Elucidation of Compositional Inhomogeneities in LiNixCoyMnzO2 by Single Particle Laser Ablation-Inductive Coupled Plasma-Mass Spectrometry

Abstract

Over the many years of cathode active material research for lithium-ion batteries, LiNixCoyMnzO2 (NCM, x+y+z = 1) have emerged as one of the most promising chemistries for mass application.[1] The exact NCM elemental composition is directly linked to the material properties which in turn influence energy density, durability, safety and cost of the final products.[2] The individual contributions of Ni, Mn and Co have been heavy investigated in both industry and academia to find the best trade-offs between all properties, nowadays settling for materials with Ni contents > 80 %.[3]The composition of NCM materials is determined in the precipitation step of the mixed transition metal hydroxide precursors.[4] At laboratory scale, precursor precipitation is typically operated in batch mode. However, the precipitation in continuous stirred tank (CSTR) reactors is industrially favourable due to the higher space-time-yield, thus reducing the overall production cost. In comparison to batch-type precursors that exhibit a homogeneous µm-sized spherical particle morphology, CSTR-type precursors can be distinguished by their broad particle size distribution due to the intrinsic residence time distribution of particles in the reactor setup. Although rarely discussed, this phenomenon is accompanied by a particle-size dependent elemental composition of the precursor particles, consequently impacting the battery performance.Besides inductive coupled plasma-optical emission spectroscopy (ICP-OES) or mass spectrometry (ICP-MS) that both determine the bulk elemental composition of NCMs and their precursors, scanning electron microscopy (SEM) or transmission electron microscopy (TEM) energy dispersive X-ray (EDX) analysis can access local elemental inhomogeneities. However, both methods are tedious in both acquisition and data evaluation.In this work, an alternative method to determine the elemental composition of individual µm sized particles is presented using a laser ablation (LA) system coupled to an ICP-MS. By the example of a Ni0.91Co0.045Mn0.045(OH)2 CSTR precursor, particle size dependent elemental inhomogeneities are demonstrated. An enrichment of Ni in larger particles with a concomitant enrichment of Co and Mn in smaller particles is identified, which persists after thermal lithiation of the precursors. The results of the LA-ICP-MS analysis are cross-validated with both SEM-EDX and TEM-EDX. Furthermore, the impact of the process conditions during precipitation on the elemental inhomogeneity are elaborated by the comparison of a series of Ni0.83Co0.12Mn0.05(OH)2 precursors precipitated at varying pH. Finally, future applications of the method are proposed. G. E. Blomgren, Journal of The Electrochemical Society, 164, A5019 (2017).H.-J. Noh, S. Youn, C. S. Yoon and Y.-K. Sun, Journal of Power Sources, 233, 121 (2013).W. Li, E. M. Erickson and A. Manthiram, Nature Energy, 5, 26 (2020).H. Dong and G. M. Koenig, CrystEngComm, 22, 1514 (2020).