Electrode/electrolyte compatibility plays a critical role to increase the energy density and longevity of lithium-ion cells. A sound understanding of the interactions between electrode and electrolyte allows us to design better electrolyte formulation with more suitable additives. Nevertheless, the study of electrode/electrolyte interaction is difficult to carry out in a full lithium-ion cell due to the interactions between positive and negative electrodes, such as the migration of oxidation products and dissolved transition metal to the negative electrode¹, and the effect of different negative electrodes on the surface layer composition of positive electrode². In recent years, there has been quite a few experimental techniques developed aiming to solve this problem^3,4. Among them, symmetric cells are excellent test vehicles where performance is exclusively determined by the compatibility of the electrode of interest with the electrolyte of choice.

In this study, the compatibility of two electrolyte additives, vinylene carbonate (VC) and prop-1-ene-1,3-sultone (PES), with uncoated polycrystalline Li[Ni_0.6Mn_0.2Co_0.2]O₂ were studied using positive-positive symmetric cells with electrodes hand-made from powder. Each symmetric cell was constructed with one fresh electrode and one delithiated electrode prepared using a coin-cell half cell. These symmetric cells were then cycled at 40^oC with 10 cycles of C/10 followed by 1 cycle of C/30 repeatedly between ±0.9 V, ±0.8 V, ±0.7 V, ±0.6 V, ±0.5 V, which correspond to half cell voltages of 4.39 V, 4.35 V, 4.29 V, 4.22 V and 4.15 V vs. Li/Li⁺, respectively. It was found that for all symmetric cells cycled with the chosen voltage cut-offs, the cells containing VC showed better capacity retention compared to those with PES. This is surprising because in full Li-ion cells operated above 4.3 V, the opposite was found to be true by Lin Ma et al. ⁵

The symmetric cell building method in this work can also be used to study electrode/electrolyte compatibility when machine-made electrodes are not readily available to researchers.

References

1. A. J. Smith, J. C. Burns, D. Xiong, and J. R. Dahn, J. Electrochem. Soc., 158, A1136 (2011)
2. E. Björklund, D. Brandell, M. Hahlin, K. Edström, and R. Younesi, J. Electrochem. Soc., 164, A3054–A3059 (2017)
3. D. J. Xiong et al., J. Electrochem. Soc., 164, A340–A347 (2017)
4. C. Shen et al., J. Electrochem. Soc., 164, A3349–A3356 (2017)
5. Lin Ma et al, J. Electrochem. Soc. 161, A2250-A2254, (2014).