(Invited) Structure of NMC Family Cathodes from Monte Carlo Simulation and NMR Spectroscopy

The cathode active material Li[Ni 1/3 Mn 1/3 Co 1/3 O 2 ], NMC 111, is well known for its excellent cycling lifetime and capacity. It is also well known that varying the ratios of transition metals (TMs), Li atoms, and vacancies 1 can provide capacity increases of up to 40 %, though at the expense of structural stability. Not only are these structural-breakdown pathways not understood, full models of even the pristine materials are not available because of the significant disorder. It seems apparent that better structural characterization tools are needed to understand these materials, and therefore may finally provide the key to unlocking their potential. Here, we present a Monte-Carlo structure-solution method, using a Hamiltonian that is based on local electroneutrality [2]. The close-packed 2D TM sheet of the NMC family is partitioned according to valence-bonding principles and a state of local charge balance—for the TM atoms with respect to the neighboring (fixed) 2D oxygen sheet—is sought. This simple Hamiltonian allows rapid, yet realistic, sampling of the configuration space of the large sheets (up to 10,000 TM atoms) necessary to properly capture the (often) complex arrangements. The predicted structural models are verified as accurate using 7 Li NMR spectroscopy. The unpaired electrons of the TM atoms generate large paramagnetic chemical shifts in the neighboring Li atoms [3]. The 7 Li spectra are therefore sensitive to the identity of the 12 TM atoms neighboring each Li: 6 in the TM sheet above, 6 in the sheet below [3]. Notably, the orientation dependence of this effect is not a hindrance when using 7 Li MATPASS NMR spectroscopy under 60 kHz MAS [4]. A series of samples with compositions Li[Ni x Mn x Co 1-2 x O 2 ] are investigated, where x = 2%, 10%, and 33%. In each case, structures generated by the Monte Carlo calculations are verified through an extremely accurate matching between predicted and experimental 7 Li NMR spectra. Additionally, accurate 3D simulations of electrochemically charged versions of these samples, where transition-metal oxidation and delithiation occur, are presented. Reference s : [1] R. Shunmugasundaram, S. Arumugam, J.R. Dahn Chem. Mater. 25 (2015) 989. [2] K.J. Harris et al. Chem. Mater. 29 (2017) 5550. [3] D. Zeng et al. Chem. Mater . 19 (2007) 6277. [4] I. Hung et al. J. Am. Chem. Soc. 134 (2012) 1898. Figure 1