Uneven Aging in Graphite Electrodes of Commercial Depht Cycled Li-Ion Battery Cells

In this paper we discuss some newly reported results^1-2 on uneven aging within the graphite porous electrode of a cycled commercial cylindrical LiFePO₄/graphite cell.

Graphite is commonly used as the negative electrode material in Li-ion battery cells intended for both consumer electronic products and electrified vehicles, even though it is known that aging of this type of cells can cause capacity fade from lithium consuming side reactions on the graphite surface due to reductive decomposition of the organic electrolyte at low potentials.³ The side reactions rates decrease during the first formation cycles as the reaction products form a passive surface film, a so called solid electrolyte interphase (SEI), which in an ideal case would prevent further electrolyte reduction. However, gradual capacity fade related to loss of cyclable lithium during battery cycling is frequently reported in literature, showing that continuous film growth occurs during cycling. On the scale of porous electrodes, the SEI is often considered a rather evenly formed film on all graphite particles and of nanometer scale thickness. Recently, however, we reported on anomalous film growth on graphite electrode samples obtained from the inner parts of a commercial cylindrical LiFePO₄/graphite cell that had been subjected to 3.75 C-rate constant-current-, wide state-of-charge (SOC)‑cycling during approximately 2.5 months.¹

In order to evaluate the distribution of the apparently thick film across depth of the electrode, surface and cross-section analyses of morphological changes and elemental compositions were performed by means of scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) on manually ripped as well as on a polished, focused ion beam (FIB) milled cross sections. In addition, ion depth profiling using time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to analyze the chemical modifications on the electrode induced by cycling compared to a calendar-aged electrode. Finally, to identify the modified specific electrode properties, such as local resistances, porosity, or available surface area and the type of limitation on the electrode performance caused by the film growth, the electrochemical data was evaluated using physics-based impedance modeling.

The results show that the film of several micrometer thickness, is concentrated on top of the porous electrode and within the electrode regions close to the electrode / separator interface. The film is enriched in P-O and carbonate species but is otherwise similar in composition to the thin SEI formed on a calendar-aged electrode. FIB-SEM shows that the film is clogging the pores in the graphite electrode. A local porosity decrease, impeding the effective electrolyte transport in the electrode was also supported by performance evaluation using physics-based EIS modeling. The local variation of electrode properties implies that current distribution in the porous electrode under these cycling conditions causes inefficient material utilization and sustained uneven electrode degradation.

The study of realistic systems, regarding cell design (larger cylindrical cells and porous electrodes) and cycling conditions, is important when targeting aging phenomena in specific applications. Detailed physical and electrochemical studies of aging phenomena in these cells can highlight the relevance of different degradation mechanisms observed in laboratory model systems. 

 

References:

1. Klett, M.; Eriksson, R.; Groot, J.; Svens, P.; Ciosek Högström, K.; Wreland Lindström, R.; Berg, H.; Gustafson, T.; Lindbergh, G.; Edström, K. Non-Uniform Aging of Cycled Commercial LiFePO4//Graphite Cylindrical Cells Revealed by Post-mMortem Analysis. J. Power Sources, 2014, 257C, 126-137.

2. Matilda Klett, Pontus Svens, Carl Tengstedt, Antoine Seyeux, Jolanta Światowska, Göran Lindbergh, and Rakel Wreland Lindström, Uneven Film Formation Across Depth of Porous Graphite Electrodes in Cycled Commercial Li-Ion Batteries. J. Phys. Chem. C. in press 2015.

3. Arora, P. ; White, R.E.; Doyle, M. Capacity Fade Mechanisms and Side Reactions in Lithium-Ion Batteries. J. Electrochem. Soc. 1998, 145, 3647-3667.