Electrolyte Role on the Thermal Runaway Characteristics of LFP Chemistry
Authors: Torres Gabriel , Vaidya Rutvik , Brown Kevin , Moganty Surya 
Published: July 2022

Lithium-ion batteries have become the power source of choice in grid storage applications and electric powertrains, given its high energy density and extended cycle life. However, lithium-ion batteries carry significant safety risks and are a growing hazard due to thermal runaway that can be caused by overheating, cell deformation, overcharging and short-circuits. It is well understood that thermal runaway occurs when the self-heating rate in the cell exceeds the heat dissipation out of the cell. This self-heating is the product of undesired side-reactions in the cell, such as SEI decomposition or oxygen evolution from the cathode, triggered by abuse conditions that then result in thermal runaway. The key to inhibiting thermal runaway is to modify the cell such that the self-heating rate under abuse conditions is significantly reduced.

These risks are believed to be mitigated by compromising energy density with safety through the choice of lithium iron phosphate (LFP) as the cathode. LFP is known to demonstrate lower rates of self-heating and achieve lower maximum temperatures during thermal runaway relative to mixed metal oxide cathodes (NMCs). LFP has other reported advantages over NMCs, such as superior cycle life, rate capability, no questionably sourced metals, and significantly lower cost. However, the safety aspect of LFP is only relative to their less stable metal oxide competitors, as LFP cells are still known to evolve oxygen and enter thermal runaway, particularly in larger cells and packs. This concern will increase in the near future as the next obvious battery design choice will be to pair LFP with silicon dominant anodes or lithium metal to create cells with energy densities on par with conventional metal oxide chemistries. There is a need to reduce cell self-heating without further compromising energy density. Therefore our approach is to modify the electrolyte system to reduce the overall cell self-heating rate. This is a tunable, drop-in solution to addressing the need for improving battery safety.

We report an alternative electrolyte design to improve the robustness of the SEI and overall stability of the electrolyte system, which carries significance in improving cycle life and abuse tolerance in LFP chemistries in both conventional graphite and silicon-dominant anodes. These electrode pairings are investigated in coin cells, 2 Ah and 30 Ah pouch cells. The choice of electrolyte formulation is shown to impact the abuse tolerance of cells on both the material level, i.e. the electrolyte-electrode interaction and on the cell level.

We evaluate the thermal profiles of delithiated LFP paired with lithiated graphite and silicon combined with electrolyte via differential scanning calorimetry (DSC). Our findings suggest that the primary driver of thermal instability in LFP systems arises from the anode in both graphite as well as silicon. The presented electrolyte systems demonstrate reduced overall energy and peak heat flow in the thermal profiles. These findings correlate electrolyte choice with lower self-heating rates in an accelerating rate calorimeter (ARC) heat-wait-seek test on LFP pouch cells. This approach carries significance in not just safer cells but also in enabling next-generation lithium-ion battery technology premised on low-cost cobalt-free cathodes.