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Sep 25, 2017 - Powder Metallurgy Research Institute, Central South University, 932, South ... In the metallurgical process of Al2O3 production, the pr...
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Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10199-10205

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*,†,‡,§ †

School of Metallurgy and Environment, Central South University, 932, South Lushan Road, Changsha 410083, P. R. 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, P. R. China ‡

S Supporting Information *

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 cutoff voltage of 4.3 V and elevated cutoff 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 a better rate capability, maintaining 117 and 131 mA h g−1 in the voltage ranges 2.8−4.3 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 the Al2O3 coating layer building 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

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INTRODUCTION

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 the aqueous solution system and the organic phase are the common media. For the latter case, it is difficult to separate the organic residue from the product, and the process has a 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 medium for surface modification. However, it is hard to control the deposition rate of the coating materials or intermediate rate in aqueous

The environmental issues and the depletion of fossil fuels 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, 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 one of the most promising alternatives.13−16 Especially, LiNi0.8Co0.1Mn0.1O2 shows the most potential for its lower cost, lower 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 abovementioned problem.24−28 Various composites, such as Al2O3,29−31 SiO2,32−34 V2O5,35−37 and several fluorides(like LiF,38 CaF2,39 and AlF340−42), have been reported as coating materials to improve the cycling performance, rate capability, © 2017 American Chemical Society

Received: July 1, 2017 Revised: September 12, 2017 Published: September 25, 2017 10199

DOI: 10.1021/acssuschemeng.7b02178 ACS Sustainable Chem. Eng. 2017, 5, 10199−10205

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Schematic diagram of the Al2O3 coating process.

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

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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 the 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. With this inspiration, an metallurgical process is used to obtain a 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 the 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 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 reactions 1 and 2. After reaction for 2 h at 70 °C, the suspension was filtered and washed with deionized water several times. Finally, the obtained powders were dried at 80 °C in air for 12 h. The Al2O3coated LiNi0.8Co0.1Mn0.1O2 was synthesized by mixing the obtained precursor and LiOH·H2O at a molar ratio of 1:1.05 followed by presintering at 480 °C for 5 h and then heating up to 750 °C for 15 h in pure O2 atmosphere.

2NaAlO2 + CO2 + 2H 2O = Na 2CO3 + Al(OH)3 ↓

(1)

NaAlO2 + CO2 + 2H 2O = 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 program.45 A scanning electron microscope (SEM, JEOL, JSM10200

DOI: 10.1021/acssuschemeng.7b02178 ACS Sustainable Chem. Eng. 2017, 5, 10199−10205

Research Article

ACS Sustainable Chemistry & Engineering 5600LV) and a transmission electron microscope (TEM, JEM-2100F) were used to observe the particle morphology and surface and crosssection 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-methyl-2-pyrrolidone (NMP). The electrodes were then dried at 120 °C for 12 h. The electrochemical performances of the prepared samples were measured with CR2025-type coin cells. LiPF6 (1 M) was dissolved in a mixture of dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC)/ethylene carbonate (EC) at a volume ratio of 1:1:1 to act as electrolyte. The cells were assembled in an Ar-filled glovebox (Mikrouna) and cycled in the voltage range from 2.8 to 4.3 and 4.5 V, respectively. The electrochemical impedance spectroscopy (EIS) tests were conducted with an electrochemical workstation (CHI 660d) in a frequency range from 105 to 0.1 Hz. In addition, 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.

Co0.1Mn0.1O2 samples present a smoother surface compared with that of the pristine sample. In addition, all samples have a well-dispersed spherical shape with a particle size 12−20 μm (Figure S1). More detailed morphological information on the pristine and 3 wt % Al2O3-coated LiNi0.8Co0.1Mn0.1O2 sample is observed by TEM analysis (Figure 3e−h). In contrast to the smooth edge line without any coating layer on the surface of the pristine sample, a distinguishable coating layer with a thickness 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 3i shows the EDS spectra of the Al2O3-coated sample from the cross-section, from which it can be found that the intensity of the 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 an Al2O3 coating layer. The ICP testing results (Table S1) show that the mAl/m(Ni + Co + Mn) ratio in xAl2O3-coated samples (x = 1, 3, and 5 wt %) is 0.0017, 0.0047, and 0.0059, respectively. The HRTEM images of xAl 2 O 3 -coated LiNi0.8Co0.1Mn0.1O2 samples (x = 0, 1, 3, and 5 wt %) (Figure S2) demonstrate that the coating layer becomes thicker as the Al content is increased. Figure 4a exhibits the first charge−discharge curves of the samples at 0.1 C (1 C = 200 mA g−1) between 2.8 and 4.3 V. All samples show similar and smooth curves, indicating that the Al2O3 coating does not bring a noticeable change to the bulk of LiNi0.8Co0.1Mn0.1O2 material during the charge/discharge process. The initial discharge capacity of the samples shows a regular decrease from 205 to 188 mA h g−1 with an increase in the Al2O3 coating amount. This is associated with the fact that the Al2O3 coating layer is electrochemically inert. Figure 4b shows the cycle performance of the samples at 1 C between 2.8 and 4.3 V. The Al2O3-coated samples obtain the significantly enhanced capacity retention. In particular, after 60 cycles at 1 C, the samples with 0, 1, 3, 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 4c,d compares the galvanostatic charge−discharge results in the voltage range 2.8−4.5 V. Obviously, as the charge cutoff potential is increased to 4.5 V, the discharge capacity is improved. The initial charge−discharge curves exhibit a similar trend to the curves in the voltage window 2.8−4.3 V, implying that increasing the charge cutoff voltage does not lead to another side phase change during the charge−discharge process. The initial discharge capacities of the samples with 0, 1, 3, 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, 3, 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 capabilities of the pristine and 1 wt % Al2O3-coated samples are illustrated in parts e (2.8−4.3 V) and f (2.8−4.5 V) of Figure 4. It is obvious that the Al2O3-

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RESULTS AND DISCUSSION The design of the coating process on the surface of the Ni0.8Co0.1Mn0.1(OH)2 precursor is schematically illustrated in Figure 1. The Al(OH)3 particle forms and aggregates on the surface of the 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 byproducts are difficult to remove. However, here, the byproducts of the introduced process are Na2CO3 and NaHCO3, which can be easily removed by repeated filtration and washing. 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 the αNaFeO2 layer structure belonging to the R3̅m space group, implying that the Al2O3 coating does not change the crystalline structure of the material. The reason for the absence of Al2O3 diffraction peaks is that the Al2O3 layer is amorphous or nanosized. The lattice parameters of all samples calculated from the Rietveld refinement are summarized in Table 1. It can be Table 1. Lattice Parameters of the xAl2O3-Coated LiNi0.8Co0.1Mn0.1O2 Samples (x = 0, 1, 3, and 5 wt %) xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 x x x x

= = = =

0 1 3 5

wt wt wt wt

% % % %

a (Å)

c (Å)

c/a

I(003)/I(104)

2.8745 2.8735 2.8712 2.8707

14.2095 14.2156 14.2187 14.2240

4.9433 4.9471 4.9522 4.9548

1.6287 1.675 1.8018 1.8018

found that the value of I(003)/I(104) increases with the increase in 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. The morphological characteristics of the xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 samples (x = 0, 1, 3, and 5 wt %) are illustrated in Figure 3. It can be seen from the SEM images in Figure 3a−d that the size of primary particles exhibits a decreasing trend with an increasing coating amount, which is favorable to maintaining the integrity of the particles during the cycling process. Moreover, the Al 2 O 3 -coated LiNi 0.8 10201

DOI: 10.1021/acssuschemeng.7b02178 ACS Sustainable Chem. Eng. 2017, 5, 10199−10205

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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 the Al2O3-coated sample (x = 3 wt %).

Figure 4. Electrochemical properties of xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 samples (x = 0, 1, 3, and 5 wt %): (a) initial charge−discharge curves at 0.1 C and (b) cycle performance at 1 C in the potential range 2.8−4.3 V; (c) charge−discharge curves at 0.1 C and (d) cycle performance at 1 C in the potential range 2.8−4.5 V; rate capacity in the potential range (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.

sample maitains a higher capacity of 117 and 131 mAh g−1 in the voltage ranges 2.8−4.3 and 2.8−4.5 V, respectively, which is

coated sample delivers a much improved rate capability. In particular, at the high rate of 5 C, the 1 wt % Al2O3-modified 10202

DOI: 10.1021/acssuschemeng.7b02178 ACS Sustainable Chem. Eng. 2017, 5, 10199−10205

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ACS Sustainable Chemistry & Engineering 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). 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 attack. 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 4g,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 the middle-frequency region, and an inclined line in the lowfrequency 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 on behalf of the resistance of the passivation surface film (Rsf). The middlefrequency 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 associated with the Li+ diffusion process in the bulk material. The spots are fitted by the equivalent circuit as shown in Figure 4i. The fitting result is shown in Table S2, where the Rct of the pristine LiNi0.8Co0.1Mn0.1O2 is 226 Ω after the first cycle and 1192 Ω after the 100th cycle. The obvious increase of Rct can be ascribed to both the erosion of HF and the oxygen release from the high 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 the Al2O3-coated sample and its slower growth during cycling are beneficial to the electrochemical process, implying that the Al2O3 coating effectively suppresses the passivation of the cathode surface caused by a 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 a thermodynamic equilibrium state and based on the chronopotentiometry. The diffusion coefficient can be calculated using the following equation:47 DLi+ =

2 ⎞2 ΔEs 4 ⎛ mBVm ⎞ ⎛ ⎜ ⎟⎜ ⎟ π ⎝ MBS ⎠ ⎝ τ(dEτ /d τ ) ⎠

DLi+ =

2 2 4 ⎛ mBVm ⎞ ⎛ ΔEs ⎞ ⎜ ⎟⎜ ⎟ πτ ⎝ MBS ⎠ ⎝ ΔEτ ⎠

(4)

On the basis of eq 4, the values of the lithium ion chemical diffusion coefficients as a function of potentials are exhibited in Figure 5. The DLi+ curve versus potential of 1 wt % Al2O3-

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

coated sample shows a behavior similar to that of the pristine material, implying that the Al2O3 coating does not influence the mechanism of Li+ extraction from the bulk structure. However, it is clearly presented that the 1 wt % Al2O3-coated sample has better diffusion characteristics, showing the DLi+ value of approximately 8.5 × 10−9 cm2 s−1 between 3.8 and 4.3 V. As the voltage drops down to 3.5 V, the corresponding value of DLi+ decreases to 2.7 × 10−11 cm2 s−1. However, for the pristine sample, the value of DLi+ is 3.0 × 10−9 cm2 s−1 in the voltage ranges of 3.7 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 % Al2O3coated 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 the Al2O3-coated sample.

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CONCLUSIONS

xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 samples with different coating ratios have been successfully synthesized by the metallurgy inspired controllable Al(OH)3 deposition followed by heat treatment. NaAlO2 was used as an Al source to form a homogeneous Al(OH)3 coating layer on the surface of the Ni0.8Co0.1Mn0.1(OH)2 precursor. The XRD patterns and Rietveld refinement results demonstrate that the structural stability of the material was enhanced and cation mixing was suppressed. The electrochemical performance was remarkably improved after Al2O3 modification. Particularly, the 1 wt % Al2O3-coated sample exhibited the best comprehensive electrochemical performance, which was attributed to the Al2O3 coating layer being 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 wide usage for building a homogeneous and stable interface for cathode materials.

(τ ≪ L2 /DLi+) (3)

Here, the following variables apply: mB is the mass of the active material on the g basis, Vm is the molar volume, MB the molecular weight of the sample, S is the area of the electrode, τ is the time duration during the current pulse, ΔEs is the voltage difference in the steady-state at a single-step GITT test, and L is the distance of Li+ ion diffusion. The detailed information on the GITT testing result is shown in Figure S4. On the basis of the linear relationship between the cell potential (E) as a function of τ1/2 as shown in Figure S4a3,b3, eq 1 can be simplified as the following:47 10203

DOI: 10.1021/acssuschemeng.7b02178 ACS Sustainable Chem. Eng. 2017, 5, 10199−10205

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02178. SEM images, ICP results, and HRTEM images of xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 (x = 0, 1, 3, 5 wt %); XRD patterns of xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 (x = 0, 1, 3, 5 wt %) after 100 cycles; and fitting result of Nyquist plots for different charged state and GITT curves of the xAl2O3-coated LiNi0.8Co0.1Mn0.1O2 samples (x = 0, 1%) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xinhai Li: 0000-0001-5401-9493 Kaimin Shih: 0000-0002-6461-3207 Jiexi Wang: 0000-0001-7398-5566 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Basic Research Program of China (2014CB643406), the National Natural Science Foundation of China (51704332), the 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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DOI: 10.1021/acssuschemeng.7b02178 ACS Sustainable Chem. Eng. 2017, 5, 10199−10205