(Invited) Probing Heterogeneous Transformations in NMC Cathodes with Microbeam Diffraction

Abstract

Ideally, the redox activity in Li-ion batteries, which is associated with storage and release of Li ions, is distributed homogeneously throughout the electrode, thereby minimizing detrimental processes while maximizing battery performance. However, it appears that every electrode material displays inhomogeneous redox activity which is expected to play a dominant role in battery performance parameters such as cycle life, efficiency and rate performance.1 For example, the occurrence of localized redox activity in layered electrodes can accelerate degradation reactions through local strain and/or decomposition1-2. Further, inhomogeneous reactions render estimating the true state of charge challenging and can introduce history effects3 that make optimization of battery performance via a battery management system problematic. Inhomogeneous redox reactions in Li-ion battery electrodes can occur on different length scales, starting at the nm length in individual electrode particles progressing up to the dimensions of complete composite electrodes (tens of μm) and can have various origins. Most directly, inhomogeneous reactions in electrodes stem from charge transport limitations, generally determined by the electrode morphology (including electrode thickness, porosity and tortuosity).4-6 A second origin for inhomogeneous reactions can be a difference in insertion potential, through mixing electrode materials5 or due to a distribution of nanoparticle sizes.7 Lastly, the nature of the phase transition can also give rise to inhomogeneous reactions. For example, the particle-by-particle transformation mechanism in LiFePO4 has been associated with a hysteresis effect8 and has been suggested to give rise to a memory effect that can influence the observed potential.3 Despite the above, limited research has been performed on this subject to date and many questions remain, mainly due to the difficulty to probe these heterogeneities in realistic battery geometries and during cycling conditions.Here we present a study of the phase transformation of ten's of individual NMC crystallites under operando cycling conditions in pouch cells using microbeam diffraction (ID11, ESRF, France). Due to the small, bright and sub-micron sized synchrotron beam diffraction rings observed with powder diffraction fall apart in individual spots each representing individual NMC crystallites in the positive electrode. In this way the phase transition behaviour through time of ten's of individual NMC crystallites is monitored at the same time while varying the electrochemical conditions.9,10 Figure 1 shows snapshots of the (108) and (110) reflection of individual NMC111 crystallites during a full C/4 charge-discharge cycle, demonstrated the anticipated continuous shift. For each individual crystallite the evolution of the lattice parameters provide insight in the distribution of the composition and local potential of each crystallite, as well as the individual transformation rate. In addition the average transformation rate can be translated in the active particle fraction and average local current density. By studying this for the series NMC111, NMC622 and NMC811, we gain insight in the heterogeneous redox activity and the impact of composition, relevant for both rate performance and cycle life of this class of intercalation cathodes. Figure 1. Following the phase transformation of single NCM111 crystallites through the (108) and (110) reflection during C/4 charge and discharge. Zhang et al. Journal of Materials Chemistry A 2019, 7 (41), 23628-23661. Lin et al. Nature Communications 2014, 5 (1), 3529. Sasaki et al. Nature Materials 2013, 12 (6), 569-575. Strobridge et al. Chemistry of Materials 2015, 27 (7), 2374-2386. Sasaki et al. Advanced Science 2015, 2 (7), 1500083. Zhang et al. Advanced Energy Materials 2015, 15, 1500498. Van der Ven et al. Electrochemistry Communications 2009, 11 (4), 881-884. Dreyer et al. Nature Materials 2010, 9 (5), 448-453.van Hulzen et al. Font. Energy Res. 2018, 6 (59). 5.Zhang et al. Nature Commun. 2015, 6 Figure 1