Biofilm Nanofiber-Coated Separators for Dendrite-Free Lithium Metal

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Applications of Polymer, Composite, and Coating Materials

Biofilm Nanofibers-Coated Separator for Dendrite-Free Lithium Metal Anode and Ultrahigh-Rate Lithium Batteries Lu Nie, Yingfeng Li, Shaojie Chen, Ke Li, Yuanqi Huang, Yubo Zhu, Zhetao Sun, Jicong Zhang, Yingjie He, Mengkui Cui, Shicao Wei, Feng Qiu, Chao Zhong, and Wei Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08656 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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ACS Applied Materials & Interfaces

Biofilm Nanofibers-Coated Separator for Dendrite-Free Lithium Metal Anode and Ultrahigh-Rate Lithium Batteries Lu Nie†,#, Yingfeng Li†,‡,§,#, Shaojie Chen†, Ke Li†, Yuanqi Huang†, Yubo Zhu†, Zhetao Sun†, Jicong Zhang†, Yingjie He†, Mengkui Cui†, Shicao Wei†, Feng Qiu ∥ , Chao Zhong†,*, Wei Liu†,* †School

of Physical Science and Technology, ShanghaiTech University, Shanghai

201210, China. ‡Shanghai

Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050,

China. §University ∥School

of Chinese Academy of Sciences, Beijing 100049, China.

of Life Science and Technology, ShanghaiTech University, Shanghai 201210,

China. #These

authors contributed equally.

E-mail: [email protected]; [email protected]

ABSTRACT Rechargeable batteries that combine high energy density with high power density are highly demanded. However, the wide utilization of lithium metal anode is limited by the uncontrollable dendrite growth and the conventional lithium-ion batteries (LIBs) commonly suffer from low rate capability. Here, we for the first time develop a biofilmcoated separator for high-energy and high-power batteries. It reveals that the coating of Escherichia coli protein nanofibers can improve electrolyte wettability, lithium transference number and enhance adhesion between separators and electrodes. Thus, lithium dendrite growth is impeded due to the uniform distribution of Li-ion flux. The modified separator also enables the stable cycling of high-voltage Li|Li1.2Mn0.6Ni0.2O2 cells at an extremely high rate of 20 C, delivering a high specific capacity of 83.1 mAh g-1, which exceeds the conventional counterpart. In addition, the modified separator in 1

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the Li4Ti5O12|Li1.2Mn0.6Ni0.2O2 full cell also exhibits a larger capacity of 68.2 mAh g-1 at 10 C than the uncoated separator of 37.4 mAh g-1. Such remarkably performances of the modified separators arise from the conformal, adhesive and endurable coating of biofilm nanofibers. Our work opens up a new opportunity for protein-based biomaterials in practical application of high-energy and high-power batteries.

KEYWORDS: biofilm nanofibers, separator, lithium dendrite, rate capability, highvoltage cathode

1. INTRODUCTION Lithium-ion batteries (LIBs) have been widely used in mobile electronic devices, because of their high energy density, long cycle life, and environmental friendliness 13.

However, current LIBs cannot meet the increasing demands for energy density and

rate capability in the applications of portable power tools, power backup and hybrid electric vehicles (HEVs) 4. Most reported publications concentrated on improving energy density by the incorporation of high-energy chemistries as electrodes, including sulfur 5, oxygen 6, lithium metal

7

and oxide cathodes with high operating voltage 8.

Nevertheless, LIBs are more keenly suffering from low rate capability due to sluggish kinetic issues associated with the electrolyte resistance, the charge transfer across the electrode/electrolyte interfaces, and the solid-state Li-ion diffusion in electrode. Most proposed approaches on improving rate capability include nanocrystallization of electrode materials 9, decreasing the tortuosity of ion diffusion 10, and manufacturing porous structured electrode

11.

Noticeably, most of the existing strategies require

complex procedures and high cost.

Additionally, less attention has been focused on separator modification, although separators can particularly affect the power density of batteries

12-13.

In general, the

commercial separators are made of polyolefins, such as polyethylene (PE) or 2


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polypropylene (PP)

14.

However, due to their low surface energies and hydrophobic

surface feature, the commercial separators intrinsically have poor electrolyte wettability with conventional polar liquid electrolytes, such as ethylene carbonate (EC) and diethyl carbonate (DEC)

15-16.

Hence, the low electrolyte uptake and retention within the

separators reduce the rate performance and cycling stability of LIBs. A number of surface modifications have been proposed to solve the above problems 17-19, primarily by coating with functional hydrophilic groups, such as hydroxyl and carboxyl

20-23.

Usually, surface modifications based on polymer coatings through solution processes involve several problems such as pore blocking and toxic organic solvents. Hence, environmental-friendly and cost-effective materials have been explored, such as polydopamine 20. Unfortunately, polydopamine is unstable in many organic solvents 24. Therefore, it is urgent to develop endurable and eco-friendly coating materials and processes that can improve the performances of the separator.

Here, we report a modified separator conformally coated with Escherichia coli (E. Coli) biofilm nanofibers (BN) for LIBs with superior high-rate capability. Our strategy was inspired by the universal conformal coating feature of bacterial biofilms 25. Bacterial cells can attach to diverse biotic or abiotic substrates through adhesive extracellular matrix under a variety of harsh conditions 26. In particular, CsgA amyloid nanofibers, the major extracellular protein components, are responsible for the integrity of E. coli biofilms as well as the strong adhesion to various surfaces 27-28. The CsgA nanofibers consist of CsgA monomers, which fold into β-strand structure with five repeating strand-loop-strand motifs and self-assemble into hierarchical nanofibers owing to strong intra- and intermolecular hydrogen bonding within several conservative residues–such as glutamine and asparagine (Figure 1) 29. Previous studies suggest that both structural features and sequence enable the CsgA subunit to adhere strongly to both polar and nonpolar surfaces 25, 30. Specifically, clues to the good affinity with liquid electrolytes on BN may lie in the polar groups, such as –NH2, –OH, and imidazole groups in asparagine, serine, glutamine, tyrosine, aspartic acid and histidine residues. 3



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BN therefore can improve electrolyte wettability and electrolyte uptake. Moreover, BN can stick to electrodes by van der Waals force, like a “zipper” pulling separator with electrodes tightly in the liquid electrolyte, BN can stabilize electrode/electrolyte interfaces and subsequently enhance battery performances (Figure 1).

2. EXPERIMENTAL METHODS 2.1. Material preparation Synthesis of Li1.2Mn0.6Ni0.2O2: The lithium-rich layered oxides Li1.2Mn0.6Ni0.2O2 was synthesized by a sol−gel method. Required amounts of lithium acetate LiCH3COO·2H2O (99.0% pure, Sigma-Aldrich), Mn(CH3COO)2·4H2O (99.0% pure, Sigma-Aldrich) and Ni(CH3COO)2·4H2O (99.0% pure, Sigma-Aldrich) were dissolved in deionized water. The citric acid (Sigma-Aldrich, 99% pure) solution acting as complexing agents, which was slowly added to the metal acetate solution, NH4OH (Sigma-Aldrich, 28−30% in water) were added to form a homogeneous aqueous solution with a pH of ∼7. Then the mixture was heated at 75 °C until a gel was formed, which was then fired at 450 °C for 6 h to remove the organic residues, and finally at 900 °C for 12 h to obtain the final Lithium-rich layered cathode material. All the procedures were carried out in air. Commercial LiFeO4 powder (Titan) and Li4Ti5O12 powder (Titan) was used.

Protein expression and purification: The recombinant plasmid pET-11d/CsgA was transformed in NEB C3016 E. coli. The strain was grown to OD600=1 in LB broth containing 50 mg/mL ampicillin at 37°C. Protein expression was induced with 1mM IPTG at 37°C for 1 h. Cells were collected by centrifugation at 4,000g. Every 5g cell pellet was resuspended and lysed by 50 mL extraction solution (8 M guanidine hydrochloride, in KPI buffer, 300mM NaCl, 50mM K2HPO4/KH2PO4, pH 7.2 pH=8). Lysates were incubated for 24h at room temperature. The insoluble portions of the lysates were removed by centrifuging at 10,000 g for 30 min before incubating with 4


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His-Select Ni-NTA resin for 1 h at room temperature. The mixed solutions were then loaded on the gravity column. Guanidine hydrochloride was further washed away by adding another 20mL KPI buffer. The CsgA proteins were then eluted with 4mL 300mM imidazole KPI buffer after washing with 20mL 40mM imidazole KPI buffer.

Surface modification of separator: Commercial separator (Celgard 2325) was directly immersed in fresh made CsgA monomer (~ 1 mg/mL) solution. After CsgA forming nanofiber coatings, separator was washed by deionized H2O and dried under a stream of nitrogen.

2.2. Material characterization Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE): CsgA protein solutions mixed with loading buffer (Life Technology) were loaded into the lanes of mini gels (Genscript). After running at 165 volts for 50 min, the gels were stained with Coomassie Blue solution (Amresco) for 1 h and then destained with washing solution for 1 h twice. The gels were then imaged using a Bio-Rad ChemiDoc MP system.

Thioflavin T assay (ThT assay): Purified CsgA proteins were loaded on 96-well black plates with transparent bottoms. ThT was added to a concentration of 20 μM. Fluorescence was measured every 5 min after shaking 5 s by a BioTek Synergy H1 Microplate Reader using BioTek GEN5 software set to 438 nm excitation and 495 nm emission with a 475-nm cut off.

Transmission electron microscopy (TEM): 20 mL sample of CsgA nanofiber was dropped onto a carbon coated nickel TEM grid (Zhongjing Keyi). After incubating for 30 s, the sample was rinsed with 30 mL distilled water for three times and then stained 5



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with 2% uranyl acetate (Zhongjing Keyi). TEM images were obtained on a FEI T12 transmission electron microscope operated at 120 kV accelerating voltage.

Circular dichroism (CD): CD spectra for CsgA nanofiber solution were recorded using an Applied Photophysics chirascan spectrometer (Great Britain). Spectra were recorded as an average of 5 to 10 scans from 180 to 260 nm with a scan rate 0.5 step with a response time of 1 s at 25 °C.

Atomic force microscopy (AFM): Uncoated and coated separator were tested by an Asylum MFP-3D-Bio using the tapping mode with AC160TS-R3 cantilevers (Olympus, k ≈ 26 N/m, ν≈300 kHz).

X-ray photoelectron spectroscopy (XPS): XPS spectrum of Uncoated and coated separator were performed on a Thermo Fisher ESCALAB 250 Xi.

Attenuated total reflection-Flourier transformed infrared spectroscopy (ATRFTIR): Uncoated and coated separator were put on the ATR crystal directly. Spectra were recorded from 1,700 to 1,600 cm−1 using a nominal resolution of 2 cm−1 with Spectrum Two™ (PerkinElmer).

X-ray diffraction (XRD): XRD data were collected with a Bruker D8 Advance and Cu Kα radiation in the 2θ range of 10−80° at an interval of 0.02°.

Scanning electron microscopy (SEM): The morphology and particle size of the lithium-rich layered cathode material were investigated with a JEOL JSM-7800F SEM.

6


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Contact angle measurements: The contact angles of the uncoated and coated separator were tested by a contact angle goniometer (SL200KS). The separator is first placed on the stage and droplets of the electrolyte (1 M LiPF6 in a mixture of ethylene carbonate (EC) / diethyl carbonate (DEC) 1:1 by volume) were dropped onto the surfaces of the separators. The volume for each droplet is 5 μL with ten times on different separators to give the final value.

The electrolyte uptake measurements: The electrolyte uptake U (%) was calculated according to the equation31: U=

𝑤2 ― 𝑤1 𝑤1

ⅹ100%

(1)

where w1 and w2 are the weight of the separator before and after absorbing the electrolyte.

Electrochemical tests: The electrode slurry consisted of 80 wt% active materials, 10 wt% Super P, and 10 wt% polyvinylidene fluoride (PVDF) binder in nmethyl-2pyrrolidone (NMP) solvent were casted onto an aluminum foil. The casted cathode was dried under vacuum at 100 °C for 12 h. The cathode disks of 1.13 cm2 area were then punched out to assemble the 2032-type coin cells with a similar active material loaded at 1−2 mg cm−2. The 60 μL volume of electrolyte was carefully controlled. All of the cells were assembled in a glovebox filled with argon. Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical working station (VMP 300, Bio-Logic) at a frequency range of 1 MHz to 100 mHz. The Li|LNMO half cells, the LTO|LNMO full-cells, the Li|LFP half cells were cycled between 2.0 and 4.8 V, 0.5 and 3.3 V, 2.5 and 4.2 V, respectively, and activated for 1 cycle at 0.1 C, then cycled at different current densities. The ionic conductivity measurements were carried out by sandwiching a separator between two stainless steel electrodes. The lithium transference number were test by Li symmetrical cells.

7



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3. RESULTS AND DISCUSSION CsgA proteins were expressed as inclusion bodies in BL21 (DE3) competent E. coli strains and purified following a denaturing protocol described in previous publication 25, 32. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) confirmed the molecular weight (Figure 2a). Moreover, thioflavin T (ThT) fluorescence assay, a dye assay to monitor amyloid protein assembly, indeed revealed that fresh eluted CsgA monomers could self-assemble into amyloid aggregates within 14 hours (Figure 2b). The nanofiber feature with typical high aspect ratio of those aggregates was further confirmed with transmission electron microscopy (Figure 2c). In addition, circular dichroism (CD) spectrum also indicated that the self-assembled nanofibers were rich in β-sheet structures (Figure 2d).

Surface modification of separator was carried out by directly incubating the separator in CsgA protein monomer aqueous solution (0.5 mg mL-1) at ~20 °C for 14 hours. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were applied to assess and compare the morphology of both BN-coated and uncoated separators. No apparent difference in surface morphology between the modified separators and control sample was detected, implying that the nano-feature of fiber coating would not cause pore blocking (Figure 2e and Figure S1). In addition, the cross sectional SEM images of the separator before and after coating process showed almost no changes of thickness, highlighting the conformal and ultra-thin attributes of our coatings (Figure S2).

To confirm the coating effect of BN on the separator, we subjected both the coated- and uncoated separators to aqueous solution containing nitrilotriacetic acid-decorated redemitting CdSeS@ZnS quantum dots (QDs). The BN-coated separator indeed showed brighter red fluorescence compared with unmodified separator under the excitation of ultraviolet light (254 nm), as the CsgA protein coatings containing Hisditine tags could specifically bind the nitrilotriacetic acid-decorated red-emitting CdSeS@ZnS QDs on 8


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the BN-coated sample (Figure 2f). The homogeneous fluorescence in the modified sample indicated that the whole separator was uniformly coated with BN. X-ray photoelectron spectroscopy (XPS) and attenuated total reflection-flourier transformed infrared spectroscopy (ATR-FTIR) analysis further confirmed that the surface signal changes on the BN-coated separator resulted from the protein coatings (Figure 2g and Figure S3). Specifically, the XPS spectra of unmodified separator only showed C1s peak, while the coated separator exhibited extra O1s and N1s originating from BN (Figure 2g). The new peak around 531 eV is attributed to O1s, originating from hydroxyl group, carboxyl group, and acylamino group within CsgA proteins in the BN, while the other newly peak around 400 eV is attributed to N1s, originating from amino group, acylamino group, and imidazole group within CsgA proteins in the BN (Figure 2g and Figure S4). In addition, the strong absorption peak at ~1625 cm-1 and minor shoulder at 1660 cm-1 in ATR-FTIR spectrum of the coated separator consistently indicated the existence of the β-sheet structures arising from CsgA nanofibers (Figure S3)

33.

Taken together, the above results confirmed that CsgA nanofibers could

uniformly coat the separator.

Furthermore, Quantitative nanoscale mechanical AFM characterization (Figure S5) demonstrated that the BN coating significantly increased the adhesion of separator from 6.90 ± 2.53 nN to 11.99 ± 3.46 nN. And, the separators after 120 cycles in half cells have also been collected for XPS chacterization and analysis, revealing that BN can be survived with high durability in the high-voltage LIBs after long cycling (Figure 2g). Hence, the tight attachment of CsgA proteins on the separator could guarantee a good cycling performance.

The separator’s affinity towards liquid electrolyte directly affects the performance of LIBs. Contact angle measurements with liquid carbonate electrolytes on separators were thus conducted. As shown in Figure 2h, the contact angle largely decreased from 9



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65.8° ± 1.2° to 13.0° ± 1.1° by coating BN, suggesting that the presence of CsgA proteins indeed significantly improved the wettability of liquid electrolyte and thus its facile penetration into the porous separators. To quantify the electrolyte wettability, we measured the electrolyte uptake of the separators. The uptake amount increased from 113.42% ± 6.52% to 184.04% ± 9.48% (Table S1 and Figure S6) with the BN coating, owing to the presence of a large number of polar groups on the surface that are known for their good affinity with liquid electrolyte.

In order to confirm the improved electrical performances of the separators by BN coating, the ionic conductivity was calculated using the below equation 34-35: L

(2)

σ = Rb ⋅ A

where 𝜎 is the ionic conductivity, Rb is the bulk resistance, L is the thickness of the separator and A is the area of the copper electrode (because the area of the separator and the steel electrode are larger than that of the copper electrode). The ionic conductivity for the modified separator was increased from 8.6 × 10-4 S cm-1 to 1.3 × 10-3 S cm-1 due to the biofilm nanofibers coatings. In addition, lithium transference number (t+) was also tested and calculated according to the equation 36-37: 𝐼𝑆𝑆(Δ𝑉 ― 𝐼0𝑅0)

𝑡 + = 𝐼0(Δ𝑉 ― 𝐼𝑆𝑆𝑅𝑆𝑆)

(3)

where 𝛥𝑉 is the potential applied across the cells, 𝐼0 and 𝐼𝑆𝑆 are the initial and steady state currents value, and 𝑅0 and 𝑅𝑆𝑆 are the initial and steady-state resistance of the passivation layer. The change of current with time and the impedance spectrum before and after polarization for the modified separator are shown in Figure S7. t+ for the separator was increased from 0.32 to 0.49 by biofilm coating, which is mainly due to amino groups within the coatings hinders the migration of anions. Previous studies38 have confirmed that amide groups could interact with anions, fascinating ion pair dissociation, thus the transfere of Li+ ion is promoted. Furthermore, the interaction between nitrogen or oxygen and Li+ ion may facilitate the ion pair dissociation between anions and Li-ions. Therefore, Li-ions tend to be released from original anion pairs. 10


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The Li transference number of the control separator is in good agreement with the literature values 39. Theoretically, the polar functional groups presented on the separator comes from side chains of typical amino acids, including hydroxyl group in serine, threonine, and tyrosine, carboxyl group in aspartic acid and glutamic acid, amino group in lysine and arginine, acylamino group in asparagine and glutamine, imidazole group in histidine (Figure S4). All these groups within BN make separator more hydrophilic, improving the wettability of electrolyte. And, amino groups could enhance the Li transference number. Collectively, these results suggest that the BN coating could improve energy density and power performance of LIBs.

Next, the electrochemical measurements of Li symmetrical cells have been carried out. Electrochemical impedance spectra (EIS) are shown in Figure 3a. The electrolyte resistance and interface resistance is smaller for the modified separator compared with the control separator. In addition, Figure 3c indicates that the modified separator render a reduced polarization and stable cycling compared to the control separator with increased polarization under the current density of 1.0 mA cm−2 at the deposited capacity of 1.0 mAh cm−2. At a current density of 3.0 mA cm−2, the difference in polarization is more significant (Figure S8). After about 400 cycles, a short circuit occurs within the cell using unmodified separator due to the dendrite piercing. In contrast, the cell based on BN has much longer cycling life. In addition, we also provide the Li|Cu cells by comparing their Coulombic efficiencies (CEs) at different current densities (Figure S9). The Coulombic efficiencies of the modified separator remained 87.4% and 84.1% at the current density of 1.0 mA cm-2, 3.0 mA cm-2 after 70 cycles. In comparison, the control separator had a fluctuant CE of only 85.3% and 40.2% at the same cycles. The SEM images (Figure 3b) indicate that the surface of the deposited lithium metal using coated separator is smoother and denser than the unmodified one. The high electrolyte wettability and uptake of the modified separator could improve electrolyte conductivity, which results in reduced voltage hysteresis. Furthermore, the adhesive feature of the BN coating with separator and electrode could also provide a 11



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uniform distribution of Li-ion flux at the electrolyte/electrode interface and therefore prevent Li dendrite growth.

In order to evaluate the influence of the surface modification for the separator, the electrochemical performance tests of Li|Li1.2Mn0.6Ni0.2O2 (LNMO) half-cells are conducted at the voltage range of 2.0-4.8 V. We choose LNMO as a cathode material because it can endure high voltage and possess remarkable properties related to high specific capacities, good safety and low cost 40-42. The X-ray diffraction (XRD) of the Li-rich layered oxides was carried out in Figure S11. The main patterns could be well indexed on α-NaFeO2 layered structure with the space group R3m, while the weak superstructure reflections at 20-23°correspond to the Li2MnO3 component (C/2m symmetry), indicating cation ordering in the transition-metal layer. The morphology and size of the layered oxides particles are nearly the same (100–300 nm), as revealed by the SEM images (Figure S12). The Nyquist plots with equivalent circuit are shown in Figure 4a. The first intersection with x-axis at high frequency represents the resistance of electrolyte (R1) and the semicircle can be ascribed to the electrode/electrolyte interface resistance (R2), and the sloping line is related to Li+ diffusion in electrodes (Wo). Obviously, the electrolyte resistance is smaller for the modified separator, which is ascribed to improved electrolyte wettability by BN coating. Additionally, the modified separator shows reduced interface resistance compared with the control separator, due to the adhesive ability of the modified separator that enables better contact between the electrode and the electrolyte 43.

Figure 4b exhibits the first charge/discharge curves of the half-cells at current densities of C/10 (25 mA g−1), which indicate the typical electrochemical behavior of the Li-rich layered cathode material. The slope potential region below 4.5 V correspond to the extraction of Li+ from the layered oxide structure, which accompanied by Ni2+ oxidation, and the peaks above 4.5 V represent the oxygen loss from Li2MnO3 component, which 12


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leads to a serious initial irreversible capacity loss 44-48. The modified separator shows a higher discharge capacity of 235.9 mAh g-1 at 0.1 C than the control separator (224.5 mAh g-1). The charge/discharge curves of the half-cells at different cycles after the first cycle activation (Figure S14) indicate that the voltage drops for the modified separator is obviously less than that of the control separator. In addition, the modified separator could achieve a higher discharge specific capacity of 143.3 mAh g−1 after 50 cycles, compared with the control separator with 131.4 mAh g−1.

Furthermore, the rate-performances for modified separator have been improved compared with control separator (Figure 4c). The cell was initially activated for 1 cycle under a C/10 rate, and then cycled at 1 C, 3 C, 6 C, 9 C, 12 C, 15 C, 20 C and back to 1 C for 5 cycles per step. The specific capacity of the LNMO cathode using modified separator and the control separator at 1 C is 166.3 mAh g-1 and 156.4 mAh g-1, respectively. As the C-rate increases, the difference between their discharge specific capacities becomes more obvious, the modified separators deliver a higher capacity retention of C-rate compared with the control separator (Figure S15). It should be noted that at an extremely high rate of 20 C, the modified separator delivered a high specific capacity of 83.1 mAh g-1 which exceeds the conventional separator (62.7 mAh g-1). In addition, after 135 cycles, the modified separators deliver a much higher specific capacity of 126.9 mAh g-1 with higher capacity retention compared with the control separator with 108.8 mAh g-1. Furthermore, the electrochemical performance in terms of specific capacity at high current for the modified separator is compared with the data related to Li-rich layered cathode materials in the most recently reported publications4954

(Figure 4d), which demonstrates improved rate performance by the use of biofilm

coating.

To further verify the universality of BN coatings for various separators, the electrochemical behavior of the Li|LiFePO4 half cell is also carried out. showing 13



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improved performances with the use of modified separator (Figure S16). We also assembled the full cells of LFP coupled with a bare copper current collector to study lithium retention rates (Figure S10). It can be seen that the cells based on the biofilm treated separator deliver a discharge capacity of 101.3 mAh g-1, which is higher than that using control separator (96.6 mAh g-1). It also can be seen that the specific capacity, capacity retention and Coulombic efficiency of modified separator have been improved compared with control separator.

In order to further evaluate the BN modification effect of the separator on the electrochemical properties particularly the rate capability, the Li4Ti5O12 (LTO)|LNMO full cells have been studied. As shown in Figure 5a, the BN-coated separator showed much smaller electrolyte and interface resistance. The charge/discharge curve (Figure 5b) indicates a higher discharge capacity of the modified separator (146.5 mAh g-1) compared to the control separator (128.5 mAh g-1) at 1 C after being activated by a cycle of low-rate (0.1 C). The rate performance of the full cell is shown in Figure 5c. The modified separators show much improved capacities of 146.5 mAh g-1 at 1 C and 68.2 mAh g-1 at 10 C, while 133.1 mAh g-1 and 41.6 mAh g-1 were obtained for the control separator. At the same time, the modified separators also exhibit superior cycle performance after the rate cycle.

In addition, the diffusion coefficient of lithium ion (𝐷𝐿𝑖 + ) can be calculated according to the equation below 55-56: DLi + = R2T2/2A2n4F4C2α2

(4)

where R is the gas constant, T is the absolute temperature, A is the surface area, n is the number of electron per molecule oxidized, F is Faraday’s constant, C is the concentration, and α is the Warburg factor which is the slope of linear fitting of Zre vs. ω-1/2 (Figure S13). The diffusion coefficients of Li ion for the modified separator and control separator can be calculated and the values are 1.5 × 10-16 and 6.8 × 10-17 cm2 14


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s-1, respectively. The diffusion coefficient of the control separator is in good accordance with the literature values 57. 𝐷𝐿𝑖 + of the modified separator is 2 times higher than that of the control separator, indicating that the improved electrode kinetics with the introduction of biofilm coatings. The enhanced rate capability and cycling performance of the modified separator mainly arise from the improved electrolyte wettability and uptake, ionic conductivity and Li transference number. Moreover, the adhesive ability of the BN coating with separator and electrodes may guarantee a stable electrode/electrolyte interface. Last but not least, the strong endurable ability of BN in batteries after long cycles also contributes to the good cycling performance.

4. CONCLUSIONS In summary, inspired by the adhesive extracellular biofilms for attachment to diverse biotic or abiotic substrates, we developed an advanced separator for LIBs by coating conventional separator surface with CsgA nanofibers. Owing to the hydrophobic feature of the coating and the presence of polar groups on its surface, the modified separators exhibit improved electrolyte wettability, ionic conductivity and Li transference number. In addition, as a “zipper” at the electrode/separator interfaces, the adhesive and endurable BN coating can guarantee the stable electrode/electrolyte interfaces. As a result, the suppression of lithium dendrite growth and longer stable cycling could be achieved in Li symmetrical cells. The half-cell and full-cell tests both indicate that the cells using modified separator deliver higher specific capacity, rate capability and stable cycling than control cells. Thus, the surface modification of separator via biofilm provides a simple and feasible method to improve rateperformance for high-energy batteries.

Supporting Information Experimental section including details about electrolyte uptake (Table S1), separator characterization (Figures S1-S3, Figures S5-S6), amino acid sequence polar group 15



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analysis of CsgA monomer protein (Figures S4); the measurement of lithium transference number and diffusion coefficient (Figures S7, Figures S13); the electrochemical performances of Li symmetrical cells (Figures S8), Li|Cu cells (Figures S9), Cu|LFP full cells (Figures S10), LNMO characterization (Figures S11-S12), Li|LNMO half cells (Figures S14-S15), and Li|LFP half cells (Figures S15-S16).

ACKNOWLEDGEMENTS This work was partially supported by ShanghaiTech University start-up funding and National Natural Science Foundations of China (21805185) for Wei Liu. This work was also partially sponsored by the Commission for Science and Technology of Shanghai Municipality (17JC1403900) and National Natural Science Foundations of China (31570972) for Chao Zhong. We acknowledge also CℏEM, SPST of ShanghaiTech University (#EM02161943) for support.

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Figure 1. Schematic illustration of CsgA protein modified separator for dendrite-free Li metal anode and high-rate LIBs. The CsgA coating can improve the electrolyte wettability of separator. As a “zipper” to pull separator and electrode tightly in the liquid electrolyte, the CsgA coating can also enhance adhesion between the separators and electrodes, resulting in stable electrode/electrolyte interfaces.

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Figure 2. Characterization of E. coli biofilm adhesive CsgA proteins and bacterial BNcoated separator. (a) SDA-PAGE showing CsgA monomer (left line) and protein ladder (right line). (b) ThT fluorescence curve during CsgA self-assembly process. (c) TEM image of self-assembled BNs. (d) CD spectra of BNs. (e) SEM images of uncoated separator (top) and BN-coated separator (bottom). (f) Digital images of uncoated separator (left) and BNcoated separator (right) after incubating with CdSeS@ZnS QDs solution under UV light. (g) XPS spectra of uncoated separator, BN-coated separator before and after cycling. (h) Electrolyte contact angles of uncoated separator (top) and BN-coated separator (bottom).

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Figure 3. Electrochemical performances of Li symmetrical cells using the separators with and without BN coating. (a) Nyquist plots for Li symmetrical cells. The inset shows the same plots but with a smaller scale. (b) SEM images of the Li metal surface after 5 cycles. (c) The cycling performances of Li symmetrical cells at 1.0 mA cm−2.

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Figure 4. Electrochemical performances of Li|LNMO half cells using the separators with and without BN coating. (a) Nyquist plots for the half cells. The inset shows the same plots but with a smaller scale. (b) Voltage profiles of the half cells during the first cycle at a rate of 0.1 C. (c) Rate capability and long cycling performance of the half cells, all cells were activated by a cycle of low-rate charge/discharge at 0.1 C. (d) The comparison of specific capacity at different C-rate from the present work and published results.

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Figure 5. Electrochemical performances of LTO|LNMO full-cells using the separators with and without BN coating. (a) Nyquist plots for the full cells. (b) Voltage profiles of the full cells using the separators at 1 C. (c) Rate capability and long cycling performance of the half-cells, all cells were activated by a cycle of low-rate charge/discharge at 0.1 C.

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The table of contents (TOC)

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Biofilm Nanofiber-Coated Separators for Dendrite-Free Lithium Metal

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