With the rapid development of electronic devices and hybrid-electric/electric vehicles (H/EVs), lithium-ion batteries have become the most promising electrical storage system due to its high volumetric and gravimetric energy density. However, the high energy density of lithium-ion batteries is difficult to maintain after extended cycles due to capacity fading, which can be attributed to multiple possible reasons including SEI formation,^[1] electrolyte decomposition,^[2] and structural changes in electrode materials.^[3] These issues generate important concerns to the stability and lifetime of the battery. Layered lithium-transition metal oxides represent a major type of cathode materials that are widely used in the commercial market. However, these materials are suggested to suffer structural transformation during electrochemical cycling. Previous studies suggest the possible surface structural transitions from the layered structure to spinel^[4], and/or rock-salt structures,^[5] or possible further decomposition of the transformed phase.^[6]A clear explanation of these surface phenomenon is still under debate. An in-depth investigation of the structural reconstruction is necessary to elucidate the degradation mechanisms of the layered cathode materials.

In the present study, we investigated the chemical evolution and structural transformation of a prominent layered cathode material for lithium-ion batteries, LiNi_1/3Mn_1/3Co_1/3O₂ (NMC). The redox reaction of NMC cathode during charge-discharge process was analyzed in detail using high-resolution electron-energy loss spectroscopy (EELS) and ⁷Li magic-angle spinning (MAS) NMR. Our results suggest that the charge compensation of the NMC cathode during the delithiation-lithiation process is mainly achieved by the oxidation and reduction of Ni²⁺↔Ni⁴⁺, whereas Mn⁴⁺ and Co³⁺ remain mostly unchanged. This is consistent with the NMR findings, from which a trend to lower chemical shift upon lithium extraction is well-correlated with the oxidation change from paramagnetic Ni²⁺ to diamagnetic Ni⁴⁺. Furthermore, the electronic structure of the NMC cathode during initial charging process is found to be inhomogeneous from the particle surface to the bulk using spatially resolved STEM-EELS technique (Figure 1a-1d). It is revealed that the particle surface is at lower oxidation state compared with the bulk region, indicating that the surface evolution of NMC cathode material occurs during the initial cycle.

This surface evolution is further analyzed at different numbers of cycles using atomic-resolution high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) imaging combined with EELS and nano-beam electron diffraction (NBED). Figure 1e shows a HAADF-STEM image of the NMC particle after 50 cycles. It can be clearly seen from the image that the bulk of the cycled NMC particle shows identical layered structure symmetry, the NBED (Figure 1h) obtained from the bulk can be indexed to the R-3m space group. In contrast, the Li layers are occupied by transition metal ions at the particle surface, and this surface reconstruction layer thickens with extended cycles. When probing to the outer-most surface, the corresponding NBED can be indexed to an Fm-3m rock-salt structure, as shown in Figure 1f.

The valence change of the transition metal cations is analyzed using STEM-EELS. The results indicate that the transition metal ions are reduced to divalent states at the surface. In addition, the TM:O ratio increases almost twice at the surface than the bulk. The results indicating that the surface layer is a MO-type rock-salt phase with small amount of residual Li ions and/or vacancies. In addition, a transition zone is observed (Figure 1e) between the bulk layered region and the surface rock-salt layer, where the Li sites are partially occupied by the transition metal ions (some are marked by the red arrows), and the diffraction spots corresponding to the alternating arrangement of Li-containing layers and transition metal layers (some are marked with arrows) become weaker (Figure 1g). ⁷Li NMR studies of the local Li-environments, following ten cycles and ending at the top of charge, indicate that the order of the TM layers remains unchanged in the bulk of the material. This highlights the importance of complementary studies that are sensitive to different length scales. Further data, including EELS and NMR of the NMC cathode at different cycling state and electrolyte effect will be discussed in the presentation.

[1]         K. Edström, T. Gustafsson, J. Thomas, Electrochim. Acta 2004, 50, 397.

[2]         L. Terborg, S. Weber, F. Blaske, et al. J. Power Sources 2013, 242, 832.

[3]         B. Xu, C.R. Fell, M. Chi, et al. Energy Environ. Sci. 2011, 4, 2223.

[4]         A. Boulineau, L. Simonin, J.F. Colin, et al. Chem. Mater. 2012, 24, 3558.

[5]         F. Lin, I. M. Markus, D. Nordlund, et al. Nat. Commun. 2014, 5, 3529.

[6]         J. Zheng, M. Gu, J. Xiao, et al. Nano Lett. 2013, 13, 3824.