Results
Local phase transition and lattice defects formation. The lattice structure of the cathode material is vital to the transportation of lithium-ion in the solid phase. The successful and/or promising choices of cathode materials for lithium-ion batteries can be grouped into three categories, which are of 1D (e.g. LiFePO4\cite{Chen_2006},\cite{Lim_2016},\cite{Shapiro_2014},\cite{Yu_2018} with olivine structure), 2D (e.g. LiCoO2\cite{Zhang_2017},\cite{Xu_2017a}, LiNixMnyCozO2 (NMC)\cite{Lin_2016},\cite{Gent_2016}, and Li- and Mn- rich NMC material\cite{Hu_2018},\cite{Yang_2014} with layered structure), and 3D (e.g. Li1.211Mo0.467Cr0.3O2\cite{Lee_2014} and Li1.3Nb0.3Mn0.4O2\cite{Kan_2018} with disordered rocksalt structure, and LiMn1.5Ni0.5O4\cite{Kuppan_2017} with spinel structure) lithium diffusion channels, respectively. The layered transition metal oxides, which is the focus of this work, are currently a key player on the market thanks to its excellent performance that is superior in many different aspects. The layered lattice structure offers 2D open space between the layers that hosts the lithium-ions in the process of intercalation and de-intercalation. It has been reported that upon prolonged cycling\cite{Lin_2014},\cite{Jung_2013} or under thermal abuse conditions\cite{Mu_2018},\cite{Mu_2018a},\cite{Yan_2018},\cite{Wei_2018} the surface of the material could undergo undesired structural transitions, forming a mixture of spinel and rocksalt structure that hinders the fast lithium-ion diffusion. The participation of liquid electrolyte is found to play a key role and to accelerate such a side reaction\cite{Tian_2018}. Under fast charging and/or deep charging conditions, the redox reaction of transition metals cations\cite{Yang_2014},\cite{Nelson_Weker_2017} and the oxygen anions\cite{Mueller_2015},\cite{Rong_2018},\cite{Singer_2018},\cite{Yang_2018} could cause migration of the lattice atoms and, subsequently, trigger the formation of local lattice defects and local structural transition.
To systematically investigate the degradation mechanism of the NMC622 cathode material under fast charging conditions, we start at the atomic scale and show, in Figure 1, a transmission electron microscopy (TEM) image of a cracked area on the extensively cycled NMC622 cathode. While different atomic structures (see highlighted regions of interest, (e) and (h)) can already be visualized, the (inverse) fast Fourier transform analysis [ref] of the selected areas (Figures 1d to 1h) and the whole image (Figures 1b and 1c) provides strong evidences, showing the intimate co-presence of layered and rocksalt phases over the localized region. The observed local lattice defect formation and phase transition can serve as nucleation points for further developments of micro-cracks that could propagate throughout the cathode particles.
Figure 1. NMC electrode’s morphological defects at nano and meso scales. Panel (a) shows a transmission electron microscopy image that reveals the mixed rocksalt and layered structure along the region of local defects in NMC cathode material. Panels (b) and (c) are the fast Fourier transform (FFT) and inversed FFT (IFFT) of the entire high-resolution image (a), showing the intimate co-presence of layered and rocksalt phases. Panels (d) to (h) are the localized FFT ((d) and (f)) and IFFT ((e) and (h)), showing the presence of layered and rocksalt phases, respectively. Panels (i) to (l) show the mesoscale structural and chemical degradation in a charged secondary NMC particle. Panel (i) shows the 3D rendering of the particle morphology with a few xz slices through a number of different positions displayed in the center. Panel (j) is an xy slice through the center of the particle. Two different types of cracks, e.g. the wide open cracks (black; red arrows in (j)) and fine cracks (gray pattern with lower contrast; green arrows in (j)), can be observed. Panel (k) is the false-colored map (colormap shown in the inset) of local Ni K-edge energy over the same xy slice in (j). The local edge energy over the fine cracks was segmented and displayed in panel (l). The scale bar in (a) is 10 nm, the ones in (i) and (j) are both 5 μm.
Mesoscale morphological defects and the crack surface chemistry. While the unwanted lattice defects formation and the local phase transition are often considered as the root causes of the cathode degradation, the structural and chemical heterogeneity at the mesoscale adds further complexity to the system\cite{Wei_2018a}. At the mesoscale, the secondary particles of the NMC622 cathode are populated with grain boundaries and micro-porosity, which not only critically influence the electric and ionic transportation in the system, but also govern the propagation of the reaction fronts when a reaction driving force is applied externally\cite{Mu_2018a}.
We employed nano-resolution x-ray spectro-microscopy\cite{Meirer_2011} to visualize the mesoscale morphological defects (i.e. micro-cracks) and the corresponding chemical responses in a charged NMC622 particle after it was cycled extensively (50 cycles with a rate of 5C). As shown in the three-dimensional (3D) rendering of the particle morphology in Figure 1i, a severe disintegration of the particle can be clearly observed. The electrochemical cycling induced disintegration of NMC secondary particle can be attributed to the repeated lattice breathing [ref] and SoC heterogeneity\cite{Tian_2018}. There are a number of factors, e.g. the cycling rate, the cycling voltage window, the environmental temperature, and the particle size and shape, which could influence the degree of the particle cracking. We refer to our recent paper for a more detailed discussion of the causes and consequences of the mesoscale cracks\cite{Xia_2018}.
The chemical sensitivity provided by the nano-resolution x-ray spectro-microscopy offers the opportunity to investigate the correlation between the mesoscale morphological defects and the local chemical responses. We show in Figure 1j an xy slice through the center of the particle with the corresponding edge energy map (over the Ni K-edge) displayed in Figure 1k. A close look at Figure 1j suggests that there are two types of micro-cracks co-existing in the imaged NMC622 particle. Some of the cracks are well-developed and appear to be of excellent image contrast (red arrows); while the other cracks are less visible (green arrows), likely due to the fact that they are in the early stage of the crack development and there is a significant amount of sub-pixel-level porosity caused by fine micro-cracks over the corresponding regions. These fine micro-cracks are often beyond the x-ray spectro-microscopy’s spatial resolution limit at ~30 nm . It is anticipated that these two different types of cracks could have different local chemical responses, which can be further elucidated by detailed analysis of the XANES mapping data.
The redox of Ni cation is the major charge compensation mechanism for lithium intercalation/deintercalation. As a result, the Ni K-edge energy is often used as a proxy for the local SoC. The surface of the particle and the well-developed cracks appear to be more oxidized in Figure 1k as indicated by the red contour. This is likely caused by the delithiation process, which extracts the lithium-ions from the hosting material through the reaction-active solid electrolyte interphase (SEI). The well-developed cracks facilitate the infiltration of the liquid electrolyte, activating the lithium-ion exchange at the crack surface. On the other hand, the change in the spectroscopic fingerprint over the newly developed cracks is less obvious. We extracted the cracks with weaker visibility based on the grayscales in Figure 1j and segmented the Ni valence map accordingly. As shown in Figure 1l, the newly developed cracks are scattered throughout the particle. There isn’t an obvious contrast (in terms of the Ni oxidation state) between the bulk and the cracks at their early stage of development. This is likely due to the lack of liquid electrolyte wetting at these locations.
The wetting effect of liquid electrolyte is a topic of great interest to both industry and academia\cite{solution}. The wettability can be influenced by the properties of the liquid electrolyte (e.g. the viscosity and surface tension) and the micro-texture of the solid electrode (e.g. the local surface curvature etc.). At the early stage of the micro-crack development, the formation of local lattice defects and phase transition could act as physical barriers that cause detouring of the lithium-ions. When the cracks are further developed, infiltration of the liquid electrolyte takes place, affecting the local lithium diffusion kinetics, and the redox events can then be initiated over the wetted crack surface.