Polydopamine-coated cellulose microfibrillated membrane as high performance lithium-ion battery separator

Abstract

Eco-friendly cellulose microfibers were modified by mussel-inspired polydopamine coating layer and a cellulose/polydopamine (CPD) membrane was fabricated by a facile and cost-effective papermaking process. It was demonstrated that CPD membrane possessed a compact porous structure, superior mechanical strength and excellent thermal dimensional stability. CPD membrane was explored as a lithium-ion battery separator with its superior mechanical strength, low-cost and favorable electrochemical properties. Lithium cobalt oxide/graphite cell using CPD separator displayed the best cycling stability and rate capability compared with those of commercialized polypropylene separator and pristine cellulose separator. Furthermore, alternating-current impedance of the cell with CPD separator presented minor variation of 9 Ω after the 100^th cycle indicating an excellent interfacial stability of the cell. These excellent performances could endow CPD membrane as a very promising application as a high performance lithium-ion battery separator.

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1. Introduction

With the development of portable electronics and electric vehicles, the market of high-performance lithium-ion battery (LIB) has grown tremendously in recent years. Furthermore, the development of sustainable energy has attracted more and more attention due to environmental pollution and exhausted fossil oil.^1–6 In order to meet these demands, excellent safety performance and low-cost of LIB separator is one of the main goals in the future research.^7–9

At present, polyolefin microporous separators have been widely applied in LIB due to their electrochemical stability, mechanical strength and thermal shutdown properties.^8–10 Nevertheless, there is still some disadvantages such as intrinsically hydrophobic characteristics, inferior dimensional stability and low porosity.¹⁰ In order to tackle these disadvantages, extensive efforts are focused on surface modification and surface polymer coating.^11–15 However, these solutions are limited in practical applications due to their high manufacturing cost, environmental problems and inherent defects of polyolefin separators. In addition, the polyolefin separators were usually fabricated by axial stretching and thermally induced phase separation processes which used low-cost polyolefin materials, however, the separators accounted for a fair portion of the total cost of LIB which could exceed by about 20%. Therefore, the majority of the separator cost was ascribed to manufacturing process.^9,10 It is mandatory to develop facile manufacturing processes and explore new materials with low cost, high thermal dimensional stability and superior wettability.^5,9 Natural cellulose, the most important skeletal component in plants, is an inexhaustible and renewable raw material with fascinating properties.^5,16,17 Cellulose membrane could be fabricated by a facile and low-cost papermaking technology. In recent years, cellulose membrane has been successfully explored as lithium-ion battery separator.^18–21 However, these cellulose membranes still have some disadvantages to tackle, such as large-sized pores and low mechanical strength which are not beneficial to the safety and cycle performances of lithium-ion batteries due to the dendritic Li and self-discharge. To tackle the above mentioned issues, cellulose separators are desired to possess a thickness more than 60 μm. However, the thickness is not beneficial to excellent rate performance.^5,9,10 In order to enhance battery safety and its rate performance, cellulose derivatives and cellulose-based materials were attempted to improve the performance of cellulose membrane.^5,20–25 Unfortunately, the fabrication process was complicated and time consuming.

Inspired by adhesive proteins in mussels, dopamine was widely exploited in many fields because of extraordinary properties.^15,26–31 In this paper, dopamine was motivated to bind the surface of microfibrillated cellulose to form CPD membrane by self polymerization with more compact tortuous nanopores which could prevent internal short circuits and enhance mechanical strength. CPD membrane was fabricated by a facile papermaking process which was favorable for alleviating manufacturing cost. Meanwhile, CPD membrane possesses homogeneous mechanical strength (33 MPa) in all directions and excellent thermal dimensional stability below 200 °C. These superior properties are essential to enhance the battery safety, especially for high-power lithium-ion batteries. Lithium cobalt oxide (LiCoO₂)/graphite cells employing CPD separator display better cycle performance and rate capability compared with that of commercialized PP separator and pristine cellulose separator. The cost-effective, superior thermal dimensional stability and excellent electrochemical properties of CPD separator would endow it as a very promising separator for high-performance lithium-ion batteries.

2. Experimental section

2.1. Materials

1 M lithium hexafluorophosphate (LiPF₆) in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1/1, v/v) (Guotai-huarong New Chemical Materials Co., Ltd.), polypropylene (PP) separator (Celgard 2500), Cotton cellulose pulp (DP 750-1000, Shandong Yinying Co., Ltd), tris(hydroxymethyl)aminomethane (Aladdin), dopamine hydrochloride (Adamas Reagent Co,. Ltd). Other chemical reagents were all purchased commercially and used without further purification.

2.2. Preparation of the CPD separator

A 12 wt% suspension of cellulose pulp was beaten in a PFI beater (PL11-00, Xianyang Taisite) at 20 000 rpm, and then grinded into fibrillated fibers with an ultra-fine friction grinding machine (MKZA6-5, Masuko Sangyo Co., Ltd). The obtained homogeneous cellulose slurry was immersed into the dopamine solution (2 g L⁻¹) containing the tris(hydroxymethyl)aminomethane buffer (10 mM) at room temperature for 24 h. Then the slurry was washed with water. Finally, the homogeneous slurry was added into a papermaking machine (Rapid Koethen BB) to fabricate a wet CPD membrane. The wet CPD membrane was subsequently dried in a hot-oven at 95 °C for 30 min, and then calendered at 120 °C under 20 MPa. After that, the final CPD membrane was dried under vacuum at 120 °C for 12 h. The average thickness of the obtained membrane was 40 ± 1 μm.

2.3. Membrane characteristics

The surface morphology of CPD membrane was analyzed by a Hitachi S-4800 field emission scanning electron microscope (SEM) and Hitachi H-7650 transmission electron microscope (TEM). The mechanical strength was examined using an Inston-3300 universal testing machine (USA) at a stretching speed of 1.66 mm s⁻¹ with the sample straps of 1 cm wide and 4 cm long. The air permeability of the separator was measured with a Gurley densometer (4110N, Gurley) by measuring the time for air to pass through a determined volume (100 cm³). The porosity of the separator was evaluated by immersing in n-butanol for 1 h, and then calculated using the equation: porosity = (m_a/ρ_a)/(m_a/ρ_a + m_b/ρ_b) × 100%, where m_a and m_b were the mass of n-butanol and the separator, ρ_a and ρ_b were the density of n-butanol and the separator, respectively. The electrolyte uptake was obtained through the weight of separators before and after liquid electrolyte (1 M LiPF₆-EC/DMC (1/1, v/v)) soaking for 1 h and calculated using the following equation: Electrolyte uptake = (W_d − W_c)/W_c × 100%, where W_c and W_d were the weights of the separator before and after soaking in the liquid electrolyte, respectively. The thermal dimensional stability of the separator was determined by placed in an oven operating at 200 °C for 0.5 h. The thermal stability of separator was tested by a differential scanning calorimeter (Diamond DSC, PerkinElmer) ranging from 50 °C to 250 °C at 10 °C min⁻¹ under N₂ atmosphere.

2.4. Electrochemical measurements

For measuring electrochemical performances, LiCoO₂ cathode (LiCoO₂/carbon black/PVDF 90/5/5 w/w/w) and a natural graphite anode (natural graphite/carbon black/CMC/SBR 93/2/1.5/3.5 w/w/w/w) were prepared by a doctor-blading and dried under vacuum at 120 °C for 12 h. A liquid electrolyte of 1 M LiPF₆ in EC/DMC (1/1, v/v) was prepared for all electrochemical measurements. The ionic conductivity of the liquid electrolyte-soaked separator between two stainless-steel plate electrodes and the interfacial resistances between the separator and lithium metal electrodes were evaluated utilizing the electrochemical impedance spectroscopy (EIS) instrument, respectively. Alternating current (AC) impedance measurement of the cells were investigated by applying an AC voltage of 10 mV amplitude in the frequency range of 1–10⁶ Hz. All cells were assembled in an argon-filled glove box. The charge/discharge rate capability and cycle performance of cells were investigated using a LAND battery test system at room temperature. The discharge current densities were varied from 0.2 C to 8.0 C under a voltage range between 2.75 V and 4.2 V. The cells were cycled at a fixed charge/discharge current density of 0.5 C/0.5 C for cycling performance test. (1 C= 140 mA h g⁻¹).

3. Results and discussion

The fabrication process of polydopamine-coated cellulose microfibers were schematically illustrated in Fig. 1. Microfibrillated cellulose was immersed in an aqueous solution of dopamine (2 g L⁻¹) at pH 8.5. After 24 h dipping, the color of aqueous solution changed to black-brown which was ascribed to self-polymerization of dopamine (Fig. S1†).¹⁵ As reported in previous studies, catechols tended to react with the hydroxyl groups resulting in the dehydration and formation of a charge-transfer complex.^26,29,30 Then, these fibrillated fibers were coated with homogeneous dopamine solution. Layer-by-layer assembly of dopamine eventually formed a polydopamine cladding layer with the thickness of 20 nm on the surface of cellulose (Fig. S2†). The catechols would continue to react with hydroxyl groups of fibrillated fibers to enhance the mechanical strength of cellulose membrane after the polydopamine-coating. The polydopamine coating layer endowed microfibrillated cellulose with strong bonding which was beneficial to enhance the mechanical strength of cellulose membrane.

Fig. 1 Schematic illustration of the polydopamine-coated microfibers.

Fig. 2 shows the morphologies of PP separator, cellulose separator and CPD separator. It was observed that PP separator possessed elliptic pores which were formed via a uniaxial stretching technology.^9,10 Cellulose separator and CPD separator possessed tortuous pores caused by the papermaking process. The pores of cellulose separator without the polydopamine coating was large-sized loosely connected, resulting in low mechanical strength and poor battery safety. From Fig. S3,† it was observed that 79% of its pore dimension was less than 300 nm. As shown in Fig. 2d, the CPD separator had intensive interwoven nanopores due to the bonding between CPD microfibers. The tortuous nanopores played a critical role in preventing internal short circuits and avoiding self-discharge which were favorable to enhance the safety performance of the battery.^5,10

Fig. 2 Typical SEM images of (a) PP separator; (b) cellulose separator; (c) and (d) CPD separator.

The basic physical parameters of separators are shown in Table 1. CPD separator was more compact than cellulose separator which caused the relatively larger Gurley value, however, these properties were still better than that of PP separator. The porosity of CPD separator (62%) was higher than that of PP separator (55%). It was well known that highly porous structure was beneficial to increase permeability.³² Although PP separator was thinner than CPD separator, the Gurley value of PP separator (235 s) were much higher than that of CPD separator (97 s) which could be ascribed to better porosity of the CPD separator. Meanwhile, CPD separator was easily accessible to liquid electrolytes and retained the electrolyte efficiently owing to better electrolyte-philicity.^10,33 The electrolyte uptake and electrolyte wettability of separators were vital for the cycle performance and rate capability of LIB.⁸ The CPD separator displayed a superior electrolyte uptake up to 200% and a rapid soaking time of 30 s (Fig. S3†) because of its higher porosity and lyophilic surface. However, the electrolyte wettability of PP separator was poor due to its lyophobic surface and low surface energy. These excellent physical properties and electrolyte wettability were beneficial to enhance electrochemical properties of high performance lithium-ion batteries.^8–10

Table 1 Physical parameters of separators

Separators	Thickness (μm)	Porosity (%)	Gurley value (s per 100 cm^3)	Electrolyte uptake (%)
PP	25	55	235	120
Cellulose	40	70	43	275
CPD	40	62	97	200

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The thermal dimensional stability and mechanical strength of separators were vital aspects for the battery safety because separator could prevent short-circuits between anode and cathode. PP separator suffered from severe thermal shrinkage at higher temperature which could cause internal short-circuits, more seriously, it could lead to fire outbreaks and even explosions. In Fig. 3b, the cellulose and CPD separator maintained original dimension after thermal treatment at 200 °C while the PP separator showed severe, more than 50%, thermal shrinkage. The superior dimensional stability of the CPD separator was beneficial to enhance safety characteristics of LIB. In order to investigate this phenomenon, DSC measurement was carried out. It was observed in Fig. 3c that PP separator began to melt at 150 °C and had an endothermic peak at 165 °C, however, the CPD separator did not show any obvious thermic peak below 250 °C. This significantly improved thermal shrinkage of the CPD separator, was mainly attributed to close-packed polysaccharide chains which were highly stabilized by strong hydrogen bonding.^20–22 Meanwhile, a papermaking process was also favorable to the improved thermal stability.^9–11 The stress–strain curves were depicted in Fig. 3d. The maximum stress of the CPD separator was up to 33 MPa while that of cellulose separator was only 15 MPa. The enhanced mechanical strength of the CPD separator was mainly ascribed to polydopamine bonding between microfibrillated cellulose from the mussel-inspired dopamine.^26,29 Compared with that of PP separator, the strength of CPD separator was more homogenous in all directions and much better than the transverse strength of PP separator (12 MPa), but it was lower than that of PP separator in the machine direction (120 MPa) as depicted in Fig. S5.† Remarkably, the electrolyte soaked CPD separator still possessed the superior mechanical property (21 MPa). CPD separator presented a considerably higher Young's modulus (400 MPa) than that of the PP separator (260 MPa). High Young's modulus is advantageous to retain mechanical integrity and avoid the rupture of the separator when it encounters accident collisions. The excellent thermal stability and superior mechanical strength of the CPD separator could greatly enhance the safety of the lithium-ion battery.

Fig. 3 (a) Photographs of PP separator, cellulose separator and CPD separator at 20 °C, and (b) after exposure to 200 °C for 0.5 h, (c) DSC curves of PP separator, cellulose separator and CPD separator, (d) stress–strain curves of cellulose separator and CPD separator in dry and wet states.

The ionic conductivity and interfacial resistance influence the performance of lithium-ion batteries significantly. The ionic conductivity of electrolyte-soaked CPD separator was calculated to be 0.95 × 10⁻³ S cm⁻¹ which was higher than that of the PP separator (0.72 mS cm⁻¹), according to the equation: σ = L/RS, where L was the thickness of the separator and S was the contact area between the separator and stainless steel blocking electrodes. However, it was lower than that of cellulose separator (1.08 mS cm⁻¹). The result is consistent with the above-mentioned porosity and permeability data which signified comparable ionic conductivity.^8,33 In addition, higher ionic conductivity was beneficial to ensure better rate capability of the LIB. In Fig. S4,† the CPD separator possessed the lowest interfacial resistance (168 Ω) which could be attributed to the lyophilic surface and better compatibility between polydopamine coating layer and electrode materials. A favorable interfacial contact was critical to the long-term reliable performance of the rechargeable battery.⁹ The cell using CPD separator exhibited excellent interfacial compatibility by the lyophilic and adhesive polydopamine coating layers which were beneficial to improve the electrochemical performance of the lithium-ion battery.

LiCoO₂/graphite cells were assembled to investigate the effect of polydopamine coating layers on electrochemical properties. As shown in Fig. 4a, rate capability was evaluated at varied current densities ranging from 0.2 C to 8 C. As clearly shown in Fig. S6,† the discharging capacity of cells dropped with the increasing discharging current densities. However, the cell with CPD separator exhibited the best capacity retention at different discharging current densities. The superior rate capability could be ascribed to higher ionic conductivity and better interfacial compatibility of electrolyte-soaked CPD separator. It was observed that the LiCoO₂/graphite cells using CPD separator displayed stable cycling performance and better capacity retention (84.9%) than that of PP separator (73.2%) and cellulose separator (77.8%). In order to investigate the enhanced cycle performance and rate capability of cells with CPD separator, AC impedance measurements were carried out after the first cycle and after 100^th cycle when discharged to 2.75 V. As shown in Fig. 4c and 4d, it is well known that the semicircle in the high frequency zone represents the charge-transfer resistance accompanied with migration of lithium ion.^15,34 The charge transfer impedance of the cell using CPD separator increased from 14.6 Ω to 23.6 Ω while that of PP separator increased from 20.9 Ω to 43.2 Ω. The lower charge-transfer resistance was closely related to the higher electrolyte uptake and better interface compatibility. This indicated that the cell using CPD separator had superior electrolyte retention and interfacial compatibility which were beneficial to improve cycling performance and rate capability.

Fig. 4 (a) Cycle performance and (b) rate capability of LiCoO₂/graphite cells using PP separator, cellulose separator and CPD separator; Nyquist plots for cells measured after the (c) first cycle and (d) 100^th cycle.

4. Conclusions

In this paper, we explored polydopamine-coated cellulose microfibrillated membrane via a facile and cost-effective papermaking process. It was demonstrated that the CPD membrane possessed not only better electrolyte wettability and tortuous nanopores, but also more favorable excellent thermal dimensional stability and mechanical strength, which could significantly enhance the safety of lithium-ion batteries. The proper interfacial compatibility between CPD separator and electrode materials alleviated the interfacial resistance and charge transfer impedance. Compared with that of PP separator, the cell using CPD separator showed superior electrochemical properties such as cycle performance and rate capability. Therefore, CPD membrane is a very promising alternative separator for high performance lithium-ion battery.

Acknowledgements

This work was financially supported by the National Program on Key Basic Research Project of China (973 Program) (no. MOST2011CB935700), the Instrument Developing Project of the Chinese Academy of Sciences (no. YZ201137) and the National High Technology Research and Development Program of China (863 Program, no. 2013AA050905).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra45879b

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