Conference-ISSS-7-Degradation Analysis of LiCoO 2 Positive Electrode Material of a Li-Ion Battery Using the Li K-Edge Signal Obtained from STEM-EELS Measurements

The Li distribution in positive electrode material (LiCoO2) was observed after a charge-discharge cycling test by analytical transmission electron microscopy (TEM). The electron energy-loss spectroscopy (EELS) imaging method revealed an inhomogeneous Li distribution in the LiCoO2 particles after the charge-discharge cycling test. Using the alternating least-squares (ALS) method for the data matrix of the EELS spectra, we obtained LiCoO2 and Li-poor LiCoO2 spectra from the LiCoO2 particles. Around the degraded LiCoO2 particles, Co3O4 and fluoride components were detected. It is suggested that inhomogeneous electrochemical reactions are one cause of LiCoO2 and electrolyte degradation. [DOI: 10.1380/ejssnt.2015.284]


INTRODUCTION
To study the degradation mechanism of Li-ion battery materials, it is important to understand the atomic structures and chemical states of these materials.Analytical transmission electron microscopy (TEM) is useful for studying these materials because of its high spatial resolution.In particular, direct analysis of Li ions using electron energy-loss spectroscopy (EELS) can be a powerful method of investigating the chemical state of Li in Li-ion battery materials [1][2][3].However, M-edge signals from Mn, Co, and Ni appear near the energy region of the Li K-edge signal in EELS images, and the overlap between the Li K-edge and M-edges of these transition metals often reduces the accuracy of the quantitative analysis or interferes with the chemical analysis of the spectrum.The aforementioned transition metals are widely used as electrodes in Li-ion batteries for charge compensation during the Li insertion and extraction reactions.High-energyresolution EELS is effective in reducing the overlapping problem in Li K-edge analysis.
By using EELS with a monochromator, the Li-ion distributions inside LiCoO 2 particles in positive electrode material with and without surface modification during the charge-discharge cycle have been reported [4,5].However, the products generated from the electrochemical reactions in a LiCoO 2 system have not been previously examined by analytical TEM.
In the present study, Li K-edge analysis using monochromated STEM-EELS was performed.In addition, maps of the Li intensity and of the reaction products were also reconstructed from EELS images [4,6].To obtain chemical information about the sample, analyses not only of the intensities of the EELS distributions but also of their shapes were conducted as post-analysis.Such a large matrix of spectral data is suitable for analysis via a chemometrics approach.To obtain individual pure spectra from the series of spectra without a reference spectrum, we applied a self-modeling curve-resolution technique (the alternating least-squares (ALS) method) [7][8][9][10][11] to the series of spectra to perform spectral decomposition.The degradation process of LiCoO 2 was analyzed based on the series of decomposed spectra.

II. EXPERIMENTAL
Small LiCoO 2 particles, which are suitable for TEM and EELS analyses, were synthesized by the Pechini process [12].
and citric acid were mixed at 165 • C in an ethylene glycol solution (Co/Li = 1:1 atomic ratio).The obtained gel was pre-calcined at 400 • C in air to decompose the precursor.The precursor was calcined again at 750 • C for 1 h in air.The LiCoO 2 particles were about 200 nm in size.
A composite containing 84 wt% active material (LiCoO 2 ), 8 wt% acetylene black (AB), and 8 wt% polyvinylidene fluoride (PVDF) was used as the positive electrode.A sheet-type two-electrode cell with 16 mmϕ electrodes sealed in Al-laminated type film was assembled using Li metal as the counter electrode and 1.0 mol•dm −3 LiPF 6 /EC (ethylene carbon-ate+DMC(dimethyl carbonate) (1:1 in volume) as the electrolyte.The cell was cycled in the voltage range between 3.0 V and 4.2 V with a 0.1 C charge-discharge rate (1 C = 150 mA/g).The temperature around the cell in the charge-discharge tests was 20 • C.
For the TEM observations, LiCoO 2 powder was directly dispersed on a carbon film supported by a Cu mesh.A TITAN 3 G2 60-300 electron microscope (FEI Company) equipped with a Super X (Bruker Corporation) for energy-dispersive X-ray spectroscopy (EDS) and a GIF Quantum ERS (Gatan Inc.) for EELS was operated at an accelerating voltage of 200 kV.The EDS and EELS measurements were performed on LiCoO 2 after the 10 th discharge by a spectrum-imaging scheme based on STEM observations.The process of spectrum imaging refers to the acquisition of three-dimensional data that contain both spectral and spatial information about the specimen.Each EELS image integrates the signal from the whole region along the beam direction at each pixel.In the EELS acquisition for Li-K mapping, a monochromator was applied to achieve a higher energy resolution.The FWHM of the zero-loss peak was less than 0.3 eV with an energy dispersion of 0.05 eV/channel.

III. RESULTS AND DISCUSSION
Figure 1 (a) shows the SEM image of the synthesized LiCoO 2 particles.The primary particles display a partial agglomerate formation, and the size of primary particles is about 200 nm.Thus, smaller-sized particles, which are suitable for TEM observations, were obtained.The typical layered structure of LiCoO 2 was also confirmed by using X-ray diffraction (XRD) (not shown) and ADF-STEM observations (Fig. 1 (b)).
Figure 2 shows the cycling performance of the cell.The discharge capacity of LiCoO 2 depends on the particle size [18].Much larger particles (∼300 nm) generally show much better electrochemical properties [4].In the present study, we treated the smaller particles as the typical model sample to represent the capacity fading and irreducible capacity in the TEM analyses.
The Li K-edge EELS plots of the LiCoO 2 particles after 10 th discharge are shown in Fig. 3 (A-D) along with the Li distribution.We obtained the spectra around the Li K-edge in each pixel of the ADF-STEM image.This information enabled the spatial distribution of Li to be reconstructed from the Li K-edge spectra [4,5].We reconstructed the Li/Co intensity-ratio map using the series of spectra in Fig. 3. Inhomogeneous Li distributions in the particles were clearly observed.At spot D, the shape of the spectra is the same as that of bare LiCoO 2 [4,19].Compared with the intensity of the Li K-edge at spot D, the corresponding intensity at spot A is slightly decreased.Considerably decreased Li K-edge intensities are observed at spots B and C, and the regions around these spots are believed to have been damaged during the chargedischarge cycling.
In our previous study, the homogeneity of the Li-ion distribution in Al-and Si-oxide-coated LiCoO 2 agreed well with the capacity retention after the cycling test [4].Specifically, LiCoO 2 with poor cycling had a decidedly http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/)   intensity changes due to variations in sample thickness can be seen.Thus, the amounts of Co and O do not change drastically during charge-discharge cycling.
In the present sample, the degraded LiCoO 2 area can be clearly seen from the Li K-edge EELS and Li distribution images.The information about the chemical state near the damaged region is quite important in order to investigate the reaction products.In addition to the Li intensity analysis shown in Fig. 3, the fine structures of the EELS distributions also yield chemical information about the sample.On these data, we performed ALS analysis in order to identify the primary spectral compohttp://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology Volume 13 (2015) nents, especially in the degraded regions.From the ALS analysis, we identified five types of components from the spectral images, and their intensity maps are shown in Fig. 5.The intensity map of comp. 1 corresponds closely to a Li-poor region.In contrast, comp. 2 corresponds to a bare LiCoO 2 region.In comps.3 and 5, very similar regions are observed, as these components were mainly obtained around the same particle edge.Similarly, comp. 4 was also obtained from the region surrounding the particle.
The decomposed spectra for all of these components are shown in Fig. 6.In the spectrum of comp. 1 (Fig. 6 (a)), a weak Li K-edge peak can be seen around 62 eV.The Li K-edge spectrum of delithiated LiCoO 2 (Li 0.5 CoO 2 ) corresponds closely to the shape of the comp. 1 spectrum [19].The Co and O intensities did not drastically change from the EDS results after the 10th discharge, which are shown in Fig. 4.This result indicates that the Li-poor region does not transform pure Co-oxide completely as a result of the Li absence but rather creates Li-poor states, such as a charged state in which Li is partially extracted.In the BF-STEM image, the region with greater particle contrast corresponds closely to the region in which comp. 1 is mainly observed.In general, BF-STEM images are sensitive to diffraction contrast, such as that introduced by local strain or dislocation in the crystal.In LiCoO 2 , significant Li extraction causes substantial change in the lattice constant.Local states are considered to induce more particle damage than are charged states.Thus, local area deformation is believed to prevent normal diffusion of Li ions in materials.
In comp.3, the Li K-edge peak completely disappears, as shown in Fig. 6 (c), in which there is no peak near 62 eV.This spectrum corresponds closely to that of Co 3 O 4 [19].Based on the present EELS results, the main product around the LiCoO 2 particle is believed to be Co 3 O 4 .The spectrum of comp. 2 in Fig. 6 (b) corresponds to that of LiCoO 2 and includes the sharp Li K-edge peak.The intensity distribution of comp. 4 is that of the particles surrounding the LiCoO 2 , and the same distribution is obtained from EDS analysis of carbon.Thus, comp. 4 is thought to be a background signal from carbon from AB (conductive additive), electrolytes, or contamination from TEM sample preparation, among other sources (Fig. 6  (d)).Finally, the peak of comp.5 in Fig. 6 (e) agrees well with the LiF x and LiO x F y peaks previously reported by Muto et al. [10].These F compounds are believed to be decomposition products from the electrolyte LiPF 6 .The reaction products around the particle surface were determined from the results of TEM-EELS analysis of a LiNi 0.8 Co 0.15 Al 0.05 O 2 sample, which was fabricated by a focused ion beam (FIB).Results similar to those observed in the degradation analysis of LiNi 0.8 Co 0.15 Al 0.05 O 2 using STEM-EELS [8][9][10][11] were obtained even in the case of LiCoO 2 .
Considering these ALS results, we can draw a schematic image of the degradation process around a LiCoO 2 particle, as is shown in Fig. 7. First, LiCoO 2 (positive electrode) is delithiated (charge process), and Li is diffused to Li metal (anode).In this delithiation process, excess Li extraction may occur locally.Crystal deformation is induced in the local area, and thus further Li diffusion is prevented.Then, side reactions occur around the surface of the damaged region, and Co 3 O 4 particles or other electrolyte decomposition products are generated (Fig. 7 (a)).In the discharge (Li insertion) process, Li returns to the positive electrode.However, such reaction products and deformed areas are considered unfavorable for the diffusion of Li ions.Finally, a Li-poor region (comp.1) appears in the LiCoO 2 particle (Fig. 7 (b)).

IV. CONCLUSION
STEM-EELS analysis of the Li K-edge in LiCoO 2 particles was conducted after charge-discharge cycling by using a monochromator.From the resulting spectra, the LiCoO 2 particles were confirmed to have inhomogeneous Li distributions.ALS analysis revealed that the Li-poor region was not Li-free Co oxide and that Co 3 O 4 and electrolyte decomposition products were distributed across the surface.Local overcharge states induced side reactions and particle deformation, preventing Li diffusion in the Li insertion and exertion processes during the chargedischarge cycling.STEM-EELS imaging with ALS analysis was successfully applied in the degradation analysis of LiCoO 2 positive electrode material.

FIG. 3 .
FIG. 3. Li-K/Co-M intensity ratio map for LiCoO2 after cycling test.Image contrast is normalized to Li K-edge/Co M-edge intensity ratio of as-prepared LiCoO2.Intensity scale corresponds to x in LixCoO2.Raw spectra of Li-K edge are shown for all spots (A-D).

FIG. 6 .
FIG.6.EELS distributions obtained from ALS analysis of LiCoO2 after cycling test.

FIG. 7 .
FIG. 7. Schematic image of sample degradation process during cycling test.