Engineering Hollow Carbon Architecture for High-performance K-ion

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Engineering Hollow Carbon Architecture for High-performance K-ion Battery Anode De-Shan Bin, Xi-Jie Lin, Yong-Gang Sun, Yan-Song Xu, Ke Zhang, An-Min Cao, and Lijun Wan J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02178 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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Journal of the American Chemical Society

Engineering Hollow Carbon Architecture performance K-ion Battery Anode

for

High-

De-Shan Bin1,2, Xi-Jie Lin1,2, Yong-Gang Sun1,2, Yan-Song Xu1,2, Ke Zhang2,3, An-Min Cao1,2*, Li-Jun Wan1,2* 1

CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, and CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, People’s Republic of China. 2

University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China.

3

State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China.

ABSTRACT: K-ion batteries (KIBs) are now drawing increasing research interest as an inexpensive alternative to Li-ion batteries (LIBs). However, due to the large size of K+, stable electrode materials capable of sustaining the repeated K+ intercalation/deintercalation cycles are extremely deficient especially if a satisfactory reversible capacity is expected. Herein, we demonstrated that the structural engineering of carbon into a hollow interconnected architecture, a shape similar to the neuron-cell network, promised high conceptual and technological potential for a high-performance KIBs anode. Using melamine-formaldehyde resin as the starting material, we identify an interesting glass blowing effect of this polymeric precursor during its carbonization, which features a skeleton-softening process followed by its spontaneous hollowing. When used as a KIBs anode, the carbon scaffold with interconnected hollow channels can ensure a resilient structure for a stable potassiation/depotassiation process, and deliver an extraordinary capacity (340 mAh g-1 at 0.1C) together with a superior cycling stability (No obvious fading over 150 cycles at 0.5 C ).

INTRODUCTION New rechargeable battery systems beyond Li-ion batteries (LIBs) are being intensively pursued to alleviate the concern on the sustainability of Li resource1-4. K-ion batteries (KIBs) emerges as a promising candidate in benefit of the high abundance and low coast of potassium, which become especially attractive for their applications in largescale electricity storage and the power grid5-7. Sharing similar work mechanism with LIBs, KIBs benefit from the lower redox of the K+/K redox couple (-2.93 V vs standard hydrogen electrode (SHE)), a value close to -3.04 V (vs SHE) of Li+/Li and much lower than that of Na+/Na (-2.71 V vs SHE), which ensures a wide voltage window and a high energy density7,8. Meanwhile, the intercalation potential of K+ in carbon-based anodes, which are the widely-used materials, sits at ∼ 0.2 V vs K+/K9,10. Such a potential can ensure a much safer charge/discharge process by avoiding dendrites formation from K metal plating10. For comparison, for those metal-ion batteries systems with lower intercalation potentials, typically ~0.05V (vs Na+/Na) for Na+ intercalation, it has been a major safety concern related to the high risk of metal plating when carbonaceous anodes are considered9-11. Despite its advantages, the development of KIBs is challenged by the extreme deficiency in stable electrode ma-

terials which can endure the repeated intercalation/deintercalation of K+ especially when a satisfactory reversible capacity is expected. Different from a small size of Li+ (1.52 Å ), K+ is much larger (2.76 Å) and will cause serious structural deformation during the potasiation/depatasiation process6,9,10. For example, the theoretical volumetric expansion reaches ~60% for graphite upon K+ insertion (~10% for a lithiation process)10. The continuous electrochemical cycles will aggravate the damage and easily result in a structural failure of the electrode. In terms of anode materials which are still in a nascent stage for their development, researchers recently succeed in identifying different species5-7,9,12-17, particularly carbonbased ones6,12,15-19, as possible candidates for K+ storage. Unfortunately their development is challenged by the severe stability issue as revealed by the continuous capacity fading of these anodes upon extended cycling5-7. Moreover, it noted that the materials innovation of KIBs should take an overall consideration of other key parameters such as reversibly capacity, Columbic efficiency, and high rate capacity so as to have a systematic evaluation of their electrochemical performance, which are important factor related to their practical applications but so far has been given insufficient attention9,12-17. Equally importantly, it is highly desirable that the synthesis approaches for electrodes are those compatible with an affordable and

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scalable process suited for their future applications in large-scale energy storage. The design and engineering of carbon architectures have witnessed numerous successes in structure-function studies for their advanced applications20-25. Our preliminary efforts on KIBs anode confirmed that hollow carbon nanospheres are favorable in capacity retention over those solid ones20. The inner cavity of nanoparticles is expected to buffer the volume change caused by K+ insertion, thereby providing a stabilization mechanism for prolonged cycling capability of KIBs. Despite this significant benefit, we are also aware of a relatively low Columbic efficiency (~60% for the initial cycle) of this nanostructured carbon anode and the troublesome operation for its synthesis process20. On the other hand, carbon scaffolds with a built-in three-dimensional network have recently inspired interests in different electrochemical systems4,2630 . As far as structural integrity and failure are concerned, the interpenetrating network can not only ensure an improved mechanical stability against structural deformation, but also facilitate the ion and electron transport for improved electrode kinetics and mass transport31,32, which should be of special importance for those sluggish metal ions like K+. Herein, we developed a high performance anode for KIBs by structural engineering the electrode materials, which existed as hollow interconnected neuron-like carbon architecture (HINCA). Starting from a commerciallyavailable melamine-formaldehyde (MF) resin, we identified an interesting glass blowing mechanism accounting for the spontaneous hollowing of its carbonaceous components. Specifically, a two-step structural transformation was confirmed depending on the chemical nature of the resin during its pyrolysis: The scissoring of the ether linkage at the initial heating stage would soften the resin skeleton, and then the intense gas release at higher temperatures could self-inflate the tetrapod-shaped building blocks into a hollow structure, forming an interesting HINCA-type material which could not only facilitate the transportation of K-ion/electron, but also ensured a resilient structure for a stable potassiation/depotassiation process. When used as a KIBs anode, the electrode was able to deliver an extraordinary reversible capacity (340 mAh g-1 at 0.1C (28 mA g-1), one of the highest values for a hard carbon anode), superb cycling stability (almost no capacity fading over 150 cycles at 0.5C), and excellent Coulombic efficiency (72.1% in the initial cycle, and over 99% in the cycling). Our work not only provided insights into a previously unexplored hollowing mechanism in carbon with potential for large scale production, but also opened new avenue in the structure design and optimization of functional materials towards high performance anode materials for KIBs. EXPERIMENTAL SECTION Preparation of HINCA-type electrode samples: Commercially-available melamine-formaldehyde (MF) resin foam with higher amount of ether linkage was purchased from BEIJIN KELINMEI NEW MATERIAL Co. Ltd (Beijing,

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China) The HINCA-type electrodes were fabricated from the MF resin through a simple one-step pyrolysis process at 1300 °C in flowing N2 atmosphere for 1h in a tube furnace. After the carbonized procedure and cooled down naturally to room temperature, HINCA-type products can be obtained. To obtain the control sample, carbon foam without HINCA-type characteristic, the MF was first pretreated at around 360 oC for 10h to remove the ether bridge (–CH2–O–CH2–) and cooled down naturally to room temperature, and then the pre-treated MF was carbonized through the same one-step pyrolysis process at 1300 °C in flowing N2 atmosphere for 1h. Besides, commercially-available MF resin sample purchased with low ether links from SHANGHAI BEIYOU JIANCAI Co. Ltd (Shanghai, China) was carbonized same pyrolysis process at 1300 °C (5 oC min-1) in flowing N2 atmosphere for 1h. General characterization. Field emission scanning electron microscopy (FESEM) images were acquired on a SU8020 microscope. The SEM images and prepared by the focused ion beam (FIB) technique were collected on a Helios Nanolab G3 CS. The High-resolution transmission electron microscopy (HRTEM) images were recorded on a JEOL-2100F microscope. X-ray diffraction (XRD) patterns were collected on a Rigaku D.MAX-2500 with Cu Kα radiation (λ = 1.5406 Å). Raman spectra were obtained with a Thermo Fisher spectra system (EXR). N2 sorption isotherms were carried out on Quadrasorb SI-MP with samples pre-degassed at 300 oC. TGA-Mass test was carried out with a Hyphenation of TGA/MS system (PerKinElmer) over the range of 30-700 °C under helium (He) flow at a heating rate of 15 °C min-1. Electrochemical measurement. The electrochemical measurements were tested with CR2032 coin cells at room temperature for half-cell. The self-supported, carbon foam were mechanically compressed and then assembled into coin cells. These free standing electrodes served as the working electrodes without any binders or conductive additives. The powder electrode (another control sample) was prepared by mixing 90 wt% active materials and 10 wt% CMC/SBR (mass ratio of CMC/SBR is 1:1) binder dissolved in deionized water. The slurry was coated on Cu foil and dried at 80 oC for 12 h under vacuum. The electrolyte was a solution of 0.8 M KPF6 in EC: DEC (v/v 1:1). The mass loadings of the electrode materials are 1.1~2 mg cm-2. A potassium foil was used as the counter electrode and glass fiber was used as the separator. All the operations were performed in the Argon-filled glove box. The electrochemical measurements were carried out on a LAND CT2001A battery test system at room temperature, where the voltage range was from 0.01 to 2 V versus K+/K. EIS was measured using Autolab PGSTAT 302N (Metrohm, Switzerland) over the frequency range from 100 kHz to 100 mHz. Cyclic voltammetry (CV) was measured using Autolab PGSTAT 302N (Metrohm, Switzerland) at scan rate of 0.1 mV s-1 within potential of 0.01-2.0 V The K-ion full-cell were constructed by using HINCA as anode and 3, 4, 9, 10-perylene–tetracarboxylicacid–dianhydride (PTCDA) as cathode in a CR2032 coin-type cell. The cathode were prepared by mixing 70 wt% PTCDA, 20 wt%


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Super P and 10 wt% PVDF and PTCDA dissolved in NMP. The slurry was coated on Al foil and dried at 110 oC for 12 h under vacuum. The PTCDA cathodes were prepotassiated before assembly. The weight ratio of the two electrodes (negative/positive) was 1: 3.3.The full cells were charged and discharged in a voltage range of 1–3.5 V at room temperature. RESULTS AND DISCUSSION Fabrication and characterization of HINCA-type samples. We used a commercially-available melamineformaldehyde (MF) resin foam as the polymeric precursor. MF resin itself was known as a very durable and versatile thermosetting plastic. As a special form of MF resin, MF foam has been mass produced with wide applications in different areas such as kitchen materials, construction materials and flame retardant33. After its carbonization, the prepared carbon foam showed promising potential as capacitor material, current collector, or efficient support for electrochemical catalysts, all of which took full advantage of its high structural flexibility and the interconnected cavities embedded inside34-36. Our interest in carbon foam as a KIBs anode was initially sparked by its unique structural flexibility, which we considered a favorable character to provide a possible stabilization mechanism to combat the material deformation caused by the potassiation/ depotassiation process. Fortunately, the persistent efforts on structure control of carbon foam from its resin precursors gave us an opportunity to disclose an interesting self-hollowing process of the resin skeleton during its pyrolysis, whose final structure was found to be highly dependent on the chemical compositions of its resin precursor.

Figure 1. SEM images of the MF precursor and the carbon product after pyrolysis. (a) SEM images of the MF resin foam, which showed interconnected network with tetrapod joints. The inset showed the SEM image of the cross-section of the tetrapod joint. (b) SEM image of the prepared carbon product which was prepared by heating the resin precursor to 1300 oC. The tetrapod joint turned spherical while the network as a whole remains almost unchanged, showing a structure similar to a biological neural network. The inset was a magnified SEM image of the tetrapod center of the carbon product. (c) SEM image of the cross section of a representative tetrapod joint of the formed carbon, which was cut open by the FIB technique and was found hollow inside. (d) SEM image of a broken arm of the tetrapod unit. (e) Cartoon for a typical neuron structure.

Figure 1a showed a representative scanning electron microscopy (SEM) image of the MF resin foam we have used, which featured a reticulated porous structure partitioned by interconnected tetrapod-shaped backbones. The center of the tetrapod unit had a size around 15 μm and showed a slightly-concaved surface in line with the spatial curvature of its arms. By using the technique of focused ion beam (FIB), we were able to cut the center open and the joint is found solid inside (inset of Figure 1a). A carbonization process was then carried out by heat the resin to a high temperature, for example 1300 oC, in a nitrogen flow at a heating rate of 5 oC min-1. As shown in Figure 1b, the key morphological characters of the precursors, particularly the interconnected tetrapod backbones, were readily preserved in the carbonized product. We did not observe a noticeable collapse of the resin motif while the carbonaceous chunk did shrink substantially in size (Figure S1), which corresponded to the large mass loss from the removal of the organics. The obtained carbon foam was highly flexible and showed an elastic behavior as evidenced by our repetitious bending test. A severe bending with an angle of 180 oC caused no obvious damage to the foam and showed its outstanding mechanical flexibility (Figure S2 and the Video S1). Despite the successful inheritance in morphology, a distinct feature emerged in the shape of the tetrapod center. As shown in Figure 1b, every joint turned spherical, which was intriguing since a contraction of the resin ligaments was expected as also widely observed in the literatures33-35. A magnified SEM picture manifested the shape details (inset of Figure 1b). It was evident that the building blocks have downsized and the spherical joint has a diameter around 9.1 μm. The joint was then cut open by FIB, whose SEM analysis revealed the emergence of an inner cavity inside (Figure 1c). Such a hollow nature of the building blocks was further revealed after a mechanical grinding process, through which the sample turned broken for a direction observation of its inner details (Figure S3). The SEM analysis on the arms of the tetrapod units confirmed the formation of tubular structure after heating (Figure 1d). It therefore concluded that a simple carbonization process could turn the resin foam into a threedimensional carbon scaffold with interconnected hollow channels, forming a hollow interconnected neuron-like carbon architecture (HINCA) as shown in Figure 1e. It is noted that the creation of hollow carbon structure has been a well-known challenge in materials synthesis23,37. Current techniques largely relied on the template-based protocols but were impeded by tedious operating process, limited desired products, and inadequate capability to build complex structure23,37. Moreover, the environmental impact associated with the usage of hazardous etching agents such as NaOH and HF has also become a serious concern23. Interestingly, our result showed that a spontaneous hollowing of the polymeric ligaments could be achieved to form a complicate architecture, which showed an outstanding mechanical flexibility and good electronic conductivity for its further electrochemical applications.



The X-ray diffraction (XRD) pattern of the carbon sample showed broad peaks centered at ~25o and ~44o, respectively, which agreed well with the (002) and (101) crystal planes of hard carbon (Figure 2a). A calculation based on the Bragg's law gave an averaged D002 interlayer distance of 3.56 Å, a value much larger than that of the nature graphite (3.35 Å). Such an increase in layer distance was a widely-acknowledged character highly favorable for the insertion/extraction of large-size metal ions, particularly potassium ion15. Raman spectroscopy was used to evaluate the chemical functionalization of the carbon foam (Figure 2b). The intensity ratio between the defect-induced band

Figure 2. Structural characterizations of the formed carbon product. (a) XRD pattern, (b) Raman spectra, (c) HRTEM image, the inset showed selected-area electron diffraction (SAED) pattern and (d) N2 adsorption and desorption isotherms, the insets showed the detailed pore size distribution.

and the crystalline graphite band, namely ID/IG, showed a much higher value of 0.94 when compared to the commercial synthetic graphite with well-defined crystal structure6, revealing a low degree of graphitization with more defects in the obtained carbon foam. The high resolution transmission electron microscopy (HRTEM) characterization on a randomly-selected piece from the grounded carbon sample showed turbostratic lattice (Figure 2c). The selected-area electron diffraction (SAED) pattern (inset of Figure 2c) existed as dispersed diffraction rings, which confirmed a short-range order with low crystalline degree of carbon. Both elemental mapping and XPS analysis revealed that the carbon product contained N and O (Figure S4 for elemental mapping, Figure S5 for XPS). According to the XPS result, the content of N and O were 1.27 atomic% and 2.04 atomic%, respectively. The pore characteristics of this HINCA-type sample were tested by N2 adsorption/desorption experiment (Figure 2d). The specific surface area of the tested sample was 171 m2 g-1. The adsorption–desorption isotherm in Figure 2d showed two interesting characteristics: a high nitrogen uptake at lower pressure caused by micropores and the hysteresis loop at higher pressure due to mesopores21. The pore volume date is 0.12 cm3 g-1. The detailed pore size distribu-

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tion was analyzed in the inset in Figure 2d, which highlighted the co-existence of the two different pores. The hollowing mechanism. Intrigued by the spontaneous hollowing of the resin components, we paid special attention to the shape evolution inside the tetrapod joints so as to catch the key steps during their structural transformation. As the heating proceeded at a rate of 5 oC min-1, initially there was no obvious shape change before 365 oC (Figure 3a and its inset). A conspicuous swelling of the tetrapod joint was noticed at 400 oC (Figure 3b). At this stage we were already able to confirm the emergence of small cavities inside the joint (inset in Figure 3b). At 450 o C, the joint was found to be nearly spherical as compared to its relatively flat surface at the beginning (Figure 3c). The inner cavity would also increase its size while an obvious shrinkage of the wall thickness was noted (inset in Figure 3c). Such a shape evolution continued until 600 oC (Figure S6). Thereafter we did not perceive an obvious morphological change as the temperature ramps up to 1300 oC.

Figure 3. Morphological characteristics of the carbon product as a function of the heating parameters. (a-c) The shape change of the tetrapod joint when the sample was heated to different temperatures at temperatures rate of 5 o C min-1: (a) 365 oC, (b) 400 oC, (c) 450 oC. The inset of (a-c) each showed the corresponding SEM image of the FIBsectioned joint to reveal its innate structure. (d-f) The SEM images of the carbon products which were formed by heating the resin precursor to 1300 oC at different temperatures rates: (d) 0.5 oC min-1, (e) 10oC min-1, (f) 15 oC min-1, the broken particles were marked in white arrows. The inset in each picture showed the magnified SEM image of the tetrapod join. The heating rate was found to be a critical factor for the hollowing effect. As we slowed down the heating to an extremely low level, for example 0.5 oC min-1, the swelling effect and the hollowing became not significant for the same resin precursor (Figure 3d). The inset in Figure 3d confirmed that the joint remained almost unchanged in shape. A higher ramping rate of 10 oC min-1 showed almost the same effect at 5 oC min-1 for the formation of the HINCA-type product (Figure 3e). A close SEM examination (inset in Figure 3e) revealed that the tetrapod joint became much rounder in shape and the size was slightly larger (10.6 μm for 10 oC min-1 vs 9.1 μm for 5 oC min-1). We found that an even faster heating speed of 15 oC min-1 would cause damage to the interconnected architecture of


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the carbon foam. The original resin network turned into ruptured segments (Figure 3f) while a large portion of the joints were broken with its inner cavity exposed (marked by the white arrows in Figure 3f). The critical role played by the heat treatment inspired us to take a close look on the thermal degradation process so as to unravel the driving force behind the structural hollowing. Firstly, the thermogravimetric-mass spectrometric analysis (TG-MS) was used to monitor the pyrolysis process. As shown in Figure 4a, the TGA curve indicates the existence of three different degradation regions. The first one corresponded to the slow mass low with a total amount of about 18 wt% below 365 oC (Stage I in Figure 4a). The MS signal identified that the gas released at this stage was mainly formaldehyde (Figure 4b). Starting from 365 oC to 400 oC (Stage II in Figure 4a), a sharp mass loss (around 30 wt%) occurred. The gases were mainly ammonia with a small portion of formaldehyde and hydrogen cyanide (Figure 4b). For a temperature higher than 400 oC (Stage III in Figure 4a), an extensive degradation of resin towards the formation of hard carbon structure would continue to release ammonia and hydrogen cyanide (Figure 4b), which was also known as a process associated with a total destruction of the polymeric species.

Figure 4. Characterizations on thermal degradation process of the MF resin. (a) TGA curve of the MF resin. (b) Mass spectrometric analysis of the gas released at different heating stages. (c) FTIR spectra of the MF resin foam after being heated at different temperature. The ether linkage located at a wavelength of 1000 cm-1 gradually decreased its intensity upon heating. (d) SEM image of the carbon product formed when an extra preheating procedure was introduced: The resin was firstly heated at 360 oC for 10 h and then cooled down to room temperature, and then the precursor was heated to 1300 oC (5 oC min-1) for the carbon sample. The prepared product showed a flat surface at the tetrapod joints as compared to those spherical ones prepared without a pretreatment of the same MF resin foam.

Due to the complicated chemistry of resins themselves, the bonding patterns of MF usually existed as complex mixtures, which made it hardly possible to achieve an accurate description and determination of the MF resin structure during its pyrolysis38. Fortunately, significant efforts have been devoted to the study of the thermal deg-

radation of different MF resins so as to optimize its application as a fire-retardant material39,40. In the 1960s, researches have proved, although not completely conclusive, that the pyrolytic broken of the ether links in MF resins was probably responsible for the formaldehyde evolution40,41. We therefore scrutinized the ether group in the MF foam by using the Fourier transfer infrared (FTIR) spectroscopy. As shown in Figure 4c, there existed obvious ether group of -CH2-O-CH2- located at a wavelength of 1000 cm-1 42,43, which would gradually decrease its intensity when the sample is heated to 365 oC (Figure 4c). As for the MF resins, there usually exist two different kinds of linking bridges in its molecule, namely methylene and methylene ether, depending on different synthesis conditions38,43,44. It is clear that the MF resin we were using possessed higher amount of methylene ether group, which would be broken at the early stage of the thermal treatment and release formaldehyde as the gaseous product. We noticed that the sharp gas release and the bubble formation in tetrapod backbones started almost simultaneously at a temperature around 365 oC (Figure S7). Such a coincidence inspired us to expect that the gas release played an important role in the hollowing process. This correlation became more indicative considering the fact that the tetrapod joints transform their shape in a way like being puffed up into hollow spheres. It is not surprising that the heating rate would largely effect shape and structure evolution, because the fast heating rate would lead to the sharp and forceful gas release in a short time, giving an stronger outward bulging force to form spherical shape and hollow structure; On the contrary, at a slow heating rate, the gas release was too slow to initiate the bulging effect. For the thermal degradation of resins, it has been widely acknowledged that continuous reactions would happen to the polymers: Initially the bond scission of those weak links would dominate the structure change, which would cause ruptures in the macromolecules and accordingly make the resins softened and highly deformable45. Thereafter a major decomposition stage at higher temperature took place and resulted in further molecular condensation and restructuring, finally forming chars as the carbonaceous main product together with volatile byproducts. Notably, the morphological and structural change recorded on the MF resin above clearly showed such a step-wise degradation mode with different reactions included during the progressive thermal decomposition of the resin. The sum of the observations enlightened us a glassblowing mechanism for the spontaneous hollowing of the MF resin backbones. As schemed in Figure 5, the initial decomposition related to the scission of ether linkages with formaldehyde evolution was able to soften the MF resin45, and then a large and sharp gas release at higher temperatures automatically inflated the skeleton up. Such a route highlights the importance of the bond scissoring of ether group so that the following gas-blowing process could become possible. Our control experiments confirmed that the ether group was indispensable for the



hollowing process. For example, by curing the MF resin at ~365 oC and then cooling down the sample to room temperature, the majority of the ether bridge (–CH2–O–CH2–) could be successfully removed (Figure 4c)40,42,46. The SEM analysis showed that such a pre-cured MF resin would not able to form the HINCA-typed product when using the same heating protocol (Figure 4d). Furthermore, we have also tested several purchased MF resins with different contents of methylene ethers linkages. Those resin samples with low ether linkages will inevitably be able to produce only solid carbon structures (Figure S8). Such a crucial dependence on the MF resin structure together with the strict requirement on the heating parameters explained well the fact that hollow carbon architecture was intriguingly not paid much attention to in the previous research.

Figure 5. Schematic illustration for the spontaneous formation of hollow carbon structure. A two-step structural transformation was proposed during the pyrolysis of the MF resin with abundant ether linkages. First, at the initial heating stage with the temperature lower than 365 oC, the scissoring of the ether linkage happened, which resulted in ruptures in macromolecules and soften the resin. The MF skeleton would become highly deformable after this heating stage. Second, a glass-blowing process was initiated by the thermal decomposition of the resin when temperature went higher than 365 oC. The fast gas release would inflate the tetrapod-shaped building blocks into a hollow spherical structure. Finally, hard carbon would form after the high temperature carbonization process. Electrochemical performances of the HINCA-typed electrode in KIBs. The HINCA-typed sample has been used as an anode material in half cells to reveal its potential in KIBs. Due to its unique structural flexibility and electronic conductivity, we can use the foam directly as the working anode without the need for extra current collector, conductive additive, and binder. The thickness of a typical HINCA-type electrode was ~230μm (with an area density of ~1.5 mg cm-2), which would become thinner after cycling owing to the long-time compression in the coin cell (Figure S9). Two control samples have been prepared and tested to facilitate our understanding on the contributions of two key shape characters, namely the skeleton hollowness and the interconnected architecture, to the electrochemical performance in KIBs. Specifically, a powder sample was obtained by mechanically grinding the HINCA-typed one with its three dimensional architecture totally destroyed. Meanwhile, a solid carbon foam was synthesized from a pre-cured MF resin precursor.

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Figure 6a showed the charge/discharge profiles of these three different samples in the first cycle. The test was carried out at a current of 28 mA g-1, which corresponded to a rate of 0.1 C as calculated by the theoretical capacity of graphite (279 mAh g-1)6. Surprisingly, the HINCA-typed electrode was able to deliver a very high reversible capacity of 340 mAh g-1, which is an unprecedented output for a hard carbon anode. The initial Coulombic efficiencywas 72.1%, which is also among one of the highest values as reported so far (See table S1 for a summary of the KIBs anodes reported in the literatures). The irreversible capacity loss would diminish in the second cycle and the Coulombic efficiency promptly increases to 93.7%, suggesting a process usually referred to the development of a solid electrolyte interphase (SEI) layer on the electrode6. For comparison, the samples of solid carbon foam and powder showed much low reversible capacities at 234 and 186 mAh g-1, respectively. The cyclic voltammetry (CV) analysis of HINCA-type electrode was also performed (Figure S10). The sharp reduction peak around 0.04 V and the anodic peak around 0.38V are highly reversible and corresponded to the reversible potassiation/depotassiation process. The advantage of the HINCA-typed sample became more prominent at higher charge/discharge rates, where faster K+ movement was enabled and accordingly a higher requirement would be put forward on the anode structures. Figure 6b compared the C-rate capability of different samples, among which the HINCA-typed one showed obvious advantage with a much higher capacity delivered at different current densities. The diagnostic tool of electrochemical impedance spectrum (EIS) was then used to probe the electrochemical interface in the electrode. A much lower charge-transfer impedance for the HINCAtyped sample was identified as shown in Figure 6c, corresponding to a well-acknowledged character favorable for a faster K+ transportation. As a matter of fact, both the hollow character of the carbon ligaments and the interconnected cavities embedded inside have been widely confirmed as beneficial factors to enhance the electrode kinetics20,30. Particularly, a hollow structure with thickness-reduced wall would offer a short diffusion distance to facilitate ion diffusion and electron transport, thus largely shortening the diffusion time (t), whose value was known to be proportional to the square of the diffusion length L (t ≈ L2/D, where L and D represent the diffusion length and the diffusion constant, respectively)32,47.


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Journal of the American Chemical Society Coulombic efficiency over 99.5%. Considering that all these three samples originated from the same MF resin precursor, such a big difference in their electrochemical performance manifested the critical role played by the structural factors in delivering a satisfactory reversible capacities and superior cycling stability. We have also tested the full-cell performance by using the 3, 4, 9, 10perylene–tetracarboxylicacid–dianhydride (PTCDA) as cathode and HINCA-type sample as anode48. In the PTCDA-HINCA configuration, the reversible capacity for HINCA was around 300 mAh g-1 at 0.2C (Figure S12a). The constructed PTCDA-HINCA full-cell battery was able to light up a light emitting diode (LED) bulb (inset of Figure S12a). The reversible capacity is around 260 mAh g-1 at 0.5C and keep at around 180 mAh g-1 after 20 cycles (Figure S12b), showing promising potential for its future application.

Figure 6. Electrochemical characterization of the HINCAtyped product in KIBs. Three different include the HINCA-typed one, the solid carbon foam, and the carbon powder have been tested as anode materials in KIBs. (a) The charge/discharge curves at 0.1 C (28 mA g-1) for the first cycle, (b) rate capability test, (c) Electrochemical impedance spectra, (d) Reversible capacity and Coulombic efficiency at 0.5C for 150 cycles. (e) Reversible capacity and Coulombic efficiency of the HINCA-typed product tested at 1C for 500 continuous cycles. The HINCA-typed electrodes used here were obtained by 1300 o C carbonization at 5 oC min-1. The prepared HINCA-typed electrode showed extraordinary cyclability when long time cycles were examined. Figure 6d showed the electrochemical performance of the HINCA sample test at 0.5 C. The slight increase in capacity in the early stage of the cycling was probably related to the activation of the electrode in the initial charge/discharge cycles22. A high reversible capacity at 250 mAh g-1 maintained with almost no capacity decay upon 150 continuous cycles. On the contrary, the capacities of those two control samples would fade to 83 % and 72% of the initial capacity, respectively. Meanwhile, a Coulumbic efficiency higher than 99% was quickly achieved for the HINCA-typed sample, revealing a highly reversible and stable potassiation/depotassiation process. The EIS of the tested samples after cycling were also evaluated (Figure S11).We observed an obvious increase in the diameters of the irregular semicircles in the highfrequency region for all the samples after cycling, showing an increase of the electrochemical impedance after the electrochemical process. The HINCA-type electrode showed the smallest impedance change, which explained well a much stable electrochemical performance as well as the improved electrode kinetics compared to the other control samples. Further tests with over 500 continuous cycles at 1C also showed a stable battery performance for the HINCA-tape electrode (Figure 6e). Only slight capacity decay of ~0.05% per cycle was observed with a high

Ex situ SEM and TEM examination on the HINCA-typed samples after 500 cycles at 1C showed a well-preserved morphology of the electrode (Figure S13), in which the interconnected network with its hollow character remained almost unchanged. The electrode after 500 cycles was able to maintain an outstanding mechanical flexibility and sustain further bending test (Video S2), showing a highly-resilient structure strong enough to endure the long term reaction with K+. On the contrary, for the electrode prepared by the carbon powder, large cracks emerged after cycling (Figure S14), suggesting the repeated K+ insertion/extraction would result in serious structural change and degradation of the electrode, which explained well the continuous capacity fading of the control sample. We did also notice the retaining of N element in the HINCA-type electrode after cycling via elemental mapping (Figure S4b) and XPS analysis (Figure S5). Considering that the HINCA-typed sample was prepared through a simple pyrolysis process on a compositionallycontrolled precursor which has already been in a largescale production, our work showed promising potential for the development of high-performance electrode materials for their future application in KIBs. CONCLUSION In conclusion, we demonstrated that the structural engineering of carbon provided an effective route to achieve a high capacity and stable anode for KIBs. Using MF resin as the starting material, we succeeded in transforming this polymeric precursor into a hollow interconnected neuron-like carbon architecture through a simple pyrolysis process. An interesting glass blowing mechanism was proposed for the self-hollowing of the resin ligaments decided by both the chemical compositions and the heating parameters: The bond scissoring of the ether linkage to soften the resin skeleton, and then an intense gas release at higher temperatures to inflate the tetrapodshaped building blocks into a hollow structure. We confirmed that the unique carbon architecture and the hollow character together ensure a high stable and effective structure to achieve a high-performance anode material for KIBs. The prepared HINCA-typed electrode was able



to deliver a reversible capacity of 340 mAh g-1 at 0.1C, one of the highest values for a hard carbon anode, with superb cycling stability (no capacity fading over 150 cycles at 0.5C) and excellent Coulombic efficiency (72.1% in the initial cycle, and over 99% in the long-life cycling). Our work not only provided insights into a previously unexplored hollowing mechanism with potential for large scale production, but also opened new avenue in the structure design and optimization of functional materials towards a much stable and promising anode material for KIBs.

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(12) Luo, W.; Wan, J.; Ozdemir, B.; Bao, W.; Chen, Y.; Dai, J.; Lin, H.; Xu, Y.; Gu, F.; Barone, V. Nano Lett. 2015, 15, 7671. (13) Sultana, I.; Ramireddy, T.; Rahman, M. M.; Chen, Y.; Glushenkov, A. M. Chem. Commun. 2016, 52, 9279. (14) Zhao, Q.; Wang, J.; Lu, Y.; Li, Y.; Liang, G.; Chen, J. Angew. Chem. Int. Ed. 2016, 55, 12528. (15) Xie, Y.; Chen, Y.; Liu, L.; Tao, P.; Fan, M.; Xu, N.; Shen, X.; Yan, C. Adv. Mater. 2017, 29, 1702268. (16) Jian, Z.; Hwang, S.; Li, Z.; Hernandez, A. S.; Wang, X.; Xing, Z.; Su, D.; Ji, X. Adv. Funct. Mater. 2017, 27, 1700324.

ASSOCIATED CONTENT

(17) Share, K.; Cohn, A. P.; Carter, R.; Rogers, B.; Pint, C. L. Acs

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Additional data from digital photos, SEM, elemental mapping, XPS, CV, EIS, PTCDA-HINCA K-ion full cell electrochemical performance.

Nano 2016, 10, 9738. (18) Zhao, J.; Zou, X.; Zhu, Y.; Xu, Y.; Wang, C. Adv. Funct. Mater. 2016, 26, 8103. (19) Yang, J.; Ju, Z.; Jiang, Y.; Xing, Z.; Xi, B.; Feng, J.; Xiong, S. Adv. Mater. 2018, 30, 1700104.

AUTHOR INFORMATION

(20) Bin, D.-S.; Chi, Z.-X.; Li, Y.; Zhang, K.; Yang, X.; Sun, Y.-G.;

Corresponding Author

Piao, J.-Y.; Cao, A.-M.; Wan, L.-J. J. Am. Chem. Soc. 2017, 13,

*[email protected], * [email protected]

13492.

Notes The authors declare no competing financial interests.

S. Z. Nat. Commun. 2013, 4, 2798.

ACKNOWLEDGMENT

Zhong, H.; Fu, R.; Wu, D. Nat. Commun. 2015, 6, 7221.

(21) Liu, J.; Yang, T.; Wang, D.-W.; Lu, G. Q. M.; Zhao, D.; Qiao, (22) Xu, F.; Tang, Z.; Huang, S.; Chen, L.; Liang, Y.; Mai, W.;

The authors acknowledge funding support from the National Natural Science Foundation of China (Grant No 51672282), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA09010101). The authors also thank Prof. Ji-Tao Chen and Dr. Xu-Sheng Wang from Peking University for their assistance in TG-MS tests.

(23) Liu, J.; Wickramaratne, N. P.; Qiao, S. Z.; Jaroniec, M. Nat. Mater. 2015, 14, 763. (24) Zheng, G.; Lee, S. W.; Liang, Z.; Lee, H.-W.; Yan, K.; Yao, H.; Wang, H.; Li, W.; Chu, S.; Cui, Y. Nat. Nanotech. 2014, 9, 618. (25) Wang, D.-W.; Li, F.; Liu, M.; Lu, G. Q.; Cheng, H.-M. Angew. Chem. Int. Ed. 2008, 47, 373

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Engineering Hollow Carbon Architecture for High-performance K-ion

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