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LiV3O8 nanoflakes with superior cycling stability as cathode material for Li-ion battery
Electrochimica Acta 157 (2015) 211–217
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Multi-layered Al2O3/LixV2O5/LiV3O8 nanoflakes with superior cycling stability as cathode material for Li-ion battery Dan Sun a , Guoqing Xu a , Haiyan Wang a,b, * , Xianguang Zeng c, Yong Ma a , Yougen Tang a , Younian Liu a, * , Yingfen Pan a,d a
College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, PR China Hunan Dahua New Energy Co., Ltd, Changsha, 410600, PR China Material Corrosion and Protection Key Laboratory of Sichuan Province, Zigong, 643000, PR China d Dongshan Entry-Exit Inspection & Quarantine Bureau of P.R.C, Dongshan, 363401, P.R China b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 25 October 2014 Received in revised form 17 January 2015 Accepted 18 January 2015 Available online 20 January 2015
Al2O3 coating is utilized in this work to further improve the cycling stability of LiV3O8 nanoflakes. Surprisingly, three layered Al2O3/LixV2O5/LiV3O8 nanostructure is well formed as confirmed by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) results. When used as a cathode for Li-ion battery, the hybrid demonstrates significantly improved cycling stability with a discharge capacity of 203 mAh g 1 remaining after 200 cycles at 150 mA g 1 and 87.5% of the initial capacity maintaining after 500 cycles at 300 mA g 1. Such superior cycling performance should be attributed to the mutual protection of Al2O3 and LixV2O5 layer between Al2O3 and LiV3O8, which can well suppress the damage of LiV3O8 during the long-term cycling process. More importantly, the LixV2O5 middle layer could contribute to the improvement of interfacial electrochemical properties of the hybrid electrode. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Li-ion battery Lithium trivanadate oxide Aluminium oxide coating Double protection Cycling stability
1. Introduction Li-ion batteries (LIBs) have been widely applied in portable mobile devices and also hold great potential for powering electric vehicles (EVs) and large-scale stationary energy storage [1,2]. In the latest two decades, intensive research efforts have focused on the development of new lithium intercalation materials with higher capacity as alternatives to LiCoO2 because of its limited reversible capacity (around 140 mAh g 1) [3]. The layered lithium vanadium oxides, LiV3O8 and its derivatives have received considerable attention as appealing cathode materials for rechargeable LIBs due to their high specific capacity, ease of fabrication, low cost and so on [4–6]. It is well known that the electrochemical properties of vanadates greatly depend on the morphology and synthesis strategy. Up to now, a great deal of methodologies, such as solgel method [7–11], hydrothermal synthesis [12], spray-drying method [6,13] and microwave-assisted synthesis [14] have been developed for high-performance LiV3O8. Recently, ultralong LiV3O8 nanowires with superior high-rate capability and cycling life have
* Corresponding authors. Tel.: +86 731 88830886; fax: +86 731 88879616. E-mail addresses: [email protected] (H. Wang), [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.electacta.2015.01.081 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
been synthesized by Xu et al. using H2V3O8 nanowires as precursor [15]. However, this kind of material still suffers from irreversible phase transformation, deterioration of crystal structure and dissolution of active materials in electrolyte, leading to fast capacity loss during cycling process [16–18]. Surface modification is a very prevalent approach to improve the electrochemical properties of electrode materials via restricting the dissolution of active materials and stabilizing the crystal structure of active materials. A number of efforts have been devoted to LiV3O8 [5,19–26]. Idris et al. [19] reported carboncoated LiV3O8, which could be cycled up to 250 cycles without considerable capacity loss at 20C. Lee et al. [21] showed Cr-coated LiV3O8 with a capacity retention of 89% after 50 cycles at 0.2C. Generally, although the cycling performance was improved, the reversible capacity of modified LiV3O8 was decreased obviously. The discharge capacity of LiV3O8 went down from 335 mAh g 1 to 227 mAh g 1 after carbon coating [19] and from 283 mAh g 1 to 243 mAh g 1 after polyaniline (PAn) coating [5]. In our opinion, the V5+ in LiV3O8 is susceptible to the coating material during the hightemperature sintering process. Accordingly, it is believed that inactive inorganic salts, for examples, metal oxides are more suitable. Among them, Al2O3 has been successfully applied to modify electrode materials like LiMn2O4 [27], LiFePO4 [28], LiCoO2 [29,30]. Al2O3 modified LiV3O8 was once reported in ref. [25] via a
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thermolysis process and the improved electrochemical properties were mainly attributed to the joint action of Al2O3 coating and Li-V-Al-O solid solution at the LiV3O8/Al2O3 interface which could provide a fast Li-ion diffusion path. However, it should be noted that the capacity retention ratio of Al2O3/LiV3O8 after 100 cycles at 2000 mA g 1 was just 71.3%, which was far insufficient. As we know, coating strategy affects the electrochemical properties seriously. In current work, precipitation method was employed to obtain Al2O3-coated LiV3O8. It is interesting to note that a three layered Al2O3/LixV2O5/LiV3O8 nanostructure was well formed, which is much different from that in ref. [25]. When used as a cathode for Li-ion battery, the composite electrode exhibits significantly improved cycling stability with a discharge capacity of 203 mAh g 1 remaining after 200 cycles at 150 mA g 1. At a current density of 300 mA g 1, it can be even cycled up to 500 cycles with a capacity retention of 87.5%. Although Al2O3 coating was once reported for LiV3O8, a special three layered Al2O3/LixV2O5/LiV3O8 nanostructure is highlighted in this work and superior cycling stability is exhibited probably due to its unique hybrid structure. 2. Experimental 2.1. Synthesis of LiV3O8 nanoflakes All the starting materials were analytical reagent grade and used directly without any purification. (NH4)0.5V2O5 precursor was prepared by a typical hydrothermal method. Oxalic acid (2.28 g, 99.5%, Sinopharm Chemical Reagent Co., Ltd.) and NH4VO3 (2.55 g, 99%, Tianjin Guangfu Institute of Fine Chemicals) were first dissolved in distilled water with rapid stirring. Then, the yellowgreen solution was transferred into a 100 ml Teflon lined stainless steel autoclave and heated at 180 C for 12 h. After that, the autoclave was cooled down to room temperature naturally. The precipitate was filtered, washed with distilled water several times and then dried at 80 C overnight. To obtain LiV3O8, a proper amount of (NH4)0.5V2O5 was added into lithium hydroxide (Tianjin Institute of Chemical Reagents) solution (the molar ratio of Li: V in theory is 1: 3) with 0.2 g of PEG4000 as dispersing agent. The mixture was stirred for 2 h at room temperature and then heated at 80 C in a hotplate under stirring to evaporate the distilled water. The collected powder was annealed at 450 C for 8 h in air to obtain the LiV3O8 nanoflakes.
Fig. 1. XRD patterns of the bare LiV3O8 and Al2O3/LixV2O5/LiV3O8 composite.
was treated under vaccum to remove the residual DMC. The operations were carried out in an Ar-filled MBraun glove box. Finally, the whole electrode was used to examine the XRD test and no obvious signal of stainless steel mesh was observed probably due to the thick electrode film. X-ray photoelectron spectroscopy (XPS) measurement was performed on the K-Alpha1063 spectrometer. The XPS patterns were collected using Al Ka radiation at a voltage of 12 kV and current of 6 mA. Charging effect was corrected by adjusting the binding energy of C1s peak from carbon contamination to 284.5 eV. Morphological studies were conducted using a JEOL JEM-2100F transmission electron microscope (TEM), employing a LaB6 filament as the electron source and an accelerating voltage of
2.2. Synthesis of Al2O3/LixV2O5/LiV3O8 composite To prepare Al2O3/LixV2O5/LiV3O8 composite, 0.059 g of Al (NO3)39H2O and 0.020 g of NaOH were separately dissolved in distilled water (the coating content in theory is 2 wt%). Then 0.400 g of LiV3O8 powder was dispersed in Al(NO3)3 solution with constantly stirring by a magnetic force stirrer for 2 h and then the as-prepared NaOH solution was dropped into LiV3O8/Al(NO3)3 suspension slowly. After stirred for 4 h, the temperature was increased to 50 C to precipitate Al3+ completely. The as-obtained precipitate after filtering was dried at 80 C overnight and further calcined at 450 C for 6 h under air atmosphere. 2.3. Characterizations X-ray diffraction (XRD) data were examined by the X-ray diffractometer (DX-2700, Dandong Haoyuan) utilizing a CuKa1 source with a step of 0.02 . For the XRD test of cycled electrode, the cells were first disassembled and the electrodes were soaked in dimethyl carbonate (DMC) for 1 h and then rinsed several times with DMC to remove the electrolyte. Then the electrode film consisting of active material, Super P carbon and polytetrafluoroethylene (PTFE) pressed on the stainless steel mesh
Fig. 2. XPS of Al2O3/LixV2O5/LiV3O8 composite: (a) survey spectrum; (b) highsolution V2p.
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200 keV. The high resolution TEM (HRTEM) images were obtained from JEOL JEM-2100F TEM.
electrochemical station (Shanghai Chenhua, China) with a scan rate of 0.1 mV s 1 at room temperature.
2.4. Electrochemical measurements
3. Results and discussion
The electrodes were fabricated by mixing the active material, polyvinylidene fluoride (PVDF), and Super P carbon in a weight ratio of 80: 10: 10 using tetrahydrofuran (THF) as solvent. The mixture was stirred for 6 h and then cast onto the Al foil. After solvent evaporation at room temperature, the electrodes were dried at 110 C under vacuum for 12 h. The loading mass of each electrode was 3.0 mg cm 2. The construction of electrodes after different cycles for XRD testing is different from that for electrochemical measurement since some of powder in electrode after cycling tends to exfoliate. To address this issue, the electrodes were fabricated by pressing a mixture of the active material, Super P carbon, and PTFE in a weight ratio of 80:10:10 using distilled water as solvent on a stainless steel mesh collector firstly and then dried at 110 C under vacuum for 8 h. The electrodes were assembled into CR2016 coin-type cells with commercial electrolyte (Guangzhou Tinci Materials Technology Co., Ltd; 1 M LiPF6 in 1:1 v/v ethylene carbonate/dimethyl carbonate) and a Li metal as counter electrode. The cells were constructed in an Ar-filled MBraun glovebox and then cycled galvanostatically between 1.5 and 4.0 V (versus Li+/Li) at a desired current density using a Neware battery testing system (CT-3008W) at room temperature. Cyclic voltammetry (CV) test was carried out using the CHI 660D
The XRD patterns of bare LiV3O8 and Al2O3/LixV2O5/LiV3O8 composite are displayed in Fig. 1. As seen, the main diffraction lines of the bare LiV3O8 can be well indexed to the standard monoclinic LiV3O8 (JCPDS card No. 72-1193, space group: P21/m). The indexed lattice parameters of a = 0.66931 nm, b = 0.3597 nm, c = 1.2085 nm and b=108.03 are well consistent with those of previous literature [31–34]. There is no other impurity diffraction line, indicating the high purity of LiV3O8. In the case of the modified LiV3O8, although LiV3O8 phase is maintained, two differences should be pointed out. One is the intensity decrease of the diffraction lines, particularly (1 0 0) peak, which may have a relationship with the coating and annealing process [24]. According to previous studies [35,36], the intensity decrease of (1 0 0) peak indicates the shortening of diffusion paths for lithium ions inserting between the (1 0 0) planes, which will be advantageous to the electrochemical properties of electrode. The other is the appearance of impurity lines as marked with asterisks, which are basically indexed to Li0.3V2O5 (JCPDS card No. 73-1670). Since it is very difficult to ensure the real amount of LixV2O5 in composite, the impurity phase is written as LixV2O5 (x0.3) for more accuracy. It is worthy to highlight that the bare LiV3O8 in ref. [25] was accompanied with certain Li0.3V2O5. While in our work, the Li0.3V2O5 impurity
Fig. 3. (a) TEM images of LiV3O8; (b) TEM and HRTEM (c, d, f) of Al2O3/LixV2O5/LiV3O8 composite; (f) EDX of Al2O3/LixV2O5/LiV3O8 composite.
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Fig. 4. Schematic model of the as-formed Al2O3/LixV2O5/LiV3O8 composite.
appeared after annealing. So the effect of LixV2O5 impurity to the bare LiV3O8 is much different. It is sure that partial reduction of superficial LiV3O8 to LixV2O5 happened during the sintering process. The lattice parameters of modified LiV3O8 are a = 0.6647 nm, b = 0.3605 nm, c = 1.1909 nm and b=107.85 . And the cell volume is 0.2716 nm3, less than that of the bare LiV3O8 (0.2776 nm3), indicating the contracted crystal volume after an annealing process. Note that there is no peak of Al2O3 owing to the low content and amorphous feature of Al2O3 as shown in HRTEM. A similar phenomenon was also reported in Al2O3-coated LiMn2O4 [27]. XPS analysis was employed to further investigate the surface chemical composition and vanadium valance state of as-modified LiV3O8. As revealed in XPS survey spectrum (Fig. 2a), the element signals of vanadium, oxygen, lithium and aluminum are clearly found. The appearance of aluminum and oxygen indicates the presence of Al2O3 on the surface of LiV3O8. Fig. 2b is the high resolution of V2p after fitting. Peak at 517.4 eV and 524.6 eV are attributed to the spin-orbit splitting of V5+2p3/2 and V5+2p1/2, which are well consistent with the valence in references [37,38]. More importantly, the peak of V4+ is observed at 516.4 eV, though it is of less intensity, confirming the existence of LixV2O5 as demonstrated by XRD. XPS results futher imply the coexistence of Al2O3 and LixV2O5 in the surface of modified LiV3O8 sample. The morphology of LiV3O8 and Al2O3/LixV2O5/LiV3O8 composite are shown in Fig. 3. The bare LiV3O8 (Fig. 3a) is of nanoflake morphology with very smooth surface. For Al2O3 coated one (Fig. 3b), the surface is apparently coated with small particles. The HRTEM images of Al2O3 coated LiV3O8 are demonstrated in Fig. 3(c, d, e). It is interesting to note that a clear three-layer structure is observed. In Fig. 3c, the inner host with periodic fringe space of 0.381 nm agrees well with the interplanar spacing between {0 0 3} planes of monoclinic LiV3O8. The interplanar spacing of middle layer is 0.366 nm, which could be indexed to the {11-1} plane of LixV2O5. There is no doubt that the outer amorphous layer is Al2O3. A similar structure is also observed in Fig. 3d, the fringe space of 0.293 nm in middle layer matches well with the interplanar spacing between {30-4} planes of LixV2O5 and the interplanar spacing of 0.262 nm in inner layer could be indexed to the {-113} planes of monoclinic LiV3O8. Fig. 3f shows the EDX pattern of Al2O3/LixV2O5/LiV3O8 composite. Elements of vanadium, oxygen and aluminum are detected, in agreement with the XPS results. Thus, combining the XRD, XPS and TEM results, it is sure that a three-layered Al2O3/LixV2O5/LiV3O8 appears. Schematic model of the as-formed Al2O3/LixV2O5/LiV3O8 composite is proposed in Fig. 4. LiV3O8 nanoflakes are coated by a LixV2O5 nanolayer firstly and then amorphous Al2O3. The thickness of both coating layer are 5–10 nm. As we know, Al2O3 layer can act as a shield to suppress the irreversible phase transformation and protect the inner active
materials from direct contacting with electrolyte [25]. According to our previous work, a proper thickness of LixV2O5 was effective to facilitate the interfacial electrochemical properties of LiV3O8 electrode [38]. Therefore, the three layered Al2O3/LixV2O5/LiV3O8 composite is expected to achieve superior electrochemical performance. Fig. 5 gives the CV curves of bare LiV3O8 (a) and Al2O3/LixV2O5/ LiV3O8 composite (b) at a scan rate of 0.1 mV s 1. There are three anodic peaks at 2.48, 2.90 and 3.71 V for the bare LiV3O8. Four obvious peaks at 2.39, 2.60, 2.68 and 3.61 V are observed in the cathodic scan. According to previous literature, the peaks at 2.68 and 2.60 V correspond to Li-ion insertion in the empty tetrahedral site with a single-phase reaction. While the peak around 2.39 V may relate to the two-phase transformation of Li3V3O8/Li4V3O8 [15]. The appearance of multi-peaks is ascribed to the complicated Li-ion insertion/extraction process in LiV3O8 [15,33], which is also a normal phenomenon for vanadate
Fig. 5. Cyclic voltammetry (CV) curves of the bare LiV3O8 (a) and Al2O3/LixV2O5/ LiV3O8 composite (b) at 0.1 mV s 1.
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Fig. 6. (a) Cyclic performance of the bare LiV3O8 and Al2O3/LixV2O5/LiV3O8 electrode at 0.5C-rate (150 mA g 1) between 1.5 and 4.0 V; (b) The corresponding charge-discharge curves at different cycles (1st, 50th, 100th, 150th, 200th).
compounds, such as NaV3O8 [39], Na2V6O16xH2O [40], NH4V3O8 [41,42]. Note that the obvious peak area loss in the first three curves implies the instability of the bare LiV3O8. Al2O3/LixV2O5/ LiV3O8 composite shows much different electrochemical behaviors in comparison with the bare one. During the cathodic scan, four split peaks at 2.48, 2.70, 2.79, and 2.90 V are observed. For anodic scan, the original peak of the bare LiV3O8 at 2.90 V splits into two peaks at 2.89 and 2.97 V. Moreover, a broad peak at 3.32 V appears. Obviously, the differences of redox peaks further confirm the slight structure change of LiV3O8 after Al2O3 coating. It is worth noting that Al2O3/LixV2O5/LiV3O8 electrode shows a much smaller area variation than the bare LiV3O8 in the initial three curves, in good agreement with the better cycling performance as shown in Fig. 6a. Fig. 6a compares the cycling performance of Al2O3/LixV2O5/ LiV3O8 electrode with the bare LiV3O8 at 0.5 C. As seen, the bare LiV3O8 shows an initial discharge capacity of 244.7 mAh g 1. However, it sharply drops to 192.9 mAh g 1 after ten cycles and then keeps stable for about 40 cycles. After that, the capacity is deceased to 126.9 mAh g 1 after 200 cycles. The quick capacity loss in the initial cycles has been reported for LiV3O8 [14,23,43]. LiV3O8 with an initial discharge capacity of 176 mAh g 1 at 1C was reported in ref. [43], but it faded to 125 mAh g 1 only after 15 cycles. According to previous reports, the capacity fading may have relationships with the dissolving of vanadium, irreversible phase transition between LiV3O8 and Li4V3O8, deterioration of crystal structure and so on [18,44,45]. For Al2O3/LixV2O5/LiV3O8, although a lower initial capacity of 170 mAh g 1 is delivered, it is gradually increased to 211.9 mAh g 1 at the 36th cycle with a discharge capacity of 203.3 mAh g 1 maintaining after 200 cycles. Comparatively, the modified electrode exhibits significantly improved cycling stability. Actually, the capacity increasing in the initial stage is not an accidental phenomenon [4,37,46].
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A similar capacity rising in the initial cycles were observed for VO2(B) [46], which was probably due to the relaxation of the nearly amorphous structure to create a smooth pathway during Li-ion insertion and extraction. In the present case, it could be partially due to the relaxation of contracted crystal volume. As shown above, the cell volume of modified LiV3O8 contracted after the annealing process. Generally, there are octahedral sites and tetrahedral sites in LiV3O8 crystallite, and the pristine Li-ions occupy the octahedral sites. The migrated Li-ions after the discharge process are accommodated at the tetrahedral sites [47]. Accordingly, the crystal shrink will decrease the available accommodation sites for migrated Li-ions slightly. With the cycles increasing, the crystal structure relaxes and the pathways of lithium insertion and extraction become more and more smooth, leading to a capacity increase. A similar discharge capacity rising was reported for high-temperature sintered NaV3O8 nanoflakes [48]. In addition, the activation of multi-layered morphology was considered as another reason. As we know, the outer tight and nonactive Al2O3 layer will hinder the migration of Li-ions to some extent and not all the active material could take part in the reaction rapidly during the initial cycling. With the cycle number increasing, more active materials may be involved due to electrochemical grinding, leading to the increase in capacity [19]. Carbon-PPy-coated LiFePO4 [49], carbon-coated LiV3O8 [19] and PPy-coated LiV3O8 [20] exhibited similar phenomena. To the best of our knowledge, the cycling performance of Al2O3/ LixV2O5/LiV3O8 composite here is superior to most of surface modified LiV3O8 like Co0.58Ni0.42 oxide-coated LiV3O8, AlPO4coated LiV3O8, PPy-coated LiV3O8, carbon coated LiV3O8 and so on [5,19,20,22–24]. PAn-coated LiV3O8 has been synthesized via chemical oxide polymerization [5] and the discharge capacity of the composite after 100 cycles at 0.1C was 204 mAh g 1. LiV3O8-PPy composite with a specific capacity of 183 mAh g 1 maintaining after 100 cycles at 40 mA g 1 was reported in ref. [20]. A discharge capacity of 194 mAh g 1 after 100 cycles at 0.2C was also reported for carbon coated LiV3O8. In our work, with no doubt, the protection of Al2O3 coating layer should be the key reason for the improvement of electrochemical properties. Meanwhile, the middle LixV2O5 layer should be another important reason. It is well known that LixV2O5 has a much higher Li-ion diffusion coefficient (10 10 cm2 s 1) [38] than LiV3O8 (10 13 cm2 s 1) [50]. So a proper thickness of LixV2O5 layer between Al2O3 and LiV3O8 layer might be beneficial to the electrochemical interfacial properties of LiV3O8. Our latest work could well support such deduction. LixV2O5/LiV3O8 composite was fabricated by a facial H2 treatment of LiV3O8 and the composite demonstrated significantly improved electrochemical properties [31]. The results revealed that the surface LixV2O5 with certain thickness could not only protect the
Fig. 7. CV curves of Al2O3/LixV2O5/LiV3O8 composite electrode after 200 cycles. The scan rate is 0.1 mV s 1.
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Fig. 8. XRD patterns of Al2O3/LixV2O5/LiV3O8 composite electrode after different cycles (5th, 200th) at 0.5C-rate during 1.5 4.0 V. Fig. 9. Long cycling performance of Al2O3/LixV2O5/LiV3O8 composite electrode at 1C-rate. Before the cycling, the electrode was first activated for several cycles at 0.1C-rate.
internal LiV3O8 from dissolution, but also increase the Li-ion diffusion coefficient and suppress the charge-transfer resistance [31]. Therefore, the improved cycling stability of Al2O3/LixV2O5/ LiV3O8 electrode could be attributed to the double protection of Al2O3 and LixV2O5 layers. Such a novel three-layered structure could provide more rigorous protection than simple single protection. Fig. 6b shows the charge-discharge curves of Al2O3/ LixV2O5/LiV3O8 composite. As observed, the plateaus around 2.70 V in the charge curves decrease obviously, while the plateau around 3.25 V increases. For the discharge curves, a plateau around 2.1 V appears with cycling number increasing. The variations of the plateaus imply possible structure rearrangement during the cycling, which was also observed in our previous paper [31]. The CV curves of Al2O3/LixV2O5/LiV3O8 composite electrode after 200 cycles are presented in Fig. 7. As seen, the anodic peak at 3.32 V after 200 cycles become stronger in comparison with fresh Al2O3/LixV2O5/LiV3O8 composite electrode and a broad new peak around 2.1 V is observed in the cathodic scan. The variations in CV curves are in good agreement with the change of charge-discharge plateaus, implying the appearance of structure rearrangement during the cycling. XRD patterns of Al2O3/LixV2O5/LiV3O8 electrode after different cycles (5th, 200th) at 0.5C-rate were investigated (Fig. 8). The electrode consisting of active material, carbon and binder was directly used for structure measurement. As displayed, both electrodes could be indexed to the LiV3O8 phase, associated with a slightly amount of LixV2O5. Meanwhile, both electrodes indicate almost the same diffraction lines, implying the absence of phase transition or structure deterioration during the cycling. The lattice parameters of Al2O3/LixV2O5/LiV3O8 composite electrodes after different cycles are compared in Table 1. The lattice parameters of Al2O3/LixV2O5/LiV3O8 composite electrode after 5 cycles are a = 0.6658 nm, b = 0.3583 nm, c = 1.2022 nm and b=107.99 , matching well with the hybrid powder. Compared with electrode after 5 cycles, the change ratio of lattice parameters of hybrid electrode
after 200 cycles is less than 1.5%. Tiny variation indicates good structure stability of Al2O3/LixV2O5/LiV3O8 composite. According to previous reports [18], an important reason for capacity fading of LiV3O8 was the local damage of crystal structure caused by drastic change in crystal lattice parameters. In this work, apparently, the excellent structure stability of Al2O3/LixV2O5/LiV3O8 composite benefits from the double protection of Al2O3 and LixV2O5 layers. To get further insights into the superior stability of Al2O3/ LixV2O5/LiV3O8 composite, cycling life up to 500 cycles at 1C-rate was investigated (Fig. 9). An initial discharge capacity of 161 mAh g 1 is exhibited and the capacity gradually increases to 181.3 mAh g 1 firstly and then stabilizes at 160–170 mAh g 1. The capacity variation tendency matches well with that at 0.5 C. After 500 cycles, about 87.5% of the initial capacity was maintained. Superior cycling stability further confirms that the double protection of Al2O3 and LixV2O5 layers is effective to improve the cycling stability of LiV3O8. 4. Conclusions In summary, a three layered Al2O3/LixV2O5/LiV3O8 nanostructure was constructed and it was well characterized by XRD, HRTEM and XPS. Electrochemical results showed that the hybrid electrode possessed significantly improved cycling performance in comparison with the bare LiV3O8. A specific discharge capacity of 203 mAh g 1 was maintained after 200 cycles at a current density of 150 mA g 1 for Al2O3/LixV2O5/LiV3O8 electrode. When the current density increased to 300 mA g 1, the electrode was able to be cycled up to 500 cycles with capacity retention of 87.5%. XRD and CV results confirmed that the superior cycling performance was mainly due to the double protection of Al2O3 and LixV2O5 in such unique three-layered composite. It is noted that LixV2O5 middle layer could be beneficial to the improvement of rate performance for the composite. Acknowledgements
Table 1 The calculated parameters of Al2O3/LixV2O5/LiV3O8 composite electrodes after different cycles. Samples
a/nm
b/nm
c/nm
b
Electrode after 5 cycles Electrode after 200 cycles Variation ratio
0.66584 0.67555 1.46%
0.35828 0.35657 0.04%
1.20218 1.21270 0.88%
107.99 108.94 0.88%
Financial supports from the National Nature Science Foundation of China (No. 21301193), Hunan Provincial Natural Science Foundation of China (No. 14JJ3022), Project of General Administration of Quality Supervision,Inspection and Quarantine of P.R China (2014IK189), the Opening Project of Material Corrosion and Protection Key Laboratory of Sichuan province (2014CL03) and the Open-End Fund for Valuable and Precision Instruments of Central South University (CSUZC2014018) are greatly appreciated.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Multi-layered Al2O3/LixV2O5/LiV3O8 nanoflakes with superior cycling stability as cathode material for Li-ion battery Dan Sun a , Guoqing Xu a , Haiyan Wang a,b, * , Xianguang Zeng c, Yong Ma a , Yougen Tang a , Younian Liu a, * , Yingfen Pan a,d a
College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, PR China Hunan Dahua New Energy Co., Ltd, Changsha, 410600, PR China Material Corrosion and Protection Key Laboratory of Sichuan Province, Zigong, 643000, PR China d Dongshan Entry-Exit Inspection & Quarantine Bureau of P.R.C, Dongshan, 363401, P.R China b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 25 October 2014 Received in revised form 17 January 2015 Accepted 18 January 2015 Available online 20 January 2015
Al2O3 coating is utilized in this work to further improve the cycling stability of LiV3O8 nanoflakes. Surprisingly, three layered Al2O3/LixV2O5/LiV3O8 nanostructure is well formed as confirmed by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) results. When used as a cathode for Li-ion battery, the hybrid demonstrates significantly improved cycling stability with a discharge capacity of 203 mAh g 1 remaining after 200 cycles at 150 mA g 1 and 87.5% of the initial capacity maintaining after 500 cycles at 300 mA g 1. Such superior cycling performance should be attributed to the mutual protection of Al2O3 and LixV2O5 layer between Al2O3 and LiV3O8, which can well suppress the damage of LiV3O8 during the long-term cycling process. More importantly, the LixV2O5 middle layer could contribute to the improvement of interfacial electrochemical properties of the hybrid electrode. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Li-ion battery Lithium trivanadate oxide Aluminium oxide coating Double protection Cycling stability
1. Introduction Li-ion batteries (LIBs) have been widely applied in portable mobile devices and also hold great potential for powering electric vehicles (EVs) and large-scale stationary energy storage [1,2]. In the latest two decades, intensive research efforts have focused on the development of new lithium intercalation materials with higher capacity as alternatives to LiCoO2 because of its limited reversible capacity (around 140 mAh g 1) [3]. The layered lithium vanadium oxides, LiV3O8 and its derivatives have received considerable attention as appealing cathode materials for rechargeable LIBs due to their high specific capacity, ease of fabrication, low cost and so on [4–6]. It is well known that the electrochemical properties of vanadates greatly depend on the morphology and synthesis strategy. Up to now, a great deal of methodologies, such as solgel method [7–11], hydrothermal synthesis [12], spray-drying method [6,13] and microwave-assisted synthesis [14] have been developed for high-performance LiV3O8. Recently, ultralong LiV3O8 nanowires with superior high-rate capability and cycling life have
* Corresponding authors. Tel.: +86 731 88830886; fax: +86 731 88879616. E-mail addresses: [email protected] (H. Wang), [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.electacta.2015.01.081 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
been synthesized by Xu et al. using H2V3O8 nanowires as precursor [15]. However, this kind of material still suffers from irreversible phase transformation, deterioration of crystal structure and dissolution of active materials in electrolyte, leading to fast capacity loss during cycling process [16–18]. Surface modification is a very prevalent approach to improve the electrochemical properties of electrode materials via restricting the dissolution of active materials and stabilizing the crystal structure of active materials. A number of efforts have been devoted to LiV3O8 [5,19–26]. Idris et al. [19] reported carboncoated LiV3O8, which could be cycled up to 250 cycles without considerable capacity loss at 20C. Lee et al. [21] showed Cr-coated LiV3O8 with a capacity retention of 89% after 50 cycles at 0.2C. Generally, although the cycling performance was improved, the reversible capacity of modified LiV3O8 was decreased obviously. The discharge capacity of LiV3O8 went down from 335 mAh g 1 to 227 mAh g 1 after carbon coating [19] and from 283 mAh g 1 to 243 mAh g 1 after polyaniline (PAn) coating [5]. In our opinion, the V5+ in LiV3O8 is susceptible to the coating material during the hightemperature sintering process. Accordingly, it is believed that inactive inorganic salts, for examples, metal oxides are more suitable. Among them, Al2O3 has been successfully applied to modify electrode materials like LiMn2O4 [27], LiFePO4 [28], LiCoO2 [29,30]. Al2O3 modified LiV3O8 was once reported in ref. [25] via a
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thermolysis process and the improved electrochemical properties were mainly attributed to the joint action of Al2O3 coating and Li-V-Al-O solid solution at the LiV3O8/Al2O3 interface which could provide a fast Li-ion diffusion path. However, it should be noted that the capacity retention ratio of Al2O3/LiV3O8 after 100 cycles at 2000 mA g 1 was just 71.3%, which was far insufficient. As we know, coating strategy affects the electrochemical properties seriously. In current work, precipitation method was employed to obtain Al2O3-coated LiV3O8. It is interesting to note that a three layered Al2O3/LixV2O5/LiV3O8 nanostructure was well formed, which is much different from that in ref. [25]. When used as a cathode for Li-ion battery, the composite electrode exhibits significantly improved cycling stability with a discharge capacity of 203 mAh g 1 remaining after 200 cycles at 150 mA g 1. At a current density of 300 mA g 1, it can be even cycled up to 500 cycles with a capacity retention of 87.5%. Although Al2O3 coating was once reported for LiV3O8, a special three layered Al2O3/LixV2O5/LiV3O8 nanostructure is highlighted in this work and superior cycling stability is exhibited probably due to its unique hybrid structure. 2. Experimental 2.1. Synthesis of LiV3O8 nanoflakes All the starting materials were analytical reagent grade and used directly without any purification. (NH4)0.5V2O5 precursor was prepared by a typical hydrothermal method. Oxalic acid (2.28 g, 99.5%, Sinopharm Chemical Reagent Co., Ltd.) and NH4VO3 (2.55 g, 99%, Tianjin Guangfu Institute of Fine Chemicals) were first dissolved in distilled water with rapid stirring. Then, the yellowgreen solution was transferred into a 100 ml Teflon lined stainless steel autoclave and heated at 180 C for 12 h. After that, the autoclave was cooled down to room temperature naturally. The precipitate was filtered, washed with distilled water several times and then dried at 80 C overnight. To obtain LiV3O8, a proper amount of (NH4)0.5V2O5 was added into lithium hydroxide (Tianjin Institute of Chemical Reagents) solution (the molar ratio of Li: V in theory is 1: 3) with 0.2 g of PEG4000 as dispersing agent. The mixture was stirred for 2 h at room temperature and then heated at 80 C in a hotplate under stirring to evaporate the distilled water. The collected powder was annealed at 450 C for 8 h in air to obtain the LiV3O8 nanoflakes.
Fig. 1. XRD patterns of the bare LiV3O8 and Al2O3/LixV2O5/LiV3O8 composite.
was treated under vaccum to remove the residual DMC. The operations were carried out in an Ar-filled MBraun glove box. Finally, the whole electrode was used to examine the XRD test and no obvious signal of stainless steel mesh was observed probably due to the thick electrode film. X-ray photoelectron spectroscopy (XPS) measurement was performed on the K-Alpha1063 spectrometer. The XPS patterns were collected using Al Ka radiation at a voltage of 12 kV and current of 6 mA. Charging effect was corrected by adjusting the binding energy of C1s peak from carbon contamination to 284.5 eV. Morphological studies were conducted using a JEOL JEM-2100F transmission electron microscope (TEM), employing a LaB6 filament as the electron source and an accelerating voltage of
2.2. Synthesis of Al2O3/LixV2O5/LiV3O8 composite To prepare Al2O3/LixV2O5/LiV3O8 composite, 0.059 g of Al (NO3)39H2O and 0.020 g of NaOH were separately dissolved in distilled water (the coating content in theory is 2 wt%). Then 0.400 g of LiV3O8 powder was dispersed in Al(NO3)3 solution with constantly stirring by a magnetic force stirrer for 2 h and then the as-prepared NaOH solution was dropped into LiV3O8/Al(NO3)3 suspension slowly. After stirred for 4 h, the temperature was increased to 50 C to precipitate Al3+ completely. The as-obtained precipitate after filtering was dried at 80 C overnight and further calcined at 450 C for 6 h under air atmosphere. 2.3. Characterizations X-ray diffraction (XRD) data were examined by the X-ray diffractometer (DX-2700, Dandong Haoyuan) utilizing a CuKa1 source with a step of 0.02 . For the XRD test of cycled electrode, the cells were first disassembled and the electrodes were soaked in dimethyl carbonate (DMC) for 1 h and then rinsed several times with DMC to remove the electrolyte. Then the electrode film consisting of active material, Super P carbon and polytetrafluoroethylene (PTFE) pressed on the stainless steel mesh
Fig. 2. XPS of Al2O3/LixV2O5/LiV3O8 composite: (a) survey spectrum; (b) highsolution V2p.
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200 keV. The high resolution TEM (HRTEM) images were obtained from JEOL JEM-2100F TEM.
electrochemical station (Shanghai Chenhua, China) with a scan rate of 0.1 mV s 1 at room temperature.
2.4. Electrochemical measurements
3. Results and discussion
The electrodes were fabricated by mixing the active material, polyvinylidene fluoride (PVDF), and Super P carbon in a weight ratio of 80: 10: 10 using tetrahydrofuran (THF) as solvent. The mixture was stirred for 6 h and then cast onto the Al foil. After solvent evaporation at room temperature, the electrodes were dried at 110 C under vacuum for 12 h. The loading mass of each electrode was 3.0 mg cm 2. The construction of electrodes after different cycles for XRD testing is different from that for electrochemical measurement since some of powder in electrode after cycling tends to exfoliate. To address this issue, the electrodes were fabricated by pressing a mixture of the active material, Super P carbon, and PTFE in a weight ratio of 80:10:10 using distilled water as solvent on a stainless steel mesh collector firstly and then dried at 110 C under vacuum for 8 h. The electrodes were assembled into CR2016 coin-type cells with commercial electrolyte (Guangzhou Tinci Materials Technology Co., Ltd; 1 M LiPF6 in 1:1 v/v ethylene carbonate/dimethyl carbonate) and a Li metal as counter electrode. The cells were constructed in an Ar-filled MBraun glovebox and then cycled galvanostatically between 1.5 and 4.0 V (versus Li+/Li) at a desired current density using a Neware battery testing system (CT-3008W) at room temperature. Cyclic voltammetry (CV) test was carried out using the CHI 660D
The XRD patterns of bare LiV3O8 and Al2O3/LixV2O5/LiV3O8 composite are displayed in Fig. 1. As seen, the main diffraction lines of the bare LiV3O8 can be well indexed to the standard monoclinic LiV3O8 (JCPDS card No. 72-1193, space group: P21/m). The indexed lattice parameters of a = 0.66931 nm, b = 0.3597 nm, c = 1.2085 nm and b=108.03 are well consistent with those of previous literature [31–34]. There is no other impurity diffraction line, indicating the high purity of LiV3O8. In the case of the modified LiV3O8, although LiV3O8 phase is maintained, two differences should be pointed out. One is the intensity decrease of the diffraction lines, particularly (1 0 0) peak, which may have a relationship with the coating and annealing process [24]. According to previous studies [35,36], the intensity decrease of (1 0 0) peak indicates the shortening of diffusion paths for lithium ions inserting between the (1 0 0) planes, which will be advantageous to the electrochemical properties of electrode. The other is the appearance of impurity lines as marked with asterisks, which are basically indexed to Li0.3V2O5 (JCPDS card No. 73-1670). Since it is very difficult to ensure the real amount of LixV2O5 in composite, the impurity phase is written as LixV2O5 (x0.3) for more accuracy. It is worthy to highlight that the bare LiV3O8 in ref. [25] was accompanied with certain Li0.3V2O5. While in our work, the Li0.3V2O5 impurity
Fig. 3. (a) TEM images of LiV3O8; (b) TEM and HRTEM (c, d, f) of Al2O3/LixV2O5/LiV3O8 composite; (f) EDX of Al2O3/LixV2O5/LiV3O8 composite.
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Fig. 4. Schematic model of the as-formed Al2O3/LixV2O5/LiV3O8 composite.
appeared after annealing. So the effect of LixV2O5 impurity to the bare LiV3O8 is much different. It is sure that partial reduction of superficial LiV3O8 to LixV2O5 happened during the sintering process. The lattice parameters of modified LiV3O8 are a = 0.6647 nm, b = 0.3605 nm, c = 1.1909 nm and b=107.85 . And the cell volume is 0.2716 nm3, less than that of the bare LiV3O8 (0.2776 nm3), indicating the contracted crystal volume after an annealing process. Note that there is no peak of Al2O3 owing to the low content and amorphous feature of Al2O3 as shown in HRTEM. A similar phenomenon was also reported in Al2O3-coated LiMn2O4 [27]. XPS analysis was employed to further investigate the surface chemical composition and vanadium valance state of as-modified LiV3O8. As revealed in XPS survey spectrum (Fig. 2a), the element signals of vanadium, oxygen, lithium and aluminum are clearly found. The appearance of aluminum and oxygen indicates the presence of Al2O3 on the surface of LiV3O8. Fig. 2b is the high resolution of V2p after fitting. Peak at 517.4 eV and 524.6 eV are attributed to the spin-orbit splitting of V5+2p3/2 and V5+2p1/2, which are well consistent with the valence in references [37,38]. More importantly, the peak of V4+ is observed at 516.4 eV, though it is of less intensity, confirming the existence of LixV2O5 as demonstrated by XRD. XPS results futher imply the coexistence of Al2O3 and LixV2O5 in the surface of modified LiV3O8 sample. The morphology of LiV3O8 and Al2O3/LixV2O5/LiV3O8 composite are shown in Fig. 3. The bare LiV3O8 (Fig. 3a) is of nanoflake morphology with very smooth surface. For Al2O3 coated one (Fig. 3b), the surface is apparently coated with small particles. The HRTEM images of Al2O3 coated LiV3O8 are demonstrated in Fig. 3(c, d, e). It is interesting to note that a clear three-layer structure is observed. In Fig. 3c, the inner host with periodic fringe space of 0.381 nm agrees well with the interplanar spacing between {0 0 3} planes of monoclinic LiV3O8. The interplanar spacing of middle layer is 0.366 nm, which could be indexed to the {11-1} plane of LixV2O5. There is no doubt that the outer amorphous layer is Al2O3. A similar structure is also observed in Fig. 3d, the fringe space of 0.293 nm in middle layer matches well with the interplanar spacing between {30-4} planes of LixV2O5 and the interplanar spacing of 0.262 nm in inner layer could be indexed to the {-113} planes of monoclinic LiV3O8. Fig. 3f shows the EDX pattern of Al2O3/LixV2O5/LiV3O8 composite. Elements of vanadium, oxygen and aluminum are detected, in agreement with the XPS results. Thus, combining the XRD, XPS and TEM results, it is sure that a three-layered Al2O3/LixV2O5/LiV3O8 appears. Schematic model of the as-formed Al2O3/LixV2O5/LiV3O8 composite is proposed in Fig. 4. LiV3O8 nanoflakes are coated by a LixV2O5 nanolayer firstly and then amorphous Al2O3. The thickness of both coating layer are 5–10 nm. As we know, Al2O3 layer can act as a shield to suppress the irreversible phase transformation and protect the inner active
materials from direct contacting with electrolyte [25]. According to our previous work, a proper thickness of LixV2O5 was effective to facilitate the interfacial electrochemical properties of LiV3O8 electrode [38]. Therefore, the three layered Al2O3/LixV2O5/LiV3O8 composite is expected to achieve superior electrochemical performance. Fig. 5 gives the CV curves of bare LiV3O8 (a) and Al2O3/LixV2O5/ LiV3O8 composite (b) at a scan rate of 0.1 mV s 1. There are three anodic peaks at 2.48, 2.90 and 3.71 V for the bare LiV3O8. Four obvious peaks at 2.39, 2.60, 2.68 and 3.61 V are observed in the cathodic scan. According to previous literature, the peaks at 2.68 and 2.60 V correspond to Li-ion insertion in the empty tetrahedral site with a single-phase reaction. While the peak around 2.39 V may relate to the two-phase transformation of Li3V3O8/Li4V3O8 [15]. The appearance of multi-peaks is ascribed to the complicated Li-ion insertion/extraction process in LiV3O8 [15,33], which is also a normal phenomenon for vanadate
Fig. 5. Cyclic voltammetry (CV) curves of the bare LiV3O8 (a) and Al2O3/LixV2O5/ LiV3O8 composite (b) at 0.1 mV s 1.
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Fig. 6. (a) Cyclic performance of the bare LiV3O8 and Al2O3/LixV2O5/LiV3O8 electrode at 0.5C-rate (150 mA g 1) between 1.5 and 4.0 V; (b) The corresponding charge-discharge curves at different cycles (1st, 50th, 100th, 150th, 200th).
compounds, such as NaV3O8 [39], Na2V6O16xH2O [40], NH4V3O8 [41,42]. Note that the obvious peak area loss in the first three curves implies the instability of the bare LiV3O8. Al2O3/LixV2O5/ LiV3O8 composite shows much different electrochemical behaviors in comparison with the bare one. During the cathodic scan, four split peaks at 2.48, 2.70, 2.79, and 2.90 V are observed. For anodic scan, the original peak of the bare LiV3O8 at 2.90 V splits into two peaks at 2.89 and 2.97 V. Moreover, a broad peak at 3.32 V appears. Obviously, the differences of redox peaks further confirm the slight structure change of LiV3O8 after Al2O3 coating. It is worth noting that Al2O3/LixV2O5/LiV3O8 electrode shows a much smaller area variation than the bare LiV3O8 in the initial three curves, in good agreement with the better cycling performance as shown in Fig. 6a. Fig. 6a compares the cycling performance of Al2O3/LixV2O5/ LiV3O8 electrode with the bare LiV3O8 at 0.5 C. As seen, the bare LiV3O8 shows an initial discharge capacity of 244.7 mAh g 1. However, it sharply drops to 192.9 mAh g 1 after ten cycles and then keeps stable for about 40 cycles. After that, the capacity is deceased to 126.9 mAh g 1 after 200 cycles. The quick capacity loss in the initial cycles has been reported for LiV3O8 [14,23,43]. LiV3O8 with an initial discharge capacity of 176 mAh g 1 at 1C was reported in ref. [43], but it faded to 125 mAh g 1 only after 15 cycles. According to previous reports, the capacity fading may have relationships with the dissolving of vanadium, irreversible phase transition between LiV3O8 and Li4V3O8, deterioration of crystal structure and so on [18,44,45]. For Al2O3/LixV2O5/LiV3O8, although a lower initial capacity of 170 mAh g 1 is delivered, it is gradually increased to 211.9 mAh g 1 at the 36th cycle with a discharge capacity of 203.3 mAh g 1 maintaining after 200 cycles. Comparatively, the modified electrode exhibits significantly improved cycling stability. Actually, the capacity increasing in the initial stage is not an accidental phenomenon [4,37,46].
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A similar capacity rising in the initial cycles were observed for VO2(B) [46], which was probably due to the relaxation of the nearly amorphous structure to create a smooth pathway during Li-ion insertion and extraction. In the present case, it could be partially due to the relaxation of contracted crystal volume. As shown above, the cell volume of modified LiV3O8 contracted after the annealing process. Generally, there are octahedral sites and tetrahedral sites in LiV3O8 crystallite, and the pristine Li-ions occupy the octahedral sites. The migrated Li-ions after the discharge process are accommodated at the tetrahedral sites [47]. Accordingly, the crystal shrink will decrease the available accommodation sites for migrated Li-ions slightly. With the cycles increasing, the crystal structure relaxes and the pathways of lithium insertion and extraction become more and more smooth, leading to a capacity increase. A similar discharge capacity rising was reported for high-temperature sintered NaV3O8 nanoflakes [48]. In addition, the activation of multi-layered morphology was considered as another reason. As we know, the outer tight and nonactive Al2O3 layer will hinder the migration of Li-ions to some extent and not all the active material could take part in the reaction rapidly during the initial cycling. With the cycle number increasing, more active materials may be involved due to electrochemical grinding, leading to the increase in capacity [19]. Carbon-PPy-coated LiFePO4 [49], carbon-coated LiV3O8 [19] and PPy-coated LiV3O8 [20] exhibited similar phenomena. To the best of our knowledge, the cycling performance of Al2O3/ LixV2O5/LiV3O8 composite here is superior to most of surface modified LiV3O8 like Co0.58Ni0.42 oxide-coated LiV3O8, AlPO4coated LiV3O8, PPy-coated LiV3O8, carbon coated LiV3O8 and so on [5,19,20,22–24]. PAn-coated LiV3O8 has been synthesized via chemical oxide polymerization [5] and the discharge capacity of the composite after 100 cycles at 0.1C was 204 mAh g 1. LiV3O8-PPy composite with a specific capacity of 183 mAh g 1 maintaining after 100 cycles at 40 mA g 1 was reported in ref. [20]. A discharge capacity of 194 mAh g 1 after 100 cycles at 0.2C was also reported for carbon coated LiV3O8. In our work, with no doubt, the protection of Al2O3 coating layer should be the key reason for the improvement of electrochemical properties. Meanwhile, the middle LixV2O5 layer should be another important reason. It is well known that LixV2O5 has a much higher Li-ion diffusion coefficient (10 10 cm2 s 1) [38] than LiV3O8 (10 13 cm2 s 1) [50]. So a proper thickness of LixV2O5 layer between Al2O3 and LiV3O8 layer might be beneficial to the electrochemical interfacial properties of LiV3O8. Our latest work could well support such deduction. LixV2O5/LiV3O8 composite was fabricated by a facial H2 treatment of LiV3O8 and the composite demonstrated significantly improved electrochemical properties [31]. The results revealed that the surface LixV2O5 with certain thickness could not only protect the
Fig. 7. CV curves of Al2O3/LixV2O5/LiV3O8 composite electrode after 200 cycles. The scan rate is 0.1 mV s 1.
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Fig. 8. XRD patterns of Al2O3/LixV2O5/LiV3O8 composite electrode after different cycles (5th, 200th) at 0.5C-rate during 1.5 4.0 V. Fig. 9. Long cycling performance of Al2O3/LixV2O5/LiV3O8 composite electrode at 1C-rate. Before the cycling, the electrode was first activated for several cycles at 0.1C-rate.
internal LiV3O8 from dissolution, but also increase the Li-ion diffusion coefficient and suppress the charge-transfer resistance [31]. Therefore, the improved cycling stability of Al2O3/LixV2O5/ LiV3O8 electrode could be attributed to the double protection of Al2O3 and LixV2O5 layers. Such a novel three-layered structure could provide more rigorous protection than simple single protection. Fig. 6b shows the charge-discharge curves of Al2O3/ LixV2O5/LiV3O8 composite. As observed, the plateaus around 2.70 V in the charge curves decrease obviously, while the plateau around 3.25 V increases. For the discharge curves, a plateau around 2.1 V appears with cycling number increasing. The variations of the plateaus imply possible structure rearrangement during the cycling, which was also observed in our previous paper [31]. The CV curves of Al2O3/LixV2O5/LiV3O8 composite electrode after 200 cycles are presented in Fig. 7. As seen, the anodic peak at 3.32 V after 200 cycles become stronger in comparison with fresh Al2O3/LixV2O5/LiV3O8 composite electrode and a broad new peak around 2.1 V is observed in the cathodic scan. The variations in CV curves are in good agreement with the change of charge-discharge plateaus, implying the appearance of structure rearrangement during the cycling. XRD patterns of Al2O3/LixV2O5/LiV3O8 electrode after different cycles (5th, 200th) at 0.5C-rate were investigated (Fig. 8). The electrode consisting of active material, carbon and binder was directly used for structure measurement. As displayed, both electrodes could be indexed to the LiV3O8 phase, associated with a slightly amount of LixV2O5. Meanwhile, both electrodes indicate almost the same diffraction lines, implying the absence of phase transition or structure deterioration during the cycling. The lattice parameters of Al2O3/LixV2O5/LiV3O8 composite electrodes after different cycles are compared in Table 1. The lattice parameters of Al2O3/LixV2O5/LiV3O8 composite electrode after 5 cycles are a = 0.6658 nm, b = 0.3583 nm, c = 1.2022 nm and b=107.99 , matching well with the hybrid powder. Compared with electrode after 5 cycles, the change ratio of lattice parameters of hybrid electrode
after 200 cycles is less than 1.5%. Tiny variation indicates good structure stability of Al2O3/LixV2O5/LiV3O8 composite. According to previous reports [18], an important reason for capacity fading of LiV3O8 was the local damage of crystal structure caused by drastic change in crystal lattice parameters. In this work, apparently, the excellent structure stability of Al2O3/LixV2O5/LiV3O8 composite benefits from the double protection of Al2O3 and LixV2O5 layers. To get further insights into the superior stability of Al2O3/ LixV2O5/LiV3O8 composite, cycling life up to 500 cycles at 1C-rate was investigated (Fig. 9). An initial discharge capacity of 161 mAh g 1 is exhibited and the capacity gradually increases to 181.3 mAh g 1 firstly and then stabilizes at 160–170 mAh g 1. The capacity variation tendency matches well with that at 0.5 C. After 500 cycles, about 87.5% of the initial capacity was maintained. Superior cycling stability further confirms that the double protection of Al2O3 and LixV2O5 layers is effective to improve the cycling stability of LiV3O8. 4. Conclusions In summary, a three layered Al2O3/LixV2O5/LiV3O8 nanostructure was constructed and it was well characterized by XRD, HRTEM and XPS. Electrochemical results showed that the hybrid electrode possessed significantly improved cycling performance in comparison with the bare LiV3O8. A specific discharge capacity of 203 mAh g 1 was maintained after 200 cycles at a current density of 150 mA g 1 for Al2O3/LixV2O5/LiV3O8 electrode. When the current density increased to 300 mA g 1, the electrode was able to be cycled up to 500 cycles with capacity retention of 87.5%. XRD and CV results confirmed that the superior cycling performance was mainly due to the double protection of Al2O3 and LixV2O5 in such unique three-layered composite. It is noted that LixV2O5 middle layer could be beneficial to the improvement of rate performance for the composite. Acknowledgements
Table 1 The calculated parameters of Al2O3/LixV2O5/LiV3O8 composite electrodes after different cycles. Samples
a/nm
b/nm
c/nm
b
Electrode after 5 cycles Electrode after 200 cycles Variation ratio
0.66584 0.67555 1.46%
0.35828 0.35657 0.04%
1.20218 1.21270 0.88%
107.99 108.94 0.88%
Financial supports from the National Nature Science Foundation of China (No. 21301193), Hunan Provincial Natural Science Foundation of China (No. 14JJ3022), Project of General Administration of Quality Supervision,Inspection and Quarantine of P.R China (2014IK189), the Opening Project of Material Corrosion and Protection Key Laboratory of Sichuan province (2014CL03) and the Open-End Fund for Valuable and Precision Instruments of Central South University (CSUZC2014018) are greatly appreciated.
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