State of Charge When Cycling LiFeO4/Graphite Cells Affects Lifetime

Lithium iron phosphate (LFP) battery cells are used ubiquitously in electric vehicles and stationary energy storage because they are cheap and have a relatively long lifetime, but they have low energy and power density 1 . This work compares the LFP / Graphite pouch cells undergoing charge-discharge cycles over three state of charge (SOC) windows: 0 – 25%, 0 – 100%, and 75 – 100%. Cycling LFP cells across a lower average SOC provides longer cell lifetime than a higher average SOC, regardless of depth of discharge. There are two capacity fade degradation mechanisms that are prominent at high SOC: iron dissolution and deposition onto the negative electrode, and lithiated graphite reactivity with electrolyte increasing incrementally with SOC and with the amount of deposited Fe. It is commonly known that cell voltage and depth of discharge should be minimized to improve lifetime, but this is based on cell types that operate at higher voltages than 3.65 V in LFP/AG cells. Our results show that even low voltage LFP systems have this trade-off between state of charge and lifetime. We show that the average SOC of operation is more critical for capacity fade than the variables of electrolyte salt choice (LiPF 6 vs LiFSI), graphite choice or temperature (40°C vs 55°C). This work employed isothermal microcalorimetry to investigate the heat flow from lithiated graphite-electrolyte reactivity 2 , scanning micro-xray fluorescence to quantify iron deposition on the negative electrodes 3 , and liquid electrolyte analysis techniques NMR and GC-MS to investigate electrolyte degradation products. Figure 1: LFP/AG pouch cell cycling capacity fade across different SOC ranges in long term cycling over 2500 hours. For best comparison, only the C/20 checkup cycles are plotted, where all cells cycle 0-100% every 100 cycles or every ~300 hours. The regular cycles are at C/3 rate, and only 60 mAh out of 240 mAh nominal are cycled. Discharge capacity vs cycling time is plotted for (a) 40°C and (b) 55°C testing of LFP/AG cells with LiPF 6 and LiFSI electrolyte salts at various SOC operation windows. The cell barcode IDs are indicated in the legends. The normalized discharge capacity fade per hour at (c) 40°C and (d) 55°C is summarized after 2500 hours of cycling for the different SOC windows cycled. Electrolyte is EC:DMC 3:7 2VC 1.5M LiFSI or LiPF 6. References: [1] Safari, M. & Delacourt, C. Aging of a Commercial Graphite/LiFePO4 Cell. J. Electrochem. Soc. 158 , A1123 (2011). [2] Logan, E. R.; Dahn, J. R. Measuring Parasitic Heat Flow in LiFePO 4 /Graphite Cells Using Isothermal Microcalorimetry. J. Electrochem. Soc. 2021 , 168 (12), 120526. https://doi.org/10.1149/1945-7111/ac405b. [3] Eldesoky, A., Logan, E. R., Johnson, M. B., McFarlane, C. R. M. & Dahn, J. R. Scanning Micro X-ray Fluorescence (μXRF) as an Effective Tool in Quantifying Fe Dissolution in LiFePO 4 Cells: Toward s a Mechanistic Understanding of Fe Dissolution. J. Electrochem. Soc. 167 , 130539 (2020). Figure 1