Towards Ni-Rich Single Crystal Materials: Synthesis of the Model Material LiNiO2 and Their Electrochemical Performance Trade-Offs

Abstract

Layered transition metal oxide materials like NCMs (Li1+δ[NixMnyCoz]1-δO2, with x+y+z = 1 and δ » 0.005-0.01) are the most prominent candidates for the large-scale implementation as cathode active materials (CAMs) for lithium-ion batteries. The development of NCM materials is heavily focused on the optimization of the material composition, whereby an increase of the Ni content leads to an increase of the reversible specific capacity, but generally also to a poorer capacity retention and lower thermal stability.[1] More recent approaches to optimize the stability and safety of these materials include the transition from the typical poly-crystalline particle morphology (i.e., primary particle crystallites in the size range of a few hundred nm aggregated to µm-sized secondary particle agglomerates) to a single-crystal morphology (monolithic particles in the low µm size range). Over the course of cycling, cracking of the secondary agglomerates of poly-crystalline materials leads to a substantial increase of the CAM surface area that is exposed to the electrolyte, while a low exposed CAM surface area can be maintained over cycling with single-crystal particles, so that they exhibit a lower extent of parasitic side reactions at the interface to the electrolyte.[2] However, a combination of both approaches, namely to increase the nickel content and to obtain single-crystal morphology proves troublesome, as the high annealing temperatures required to get single-crystal materials are difficult to achieve with Ni-rich NCMs due to their lower temperature stability and due to their reported susceptibility towards the migration of Ni ions into the Li layer.[3, 4]LiNiO2 can serve as a model material for Ni-rich NCMs, for which the Ni content has been pushed towards or even beyond 90 % threshold by now. In the present study, the synthesis conditions of LiNiO2 have been systematically varied with respect to the annealing temperature (700 – 850 °C) and the annealing duration (1 – 24 h). A combination of X-ray diffraction (XRD), particle size distribution (PSD) analysis by scanning electron microscopy, titration of the residual Li-impurities as well as electrochemical testing has been performed on the thus synthesized materials. Based on these studies, the following observations could be made: i) the particle size of the LNO primary crystallites is mainly dependent on the annealing temperature; ii) an increase of the Ni occupancy on the Li sites only sets in above a certain temperature threshold, but does not significantly affect the performance up to ~ 5 %; iii) the lower specific surface area attained at high annealing temperatures comes with increased kinetic limitations of the cathode active material at high degrees of lithiation; and, iv) further protection of the surface is necessary to achieve a stable cycling behavior even for single-crystalline LiNiO2 materials. A decoupling of particle growth and material decomposition is presented by introduction of multiple-temperature-step annealing strategies. H.-J. Noh, S. Youn, C. S. Yoon and Y.-K. Sun, Journal of Power Sources, 233, 121 (2013). J. Langdon and A. Manthiram, Energy Storage Materials, 37, 143 (2021). A. Liu, N. Zhang, J. E. Stark, P. Arab, H. Li and J. R. Dahn, Journal of The Electrochemical Society, 168, 050506 (2021). A. Liu, N. Zhang, J. E. Stark, P. Arab, H. Li and J. R. Dahn, Journal of The Electrochemical Society, 168, 040531 (2021).