Lithium Borate Containing Bifunctional Binder To Address Both Ion

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Lithium Borate Containing Bifunctional Binder to Address Both Iontransporting and Polysulfide-trapping for High Performance Li-S Batteries Lei Zhong, Yudi Mo, Kuirong Deng, Shuanjin Wang, Dongmei Han, Shan Ren, Min Xiao, and Yuezhong Meng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09604 • Publication Date (Web): 23 Jul 2019 Downloaded from pubs.acs.org on July 23, 2019

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ACS Applied Materials & Interfaces

Lithium Borate Containing Bifunctional Binder to Address Both Ion-transporting and Polysulfide-trapping for High Performance Li-S Batteries Lei Zhonga, Yudi Moa, Kuirong Denga, Shuanjin Wanga, Dongmei Hanb, Shan Rena, Min Xiaoa*, Yuezhong Menga*

The Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong Province/State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China a

School of Chemical Engineering and Technology, Sun Yat-sen Univeristy, Zhuhai 519082, P. R. China. b

ABSTRACT: A slightly cross-linked lithium borate containing single ion-conducting polymer (LBSIP) as a bifunctional binder for lithium sulfur batteries is designed and fabricated via one-step thiol-ene click reaction. The LBSIP binder exhibits an over 600 mN mm-1 of the maximum peeling strength between sulfur cathode and aluminium foil, together showing a high lithium ions diffusion coefficient of 2.1×10-12 cm2 s-1. Owing to the unique electron-donating groups, the binder can provides a good ionic conductive network and dramatically enhanced polysulfides-trapping feature. The strong interaction between electrondonating groups of LBSIP with Li2S6 was confirmed by 7Li-NMR analysis and densityfunctional theory calculation. The cathode using LBSIP binder exhibited high Coulombic efficiency of ~100%, and initial specific capacity of 712 mAh g−1 with a capacity fading rate of 0.06% per cycle after 500 cycles at 0.5 C. Even at a high current rate of 2 C, the reversible capacity of over 500 mAh g−1 was still obtained. However, the capacity of the cathode using PVDF binder decreased to 331 mAh g-1 after 180 cycles at 0.5 C. This work is very attractive for the rational design of functional binders for Li-S batteries with both ion-transporting and polysulfide-trapping features.

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KEYWORDS: Binder; Lithium borate; Ion-transporting network; Polysulfide; Lithiumsulfur battery INTRODUCTION With the development of electronic products and targeting the improvement of energy density, lithium-sulfur (Li-S) battery affords the compelling blueprint owing to its attractive theoretical energy density (2600 Wh kg-1), inexpensive and environmentally benign feedstock.1-3 Consequently, in the past decades, Li-S battery has received overwhelming attention as one of the most promising candidates for high energy density storage system.4 However, the commercialization process of this system is still impeded by the following challenges: the inferior electrical conductivity of sulfur and final discharge products (Li2S1-2), shuttle effect of polysulfides (Li2S4−8) and deteriorating surface of lithium metal anode.5 Using various carbon materials in cathode is a popular strategy to improve electrical conductivity.6-10 Yu et al.11 reviewed 3D carbon nanofiber in electrochemical energy storage devices. Li et al.12 and Manthiram et al.13 disclosed a carbon-cotton cathode with super high sulfur loading and content by a simple carbonization process. Other strategies, such as adopting heteroatom (N, O, S, B, P) doped carbon materials for sulfur cathodes14-19, modifying separators20, 21 and adding polysulfide shielding interlayer22, were used to suppress the shuttle effect of Li2S4−8 by a synergistic effect of physical adsorption of porous carbon and chemical adsorption of heteroatom doping. In addition, both the electron and ion transporting phases in electrode are of equal significance to improve the electrochemical performance of Li-S battery.23 Therefore, some efforts have been devote to rationally designing various functional materials to achieve the fast electron/ion-transporting and sufficient polysulfides restraining for Li-S battery.24-26 It's worth noting that an effective strategy is to introduce ionic conduction networks in Li-S battery by using functional polymers.27 And lots of polymers have been used as novel type 2


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binders to improve cycling performance due to their high binding strength, good ductility and oxygen-containing functional groups (-OH, -COOH)28, 29. Poly(acryl acid) (PAA, containing R-COO- group)30 as binder in sulfur cathode shows a high capacity of 325 mAh g−1 with capacity retention of 89.5% at the current density of 335 mA g−1 after 50 cycles. Wang et al.31 reported that the guar gum binder with hydroxyl functional group, exhibits excellent electrochemical performance for Li-S battery. And Lin et al.32 prepared a 3D network binder with mechanically robust by the intermolecular binding effect between guar gum (containing R–OH group) and xanthan gum (containing R-COO- group). The 3D network binder affords a high-sulfur-loading electrode (19.8 mg cm-2, Ni foam as current collector). Yan et al.33, 34 also reported that the amino functional group containing binder could effectively reduce Li2S4−8 dissolution and enhance the cycling stability of the battery. Therefore, a high-performance LiS battery can be obtained using effective binder, which could improve the electron/ionic conductivity and weaken the shuttle effect of Li2S4−8. In previous work, we have prepared a single-ion conducting polymer with rich functional groups (-COO- and -C-S-C-), which can show an excellent ionic conductivity (~10−3 S cm−1) as gel electrolyte for LiFePO4 battery.35 Single-ion conducting polymer can be used not only in lithium ion batteries34 but also in Li-S batteries. Moreover, Goodenough et al.36 reported that the polymer with electron-donating groups (-COO-) can provide effective anchoring sites for the Li2S4−8 by forming lithium bonds. In 1959, Shigorin has proposed the lithium bond that is an analogue of hydrogen bond,37 thereafter, Zhang et al.38 also confirmed the lithium bond between electron-donating group and lithium ions by solid state 7Li-NMR. And an excellent functional binder for lithium sulfur battery should have good adhesion, high mechanical strength and conductivity.39 Enlightened by these works, we designed a bifunctional binder (LBSIP) with abundant functional groups (-COO-, -C-O-C- and -C-S-C-), which was synthesized by the one-step thiol-ene click reaction between diene lithium borate (LiBAMB) and dithiol ended monomers (DODT) with tetrathiol compound (PETMA) as 3



crosslinking agent. The unique slightly cross-linked structure of the as-prepared LBSIP endows the sulfur cathode with favorable ions-transporting network and the sufficient polysulfides-trapping sites in Li-S batteries. RESULTS AND DISCUSSION Structure Design and Adhesive Performance The LBSIP binder can be readily prepared by a thiol-ene click reaction, which can be completed by about 40 seconds as shown in the in-situ DSC analysis (Figure 1a). The synthesized LBSIP binder has an uniformly and homogenous cross-linked network attributing to the step growth polymerizations mechanisms of this reaction.40 The schematic synthesis process of LiBAMB monomer is depicted in Figure S1, and the corresponding structure is confirmed by 1H-NMR analysis with the signals of [H2C=CH-] δ = 5.0 and 5.8 ppm as shown in Figure S2. The LBSIP binder in cathode should simultaneously possess superior mechanical and solubility in solvent for easy preparing slurry, whilst insolubility in electrolytes, thus a slightly cross-linked polymeric structure is designed and fabricated via one-step photoinitiated thiol-ene click reaction to meet all these requirements. In Figure S3, 1H-NMR

spectrum shows no signals of [H2C=CH-], but other signals (a-i), demonstrating are

the structural of LBSIP with a slightly cross-linked structure. And LBSIP film is insoluble in electrolyte due to the unique structure (Figure S4). The adhesive performances of LBSIP are characterized with different methods as shown in Figure 1b-e, respectively. It can be seen that the LBSIP binder exhibits more cohesive than PVDF binder at same usage level (Figure 1c), and the LBSIP can easily stick to a 200 g weight, which is a thousand times larger than its own weight (the inset in Figure 1d). Moreover, Figure 1d-e show the excellent adhesion of the LBSIP-based sulfur cathode (LSC), which intuitively reveals that the LSC cathode is more difficult to be scraped off aluminium foil current collector than PVDF-based sulfur cathode (PSC) by using blade to scrape the two 4


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cathodes, respectively. The more details are recorded in Video S1-2. Furthermore, the maximum peeling strength (over 600 mN mm-1) and average peeling strength (over 100 mN mm-1) of the LSC cathode are higher than those of PSC cathode (Figure 1b), as shown in Figure S5. Finally, the surface morphology of both PSC and LSC cathodes were examined with SEM in Figure S6, showing that the former has rough and obvious cracks, while the latter exhibits smoother surface and fewer cracks. These results indicate that LBSIP can be used as an alternative binder when compared with PVDF in sulfur cathode. Theoretical Calculation and Prediction Conceptually, lithium bond theory37 means that electron-rich donors [-COO-, -CO-]36, 41, [-NH- ， -NH2]38, 42 interact with Li2S4-8. Based on the theory, LBSIP binder is expected to provide effectively anchoring sites for Li2S4−8 by forming lithium bonds due to its functional groups of [-CO-], [-C-S-C-] and [-C-O-C-]. The results of density functional theory (DFT) calculations are shown in Figure 2, which can prove the lithium bonds in a theoretical way. The interaction of Li2S6 and [-CO-], [-C-S-C-], [-C-O-C-], [-F] are investigated by both theoretical calculation of atomic configurations and binding energies based on the DFT within the Perdew-Burke-Ernzerhof generalized gradient approximation (GGA-PBE).43-46 Compared with the interaction of Li2S6 with the two binders, a more negative value depicts a stronger interaction of Li2S6 and the LBSIP (-1.84, -1.55, -1.13, and -0.85 eV), indicating that LBSIP has a better ability to capture Li2S6 than PVDF (-0.45 eV). Lithium bonds can be characterized by experimental method by FTIR signals,36 XPS42 and 7Li-NMR38, of which 7Li-NMR spectroscopy is a better approach to accurately explore possible lithium bonds. The slight change in the surrounding environment can affect the chemical shift in 7Li-NMR spectra. In Figure 3, the 7Li-NMR of LiNO3 (20 mg LiNO3 in 0.6 mL THF) was used as a reference (δ =0 ppm). By comparison, the pristine LBSIP shows a 7Li-NMR

signal at δ = -0.07 ppm, whilst the 7Li-NMR signals of fresh Li2S6-8 locate at δ = 5



0.20 and -0.05 ppm. To further investigate the interaction of the two binders with Li2S6-8, LBSIP and PVDF with same usage were added into Li2S6-8 solution and designated as Li2S6-8PVDF and Li2S6-8-LBSIP, respectively. Compared with the 7Li-NMR signal of Li2S6-8, there is an obvious signal shift (Δδ>7.8 ppm) in the Li2S6-8-LBSIP, however, the Li2S6-8-PVDF has a slight change (Δδ ≈ 0.05 ppm). The result indicates that the binding energy of the lithium bond of the Li2S6-8-LBSIP is much stronger than that of the Li2S6-8-PVDF. Combined with above theoretical analysis of DFT calculations and experiment results of 7Li NMR spectra, it could be predicted that LBSIP as a bifunctional binder for Li-S battery, may effectively anchor Li2S4−8 due to its electron-rich donors. Bifunctional Nature of as-synthesized LBSIP Studies showed47, 48 when PVDF was used as a traditional binder in sulfur cathode, there is a large amount of Li2S4−8 dissolved in electrolyte, leading to the serious shuttle effect of Li2S4−8 during the charge/discharge processes. As discussed above, the bifunctional LBSIP binder, the shuttle effect of polysulfides can be inhibited due to the fixation of Li2S4−8 within cathode. A schematic illustration of charge/discharge process in LSC cathode is depicted in Figure 4a. Figure 4b shows the details in which Li2S4−8 are well kept within the cathode by LBSIP. This may also helps to further understand the principle of lithium bonds. The effective capture of Li2S4−8 using LBSIP as bifunctional binder is fully validated by a simple and intuitive experiment as depicted in Figure 5. In a V-shaped visible electrolytic battery with 3 mL 1 M LiTFSI (2% LiNO3) electrolyte, PSC or LSC cathode, lithium anode and a red LED lamp. The visible Li-S battery with the PSC or LSC cathode can readily light up the red LED lamp. The color of electrolyte changed from colourless to orange with PSC cathode after light the red LED. On the contrary, the color of electrolyte changed weakly with LSC cathode after light the red LED for 30 min, and the video details of discharge about one hour is clearly observed in Video S3. The electrolyte with dissolution of polysulfides in the 6


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V-shaped container has been tested by UV-vis absorption spectra, as shown in Figure S7. The UV-vis absorption spectra give obvious characteristic absorption peaks of S82-/S62- between 260-280 nm25, 49, 50 in PSC. By contrast, the characteristic absorption peaks almost disappear in LSC. The UV-vis results further suggest that the “shuttle effect” of Li2S4−8 was greatly suppressed in LSC cathode. Additionally, the polymer with the repeating units of [-C-O-C-] or [-C-S-C-] has the ability to conduct lithium ions.51, 52 In this sense, therefore, LSC cathode is expected to exhibit excellent electrochemical performances due to both the ion-transporting and polysulfides-trapping architecture of the LBSIP. Performance with Bifunctional LBSIP Binder It can be seen from Figure 6 that LSC cathode shows superior electrochemical performance compared with PSC cathode. Figure 6a-b display the charge/discharge curves of batteries with a sulfur content of 70% and the loading of 1.5 mg cm-2 in the PSC and LSC cathodes (aluminium foil as current collector). Discharge curves show two plateaus which corresponds to the two steps reduction of sulfur including S8 to higher-order Li2Sn (n≥4), and lower-order Li2S2 or Li2S during the discharge process.53 At the low current density of 0.05 and 0.1 C, two discharge plateaus are clearly detected at 2.3 and 2.1 V respectively. And the voltage values of the discharge plateaus decrease with increasing the current density because of the high polarizaiton. In addition, an oxidation plateau of 2.22-2.42 V is observed at 0.05 C, which reflects Li2S to the formation of Li2Sn (n>2) and the ultimate oxidation to S8.54 Similarly, the oxidation plateaus also gradually increase with increasing current rate. Comparing Figure 6a with b, PSC cathode has a more serious polarization than LSC cathode with the increase of charge/discharge current rate. Even the lower cut off voltage (1.5 V) is selected to enable an adequate discharge of the PSC cathode, the only 600 mAh g-1 is obtained at 0.5 C. However, LSC cathode reveals a capacity of 712 mAh g-1 with a higher cut off

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voltage (1.7 V) at 0.5 C. And the cells show a high Coulombic efficiency of ~100% with 2% LiNO3 additive in the electrolyte, as shown in Figure S8. Moreover, during discharge of Li-S battery, the electrode reaction may be as follows: solid phase S8 → liquid phase Li2S4-8 → solid phase Li2S1-2. And the liquid-solid phase transition contributes most of the specific capacity (3/4 of the total specific capacity in theory).55 Intermediate products of Li2S4-8 are soluble in DOL/DME (1:1 v/v), which results in fast reaction kinetics for high capacity, whilst leading to severe shuttle effect and inferior capacity retention.56, 57 Therefore, we have calculated the specific capacity ratio (δ) of the two platforms of PSC and LSC by Equation 1, which further proves that LBSIP as a functional binder can inhibit the shuttle effect and improve performance of Li-S battery, and the detail as shown in Figure S9. C1 corresponds to the solid-liquid phase transition, and C2 corresponds to the liquid-solid phase transition. δ = C2/ C1

Equation (1)

Cyclic voltammetry (CV) measurement is used to further investigate stable redox behaviour of the PSC and LSC cathodes. In Figure 6c, the two cathodes have double typical reduction peaks, which are assigned to soluble Li2S4-8 and insoluble Li2S2 or Li2S, and a oxidization peak is attributed to the oxidization of Li2S to Li2Sn (n > 2), respectively. The reduction and oxidization peaks are consistent with discharge/charge plateaus from Figure 6a-b during the lithiation-delithiation process. However, there are also some clearly differences in Figure 6c, which are potential regions of reduction and oxidization peaks between the PSC and LSC cathodes. The two reduction peaks of the PSC cathode at around 2.25 and 1.97 V, and an overlapped oxidization peak at 2.60 V can be seen in the CV curve. But the two reduction peaks of the LSC shift towards high potential region (at 2.29 and 2.01 V), whilst bifurcated oxidation peaks shift to low potential region (at 2.42 and 2.48 V), indicating a smaller polarized of LSC cathode.58 Previous literatures59, 8


60

report that there

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should be two oxidation peaks in CV curve of Li-S battery. The one corresponds to the conversion of Li2S1-2 to Li2S4, another corresponds to the conversion of Li2S4 to Li2S6-8 and to S8. Normally, the two oxidation peaks merge into one when a large polarization of the battery, but the two separate oxidation peaks can be observed when a small polarization. From the Figure 6 in the manuscript, the two oxidation peaks can be clearly observed of LSC, which indicating that the battery has a small polarization. The electrode reaction may be as follows: 2Li2S1-2→Li2S4 +2Li++2e-

Equation (2)

2Li2S4→Li2S6-8 +2Li++2e- and/or Li2S8→S8 + 2Li++2e-

Equation (3)

Nyquist plots of PSC and LSC fresh cells were tested by Electrochemical Impedance Spectroscopy (EIS) to further explain the reaction kinetics as shown in Figure 6d, both PSC and LSC cathodes have a single semicircle at high-to-medium frequency and an inclined line at low-frequency region, respectively. Generally, the single semicircle can be ascribed to the charge-transfer impedance (Rct),61 which expresses the electrode reaction kinetics. The inclined line is Warburg impedance (Wo), which represents lithium ions diffusion process.62 At higher frequency, X-intercept is ohmic resistance (Ro),63 which is currently attributed to the electrolyte and intrinsic electrode impedance, where Ro and Rct are co-contributor for Rtotal of cells.64 As described in Figure 6d and Table S1, the Ro and Rct values of the LSC cathode (3.20 Ω, 74.4 Ω) are smaller than that of the PSC cathode (6.30 Ω, 122.5 Ω). The diffusion coefficient values of lithium ions (

) of the two cells are calculated by Equation (S2)65 to

evaluate the contribution of lithium ions conduction by LBSIP. The relations between Z’ and ω-1/2 for the PSC and LSC cathodes are shown in Figure S10 (the more details can be seen in experimental of supporting information), and the calculated in Table S2. The

values are also summarized

of LSC cathode (2.1×10-12 cm2 s-1) is larger than that of PSC cathode

(1.5×10-12 cm2 s-1), indicating that LBSIP can effectively improve the diffusion capability of lithium ions. This suggests that the bifunctional LBSIP binder in Li-S battery results in a

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lower Rct and higher

values, whilst boosting the transport of electrons and ions as well as

accelerating the reaction kinetics of the LSC electrode. Figure 7a shows the galvanostatic discharge-charge cycling performance of the PSC and LSC cathodes at 0.5 C after several activations (0.05-0.3 C). PSC cathode delivers a higher initial discharge specific capacity of 1134 mAh g-1 than that of the LSC cathode (966 mAh g-1) at 0.05 C. However, when discharge rate increases to 0.5 C, the capacity of PSC cathode decreases to 615 mAh g-1, whilst LSC cathode maintains a higher capacity of 712 mAh g-1. In subsequent cycling, the capacity of PSC decreases to 331 mAh g-1 after 180 cycles with appreciable capacity fading rate of 0.26% per cycle. LSC cathode shows excellent advantages in capacity retention with a reversible capacity of 503 mAh g-1 after 500 cycles, and the decay rate was 0.06% per cycle in Figure 7c. Additionally, as shown in Figure 7b, the LSC cathode exhibits excellent rate performance (0.05-2 C), which affords reversible capacity over 500 mAh g−1 at a high rate up to 2 C, followed by smoothly cycling for another continuous 60th with capacity retention of 95% when the rate of charge/discharge switched back to 0.5 C. LSC cathode also can directly perform long-term cycling tests (500th) at a higher rate of 2 C (Figure 7c). Figure S11 shows the digital photographs of the disassembled batteries with two different binders after 200 cycles at 0.5 C. Both cathodes show good integrity after cycling. Nevertheless, the yellow-colored surface of the separator membrane in PSC battery indicates that it suffers from an obvious contamination by soluble Li2S4-8 in the electrolyte, and also a severe anodic corrosion can be observed on the lithium foil surface faced to the cathode side. By contrast, LSC battery shows a relatively shiny membrane and only slight anodic corrosive behavior, re-confirming the merits of LSC cathode. Consequently, the LBSIP can ctransport ions and keep Li2S4-8 within the cathode, hence enhance the electrochemical performance of Li-S batteries. Compared with 2D Al foil current collector shown in Figure 6 and Figure 7, the electrochemical performances of PSC and LSC cathodes with 3D network current collector of 10


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carbon paper (in Figure 8) have smaller polarization and higher discharge capacity. Certainly, the electrochemical performance of LSC cathode is better than PSC cathode. In Figure 8a-b, the charge/discharge curves show that the polarization of battery increases gradually with increasing charge/discharge rate, but the increase level of LSC cathode is smaller than PSC cathode. Moreover, Figure 8c shows that the discharge specific capacity of LSC cathode is higher than that of PSC cathode, even at a high rate of 2 C, LSC cathode also delivers a reversible discharge capacity of 720 mAh g−1, but PSC cathode shows only a discharge capacity of 560 mAh g−1. These results suggest that the electrochemical performance of batteries can be improved in the presence of both electron and ion conduction networks in the cathodes. Finally, it is worthy to point out that the cycling stability of LSC cathode is much better than those recently reported functional binders for sulfur electrode in Li-S batteries (see Supporting Information, Table S3). CONCLUSIONS In summary, we have demonstrated a bifunctional LBSIP binder containing lithium borate synthesized by one-step in situ photoinitiated thiol-ene click reaction. The unique slightly cross-linked structure of as-prepared LBSIP binder can endow the sulfur cathode with strong mechanical properties, favorable ions-transporting network and excellent polysulfidetrapping ability in Li-S batteries. Meanwhile, the anchoring Li2S4-8 by LBSIP due to rich electron-donating groups [-CO-], [-C-S-C-], [-C-O-C-] can be proved by DFT calculations and 7Li NMR analysis. Using the novel binder, LSC cathode with an areal sulfur loading of 1.5 mg cm−2 (70 wt% in sulfur content) released an initial discharge specific capacity of 712 mAh g-1 at 0.5 C, and retained a reversible capacity of 503 mAh g-1 after 500 cycles (0.06% in capacity decay rate). Contrarily, the capacity of PSC cathode decreased to a very lower value

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of 331 mAh g-1 after 180 cycles. The bifunctional binder can provide a new approach for the development of cathode binder used in Li-S batteries. METHODS Materials Allylmalonic acid (H2C=CHCH2CH(COOH)2, Alfa-Aesar), boric acid (H3BO3, Aladdin), and lithium carbonate (Li2CO3, Aladdin), anhydrous acetonitrile (CH3CN, Aladdin), 2,2dimethoxy-2-phenylacetophenone (DMPA， Aladdin),. 3,6-Dioxa-1,8-octanedithiol (DODT, Sigma-Aldrich), pentaerythritol tetrakis(2-mercaptoacetate) (PETMA, Sigma-Aldrich), N,NDimethylformamide (DMF, Aladdin), sublimed sulfur powder (S, Aladdin), Super P (C, kejing), polyvinylidene fluoride (PVDF, Arkema HSV900). Preparation of LiBAMB The diene lithium borate (LiBAMB) was synthesized by the same procedure in our previous report.35 60 mmol of allylmalonic acid, 30 mmol of H3BO3, 15 mmol of Li2CO3 and 150 mL of CH3CN were mixed and heated (80 oC) under nitrogen gas flow for 15 h. After cooling down, it was filtered and the solvent was removed by vacuum distillation. White solid was obtained after continuously drying under high vacuum at 70 oC for 2 h with a yield over 90%. It was transferred into an argon filled glove box and stored in it (H2O and O2 content

Lithium Borate Containing Bifunctional Binder To Address Both Ion

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