Degradation is one of the most important battery characteristics in the development of (Hybrid) Electric Vehicles ((H)EV). Many studies are devoted to the cycling performance of Li-ion batteries in order to understand the underlying aging mechanisms [1,2]. Cyclable Li loss and electrode materials decay are considered to be the most important factors leading to battery degradation. C₆/LiFePO₄ (LFP) batteries are excellent candidates to study the C₆ anode aging since the cathode has an outstanding (electro)chemical stability at moderate temperatures.

     In the present work, the aging mechanisms of LFP batteries have been systematically investigated as a function of temperature under both long-term storage and cycling conditions. Electromotive force (EMF) curves, regularly determined by mathematical extrapolation of the measured voltage curves under various discharge currents, are used to obtain an in-depth understanding of the aging mechanisms. The irreversible capacity loss (ΔQ_ir) under various aging conditions have been accurately determined on the basis of the maximum capacities estimated from the EMF curves. To distinguish between calendar ageing and degradation resulting from cycling, a new method is proposed. Information about the individual contributions is elegantly obtained by extrapolating the total irreversible capacity loss on the basis of current and time as shown in Fig. 1a and b, respectively. Extrapolating to cycle number 0 in Fig. 1a now exclusively unravels the calendar ageing performance. On the other hand, extrapolating to time 0 in Fig. 1b exclusively reveals the ageing characteristics upon cycling.

     A non-destructive analyses of the graphite electrode decay is carried out on the basis of the extrapolated dV_EMF/dQ curves. The graphite electrode decay has been quantified and has been attributed to (i) Li-immobilization inside the SEI layer, (ii) structural deformation of the graphite electrode and (iii) iron deposition onto the graphite electrode.The structural deformation of the graphite electrode has been confirmed by the Raman analyses. Iron deposition onto the graphite electrode at elevated temperatures under both storage and cycling conditions has been confirmed by X-ray photoelectron spectroscopy (XPS), indicating that iron dissolution from the cathode is strongly temperature dependent. The Fe deposits onto the graphite electrode are shown to accelerate the SEI formation due to the enhanced electron transport.

     All irreversible capacity losses are modeled on the basis of the recently proposed SEI electron tunneling model [3,4]. Both the experimental and simulation results show that the temperature, the (dis)charge current and the State-of-Charge (SoC) play an important role in the degradation of LFP batteries.

Fig. 1.The development of under various cycling conditions as a function of both cycle number and time. The storage capacity curves are extrapolated to cycle number 0 implying a current of 0 C-rate and representing calendar aging (a). Extrapolating to cycle time 0 then exclusively represents ageing characteristics upon cycling (b).

References

[1] M. Safari, C. Delacourt, J. Electrochem. Soc., 158(2011) A1123.

[2] M. Dubarry, B.Y. Liaw, M.S. Chen, S.S. Chyan, K.C. Han, W.T. Sie, S.H. Wu, J. Power Sources, 196      (2011) 3420.

[3] D. Li, D. Danilov, Z. Zhang, H. Chen, Y. Yang, P.H.L. Notten, ECS Transactions, 62(2014) 8.

[4] D.J. Li, D. Danilov, Z.R. Zhang, H.X. Chen, Y. Yang, P.H.L. Notten, J. Electrochem. Soc., 162 (2015) A858.