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Combining two active materials in a single electrode is an established strategy to improve lithium-ion battery performance, reduce cost, and tailor properties such as the voltage profile and thermal stability [1,2,3]. Many commercial EVs employ blended positive electrodes (e.g., NMC–LMO, where NMC is LiNiₓMnᵧCozO₂, x+y+z = 1, and LMO is LiMn₂O₄) and negative electrodes (e.g., silicon–graphite). However, the origin of the synergistic behavior of blends remains poorly understood, and their design is largely empirical. Furthermore, very few studies address the effect of temperature on such systems, a critical factor, since EV batteries must operate over a wide and constantly varying temperature range.
Here we investigate the temperature dependence of the reaction mechanism in NMC–LMO blended cathodes, one of the most commercially relevant blend chemistries. Operando synchrotron X-ray diffraction at the MSPD beamline and operando X-ray absorption spectroscopy at the CLAESS beamline were performed on 3 coin cells at 0ºC, 25ºC and 45ºC. XRD follows the structural evolution of each phase individually as seen in Figure 1 for NMC (003) and LMO (111) reflections. In the XANES data the Mn K-edge was analyzed by multivariate curve resolution (MCR) the redox response of which is dominated by LMO. Both techniques reveal a clear alteration of the reaction mechanism with temperature. At elevated temperature, NMC shows a more solid-solution-like behavior on both charge and discharge, as well as a larger c-axis contraction at the end of charge, which could impact its structural stability upon extended cycling. At 0 °C, the LMO (111) reflection develops a markedly broader distribution of lattice parameters, indicating a more inhomogeneous reaction across the electrode (Figure 1). Most strikingly, both XRD and the MCR-resolved XANES components (Figure 2) show that at 0 °C, during the first charge, it is LMO that reacts first, despite operating at higher potentials and therefore being expected to react at a later stage. To the best of our knowledge, this temperature-induced inversion of the reaction sequence has not been previously reported.
These results provide mechanistic insight into how blended electrodes function away from ambient conditions, contributing to both the fundamental understanding of such synergistic systems as well as testing the systems in conditions closer to their real applications. In addition, this study serves as a proof of concept for the simultaneous measurement of three coin cells at three different temperatures, with a setup [4] adapted to both MSPD and CLAESS, increasing the throughput and ease of temperature-dependent operando studies.
Figure 1: Operando X-ray diffraction patterns showing the (003) reflection of NMC (lower angles) and the (111) reflection of LMO (higher angles). A clear difference in the reaction mechanism is observed as temperature is varied.
Figure 2: Left: MCR-ALS spectral components resolved from the operando Mn K-edge XANES. Middle and right: concentration profiles of the two components during cycling of NMC–LMO cells at 0 °C, 25 °C, and 45 °C, together with a pure LMO reference.
References
[1] J. Smith et al 2012 J. Electrochem. Soc. 159 A1696
[2] Hideki Kitao et al 2005 Electrochem. Solid-State Lett. 8 A87
[3] ACS Appl. Mater. Interfaces 2016, 8, 31, 20147–20156
[4] R. G. Houdeville et al 2024 J. Electrochem. Soc. 171 020533