In order to obtain a MOF with excellent Li+ conductivity, H2BPDC-2SO3H (2',5'-disulfo-[1,1':4',1''-terphenyl]-3,3'',5,5''-tetracar-boxylic acid), with high-density electronegative groups (-SO3H) and Zr4+ ions, were chosen as the ligand and metal nodes to construct a stable MOF framework. The MOF (Zr-BPDC-2SO3H) with an average particle size of 500 nm was obtained via solvothermal reaction (Supplementary Fig. 1). As shown in Fig. 2a and b, the MOF is isostructural to the reported UiO-67 with a fcu topology.19 Four weak peaks at 2θ of 2 to 8 degrees (indicated by arrows) can be observed in the XRD pattern of Zr-BPDC-2SO3H (Fig. 2b), which can be ascribed to the existence of correlated defect nano regions in crystals30. The N2 adsorption isotherm collected at 77 K shows a much smaller Brunauer-Emmett-Teller (BET) surface area of 231.2 m2 g− 1 compared with that of UiO-67 (2500 m2 g− 1)31, due to the high density -SO3H groups in pores (Supplementary Fig. 2).
The large amounts of -SO3H groups are expected to serve as ion hopping sites because of its electronegativity and possible coulomb force toward metal ions, which can then enhance the ability of MOF for ion transport. The ionic conductivities of Zr-BPDC-2SO3H for various metal ions were thus investigated. Zr-BPDC-2SO3H powders were treated in PC (propylene carbonate) solutions with different metal ions and pressed under 6 Mpa to obtain pellets of Zr-BPDC-2SO3M (M = Li+, Na+, K+, Zn2+). The electrochemical impedance spectroscopy (EIS) of Zr-BPDC-2SO3M was collected at a wide range of temperatures (from − 20 to 60 ℃) (Supplementary Fig. 3). Figure 2c summarizes the EIS of Zr-BPDC-2SO3M at room temperature and the corresponding ionic conductivities of MOFs for Li+, Na+, K+ and Zn2+ are determined to be 2.65 × 10− 4, 1.68 × 10− 4, 5.54 × 10− 5 and 1.9 × 10− 4 S cm− 1, respectively. Moreover, the ionic conductivities gradually rise as the increment of temperature, and the activation energy (Ea) for corresponded ions derived from Fig. 2d are calculated to be 0.16, 0.19, 0.27 and 0.21 eV, respectively. To reveal the role of the functional groups in ion transport, the interaction between Li+ and MOF was studied by X-ray photoelectron spectra (XPS) with dry Zr-BPDC-2SO3Li (Supplementary Fig. 4). The characteristic signal of Li-N group in LiTFSI can be observed at 56.2 eV, however, it disappeared in Zr-BPDC-2SO3Li while a new peak at 54.3 eV emerged, which can be assigned to Li-O bond, indicating the interaction between Li+ and -SO3−.
As ionic conductivity is determined by the movement of both positive and negative ions, ion transference number refers to the conductivity contributed solely by the movement of positive ions. In this work, ion transference number of Li+ (tLi+) was characterized as an example with a composite membrane, Zr-BPDC-2SO3Li+/PVDF-HFP, consisting of 20 wt% of PVDF-HFP (poly(vinylidene-fluoride-co-hexafluoropropylene)) and 80% Zr-BPDC-2SO3Li+. The Zr-BPDC-2SO3Li+/PVDF-HFP membrane exhibits good flexibility and mechanical strength (Supplementary Fig. 5a). tLi+ was calculated from the current-time curve and the ac impedance spectra before and after polarization using Li|Li symmetric cells referring to the Evans method at room temperature32. As shown in Supplementary Fig. 6, the initial current reached 5.07 µA and then stabilized at 4.01 µA after 3600s. The corresponding tLi+ of Zr-BPDC-2SO3H is 0.86, which is excellent compared with other reported porous Li+ conductors (Supplementary Table 1 and Supplementary Fig. 7a). Considering the design strategy and the interaction between Li+ and -SO3−, it is assumed that the high-density electronegative groups (-SO3H) in pores offers abundant hopping sites for Li+, which can greatly promote the transport of Li+ without the movement of electronegative -SO3− under the electric field.
To apply MOFs as SEs in solid batteries, mixing MOFs with polymers is the most typical way to fabricate a flexible SE membrane23,33. However, in this case, there is hardly chemical contact between MOF particles and thus the ion transport through the interface between MOF particles becomes challenging due to the lack of conductive medium. To solve this problem, in this work, the ion conductive MOF was in-situ synthesized on the framework of BC nanofibers and an interconnected MOFs network was obtained. BC was chosen here for its well-arranged linear chain and 3D structure. Both the peaks of Zr-BPDC-2SO3H and BC nanofiber can be observed in the PXRD pattern of interconnected MOFs network (Supplementary Fig. 8), suggesting that the structure of MOF particles in the network remains the same as the separately synthesized Zr-BPDC-2SO3H crystals. As shown in Fig. 3a, BC serves as a template, and Zr-BPDC-2SO3H completely covers the nanofibers of BC in the form of particle arrays without intervals, which is expected to be exempt from the long-distance ion transport and high interfacial resistance between the physically contacted individual particles in the mixture of MOF/polymer. Moreover, nodes can be observed between MOF arrays in the enlarged view, which results in an interconnected 3D network. The mass ratio of MOF and BC nanofiber in the interconnected MOFs network was determined to be 7:3. The cross-sectional view in Fig. 3b shows that the interconnected MOFs network has a uniform thickness of about 86.9 µm. The ion transport ability of the interconnected MOFs network was also investigated. The interconnected MOFs network shows an Li+ conductivity of 7.88 × 10− 4 S cm− 1 with less than 17.63 wt% PC in pores (Supplementary Fig. 9b, d), which is higher than that of the pellet of Zr-BPDC-2SO3H powders (2.65 × 10− 4 S cm− 1, 27.62 wt% PC in pores, Supplementary Fig. 9a, c). Moreover, the ion transference number of the interconnected MOF networks is as high as 0.88, indicating that the network inherits the excellent single ion transport ability of Zr-BPDC-2SO3H (Fig. 3d) and the transference number is higher than that of previously reported MOFs based SEs (Supplementary Fig. 7a, Supplementary Table 1).
The electrochemical stability of the interconnected MOFs network was investigated by studying the electrochemical window determined by linear sweep voltammetry (LSV) using a stainless steel (ss)|SE|Li cell at room temperature. Zr-BPDC-2SO3H/BC composite membrane (Supplementary Fig. 5b) with the same composition as interconnected MOFs network (MOF wt%: BC wt% =7:3), BC nanofiber and Zr-BPDC-2SO3Li+/PVDF-HFP were also investigated for comparison. As shown in Fig. 3e, the electrochemical window of interconnected MOFs network is 5.15 V (vs. Li/Li+), which is much wider than that of BC nanofiber (4.68 V), Zr-BPDC-2SO3H/BC (4.91 V) and Zr-BPDC-2SO3 Li+/PVDF-HFP (4.4 V). These results obviously indicate that the interconnected MOFs network has wider electrochemical window, which can be attributed to the good electrochemical stability of Zr-BPDC-2SO3H. The higher electrochemical stability of the interconnected MOFs network will guarantee its stable working as SE in SSB. The EIS Nyquist plots of Li|SE|Li symmetric cells were measured to investigate the interfacial compatibility between electrode and different Zr-BPDC-2SO3H-based SEs. It can be seen in Fig. 3f that the Li|MOF-based network|Li cell possesses the lowest interface resistance of 74 Ω, smaller than 183 Ω of Li|Zr-BPDC-2SO3Li+/PVDF-HFP|Li cell and 232 Ω of Li|Zr-BPDC-2SO3H/BC|Li cell. The reason can be ascribed to the different surface morphologies of the SEs (Supplementary Fig. 10). A smooth surface formed by uniform nanosized MOF chains can be observed in the interconnected MOFs network, which is supposed to benefit the interfacial contact with the electrodes. Whereas, numerous potholes can be observed in Zr-BPDC-2SO3Li+/PVDF-HFP, and aggregation of particles leads to uneven surfaces for Zr-BPDC-2SO3Li+/PVDF-HFP and Zr-BPDC-2SO3H/BC (Supplementary Fig. 5e, f), resulting in the higher interfacial resistance.
Critical current density (CCD) is determined by the current density at voltage drop during the step increased galvanostatic test and higher CCD represents better capability for suppressing dendrites formation. To evaluate the CCD of different SEs, the current densities of galvanostatic test were step increased and the holding time for one cycle was 1 h using a symmetric Li|SE|Li cell. As shown in Fig. 4a, the voltage increases as the increasing of current density until the short circuit of the cells. Specifically, the Li|MOF-based network|Li shows the lowest voltage at the same current density due to its least interfacial resistance (Fig. 3f). The CCD of the Li|Zr-BPDC-2SO3Li+/PVDF-HFP|Li cell, Li|BC|Li cell and Li|Zr-BPDC-2SO3H/BC|Li cell are 0.3 mA cm− 2, 0.5 mA cm− 2 and 0.5 mA cm− 2, respectively, while the Li|MOF-based network|Li cell shows an obviously improved CCD of 1.3 mA cm− 2. The Li plating/stripping process during galvanostatic tests were further conducted at RT with Li|SE|Li symmetric cells to study the dendrites suppressing capability of different SEs soaked with PC. As represented in Fig. 4b, the cycling performance of Li-Li symmetric cells fabricated with BC, Zr-BPDC-2SO3Li+/PVDF-HFP and interconnected MOFs network was studied at a fixed current areal capacity of 0.10 mAh cm-2. For the Li|MOF-based network|Li cell, lowest voltage can be observed and stable lithium plating/stripping can be realized, and no sign of short circuit is observed after 2000 h. By contrast, the Li|BC|Li and Li|Zr-BPDC-2SO3 Li+/PVDF-HFP|Li symmetric cell can only work steadily for 200 h and 730 h respectively at a current areal capacity of 0.10 mAh cm− 2, then the polarization voltage becomes unstable and the cell short-circuited. Moreover, Li|MOF-based network|Li symmetric cell can also work for over 2000 h at a current areal capacity of 0.20 mAh cm− 2, as proved in Fig. 4c. All this results certificate that the interconnected MOFs network has remarkably improved capacity for suppressing the growth of lithium dendrites, which matches well with its better interfacial compatibility and electrochemical stability than other compared SEs (Fig. 3e and f).
To interview how the different SEs influence the deposition of Li+ on lithium anode, the surface morphologies of lithium plates after 50 cycles of Li plating/stripping at current areal capacity of 0.10 mAh cm− 2 were investigated. Compared with the origin lithium plate (Supplementary Fig. 11a1 and a2), the surface of lithium plate in the Li|interconnected MOFs network|Li symmetric cell is flat and no lithium dendrites are observed (Supplementary Fig. 11d1 and d2). However, lots of protuberance can be observed on the uneven surface of lithium plates in the symmetric cells with BC and Zr-BPDC-2SO3Li+/PVDF-HFP as SEs (Supplementary Fig. 11b1, b2, c1 and c2). The better capability of interconnected MOFs network for optimizing lithium deposition can be ascribed to the smooth and homogeneous surface of interconnected MOFs network. Its homogeneous surface created an even potential energy surface by -SO3− which can interact with Li+ (proved by XPS results in Supplementary Fig. 4) and thus prohibited the inhomogeneous deposition of Li+ at interface34,35, resulting in better cycling performance in Li-Li symetric cells.
Finally, Li metal SSBs were fabricated and the widely used commercial LiFePO4, super P and PVDF were mixed as the cathode to investigate the influence of different SEs for battery performance. As shown in Fig. 5a and b, the average discharge capacities of SSB with interconnected MOFs network as SE at 0.2, 0.5, 1, 2, 3 and 5 C are 159, 145, 143, 140, 119 and 108 mA h g− 1, respectively. In comparison, the Zr-BPDC-2SO3Li+/PVDF-HFP and Zr-BPDC-2SO3H/BC based SSBs have similar discharge capacities at low C-rates. However, their discharge capacities decline severely at high C-rates. For instance, the discharge capacity of the SSB with interconnected MOFs network as SE at 3 C is 100% and 200% higher than that of SSBs with Zr-BPDC-2SO3Li+/PVDF-HFP and Zr-BPDC-2SO3H/BC as SEs. The cycling performance of the SSB fabricated with interconnected MOFs network at 1 C is exhibited in Fig. 5c and d, which retains a stable discharge specific capacity of around 140 mA h g− 1 after 500 cycles without decay. The cycling performances of SSBs with different SEs at 3 C are compared in Fig. 5e. The SSB fabricated with interconnected MOFs network remains a specific capacity of 119 mA h g− 1 at 3 C after 600 cycles with a decay rate of 0.02% per cycle (Fig. 5f). However, the SSBs fabricated with BPDC-2SO3Li+/PVDF-HFP and Zr-BPDC-2SO3H/BC show low and fast decayed specific capacity, indicating their disability for running at high C-rate. Moreover, the unchanged PXRD of interconnected MOFs network after cycling at 3 C proves its electrochemical stability during cycling (Supplementary Fig. 8). Compared with the reported SSBs based on MOFs, the SSB fabricated with interconnected MOFs network shows excellent rate performance and cycling performance especially at high C-rate over 1 C (Supplementary Fig. 7b, Supplementary Table 2). The much better performance of interconnected MOFs network can be ascribed to its integrated linear channels for ion transport, optimized interfacial compatibility and ability for suppressing lithium dendrites certificated by above-mentioned experiments.