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Cu2O Hybridized Titanium Carbide with Open Conductive Frameworks for Lithium-ion Batteries
Electrochimica Acta 202 (2016) 24–31
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Cu2O Hybridized Titanium Carbide with Open Conductive Frameworks for Lithium-ion Batteries Huang Zhanga,* , Hui Dongb , Xuan Zhanga , Yunlong Xub , Jan Fransaera,* a
Department of Materials Engineering (MTM), KU Leuven, Kasteelpark Arenberg 44, B-3001 Leuven, Belgium Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, PR China b
A R T I C L E I N F O
Article history: Received 6 February 2016 Received in revised form 21 March 2016 Accepted 3 April 2016 Available online 4 April 2016 Keywords: MXene Hybridization Three-dimensional Carbon black-free Lithium-ion batteries
A B S T R A C T
Though MXenes, a new family of 2D transition metal carbides, are generating considerable interests as electrode materials for batteries and supercapacitors, further application is hindered by their low capacities and poor rate capabilities. Here we propose a simple route for the synthesis of Cu2O particle hybridized titanium carbide Ti2CTx (T = O, OH) composites via a solvothermal method. Electrodes containing Cu2O/MXene were fabricated without carbon black, and tested as anodes for lithium ion batteries. A discharge capacity of 143 mAh g1 was obtained at a discharge current density of 1000 mA g1 and the capacity retention was near 100% after 200 cycles. The hybrid electrodes with open conductive frameworks exhibited significantly improved electrochemical performance, suggesting a new method for preparing MXene-based composites for energy storage application. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Lithium-ion batteries (LIB) are the most attractive rechargeable batteries and have been widely used for powering electronic devices in our daily life [1,2]. Addressing the challenges to develop high-performance battery materials, two-dimensional (2D) materials have historically been one of the most extensively studied classes of materials for batteries, among which graphene (GN) is a typical representative with a 2D structure and strongly bonded carbon networks, and can inevitably contribute to their intriguing electronic conductivity, high surface area and short diffusion path, rendering them interesting electrode materials for LIBs. Such properties also suggest a wide-range of hybridization strategies with other materials, such as CNTs, Fe3O4, TiO2, and carbon fibers, to further optimize their performances in LIBs [3,4]. MXenes are a novel family of two-dimensional (2D) metal carbides, which are synthesized by the exfoliation of ternary carbides MAX phases [5,6]. Generally, MXenes are produced by etching away the A layers from Mn+1AXn (MAX phases), in which ‘M’ represents a transition metal, ‘A’ is the element in the group (Al, Ge, Si, Sn, etc.), and ‘X’ represents carbon and/or nitrogen and n = 1 to 3 [7]. Due to their high specific surface area, metallic conductivity and hydrophilic surface, MXenes have shown
* Corresponding authors. Tel.: +32 16 32 12 60; fax: +32 16 32 19 91. E-mail addresses: [email protected] (H. Zhang), [email protected] (J. Fransaer). http://dx.doi.org/10.1016/j.electacta.2016.04.009 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
outstanding performance as electrodes for supercapacitors [8–11] and lithium-ion batteries [12–14], with superior volumetric capacitances that exceed most previously reported materials. Using density functional calculations, Er et al. predicted that Ti3C2 can increase the performance of Li, Na, K, Ca ion batteries as anode material [15]. Sun et al. exfoliated Ti3AlC2 with HF solution to obtain the two-dimensional Ti3C2 MXene. At a rate of 3C and 10C, the intercalated Ti3C2 possessed specific capacities of 88 mAh g1 and 69 mAh g1 after 100 cycles, respectively, without obvious capacity loss (the theoretical specific capacity of 130 mAh g1 provided by Ti3C2F2Li) [16]. Moreover, the terminated-Ti3C2Tx (T = F, O, OH, etc.) MXenes have also been used as electrodes for Li-ion batteries with and without carbon black as conductive agent. These studies reveal that the electrodes fabricated without carbon black have an initial capacity of 125 mAh g1 at a current density of 30 mA g1, but show a capacity of less than 30 mAh g1 after 50 cycles [17]. Ti2AlC is one of the most common and cheapest MAX phases which are commercially available [18]. The MXene Ti2CTx derived from this MAX phase materials has good thermal and electric properties [19,20]. In 2012, Naguib et al. reported the Li insertion of Ti2C-based material (MXene) with an oxidized surface, exhibiting a capacity of 70 mAh g1 after 200 cycles at 10C, suggesting a potentially promising anode material for lithium ion batteries [21]. From this we can realize that the pristine MXenes are limited by their low specific capacity and poor cycling when directly used as anode materials for batteries, especially at a high current density.
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Therefore, in order to further develop the MXene into practical battery application, new strategies should be debuted to optimize the overall performance as electrodes. Results demonstrate that M2X structures have higher Li-ion specific capacities, compared to their M3X2 and M4X3 counterparts, as the numbers of atomic layers per MXene sheet in the former are 3 instead of 5 or 7 for the latter [22]. The introduction of carbon additives, such as carbon nanotubes (CNTs) into the electrode structure can improve ion accessibility into the MXene layers and boost both specific capacities and rate performance. [23]. Meanwhile, delighted by the previous research, the combination of MXenes with carbon nanotubes or carbon nanofibers can improve the electrochemical performance [24,25]. This can be attributed to the high conductivity, large surface area and short diffusion path, namely the intrinsic properties of these combined nanomaterials, which will be beneficial to the electrochemical activity of fabricated electrodes. However, compared with the currently commercialized carbon materials, transition metal oxide has been demonstrated with various advantages as anode materials for lithium ion batteries, such as high theoretical capacity, widespread availability, and environmental benignity [26–28]. Therefore, the performance of MXene can be improved When hybridized with high capacity transition metal oxides, and assembled into conductive additive-free electrodes. Using a hydrothermal method, the synthesis of Cu2O nanospheres can be achieved on a large scale [29–31]. Mesoporous Cu2O films with an open 3D framework can achieve superior reversible electrochemical performance as anodes for lithium ion batteries, [32] and the performance can be enhanced by hybridization with graphene nanosheets [33]. In this work, we propose an easy solvothermal method to grow Cu2O nanoparticles on MXenes and construct an open conductive framework, in order to achieve high performance anode materials for lithium ion batteries. 2. Experimental 2.1. Preparation of Ti2CTx MXenes, Cu2O spheres and Cu2O/Ti2CTx composites The Ti2AlC MAX phase powders used in this work were purchased from Kanthal Company (Sweden) and used after wet ball-milling to reduce the particle size from 10 um to 50 um. The MXene materials were synthesized through a green and published method according to the work by Gogotsi [8,9]. Specifically, stoichiometric amount of MAX phase Ti2AlC powders were dispersed in 6 M HCl solution and LiF powders were slowly added into the suspension. The prepared suspension was transferred into a sealed plastic container and heated at 50 C for 48 h under constant stirring. After that, the product was washed several times using deionized water and separated by centrifuging. Then the final MXene product was dried in a vacuum oven at 80 C for 10 h. The Cu2O modified MXenes were synthesize in a single-step experiments by adding stoichiometric amounts of Cu(CH3COO)2H2O and Ti2CTx powders to N, N-dimethylformamide (DMF) followed by ultrasonic treatment for 15 min. After that, the black-green suspension was put in a sealed container and heated to 150 C for 10 h while stirring. The final black powders were obtained using centrifuging after washing with deionized water and ethanol. The theoretical weight ratio of Cu2O and Ti2CTx in the composites is 25:100. The pure Cu2O was also synthesized through the same method described above. 2.2. Materials characterization The phase identification of powders was conducted with X-ray diffraction (XRD, Seifert3003) using Cu Ka radiation (l=0.15418 nm). The morphology and structure were evaluated
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by field emission transmission electron microscopy (FETEM, JEM2010F, Japan) and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX, Philips XL 30 FEG). 2.3. Electrochemical measurements Electrochemical studies were conducted in standard CR2032 coin cells. The electrode slurries were prepared by dispersing 90 wt.% active materials and 10 wt.% polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) solvent and coated onto Cu foil, and then dried in a vacuum oven. The cells were assembled and performed in an argon-filled glove box using lithium metal as the anode electrode, and 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1, v/v) as electrolyte. The lithiation/delithiation behavior and long-term stability were performed on a Maccor Battery Tester 4300. All the cells were tested galvanostatically between 0.05 and 2.5 V (vs. Li+/Li) at current densities of 10, 100, 500 and 1000 mA g1, respectively. Capacity was calculated on the basis of the mass of Cu2O/MXene. CV and EIS were performed on an Autolab PGSTAT 301 N electrochemical workstation. Cyclic voltammograms (CV) were recorded at a scan rate of 0.1 mV s1 between 0.01 and 3.5 V (vs. Li+/Li). The EISs were measured potentiostatically at the open circuit potential (OCP) with an AC amplitude of 5 mV from 102 Hz to 105 Hz. 3. Results and Discussion A schematic of Ti2CTx synthesis and solvothermal treatment with DMF t is shown in Fig. 1. When the Ti2AlC powders were added to the HCl/LiF solution, bubbles, presumably H2, were observed. As shown in Fig. 1a, the Al can be removed by LiF/HCl treatment. The absence of Al suggests the following reaction: Ti2AlC + 3HCl + 3LiF = Ti2CTx + AlF3 + 3/2H2 + 3LiCl
(1)
After the removal of Al, the exposed Ti was terminated by oxygen, hydroxyls, fluoride and chloride surface groups. Followed by the solvothermal process, most of the F and Cl impurities are removed and replaced by oxygen or hydroxyls through the solvothermal treatment with DMF which can benefit to their performance as electrodes. For the growth of nano Cu2O/Ti2CTx synthesis as shown in Fig. 1b, since no reducing agents were added into the starting solution, it is believed that N, N-dimethylformamide (DMF) acts as a weak reducing agent in the Cu2O synthesis, according to: [29] 4HCON(CH3)2 + H2O ! HCOOH + NH(CH3)2
(2)
Cu2+ + H2O + 2NH(CH3)2 ! CuO(s) + 2NH2(CH3)2+
(3)
2CuO(s) + HCOOH ! Cu2O(s) + H2O + CO2
(4)
Fig. 2 shows the XRD patterns of Cu2O, Ti2AlC, Ti2CTx and Cu2O/ Ti2CTx. It is clear that the solvothermally synthesized Cu2O powders have a cubic crystal structure (PDF No. 77-0199) without other crystal structures such as Cu and CuO. This indicates that the single crystal Cu2O can be successfully synthesized through solvothermal method. The XRD patterns of etched Ti2AlC show that most of the nonbasal plane peaks of Ti2AlC, notably the most intense peak at 39.2 2u (PDF No. 29-0095), disappears, indicating that the Al was selectively etched away [20]. After the solvothermal process, we can find the shoulder peaks appearing at 40 2u. Since in our case, DMF was primarily
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H. Zhang et al. / Electrochimica Acta 202 (2016) 24–31
Fig. 1. Schematic of synthesis and intercalation of Ti2CTx (a), and hybridization growth of Cu2O/Ti2CTx (b).
regarded as an intercalant and a reducing agent as well at a relatively high temperature of 150 C [35]. This could directly induce to the intercalation of MXenes [14]. It has been reported that when cold-pressing the MXene, there are shoulder peaks near 40 2u appearing resulting from the restacking/reorienting in their preferred orientation, and a weak broad peak at 28 2u as well [34]. Our result corresponds well with the reported literature. So the diffraction peaks marked as "*" can be ascribe to the restacked, reoriented, and intercalated Ti2CTx from the DMF treatment. Another important factor is that the non-basal peaks at 60 2u remarkably weakened after DMF treatment, which is always
explained as the result of delamination [14]. Therefore, we can conclude that the solvothermal process would force the Ti2CTx to restack and reorient just like cold-pressing, and this behavior might be enhanced after the removal of terminating groups (F, Cl, or OH) by DMF solvothermal treatment, resulting in the final intercalation and delamination of MXenes. In agreement with the disappearance of the MAX phase peaks in the X-ray diffraction (XRD) patterns after etching, the Energy Dispersive X-ray Spectra in Fig. 3(a) (EDS) show a significant drop in the Al signal indicating that the Al layers were removed and replaced by O, F and Cl. Specifically, the etched Ti2CTx shows an
Fig. 2. XRD patterns of (a) Cu2O/Ti2CTx (MXene), (b) Ti2CTx, (c) Ti2AlC (Max phase) and (d) Cu2O.
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Fig. 3. EDS spectra of Ti2CTx (a) and Cu2O/Ti2CTx (b).
atomic ratio of Ti/C/F/Cl/O to be 40:21:10:7:24, respectively. The content of Al in the materials reduced from 17.4 wt% to 0.33 wt% after LiF/HCl etching. By solvothermal treatment with DMF, the vast majority of F, Cl elements are removed in the Cu2O/Ti2CTx composites, and the content of O increased from 12.73 to 22.51 wt% due to the formation of Cu2O and re-termination of Ti after removal of F, Cl. Moreover, the Al remained in a weight ratio of 0.24%, much lower than that of MAX phase (17.4 wt %). Fig. 4 shows the SEM images of Ti2CTx, pure Cu2O spheres and Cu2O/Ti2CTx composites. SEM image of Ti2AlC particles after etching in Fig. 2a resembles the images of exfoliated transition metal oxides and even graphite, and clearly shows the featured layered structure of MXene with typical open frameworks [20]. SEM image of the pure Cu2O particles synthesized by the same solvothermal method is shown in Fig. 4b, and it reveals that the particles are microspheres with a diameter of 1–2 um, whose shell
wall is composed of small nanoparticles (as shown in the inset of Fig. 4b) [30]. The Cu2O/Ti2CTx composites consist of Cu2O particles growing both on the surface and between the layers of Ti2CTx MXenes to form an open conductive framework (Fig. 4d). The Cu2O spheres on the MXene layers exhibit a smaller particle size than the pure Cu2O spheres synthesized with the absence of MXene layers. It is believed that the smaller Cu2O particles and 3D conductive frameworks are beneficial to the performance of this material as anode for lithium ion batteries. To confirm the structure of this material, TEM analysis was performed on the Cu2O/Ti2CTx composites after ultrasonication treatment in ethanol. The Fig. 5a is the TEM picture of Ti2CTx sheets separated from the Cu2O/Ti2CTx composites. Notably, the Ti2CTx particle consists of restacked exfoliated MXenes and can be delaminated by ultrasonication. After ultrasonication, MXene sheets consisting of only a few layers can be obtained. The
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Fig. 4. SEM images of pristine Ti2CTx (a), pure Cu2O spheres (b) and Cu2O/Ti2CTx composite materials (c, d).
Fig. 5. TEM images of Cu2O/Ti2CTx composites after ultrasonication: (a) the Ti2CTx particles; (b) the Cu2O.
individual Cu2O particles are found to be several hundred nanometers in diameter and have an uneven surface providing more contact area with the electrolyte, which can be identified from Fig. 5b. The charge-discharge profiles of the pristine Cu2O, Ti2CTx (MXene), Cu2O/Ti2CTx composite electrodes at current densities of 10 mA g1, 100 mA g1, 500 mA g1and 1000 mA g1 in the voltage
range from 0.005 to 2.5 V (vs. Li+/Li) are shown in Fig. 6a, b and c, respectively. The 1st discharge capacities of pristine Cu2O, Ti2CTx and Cu2O/Ti2CTx composite were 628 mAh g1, 506 mAh g1 and 790 mAh g1, respectively, at a current density of 10 mA g1, which are higher than both the theoretical capacities of Cu2O (375 mAh g1) and Ti2CTx (225 mAh g1 corresponding to one reversible Li ion) [21]. Normally, the much higher first-cycle discharge behavior
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Fig. 6. Charging/discharging curves of Cu2O (a), Ti2CTx (MXene) (b), Cu2O/Ti2CTx composites (c) and cycling performance of Cu2O/Ti2CTx composites at different current densities (d).
than the theoretical capacity is supposed to be originated from the electrolyte reduction on the porous materials with high specific surface areas, i.e. solid electrolyte interface (SEI) formation, interfacial absorption and residual electrochemically active surface groups. In our case, it can be attributed to the mesoporous
structure with higher surface area as well as to the irreversible reduction of electrochemically active surface groups in Ti2CTx MXenes such as hydroxyls [36–39]. Typically, in the 1st cycle of Cu2O, plateaus appear around 1.0 V due to the reversible redox Cu+/Cu (vs. Li+/Li). From the second cycle, the discharge capacities
Fig. 7. (a) CV curves of Cu2O and Cu2O/Ti2CTx from 3.5 V to 0.01 V vs. Li+/Li at a scan rate of 0.1 mV s1; (b) Nyquist plots of the Cu2O and Cu2O/Ti2CTx samples.
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of Cu2O, Ti2CTx (MXene) and Cu2O/Ti2CTx composites remain 264 mAh g1, 244 mAh g1, 369 mAh g1 at a current density of 10 mA g1, 177 mAh g1, 122 mAh g1 and 260 mAh g1 at 100 mA g1, and 110 mAh g1, 98 mAh g1 and 203 mAh g1 at 500 mA g1, and 53 mAh g1, 65 mAh g1 and 145 mAh g1, at 1000 mA g1, respectively. The initial charge/discharge efficiency values for Cu2O, Ti2CTx, and Cu2O/Ti2CTx composite are calculated as 41%, 64%, and 46%, and the initial columbic efficiencies (second discharge capacity/first charge capacity) are 101%, 76% and 102%, respectively. These results show that the Cu2O/Ti2CTx composites have a superior electrochemical performance than either Cu2O or Ti2CTx, resulting from the introduction of Cu2O which can improve the capacity of Ti2CTx and the combination of Ti2CTx sheets with higher electronic conductivity, working as graphene, which can facilitate continuous conducting pathways for electrons through the electrodes and restrain the structure changes of Cu2O particles during the lithiation–delithiation process. To further investigate the electrochemical performance of the as-prepared Cu2O/Ti2CTx composites, the cycling performance of the Cu2O/Ti2CTx composites is shown in Fig. 5d for the first 250 cycles. During the first few cycles, the electrode exhibits a slight capacity loss, but stabilizes at 203 mAh g1 at a current density of 500 mA g1 because nanostructured composite needs several charge–discharge cycles for stable SEI formation and interaction of electrolyte with active particles and current collector in a no conducting carbon employed electrode [39]. When the current density was decreased from 1000 mA g1 to 100 mA g1, the reversible capacity of Cu2O/Ti2CTx could be recovered to 240 mAh g1, revealing a good reversibility of the electrode. Especially, the composite shows an excellent cycling stability, with capacity retention of near 100% over 250 cycles at a current density of 1000 mA g1. This result can be due to the special open structures of the Cu2O/Ti2CTx composites and the capacity contribution from the Cu2O nanoparticles. Typical cyclic voltammetry curves of the pure Cu2O and hybridized Cu2O/Ti2CTx composites, at a rate of 0.1 mV s1, are shown in Fig. 7a. The lithiation of Cu2O at the surface level started at about 1.6 V in the first cycle, leading to a prominent peak at 1.5 V. After this, a peak appeared at 0.7 V was identified as the second step of lithiation. On the reverse scan, the oxidation process was evidenced by a broad peak at about 2.5 V, which were attributed to the decomposition of the lithium oxides, SEI phases, and partially lithiated phases. In the case of the first lithiation process of Cu2O/ Ti2CTx composites, a broad, irreversible peak located at around 0.75 V was observed and it was absent in subsequent cycles. It is reasonable to assign this irreversible peak to the formation of a solid electrolyte interphase (SEI) and irreversible reaction of electrolyte with the electrode material. In the subsequent two cycles, broad reversible peaks were observed at 1.5 V and 1.9 V (vs. Li+/Li) during lithiation and delithiation, respectively. Overall, we can find that our results correspond well with the featured broad reversible peaks of pure Ti2C MXene observed at 1.6 V and 2.0 V vs. Li+/Li [21]. To better understand the lithiation/delithiation processes at the electrode/electrolyte interface, electrochemical impedance spectroscopy (EIS) measurements were carried out, and Fig. 7b shows the Nyquist plots of pristine Cu2O and the Cu2O/Ti2CTx electrodes. The EIS spectra consist of semi-circles ending with sloped lines. The semicircle in the middle to high frequency region is attributed to the lithium-ion migration through the SEI film and charge transfer reaction. The slope in the low frequency region is attributed to lithium-ion diffusion in the bulk electrode. In general, the diameter of the semicircle is related to the charge transfer resistance at the electrode. From the Nyquist plots, it is clear that the Cu2O/Ti2CTx electrode has a smaller semicircle diameter compared with the pure Cu2O electrode, indicating the faster charge transfer reaction of Cu2O/Ti2CTx composites which is in
favor to enhance the rate performance of the electrode. This result convincingly demonstrates the excellent reversibility and rate capability of Cu2O/Ti2CTx composites with open 3D nano/microstructures. These electrochemical results show that the Cu2O/Ti2CTx composites have a higher electrical conductivity and lithium ion mobility resulting from the Ti2CTx, which improves the chargedischarge performance and rate stability. As shown in SEM and TEM images, Cu2O nanoparticles grow between the stacked Ti2CTx sheets and are electrically connected by the exfoliated MXene sheets, forming open conductive frameworks. In this architecture, Cu2O contributes more specific capacity to Ti2CTx MXenes while conductive Ti2CTx sheets also provide the sites for the growth of Cu2O hollow nanoparticles, forming a 3D conductive network and strongly reducing the capacity loss of Cu2O. The symbiosis of Cu2O particles combined with Ti2CTx sheets benefits both the lithium insertion/extraction ability and the storage capacity of lithium ions within the whole electrode, as exhibited by the excellent electrochemical performances. 4. Conclusions A simple way to synthesize Cu2O/Ti2CTx hybrid materials with an open conductive structure was reported. The resulting hybrid materials were tested as anode for lithium ion batteries without carbon black for the electrode fabrication, showing high capacity and cycle stability. At 1000 mA g1, the Cu2O/Ti2CTx materials exhibited a specific capacity of 143 mAh g1, with no degradation after 250 cycles. The performances reported here can be further improved by optimizing the composites of MXene and transition metal oxide, and developing the microstructure of both the MXene and oxide. This kind of materials with high surface area, easy preparation, good conductivity also can be exploited in other electronics and photovoltaic applications, such as photo-induced water splitting, catalysis, as gas sensors and supercapacitor, and provide an avenue for combining other metal oxides and 2D materials. Acknowledgements The authors gratefully acknowledge the financial support from Chinese State Scholarship Fund (No. 201408310121). Financial support from the Shanghai Leading Academic Discipline Project (B502) and Shanghai Key Laboratory Project (08DZ2230500) is also acknowledged. The authors also acknowledge Dr. ir. Minxian Wu, ir. Thomas Lapauw for the help with the experiments. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Cu2O Hybridized Titanium Carbide with Open Conductive Frameworks for Lithium-ion Batteries Huang Zhanga,* , Hui Dongb , Xuan Zhanga , Yunlong Xub , Jan Fransaera,* a
Department of Materials Engineering (MTM), KU Leuven, Kasteelpark Arenberg 44, B-3001 Leuven, Belgium Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, PR China b
A R T I C L E I N F O
Article history: Received 6 February 2016 Received in revised form 21 March 2016 Accepted 3 April 2016 Available online 4 April 2016 Keywords: MXene Hybridization Three-dimensional Carbon black-free Lithium-ion batteries
A B S T R A C T
Though MXenes, a new family of 2D transition metal carbides, are generating considerable interests as electrode materials for batteries and supercapacitors, further application is hindered by their low capacities and poor rate capabilities. Here we propose a simple route for the synthesis of Cu2O particle hybridized titanium carbide Ti2CTx (T = O, OH) composites via a solvothermal method. Electrodes containing Cu2O/MXene were fabricated without carbon black, and tested as anodes for lithium ion batteries. A discharge capacity of 143 mAh g1 was obtained at a discharge current density of 1000 mA g1 and the capacity retention was near 100% after 200 cycles. The hybrid electrodes with open conductive frameworks exhibited significantly improved electrochemical performance, suggesting a new method for preparing MXene-based composites for energy storage application. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Lithium-ion batteries (LIB) are the most attractive rechargeable batteries and have been widely used for powering electronic devices in our daily life [1,2]. Addressing the challenges to develop high-performance battery materials, two-dimensional (2D) materials have historically been one of the most extensively studied classes of materials for batteries, among which graphene (GN) is a typical representative with a 2D structure and strongly bonded carbon networks, and can inevitably contribute to their intriguing electronic conductivity, high surface area and short diffusion path, rendering them interesting electrode materials for LIBs. Such properties also suggest a wide-range of hybridization strategies with other materials, such as CNTs, Fe3O4, TiO2, and carbon fibers, to further optimize their performances in LIBs [3,4]. MXenes are a novel family of two-dimensional (2D) metal carbides, which are synthesized by the exfoliation of ternary carbides MAX phases [5,6]. Generally, MXenes are produced by etching away the A layers from Mn+1AXn (MAX phases), in which ‘M’ represents a transition metal, ‘A’ is the element in the group (Al, Ge, Si, Sn, etc.), and ‘X’ represents carbon and/or nitrogen and n = 1 to 3 [7]. Due to their high specific surface area, metallic conductivity and hydrophilic surface, MXenes have shown
* Corresponding authors. Tel.: +32 16 32 12 60; fax: +32 16 32 19 91. E-mail addresses: [email protected] (H. Zhang), [email protected] (J. Fransaer). http://dx.doi.org/10.1016/j.electacta.2016.04.009 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
outstanding performance as electrodes for supercapacitors [8–11] and lithium-ion batteries [12–14], with superior volumetric capacitances that exceed most previously reported materials. Using density functional calculations, Er et al. predicted that Ti3C2 can increase the performance of Li, Na, K, Ca ion batteries as anode material [15]. Sun et al. exfoliated Ti3AlC2 with HF solution to obtain the two-dimensional Ti3C2 MXene. At a rate of 3C and 10C, the intercalated Ti3C2 possessed specific capacities of 88 mAh g1 and 69 mAh g1 after 100 cycles, respectively, without obvious capacity loss (the theoretical specific capacity of 130 mAh g1 provided by Ti3C2F2Li) [16]. Moreover, the terminated-Ti3C2Tx (T = F, O, OH, etc.) MXenes have also been used as electrodes for Li-ion batteries with and without carbon black as conductive agent. These studies reveal that the electrodes fabricated without carbon black have an initial capacity of 125 mAh g1 at a current density of 30 mA g1, but show a capacity of less than 30 mAh g1 after 50 cycles [17]. Ti2AlC is one of the most common and cheapest MAX phases which are commercially available [18]. The MXene Ti2CTx derived from this MAX phase materials has good thermal and electric properties [19,20]. In 2012, Naguib et al. reported the Li insertion of Ti2C-based material (MXene) with an oxidized surface, exhibiting a capacity of 70 mAh g1 after 200 cycles at 10C, suggesting a potentially promising anode material for lithium ion batteries [21]. From this we can realize that the pristine MXenes are limited by their low specific capacity and poor cycling when directly used as anode materials for batteries, especially at a high current density.
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Therefore, in order to further develop the MXene into practical battery application, new strategies should be debuted to optimize the overall performance as electrodes. Results demonstrate that M2X structures have higher Li-ion specific capacities, compared to their M3X2 and M4X3 counterparts, as the numbers of atomic layers per MXene sheet in the former are 3 instead of 5 or 7 for the latter [22]. The introduction of carbon additives, such as carbon nanotubes (CNTs) into the electrode structure can improve ion accessibility into the MXene layers and boost both specific capacities and rate performance. [23]. Meanwhile, delighted by the previous research, the combination of MXenes with carbon nanotubes or carbon nanofibers can improve the electrochemical performance [24,25]. This can be attributed to the high conductivity, large surface area and short diffusion path, namely the intrinsic properties of these combined nanomaterials, which will be beneficial to the electrochemical activity of fabricated electrodes. However, compared with the currently commercialized carbon materials, transition metal oxide has been demonstrated with various advantages as anode materials for lithium ion batteries, such as high theoretical capacity, widespread availability, and environmental benignity [26–28]. Therefore, the performance of MXene can be improved When hybridized with high capacity transition metal oxides, and assembled into conductive additive-free electrodes. Using a hydrothermal method, the synthesis of Cu2O nanospheres can be achieved on a large scale [29–31]. Mesoporous Cu2O films with an open 3D framework can achieve superior reversible electrochemical performance as anodes for lithium ion batteries, [32] and the performance can be enhanced by hybridization with graphene nanosheets [33]. In this work, we propose an easy solvothermal method to grow Cu2O nanoparticles on MXenes and construct an open conductive framework, in order to achieve high performance anode materials for lithium ion batteries. 2. Experimental 2.1. Preparation of Ti2CTx MXenes, Cu2O spheres and Cu2O/Ti2CTx composites The Ti2AlC MAX phase powders used in this work were purchased from Kanthal Company (Sweden) and used after wet ball-milling to reduce the particle size from 10 um to 50 um. The MXene materials were synthesized through a green and published method according to the work by Gogotsi [8,9]. Specifically, stoichiometric amount of MAX phase Ti2AlC powders were dispersed in 6 M HCl solution and LiF powders were slowly added into the suspension. The prepared suspension was transferred into a sealed plastic container and heated at 50 C for 48 h under constant stirring. After that, the product was washed several times using deionized water and separated by centrifuging. Then the final MXene product was dried in a vacuum oven at 80 C for 10 h. The Cu2O modified MXenes were synthesize in a single-step experiments by adding stoichiometric amounts of Cu(CH3COO)2H2O and Ti2CTx powders to N, N-dimethylformamide (DMF) followed by ultrasonic treatment for 15 min. After that, the black-green suspension was put in a sealed container and heated to 150 C for 10 h while stirring. The final black powders were obtained using centrifuging after washing with deionized water and ethanol. The theoretical weight ratio of Cu2O and Ti2CTx in the composites is 25:100. The pure Cu2O was also synthesized through the same method described above. 2.2. Materials characterization The phase identification of powders was conducted with X-ray diffraction (XRD, Seifert3003) using Cu Ka radiation (l=0.15418 nm). The morphology and structure were evaluated
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by field emission transmission electron microscopy (FETEM, JEM2010F, Japan) and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX, Philips XL 30 FEG). 2.3. Electrochemical measurements Electrochemical studies were conducted in standard CR2032 coin cells. The electrode slurries were prepared by dispersing 90 wt.% active materials and 10 wt.% polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) solvent and coated onto Cu foil, and then dried in a vacuum oven. The cells were assembled and performed in an argon-filled glove box using lithium metal as the anode electrode, and 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1, v/v) as electrolyte. The lithiation/delithiation behavior and long-term stability were performed on a Maccor Battery Tester 4300. All the cells were tested galvanostatically between 0.05 and 2.5 V (vs. Li+/Li) at current densities of 10, 100, 500 and 1000 mA g1, respectively. Capacity was calculated on the basis of the mass of Cu2O/MXene. CV and EIS were performed on an Autolab PGSTAT 301 N electrochemical workstation. Cyclic voltammograms (CV) were recorded at a scan rate of 0.1 mV s1 between 0.01 and 3.5 V (vs. Li+/Li). The EISs were measured potentiostatically at the open circuit potential (OCP) with an AC amplitude of 5 mV from 102 Hz to 105 Hz. 3. Results and Discussion A schematic of Ti2CTx synthesis and solvothermal treatment with DMF t is shown in Fig. 1. When the Ti2AlC powders were added to the HCl/LiF solution, bubbles, presumably H2, were observed. As shown in Fig. 1a, the Al can be removed by LiF/HCl treatment. The absence of Al suggests the following reaction: Ti2AlC + 3HCl + 3LiF = Ti2CTx + AlF3 + 3/2H2 + 3LiCl
(1)
After the removal of Al, the exposed Ti was terminated by oxygen, hydroxyls, fluoride and chloride surface groups. Followed by the solvothermal process, most of the F and Cl impurities are removed and replaced by oxygen or hydroxyls through the solvothermal treatment with DMF which can benefit to their performance as electrodes. For the growth of nano Cu2O/Ti2CTx synthesis as shown in Fig. 1b, since no reducing agents were added into the starting solution, it is believed that N, N-dimethylformamide (DMF) acts as a weak reducing agent in the Cu2O synthesis, according to: [29] 4HCON(CH3)2 + H2O ! HCOOH + NH(CH3)2
(2)
Cu2+ + H2O + 2NH(CH3)2 ! CuO(s) + 2NH2(CH3)2+
(3)
2CuO(s) + HCOOH ! Cu2O(s) + H2O + CO2
(4)
Fig. 2 shows the XRD patterns of Cu2O, Ti2AlC, Ti2CTx and Cu2O/ Ti2CTx. It is clear that the solvothermally synthesized Cu2O powders have a cubic crystal structure (PDF No. 77-0199) without other crystal structures such as Cu and CuO. This indicates that the single crystal Cu2O can be successfully synthesized through solvothermal method. The XRD patterns of etched Ti2AlC show that most of the nonbasal plane peaks of Ti2AlC, notably the most intense peak at 39.2 2u (PDF No. 29-0095), disappears, indicating that the Al was selectively etched away [20]. After the solvothermal process, we can find the shoulder peaks appearing at 40 2u. Since in our case, DMF was primarily
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Fig. 1. Schematic of synthesis and intercalation of Ti2CTx (a), and hybridization growth of Cu2O/Ti2CTx (b).
regarded as an intercalant and a reducing agent as well at a relatively high temperature of 150 C [35]. This could directly induce to the intercalation of MXenes [14]. It has been reported that when cold-pressing the MXene, there are shoulder peaks near 40 2u appearing resulting from the restacking/reorienting in their preferred orientation, and a weak broad peak at 28 2u as well [34]. Our result corresponds well with the reported literature. So the diffraction peaks marked as "*" can be ascribe to the restacked, reoriented, and intercalated Ti2CTx from the DMF treatment. Another important factor is that the non-basal peaks at 60 2u remarkably weakened after DMF treatment, which is always
explained as the result of delamination [14]. Therefore, we can conclude that the solvothermal process would force the Ti2CTx to restack and reorient just like cold-pressing, and this behavior might be enhanced after the removal of terminating groups (F, Cl, or OH) by DMF solvothermal treatment, resulting in the final intercalation and delamination of MXenes. In agreement with the disappearance of the MAX phase peaks in the X-ray diffraction (XRD) patterns after etching, the Energy Dispersive X-ray Spectra in Fig. 3(a) (EDS) show a significant drop in the Al signal indicating that the Al layers were removed and replaced by O, F and Cl. Specifically, the etched Ti2CTx shows an
Fig. 2. XRD patterns of (a) Cu2O/Ti2CTx (MXene), (b) Ti2CTx, (c) Ti2AlC (Max phase) and (d) Cu2O.
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Fig. 3. EDS spectra of Ti2CTx (a) and Cu2O/Ti2CTx (b).
atomic ratio of Ti/C/F/Cl/O to be 40:21:10:7:24, respectively. The content of Al in the materials reduced from 17.4 wt% to 0.33 wt% after LiF/HCl etching. By solvothermal treatment with DMF, the vast majority of F, Cl elements are removed in the Cu2O/Ti2CTx composites, and the content of O increased from 12.73 to 22.51 wt% due to the formation of Cu2O and re-termination of Ti after removal of F, Cl. Moreover, the Al remained in a weight ratio of 0.24%, much lower than that of MAX phase (17.4 wt %). Fig. 4 shows the SEM images of Ti2CTx, pure Cu2O spheres and Cu2O/Ti2CTx composites. SEM image of Ti2AlC particles after etching in Fig. 2a resembles the images of exfoliated transition metal oxides and even graphite, and clearly shows the featured layered structure of MXene with typical open frameworks [20]. SEM image of the pure Cu2O particles synthesized by the same solvothermal method is shown in Fig. 4b, and it reveals that the particles are microspheres with a diameter of 1–2 um, whose shell
wall is composed of small nanoparticles (as shown in the inset of Fig. 4b) [30]. The Cu2O/Ti2CTx composites consist of Cu2O particles growing both on the surface and between the layers of Ti2CTx MXenes to form an open conductive framework (Fig. 4d). The Cu2O spheres on the MXene layers exhibit a smaller particle size than the pure Cu2O spheres synthesized with the absence of MXene layers. It is believed that the smaller Cu2O particles and 3D conductive frameworks are beneficial to the performance of this material as anode for lithium ion batteries. To confirm the structure of this material, TEM analysis was performed on the Cu2O/Ti2CTx composites after ultrasonication treatment in ethanol. The Fig. 5a is the TEM picture of Ti2CTx sheets separated from the Cu2O/Ti2CTx composites. Notably, the Ti2CTx particle consists of restacked exfoliated MXenes and can be delaminated by ultrasonication. After ultrasonication, MXene sheets consisting of only a few layers can be obtained. The
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Fig. 4. SEM images of pristine Ti2CTx (a), pure Cu2O spheres (b) and Cu2O/Ti2CTx composite materials (c, d).
Fig. 5. TEM images of Cu2O/Ti2CTx composites after ultrasonication: (a) the Ti2CTx particles; (b) the Cu2O.
individual Cu2O particles are found to be several hundred nanometers in diameter and have an uneven surface providing more contact area with the electrolyte, which can be identified from Fig. 5b. The charge-discharge profiles of the pristine Cu2O, Ti2CTx (MXene), Cu2O/Ti2CTx composite electrodes at current densities of 10 mA g1, 100 mA g1, 500 mA g1and 1000 mA g1 in the voltage
range from 0.005 to 2.5 V (vs. Li+/Li) are shown in Fig. 6a, b and c, respectively. The 1st discharge capacities of pristine Cu2O, Ti2CTx and Cu2O/Ti2CTx composite were 628 mAh g1, 506 mAh g1 and 790 mAh g1, respectively, at a current density of 10 mA g1, which are higher than both the theoretical capacities of Cu2O (375 mAh g1) and Ti2CTx (225 mAh g1 corresponding to one reversible Li ion) [21]. Normally, the much higher first-cycle discharge behavior
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Fig. 6. Charging/discharging curves of Cu2O (a), Ti2CTx (MXene) (b), Cu2O/Ti2CTx composites (c) and cycling performance of Cu2O/Ti2CTx composites at different current densities (d).
than the theoretical capacity is supposed to be originated from the electrolyte reduction on the porous materials with high specific surface areas, i.e. solid electrolyte interface (SEI) formation, interfacial absorption and residual electrochemically active surface groups. In our case, it can be attributed to the mesoporous
structure with higher surface area as well as to the irreversible reduction of electrochemically active surface groups in Ti2CTx MXenes such as hydroxyls [36–39]. Typically, in the 1st cycle of Cu2O, plateaus appear around 1.0 V due to the reversible redox Cu+/Cu (vs. Li+/Li). From the second cycle, the discharge capacities
Fig. 7. (a) CV curves of Cu2O and Cu2O/Ti2CTx from 3.5 V to 0.01 V vs. Li+/Li at a scan rate of 0.1 mV s1; (b) Nyquist plots of the Cu2O and Cu2O/Ti2CTx samples.
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of Cu2O, Ti2CTx (MXene) and Cu2O/Ti2CTx composites remain 264 mAh g1, 244 mAh g1, 369 mAh g1 at a current density of 10 mA g1, 177 mAh g1, 122 mAh g1 and 260 mAh g1 at 100 mA g1, and 110 mAh g1, 98 mAh g1 and 203 mAh g1 at 500 mA g1, and 53 mAh g1, 65 mAh g1 and 145 mAh g1, at 1000 mA g1, respectively. The initial charge/discharge efficiency values for Cu2O, Ti2CTx, and Cu2O/Ti2CTx composite are calculated as 41%, 64%, and 46%, and the initial columbic efficiencies (second discharge capacity/first charge capacity) are 101%, 76% and 102%, respectively. These results show that the Cu2O/Ti2CTx composites have a superior electrochemical performance than either Cu2O or Ti2CTx, resulting from the introduction of Cu2O which can improve the capacity of Ti2CTx and the combination of Ti2CTx sheets with higher electronic conductivity, working as graphene, which can facilitate continuous conducting pathways for electrons through the electrodes and restrain the structure changes of Cu2O particles during the lithiation–delithiation process. To further investigate the electrochemical performance of the as-prepared Cu2O/Ti2CTx composites, the cycling performance of the Cu2O/Ti2CTx composites is shown in Fig. 5d for the first 250 cycles. During the first few cycles, the electrode exhibits a slight capacity loss, but stabilizes at 203 mAh g1 at a current density of 500 mA g1 because nanostructured composite needs several charge–discharge cycles for stable SEI formation and interaction of electrolyte with active particles and current collector in a no conducting carbon employed electrode [39]. When the current density was decreased from 1000 mA g1 to 100 mA g1, the reversible capacity of Cu2O/Ti2CTx could be recovered to 240 mAh g1, revealing a good reversibility of the electrode. Especially, the composite shows an excellent cycling stability, with capacity retention of near 100% over 250 cycles at a current density of 1000 mA g1. This result can be due to the special open structures of the Cu2O/Ti2CTx composites and the capacity contribution from the Cu2O nanoparticles. Typical cyclic voltammetry curves of the pure Cu2O and hybridized Cu2O/Ti2CTx composites, at a rate of 0.1 mV s1, are shown in Fig. 7a. The lithiation of Cu2O at the surface level started at about 1.6 V in the first cycle, leading to a prominent peak at 1.5 V. After this, a peak appeared at 0.7 V was identified as the second step of lithiation. On the reverse scan, the oxidation process was evidenced by a broad peak at about 2.5 V, which were attributed to the decomposition of the lithium oxides, SEI phases, and partially lithiated phases. In the case of the first lithiation process of Cu2O/ Ti2CTx composites, a broad, irreversible peak located at around 0.75 V was observed and it was absent in subsequent cycles. It is reasonable to assign this irreversible peak to the formation of a solid electrolyte interphase (SEI) and irreversible reaction of electrolyte with the electrode material. In the subsequent two cycles, broad reversible peaks were observed at 1.5 V and 1.9 V (vs. Li+/Li) during lithiation and delithiation, respectively. Overall, we can find that our results correspond well with the featured broad reversible peaks of pure Ti2C MXene observed at 1.6 V and 2.0 V vs. Li+/Li [21]. To better understand the lithiation/delithiation processes at the electrode/electrolyte interface, electrochemical impedance spectroscopy (EIS) measurements were carried out, and Fig. 7b shows the Nyquist plots of pristine Cu2O and the Cu2O/Ti2CTx electrodes. The EIS spectra consist of semi-circles ending with sloped lines. The semicircle in the middle to high frequency region is attributed to the lithium-ion migration through the SEI film and charge transfer reaction. The slope in the low frequency region is attributed to lithium-ion diffusion in the bulk electrode. In general, the diameter of the semicircle is related to the charge transfer resistance at the electrode. From the Nyquist plots, it is clear that the Cu2O/Ti2CTx electrode has a smaller semicircle diameter compared with the pure Cu2O electrode, indicating the faster charge transfer reaction of Cu2O/Ti2CTx composites which is in
favor to enhance the rate performance of the electrode. This result convincingly demonstrates the excellent reversibility and rate capability of Cu2O/Ti2CTx composites with open 3D nano/microstructures. These electrochemical results show that the Cu2O/Ti2CTx composites have a higher electrical conductivity and lithium ion mobility resulting from the Ti2CTx, which improves the chargedischarge performance and rate stability. As shown in SEM and TEM images, Cu2O nanoparticles grow between the stacked Ti2CTx sheets and are electrically connected by the exfoliated MXene sheets, forming open conductive frameworks. In this architecture, Cu2O contributes more specific capacity to Ti2CTx MXenes while conductive Ti2CTx sheets also provide the sites for the growth of Cu2O hollow nanoparticles, forming a 3D conductive network and strongly reducing the capacity loss of Cu2O. The symbiosis of Cu2O particles combined with Ti2CTx sheets benefits both the lithium insertion/extraction ability and the storage capacity of lithium ions within the whole electrode, as exhibited by the excellent electrochemical performances. 4. Conclusions A simple way to synthesize Cu2O/Ti2CTx hybrid materials with an open conductive structure was reported. The resulting hybrid materials were tested as anode for lithium ion batteries without carbon black for the electrode fabrication, showing high capacity and cycle stability. At 1000 mA g1, the Cu2O/Ti2CTx materials exhibited a specific capacity of 143 mAh g1, with no degradation after 250 cycles. The performances reported here can be further improved by optimizing the composites of MXene and transition metal oxide, and developing the microstructure of both the MXene and oxide. This kind of materials with high surface area, easy preparation, good conductivity also can be exploited in other electronics and photovoltaic applications, such as photo-induced water splitting, catalysis, as gas sensors and supercapacitor, and provide an avenue for combining other metal oxides and 2D materials. Acknowledgements The authors gratefully acknowledge the financial support from Chinese State Scholarship Fund (No. 201408310121). Financial support from the Shanghai Leading Academic Discipline Project (B502) and Shanghai Key Laboratory Project (08DZ2230500) is also acknowledged. The authors also acknowledge Dr. ir. Minxian Wu, ir. Thomas Lapauw for the help with the experiments. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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