Understanding Performance Issues in Gr/Lfp: An Exploration of Electrochemical Testing Parameters and Origins of Gassing

The majority of Li-ion batteries in use today make use of NMC cathode systems, due to their ability to meet the power and energy requirements demanded of them. For applications that don’t require high energy density and instead opt for battery systems with a higher emphasis towards cost and safety metrics, LFP is often utilized, as LFP is considered a much safer chemistry than NMC due to enhanced chemical stability, reduced toxicity, and greater abundance of raw materials for synthesis, leading to lower cost. However, implementation of LFP systems faces its own issues- at elevated temperatures, LFP suffers from reduced stability manifesting as reduced capacity retention and elevated gassing during high temperature testing 1 . Performance of lithium-ion cells using this chemistry, particular at higher temperatures and higher energy density applications, prevents widespread adoption of Gr/LFP due to observed performance detriments. Creating a thorough understanding of the effects of temperature and voltage is critical to solving the problems associated with implementation of Gr/LFP systems. In this presentation, studies were conducted to explore the effect of different electrochemical testing parameters on the stability of Gr/LFP systems. Testing included storage testing at 30°C, 45°C, and 60°C and at 3.4 and 3.65V. Development and optimization of electrolyte components have been successfully employed to improve performance and abate key stability issues by both increasing capacity retention and reducing gassing at higher temperatures. Previously, isotopically labeled carbonates were used in Gr/NMC811 multilayer pouches aged at elevated temperatures, and the resulting gases were analyzed by GC-MS to identify and quantify the specific solvent source(s) of each gas species 2 . This presentation builds on this knowledge base by investigating whether gas mechanisms observed in the NMC811 system persist in other chemistries, or if Gr/LFP system exhibits fundamentally different electrolyte decomposition mechanisms. 13C-labeled carbonate solvents, including labeled DMC, DEC, and EC, are used in conjunction with simple binary carbonate blends to understand key gassing mechanisms and identify markers for electrolyte solvent decomposition. These studies offer insights that can directly inform electrolyte-based modifications to improve the high temperature stability of Gr/LFP batteries. Logan, E.R.; Hebecker, H.; Eldesoky, A.; Luscombe, A.; Johnson, M.; Dahn, J.R.Performance and Degradation of LiFePO 4 /Graphite Cells: The Impact of Water Contamination and an Evaluation of Common Electrolyte Additives. Journal of the Electrochemical Society 2020. 167, 130543 Guillot, S.L.; Kerber, B.; Du, P.; Zhou, L.; Garg, S.; Usrey, M.L. Isotopic Labeling of Carbonate Solvents for Mechanistic Insights into Battery Gassing and Gas Reduction By Fluorinated Organosilanes. 243 rd Electrochemical Society Meeting. Boston, MA. May 29 th , 2023.