Ultra-High Energy Density LiFePO4 Electrodes

Abstract

Olivine structured LiFePO4 (LFP) has attracted significant attention as a promising cathode material for LIBs. An advantage of using LFP compared to other cathode chemistries (i.e., NMC, LCO & LNO) is that iron is naturally abundant, making LFP low in cost, environmental impact, and toxicity.1,2 Though LFP has high thermal stability and excellent cycle life, it suffers from poor electronic conductivity and low Li+ ion diffusion which have been improved by carbon coating and reducing particle sizes to nanometres (<1 µm), respectively.3 To improve the energy density of LFP electrodes we have developed ultra-dense LFP electrodes with densities in excess of 2.7 g/cm3 to be achieved with high loadings (15 mg/cm3) and with electrode formulations containing 90% LFP active material. This corresponds to an electrode porosity of only 14% and, as shown in Figure 1(a), represents a volumetric energy density increase of about 27% compared to conventional LFP. Moreover, as shown in Figure 1(b), ultra-high density LFP electrodes were found to have higher coulombic efficiency than conventional calendered LFP electrodes with lower densities. Voltage polarization was also found to be reduced. Lastly, Figure 1(c) shows an approximate 4 times reduction in charge transfer resistance for an ultra-dense LFP electrode with respect to a conventional calendered LFP electrode. The ability to make highly dense LFP electrodes could have profound impacts, allowing for Li-ion cells to be made with low costs and low environmental impact LFP, while achieving volumetric energy densities approaching that of Li-ion cells employing layered oxide cathodes. References X. Ren et al., J. Electrochem. Soc., 167, 130523 (2020). L. Wen, X. Hu, H. Luo, F. Li, and H. Cheng, Particuology, 22, 24–29 (2015). L.-X. Yuan et al., Energy Environ. Sci., 4, 269–284 (2011). Figure 1