Flexible Quasi-Solid-State Sodium-Ion Batteries Built by Stacking Two

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Flexible Quasi-Solid-State Sodium-Ion Batteries Built by Stacking Two-Dimensional Titania Sheets with Carbon Nanotube Spacers Hongbin Chu, Kun Jiang, Guohui Li, Zhigang Zhao, Qingwen Li, and Fengxia Geng ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00852 • Publication Date (Web): 19 Jul 2019 Downloaded from pubs.acs.org on July 20, 2019

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ACS Applied Energy Materials

Flexible Quasi-Solid-State Sodium-Ion Batteries Built by Stacking Two-Dimensional Titania Sheets with Carbon Nanotube Spacers Hongbin Chu,1 Kun Jiang,1 Guohui Li,1 Zhigang Zhao,2 Qingwen Li,2 Fengxia Geng*1 1

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China

2

Suzhou Institute of Nanotech and Nanobionics, Chinese Academy of Sciences, 398 Ruoshui Road, Suzhou Industry Park, Suzhou 215123, China E-mail: [email protected]

Abstract: The rapid advances in portable and wearable electronics have triggered an ever increasing demand for flexible energy storage devices. Due to concerns on the limited lithium resources and constantly increasing lithium prices, it is predicable that flexible rechargeable sodium (Na)-ion batteries could pave the way for broader acceptance of wearable energy storage devices. However, developing flexible electrodes for rechargeable Na-ion batteries has been seriously limited on account of the reduced availability of electrodes that can host bulky Na ions while maintaining structural integrity. Here, a stacking assembly of Na-ion intercalation materials, titania sheets, was designed and fabricated by a simple vacuum-assisted filtration method. Chemically reduced graphene oxide was molecularly hybridized and served as the current collector, and trace carbon nanotubes were introduced between the sheets for improved electrolyte infiltration and ion intercalations. As a result, the designed electrode manifested good mechanical flexibility integrated with excellent electrochemical Na-ion storage performance, delivering an initial discharge capacity of 130 mA h g-1 at 15 mA g-1 and still preserving 74% after 500 cycles. A quasi-solid-state Na-ion full battery employing the designed electrode coupled with Prussian blue maintained excellent Na-ion storage performance. Importantly, the electrochemical behavior was not affected by the battery shape or external mechanical deformation, demonstrating practical applicability to powering various wearable electronic devices. The work is expected to accelerate the utilization of two-dimensional sheet materials in flexible energy storage systems. Keywords: Two-dimensional sheets; Titania; sodium ion; Quasi-solid-state battery; Flexible device

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Introduction The emergence and rapid development of next-generation flexible electronics, including foldable smart phones, curved tablets, and implantable medical devices, has sparked unprecedented research in the construction of flexible and deformable energy storage devices, especially flexible batteries.1-3 Sodium-ion (Na-ion) batteries possess the same operating principles as Li-ion batteries and have drawn tremendous research focus in recent years due to their numerous merits, especially the wide availability and low cost of Na resources.4-8 However, sharply contrasting to the vast research attention devoted to flexible Li-based battery systems,9-12 the exploration of flexible Na-ion batteries,13 especially those employing solid-state electrolytes, is still very limited and challenging. In analogy to flexible Li-ion battery, a flexible Na-ion battery consists of three main components, a cathode, an anode, and an electrolyte, and each component needs to be appropriately designed showing mechanical flexibility and sustainability.13-14 Considering the serious leakage and safety problems of liquid electrolyte, a gel polymer electrolyte is usually employed, which composes conventional liquid electrolyte and a polymer matrix and displays excellent electrolyte leakage-proof ability, mechanical flexibility, and decent ionic conductivity.9,15 For the flexible electrodes, the existing protocols are mainly focused on embedding active materials into a flexible and conductive matrix, including pre-treated carbon cloth, graphene, carbon nanotubes (CNTs).16-19 It should be mentioned that most materials undergo a volume expansion/shrinkage upon intercalation/de-intercalation of bulky Na-ions (1.02 Å for Na+ vs 0.76 Å for Li+),20 thus typically generating large strain or even cracks at interface with the substrate, which would definitely destroy integrity and stability of the electrodes. Further, the substrate would introduce “dead” weight and then influence energy density of the whole device. Therefore the biggest challenge in realizing flexible Na-ion battery is to design flexible and integrated electrodes that can stably accommodate bulky Na-ions. Molecularly thin two-dimensional sheets of electrochemically active materials have been proven to be the most promising way to construct high-performance paper electrodes integrated with good mechanical strength and can be easily produced by a simple vacuum filtration method.21-23 Such a structure has three important advantages: (1) a large overlapping area and close interaction between the sheets ensures mechanical stability, enabling the direct use of the films as electrodes without additional polymer binders; (2) an ultrathin structure exposing high dose of reactive metal centers on surface maximizes 2 ACS Paragon Plus Environment

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the electrochemical performance of the obtained electrode; and (3) a space between the stacking sheets is elastic and can be finely tuned by foreign materials, which facilitates electrolyte infiltration and allows ion intercalations without structural damage. Therefore, stacking two-dimensional sheets of electrochemically active materials would be an ideal route for constructing flexible energy storage electrodes. Although some attempts have been made toward Li-ion storage by, for example, assembling graphene and metal dichalcogenide sheets,24-27 successful examples for Na-ion storage have been very rare. This rarity may be due to the low availability of materials that can work properly under mechanical deformation while withstanding a large volume change during the Na+-intercalation/deintercalation process.28-30 Transition metal oxides, particularly titania, have been regarded as an excellent anode active material for Na-ion battery with the merits of low redox potential, high stability, and low cost and toxicity.31-32 The titania in two-dimensional sheet morphology can be achieved either by bottom-up molecular assembly or through top-town delamination.33 The former approach involved solvothermal reaction of Ti-based oligomers under the presence of lamellar reverse micelles, yielding TiO2 of confined thickness.34 For the latter, the sheets are derived from delamination of a layered parent compound composing infinite two-dimensional layers of edge-sharing Ti–O6 octahedra and alkali ions between layers, the disintegration of which would yield titania sheets with compositions of Ti1-x□xO2 (□ represents vacancies).35-37 Here, a flexible high-performance Na-ion storage electrode was constructed by stacking titania sheets of Ti0.87O20.52-, which was achieved by top-down delamination. To facilitate electron transfer, in situ molecularly hybridized reduced graphene oxide (rGO) was employed as the current collector. Meanwhile, to avoid compact stacking between the neighboring sheets, which would seriously sacrifice active surface area and block ion intercalations, CNTs were introduced between sheets as spacers. It should be mentioned that the presence of such carbon materials, rGO and CNT, may additionally generate synergistic effect and make the two-dimensional titania sheets perform better,38 thus producing high-capacity and long-term stable Na-ion storage. A full battery using the so-obtained electrode coupled with Prussian blue as the electrodes and a solid gel-polymer as the electrolyte was assembled, which could be used to power various electronic devices even under serious deformation, demonstrating a promising prospect for practical applications. This work highlighted the importance of designing an optimal structure for Na-ion batteries. 3 ACS Paragon Plus Environment


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Figure 1. Schematic showing the designed structure of the battery electrode that involves stacking titania sheets. rGO is molecularly hybridized to ensure efficient electron transfer; CNTs between the sheets serve as spacers to allow electrolyte infiltration and bulky Na-ion intercalation. Results and discussion The fabrication strategy for the self-standing flexible hybrid electrode is schematically illustrated in Figure 1, which could be obtained through the simple filtration of a colloidal mixture of titania, graphene oxide (GO), and carbon nanotubes (CNTs), followed by reduction of the GO through hydriodic acid (HI) treatment. Titania sheets were obtained by a reported procedure,35,39 namely, delaminating protonated titanate in TMAOH and were observed to be a two-dimensional sheet configuration with lateral dimensions of 3-5 μm and a uniform height of approximately 1.0 nm, as confirmed by the atomic force microscopy (AFM) characterization displayed in Figure 2a. GO was produced in a similar intercalation-delamination protocol via an improved Hummer’s method.40 Figure 2b shows the corresponding AFM image of the GO with a transverse size of 2-4 μm and a longitudinal size of 1.1 nm. Both the titania sheets and GO possessed net negative charges, enabling stable aqueous colloids of high concentrations, namely, 4.0 mg ml-1, respectively. The charges originated from the nonstoichiometric composition, Ti0.87O2, for the delaminated titania sheets and from the surface oxygen-rich moieties for the GO.41-42 For intimately mixing with CNTs, an aqueous dispersion of CNTs was commercially obtained, and the CNTs had a diameter of 30-50 nanometers and a length of several micrometers (Figure 2c). All three colloids showed negative zeta-potentials of -39.7, -27.5, and -35.0 mV, and the mixed solution was stable enough with a zeta-potential of -37.9 mV 4 ACS Paragon Plus Environment

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(Supporting Information, Figure S1). The system could remain unchanged even when stored for months (Supporting Information, Figure S2). In addition, the high horizontal-to-vertical ratio along with the high colloidal concentration made the titania sheets and GO form a stable liquid crystal phase that showed brilliant colors under optical polarizing microscopy, as displayed in Figure 2d and e, which indicated the presence of a microscopic alignment. The colloid mixture still showed vivid colors under polarizing microscope observations (Figure 2f), suggesting that the microscopic alignment was well maintained in the mixing process and would facilitate the formation of an orderly stacking structure.

Figure 2. (a, b) Typical AFM images for titania sheets and GO. (c) TEM image of CNTs. Insets are the corresponding digital images of each colloid. (d-f) Polarizing optical microscope images for colloids of titania sheets, GO, and the mixture of the three components, respectively. The colloid mixture still produced the vivid colors typical of a liquid crystalline phase, implying that microscopic alignment was well maintained in the mixing process. After filtrating the colloid mixture through a cellulose acetate membrane, a thin film of high flexibility could be obtained. The film composition and thickness could be tuned by adjusting the formula and volume of the colloid mixture (Supporting Information, Figure S3). After optimization, a formula with titania:GO:CNTs in a mass ratio of 7:2:1 and a film thickness of 6 m was chosen (Supporting Information, Figure S4-5). GO in the film would be reduced to rGO by chemical reduction in HI, which was confirmed by TG and Raman characterizations (Supporting Information, Figure S6-7). The microstructure of the so-constructed film electrode was first examined with scanning electron microscopy 5 ACS Paragon Plus Environment


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(SEM). From the cross-sectional SEM images shown in Figure 3a and b, it was observed that the hybrid paper structure was formed by the regular stacking of titania sheets and rGO sheets, and CNTs were distributed uniformly in the space between them. The corresponding elemental mapping showed a uniform distribution of the elements of Ti, O, and C (Supporting Information, Figure S8). In such a construction, each individual 2D sheet possessed relatively high mechanical durability and was largely overlapped, endowing superior integral mechanical flexibility. Per the digital images shown in Figure 3c, the hybrid film was capable of being rolled up on a glass bar or folded without the creation of any obvious cracks. Figure 3d shows the magnified cross-sectional SEM view of the folded area, and Figure 3e represents the typical surface morphology when spread to the flat state, from which it is obvious that the stacking structure was well maintained and no structural damage occurred even upon severe folding. The regular sheet stacking structure was corroborated with X-ray diffraction (XRD), and an example pattern is shown in Figure 3f. The presence of diffraction in the low angle region of 8.56° suggested a sheet-to-sheet distance of 1.03 nm. The broad peak at 25° should originate from the carbon structure,43-44 which may be related to CNT incorporation or successful reduction of the GO. In contrast, without the CNTs, the sheets were tightly stacked together, and the sheet-to-sheet spacing was only 0.98 nm (Supporting Information, Figure S9-10), which could result in impedance of the electrolyte infiltration and thus a substantial decay of the electrochemical performance. The presence of CNTs also helped the infiltration of the HI into the bulk of the film, thus greatly improving the reduction efficiency (Supporting Information, Figure S5). As a result, the sheet conductivity was comprehensively increased (Supporting Information, Figure S11), giving a good value of 0.142 S cm-1, which would ensure efficient charge transfer during electrochemical reactions. The sheet resistance of the final hybrid paper structure was examined while progressively varying bending angles, as shown in Figure 3g. With the change in bending angle, the value of the resistance remained almost unchanged, ensuring its application as a flexible energy storage electrode.


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Figure 3. (a) Low- and (b) high-magnification SEM image of the film cross-section showing the well-aligned stacking of sheet components with CNTs dispersed between sheets. (c) Digital images of the obtained hybrid film being rolled onto a glass bar or harshly folded without the occurrence of obvious damage. SEM images of (d) cross-section and (e) surface morphologies of the folded area, showing no obvious structural damage. (f) XRD pattern for the hybrid film. (g) Resistance variation (R/R0) at a range of bending curvatures of the hybrid film, where R and R0 are sheet resistance at a certain bending angle and in a flat state, respectively. To investigate the Na-ion storage behavior of the designed hybrid paper electrode, half-cells in CR2032-type coin configuration were assembled that used the self-standing paper structure as the working electrode and Na metal foils as the reference and counter electrode. As the sheets were hydrophilic and the synthesis was performed in an aqueous media, some water molecules may be inevitably remained in the structure.35 While battery electrodes are normally calcined at a temperature above 500 ℃ to remove all water and 7 ACS Paragon Plus Environment


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avoid possible side reactions with electrolyte, some recent studies have shown that water may not necessarily be bad factor for cell behavior if they are deeply trapped and do not enter into the electrolyte during the electrochemical process.45-46 Instead, in some cases, for example, lithium titanate hydrate,47 iron phosphate hydrate,48 etc., it was proved that the presence of deeply trapped water was beneficial to battery performance by enlarging channel dimensions and efficiently screening the electrostatic interactions between guest ion and host charge. Taking the facts into considerations, sample treatment before use was done by immersing in electrolyte overnight followed by heating at a temperature of 80 ℃, intending to exchange or removing the loosely bound surface adsorbed and crystallographic water. A working potential range from 0.01 to 3.0 V vs Na+/Na was carefully selected, where both the rGO and CNTs did not provide a capacity contribution (Supporting Information, Figure S12); thus, the major capacity contribution came from the titania. For comparison purposes, the film electrode without introducing CNTs was also studied. Figure 4a presents the example galvanostatic charge-discharge profiles of the paper electrodes with current densities ranging from 15 to 750 mA g-1. No obvious plateau was observed and there were sloping profiles, which indicated a continuous intercalation/deintercalation of the Na-ion.49 To better understand either the charge transfer or the ion migration behavior of Na-ions in the bulk host, electrochemical impedance spectroscopy (EIS) measurements were performed and compared with an electrode with no CNT introduction, and the results are depicted in Figure 4b. Clearly, the spectra for both electrodes consisted of a straight line in the low frequencies and a semicircle in the high frequencies, with our designed electrode exhibiting a smaller Ohmic resistance (3 vs 15 Ω), better charge transfer resistance (233 vs 330 Ω and faster ion diffusion.50 The Na-ion diffusion coefficient for our designed electrode achieved a value of 2.67 × 10-12 cm2 s-1, which was nearly an order of magnitude higher than that for the electrode without CNTs (Supporting Information, Figure S13), indicating a much improved Na-ion diffusion with the CNT spacers. The rate performance examined by varying current densities from 15 to 750 mA g-1 is displayed in Figure 4c. Our electrode showed a high capability of electrochemical storage of Na ions, delivering a decent reversible specific capacity of 130 mAh g-1 at a low current of 15 mA g-1. On increasing the current to 30, 75, 150, 300, 750 mA g-1, capacities of 119, 107, 76, 53 and 36 mAh g-1,respectively, were maintained. Remarkably, upon the current density was switched back to 15 mA g-1, a capacity of 128 mAh g-1 was recovered, almost 8 ACS Paragon Plus Environment

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100% of the initial value, demonstrating the excellent structural stability of our designed hybrid, even at high rates. In sharp contrast, although the electrode without the CNT spacers could provide a comparable capacity at a low current of 15 mA g-1, possibly owing to the sufficient electrolyte infiltration and electrochemical reaction, a very quick capacity decay was observed with increasing current, and the capacity retention was only 17.0% at a 50-fold current of 750 mA g-1. When reversing back to 15 mA g-1, a continuous capacity drop was found, which could be explained by possible structural damage during the high-rate operations. Figure 4d shows the cycling behavior at 15 mA g-1 for a long run of 500 loops. The capacity retention of our designed electrode was maintained at 74.0% after extended operation and showed a reversible capacity of 96 mAh g-1. Employing the same measurement conditions, the specific capacity of the electrode without CNT spacers showed a sharp decrease after cycling only 100 loops, and the capacity retention was as low as 30.0% (38 mAh g-1) after an extended run of 500 cycles. Such a strong contrast further demonstrated that the presence of CNTs can greatly enhance the electrode conductivity

and

maintain

structural

integrity

during

the

Na-ion

intercalation/deintercalations, thus endowing the materials with a high-rate stability and long lifetime. In addition, the morphological evolutions of the electrodes after durable cycling process were recorded by SEM (Supporting Information, Figure S14). In the electrode without a CNT spacers, obvious cracks were observed, which may have resulted from the iterative intercalation/deintercalation of bulky Na ions; in sharp contrast, the initial appearance and structure for our designed electrode remained almost intact, which explained the high-rate capability and superior cyclic stability of our designed electrode.



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Figure 4. Electrochemical performance of our designed film electrode compared with that having no CNT spacers. (a) Potential profiles at different current densities increasing from 15, 30, 75, 150, 300, to 750 mA g-1, (b) EIS spectra measured from 0.1 to 105 Hz, (c) rate behavior, and (d) cycling performance. For verifying the feasibility of real applications of our designed flexible electrode, full cells using the designed paper electrode as the anode and Prussian blue coated on a carbon cloth as the cathode were assembled. Note that Prussian blue has been considered a suitable candidate of cathode material for Na-ion batteries on account of its high theoretical specific capacity, satisfactory long-term cycling stability, moderate price, and safe operation.51 The Prussian blue wrapped with Ketjen Black was synthesized according to a previous report, and its electrochemical performance examined in the half-cell configuration is presented in Supporting Information, Figure S15 exhibiting high reversible specific capacities of 130, 128, 124, 120, 110, and 100 mAh g-1 at current densities of 15, 30, 75, 150, 300, and 750 mA g-1, respectively. Additionally, a long 10 ACS Paragon Plus Environment

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cycling stability for a duration of 100 cycles (>80% capacity retention) and a high coulombic efficiency were observed, indicating its reliability as the cathode for full cells. Meanwhile, due to the intrinsic mobility and flammability, organic liquid electrolytes that are commonly used in coin cells always have safety issues, such as leakage and even short circuits, especially under mechanical deformation.52 As an alternative, a quasi-solid-state

Na-ion

conducting

gel

polymer,

poly(vinylidene

difluoride-co-hexafluoropropylene) [P(VDF-HFP)], that is rich in porosity with an average pore size of ca. 2 m was chosen as the electrolyte (Supporting Information, Figure S16).53 With the rigid mechanical flexibility of the polymer backbones, this gel polymer could be easily bent and folded without significant signs of breaking. The ion mobility in the polymer was 0.785 mS cm-1, which was sufficient for battery operation at room temperature.54 By placing the quasi-solid-state polymer electrolyte between the cathode and anode, followed by using the commercially available Al-plastic film as the package, both planarand belt-shaped quasi-solid-state Na-ion full batteries were constructed, as illustrated in Figure 5a. The cycling performance of both battery configurations operating in the voltage range of 0.01-3.5 V and at the current density of 30 mAh g-1 was evaluated. After the initial few cycles, the reversible discharge capacity of the planar-shaped pouch battery (based on cathode material mass) was stable at 110 mAh g-1 (Figure 5b). When estimating by the mass of sum electrodes, the capacity was 50 mAh g-1 and the corresponding specific energy density was ~100 Wh kg-1, exceeding that of lead-acid and nickel-metal hydride batteries. After an extended run of up to 200 cycles, a negligible loss in capacity was noticed while the charge-discharge curve shapes were maintained (Supporting Information, Figure S17), indicating the excellent electrochemical stability of our assembled full battery. Even when bent into different angles, including 60°, 120°, 180° and rolling into a circle, no significant capacity fluctuation or fading was observed during continuous cycling at each configuration for 15 cycles. The shape of the galvanostatic charge-discharge profiles was preserved with no obvious change when bending the battery, even at a bending angle up to 180°, as displayed in Figure 5c, demonstrating the excellent flexibility of our designed electrode and the intimate contact between the electrode and the polymer electrolyte. Similar results were also observed when the battery was customized into a belt shape, with the corresponding cycling performance at various bending angles and galvanostatic charge-discharge profiles shown in Figure 5d-e. Because of the high 11 ACS Paragon Plus Environment


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uniformity of our electrode, it could actually be cut into any shape while maintaining the electrochemical Na-ion storage behavior, implying the versatility of our designed hybrid film as a flexible battery electrode. After being fully charged to 3.5 V, the battery could continuously power various electron devices, for example, an electroluminescent panel, a thermometer, and a smart watch as demonstrations, without sacrificing the luminescent intensity even under mechanical deformation (Figure 5f-h).

Figure 5. (a) Schematic illustration of soft-packaged (a) planar- (above) and belt-shaped (below) quasi-solid-state full battery. Cycling performance and corresponding galvanostatic charge-discharge curves of (b-c) planar- and (d-e) belt-shaped full battery at a current density of 30 mA g-1 under different bending angles. Photographs of the flexible batteries to power (f) electroluminescent panel, (g) thermometer, and (h) smart watch. The electrochemical behaviors and practical operations were little affected by changes in the battery shape or any external deforming stresses, proving the excellent versatility and mechanical flexibility of our assembled full battery. Conclusion In summary, we developed a new Na-ion flexible battery electrode based on the stacking of titania sheets. GO was molecularly hybridized with titania sheets, followed by 12 ACS Paragon Plus Environment

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a chemical reaction, and served as an efficient current collector. CNTs were introduced between the sheets to further facilitate electrolyte infiltration and ion diffusion. With the unique hieratical stacking structure, highly reversible and stable electrochemical intercalation/deintercalation of bulky Na-ions was enabled, showing a competitive reversible capacity of 130 mA g-1 and a high capacity retention of 73.1% after an extended run of 500 cycles, surpassing most previously reported flexible electrodes for Na-ion storage. Correspondingly, a quasi-solid-state full battery employing the designed structure coupled with Prussian blue as the electrodes and a P(VDF-HFP) gel polymer as the electrolyte showed high mechanical durability and maintained the electrochemical behavior at various deformation states, indicating that this approach could be used to power various electronic devices, such as electroluminescent panels, thermometers, and smart watches. We believe that this success could offer an effective concept for fabricating high-performance flexible electrodes for bulky-ion storage. Experimental section: Materials. Titanium oxide (TiO2, ≥98%), potassium carbonate (K2CO3, ≥98%), lithium carbonate (Li2CO3, ≥98%), N,N-dimethylformamide (DMF, ≥99%), sodium ferrocyanide (Na4Fe(CN)6, ≥99%), poly(vinylidene difluoride-co-hexafluoropropylene) [P(VDF-HFP), molecular weight: ~13000], graphene oxide (GO, 4 mg mL-1) dispersion, and carbon nanotube (CNT, 9-10wt%) dispersion were purchased from Aladdin Industrial Co.; carbon powder (Ketjen Black, ≥99%) was purchased from Lion Co. Hydroiodic acid (HI, ≥45%) and hydrochloric acid (HCl, 38%) were obtained from Sinopharm Chemical Reagent Co., Ltd. Preparation of titania colloid solution. The colloid dispersion containing monolayer titania sheets was produced using our previously reported strategy.35,39 Briefly, a homogeneous compound composed of TiO2, K2CO3, and Li2CO3 (molar ratio in 10.4:2.4:0.8) was placed in a tube furnace followed by a calcinations treatment at 1173 K for a period 20 h, and the layered K0.8[Ti1.73Li0.27]O4 was successfully obtained. The following protonation process was performed by immersing the K0.8[Ti1.73Li0.27]O4 in 0.5 mol L-1 HCl and stirring at ambient environment for 2 days, with the solvent being refreshed every day. The protonated H1.07Ti1.73O4 was separated using a filtration method and cleaned with deionized water and ethanol repeatedly to remove the excess acid. After 13 ACS Paragon Plus Environment


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air drying, the product was redispersed in a certain amount of aqueous TMAOH solution and shaken at 180 rpm for a few days, by which the single-layer titania sheet colloidal solution with a solid-to-solution ratio of 4 mg mL-1 was obtained. Preparation of the hybrid film. Colloidal dispersions of individual titania, GO, and CNTs were homogeneously mixed and filtrated through a selected filter membrane via a vacuum-assisted filtration process. After air-drying for a few minutes, the film was peeled off and then treated with HI at 90 ℃ for 4.5 h, upon which the GO could be reduced to rGO. After washing with deionized water and drying in a vacuum oven overnight at 80℃, the hybrid film was obtained and directly used as the electrode. The film composition and thickness could be conveniently adjusted by varying the volume of each solution. The control film of titania/rGO was produced employing the same procedures without adding the CNTs in the initial recipe. Preparation of Prussian blue cathode. To optimize the electrochemical performance of Prussian blue, a nanostructure wrapped with carbon derived from Ketjen Black was prepared using a modified strategy described previously. Briefly, a certain weight (3.5 g) of a commercially obtained Ketjen Black powder was added to 200 mL of deionized water, followed by vigorous stirring at ambient temperature until achieving a homogenous solution, to which 40 mmol of Na4Fe(CN)6 and 2 mL of HCl were then dissolved in order. After stirring at 65 ℃ for 6 h, the reaction product was separated by filtration, cleaned with deionized water several times, and vacuum dried at 90 ℃ for 2 days. Preparation of P(VDF-HFP)-based gel polymer electrolyte. The P(VDF-HFP) gel polymer electrolyte membrane was prepared following a previously reported strategy.53 Typically, 15 mg of P(VDF-HFP) powder was dissolved in a mixture containing 85 mg of DMF and 3 mL of deionized water at 80 ℃, the solution of which was coated onto a clean glass slide. The slide was then immersed into a hot water bath at approximately 80 ℃, yielding a homogeneous white membrane. After drying under vacuum at 90 ℃ for 24 h, the membrane was cut into smaller pieces according to the sizes of the prepared flexible electrodes. Upon immersing the gel polymer membrane pieces into the liquid organic electrolyte of 1 M sodium perchlorate (NaClO4) dissolved in ethylene carbonate-diethyl carbonate (EC-DEC, 1:1 v/v%) with 5 wt% fluorinated ethylene carbonate as an additive in an argon-filled glovebox for 12 h, the membrane color rapidly changed from white to transparent, corresponding to the penetration of the organic electrolyte into the pores of the polymer network, upon which the gel polymer electrolyte was finally obtained. 14 ACS Paragon Plus Environment

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Preparation of electrodes and battery assembly. The half-cells in coin configuration were assembled using the as-fabricated freestanding hybrid films as the working electrodes without the necessity of any binders or additives; Na metal foils as reference/counter electrodes; glass fibers (Whatman GF) as the separator; and 1 M NaClO4 in EC/DEC, 1:1 v/v% with 5 wt% of fluoroethylene carbonate additive as the electrolyte. All procedures were conducted in an argon-filled glovebox. In terms of soft-packaged full cells, the Prussian blue cathode was coated onto a carbon cloth to achieve flexibility, and the designed hybrid film was directly used as an anode with an optimal mass ratio of cathode to anode of approximately 1:1.2. The liquid electrolyte was replaced with the Na-ion conducting gel polymer electrolyte of P(VDF-HFP), which simultaneously served as a separator. The flexible cathode was made by blading a homogeneous slurry composed of prepared Prussian blue and PVDF with a weight ratio of 9:1 onto a piece of carbon cloth followed by vacuum drying at 90℃ for 12 h. The cathode-separator-anode sandwich structure was then encapsulated using aluminum plastic laminated films, upon which the Na-ion full cell was assembled. The energy density (Wh kg-1) was calculated by multiplying the maximum capacity (mAh g-1) and the mid-point potential (V), that is, the potential when the battery is discharged to 50% of its capacity. Characterization. The lateral dimensions and thickness of the titania and GO sheets were determined by an atomic force microscope (AFM; MFP-3D Origin, Asylum Research), where the solutions were deposited on a silicon substrate followed by air-drying. A polarized optical microscope (IX73, Olympus) was used to investigate the birefringence of the titania and GO solutions. The microstructure information and elemental compositions were obtained using a field emission scanning electron microscope (SEM, FEI/S-4700) equipped with an energy dispersive X-ray (EDS) spectrometer. X-ray diffraction (XRD) examinations of the hybrid films were conducted on a Rigaku Ultima IV powder diffractometer using Cu Kα radiation (λ= 0.15405 nm) over the 2θ range of 5 to 40°. Raman spectra were recorded on a confocal LabRAM HR800 Raman spectrometer with a He-Ne laser providing an excitation wavelength of 633 nm. Thermogravimetry curves were detected by a NETZSCH STA 449F3 instrument in the range of 30-800℃ under air flow with a heating speed of 10 ℃ min-1. Mechanical properties were measured on an Instron 3365 machine. Galvanostatic discharge/charge tests were carried out using a Land Battery system (Land CT2001A, China). Both cyclic voltammetry (CV) and 15 ACS Paragon Plus Environment


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electrochemical impedance spectroscopy (EIS) were characterized on an electrochemical analyzer (CHI660E, Shanghai). Acknowledgements. The authors acknowledge financial support from the National Natural Science Foundation of China (51772201), Thousand Young Talents Program, Jiangsu Specially-Appointed Professor Program, and a project funded by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions. Supporting Information. More characterization data on the sheets and optimized films, details on cathode Prussian blue and gel electrolyte, electrode morphology after electrochemical cycling, and more performance data of the flexible batteries. References 1.

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Table of Contents Graphic


Flexible Quasi-Solid-State Sodium-Ion Batteries Built by Stacking Two

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