In this work, we present a detailed investigation of the aging behavior in commercially produced 18650-type cells with a graphite/LFP cell chemistry. Two different kinds of cells are investigated, which only differ in the graphite type used as anode active material, namely mesocarbon microbeads (MCMB) or needle coke (NC). These graphite materials have a similar BET surface area (2.4 m2 g‑1 for MCMB, 1.9 m2 g‑1 for NC) but differ in the particle shape and morphology (inset Figure 1) and in the ratio of exposed basal and edge planes. Both cells showed good cycling stabilities with relative capacity losses of 8 % (MCMB) and 23 % (NC) after 4750 cycles with a 1C rate.
In the first part, in situ neutron diffraction at the instrument SPODI of the Heinz Maier-Leibnitz Zentrum is used to study the cell aging. We demonstrate that, based on a careful comparison of neutron diffraction and electrochemical data, it is possible to differentiate several aging mechanisms like i) active lithium loss, ii) particle isolation or particle deactivation due to loss of ionic and/or electronic contact, and, iii) destruction of bulk electrode material (metal dissolution or irreversible phase transformation). In all investigated cells, active lithium loss is the only observable capacity loss mechanism.
The second part focuses on the contribution of lithium plating to capacity fading. The enhanced capacity fading rate during the initial five hundred cycles for the needle coke cell in comparison to the MCMB cell (Figure 1) is assigned to partially irreversible lithium plating during 1C charging. The two carbons differ in particle shape and corresponding electrode properties (porosity and tortuosity), affecting Li-ion transport within the porous electrode.[1] They also differ in the nature of exposed graphite planes (edge vs. basal), affecting the charge transfer kinetics and solid electrolyte interphase (SEI) formation.[2] The question to what extent the differences in mass transport and electrode kinetics are responsible for the different lithium plating behavior of the MCMB and needle coke cells is investigated using several electrochemical techniques. Furthermore, the fundamental lithium plating mechanism and re-intercalation kinetics are studied in more detail using recently developed electrochemical operando electron paramagnetic resonance spectroscopy (EPR).[3]
Figure 1 Capacity loss of 18650-type graphite/LFP cells upon cycling between 2.0 and 3.6 V with a rate of 1C for discharge and charge. Anode active materials are mesocarbon microbeads (MCMB) or needle coke (NC). Inset: SEM images of the different carbon materials.
References
[1] M. Ebner, D. W. Chung, R. E. García, V. Wood, Adv. Energy Mater. 2014, 4, 1–6.
[2] J. P. Olivier, M. Winter, J. Power Sources 2001, 97–98, 151–155.
[3] J. Wandt, C. Marino, P. Jakes, R. Eichel, H. A. Gasteiger, J. Granwehr, Energy Environ. Sci. 2015, 8, 1358–1367.
Acknowledgement
TUM gratefully acknowledges financial support by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology under the auspices of the EEBatt project. Beam time at the Heinz Maier-Leibnitz Zentrum is acknowledged.