An Efficient and Stable Lithium-Oxygen Battery Based on Metal-Organic Framework Separator Operating at 160 ° C

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Introduction
A lithium oxygen battery (LOB) is regarded as one of the most promising next-generation energy storage devices due to its high theoretical specific energy. [1]Conventional aprotic LOB is restricted by the organic electrolytes, which are flammable, raise safety concerns, and operate mostly at temperatures lower than 100 °C. [2]Giordani et al. [3] adopted a LiNO 3 -KNO 3 molten salt electrolyte, exhibiting a Li + ionic conductivity up to 88 mS cm −1 and a Li + transfer number 0.68, with a porous glass fiber separator and operated the battery at an elevated temperature, i.e., 150 °C.The LOB delivered a high energy efficiency with an overpotential as low as 50 mV at 80 mA g −1 .Li 2 O 2 as the discharge product was generated during discharge and decomposed in charge.During cycling, Li 2 O 2 was partially dissolved in molten salt and caused capacity loss because of the uncontrolled diffusion and precipitation of soluble Li 2 O 2 .Xia et al. [4] proved that Li 2 O can be the discharge product when a nano Ni catalyst cathode was employed at 150 °C, which enabled a 4e − /O 2 pathway for the LOB delivering a theoretical energy density of 5200 Wh kg −1 , much higher than Li 2 O 2 as the discharge product, with which the limit would be 3500 Wh kg −1 .However, they also noticed that Li 2 O would cross over the glass fiber separator, which affects the reversibility of the battery.They then incorporated a Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 (LAGP) solid electrolyte above the glass fiber separator to limit the crossover of the suspected species.
Metal-organic frameworks (MOFs) with inorganic central metallic atoms and organic ligands, possessing regular micropores with high porosity and crystallinity, are widely applied in energy storage devices and have played a vital role in restricting the crossover of anode and cathode species, such as in redox flow battery and lithium batteries. [5]The separators should facilitate the transport of Li + effectively, which is crucial for LOB to achieve high performance and long-term stability. [6]Based on that, Qiao et al. [7] adopted a vacuum filtration method to decorate the Celgard separators with copper (II) benzene-1,3,5tricarboxylate MOF.Although this MOF@Celgard separator could restrict the shuttling of the redox mediator in a LOB at room temperature, the stability needs further enhancement.
Herein, we report a zirconium (II) 1,4-benzenedicarboxylate (UiO-66) based membrane fabricated with a facile "rolling dough" method for an elevated temperature LOB.UiO-66 can be easily functionalized, for instance, UiO-66-SO 3 H is a typical isostructural analog of UiO-66.It is selected to fabricate the membrane because of the ionic sieve effect (pore size of ≈6 Å), as well as the high electrochemical and thermal stability. [8]Furthermore, UiO-66-SO 3 H is transferred into UiO-66-SO 3 Li after lithiation.The UiO-66-SO 3 Li provides well-defined Li + channels, accelerated by the built in ─SO 3 groups in the UiO-66-SO 3 Li framework.This Li + transfer is selective, thus superior to the nonselective diffusion in glass fiber separator.This MOF membrane with well-defined nano anionic channels not only impedes the crossover of the discharge products but also facilitates high Li + flux at 160 °C, at which the cathode catalyst is well activated.Consequently, the LOB with this MOF membrane delivered high capacity and long-term stability.

Results and Discussion
As shown in Figure 1a.our work adopted a MOF membrane as the separator in a LOB operating at 160 °C, facilitating the Li + conduction and hindering the crossover of the discharge product.A "rolling dough" method is employed, detailed process is shown in Figure S1 (Supporting Information).To overcome the inherent mechanical brittleness of MOF, polytetrafluoroethylene (PTFE) is applied as the binder, with UiO-66-SO 3 Li particles closely packed in the bulk phase.The spherical UiO-66-SO 3 Li particles with a uniform morphology and size of 100-200 nm are conducive to the preparation of a dense and uniform membrane (Figure 1b; Figure S2, Supporting Information).The inserted selected area electron diffraction (SAED) pattern shows that the UiO-66-SO 3 Li phase has a diffraction feature with a dspacing of 10.5 Å, which is consistent with its (200) reflection. [9]he cross-section of the membrane is depicted in Figure 1c and Figure S11a (Supporting Information) and the dense and uniform membrane without cracks is ≈120 μm in thickness.The corresponding EDS mapping micrographs reveal that all the elements are distributed uniformly without delamination and fracture.After loading LiNO 3 -KNO 3 , the EDS mapping shows that K and N elements are distributed uniformly on the separator, illustrating adequate molten salt electrolyte formation (Figure S3, Supporting Information).
In Figure 1d, the XRD patterns show that UiO-66-SO 3 H powder was successfully synthesized via the solvothermal technique and agrees with the pattern obtained via simulation.A strong diffraction peak at ≈7.4°exists for all the samples, representing the porous structure of MOF. [10]An extra peak at ≈18.1°is observed for the membrane samples, indicating the existence of PTFE. [11]For the membrane filled with LiNO 3 -KNO 3 , the diffraction peaks also correspond to the MOF, the PTFE, and the mixed salt LiNO 3 -KNO 3 , while the peaks remain the same for the membrane after the test.After cationic exchange in LiOH solution, the diffraction peaks of UiO-66-SO 3 Li are shifted to the left for ca.0.1°b ecause the charge-induced dipole interaction of Li + leads to the lattice expansion in the structure (Figure 1e). [12]Moreover, the XPS result shows that the peak at binding energies of 54.6 and 57.4 eV corresponds to Li 1s, which further indicates the success of the cationic exchange of H + to Li + (Figure 1f). [12]o fully demonstrate the advantages of this MOF membrane, the ionic conductivity and Li + transference number (t Li + ) of MOF membrane in LiNO 3 -KNO 3 electrolyte is evaluated with EIS measurements (Supporting information).The ionic conductivity dependence of temperature is presented in Figure 2a.There is a significant increase from 2.9 to 4.1 mS cm −1 since the temperature rises from 150 to 160 °C, which is followed by linear increase of the values as the temperature further increases.The attractive ionic conductivity is attributed to the sufficient Li + pathways in MOF phase.Furthermore, Li + transference number (t Li + ) was demonstrated by the Bruce and Vincent method. [13]As shown in the inserted figure in Figure 2b, the EIS result indicates R of 292 and 454 Ω before and after adopting DC bias, respectively.The initial current is 171 uA, which decreases to 110 uA as time progresses because of the growth of the passivation layers.The above results indicate a t Li + of 0.73 is obtained with MOF membrane, which is comparable with the glass fiber membrane. [3]e membrane then was tested as the separator for LOBs to demonstrate the advantages, with LaNi 0.5 Co 0.5 O 3 (LNCO) as the highly active cathode catalyst.Figure 3a shows the calculated open circuit voltage (OCV) of a LOB with Li 2 O as the discharge product, which decreases linearly with the increase of temperature because of the decrease of Gibbs energy for the battery reaction.The theoretical OCV at 160 °C is 2.83 V, consistent with the experimental result obtained with a galvanostatic intermittent titration technique (GITT).By interrupting the discharge and charge steps with extended periods of OCV, GITT demonstrates the electrochemical potential of the reaction and confirms the reversibility of the battery reactions (Figure 3b). Figure 3c shows that the LOB with MOF membrane delivers a discharge capacity of 5.1 mAh cm −2 , which is ca.1.1 mAh cm −2 higher than that of the LOB with glass fiber membrane.For LOB with MOF membrane, the first discharge plateau appears at 2.87 V, contributing a low capacity of less than 0.2 mAh cm −2 , and followed by a longer plateau contributing a major capacity at 2.80 V.The main charge plateau exists at 2.84 V, which means a super-low 40 mV overall overpotential is achieved.In comparison, the overall overpotential of LOB with glass fiber is 90 mV, which is 50 mV higher than that of LOB with MOF membrane.Furthermore, the MOF membrane provides sufficient Li + transfer channels, and its highly ordered porous structure exhibits a narrow pore size window, effectively blocking the discharge product Li 2 O crossover to the anode side, realizing the higher coulombic efficiency (CE) 97.6% than that with glass fiber as the separator, which is 96.5%.Besides the effective separator, the LNCO cathode also contributes to the superior electrochemical performance.Both high oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic activities are achieved with LNCO. [14]This catalyst here facilitates the reduction of the overpotential and achieving high round-trip efficiency.
We subsequently examined the rate performance, and LOB exhibits overpotentials of 40, 54, 98, and 164 mV at current densities of 0.1, 0.2, 0.5, and 1 mA cm −2 , respectively, see Figure 3d.For a high current density of 2 mA cm −2 , a moderate overpotential 320 mV is delivered.This small overpotential is ≈50% of that with LAGP electrolyte as a separator at the same current density. [14]Furthermore, despite the overpotential increase at 2 mA cm −2 , the value reduced back to 50 mV when the current   density reduced to 0.1 mA cm −2 again, suggesting a superior structural stability of the MOF membrane.
To examine the long-term capability of the MOF membrane, a limited capacity cycling strategy is adopted.As shown in Figure 3e, the LOB runs stable for 180 cycles with a small overpotential of 120 mV at a current density of 0.5 mA cm −2 with a limited capacity of 0.5 mAh cm −2 .In Figure 3f, the CE increases at the first three cycles and follows with a superior CE of 99.9% during the following 177 cycles.The energy efficiency (EE) was calculated with the middle voltage of discharge and charge plateaus and the LOB has an impressively high EE, above 90% during cycling.It demonstrates the long-term stability of the separator in LOB operating at 160 °C.Furthermore, the MOF membrane was kept stable without obvious cracking after the stability test (Figures S4 and S5, Supporting Information).
The promising performance of the MOF membrane is closely related to the well-defined Li + transport channels in the pore structure.Figure 4a shows that micropores exist in MOF powder and membrane and concentrate at ≈0.6 nm, The pore size distribution is calculated with the non-linear density functional theory (NLDFT) and is consistent with the structural nanopores of UiO-66.The average pore size here is much smaller than that in glass fiber (1.6 μm) and LAGP (0.9-2.8 nm) separators, therefore can impede more effectively the crossover of discharge products. [15]he TG curves presented in Figure 4b show no obvious thermal decomposition from 30 to 200 °C for all the MOF membrane samples, which assures that the MOF membrane is stable and is compatible with the other components in the LOB working at 160 °C.
The slope in Figure 4d, derived with MD calculation, represents the diffusion rate of the Li + and O 2 in the pores of UiO-66-SO 3 Li.The Li + diffusion coefficient is 1.46 × 10 −5 Å 2 ps −1 , which is one order of magnitude higher than the diffusion coefficient of 4.86 × 10 −6 Å 2 ps −1 of O 2 .The permeability test was carried out and the concentration of Li 2 O was measured by titration (Figure S6, Supporting information).As shown in Table 1, compared with a glass fiber separator, the MOF-based separator delivers a lower permeation of Li 2 O all the time.The MOF membrane exhibits obvious blocking effects toward Li 2 O, benefiting the reversibility of the LOB during cycling.Overall, the superior performance of this LOB is attributed to the deliberately designed MOF-based separator.Enabled by the success of the cationic exchange of H + to Li + , the Li + diffuses efficiently through the welldefined Li + transport tunnel.The unique pore structure of the UiO-66-SO 3 Li impedes the crossover of the discharge product Li 2 O as well.

Conclusion
In summary, we designed and fabricated a MOF membrane for a lithium-oxygen battery operating at elevated temperature.Prepared through a facile "rolling dough" approach, 100% UiO-66-SO 3 Li material utilization was realized, and the membrane demonstrated good electrochemical performance.With this MOF membrane, the LOB delivered a high capacity of 5.1 mAh cm −2 at 0.1 mA cm −2 with a low overpotential of 40 mV and ran stably for 180 cycles at 0.5 mA cm −2 without obvious degradation.The well-defined Li + transport channels in UiO-66-SO 3 Li facilitate sufficient Li + pathways and impede the discharge product migration.This work indicates a great potential for the future LOB with a specifically designed MOF-based membrane.LOB with MOF membrane maximizes the advantages of the cathode activity and the battery performance, delivering high specific capacity, long-term stability, high energy efficiency, and high coulombic efficiency.Furthermore, it is much more flexible compared to the so-called solid-state electrolytes and is more favorable for fabricating practically large-scale LOBs.

Figure 1 .
Figure 1.a) Schematic of the synthesis of MOF membrane and the elevated temperature LOB.b) TEM image and SAED pattern of UiO-66-SO 3 Li.c) Cross-section of the MOF membrane.d) XRD patterns of simulated, synthesized MOF powder, pristine membrane, a membrane with LiNO 3 -KNO 3, and membrane after test.e) the region of 6-10°.f) Li 1s XPS spectrum.

Figure 2
Figure 2. a) the Arrhenius plots of the ionic conductivity versus temperature; The insert is the impedance spectra.b) DC/impedance analysis to measure the Li + transference number of the MOF membrane; The insert is the EIS before and after applying DC bias of 50 mV for 6 h.

Figure 3 .
Figure 3. a) Gibbs free energy for formation Li 2 O as a function of temperature and the corresponding OCV.The thermodynamic data were calculated according to the database of HSC chemistry version 9.4.b) GITT measurement of LOB with a MOF membrane at 160 °C.c) Fully discharge and charge the LOB with MOF membrane and glass fiber, respectively.d) Rate performance.e) Cycling performance at 0.5 mA cm −2 .f) The corresponding EE and CE.

Figure 4 .
Figure 4. a) Pore size distribution of powders and membrane.b) TG analysis of MOF powder, pristine membrane, and membrane with LiNO 3 -KNO 3 .c) The model of Li + and O 2 diffusions in the pore of UiO-66-SO 3 Li.d) The diffusion coefficients of Li + and O 2 in the pore of UiO-66-SO 3 Li.

Table 1 .
The concentration of Li 2 O crossed over the separator at 160 °C.