Editors' Choice—Capacity Fading Mechanisms of NCM-811 Cathodes in Lithium-Ion Batteries Studied by X-ray Diffraction and Other Diagnostics

Abstract

Ni-rich layered oxides, like NCM-811, are promising lithium-ion battery cathode materials for applications such as electric vehicles. However, pronounced capacity fading, especially at high voltages, still lead to a limited cycle life, whereby the underlying degradation mechanisms, e.g. whether they are detrimental reactions in the bulk or at the surface, are still controversially discussed. Here, we investigate the capacity fading of NCM-811/graphite full-cells over 1000 cycles by a combination of in situ synchrotron X-ray powder diffraction, impedance spectroscopy, and X-ray photoelectron spectroscopy. In order to focus on the NCM-811 material, we excluded Li loss at the anode by pre-lithiating the graphite. We were able to find a quantitative correlation between NCM-811 lattice parameters and capacity fading. Our results prove that there are no considerable changes in the bulk structure, which could be responsible for the observed ≈20% capacity loss over the 1000 cycles. However, we identified the formation of a resistive surface layer, which is responsible for (i) an irreversible loss of capacity due to the material lost for its formation, and (ii) for a considerable impedance growth. Further evidence is provided that the surface layer is gradually formed around the primary NCM-811 particles.

Phase Behavior during Electrochemical Cycling of Ni‐Rich Cathode Materials for Li‐Ion Batteries

“…After prolonged electrochemical cycling, no bulk phase transformation from layered to spinel and/or rocksalt has been reported in the literature, and no obvious increase in the antisite defects is observed after ageing at room temperature. [ 49,71 ] Moreover, the overall change in the lattice parameters as a function of SoC is in good agreement with what is observed during initial cycles, that is the a ‐parameter decreases while the c ‐parameter increases initially and then collapse at high SoCs. [ 49,71 ]…”

“…[ 49,71 ] Moreover, the overall change in the lattice parameters as a function of SoC is in good agreement with what is observed during initial cycles, that is the a ‐parameter decreases while the c ‐parameter increases initially and then collapse at high SoCs. [ 49,71 ]…”

“…As a result, the patterns can be fitted with one phase throughout the cycling (up to 4.8 V vs Li) as demonstrated in numerous studies. [ 29,48,49 ] The extracted cell parameter evolution is generally similar to the LNO: upon delithiation, a hex continuously decreases and c hex increases initially but then decreases rapidly as shown in Figure 2e. One distinct difference lies in the c ‐parameter.…”

“…By contrast, for NCAs and NMCs, the change in the c ‐parameters is continuous. [ 29,49,50 ] Of note, the onset of the c ‐parameter collapse is heralded by high voltage peaks in the d Q /d V plots, which some authors have assigned to an H2–H3 transformation. [ 51–53 ] At this stage, we stress that electrochemical features cannot be used independently to assign certain phase transformations and should be complemented by diffraction characterization.…”

“…For materials which show rapid capacity fading, both NCA [ 50,78,91 ] and NMC‐811 [ 29,49 ] show remarkably reversible long‐range structure evolution on charge and discharge; the lattice parameters for NMC‐811 cycled between 3.0 and 4.4 V show very little hysteresis between charge and discharge and excellent reversibility between first and second cycles. [ 29 ] This aligns well with the observation that degradation in Ni‐rich LiTMO 2 cathodes is a gradual and continuous process.…”