Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/NanoLett
Mg2B2O5 Nanowire Enabled Multifunctional Solid-State Electrolytes with High Ionic Conductivity, Excellent Mechanical Properties, and Flame-Retardant Performance Ouwei Sheng, Chengbin Jin, Jianmin Luo, Huadong Yuan, Hui Huang, Yongping Gan, Jun Zhang, Yang Xia, Chu Liang, Wenkui Zhang, and Xinyong Tao* College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China S Supporting Information *
ABSTRACT: High ionic conductivity, satisfactory mechanical properties, and wide electrochemical windows are crucial factors for composite electrolytes employed in solid-state lithium-ion batteries (SSLIBs). Based on these considerations, we fabricate Mg2B2O5 nanowire enabled poly(ethylene oxide) (PEO)-based solid-state electrolytes (SSEs). Notably, these SSEs have enhanced ionic conductivity and a large electrochemical window. The elevated ionic conductivity is attributed to the improved motion of PEO chains and the increased Li migrating pathway on the interface between Mg2B2O5 and PEO-LiTFSI. Moreover, the interaction between Mg2B2O5 and −SO2− in TFSI− anions could also benefit the improvement of conductivity. In addition, the SSEs containing Mg2B2O5 nanowires exhibit improved the mechanical properties and flame-retardant performance, which are all superior to the pristine PEO-LiTFSI electrolyte. When these multifunctional SSEs are paired with LiFePO4 cathodes and lithium metal anodes, the SSLIBs show better rate performance and higher cyclic capacity of 150, 106, and 50 mAh g−1 under 0.2 C at 50, 40, and 30 °C. This strategy of employing Mg2B2O5 nanowires provides the design guidelines of assembling multifunctional SSLIBs with high ionic conductivity, excellent mechanical properties, and flame-retardant performance at the same time. KEYWORDS: Solid-state lithium ion batteries, Mg2B2O5 nanowires, mechanical property, flame-retardant performance near 10−6 S cm−1 at room temperature and relatively poor security compared with sulfide and oxide electrolytes. However, the advantages of polymer electrolytes such as reliable stability, low interfacial resistance, flexibility, stretchable property, and low fabricating cost are all very important for assembling the commercial SSLIBs.2,15,28−30 Moreover, the polymer electrolyte also has relative stability to pair with Li metal anode. Therefore, many scientists have been devoted to the research of polymer electrolytes and aimed to improve the ionic conductivity and mechanical properties of SSEs.15,31,32 Methods such as introducing inorganic nanomaterials, adding plasticizer, crosslinking, and block copolymerization are all beneficial to optimizing the performance of polymer electrolytes.33−35 The design of introducing nanomaterials to SSEs began in 1982. Weston and Steele reported improved properties of polymer electrolyte upon the addition of an inert filler.36,37 Numerous research has been done since then, such as adding Al2O3,38 TiO2,38,39 ZnAl2O4,40 Fe2O3,41 CeO2,42 SiO2,39 halloysite nanoclay (HNT),43 et al. to polymer electrolyte. The
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raditional lithium-ion batteries (LIBs) using liquid electrolyte are widely used in daily life as power devices for electric vehicles, mobile phones, hand-held computers, et al.1−6 The liquid electrolyte usually consists of salts such as LiPF6 and organic solvents such as ethylene carbonate (EC) and dimethyl carbonate (DMC),7,8 which suffer from a safety hazard due to the volatility and combustion of organic solvent.9,10 Recently, frequent accidents such as the combustion of electrical automobiles and the explosion of mobile phone all urge scientists to optimize the safety of the battery. Distinguished from the leakage and combustion of organic liquid electrolyte, solid-state electrolytes (SSEs) including sulfide, oxide, and polymer with relatively satisfactory safety have thus have caught people’s eyes as promising nextgeneration battery technology.11−21 Sulfide and oxide electrolytes have extremely high ionic conductivity near 10−2 or 10−3 S cm−1 at room temperature.22−26 However, the undesired nature of these two electrolytes hinders their practical application. Sulfide is unstable in air and easy to react with moisture, causing the decomposition and release of poisonous byproducts. As for oxide electrolytes, the extremely large interfacial resistance restricts their large-scale industrialization.27 Polymer electrolytes such as PEO may show low conductivity © XXXX American Chemical Society
Received: February 14, 2018 Revised: March 22, 2018
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DOI: 10.1021/acs.nanolett.8b00659 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic illustration of the synthetic route of Mg2B2O5 nanowires. Characterization of the Mg2B2O5 nanowires. (b) Typical XRD spectra. (c) SEM image. (d, e) TEM images. (f) SAED pattern and (g) high-resolution TEM image.
Figure 2. (a) Ionic conductivity of composite SSEs with different Mg2B2O5 additives (0, 5, 10, 15, and 20 wt %). (b) The mechanical property (stress−strain curves) of different composite SSEs with various Mg2B2O5 additives. (c) Flammability tests of PEO-LiTFSI, PEO-LiTFSI-10 wt % Mg2B2O5, and PEO-LiTFSI-20 wt % Mg2B2O5 composite SSEs with a burning torch heater. (d) The XRD patterns of SSEs residue (0, 10, and 20 wt % Mg2B2O5) after burning tests.
gradually being considered.18,44,45 Cui’s group reported that the conductivity enhancement of the polyacrylonitrile-Y2O3-doped
performance is improved to some degree. Meanwhile, the uncertain mechanism of ion transport in polymer electrolyte is B
DOI: 10.1021/acs.nanolett.8b00659 Nano Lett. XXXX, XXX, XXX−XXX
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polymer matrix will lead to the aggregation and free-volume depletion of the nanowires, causing the decrease of ionic conductivity. These two phenomena would further reduce the possibility of forming the ionic percolation.19,45 The structural stability of the composite SSEs is also very important and is closely related with the cycle life of battery. Thus, the mechanical properties measurements based on strain−stress curves were conducted. The film used in this test was controlled within the length of 18.7 mm, width of 10.0 mm, and thickness of 0.22 mm. From Figures 2b and S2c, it is found that the maximum stress and strain is 1.31 MPa and 940% for SSEs without Mg2B2O5 nanowires (Figure S2c). When Mg2B2O5 nanowires are added to composite SSEs, the stresses of various samples are all increased (Figure S3). This indicates that the introduction of Mg2B2O5 nanowires really improves the mechanical properties of SSEs. Among all the samples, SSEs film with 5 wt % Mg2B2O5 has the largest maximum stress of 2.29 MPa. When the content of Mg2B2O5 keeps increasing, the maximum stress decreases. In addition, the elongation of composite SSEs containing small quantity (5 and 10 wt %) Mg2B2O5 nanowires were enhanced in comparison to PEO-LiTFSI electrolyte. However, when the Mg2B2O5 nanowires quantity reaches 15 and 20 wt %, the elongation was decreased. This may be related with the aggregation of Mg2B2O5 nanowires, which increases the tortuosity of the electrolyte.18,50 The digital photos of composite SSEs containing 10 wt % Mg2B2O5 nanowires before and after stretching are shown in Figure S2d. This photo visual proof than enhancement of mechanical properties can be attributed to the introduction of Mg2B2O5 nanowires. Flammability tests of liquid electrolyte and SSEs with or without Mg2B2O5 were compared in Figures 2c and S4. The liquid electrolyte was typical LiPF6 in EC/DMC (v/v = 1:1), which burned immediately and fiercely as in touch with the lighter.51 The PEO-LiTFSI electrolyte began burning with small fire after being close to the burning torch heater for 30 s, indicating the superior security of SSEs against liquid electrolyte. When we remove the burning torch heater, the fire quenched immediately. After the burning test, the PEOLiTFSI electrolyte film could not maintain its original shape and became a kind of sticky liquid. In contrast, the PEOLiTFSI-Mg2B2O5 electrolyte can hardly burn. Furthermore, as the content of Mg2B2O5 nanowires increased, the properties of flame-resistance also improved (Figure 2c). This is because the carbon protective layer against heat and fire was formed during the burning process. And the presence of Mg2B2O5 not only benefits the formation process of carbon but also increases the stability of the carbon layer. Therefore, it prevents heat from transferring and being passed into SSEs.52,53 Although the flame-retardant performance of polymer electrolyte additives, such as Mg(OH)2, has been studied in the previous literature,54 for succinonitrile flame retardant,55 no universal mechanism has been proposed and proven. It is supposed that the flameretardant mechanism of Mg2B2O5 is related with promotion of the formation of protective carbon layer.52,53,56−58 To confirm this assumption, we performed XRD to characterize the chemical component of various SSEs samples after the burning test. As can be seen from Figure 2d, the pure PEO-LiTFSI only delivered the peaks of PEO and an extremely weak peak of amorphous carbon. As for the SSEs with 10 wt % Mg2B2O5, the sample after burning showed the peaks of both Mg2B2O5 and PEO. Notably, an obvious peak of amorphous carbon appeared. When the amount of Mg2B2O5 increased to 20 wt %, the
ZrO2 (PAN-YSZ) electrolyte is due to the presence of positivecharged oxygen vacancies on the YSZ surfaces.45 The aligned Li0.33La0.557TiO3 (LLTO) nanowires could provide a fast ion transport channel without crossing junctions.18 Fruitful research results all shed light on optimizing the comprehensive performance of polymer electrolyte and gradually exploring the mechanism through various in situ and ex situ characterization methods. Mg2B2O5 nanowires, which have cheap fabricating cost and a facile preparing process, are also lightweight, high-tenacity, have high wear resistance, and are flame-retardant.46,47 Therefore, it is widely used as composite strengthening materials, membrane materials, flame-retardant materials, insulating materials, et al. In 2008, we synthesized a kind of low-cost Mg2B2O5 nanowires with remarkable mechanical properties and that exhibited a hardness of 15.4 GPa and a Young’s modulus of 125.8 GPa.46 Considering all of these outstanding characteristics, we fabricate Mg2B2O5 nanowires with excellent performance and originally employed them in the poly(ethylene oxide) (PEO)based SSEs as additive, which effectively improves the ionic conductivity, increases the mechanical properties, and enhances the flame-retardant performance of the SSEs. In addition, we investigate the mechanism of improved lithium ion transport in SSEs. When the multifunctional composite SSEs are used, the SSLIBs cycled at 30−50 °C with different current densities could deliver enhanced rate performance and cyclic stability. Hence, this novel multifunctional additive and the Li migration mechanism within these nanowires embedded system may provide SSEs design criteria for constructing high-performance SSLIBs with satisfactory security. The schematic diagram illustrates the synthetic route of Mg2B2O5 nanowires is shown in Figure 1a. These nanowires were one-step prepared by mixing MgCl2·6H2O, Na2B4O7· 10H2O, KCl, and NaCl with a certain proportion. After grinding uniformly, the mixture was calcined at 800 °C for 6 h. The typical XRD pattern of the obtained Mg2B2O5 powder is shown in Figure 1b. All of the diffraction peaks fit well with the Mg2B2O5 (PDF no. 86-0531), indicating the high quality of the obtained sample.46 Figure 1c show the SEM morphologies of the Mg2B2O5, which reveals Mg2B2O5 as uniform smooth, straight nanowires. The average diameter is about 270 nm. This can be observed from TEM images in Figure 1d, e. The detailed diameter distribution of these nanowires is evaluated and presented in Figure S1. The selected area electron diffraction (SAED) pattern (Figure 1f) and high-resolution TEM image (Figure 1g) indicate the Mg2B2O5 nanowires have high crystallinity. Meanwhile, the interplanar spacing of 0.25 nm were calibrated from lattice fringes in Figure 1g, which is consistent with standard value of ∼0.25 nm for (112) plane of Mg2B2O5 nanowires (PDF no. 86-0531). Ionic conductivity is a crucial parameter to assess the Li+ ion migration ability in composite SSEs.48,49 To obtain high ionic conductivity of SSEs with different Mg2B2O5 additive contents, AC impedance spectroscopy measurements with two stainlesssteel electrodes were conducted, and the results are presented in Figures 2a and S2a. Compared with the SSEs without Mg2B2O5 nanowires, the ionic conductivity of composite SSEs with Mg2B2O5 (5, 10, 15, and 20 wt %) were all increased. Among all of the composite SSEs, the sample including 10 wt % Mg2B2O5 nanowires has the highest ionic conductivity of 1.53 × 10−4 S cm−1 at 40 °C and 3.7 × 10−4 S cm−1 at 50 °C (Figure S2a). The nonmonotonic behavior of conductivity is understandable because excessive Mg2B2O5 nanowires filling in C
DOI: 10.1021/acs.nanolett.8b00659 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 3. (a) Schematics of lithium ion migration in Mg2B2O5 enhanced composite SSEs. (b) FTIR spectra of the PEO, LiTFSI, Mg2B2O5, PEOLiTFSI, and PEO-LiTFSI-Mg2B2O5 at 4000−600 cm−1. (c) FTIR spectra of the PEO-LiTFSI and PEO-LiTFSI-Mg2B2O5 at 1400−1160 cm−1.
−SO2− group in the TFSI− anion, which indicated the enhanced dissociation of lithium salts to release more Li+ and further improved ionic conductivity. Second, the additive Mg2B2O5 nanowires could reduce the crystallization degree of the PEO and inhibit the recrystallization of PEO chains, which will enhance the segmental motion of the polymer and further promote the migration of lithium ions. Third, the added Mg2B2O5 nanowires enable the migration of lithium ions in the two phase interfaces between electrolyte and Mg2B2O5 due to the existence of abundant Lewis acid sites. The second and third aspect will be discussed combining with the later DSC and XRD results in the article. To confirm that the additive Mg2B2O5 nanowires could reduce the crystallization degree of the PEO as mentioned before, the XRD patterns and DSC curves of PEO, PEOLiTFSI film, and PEO-LiTFSI-10 wt % Mg2B2O5 film are demonstrated in Figure 4a,b. In Figure 4a, the PEO has two diffraction peaks at around 19° and 23°. In comparison to LiTFSI, the diffraction peak intensity of PEO is obviously reduced, and the diffraction peaks of LiTFSI are not observed in PEO-LiTFSI film. This indicates that no LiTFSI particles exist and that the crystallinity degree of PEO is decreased.19 In comparison, the composite SSEs containing 10 wt % Mg2B2O5 with SSEs without Mg2B2O5, the peaks at 19° and 23° of PEO are decreased to some degree, and the diffraction of Mg2B2O5 is obviously observed. It confirms that with the increase of Mg2B2O5, the crystallinity of PEO shows certain decrease. Figure 4b shows the DSC thermograms of the PEO, PEOLiTFSI, and PEO-LiTFSI-10 wt % Mg2B2O5 composite SSEs. The melting transition of bare PEO is near 66.5 °C. After the introduction of LiTFSI, the peak shifted to lower temperature and the shape of the endothermic peak changed.50 The melting transition of PEO-LiTFSI-10 wt % Mg2B2O5 composite electrolyte is nearly 48.3 °C, which is smaller than PEOLiTFSI electrolyte (51.2 °C). This further illustrates that Mg2B2O5 added to SSEs would indeed decrease the crystallinity of PEO and increase PEO segmental mobility. From the XRD
intensity of carbon became stronger, which indicated the existence of Mg2B2O5 indeed played a positive role in the formation process of carbon layer. This result may account for the excellent flame-retardant performance of Mg 2 B 2 O 5 enhanced SSEs. In comparison, all of the SSEs samples with different Mg2B2O5, the composite SSEs adding 10 wt % Mg2B2O5 delivered the highest ionic conductivity as well as the satisfactory mechanical properties and flame-retardant performance. Therefore, the PEO-LiTFSI-10 wt % Mg2B2O5 electrolyte as an optimized sample was selected for further study. These above results may provide a reference for preparing multifunctional SSEs that give consideration to ionic conductivity, mechanical properties, and flame-retardant performance at the same time. As mentioned above, the ionic conductivity of SSEs containing Mg2B2O5 nanowires was effectively increased, which could be explained by three aspects as follows. First, the Lewis acid center of Mg2B2O5 nanowires could interact with lithium salts anion TFSI−, thus weakening the interaction between Li+ and TFSI− and promoting the dissociation of lithium salts to release more Li+ (Figure 3a). To confirm this assumption, a Fourier transform infrared (FTIR) test was performed.59 Figure 3b shows the FTIR spectra of PEO, LiTFSI, Mg2B2O5, PEO-LiTFSI, and PEO-LiTFSI-Mg2B2O5 from 4000 to 650 cm−1, and Figure 3c shows the detailed spectra of PEO-LiTFSI and PEO-LiTFSI-Mg2B2O5 within 1400−1160 cm−1. In Figure 3c, the peaks at 1186, 1205, and 1229 cm−1 can be observed in both PEO-LiTFSI and PEOLiTFSI-Mg2B2O5 electrolytes, which are corresponding to asymmetric stretching and symmetric stretching of −CF3. The other peaks at 1342, 1280, and 1241 cm−1 belonging to PEO are also observed. In addition, the peaks corresponding to asymmetric −SO2− stretching are observed at 1298 and 1327 cm−1 in a PEO-LiTFSI electrolyte. However, there, the peaks shift to 1295 and 1332 cm−1 in PEO-LiTFSI-Mg2B2O5 electrolyte. These shifts of specific peaks in FTIR resulted from the interaction between Mg2B2O5 nanowires and the D
DOI: 10.1021/acs.nanolett.8b00659 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 4. (a) Typical XRD patterns of PEO, PEO-LiTFSI film, and PEO-LiTFSI-10 wt % Mg2B2O5 film. (b) DSC curves of the thermal property of PEO and composite film with and without Mg2B2O5. (c) The chronoamperometry curves of a Li/electrolyte/Li cell under a potential step of 10 mV at 50 °C. “Electrolyte” refers to a PEO-LiTFSI electrolyte or a PEO-LiTFSI-Mg2B2O5 electrolyte. (d) LSV curves of PEO-LiTFSI film and PEOLiTFSI-10 wt % Mg2B2O5 film at a scanning rate of 1 mV s−1 at 50 °C. (e) Voltage−time profiles of Li metal plating and stripping in Li/SSEs/Li cells at 0.1 mA cm−2 and 50 °C. The electrolyte is a PEO-LiTFSI electrolyte or PEO-LiTFSI-10 wt % Mg2B2O5 electrolyte film.
SSEs at 50 °C are shown in Figures 4c and S5, respectively. It can be seen that the PEO-LiTFSI-Mg2B2O5 electrolyte exhibits improved tLi+ of 0.44. In contrast, the tLi+ of PEO-LiTFSI electrolyte is only 0.19. The improvement of tLi+ may be beneficial from the interaction between the Mg2B2O5 and −SO2− groups in the TFSI− anion, which we have proven by FTIR (Figure 3b,c). In addition, the Li diffusion coefficient in PEO-LiTFSI electrolyte and PEO-LiTFSI-Mg2B2O5 electrolyte is near 3.2 × 10−13 and 4.0 × 10−13 m2 s−1 at 50 °C based on PFG NMR results, respectively (Figure S6). It is indicated that there is a small increase of lithium ion migration rate in PEOLiTFSI-Mg2B2O5 electrolyte. Therefore, the enhancement of ionic conductivity in composite electrolyte should result from both the increasing ion migration number and the improving ion migration rate. The increase of ion migration numbers plays a dominant role in the improvement of ionic conductivity. To investigate the stability of SSEs against high voltage, we measured the electrochemical window of various SSEs samples using stainless steel/SSEs (with and without Mg2B2O5)/Li cells at 50 °C (Figure 4d). The electrochemical window of SSEs
and DSC results combining with the ionic conductivity data, it is known that the ionic conductivity of composite electrolyte improved significantly but the melting transition (DSC, Figure 4b) and PEO peaks intensity (XRD, Figure 4a) decreased slightly. These results show that adding Mg2B2O5 to SSEs could, to some extent, promote migration of lithium ions through PEO segmental mobility. However, considering the significant improvement of ionic conductivity for composite SSEs, there must be some other reasons for this great enhancement. This would strongly indicate the increased that the two-phase interfacial (PEO-LiTFSI and Mg2B2O5) lithium ion transport pathway may play a greater role in optimizing ionic conductivity of the composite SSEs. This perspective is consistent with previous works in literature and our previous assumption.60−62 The lithium transference number in two SSEs (PEO-LiTFSI electrolyte and PEO-LiTFSI-Mg2B2O5 electrolyte) was also tested by chronoamperometry (Figure 4c) and calculated. The chronoamperometry curves and AC impedance spectra of the Li/SSEs/Li cells before and after polarization for two different E
DOI: 10.1021/acs.nanolett.8b00659 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 5. (a) SEM images of PEO-LiTFSI electrolyte. (b) SEM images of SSEs with 10 wt % Mg2B2O5. (c) The cross-sectional SEM image of 80 μm thick SSEs. (d) Elemental mappings of LiFePO4 cathode and SSEs.
Figure 6. (a) Typical charge−discharge curves of LiFePO4/Li SSLIBs using PEO-LiTFSI-10 wt % Mg2B2O5 electrolyte at 50 °C. (b) Rate performance of LiFePO4/Li SSLIBs using SSEs with and without Mg2B2O5 at 50 °C. (c) EIS spectra of battery using SSEs with and without Mg2B2O5 at 50 °C. (d) Cycling performance of LiFePO4/Li SSLIBs with PEO-LiTFSI-10 wt % Mg2B2O5 electrolyte at 1.0 C and 50 °C. The inset is a digital photograph of LEDs lightened by SSLIBs.
stripping over-potential of cell use of the PEO-LiTFSIMg2B2O5 electrolyte is near 55 mV at 0.1 mA cm−2 and remains stable during the 650 h of cycling. However, the cell with PEO-LiTFSI electrolyte delivers a much higher overpotential of 100 mV at the first 200 h and continuously increases in the following Li plating/stripping process (Figure 4e). At other current densities, the cell with PEO-LiTFSIMg2B2O5 electrolyte also shows lower over-potential and better cycling stability than that of PEO-LiTFSI (Figure S8). The Li symmetric test confirms that the as-prepared Mg2B2O5-enabled SSEs has a superior stability against Li metal anode. Hence, this electrolyte is able to pair with Li metal anodes and high-voltage cathodes to construct advanced lithium rechargeable battery.
containing 10 wt % Mg2B2O5 nanowires is nearly 4.75 V, which is superior to SSEs without Mg2B2O5 nanowires (4.25 V). The enhancement in electrochemical stability of SSEs over a wide voltage range indicates the applicability of these SSEs to be matched well with high-voltage cathode materials. Meanwhile, the thermal stability of Mg2B2O5-enabled SSEs was also increased, which can be seen in Figure S7. Besides electrochemical and thermal stability, these composite SSEs also could stay stable with Li metal anode and may have a positive effect in inhibiting the uncontrollable formation and growth of Li dendrite.63−67 In Figures 4e and S8, the stability between lithium and SSEs was explored using Li/SSEs (with or without Mg2B2O5) /Li cells at 50 °C. It is seen that the Li plating and F
DOI: 10.1021/acs.nanolett.8b00659 Nano Lett. XXXX, XXX, XXX−XXX
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performance of SSLIBs using LiCoO2 and LiCo1/3Ni1/3Mn1/3O2 cathodes. The specific capacity was 130 and 140 mAh g−1 under 0.2 C at 50 °C, respectively. The results proved that these composite SSEs can also work at low temperatures and match with various cathodes, which again presents the advances of these SSEs for possible commercial application. In summary, the Mg2B2O5 nanowires were directly used as SSEs additives. The FTIR results revealed the interaction between Mg2B2O5 nanowires and −SO2− in the TFSI− anion, which could promote the release and transport of lithium ions, thus improving the ionic conductivity. However, the XRD and DSC results confirmed that the migration of lithium ions was not only via PEO segmental mobility but also via the special lithium ion transport pathway, such as two phase interfaces (PEO-LiTFSI and Mg2B2O5). Hence, the ionic conductivity of SSEs was effectively improved. In addition, due to the high strength and flame-retardant characteristic of the Mg2B2O5 nanowire additive, both the mechanical properties and the flame-retardant performance of SSEs were enhanced. Consequently, the SSLIBs assembled using PEO-LiTFSI-Mg2B2O5 electrolyte could show effectively improved cyclic performance and rate capability with a high specific capacity of 150, 106, and 50 mAh g−1 at the current density of 0.2 C at 50, 40, and 30 °C. This work demonstration of employing Mg2B2O5 in SSEs provides a guidance of high-strength flame-retardant materials design for optimizing SSLIBs, which also opens up opportunities to design and engineer the big family of onedimensional nanostructure metal borate materials for safe SSLIBs.
The morphology and structure of SSEs and electrode (LiFePO4 cathode) is related with their performance, thus SEM and elemental mapping were conducted. Figure 5a,b exhibited the SEM images of PEO-LiTFSI electrolyte and PEOLiTFSI-10 wt % Mg2B2O5 electrolyte, respectively. It was noted that the surface of PEO-LiTFSI electrolyte (Figure 5a) is smooth and homogeneous. After the addition of 10 wt % Mg2B2O5 into PEO-LiTFSI electrolyte, the Mg2B2O5 nanowires distributed randomly in PEO matrix, presenting relatively flat and uniform surface. Similarly, the uniformly distributed Mg2B2O5 nanowires in the polymer matrix can also be observed from the cross-sectional SEM image (Figure S9). In addition, the thickness of the SSEs is among 80−100 μm, as shown in Figure 5c. The elemental mappings of LiFePO4 cathode and SSEs layer were performed and are shown in Figure 5d. Very uniform distributions of element Mg, B, S, C, O, and F in SSE layers and elements Fe and P in the cathode layer could be observed. Notably, we could see that the interface between cathode and SSEs is very good and dense. The electrochemical performance of these SSLIBs using PEO-LiTFSI-Mg2B2O5 or PEO-LiTFSI electrolytes, LiFePO4 cathodes, and lithium anodes is shown in Figure 6a−d. Figure 6a shows the typical discharge and charge curves of SSLIBs with PEO-LiTFSI-Mg2B2O5 electrolyte at different C-rates. The discharge and charge voltage plateaus are around 3.35 and 3.50 V at 0.2 C (Figure 6a), respectively. As the current density increases, the over-potential between discharge and charge plateaus enlarges. Figure 6b presents the rate performance of SSLIBs using different electrolyte (with and without Mg2B2O5 nanowires). The specific discharge capacity of SSLIBs (SSEs with Mg2B2O5) was 158, 140, 124, 117, and 72 mAh g−1 at 0.2, 0.4, 0.8, 1.0, and 2.0 C, respectively. When current density returned to 0.2 C, the discharge capacity could reach 160 mAh g−1. In contrast, the specific discharge capacity of SSLIBs (SSEs without Mg2B2O5) was 139, 121, 92, 75, and 37 mAh g−1 at 0.2, 0.4, 0.8, 1.0, and 2.0 C, respectively. When the current density went back to 0.2 C, the discharge capacity decreased to be only 128 mAh g−1. In conclusion, whatever the capacity or the cyclic stability, the SSLIBs with PEO-LiTFSI-Mg2B2O5 electrolyte are much better than that of battery with pure PEO-LiTFSI electrolyte. EIS of PEO-LiTFSI-10 wt % Mg2B2O5 electrolyte and PEO-LiTFSI electrolyte for SSLIBs were conducted and shown in Figure 6c. The semicircle in the high-frequency region and the intersection at high frequency of a real axis represent the charge-transfer resistance (Rct) and the ohmic resistance (Ro), respectively.68 In Figure 6c, the Rct and Ro of PEOLiTFSI-10 wt % Mg2B2O5 are nearly 478.3 and 317.1Ω, respectively, smaller than that of PEO-LiTFSI (550.1 and 359.1Ω). This result reflects the higher ionic conductivity of PEO-LiTFSI-10 wt % Mg2B2O5, which again confirms well the positive effect of Mg2B2O5 nanowire additives. The cycling performance of SSLIBs at 1.0 C and 50 °C is presented in Figure 6d. The SSLIBs present stable specific capacity nearly 120 mAh g−1 in 230 discharge−charge cycles. The Coulombic efficiency of each cycle is close to 100%. This excellent electrochemical performance is attributed to the excellent conduction properties and electrochemical stability. The inset in Figure 6d would confirm that one such SSE-based SSLIBs could light eight LEDs with the shape of an “I”. In addition, Figure S10 present the electrochemical performance for LiFePO4-based SSLIBs tested at 30−50 °C at 0.2 C. The discharge capacity was 150, 106, and 50 mAh g−1 at 50, 40, and 30 °C, respectively. Figure S11 shows the electrochemical
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b00659. Additional details on synthetic procedures, the preparation of SSEs film and assembly of the SSLIBs, characterization, and electrochemical measurements. Figures showing SEM images, corresponding diameter distributions and average diameters, the conductivity of composite SSEs, impedance spectroscopy, mechanical properties, digital photographs of films, stress−strain curves, flammability tests, AC impedance spectra, NMR spectra, TG curves, voltage−time profiles, electrochemical performance, charge and discharge performance, electrochemical characterization, nanowires, cycle performance, ionic conductivity. A table showing activation energy. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Wenkui Zhang: 0000-0002-6416-6275 Xinyong Tao: 0000-0002-6233-4140 Author Contributions
O.W.S and X.Y.T. conceived the idea and designed the experiments. O.W.S synthesized the materials and performed electrochemical measurement. C.B.J., J.M.L., and H.D.Y. carried out the characterization. W.K.Z., H.H., and other authors G
DOI: 10.1021/acs.nanolett.8b00659 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
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analyzed the electrochemical results. All authors discussed the results and co-wrote the paper. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge financial support by the National Natural Science Foundation of China (grant nos. 51722210, 51677170, 21403196, 51777194, and 51572240), the Natural Science Foundation of Zhejiang Province (grant nos. LD18E020003, LY16E070004, LY18B030008, and LY17E020010), and the Xinmiao Talents Program of Zhejiang Province (grant no. 2017R403081).
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