Metallurgy Inspired Formation of Homogeneous Al2O3 Coating Layer

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Metallurgy inspired formation of homogeneous Al2O3 coating layer to improve the electrochemical properties of LiNi0.8Co0.1Mn0.1O2 cathode material Mingxia Dong, Zhixing Wang, Hangkong Li, Huajun Guo, Xinhai Li, Kaimin Shih, and Jiexi Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02178 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017

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ACS Sustainable Chemistry & Engineering

Metallurgy inspired formation of homogeneous Al2O3 coating layer to improve the electrochemical properties of LiNi0.8Co0.1Mn0.1O2 cathode material Mingxia Dong,† Zhixing Wang,† Hangkong Li,‡ Huajun Guo,† Xinhai Li,† Kaimin Shih,‡ Jiexi Wang,†,‡,§,* †

School of Metallurgy and Environment, Central South University, 932, South Lushan

Road, Changsha 410083, PR China ‡

Department of Civil Engineering, The University of Hong Kong, Pok Fu Lam Road, Hong

Kong §

Powder Metallurgy Research Institute, Central South University, 932, South Lushan

Road, Changsha 410083, PR China

* Corresponding author, E-mail address: [email protected]

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Abstract: Inspired by the metallurgical process of aluminum production, a controllable and cost-effective Al2O3 coating strategy is introduced to improve the surface stability of LiNi0.8Co0.1Mn0.1O2. The CO2 is introduced to NaAlO2 aqueous solution to generate a weak basic condition that is able to decrease the deposition rate of Al(OH)3 and is beneficial to the uniform coating of Al(OH)3 on the surface of commercial Ni0.8Co0.1Mn0.1(OH)2 precursor. The electrochemical performance of Al2O3-coated LiNi0.8Co0.1Mn0.1O2 is improved at both ordinary cut-off voltage of 4.3 V and elevated cut-off voltage of 4.5 V. With the optimized Al2O3 coating amount (1%), the capacity retention of the material after 60 cycles increases from 90% to 99 % at 2.8-4.3 V and from 86% to 99% at 2.8-4.5 V, respectively. The Al2O3-coated sample also delivers better rate capability, maintaining 117 mA h g-1 and 131 mA h g-1 in the voltage range of 2.8 V - 4.3 V and 2.8 V - 4.5 V at the current density of 5 C, respectively. The enhanced properties of as-prepared Al2O3-coated LiNi0.8Co0.1Mn0.1O2 are due to that the Al2O3 coating layer builds up a favorable interface, preventing the direct contact between the active material and electrolyte and promoting Li+ transmission at the interface. Keywords: NaAlO2; Al2O3 coating; Ni-rich cathode material; Li+ diffusion coefficient; interface stability. INTRODUCTION The environmental issues and the use up of fossil fuel promote the development of clean energy.1-3 In recent years, with the large application of rechargeable Li-ion batteries in electric vehicles and energy storage systems, the electrode materials with high energy, fast charging rate and low cost have attracted increasing attention.4-12 Among the several kinds of cathode materials, the layered Ni-rich cathode material LiNi1-x-yCoxMnyO2 (1-x-y≥0.5) is considered as one of the most promising alternatives.13-16 Especially, LiNi0.8Co0.1Mn0.1O2 shows the most potential for its lower cost, less toxicity and higher capacity compared to the commercial LiCoO2.17-20 However, the high content of Ni causes a deterioration of structure due to the increased Li/Ni disorder and the production of highly-reactive Ni4+ ions in the delithiated state that can react with the organic electrolyte.6, 21-23 Surface coating is an effective method to solve the above-mentioned problem.24-28 Various composites, such as Al2O3,29-31 SiO2,32-34 V2O5,35-37 and several fluorides( like LiF38, CaF239 and 2


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AlF340-42), have been reported as coating materials to improve the cycling performance,

rate capability and thermal stability of the cathode materials, which are mainly attributed to that these materials can protect the host structure from the attack of HF and inhibit the side reaction between the electrode and the electrolyte. Al2O3 is a good choice for its chemically inert features. Moreover, Al2O3 itself does not react with electrode or electrolyte, therefore providing a cover layer to avoid the early side reaction between the electrode and electrolyte. During the coating process, both aqueous solution system and organic phase are the common media. For the latter case, it is difficult to separate the organic residue from the product and the process consumes high cost and long reacting time. While achieving the coating process in aqueous solution can effectively avoid the above problems. Therefore, the aqueous solution is a better media for surface modification. However, it is hard to control the deposition rate of the coating materials or intermediate rate in aqueous solution. As a result, it is hard to obtain a homogeneous coating layer. In the metallurgical process of Al2O3 production, the process of seed precipitation is one of crucial steps. In this process, the settlement of Al(OH)3 is controlled by adjusting the concentration of CO2 and the products are crystalline material and colloidal material

43-44.

The second one is undesirable because it is bad for the filtration and separation of the product. However, the production of colloidal material Al(OH)3 is a low-rate process. Inspired by this, an metallurgical process is used to obtain uniform colloidal Al(OH)3 layer on the surface of Ni-rich precursor Ni0.8Co0.1Mn0.1(OH)2. We did not conduct the coating process directly on the surface of LiNi0.8Co0.1Mn0.1O2 because the Ni-rich cathode materials suffer from the rapid moisture uptake caused by the formation of the LiOH/Li2CO3 impurity phase. The effects of Al2O3 layer on the structure, surface stability and electrochemical performance of the LiNi0.8Co0.1Mn0.1O2 cathode material are investigated in detail. EXPERIMENTAL SECTION

The pristine Ni0.8Co0.1Mn0.1(OH)2 precursor was obtained by commercial supply. The Al(OH)3-coated Ni0.8Co0.1Mn0.1(OH)2 materials were prepared by a hydroxide precipitation method. Specifically, commercial Ni0.8Co0.1Mn0.1(OH)2 precursor was 3



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dissolved in deionized water with continuous stirring. Subsequently, the sodium aluminate (NaAlO2) was added into the mixture. After that, the carbon dioxide was introduced into the system to produce Al(OH)3 according to reaction (1) and (2). After reacted for 2 hours at 70 oC, the suspension was filtered and washed with deionized water for several times. Finally, the obtained powders were dried at 80 oC in air for 12 hours. The Al2O3-coated LiNi0.8Co0.1Mn0.1O2 was synthesized by mixing the obtained precursor and LiOH·H2O at a molar ratio of 1:1.05 followed by pre-sintering at 480 oC for 5h and then heating up to 750 oC for 15 h in pure O2 atmosphere. 2NaAlO2+CO2+2H2O= Na2CO3+Al(OH)3↓

(1)

NaAlO2+CO2+2H2O=NaHCO3+Al(OH)3↓

(2)

The X-ray diffraction (XRD, Rigaku, Rint-2000) with Cu-Kα radiation (1.54056 Å) was carried out to identify the crystal structure of the material. XRD data was attained in the 2θ range from 10 to 90°with a step size of 0.01°. The collected XRD intensity data was fitted by the GSAS program45. Scanning electron microscope (SEM, JEOL, JSM-5600LV), transmission electron microscope (TEM, JEM-2100F) were used to observe the particle morphology and surface and cross-section element species of the material respectively. The working electrodes were prepared by covering Al foils with the slurry containing 80 wt% of active material, 10 wt% of carbon black (Super P) and 10 wt% of polyvinylidene fluoride (PVDF) dissolved in n-nmethyl-2-pyrrolidone (NMP). The electrodes were then dried at 120 oC for 12 h. The electrochemical performances of the prepared samples were measured with CR2025-type coin cells. 1M LiPF6 was dissolved in a mixture of dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC)/ethylene carbonate (EC) at a volume ratio of 1:1:1 act as electrolyte. The cells were assembled in an Ar-filled glove box (Mikrouna) and cycled in the voltage range from 2.8 V to 4.3 V and 4.5 V, respectively. The electrochemical impedance spectroscopy (EIS) tests were conducted with electrochemical workstation (CHI 660d) in a frequency range from 105 to 0.1 Hz. Besides, the galvanostatic intermittent titration technique (GITT) measurement was programmed by supplying a constant current flux of 0.1 C for 10 min followed by an open circuit stand for 50 min, respectively. The sequence was continued for the composition (x in Li1-xNi0.8Co0.1Mn0.1O2) or voltage (2.8~4.3 V) of interest. 4


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RESULTS AND DISCUSSION

The design of the coating process on the surface of Ni0.8Co0.1Mn0.1(OH)2 precursor and its reaction mechanism is schematically illustrated in Figure 1. The Al(OH)3 particle forms and aggregates on the surface of Ni0.8Co0.1Mn0.1(OH)2 precursor in NaAlO2 aqueous solution. As we all know, the traditional coating process carried out in organic solvent is dangerous sometimes and the by-products are difficult to remove. However, here, the by-products of the introduced process are Na2CO3 and NaHCO3, which can be easily removed by repeated filtration and washing.

Figure 1 Schematic diagram of the Al2O3 coating process

Figure 2 XRD and Retiveld refinement of the xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 samples 5



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(x=0, 1 wt%, 3 wt% and 5 wt%) Figure 2 shows the XRD and Rietveld refinement patterns of the prepared samples. All the diffraction peaks of the pristine and modified samples are in good accordance with α-NaFeO2 layer structure belonging to the R3 space group, implying that the Al2O3

coating does not change the crystalline structure of the material. The reason for absence of Al2O3 diffraction peaks is that the Al2O3 layer is amorphous or in nano-size. The lattice parameters of all samples calculated from the Rietveld refinement are summarized in Table 1. It can be found that the value of I(003)/I(104) increases with the increase of coating amount, indicating an effectively suppressed cation mixing after coating. Moreover, as illustrated in Table 1, the Al2O3-coated samples show a higher c/a value, which is in favor of the well-defined layer structure as well. Table 1 lattice parameters of the xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 samples (x=0, 1wt%, 3wt% and 5wt%) xAl2O3-coated LiNi0.8Co0.1Mn0.1O2

a(Å)

c(Å)

c/a

I(003)/I(104)

x=0wt%

2.8745

14.2095

4.9433

1.6287

x=1wt%

2.8735

14.2156

4.9471

1.675

x=3wt%

2.8712

14.2187

4.9522

1.8018

x=5wt%

2.8707

14.2240

4.9548

1.8018

The morphological characteristics of the xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 samples (x=0wt%, 1wt%, 3wt% and 5wt%) are illustrated in Figure 3. It can be seen from the SEM images Figure 3(a-d) that the size of primary particles exhibits a decrease trend with the increase of the coating amount, which is favorable to maintaining the integrity of the particles during the cycling process. Moreover, the Al2O3-coated LiNi0.8Co0.1Mn0.1O2 samples present a smoother surface compared with the pristine sample. In addition, all samples have well-dispersed spherical shape with a particle size of 12-20 μm (Figure S1). More detailed morphological information of the pristine and 3 wt% Al2O3-coated LiNi0.8Co0.1Mn0.1O2 sample is observed by TEM analysis [Figure 3(e-h)]. In contrast to the smooth edge line without any coating layer on the surface of the pristine sample, a 6


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distinguishable coating layer with a sickness of ~10 nm can be clearly observed on the surface of the modified LiNi0.8Co0.1Mn0.1O2 particles, indicating the existence of Al2O3 coating layer in the modified sample. Figure 3(i) shows the EDS spectra of the Al2O3-coated sample from the cross-section, from which it can be found that the intensity of Al peak at the edge is much higher than that in the core, which indicates that the elemental Al mainly concentrates at the surface of the particles, further confirming the formation of Al2O3 coating layer. The ICP testing results (Table S1) show the mAl/m(Ni+Co+Mn) ratio in xAl2O3-coated samples (x=1 wt%, 3 wt% and 5 wt%) samples is 0.0017, 0.0047 and 0.0059, respectively. The HRTEM images of xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 samples (x=0, 1 wt%, 3 wt% and 5 wt%) [Figure S2] demonstrate that the coating layer becomes thicker as the Al content is increased.

Figure 3 (a-d) SEM images of the xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 samples [(a) x=0, (b) x=1 wt%, (c) x=3 wt%, (d) x= 5 wt%]; (e) TEM and (f) HRTEM images of pristine LiNi0.8Co0.1Mn0.1O2; (g) TEM and (h) HRTEM images of Al2O3-coated LiNi0.8Co0.1Mn0.1O2 (x=3 wt%); (i) EDS liner spectrum of Ni and Al from the cross-section of of Al2O3-coated sample (x=3 wt%). Figure 4(a) exhibits the first charge-discharge curves of the samples at 0.1 C (1 C=200 mA g-1) between 2.8 V and 4.3 V. All samples show similar and smooth curves, indicating that the Al2O3 coating does not bring noticeable change to the bulk of LiNi0.8Co0.1Mn0.1O2 7



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material during charge/discharge process. The initial discharge capacity of the samples show a regular decrease from 205 mA h g-1 to 188 mA h g-1 with the increase of Al2O3 coating amount. This is in associate with the fact that the Al2O3 coating layer is electrochemically inert. Figure 4(b) shows the cycle performance of the samples at a 1 C between 2.8 V and 4.3 V. The Al2O3 coated samples obtain the significantly enhanced capacity retention. Particularly, after 60 cycles at 1 C, the samples with 0, 1 wt%, 3 wt% and 5 wt% of Al2O3 coating deliver the discharge capacities of 164, 179, 175 and 171 mA h g-1, corresponding to the capacity retentions of 85.8%, 96.1%, 98.9% and 99.6%, respectively. Note that, when the coating amount reaches beyond 3 wt%, the discharge capacity shows rapid decay, which is not helpful to the electrochemical property of the material. Figure 4(c) and 4(d) compares the galvanostatic charge-discharge results in the voltage range of 2.8~4.5 V. Obviously, as the charge cut-off potential is increased to 4.5 V, the discharge capacity is improved. The initial charge-discharge curves exhibit the similar trend to the curves in the voltage window of 2.8~4.3 V, implying that increasing the charge cut-off voltage does not lead to other side phase change during charge-discharge process. The initial discharge capacities of the samples with 0, 1 wt%, 3 wt% and 5 wt% of Al2O3 coating are 210, 215, 201 and 191 mA h g-1, respectively. Meanwhile, the sample with 1 wt% coating achieves the highest coulomb efficiency of 84%. More importantly, the capacity retention is greatly enhanced after coating, delivering 173, 195, 186 and 173 mAh g-1 after 60 cycles at 1 C for the samples with 0, 1 wt%, 3 wt% and 5 wt% of Al2O3 coating, respectively. It can be concluded that the sample with 1 wt% Al2O3 coating exhibits the best comprehensive electrochemical properties including high reversible discharge capacity and large capacity retention. To further compare the performance of the samples, the rate capability of the prinsine and 1 wt% Al2O3 coated samples are illustrated in Figure 4(e) (2.8~4.3 V) and (f) (2.8~4.5 V). It is obvious that the Al2O3-coated sample delivers a much improved rate capability. Particularly, at the high rate of 5 C, the 1wt % Al2O3 modified sample maitains a higher capacity of 117 and 131 mAh g-1 in the voltage range of 2.8~4.3 V and 2.8~4.5 V,

8


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respectively, which is are better than that possessed by the pristine one (37 mAh g-1 at 2.8~4.3 V; 68 mA h g-1 at 2.8~4.5 V).

Figure 4 The electrochemical properties of xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 samples (x=0, 1 wt%, 3 wt% and 5 wt%): (a) initial charge-discharge curves at 0.1 C and (b) cycle performance at 1 C in the potential range of 2.8~4.3 V; (c) charge-discharge curves at 0.1 C and (d) cycle performance at 1 C in the potential range of 2.8~4.5 V; rate capacity in the potential range of (e) 2.8~4.3 V and (f) 2.8~4.5 V; Nyquist plot of (g) pristine and (h) 1 wt% Al2O3 coated samples before and after cycle; (i) the equivalent circuit used to fit the EIS plots. The reason for the improvement of cycle performance and rate capability is associated with the Li+ diffusion at the interface during the charge/discharge process. On one hand, the uniform Al2O3 coating layer on the surface of the particle effectively prevents the direct contact between the active material and electrolyte, leading to an improved structural and surface stability, suppressing the oxygen generation and the HF attacking. 9



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As a result, the structure of the Al2O3 coated sample can be stabilized after cycling 29, 46, as shown in Figure S3. Moreover, an appropriate thin coating layer shows relatively small transfer impedance during the charge-discharge process, which results in the improved electrochemical performance. To further investigate the impedance difference of the samples before and after Al2O3 coating, the EIS testing results of the pristine and 1 wt% Al2O3 coated samples at the charge state before and after cycles are shown in Figure 4(g) and (h), respectively. The Nyquist plots consist of an intercept in the high frequency region, a small semicircle in high frequency region, a large semicircle in middle– frequency region and an inclined line in the low frequency region. Generally, the intercept impedance on the Z’ real axis represents ohmic resistance (Re), corresponding to the total resistance of the electrolyte, current collector and electric leads. The high-frequency semicircle is in behalf of the resistance of the passivation surface film (Rsf). The middle-frequency semicircle stands for the charge-transfer resistance (Rct), reflecting the lithium transfer rate and the capacitance of the electrode/electrolyte interface double-layer. The slope in the low frequency region is in associated with the Li+ diffusion process in the bulk material. The spots are fitted by the equivalent circuit as shown in Figure 4(i). The fitting result is shown in Table S2, where the Rct of the pristine LiNi0.8Co0.1Mn0.1O2 are 226 Ω after 1st cycle and 1192 Ω after 100th cycle. The obvious increase of Rct can be ascribed to both the erosion of HF and the oxygen release from the highly lithium deintercalation. In contrast, the Rct of the 1 wt% Al2O3-coated sample exhibits a smaller Rct value of 796 Ω after 100 cycles. The smaller Rct of Al2O3 coated sample and its slower growth during cycling is beneficial to the electrochemical process, implying that Al2O3 coating effectively suppresses the passivation of the cathode surface caused by side reaction between the electrode and electrolyte. GITT technology is known as a common and effective way to estimate the chemical diffusion coefficient, which is tested under thermodynamic equilibrium state and based on the chronopotentiometry. The diffusion coefficient can be calculated using the following equation:47:

=

△ √

! τ ≪ % &

10


(3)

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Where mB is the mass of the active material on the g basis, Vm is the molar volume and MB the molecular weight of the sample, S is the area of the electrode, τ is the time duration during the current pulse, △Es is the voltage difference in the steady-state at a single-step GITT test, L is the distance of Li+ ions diffusion.

The detailed information of GITT testing result is shown in Figure S4. Based on the linear relationship between the cell potential (E) as a function of τ1/2 as shown in Figure S4(a3, b3), the Equation (1) can be simplified as following:47:

=

△

△

(4)

Based on the Equation (4), the value of the lithium ion chemical diffusion coefficients as a function of potentials are exhibited in Figure 5. The curves versus potential of 1 wt% Al2O3 coated sample is of similar behavior to the pristine material, implying that Al2O3 coating does not influence the mechanism of Li+ extracting from the bulk structure. However, it is clearly presented that the 1wt% Al2O3-coated sample owns a better diffusion characteristics, showing the value of approximately 8.5×10-9 cm2·s-1 between 3.8 V and 4.3 V. As the voltage drops down to 3.5 V, the corresponding value of decreases to 2.7×10-11 cm2 s-1. While for the pristine sample, the value of is 3.0 ×10-9 cm2 s-1 at the voltage range of 3.7 V and 4.3 V, and decreases to 1.08×10-11 cm2 s-1 when the voltage drops down to 3.6 V. The 1 wt % Al2O3 coated sample shows an enhanced lithium ion diffusion capability, indicating that a small amount of Al2O3-coating is helpful to enhance the Li+ diffusion coefficient. The improvement of lithium ion diffusion coefficient further proves the enhanced rate capacity of Al2O3 coated sample.

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Figure 5 The chemical diffusion coefficients Li+ as function of potential for xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 samples (x=0, 1wt%) CONCLUSIONS

xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 samples with different coating ratio have been successfully synthesized by the metallurgy-inspired controllable Al(OH)3 deposition followed by heat treatment. NaAlO2 was used as Al source to form a homogeneous Al(OH)3 coating layer on the surface of Ni0.8Co0.1Mn0.1(OH)2 precursor. The XRD patterns and Rietveld refinement results demonstrate that the structure stability of the material was enhanced and cation mixing was suppressed. The electrochemical performance was remarkably improved after Al2O3 modification. Particularly, the 1wt% Al2O3-coated sample exhibited the best comprehensive electrochemical performance, which was attributed to that the Al2O3 coating layer was able to build up a protective barrier preventing the direct contact between the active material and electrolyte, and the simultaneously formed fast ion conductor LixAl2O3 was in favor of Li+ transmission at the interface. This coating strategy showed a wide usage for building a homogenous and stable interface for cathode materials.

SUPPORTING INFORMATION SEM images, ICP results and HRTEM images of xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 (x=0, 1 wt%, 3 wt%, 5 wt%); XRD patterns of xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 (x=0, 1 wt%, 3 wt%, 5 wt%) after 100 cycles. Fitting result of Nyquist plots for different charged state and GITT curves of the xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 samples (x=0, 1%). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding author *E-mail: [email protected]

Notes The authors declare no completing financial interest. ACKNOWLEDGMENT

This research was supported by the National Basic Research Program of China (2014CB643406), the National Natural Science Foundation of China (51704332), the 12


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National Science-Technology Support Plan of China (2015BAB06B00), the National Postdoctoral Program for Innovative Talents (BX201700290), and T21-711/16R and 17212015 from the Research Grants Council (RGC) of the Government of Hong Kong SAR.

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For Table of Contents Use Only

Al Metallurgy-inspired Al(OH)3 controllable deposition realized uniform Al2O3 coating to improve the high-voltage cycle performance of Ni-rich cathode materials for lithium ion batteries.

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Figure 1 Schematic diagram of the Al2O3 coating process 254x69mm (96 x 96 DPI)



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Figure 2 XRD and Retiveld refinement of the xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 samples (x=0, 1 wt%, 3 wt% and 5 wt%) 192x146mm (300 x 300 DPI)


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Figure 3 (a-d) SEM images of the xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 samples [(a) x=0, (b) x=1 wt%, (c) x=3 wt%, (d) x= 5 wt%]; (e) TEM and (f) HRTEM images of pristine LiNi0.8Co0.1Mn0.1O2; (g) TEM and (h) HRTEM images of Al2O3-coated LiNi0.8Co0.1Mn0.1O2 (x=3 wt%); (i) EDS liner spectrum of Ni and Al from the cross-section of of Al2O3-coated sample (x=3 wt%). 1069x603mm (96 x 96 DPI)



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Figure 4 The electrochemical properties of xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 samples (x=0, 1 wt%, 3 wt% and 5 wt%): (a) initial charge-discharge curves at 0.1 C and (b) cycle performance at 1 C in the potential range of 2.8~4.3 V; (c) charge-discharge curves at 0.1 C and (d) cycle performance at 1 C in the potential range of 2.8~4.5 V; rate capacity in the potential range of (e) 2.8~4.3 V and (f) 2.8~4.5 V; Nyquist plot of (g) pristine and (h) 1 wt% Al2O3 coated samples before and after cycle; (i) the equivalent circuit used to fit the EIS plots. 204x162mm (300 x 300 DPI)


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Figure 5 The chemical diffusion coefficients Li+ as function of potential for xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 samples (x=0, 1wt%) 180x130mm (300 x 300 DPI)


Metallurgy Inspired Formation of Homogeneous Al2O3 Coating Layer

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