Stabilizing the (003) Facet of Micron-Sized LiNi 0.6 Co 0.2 Mn 0.2 O 2 Cathode Material Using Tungsten Oxide as an Exemplar

: The structural stability of layered LiNi 1-x-y Co x Mn y O 2 cathode materials is critical for guar-anteeing their excellent electrochemical cycling performance, particularly at elevated temperatures. However, the notorious H 2 –H 3 phase transition along with associated large changes in the c -axis or (003) facet is the fundamental origin of the anisotropic and abrupt change in the unit cell and the degradation of the cycling performance. In this study, we coat micron-sized LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM) with tungsten oxide via atomic layer deposition and investigate the atomic-to-microscopic structures in detail via advanced characterization techniques, such as Cs-corrected scanning transmission electron microscopy. The results reveal that coated tungsten oxide is predominately accumulated on the (003) facet of NCM, with the migration of a small amount of W 6+ into this facet, resulting in a reduction of Ni 3+ to Ni 2+ and the formation of a rock-salt-like structure on the surface. The electrochemical cycling performance of tungsten-oxide-coated NCM is signiﬁcantly improved, showing a capacity retention of 86.8% after 300 cycles at 55 ◦ C, compared to only 69.4% for the bare NCM. Through further structural analysis, it is found that the initial tungsten-oxide-coating-induced (003) facet distortion effectively mitigates the expansion of the c -lattice during charge, as well as oxygen release from the lattice, resulting in a lowered strain in the cathode lattices and a crack in the cathode particles after prolonged cycling.


Introduction
Since the first commercialization of lithium-ion batteries (LIBs) in 1991, LIBs have attracted great attention for the applications in consumer electronics and electric vehicles [1,2]. With the increasing demand for high-energy-density and high-power-density batteries, it is necessary to develop new cathode materials to replace commercially expensive LiCoO 2 . LiNi 1-x-y Co x Mn y O 2 cathode materials have the advantages of high energy density and relatively low cost; nickel oxide is considered one of the most promising cathode materials for LIBs [3,4]. However, the cycle performance of the LiNi 1-x-y Co x Mn y O 2 cathode materials shows a gradual decrease with increasing the nickel content [5,6]. The mechanisms behind the degradation of performance have been extensively investigated and are attributed to the surface irreversible formation of NiO rock-salt-like phases, intergranular cracks, micro-strains, transition metal dissolution, etc. [7][8][9][10][11][12]. Among nickel-rich

Structure and Morphology Characterization
A scanning electron microscope (SEM, Zeiss Gemini SEM 300) combined with energydispersive spectroscopy (EDS) was used to analyze the chemical composition on the surface of the NCM particles.
X-ray diffraction (XRD) was applied to study the crystal structure of the bare and WO 3 -coated NCM powders. The XRD patterns were collected between 10 • and 100 • (2θ) at a rate of 0.02 • per minute. The diffractometer was a Bruker D8 ADVANCE diffractometer with a Cu K α radiation source (λ 1 = 1.54056 Å, λ 2 = 1.54439 Å). Rietveld refinement for XRD patterns was performed via the TOPAS Academic software, with a fitting model similar to that reported by Dahn et al. [35] and Delmas et al. [36] Chebychev background and the modified Thompson-Cox-Hastings pseudo-Voigt peak type were used during the refinement. To understand the change in the lattice parameters of the cathode materials in the fully charged state (first cycle), cathode powders were scraped from electrodes dissembled from coin-type half-cells and mixed with internal Si standard for XRD analysis. The lattice parameters of the electrochemically cycled samples were fitted by the Pawley method.
The surface element information was investigated with X-ray photoelectron spectroscopy (XPS) conducted via a Kα X-ray photoelectron spectrometer (Thermo Fisher) with an Al Kα source (1486.6 eV). The binding energy was calibrated with the C1s peak (284.8 eV) of an external carbon in a vacuum chamber. Peak fitting and quantitative evaluation were conducted using the XPSPEAK software, and the background was corrected using the Shirley method.
A ThermoFisher ICAP 7200 inductively coupled plasma optical emission spectroscopy (ICP-OES) instrument was used for the determination of elements, particularly W in the NCM cathode materials. The experiment was performed five times to obtain an averaged value.
To precisely understand the atomic-scale structure of NCM before and after the ALD coating of tungsten, a Cs-corrected scanning transmission electron microscope (JEOL ARM200, 200 kV) was used to characterize the particles prepared with a focused ion beam. EDS and electron energy loss spectroscopy (EELS) were also employed to investigate element information and oxidation. The collection semi-angle of the HAADF detector was 12−24 and 75-250 mrads.

Electrode Preparation
The electrodes for electrochemical cycling were prepared similar to that reported in the literature [34].

Electrochemical Cycling Test
Electrochemical cycling was performed at room temperature and 55 • C in the voltage range of 2.75-4.3 V on a LAND automatic battery tester (Wuhan Land Electronics, Wuhan, China).

Results
The morphology and crystal structure of the NCM cathode material before and after ALD coating with tungsten oxide were initially studied. The SEM images indicate that both the bare (Figure 1a 1 ,a 2 ) and the tungsten-oxide-coated (WO 3 -coated) (Figure 1b 1 ,b 2 ) NCM particles were irregular, with size ranging from 1 to 4 µm. There was no obvious morphology change after tungsten oxide coating. The XRD patterns of the bare and WO 3coated NCM cathode materials are shown in Figure 1c. In general, all the cathode materials exhibited similar diffraction patterns, with the main peaks indexed to the standard layered LiNiO 2 structure (space group R-3m) [37]. Typical features, such as the splitting of the (006)/(012) and (108)/(110) peaks, were evident for both the cathode materials, indicating the highly ordered layered structure [38]. To obtain more detailed lattice parameters of the bare and WO 3 -coated NCM cathode materials, Rietveld refinement was performed using XRD data. The refined unit-cell parameters were a = 2.8666(3) Å, c = 14.2066(1) Å, and V = 101.107(2) Å 3 for the bare NCM, which are in good agreement with those reported previously [39,40]. After tungsten oxide coating, the refined unit-cell parameters were a = 2.8701(1) Å, c = 14.2133(2) Å, and V = 101.396(2) Å 3 . The detailed refinement parameters are shown in Table S1. The slightly increased unit cell is attributed to the migration of a small amount of W 6+ into the lattice of NCM during the post-ALD heat treatment at 500 • C, as will be shown later. Note that no additional reflections related to WO 3 were observed for WO 3 -coated NCM, possibly due to the small coating amount.
parameters of the bare and WO3-coated NCM cathode materials, Rietveld refinement was performed using XRD data. The refined unit-cell parameters were a = 2.8666(3) Å, c = 14.2066(1) Å, and V = 101.107(2) Å 3 for the bare NCM, which are in good agreement with those reported previously [39,40]. After tungsten oxide coating, the refined unit-cell parameters were a = 2.8701(1) Å, c = 14.2133(2) Å, and V = 101.396(2) Å 3 . The detailed refinement parameters are shown in Table S1. The slightly increased unit cell is attributed to the migration of a small amount of W 6+ into the lattice of NCM during the post-ALD heat treatment at 500 °C, as will be shown later. Note that no additional reflections related to WO3 were observed for WO3-coated NCM, possibly due to the small coating amount. The element distribution of the bare and WO3-coated NCM cathode materials were investigated with SEM-EDS mapping, which showed a homogenous distribution of Ni, Co, Mn, and W elements (Figure 2a1,a2,b1,b2). To further understand the surface element information, particularly of tungsten, XPS was performed, and the results of the WO3coated NCM materials are shown in Figure 2c1,c2. After tungsten oxide coating, two characteristic peaks at 35.43 and 37.66 eV associated with W 6+ appeared, indicating the presence of WO3 on the NCM particle surface after ALD coating [15]. The detailed tungsten composition in the WO3-coated NCM was 0.48 ± 0.02 wt%, as verified through ICP-OES.
The above results confirm the presence of a small amount of WO3 on the surface of the NCM after ALD coating, without significantly altering the general morphology and layered structure of NCM. To understand the exact atomic-scale structure and element information before and after the ALD coating of tungsten oxide, the micron-size particles of bare and WO3-coated NCM were cut by a focused ion beam, and the regions of interest (ROIs) were precisely selected ( Figure 3a) and investigated in detail by STEM. In general, two ROIs from each sample were investigated, i.e., a purple-boxed region (ROI-A) and a green-boxed region (ROI-B) for the bare NCM. The ROI-A exhibited a standard arrangement of transition metal (TM) atoms viewed along the [100] zone axis (Figure 3b1) [41][42][43][44]. There was no evidence of any structural distortion via the migration of TM ions into the Li layer, as shown by the magnified region of Figure 3b2. Similarly, STEM images of ROI-B also show a standard arrangement of TM atoms viewed along the [100] zone axis, without strong evidence of a contrast in the Li layer in the surface region (Figure 3c1,c2). The standard arrangements of the atoms in the ROI-A and B were also confirmed by line profiles (Figure 3d). In addition, based on the arrangement of TM atoms, the crystallographic [003] direction and (003) plane are indexed, as shown in Figure S4. This indicates that TM atoms are also arranged parallel to the (003) plane. The element distribution of the bare and WO 3 -coated NCM cathode materials were investigated with SEM-EDS mapping, which showed a homogenous distribution of Ni, Co, Mn, and W elements (Figure 2a 1 ,a 2 ,b 1 ,b 2 ). To further understand the surface element information, particularly of tungsten, XPS was performed, and the results of the WO 3coated NCM materials are shown in Figure 2c 1 ,c 2 . After tungsten oxide coating, two characteristic peaks at 35.43 and 37.66 eV associated with W 6+ appeared, indicating the presence of WO 3 on the NCM particle surface after ALD coating [15]. The detailed tungsten composition in the WO 3 -coated NCM was 0.48 ± 0.02 wt%, as verified through ICP-OES.
The above results confirm the presence of a small amount of WO 3 on the surface of the NCM after ALD coating, without significantly altering the general morphology and layered structure of NCM. To understand the exact atomic-scale structure and element information before and after the ALD coating of tungsten oxide, the micron-size particles of bare and WO 3 -coated NCM were cut by a focused ion beam, and the regions of interest (ROIs) were precisely selected ( Figure 3a) and investigated in detail by STEM. In general, two ROIs from each sample were investigated, i.e., a purple-boxed region (ROI-A) and a green-  Figure S4. This indicates that TM atoms are also arranged parallel to the (003) plane.  Figure 3f4) maps for ROI-B, the compositions of these elements in both regions were distributed homogeneously. The oxidation state of the transition metal (TM) ions and the bonding nature between the TM ions could be detected by monitoring the fine structure changes of the core edges of these elements. The fitting of the as-obtained electron energy loss spectra (EELS) to the standard models obtained from the sample materials was performed to obtain the Ni concentration for each oxidation state. The details of the fitting procedure were reported previously [41,43]. The results show that the concentration of Ni 2+ in ROI-A is 5.4% ( Figure S1a), similar to that of 5.8% in ROI-B ( Figure S1b). The limited presence of Ni 2+ is also in agreement with the oxygen pre-peak in the K edge EELS spectra, showing a high intensity of the pre-peak ( Figure S1c,d), suggesting limited oxygen deficiency in the bare NCM lattice.
Further STEM analysis was performed for WO3-coated NCM cathode material to understand the effect of tungsten oxide coating on the atomic-scale structure. Two ROIs, i.e., purple-boxed region ROI-C and green-boxed region ROI-D, as shown in Figure 4a1, were selected for detailed analysis. Unlike the bare NCM, as shown above, both the ROI-C and D regions exhibited increased atomic-scale structure distortion by the migration of TM atoms to the Li-layer to form the rock-salt-like structure (Figure 4b1,b2,c1,c2, respectively) [42,43,45]. Furthermore, the rock-salt-like structure was ~10 nm in ROI-D (Figure 4c1 Figure 4f4) maps for ROI-D, the compositions of these Mn, Co, and Ni elements in both regions were distributed homogeneously; however, the W element predominated and was accumulated on the surface of ROI-D (Figure 4f). The fitting of the as-obtained EELS to the standard models showed that the concentration of Ni 2+ in ROI-C was 8.4% ( Figure S2a), while it was 25.8% in ROI-D ( Figure S2b). The increased Ni 2+ was also in agreement with the oxygen K-edge EELS spectra, showing a reduced intensity in the oxygen pre-peak, particularly in ROI-D ( Figure S2c,d), suggesting an oxygen deficiency in the lattice in this region [46].
To further explore the effect of tungsten oxide coating on WO3-coated NCM for practical use, electrochemical tests were conducted on the bare and WO3-coated NCM halfcells. The results of the electrochemical results are shown in Figure 5. The half-cells were initially cycled at room temperature (RT) with a C-rate of 0.1 C for five cycles at a voltage range of 2.75-4.3 V. The initial discharge capacity of the bare NCM was 174.5 mAh/g at 0.1 C at RT (Figure 5a). After tungsten oxide coating, WO3-coated NCM displayed an initial discharge capacity of 165.1 mAh/g at 0.1 C (RT). The prolonged RT cycling performance of the bare and WO3-coated NCM half-cells are compared in Figure 5b. The bare NCM presented an initial 1 C discharge capacity of 165.5 mAh/g and a capacity retention of 93.7% after 100 cycles at RT. By contrast, WO3-coated NCM displayed a slightly lower 1 C discharge capacity of 158 mAh/g but a higher capacity retention of 96.8% after 100 cycles at RT. The oxidation state of the transition metal (TM) ions and the bonding nature between the TM ions could be detected by monitoring the fine structure changes of the core edges of these elements. The fitting of the as-obtained electron energy loss spectra (EELS) to the standard models obtained from the sample materials was performed to obtain the Ni concentration for each oxidation state. The details of the fitting procedure were reported previously [41,43]. The results show that the concentration of Ni 2+ in ROI-A is 5.4% ( Figure S1a), similar to that of 5.8% in ROI-B ( Figure S1b). The limited presence of Ni 2+ is also in agreement with the oxygen pre-peak in the K edge EELS spectra, showing a high intensity of the pre-peak ( Figure S1c,d), suggesting limited oxygen deficiency in the bare NCM lattice.
Further STEM analysis was performed for WO 3 -coated NCM cathode material to understand the effect of tungsten oxide coating on the atomic-scale structure. Two ROIs, i.e., purple-boxed region ROI-C and green-boxed region ROI-D, as shown in Figure 4a 1 , were selected for detailed analysis. Unlike the bare NCM, as shown above, both the ROI-C and D regions exhibited increased atomic-scale structure distortion by the migration of TM atoms to the Li-layer to form the rock-salt-like structure (Figure 4b 1 ,b 2 ,c 1 ,c 2 , respectively) [42,43,45]. Furthermore, the rock-salt-like structure was~10 nm in ROI-D (Figure 4c 1 ), while it was only a few atomic layers in ROI-C (Figure 4b 2 ). The arrangement of atoms in ROI-C and D are shown by line profiles (Figure 4d), demonstrating that the clear rock-salt-like structure induced a sharp contrast in the Li-layer (L4) in ROI-D.   Elemental distribution mapping of ROI-C and ROI-D regions was also performed with STEM-EDS. As shown by the Mn (Figure 4e (Figure 4f 4 ) maps for ROI-D, the compositions of these Mn, Co, and Ni elements in both regions were distributed homogeneously; however, the W element predominated and was accumulated on the surface of ROI-D (Figure 4f). The fitting of the as-obtained EELS to the standard models showed that the concentration of Ni 2+ in ROI-C was 8.4% ( Figure  S2a), while it was 25.8% in ROI-D ( Figure S2b). The increased Ni 2+ was also in agreement with the oxygen K-edge EELS spectra, showing a reduced intensity in the oxygen pre-peak, particularly in ROI-D ( Figure S2c,d), suggesting an oxygen deficiency in the lattice in this region [46].
To further explore the effect of tungsten oxide coating on WO 3 -coated NCM for practical use, electrochemical tests were conducted on the bare and WO 3 -coated NCM halfcells. The results of the electrochemical results are shown in Figure 5. The half-cells were initially cycled at room temperature (RT) with a C-rate of 0.1 C for five cycles at a voltage range of 2.75-4.3 V. The initial discharge capacity of the bare NCM was 174.5 mAh/g at 0.1 C at RT (Figure 5a). After tungsten oxide coating, WO 3 -coated NCM displayed an initial discharge capacity of 165.1 mAh/g at 0.1 C (RT). The prolonged RT cycling performance of the bare and WO 3 -coated NCM half-cells are compared in Figure 5b. The bare NCM presented an initial 1 C discharge capacity of 165.5 mAh/g and a capacity retention of 93.7% after 100 cycles at RT. By contrast, WO 3 -coated NCM displayed a slightly lower 1 C discharge capacity of 158 mAh/g but a higher capacity retention of 96.8% after 100 cycles at RT.
The corresponding dQ/dV curves in the first cycle at 0.1 C for the bare and WO 3coated NCM are shown in Figure 5c, which exhibits similar redox peaks, indicating that the ALD coating of tungsten oxide does not have a significant effect on the electrochemical reaction during the charge-discharge process.
The rate capability of the bare and WO 3 -coated NCM cathode materials between 0.1 and 5 C at RT are shown in Figure 5d. The bare NCM showed capacities of 174.5, 173.2, 169.4, 165.1, 155.2, and 127.6 mAh/g at 0.1, 0.2, 0.5, 1, 2, and 5 C, respectively. By contrast, WO 3 -coated NCM exhibited lower capacities at lower C rates but higher capacities at higher C rates. For example, the capacities were 168.1 mAh/g, 166.3, 163.1, 158.2, and 153.2 mAh/g at 0.1-2 C but 134.2 mAh/g at 5 C. When the current density changed back to 0.1 C, WO 3 -coated NCM half cells exhibited a strong structural tolerance for rapid lithiumion exchange, showing a nearly complete recovery of discharge capacity. The improvement of the rate performance of NCM through tungsten oxide coating is mainly attributed to an improved structural stability, as will be shown later. By contrast, the reversible capacity for the bare NCM counterpart exhibited a recovery to 159.4 mAh/g when the current density changed back to 0.1 C, owing to the poor structural stability and associated decrease in the lithium-ion diffusion.
The bare and WO 3 -coated NCM were further cycled at 55 • C to accelerate the degradation of cycling performance for better characterization, and the results are shown in Figure 5e,f. It is known that the reactions between cathode materials and trace amounts of HF in the electrolyte are enhanced at elevated temperatures [43,47,48], causing significant degradation of the cycling performance if the cathode structure is not stable. Both the bare and the WO 3 -coated NCM showed a slightly increased discharge capacity during the first cycle at 55 • C because of the increased lithium-ion kinetics at elevated temperatures (Figure 5f). For the WO 3 -coated NCM, the discharge capacity demonstrates an excellent retention of 86.9% after 300 cycles at 1 C (55 • C), compared to 69.4% for the bare NCM. The corresponding dQ/dV curves in the first cycle at 0.1 C for the bare and WO3coated NCM are shown in Figure 5c, which exhibits similar redox peaks, indicating that the ALD coating of tungsten oxide does not have a significant effect on the electrochemical reaction during the charge-discharge process. The rate capability of the bare and WO3-coated NCM cathode materials between 0.1 and 5 C at RT are shown in Figure 5d. The bare NCM showed capacities of 174.5, 173.2, Figure 5. Electrochemical properties of the bare and WO 3 -coated NCM half-cells. (a) Chargedischarge curves in the first cycle at 0.1 C (RT), (b) discharge capacity retention for the 6th-105th cycles at RT (1-5th at 0.1 C), (c) dQ/dV curves for the first cycle at RT, (d) discharge capacity at 0.1-5 C for 5 cycles at each C-rate, (e) charge-discharge curves in the first cycle at 55 • C and (f) discharge capacity retention at 55 • C for the 6th-305th cycles (0.1 C for the 1st-5th cycles at RT).
To understand the structural degradation of the NCM after prolonged cycling, typical electrodes were disassembled after 300 cycles at 55 • C in an argon-filled glove box for morphology analysis with SEM. As shown in Figure 6a 1 ,a 2 , after 300 cycles at 55 • C (1 C), clear cracks were evident for the bare NCM cathode particles, ranging from the center to the edge of the particles. STEM-HAADF images of the 55 • C cycled bare NCM (Figure 6c 1 ,c 2 ) also reveal that the atomic-scale crack originating in the bulk region was parallel to the (003) plane. The oxygen K-edge EELS spectra showed that the intensity of the pre-peak was diminished, indicating significant oxygen loss from the lattice in the bulk region near the crack ( Figure S3a). Compared with bare NCM, there was no evidence of cracks observed through SEM in the WO 3 -coated NCM cathode particles (Figure 6b 1 ,b 2 ). STEM-HAADF images of the 55 • C cycled WO 3 -coated NCM (Figure 6d 1 ,d 2 ) also revealed excellent atomicscale arrangement of atoms in the bulk region. The oxygen K-edge EELS spectra showed that the intensity of the pre-peak was high, indicating no significant oxygen loss from the lattice in the bulk region ( Figure S3b). These observations confirm that the ALD coating of tungsten oxide significantly improved the stability of the NCM cathode material, alleviating particle breakage during cycling and consequently improving the cycling stability.
bulk region near the crack ( Figure S3a). Compared with bare NCM, there was no evidence of cracks observed through SEM in the WO3-coated NCM cathode particles (Figure 6b1,b2). STEM-HAADF images of the 55 °C cycled WO3-coated NCM (Figure 6d1,d2) also revealed excellent atomic-scale arrangement of atoms in the bulk region. The oxygen K-edge EELS spectra showed that the intensity of the pre-peak was high, indicating no significant oxygen loss from the lattice in the bulk region ( Figure S3b). These observations confirm that the ALD coating of tungsten oxide significantly improved the stability of the NCM cathode material, alleviating particle breakage during cycling and consequently improving the cycling stability.

Discussion
Based on the above detailed atomic-to-microscopic structure analysis and electrochemical cycling of the bare and WO 3 -coated NCM, the improved cycling performance of NCM after ALD coating of tungsten oxide is mainly attributed to its effective inhibition of cycling-induced cracks [49,50]. The formed cracks expose fresh surfaces to the electrolyte, causing the formation of cycling-induced rock salt and the dissolution of TM ions in the electrolyte, and consequently, the rapid degradation of the cycling performance [51][52][53]. These adverse effects are further enhanced at elevated temperatures, owing to the increased amount of HF in the electrolyte, thus causing the rapid degradation of the cathode structure, and consequently, the cycling performance [47,48]. Furthermore, the loss of oxygen from the lattice, as clearly observed in the crack region ( Figure S3a), results in the oxidation of the electrolyte and further formation of HF [46,54]. This, in turn, accelerates the structural distortion and degradation of the cycling performance. Thus, the bare NCM exhibits a much poorer capacity retention than WO 3 -coated NCM, as observed in Figure 5f.
The formation of cracks in layered cathode materials may be attributed to many factors, with the main factor being anisotropic and abrupt lattices changes, which in turn induce stains and the formation of cracks. The anisotropic lattice changes during cycling are usually much greater along the c-axis than the a-axis. In an extreme case, pure LiNiO 2 showed ∆a = 2.0% and ∆c = 7.9% in the fully charged state. The repeated expansion/shrinkage of the (003) facet during lithium extraction (charging)/insertion (discharging) is the fundamental origin for cracks formed perpendicular to the c-direction or parallel to the (003) facet [3,49,50]. Though the lattice changes were less significant in NCM here, which contained 20 at % Co and 20 at % Mn, the change in ∆c was still 1.81% in the fully charged state (14.2066 to 14.4641 Å), Figure S5a, which led to cracks parallel to the (003) facet after repeated cycles, as may be clearly observed through the STEM-HAADF images in Figure 6. In shape contrast, after the ALD coating of tungsten oxide, a small amount of W 6+ migrated to the lattice, forming a~10 nm-surface distorted rock-salt structure parallel to the (003) facet ( Figure 4). Note that the exact origin for the (003) facet preferred for W 6+ migration is not clear but is likely attributed to the minimization of surface energy, as suggested by previous theoretical modeling works [55]. Though this surface structural distortion led to a sacrifice of the initial capacity at RT (~9 mAh/g lower at 0.1 C, RT, Figure 5a), the changes in the c-axis were significantly reduced, with a ∆c of only 0.7% in the fully charged state (14.2133 to 14.3070 Å), as shown in Figure  S5b, and resulting in a reduced formation of cracks, as observed above. Meanwhile, the surface distorted rock-salt structure also impeded the release of oxygen during cycling (see Figure S3b) and improved the structural stability [56]. The effect of tungsten oxide coating on the NCM cathode material during cycling is summarized in Figure 7.

Conclusions
In this work, a micron-sized LiNi0.6Co0.2Mn0.2O2 cathode material was prepared and coated with 0.48 ± 0.02 wt% WO3 via atomic layer deposition using WCl6 as the metal source. By employing advanced structural characterization techniques, WO3 was mainly accumulated on the (003) facet, with a small amount of W 6+ entering the ~10 nm surface, causing structural distortion and the formation of a rock-salt-like structure. This tungsten oxide coating induced initial structural distortion; however, it effectively mitigated a caxis change during cycling and oxygen release from the lattice. Consequently, the electrochemical cycling performance of tungsten-oxide-coated NCM was significantly improved, showing a capacity retention of 86.8% after 300 cycles at 55 °C, compared to only 69.4% for the bare NCM.
Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Table S1. Rietveld refinement parameters for the bare and WO3-coared NCM cathode materials before cycling. Figure S1. EELS Ni L edge and O K edge spectra for uncycled bare NCM from (a,c) ROI-A and (b,d) ROI-B. Figure S2. EELS Ni L edge and O K edge spectra for uncycled WO3-coated NCM from (a,c) ROI-C and (b,d) ROI-D. Figure S3. EELS oxygen K edge spectra from bulk region of cycled (a) bare and (b) WO3-coated NCM. Figure S4

Conclusions
In this work, a micron-sized LiNi 0.6 Co 0.2 Mn 0.2 O 2 cathode material was prepared and coated with 0.48 ± 0.02 wt% WO 3 via atomic layer deposition using WCl 6 as the metal source. By employing advanced structural characterization techniques, WO 3 was mainly accumulated on the (003) facet, with a small amount of W 6+ entering the~10 nm surface, causing structural distortion and the formation of a rock-salt-like structure. This tungsten oxide coating induced initial structural distortion; however, it effectively mitigated a c-axis change during cycling and oxygen release from the lattice. Consequently, the electrochemical cycling performance of tungsten-oxide-coated NCM was significantly improved, showing a capacity retention of 86.8% after 300 cycles at 55 • C, compared to only 69.4% for the bare NCM.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/inorganics10080111/s1, Table S1. Rietveld refinement parameters for the bare and WO 3 -coared NCM cathode materials before cycling. Figure S1. EELS Ni L edge and O K edge spectra for uncycled bare NCM from (a,c) ROI-A and (b,d) ROI-B. Figure S2. EELS Ni L edge and O K edge spectra for uncycled WO 3-coated NCM from (a,c) ROI-C and (b,d) ROI-D. Figure  S3. EELS oxygen K edge spectra from bulk region of cycled (a) bare and (b) WO 3 -coated NCM. Figure  S4.