell balancing evolution. That loss may have been caused by parasitic reactions occurring at both electrodes, in accordance with their rich surface chemistry as evidenced by MAS-NMR.