Comprehensive Study of LFP and LMFP/Graphite Lithium-ion Batteries
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Abstract
Although phosphate-based cathodes are valued for their safety and low cost, their lifetime remains limited by capacity fade associated with lithium inventory loss and transition-metal dissolution. This thesis integrates the author’s published and unpublished work to investigate the degradation of LFP/graphite and LMFP/graphite lithium-ion cells. Electrochemical, physical, and chemical techniques, including cycling tests, impedance spectroscopy, X-ray fluorescence, and liquid-phase NMR, were used to study degradation at elevated temperatures. The results show that cell degradation is strongly influenced by electrolyte additives, salt chemistry, electrolyte quantity, and residual N-methyl pyrrolidone from electrode manufacturing. Increasing vinylene carbonate (VC) suppresses Fe deposition on graphite and improves the lifetime of LFP/graphite cells. In contrast, higher VC content accelerates Mn deposition in LMFP/graphite cells, highlighting the greater complexity of transition-metal dissolution in LMFP systems and the need for further electrolyte optimization to extend cell lifetime.
Description
Chapter 3 investigated the degradation in LFP/graphite cells by correlating VC consumption, lithium-alkoxide (DEOHC) generation, Fe deposition on the graphite, gas evolution, and impedance growth. VC can suppress lithium-alkoxide formation. But once VC is depleted, the generation of lithium alkoxide causes DEOHC, triggers accelerated Fe dissolution and subsequent deposition on the graphite anode. The Fe deposition compromises the SEI and accelerates the lithium inventory loss which causes faster cell degradation. Increasing VC content effectively delays the acceleration of Fe deposition and thus improves lifetime, but at the cost of increased gas evolution and charge-transfer resistance.
Chapter 4 compared cells with LiPF6 or LiFSI electrolytes to develop the understanding based on LiFSI. While higher VC content improved lifetime for cells with LiPF6 or LiFSI, significant differences were identified. For cells with high content of VC, cells with LiPF6 showed continued salt consumption and much higher impedance growth. Cells with LiFSI showed a consistent relationship between VC consumption and capacity loss. For cells with low content of VC, although the Fe deposition rates were similar, LiFSI cells demonstrated superior lifetime, indicating the difference in SEI quality.
Chapter 5 further explored the LFP/graphite cell degradation by examining cells with higher VC content, alternative additives, or a different commercial LFP material. Cells with excessive VC content had serious detrimental effects such as increased impedance and gas evolution, but without obvious improvement of life-time. This confirms that VC content should be within an optimal concentration range. For alternative additives, FEC exhibited similar behavior to VC in terms of cycling stability and Fe deposition. However, DTD was less effective compared with FEC and VC. For the cells with a different commercial LFP material, similar degradation trends were found in terms of VC consumption, DEOHC generation and acceleration of Fe deposition. This shows that lithium-alkoxide-driven Fe dissolution is likely to be a general phenomenon. This chapter highlights both the validity of the lithium alkoxide–driven degradation mechanism and the effects of different electrolyte additives on LFP cell performance.
Chapter 6 continued the investigation into LMFP/graphite cells, and fundamentally different behavior was found compared to LFP. Unlike in LFP/graphite cells, increasing VC content did not improve cycle life in LMFP cells using LiPF6. Instead, higher VC content accelerated Mn deposition on the graphite anode. However, this did not significantly worsen cycle life apparently due to the formation of a more robust VC-derived SEI. The direct comparison between LFP and LMFP/graphite cells shows that with identical electrolyte, the cell life-time is highly dependent on the amount of transition metal deposition, no matter Fe or Mn. These findings suggest that the proper electrolyte design for LFP/graphite cells cannot be directly transferred to LMFP.
Chapter 7 investigated the impact of residual NMP from electrode processing. It was shown that NMP can persist in cells within cathodes of high BET. Studies using NMC532/graphite cells revealed that NMP can increase cathode impedance, with additional degradation arising from its interaction with electrolyte additives DTD. Small amounts of NMP might not cause severe capacity loss, but they can increase charge transfer impedance and degrade power performance. These results suggest the importance of the electrode drying process in both laboratory and industrial manufacturing.
Keywords
LFP, LMFP, battery