Free-standing 3D sponged nano-fibre electrodes for ultrahigh-rate

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Free-standing 3D sponged nano-fibre electrodes for ultrahigh-rate energy storage devices Marco Agostini, Du Hyun Lim, Sergio Brutti, Niklas Lindahl, Jou-Hyeon Ahn, Bruno Scrosati, and Aleksandar Matic ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09746 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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ACS Applied Materials & Interfaces

Free-standing 3D sponged nano-fibre electrodes for ultrahigh-rate energy storage devices Marco Agostini*1, Du Hyun Lim1, Sergio Brutti2, Niklas Lindahl1, Jou Hyeon Ahn3, Bruno Scrosati*4, Aleksandar Matic*1 1 2

3

Department of Physics, Chalmers University of Technology, SE41296 Göteborg, Sweden CNR-ISC, U.O.S. Sapienza, Piazzale A. Moro 5, 00185, Roma, Italy Department of Chemical Engineering and Research Institute for Green Energy Convergence

Technology, Gyeongsang National University, 900 Gajwa-dong, Jinju 660-701, Republic of Korea 4

Helmholtz-Institut Ulm (HIU), Ulm, Germany

*Corresponding authors: [email protected]; [email protected]; [email protected]

Keywords Fast charging Li-batteries anode, 3D sponged nanofibers electrode, Li-ion batteries, Freestanding electrodes materials, high gravimetric energy density.

Abstract We have designed a self-standing anode built-up from highly conductive 3D sponged nano-fibres, i.e. with no current collectors, binders, or additional conductive agents. The small diameter of the fibres combined with an internal sponge-like porosity results in short distances for lithium-ions diffusion and 3D pathways that facilitates the electronic conduction. Moreover, functional groups at the fibre surfaces lead to the formation of a stable solid-electrolyte interphase. We demonstrate that this anode enables the operation of Li-ion cells at specific

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currents as high as 20 A g-1 (approx. 50C) with excellent cycling stability and an energy density which is >50% higher than what is obtained with a commercial graphite anode.

Introduction Climate change is unquestionably modern societies’ greatest challenge1,2. As a result, the interest in renewable energy sources and the development of electric vehicles has increased with the aim to drastically reduce CO2 emissions in urban areas over the next 20 years3. The production of electric energy from renewable sources (e.g. sun and wind) is mostly intermittent and also dependent on seasons. Thus, in parallel with the development of energy conversion technologies, large scale and high capacity energy storage systems need to be developed in in order to allow load levelling in grid applications and electrification of the transport sector. Key performance of next generation energy storage technologies are obviously a high energy density and long cycle life, but also high rate of charge and discharge, i.e. high power density. In addition, improvement of sustainability aspects, in terms of materials used and recycling, is crucial when mass implementation in large scale applications is around the corner. During the last decades there has been a large focus on the development of rechargeable lithium ion batteries, in particular for transport applications4. Although commercially successful, Li-ion battery technology is rapidly reaching its limits, due to the intrinsic limitation in energy density of the chemistry involved and the power density due to the design of the battery componets5,6. Looking inside the core, Li-ion batteries exploit active materials in the anode and cathode which are mixed with conducting agents (i.e. different types of carbons) and polymers as binders and the resulting composite material is casted on a metal current collector. Thus, the electrode often contains only around 40%-60% of active material, which limits the practical energy density. In


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addition, this configuration results in a complex and sluggish diffusion of both lithium and electrons when the loading is increased which severely limits the charge/discharge rates. To overcome the limitations in rate capability new nanostructured materials have been designed7,8. The engineering of materials at the nanoscale allows the reduction of ion/electron transport distances. Successful examples of this approach are provided by the development of new nanostructured anodes9,10 using for instance lithium alloy materials, such as Si11,12, Sn13,14, Al15, P16,17. Despite demonstrating promising specific capacity and rate capability, and addressing problems related to volume expansion/cracking, the nano-materials are still far from practical applications. When the loading of the active material is increased, to meet the demands in energy density in practice, the thickness of the electrode increases and the nanostructured morphology is stacked in micrometric aggregates, increasing diffusion lengths of ions and electrons. As an example, it has been demonstrated that tuning the morphology of graphite can lead to improved electrochemical performance18,19. By introducing disorder in the graphite structure20,21 or by exfoliating the graphite to just a few graphene layers22, higher specific capacity and rate performance can be achieved. However, these strategies lead to anodes with low mass loading and when increasing the tap-density the graphene layers re-stack in a graphitic morphology strongly reducing the electrodes performance. Here, we demonstrate how 3D carbon nano-fibre anodes can provide a solution. Our strategy is to design, using an easily scalable route, a simple low-cost material that is a selfstanding carbo-fibre based anode. The morphology is engineered on several length scales starting from the nano-fibre network and down to a sponge-like inner porosity of the fibres. The highly conductive and self-supporting structure requires neither current collector, binder nor conductive agent thus maximizing the use of active material. We show that anodes are able to deliver and



take up lithium at current rates as high as 20 A g-1 due to the small diameter and sponge-like morphology of the fibres with short transport distance for lithium-ion diffusion and conducting 3D pathways that facilitates the electronic conduction. The free-space between the fibres acts as a buffer for the electrode expansion during lithium uptake/delivery.

Results and discussion The preparation of the anode starts from electrospinning of a polymer composite, polyacrylonitrile solution containing silica nanoparticle (PAN/SiO2), membrane with randomly aligned fibres (Fig. 1a).

Figure 1| Schematic synthesis of the free standing 3D sponged nano-fibre membranes. a Electrospinning of PAN/DMF/SiO2 solution through syringe needle (0.6 mm) at constant flow rate of 0.1 ml/min and DC voltage of 20 kV. b Carbonization of the fibre mat at 1000 °C under N2 flow (800 ml min-1) for 12 h and c following by HF etching for 24 h. d 3D self-standing sponged nano-fibre membrane, width 2 cm, length 6 cm, thickness 50 to 100 µm. The presence of SiO2 nanoparticles play a central role when designing the self-supporting CNF anode. Indeed, during the electrospinning phase the nanoparticles are incorporated in the carbon-fibre.

Subsequently, the membrane is carbonized at high temperature and the SiO2 nanoparticles removed by HF-etching (Fig. 1b-c), introducing the sponge-like inner morphology of the fibres.


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The resulting 3D carbon nano-fibre membrane is self-standing, flexible and highly conductive (Fig. 1d). Typically, the membranes have a thickness in the range 50-100 µm and a mass loading between 3 to 6 mg cm-2 and an electrical conductivity around 1 S cm-1, a value 3-times higher than that of carbon/graphite based electrodes. Field emission scanning electron microscopy (FE-SEM) images show that there is free volume between the fibres in the membrane (Fig. 2a) and a nano-porous morphology of the fibres (Fig. 2b). Transmission electron microscopy (TEM) shows that this porosity extends to the interior of the fibres forming a sponge like structure (Fig. 2c). From the TEM images the presence of graphene layers building up the walls of the pores at the nanoscale is revealed (Fig. 2d). The specific surface area of the 3D sponged fibres is 82 m2 g-1 (Fig. 2d, top) while the pore size is distributed between 5 to 15 nm (Fig. 2d bottom). For the pristine fibers electrode the surface is only 8 m2 g-1, while the low presence of pores is distributed between 10 to 50 nm (see Fig. S1 in the supplementary information section). The rather low surface area is desirable to reduce electrolyte decomposition and the consequent irreversible capacity during the first discharge of the anode, an important parameter when designing full Li-ion cell configurations. While the nano-porosity of the fibres allows fast uptake/release of lithium during the electrochemical process, the presence of graphene bi-layers at the nanoscale guarantees fast Li+ transport in the carbon structure23. There is a considerable disorder of the graphene layers as revealed by Raman spectroscopy from the I(D)/I(G) intensity ratio (Fig. 2e). The high intensity ratio (〜 2.6) indicates the presence structural defects in the carbon nano-fibres24. The presence of extended structural defects is also confirmed by synchrotron X-ray diffraction (see the supplementary material Fig. S2) that highlights the presence of the broad γ and Π bands in the



2Θ range between 10°-20° due to the occurrence of turbostratic in-layer corrugations, stacking disorder24 and covalently bonded hydrogen25,26.

Figure 2| Characterization of the free standing 3D sponged nano-fibres. a,b Field emission scanning electron microscopy (FE-SEM) images at low and high magnification. c Transmission electron microscopy (TEM) and d Nitrogen adsorption desorption isotherms (top) and corresponding pore size distribution (down). e Raman spectrum measured using a 532 nm laser excitation wavelength. f XPS-analysis for C 1s, N 1a, O 1s, and F 1s core levels. As a result of the synthesis procedure, with a polymer precursors and thermal and etching treatments we expect that the fibre surface contains functional groups. X-ray photoelectron spectroscopy (XPS) analysis was thus performed to gain a better understanding of the composition of the surface. Fig. 2f shows the XPS spectra and we observe C1s core level peaks at 284.8 eV and 286.3 eV, related with C-C and C-N groups respectively27,28, N1s peaks at 398 eV and 400 eV, assigned to pyridinic (N-6) and quaternary nitrogen (N-Q) which are products of PAN decomposition29. The O1s core level peak is found at 531.5 eV confirming the presence of


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adsorbed O species27, while the F1s peaks at 685 eV and 688 eV reveal the presence of semiionic and covalent C-F groups respectively30. The fluorination of the surface can be traced back to the HF treatment and is expected to mitigate the decomposition of the electrolyte in the first discharge and to promote the formation of a stable solid electrolyte interphase (SEI)31,32. The elemental composition of the nano-fibre membrane is further confirmed by SEM-EDS analysis (see supplementary Fig. S3 and table S1). For the design of efficient, safe and stable Li-ion batteries, an important parameter is given by the working voltage vs. Li+/Li for both cathode and anode. Since the cathode should have high working voltage in order to achieve high energy density, the anode should have a working voltage as low as possible to increase the potential difference. However, when the anode works at voltages close to 0 V lithium metal plating could take place at the surface of the anode, in particular at high current-rates, giving rise to safety issues. Our nano-fibres anode works at a safe average voltage, ranging between 0.5 to 1.2 V at 0.1 and 20 A g-1 respectively, during galvanostatic charge. The rather disordered structure of the carbon matrix undergoes a partial ordering after the first lithium intercalation/de-intercalation cycle, with a decrease of the turbostratic/stacking disorder and hydrogen-content33,34, as shown by post mortem synchrotron XRD and Raman experiments (see supplementary Figs. S2 and S4). After this restructuring during the first galvanostatic cycle the structure of the carbon matrix is stable. The processes involved in lithium ion storage and transfer kinetics are further investigated by cyclic voltammetry (CV) analysis at increasing scan rates (supplementary Fig. S5), in order to decouple the capacitive (non-Faradic, Li+ adsorbed onto the 3D sponged nanofibers surface) and the diffusion controlled (Faradic, Li+ intercalation) contributions to the overall capacity (see Methods section for technical details)35. In CV the current response is a function of the scan rate



and follows the expression i(V)=avb, where i is the current, V the potential, v the scan rate (mV/s), a and b constants. In particular, the b constant ranges between 0.5 (fully Faradic) and 1.0 (fully Capacitive) and its experimental quantification allows to evaluate and compare the capacitive and faradic responses of the electrode upon lithium incorporation/de-incorporation at various scan rates and potentials (Supplementary Fig. S6). The 3D sponged nanofibers show a dual capacitive/diffusion controlled electrochemical response depending on working potential. Below 1.0 V the b values approach 0.5, and thus diffusion-controlled process dominates, whereas above 1.0 V the capacitive contribution takes over (0.75 > b > 1). Overall, at low scan rate only a small portion of the process is capacitive (Figure 3d, top) and almost all capacity is diffusion controlled. On the contrary by increasing the scan rate, the electrochemical response of the electrode is more and more pseudo-capacitive (Fig. 3d, bottom). This is ascribed to the presence of defects at the surface of the fibers (see Raman analysis in Fig. 2e) and to the presence of graphene layers at the nanometric scale (see TEM of Fig. 2c). Prolonged electrochemical performance tests were performed at high current rates in a lithium half-cell configuration (Fig 3a). The stability of the electrode is demonstrated over 1000 cycles. At the lowest current rate (1 A g-1) the electrode exhibits a practical specific capacity of 250 mAh g-1 while when increasing the current to 4 A g-1 the practical capacity is still around 200 mAh g-1. When the current is boosted to a value as high as 20 A g-1, the electrode is still able to deliver a stable capacity of 125 mAh g-1, i.e. with a charging time of only few seconds, reflecting the excellent fast charging behaviour of the nano-fibre electrode. Furthermore, the electrode shows a Coulombic efficiency increasing towards 100% at increasing current rates, a value not simple to reach at high rates (Fig 3b).


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Figure 3| Electrochemical characteristics of the free-standing 3D sponged nano-fibre membrane. a Prolonged cycling performance of the 3D sponged nano-fibre anode in lithium half-cell configuration at different current rates: 1 Ag-1 (red), 4 Ag-1 and 20 Ag-1 (blue) and b corresponding Coulombic efficiency. c Rate capability (top) and corresponding voltage profiles (bottom). d Cyclic voltammetry curves at low scan rate (top) and high scan rate (bottom) and corresponding capacitive curves (black lines). Rate capability tests are further performed to investigate the capacity retention of the nanofibre anode. The test starts at a current of 100 mA g-1 (a value normally acceptable for cycling of



graphite-based anodes (see supplementary Fig. S7)) and with a subsequent increase in 16 steps to a current rate as high as 20 A g-1 (Fig. 3c top). The delivered capacity, and the corresponding voltage profiles (despite increase in voltage polarization), are still in a range suitable for battery application. Furthermore, when the current is lowered back to the initial value of 100 mA g-1, the electrode recovers 100% of the capacity delivered at the 5th cycle, further demonstrating stability and high Coulombic efficiency. In contrast, at current rates larger than 200 mA g-1 a commercial graphite electrode shows poor performance (see Supplementary Fig. S7). Turning to the overall electrode performance, thus also comprising the copper collector in the case of the graphite benchmark, the specific capacities of the 3-D sponged CNF electrodes are six to five times larger compared to graphite at 200 mA g-1, and show excellent performance stability upon cycling (see Supplementary Fig. S8). Additionally, comparing the 3D sponged fibers with the pristine CNFs electrode, the voltage shape differs between 1.2 to 2V, where no storage mechanism of Li+ is highlighted for the latter (see Supplementary Fig. S9). It is interesting to observe that the morphology of the CNF-based electrodes (before and after cycling in batteries) is only marginally altered upon cycling, as demonstrated by the ex situ TEM images recorded on post mortem electrodes (see Supplementary Fig. S10 and S11), and the composition of the SEI layer (mainly constituted by a mixture of organic carbonates and Li2CO3) shows minor changes between cycle 1 and 30 thus suggesting a remarkable stability. On the contrary the pristine CNFs, prepared without SiO2 addition and thus not treated with HF, show a remarkable thickening of the SEI layer upon cycling and an evolution of its composition between cycle 1 and 30. One may speculate that the fluorinated surface, resulting from the preparation procedure


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invoking HF-etching (see Fig. 2f), tunes the SEI layer formation and stabilizes the electrolyte/electrode interface. To demonstrate the applicability of the nano-fibre anode in a full cell we selected a carbon coated lithium iron phosphate (ccLiFePO4) as cathode, with theoretical capacity of 170 mAh g-1. A schematic of this Li-ion cell is shown in Fig. 4a.

Figure 4| Design and performance in a Li-ion battery. a Schematic of the Li-ion battery exploiting Al-supported ccLiFePO4 cathode and free standing 3D sponged nano-fibre anode. b Prolonged cycling performance at a current rate of 170 mA g-1 and c corresponding voltage profiles during stable cycles. d Electrochemical Impedance Spectroscopy (EIS) at the 1st and 1000th cycle. e Specific energy density comparison between graphite/LiFePO4 (red) and the here designed 3D CNFs/LiFePO4 (blue) Li-ion batteries calculated by considering the total weight of the battery components (anode, cathode, current collectors and electrolyte amount).



The LFP cathode is supported by an Al current-collector and delivers a stable capacity of 170 mAh g-1(see Fig. S12 in the Supporting Information section), while the nano-fibre anode works without the use of any current collector, thus reducing the total weight and the cost of the Li-ion battery. To reach an optimal balance of anode and cathode a Negative to Positive (N/P) ratio of 0.65 is selected considering the specific capacity (mAh g-1) of the two electrodes, to achieve a cell capacity (mAh) balance of 1:1; a value needed to ensure cycling stability. The cell shows a very stable performance, delivering a specific capacity of 150 mAh g-1 for more than 1000 cycles, using a current density as high as 170 mA g-1, i.e. with a charging time less then 1h (Fig. 4b). The battery operates at 2.4 V with a voltage profile matching the combination of the flat voltage of LFP and of the sloping shape of the nano-fibre anode (Fig. 4c), with a plateau-free process, as confirmed by Cyclic Voltammetry (CV) of Fig. S13 in the Supplementary Section. The cycling stability is underlined by Electrochemical Impedance Spectroscopy (EIS) tests at the 1st and 1000th cycle. By calculating the interface resistance (through the analysis of the semi circles of Fig. 4d) we found only a slight increase, from 40 Ω at the 1st cycle to 50 Ω at the 1000th, confirming the ability of the nano-fibre electrode to prevent electrolyte decomposition and parasitic reactions, such as lithium plating, at the surface upon cycling (see also Supplementary Fig. S2, Fig. S4, Fig. S10, Fig. S11 and FTIR in Fig. S14). Comparing the here designed Li-ion cell to a Li-ion cell using a Cu/Graphite based anode and taking into account the total weight of the components, practical energy densities of 205 Wh kg-1 and 135 Wh kg-1 are estimated respectively, Fig. 4e. Thus, exploiting the self-standing nature of the 3D sponged nano-fibre anode, i.e. omitting current collector, binders and conductive agents, we raise the practical energy density by 52% compared to Li-ion cells employing commercial Cu/Graphite anodes, and in addition lowering the cost by removing the Cu-foil.


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Conclusions By tuning the morphology of carbon-based anodes higher capacities and power capabilities of Li-ion batteries can be achieved. We show that a self-standing 3D sponged nano-fibre anode is able to deliver and take up lithium at current rates as high as 20 A g-1. The anode is designed at the nanoscale with highly conductive porous carbon fibres with a small diameter enabling short transport distance for lithium-ion diffusion and high electronic conduction in 3D pathways. We demonstrate that the self-standing nano-fibre anode is suitable for Li-ion batteries applications with a projected increase of the energy density by more than 50% and a reduction in the cost by removing the need of Cu current collector. Due to the superior electrochemical performance, low cost and easily scalable synthesis path we believe that the self-standing 3D sponged nano-fibre anode opens a new route in the development of high-energy and fast charging energy storage devices.

Experimental Section Materials Synthesis. The 3-D sponged nano-fibre anode was prepared through an electrospinning procedure. Polyacrylonitrile (Mw=15000, Aldrich)/silica nanoparticles (10-20 nm, Aldrich) 8:2 w:w were dispersed in N,N-dimethylformamide (DMF, Aldrich) at room temperature by ball milling (1000 rpm), for 1h. The solution was fed through a syringe needle (Ø=0.6 mm) by means of a vacuum pump, applying a DC voltage of 20 kV. The distance between needle and collector (grounded stainless steel rotating drum at 150 rpm covered by Al foil) was 20 cm. The polymer composite membranes were dried at 70 °C for 12 h and stabilized at 250 °C for 1 h. Subsequently, carbonization was carried out at 1000 °C for 1h under N2 flow (800 mL/min). Finally, the membrane was treated by a HF solution (50 wt%) for 1h and then



rinsed with distilled water until pH 7.0 was achieved, with a final drying step of 24 h under vacuum. The thickness of as-prepared polymer composite membranes ranged between 50-100 µm and a mass loading between 3 to 6 mg cm-2. The carbon coated LFP was synthesized by high energy ball milling (10 h under Argon atmosphere, Zr balls, 5mm) of 0.5M Li2CO3, 1M FeC2O4·H2O, 1M NH4H2PO4 and 5wt% acetylene black; weight ratio Zr balls to powders 6:1. The resulting powder was annealed at 600 °C under N2 flow, 800 mL/min, for 10 h. The cathode was prepared by mixing LFP powder with 5% carbon Super P and 5% polyvinylidene fluoride (PVDF) in N-methylpyrrolidone (NMP) solvent and casting the slurry on Al foil. Materials Characterization. SEM images were collected using a Field Emission Scanning Electron Microscope AURIGA Zeiss equipped with a Bruker EDS probe, whereas transmission electron microscopy (TEM) images were recorded using a FEI G2 20 HR-TEM instrument equipped with a LaB6 electron beam source and two 2D flat cameras (low and high resolution) at 200 kV electron-beam acceleration. Xray photo-electron spectroscopy (XPS) was performed on a PHI 5800 Physical Electronics instrument under argon atmosphere using Al Kα radiation (200W, 13kV), the chamber pressure 10−9 Torr, and the diameter of the analyzed surface 800 µm. The spectra were calibrated by the binding energy of the C 1s peak (BE = 284.5 eV). Raman analysis was performed using a Dilor LabRam Confocal micro-Raman Spectrometer utilizing a HeNe 632.8 nm, 4.7 mW laser, an 1800 grooves mm-1 grating and X50 objective. The specific surface area, the pore volume, and the pore size/distribution were measured with a Brunauer–Emmett– Teller analyzer (BET, ASAP 2010). The XRD measurements were performed at the MCX beamline in the ELETTRA synchrotron radiation facility (beam energy 13 eV) using a flat aluminium holder sealed with a Kapton tape, in order to avoid exposure to air during the


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experiment. The synchrotron XRD experiments have been carried out at the MCX beamline in ELETTRA within the frame of the project 20165060. Fourier Transformed Infra-Red Spectroscopy (FTIR) was carried out with a JascoFTIR-300. The electronic conductivity was measured by a direct volt–ampere method (CMT- SR1000, AIT Co.) in which a disk sample of 1 cm2 was contacted with a four-point probe. Electrochemical characterization. 3-D sponged nano-fibre membranes were used as electrodes (10 mm) in Swagelok type cells together with Celgard 2400 (12 mm; 1.26 mg cm-2) separator soaked with 10 µl cm-2 of LP30 (EC:DMC 1:1 w:w, LiPF6 1M) electrolyte and a lithium disk (11 mm; 11 mg cm-2). The cells were cycled between 0.05 – 2V using a current density between 0.1 Ag-1 to 20 Ag-1. Lithium ion cells were assembled using an LiFePO4 (LFP) cathode with a mass density of 11 mg cm-2. The cells were cycled between 3 to 1.2V using a current density of 175 mA g-1, considering the mass loading of the LFP cathode. The negative to positive (N/P) ratio was of about 0.65 and the electrolyte amount was keep to 10 µl cm-2. The practical energy density of the Li-ion cell was calculated by considering the total weight of the all components (LFP-pvdf-Super P: 10.5 mg cm-2; Celgard Separator: 1.26 mg cm-2; LP30: 10 mg cm-2; 3-D sponged fibre anode 6.3 mg cm-2; Cu/Graphite 18 mg cm-2) a working potential of 2.6V for the Li-ion cell using 3D-sponged fibres and 3.2V for the one using Cu/Graphite anode36. All of the cycling tests were performed using a Scribner 585 battery test system. EIS and CV measurements were carried out using a VSP Biologic instruments. For EIS measurements AC signal with amplitude of 10 mV and frequency ranging from 100 kHz to 1 Hz was applied.



Supporting Information Low angle X-ray, FTIR analysis, ex-situ Raman spectroscopy, ex-situ TEM, additional electrochemical tests and supplemented figures and table, and comparison with previously reported CNFs based anodes.

Acknowledgements M.A and A.M. acknowledge the support from the Chalmers Areas of Advance Materials Science and Energy, FORMAS, and the Swedish Energy Agency. The authors kindly thanks Dr. Jasper Plaisier, Dr. Lara Gigli and Dr. Laura Silvestri for the support in the experimental campaign at the ELETTRA synchrotron. This research was also supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (No. NRF-2017R1A4A1015711).

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Free-standing 3D sponged nano-fibre electrodes for ultrahigh-rate

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