Freestanding Electrode Pairs with High Areal Density Fabricated

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Freestanding Electrode Pairs with High Areal Density Fabricated under High Pressure and High Temperature for Flexible Lithium Ion Batteries Rahim Shah, Jinyu Gu, Amir Razzaq, Xiaohui Zhao, Xiaowei Shen, Lixiao Miao, Chenglin Yan, Yang Peng, and Zhao Deng ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00388 • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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ACS Applied Energy Materials

Freestanding Electrode Pairs with High Areal Density Fabricated under High Pressure and High Temperature for Flexible Lithium Ion Batteries Rahim Shah,†,‡ Jin-Yu Gu,†,‡ Amir A. Razzaq,†,‡ Xiaohui Zhao,†,‡ Xiao-Wei Shen,†,‡ Lixiao Miaoǀǀ, Cheng-Lin Yan,†,‡ Yang Peng,*,†,‡ Zhao Deng,*,†,‡

† Soochow

Institute for Energy and Materials Innovations, College of Physics,

Optoelectronics and Energy, Soochow University, Suzhou 215006, China. ‡

Provincial Key Laboratory for Advanced Carbon Materials and Wearable Energy

Technologies, Soochow University, Suzhou 215006, China. ǀǀ

Sound Group Institute of New Energy, Beijing, 101102, China

1

ACS Paragon Plus Environment

ACS Applied Energy Materials 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

ABSTRACT The emerging market of wearable smart electronics calls for flexible energy storage solutions with high performance but low cost. To fulfil this demand, a novel method involving high pressure and high temperature filtration has been developed to fabricate flexible and freestanding thin-film electrodes with high packing density for lithium ion batteries. The method is universal and can be used for fabricating both cathodes and anodes, and is applicable for most electrochemically active materials. No organic solvent is involved throughout the fabrication process, demonstrating excellent environmental benignity. Despite of the absence of current collectors, electrochemical characterizations on a pair of proof-of-concept anode (graphite) and cathode (lithium iron phosphate) reveal good areal capacity and rate capability. Prototype full pouch cells constructed using the electrode pair demonstrate excellent deformability, durability and power output, promising for practical implementation with low fabrication cost.

KEYWORDS: Flexible energy storage devices; freestanding graphite anode; freestanding lithium iron phosphate cathode; elastomer binder; areal capacity; full battery.

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The ever-increasing popularity of wearable smart electronics (WSE) demands high-performance but cost-effective flexible energy storage solutions, which become one of the major bottlenecks in this emerging industry.1,2 The maximization of electrochemical and ergonomic performances while minimizing material and production costs is highly desired, and often practically balanced. Lithium ion batteries (LIBs), thanks to their relatively high capacity, light weight, and long life-span, have been the mainstream energy storage devices for powering portable electronics.3-5 However, conventional LIBs that utilize metal current collectors and rigid electrode materials could hardly satisfy the ergonomic and durability requirements of wearable devices due to large Young’s moduli, as well as high internal and interfacial stresses of/between each battery component during repeated device cycling and deformation.6,7 First, the internal stress of active materials caused by lithium intercalation and thus volume expansion severely plagues the electrode stability and should be always concerned when designing the electrode structure and composition. Second, the interfacial stress among various battery components such as the binder/active material interface and the current collector/electrode interface should be considered when tackling problems of electrochemical and mechanical stability. Lastly, materials with high elasticity and low Young’s modulus should be always a preferred choice when constructing such flexible devices to achieve the desired ergonomics.

Till now, only a few strategies have been developed to realize the flexibility of LIBs at the system level (full-cell level) although many efforts have been documented to develop high-performance and freestanding anode/cathode materials alone involving sophisticatedly synthesized nanomaterials and composite architecture.4,8,9 In 3



order to construct bendable, twistable and even stretchable LIBs at the system level, four major strategies have been exploited, including 1) lamination with flexible substrates; 2) soft matric connection among micro-sized unit cells; 3) interweaving of 1D fibers into 2D textiles; and 4) paper origami. For example, Hu et al. took the regular copy paper as both flexible support and separator membrane, and laminated LCO/CNT and LTO/CNT on both sides of the paper to construct a full cell with robust mechanical flexibility and high energy density.10 Simon et al. fabricated flexible paper batteries using nano-fibrillated cellulose paper as both separator and electrode binder through a paper-making type process by vacuum filtration of aqueous dispersions containing battery components.11 Similar filtration techniques, allow obtaining electrodes with high Young modulus and with excellent conductivity up to 100 MPa and 15 S m-1, respectively.12-13 Using low-modulus silicone elastomers as substrates, Xu et al. successfully fabricated flexible full batteries with high stretchability incorporating a segmented design in active materials and a ‘self-similar’ interconnect structure among the segments.14 Kwon et al. introduced a cable-type LIB with high mechanical flexibility through a coaxial battery structure with hollow-spiral and multiple-helix anode design and conventional cathode coating.15 These 1D batteries can then be woven into textiles, further promoting the power and energy density.16-20 Taking advantage of the art of origami, Song et al. fabricated a deformable LIB allowing folding, bending and twisting of the full battery, in which CNT-coated Kimwipes were used as the current collectors to load active material layers.21 It is remarkable that all the above methods used to construct flexible LIBs are highly 4


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innovative, each representing a genuine structural design and unique fabrication strategy. Nevertheless, further optimization on performance, safety and cost are imperial to bring these prototype devices into commercialization. Herein, we present another facile method to realize the full flexibility of LIBs with high perspective for practical implementation, adding a new strategic category to the above-mentioned fabrication methodologies for flexible LIBs. Briefly, a high pressure and high temperature (HPHT) filtration apparatus has been employed to fabricate both flexible LIB cathodes and anodes using an elastic latex binder, which enables to form freestanding electrode membranes with high packing density. The fabrication process is environmental-friendly without any organic solvents involved and can be easily scaled up. All raw materials are conventional and sourced from existing industries with no need of any high-cost and sophisticatedly-synthesized components.

It

constitutes

as

a

universal

method

applicable

for

most

electrochemically active materials to fabricate both LIB cathodes and anodes. The adoption of the elastic binder and elimination of current collectors enable to alleviate both internal and interfacial stresses during repeated device cycling and deformation.22 Electrochemical characterizations on both half cells and prototype full cells demonstrated good performance promising for applications in wearable electronics. With all these advantages and further efforts in material and process refinement, it can be expected that this newly invented approach for fabricating flexible LIBs should create a new arena for driving the fabrication of flexible energy storage devices towards ultimate commercialization. An HPHT filtration method, inspired from the Oil & Gas industry, has been employed to fabricate both flexible and freestanding thin-film composites for 5



assembly of LIBs. Figure 1a shows a schematic drawing and a picture of the HPHT filtration apparatus, which in the oil field is typically used for testing the wellbore filtration control of downhole fluids. A downhole fluid with good filtration control is characterized with reduced filtration rate, filtrate volume, filter cake thickness and permeability. Herein, we creatively exploit the filter cake as a means of composite thin film deposition under high pressure and high temperature conditions. This new invention brings several benefits that conventional vacuum filtration cannot achieve. First, many high-viscosity slurries that are impossible to be filtered under regular vacuum pressure (Figure S1) can now be easily and instantaneously filtered under elevated pressure. Second, the high temperature helps dry the deposited composite film much faster, and in some cases even promotes desired hydrolysis or cross-linking reactions. Third and most importantly, the packing density of the deposited composite films is much higher than those made by other film-casting methods owing to the high pressure applied. This is particularly useful when fabricating electrodes for energy storage applications, as a high loading mass/volume ratio is often desired. Lastly, the HPHT filtration setup for thin film deposition is simple, sourced from existing industry, and therefore can be easily scaled up for mass production.

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Figure 1. The HPHT filtration method to fabricate flexible freestanding electrodes. (a) Schematic drawing and picture of the HPHT filtration apparatus. (b) Procedure flowchart for fabricating an exemplary graphite anode. (c) Resulted thin-film graphite anode under various mechanical manipulation. A key aspect of the above film-forming process is the choice of the polymer binder, which is used to coalesce inorganic active and additive materials within the electrode. In this study, latex of polyacrylonitrile-butadiene (NBR) has been chosen to serve as the aqueous binder for LIB electrodes, which brings the following benefits: 1) the use of elastomer helps alleviate the internal stress caused by lithium intercalation of active electrode materials;22 2) the strong adhesion of NBR latex to other components enables to form flexible freestanding thin films with high mechanical strength; 3) in comparison to many other polymeric binders, NBR rubber has higher solvent tolerance and relatively lower electric resistivity, making it possible to fabricate freestanding electrodes without current collectors; 4) latex of NBR has good 7



thermal stability up to 400 °C (Figure S2) and low glass transition temperature down to -40 °C (dependent on the nitrile content), allowing wide-range operational temperature of fabricated LIBs. It is also worth to note that although another similar rubbery material, polystyrene-butadiene (SBR), has been widely used for fabricating LIB anodes, few reports have been seen on using NBR for either LIB cathodes or anodes. Taking the graphite anode as an example, Figure 1b lays out the fabrication procedure involving the HPHT filtration apparatus and the aqueous NBR latex binder. First, both commercial grade graphite (Figure S3) and carbon black are ball-milled together to obtain a homogenous powder mixture, followed by dispersing it into an aqueous carboxy methyl cellulose (CMC) solution through high speed shear mixing. The purpose of CMC is to maintain a sufficient viscosity during the fabrication process ensuring good dispersion of electrode components. Then, the commercial NBR latex concentrate is weighed and added into the above mixture to obtain a film-casting slurry, which is further poured into the stainless-steel vessel of the HPHT filtration apparatus for deposition. The HPHT filtration apparatus has a feedback control for both temperature and pressure, enabling to regulate the thickness and structure of the film. After drying under raised temperature and nitrogen atmosphere, a thin composite film with high packing density and excellent mechanical flexibility (Figure 1c) can be peeled off from the filter paper and tailored into smaller freestanding electrodes for further studies.

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Figure 2. FE-SEM images at various magnifications for both thin-film graphite anode and LiFePO4 cathode. (a and d) Low-magnification bird’s view of the anode and cathode surface, respectively. (b and e) High magnification surface morphology of the anode and cathode, respectively. (c and f) Cross-section view of the thin film anode and cathode, respectively. Fabrication of flexible freestanding thin-film cathodes using active materials such as lithium metal oxides and salts follows exactly the same procedure as Figure 1b except that a lower amount of latex binder can be used when the particle size is smaller (Table S1). To investigate the microstructure of these HPHT-fabricated electrode materials, morphological images were taken using the scanning electron microscope (SEM). Figure 2a and 2d present the bird’s view of the graphite anode and LiFePO4 cathode surface, respectively, captured at low SEM magnification. From both images, densely packed surface morphology with striped patterns can be clearly visualized. These images were taken on the bottom side of the electrode film in contact with the filter paper, and hence the striped pattern could be a replica of the underlying filter paper surface owing to the applied high pressure. On the top side of these electrode films, similar stripped pattern was not observed and the carbon black 9



and latex are homogenously dispersed in the electrodes as shown in Figure 2b and 2e for the graphite anode and LiFePO4 cathode, respectively. EDX mapping further confirmed the homogeneity of the latex distribution within the electrodes (Figure S5). Figure 2c and 2f respectively shows the cross-section morphology of a thin-film graphite anode and LiFePO4 cathode. As a result of the high pressure applied, these thin-film electrodes are densely packed. Due to the larger flake size and lower material density, graphite anodes are typically thicker than LiFePO4 cathodes. We also note that the thickness of the fabricated thin-film electrodes is pressure dependent, with higher applied pressure resulting in lower thickness (Figure S6). As a result, at the applied pressure of 600 psi, the average film thickness for graphite anodes is 140 ± 30 µm, whereas for LiFePO4 cathodes is 110 ± 20 µm. The high packing density caused by high positive pressure applied helps lower the overall electrode volume, enhance electrode conductivity, and therefore promote charge transportation (Table S2). As a significant benefit brought by adopting the HPHT filtration method, as well as the NBR latex binder, fabrication of freestanding and conductive thin-film electrodes with superior mechanical flexibility was enabled. To testify this, stress-strain profiles were acquired using a tensile tester for both graphite anodes and LiFePO4 cathodes. Figure 3a presents a representative stress-strain curve taken on the thin-film graphite anode. Three regions exhibiting different stress-strain behavior (marked by different color) are clearly seen. The green section of the curve refers to the beginning of the experiment when the sample is still loose and bent without tensile load. We notice that even the film is relaxed and bent, a very small force is still necessary to pull it straight, probably to counter against the gravitational centroid 10


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change.

Figure 3. Stress-strain profiles of the freestanding electrode films. (a) Stress-strain curve of the graphite anode is divided into three regions, representing pre-tensile, tensile and fractured status, respectively. (b) Stress-strain curve of the LiFePO4 cathode with pictures of the pre-tensile, tensile and fractured thin film, respectively. On both graphs, the dotted black curves are Ramberg-Osgood fittings of the tensile region. As soon as a tensile load is applied onto the film, a quasi-linear behavior can be observed in the beginning, but soon turns into a power law relationship, as shown by the pink region of Figure 3a. This behavior is consistent with typical stress-strain relationship of hardened materials, of which the plastic strain composes a significant portion of the stress-strain profile. It is well known that for most materials, the stress-strain behavior can be described using the non-linear Ramberg-Osgood relationship,23 which is constituted by two parts of strain, i.e., the elastic part and plastic part. By fitting the pink region of all acquired stress-strain curves to the Ramberg-Osgood equation (Eq. S1), we were able to extract an average Young’s 11



modulus of 84 ± 8 MPa for thin film graphite anodes, and 312 ± 29 MPa for the LiFePO4 cathodes. Similar stress-strain behavior and mechanical strength were observed on a binder-free composite electrode composed of LiCoO2 and super-aligned CNTs.24 In plastic elongation of the thin film, the electrode material starts to yield and its deformation is no longer reversible. Once the fracture point is reached, the electrode film begins to break and the tensile stress drops accordingly, as indicated by the purple region in Figure 3a and shown by the rightmost picture in Figure 3b. Comparing the stress-strain profiles of graphite anodes with those of LiFePO4 cathodes, one can see that both Young’s modulus and fracture stress of the later is significantly higher, whereas the former has higher strain values. The high modulus of LiFePO4 cathodes is likely due to their more compact microstructure resulted from finer particle size (Figure 2e) and the less elastomer binder used (Table S1). Moreover, mechanical durability tests were conducted using the tensile tester on both graphite and LiFePO4 electrodes by repeatedly bending and pulling the clamped thin film within the predefined elastic deformation region, set to 1.5% strain for both materials. All tested materials lasted over 500 bending and stretching cycles without any sign of fatigue damage. To assess the potential of the as-prepared thin-film materials for using as LIB electrodes, their electrochemical properties were carefully characterized through half-cell assemblies using lithium metal as the counter electrode. No current collector was used throughout all these tests. Cyclic voltammograms (CV) of half-cell vs Li/Li+ in a potential range of 0.01 - 2 and 2 - 4.2 V were acquired for the graphite and LiFePO4 electrodes, respectively. At a scan rate of 0.05 mV s-1, the first cycle of 12


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graphite lithiation shows two broad peaks at 0.6 and 1.7 V, well known as a result of SEI formation.25 Towards more negative potential, another two cathodic peaks at 0.01 and 0.2 V can be ascribed to different stages of Li+ interaction into graphite.26 At positive sweep, one split anodic peak is located at around 0.25 V, associated with graphite delithiation. In subsequent cycles, while both of the cathodic peaks at 0.6 and 1.7 V disappear, indicating good SEI stability, other peaks are well superimposed, indicating good stability and reversibility of the graphite electrode during cycling. In contrast, on CV curves taken on the LiFePO4 electrode (Figure 4b), two broad peaks are prominent, corresponding to a reversible two-phase reaction between the alternating lithiated LiFePO4 phase and delithiated Li1-xFePO4 phase.27,28 Figure 4c and d show the early first, second and tenth cycle of charge-discharge curves for the graphite and LiFePO4 electrodes vs lithium metal, respectively. In both cases, a charging and discharging rate of 0.1 C was used. It is important to note here that due to the high mass loading of both Graphite and LiFePO4 electrodes, 11.49 mg cm-2 and 17.59 mg cm-2 respectively, the current rate of 0.1 C corresponds to an areal current density of 0.32 mA cm-2 for the graphite electrode and 0.23 mA cm-2 for the LiFePO4 electrode. These are indeed very high current rate when compared with most literature studies involving nanomaterials with smaller mass loading.8,29 On the graphite electrode, the first discharging and charging capacity are 512.5 and 350.0 mAh g-1, respectively, corresponding to a calculated Coulombic efficiency of 68.3%. Apparently, although most of the graphite active material is exploited contributing to the reversible electrode capacity (~350 mAh g-1), some irreversible portion (more than 150.0 mAh g-1) is given by other electrode components.25,30,31 This significantly lowers the initial Coulombic efficiency when compared to the state-of-art LIB graphite anodes with current collector (>90%), most likely resulted from the larger 13



amount of binder used, the thicker electrode film and the lack of current collector. In sequential cycles, the plateau at 0.7 V observed from the first cycle disappears and is replaced by gradual discharge slopes, resonating with the CV analysis described above. From the second cycle onward, the discharging capacity become stable and the Coulombic efficiencies are increasingly approaching to 100%, which confirms no significant SEI formation in subsequent cycles. On the other side, the first cycle of the LiFePO4 electrode delivers an initial charging capacity of 160.6 mAh g-1 and discharging capacity of 158.4 mAh g-1 (with an initial Coulombic efficiency of 98.6%), very close to its theoretical capacity (170 mAh g-1). The long plateau of the charging voltage at 3.5 V and discharging voltage at 3.4 V indicates a steady operational performance of the cathode.32,33

Both Graphite and LiFePO4 electrodes gave satisfactory rate capabilities when tested from 0.1 to 1 C as exemplified in Figure 4e and f. Again here 1 C equals to 3.2 mA cm-2 for the graphite electrode and 3.0 mA cm-2 for the LiFePO4 electrode. Upon switching back to the initial current rate, the capacity was well recovered for both electrodes. As for the cycling stability, with no current collector used both graphite and LiFePO4 electrodes exhibit fairly good cycle performance in half-cells against lithium metal. For example, at a current density of 0.32 mA cm-2, the graphite electrode exhibited an initial areal capacity of 3.02 mAh cm-2 and 2.47 mAh cm-2 after 50 cycles, accounting for a capacity retention of 81.5% (Figure 4g). On the other side, the LiFePO4 electrode has a starting capacity of 2.12 mAh cm-2 and 1.47 mAh cm-2 after cycling 50 times with 69.3% capacity retention (Figure 4h). We noted a drop of capacity for all examined LiFePO4 electrodes after about 20 cycles of stable operation 14


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(despite of a very stable operation in the first 20 cycles), which might be attributed to the expedited electrolyte consumption by the high mass-loading electrodes at the high operational voltage range applied.25 A high-volume cell assembly capable of accommodating more electrolytes and a milder operational condition would give a more stable cycling performance for these electrodes. To further examine the structural stability of electrodes after the electrochemical tests, coin cells composed of graphite and LiFePO4 electrodes were disassembled in the glove box, followed by cleaning and drying, and then subjected to SEM characterizations. Both graphite and LiFePO4 electrodes after rate tests retain an integral structure without any cracks or fractures, but exhibit an apparent SEI layer in comparison to their fresh status (Figure S7). To understand the ohmic resistivity and Li+ diffusion in the freestanding graphite and LiFePO4 electrodes, electrochemical impedance spectroscopy (EIS) was taken on half-cells before the cycling test, as well as right after the 20th cycle in their fully delithiated states at 0.1C. As shown in Figure 4i and j, Nyquist plots for both graphite and LiFePO4 electrodes exhibit similar features, consisting of a depressed semi-circle at high frequency and a long slope in the low frequency region. The large semi-circles for the fresh electrodes most likely come from the SEI layers (RSEI), whereas the long slopes are associated with the impedance of lithium diffusion inside the electrodes.34,35 After 20 cycles, the semi-circles of both electrodes decreased significantly, likely due to better electrolyte infiltration and well-established electrolyte-electrode interface. Meanwhile, the semi-circle onset values for both graphite and LiFePO4 electrodes before cycling are around 3 ohms (inserts in Figure 15



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4i, j), indicating similar electrolyte conditions (Re). After the 20th charge-discharge cycle, Re increased to 3.9 ohms for the graphite electrode and 5.8 ohms for the LiFePO4

electrode,

respectively,

which

may

help

explain

the

expedited

decomposition/consumption of electrolyte by the LiFePO4 electrode as mentioned above.36

16


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Figure 4. Electrochemical performance of the freestanding electrodes in half-cell assemblies. (a and b) Cyclic voltammograms. (c and d) First 10 cycles of 17



galvanostatic charge-discharge profiles at 0.1C. (e and f) Rate performance. (g and h) Cycling performance. (i and j) Nyquist plots of the as-prepared graphite and LFP electrodes and after 20 cycles at 0.1C. From the above electrochemical characterizations of half-cells, the high areal capacity obtained at the high operational current density in this study is above of most reported literature values (Figure 5),29 which, together with the good mechanical strength, endorses potential realistic applications in flexible LIBs. The uniform dispersion of the NBR latex binder and conductive carbon within the slurry of active materials endows the electrodes with sufficient electric conductivity, enabling the elimination of current collectors, which in turn avoids the interfacial stress between the electrode material and the current collector in a conventional cell setup. The relatively lower initial Coulombic efficiencies observed on the graphite anodes than on those LiFePO4 cathodes are likely caused by the irreversible capacity loss from the larger amount of binder used. From the cycling and EIS studies, it is possible that the same kind of electrolyte in the LiFePO4 half-cells consumes faster than in the graphite half-cells, resulting in the slightly inferior stability of the cathodes. Nevertheless, it is worth to note that these are still proof-of-concept studies, and it is not fair to compare our conceptual results targeted for flexible batteries with other low mass-loading studies using current collectors. It is expected that with more efforts gathered to further optimize the electrodes, a further leap of performance should be on its way.

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Figure 5. Comparison of the areal capacity vs. current density of graphite anodes in this study with other freestanding anodes in literature. With both flexible cathodes and anodes successfully made, demonstration of a prototype full cell capable of withstanding rigorous deformation would naturally be the next task. For this purpose, pouch cells based on the simple structure illustrated in Figure 6a containing no current collector were fabricated, giving a good voltage reading after charging (Figure 6b). Figure 6d shows the charge/discharge profile for the initial cycle at 17 mA g-1 between 2.8 to 4.2 V, exhibiting a first discharge capacity up to 91.1 mAh g-1 with an initial Coulombic efficiency of 94.1% based on the weight of cathode active materials. In addition, the battery retained a specific capacity of 68.5 mAh g-1 after 20 cycles (Figure 6e). Figure 6c demonstrate lighting of an LED using the fabricated pouch cell under bent conditions. Encouragingly, the full cell can be deformed in any fashion without significant loss of electrochemical performance (Supporting Video), signifying its superior mechanical flexibility and electrochemical stability. 19



Figure 6. Demonstration of flexible full-cell LIBs. (a) Schematic illustration of the structure of fabricated full pouch cells. (b and c) Charge-discharge and cycle performance of full cell, respectively. (d) A pouch cell showing steady voltage output at the flat form. (e) Lighting of an LED by a pouch cell under bending curvature. To summarize, a novel method involving HPHT filtration has been developed to fabricate thin film electrodes with high packing density. The method is universal and can be employed for fabricating both LIB cathodes and anodes without much modification, and is applicable for most electrochemically active materials. Utilization of NBR latex as the aqueous binder enables to achieve freestanding electrodes with good conductivity and mechanical strength. The intrinsic elasticity of the binder not only helps alleviate the internal stress caused by cyclic volume expansion of active materials, but also mitigate the interfacial stress during repeated deformation through elimination of current collectors. Throughout the electrode 20


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fabrication process, there is no any organic solvent involved, demonstrating excellent environmental benignity. In addition, the fabrication process and equipment can be easily scaled up, and all materials used in this study are conventional and standard from existing LIB and chemical industries with no sophisticatedly-synthesized components required. Using half-cell assemblies vs. Lithium metal, electrochemical characterizations on a pair of proof-of-concept graphite anode and LiFePO4 cathode revealed large areal capacity at high current density with high Coulombic efficiency. Prototype full pouch cells constructed using the graphite anode and LiFePO4 cathode demonstrate excellent deformability and power output, promising for applications in wearable electronics upon further optimization in component chemistry, fabrication process, electrode parameters, and battery structure. ASSOCIATED CONTENT Supporting Information Available: Detailed experimental steps; TGA of the latex; SEM and XRD of LiFePO4 and graphite; EDX mapping of graphite and LiFePO4 electrodes; cross section SEM images of graphite and LiFePO4 electrodes under different pressure; SEM morphology of graphite and LiFePO4 after cycling performance. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ; [email protected]. ORCID Xiaohui Zhao: 0000-0002-2060-6961 Chenglin Yan: 0000-0003-4467-9441 21



Yang Peng: 0000-0002-6780-2468 Zhao Deng: 0000-0002-0008-5759

Notes There is no conflict of interests to declare.

ACKNOWLEDGEMENTS This work was supported by the 1000 Young Talents Program of China, Natural Science Foundation of China (No. 21701118), and Natural Science Foundation of Jiangsu Province (No. BK20161209 and No. BK20160323). We also extend our sincere appreciation to the support by Suzhou Key Laboratory for Advanced Carbon Materials and Wearable Energy Technologies.

REFERENCES

(1) Nishide, H.; Oyaizu, K. Toward flexible batteries. Science 2008, 319, 737-738, DOI: 10.1126/science.1151831 (2) Stoppa, M.; Chiolerio, A. Wearable electronics and smart textiles: a critical review. Sensors 2014, 14, 11957-11992, DOI: 10.3390/s140711957 (3) Scrosati, B.; Garche, J. Lithium batteries: Status, prospects and future. J. Power

Sources 2010, 195, 2419-2430, DOI: 10.1016/j.jpowsour.2009.11.048 (4) Zhou, G.; Li, F.; Cheng, H.-M. Progress in flexible lithium batteries and future prospects. Energy Environ. Sci. 2014, 7, 1307-1338, DOI: 10.1039/C3EE43182G 22


Page 22 of 29

Page 23 of 29 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


(5) Armand, M.; Tarascon, J.-M. Building better batteries. Nature 2008, 451, 652-657, DOI: 10.1038/451652a (6) Gaikwad, A. M.; Arias, A. C.; Steingart, D. A. Recent progress on printed flexible batteries: mechanical challenges, printing technologies, and future prospects.

Energy Technol. 2015, 3, 305-328, DOI: 10.1002/ente.201402182 (7) Harris, K.; Elias, A.; Chung, H.-J. Flexible electronics under strain: a review of mechanical characterization and durability enhancement strategies. J. Mater. Sci. 2016,

51, 2771-2805, DOI 10.1007/s10853-015-9643-3 (8) Sun, Y.; Liu, N.; Cui, Y. Promises and challenges of nanomaterials for lithium-based

rechargeable

batteries.

Nat. Energy

2016,

1,

16071,

DOI:

10.1038/nenergy.2016.71 (9) Li, N.; Chen, Z.; Ren, W.; Li, F.; Cheng, H.-M. Flexible graphene-based lithium ion batteries with ultrafast charge and discharge rates. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 17360-17365, DOI: 10.1073/pnas.1210072109 (10) Hu, L.; Wu, H.; La Mantia, F.; Yang, Y.; Cui, Y. Thin, flexible secondary Li-ion paper batteries. ACS nano 2010, 4, 5843-5848, DOI: 10.1021/nn1018158 (11) Simon, L.; Cornell, A.; Lindbergh, G.; Wågberg, L. Single-paper flexible Li-ion battery cells through a paper-making process based on nano-fibrillated cellulose. J. Mater. Chem. A 2013, 1, 4671-4677, DOI: 10.1039/C3TA01532G (12) Lara, J.; Destro, M.; Chaussy, D.; Gerbaldi, C.; Penazzi, N.; Bodoardo, S.; 23



Page 24 of 29

Beneventi, D. "Flexible cellulose/LiFePO4 paper-cathodes: toward eco-friendly all-paper

Li-ion

batteries. Cellulose 2013,

20,

571-582,

DOI:

10.1007/s10570-012-9834-x (13) Shaomei, C.; Feng, X.; Song, Y.; Xue, X.; Liu, H.; Miao, M.; Fang, J.; Shi, L. Integrated fast assembly of free-standing lithium titanate/carbon nanotube/cellulose nanofiber hybrid network film as flexible paper-electrode for lithium-ion batteries. ACS

Appl.

Mater.

Interfaces. 2015,

7,

10695-10701,

DOI: 10.1021/acsami.5b02693 (14) Xu, S.; Zhang, Y.; Cho, J.; Lee, J.; Huang, X.; Jia, L.; Fan, J. A.; Su, Y.; Su, J.; Zhang, H. Stretchable batteries with self-similar serpentine interconnects and integrated wireless recharging systems. Nat. Commun. 2013, 4, 1543, DOI: 10.1038/ncomms2553 (15) Kwon, Y. H.; Woo, S. W.; Jung, H. R.; Yu, H. K.; Kim, K.; Oh, B. H.; Ahn, S.; Lee, S. Y.; Song, S. W.; Cho, J. Cable‐type flexible lithium ion battery based on hollow multi ‐ helix electrodes. Adv. Mater. 2012, 24, 5192-5197, DOI: 10.1002/adma.201202196 (16) Jing, R.; Li, L.; Chen, C.; Chen, X.; Cai, Z.; Qiu, L.; Wang, Y.; Zhu, X.; Peng, H. Twisting carbon nanotube fibers for both wire‐shaped micro‐supercapacitor and micro‐battery Adv. Mater. 2013, 25, 1155-1159, DOI: 10.10.1002/adma.201203445 (17) Wei, W.; Sun, Q.; Zhang, Y.; Lin, H.; Ren, J.; Lu, X.; Wang, M.; Peng, H. "Winding aligned carbon nanotube composite yarns into coaxial fiber full batteries 24


Page 25 of 29 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


with high performances." Nano Lett. 2014, 14, 3432-3438, DOI: 10.1021/nl5009647 (18) Zeng, W.; Shu, L.; Li, Q.; Chen, S.; Wang, F.; Tao, X. M. Fiber‐based wearable electronics: a review of materials, fabrication, devices, and applications. Adv. Mater. 2014, 26, 5310-5336, DOI: 10.1002/adma.201400633 (19) Zhang, Y.; Zhao, Y.; Ren, J.; Weng, W.; Peng, H. Advances in Wearable Fiber‐ Shaped Lithium ‐ Ion Batteries. Adv. Mater. 2016, 28, 4524-4531, DOI: 10.1002/adma.201503891 (20) Ren, J.; Zhang, Y.; Bai, W.; Chen, X.; Zhang, Z.; Fang, X.; Weng, W.; Wang, Y.; Peng, H. Elastic and wearable wire ‐ shaped lithium ‐ ion battery with high electrochemical performance. Angew. Chem. 2014, 126, 7998-8003, DOI: 10.1002/ange.201402388 (21) Song, Z.; Ma, T.; Tang, R.; Cheng, Q.; Wang, X.; Krishnaraju, D.; Panat, R.; Chan, C. K.; Yu, H.; Jiang, H. Origami lithium-ion batteries. Nat. Commun. 2014, 5, 3140, DOI: 10.1038/ncomms4140 (22) Choi, S.; Kwon, T.-w.; Coskun, A.; Choi, J. W. Highly elastic binders integrating polyrotaxanes for silicon microparticle anodes in lithium ion batteries.

Science 2017, 357, 279-283, DOI: 10.1126/science.aal4373 (23) Patel, S. A.; Venkatraman, B.; Bentson, J. A stress-strain relation for inelastic material behavior. J. Frankl. Inst-Eng. Appl. Math. 1963, 275, 98-106, DOI: 10.1016/0016-0032(63)90856-1

25



Page 26 of 29

(24) Luo, S.; Wang, K.; Wang, J.; Jiang, K.; Li, Q.; Fan, S. Binder ‐ free LiCoO2/carbon nanotube cathodes for high‐performance lithium ion batteries. Adv.

Mater. 2012, 24, 2294-2298, DOI: 10.1002/adma.201104720 (25) An, S. J.; Li, J.; Daniel, C.; Mohanty, D.; Nagpure, S.; Wood, D. L. The state of understanding of the lithium-ion-battery graphite solid electrolyte interphase (SEI) and its relationship to formation cycling. Carbon 2016, 105, 52-76, DOI: 10.1016/j.carbon.2016.04.008 (26) Reynier, Y.; Yazami, R.; Fultz, B. Thermodynamics of lithium intercalation into graphites and disordered carbons. J. Electrochem. Soc. 2004, 151, A422-A426, DOI: 10.1149/1.1646152 (27) Hu, L.-H.; Wu, F.-Y.; Lin, C.-T.; Khlobystov, A. N.; Li, L.-J. Graphene-modified LiFePO4 cathode for lithium ion battery beyond theoretical capacity. Nat. Commun. 2013, 4, 1687, DOI: 10.1038/ncomms2705 (28) Zhou, Y.; Wang, J.; Hu, Y.; O’Hayre, R.; Shao, Z. A porous LiFePO4 and carbon nanotube composite. Chem. Commun. 2010, 46, 7151-7153, DOI: 10.1039/C0CC01721C (29) Sun, H.; Mei, L.; Liang, J.; Zhao, Z.; Lee, C.; Fei, H.; Ding, M.; Lau, J.; Li, M.; Wang, C. Three-dimensional holey-graphene/niobia composite architectures for ultrahigh-rate

energy

storage.

Science

2017,

356,

599-604,

DOI:

10.1126/science.aam5852 (30) Verma, P.; Maire, P.; Novák, P. A review of the features and analyses of the 26


Page 27 of 29 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


solid electrolyte interphase in Li-ion batteries. Electrochim. Acta 2010, 55, 6332-6341, DOI: 10.1016/j.electacta.2010.05.072 (31) Masaki, Y.; Nishigaki, Nobuhide.; Nishishita, S.; Matsui, Y.; Sugimoto, T.; Kikuta, M.; Higashizaki, T.; Kono, Michiyuki.; Ishikawa. M. Charge–discharge behavior of graphite negative electrodes in bis (fluorosulfonyl) imide-based ionic liquid and structural aspects of their electrode/electrolyte interfaces. Electrochim Acta 2013, 110, 181-190, DOI: 10.1016/j.electacta.2013.03.018 (32) Cabán-Huertas, Z.; Ayyad, O.; Dubal, D. P.; Gómez-Romero, P. Aqueous synthesis of LiFePO4 with Fractal Granularity. Sci. Rep. 2016, 6, 27024, DOI: 10.1038/srep27024 (33) Kim, J.-K.; Choi, J.-W.; Chauhan, G. S.; Ahn, J.-H.; Hwang, G.-C.; Choi, J.-B.; Ahn, H.-J. Enhancement of electrochemical performance of lithium iron phosphate by controlled sol–gel synthesis. Electrochim. Acta 2008, 53, 8258-8264, DOI: 10.1016/j.electacta.2008.06.049 (34) Tang, J.; Yang, J.; Zhou, X.; Yao, H.; Zhou, L. A porous graphene/carbon nanowire hybrid with embedded SnO2 nanocrystals for high performance lithium ion storage. J. Mater. Chem. A 2015, 3, 23844-23851, DOI: 10.1039/C5TA06859B (35) Levi, M.; Aurbach, D. Simultaneous measurements and modeling of the electrochemical impedance and the cyclic voltammetric characteristics of graphite electrodes doped with lithium. J. Phys. Chem. B 1997, 101, 4630-4640, DOI: 10.1021/jp9701909 27



(36) Wan, J.; Kaplan, A. F.; Zheng, J.; Han, X.; Chen, Y.; Weadock, N. J.; Faenza, N.; Lacey, S.; Li, T.; Guo, J. Two dimensional silicon nanowalls for lithium ion batteries. J. Mater. Chem. A 2014, 2, 6051-6057, DOI: 10.1039/C3TA13546B (37) Billaud, J.; Bouville, F.; Magrini, T.; Villevieille, C.; Studart, A. R. Magnetically aligned graphite electrodes for high-rate performance Li-ion batteries.

Nat. Energy 2016, 1, 16097, DOI: 10.1038/nenergy.2016.97 (38) Xiao, Q.; Fan, Y.; Wang, X.; Susantyoko, R. A.; Zhang, Q. A multilayer Si/CNT coaxial nanofiber LIB anode with a high areal capacity. Energy Environ.

Science 2014, 7, 655-661, DOI: 10.1039/C3EE43350A (39) Wang, X.; Sun, L.; Susantyoko, R. A.; Zhang, Q. A hierarchical 3D carbon nanostructure for high areal capacity and flexible lithium ion batteries. Carbon 2016,

98, 504-509, DOI: 10.1016/j.carbon.2015.11.049 (40) Wang, X.; Yan, C.; Yan, J.; Sumboja, A.; Lee, P. S. Orthorhombic niobium oxide nanowires for next generation hybrid supercapacitor device. Nano Energy 2015,

11, 765-772, DOI: 10.1016/j.nanoen.2014.11.020 (41) Jeong, S.; Li, X.; Zheng, J.; Yan, P.; Cao, R.; Jung, H. J.; Wang, C.; Liu, J.; Zhang, J.-G. Hard carbon coated nano-Si/graphite composite as a high performance anode for Li-ion batteries. J. Power Sources 2016, 329, 323-329, DOI: 10.1016/j.jpowsour.2016.08.089 (42) Zeng, W.; Zheng, F.; Li, R.; Zhan, Y.; Li, Y.; Liu, J., Template synthesis of SnO2/α-Fe2O3 nanotube array for 3D lithium ion battery anode with large areal 28


Page 28 of 29

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capacity. Nanoscale 2012, 4, 2760-2765, DOI: 10.1039/C2NR30089C

TOC Graphic.

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Freestanding Electrode Pairs with High Areal Density Fabricated

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