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Energy, Environmental, and Catalysis Applications
Combination Effect of Bulk Structure Change and Surface Rearrangement on the Electrochemical Kinetics of LiNi0.80Co0.15Al0.05O2 During Initial Charging Processes Hai Li, Dongqing Liu, Lihan Zhang, Kun Qian, Ruiying Shi, Feiyu Kang, and Baohua Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15131 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018
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ACS Applied Materials & Interfaces
Combination Effect of Bulk Structure Change and Surface Rearrangement on the Electrochemical Kinetics of LiNi0.80Co0.15Al0.05O2 During Initial Charging Processes Hai Li a, b, Dongqing Liu a, *, Lihan Zhang a, b, Kun Qian b, c, Ruiying Shi a, b, Feiyu Kang a, b, c, Baohua Li a, * a
Engineering Laboratory for the Next Generation Power and Energy Storage Batteries, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, P.R.China. b
Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P.R.China. c
Shenzhen Environmental Science and New Energy Technology Engineering Laboratory, Tsinghua-Berkeley Shenzhen Institute, Shenzhen 518055, P.R.China. * Corresponding authors. E-mail:
[email protected] (Baohua Li). E-mail:
[email protected] (Dongqing Liu).
Keywords: LiNi0.80Co0.15Al0.05O2; surface rearrangement; phase evolution; microcrack; electrochemical kinetic.
Abstract The correlations between bulk/surface structure change and electrochemical kinetics of LiNi0.80Co0.15Al0.05O2 (NCA) is systematically investigated at atomic level, including the initial charge, half charge and over charge states. In the initial stage of charge, surface rearrangement occurs and amorphous Li2CO3 layer forms on the surface, which can release stress and provides stable interface. The Li2CO3 surface layer decomposes upon charging, resulting in decreased interface resistance for charge transfer. Meanwhile, the bulk structure goes through the two phase reaction region towards solid solution region, which demonstrate higher electrical conductivity and faster Li-ion mobility. Along with the charging process, more substantial surface rearrangement and decomposed Li2CO3 layer
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lead to surface degradation. Together with the anisotropic volume change induced mechanical stress, microcracks stem from the surface and provide access for electrolyte penetration. All of these cause high kinetic barriers for Li-ion extraction, as demonstrated by the high interface and charge transfer resistance, slow lithium diffusion in this region.
1. Introduction With the rapid development of large scale vehicles with electric mobility, the Li-ion batteries (LIBs) are getting very important in energy storage because of their high energy and power density with long cycle life 1. To date, many researchers have focused their attention on layered transition metal oxide cathode. Nickel-rich cathode material LiNi0.80Co0.15Al0.05O2 (NCA) is an important candidate cathode material for application with respect to the reversible capacity, rate capability, and capital cost 2. However, these materials suffer from capacity fading and impedance rise over long term cycles 3-5. A great deal of effort has been devoted to explain the fading mechanism by correlating the chemistry with the structure changes. During the first charge of NCA, it’s well know that there is an expansion along the c-axis with contractions along the a-/b-axis during initial charge, followed by a contraction along the c-axis and expansions along the a-/b-axis at the end of charge 6. Robert et al. revealed that it’s an irreversible two-phase transition initially followed by a reversible solid solution reaction7. However, Grenier et al. argued that the irreversible "two-phase" transition is not an intrinsic behavior of NCA, they suggested that it’s the Li2CO3 surface layer induced reaction heterogeneity 8. In a further step, Hwang et al tracked the surface structure changes during the initial charge 9. They found that the structure of surface region changed from the layered (R-3m) structure to the disordered spinel (Fd-3m) structure, and finally the rock salt (Fm-3m) structure with the charge process. The Ni4+ reduction of the surface region will lead to charge imbalance resulting in oxygen vacancy and surface porosity formation
10.
The anisotropic volume change with
surface reconstruction during cycling could lead to microcrack formation at grain boundaries and eventually capacity fading 11-12.
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Beside the structure changes, the electrochemical kinetics is also important for the electrochemical performance study of cathode material. The Li+ insertion/extraction induced structural changes should influence on the electrochemical kinetics of NCA significantly. Nevertheless, most of previous reports either study the structural change or the kinetic process separately, simultaneous measurements including both the structural changes and the kinetic process, and the interpretation of their relationship have rarely been reported. Therefore, in-depth elucidation of the structure change on the electrochemical kinetics are very important and will provide a much more comprehensive understanding of the cathode degradation mechanism. In this study, the bulk structural changes were characterized by in situ X-ray diffraction (XRD) together with the surface characterization by scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) during the first charge process. Electrochemical impedance spectroscopy (EIS) and the galvanostatic intermittent titration technique (GITT) were used to study the electrochemical kinetics. This work will establish an relationship between bulk and surface structure changes with the electrochemical kinetics of NCA, which broaden the understanding of inner mechanism of lithium transition metal oxide layered cathode materials. 2. Experimental Section 2.1. Electrochemical characterization Industry-level NCA cathode was used in this study (CATL, China) and inductively coupled plasma spectrometry (ICP) was applied to analyzed the average chemical composition (Supporting information). The cathode preparation and coin cell (2032-type) assemble have been introduced in detail in our previous publication 13. All the tests were performed at room temperature 25°C. Galvanostatic charge-discharge was cycled on a LAND CT2001A instrument at C/20 (theoretical specific capacity: 279 mAh g-1) with the cutoff voltage window of 3.0-4.5 V. The cyclic voltammetry (CV), EIS and GITT were tested using VMP3 Multi-channel potentiostat (Bio-Logic-Science Instruments SA,
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France). CV measurements were conducted at sweep rate of 0.1 mV s-1 in the voltage range of 3.0 V-4.5 V, while EIS was carried out in the frequency range from 100 kHz to 10 mHz. For GITT measurements, the cell was charged at C/10 rate for 1 hour’s intermittent time, followed by 3 hours’ open-circuit relaxation to allow the cell voltage relaxation to the steady-state (Es). 2.2. Material characterization Field emission scanning electron microscope (FE-SEM, Hitachi SU8010) at 5 kV was used to analysis the surface morphology of the samples. For the cross section observation by SEM, the particles were cut using Ar+ (2 kV, 5°). High-resolution transmission electron microscopy (HRTEM) and High angle annular dark field (HAADF) images, selected area electron diffraction pattern (SADP) and element mapping were obtained with a Tecnai G2 F30 (FEI) performed at 300 kV accelerating voltage and room temperature. X-ray diffraction (D8 Advance, Bruker, λ=1.5406Å, LYNXEYE XE-T diffractometer with CuKa radiation) was employed to identify the crystal structure of the NCA powder. The XRD powder diffraction pattern were performed between 10° to 80° and analyzed by the Topas software. In situ XRD measurements were performed at C/10 in the voltage range of 3.0-4.5V, and the relative lattice constants were discussed by software Topas. XPS (PHI 5000 VersaProbe II) using X-rays aluminum anode source (monochromatic Kα Xrays at 1486.6 eV) was conducted to analysis the surface elemental composition. During the depth profiling, an etch rate of 29nm min-1 of Ar+ beam was confirmed by the SiO2 standard sample. For SEM and XPS characterization, the electrode samples charged to the corresponding voltage were disassembled and rinsed with DMC in the glove box. To keep an vacuum environment, the rinsed samples were then transferred using an air-tight transfer device to the analysis chambers. The TEM samples were prepared by abrading the active materials particles from the Al foil current collector, and then the sample particles were dissolved in DMC solution for sonication. All these procedures were performed in the glovebox. 3 Results and discussion
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3.1 Three typical states during the initial delithiation of NCA The characterizations of pristine NCA materials including morphology and crystal structure are as shown in Figure S1. The size of the secondary particles of NCA are in the range of 10-30 μm in diameter with spherical morphology. The NCA materials assembled into coin type half-cells are used for the electrochemical performance tests. Figure 1a shows the charge and discharge curves for the first three cycles from 3.0V to 4.5V vs. Li+/Li at a rate of C/20. Apparently, the charge profile show distinctive difference between the first and subsequent cycles especially at the initial stage, indicating that the reaction mechanisms are different between the first and subsequent cycles. To analysis the chemical reactions qualitatively, the CV curves were recorded for the first two cycles at 0.1 mV s-1 with upper cutoff voltage of 4.5 V (Figure 1b). Obviously, the CV curves are different between the first and the subsequent cycle. During the first charge process, oxidation peaks are observed at 3.8 V, 3.98 V and 4.18 V. In the second cycle, the oxidation peak at 3.80 V decreases to lower potential of 3.76 V. To clarify the structural change of NCA during the first charge, in situ XRD were performed for the first cycle at 1/10 C with potential window of 3.0 V-4.5 V. Figure S2 show the contour plot of in situ XRD for the first cycle. X-ray patterns with selected 2θ regions and the corresponding galvanostatic profile of the first charge process are displaced in Figure 1c. The calculated lattice parameters using least-square refinement were plotted as a function of x in Li1-xNCA (Figure 1d), which process is divided into three stages from the change of lattice parameters 6, 14. At the beginning (stage I, 0
