Evolution of Structure and Lithium Dynamics in LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) Cathodes during Electrochemical Cycling

Abstract

The nickel-rich layered oxide LiNi0.8Mn0.1Co0.1O2 (NMC811) is a promising future cathode material for lithium-ion batteries in electric vehicles due to its high specific energy density. However, it exhibits fast voltage and capacity fading. In this article, we combine electrochemistry, operando synchrotron X-ray diffraction (XRD), and ex situ solid-state NMR spectroscopy to provide new insights into the structural changes and lithium dynamics of NMC811 during electrochemical charge and discharge, which are essential for a better understanding of its fast degradation. The evolution of the interlayer spacing is tracked by XRD, showing that it gradually increases upon delithiation before collapsing at high state-of-charge (SOC). Importantly, no two-phase O3→O1 transition is observed at high SOC, demonstrating that this cannot be a major cause of degradation. A strong increase of Li dynamics accompanies the increase of the interlayer spacing, which is shown by 7 Li NMR and electrochemical characterization. At high SOC, Li mobility drops considerably, and Li/vacancy ordering can be observed by NMR. A detailed analysis of 7 Li NMR spectra at different SOC is provided, demonstrating how Li NMR can be used to extract information on the dynamics of such challenging paramagnetic samples with several hundred different local Li environments. The insights on the evolution of structure and dynamics of NMC811 will further the understanding of its cycling behavior and contribute to the efforts of mitigating its performance fade.

Surface/Interface Structure Degradation of Ni‐Rich Layered Oxide Cathodes toward Lithium‐Ion Batteries: Fundamental Mechanisms and Remedying Strategies

“…Of particular note, at discharged state below 3.6 V, the disorder of TM/Li + increases, which causes an AB 2 O 4 ‐like full spinel phase to rise based on the foundation of defective spinel associated with the existence of tetrahedral Li + . However, a new insight into the structural changes and lithium dynamics of the LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) at high SOC is revealed by Grey and co‐workers, showing that no two‐phase O3 → O1 transition occurs for the NCM811 at high SOC, signifying that it cannot be a major reason for the structural degradation, while the pronounced change in the c lattice parameter is still observed, and most notably, its collapse is above ≈70% SOC . Similarly, the novel observation into the structural and chemical evolution in surface phases of the LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA801505) evaluated at a high cutoff voltage up to 4.75 V ( Figure a) is also presented, The formation of the disordered phases with cation and oxygen vacancies is driven by surface oxygen loss caused by reactions with the electrolyte followed by cation migration from the octahedral 3 a TM layer to the octahedral 3 b Li layer.…”

“…However, a new insight into the structural changes and lithium dynamics of the LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) at high SOC is revealed by Grey and co-workers, showing that no two-phase O3 → O1 transition occurs for the NCM811 at high SOC, signifying that it cannot be a major reason for the structural degradation, while the pronounced change in the c lattice parameter is still observed, and most notably, its collapse is above ≈70% SOC. [147] Similarly, the novel observation into the structural and chemical evolution in surface phases of the LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA801505) evaluated at a high cutoff voltage up to 4.75 V (Figure 6a) is also presented, [41] The formation of the disordered phases with cation and oxygen vacancies is driven by surface oxygen loss caused by reactions with the electrolyte followed by cation migration from the octahedral 3a TM layer to the octahedral 3b Li layer. Aside from the irreversible microstructure transformation induced at high cutoff voltages and temperatures, the side reactions at the CEI are easily triggered, thus deducing an electrochemical inert interface containing such as Li 2 CO 3 , LiF, and reduced metal-ion species as well as some salt decomposition species.…”

Identifying the Origins of Microstructural Defects Such as Cracking within Ni‐Rich NMC811 Cathode Particles for Lithium‐Ion Batteries

“…Moreover, when an NMC811 electrode is de-lithiated, the interlayer spacing gradually increases with the cell's state of charge (SoC), before collapsing at high SoC; this is also thought to induce further strain between primary particles within the secondary particle agglomerates. [20] In conclusion, the literature reports a myriad of mechanisms that may act as precursors for operational particle cracking in NMC [21] and although promising efforts are being made to pursue new materials to combat these issues, [22] further knowledge is required of cracking within NMC811 cathodes. Firstly, minimal cracking is anticipated in raw powders [18] but after operation, many particles are expected to possess defects, accompanied by a loss of electrochemical capacity.…”

A Solid‐State Battery Cathode with a Polymer Composite Electrolyte and Low Tortuosity Microstructure by Directional Freezing and Polymerization

“…[ 88 ] The EIS results in Figure 5d show the high resistance of the [DFP+P] cathode and no useful capacity was obtained. Figure 6 a compares the galvanostatic charge and discharge profiles of the SSLMB full cells using the [IFP+IP] and [DFP+IP] cathodes at 0.05 C (assuming the theoretical capacity of NMC811 is 200 mAh g −1 [ 88 ] ), at the first and tenth cycle and 25 o C. The [IFP+IP] cathode delivered discharge capacities of 152 and 149 mAh g −1 at the first and tenth cycles, respectively, corresponding to areal capacities of 12.5 and 12.2 mAh cm −2 . The [DFP+IP] cathode delivered higher discharge capacities of 199 and 196 mAh g −1 at the first and tenth cycles, respectively, corresponding to areal capacities of 16.7 and 16.4 mAh cm −2 .…”