LFP/Graphite cells are attractive because they are cheaper^[1], safer^[2,3] and can achieve acceptable energy density for most applications. A limitation of the LFP/Graphite cells is their inferior capacity retention at elevated temperature when compared to NMC/Graphite cells especially in the absence of electrolyte additives, e.g., VC (Vinylene carbonate), as observed by our group in a recent study^[4].The time it takes for a LFP graphite cell to completely self-discharge at 60^oC is around 500 hours with a base electrolyte of 1.5 M LiPF₆ (Lithium hexafluorophosphate) dissolved in 7:3 DMC:EC (Dimethyl carbonate/ethylene carbonate)^[4]. It also has been observed that during cycling some Fe will accumulate on the anode which can be explained by dissolution of Fe from the cathode and subsequent deposition on the anode. With the same electrolyte as mentioned before we can observe up to 0.2 μg/cm² on the anode after 60 cycles at 40⁰C^[4].

The goal of this research is to understand what is happening inside the LFP/Graphite cells by analyzing the electrolyte from cells that only did a formation procedure. We were able to extract different electrolytes from LFP/Graphite and NMC811/Graphite pouch cells. We expected the electrolyte to stay clear as it was just after preparation, but we observed yellow and red colors depending on the temperature of formation. If the electrolyte contained 2%wt VC, no color change was observed. The different electrolytes used for the experiments were LiPF₆, LiFSI (Lithium bis(fluorosulfonyl)imide) and LiPF₆+2%wt VC all dissolved in 7:3 DMC:EC. The extracted electrolytes were put inside coin cells with an Al foil working-electrode (WE) and a Li foil counter-electrode (CE) and cyclic voltammetry (CV) was done on them from 2.6 V - 3.75 V vs. Li⁺/Li. The CV traces show the presence of current in the μA range for electrolyte without VC extracted from LFP cells, indicating the presence of a reversible shuttle species. The electrolyte with no VC expected from LFP cells showed more current than the corresponding electrolyte extracted from NMC811 cells. There was almost no current in the coin cells using electrolytes with 2% VC extracted from the LFP and NMC811 cells. We also made systematic experiments at different formation temperature and different wait times before extraction.

Figure 1: Observation of the different electrolytes extracted from LFP/Graphite cells with a) 1.5 M LiPF₆ EC:DMC 3:7 and b) 1.5 M LiPF₆ +2%wt VC EC:DMC 3:7 that did formation at 25, 40 ,55 and 70⁰C (left to right).

References

1. W. Li, Y. Cho, W. Yao, Y. Li, A. Cronk, R. Shimizu, M. A. Schroeder, Y. Fu, F. Zou, V. Battaglia, A. Manthiram, M. Zhang, and Y. S. Meng. “Enabling high areal capacity for Co-free high voltage spinel materials in next-generation Li-ion batteries”, Journal of Power Sources, 473 (2020).
2. D. Jian, T. Xuan, D. Haifeng, Y. Ying, W. Wangyan, W. Xuezhe, and H. Yunhui. “Building Safe Lithium-Ion Batteries for Electric Vehicles: A Review”, Electrochem. Energ. Rev. 3, 1–42 (2020).
3. W. Li, H. Wang, Y. Zhang, and M. Ouyang. “Flammability characteristics of the battery vent gas: A case of NCA and LFP lithium-ion batteries during external heating abuse”, Journal of Energy Storage, 24 (2019)

4. E. R. Logan, H. Hebecker, A. Eldesoky, A. Luscombe, M. B. Johnson, and J. R. Dahn. “Performance and Degradation of LiFePO4/Graphite Cells: The Impact of Water Contamination and an Evaluation of Common Electrolyte Additives”, Journal of The Electrochemical Society, 167, 13 (2020)