Abstract
The increasing demand for clean energy storage and electric vehicles is highlighting the need to develop Li-ion battery materials with low cost and abundant elements.1 , 2 Positive electrode materials such as LiFePO4 (LFP) are good candidates to replace layered oxide materials due to the high relative abundance of Fe compared to other transition metals, such as Ni and Co.1 Unfortunately, LFP suffers from low energy density.2 LiMnxFe1-xPO4 (LMFP) has been proposed as an alternative to LFP. It shares the same crystal structure as LFP with a portion of Fe sites substituted with Mn. The Mn2+/3+ redox takes place at higher potential, improving the energy density.2 In this work, we set to decouple the contribution of the Fe and Mn to cell failure in order to better understand this class of olivine materials. LiMn0.8Fe0.2PO4/artificial graphite (AG) cells were cycled over varying voltage ranges to determine the capacity fade behaviour of each plateau, Fe (2.5-3.6 V) and Mn (3.6-4.2 V), as well as the full voltage range of the cell. The worst capacity fade was seen for cells cycled over the full voltage range, while very little capacity fade was seen for the Fe plateau and no capacity fade was seen for the Mn plateau, due to the Li reservoir from the negative electrode. In order to determine the capacity fade mechanism, differential voltage analysis was utilized.3 dV/dQ analysis shows no mass loss over any of the voltage ranges, large Li inventory loss for the full voltage range and Mn voltage region, and small Li inventory loss for the Fe region. X-ray fluorescence spectroscopy was used to determine the degree of Mn deposition on the negative electrode. The degree of Mn deposition was found to be well-correlated with the amount of Li inventory loss, demonstrating that the primary failure mode of LMFP/AG cells is Mn-catalyzed Li inventory loss. In order to fix that problem, we synthesized advanced LiMnxFe1-xPO4 with different values of x and optimized material properties. We also developed novel electrolytes that reduce Li inventory loss even at elevated temperature. Overall, our work shall enable long-lived LMFP/graphite cells for longer cost electric vehicles and stationary energy storage.(1) Nitta, N.; Wu, F.; Lee, J. T.; Yushin, G. Li-Ion Battery Materials: Present and Future. Materials Today 2015, 18 (5), 252–264. https://doi.org/10.1016/j.mattod.2014.10.040.(2) Ryu, H.-H.; Hohyun Sun, H.; Myung, S.-T.; S. Yoon, C.; Sun, Y.-K. Reducing Cobalt from Lithium-Ion Batteries for the Electric Vehicle Era. Energy & Environmental Science 2021, 14 (2), 844–852. https://doi.org/10.1039/D0EE03581E.(3) Dahn, H. M.; Smith, A. J.; Burns, J. C.; Stevens, D. A.; Dahn, J. R. User-Friendly Differential Voltage Analysis Freeware for the Analysis of Degradation Mechanisms in Li-Ion Batteries. J. Electrochem. Soc. 2012, 159 (9), A1405. https://doi.org/10.1149/2.013209jes.
Published Version
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