Between Scylla and Charybdis: Balancing Among Structural Stability and Energy Density of Layered NCM Cathode Materials for Advanced Lithium-Ion Batteries

Abstract

Two major strategies are currently pursued to improve the energy density of lithium-ion batteries using LiNi x Co y Mn z O 2 (NCM) cathode materials. One is to increase the fraction of redox active Ni (≥80%), which allows larger amounts of Li to be extracted at a given cutoff voltage (U max ). The other is to increase U max , in particular for medium-Ni content NCM materials. However, the accompanying lattice changes ultimately lead to capacity fading in both cases. Here the structural changes occurring in Li 1.02 Ni x Co y Mn z O 2 (with x = 1 / 3 , 0.5, 0.6, 0.7, 0.8 and 0.85) during cycling operation in the voltage range between 3.0 and 4.6 V vs Li are quantified by means of operando X-ray diffraction combined with detailed Rietveld analysis. All samples show a large decrease in unit cell volume upon charging, ranging from 2.4% for NCM111 (33% Ni) to 8.0% for NCM851005 (85% Ni). To make a fair comparison of the structural stability of the different NCM materials, energy densities as a function of U max are estimated and correlated with X-ray diffraction results. It is shown that NCMs with a lower Ni content allow for specific energies similar to that of, e.g., Ni-rich NCM811 (80% Ni) when operated at sufficiently high U max , but still undergo less pronounced changes in structure. Nevertheless, as indicated by charge/discharge tests, the capacity retention of low-and medium-Ni content NCMs cycled to high U max is also strongly affected by factors other than stability of the layered crystal lattice (electrolyte decomposition etc.). Overall, it is demonstrated that the complexity of the degradation processes needs to be better understood to identify optimal cycling conditions for specific cathode compositions.

Chemical, Structural, and Electronic Aspects of Formation and Degradation Behavior on Different Length Scales of Ni‐Rich NCM and Li‐Rich HE‐NCM Cathode Materials in Li‐Ion Batteries

“…For more detailed information on the pristine structures of both NCM [2,17,18,28,61,115,116] and HE-NCM, [34,41,48,51,112,[117][118][119][120][121][122][123][124][125] we refer to other reports available in the literature. Note that by this evaluation method, the number of subsequently stacked defect-free TM layers is determined.…”

“…[129] Operando XRD is well suited to analyze crystallographic changes under dynamic conditions, for example during electrochemical cycling, and several studies have reported on the lattice parameter changes of NCM CAMs with cycling, [17,18,53,61,63,104,[129][130][131][132][133][134] but only few of them revealed sufficient time and angular resolutions. Crystallographic volume changes and multiphase transformations are assumed to be responsible for the capacity fading and to be one of the main factors governing the material fatigue.…”

“…[135] The high-flux beam (Mo Kα 1,2 ) and the fast 2D detector of the setup allowed performing operando XRD experiments with good time resolution and tracking crystallographic parameters, such as unit cell volume, atomic positions, and crystallographic strain, directly as a function of Li content in the electrode. [17,18,61,63,104,[130][131][132][133] An illustration/structural description of the NCM lattice with delithiation is given by Lee et al [136] Overall, this indicates the stress that NCM CAMs experience when the Ni content is increased (and with that the specific (Li) was calculated from the electrochemical data. The corresponding diffraction patterns were acquired with a time resolution of 150 s, being equivalent to Δx(Li) ≈ 0.003 per pattern.…”

“…The corresponding diffraction patterns were acquired with a time resolution of 150 s, being equivalent to Δx(Li) ≈ 0.003 per pattern. [61] Copyright 2017, American Chemical Society. [61,104] In particular, at high SOC, the overall lattice parameter changes are highly anisotropic.…”

“…For reaching the ambitious goals in the electric vehicle and largescale energy storage sectors, sustainable and environmentally friendly solutions providing higher energy density and lower impedance rise in cells using Ni-rich NCM [28,29,[54][55][56][57][58][59][60][61][62][63] and Li-rich HE-NCM CAMs. For reaching the ambitious goals in the electric vehicle and largescale energy storage sectors, sustainable and environmentally friendly solutions providing higher energy density and lower impedance rise in cells using Ni-rich NCM [28,29,[54][55][56][57][58][59][60][61][62][63] and Li-rich HE-NCM CAMs.…”