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Graphene nanocomposite for high-performance K-ion battery anode
Nano Energy 60 (2019) 912–918
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Full paper
Structural engineering of SnS2/Graphene nanocomposite for highperformance K-ion battery anode
T
De-Shan Bina,b, Shu-Yi Duana,b, Xi-Jie Lina,b, Lin Liua, Yuan Liua,b, Yan-Song Xua,b, Yong-Gang Suna, Xian-Sen Taoa,b, An-Min Caoa,b,∗, Li-Jun Wana,b a
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 b University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China
ARTICLE INFO
ABSTRACT
Keywords: K-ion battery anode SnS2/Graphene composite Sub-5 nm nanoparticles Anodic aluminium current collector High-peel-strength electrode
K-ion batteries (KIBs) are drawing increasing research interest as a promising supplement of Li-ion batteries due to the natural abundance of K resource. However, due to the large size of K+, high-capacity anodes are challenged by the structural stability of the active materials which are susceptible to large volumetric deformation after incorporating with a sufficient number of K+. Herein, using SnS2/graphene as an example, we demonstrated that high-performance KIBs anode could be achieved through collaborative efforts targeting on both the active material and the prepared electrode film. The electrochemically-active species of SnS2 were controlled into small nanoparticles with their size below 5 nm to provide sufficient reactive sites for K+ storage. Meanwhile, highly-resilient electrode film based on the prepared SnS2/graphene nanocomposite was built on aluminum (Al) current collector rather than the widely-used copper foil, forming a strong anode film with high peel strength to endure the potassiation/depotassiation process. In this way, the active species was able to deliver an extraordinary reversible capacity of 610 mAh g−1 with unprecedented high-rate capability (around 290 mAh g−1 at 2A g−1) and promising cycling stability. This contribution sheds light on the rational design of high-performance electrode for KIBs and beyond.
1. Introduction Lithium-ion batteries (LIBs) are now the leading energy storage systems, but the insufficiency, fast consumption, and high cost of lithium resources cause concern on its sustainability [1,2]. K-ion batteries (KIBs), sharing a similar working mechanism with LIBs, have been considered as a promising supplement of LIBs in the energy storage field owing to the natural abundance of K and the low K+/K redox potential of −2.93 V vs standard hydrogen electrode (SHE), which is close to −3.04 V (vs SHE) of Li+/Li, suggesting the technological potential of achieving a higher voltage cell [3–6]. However, K+ is much larger and heavier than Li+, which would lead to serious structural deformation and sluggish kinetics for K+ transfer during the potassiation/depotassiation process, resulting in poor cycling performance and limited rate capability [7,8]. In terms of KIBs anode, the inexpensive carbon-based materials have been widely studied as one of the leading candidates [9–12]. Our previous efforts on carbon-based anodes demonstrated that hollow carbon materials could act as promising KIBs
anodes with excellent cycling performance [13,14]. However, carbon anode, which usually based on an intercalation storage mechanism, could only deliver limited reversible capacity [6,10]. Therefore, the development of high-capacity KIBs anodes with outstanding rate capability and promising cycling performance is essential for the innovation of advanced KIBs technology [15,16]. Tin (Sn)-based alloy materials, like Sn metal, SnS2, and Sn4P3, are drawing considerable attention as promising KIBs anodes owing to the high theoretical capacity based on an alloying mechanism for K+ storage [7,17–19]. However, there is usually a pivotal challenge associated with the structural degradation of the alloy materials caused by the large volume change after fully reacting with K+ for high capacity [7,15,16,18–20], subsequently leading to particle fracture and then losing electrical contact with the current collector, finally fading the capacity rapidly [7,16,21,22]. For example, a Sn4P3/carbon nanocomposite for KIBs anode have been recently developed, which delivered a promising reversible capacity of 384.8 and 221 mAh g−1 at current density of 50 and 1000 mAg−1, respectively [7]. Although it
∗ Corresponding author. 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. E-mail address: [email protected] (A.-M. Cao).
https://doi.org/10.1016/j.nanoen.2019.04.032 Received 27 January 2019; Received in revised form 28 March 2019; Accepted 6 April 2019 Available online 11 April 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
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has exceeded the capacity for most of the carbon anodes, the potential of its high capacity and high rate capability was not fully exploited. Meanwhile, the capacity faded rapidly to around 307 mAh g−1 after 50 cycles perhpers owing to the lack of rational design of the electrode with inherent high-volume-change alloying materials as anode. Such an unsatisfied battery performance may be ascribed to the limited capability of the electrode film for resisting the serious structure deformation and losing electrical contact after storing numerous large-size K+ [7,16,21,22]. It suggests that there is still a huge room but also a very formidable challenge for fully exploiting the potential of the alloy species to boost their electrochemical performance in KIBs anodes. Therefore, the electrode innovation of high-capacity alloy anode materials for KIBs should take an overall consideration of both the active material and the electrode film including: 1) Favorable architecture of active materials with size-control, high-dispersity alloy nanoparticles incorporated with the conductive matrix to avoid particle fracture and provide sufficient reactive sites for K+ storage as well as excellent K+/e− transport kinetics [7,15,16]. Especially, the control of the particle size of the alloys particles have long been identified as an effective strategy to improve the cycling stability in LIBs but it was overlooked in KIBs [23]. 2) High-peel-strength of the electrode to withstand the large potassiation/depotassiation strains and ensure a resilient electrode structure to mitigate the active materials peeling from the current collector and losing electrical contact [24,25]. 3) The cost of the passive component of the prepared electrode, for example, current collector, should be taken into consideration for KIBs for their future application in large-scale energy storage. Aluminum (Al) foil, which is much lighter and lower-cost than the widely-used copper foil, is electrochemically inactive with K at a low potential [4,26–28], suggesting it is a promising anodic current collector. Using Al foil as a current collector for KIBs anode would be beneficial for reducing the cost of the device and meanwhile enhancing energy density of the whole KIBs system [4,26], which would be advantages related to their practical applications but so far have been given insufficient attention. Herein, we demonstrated that the collaborative efforts in engineering both the SnS2/graphene nanocomposite and the electrode film could promise high conceptual and technological potential for a high-performance KIBs anode. We identified that the SnS2 species in the SnS2/graphene composite could be engineered into small-size nanoparticles below 5 nm with high dispersity to provide sufficient reactive sites and facilitate the K+/e transfer for K+ storage. Meanwhile, highlyresilient electrode film based on the prepared sub-5 nm SnS2/graphene nanocomposite could be built on an aluminum current collector instead of the widely-used copper foil using a hybrid binder of sodium carboxmethyl cellulose and styrene butadiene rubber (CMC/SBR), forming a strong anode film with high peel strength to endure the potassiation/ depotassiation process. As a result, the designed electrode was able to deliver a reversible capacity of 610 mAh g−1 at 50 mA g−1 (one of the highest reported values in the literature), unprecedentedly high rate capability of around 290 mAh g−1 at 2A g−1, and promising cycling stability in KIBs anode. Furthermore, a full-cell KIBs constructed by the prepared Al-based anode and Al-based cathode of 4,9,10-perylene-tetracarboxylic acid−dianhydride (PTCDA) demonstrated the feasibility of a KIBs based on dual-Al current collectors for both anode and cathode. We believe this contribution would shed light on the rational design of high-performance electrode for KIBs and beyond.
for around 10 min. Then 0.342 g thioacetamide (TAA) added into the solution and stirred for another 10 min. Then the mixed aqueous solution was transferred to a Teflon-lined stainless steel autoclave for the hydrothermal process, the temperature is 160 and 100 °C for 12 h to control the particles size of SnS2 to be sub-5 nm and above 15 nm, respectively. After the reaction and natural cool to room temperature, the synthesized product was collected by centrifugation, and then washed with deionized water at least three times. To keep the product well dispersity, a freeze-drying process was conducted for the drying of the collected SnS2/graphene. The obtained dried product was then heated to 400 °C for 4 h at Ar atmosphere. SnS2 nanoparticles were also synthesized without the presence of graphene under the same process and condition. The control sample of graphene was also performed the hydrothermal process and 400 °C heated treatment. 2.2. Materials characterizations Scanning electron microscopy (SEM) images were recorded on a SU8020 microscope. The transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and element mapping images were acquired on a JEOL-2100F microscope. Xray diffraction (XRD) patterns were collected by a Rigaku D.MAX-2500 system with Cu Kα radiation (λ = 1.5406 Å). Raman spectra were obtained with a HORIBA spectra system (LabRAM HR Evolution). N2 sorption isotherms were carried out on Quantachrome nitrogen sorption instrument (NOVA3200e and iQ). Thermal gravimetric analysis was carried out with a heating ramp of 10 °C min−1 in air atmosphere. The peel strength was measured with an electrode peel tester (HY-0580, Shanghai Hengyi Precious Instrument Co., Ltd). A 20 mm wide and 160 mm long electrode sample was attached to an adhesive tape. Then the adhesive tape was removed by peeling at the angle of 180° at a rate of 50 mm min−1. The peel strength was recorded by the computer system. 2.3. Electrochemical measurements The electrochemical measurements were tested with CR2032 coin cells (half-cell) at room temperature. The working electrode was prepared by coated the mixed slurry of active material, binder and Super P with a weight ratio of 80:15:5 onto the current collector of commercial Al foil or Cu foil which were purchased and use directly, and then dried at 80 °C in vacuum for overnight. Water was used as the solvent for the slurry when CMC/SBR (mass ratio 1:1) or CMC were used as binder. NMethyl pyrrolidone (NMP) was used as the solvent for the slurry when PVDF was used as binder. For KIBs, the electrolyte was a solution of 0.8 M KPF6 in ethylene carbonate and diethyl carbonate (1:1 in volume). K foil and glass fiber were used as the counter electrode and separator, respectively. All the operations were performed in the Argonfilled 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 2.5 V versus K+/K for KIBs. The specific capacities in the half-cell were calculated based on the mass of SnS2 active materials only with the extraction of the graphene contribution. The K-ion full-cells were constructed by using SnS2/grephene as anode and 3, 4, 9, 10-perylene–tetracarboxylicacid–dianhydride (PTCDA) as cathode in a CR2032 coin-type cell. The cathodes were prepared by mixing 70 wt% PTCDA, 20 wt% Super P and 10 wt% PVDF (binder) dissolved in NMP. The slurry was coated on Al foil and dried at 100 °C for 12 h under vacuum. The PTCDA cathodes were prepotassiated before assembly. The full cells were charged and discharged in a voltage range of 0.5–3.5 V at room temperature. The specific capacities of the full cell K-ion battery were calculated based on the total mass of SnS2/ graphene so as to have a better evaluation of its feasibility for realistic application. Cyclic voltammetry (CV) was tested using an AutoLab PGSTA302N electrochemical workstation (Metrohm, Switzerland). EIS was also measured using the Autolab PGSTAT 302N with the frequency
2. Experimental Section 2.1. Materials synthesis The SnS2/graphene nanocomposite was synthesized through a simple hydrothermal process [29], and then the freeze-drying step was carried out to keep the dispersity of the hybrid nanoparticles. The used graphene is a commercial product. Typically, 0.4 g SnCl4·5H2O were added to 60 ml of ∼1 mg ml−1 graphene aqueous solution and stirred 913
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Fig. 1. SEM image (A), TEM image (B–C) and HR-TEM image (D) of SnS2/ graphene nanocomposite. The reaction temperature for the hydrothermal process is 160 °C.
Fig. 2. XRD pattern (A), Raman spectra (B), elemental mappings (C), and N2 adsorption-desorption isotherms (D) of the SnS2/graphene nanocomposite. The reaction temperature for the hydrothermal process is 160 °C. The pure SnS2 nanoparticles was synthesized via the same method without adding graphene and also tested by XRD and Raman spectra for comparison.
range from 100 kHz to 10 mHz.
SnS2 [29]. The banks at ∼1585 cm−1 and ∼1348 cm−1 for the hybrid SnS2/graphene nanocomposite could be assigned to the typical graphitic-induced G band and disorder-induced D band for the carbon component of graphene [31]. Further evaluation was carried out on the SnS2/graphene nanocomposite with a focus on its compositional distribution with the elemental mapping analysis (Fig. 2C). As shown in Fig. 2C, the elemental mapping images of the randomly picked SnS2/ graphene particle illustrated the homogeneous distribution of three concerned elements (Sn, S, C), further verifying the successful incorporation of the ultrafine SnS2 nanoparticles onto the functional graphene with a high dispersity. The nitrogen adsorption-desorption isotherms analysis was performed to characterize the porous structure of the SnS2/graphene architectures (Fig. 2D). The specific surface area of the nanocomposite is measured and calculated to be 78.18 m2 g−1. The result of the porous structure revealed the simultaneous existence of mesopores and macropores with a pore volume of 0.885 cm3 g−1 and a broad distribution of the pore size for such hybrid nanocomposites (inset of Fig. 2D). Such a porous structure is expected to be favorable for its applications in KIBs for facile access of the electrolyte and promoting the kinetics of large K+ transport [32], which is key for the high-performance KIBs anodes [11].
3. Results and discussion 3.1. Synthesis and characterization of the SnS2/graphene nanocomposites The SnS2/graphene nanocomposites were synthesized through a simple hydrothermal process, and then the subsequent freeze-drying step was carried out to keep the dispersity of the hybrid nanoparticles (see Experimental Section for the detail). Fig. 1A illustrated a scanning electron microscopy (SEM) image of the prepared hybrid nanocomposite synthetized under a hydrothermal process of 160 °C, which showed the sheet-stacking porous architecture. Transmission electron microscopy (TEM) analysis was performed to evaluate the morphology of the synthesized hybrid nanoparticles (Fig. 1B–C). As showed in Fig. 1B, the ultrafine and well-dispersed SnS2 nanoparticles are homogeneously incorporated in the graphene matrix. A magnified observation could clearly identify the nanoparticles size of the nanocomposites (Fig. 1C), which is less than 5 nm with a narrow size distribution (Figure S1). Such well-defined sub-5 nm nanoparticles anchored onto the graphene sheet was expected to provide sufficient reactive sites and excellent ion/electron transport kinetics for the K+ storage, and would effectively alleviate the aggregation of the nanoparticles [30]. The X-ray diffraction (XRD) pattern of this hybrid nanocomposite sample identified the emergence of berndtite-4H SnS2 (PDF #21–1231) (Fig. 2A). High-resolution TEM (HR-TEM) test illustrated that the SnS2 nanoparticles exhibited a well crystalline structure with clear lattice fringes (Fig. 1D). The lattice spacing of 3.25 and 2.70 Å correspond to the (100) and 102 planes of the berndtite-4H SnS2 nanoparticles, respectively. The content of the SnS2 in the nanocomposite is calculated to be 76.8 wt % based on the thermogravimetric analysis (TGA) (Figure S2). We identified that the particle size and dispersity of SnS2 nanoparticles in the hybrid composites could be facilely controlled by tuning the reaction temperature in the hydrothermal process. When decreasing the temperatures from 160 to 100 °C, not only the size of the SnS2 nanoparticles would raise from sub-5nm (Figure 1C) to 15–50 nm, but also the agglomeration of the SnS2 nanoparticles with poor dispersity could be observed (Figure S3). The sub-5 nm SnS2/graphene nanocomposite was further analyzed by Raman spectroscopy (Fig. 2B) with pure SnS2 as a comparison. As illustrated in Fig. 2B, the band at ∼310 cm−1 for both pure SnS2 and SnS2/graphene nanoparticles could be ascribed to the A1g mode of the
3.2. Electrochemical performance in K-ion batteries For the electrochemical performance evaluations of the prepared SnS2/graphene nanocomposite as an anode in KIBs, we have a special interest in using aluminum foil as current collector rather than the widely-used copper foil, which was initially sparked by that aluminum foil would not form an alloy with K at low potential [4,26–28]. Meanwhile, aluminum foil is much lighter and lower-cost than copper foil. It would be beneficial for reducing the cost and in the meantime enhancing the energy density of the whole KIBs system for their future applications [4,26]. For the binder used for preparing the electrode, we employed a hybrid one, namely, sodium carboxmethyl cellulose (CMC) and styrene butadiene rubber (SBR) (CMC/SBR), which is the most commonly-used binder in the graphite and silica-based anode material for commercial LIBs owes to its high elasticity and strong adhesion [33,34]. Fig. 3A illustrated the typical charge/discharge profiles of the synthesized sub-5nm SnS2/graphene nanocomposites obtained from the first two cycles using Al foil and CMC/SBR as the current collector and 914
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Fig. 3. Applications of sub-5nm SnS2/ graphene nanocomposite as an anode for KIB. (A) Discharge/charge curves at the first two cycles for SnS2/graphene nanocomposite based on Al-CMC/SBR at 0.05 A g−1. (B) Cyclability tests for SnS2/graphene nanocomposite based on Al-CMC/SBR and Cu-CMC/SBR at 0.1A g−1. (C) Comparison of the cycling performance SnS2/graphene nanocomposite based on Al-CMC/SBR with various representative researched high-capacity KIBs anodes [42]. (D) EIS spectra, the inset is the equivalent circuit for fitting the EIS spectra. (E) Rate capabilities SnS2/graphene nanocomposite based on Al-CMC/SBR. (F) Comparison of the rate capability of SnS2/graphene nanocomposite based on Al-CMC/SBR and the representative KIBs anodes reported in the literature [43–44].
binder, respectively. A high reversible capacity of 610 mAh g−1 based on SnS2 could be achieved at a current of 50 mA g−1, which is one of the highest outputs for KIBs anodes reported in the literature as shown in Table S1. The previous reports of Sn and S for K+ storage showed that S can form K2S and Sn can form KSn during the potassiation process [7,35], resulting in the possible theoretical capacity of 733 mAh g−1. The cyclic voltammetry (CV) analysis of the Al-CMC/SBR (sub5nm) electrode was also performed (Figure S4). The obvious reduction peak around 0.72, 0.36, and 0.16 V and the anodic peak around 1.01, 1.28, and 1.79V are highly reversible and corresponded to the reversible potassiation/depotassiation process. The SnS2/graphene with the particles size of SnS2 above 15 nm was also tested for comparison (Figure S5), which could only deliver a reversible capacity of 454.5 mAh g−1. It is expected that the well-dispersity of the sub-5 nm SnS2/ graphene nanocomposites could provide sufficient reactive sites for K+ storage, thus resulting in a higher capacity. To evaluate the stability of the SnS2/graphene nanocomposites during the repeated potassiation/depotassiation processes, cyclability tests were performed. Both the sub-5nm SnS2/graphene composite and the SnS2/graphene sample with large size (> 15 nm) SnS2 nanoparticles were tested with the same current collector of Al foil and hybrid binder of CMC/SBR. Interestingly, the sub-5 nm SnS2/graphene nanoparticles coated on the Al foil (denoted as Al-CMC/SBR (sub-5nm)) delivered an improved cycling capability than the large size (> 15 nm) SnS2/graphene composite (denoted as Al-CMC/SBR (> 15 nm) (Fig. 3B). A high reversible capacity at around 559 mAh g−1 maintained with a capacity retaining of 94% upon over 50 continuous cycles at a current density of 100 mA g−1 for the Al-CMC/SBR (sub-5nm) electrode (Fig. 3B), which was much superior than that of the representational high-capacity KIBs anodes reported in the literature (Fig. 3C). On the contrary, the Al-CMC/SBR (> 15 nm) electrode showed a rapid-fading cycling performance with low capacity retaining of only 35% after 50 cycles, highlighting the success of the structurefunction design of the sub-5 nm SnS2/graphene nanocomposite for advanced KIBs materials. Meanwhile, we also used copper (Cu) foil as the current collector for the sub-5 nm SnS2/graphene nanocomposite anode for comparison, as copper (Cu) foil is the most-frequently used anodic current collector in LIBs, SIBs, and KIBs. Our results showed that the sub 5 nm SnS2/graphene electrode based on Cu foil with the same binder of CMC/SBR (denoted as Cu-CMC/SBR (sub-5nm)) showed a rapid fading of capacity after the same cycling compared with the AlCMC/SBR (sub-5nm) electrode. The electrochemical impedance spectrum (EIS), which has long been known to be a useful technique for
probing the electrochemical interface in batteries [32,36], was carried out to evaluate the electrochemical interface in the electrode. The Nyquist plots for these two different samples of Al-CMC/SBR (sub-5nm) and Cu-CMC/SBR (sub-5nm) were illustrated in Fig. 3D. In Nyquist plots, the radius of the emerging semicircle in high-to-medium frequency region is known as the charge transfer and surface film resistance for the electrode [36]. As shown in Fig. 3D, the charge transfer resistance of the Al-CMC/SBR (sub-5nm) electrode was much smaller than that of Cu-CMC/SBR (sub-5nm), suggesting the low electrochemical impedance for the Al-CMC/SBR (sub-5nm) electrode. The rate capability tests showed that such an Al-CMC/SBR (sub5nm) electrode could deliver an outstanding high-rate capability (Fig. 3E). Not only could it show a high reversible capacity of 583 and 495 mAh g−1 at 0.1 and 0.5 A g−1, respectively, but also could deliver high reversible capacity of 412 and around 290 mAh g−1 at a high current densities of 1 and 2 A g−1, respectively. The charge/discharge profiles in different current density were illustrated in Figure S6. Such a rate performance of the SnS2/graphene sample was much superior compared to various representative researched KIBs anodes (Fig. 3F), highlighting the functionality of such a SnS2/graphene electrode. The rate capability of graphene (with the same hydrothermal process and heated treatment of preparing SnS2/graphene) and pure SnS2 was also tested for comparison, respectively. The pure graphene delivered revisable capacity of 174 mAh g−1 at 0.05 A g−1 and contributed very negligibly to K+ storage with almost no revisable capacity at 1 A g−1 (Figure S8). For the pure SnS2, it also delivers low reversible capacity (Figure S9). The cyclic voltammetry of the sub-5 nm SnS2/graphene based on Al-CMC/SBR was also tested at different sweep rates (Figure S7A). A detailed analysis on the CV result revealed that a large part of capacity of SnS2/graphene electrode was contributed by the capacitive (Figure S7B). The capacitive contribution gradually increases from 46.3% to 90.3% with the increasing scan rate increase from 0.1 to 1 mV s−1, which explains well its outstanding rate capability of the design electrode of Al-CMC/SBR (< 5 nm). The distinguished electrochemical performances of SnS2/grapnene in KIBs anode suggested the success of structural engineering of such a nanocomposite of small highdispersity SnS2 particles incorporated in the conductive graphene matrix for the boosting of the K-storage capability. To demonstrate the high conceptual and technological potential of the designed Al-based anode of SnS2/grapnene, the full-cell K-ion batteries were constructed and evaluated by employing the sub-5nm SnS2/ graphene anode based on Al-CMC/SBR and Al-based cathode of 4,9,10perylene-tetracarboxylic acid−dianhydride (PTCDA) [11,37]. The 915
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PTCDA cathode is a commercial product (Alfa Aesar, 98%), which features microparticles (Figure S10). In the dual-Al full-cell configuration, the SnS2/graphene electrode showed a high reversible capacity of 465 mAh g−1 at 50 mA g−1 based on the mass of SnS2/graphene (Figure S11A). The initial Coulombic efficiency of the K-ion full cell is 40.5% and increases to 89.2% in the second cycle, suggesting a typical process probably related to the formation of a solid electrolyte interphase (SEI) in the anode during the initial potassiation/depotassiation process [10]. It delivered a reversible capacity of around 390 mAh g−1 at 100 mA g−1 and kept at around 230 mAh g−1 after 25 cycles (Figure S11B). The constructed full-cell battery could deliver an initial voltage around 3.1 V after charging (Figure S11C). It was able to simultaneously power up more than fifty light-emitting diodes (LEDs) bulbs (Figure S11D), showing the applicability of the SnS2/graphene anode in the K-ion full cell application. Although the results were preliminary, it envisions new perspectives on the development of low cost and high-energy-density dual-Al-current collector battery for application in large-scale energy storage.
capacity retention of 83.3% after 50 cycles, which was a moderate capacity retention compared to the value of 94% for the CMC/SBR hybrid binder. Meanwhile, capacity for the electrode based on PVDF showed a fast fading with only capacity retention of 57.6% after the same cycles. Such a result highlighted the uniqueness of the Al-CMC/ SBR protocol for the high-capacity SnS2/graphene anode in KIBs. Such an obvious difference between the batteries performances of the same sub-5 nm SnS2/graphene active materials affected by the current collectors or binders is interesting. In general, binders and current collectors could affect the adhesion properties of the electrode film, reflected by the peel strength of the current collector and active materials [24,25]. We thus carried the peel test of the electrodes, which has long been considered as an effective technique to evaluate the adhesion strength for the active materials and current collectors [24,41]. As shown in Fig. 4B, the SnS2/graphene materials with CMC/SBR as binder could tightly adhere to the Al foil with better adhesive property than other three electrodes. Such favorable adhesion strength is expected to be very helpful to alleviate the stripping of the high-volume change of the electrode materials during the repeated potassiation/ depotassiation [24,41], which is consistent with the cycling performance discussed above. On the contrary, the electrode of control samples delivered poorer cycling stability, showing lower adhesion strength (Fig. 4B). To evaluate the structure change of the electrodes after repeated potassiation/depotassiation processes, SEM examinations were conducted. All the primary electrodes observed by SEM were well-intact without cracks (Fig. S12). After repeated potassiation/depotassiation cycling, only the SnS2/graphene electrode based on Al-CMC/SBR was able to keep a well-preserved morphology of the electrode film (Fig. 4C), suggesting a promising durability of the electrode for accommodating the serious volume deformation. On the contrary, we could observe cracks imprinted on the electrode prepared by Al-CMC (Fig. 4D), large cracks emerged for the electrode based on Al-CMC/SBR (Fig. 4E), and obvious wrinkle and peeling of the electrode prepared by Al-PVDF (Fig. 4F), suggesting the repeated K+ insertion/extraction processes would result in structural degradation of the electrode film, which well agreed with the lower peel strength of these control electrode and explained well the continuous capacity fading of these samples (Fig. 4B). As simple as the strategy might look like, it is interesting that no previous researchers have paid sufficient attention to the Al current collector as anodic current collector together with an appropriate CMC/ SBR hybrid binder to construct a high-peel-strength electrode film for improving the electrochemical performance of high-volume-change anode for KIBs. It might be related to two facts: Firstly, the Al foil is always employed as the current collector for the cathode materials thus it can be easily overlooked in anode, considering Cu foil is the widelyused anodic current collectors. Secondly, a suitable binder of hybrid CMC/SBR also plays a key role and good care for the binder selection should be taken if an electrode with stronger peel strength is expected (See Fig. 4B). Fortunately, our persistent efforts on the aiming to seek the low cost and light current collector and binder endowed us an opportunity to find this interesting strategy for improving the electrode performances.
3.3. The evaluation of peel strength and structure of the designed electrodes To have a better understanding on the beneficial effects of Al current collector combined with the binder of CMC/SBR for the enhanced performance of such sub-5 nm SnS2/graphene anode for KIBs, we carried out the controlled experiments by using other binders. Based on the same Al current collector, we employed another two binders, namely, polyvinylidene fluoride (PVDF) and pure CMC, which were also widely used as the binder for anode materials in LIBs, SIBs, and KIBs [36,38–40]. The results illustrated in Fig. 4A revealed that the sub-5 nm SnS2/graphene electrode with pure CMC as binder could deliver a
4. Conclusions In summary, we confirmed that the collaborative engineering of both the active materials and the electrode film provided an effective route to achieve a high-performance anode for KIBs. Using SnS2/graphene nanocomposite as an example, we demonstrated that electrochemically-active species of SnS2 could be controlled into small nanoparticles with the size below 5 nm and maintained high dispersity, which provided sufficient reactive sites and excellent K+/e− transport kinetics for fully exploiting its potential in K+ storage. Meanwhile, we identified that a high-peel-strength electrode was key in ensuring a
Fig. 4. The evaluation of peel strength and structrue of the designed electrode. A) Cyclability tests for sub-5 nm SnS2/graphene nanocomposite based on AlCMC and Al-PDVF at 0.1A g−1. B) Peel test of sub-5 nm SnS2/graphene nanocomposite based on Al-CMC/SBR, Cu-CMC/SBR, Al-CMC, Al-CMC, Al-PVDF. C-F) SEM images of the electrode after 10 cycles at 0.1A g−1, c) Al-CMC/SBR, d) Al-CMC, e) Cu-CMC/SBR, f) Al-PVDF. 916
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resilient electrode structure for a durable potassiation/depotassiation process. We found that building the synthesized sub-5 nm SnS2/graphene nanocomposite on the aluminum current collector, rather than the frequently-used copper one, could promise highly-resilient electrode film with high peel strength to endure the potassiation/depotassiation process. As a result, the sub-5 nm SnS2/graphene anode was able to deliver an extraordinary reversible capacity (610 mAh g−1 at 50 mA g−1, one of the highest reported values in the literature), unprecedentedly high rate capability (around 290 mAh g−1 at 2A g−1), and promising cycling stability in KIBs anode. Moreover, a full-cell KIBs constructed by the Al-based anode and Al-based cathode demonstrated the feasibility of a dual-Al-current collector KIBs. Our work would sheds light on the rational design of high-performance electrode for KIBs and beyond.
[18] [19]
[20] [21] [22]
Acknowledgment
[23]
This work was supported by the National Natural Science Foundation of China (Grant No. 51672282), and the joint fund of Beijing Municipal Natural Science Foundation and Haidian district for original innovation (No. L182050)
[24]
Appendix A. Supplementary data
[26]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.04.032.
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D.-S. Bin, et al. De-Shan Bin earned his Ph.D. degree from Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in 2018. Now he works at AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory in Japan as a postdoctoral research fellow. His current research interests mainly focus on the design and synthesis of electrode materials for different battery systems including Li-ion batteries, Na-ion batteries, K-ion batteries, and Zin-Air batteries.
Yan-Song Xu is a Ph.D. student at Key Laboratory of Molecular Nanostructure and Nanotechnology, ICCAS. His main research topics include synthesis advanced cathode materials for potassium-ion batteries and design functional carbon materials for electrochemical energy storage.
Shu-Yi Duan is a Ph.D. candidate at Key Laboratory of Molecular Nanostructure and Nanotechnology, ICCAS. Her research interest mainly focuses on designing and constructing core-shell structured materials for sodium ion batteries.
Yong-Gang Sun is a Lecturer at Yancheng Institute of Technology. He received his B.S. (2008) and M.S. (2014) degrees in chemistry from Soochow University and received his Ph.D. degree from ICCAS in 2018. His research interest is primarily concerned with the design and synthesis of nanostructured materials for energy storage and conversion.
Xi-Jie Lin is a Ph.D. candidate at Key Laboratory of Molecular Nanostructure and Nanotechnology, ICCAS. Her research interest mainly focuses on designing and constructing cathode materials for Li-ion batteries.
Xiansen Tao is a Ph.D student in CAS Key Laboratory of Molecular Nanostructure and Nanotechnology and CAS Research/Education Center for Excellence in Molecular Sciences, ICCAS. His research focuses on cathode materials of lithium-ion batteries and solid electrolyte.
Lin Liu received her Ph.D. degree from ICCAS in 2018. She joined Nanyang Technological University in Singapore as a research fellow in 2018. Her current research interests mainly focus on the design of electrode materials and solid electrolyte for Li-ion batteries.
An-Min Cao is a Professor of Chemistry at ICCAS. He earned his Ph.D. degree from ICCAS in 2006. He worked in Prof. Götz Veser's group on industrial catalysis at University of Pittsburgh during 2007–2010. After two more years' postdoctoral research at the University of Texas at Austin with Prof. Arumugam Manthiram on lithium ion batteries, he started his current position as a PI in ICCAS in 2012. His research currently focuses on the surface and interface control of electrode materials for different battery systems.
Yuan Liu is a Ph.D. student at Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences (ICCAS). Her research interest mainly focuses on synthesis of anode materials for Potassium ion battery.
Li-Jun Wan is a Professor of Chemistry at ICCAS. He received his B.S. and M.S. degrees in Materials Science from Dalian University of Technology in 1982 and 1987, respectively, and Ph.D. in Materials Chemistry from Tohoku University (Japan) in 1996. His research focuses on the physical chemistry of single molecules and molecular assemblies, nanomaterials for applications in energy and environmental science, and scanning probe microscopy.
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Full paper
Structural engineering of SnS2/Graphene nanocomposite for highperformance K-ion battery anode
T
De-Shan Bina,b, Shu-Yi Duana,b, Xi-Jie Lina,b, Lin Liua, Yuan Liua,b, Yan-Song Xua,b, Yong-Gang Suna, Xian-Sen Taoa,b, An-Min Caoa,b,∗, Li-Jun Wana,b a
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 b University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China
ARTICLE INFO
ABSTRACT
Keywords: K-ion battery anode SnS2/Graphene composite Sub-5 nm nanoparticles Anodic aluminium current collector High-peel-strength electrode
K-ion batteries (KIBs) are drawing increasing research interest as a promising supplement of Li-ion batteries due to the natural abundance of K resource. However, due to the large size of K+, high-capacity anodes are challenged by the structural stability of the active materials which are susceptible to large volumetric deformation after incorporating with a sufficient number of K+. Herein, using SnS2/graphene as an example, we demonstrated that high-performance KIBs anode could be achieved through collaborative efforts targeting on both the active material and the prepared electrode film. The electrochemically-active species of SnS2 were controlled into small nanoparticles with their size below 5 nm to provide sufficient reactive sites for K+ storage. Meanwhile, highly-resilient electrode film based on the prepared SnS2/graphene nanocomposite was built on aluminum (Al) current collector rather than the widely-used copper foil, forming a strong anode film with high peel strength to endure the potassiation/depotassiation process. In this way, the active species was able to deliver an extraordinary reversible capacity of 610 mAh g−1 with unprecedented high-rate capability (around 290 mAh g−1 at 2A g−1) and promising cycling stability. This contribution sheds light on the rational design of high-performance electrode for KIBs and beyond.
1. Introduction Lithium-ion batteries (LIBs) are now the leading energy storage systems, but the insufficiency, fast consumption, and high cost of lithium resources cause concern on its sustainability [1,2]. K-ion batteries (KIBs), sharing a similar working mechanism with LIBs, have been considered as a promising supplement of LIBs in the energy storage field owing to the natural abundance of K and the low K+/K redox potential of −2.93 V vs standard hydrogen electrode (SHE), which is close to −3.04 V (vs SHE) of Li+/Li, suggesting the technological potential of achieving a higher voltage cell [3–6]. However, K+ is much larger and heavier than Li+, which would lead to serious structural deformation and sluggish kinetics for K+ transfer during the potassiation/depotassiation process, resulting in poor cycling performance and limited rate capability [7,8]. In terms of KIBs anode, the inexpensive carbon-based materials have been widely studied as one of the leading candidates [9–12]. Our previous efforts on carbon-based anodes demonstrated that hollow carbon materials could act as promising KIBs
anodes with excellent cycling performance [13,14]. However, carbon anode, which usually based on an intercalation storage mechanism, could only deliver limited reversible capacity [6,10]. Therefore, the development of high-capacity KIBs anodes with outstanding rate capability and promising cycling performance is essential for the innovation of advanced KIBs technology [15,16]. Tin (Sn)-based alloy materials, like Sn metal, SnS2, and Sn4P3, are drawing considerable attention as promising KIBs anodes owing to the high theoretical capacity based on an alloying mechanism for K+ storage [7,17–19]. However, there is usually a pivotal challenge associated with the structural degradation of the alloy materials caused by the large volume change after fully reacting with K+ for high capacity [7,15,16,18–20], subsequently leading to particle fracture and then losing electrical contact with the current collector, finally fading the capacity rapidly [7,16,21,22]. For example, a Sn4P3/carbon nanocomposite for KIBs anode have been recently developed, which delivered a promising reversible capacity of 384.8 and 221 mAh g−1 at current density of 50 and 1000 mAg−1, respectively [7]. Although it
∗ Corresponding author. 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. E-mail address: [email protected] (A.-M. Cao).
https://doi.org/10.1016/j.nanoen.2019.04.032 Received 27 January 2019; Received in revised form 28 March 2019; Accepted 6 April 2019 Available online 11 April 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
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has exceeded the capacity for most of the carbon anodes, the potential of its high capacity and high rate capability was not fully exploited. Meanwhile, the capacity faded rapidly to around 307 mAh g−1 after 50 cycles perhpers owing to the lack of rational design of the electrode with inherent high-volume-change alloying materials as anode. Such an unsatisfied battery performance may be ascribed to the limited capability of the electrode film for resisting the serious structure deformation and losing electrical contact after storing numerous large-size K+ [7,16,21,22]. It suggests that there is still a huge room but also a very formidable challenge for fully exploiting the potential of the alloy species to boost their electrochemical performance in KIBs anodes. Therefore, the electrode innovation of high-capacity alloy anode materials for KIBs should take an overall consideration of both the active material and the electrode film including: 1) Favorable architecture of active materials with size-control, high-dispersity alloy nanoparticles incorporated with the conductive matrix to avoid particle fracture and provide sufficient reactive sites for K+ storage as well as excellent K+/e− transport kinetics [7,15,16]. Especially, the control of the particle size of the alloys particles have long been identified as an effective strategy to improve the cycling stability in LIBs but it was overlooked in KIBs [23]. 2) High-peel-strength of the electrode to withstand the large potassiation/depotassiation strains and ensure a resilient electrode structure to mitigate the active materials peeling from the current collector and losing electrical contact [24,25]. 3) The cost of the passive component of the prepared electrode, for example, current collector, should be taken into consideration for KIBs for their future application in large-scale energy storage. Aluminum (Al) foil, which is much lighter and lower-cost than the widely-used copper foil, is electrochemically inactive with K at a low potential [4,26–28], suggesting it is a promising anodic current collector. Using Al foil as a current collector for KIBs anode would be beneficial for reducing the cost of the device and meanwhile enhancing energy density of the whole KIBs system [4,26], which would be advantages related to their practical applications but so far have been given insufficient attention. Herein, we demonstrated that the collaborative efforts in engineering both the SnS2/graphene nanocomposite and the electrode film could promise high conceptual and technological potential for a high-performance KIBs anode. We identified that the SnS2 species in the SnS2/graphene composite could be engineered into small-size nanoparticles below 5 nm with high dispersity to provide sufficient reactive sites and facilitate the K+/e transfer for K+ storage. Meanwhile, highlyresilient electrode film based on the prepared sub-5 nm SnS2/graphene nanocomposite could be built on an aluminum current collector instead of the widely-used copper foil using a hybrid binder of sodium carboxmethyl cellulose and styrene butadiene rubber (CMC/SBR), forming a strong anode film with high peel strength to endure the potassiation/ depotassiation process. As a result, the designed electrode was able to deliver a reversible capacity of 610 mAh g−1 at 50 mA g−1 (one of the highest reported values in the literature), unprecedentedly high rate capability of around 290 mAh g−1 at 2A g−1, and promising cycling stability in KIBs anode. Furthermore, a full-cell KIBs constructed by the prepared Al-based anode and Al-based cathode of 4,9,10-perylene-tetracarboxylic acid−dianhydride (PTCDA) demonstrated the feasibility of a KIBs based on dual-Al current collectors for both anode and cathode. We believe this contribution would shed light on the rational design of high-performance electrode for KIBs and beyond.
for around 10 min. Then 0.342 g thioacetamide (TAA) added into the solution and stirred for another 10 min. Then the mixed aqueous solution was transferred to a Teflon-lined stainless steel autoclave for the hydrothermal process, the temperature is 160 and 100 °C for 12 h to control the particles size of SnS2 to be sub-5 nm and above 15 nm, respectively. After the reaction and natural cool to room temperature, the synthesized product was collected by centrifugation, and then washed with deionized water at least three times. To keep the product well dispersity, a freeze-drying process was conducted for the drying of the collected SnS2/graphene. The obtained dried product was then heated to 400 °C for 4 h at Ar atmosphere. SnS2 nanoparticles were also synthesized without the presence of graphene under the same process and condition. The control sample of graphene was also performed the hydrothermal process and 400 °C heated treatment. 2.2. Materials characterizations Scanning electron microscopy (SEM) images were recorded on a SU8020 microscope. The transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and element mapping images were acquired on a JEOL-2100F microscope. Xray diffraction (XRD) patterns were collected by a Rigaku D.MAX-2500 system with Cu Kα radiation (λ = 1.5406 Å). Raman spectra were obtained with a HORIBA spectra system (LabRAM HR Evolution). N2 sorption isotherms were carried out on Quantachrome nitrogen sorption instrument (NOVA3200e and iQ). Thermal gravimetric analysis was carried out with a heating ramp of 10 °C min−1 in air atmosphere. The peel strength was measured with an electrode peel tester (HY-0580, Shanghai Hengyi Precious Instrument Co., Ltd). A 20 mm wide and 160 mm long electrode sample was attached to an adhesive tape. Then the adhesive tape was removed by peeling at the angle of 180° at a rate of 50 mm min−1. The peel strength was recorded by the computer system. 2.3. Electrochemical measurements The electrochemical measurements were tested with CR2032 coin cells (half-cell) at room temperature. The working electrode was prepared by coated the mixed slurry of active material, binder and Super P with a weight ratio of 80:15:5 onto the current collector of commercial Al foil or Cu foil which were purchased and use directly, and then dried at 80 °C in vacuum for overnight. Water was used as the solvent for the slurry when CMC/SBR (mass ratio 1:1) or CMC were used as binder. NMethyl pyrrolidone (NMP) was used as the solvent for the slurry when PVDF was used as binder. For KIBs, the electrolyte was a solution of 0.8 M KPF6 in ethylene carbonate and diethyl carbonate (1:1 in volume). K foil and glass fiber were used as the counter electrode and separator, respectively. All the operations were performed in the Argonfilled 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 2.5 V versus K+/K for KIBs. The specific capacities in the half-cell were calculated based on the mass of SnS2 active materials only with the extraction of the graphene contribution. The K-ion full-cells were constructed by using SnS2/grephene as anode and 3, 4, 9, 10-perylene–tetracarboxylicacid–dianhydride (PTCDA) as cathode in a CR2032 coin-type cell. The cathodes were prepared by mixing 70 wt% PTCDA, 20 wt% Super P and 10 wt% PVDF (binder) dissolved in NMP. The slurry was coated on Al foil and dried at 100 °C for 12 h under vacuum. The PTCDA cathodes were prepotassiated before assembly. The full cells were charged and discharged in a voltage range of 0.5–3.5 V at room temperature. The specific capacities of the full cell K-ion battery were calculated based on the total mass of SnS2/ graphene so as to have a better evaluation of its feasibility for realistic application. Cyclic voltammetry (CV) was tested using an AutoLab PGSTA302N electrochemical workstation (Metrohm, Switzerland). EIS was also measured using the Autolab PGSTAT 302N with the frequency
2. Experimental Section 2.1. Materials synthesis The SnS2/graphene nanocomposite was synthesized through a simple hydrothermal process [29], and then the freeze-drying step was carried out to keep the dispersity of the hybrid nanoparticles. The used graphene is a commercial product. Typically, 0.4 g SnCl4·5H2O were added to 60 ml of ∼1 mg ml−1 graphene aqueous solution and stirred 913
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Fig. 1. SEM image (A), TEM image (B–C) and HR-TEM image (D) of SnS2/ graphene nanocomposite. The reaction temperature for the hydrothermal process is 160 °C.
Fig. 2. XRD pattern (A), Raman spectra (B), elemental mappings (C), and N2 adsorption-desorption isotherms (D) of the SnS2/graphene nanocomposite. The reaction temperature for the hydrothermal process is 160 °C. The pure SnS2 nanoparticles was synthesized via the same method without adding graphene and also tested by XRD and Raman spectra for comparison.
range from 100 kHz to 10 mHz.
SnS2 [29]. The banks at ∼1585 cm−1 and ∼1348 cm−1 for the hybrid SnS2/graphene nanocomposite could be assigned to the typical graphitic-induced G band and disorder-induced D band for the carbon component of graphene [31]. Further evaluation was carried out on the SnS2/graphene nanocomposite with a focus on its compositional distribution with the elemental mapping analysis (Fig. 2C). As shown in Fig. 2C, the elemental mapping images of the randomly picked SnS2/ graphene particle illustrated the homogeneous distribution of three concerned elements (Sn, S, C), further verifying the successful incorporation of the ultrafine SnS2 nanoparticles onto the functional graphene with a high dispersity. The nitrogen adsorption-desorption isotherms analysis was performed to characterize the porous structure of the SnS2/graphene architectures (Fig. 2D). The specific surface area of the nanocomposite is measured and calculated to be 78.18 m2 g−1. The result of the porous structure revealed the simultaneous existence of mesopores and macropores with a pore volume of 0.885 cm3 g−1 and a broad distribution of the pore size for such hybrid nanocomposites (inset of Fig. 2D). Such a porous structure is expected to be favorable for its applications in KIBs for facile access of the electrolyte and promoting the kinetics of large K+ transport [32], which is key for the high-performance KIBs anodes [11].
3. Results and discussion 3.1. Synthesis and characterization of the SnS2/graphene nanocomposites The SnS2/graphene nanocomposites were synthesized through a simple hydrothermal process, and then the subsequent freeze-drying step was carried out to keep the dispersity of the hybrid nanoparticles (see Experimental Section for the detail). Fig. 1A illustrated a scanning electron microscopy (SEM) image of the prepared hybrid nanocomposite synthetized under a hydrothermal process of 160 °C, which showed the sheet-stacking porous architecture. Transmission electron microscopy (TEM) analysis was performed to evaluate the morphology of the synthesized hybrid nanoparticles (Fig. 1B–C). As showed in Fig. 1B, the ultrafine and well-dispersed SnS2 nanoparticles are homogeneously incorporated in the graphene matrix. A magnified observation could clearly identify the nanoparticles size of the nanocomposites (Fig. 1C), which is less than 5 nm with a narrow size distribution (Figure S1). Such well-defined sub-5 nm nanoparticles anchored onto the graphene sheet was expected to provide sufficient reactive sites and excellent ion/electron transport kinetics for the K+ storage, and would effectively alleviate the aggregation of the nanoparticles [30]. The X-ray diffraction (XRD) pattern of this hybrid nanocomposite sample identified the emergence of berndtite-4H SnS2 (PDF #21–1231) (Fig. 2A). High-resolution TEM (HR-TEM) test illustrated that the SnS2 nanoparticles exhibited a well crystalline structure with clear lattice fringes (Fig. 1D). The lattice spacing of 3.25 and 2.70 Å correspond to the (100) and 102 planes of the berndtite-4H SnS2 nanoparticles, respectively. The content of the SnS2 in the nanocomposite is calculated to be 76.8 wt % based on the thermogravimetric analysis (TGA) (Figure S2). We identified that the particle size and dispersity of SnS2 nanoparticles in the hybrid composites could be facilely controlled by tuning the reaction temperature in the hydrothermal process. When decreasing the temperatures from 160 to 100 °C, not only the size of the SnS2 nanoparticles would raise from sub-5nm (Figure 1C) to 15–50 nm, but also the agglomeration of the SnS2 nanoparticles with poor dispersity could be observed (Figure S3). The sub-5 nm SnS2/graphene nanocomposite was further analyzed by Raman spectroscopy (Fig. 2B) with pure SnS2 as a comparison. As illustrated in Fig. 2B, the band at ∼310 cm−1 for both pure SnS2 and SnS2/graphene nanoparticles could be ascribed to the A1g mode of the
3.2. Electrochemical performance in K-ion batteries For the electrochemical performance evaluations of the prepared SnS2/graphene nanocomposite as an anode in KIBs, we have a special interest in using aluminum foil as current collector rather than the widely-used copper foil, which was initially sparked by that aluminum foil would not form an alloy with K at low potential [4,26–28]. Meanwhile, aluminum foil is much lighter and lower-cost than copper foil. It would be beneficial for reducing the cost and in the meantime enhancing the energy density of the whole KIBs system for their future applications [4,26]. For the binder used for preparing the electrode, we employed a hybrid one, namely, sodium carboxmethyl cellulose (CMC) and styrene butadiene rubber (SBR) (CMC/SBR), which is the most commonly-used binder in the graphite and silica-based anode material for commercial LIBs owes to its high elasticity and strong adhesion [33,34]. Fig. 3A illustrated the typical charge/discharge profiles of the synthesized sub-5nm SnS2/graphene nanocomposites obtained from the first two cycles using Al foil and CMC/SBR as the current collector and 914
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Fig. 3. Applications of sub-5nm SnS2/ graphene nanocomposite as an anode for KIB. (A) Discharge/charge curves at the first two cycles for SnS2/graphene nanocomposite based on Al-CMC/SBR at 0.05 A g−1. (B) Cyclability tests for SnS2/graphene nanocomposite based on Al-CMC/SBR and Cu-CMC/SBR at 0.1A g−1. (C) Comparison of the cycling performance SnS2/graphene nanocomposite based on Al-CMC/SBR with various representative researched high-capacity KIBs anodes [42]. (D) EIS spectra, the inset is the equivalent circuit for fitting the EIS spectra. (E) Rate capabilities SnS2/graphene nanocomposite based on Al-CMC/SBR. (F) Comparison of the rate capability of SnS2/graphene nanocomposite based on Al-CMC/SBR and the representative KIBs anodes reported in the literature [43–44].
binder, respectively. A high reversible capacity of 610 mAh g−1 based on SnS2 could be achieved at a current of 50 mA g−1, which is one of the highest outputs for KIBs anodes reported in the literature as shown in Table S1. The previous reports of Sn and S for K+ storage showed that S can form K2S and Sn can form KSn during the potassiation process [7,35], resulting in the possible theoretical capacity of 733 mAh g−1. The cyclic voltammetry (CV) analysis of the Al-CMC/SBR (sub5nm) electrode was also performed (Figure S4). The obvious reduction peak around 0.72, 0.36, and 0.16 V and the anodic peak around 1.01, 1.28, and 1.79V are highly reversible and corresponded to the reversible potassiation/depotassiation process. The SnS2/graphene with the particles size of SnS2 above 15 nm was also tested for comparison (Figure S5), which could only deliver a reversible capacity of 454.5 mAh g−1. It is expected that the well-dispersity of the sub-5 nm SnS2/ graphene nanocomposites could provide sufficient reactive sites for K+ storage, thus resulting in a higher capacity. To evaluate the stability of the SnS2/graphene nanocomposites during the repeated potassiation/depotassiation processes, cyclability tests were performed. Both the sub-5nm SnS2/graphene composite and the SnS2/graphene sample with large size (> 15 nm) SnS2 nanoparticles were tested with the same current collector of Al foil and hybrid binder of CMC/SBR. Interestingly, the sub-5 nm SnS2/graphene nanoparticles coated on the Al foil (denoted as Al-CMC/SBR (sub-5nm)) delivered an improved cycling capability than the large size (> 15 nm) SnS2/graphene composite (denoted as Al-CMC/SBR (> 15 nm) (Fig. 3B). A high reversible capacity at around 559 mAh g−1 maintained with a capacity retaining of 94% upon over 50 continuous cycles at a current density of 100 mA g−1 for the Al-CMC/SBR (sub-5nm) electrode (Fig. 3B), which was much superior than that of the representational high-capacity KIBs anodes reported in the literature (Fig. 3C). On the contrary, the Al-CMC/SBR (> 15 nm) electrode showed a rapid-fading cycling performance with low capacity retaining of only 35% after 50 cycles, highlighting the success of the structurefunction design of the sub-5 nm SnS2/graphene nanocomposite for advanced KIBs materials. Meanwhile, we also used copper (Cu) foil as the current collector for the sub-5 nm SnS2/graphene nanocomposite anode for comparison, as copper (Cu) foil is the most-frequently used anodic current collector in LIBs, SIBs, and KIBs. Our results showed that the sub 5 nm SnS2/graphene electrode based on Cu foil with the same binder of CMC/SBR (denoted as Cu-CMC/SBR (sub-5nm)) showed a rapid fading of capacity after the same cycling compared with the AlCMC/SBR (sub-5nm) electrode. The electrochemical impedance spectrum (EIS), which has long been known to be a useful technique for
probing the electrochemical interface in batteries [32,36], was carried out to evaluate the electrochemical interface in the electrode. The Nyquist plots for these two different samples of Al-CMC/SBR (sub-5nm) and Cu-CMC/SBR (sub-5nm) were illustrated in Fig. 3D. In Nyquist plots, the radius of the emerging semicircle in high-to-medium frequency region is known as the charge transfer and surface film resistance for the electrode [36]. As shown in Fig. 3D, the charge transfer resistance of the Al-CMC/SBR (sub-5nm) electrode was much smaller than that of Cu-CMC/SBR (sub-5nm), suggesting the low electrochemical impedance for the Al-CMC/SBR (sub-5nm) electrode. The rate capability tests showed that such an Al-CMC/SBR (sub5nm) electrode could deliver an outstanding high-rate capability (Fig. 3E). Not only could it show a high reversible capacity of 583 and 495 mAh g−1 at 0.1 and 0.5 A g−1, respectively, but also could deliver high reversible capacity of 412 and around 290 mAh g−1 at a high current densities of 1 and 2 A g−1, respectively. The charge/discharge profiles in different current density were illustrated in Figure S6. Such a rate performance of the SnS2/graphene sample was much superior compared to various representative researched KIBs anodes (Fig. 3F), highlighting the functionality of such a SnS2/graphene electrode. The rate capability of graphene (with the same hydrothermal process and heated treatment of preparing SnS2/graphene) and pure SnS2 was also tested for comparison, respectively. The pure graphene delivered revisable capacity of 174 mAh g−1 at 0.05 A g−1 and contributed very negligibly to K+ storage with almost no revisable capacity at 1 A g−1 (Figure S8). For the pure SnS2, it also delivers low reversible capacity (Figure S9). The cyclic voltammetry of the sub-5 nm SnS2/graphene based on Al-CMC/SBR was also tested at different sweep rates (Figure S7A). A detailed analysis on the CV result revealed that a large part of capacity of SnS2/graphene electrode was contributed by the capacitive (Figure S7B). The capacitive contribution gradually increases from 46.3% to 90.3% with the increasing scan rate increase from 0.1 to 1 mV s−1, which explains well its outstanding rate capability of the design electrode of Al-CMC/SBR (< 5 nm). The distinguished electrochemical performances of SnS2/grapnene in KIBs anode suggested the success of structural engineering of such a nanocomposite of small highdispersity SnS2 particles incorporated in the conductive graphene matrix for the boosting of the K-storage capability. To demonstrate the high conceptual and technological potential of the designed Al-based anode of SnS2/grapnene, the full-cell K-ion batteries were constructed and evaluated by employing the sub-5nm SnS2/ graphene anode based on Al-CMC/SBR and Al-based cathode of 4,9,10perylene-tetracarboxylic acid−dianhydride (PTCDA) [11,37]. The 915
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PTCDA cathode is a commercial product (Alfa Aesar, 98%), which features microparticles (Figure S10). In the dual-Al full-cell configuration, the SnS2/graphene electrode showed a high reversible capacity of 465 mAh g−1 at 50 mA g−1 based on the mass of SnS2/graphene (Figure S11A). The initial Coulombic efficiency of the K-ion full cell is 40.5% and increases to 89.2% in the second cycle, suggesting a typical process probably related to the formation of a solid electrolyte interphase (SEI) in the anode during the initial potassiation/depotassiation process [10]. It delivered a reversible capacity of around 390 mAh g−1 at 100 mA g−1 and kept at around 230 mAh g−1 after 25 cycles (Figure S11B). The constructed full-cell battery could deliver an initial voltage around 3.1 V after charging (Figure S11C). It was able to simultaneously power up more than fifty light-emitting diodes (LEDs) bulbs (Figure S11D), showing the applicability of the SnS2/graphene anode in the K-ion full cell application. Although the results were preliminary, it envisions new perspectives on the development of low cost and high-energy-density dual-Al-current collector battery for application in large-scale energy storage.
capacity retention of 83.3% after 50 cycles, which was a moderate capacity retention compared to the value of 94% for the CMC/SBR hybrid binder. Meanwhile, capacity for the electrode based on PVDF showed a fast fading with only capacity retention of 57.6% after the same cycles. Such a result highlighted the uniqueness of the Al-CMC/ SBR protocol for the high-capacity SnS2/graphene anode in KIBs. Such an obvious difference between the batteries performances of the same sub-5 nm SnS2/graphene active materials affected by the current collectors or binders is interesting. In general, binders and current collectors could affect the adhesion properties of the electrode film, reflected by the peel strength of the current collector and active materials [24,25]. We thus carried the peel test of the electrodes, which has long been considered as an effective technique to evaluate the adhesion strength for the active materials and current collectors [24,41]. As shown in Fig. 4B, the SnS2/graphene materials with CMC/SBR as binder could tightly adhere to the Al foil with better adhesive property than other three electrodes. Such favorable adhesion strength is expected to be very helpful to alleviate the stripping of the high-volume change of the electrode materials during the repeated potassiation/ depotassiation [24,41], which is consistent with the cycling performance discussed above. On the contrary, the electrode of control samples delivered poorer cycling stability, showing lower adhesion strength (Fig. 4B). To evaluate the structure change of the electrodes after repeated potassiation/depotassiation processes, SEM examinations were conducted. All the primary electrodes observed by SEM were well-intact without cracks (Fig. S12). After repeated potassiation/depotassiation cycling, only the SnS2/graphene electrode based on Al-CMC/SBR was able to keep a well-preserved morphology of the electrode film (Fig. 4C), suggesting a promising durability of the electrode for accommodating the serious volume deformation. On the contrary, we could observe cracks imprinted on the electrode prepared by Al-CMC (Fig. 4D), large cracks emerged for the electrode based on Al-CMC/SBR (Fig. 4E), and obvious wrinkle and peeling of the electrode prepared by Al-PVDF (Fig. 4F), suggesting the repeated K+ insertion/extraction processes would result in structural degradation of the electrode film, which well agreed with the lower peel strength of these control electrode and explained well the continuous capacity fading of these samples (Fig. 4B). As simple as the strategy might look like, it is interesting that no previous researchers have paid sufficient attention to the Al current collector as anodic current collector together with an appropriate CMC/ SBR hybrid binder to construct a high-peel-strength electrode film for improving the electrochemical performance of high-volume-change anode for KIBs. It might be related to two facts: Firstly, the Al foil is always employed as the current collector for the cathode materials thus it can be easily overlooked in anode, considering Cu foil is the widelyused anodic current collectors. Secondly, a suitable binder of hybrid CMC/SBR also plays a key role and good care for the binder selection should be taken if an electrode with stronger peel strength is expected (See Fig. 4B). Fortunately, our persistent efforts on the aiming to seek the low cost and light current collector and binder endowed us an opportunity to find this interesting strategy for improving the electrode performances.
3.3. The evaluation of peel strength and structure of the designed electrodes To have a better understanding on the beneficial effects of Al current collector combined with the binder of CMC/SBR for the enhanced performance of such sub-5 nm SnS2/graphene anode for KIBs, we carried out the controlled experiments by using other binders. Based on the same Al current collector, we employed another two binders, namely, polyvinylidene fluoride (PVDF) and pure CMC, which were also widely used as the binder for anode materials in LIBs, SIBs, and KIBs [36,38–40]. The results illustrated in Fig. 4A revealed that the sub-5 nm SnS2/graphene electrode with pure CMC as binder could deliver a
4. Conclusions In summary, we confirmed that the collaborative engineering of both the active materials and the electrode film provided an effective route to achieve a high-performance anode for KIBs. Using SnS2/graphene nanocomposite as an example, we demonstrated that electrochemically-active species of SnS2 could be controlled into small nanoparticles with the size below 5 nm and maintained high dispersity, which provided sufficient reactive sites and excellent K+/e− transport kinetics for fully exploiting its potential in K+ storage. Meanwhile, we identified that a high-peel-strength electrode was key in ensuring a
Fig. 4. The evaluation of peel strength and structrue of the designed electrode. A) Cyclability tests for sub-5 nm SnS2/graphene nanocomposite based on AlCMC and Al-PDVF at 0.1A g−1. B) Peel test of sub-5 nm SnS2/graphene nanocomposite based on Al-CMC/SBR, Cu-CMC/SBR, Al-CMC, Al-CMC, Al-PVDF. C-F) SEM images of the electrode after 10 cycles at 0.1A g−1, c) Al-CMC/SBR, d) Al-CMC, e) Cu-CMC/SBR, f) Al-PVDF. 916
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resilient electrode structure for a durable potassiation/depotassiation process. We found that building the synthesized sub-5 nm SnS2/graphene nanocomposite on the aluminum current collector, rather than the frequently-used copper one, could promise highly-resilient electrode film with high peel strength to endure the potassiation/depotassiation process. As a result, the sub-5 nm SnS2/graphene anode was able to deliver an extraordinary reversible capacity (610 mAh g−1 at 50 mA g−1, one of the highest reported values in the literature), unprecedentedly high rate capability (around 290 mAh g−1 at 2A g−1), and promising cycling stability in KIBs anode. Moreover, a full-cell KIBs constructed by the Al-based anode and Al-based cathode demonstrated the feasibility of a dual-Al-current collector KIBs. Our work would sheds light on the rational design of high-performance electrode for KIBs and beyond.
[18] [19]
[20] [21] [22]
Acknowledgment
[23]
This work was supported by the National Natural Science Foundation of China (Grant No. 51672282), and the joint fund of Beijing Municipal Natural Science Foundation and Haidian district for original innovation (No. L182050)
[24]
Appendix A. Supplementary data
[26]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.04.032.
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References
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D.-S. Bin, et al. De-Shan Bin earned his Ph.D. degree from Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in 2018. Now he works at AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory in Japan as a postdoctoral research fellow. His current research interests mainly focus on the design and synthesis of electrode materials for different battery systems including Li-ion batteries, Na-ion batteries, K-ion batteries, and Zin-Air batteries.
Yan-Song Xu is a Ph.D. student at Key Laboratory of Molecular Nanostructure and Nanotechnology, ICCAS. His main research topics include synthesis advanced cathode materials for potassium-ion batteries and design functional carbon materials for electrochemical energy storage.
Shu-Yi Duan is a Ph.D. candidate at Key Laboratory of Molecular Nanostructure and Nanotechnology, ICCAS. Her research interest mainly focuses on designing and constructing core-shell structured materials for sodium ion batteries.
Yong-Gang Sun is a Lecturer at Yancheng Institute of Technology. He received his B.S. (2008) and M.S. (2014) degrees in chemistry from Soochow University and received his Ph.D. degree from ICCAS in 2018. His research interest is primarily concerned with the design and synthesis of nanostructured materials for energy storage and conversion.
Xi-Jie Lin is a Ph.D. candidate at Key Laboratory of Molecular Nanostructure and Nanotechnology, ICCAS. Her research interest mainly focuses on designing and constructing cathode materials for Li-ion batteries.
Xiansen Tao is a Ph.D student in CAS Key Laboratory of Molecular Nanostructure and Nanotechnology and CAS Research/Education Center for Excellence in Molecular Sciences, ICCAS. His research focuses on cathode materials of lithium-ion batteries and solid electrolyte.
Lin Liu received her Ph.D. degree from ICCAS in 2018. She joined Nanyang Technological University in Singapore as a research fellow in 2018. Her current research interests mainly focus on the design of electrode materials and solid electrolyte for Li-ion batteries.
An-Min Cao is a Professor of Chemistry at ICCAS. He earned his Ph.D. degree from ICCAS in 2006. He worked in Prof. Götz Veser's group on industrial catalysis at University of Pittsburgh during 2007–2010. After two more years' postdoctoral research at the University of Texas at Austin with Prof. Arumugam Manthiram on lithium ion batteries, he started his current position as a PI in ICCAS in 2012. His research currently focuses on the surface and interface control of electrode materials for different battery systems.
Yuan Liu is a Ph.D. student at Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences (ICCAS). Her research interest mainly focuses on synthesis of anode materials for Potassium ion battery.
Li-Jun Wan is a Professor of Chemistry at ICCAS. He received his B.S. and M.S. degrees in Materials Science from Dalian University of Technology in 1982 and 1987, respectively, and Ph.D. in Materials Chemistry from Tohoku University (Japan) in 1996. His research focuses on the physical chemistry of single molecules and molecular assemblies, nanomaterials for applications in energy and environmental science, and scanning probe microscopy.
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