Research Article www.acsami.org
Separators with Biomineralized Zirconia Coatings for Enhanced Thermo- and Electro-Performance of Lithium-Ion Batteries Jun-Ke Pi,† Guang-Peng Wu,*,† Hao-Cheng Yang,† Christopher G. Arges,*,‡ and Zhi-Kang Xu*,† †
MOE Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States S Supporting Information *
ABSTRACT: Porous separators are key components for lithium-ion batteries (LIBs) and they have drawn considerable attention because of their vital role in governing battery cost and performance (e.g., power density, safety, and longevity). Here, zirconia-coated separators were fabricated via a facile biomineralization process with the aim to improve the performance of commercialized polypropylene separators. The as-prepared organic−inorganic composite separators show excellent thermal stability, even at the melting temperature (160 °C) of polypropylene. This is due to the welldistributed zirconia coatings on the separator surfaces. Furthermore, the interfacial impedance of the composite separators is only 343.8 Ω, which is four times lower than the pristine polypropylene ones. The results demonstrate an attractive method to prepare organic−inorganic composite separators with outstanding properties, which makes them promising candidates for high-performance LIBs. KEYWORDS: polyolefin separators, polydopamine, zirconia coating, lithium-ion battery, thermal stability
1. INTRODUCTION Lithium-ion batteries (LIBs) are ubiquitous power sources for various portable electronic devices and electric vehicles because of their high energy density and long cycle life.1−3 As one of the core components of LIBs, the separators permit lithium-ion transport between anode and cathode while preventing direct contact of the two electrodes (which can short the LIB cell). Hence, the separators play an instrumental role in LIB performance. In this context, an eligible separator must satisfy electrochemical stability in addition to thermal tolerance and deformation resistance to keep the cell running normally.4 Most of the commercially available separators today for LIBs are based on microporous polyethylene (PE) and polypropylene (PP) materials because of their commercial availability (e.g., Celgard), low cost, and chemical resistance. Despite their success in current LIB technology, there is still plenty of room to improve upon the performance of polyolefin-based separators. A few areas of improvement that warrant attention: (1) thermal stability, such as thermal-deformation which risks the internal short-circuiting of the batteries or even fire and explosion; (2) intrinsic hydrophobicity that results in the poor wettability and retention of electrolyte, compromising lithiumion transport, cell capacity, and cycle life.5 Many strategies have been devised to overcome the current limitations for polyolefin-based separators.6−8 For instance, polymer-coating or grafting of hydrophilic brushes via highenergy irradiation or e-beam treatments could promote surface © 2017 American Chemical Society
wettability of the separators with organic electrolytes (e.g., organic carbonates). 9−11 Advances have been made in promoting the hydrophilicity of commercially available separators, however, these strategies often alter material resistance integrity to thermal deformation because of the incorporated soft organic hydrophilic polymers or/and the separator matrix damage caused by the high energy radiation.12 Furthermore, the strategies are constantly subjected to several steps requirement, toxic organic solvents usage, and complex instrumentation, hindering their high-throughput scalability.13 Coating an inorganic layer onto the polymeric separators (i.e., organic−inorganic composite separators) was shown to be an effective method to enhance the performance of LIBs because the inorganic material bestowed both the wettability and rigidity of the separator simultaneously.14,15 For example, Park and Xiao groups reported that the ZrO2-composite separator could highly improve the thermo-stability, discharge C-rate capability and cycling performance, respectively.16,17 Yang, Zhao and co-workers developed an approach to improve the rate performance and the thermal stability by coating elaborate silica core particles on the surfaces of PE separator.18 Lee et al. utilized polyvinylidene fluoride-hexafluoro propylene (PVDFHFP) as a polymeric binder to adhere a layer of ceramic Received: March 30, 2017 Accepted: June 14, 2017 Published: June 14, 2017 21971
DOI: 10.1021/acsami.7b04505 ACS Appl. Mater. Interfaces 2017, 9, 21971−21978
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic representation and the proposed mechanism of the procedure used to fabricate ZrO2-coated PP separators.
nanoparticles on the separator surface.19 However, the development of these process have been held back by the increasing of separator thickness and the blocking of separator pores, which depress the transport of Li+ ion.19 In contrast, bioinspired coatings provide an alternative strategy for addressing the aforementioned shortcomings because they take advantage of recent advances in surface chemistry.20−26 For example, Park, Choi, and co-workers reported a surface modification method for polyolefin separators by using polydopamine (PDA).27 In their work, the silica-composite polyolefin separator yielded improved electrochemical and thermal stability.28 Subsequently, the same group also investigated the roles of PDA coatings on PE separators for LIBs.29 Their work revealed that hydrophilic PDA coatings, with strong adherence to the separator surface, suppressed the growth of Li-dendrites fostering greater battery cycle life. More recently, our group has reported robust strategies to mineralize various microporous membranes with enhanced curling resistance and surface wettability via a musselinspired PDA/polyethylenimine (PEI) intermediate layers.30 Compared with the PDA use only, the PDA/PEI intermediate layer is thinner, smoother, and more stable. This robust modification derived from the abundant amino groups on the PEI main chains that significantly accelerated the deposition process via Michael addition or Schiff-base reaction. This scheme enabled uniform distribution of inorganic particles throughout the entire membrane.31,32 Drawing inspiration from this codeposition methodology, herein we report ZrO2-coated PP separators with excellent electrochemical performances and thermal stability for LIBs. ZrO2-coated separators in our work were easily fabricated by sequentially immersing those PP separators into dopamine PDA/PEI and Zr(SO4)2 aqueous solution (Figure 1). The abundant amino groups in the PEI chains accelerate the self-polymerization of dopamine and aid the formation of a stable cross-linked PDA/PEI layer on the separator surface. The catechol groups on the PDA/PEI layer then chelate Zr4+ from aqueous solution causing the growth of
ZrO2. Given the highly improved permeability to aqueous solutions of the PDA/PEI-modified separators, the mineralization process occurred throughout the entire separator. Furthermore, the thickness of the ZrO2 coating was wellcontrolled by mineral concentration, solution pH value or/and the mineralization time.33,34
2. EXPERIMENTAL SECTION 2.1. Materials. Polypropylene microporous separators (porosity 45%, thickness 20 μm) were purchased from Beijing Normal University (China) and were cut into rounds with a diameter of 19 mm. The samples were washed by acetone for 0.5 h to remove impurities absorbed on the separator surfaces and then dried in a vacuum oven at 40 °C to a constant weight. Dopamine hydrochloride was purchased from Sigma-Aldrich and used without further purification. Polyethylenimine (PEI, Mw = 600 Da) was purchased from Aladdin. Lithium iron phosphate (LiFePO4), conductive carbon black, polyvinylidene fluoride (PVDF), liquid electrolyte (1 M LiPF6 in EC/DEC/DMC, 1:1:1 by volume), and other battery materials were purchased from Hefei Ke Jing Materials Technology Co. Ltd. Tris (hydroxymethyl) aminomethane (Tris), zirconium(IV) sulfate tetrahydrate (Zr(SO4)2) and other reagents were obtained from Sinopharm Chemical Reagent Co. Ltd. Other reagents were of analytical grade and used as received. 2.2. Deposition of the PDA/PEI Layer on the Separator Surface. The dried separators were prewetted by ethanol and then immersed into a dopamine hydrochloride/polyethylenimine solution (tris-buffer solution, 50 mM, pH 8.5) with a respective concentration of 2 mg/mL for 4 h. Then, the as-prepared separators were rinsed by deionized water with a vibrator and then dried in a vacuum oven to constant weight. 2.3. Decoration of ZrO2 Coatings on Separator Surface. Fourteen mg Zr(SO4)2·4H2O was dissolved in 10 mL of HCl solution (10 mM). The as-prepared separators were immersed into the above solution and mineralized for 4 h under 25 °C. The separators were washed by deionized water 5 times and dried for further characterization. 2.4. Characterization. The surface morphologies of separator were characterized by field emission scanning electron microscopy (FESEM, Hitachi, S4800, Japan) with a voltage of 3.0 kV. Element distribution of the separators was analyzed by energy dispersive X-ray 21972
DOI: 10.1021/acsami.7b04505 ACS Appl. Mater. Interfaces 2017, 9, 21971−21978
Research Article
ACS Applied Materials & Interfaces spectrometer (EDX, Hitachi, S4800, Japan) with voltage of 20.0 kV. The separator samples were dried overnight in a vacuum oven at room temperature, then attached to the sample supports, and coated with a gold layer. The surface chemistries were revealed by X-ray photoelectron spectrometer (XPS, PerkinElmer, USA) using Al Kα excitation radiation (1486.6 eV). The whole spectra were collected ranging from 0 to 1000 eV with a survey depth of 5−10 nm. The surface wettability was characterized by measuring static contact angles from a contact angle system (MAIST Vision Inspection & Measurement Co. Ltd., DropMeter A-200, China) at room temperature. One drop of water (2 μL) was dropped onto the separator surface with an automatic piston syringe and photographed, then water contact angles were calculated with a circle-fitting algorithm. The thermal shrinkage was evaluated by measuring the area of separators before and after thermal treatment under 100−160 °C for 1 h. The degree of thermal shrinkage was calculated by using eq 1#tab; ⎡W − W ⎤ f shrinkage (%) = ⎢ i ⎥100 ⎣ Wi ⎦
LiFeO4, SUPER-P (Conductive carbon black) and PVDF in a weight ratio of 8:1:1 on aluminum foil (coating thickness: 0.015 mm), and the cast foils were then punched into circular pieces (d = 14.0 mm) and dried at 60 °C for 24 h. The coin cells were cycled between 3.0 and 4.2 V at room temperature on a LAND cycler (Wuhan, China). All the cells were assembled in an argon-filled glovebox. For the measurement of cycling performance, the cells were cycled at a constant charge/ discharge current density of 0.1 C/0.1 C. For the measurement of Crate performance, the cells were cycled at varied charge/discharge current density.
3. RESULTS AND DISCUSSION The surface morphology and chemistry were initially evaluated before testing the thermo- and electro-characteristics of the ZrO2-coated PP separators demonstrated herein. To confirm our strategy, we comparatively studied the nonmodified, the PDA-modified,27 and the ZrO2-coated PP separators. X-ray photoelectron spectroscopy (XPS) (Figure S1) was used to analyze the surface modification of PP separators with PDA/ PEI and ZrO2 coatings. It can be seen that the peaks of N 1s and O 1s present in the XPS spectrum result from PDA/PEI deposition. The sharp peak present at 180 eV originates from Zr 3d5. The ZrO2-coated separators demonstrate a larger O 1s peak intensity when compared to the non- and PDA-modified separators. The presence of zirconium and the high peak intensity for O 1s in the XPS spectrum substantiate the formation of ZrO2 coating.33 To complement the structural information obtained from XPS spectra, we used SEM analysis to observe the surface morphology of separators. The nonmodified separators exhibit stretching-induced pores with a pore size around 200 nm. For the PDA-modified ones, many particles are clearly observed on the separator surface causing unwanted blockage of the pores (Figure 2a). In sharp contrast, the pores of the ZrO2-coated separators remain intact (∼180 nm), and the surface is continuous and smooth without detection of particles. Compared with the nonmodified separator with a thickness of 20 μm, the invisible ZrO2 particles is too small to affect the
(1)
where Wi is the initial area and Wf is the final area of the separator after the storage test. The electrolyte uptake of separator was measured by immersing them into the liquid electrolyte separately for 1 h and calculating with the eq 2#tab; ⎡ W − Wo ⎤ electrolyte uptake (%) = ⎢ i ⎥100 ⎣ Wo ⎦
(2)
where Wi represents the weight of the electrolyte-soaked separator, and the extra electrolyte on the surface of separator was wiped with a filter paper before measuring the weight; W0 is the weight of dry separator. The porosity of separator was examined by immersing them into the n-butanol for 1 h, and calculating with the eq 3
⎡ρ − ρ⎤ e ⎥100 porosity (%) = ⎢ c ⎢⎣ ρc ⎥⎦
(3)
where ρc is the calculated density of the n-butanol soaked separators, and the extra n-butanol on the surface of separators was wiped with a filter paper before measuring the weight; ρe is the calculated density of the pristine separator. 2.5. Electrochemical Measurements. The ionic conductivities of the separators were measured by electrochemical workstation (Chenhua, CHI660E); 2025 coin-type test cells were assembled by sandwiching the separators between two stainless steel electrodes and filling with the liquid electrolyte (1 M LiPF6 in EC/DEC/DMC, 1:1:1 by volume) for AC impedance measurements in the frequency range of 1 × 10−2 to 1 × 106 Hz with an amplitude of 10 mV at room temperature. The ionic conductivity (σ) was calculated with the eq 4 σ (mS/cm) =
d RA
(4)
where d is the thickness of the separators, A is the area of the electrode, and R is the electrolyte resistance measured by AC impedance. The interfacial impedance of the separators was also measured using an electrochemical workstation (Chenhua, CHI660E). 2025 coin-type test cells were assembled by sandwiching the separators between two lithium metal electrodes and filling with the liquid electrolyte (1 M LiPF6 in EC/DEC/DMC, 1:1:1 by volume) for AC impedance measurements in the frequency range of 1 × 10−2 to 1 × 106 Hz with an amplitude of 10 mV at room temperature. The electrochemical performance of the separators was examined using 2025 coin-type half-cells, where the separators (d = 19.0 mm) were sandwiched between a lithium metal anode (d = 15.4 mm) and a LiFePO4 cathode and activated by filling the electrolyte (1 M LiPF6 in EC/DEC/DMC, 1:1:1 by volume). The cathode was prepared by coating the N-methyl-2-pyrrolidone (NMP)-based slurry consisting of
Figure 2. Characterization of the nonmodified (left), the PDAmodified (middle), and the ZrO2-coated (right) separators. (a) SEM images. The scale bar is 1 μm. (b) Electrolyte contact angles. (c) Electrolyte uptake (%). 21973
DOI: 10.1021/acsami.7b04505 ACS Appl. Mater. Interfaces 2017, 9, 21971−21978
Research Article
ACS Applied Materials & Interfaces
combination resistance Rb related to the intrinsic resistance of the soaked separator.36,37 Interfacial resistance is defined as the summation of RSEI and Rct (Rint = RSEI + Rct). As shown in Figure 3, for initial separator (after 0 cycle), a significantly smaller interfacial resistance (343.8 Ω) is observed with cells assembled with the ZrO2-coated separators over cells assembled with the nonmodified (1600.0 Ω) and the PDA-modified separators (1607.4 Ω). The lower resistance for the ZrO2coated separators indicates a great compatibility of the electrolyte-soaked separator with the lithium electrode.38 To show the quality of our separator, we show the Nyquist plot of the Li/separator/Li cells assembled with various separators after 100th cycle in Figure 3, the resistance is also provided in Table 1. The interfacial resistance of cells
thickness of separators. Thus, the thickness of separators before and after modification shows no significant change. Energydispersive X-ray spectrometry (EDX) (Figure S2) was performed to analyze the Zr element distribution, which indicates the uniform presence of ZrO2 layer on the PP matrix. We attribute the uniform deposition to the slow sol−gel process and amorphous mineral formation.35 The electrolyte contact angle and electrolyte uptake experiments also confirm the successful modification of the PP separator (see Figure 2b). For the ZrO2-coated ones, the electrolyte drop deposited on the surface permeates through the pores rapidly, displaying a contact angle of 0°. In contrast, the electrolyte contact angle of the nonmodified and the PDAmodified separators are ∼43 and ∼20°, respectively. These results clearly suggest that the ZrO2-coated separators own excellent electrolyte wettability. The electrolyte uptake of the ZrO2-coated separators is 312.9 ± 21.1%, which is significantly greater than those of the nonmodified (138.6 ± 15.6%) and the PDA-modified (203.2 ± 17.3%) separators under the same conditions (see Figure 2c). On the basis of the electrolyte contact angle and uptake results, the enhanced hydrophilicity and electrolyte retention of the ZrO2-coated separators are posited for enhancing Li+ ion transport in LIBs and promoting enhanced battery performance. The interfacial resistance of the separators was quantified by electrochemical impedance spectroscopy (EIS). In Figure 3, the
Table 1. Resistance of the Li/Separator/Li Cells Assembled with Various Separators after Different Cycles entry
separators
cycle
Rb (Ω)
RSEI (Ω)
Rct (Ω)
R (Ω)
1 2 3 4 5 6
nonmodified nonmodified PDA-modified PDA-modified ZrO2-coated ZrO2-coated
0 100th 0 100th 0 100th
7.2 11.6 7.3 10.5 3.6 7.1
1248.0 1854.0 990.0 1368.1 289.9 347.6
344.8 624.8 610.1 829.6 50.3 80.2
1600.0 2490.4 1607.4 2208.2 343.8 434.9
assembled with the non- and PDA-modified separators increases with battery cycling (from 1600.0 Ω to 2490.4 Ω for the nonmodified separators, entries 1−2, and 1607.4 Ω to 2208.2 Ω for the PDA-modified separators, entries 3−4), whereas the interfacial resistance of cells assembled with the ZrO2-coated separators displays a smaller increase over the same battery cycling conditions (from 343.8 Ω to 434.9 Ω, entries 5−6). The smaller change in interfacial resistance is attributed to the improved electrolyte wettability and the unchanged pore structure of ZrO2-coated separators in contrast with the non- and PDA-modified analogues. The ZrO2-coated separators enable more efficient transport of Li+ ions in the SEI layer and electrolyte/electrode interface yielding a substantial reduction in interfacial resistance in LIB cells.39 Figure 4a reports battery performance with different separators by plotting the charge and discharge polarization at 0.1 C. The battery was sandwiched between a lithium metal anode and a LiFePO4 cathode and soaked with the electrolyte. Discharge capacity of cells assembled with the non- and PDAmodified separators is 130.9 mAh/g and 138.9 mAh/g, respectively. The LIB cells with the ZrO2-coated separators exhibit a larger discharge capacity (159.4 mAh/g), indicating the ZrO2-coated separators facilitate a lower internal cell resistance. Figure 4b gives the discharge C-rate capability of the three separators, and the cells were charged under a voltage range between 3.0 and 4.2 V at a constant C-rate (0.1, 0.2, 1.0, 2.0, and 5.0 C). The discharge capacity gradually decays with increasing current density. However, the decay is smaller for the ZrO2-coated separators compared with the non- and PDAmodified ones because of the separator’s lower interfacial resistance. Additionally, it is noteworthy to point out that the discharge capacity of cells assembled with the ZrO2-coated separators is substantially higher than that with the non- and PDA-modified ones at 5.0 C (i.e., the ZrO2-coated separators yield a 60.8% of the initial discharge capacity, whereas the cells assembled with non- and PDA-modified separators demonstrate 6.0 and 41.3%, respectively).
Figure 3. Variation in AC impedance spectra (0 cycle−100th cycle, Crate 0.1 C) of cells assembled with the nonmodified (green squares), the PDA-modified (blue triangles) and the ZrO2-coated separators (orange circles) at the open-circuit potential. The green square mark for the nonmodified, blue triangle mark for the PDA-modified and orange cycle mark for the ZrO2-coated separators applies to all figures.
Nyquist plot from EIS for the Li/separator/Li cells is presented at open-circuit potential. Two semicircles are clearly observed in the Nyquist plot. The first semicircle, in the high frequency region, represents Li-ion migration through the solid electrolyte interphase (SEI) layer (RSEI). The second semicircle, in the middle to low frequency region, corresponds to the charge transfer process between the electrode and the electrolyte (Rct). The intercept of Nyquist curves at Z′ axis represents the 21974
DOI: 10.1021/acsami.7b04505 ACS Appl. Mater. Interfaces 2017, 9, 21971−21978
Research Article
ACS Applied Materials & Interfaces
Figure 5. (a) Thermal shrinkage of all the separators stored at different temperatures for 1 h. (b) Photos of all the separators treated at 140 °C, 1 h.
modified separators shrink to less than half of the original size after heating at 140 °C for 1 h. By contrast, the ZrO2-coated separators display outstanding resistance to thermal deformation because the shrinkage of the separator is
