Aqueous aluminide ceramic coating polyethylene separators for lithium-ion batteries

Solid State Ionics 345 (2020) 115188

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Aqueous aluminide ceramic coating polyethylene separators for lithium-ion batteries

T

Yuan Wang, Qiulin Wang, Yu Lan, Zhenglin Song, Jinpeng Luo, Xiuqin Wei, Fugen Sun, ⁎ Zhihao Yue, Chuanqiang Yin, Lang Zhou, Xiaomin Li Institute of Photovoltaics, Nanchang University, Nanchang 330031, China

ARTICLE INFO

ABSTRACT

Keywords: Lithium-ion batteries Polyethylene separator Aluminide ceramic coating Boehmite Thermal stability

A new type of aqueous aluminide ceramic coating polyethylene (ACP) separators is prepared by a simple spreader coating process using aluminum oxide (Al2O3) and boehmite (AlOOH) as the ceramic coating particles. During the preparation of the separator, water is used as the solvent and BYK (Beck Chemical Co., Ltd.) additives are used as the aqueous assistant. The as-prepared ACP separators present more outstanding synthetic properties than commercial polyethylene (PE) separators. In addition, compared with Al2O3-coated PE (PE-Al2O3) separators, AlOOH-coated PE (PE-AlOOH) separators have superior synthetic properties, including a better interface stability and a higher liquid electrolyte uptake due to the presence of hydroxyl groups (-OH). As a consequence, the LiNi0.3Co0.3Mn0.3O| Li metal half-cells with PE-AlOOH separators have a good cycling stability and a high rate performance. The advantages of the PE-AlOOH separators make them promising candidates for applications in high power lithium-ion batteries.

1. Introduction In recent years, the decreasing reserves in traditional energy sources make it urgent to find new alternative sources. Lithium-ion batteries (LIBs) are the most promising high-efficiency secondary battery and the fastest-developing chemical energy storage power source used in electronic digital devices, energy storage systems, and in aerospace because of their high energy density, their low self-discharge, their good cycle performance, the lack of memory effect, and their low environmental impact [1–4]. The separator is one of the key inner-layer components in LIBs. It is often referred to as “the third pole” of the battery [5]. The separator is a porous membrane that plays an essential role in LIBs since it separates the cathode and the anode to prevent short-circuits and allows the lithium ions to transfer between the electrodes [6]. Commercial microporous membranes, such as polyethylene (PE), polypropylene (PP) and PP/PE/PP multilayer membranes, are widely used as separators in LIBs because of their good overall performance along with their low cost, good chemical stability, and excellent mechanical properties. However, polyolefin-based separators have known weaknesses, such as the poor thermal stability and insufficient liquid electrolyte wettability, which is detrimental to the electrochemical performance of the LIBs [7,8]. LIBs are becoming increasingly sophisticated so that the quality requirements for the separators are getting more

⁎

drastic. High-performance separators make a significant difference to improve the overall operation efficiency of LIBs [9,10]. In the past decade, most research has focused on the modification of the surface of polyolefin separators. It has been reported that using organic or inorganic materials to modify the surface of the polyolefin separators greatly enhances the wettability and the thermal dimensional stability of LIBs [11,12]. Among the surface modification strategies employed, the ceramic coating of separators has raised substantial interest, since it combines the superiority of the separators with hydrophilic inorganic materials such as Silica (SiO2) [13,14], Aluminum oxide (Al2O3) [15,16], Titanium dioxide (TiO2) [17,18], and Magnesium oxide (MgO) [19], which significantly improve the thermal stability of the separator and the wettability to the electrolyte. Al2O3 particles have been adopted in the industry as the first generation of ceramic coating materials to modify polyolefin separators with a good overall performance [20]. Zhang et al. [21] found that the layer of Al2O3 ceramic particles applied on the surface of PE separator improves the thermal stability and the electrochemical performance of the LIBs. Shi et al. [22] reported about the effect of the thickness of the Al2O3 coating layer on the performance of LIBs. When the thickness increases, the thermal stability of the separator improves, but it negatively affects the electrochemical performance of the LIBs. However, the hardness of the Al2O3 particles is so high that the coating equipment can be

Corresponding author. E-mail address: [email protected] (X. Li).

https://doi.org/10.1016/j.ssi.2019.115188 Received 11 October 2019; Received in revised form 25 November 2019; Accepted 9 December 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.

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seriously worn out after multiple uses, which restricts its further application as a coating layer for LIB separators. Boehmite (AlOOH) has crystalline water in its structure and is the main precursor for the industrial production of alumina [23]. AlOOH is a lighter and softer ceramic coating for LIBs separators and has a better dispersion than Al2O3 in an aqueous slurry. Furthermore, AlOOH is cheaper than Al2O3, which is also of great significance for large-scale industrial applications. Only a few studies have reported the surface modification of polyolefin separators with AlOOH [24–26]. Holtann et al. [24] reported a boehmite-based ceramic separator for lithium-ion batteries using Nethyl pyrrolidone (NEP) as the solvent, but it is not environmentfriendly and the price is higher compared with the aqueous ceramiccoating slurry. Yang et al. [25] reported the use of AlOOH particles as the inorganic ceramic coating to modify the surface of the traditional PE separator in order to improve its thermal stability. After a heat treatment at 180 °C for 0.5 h, the heat shrinkage rate of the modified PE separator was only 3%. However, the improvement of the thermal stability of the modified PE separator was attributed to the formation of an interlocking interface between the molten PE separator and the AlOOH particles, without further analyzing the crystal water in the AlOOH structure and how it can affect the thermal performance. At present, the comparison between AlOOH and Al2O3 mainly focuses on the hardness, the density, the water content, and the cost difference, but rarely involves any comprehensive research on the interface bonding, the electrochemical stability, and other electrochemical performance parameters. In the current study, traditional aluminum oxide (Al2O3) and lowmoisture boehmite (AlOOH) were both used to provide aluminide ceramic coating particles on the PE separator. Furthermore, non-polluting BYK (Beck Chemical Co., Ltd.) aqueous additives are used to form the slurry with the aluminide particles via a ball milling method. The effects of AlOOH and Al2O3 ceramic coatings layer on the battery performance were compared through various performance characterizations. The results show that the polar group (-OH) in the AlOOH structure improves the bonding force between the coating layer and the matrix PE separator and increases the liquid electrolyte uptake, which then improves the overall electrochemical performance of the battery.

2.2. Material characterization 2.2.1. Structure and morphology The surface and the cross-sectional morphologies of the pristine PE and the aluminide ceramic coating PE separators were investigated with a field emission scanning electron microscope (FE-SEM, JSM 6701F, Japan). The chemical composition of the separators was determined by Fourier-transform infrared spectroscopy (FT-IR, iS50, Thermo Fisher Scientific, Inc.) within 400–4000 cm−1. 2.2.2. Physical and mechanical properties The mechanical properties, including the tensile properties, the puncture strength, and the peel strength were measured on an intelligent electronic tension machine (XLM, Labthink, China) with a stretching rate of 25 mm/min. The peel strength test samples were obtained by bonding different separators and steel sheets with superglue. To investigate the thermal stability, the thermal shrinkage of disc separators with a diameter of 19 mm, was investigated by observing the dimensional changes after exposure to heat in an oven at temperatures between 130 °C and 170 °C for 0.5 h. The shrinkage was calculated as:

Thermal shrinkage (%) =

S0

ST S0

× 100%

(1)

where S0 and ST are the area of the separators before and after thermal treatment, respectively. The wetting properties of the separators by deionized water with a similar polarity to the electrolyte used were determined by placing a water droplet placed on the surface and measuring the contact angle on a drop shape analyzer (DSA100, Japan). The thickness of the samples was measured by a digital thickness gauge. The porosity of the separators was obtained by measuring the weight change before and after the samples were fully saturated with n-hexadecane for 6 h. The porosity was calculated by Eq. (2) [27,28]:

Porosity (%) =

m1

m0 × 100% v n

(2)

where, mo and m1 indicate the weight of the separators before and after the absorption of n-hexadecane, respectively. ρn is the density of nhexadecane and v represents the volume of the separator, including the polymer volume and the porous volume. The electrolyte uptake of the separators was calculated as [28]:

2. Materials and methods

m1

m0

2.1. Preparation of the separators

Electrolyte uptake (%) =

The aluminide coating solutions were prepared by mixing aluminide ceramic particles with BYK aqueous additives, including an ammonium acrylate type dispersing agent (BYK-LPC20992, ALTANA, Germany), a silicone surfactant type wetting agent (BYK-LPX20990, ALTANA, Germany), and a polyacrylate binder (BYK-LPC22346, ALTANA, Germany) at a certain percentage in deionized water. In this work, the aluminide ceramic particles mainly refers to Al2O3 particles with an average diameter of 1.5 μm (VK-L500G, Xuan Cheng Jing Rui New Material Co., Ltd., China) and AlOOH particles with an average diameter of 1.4 μm (BG-611, Anhui Estone Material Technology Co., Ltd., China). First, 0.1 wt% carboxymethyl cellulose (CMC) was completely dissolved in deionized water, stirred magnetically to form a CMC aqueous solution with a given viscosity. A fixed amount of the Al2O3 particles or AlOOH particles was then added into the CMC aqueous solution. Then, a fixed amount of BYK adhesive, dispersing agent, and wetting agent was dispersed into the mixed solution by planetary ballmilling at 400 rpm for 6 h. The PE separators (20 μm thick, SK cop.) were coated via a simple spreader coating process at a specific temperature and dried at 55 °C for 6 h. To investigate the effect of the aluminide ceramic of Al2O3 and AlOOH, a bare PE separator was used as a reference and the samples were named PE separator, PE-Al2O3 separator, and PE-AlOOH separator.

where m0 and m1 are the weight of the separators before and after soaking in the liquid electrolyte for 6 h, respectively.

m0

× 100%

(3)

2.3. Electrochemical performance To measure the ionic conductivity of a pristine PE and the aluminide ceramic modified PE separators, the separators with the liquid electrolyte (1 M LiPF6 in EC:DEC:DMC = 1:1:1 v/v) were tested by sandwiching them between two stainless steels plates (stainless steel| stainless steel). The ionic conductivity was calculated by:

=

d Rb × S

(4)

where d is the thickness of the separator, S is the area of stainless steel, and Rb is the bulk resistance measured by electrochemical impedance spectroscopy (EIS) on an electrochemical workstation (VersaSTAT200, PAR-Ametek. Cop.) with a frequency from 1000 kHz to 1 Hz an amplitude of 10 mV. The electrochemical stability of the separators were characterized using linear sweep voltammetry (LSV) from 3 V to 6 V vs. Li/Li+ at a rate of 5 mV/s using cells assembled by sandwiching the separator soaking in liquid electrolyte between stainless steel and Li metal (stainless steel| Li). The EIS spectra of CR2032 coin-type half cells were 2

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obtained by sweeping the frequency from 100 kHz to 0.1 Hz with an amplitude of 5 mV at room temperature. When preparing the half-cell, the cathode was composed of 80 wt% active material LiNi0.3Co0.3Mn0.3O (NCM-111), 10 wt% conductive agent super-P, and 10 wt% of a Polyvinylidene fluoride (PVDF) binder, then assembled in a glove box into a battery (NCM-111| Li) where Li metal served as the anode with different separators and liquid electrolytes. To investigate the electrochemical performance of a cell using these prepared separators, CR2032 coin-type half cells (NCM-111| Li) were packaged in a glove box filled with argon gas. After assembly, the cells were left for 12 h to allow the liquid electrolyte to sufficiently infiltrate the voids of the separator. To evaluate the stability during a chargedischarge cycle at a high current density, the cells were cycled with potential window from 2.5 V to 4.3 V vs. Li/Li+ in the constant current density of 1C for 200 cycles at room temperature in a Neware battery test system (CT-4008, ShenZhen, China). The rate capacity of the half cells at room temperature was evaluated from five successive cycles at various discharging current densities from 0.2C to 3C (0.2, 0.5, 1, 2, and 3C) and 0.2C.

determine their microstructure, SEM images of the surface and the cross-section of the samples were acquired, as shown in Fig. 1(b)–(d) and (e)–(f). Fig. 1(b) shows the typical morphology of the PE separator obtained by a wet process. It has a large number of interconnected submicron pores that serve as storage space that retains a large amount of electrolyte and provides channels for the lithium ions passing through during the charging and discharging of the LIBs. The surface morphology of the PE-Al2O3 and PE-AlOOH separators are presented in Fig. 1(c)–(d), respectively. The morphology of the two aluminide particles is quite different. The Al2O3 particles are elliptical whereas the AlOOH particles are cubic. However, both Al2O3 and AlOOH particles are uniformly distributed on the surface of the PE separator without any agglomeration. The distribution of the Al2O3 and AlOOH particles after ball milling are shown in the inset of Fig. 1(c) and (d). The Al2O3 and AlOOH particles have very similar size distributions between 0.12 μm and 2 μm. The D50 of the Al2O3 particles is 0.59 μm and that of the AlOOH particles is 0.64 μm, which is consistent with the observations made from the SEM images. The coating also presents a porous structure, which is formed by the aluminide particles. It plays a key role in improving the electrolyte wettability and uptake as well as the ionic conduction of the separator with the electrolyte. The cross-section morphology of the PE-Al2O3 and PE-AlOOH separators are shown in Fig. 1(e) and (f). The thickness of the coating layers is approximately 3 μm on one side of both the PE-Al2O3 and PE-AlOOH separators. All coating layers are uniform and have a good adhesion to the matrix

3. Results and discussion Fig. 1(a) presents a photograph of the pristine PE, PE-Al2O3, and PEAlOOH separators. All of them present a white color and the aluminide ceramic coating on the PE separators are all thin and homogeneous. To

Fig. 1. (a) Photographs of the pristine PE separator, the PE-Al2O3 separator, and the PE-AlOOH separator. SEM images of the surface of (b) the pristine PE separator, (c) the PE-Al2O3 separator, and (d) the PE-AlOOH separator. SEM images of the cross-section of (e) the PE-Al2O3 separator and (f) the PE-AlOOH separator. 3

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really affects the performance of the PE-AlOOH separator is the -OH in the structural water. As expected, these peaks indicated that the Al2O3 and AlOOH particles were well-coated on the surface of the pristine PE separator with the aqueous binder system. The separator should have sufficient strength to accommodate the mechanical stresses generated by the LIBs during assembly and extrusion and maintain the dimensional stability of the separator as well as to reduce the chemical side reactions within the LIBs. In addition, the peel strength was measured to characterize the interface bonding force between the coating layer and the matrix separator. The stress-strain curve of the pristine PE separator, the PE-Al2O3 separator, and the PEAlOOH separator are shown in Fig. 3(a). The tensile properties including the tensile strength test, and the elongation at break increase gradually when aluminide ceramic particles of Al2O3 or AlOOH are coated on the PE separators. The specific data for the tensile properties are listed in Table 2. The pristine PE separator has a tensile strength of 55.27 MPa, but the PE-Al2O3 separator and the PE-AlOOH separator have a tensile strength of 59.30 MPa and 61.78 MPa, respectively. Compared with the pristine PE separator, the tensile strength of the samples after ceramic coating with Al2O3 and AlOOH increased by 7.3% and 11.8%, respectively. The pristine PE separator has an elongation at break of 81%, whereas the values increase to 161% and 171% after ceramic coating with Al2O3 and AlOOH, respectively. The results indicate that the PE-AlOOH separator is superior to the PE-Al2O3 separator for tensile properties. Additionally, the puncture strength and peel strength are also improved, as shown in Table 2. In particular, the peel strength of the PE-AlOOH separator (0.3KN/m) is 3 times that of the PE-Al2O3 separator (0.1KN/m), which means that the AlOOH coating layer has a better interfacial adhesion to the matrix separator than the Al2O3 coating layer. The superior mechanical properties of the PE-AlOOH separator are mainly due to the polar –OH groups forming more hydrogen bonds with the matrix separator, which enhances the interface bonding force between the matrix separator and the AlOOH coating layer. The thermal stability is one of the most important factors for the safety of LIBs. To investigate the thermal stability of the pristine PE separator, the PE-Al2O3 separator, and the PE-AlOOH separator, the thermal shrinkage after storage at 130 °C and 170 °C for 0.5 h were measured, as shown in Fig. 3(b). The pristine PE separator already underwent a high degree of shrinkage to its elliptical shape even at a relatively low temperature of 130 °C for 0.5 h. However, the aluminide ceramic coating PE-Al2O3 and PE-AlOOH separators kept their original state without any distortion. When the temperature was increased to 170 °C, the pristine PE separator shrunk into a small transparent elliptical molten mass and the PE-Al2O3 separator also shrunk slightly less, but the dimensions of the PE-AlOOH separator almost didn't change, which improves the safety of LIBs. The AlOOH coating layer has a better interfacial bonding force with the matrix separator, which allows the AlOOH coating layer to act as a support skeleton to resist the melt shrinkage of the PE at high temperatures. Separators with excellent wettability to the electrolyte can improve the capacity of electrolyte uptake and speed up the lithium ion transfer. This creates a significant difference in the LIBs electrochemical performance. To evaluate the wettability of the separators before and after treatment by aluminides like Al2O3 and AlOOH, the contact angle was measured. Fig. 4(a) shows that the measured contact angles for a droplet of deionized water on the pristine PE separator was higher than 117° due to its hydrophobic character and the low surface energy, indicating a poor compatibility with polar liquid electrolytes that limits the performance of the LIBs and causes damages in the manufacturing of LIBs [30,36]. In contrast, the aluminide-coated separators showed an improved wettability for deionized water. The contact angle with the electrolyte decreased from 117° to 11.6° and 5.7° for the Al2O3 and AlOOH ceramic coating, respectively (Fig. 4(b) and (c)), as listed in Table 3. The difference is mainly attributed to the high specific surface energy of the aluminide particles and the similar polarity with the

Table 1 Physical and chemical properties of aluminides. Aluminides

Aluminum oxide

Boehmite

Crystal structure Specific surface area (m2/g) Mohs hardness Density (g/m2) Na+ (water-soluble in %)

α-Al2O3 6.3 9 3.9 0.036

γ-AlOOH 5 3 3 0.002

separator, which is very important for practical applications. To better differentiate between Al2O3 and AlOOH, their respective physical and chemical properties are listed in Table 1. The difference in the crystal structure, the α phase in Al2O3, and the γ phase in AlOOH lead to a different morphology, which will cause different characteristics for the coating layer on the PE separator. Table 1 clearly shows that the Mohs hardness value of Al2O3 is three times higher than for AlOOH. AlOOH is therefore a better choice in terms of mechanical abrasion. At the same time, AlOOH can reduce the moisture uptake due to its lower specific surface area and the content of Na+ in the aqueous solution, which has a great influence on the electrochemical performance of the LIBs. Furthermore, the density of Al2O3 is 3.9 g/m2 compared to only 3 g/m2 for AlOOH, which reduces the total weight of the ceramic coating and the total cost of the LIBs. To confirm the changes in the structure of the separator samples before and after the aluminide ceramic coating, an FT-IR characterization was carried out, as shown in Fig. 2. The pristine PE separator shows strong absorption peaks at 2850 cm−1 and 2920 cm−1 corresponding to the symmetric and asymmetric stretching vibration of the CeH bond [29]. The PE-Al2O3 separator exhibited all the peaks from the pristine PE separator and preserved Al2O3 bands near 700 and 600 cm−1 that can be attributed to the symmetric stretching of the AleO bond [30,31]. The PE-AlOOH separator had all the characteristic peaks of the PE separator and the AleO bonds were also preserved, but strong absorption peaks near 3000 cm−1 appeared. The peaks at 2850 cm−1 and 2920 cm−1 were assigned to the symmetric and asymmetric stretching of hydroxyl groups (-OH), present in AlOOH [32–34]. Other studies have shown that ƴ-AlOOH is a type of hydrated alumina with incomplete crystals, which has a layered structure similar to graphene, and a large number of -OH are located on the surface of the layered structure. The crystal water of ƴ-AlOOH is mainly structural water in which -OH between layers is hydrogen-bonded [35]. Therefore, what

Fig. 2. FT-IR spectra of the pristine PE separator (black), the PE-Al2O3 separator (red), and the PE-AlOOH separator (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 4

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Fig. 3. Stress-strain curves (a) and digital photographs of the separators after heat treatment at 25 °C, 130 °C, and 170 °C for 0.5 h (b). Table 2 Tensile strength, puncture strength, and other mechanical properties of the separators.

Table 3 Thickness, contact angle, porosity, and electrolyte uptake of the PE separator, the PE-Al2O3 separator, and the PE-AlOOH separator.

Parameters

PE

PE-Al2O3

PE-AlOOH

Parameters

PE

PE-Al2O3

PE-AlOOH

Tensile strength (MPa) Elongation (%) Puncture strength (N) Peel strength (kN/m)

55.27 81 5.67 \

59.30 161% 6.14 0.1

61.78 171% 6.55 0.3

Thickness (μm) Contact angles (°) Porosity (%) Electrolyte uptake (%)

20 117° 55.74 126

26 11.6° 55.20 144

26 5.7° 55.63 187

electrolyte. Moreover, the polar -OH groups in AlOOH improve its compatibility with polar electrolytes, which confers them with better wetting properties. Table 3 also lists the thickness, the porosity, and the electrolyte uptake of the pristine PE separator as well as the PE-Al2O3 and the PEAlOOH separators. First, the thickness of the separators was measured with a digital gauge. The thickness of the ceramic coating of the PEAl2O3 and the PE-AlOOH separators were both approximately 3 μm on each side, as indicated in Fig. 1(e), (f). The porosity is defined as the ratio of the volume of micropores in the separator to the total volume of the separator. Typically, the porosity of polyolefin separator is 40%–50% [37]. The aqueous aluminide ceramic coating did not negatively affect the porosity of the separator, according to our calculations. For example, the porosity is 55.74%, 55.20%, and 55.63% for the pristine PE, PE-Al2O3, and PE-AlOOH separators, respectively. The performance of the separators is also determined by the electrolyte uptake, which will have a large effect on the migration of the lithium ions and the electrochemical performance of LIBs. Table 3 gives an electrolyte uptake of 126% for the pristine PE separator compared to a value of 144% for the PE-Al2O3 separator and 187% for the PE-AlOOH separator. The change in electrolyte up-take is related to the improved

hydrophilicity and interfacial compatibility of the separators modified by an aqueous aluminide ceramic coating from Al2O3 and AlOOH submicron particles. The AlOOH-modified separator allows a better wetting of the liquid electrolyte than the Al2O3-modified PE separator, which results in a better liquid electrolyte uptake. The ionic conductivity reflects the ion transit in the LIBs separators. Fig. 5(a) shows the ionic conductivity of the cells (stainless steel| stainless steel) with the pristine PE, PE-Al2O3, and PE-AlOOH separators for impedance measurements at room temperature. The intercepts on the real axis indicate bulk resistances (Rb) of 18.19 Ω, 17.55 Ω, and 12.31 Ω for the pristine PE separator, the PE-Al2O3 separator, and the PE-AlOOH separator, respectively. Eq. (4) can be used to calculate an ionic conductivity of 0.55 mS/cm for the pristine PE separator, whereas it increases to 0.75 mS/cm and 1.00 mS/cm for the Al2O3- and the AlOOH-modified separators, respectively. It is necessary to further evaluate the electrochemical characteristics of the LIBs before and after the surface modification of the separator. Therefore, we carried out EIS measurements after pre-cycling (Fig. 5(b)) using the cells (NCM-111| Li) with the pristine PE, the PEAl2O3, and the PE-AlOOH separators to investigate the interfacial resistance in the coin cell. The equivalent circuit shown in the inset of

Fig. 4. Contact angle determination for (a) the PE separator, (b) the PE-Al2O3 separator, and (c) the PE-AlOOH separator.

5

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Fig. 5. (a) Ionic conductivity of the liquid electrolyte-soaked samples placed on two stainless steel plates and (b) Nyquist plots of the cells containing the pristine PE (black) and the PE-Al2O3 (red) and PE-AlOOH (blue) separators after pre-cycling. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

[26,39]. Our results let us to conclude that the AlOOH coating improves the electrochemical stability of the cell. Other studies have also shown that Al element is suitable in LIBs [21]. To further evaluate the effect of the aluminide coating layer on the electrochemical characteristics of the cells (NCM-111 | Li), cyclic voltammetry was carried out between 2.5 V and 4.3 V at a scan rate of 0.5 mV/s. Fig. 6(b) shows the initial cyclic voltammogram and that after 20 cycles for cells assembled with the PE, PE-Al2O3 and PE-AlOOH separators. The initial cycle for all samples has the same oxidation and reduction potentials of 4.10 V and 3.60 V, respectively. After the 20th cycle, the positions of the oxidation and reduction peaks shoft and become closer to each other, indicating that the reaction kinetics of the active material is improved from the initial cycle [21]. Moreover, the redox peaks of all samples were shifted to the same potential with an oxidation peak of 3.97 V and a reduction peak of 3.63 V. These results indicate that the aluminide coating layer does not have an obvious influence on the reversibility of the cells at normal working voltage 2.5 V–4.3 V. The rate capability and the cycle performance are critical parameters for the electrochemical performances of LIBs. The cells (NCM111| Li) with pristine PE, PE-Al2O3, and PE-AlOOH separators were packed to investigate the rate capability and the cycle performance from 2.5 V to 4.3 V at room temperature. In order to test the rate capability, the cells were discharged continuously at various current densities from 0.2C to 3C (0.2, 0.5, 1, 2, and 3C) and charged back to 0.2C for 5 cycles. Fig. 7(a) shows that the discharge capacity gradually decreased with the increasing discharge current density for all cells. At a current density of 0.2C, the specific discharge capacity of all cells was the same. When the discharge rate increased, the discharge performance of the cells with the coated separators were better than for the cell assembled with the pristine PE separator. The cells using the pristine PE separator had an initial discharge capacity of about 147.5 mAh/ g at 0.2C, 122.6 mAh/g at 1C, and 88.6 mAh/g at 3C. The PE-Al2O3 separator had a better performance than the pristine PE separator, which agrees with the results of Lee et al. [16,40]. Furthermore, the cell with the PE-AlOOH separator had the best discharge performance regardless of the discharge capacity and the capacity retention when the current increased. The cell with the PE-AlOOH separator had the largest initial discharge capacity: 147.1 mAh/g at 0.2C, 129.5 mAh/g at 1C, and 107.6 mAh/g at 3C. The discharge capacity also presented the largest differences when the current increased. When the current density increased to 3C, the cell with the PE-AlOOH separator maintained 72.1% of its initial discharge capacity, which is much higher than for the pristine PE separator (59.8%) and the PE-Al2O3 separator (66.7%). The rate capability of the PE, PE-Al2O3, and PE-AlOOH separators

Table 4 Ionic conductivity and impedance of the PE separator, the PE-Al2O3 separator, and the PE-AlOOH separator. Parameters

PE

PE-Al2O3

PE-AlOOH

Rb (Ω) Ionic conductivity (mS cm−1) Rct (Ω)

18.19 0.55 75.33

17.55 0.75 70.22

12.31 1.00 38.50

Fig. 5(b) agrees well with the measurements. Generally, the impedance spectra of the cell consists of two partially overlapped semi-circles and a straight line [38]. The semicircle in the higher frequency corresponds to the migration of the lithium ions through the solid electrolyte interface (SEI) determined by the RSEI resistance, an important parameter for the electrochemical interfacial reaction. The second semi-circle in the medium frequency range corresponds to the charge transfer resistance (Rct) between the electrolyte and electrodes. The straight line at lower frequencies is related to diffusion of lithium ions into the active mass and is characterized by the Warburg impedance (W). The measured Rct values were 72.33 Ω, 70.22 Ω, and 38.50 Ω for the pristine PE separator, the PE-Al2O3 separator, and the PE-AlOOH separator, respectively (Table 4). The liquid electrolyte is the carrier for lithium ion migration and ta larger liquid electrolyte uptake helps the charge transfer on the surface of the lithium ion electrolyte and the electrode material, thereby reducing the charge transfer resistance [16]. To determine the electrochemical stability of the separators before and after the surface modification by the aluminide ceramic coating under the operating conditions of the cell, linear sweep voltammetry (LSV) was performed on the pristine PE, PE-Al2O3, and PE-AlOOH separators at room temperature (25 °C). Fig. 6(a) showed the LSV curves of the cells (stainless steel| Li) with the different separators. There is no current change during the anodic potential sweep for all samples below 4.0 V, which indicates that all cells are stable up to 4.0 V. When the scanned voltage was increased from 4.0 V to 5.0 V, the current-voltage curves of the separators before and after the surface modification had different shapes. A small peak appeared for the pristine PE, whereas the PE-Al2O3 separator had a large sharp peak from 4.1 V to 4.7 V. However, the PE-AlOOH separator curve remained flat and indicated a better electrochemical stability. Furthermore, the stable plateau remained until almost 5.0 V for the PE-AlOOH separator, which helps obtain a large current in the charge and discharge of the LIBs. When the scanned voltage was higher than 5.0 V, distinct oxidation peaks related to electrolyte decomposition occurred in the PE separator, whereas the coated separators had wider working windows of 5.6 V in this case 6

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Fig. 6. (a) the Linear sweep voltammogram of cells (stainless steel| Li), (b) the cyclic voltammetry curves of the original and after 20 cycles (NCM-111 | Li) of the cells with PE, PE-Al2O3 and PE-AlOOH separators.

agreed well with the values of the ion conductivity and the electrolyte uptake. The cycling performance of the CR2032 half-cells for a charge and discharge rate of 1C was evaluated over 200 cycles to further study the effect of the aluminide ceramic coating on the electrochemical stability of cells (as shown in Fig. 7(b)). The PE-Al2O3 and PE-AlOOH separators, especially the PE-AlOOH separator, produced an excellent cycling performance compared to the pristine PE separator. The initial discharge capacity of the cell with the pristine PE, the PE-Al2O3, and the PE-AlOOH separators were 129.4 mAh/g, 130.6 mAh/g and 126.6 mAh/g, respectively. The cell with the PE-AlOOH separator has a small specific capacity because of the presence of the -OH groups. Wang et al. [27] showed that the -OH groups reduce the first coulombic efficiency of the battery. After 200 cycles, the cell with the PE-AlOOH separator had the highest capacity retention of 75.1% (95.1 mAh/g) whereas the capacity retention for the PE and PE-Al2O3 separators were 63.1% (81.6 mAh/g) and 68.4% (89.3 mAh/g), respectively. The better performance of the PE-AlOOH separator can be attributed to the presence of -OH groups in AlOOH that yields a higher electrolyte uptake, electrolyte wettability, and higher ionic conductivity.

To further investigate the interfacial stability between the aluminide coating layer and the matrix separator, we observed the surface morphology of the different separators after cycling. Fig. 8(a)–(b) shows the SEM images of the PE-Al2O3 and PE-AlOOH separators after a current density of 1C and 200 cycles. A small number of Al2O3 particles was detached from the surface of the PE-Al2O3 separator after 200 cycles. On the contrary, the surface of the PE-AlOOH separator after 200 cycles was uniform without any AlOOH particles falling off from the matrix separator. The results indicated that the polar -OH groups in AlOOH improved the interfacial stability between the coating layer and the matrix separator during the operation of the battery. At the same time, the different separators were characterized by FT-IR after the cycling treatment to further prove the chemical stability of the polar -OH groups in the electrolyte. Fig. 8(c) shows that the separator almost retains the characteristic peaks of the original separator (Fig. 2) after circulation. The FT-TR spectrum of the PE-AlOOH separator showed characteristic -OH peaks at 2850 cm−1 and 2920 cm−1, indicating that the -OH group in the AlOOH structure did not react with the electrolyte. As expected, the aqueous AlOOH ceramic coating separator to be a promising candidate for large-scale use in high-power LIBs.

Fig. 7. Cycle performance (a) and rate capability (b) of the cells with the different separators (pristine PE in black, PE-Al2O3 in red, and PE-AlOOH in blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 8. SEM images of the surface of (a) the PE-Al2O3 separator, (b) the PE-AlOOH separator after 200 cycles at 1C. FT-IR spectra (c) of the pristine PE separator (black), the PE-Al2O3 separator (red), and the PE-AlOOH separator (blue) after 200 cycles at 1C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4. Conclusions

Wei:Supervision.Fugen Sun:Validation.Zhihao Yue:Visualiza tion.Chuanqiang Yin:Formal analysis.Lang Zhou:Funding acqui siion.Xiaomin Li:Writing - review & editing, Resources.

In summary, novel aqueous aluminide ceramic coating polyolefin separators for LIBs were successfully prepared by a coating method using a series of aqueous BYK additives. In this work, the aluminide ceramic-coated particles used in the polyolefin separators were mainly Al2O3 and AlOOH particles. Compared with a pristine PE separator, the PE-Al2O3 and the PE-AlOOH separators had a better thermal stability, a higher electrolyte wettability, and a higher lithium ion conductivity. Moreover, due to the presence of hydroxyl (-OH) groups in the internal structure of AlOOH, the PE-AlOOH separator had the best properties. The better interface adhesion and better stability between the AlOOH particles and the matrix separator led to better mechanical properties (peel strength of 0.3 KN/m) and thermal stability. The polar –OH groups in AlOOH are more compatible with the electrolyte and the PEAlOOH separator has a better liquid electrolyte uptake (187%), a higher electrolyte wettability (contact angle of 5.7°), and a fairly high ionic conductivity (1.0 mS cm−1). These higher values all led to a significantly increased electrochemical performance, especially the rate capability and the cycle performance. AlOOH-coated separators are expected to replace commercial Al2O3-coated ones to be the next generation of power battery separator. Furthermore, we have reasons to believe that the PE-AlOOH separator is a good candidate to meet the environmental and safety performance requirements for the next generation of LIBs.

Declaration of competing interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. This work was done by my research group. We declare that the supporting source had no such involvement. Acknowledgements This work was financially supported by the Youth Long-term Project of Jiangxi Province to Introduce Leading Innovative Talents, China (No. jxsq2018106023), National Natural Science Foundation of China (No. 51502090), Natural Science Foundation of Jiangxi Province (No. 20171BAB216007 and 20181BAB206006), Innovation and Entrepreneurship Training Program for College Students of Nanchang University, China (No. 20190402). References [1] Y.G. Guo, J.S. Hu, L.J. Wan, Nanostructured materials for electrochemical energy conversion and storage devices, Adv. Mater. 20 (2008) 2878–2887, https://doi.org/ 10.1002/adma.200800627. [2] J. Hassoun, S. Panero, P. Reale, High-rate and high-energy polymer lithium-ion battery, Adv. Mater. 21 (2009) 4807–4810, https://doi.org/10.1002/adma. 200900470. [3] A. Manthiram, A. Vadivel Murugan, A. Sarkar, Nanostructured electrode materials for electrochemical energy storage and conversion, Energy Environ. Sci. 1 (2008)

CRediT authorship contribution statement Yuan Wang:Conceptualization, Writing - original draft.Qiulin Wang:Data curation.Yu Lan:Investigation.Zhenglin Song:Meth odology, Project administration.Jinpeng Luo:Software.Xiuqin 8

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