Preparation of Composite Single‐Ion Conductor Membrane and Its Application in Lithium–Oxygen Battery

Redox mediators (RMs) are reported to effectually encourage the inactive decomposition of discharge products in lithium–oxygen batteries (LOBs); however, oxidized RMs and other injurious species still attack the Li anodes. An ionic liquid (IL)‐modified PVDF single‐lithium‐ion conducting composite membrane that can permit Li+ only but blocks anions and other molecules is prepared. The PVDF is combined with the strong‐acid cation styrene exchange resin with cation exchange characteristics to make a heterogeneous composite single‐ionic conductor membrane and the IL modification causes the formation of a more‐dense groove inside the PVDF membrane apparently strengthening the conductivity of the membrane without losing its impermeability to cations and discharge intermediates. The utility of the modified composite PVDF (MCP) separator greatly extends the cycle life of the LOB from 66 to 347 cycles at 1000 mAg−1 and 1000 mAhg−1 compared to cells using glass fiber separators. Rate performance is significantly enhanced at 3000 and 5000 mAg−1, increasing from 33 and 21 cycles to 171 and 137 cycles. The full discharge capacity also expands from 3702 to 51 205 mAhg−1. The MCP separator enables iodide‐assisted cathode processes and improves lithium anode stripping/plating, which is essential to improve the performance of nonelectrolytic LOBs.

Redox mediators (RMs) are reported to effectually encourage the inactive decomposition of discharge products in lithium-oxygen batteries (LOBs); however, oxidized RMs and other injurious species still attack the Li anodes.An ionic liquid (IL)-modified PVDF single-lithium-ion conducting composite membrane that can permit Li þ only but blocks anions and other molecules is prepared.The PVDF is combined with the strong-acid cation styrene exchange resin with cation exchange characteristics to make a heterogeneous composite single-ionic conductor membrane and the IL modification causes the formation of a moredense groove inside the PVDF membrane apparently strengthening the conductivity of the membrane without losing its impermeability to cations and discharge intermediates.The utility of the modified composite PVDF (MCP) separator greatly extends the cycle life of the LOB from 66 to 347 cycles at 1000 mAg À1 and 1000 mAhg À1 compared to cells using glass fiber separators.Rate performance is significantly enhanced at 3000 and 5000 mAg À1 , increasing from 33 and 21 cycles to 171 and 137 cycles.The full discharge capacity also expands from 3702 to 51 205 mAhg À1 .The MCP separator enables iodideassisted cathode processes and improves lithium anode stripping/plating, which is essential to improve the performance of nonelectrolytic LOBs.products; 3) Develop a more stable electrolyte to inhibit the decomposition of the electrolyte during charging/discharging. [4] 4) Take protective measures for the lithium anode to inhibit the formation of lithium dendrites and dead lithium, and prevent the attack on the negative electrode from discharge intermediates, redox intermediates, moisture, CO 2 and O 2 corrosion, etc. [5,6] The distinctive point of the redox mediator (RM) catalyst is that it acts as a charge carrier to participate in the positive reaction.In the electrochemical step, the soluble RM is oxidized (Equation ( 1)) and it then reacts chemically with Li 2 O 2 (Equation ( 2)) A technical obstacle to the application of RMs is the shuttling phenomenon, which leads to the reduction of coulombic efficiency, the degradation of lithium metal, and the decomposition of RMs. [7]In RMs catalysts, LiI has is now being intensively studied, [8,9] and the contribution of iodide to LOB was demonstrated by Liu et al. using 0.05 mol L À1 of LiI and 1,2-dimethoxyethane (DME) as electrolyte: the potential remained below 3.2 V for 3000 cycles. [10]16][17] Sun et al. used a diaphragm modified with a negatively charged polymer to prevent migration of the redox intermediate to the lithium negative side. [18]When 5,10-dihydrogen-5,10-dimethylphenazine (DMPZ) was used as a redox intermediate, it was found that the modified diaphragm inhibited the migration of DMPZ to the lithium metal anode, improving the battery performance.Our research group used nano-SiO 2 /GF composite ultrathin PU membrane as the skeleton, and its ionic conductivity could reach 2.5 Â 10 À3 S cm À1 ; it effectively blocked O 2 and H 2 O. [7] In order to run out RM-assisted LOBs with excellent performance, there have been quite a few studies on conductive polymer electrolytes with single lithium ions. [14,15,19]22] In this study, single-ion conductor membranes were prepared using PVDF doped with SCER, a cation-exchange resin with a sulfonic acid group (-SO 3 H) on a styrene-diethylene copolymer matrix with high exchange capacity and fast exchange rates; they were used as a nonpermeable separator in an iodide-assisted LOB.Additionally, IL is added during membrane preparation to further improve the inherent ionic conductivity of the membrane by increasing the roughness of the membrane surface. [23]The I À /I 3 À couple exhibited different potentials in different electrolytes, with a large potential of 3.33 V in DMSO. [19]n this article, cells are assembled from DMSO electrolytes with the I À /I 3 À couple tested.The cycle life of the iodide-assisted LOB was amplified by a factor of %5 at different rates of 1000, 3000, and 5000 mA g À1 and the complete discharge capacity was amplified by a factor of 5 at a capacity of 1000 mAh g À1 .The work of IL-modified composite PVDF (MCP) separators in the iodideassisted anode process is also discussed based on the membrane structure and electrochemical analysis of the cells.

Manufacturing of the Modified Composited PVDF (MCP) Membrane
Scheme 1 demonstrates the manufacture process of IL-modified composited PVDF (MCP) membrane.The first step was to grind the SCER balls into a powder with a pore size of 0.075 mm.Then, 4.0 g of PVDF powder was dissolved into 22.0 g of DMF under supersonic stirring, followed by the addition of 3.0 g of [BMIM]Cl ionic liquid (IL) and 2.0 g of SCER to form a mixture for casting the membrane.After degassing, the liquid mixture was spread on a glass sheet with controlled thickness of the membrane of about 200 μm and then dried in an oven at 80 °C.After that, the membrane was immersed in deionized water to dissolve [BMIM]Cl, thereby obtaining an IL-modified composite conductor membrane (MCP) with rough holes on its surface for use as a separator of LOB.

Assembly and Testing of LOBs
20 mg of MWNTs was added into 40 mL of ethanol, stirred with ultrasound to disperse them fully, and then sprayed evenly on carbon paper with a loading of 0.1 mg cm À2 .After drying the carbon paper in a vacuum oven at 80 °C for 10 h, it was cut into 1.0 cm 2 and used as a positive electrode.The cells (CR2032 with holes) were assembled in an argon-filled glove box (O 2 < 0.1 ppm, H 2 O < 0.1 ppm) in the following order: Li anode-injection of anolyte-GF or MCP separator-injection of catholyte-MWNT cathode.The assembled nonporous battery was tested under the conditions of current density of 1000 mA g À1 , fixed capacity of 1000 mAh g À1 , cutoff voltage of 2.0 V, and charging voltage of 4.5 V.The cyclic rate performance of three times and five times was also tested under fixed capacity, in which the current density values were 3000 and 5000 mA g À1 .As for the complete discharge test of LOB, the discharge current was set to 0.1 mA and the cutoff voltage was 2.0 V.

Assembly and Testing of Symmetrical Cells
SS-SS symmetrical battery was used to research the difference of ionic conductivity among GF and MCP separators.A nonporous coin battery (CR2032) was used to complete the symmetrical battery experiment, in which the anode and cathode were SS plates, and the battery separator was GF and MCP respectively.

Characterizations
Phenom LE (Thermo Fisher Scientific) and JEM-2100 (JEOL) were used to characterize the morphological changes of the separator and anode, and X-ray diffractometer (XRD, D/max 2500PC, Rigaku) was used to analyze the components of the separator and the failed anode.Raman spectrometer (DXR2, Thermo Scientific) and electrochemical workstation (CHI 760 E, Shanghai Brilliance Instrument Co., Ltd.) were equipped to analyze the discharge products and electrochemical processes in LOB.The frequency range of electrochemical impedance spectroscopy (EIS) was from 1 MHz to 0.1 Hz, and it was 5 mV at open-circuit potential.By testing SS|SS symmetrical battery, the ionic conductivity of the electrolyte was calculated, as shown in Equation ( 3), where Rb is the solution resistance, d represents the thickness of the separator, and S represents the area of SS grade sheet.

Modification of PVDF Membranes
First, PVDF was modified with 12% [BMIM]Cl IL to improve its ionic conductivity.The scanning electron microscope (SEM) of the MCP in Figure 1a shows the longitudinal channels on the surface, and the inset highlights its unevenness by removing the components of IL. Figure 1b shows the SEM of the SCERadapted composite PVDF (MCP) membrane with cationexchange properties, showing that the SCER was successfully incorporated into the PVDF membrane with relatively uniform distribution.This allows the membrane to be applied as a singleion conducting separator with selectivity for LOBs, dividing the cell into different anode and cathode compartments.Figure 1c shows the Nyquist plots obtained in SS | SS symmetric cells, which compare the ionic conductivity of GF and MCP separator with different proportions of SCER.The conductance of the GF separator, 0%, 2%, 4%, 6%, and 8% MCP separator was 8.0 Â 10 À3 , 1.93 Â 10 À3 , 3.27 Â 10 À3 , 7.74 Â 10 À3 , 4.30 Â 10 À3 , and 2.34 Â 10 À3 S cm À1 (see Figure 1d) respectively, demonstrating that the addition of SCER increases the ion conductivity by nearly four times, which greatly improves the energy competence of the LOBs.The preparation process of the MCP membrane from the IL-modified PVDF (the untreated PVDF) was characterized by XRD and flight test instrumentation requirements (FTIR) analysis.Figure 1e shows the XRD spectrum of the IL-modified PVDF separator mixed with different proportions of SCER,where the peaks centered around %20.5°are seen in the patterns, assigned to the β-phase PVDF ((110) and (200) crystal surface diffraction).[26] Figure 1f shows that unmodified PVDF exhibits -CH 2 bending vibration and -CF 2 stretching vibration at 1402 and 1170 cm À1 , and the characteristic peaks of crystalline and amorphous phase of PVDF are at 1073 and 880 cm À1 , which are the characteristic peaks of PVDF, [27,28] while the modified MCP membrane is doped with SCER with sulfonic acid groups, There is a characteristic peak at 1127 cm À1 , which is the vibration of sulfonic acid group (-SO 3 H) on benzene ring. [22,27]The band at 1034 cm À1 is the in-plane C-H bending of disubstituted benzene ring, and the band at 1006 cm À1 is the stretching of symmetric S=O. [29]he FTIR analysis showed that SCER was successfully adulterated in PVDF film.Figure 2a represents the total measured spectrum of the MCP membrane.Figure 2b shows two fitted peaks of the O 1s spectrum at 532.58 and 531.68 eV, designated as S=O and S-OH, respectively.Accordingly, the resolved S 2p spectrum fit shows two peaks of S 2p 1/2 at 169.18 eV and S 2p 3/2 at 167.98 eV (see Figure 2c), indicating S=O and S-OH groups. [30]These Scheme 1. Fabrication of the IL-modified composited PVDF (MCP) membrane.
results are consistent with XRD and FTIR analyses, indicating that SCER with sulfonic acid groups was successfully doped into the PVDF membrane.As shown in Figure 2d,e, the selective permeability and Ion exchange capacity (IEC) of the membranes increased with the amount of SCER compared to the unmodified PVDF single-ion conductor membrane.At 4% addition, membrane permeability (96.26%) and IEC (2.98 mmol g À1 ) were highest.This is mainly because the increase in the number of sulfonic acid groups on the surface and inside of the membrane will improve the selection of membrane permeability and IEC ability.However, when the added amount is more than 6%, the IEC of the membrane begins to decrease, wherein the rate of membrane to electrolyte gradually increased.Among them, 4% SCER addition amount of single-ion conductor membrane absorption rate is the best.may be because of the excessive strong-acid resin accumulation distribution on the PVDF membrane, which affects the formation of the membrane and holes; so, it affects the selection of permeability.As shown in Figure 2d-f, the selective permeability, IEC, and electrolyte uptake of the membranes increased with the addition of SCER compared to the unmodified PVDF single-ion conductor membrane.The highest selective permeability (96.26%),IEC (2.98 mmol g À1 ), and electrolyte uptake (96.56%) of the membranes were obtained at 4% addition.This is mainly due to the increase in the number of sulfonic acid groups inside the membrane, which enhances the roughness of the MCP membrane and also imparts specific selective permeability properties to cations and anions.However, when the addition amount exceeded 6%, the properties started to decrease, probably due to the excessive accumulation of SCER distribution on the PVDF membrane, which affected the membrane formation and increased the pores.
Figure 3a-d shows the contact angle test of PVDF single-ion conductor membrane on water and electrolyte (LiClO 4 /DMSO).The results show that the SCER has good infiltration of the electrolyte and repel water.4% added amount of the single-ion conductor membrane has the best performance.The contact angle and electrolyte absorption rate indicate that the PVDF single-ion conductor membrane is diaphragm as an organic electrolyte.The contact angle and electrolyte uptake rate suggest that the PVDF single-ion conductor membrane is an excellent cell diaphragm.It is well known that lithium metal is actual energetic; therefore, the ionic selectivity of the MCP separator is important to protect the lithium anode, and we tested the routine GF separator by means of an H-type electrolytic cell.The promotion of I À and I 3 À transfer on the MCP separator by electric field was also tested by considering the operating environment during the charging/discharging processes of the cell.Figure 3e-g shows the occlusion effect of MCP membranes on I À , where the LiClO 4 /DMSO electrolyte was added in the left column and the LiI/LiClO 4 /DMSO electrolyte was added in the right column.Figure 3e displays that the GF separator cannot prevent transmission of I À , and the samples in the left column extend to 90 min when the color turns dark blue.In contrast, Figure 3f shows that the transmission of I À from the left column electrolyte after 8 days is unchanged when using the MCP separator, even when cured by the electric field.The electrode in the left column is positively charged and the electrode on the right is negatively charged.Every once in a while, the electrolyte in the left H cell  is sampled, and then it is put into the prepared starch/hydrogen peroxide solution.When the solution turns blue, it means that the side I À in the right H cell can penetrate the membrane and reach the left polar chamber (see Figure 3g).As for the permeability test of I 3 À , Figure 3h turns deep purple after 90 min, indicating that the solution on the left contains starch and I 3 À , which shows that GF still can't prevent I 3 À from passing through, but the MCP membrane can still strongly prevent I 3 À from passing through, and there is no color alteration in the left column after 8 days (see Figure 3i), even though the transfer of I 3 À is spurred by electric field (see Figure 3j).

Iodide-Assisted LOB with the MCP Separator
The effective difference between GF and MCP separator in iodide-assisted LOB was studied.The LOB divided the battery into anode chamber and cathode chamber, in which LiClO 4 /DMSO anode electrolyte contained 1.0 mol L À1 LiClO 4 , and LiI/LiClO 4 /DMSO cathode electrolyte contained 1.0 mol L À1 LiClO 4 and 0.05 mol L À1 LII electrolyte.Figure 4a shows that the LOB with GF separator can only run for 66 cycles at a current density of 1000 mA g À1 and a capacity of 1000 mAh g À1 .Figure 4c shows that the charging potential rises rapidly to about 4.5 V, while the discharging potential and the discharging capacity also drop more rapidly.In contrast, using the MCP separator (see Figure 4b), the charge potential remained very low for the first 200 cycles, which meaningfully extended the cycle life to 347 rounds, as shown in Figure 4b.In addition, the cell with MCP separator similarly shows better rate properties than the one with GF separator.As shown in Figure 4e, the LOB with GF separator can only run for 33 and 21 cycles at current densities of 3000 and 5000 A g À1 , while the LOB with MCP separator can run for 171 and 137 cycles, respectively, with more than five times longer cycle life.Figure 4f reveals that the total discharge capacity can be developed by nearly 14 times using MCP separator: the total discharge capacity of the battery with GF separator is 3702 mAh g À1 , while the one with MCP separator achieves 51 205 mAh g À1 .The inset shows that the complete discharge capacity of the battery using MCP separator in argon is only 14.79 mAh g À1 , which indicates that the reaction is an oxygen reduction reaction.

Cathode and Anode in the LOB with the MCP Separator
Figure 5a demonstrates the SEM of the pristine MWNTs cathode.In the LOB with GF separator, Figure 5b shows that after the first discharge, the MWNTs are covered with discharge products.After the first recharge, Figure 5c shows that the MWNTs are still partially covered by discharge products.At the 60th discharge, MWNTs were completely covered by discharge products, and the positive electrode was passivated (see Figure 5d).In the battery with MCP separator, Figure 5e shows that after the first discharge, the MWNTs are covered by membranous discharge products, but after the first recharge, most of the deposits disappear (see Figure 5f ), and the pores of the MWNTs nanotubes are still large after the 60th discharge (see Figure 5g).Most of them are covered by the discharge products after the 300th discharge (see Figure 5h).Figure 6a displays the surface of the pristine lithium anode, as shown in the inset, with a thickness of %330 μm.In the cell with GF separator, the surface of the lithium anode becomes coarse after the first discharge (see Figure 6b) and its thickness is reduced to 270 μm (see inset).After the 60th discharge, lithium metal is exhausted and white powders are formed (see Figure 6c).In the case of the MCP membrane as a separator, the lithium depletion is significantly lower in the cell cycle and its thickness is 278 μm after the first discharge (see Figure 6d and inset).After the 60th discharge, the surface of lithium remains rather flat and smooth (see Figure 6f ) with a thickness of 108 μm, as illustrated in the inset.
The white powder on the lithium anode was gathered and the XRD analysis illustrated in Figure 7a suggests that the composition of the white powder on the lithium anode is the same in the cells using GF and MCP separators and can be attributed to LiOH (PDF No. 85-1064).The lithium anode was protected due to the use of the MCP separator, while the battery failure was caused by passivation of the cathode lithium sheet, and the role of the MCP separator in improving the battery performance was further researched.
Raman analysis of the discharge products of the LOB with GF and MCP separators after the first discharge demonstrated that the use of the MCP separator apparently reduced the generation of LiOH as a byproduct of charging and discharging and enhanced the reversibility of the LOB (see Figure 7b) due to the interaction between the cathode and anode.Cyclic voltammetry (CV) analysis in Figure 7c displays that the peak currents for the formation and decomposition of the discharge products are greater with the addition of LiI to the LiClO 4 /DMSO electrolyte, indicating that both ORR and OER processes are accelerated by iodide.The batteries after the first discharge, the 60th discharge, and the 300th discharge were disassembled for generating the diaphragm and then reassembled into SS|SS symmetrical batteries to test the respective ionic conductivity.Figure 7d displays the Nyquist plot obtained in SS|SS symmetrical batteries, which compares the original conductivity of the MCP membrane (7.74Â 10 À3 S cm À1 ), with those after the 1st discharge (4.45 Â 10 À3 S cm À1 ), the 60th discharge (3.19 Â 10 À3 S cm À1 ), and the 300th discharge (2.40 Â 10 À3 S cm À1 ), indicating that the increase of charging potential associates closely with the degrading of the MCP membrane on ionic conductivity.Figure 7e describes the differences when using GF and impermeable MCP separators in iodide-assisted LOBs.In cells using GF separators, I À at the cathode and I 3 À produced at the cathode are transferred to the anode, leading to lithium metal depletion.Other destructive substances including dissolved oxygen and decomposition products of the electrolyte might as well disturb the chemical stability of the anode.In contrast, I À and I 3 À remain at the cathode during the cell cycle because the MCP membrane conducts Li þ while blocking I À and other substances produced during ORR and OER.As a result, lithium anode is protected from the attack of injurious species during charging/discharging. the MCP separator also facilitates the formation of reversible discharge products, Li 2 O 2 , at the cathode, associated with the suppression of shuttle phenomena.In addition, the modification of the IL of the MCP membrane distorts the ionic conduction path of the PVDF membrane, which helps to uniformly distribute the Li þ flux during lithium dissolution/deposition, thus inhibiting the lithium dendritic growth.All these have a crucial role in the improvement of battery performance.

Conclusion
Modified composite PVDF (MCP) membranes doped with the SCER were successfully prepared, which allowed Li þ transfer and prevented penetration of anions and other molecules in the electrolyte.The membrane was used as a single-lithium-ion conductivity separator in an iodide-assisted LOB with significantly improved performance compared to cells using the GF separators.At a current density of 1000 mA g À1 and a capacity of 1000 mAh g À1 , the number of cycles increased from 66 to 347 and the rate performance increased from 33 and 21 to 171 and 137, respectively, at rates of 3000 and 5000 mA g À1 .The fully discharged capacity expanded from 3702 to 51 205 mAh g À1 , an expansion of more than 13 times.The MCP separator effectively protects the lithium anode from the attack of I À , I 3 À , and other harmful species in the cathode and also improves the deposition/ dissolution of lithium in the cell cycle, thus enabling an iodidemediated cathodic reaction, which is essential for the design and performance optimization of inorganic LOBs.

Figure 1 .
Figure 1.a) and b) SEM of undoped and SCER-doped membranes.c) EIS analysis of the SS|SS symmetrical batteries with the GF and MCP separators.d) Ionic conductivity of batteries with MCP separators at different proportions of SCER.e) XRD characterizations of MCP membranes with different proportions of SCER.f ) FTIR characterizations of undoped and SCER-doped membranes.

Figure 2 .
Figure 2. X-ray photoelectron spectroscopy (XPS) analysis of the MCP membranes.a) Total Survey spectra.b) O 1s. c) S 2p.d) Selective permeability of MCP membranes.e) Ion exchange capacity of MCP membranes.f ) Electrolyte absorption rate of MCP membranes.

Figure 3 .
Figure 3. a,b) Contact angle of the PVDF with the water and DMSO.c,d) Contact angle of the MCP with the water and DMSO.e) and f ) Permeability of I À through the GF and MCP separators.g) Permeability of I À through the MCP separators enhanced by electric field.h,i) Permeability of I 3 À through the GF and MCP separators.j) Permeability of I 3 À through the MCP separators enhanced by electric field.

Figure 4 .
Figure 4. a,b) Cyclability of LOBs with the GF and MCP separators.c,d) Dependance of charge potential, discharge potential, and discharge capacity on cycle number with the GF and MCP separators.e,f ) Rate performance and full discharge capacity with the GF and MCP separators (inset: discharged in argon).

Figure 5 .
Figure 5. a) SEM of the pristine MWNTs cathode.b,c) and d) SEM of the MWNTs after the 1st discharge, 1st recharge, and 60th discharge in the iodide-assisted LOB with the GF separator.e-h) SEM of the MWNTs after the 1st discharge, 1st recharge, 60th discharge, and 300th discharge in the iodide-assisted LOB with the MCP separator.

Figure 6 .
Figure 6.a) Surface and cross-section (inset) images of the pristine Li anode.b,c) After the 1st and 60th discharges in the iodide-assisted LOB with the GF separator.d-f ) Li anode in the iodide-assisted cell with the MCP separator after the 1st discharge, 60th discharge, and 300th discharge.

Figure 7 .
Figure 7. a) XRD analysis of the white powder at the anode side of the LOBs with the GF separator after the 60th discharge and with the MCP separator after the 340th discharge.b) Raman analysis of the cathode product after the 1st discharge in the LOBs with the GF and MCP separators.c) CV in the LiClO 4 /DMSO and LiI/LiClO 4 /DMSO electrolytes.d) EIS analysis of the conductivity of the MCP separator after the 1st, 60th, and 300th discharges in the iodide-assisted LOB using the SS|SS symmetric cell.e) Schematic illustration of iodide-assisted LOBs with the GF and MCP separators.