Revealing the Superior Performance of Li(Mn 0.6 Fe 0.4 )PO 4 over LiFePO 4 for Thick Electrode Applications

LiFePO 4 (LFP) olivine has been widely used as cathode materials for commercial lithium-ion batteries (LIBs) due to their high safety, low cost, and overall reliable performance. Compared to LiFePO 4 , LiMnPO 4 offers a higher theoretical energy density but suffers from low electrical conductivity and lithium diffusivity due to Jahn-Teller distortion, hindering its practical application 1 . With the partial replacement of Fe with Mn, Li(Mn y Fe 1-y )PO 4 (LMFP) not only overcomes the performance limitations of LiMnPO 4 but also enables higher average intercalation potential than LFP, which provides an attractive alternative to LiFePO 4 in commercial batteries 2 . In this work, we report yet another distinct advantage of LMFP over LFP, namely, its superior performance for thick battery electrode applications. Thick electrodes have attracted significant interest because they boost the energy density at the cell level by reducing the fraction of inactive components and reduce the manufacturing cost 3 . However, the commercial application of thick electrodes is still impeded by their inferior performance and life 4 . Our study reveals that utilizing LMFP as the active material significantly increases the rate capability and cycling stability of thick electrodes than LFP. Such difference is attributed to a more uniform redox reaction in LMFP upon charging/discharging, which results from its more sloped OCV curve compared to the wide voltage plateau exhibited by LFP. We prepared LFP and LMFP electrodes with the same porosity at different thickness values. While LFP and LMFP thin electrodes (~30 μm) have similar rate performance, their thick electrode counterparts (~110 μm) exhibit notable differences with LMFP showing better capacity utilization at high rates ( Fig. 1a ). We define the critical C rate (C crit ) as the rate above which the normalized capacity of the thick electrode starts to deviate from that of the thin reference electrode ( Fig. 1b ). Fig. 1c illustrates that LMFP exhibits consistently higher C crit than LFP over a wide range of electrode thickness from 80 to 300 μm. To investigate the underlying cause for the superior performance of LMFP, time-of-flight secondary ion mass spectrometry (ToF-SIMS) was applied to characterize the reaction distribution in LFP and LMFP thick electrodes by mapping the lithium concentration across the electrode thickness. While a strong Li concentration gradient was observed in LFP, Li was much more uniformly distributed in LMFP. According to our previous theoretical study 5 , a more uniform Li (de)interaction flux reduces the severity of salt depletion upon (dis)charging, which explains the enhanced rate capability of LMFP. We also discovered that the non-uniform reaction in LFP thick electrodes creates the "memory effect" phenomenon, where the (dis)charging performance of the battery cell depends on its cycling history. In contrast, LMFP displays a minimal memory effect, which will simplify battery management, such as state of charge (SoC) determination. The improved reaction uniformity in LMFP thick electrodes also results in better cycling stability. When charged at 0.2C and discharged at 1.5C, a 150μm-thick LMFP electrode retained ~60% of its original capacity retention after 200 cycles, compared to less than 20% for the LFP electrode of the same thickness ( Fig. 1d ). Our study demonstrates that a judicious selection of active material is another effective approach to achieving better thick electrode performance. In particular, electrode systems with strong SoC-dependent equilibrium potentials hold greater potential for thick electrode applications. Reference Yang, L.; Deng, W.; Xu, W.; Tian, Y.; Wang, A.; Wang, B.; Zou, G.; Hou, H.; Deng, W.; Ji, X., Olivine LiMnxFe1−xPO4 cathode materials for lithium ion batteries: restricted factors of rate performances. J Mater Chem A 2021, 9 (25), 14214-14232. Ravnsbaek, D. B.; Xiang, K.; Xing, W.; Borkiewicz, O. J.; Wiaderek, K. M.; Gionet, P.; Chapman, K. W.; Chupas, P. J.; Tang, M.; Chiang, Y. M., Engineering the Transformation Strain in LiMnyFe1-yPO4 Olivines for Ultrahigh Rate Battery Cathodes. Nano Lett 2016, 16 (4), 2375-80. Singh, M.; Kaiser, J.; Hahn, H., A systematic study of thick electrodes for high energy lithium ion batteries. J Electroanal Chem 2016, 782 , 245-249. Kim, H.; Oh, S. K.; Lee, J.; Doo, S. W.; Kim, Y.; Lee, K. T., Failure mode of thick cathodes for Li-ion batteries: Variation of state-of-charge along the electrode thickness direction. Electrochim Acta 2021, 370 . Wang, F.; Tang, M., A Quantitative Analytical Model for Predicting and Optimizing the Rate Performance of Battery Cells. Cell Reports Physical Science 2020, 1 (9). Figure 1