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carbon nanotube cathodes for high performance lithium ion batteries
Journal of Power Sources 321 (2016) 120e125
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Free-standing and flexible LiMnTiO4/carbon nanotube cathodes for high performance lithium ion batteries Yinhua Bao a, Xingyu Zhang a, Xu Zhang b, Le Yang a, Xinyi Zhang a, Haosen Chen c, *, Meng Yang a, b, **, Daining Fang a, c a b c
State Key Laboratory for Turbulence and Complex System, College of Engineering, Peking University, Beijing 100871, China College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing 100081, China
h i g h l i g h t s A free-standing cathode based on LiMnTiO4 and multiwall carbon nanotube is developed. MWCNT networks improve reversible capacity and rate capability. LiMnTiO4/MWCNT cathode exhibits the flexibility and light-weight.
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Article history: Received 20 February 2016 Received in revised form 22 April 2016 Accepted 25 April 2016 Available online 6 May 2016
A flexible, free-standing, and light-weight LiMnTiO4/MWCNT electrode has been prepared by vacuum filtration method for the first time. The as-prepared flexible LiMnTiO4/MWCNT electrode possesses a three-dimensional braiding structure in which LiMnTiO4 particles are well embedded in the twining CNT networks. The novel LiMnTiO4/MWCNT electrodes show tensile strength of 1.34 MPa and 2.04 MPa, when the percentages of MWCNTs reach to 30% and 50%, respectively. This novel flexible electrode exhibits a superior electrochemical property, especially at rate capability and cycling stability. The LiMnTiO4/ MWCNT electrode can deliver capacity of 161 mAh g1 (86.4% retention) after 50 cycles at 0.5C rate. Since the high conductivity from MWCNT networks, the LiMnTiO4/MWCNT electrode can still maintain a capacity of 77 mAh g1 at 5C rate, which is much higher than that of the conventional electrode fabricated by slurry casting method on Al foil. The features of free-standing, light-weight, and excellent electrochemical performance indicate the potential of using the LiMnTiO4/MWCNT cathode in new-generation flexible lithium ion batteries. © 2016 Elsevier B.V. All rights reserved.
Keywords: Lithium ion batteries Multiwall carbon nanotube (MWCNTs) LiMnTiO4 Free-standing Cathode
1. Introduction With rapid development in portable electronic devices such as mobile phone, flexible displays, ultra-thin laptop computer and wearable devices, higher demand in flexible energy storage applications has been raised to meet the energy requirements [1e3]. Due to the high power capability and durable cycle life, lithium ion battery is one of the most attractive and promising candidates
* Corresponding author. ** Corresponding author. College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China. E-mail addresses: [email protected] (H. Chen), [email protected] (M. Yang). http://dx.doi.org/10.1016/j.jpowsour.2016.04.121 0378-7753/© 2016 Elsevier B.V. All rights reserved.
compared with other battery technologies. However, except for meeting the ever-increasing energy density and rate capability, new requirements of lithium ion batteries, such as flexibility, high mechanical strength, and light-weight have been imposed to identify with the improvement of portable electronic equipment [4e6] and even the future electric vehicles [7]. Hence, developing the innovative lithium ion battery with flexibility and high power density is crucial for the challenges from new generation portable electronics. In order to meet the above-mentioned requirements, optimizing the properties of electrode has been demonstrated as one of the most efficient approaches [8e10]. In terms of development of electrode materials, extensive efforts have been focused on developing novel cathode materials such as surface coating and element
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substitution [11]. For instance, the LiMnTiO4 cathode material, which formed by substituting Ti4þ for Mn4þ in LiMn2O4, can provide higher capacity and excellent cycling stability than conventional cathode materials such as LiCoO2 [12] and LiFePO4 [13]. Besides, a new series of the titanate cathode material such as Li2MTiO4 (M ¼ Mn, Fe, Co, Ni) has also emerged as a promising candidate for lithium ion batteries [14,15], which has a cubic cation disordered rock salt structure and offers a high theoretical capacity and stability. In terms of realizing the properties of light-weight and flexibility, the elimination of the inactive electrode components including polymer binder, conductive additive, and metal foil has been carried out in some reports [16]. On the one hand, it will be favorable to decrease the overall weight and have higher power density [17] in a binder-free electrode structure which contains more active materials. On the other hand, with the volumetric change [8,18,19] during charge/discharge processes, debonding will occur at the interface between electrode and current collector [20,21] in the traditional electrode structure, which is more serious in flexible batteries because of the bending and rolling activities [22]. It is quite clear that this issue will be automatically solved by an integrative electrode structure design. Recently, many novel designs of electrode structure have been developed to keep the pace with the flexible lithium ion batteries [23e30]. For example, Lu et al. [24] developed an in-situ hydrothermal process to synthesize high performance LiMn2O4/CNT cathodes, which show good flexibility, high capacity and stable cycling performance. Lee et al. [26] developed the synthesis of freestanding, binder-free CNT anode electrodes of tens of microns in thickness by using the method of vacuum filtration. Wu et al. [31] provided the combination of LiNi0.5Mn1.5O4 with multiwall carbon nanotube, which achieved electronical enhancement and lightweight electrode structure. Although these flexible electrodes can provide superior electrochemical performance, most of them are synthesized by sophisticated methods, which results in complexity during preparation process and decreases the feasibility in the battery industry. In addition, to the best of our knowledge, there are few reports about the application of Ti-substituted cathode material in flexible electrode. In this study, we have synthesized the LiMnTiO4 nanoparticles by the sol-gel method. In addition, MWCNT (purity 95%, diameter 15e60 nm) is purified and functionalized for good suspension. Then a novel free-standing, flexible, binder-free electrode design is developed by the integration of LiMnTiO4 nanoparticles and MWCNT through simple vacuum filtration method. Meanwhile, the galvanostatic charge/discharge performance of LiMnTiO4/MWCNT electrodes and mechanical performances are both studied. As a free-standing cathode for lithium ion batteries, LiMnTiO4/ MWCNT electrodes are expected to have superior overall performances including high capacity, excellent rate capability, good cycle stability, lightweight, and flexibility. On the one hand, the substitution of Ti4þ for Mn4þ in LiMn2O4 can provide a higher bonding energy (TieO, 662 kJ mol1) than MneO (402 kJ mol1) [32], which reduces the concentration of Jahn-Teller Mn3þ ions. Furthermore, the Mn2þ/Mn4þ redox couple in LiMnTiO4 contributes to a high theoretical capacity of 308 mAh g1 [33]. On the other hand, the porous and twining MWCNT networks can promote electrolyte infiltration, which can accelerate both electrons and Li ions transformation. Without the inactive materials, the power density of entire electrode has been significantly improved compared with the conventional electrodes. Combining the abovementioned advantages, the free-standing LiMnTiO4/MWCNT electrode could be a promising candidate for new flexible lithium ion batteries.
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2. Experimental section 2.1. Preparation of LiMnTiO4/MWCNT electrodes LiMnTiO4 particles were synthesized using a well-known sol-gel method [34,35]. Stoichiometric amounts of lithium acetate dihydrate, manganese acetate tetrahydrate and tetra-n-butyl titanate were dissolved in ethanol, and refluxed at 353 K for 24 h with continued vigorous stirring until a transparent sol was obtained. After that, the resultant sol was dried at 393 K for 12 h under air atmosphere. Finally, the mixture was hand-milled in an agate mortar and sintered at 923 K for 10 h in air. The purified and functionalized MWCNTs dealt by means of the mild acid treatment process (see Supplementary data) were dispersed in DI water with Triton X-100 (2.5 ml) as the surfactant to form a stable suspension with probe-sonication. LiMnTiO4 powders were added to about 70% of the as-prepared MWCNT suspension and dispersed by the ultrasonic force again. LiMnTiO4/MWCNT electrodes were then prepared by vacuum filtration. About 15% of the as-prepared MWCNT suspension was filtered first through a porous membrane, the Celgard 3500 polypropylene separator, to obtain a thin MWCNT bottom layer. The mixture of LiMnTiO4/MWCNT suspension was added to form a middle layer before covering another MWCNT layer on the surface. To remove the surfactant residue thoroughly, the resultant LiMnTiO4/MWCNT film was washed with DI water (1000 mL), followed closely by methanol (100 mL). During the last step, the film was dried in a vacuum chamber at 353 K for 72 h. The LiMnTiO4/ MWCNT electrodes with two weight percentages of MWCNTs, 30% and 50%, were both obtained in this work.
2.2. Material characterization Structural and crystallographic analysis of the as-prepared materials was determined by X-ray diffraction (XRD) using a Rigaku SmartLab diffractometer with a Cu-Ka radiation. The Fourier transform infrared (FTIR) spectra were recorded to confirm presence of the functional groups in the multi-walled carbon nanotubes structure, using a NEXUS670 Fourier transform spectrometer (NICOLET, USA) over the range of 4000e400 cm1 at room temperature. The morphologies of the samples were characterized by a field emission scanning electron microscope (HITACHI S-4800). The LiMnTiO4/MWCNT films were cut into rectangular strips of approximately 4 mm 30 mm for testing. Static mechanical tests [36e38] were carried out on a Hounsfield (HS5N) tensile tester with a strain rate of 0.05 mm min1.
2.3. Electrochemical measurements The electrochemical performance was evaluated using CR2032type coin cells. The LiMnTiO4/MWCNT film was cut into small pieces with 12 mm diameter. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC)-dimethyl carbonate (DMC) (1:1 by volume). For comparison, conventional electrodes were made by slurry casting method on Al foil. The weight ratio was LiMnTiO4: carbon black: polyvinylidene fluoride ¼ 8: 1: 1. Galvanostatic charge/discharge testing was performed between 1.5 and 4.8 V at 298 K by a BT-2000 battery testing system (Arbin, USA). The cyclic voltammetry (CV) was measured using the CHI 660D electrochemical workstation in the voltage range of 1.5 Ve4.8 V at the scan rate of 0.1 mV s1. Electrochemical impedance spectra (EIS) was tested at a frequency range from 0.01 Hz to 105 Hz with an amplitude of 5 mV.
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Fig. 1. XRD patterns of LiMnTiO4 and LiMnTiO4/MWCNT electrode.
3. Result and discussion The characterization of original MWCNTs and oxidized MWCNTs was presented by FTIR in Fig. S1. The results in Fig. S1 indicate that there are some functional groups on the surface of MWCNTs. The OeH stretching mode, carbonyl stretching mode, CeC stretching mode were respectively confirmed with the corresponding peaks at 3400, 1717, and 1564 cm1. The extra peak between 1340 and 1400 cm1 belongs to the very little residual Nitric acid in MWCNTs. The XRD patterns of the oxidized MWCNTs were also obtained, as shown in Fig. S2, stronger and sharper diffraction peaks at 2q ¼ 26.3 , 43 and 54.2 were observed, comparing with the original MWCNTs. Furthermore, the peak (marked in Fig. S2) of the impurity phase was decreased significantly after the mild acid treatment process. The crystal structure of LiMnTiO4 particles was confirmed to be a spinel cubic phase (Fd-3m space group) as previously reported [32]. The rietveld refined XRD spectrum shown in Fig. S3 indicates that the sample consists of a major spinel LiMnTiO4 coexisting with a minor fraction of rutile TiO2 (9.73 wt %) phase. The LiMnTiO4 phase possesses refined cell parameters of a ¼ 8.256 Å and cell volume of V ¼ 562.7 Å3 while the TiO2 phase possesses a ¼ 4.598 Å and V ¼ 62.7 Å3. Fig. 1 shows the XRD of the patterns of LiMnTiO4/ MWCNT electrode. It is found that LiMnTiO4/MWCNT electrode
presents a similar diffraction peaks to the LiMnTiO4 sample except for the CNT peak at 26 and some ambiguous peaks between 38 and 42 . The ambiguous peaks between 38 and 42 may be attributed to the residual Triton X-100. Fig. 2a shows the photograph of the as-prepared flexible LiMnTiO4/MWCNT electrode. The LiMnTiO4/MWCNT electrode is a circular film with the diameter of approximately 40 mm, which can be bent into an arc shape without any obvious breakage. The structure schematic of free-standing LiMnTiO4/MWCNT electrode is shown in Fig. 2b. The top and bottom MWCNT layers can prevent the LiMnTiO4 particles from falling out effectively. Meanwhile, the LiMnTiO4 particles are embedded in the twining MWCNT networks, which can provide efficient electron transport pathways as well as a flexible and binder-free scaffold. Since the use of inactive material is eliminated, the LiMnTiO4/MWCNT electrode is much lighter than the conventional electrode. The total weight of an electrode (diameter ¼ 12 mm) is shown in Fig. S4, based on LiMnTiO4 particles loading of 2.5 mg cm2. The weight of LiMnTiO4/MWCNT electrode with 30% MWCNTs is only half of the conventional electrode. It should be noted that the flexible and binder-free scaffold, as well as the reduced weight of the electrode, would remain great advantages in lithium ion battery design. Fig. 3 exhibits the morphologies of LiMnTiO4/MWCNT film by a field emission scanning electron microscope (FE-SEM). As illustrated in the surface image of the film electrode (Fig. 3a), a robust network structure is formed by interlaced MWCNT bundles with diameters of 15e50 nm. The interspaces between the bundles can provide a high-efficiency path for electrolyte to infiltrate into the active materials, which is helpful to Li ion transmission. Fig. 3b shows the LiMnTiO4/MWCNT mixture layer between the two MWCNT covers. The active particles are uniformly enlaced by MWCNTs, which can be clearly presented by the high magnification image in Fig. 3c. Such network structure is greatly conducive to enhance the electrons conduction. The interface between the MWCNT layer and mixture layer is well bonded, which is vividly shown by the cross section FE-SEM image in Fig. 3d. The mechanical properties of the LiMnTiO4/MWCNT electrode were quantified by using tensile testing. Fig. 4 shows the stressstrain curves for the pure MWCNT film and the LiMnTiO4/ MWCNT electrodes with two weight percentages of MWCNTs, 30% and 50%, denoted by CNT30 and CNT50. The CNT30 and CNT50 electrodes show tensile behavior very similar to the pure MWCNT film, which was prepared by vacuum filtration. The tensile strength of the pure MWCNT film was 3.85 MPa, and 1.34 MPa, and 2.04 MPa for the CNT30 and CNT50, respectively. The ultimate strain of pure MWCNT film, CNT30, CNT50 was 6.18%, 2.07%, and 4.01%, respectively. The Young's modulus of pure MWCNT film, CNT30, CNT50
Fig. 2. (a) Digital photograph of original LiMnTiO4/MWCNT membrane to indicate the flexibility and its diameter, with the inset showing the circular LiMnTiO4/MWCNT cathode (diameter ¼ 12 mm) in a CR2032 coin cell. (b) Schematic of the structure of free-standing LiMnTiO4/MWCNT electrodes.
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Fig. 3. (a) SEM images of thin layer of MWCNT film on the surface of the LiMnTiO4/MWCNT electrodes. Inset: photograph of the higher magnification. (b) SEM images of the LiMnTiO4/MWCNT mixture layer between the two MWCNT covers. (c) High resolution SEM image indicating LiMnTiO4 particles are located in/on the voids within the MWCNTs network. (d) The interface between the MWCNT layer and mixture layer.
was 1.94 MPa, 0.84 MPa, and 1.45 MPa, which was determined by fitting the stress-strain curves ranging from initial point to 0.1%. These results indicate that CNT50 exhibited the higher ultimate strain and tensile strength than CNT30. However, with the increase of LiMnTiO4 nanoparticles, the tensile strength of the samples decreases to some extent, which may be caused by the defective of multiwall carbon nanotube network structure. To understand the electrochemical behavior of the LiMnTiO4/ MWCNT electrode, the CV tests were conducted in this work. Fig. 5a shows the CV curve of the original LiMnTiO4 electrode between 1.5 and 4.8 V. Very similar reduction and oxidation process was observed in Fig. 5b and Fig. S5, which shows the CV curves of the CNT50 and CNT30, respectively. This result indicates that the reaction mechanisms remain unchanged with the addition of multiwall carbon nanotubes. All of the CV curves show the corresponding redox peaks (Mn2þ/Mn3þ between 2.5 and 3.4 V, Mn3þ/ Mn4þ between 3.7 and 4.5 V) during the first three cycles. The structural changes during cycles cause the plateaus fade gradually. It should be noted that the peak current in Fig. 5b is higher than that in Fig. 5a in the first few cycles, which reveals a more active electrochemical process in CNT50. Moreover, CNT50 electrode shows decreased potential separations between the anodic and cathodic peaks in Fig. 5, which means a much lower electrode polarization during electrochemical reaction process for CNT50 electrode. The charge and discharge curves of the original, CNT30, and CNT50 electrodes in the voltage of 1.5e4.8 V at 0.5C (1C ¼ 308 mA g1) are presented in Fig. 6a. The curves show two charge/discharge voltage plateaus at around 4 V and 2.8 V, which are consistent with the peaks in the CV curve. The CNT30 and CNT50 electrodes can deliver the discharge capacities of
192 mAh g1 and 197 mAh g1, respectively, in the earlier stage of cycling, which is higher than that of the original LiMnTiO4 electrode (145 mAh g1). In addition, The CNT30 and CNT50 electrodes present excellent cycle capability as well as original LiMnTiO4 electrode, as shown in Fig. 6b. However, the discharge capacities of CNT30 and CNT50 electrodes are much higher than that of original LiMnTiO4 electrode. In the case of CNT50 electrodes, the initial discharge capacity is 192 mAh g1, and Coulombic efficiency 96.6%. After 50 cycles, the CNT50 electrode still maintains high capacity of 161 mAh g1, which is higher than that of the original electrode
Fig. 4. Stress-strain curve for pure CNT film, CNT30 and CNT50.
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Fig. 5. Cyclic voltammograms of the original LiMnTiO4 cathode (a) and CNT50 (b) samples in the voltage range of 1.5 Ve4.8 V at the scan rate of 0.1 mV s1.
(128 mAh g1). The improvement in the reversible capacity may be due to the three-dimensional braiding structure, which can provide superior electron and Li ion transmission capability. Meanwhile, it should be noted that the elimination of the binder and Al foil current collector can also improve the electrochemical performance of the electrode. The rate capability of the CNT30 and CNT50 electrodes was also studied and the results are presented in Fig. 6c. Evidently, the LiMnTiO4/MWCNT electrode exhibits much better than the
traditional electrodes. The discharge capacity descends gradually when the current rate increases from the 0.5C to 1, 2, 3, and 5C, the CNT50 can deliver a considerably high capability of 192, 165, 131, 106, and 77 mAh g1, respectively, which is higher than the traditional electrode. At 5C charge/discharge rate, the CNT30 and CNT50 electrodes can still maintain a capacity of 53 mAh g1 and 77 mAh g1, while the capacity of the traditional electrode almost decreased to 10 mAh g1. Meanwhile, the capacity retention at different current density was presented in Fig. S6. After 50 cycles at
Fig. 6. (a) Charge/discharge curves of the original LiMnTiO4, CNT30, and CNT50 electrodes at 0.5C. (b) Cycling performance of the original LiMnTiO4, CNT30, and CNT50 electrodes at 0.5C. (c) Rate capability of the original LiMnTiO4, CNT30, and CNT50 electrodes. (d) EIS spectra of the fresh cells with the original LiMnTiO4, CNT30, and CNT50 electrodes. Inset: the corresponding equivalent circuit.
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different rates, the stable reversible capacity from CNT30 and CNT50 is resumed to 155 mAh g1, when the current is returned to 1C. Between the CNT30 and CNT50 electrodes, there are almost no differences until the current reached 3C. It can be obviously seen that the novel LiMnTiO4/MWCNT electrode exhibits the distinct superior rate capacity performance, which may be contributed to the good electron conductivity of the MWCNTs network structure. Electrochemical impedance spectroscopy (EIS) was carried out to investigate the kinetic performance of the original, CNT30 and CNT50 electrodes. Fig. 6d shows the Nyquist plots of the impedance spectra for the original, CNT30 and CNT50 electrodes, and the equivalent circuit model, which is shown in inset figure, was used to fit the impedance spectra. On this circuit, Rs is the electrolyte's resistance, Rint is the surface layer resistance, Rct is the chargetransfer resistance, CPE1 and CPE2 are two constant phase elements corresponding to interfacial resistance and charge-transfer resistance, respectively, and Wdif indicates the impedance of lithium ion diffusion. All of the plots consist of a semicircle in the high-frequency region and a long inclined line in the low-frequency region. Obviously, the semicircles of the CNT30 and CNT50 are both smaller than the conventional electrode. Based on the circuit model, the CNT30 and CNT50 have lower charge-transfer resistance (Rct ¼ 32.9, 17.2 Ohm, respectively) compared with those for the original electrode (Rct ¼ 55.85 Ohm). These results demonstrate that the network structure can provide a fast transmission path for electron and Li ion, which is also beneficial for enhancing the conductivity of the new electrode. 4. Conclusion In this paper, the flexible, free-standing and light-weight electrodes for lithium ion batteries were successfully developed by adding LiMnTiO4 particles in the twining MWCNT networks. Compared with the conventional electrode, the LiMnTiO4/MWCNT electrode exhibits superior high rate performance and reversible charge-discharge capacities. It can deliver a capacity of 161 mAh g1 (86.4% retention) after 50 cycles at 0.5C rate. With the rate increasing from 0.5C to 5C, the LiMnTiO4/MWCNT electrode can still maintain a capacity of 77 mAh g1, which is much higher than the capacity of the traditional electrode (10 mAh g1). Moreover, as a free-standing cathode in the lithium ion batteries, the CNT50 film exhibits better electrochemical performance the CNT30, displaying better reversible charge-discharge capacities, higher rate performance, and superior mechanical properties. This may be due to the increase of the MWCNTs, which can provide more electronic conduction pathways and more tough mechanical support. In addition, these results agree well with the investigation of the electrochemical impedance spectroscopy (EIS). The excellent electrochemical performance and free-standing LiMnTiO4/MWCNT electrode would be a promising cathode for new flexible lithium ion batteries. Acknowledgements The authors gratefully acknowledge the financial supports by the National Natural Science Foundation of China (11572002, 51404142); Supports by the Natural Science Foundation of Jiangsu Province (BK20140936) and the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD) are also acknowledged.
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Free-standing and flexible LiMnTiO4/carbon nanotube cathodes for high performance lithium ion batteries Yinhua Bao a, Xingyu Zhang a, Xu Zhang b, Le Yang a, Xinyi Zhang a, Haosen Chen c, *, Meng Yang a, b, **, Daining Fang a, c a b c
State Key Laboratory for Turbulence and Complex System, College of Engineering, Peking University, Beijing 100871, China College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing 100081, China
h i g h l i g h t s A free-standing cathode based on LiMnTiO4 and multiwall carbon nanotube is developed. MWCNT networks improve reversible capacity and rate capability. LiMnTiO4/MWCNT cathode exhibits the flexibility and light-weight.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 February 2016 Received in revised form 22 April 2016 Accepted 25 April 2016 Available online 6 May 2016
A flexible, free-standing, and light-weight LiMnTiO4/MWCNT electrode has been prepared by vacuum filtration method for the first time. The as-prepared flexible LiMnTiO4/MWCNT electrode possesses a three-dimensional braiding structure in which LiMnTiO4 particles are well embedded in the twining CNT networks. The novel LiMnTiO4/MWCNT electrodes show tensile strength of 1.34 MPa and 2.04 MPa, when the percentages of MWCNTs reach to 30% and 50%, respectively. This novel flexible electrode exhibits a superior electrochemical property, especially at rate capability and cycling stability. The LiMnTiO4/ MWCNT electrode can deliver capacity of 161 mAh g1 (86.4% retention) after 50 cycles at 0.5C rate. Since the high conductivity from MWCNT networks, the LiMnTiO4/MWCNT electrode can still maintain a capacity of 77 mAh g1 at 5C rate, which is much higher than that of the conventional electrode fabricated by slurry casting method on Al foil. The features of free-standing, light-weight, and excellent electrochemical performance indicate the potential of using the LiMnTiO4/MWCNT cathode in new-generation flexible lithium ion batteries. © 2016 Elsevier B.V. All rights reserved.
Keywords: Lithium ion batteries Multiwall carbon nanotube (MWCNTs) LiMnTiO4 Free-standing Cathode
1. Introduction With rapid development in portable electronic devices such as mobile phone, flexible displays, ultra-thin laptop computer and wearable devices, higher demand in flexible energy storage applications has been raised to meet the energy requirements [1e3]. Due to the high power capability and durable cycle life, lithium ion battery is one of the most attractive and promising candidates
* Corresponding author. ** Corresponding author. College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China. E-mail addresses: [email protected] (H. Chen), [email protected] (M. Yang). http://dx.doi.org/10.1016/j.jpowsour.2016.04.121 0378-7753/© 2016 Elsevier B.V. All rights reserved.
compared with other battery technologies. However, except for meeting the ever-increasing energy density and rate capability, new requirements of lithium ion batteries, such as flexibility, high mechanical strength, and light-weight have been imposed to identify with the improvement of portable electronic equipment [4e6] and even the future electric vehicles [7]. Hence, developing the innovative lithium ion battery with flexibility and high power density is crucial for the challenges from new generation portable electronics. In order to meet the above-mentioned requirements, optimizing the properties of electrode has been demonstrated as one of the most efficient approaches [8e10]. In terms of development of electrode materials, extensive efforts have been focused on developing novel cathode materials such as surface coating and element
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substitution [11]. For instance, the LiMnTiO4 cathode material, which formed by substituting Ti4þ for Mn4þ in LiMn2O4, can provide higher capacity and excellent cycling stability than conventional cathode materials such as LiCoO2 [12] and LiFePO4 [13]. Besides, a new series of the titanate cathode material such as Li2MTiO4 (M ¼ Mn, Fe, Co, Ni) has also emerged as a promising candidate for lithium ion batteries [14,15], which has a cubic cation disordered rock salt structure and offers a high theoretical capacity and stability. In terms of realizing the properties of light-weight and flexibility, the elimination of the inactive electrode components including polymer binder, conductive additive, and metal foil has been carried out in some reports [16]. On the one hand, it will be favorable to decrease the overall weight and have higher power density [17] in a binder-free electrode structure which contains more active materials. On the other hand, with the volumetric change [8,18,19] during charge/discharge processes, debonding will occur at the interface between electrode and current collector [20,21] in the traditional electrode structure, which is more serious in flexible batteries because of the bending and rolling activities [22]. It is quite clear that this issue will be automatically solved by an integrative electrode structure design. Recently, many novel designs of electrode structure have been developed to keep the pace with the flexible lithium ion batteries [23e30]. For example, Lu et al. [24] developed an in-situ hydrothermal process to synthesize high performance LiMn2O4/CNT cathodes, which show good flexibility, high capacity and stable cycling performance. Lee et al. [26] developed the synthesis of freestanding, binder-free CNT anode electrodes of tens of microns in thickness by using the method of vacuum filtration. Wu et al. [31] provided the combination of LiNi0.5Mn1.5O4 with multiwall carbon nanotube, which achieved electronical enhancement and lightweight electrode structure. Although these flexible electrodes can provide superior electrochemical performance, most of them are synthesized by sophisticated methods, which results in complexity during preparation process and decreases the feasibility in the battery industry. In addition, to the best of our knowledge, there are few reports about the application of Ti-substituted cathode material in flexible electrode. In this study, we have synthesized the LiMnTiO4 nanoparticles by the sol-gel method. In addition, MWCNT (purity 95%, diameter 15e60 nm) is purified and functionalized for good suspension. Then a novel free-standing, flexible, binder-free electrode design is developed by the integration of LiMnTiO4 nanoparticles and MWCNT through simple vacuum filtration method. Meanwhile, the galvanostatic charge/discharge performance of LiMnTiO4/MWCNT electrodes and mechanical performances are both studied. As a free-standing cathode for lithium ion batteries, LiMnTiO4/ MWCNT electrodes are expected to have superior overall performances including high capacity, excellent rate capability, good cycle stability, lightweight, and flexibility. On the one hand, the substitution of Ti4þ for Mn4þ in LiMn2O4 can provide a higher bonding energy (TieO, 662 kJ mol1) than MneO (402 kJ mol1) [32], which reduces the concentration of Jahn-Teller Mn3þ ions. Furthermore, the Mn2þ/Mn4þ redox couple in LiMnTiO4 contributes to a high theoretical capacity of 308 mAh g1 [33]. On the other hand, the porous and twining MWCNT networks can promote electrolyte infiltration, which can accelerate both electrons and Li ions transformation. Without the inactive materials, the power density of entire electrode has been significantly improved compared with the conventional electrodes. Combining the abovementioned advantages, the free-standing LiMnTiO4/MWCNT electrode could be a promising candidate for new flexible lithium ion batteries.
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2. Experimental section 2.1. Preparation of LiMnTiO4/MWCNT electrodes LiMnTiO4 particles were synthesized using a well-known sol-gel method [34,35]. Stoichiometric amounts of lithium acetate dihydrate, manganese acetate tetrahydrate and tetra-n-butyl titanate were dissolved in ethanol, and refluxed at 353 K for 24 h with continued vigorous stirring until a transparent sol was obtained. After that, the resultant sol was dried at 393 K for 12 h under air atmosphere. Finally, the mixture was hand-milled in an agate mortar and sintered at 923 K for 10 h in air. The purified and functionalized MWCNTs dealt by means of the mild acid treatment process (see Supplementary data) were dispersed in DI water with Triton X-100 (2.5 ml) as the surfactant to form a stable suspension with probe-sonication. LiMnTiO4 powders were added to about 70% of the as-prepared MWCNT suspension and dispersed by the ultrasonic force again. LiMnTiO4/MWCNT electrodes were then prepared by vacuum filtration. About 15% of the as-prepared MWCNT suspension was filtered first through a porous membrane, the Celgard 3500 polypropylene separator, to obtain a thin MWCNT bottom layer. The mixture of LiMnTiO4/MWCNT suspension was added to form a middle layer before covering another MWCNT layer on the surface. To remove the surfactant residue thoroughly, the resultant LiMnTiO4/MWCNT film was washed with DI water (1000 mL), followed closely by methanol (100 mL). During the last step, the film was dried in a vacuum chamber at 353 K for 72 h. The LiMnTiO4/ MWCNT electrodes with two weight percentages of MWCNTs, 30% and 50%, were both obtained in this work.
2.2. Material characterization Structural and crystallographic analysis of the as-prepared materials was determined by X-ray diffraction (XRD) using a Rigaku SmartLab diffractometer with a Cu-Ka radiation. The Fourier transform infrared (FTIR) spectra were recorded to confirm presence of the functional groups in the multi-walled carbon nanotubes structure, using a NEXUS670 Fourier transform spectrometer (NICOLET, USA) over the range of 4000e400 cm1 at room temperature. The morphologies of the samples were characterized by a field emission scanning electron microscope (HITACHI S-4800). The LiMnTiO4/MWCNT films were cut into rectangular strips of approximately 4 mm 30 mm for testing. Static mechanical tests [36e38] were carried out on a Hounsfield (HS5N) tensile tester with a strain rate of 0.05 mm min1.
2.3. Electrochemical measurements The electrochemical performance was evaluated using CR2032type coin cells. The LiMnTiO4/MWCNT film was cut into small pieces with 12 mm diameter. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC)-dimethyl carbonate (DMC) (1:1 by volume). For comparison, conventional electrodes were made by slurry casting method on Al foil. The weight ratio was LiMnTiO4: carbon black: polyvinylidene fluoride ¼ 8: 1: 1. Galvanostatic charge/discharge testing was performed between 1.5 and 4.8 V at 298 K by a BT-2000 battery testing system (Arbin, USA). The cyclic voltammetry (CV) was measured using the CHI 660D electrochemical workstation in the voltage range of 1.5 Ve4.8 V at the scan rate of 0.1 mV s1. Electrochemical impedance spectra (EIS) was tested at a frequency range from 0.01 Hz to 105 Hz with an amplitude of 5 mV.
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Fig. 1. XRD patterns of LiMnTiO4 and LiMnTiO4/MWCNT electrode.
3. Result and discussion The characterization of original MWCNTs and oxidized MWCNTs was presented by FTIR in Fig. S1. The results in Fig. S1 indicate that there are some functional groups on the surface of MWCNTs. The OeH stretching mode, carbonyl stretching mode, CeC stretching mode were respectively confirmed with the corresponding peaks at 3400, 1717, and 1564 cm1. The extra peak between 1340 and 1400 cm1 belongs to the very little residual Nitric acid in MWCNTs. The XRD patterns of the oxidized MWCNTs were also obtained, as shown in Fig. S2, stronger and sharper diffraction peaks at 2q ¼ 26.3 , 43 and 54.2 were observed, comparing with the original MWCNTs. Furthermore, the peak (marked in Fig. S2) of the impurity phase was decreased significantly after the mild acid treatment process. The crystal structure of LiMnTiO4 particles was confirmed to be a spinel cubic phase (Fd-3m space group) as previously reported [32]. The rietveld refined XRD spectrum shown in Fig. S3 indicates that the sample consists of a major spinel LiMnTiO4 coexisting with a minor fraction of rutile TiO2 (9.73 wt %) phase. The LiMnTiO4 phase possesses refined cell parameters of a ¼ 8.256 Å and cell volume of V ¼ 562.7 Å3 while the TiO2 phase possesses a ¼ 4.598 Å and V ¼ 62.7 Å3. Fig. 1 shows the XRD of the patterns of LiMnTiO4/ MWCNT electrode. It is found that LiMnTiO4/MWCNT electrode
presents a similar diffraction peaks to the LiMnTiO4 sample except for the CNT peak at 26 and some ambiguous peaks between 38 and 42 . The ambiguous peaks between 38 and 42 may be attributed to the residual Triton X-100. Fig. 2a shows the photograph of the as-prepared flexible LiMnTiO4/MWCNT electrode. The LiMnTiO4/MWCNT electrode is a circular film with the diameter of approximately 40 mm, which can be bent into an arc shape without any obvious breakage. The structure schematic of free-standing LiMnTiO4/MWCNT electrode is shown in Fig. 2b. The top and bottom MWCNT layers can prevent the LiMnTiO4 particles from falling out effectively. Meanwhile, the LiMnTiO4 particles are embedded in the twining MWCNT networks, which can provide efficient electron transport pathways as well as a flexible and binder-free scaffold. Since the use of inactive material is eliminated, the LiMnTiO4/MWCNT electrode is much lighter than the conventional electrode. The total weight of an electrode (diameter ¼ 12 mm) is shown in Fig. S4, based on LiMnTiO4 particles loading of 2.5 mg cm2. The weight of LiMnTiO4/MWCNT electrode with 30% MWCNTs is only half of the conventional electrode. It should be noted that the flexible and binder-free scaffold, as well as the reduced weight of the electrode, would remain great advantages in lithium ion battery design. Fig. 3 exhibits the morphologies of LiMnTiO4/MWCNT film by a field emission scanning electron microscope (FE-SEM). As illustrated in the surface image of the film electrode (Fig. 3a), a robust network structure is formed by interlaced MWCNT bundles with diameters of 15e50 nm. The interspaces between the bundles can provide a high-efficiency path for electrolyte to infiltrate into the active materials, which is helpful to Li ion transmission. Fig. 3b shows the LiMnTiO4/MWCNT mixture layer between the two MWCNT covers. The active particles are uniformly enlaced by MWCNTs, which can be clearly presented by the high magnification image in Fig. 3c. Such network structure is greatly conducive to enhance the electrons conduction. The interface between the MWCNT layer and mixture layer is well bonded, which is vividly shown by the cross section FE-SEM image in Fig. 3d. The mechanical properties of the LiMnTiO4/MWCNT electrode were quantified by using tensile testing. Fig. 4 shows the stressstrain curves for the pure MWCNT film and the LiMnTiO4/ MWCNT electrodes with two weight percentages of MWCNTs, 30% and 50%, denoted by CNT30 and CNT50. The CNT30 and CNT50 electrodes show tensile behavior very similar to the pure MWCNT film, which was prepared by vacuum filtration. The tensile strength of the pure MWCNT film was 3.85 MPa, and 1.34 MPa, and 2.04 MPa for the CNT30 and CNT50, respectively. The ultimate strain of pure MWCNT film, CNT30, CNT50 was 6.18%, 2.07%, and 4.01%, respectively. The Young's modulus of pure MWCNT film, CNT30, CNT50
Fig. 2. (a) Digital photograph of original LiMnTiO4/MWCNT membrane to indicate the flexibility and its diameter, with the inset showing the circular LiMnTiO4/MWCNT cathode (diameter ¼ 12 mm) in a CR2032 coin cell. (b) Schematic of the structure of free-standing LiMnTiO4/MWCNT electrodes.
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Fig. 3. (a) SEM images of thin layer of MWCNT film on the surface of the LiMnTiO4/MWCNT electrodes. Inset: photograph of the higher magnification. (b) SEM images of the LiMnTiO4/MWCNT mixture layer between the two MWCNT covers. (c) High resolution SEM image indicating LiMnTiO4 particles are located in/on the voids within the MWCNTs network. (d) The interface between the MWCNT layer and mixture layer.
was 1.94 MPa, 0.84 MPa, and 1.45 MPa, which was determined by fitting the stress-strain curves ranging from initial point to 0.1%. These results indicate that CNT50 exhibited the higher ultimate strain and tensile strength than CNT30. However, with the increase of LiMnTiO4 nanoparticles, the tensile strength of the samples decreases to some extent, which may be caused by the defective of multiwall carbon nanotube network structure. To understand the electrochemical behavior of the LiMnTiO4/ MWCNT electrode, the CV tests were conducted in this work. Fig. 5a shows the CV curve of the original LiMnTiO4 electrode between 1.5 and 4.8 V. Very similar reduction and oxidation process was observed in Fig. 5b and Fig. S5, which shows the CV curves of the CNT50 and CNT30, respectively. This result indicates that the reaction mechanisms remain unchanged with the addition of multiwall carbon nanotubes. All of the CV curves show the corresponding redox peaks (Mn2þ/Mn3þ between 2.5 and 3.4 V, Mn3þ/ Mn4þ between 3.7 and 4.5 V) during the first three cycles. The structural changes during cycles cause the plateaus fade gradually. It should be noted that the peak current in Fig. 5b is higher than that in Fig. 5a in the first few cycles, which reveals a more active electrochemical process in CNT50. Moreover, CNT50 electrode shows decreased potential separations between the anodic and cathodic peaks in Fig. 5, which means a much lower electrode polarization during electrochemical reaction process for CNT50 electrode. The charge and discharge curves of the original, CNT30, and CNT50 electrodes in the voltage of 1.5e4.8 V at 0.5C (1C ¼ 308 mA g1) are presented in Fig. 6a. The curves show two charge/discharge voltage plateaus at around 4 V and 2.8 V, which are consistent with the peaks in the CV curve. The CNT30 and CNT50 electrodes can deliver the discharge capacities of
192 mAh g1 and 197 mAh g1, respectively, in the earlier stage of cycling, which is higher than that of the original LiMnTiO4 electrode (145 mAh g1). In addition, The CNT30 and CNT50 electrodes present excellent cycle capability as well as original LiMnTiO4 electrode, as shown in Fig. 6b. However, the discharge capacities of CNT30 and CNT50 electrodes are much higher than that of original LiMnTiO4 electrode. In the case of CNT50 electrodes, the initial discharge capacity is 192 mAh g1, and Coulombic efficiency 96.6%. After 50 cycles, the CNT50 electrode still maintains high capacity of 161 mAh g1, which is higher than that of the original electrode
Fig. 4. Stress-strain curve for pure CNT film, CNT30 and CNT50.
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Fig. 5. Cyclic voltammograms of the original LiMnTiO4 cathode (a) and CNT50 (b) samples in the voltage range of 1.5 Ve4.8 V at the scan rate of 0.1 mV s1.
(128 mAh g1). The improvement in the reversible capacity may be due to the three-dimensional braiding structure, which can provide superior electron and Li ion transmission capability. Meanwhile, it should be noted that the elimination of the binder and Al foil current collector can also improve the electrochemical performance of the electrode. The rate capability of the CNT30 and CNT50 electrodes was also studied and the results are presented in Fig. 6c. Evidently, the LiMnTiO4/MWCNT electrode exhibits much better than the
traditional electrodes. The discharge capacity descends gradually when the current rate increases from the 0.5C to 1, 2, 3, and 5C, the CNT50 can deliver a considerably high capability of 192, 165, 131, 106, and 77 mAh g1, respectively, which is higher than the traditional electrode. At 5C charge/discharge rate, the CNT30 and CNT50 electrodes can still maintain a capacity of 53 mAh g1 and 77 mAh g1, while the capacity of the traditional electrode almost decreased to 10 mAh g1. Meanwhile, the capacity retention at different current density was presented in Fig. S6. After 50 cycles at
Fig. 6. (a) Charge/discharge curves of the original LiMnTiO4, CNT30, and CNT50 electrodes at 0.5C. (b) Cycling performance of the original LiMnTiO4, CNT30, and CNT50 electrodes at 0.5C. (c) Rate capability of the original LiMnTiO4, CNT30, and CNT50 electrodes. (d) EIS spectra of the fresh cells with the original LiMnTiO4, CNT30, and CNT50 electrodes. Inset: the corresponding equivalent circuit.
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different rates, the stable reversible capacity from CNT30 and CNT50 is resumed to 155 mAh g1, when the current is returned to 1C. Between the CNT30 and CNT50 electrodes, there are almost no differences until the current reached 3C. It can be obviously seen that the novel LiMnTiO4/MWCNT electrode exhibits the distinct superior rate capacity performance, which may be contributed to the good electron conductivity of the MWCNTs network structure. Electrochemical impedance spectroscopy (EIS) was carried out to investigate the kinetic performance of the original, CNT30 and CNT50 electrodes. Fig. 6d shows the Nyquist plots of the impedance spectra for the original, CNT30 and CNT50 electrodes, and the equivalent circuit model, which is shown in inset figure, was used to fit the impedance spectra. On this circuit, Rs is the electrolyte's resistance, Rint is the surface layer resistance, Rct is the chargetransfer resistance, CPE1 and CPE2 are two constant phase elements corresponding to interfacial resistance and charge-transfer resistance, respectively, and Wdif indicates the impedance of lithium ion diffusion. All of the plots consist of a semicircle in the high-frequency region and a long inclined line in the low-frequency region. Obviously, the semicircles of the CNT30 and CNT50 are both smaller than the conventional electrode. Based on the circuit model, the CNT30 and CNT50 have lower charge-transfer resistance (Rct ¼ 32.9, 17.2 Ohm, respectively) compared with those for the original electrode (Rct ¼ 55.85 Ohm). These results demonstrate that the network structure can provide a fast transmission path for electron and Li ion, which is also beneficial for enhancing the conductivity of the new electrode. 4. Conclusion In this paper, the flexible, free-standing and light-weight electrodes for lithium ion batteries were successfully developed by adding LiMnTiO4 particles in the twining MWCNT networks. Compared with the conventional electrode, the LiMnTiO4/MWCNT electrode exhibits superior high rate performance and reversible charge-discharge capacities. It can deliver a capacity of 161 mAh g1 (86.4% retention) after 50 cycles at 0.5C rate. With the rate increasing from 0.5C to 5C, the LiMnTiO4/MWCNT electrode can still maintain a capacity of 77 mAh g1, which is much higher than the capacity of the traditional electrode (10 mAh g1). Moreover, as a free-standing cathode in the lithium ion batteries, the CNT50 film exhibits better electrochemical performance the CNT30, displaying better reversible charge-discharge capacities, higher rate performance, and superior mechanical properties. This may be due to the increase of the MWCNTs, which can provide more electronic conduction pathways and more tough mechanical support. In addition, these results agree well with the investigation of the electrochemical impedance spectroscopy (EIS). The excellent electrochemical performance and free-standing LiMnTiO4/MWCNT electrode would be a promising cathode for new flexible lithium ion batteries. Acknowledgements The authors gratefully acknowledge the financial supports by the National Natural Science Foundation of China (11572002, 51404142); Supports by the Natural Science Foundation of Jiangsu Province (BK20140936) and the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD) are also acknowledged.
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