The introduction of electric vehicles (EVs) in the mobility market has been gaining popularity in the last decade and is considered the next step to achieve a sustainable and environmentally friendly technology for the future. EVs rely on energy storage systems to increase its capacity range. Li-ion batteries as the main power source are key for the mass market penetration of EVs. However, they still need to be improved in terms of energy density and reduce the cost of materials. The cathode is largely responsible for the limited energy density in Li-ion batteries and has a significant contribution to the total cost of the battery. Therefore, the cathode materials represent an optimization point for battery performance and cost.
Among the active cathode materials for lithium-ion batteries, Li- and Mn-rich NCMs (x Li2MnO3 · (1−x) LiNiaCobMncO2, with a + b + c = 1; often referred to as LMRNCMs) are one of the most promising materials due to their high reversible capacity. However, they need to be improved in terms of cycling performance, it has been shown that when delithiation is carried out above 80% SOC a release of oxygen lattice start. Such oxygen lattice release causes degradation of both the active cathode material and the electrolyte of the cell leading to capacity fade. Studies of these materials have revealed that the electrolyte has a great impact on this process in LMRNCMs, especially ethylene carbonate (EC).
Owing to the effect of the electrolyte in batteries containing LMRNCMs, several efforts have been taken to identify the most suitable compounds to increase their cycling performance. For this matter, fluorinated electrolytes (e.g., FEC) have shown promising results. According to the literature, it seems that FEC is more resistant to electrochemical oxidation at high overpotentials which are reflected as a superior cycling performance. Therefore, it is of great importance to elucidate the processes involved in the degradation of the battery components to develop more stable assemblies.
In a recent report, Teufl et al performed a study to evaluate the stability of EC and FEC at high potentials and the role of lattice oxygen release in the cycling performance of full cells containing LMRNCMs (Teufl et al., 2020). They proposed the implementation of on-line electrochemical mass spectrometry to identify the processes related to capacity fade. The results showed that when the full cell was operated below the onset potential of the lattice oxygen release, the performance with both electrolytes is similar. However, at higher potentials, the lattice oxygen release starts and generates chemical reactions causing the degradation of electrode and electrolyte materials. This effect is higher in the presence of ethylene carbonate, which presents a fast capacity fading related to a dramatic impedance increase. Based on these results, the authors highlighted the importance of the use of EC-free electrolytes to avoid oxygen lattice release.
The study presented by Teufl et al helped to identify the process related to the detrimental cycling performance in Li-ion batteries in the presence of EC-based electrolytes. Nevertheless, more research needs to be done to identify the most suitable materials to be used in batteries containing LMRNCMs.
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For further details please refer to the full paper available at https://iopscience.iop.org/article/10.1149/1945-7111/ab9e7f.
Teufl, T., Pritzl, D., Krieg, P., Strehle, B., Mendez, M. A., & Gasteiger, H. A. (2020). Operating EC-based Electrolytes with Li- and Mn-rich NCMs: The Role of O 2 -Release on the Choice of the Cyclic Carbonate. Journal of The Electrochemical Society, 167(11), 110505. https://doi.org/10.1149/1945-7111/ab9e7f