High-strength Internal Crosslinking Bacterial Cellulose Network Based

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Energy, Environmental, and Catalysis Applications

High-strength Internal Crosslinking Bacterial Cellulose Network Based Gel Polymer Electrolyte for Dendrite-suppressing and High-rate Lithium Batteries† Dong Xu, Bangrun Wang, Qing Wang, Sui Gu, Wenwen Li, Jun Jin, Chunhua Chen, and Zhaoyin Wen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00034 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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ACS Applied Materials & Interfaces

High-strength Internal Crosslinking Bacterial Cellulose Network Based Gel Polymer Electrolyte for Dendrite-suppressing and High-rate Lithium Batteries Dong Xua,b, Bangrun Wang a,b, Qing Wanga,b, Sui Gua,b, Wenwen Li a,b, Jun Jina, Chunhua Chenc, and Zhaoyin Wena,* a. CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China. b. University of Chinese Academy of Sciences, Beijing 100049, P. R. China c. CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, Hefei, Anhui, 230026, P. R. China. *. Corresponding author Tel: +86-21-52411704, Fax: +86-21-52413903 E-mail: [email protected].

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Keywords: gel polymer electrolyte, bacterial cellulose, internal crosslinking, lithium battery, dendrite suppressed

Abstract Lithium is a promising anode material for high energy density batteries. However, the growth of lithium dendrite causes serious safety issues, which inhibits the application of lithium anode. Herein, a novel gel polymer electrolyte based on high-strength internal crosslinking bacterial cellulose network was prepared via an environmentally friendly and simple fast freeze-drying method. The as-obtained gel polymer electrolyte demonstrates an excellent lithium ion conductivity of 4.04×10-3 S cm-1 with an exceptional lithium ion transference number of 0.514 at 25 ˚C. The lithium metal battery with this gel polymer electrolyte shows an initial reversible capacity of 141.2 mA h g-1 with a capacity retention of 104.2% (compared with the initial reversible capacity) after 150 cycles at 0.5 C. An average reversible capacity of 75.2 mA h g-1 is maintained at high rate of 9 C. Moreover, this gel polymer electrolyte possesses superior mechanical strength of 49.9 MPa with a maximum strain of 56.33%. Therefore, the vertical growth of lithium dendrite is effectively suppressed. This research indicates the potential of applying low cost bacterial cellulose into high performance energy storage devices.

1. Introduction Developing new energy transportations, such as electric vehicles, is regarded as one of the most useful strategies to solve the problems of climate change and pollution caused by the using of non-renewable power resources.1 Most currently, the challenge is to improve the energy density of the batteries which work as the power source of electric vehicles.2-5 Lithium metal has an unparalleled high specific capacity (3860 mA h g-1) and an exceedingly low redox potential (-


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3.04 V vs standard hydrogen electrode).6-7 Thus, lithium is considered to be an ideal anode material to meet the increasing demands of high energy density batteries, aiming at the application in electronic devices and electric vehicles.8 Unfortunately, the practical application of lithium anode has been prohibited by the formation and inevitable growth of lithium dendrites during repetitive charge/discharge cycles. 6, 9-11 The lithium dendrites ultimately transfix through the separator, resulting in internal short-circuit of the battery, thereby leading to serious safety issues, such as thermal runaway and explosion. 6, 12-14 Substituting a solid polymer electrolyte (SPE) for the reactive liquid electrolyte has been proved to be an effective method to suppress the growth of lithium dendrite, which is attributed to the less reactive with lithium and good mechanical strength of SPE.15-18 However, the limited ionic conductivity of SPE at room temperature and the high interfacial resistance between SPE and the electrodes hinder its further application.18-21 On the contrary, a gel polymer electrolyte (GPE) always exists higher ionic conductivity at room temperature and better interfacial compatibility.22-27 Disappointingly, the mechanical strength of GPE is too poor to suppress the lithium dendrite.22-23, 28 Due to the strong interaction of ether groups (-C-O-C-) with lithium ions and electrolyte solvents, Poly (ethylene oxide) (PEO) and its derivatives are widely researched as the host of GPEs.17,

29

Nevertheless, the GPEs plasticized by the organic solvents lose their

mechanical strength easily, resulting in short-circuit of the batteries.

17, 30

Recently some reports

illustrate that crosslinked network built by electrospinning technology improved the mechanical properties and thermal stability of GPEs.31-33 Whereas, the low production rate and high cost restrict the further commercial application of electrospinning method. Wan et al. successfully improved the strength of GPE through a supercritical drying method, while the using of expensive ionic liquid as solvent is not feasible.34 Therefore, further researches shall be done,


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concentrating on obtaining a crosslinking network strengthened GPE with high ionic conductivity, strong interfacial adhesion and good mechanical properties through a facile and cost-effective method. Moreover, it would be much appreciated if the GPE is prepared with ecofriendly materials and environmentally harmless process, since environmental problem has been a central issue in the world.35-38 Focusing on our lives, it is easy to find numerous materials with ether groups (-C-O-C-) similar to PEO, for instance, the polysaccharides. Bacterial cellulose (BC), the metabolic product of bacteria,39 is widely used in food industry,40 medical science,41 paper industry,42 cosmetics industry43 and so on. It is made up of glucose monomers connected by glycosidic bonds, forming a linear chain structure.44 The chemical structure enables BC to dissociate the lithium slats, catch the electrolyte solvents, and enhance the transmit of ions.17,

45-47

Besides, the BC chains

assembled together to form nanofiber structure, which remarkably improves the mechanical strength of BC films.48 Taking all these characteristics of BC into consideration, we became interested in employing BC as the skeleton of GPE. In this work, we prepared a BC-based gel polymer electrolyte (BC-GPE) with high-strength internal crosslinking network derived from commercial BC through a simple and environmentally friendly method without using any toxic or expensive solvent. This BC-GPE demonstrates excellent ionic conductivity at room temperature, outstanding mechanical strength and good thermal stability. In this BC-GPE, the internal crosslinking structure of BC nanofibers greatly enhances the mechanical properties of the BC-GPE. Meantime, the linked glucose monomers guarantee the electrolyte absorbent capacity and provide abundant ionic passageways. The lithium metal battery based on this BCGPE exhibits superior electrochemical and safety performance, indicating the great possibility of using low cost bacterial cellulose in energy storage materials and devices.


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2. Experiment section 2.1 Materials The commercial BC films were purchased from Hainan Yeguo Food Co., Ltd. Anhydrous ethanol was from Sinopharm Chemical Reagent Co., Ltd. To prepare the liquid electrolyte, 1.0 M lithium hexafluorophosphate (LiPF6) was dissolved in ethylene carbonate (EC) / dimethyl carbonate (DMC) (1:1 by volume, Suzhou Fosai New Material Co., Ltd.). Commercial separators (Celgard 2320, PP/PE/PP, thickness: 20 µm) purchased from Celgard company were used for comparison. Commercial LiFePO4 without carbon coating layer was purchased from Tianjin Silante Energy Technology Co., Ltd. 2.2 Preparation of BC-GPE The BC-GPE was prepared through an eco-friendly fast freeze-drying method without involving any toxic or costly solvent，as shown in Figure 1. First, the prewashed BC films (4 cm×4 cm) were rehydrated by boiling in deionized water for 1 h. And the rehydrated BC films were immersed in anhydrous ethanol for 2 days before repeating the boiling treatment. The treated BC films were quickly frozen at -45 ˚C to -42 ˚C for 30 min before freeze-drying for 24 h. The freeze-dried BC films were punched into round pieces with a diameter of 18mm, and the thickness is about 120 µm, named BC-based gel polymer electrolyte skeleton (BC-GPES). The BC-GPESs were transferred to an argon filled glovebox (moisture level < 0.1 ppm) after dried in vacuum oven at 60 ˚C for 24 h. The BC-GPE was obtained by adding 50 µL of 1 M LiPF6/EC: DMC (1:1 by volume) onto the BC-GPES. The electrochemical measurements were performed


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after desired gelation time. For the sake of controlling the variable, the same amount of liquid electrolyte was also used in the comparisons.

Figure 1 Schematic of the preparation process of BC-GPE. 2.3 Characterization The surface morphologies of BC-GPES, BC-GPE, dried BC-GPE and lithium surface were characterized by scanning electron microscope (SEM, HITACHI S-3400 N). The detailed structure of dried BC-GPE was detected by field emission scanning electron microscope (FESEM, ZEISS SUPRA 55 SAPPHIRE) and transmission electron microscopy (TEM, JEOL JEM-2100F). The chemical components of BC-GPES were examined by Fourier infrared spectroscopy (FTIR, Bruker TENSOR 27). Raman spectra were collected using a laser with an excitation wavelength of 532 nm at laser power of 7 mW (Thermal Scientific Corporation). The thermal properties of BC-GPES and commercial separator were measured by thermal gravimetric analysis (TGA, Netzsch STA 409 PC,) from room temperature up to 500 ˚C at a heating rate of 10 ˚C per minute under air atmosphere. Stress-strain tests were done to measure the mechanical properties of BC-GPE, BC-GPES and commercial separator with universal testing system (Instron 3366) at a tensile speed of 1 mm s-1.


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The BC-GPE was exposed in argon filed glovebox for 90min to measure the electrolyte retention. The weights of BC-GPE before and after the exposure were measured to show the liquid electrolyte conservation rate (φ), φ was calculated by eq 1: φ%=

WBC-GPE -WBC-GPE` ×100% WBC-GPE

(1)

where WBC-GPE and WBC-GPE` are the mass of the BC-GPE before and after the exposure, respectively. The ionic conductivities of BC-GPE and liquid electrolyte immersed commercial separator (LE-S) were measured by AC impedance spectroscopy, using impedance/gain-phase analyzer (SI1260, Solartron) with a signal amplitude of 10 mV and the frequency from 7 MHz to 10 Hz at temperatures between 20 ˚C and 70 ˚C. BC-GPE/LE-S was sealed between two stainless steel (SS) plates to assemble a blocking cell for the ionic conductivity measurements. The ionic conductivities were calculated by eq 2:15 δ=

L Rb S

(2)

where δ is the ionic conductivity, Rb represents the bulk resistance, L and S are the thickness and area of the electrolyte film, respectively. The technology chronoamperometry (CA) with a polarization voltage of 10 mV was applied to Li /electrolyte/Li symmetrical cells to measure the lithium-ion transference number (t+) of BCGPE and LE-S. The AC impedance plots of Li /electrolyte/Li symmetrical cells before and after polarization were measured under a signal amplitude of 10 mV and the frequency range from 1 MHz to 10 Hz. The t+ was calculated with eq 3:49 t+ =

Is V-I0 R0 I0 V-Is Rs

(3)


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where I0 and Is are the initial current and the steady state current, respectively; V is the voltage applied to the symmetrical cell; and R0 and Rs are the interfacial resistance before and after polarization, respectively. Linear sweep voltammetry (LSV) was applied to measure the electrochemical window of BCGPE and LE-S. SS was used as working electrode and lithium metal as reference, the testing was done at a scan rate of 10 mV s-1 with potential voltage gradually changing from 3 V to 5.5 V. Electrochemical impedance spectroscopy (EIS) of the batteries was measured with an autolab (Metrohm AUT85167) at the frequency from 1 MHz to 0.1 Hz. LiFePO4, carbon black (Super-P), VGCF and PVDF were mixed at a ratio of 80:5:5:10 based on weight. LiFePO4 was used as the active material in the cathode, carbon black and VGCF were used to improve the electric conductivity, PVDF was chosen as binder. The mixture was casted on Al foil to prepare the cathode. The charge/discharge capacity of the batteries with the BC-GPE and LE-S were measured by Land battery tester (CT2001A Wuhan Land Electronic Co., Ltd.) in the potential range of 2.5-4.0 V. The cells were disassembled in argon filled glovebox after planned testing cycles. And the surface morphologies of the lithium anodes were shown by SEM. 3. Results and discussion The surface morphologies of BC-GPES, BC-GPE and dried BC-GPE were measured by SEM, as shown in Figure 2a-c. Furthermore, the detailed structure of dried BC-GPE was measured by Field Emission Scanning Electron Microscopy (FESEM) and transmission electron microscopy (TEM), as shown in Figure 2d and 2e, respectively. It can be seen in Figure 2a that the homogenous BC nanofibers interweave randomly to build up a porous and crosslinking structure, which is beneficial for the gelation process and the mechanical properties, while the SEM image of oven-dried BC film shows a relatively dense and ruptured morphology (Figure S1 Supporting


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information), indicating that the freeze-drying method is advantageous to the gelation and mechanical properties of the prepared BC-GPE. The SEM image of BC-GPE shows a relatively smooth and compact surface, resulting from the gelation of BC nanofibers (Figure 2b), which is beneficial to the liquid electrolyte conservation rate and the contact between BC-GPE and the electrodes. The BC-GPE was then dried for further analysis. As can be seen in Figure 2c, the dried BC-GPE shows similar crosslinking structure as BC-GPES, indicating that the crosslinking network structure of BC-GPES remains intact in BC-GPE, which ensures the superior mechanical strength of BC-GPE. The FESEM image (Figure 2d) shows that the diameter of BC nanofibers is about 20-40 nm. This structure is similar to that of the membranes prepared by electrospinning,31 while the BC nanofibers interconnect together to form an internal crosslinking structure, which is different from the simple stacking of nanofibers in electrospinning.50 TEM was applied to further figure out the internal crosslinking structure in BC-GPE, as shown in Figure 2e. It is clear that each BC nanofiber is made up of several secondary BC nanofibers. Besides, each of the secondary BC nanofibers interests in different BC nanofibers, which makes the BC nanofibers connected internally, as simulated with the read dotted lines. The specific interior crosslinking network structure of this BC-GPE consequently endows the gel polymer electrolyte with superb mechanical strength, which is expected to effectively suppress the vertical growth of lithium dendrite. Moreover, our preparation method is much more environmentally friendly and economical in comparison with other methods.


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Figure 2 SEM image of (a) BC-GPES, (b) BC-GPE and (c) Dried BC-GPE; (d) FESEM image of dried BC-GPE; (e) TEM image of dried BC-GPE; (f) FTIR of BC-GPES. The chemical components of BC-GPES were characterized with FTIR spectroscopy, as shown in Figure 2f. The broad absorption peak at 3415 cm-1 refers to the stretching vibration of -OH groups on the glucoses. The peaks at 2923 cm-1 and 2858 cm-1 represent the stretching vibration of C-H (including CH2 and CH3). The absorption band at 1625 cm-1 is corresponding to the ring stretching of C-C. The absorption peaks at 1421 cm-1 and 1379 cm-1 are due to the bending vibration of CH2 and CH respectively. The peak at 1095 cm-1 is caused by the stretching vibration of C-O. All the peaks in the FTIR spectroscopy are consistent with the absorption peaks of cellulose, thus the BC-GPES is composed of pure cellulose.51


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Figure 3 Optical images of BC-GPES and commercial separator (a) before and (b) after treated at 150˚C for 1h; (c) DSC curves of BC-GPES and commercial separator obtained in air atmosphere in the temperature range of 80˚C to 200 ˚C at a rate of 10 ˚C min-1; (d) Stress-strain curves of BC-GPE dried BC-GPE and commercial separator tested with a tensile rate of 1 mm s-1. The heating experiments of BC-GPES and commercial separator at 150 ˚C are shown in Figure 3a and 3b. The BC-GPES keeps intact, and the change of its diameter is ignorable, while the commercial separator suffers obviously shrinkage and becomes transparent after treating at 150 ˚ C for 1 h. The corresponding differential scanning calorimeter (DSC) was applied to further investigate the thermal processes during the heating experiments. As can be seen in Figure 3c, the endothermic peak Tm (166.2˚) is caused by the melting of polyethylene and polypropylene in


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commercial separator.34 Contrarily, no melting peak is found in the DSC curve of BC-GPES. The results demonstrate the higher thermal stability of BC-GPES in comparison with commercial separator. Moreover, the endothermic peak (Ts2 =99.9˚) of the evaporation of water absorbed in BC-GPES is slightly higher than that of commercial separator (Ts1=93.0˚), which further proves the higher thermal resistance of BC-GPES. The thermogravimetric analysis (TGA) is shown in Figure S2 (Supporting information), the commercial separator begins to lose weight at 200 ˚C, and a sharp weight losing is observed between 220 ˚C and 300 ˚C. Conversely, the BC-GPES shows better thermal stability, the weight loss of BC-GPE can be observed after 254 ˚C, which is higher than that of commercial separator (200 ˚C). This is related to the heat resistance and the special crosslinking network structure of BC nanofibers.52 The good thermal stability of BCGPES is beneficial for the safety of lithium metal batteries. The main disadvantage of GPEs is the poor mechanical properties, which greatly restricts the practical application of GPEs. 22-23, 28 Figure 3d displays the stress-strain curves of BC-GPES, BC-GPE and commercial separator. The commercial separator exhibits a tensile strength of 8.5 MPa with a maximum strain of 44.66%, while the BC-GPE demonstrates a tensile strength of 49.9 MPa with a maximum strain of 56.33%, which is much higher than other types of gel polymer electrolytes (Table S1 Supporting information). Thus, the BC-GPE owns higher hardness and better toughness simultaneously compared with the commercial separator. The BCGPE also demonstrates good bending properties (Figure S3 Supporting information). The excellent mechanical properties of BC-GPE are mainly owing to the internal crosslinking network of BC nanofibers, since the BC-GPES shows a high tensile strength of 39.06 MPa. Besides, the gelatination of BC nanofibers ensures superb toughness. This outstanding


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mechanical endurance is beneficial to reducing the possibility of short circuit caused by lithium dendrite and to improving the safety of batteries.

Figure 4 (a) The ionic conductivity of BC-GPE as a function of gelation time; (b) The ionic conductivities of BC-GPE and LE-S as a function of temperature; (c) The Raman spectra of BCGPE and liquid electrolyte; (d) The schematic illustration of the interactions between BC chains and the solvents. The ionic conductivity, as a basic property, greatly influences the electrochemical performance of GPEs. The ionic conductivities of BC-GPEs after different gelatinization time were measured. As Figure 3a shows, without adding liquid electrolyte, the BC-GPE only has a low ionic conductivity of 2.35×10-8 S cm-1, but the ionic conductivity immediately rises to


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2.48×10-3 S cm-1 after gelation for 5 min. With the increasing of gelation time, the ionic conductivity keeps the same order of magnitude. This result demonstrates that the BC-GPE has a strong adsorption to the liquid electrolyte. The BC-GPEs were gelatinized for 1h to ensure the complete gelation before the subsequent electrochemical measurements. The electrolyte conservation rates of both BC-GPE and LE-S were measured, as shown in Table S2 Supporting information. The electrolyte conservation rate of BC-GPE is 93.32% after exposed in glove box for 90min. Contrarily, the inherent hydrophobic feature of the commercial separator brings a quick evaporation of electrolytes in the separator, resulting in a conservation rate of only 21.29%. Table 1 The relative intensity of the scattering peaks of the Raman spectra for BC-GPE and liquid electrolyte. Electrolyte

Intensity (a. u) 716 cm-1 726 cm-1

741 cm-1

893 cm-1

902 cm-1 915 cm-1

936 cm-1

Liquid electrolyte

0.6783

0.2257

0.3195

1

0.8099

0.4386

0.1449

BC-GPE

0.4629

0.0332

0.2550

1

0.7945

0.0581

0.0262

The Raman measurement was applied to investigate the interaction between the BC chains and liquid electrolyte. As is shown in Figure 3c, the scattering peaks at 716 cm-1 and 726 cm-1 refer to the ring bending modes of free EC and Li+ associated EC (Li+ - EC) respectively; the peak at 741 cm-1 is caused by the symmetric vibration of free PF6-; the peaks at 893 cm-1 and 902 cm-1 represent the ring breathing of free EC and Li+ - EC respectively; the peaks at 915 cm-1 and 936 cm-1 are assigned to the free DMC and Li+ associated DMC (Li+ - DMC); No peak of Li+


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associated PF6- at 951 cm-1 is found in both of the BC-GPE and liquid electrolyte, indicating LiPF6 is completely dissolved in both electrolytes. As can be seen in Table 1, the BC-GPE has a relatively lower intensity peak at 716 cm-1 for free EC than the liquid electrolyte. Besides, the peak (915 cm-1) for free DMC is almost disappeared. This is related to the good affinity between the -OH groups, H atoms and the organic solvent. The solvent molecules are captured in the BC network consequently, which is beneficial to the electrolyte retention ability of BC-GPE.53 The schematic illustration of the interactions between BC chains and the solvents is shown in Figure 4d. Moreover, the peaks for Li+ - EC (726 cm-1) and Li+ - DMC (936 cm-1) in BC-GPE are also suppressed in comparison with that of the liquid electrolyte, which is owing to the strong interactions between the functional -OH groups, ether groups (EO), glycosidic bonds on the BC chains and the lithium ions. The gelatinized BC chains work as solvent to improve the dissociation of lithium salt and motivation of lithium ions.45, 47, 54 The temperature dependence of the conductivities of BC-GPE and LE-S were tested in the range of 20 ˚C to 70 ˚C (Figure 3b). Unsurprisingly, the ionic conductivities of both BC-GPE and LE-S increase along with the rising of temperature, which is in consistent with the Arrhenius formula.55 The BC-GPE has a much higher ionic conductivity (4.04×10-3 S cm-1 at 25 ˚C) than that of LE-S (6.31×10-4 S cm-1 at 25 ˚C). The functional -OH groups, ether groups (EO) and glycosidic bonds on the gelatinized BC chains work as solvent to improve the transfer of lithium ions.45, 47, 54 Moreover, the movements of the numerous ether groups, glycosidic groups and -OH groups and liquid solvents are intensified at higher temperature, generating a higher ionic conductivity. This structure endows BC-GPE a higher ionic conductivity compared with LE-S in the whole temperature range.


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Figure 5 (a) Chronoamperometry of the Li/BC-GPE/Li battery at a potential step of 10 mV at 25˚C. Inset: the AC impedance spectra of the battery before and after polarization for 5500s; (b) Linear sweep voltammetry of SS/BC-GPE/Li and SS/LE-S/Li at a scan rate of 10 mV s-1 from 3.0 V to 5.5 V. Another significant parameter of GPEs is the lithium-ion transference number (t+), which is measured by potentiostatic polarization technology. As shown in Figure 5a, the BC-GPE demonstrates a high t+ of 0.514. This can be assigned to the strong interactions between the functional -OH groups, ether groups (EO), glycosidic bonds on the BC chains and the lithium ions, which offer great amount of transportation routes for Li+ ions. While the movements of PF6- ions are suppressed since the EC and DMC are captured by the BC chains. This phenomenon was also demonstrated in other works.53, 56-58 Besides, the BC chains with cyclic glucose units have a high dielectric constant of 50, which is about 10 times higher than that of liner PEO.59-60 However, the t+ of LE-S is much lower (0.316). The better lithium-ion transfer property of BC-GPE indicates a higher battery performance. The electrochemical windows of BC-GPE and LE-S were tested through LSV method (Figure 5b). The response current of BC-GPE remains stable till 4.9V, while the current of LE-S


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increases rapidly after 4.7V. This result is attributed to the electrostatic interactions between the functional groups on the BC chains and the organic solvents, which could delay the oxidation reaction to higher voltage. Therefore, the BC-GPE shows a better electrochemical stability, indicating the possibility of using in high voltage batteries. For further investigating the stability of the electrolyte with lithium anode during charge/discharge cycles, galvanostatic cycling tests of Li/electrolyte/Li symmetrical batteries with BC-GPE and LE-S were executed. During charge/discharge cycles of the two batteries, constant current density of 0.5 mA cm-2 was applied, and each cycle lasted 1 h. It can be seen from Figure 6a that the overpotential of Li/BC-GPE/Li cell is slightly higher than the Li/LE-S/Li cell at the beginning. And this is mainly ascribed to the incomplete contact between the gel polymer electrolyte and the electrodes. We further calculated the Young`s modulus of the BC-GPE with the equation below:61

Y=

F×L S×∆L

(4)

Where Y is the Young`s modulus, F is the tensile stress, S is the cross-section area of the BCGPE, L is the length of the sample before tensile test, ∆L is the variety of length. The result of the calculated Young`s modulus of the BC-GPE is 4.95 MPa, which is 49.5% ~ 4.95% of the Young`s modulus of rubber (10.00 MPa ~ 100.00 MPa).62 This means the rigidity of the BC-GPE is still high, even though it demonstrates excellent toughness in macro scale. The cellulose fibers demonstrate a Young`s modulus of 137000.00 MPa.62 These results indicate that the rigidity of the BC fibers should still be very high in the BC-GPE. The contact between the BC-GPE and the electrodes is still a rigid solid to solid contact, resulting in incomplete contact and higher overpotential at the beginning. During the depositing/stripping cycles of lithium, volume expansion happens on the lithium electrodes and the sedimentary lithium fills the spaces between BC-GPE and the electrodes, resulting in a tighter contact between lithium electrodes


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and the gel polymer electrolyte, thus the voltage keeps decreasing during the cycling processes.56, 63-64

In contrast, the initial overpotential of Li/LE-S/Li cell is 0.08 V at the beginning whereas it

increases rapidly to 0.42 V after 385 h. And the voltage drops sharply (Figure S4 Supporting information), owing to the short circuit caused by lithium dendrites. The overpotential of Li/BCGPE/Li battery remains stable even after cycling for 600 h, without short circuit. In consequence, the BC-GPE greatly restrains the vertical growth of lithium dendrite. More comparison tests are shown in Figure S8 Supporting information.


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Figure 6 (a) Galvanostatic cycling tests of Li/BC-GPE/Li and Li/LE-S/Li at 0.5 mA cm-2; Cycling performance of the LiFePO4/BC-GPE/Li and LiFePO4/LE-S/Li batteries at (b) 0.5C and (c) 5C; (d) Rate performance of the LiFePO4/BC-GPE/Li and LiFePO4/LE-S/Li batteries Batteries with lithium anodes and LiFePO4 cathodes were assembled to further study the usage of BC-GPE. Also, LiFePO4/LE-S/Li cells were tested as comparisons. The initial charge and


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discharge curves of LiFePO4/BC-GPE/Li and LiFePO4/LE-S/Li at 0.5 C are shown in Figure S5 Supporting information. The LiFePO4/BC-GPE/Li battery shows an initial discharge capacity of 141.2 mA h g-1 with an initial Coulombic efficiency of 89.46%, which are apparently higher than that of LiFePO4/LE-S/Li battery (138.3 mA h g-1 and 87.77%, respectively). The cycling performances of both LiFePO4/BC-GPE/Li and LiFePO4/LE-S/Li batteries are illustrated in Figure 6, it is distinctly seen that the cycling performance of LiFePO4/BC-GPE/Li battery is improved remarkably. As demonstrated in Figure 6b, the LiFePO4/BC-GPE/Li battery retains 104.2% of the initial discharge capacity after 150 cycles at 0.5C and the Coulombic efficiency is higher than 98% during the 150 cycles. Nevertheless, though the LiFePO4/LE-S/Li battery shows an increasing capacity during the first cycles, its capacity retention and Coulombic efficiency gradually decreases to 100.4% and 90.79%, respectively. The lower Coulombic efficiency of LiFePO4/LE-S/Li battery is mainly caused by the decomposition of liquid electrolyte at the lithium/electrolyte interface and the growth of lithium dendrite.65 The high rate cycling performances of LiFePO4/BC-GPE/Li and LiFePO4/LE-S/Li batteries at 5 C (3.02 mA cm-2) were also investigated (Figure 6c). Obviously, the LiFePO4/BC-GPE/Li battery possesses higher capacity stability than the LiFePO4/LE-S/Li battery. The speedy capacity decay of LiFePO4/LES/Li battery is resulted from the quick decomposition of liquid electrolyte and the fast growth of lithium dendrite.6,

9

More comparison tests are shown in Figure S9 Supporting information.

Besides, rate performances of LiFePO4/BC-GPE/Li and LiFePO4/LE-S/Li batteries were also tested and displayed in Figure 6d. The LiFePO4/BC-GPE/Li battery possesses higher discharge capacity at different current rates. Especially, when the rate reaches 9C (5.42 mA cm-2), the capacity of LiFePO4/BC-GPE/Li battery reaches 75.2 mA h g-1, while the LiFePO4/LE-S/Li battery fails to finish the charge/discharge cycles and shows so little capacity (3.8 mA h g-1),


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which shall be ascribed to the quick failure of lithium anode in the liquid electrolyte-based batteries.6, 9 Moreover, the rate performance of the LiFePO4/BC-GPE/Li battery is much higher than other gel polymer electrolyte lithium battery systems, as shown in Table S3 Supporting information. And this outstanding rate performance of LiFePO4/BC-GPE/Li battery is mainly because of the superior electrochemical stability of BC-GPE with lithium anode, as mentioned in Figure 6a. On the whole, thanks to the high electrochemical properties of BC-GPE and the better interfacial compatibility between BC-GPE and the electrodes, the LiFePO4/BC-GPE/Li battery owns excellent cycling and rate performance. And this indicates that the BC-GPE has great potential in high performance lithium batteries.

Figure 7 EIS plots of the LiFePO4/BC-GPE/Li and LiFePO4/LE-S/Li batteries (a) before and (b) after 150 cycles at 0.5C (inset image is the equivalent circuit); (c-e) SEM images of the surface of lithium anode (c) the fresh lithium, (d) the lithium anode of the LiFePO4/LE-S/Li battery, and (e) the lithium anode of the LiFePO4/BC-GPE/Li battery after 150 cycles at 5C.


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During the cycling of the batteries, EIS method was used to measure the interfacial properties of the LiFePO4/BC-GPE/Li and LiFePO4/LE-S/Li batteries. The EIS plots of LiFePO4/BCGPE/Li and LiFePO4/LE-S/Li before cycling and after 150 cycles at 0.5 C are shown in Figure 7a-b. Before cycling (Figure 7a), both of the Nyquist plots show a semicircle at high-medium frequency, which is related to the interfacial resistance between the electrodes and the electrolyte (Rƒ). The fitting results are listed in Table S4 Supporting information and the equivalent circuit is inserted in Figure 7a. The initial Rƒ of LiFePO4/BC-GPE/Li battery (31.20 Ω) is lower than that of LiFePO4/LE-S/Li battery (59.10 Ω), demonstrating a higher interfacial compatibility of BCGPE. Moreover, LiFePO4/BC-GPE/Li and LiFePO4/LE-S/Li batteries were further examined after 150 cycles (Figure 7b). Each Nyquist plot consists of two semicircles, according to the equivalent circuit (inserted in Figure 7b). The semicircle at high frequency is attributed to Rƒ. And the semicircle at medium frequency is caused by the charge transfer resistance (Rct). The sloping line at low frequency is Warburg impedance, which is related to the diffusion of lithium ions into active materials. The accurate fitting results are listed in Table S4 Supporting information. It can be seen that the Rƒ of the LiFePO4/BC-GPE/Li battery after cycling (47.60 Ω) is slightly higher than the initial value (31.20 Ω), indicating a good interfacial stability of BCGPE. However, the Rƒ of the LiFePO4/LE-S/Li battery increases to 109.00 Ω, which is much higher than the original impedance (59.10 Ω). The huge increasing of Rƒ in LiFePO4/LE-S/Li battery after 150 cycles is attributed to the consuming of liquid electrolyte and the forming of low ionic conductivity SEI layers. In contrast, the excellent interfacial compatibility of BC-GPE with lithium anode in LiFePO4/BC-GPE/Li battery ensures a stable interface layer, modifies the deposition of lithium, and then make sure the stable cycling performance of the LiFePO4/BCGPE/Li battery. And the similar results were found in the EIS plots of LiFePO4/BC-GPE/Li and


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LiFePO4/LE-S/Li batteries before and after cycling for 150 cycles at 5C (Figure S6 Supporting information). The batteries were disassembled to measure the surface morphologies of the lithium anode after cycling at 0.5C and 5C for 150 cycles. The fresh lithium was tested as comparison. As shown in Figure 7c, the fresh lithium anode shows a smooth surface before cycling. However, the surface of lithium anode in LiFePO4/LE-S/Li batteries becomes loose and porous after 150 cycles at 0.5 C (Figure S7a Supporting information). Furthermore, after 150 cycles at 5 C, it is obviously to see massive lithium dendrites on the surface of lithium anode in LiFePO4/LE-S/Li batteries (Figure 7d). Diametrically opposite results are found in the LiFePO4/BC-GPE/Li batteries. No dendrite is observed after 150 cycles at 0.5 C (Figure S7b Supporting information) or 5 C (Figure 7e). These results demonstrate that using BC-GPE in the lithium battery is able to suppress the vertical growth of lithium dendrite effectively. The dense gelled BC-GPE has good surface compatibility with the lithium anode surface, which keeps a stable interface and a lower interfacial resistance during cycling. These features greatly improve the cycling performance and safety of lithium battery. A schematic illustration was drawn to show how the lithium dendrite was suppressed. The morphological change of lithium anode with commercial separator is shown in Figure S10a Supporting information. In charging cycles, Lithium ions always deposit at the sites where the surface energy is high, such as the defects, which generates the growth of lithium dendrite with solid state electrolyte layer formed simultaneously. As a result, liquid electrolyte is consumed continuously. During discharging, lithium dissolution is nonuniform, which leads to the break of solid state interface layer and fracture of lithium dendrite. Thus, liquid electrolyte is further decomposed. After cycles, the lithium surface becomes rough and loose and the liquid electrolyte


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is consumed continuously. 6, 9-10 These processes cause the fading of capacity and safety accident. However, when the liquid electrolyte is captured in the BC-GPE, the reaction between liquid electrolyte and lithium anode is limited (Figure S10b Supporting information). The dense surface of BC-GPE further improves the interfacial compatibility of electrolyte with lithium anode and brings the homogenously deposition of lithium ions on the surface of anode. Meantime, the high mechanical properties of BC-GPE greatly suppress the vertical growth of lithium dendrite on a physical aspect.

Figure 8 (a) The Voltage of LiFePO4/LE-S/Li and LiFePO4/BC-GPE/Li batteries as a function of cycling time at 1C with mutative temperature; (b) The composition of the battery used for the heat-resistant measurement; (c-e) The optical images of a pouch type LiFePO4/BC-GPE/Li battery lights bulbs.


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The heat-resistant properties of both LiFePO4/LE-S/Li and LiFePO4/BC-GPE/Li batteries were measured. As Figure 8a shows, both LiFePO4/LE-S/Li and LiFePO4/BC-GPE/Li batteries work well at room temperature. While the voltage of LiFePO4/LE-S/Li battery suddenly decreases to 0 V when the temperature reaches 150˚C, resulting from the shrinkage of commercial separator. Conversely, the voltage of LiFePO4/BC-GPE/Li battery remains at the normal level at 150˚C. This result is mainly associated with the higher thermal stability of BC-GPE as shown in Figure 3. Furthermore, the LiFePO4/LE-S/Li battery is not able to finish change/discharge at 150 ˚C, however, the LiFePO4/BC-GPE/Li battery only suffers some unstable cycles at the beginning, which might be caused by the melting of lithium anode at this high temperature, then the LiFePO4/BC-GPE/Li battery regains the normal charge/discharge ability. When the temperature is recovered to room temperature, the voltage of the LiFePO4/LE-S/Li battery is still 0 V, as a result of short-circuit. Expectedly, the LiFePO4/BC-GPE/Li battery is able to continue the charge/discharge cycles. The higher thermal resistance of BC-GPE gives the LiFePO4/BCGPE/Li battery the ability to survive at high temperature, while the LiFePO4/LE-S/Li battery is failed. The composition of the battery used for heat-resistant measurement is shown in Figure 8b. The pouch type LiFePO4/BC-GPE/Li battery is assembled to show the possibility of amplification (Figure 8c-e). 4. Conclusions A novel gel polymer electrolyte based on high-strength internal crosslinking bacterial cellulose network was prepared with a simple fast freeze-drying method. The raw material used to prepare the BC-GPE is widely used in our daily lives with very low cost. And the method is environmentally friendly without introducing any toxic solvent or high cost operation. Thus, it is easy to be enlarged in practical producing. The -OH groups, EO and glycosidic bonds on the BC


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chains capture the organic solvents and provide lithium ion channels to generate high ionic conductivity at room temperature. More importantly, the internal crosslinking network structure of BC nanofibers enables the BC-GPE an outstanding mechanical strength and thermal stability, which greatly inhibit the vertical growth of lithium dendrite. Batteries using BC-GPE show better cycling performance, rate performance and heat resistance in comparison with batteries based on liquid electrolyte. In conclusion, the cost-effective preparing method and excellent performance of BC-GPE bring it to be one of the most possible electrolytes for lithium batteries with high performance and safety. Acknowledgements The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 51772315, 51432010, and 51272267). Supporting information The SEM image of oven-dried BC film, TGA results of BC-GPES and commercial separator, bending measurement of BC-GPE, galvanostatic cycling of Li/LE-S/Li, initial cycles of batteries, EIS plots of batteries before and after 150 cycles at 5C, SEM images of lithium anodes after 150 cycles at 0.5C, schematics of lithium dendrite suppressing and tables of mechanical strength comparison, ionic conductivities, electrolyte conservation rate and EIS fitting results are shown in the supporting information.


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Table of contents


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High-strength Internal Crosslinking Bacterial Cellulose Network Based

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