Surface Structure, Morphology, and Stability of Li(Ni_1/3Mn_1/3Co_1/3)O₂ Cathode Material

Abstract

Layered Li(Ni 1−x−y Mn x Co y )O 2 (NMC) oxides are promising cathode materials capable of addressing some of the challenges associated with next-generation energy storage devices. In particular, improved energy densities, longer cycle-life, and improved safety characteristics with respect to current technologies are needed. However, sufficient knowledge on the atomic-scale processes governing these metrics in working cells is still lacking. Herein, density functional theory (DFT) is employed to predict the stability of several low-index surfaces of Li(Ni 1/3 Mn 1/3 Co 1/3 )O 2 (NMC111) as a function of Li and O chemical potentials. Predicted particle shapes are compared with those of single crystal NMCs synthesized under different conditions. The most stable surfaces for stoichiometric NMC111 are predicted to be the nonpolar (104), the polar (012) and (001), and the reconstructed, polar (110) surfaces. Results indicate that intermediate spin Co 3+ ions lower the (104) surface energy. Furthermore, it was found that removing oxygen from the (012) surface was easier than from the (104) surface, suggesting a facet dependence on surface-oxygen vacancy formation. These results give important insights into design criteria for the rational control of synthesis parameters as well as establish a foundation on which future mechanistic studies of NMC surface instabilities can be developed.

The Unified Electrochemical Band Diagram Framework: Understanding the Driving Forces of Materials Electrochemistry

“…The UEB framework allows us to understand how varying the surface termination through synthesis conditions or coating the surface with an interfacial layer will impact the material work function and surface band bending. The ability to model and understand these effects will help inform the emerging field of electrode–electrolyte interfacial engineering . The ability to understand surface versus bulk electrochemical descriptions and the depth of surface band bending within the UEB framework will aid the understanding of the electrochemical behavior of emerging nanoscale electrode materials .…”

“…For example, stabilizing the <100> surface of α‐MnO 2 is predicted to shift the conduction band edge, and therefore the cation insertion potential, from 3 to ≈4 V versus Li/Li + , whereas stabilizing one of the <110> surfaces (labeled <110> c) is predicted to shift the cation insertion potential to ≈2 V versus Li/Li + as depicted in Figure a. The application of surface engineering in this spirit has been demonstrated in parallel with the UEB framework development, for instance in β‐MnO 2 , LiNi 0.5 Mn 1.5 O 4 , LiMn 2 O 4 , and Li(Ni 1/3 Mn 1/3 Co 1/3 )O 2 . In each of these studies, the stability and rate performance of cathode materials were identified to depend on the crystal facets exposed to the electrolyte, and structural changes were identified to improve performance.…”

Single-crystal based studies for correlating the properties and high-voltage performance of Li[NixMnyCo1−x−y]O2 cathodes

“…Previously, DFT calculations reported that the equilibrium shapes of layered NMC particles are enclosed by three families of facets, namely (104), (001), and (012). 26 Among them, the surface energy of (012) and (104) is the highest and lowest, respectively, suggesting that they are the least and most thermodynamically favorable surfaces. Our recent theory-based studies showed that the surface fraction of each facet can be inuenced by the chemical potentials of oxygen and lithium during synthesis.…”

“…As the (001) surface is parallel to the TM layers, it is impermeable to Li. 26,27 On the other hand, the (012) surface has efficient Li diffusion pathways. The net result is that particles with a large (001) surface tend to have lower utilization and capacity but better stability as compared to the sample with the (012) surface.…”

“…Previous studies using density functional theory (DFT) calculations revealed the importance of the primary particle surface on cathode reactivity and cycling stability. [26][27][28][29] For example, the calculation carried out on LiCoO 2 and NMC111 determined that their equilibrium particle shapes consist of (104), (001) and (012) facets, 26,27 and the (104) family facets are more stable than the (012) facets during electrochemical cycling. Experimentally, oxygen release and thermal decomposition of LiCoO 2 were found to be facet-dependent, and structural reconstruction from the layered to spinel and rock-salt phases occurred preferably on (012) and (104) family facets, as opposed to the (001) facets.…”