Yingfei Hou,Lin Jiang,Yaoyao Zhang,Zhiwen Qin,Chi Jiang,Ming Wang

State Key Laboratory of Heavy Oil Processing,China University of Petroleum (East China),Qingdao 266580,China

Keywords: Lithium-air battery Polymer electrolyte Redox mediator P(VDF-HFP)Nafion

ABSTRACT Lithium-air (also known as lithium-oxygen) batteries have attracted considerable global attention in recent years due to their extremely high energy density (11,140 W·h·kg-1).The electrolyte is a key element in lithium-air batteries and the traditional organic electrolyte has great safety risk due to leakage.On the contrary,the polymer electrolyte has the advantages of high safety,high stability and easy processing comparing with the organic liquid electrolytes.In this paper,a new idea was proposed to coat the Nafion membrane on a layer of polymer for blocking the oxidation reduction electric (RM) and Li based on the selective permeability on lithium ion of the Nafion membrane.Self-made thicknesscontrollable Nafion membrane,polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP)and 2,2,6,6-tetramethylpiperidinooxy (TEMPO) were used to prepare a quasi solid polymer electrolyte(NSPE).Electrochemical workstation and LAND battery testing system were used to perform a galvanostatic charge/discharge test on Li-O2.The ionic conductivity of NSPE was 4.3×10-4 S·cm-1at room temperature and the discharge platform was 2.6 V and the charging voltage was 3.7 V after 50 cycles with the cut-off capacity of 500 mA·h·g-1.

1.Introduction

With the exhaustion of non-renewable energy such as petroleum,coal and natural gas,energy issues and environmental problems attract increasingly attention.Developing environmental friendly and sustainable energy has become one of the interests for scientists [1,2].Lithium-O2batteries have shown important application prospects in the new generation of electric vehicles and other areas,due to their extremely high energy density(11,140 W·h·kg-1),which is 5–10 times bigger than that of conventional lithium-ion batteries [3–5].The electrolyte is a key point in Lithium-O2batteries.The ionic conductivity of solid electrolyte is low at room temperature,and the liquid electrolyte has a high ionic conductivity but poor safety.The gel polymer electrolyte is a type of functional polymer material between the above two,in which the ionic conductivity is higher than solid electrolyte and the performance is stable.

Li2O2and LiO2are important products in Li-O2battery discharge process andandhave very strong nucleophilic capacity,which requires the electrolyte to have good stability under strong nucleophilic capacity [6,7].Hammond [8]studied the reactions of nine polymers with lithium peroxide and concluded that the reactions of PAN,PVC,PVDF,PVP with lithium peroxide are not stable,the reaction of PEO with lithium peroxide is relatively stable,and PMMA,PTFE,Nafion are stable when in contact with lithium peroxide.Moreover,the Nafion membrane is a single-ion conductor,which can selectively permeate lithium ions in a lithium-O2battery with a quasi solid electrolyte system.As a result,it can block the damage of other radicals or ions on the lithium film,thus can also block the attack of the lithium peroxide on the polymer electrolyte.Because of the excellent performance of Nafion,it was coated on a polymer matrix to design a composite structured electrolyte in order to obtain higher ionic conductivity and good cycle stability.

As a mature electrolyte,polyvinylidene fluoridehexafluoropropylene copolymer (P(VDF-HFP)) [9,10]has been widely used in batteries,which can effectively protect lithium film and improve the stability of the interface.Thus,the Nafion membrane was coated on a layer of P(VDF-HFP) to prepare a kind of quasi solid polymer electrolyte (NSPE).As shown in Fig.1,the transport mechanism of lithium ions in quasi solid polymer electrolytes is realized by the movement of molecular chains.Lithium ions were transfered from one coordination site to another,and jump from one chain segment to another.

Fig.1.Li+ transport mechanism in NSPE.

The main discharge product of Li-O2batteries is lithium peroxide(Li2O2),which is insoluble in electrolyte and has high decomposition voltage,leading to the pores of super P slowly plugged during the cycle and the failure of battery eventually [6,7].The addition of a red-ox mediator(RM)will greatly reduce the decomposition potential of Li2O2and weaken the polarization of Li-O2batteries[11],and it is necessary to be introduced in redox media.The red-ox media that have been studied include lithium iodide(LiI),tetrathiafulvalene (TTF) [12],and tetramethylpiperidine nitrogen oxide (TEMPO).Benjamin [13]used TEMPO as an oxygen evolution red-ox media in a Li-O2battery and TEMPO showed high electrochemical stability,reducing the charging voltage by 500 mV.Limet al.[14]applied LiI to a Li-O2battery and the Li-O2battery exhibit excellent cycling performance,which reached a discharge depth of 1000 mA·h·g-1at a current density of 2000 mA·g-1.The current research on the catalytic mechanism of TEMPO[12,13,15–17]is deep and TEMPO is also stable.

TEMPO is a persistent radical,which is sterically protected by four α-methyl groups.The reversible one electron oxidation leads to TEMPO+,the corresponding N-oxoammonium cation (Fig.2).

TEMPO can react with Li2O2.The principle is shown in formulas(1) and (2).Fig.3 Proposed catalytic cycle for the electrochemical charging of Li-O2cells with TEMPO.TEMPO exhibits high electrochemical stability,fast diffusion kinetics,an appropriate redox potential and enables a sufficient oxygen solubility.The use of TEMPO in Li-O2cells leads to a significantly reduced charging voltage and hence,to a distinctly higher round-trip efficiency [13].Hence,TEMPO is a highly suitable redox mediator Li-O2batteries.

Herein,we design a polymer-based bilayer-structured quasi solid electrolyte(NSPE)to isolate lithium anode from RM containing catholyte,and TEMPO was added to the catholyte as the mediator,as its stability was well demonstrated by previous work[13].

Fig.2.Redox couple TEMPO+/TEMPO.

Fig.3.The working mechanism of soluble catalyst TEMPO.

2.Experimental

2.1.Materials

Bistrifluoromethanesulfonimide lithium salt (LiTFSI),TEMPO and P(VDF-HFP) were purchased from Sigma-Aldrich.Tetraethylene glycol dimethyl ether (TEGDME),N,N-dimethylformamide(DMF),dimethyl sulfoxide (DMSO) and super conductive carbon black pearls(SP)were manufactured by Aladdin(Shanghai,China).Commercial polypropylene membrane (PP) was purchased from Celgard.

2.2.Preparation of the electrolyte

2.2.1.Preparation of the liquid electrolyte

1 mol·L-3LiTFSI/TEGDME electrolyte: 1.43 g LiTFSI and 5 ml TEGDME solution were taken in sample bottle A and stirred to form a homogeneous solution.

LiTFSI/TEMPO/TEGDME mixed electrolyte: 1.43 g LiTFSI and 5 ml TEGDME solution were taken in sample bottle B and stirred to homogeneous phase.Then 0.039 g TEMPO was added under stirring.

P(VDF-HFP)solution:1.2 g P(VDF-HFP)were dissolved into 5 ml DMF in sample bottle C.Then 2.8 ml solution in sample bottle A were taken and added to sample bottle C to form a homogenous solution.The mixture was exposed to ultrasomication to remove air bubbles.

2.2.2.Preparation of the gel polymer electrolyte

Nafion membrane: the DMSO and Nafion solution (5% (mass))were mixed at a volume ratio of 8:2.Then the mixed solution was stirred and exposed to ultrasomication to remove air bubbles.A certain amount of mixed solution was poured into a self-made glass mold and placed the mold in an oven at 80 °C to volatilize the solvent.Then the mold was put in a vacuum oven at 120 °C to further evacuate the solvent,and finally heated in an oven at 150 °C for 2 h.The film can be removed by adding warm water of 40 °C to 50 °C to the film surface.

SPE electrolyte (P(VDF-HFP) based electrolyte): the homogenous solution in sample vial C was uniformly coated on a glass plate by a spatula,and then was dried in an oven at 80 °C for 1 h to form an SPE electrolyte membrane.The SPE was cut into electrolyte membranes with diameters of 16.5 mm and 12 mm.Among them,16.5 mm electrolyte membranes were used to assembly the button cells,and the electrochemical properties such as ion conductivity and electrochemical window were tested.The 12 mm electrolyte membrane was assembled into a lithium-O2battery to test the rate and cycling performance.

NSPE electrolyte (Nafion/P(VDF-HFP) based electrolyte): the homogenous solution in sample vial C was uniformly coated on a glass plate with a spatula,and the prepared Nafion membrane was gently placed thereon,and then was dried in an oven at 80°C for 1 h to form an NSPE electrolyte membrane eventually.The NSPE was cut into 16.5 mm and 12 mm electrolyte membranes.

2.3.Preparation of electrode and assembly of battery

Coin cells (CR2032) are assembled depending on the type of test.When testing the ionic conductivity of the electrolyte,the coin cells were assembled in the order of steel sheets/polymer electrolytes/steel sheets; when the linear voltammogram scan curve was tested,the assembly was performed in the order of lithium sheet/polymer electrolyte/steel sheet; and the assembly was performed in the order of lithium sheet/polymer electrolyte/lithium sheet to test the interface impedance.

Assembly Li–O2battery: The Swagelok type lithium-O2battery manufactured by Swagelok was used in the experiment and was prepared in an Ar-flled glove box,as shown in Fig.4.The SPE or NSPE electrolyte was used as separator.The anode side was carbon electrode and the cathode side was lithium.Connect the air bag filled with pure oxygen to the cathode,and then seal the battery with a sealing film to isolate water and carbon dioxide in the air.

Notably,50 μl of LiTFSI/TEMPO/TEGDME mixed electrolyte was added to the Nafion side of the quasi-solid electrolyte(NSPE) during the assembly process of the coin cells and the Li–O2battery.

2.4.Characterizations of electrolyte

2.4.1.Scanning eletron microscope (SEM)

The morphology of the SPE or NSPE electrolyte and products of charged and discharged was characterized with SEM (HITACHI,S-4800,Japan).

2.4.2.Fourier transform infrared (FTIR) spectroscopy

FTIR spectra of the electrolyte were taken using a Nicolet Avatar 370RCT basic FTIR spectrometer (USA) within the range of 4000–400 cm-1.

2.4.3.Ultraviolet and visible spectrum (UV)

The UV–visible absorption spectrum model U-4100 (Hitachi,Ltd) was used in the experiment.

Fig.4.Schematic diagram of Li-O2 battery [18].

2.4.4.X-ray diffraction (XRD) studies

The structure of charge and discharge products of Li-O2battery was analyzed by D8 ADVANCE X-ray diffractometer of Bruker company (Germany).

2.4.5.Electrochemical workstation

Electrochemical experiments were conducted on a Zenniumtype electrochemical workstation manufactured by ZAHNER (Germany) to test the ionic conductivity,electrochemical window and interface stability of the electrolyte.

2.4.6.Charge and discharge test (Land)

In the experiment,CT2001A Land system produced by Wuhan LAND Electronic Co.Ltd.was used to test the cycling performance and rate capacity.

2.5.Electrochemical performance of electrolyte

2.5.1.Ionic conductivity

Applying an alternating current signal of different amplitude to the electrochemical system,an electrochemical impedance spectrum was obtained by measuring the impedance as a function of the frequency of the sinusoidal wave.The impedanceRbcan be obtained on the spectrum and the ionic conductivity(σ)can be calculated according to Eq.(3):

whereL,RbandSpresent the thickness (cm) of the polymer electrolyte,the bulk resistance (Ω) and the contact area (cm2) of the electrolyte with the electrode,respectively.

2.5.2.Electrochemical window

Cyclic voltammetry (CV) and linear sweep voltammetry (LSV)were used to determine the electrochemical window of the electrolyte.For LSV testing,the scan rate was 0.5 mV·s-1and the voltage range was 0–4 V.2.5.3.Interface stability

The electrolytes in the initial state and in the stable state were tested by the electrochemical workstation(VMP 300)to obtain the stability of the interface between the electrolyte and the electrodes.The test frequency is 0.1–106Hz and the amplitude is 5 mV.

2.5.4.Constant current charge and discharge

LAND battery testing system was used in the experiment to perform a galvanostatic charge/discharge test on Li-O2with a cutoff capacity of 500 mA·h·g-1.In contrast to the Li-ion battery charge/discharge program setting,the Li-O2battery was set to the discharge step at first and then the discharge step.

3.Results and Discussions

3.1.Morphology of polymer electrolytes

As shown in Fig.5(a) and (b) are SEM images of SPE and NSPE,respectively,and the surface morphology of NSPE is smoother and more uniform than that of SPE after a layer of Nafion film is coated on the surface.Comparing (a) and (b),it can be seen that the SPE film has obvious pores and the pores are penetrative.After Nafion was coated on the surface,the number of pores was reduced,while the pores were not completely covered,so that lithium ions could smoothly pass through.Fig.5 (c) is an optical photograph of SPE and NSPE,and it can be seen that the SPE film and the NSPE film are both transparent film.The thickness of SPE film measured by the spiral micrometer is 150 μm,and the thickness of NSPE film is 175 μm.

Fig.5.The SEM images of (a) SPE and (b) NSPE; (c) the photographs of SPE and NSPE.

As shown in Table 1,the SPE and NSPE membranes exhibited higher porosity than PP membrane.The porosity of SPE is 71.67%.The porosity of NSPE is slightly lower than that of SPE,but it can also reach 68.33%.Because after Nafion was coated on the surface,the number of pores was reduced,while the pores were not completely covered.

Table 1Comparison of porosity of PP,SPE and NSPE membrane

3.2.FTIR analysis

Fig.6 depicts the attenuated total reflection infrared spectra of solid P(VDF-HFP),SPE film,NSPE film,and Nafion film respectively.As shown in Fig.6,solid P(VDF-HFP) shows the peaks at 845 and 880 cm-1in the spectra,which correspond to the characteristic absorption peaks of amorphous phase in polymers.The peaks at 1239 and 1170 cm-1in the spectra correspond the asymmetrical and symmetric stretching vibration peaks of-CF2-group,respectively.From the infrared spectrum of Nafion,the stretching vibration peak of -CF2appears at 1150 cm-1; the stretching vibration peak of -SO3is at 1058 cm-1,and the stretching vibration peak of-CF2-is at 982 cm-1.SPE film shows obvious P(VDF-HFP)characteristic peaks,and NSPE film showed obvious characteristic peaks of Nafion,indicating that Nafion film was successfully coated on SPE film.

Fig.6.FTIR-ATR spectra of the P(VDF-HFP),SPE,NSPE and Nafion membrane.

3.3.Selective permeability of Nafion membranes

In order to prove the selective permeability of Nafion membrane,an experiment was designed.As shown in Fig.7,two centrifuge tubes sealed with a PP membrane and a Nafion membrane at the bottom were placed in a glass bottle vertically,and the inside solution of the centrifuge tube was an orange TEMPO/TEGDME solution,while the outside was a colorlessTEGDME solution.The color of the external solution at different times(initial state,after 24 h and 48 h,respectively)was observed,and the color of the external solution of the centrifuge tube sealed by the Nafion membrane did not change significantly and was always colorless,while the color of the external solution of the centrifuge tube sealed by the PP changed to orange and gradually deepened with time.This result indicates that Nafion membrane can better prevent the diffusion of TEMPO compared with PP membrane.The possible reason is that Nafion membrane can only selectively permeate lithium ions in a lithium-air battery with a quasi solid electrolyte system.

Fig.7.The permeability of PP membrane and Nafion membrane at the initial state,after 24 h and 48 h.

In order to further prove the selective permeability of Nafion membrane,the external solution in two glass bottles at different times was taken for UV testing.As can be seen from Fig.8,the ultraviolet absorption peak intensity of the solution isolated from the PP membrane was significantly higher than that of the Nafion membrane-isolated solution after 24 h or 48 h.The absorption peak of the solution isolated by the Nafion membrane is weak and does not completely prevent the diffusion of TEMPO from the UV.The reason is that the Nafion membrane is a porous membrane,which was verified by electron microscopy,so that TEMPO molecules can diffuse into the Nafion membrane and spread to the outside of centrifuge tube.

3.4.Electrochemical properties of polymer electrolytes

3.4.1.Ionic conductivity

Fig.9 showed the bulk impedance of SPE and NSPE at 25–70°C.It can be seen that the impedance of the two electrolytes decreased with the increasing of temperature,and the impedance dropped sharply from 25°C to 35°C,while the decrease rate in the later period slowed down.

The ionic conductivity at different temperature of the two electrolytes was calculated by Eq.(1),as shown in Fig.10.As the temperature increases,the ionic conductivity of the electrolyte also increases.The reason is that the polymer chain becomes more and more flexible,and lithium ions transmit more rapidly with the increase of temperature.Both the SPE electrolyte and the NSPE electrolyte exhibited high ionic conductivity,which ensured that the Li-O2battery can be charged and discharged normally at room temperature.In addition,at the same temperature,the ionic conductivity of SPE is higher than that of NSPE.One reason is that NSPE has one more layer of interface than SPE due to the addition of Nafion membrane.Another reason is the surface pores of NSPE are reduced due to the coating of Nafion membrane.

3.4.2.Electrochemical window and interface stability

The linear voltammetry test of NSPE was carried out using Li/NSPE/SS (stainless steel) cell (Fig.11).It can be seen from Fig.11(a) that the electrochemical window of NSPE is 0–4 V,showing good electrochemical oxidative stability.The interfacial stability between the electrolyte and the electrode was measured by testing the electrochemical impedance spectroscopy(EIS)of the Li/NSPE/Li button cell.The high frequency region in the spectrum represents the interface impedance of the polymer electrolyte.Fig.8(b)shows the impedance spectra at initial and steady state(cut-off time was 15 days).As can be seen from the figure,the interface impedance in the initial state is 600 Ω,and the interface impedance in the steady state reaches 800 Ω.The reason is that the TEGDME in the polymer electrolyte reacts with the lithium plate to form a film between the lithium plate and the electrolyte,which increases the difficulty of lithium ion transmission and increases the interface resistance between the electrolyte and the lithium plate.

Fig.8.The UV spectrum of the solution from (a) PP and (b) Nafion,outside the centrifuge tube.

Fig.9.The impedance of the (a) SPE and (b) NSPE electrolyte at the linear temperature.

Fig.10.The ionic conductivity-temperature diagram of the SPE and NSPE electrolyte.

3.5.Battery performance test

Fig.12(a) and (b) are the discharge–charge curves of the first ring of Li-O2battery based on NSPE without RM and with RM,respectively.In the assembly of Li-O2batteries,small amount of 1#or 2#electrolytes(50 μl)were added and compared with a typical half-inch Swagelok Li–O2using liquid organic electrolyte(200–300 μl) [19–21],and it was reduced by around 80%.

It can be concluded that the electrochemical window of the NSPE-based electrolyte is 0–4 V,and a weak current was observed when the voltage exceeded 4 V,demonstrating the electrolyte began to decompose.From Fig.12(a),the NSPE-based Li-O2battery without RM has a discharge platform of 2.6 V and a charging platform of 4.2 V(a litter larger than 4 V),indicating that it is necessary to introduce TEMPO to reduce the charging potential.As shown in Fig.12(b),the NSPE-based Li-O2battery with RM added has a discharge platform of 2.6 V and a charging platform of 3.5 V.Comparing Fig.12(a) and (b),it can be proved that the addition of RM greatly reduces the overcharge potential of the battery,and its performance is not affected even if a small amount of TEMPO molecules diffuses across the NSPE.

Comparing Fig.12 and Fig.13,the charge voltage of the NSPEbased electrolyte slowly increased,while the voltage of the SPEbased electrolyte steeply increased to 4.2 V,which shows that the addition of Nafion membrane inhibits the occurrence of side reactions.

Fig.14 depicts the cycle performance and rate performance of Li–O2battery using NSPE.The cycling stability was measured at a fixed capacity of 500 mA·h·g-1and a current density of 100 mA·g-1.The rate performance was measured at a fixed capacity of 500 mA·h·g-1and the different current densities of 50 mA·g-1,100 mA·g-1,150 mA·g-1.

Fig.11.(a) LSV curve obtained for the NSPE membrane; (b) The impedance spectra of Li/NGPE/Li cell.

Fig.12.First discharge–charge curves of Li-O2 battery (a) without RM of NSPE; (b) with RM of NSPE.

Fig.13.First discharge–charge profile of Li-O2 battery based on SPE with RM.

Fig.14(a) shows the charge/discharge profiles of Li-O2battery based on NSPE,it can be seen that the first cycle charge platform is 3.6 V,the discharge platform is 2.6 V,and the difference is 1.0 V.After 50 cycles,the charge platform is kept at 3.7 V; the discharge platform is still 2.6 V,and the difference is 1.1 V.The rate capability of Li–O2battery using NSPE is shown in Fig.14(b).The NSPE has good rate performance,which charge platform is 3.8 V,and discharge platform is 2.4 V at 150 mA·g-1.

3.6.Analysis of charge and discharge product

Fig.15 is the SEM photo of (a) 1st discharge and (b) 1st charge product of Li-O2cell based on NSPE with a fixed capacity of 500 mA·h·g-1at 100 mA·g-1.As shown in Fig.15(a),several erythrocyte-like Li2O2particles are scattered with a diameter about 1 μm.It can be seen from Fig.15(b)that the large particles of Li2O2disappear and the surface of the active material becomes smooth.Apart from the morphology of activated carbon in the field of view,no other charging products or by-products can be observed,indicating that Li2O2decomposed after the first charge and the reaction is reversible.

It is necessary to analyze the charge and discharge products after 25 cycles to demonstrate that the NSPE Li-O2battery reaction is cyclically reversible.Fig.16 shows the SEM photo of the(a)25th discharge and (b) 25th charge product of Li-O2cell based on NSPE with a fixed capacity of 500 mA·h·g-1at 100 mA·g-1.The box marked in Fig.16(a)is erythrocyte-like Li2O2particles with a diameter of about 2 μm.As shown in Fig.16(b),after 25 cycles,erythrocyte-like Li2O2particles disappear completely.Similarly,the surface of the active material becomes smooth.Apart from the morphology of Super P in the field of view,no other products are observed,indicating that the reaction is still reversible after 25 cycles.

Fig.14.(a) Charge/discharge profiles of Li-O2 battery based on NSPE; (b) The rate capacity for Li-O2 battery using NSPE at different current density.

Fig.15.The SEM photos of the (a) 1st discharge and (b) 1st charge product of Li-O2 cell based on NSPE.

Fig.16.The SEM photos of the (a) 25th discharge and (b) 25th charge product of Li-O2 cell based on NSPE.

Fig.17.The XRD patterns of charge and discharge product of Li-O2 battery based on NSPE at (a) 1st and (b) 25th state.

Fig.17 shows the XRD patterns of the air cathode in the NSPE cell at different discharge–charge states with a fixed capacity of 500 mA·h·g-1at 100 mA·g-1.The three strong diffraction peaks are the diffraction peaks of Super P and stainless steel,and the three weak diffraction peaks of the marking stars are Li2O2.There are only diffraction peaks of Li2O2occurring in the discharge spectra,which proves that the Li2O2is the main discharge product.The characteristic peaks corresponding to Li2O2disappear in the charge process,indicating that Li2O2is decomposed.The generation and decomposition of Li2O2after first cycle or 25 cycles shows that the reaction is reversible,consistent with the conclusions drawn in the SEM image.

4.Conclusions

In conclusion,a bilayer-structured quasi-solid polymer electrolyte(NSPE)has been designed with separate catholyte and anolyte,which compose of P(VDF-HFP)/Li-Nafion solid polymer electrolyte and TEMPO as cathodic additives.Through the UV and electrochemical performance tests,NSPE showed high ionic conductivity,good selective permeability,and low interface impedance.Through the charge and discharge test,NSPE shows good cycle stability and rate performance,which worked well for 50 cycles in Li-O2cell.From a holistic perspective,the Li-O2battery using bilayer-structured NSPE shows higher security and good cycling stability while reducing polarization.It also provided a new idea,using a combination of Nafion membranes and different electrolyte materials to promote integral properties for Li-O2batteries.

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 Fundamental Research Funds for the Central Universities (18CX05001A),National Key R&D Program of China (2019YFE0115600),National Natural Science Foundation of China (21908247) and Shandong Province Major Science and Technology Innovation Project(2018CXGC1002).