The effects of amino groups and open metal sites of MOFs onpolymer-based electrolytes for all-solid-state lithium metal batteries

Jiahao Lu,Zhimeng Wang,Qi Zhang,Cheng Sun,Yanyan Zhou,Sijia Wang,Xiangyun Qiu,Shoudong Xu,Rentian Chen,4,Tao Wei,

1 School of Energy and Power,Jiangsu University of Science and Technology,Zhenjiang 212003,China

2 Power & Energy Storage System Research Center,College of Mechanical and Electrical Engineering,Qingdao University,Qingdao 266071,China

3 College of Chemical Engineering and Technology,Taiyuan University of Technology,Taiyuan 030024,China

4 Institute of Mechanics and Energy,National Research Ogarev Mordovia State University,Saransk 430000,Russia

Keywords:Solid composite electrolytes NH2-MIL-101(Fe)All solid-state lithium metal batteries Metal-organic frameworks (MOFs)Open metal sites (OMSs)

ABSTRACT Metal-organic frameworks (MOFs) are becoming more and more popular as the fillers in polymer electrolytes in recent years.In this study,a series of MOFs (NH2-MIL-101(Fe),MIL-101(Fe),activated NH2-MIL-101(Fe)and activated MIL-101(Fe))were synthesized and added to PEO-based solid composite electrolytes(SCEs).Furthermore,the role of the—NH2 groups and open metal sites(OMSs)were both examined.Different ratios of MOFs vs polymers were also studied by the electrochemical characterizations.At last,we successfully designed a novel solid composite electrolyte containing activated NH2-MIL-101(Fe),PEO,LiTFSI and PVDF for the high-performance all-solid-state lithium-metal batteries.This work might provide new insight to understand the interactions between polymers and functional groups or OMSs of MOFs better.

1.Introduction

In the past decades,numerous industries have embraced the usage of lithium-ion batteries,including portable electronics,electric vehicles (EVs),power grid energy storage systems and so on[1-5].However,it also faces some problems that limit its development,such as the low energy density which is increasingly difficult to meet contemporary needs,and the possibility for safety issues brought on by the liquid electrolyte has increased rapidly [6,7].Recently,all-solid-state lithium-ion batteries (ASSLBs) based on lithium anode and solid electrolyte have attracted more and more attentions due to their high energy densities and excellent safeties[8,9].

Solid-state electrolytes with good thermal and electrochemical stability are the key part of ASSLBs [10-12].Among them,PEObased polymer electrolytes have been widely studied owing to their simple and convenient machining method and excellent interface contact with electrodes [13,14].However,the low Li-ion conductivity of PEO limits its application.Therefore,a series of methods have been adopted to modify PEO-based electrolytes,including copolymerization [15],grafting [16],cross-linking [17]and adding nano-sized inorganic fillers [18].Among different methods,the preparation of solid composite electrolytes (SCEs)by adding nano-sized fillers in PEO is one of the most effective ways for the reason that these inorganic fillers can inhibit the crystallization of polymer and increase the proportion of amorphous region in PEO,which can enhance ionic conductivity.Because the migration of Li+mainly depends on the movement of amorphous region chain segment [19,20].

In recent years,metal-organic frameworks (MOFs) that are nanoscale in size and have a high specific surface area and porosity have frequently been employed as fillers to improve the conductivity and other electrochemical properties of PEO based SCEs[21-24].Take the following as an example,Yuanetal.[25]reported that the introduction of MOF-5 into PEO matrix,which provided a high-speed channel for the transmission of Li+and increased its room temperature ionic conductivity to 3.16 × 10-4S.cm-1.Zhangetal.[26]reported an Al-MOF-nanorod-doped SCEs with the reduced crystallinity of PEO due to the unique structure of MOFs,which increased the Li-ion conductivity from 7.71 × 10-7S.cm-1to 2.09 × 10-5S.cm-1at 30 °C.What’s more,in addition to the advantages of high porosity and high specific area mentioned above,MOFs also have the functional groups (amino groups) on the organic ligand,which have a great impact on the performances of the SCEs (Fig.1(a)) [27,28].For example,Stephanetal.[29] reported a PEO based SCEs doped with UIO-66-NH2@SiO2.The conductivity of the electrolyte film can reach 10-4S.cm-1at 60 °C,and which has excellent performance in the full battery.In our previous work [30],a new type of SCEs based on PVDF-HFP doped with UiO-66-NH2and surface-functionalized zirconium dioxide was proposed,which also exhibited high Li-ion conductivity of 1.05×10-4S.cm-1at 60°C.At the same time,thermal activation is an effective way to expose internal unsaturated metal sites (also called open metal sites (OMSs)) of MOFs,and the OMSs could also have impact on the ionic conductivity (Fig.1(b)),some MOFs with OMSs even can be used as the Li-ion conductor which have been discussed in our previous studies [21,22].However,there are few reports talked about the amino groups and OMSs when both of them were existed in polymer-based electrolyte.

Fig.1.(a) the—NH2 groups can form hydrogen bonds with the ether oxygen atoms on the PEO chains;(b) the OMSs(Fe3+)were exposed after the activation processes and can immobilize TFSI- through Lewis acid-base interaction.

Herein,we propose a new SCEs prepared by a straightforward solution-casting way,which is consisted of PEO,PVDF,LiTFSI and activated NH2-MIL-101(Fe).For the reason that the film-forming properties of PEO are poor [31,32],the adding of a small amount of a high viscosity binder (PVDF) can result in a significant improvement in the film-forming properties of PEO [33].NH2-MIL-101(Fe) has garnered some interest in the catalysis and adsorption disciplines due to its substantial specific surface area and OMSs.Thus,activated NH2-MIL-101(Fe) was chosen here as the filler,namely the role of the—NH2groups and OMSs were both examined in this polymer-based electrolyte.

2.Experimental

2.1.The synthesis and activation of NH2-MIL-101(Fe)and MIL-101(Fe)

MOFs powders was prepared by a simple solvothermal method and the activation of MOFs are according to our previous studies[21,30],and the details are seen in the Supplementary Material.The activated NH2-MIL-101(Fe)and MIL-101(Fe)powders were denoted as a-NH2-MIL-101(Fe) and a-MIL-101(Fe),respectively.The un-activated MOFs were denoted as un-NH2-MIL-101(Fe)and un-MIL-101(Fe),respectively.

2.2.Preparation of SCEs

The electrolyte membrane was prepared by a solution casting method also based on our previous studies [21,30,32].According to different mass contents of a-NH2-MIL-101(Fe) powders (5%,10% and 15%) compared to the mass of PEO,the obtained SCEs were called 5%-a-NH2-MIL,10%-a-NH2-MIL and 15%-a-NH2-MIL in short respectively in the following text.At the same time,a-MIL-101(Fe),un-MIL-101(Fe) and un-NH2-MIL-101(Fe) (10% of the mass of PEO),were also added into the mixed solution of PEO,PVDF and LiTFSI to prepare the corresponding SCEs named 10%-a-MIL101,10%-un-MIL101 and 10%-un-NH2-MIL,and the details can be seen in the Supplementary Material.Finally,the obtained SCEs were cut into disks in diameter of 18 mm and stored in an argon-filled glove box for further use.

3.Results and Discussion

The structure of MIL-101 is composed of a corner-sharing of socalled super-tetrahedra that are linked together by inorganic trimers and PTA anions (Fig.2(a)).As Fig.2(b) shown,the trimers are found at the four vertices of the super-tetrahedron,and the organic linkers are found at its six edges.In order to create a three-dimensional crystal material with MTN (mobil thirty-nine)topology,these super-tetrahedra are next joined by cornersharing.The resulting framework then distinguishes between two different forms of mesoporous cages [34].These two cages are restricted by 12 pentagonal faces and 16 faces (12 pentagonal and 4 hexagonal) respectively.Pentagonal windows with a～1.2 nm free aperture are presented in the larger cages and larger hexagonal windows of～1.6 nm(Fig.2(c)-(e))while the smaller cages only exhibit pentagonal windows.

Fig.2.(a)The trimers of iron and the linkers;(b)super-tetrahedra;(c)the larger mesoporous cages with the pentagonal windows(d)pentagonal window and(e)hexagonal window.

The iron in the trimer is an octahedral coordination with a coordination number of six,five of which are occupied by a common vertex μ3-O and four carboxyl oxygen,and the remaining one is occupied by water or monovalent anion (typically OH-) [35,36].Thermal activation combined with vacuum is an effective way to expose the OMSs.Some of the water molecules coordinating with iron were removed and the coordination unsaturated Fe3+were exposed after 12 h of vacuum treatment at 150 °C [36].These exposed OMSs immobilize the anions of lithium salt (TFSI-)through lewis acid-base interaction,which has been confirmed by DFT calculations in our previous study [21].

The crystal structure of synthesized MIL-101(Fe)and NH2-MIL-101(Fe)powders were tested by XRD patterns,which are depicted in Fig.3(a).From the XRD patterns,the characteristic peaks of the MIL-101(Fe)and NH2-MIL-101(Fe)are in accordance with the pattern simulated by the lattice parameters of MIL-101(Cr) [34],which indicates that the synthesis of MIL-101(Fe) and NH2-MIL-101 (Fe) was successful.Moreover,the introduction of amino group(—NH2)didn’t cause deformation or destruction of the crystal structure of NH2-MIL-101(Fe).The MOFs powder size and surface morphology were characterized by SEM and the results are presented in Fig.3(b) and (c).Obviously,both NH2-MIL-101(Fe)and MIL-101(Fe) have similar regular cubic octahedral structures,but the surfaces of NH2-MIL-101(Fe) ware coarser than that of MIL-101(Fe).In addition,the size and size distribution of the two MOFs were also very consistent,with the particle size of about 1 μm.

Fig.3.(a)XRD patterns of MIL-101(Fe),NH2-MIL-101(Fe)and the simulated XRD pattern of MIL-101;the SEM images of(b)MIL-101(Fe)powders and(c)NH2-MIL-101(Fe)powders;(d) digital photographs of NH2-MIL-101(Fe) before and after activation,respectively.

Uniform particle size would make MOFs more evenly distributed in PEO matrix.Fig.3(d) and Fig.S1 (in Supplementary Material) respectively show the slight color difference of NH2-MIL-101(Fe) and MIL-101(Fe) before and after activation.

The digital photographs of 10%-a-NH2-MIL and 10%-a-MIL101 are shown in Fig.4.One major benefit of polymer-based electrolytes is that the thin and soft SCEs may be twisted and sliced into practically any shape (Fig.4(a) and (b)).The sectional SEM images of 10%-a-NH2-MIL and 10%-a-MIL101 are shown in Fig.4(c)and(d),respectively,which indicates that the NH2-MIL-101(Fe)and MIL-101(Fe) powders are evenly distributed throughout the PEO matrix without a clear pore structure or aggregation.The thickness of 10%-a-NH2-MIL and 10%-a-MIL101 are about 88 μm and 66 μm,respectively.Meanwhile,the thickness of 5%-a-NH2-MIL,15%-a-NH2-MIL,10%-un-NH2-MIL and 10%-un-MIL101 are 85,91,74 and 63 μm according to the section SEM images of these SCEs as shown in Fig.S2.

Ionic conductivity,which was calculated from the impedance spectra (Fig.S3 and Fig.S4(a)) of symmetric SS/SCEs/SS cell without liquid electrolyte,is a crucial parameter for an all-solid-state electrolyte to be applied in ASSLBs.The fluctuation of ionic conductivity of various SCEs with MOFs as a function of temperature is shown in Fig.5(a) and the ionic conductivities of the samples at 30,40,50,60,75 and 90°C(as listed in Table 1).It is observed that the ionic conductivities of all SCEs rise over an increase in temperatures as expected.Compared with the membrane without MOFs prepared by the same method(Fig.S4)and Ref.[21],the SCEs prepared in this paper has a great improvement in ionic conductivities.The improvement of the ionic conductivities of these SCEs can be attributed to the following factors.Firstly,the octahedral shaped MOFs are evenly dispersed across the PEO matrix,which reduces the crystalline phase of PEO,thus increase the proportion of amorphous region in PEO and increases the proportion of free moving chains.It should be noted that the migration of Li+in the polymer matrixes mainly rely on the motion of the polymeric chains and this happens mostly in amorphous region [31,37].Secondly,MOFs can offer quicker routes for the movement of lithium ions owing to their large specific surface area and specialized micropores [38-40].Additionally,it has been shown in our previous work [21] that OMSs (Fe3+) were exposed after the activation processes which can immobilize TFSI-through Lewis acid-base interaction(Fig.1(b)),which promotes the dissolution and dissociation of lithium salts (LiTFSI) and further increases the content of freely moving Li+in SCEs [41,42].Among these different SCEs,the 10%-a-NH2-MIL showed the best performance on Li-ion conductivity at 60 °C (5.70 × 10-4S.cm-1),which is little higher than the 5%-a-NH2-MIL (4.53 × 10-4S.cm-1) and 15%-a-NH2-MIL(4.97×10-4S.cm-1),which can be attributed to the dilution effect dominates when the content of MOFs is greater than a certain value,resulting in the decrease of conductivity instead of increase[43].It also can be obviously seen that the ionic conductivity of 10%-a-MIL101 is slightly lower (3.63 × 10-4S.cm-1) than those ones with —NH2groups.This might be due to the ether oxygen atoms on the PEO chain can form hydrogen bonds with the—NH2groups of NH2-MIL-101 because the ether oxygen atoms has lone electron pairs (Fig.1(a)),which can further suppress the crystalline of PEO so that increase the proportion of amorphous area[30,44,45].Moreover,the SCEs with un-NH2-MIL and un-MIL101 were prepared for comparing the effects of —NH2groups on ionic conductivity individually without the influence of OMS (Fe3+),which were also confirmed that the —NH2groups has great effect on the improvement of ionic conductivity.In addition,the ionic conductivities of 10%-un-NH2-MIL (2.81 × 10-4S.cm-1) and the 10%-un-MIL101 (1.12 × 10-4S.cm-1) were lower than the corresponding activated ones,which is ulteriorly prove that the positive effects of OMS(Fe3+)on improving the ionic conductivities of SCEs.

Table 1 The ionic conductivities of 5%-a-NH2-MIL,10%-a-NH2-MIL,15%-a-NH2-MIL,10%-un-NH2-MIL,10%-a-MIL101 and 10%-un-MIL101 at 30,40,50,60,75 and 90 °C

Fig.5.(a) Ionic conductivity of 5%-a-NH2-MIL,10%-a-NH2-MIL,15%-a-NH2-MIL,10%-un-NH2-MIL,10%-a-MIL101 and 10%-un-MIL101 at different temperatures from 30 to 90°C;(b)LSV curves of 5%-a-NH2-MIL,10%-a-NH2-MIL,15%-a-NH2-MIL,10%-un-NH2-MIL,10%-a-MIL101 and 10%-un-MIL101 at a scan rate of 1 mV.s-1 at room temperature.

The electrochemical stability window is a critical factor in evaluating the performances of solid polymer electrolytes and the possibilities of their practical applications in high-energy lithium-ion batteries.The linear sweep voltammetry (LSV) method is used in Fig.5(b)and Fig.S4(b)to show the electrochemical stability of various SCEs at ambient temperature.A rapid rise in current indicates electrolyte breakdown [46].It is evident that the electrochemical stability windows of four SCEs with —NH2groups are higher than those without —NH2groups.It should be noted that the electrochemical decomposition potentials for the 10%-a-NH2-MIL is around 5.1 V,which has a large promotion compared with the pure PEO electrolyte(～3.5 V,Fig.S4(b)).This might be also owing to the—NH2groups could form hydrogen bonds with the ether oxygen atoms,which can enhance the high-voltage stability of PEO effectively [24].At the same time,the LSV curves also depicted that whether MOFs is activated or not,it doesn’t affect the electrochemical window very much,this certify that the OMSs(Fe3+)don’t have interaction with the PEO chains.

Another important factor in measuring the electrochemical performance of SCEs is lithium-ion transference number (tLi+).HightLi+can effectively reduce the polarization effect of the charging/discharging cycles in ASSLBs.The symmetric Li/SCE/Li cells were assembled for testing and the corresponding results were presented in Fig.6,Fig.S5 and Table 2.As can be clearly seen in Table 2,the 10%-a-NH2-MIL (Fig.6(b)) shows the highesttLi+(0.350) than the 5%-a-NH2-MIL (0.255,Fig.6(a)) and 15%-a-NH2-MIL (0.288,Fig.6(c)).In addition,the 10%-a-MIL101 also has a high value of tLi+(0.311,Fig.6(d)).Oppositely,the 10%-un-NH2-MIL has a lowtLi+(0.251,Fig.S5(a)) and the SCE without MOFs has the lowest value oftLi+(0.193,Fig.S5(b)).These results illustrate two things:on one hand,the OMS (Fe3+) exposed after activation of MIL-101(Fe) can immobilize TFSI-through Lewis acid-base interaction,thereby promote the dissociation of lithium salt (LiTFSI) and then furtherly increasing the proportion of free-moving Li+in SCEs;on another hand,the 10%-a-MIL101 has a highertLi+than the 5%-a-NH2-MIL and 15%-a-NH2-MIL,which indicates the —NH2groups don’t make much contribution to the increase oftLi+.Moreover,the 15%-a-NH2-MIL has a relative lowertLi+than that of 10%-a-NH2-MIL,which could be ascribed the fact that the particle agglomeration caused by inorganic nanofillers at higher concentration is still a troublesome issue affecting the comprehensive performance of CSPEs and further hinder the transmission of Li+[30,47].

Table 2 tLi+ and corresponding test values of electrolytes at 60 °C

Fig.6.The lithium-ion transference number(tLi+)of different SCEs(a)5%-a-NH2-MIL;(b)10%-a-NH2-MIL;(c)15%-a-NH2-MIL;(d)10%-a-MIL101 at 60°C(the inset shows the electrochemical impedance spectroscopy curves before and after polarization).

Based on the above results of electrochemical properties,the 10%-a-NH2-MIL has the best performance in ionic conductivity(5.70 × 10-4S.cm-1at 60 °C),lithium-ion transference number(tLi+=0.350)and electrochemical stability window(～5.1 V).In addition,the 10%-a-NH2-MIL also has a satisfying ionic conductivity and electrochemical stability window compared to previous studies [1,48-50] (Table 3),which could be benefited from the synergistic effects of —NH2groups of and the exposed OMSs (Fe3+)after introducing the activated NH2-MIL-101 into SCEs.Thus,the 10%-a-NH2-MIL SCE was assembled in symmetric Li/Li cells to evaluate the electrochemical stability between lithium metal and electrolyte.As can be clearly seen in Fig.7(a),the cell with 10%-a-NH2-MIL exhibited a smooth and stable state of voltage(～22 mV)during the charging/discharging processes for more than 2000 h at 0.1 mA.cm-2,which indicates the SCE can effectively suppress the formation and growth of lithium dendrite over long-term cycles.It is worth noting that the symmetric Li/Li cell with 10%-a-MIL101 could also maintain for over 1100 h(Fig.7(b)) at the same current density,which demonstrates the excellent electrochemical stability against lithium metal.In addition,the two kinds of symmetric Li cells were also tested at 0.2 mA.cm-2(Fig.S8).It can be obviously observed that the cells including 10%-a-NH2-MIL could run stably at the current density of 0.2 mA.cm-2without any short circuit for about 1100 h,with a slight voltage rise.However,the symmetric Li/Li cell with 10%-a-MIL101 experiences a sudden voltage change and short circuit after cycling for 650 h,which also presented a higher voltage stage (～75 mV) than the Li/10%-a-NH2-MIL/Li cells (～50 mV).

Table 3 Comparison of electrochemical performance of solid electrolyte

Fig.7.Galvanostatic cycle curves of (a) Li/10%-a-NH2-MIL/Li;(b) Li/10%-a-MIL101/Li symmetric cell at 0.1 mA.cm-2 (inset diagram: Cycle curve zooming in at a specific time).Cycle curves of(c)Li/10%-a-NH2-MIL/Li and(d)Li/10%-a-MIL101/Li symmetric cell at different current densities(0.05,0.1,0.2,0.4 mA.cm-2).All the above-mentioned tests were performed at 60 °C.

At the same time,these two kinds of cells have been measured for rate performances,which were displayed in Fig.7(c)and(d).It was observed that the rate performance of 10%-a-NH2-MIL is superior than that of the 10%-a-MIL101.In particular,the 10%-a-MIL101 showed irreversible voltage change when the current densities changed from 0.4 to 0.1 mA.cm-2.In opposite,the 10%-a-NH2-MIL shows excellent electrochemical reversibility at 0.4 mA.cm-2.In summary,the 10%-a-NH2-MIL and 10%-a-MIL101 have the similar ability of suppressing the growth of lithium dendrite effectively at low current densities.Nevertheless,while the current density increasing to a higher value,the 10%-a-NH2-MIL is a better choice for the practical application in ASSLBs.The phenomenon could illustrate that the interfacial stability of 10%-a-NH2-MIL is superior than that of 10%-a-MIL101,which might because the—NH2groups can form hydrogen bonds with the ether oxygen atoms on PEO chains and this can enhance the interface interaction between —NH2groups and PEO in 10%-a-NH2-MIL.Moreover,the lone electron pairs could complex with Li+,which promotes the dissociation of LiTFSI and inhibits Li dendrite [51].

Since the 10%-a-NH2-MIL SCE exhibits the best electrochemical properties and acceptable mechanical strength,it was thus applied as the solid electrolyte for all-solid-state LiFePO4/SCE/Li cells.The electrochemical performances of the cells were tested from 2.6 V to 4.0 V at 60 °C,where the size of Li foils and LiFePO4disks are 14 mm and 15.6 mm in diameter.Additionally,the areal loading of LiFePO4was～1.0 mg.cm-2.Fig.8(a)depicted the cycling performance of the cells at 0.2C(1C=170 mA.h.g-1).It can be clearly seen that the cell assembled with 10%-a-NH2-MIL SCE exhibits an average discharge capacity of 155 mA.h.g-1with the Coulombic efficiency of 99.9%.It should be noted that the discharge capacity increases gradually in the initial several cycles because of the activated process in full cells.The cells were tested at 0.5C as well,which was shown in Fig.S7(a).The initial discharge capacity was 147.2 mA.h.g-1,and the capacity increases slowly until the 10th cycle (151.2 mA.h.g-1),which represents that the increase of the initial capacity due to the activation is not obvious.Finally,the capacity maintained at 147 mA.h.g-1after 220 cycles.Moreover,the full cells were run for about 100 cycles at a higher current density of 2C (Fig.S6),which could be further demonstrated that 10%-a-NH2-MIL has a perfect improvement on stabilizing the electrode/electrolyte interface.From the results mentioned above,the full cells with 10%-a-NH2-MIL SCE showed excellent discharge capacity,Coulombic efficiency and capacity retention.Fig.8(b)shows the rate performance of the LFP/10%-a-NH2-MIL/Li cell between 0.2C and 2C.It can be observed obviously that the capacity is still maintains a high value(159.4 mA.h.g-1)when returning to 0.2C.

Fig.8.(a) Long cycle performance of LFP/10%-a-NH2-MIL/Li at 0.2C;(b) rate performance of the cell between 2.6 and 4.0 V tested at 60 °C;(c) the corresponding chargedischarge curves of LFP/10%-a-NH2-MIL/Li at 0.2C;(d) CV profiles of the LFP/10%-a-NH2-MIL/Li (after 10 cycles at 0.2C) at 0.2 mV.s-1.All the above-mentioned tests were performed at 60 °C.

The charge-discharge curves of partial cycles were further investigated,as presented in Fig.8(c)and Fig.S7(b).It can be obviously observed that the charge-discharge curves under the two conditions show a good coincidence,indicating that the full cells using 10%-a-NH2-MIL has good electrochemical stability.As the cycle goes on,the internal polarization voltage of the cells does not increase significantly,because the high lithium-ion transference number can alleviate the polarization effect inside the cells [25].However,the polarization voltage of the full cells at 0.5C is slightly higher than that of the battery at 0.2C,which is due to the fact that the lithium-ion diffusion capacity between the electrode/electrolyte interface and inside the electrode does not meet the demand under high current density,resulting in the increase of polarization.Therefore,SCEs 10%-a-NH2-MIL has shown great potential in ASSLBs due to its excellent electrochemical properties.

To further study the activation process of the cell,the CV tests have been conducted in Fig.8(d) and Fig.S7(c).In the newly assembled cell,the redox peaks of LFP are clearly unstable(Fig.S7(c)),but after 10 cycles at 0.2C (Fig.8(d)),the reversible redox peaks are almost overlapped and becomes stable(the oxidation potential～3.71 V and reduction potential～3.16 V),which matches with the activated process in Fig.8(a),indicating that the cells are not stable in the early cycles.However,after several cycles of activation,the internal of the cells becomes stable,which presents that the capacity increases and remains stable.

4.Conclusions

In summary,a series of MOFs (a-NH2-MIL-101(Fe),a-MIL-101(Fe),un-NH2-MIL-101(Fe) and un-MIL-101(Fe)) were added to polymer SCEs and the role of the —NH2groups and OMSs were both examined.Both —NH2groups and the OMSs have positive effect on the improvement of ionic conductivity,for the reason that the ether oxygen atoms on the PEO chain can form hydrogen bonds with the —NH2groups and the OMSs (Fe3+) exposed after activation can immobilize TFSI-through Lewis acid-base interaction.While the —NH2groups don’t make much contribute to the increase oftLi+and the OMSs don’t affect the electrochemical window very much.Finally,the ionic conductivity and the electrochemical stability window of 10%-a-NH2-MIL are 5.70 × 10-4S.cm-1at 60 °C and 5.1 V,respectively.The 10%-a-NH2-MIL also exhibits an excellent electrochemical stability against lithium metal and shows good cycling and rate performances.We think that the 10%-a-NH2-MIL SCE is an appropriate option of electrolyte in ASSLBs based on the above-mentioned results.

Data Availability

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (21701083) and Postgraduate Research &Practice Innovation Program of Jiangsu Province(KYCX20_3137).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2023.01.011.