Electrospun Nanofiber Composite Polymer

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Poly(ionic liquid)/electrospun nanofibre composite polymer electrolytes for high energy density and safe Li metal batteries Xiaoen Wang, Gaetan M.A Girard, Haijin Zhu, Ruhamah Yunis, Douglas R MacFarlane, David Mecerreyes, Aninda Jiban Bhattacharyya, Patrick C. Howlett, and Maria Forsyth ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00765 • Publication Date (Web): 12 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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ACS Applied Energy Materials

Poly(ionic liquid)/electrospun nanofibre composite polymer electrolytes for high energy density and safe Li metal batteries Xiaoen Wang,a Gaetan M. A. Girard,a Haijin Zhu,a Ruhamah Yunis,a Douglas R. MacFarlane,b David Mecerreyes,c Aninda J. Bhattacharyya,d Patrick C. Howlett,a Maria Forsyth a *

a. Institute for Frontier Materials, Deakin University, Geelong, VIC 3217, Australia. b. School of Chemistry, Monash University, Clayton, VIC 3800, Australia. c. POLYMAT University of the Basque Country UPV/EHU, Joxe Mari Korta Center, Avda. Tolosa72, 20018 Donostia-San Sebastian, Spain. d. Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.

* Corresponding author: [email protected]

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Abstract Solid-state electrolytes with mechanical integrity and high ionic conductivity are important components in high performance all-solid-state lithium (Li) batteries. Relative to these electrolytes, ionic liquid-based composite polymer electrolytes exhibit high ionic conductivity and improved safety. However, the incorporation of large concentration of non-active ions and the presence of a liquid phase lead to relatively low Li+ transference number and poor mechanical properties. In this study, poly(ionic liquid)s or polymerised ionic liquids (polyILs) are combined with an electrospun fibrous support to afford electrolytes with high ionic liquid content and greatly increased Li+ transference number. The incorporation of electrospun PVDF nanofibres effectively improves the mechanical strength of the composite polymer electrolytes and consequently, flexible electrolytes with superior mechanical properties are described. Finally, we demonstrate the performance of high energy density Li-metal batteries under highvoltage operation (up to 4.5 V) using LiNiMnCoO2 (NMC) and LiNi0.8Co0.15Al0.05O2 (NCA) cathodes with areal capacity up to 1.1 mAh.cm-2.

Key words: composite polymer electrolytes (CPEs), poly(ionic liquid), high-voltage, energy density, Li-metal batteries,


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1. Introduction The design of safe and high energy density lithium (Li) batteries is viewed as one of the most important goals for advanced energy storage systems for a range of portable, entertainment, telecommunication devices and electric vehicle (EVs) applications.1, 2 Capacity enhancements can be achieved by the use of Li metal anodes coupled with high-voltage cathode materials such as Li- and Mn-rich layered oxides (LiNixMnyCozO2, NMC).3, 4 However, this strategy is challenging for traditional organic liquid electrolytes. For example, the widely used carbonate electrolytes have shown inferior anodic stability at high potential (e.g. > 4.3 V versus Li+/Li0) as well as an incapability to inhibit Li dendrite formation, which are known as the main obstacles for developing high energy density and safe Li metal batteries.5, 6 Polymer-based solid electrolytes have shown superior properties such as high mechanical properties, good flexibility and improved safety. Generally, polymer electrolytes are prepared by mixing and dissolution of Li salts into, or by chemically bonding of Li+ onto the polymer backbones.7,

8

In these systems, the Li+ diffusion/conduction is highly dependent on the

segmental relaxation of the polymer backbone. Due to the sluggish movements of Li+ within the polymer matrix, the ionic conductivity of these systems are generally too low to be used in practical devices.9, 10 In general, the incorporation of liquid electrolytes or small molecules into polymer electrolytes provides an effective strategy to enhance their ionic conductivity. The small molecular solvents can not only plasticize the polymer hosts, resulting in improved segmental dynamics of the polymer backbone, but also benefit the dissociation of Li+ anion species from the polymeric backbone, leading to improved Li+ diffusivities.11 Different plasticizers such as organic solvents and ionic liquids have thus been used to prepare highly plasticized gel electrolyte systems.12, 13 For example, Zhang et al. developed a gel polymer electrolyte with a porous PVDF-HFP matrix. Benefiting from the unique honeycomb structure of the polymer host, the prepared gel electrolyte exhibited high ionic conductivity and good



cycling performance.12 However, for these systems, although higher conductivity is achieved compared with solvent-free polymer electrolyte systems, the application of large amounts of flammable solvent raises the unfavourable safety issues for batteries, including potential fire or explosion. Compared with traditional liquid electrolytes, ionic liquids (ILs) are considered as alternative safe electrolytes for battery applications due to their low flammability, low vapour pressure as well as high thermal/electrochemical stability.14, 15 Base on the same consideration, ionic liquid containing polymer electrolytes have been widely investigated towards safe battery applications.16 However, ionic liquids-based electrolytes typically show relatively low transference number, which arises from the presence of a large amount of cations, anions and charged clusters. 17, 18 In polymer electrolytes, it has been found that the Li+ transport can be improved by using different polymer hosts. Machanic et al. proposed a loosely coordinated electrolyte system by using a poly(tetrahydrofuran) host. It was revealed that the Li+ is less bound to the polymer backbone due to weakened interactions between Li+ and oxygen, thus a higher Li+ transport number of 0.53 is achieved compared with traditional polyethylene oxide (PEO)-based polymer electrolytes.19 Other systems such as polycarbonate electrolytes also demonstrate improved Li+ transport.[7] It should be noted that, similar to PEO-based polymer electrolytes, these reported systems are all based on the interactions between polymer backbone and Li+, which means the polymer backbone can also limit the Li+ transport. Poly(ionic liquid) (polyIL) represents a promising class of polymer host showing high dielectric constant and high chemical/electrochemical stability.20, 21 Furthermore, Bhandary et al found that the polyIL, poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (PDADMA NTf2), can facilitate the dissociation of Li salt and improve Li+ transport.22 Recently, we have demonstrated a composite electrolyte (ion gel electrolyte) system containing a high concentration of Li salt. Benefiting from improved Li transport, the prepared electrolytes


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showed good Li metal stability and full cell cyclability.23, 24 However, it turns out that the increased ionic conductivity achieved by increasing the amount of ionic liquid component normally deteriorates the mechanical stability. Thus, the freestanding membranes can only be obtained when the ionic liquid mass fraction is lower than ~50 wt%.23 In this study, we demonstrate a mechanically enhanced composite polymer electrolyte system via incorporation of electrospun nano polyvinylidene fluoride (PVDF) fibres, which allows us to use more ionic liquid and further increase the Li salt content. To optimize these composite electrolytes, the effects of Li salt concentration on mechanical/physical properties, ion dynamics and electrochemical properties are investigated. Cells based on Li metal anodes, optimized composite electrolytes and high loadings of high-voltage cathodes including LiNiMnCoO2 (NMC) and LiNi0.8Co0.15Al0.05O2 (NCA) are demonstrated.



2. Experimental 2.1 Materials Lithium bis(trifluoromethanesulfonyl)imide (LiNTf2) (>99.9%, Solvay Canada), lithium bis(fluorosulfonyl) imide (LiFSI) (>99.9%, Coors tek US), poly(diallyldimethylammonium) chloride (PDADMAC) (20 wt% in H2O, 400,000-500,000 g/mol, Sigma Aldrich Australia), potassium bis(fluorosulfonyl)imide (KFSI) (≥99.9%, Suzhuo Fluolte China), poly(vinylidene fluoride) (PVDF nanoparticles) (KF850, Kureha Chemicals, Japan) and acetonitrile (99.9%, Sigma Aldrich Australia) were purchased from commercial suppliers and used as-received. The N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl) imide (C3mpyrFSI) was synthesized by the known procedure of anion metathesis of C3mpyr Br and KFSI. The purity was confirmed by 1H, 13C and 19F NMR, mass spectroscopy, ion selective electrode (bromide content below 100 ppm). The potassium content was determined by ICP-MS (below 150 ppm). Poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl) imide, PDADMA NTf2, was synthesised according to our previous report by anion metathesis of PDADMAC and LiNTf2.23 The purity was confirmed by 1H, 13C and 19F NMR, mass spectroscopy (anion) and ion selective electrode (chloride content = 0.7%). 2.2 Synthesis and electrolyte preparation PVDF electrospun membrane. The PVDF fibrous membranes were prepared by electrospinning. Firstly, the electrospinning solution (10 wt%) was made by dissolving PVDF powder in a mixed solvent N,N-dimethylformamide (DMF) and acetone (1 : 1 by volume), then the solution was transferred into a 10 mL syringe with a pre-polished needle (Terumo, 20 G×1.5”). A grounded rotating drum collector (100 r/min) was used in order to obtain a uniform membrane. The horizontal distance between the needle tip and the rotating drum was 12.5 cm,


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the voltage was 20 kV and the solution feed rate was 1 mL/h. The thickness of the electrospun matrix was around 100 μm. Preparation of composite polymer electrolytes. The solution casting method was used to prepare composite electrolytes incorporating with PVDF fibres, as shown in Figure 1a. Firstly, the ionic liquid C3mpyrFSI containing 3.2 m of LiFSI was mixed with PDADMA NTf2 (60 wt%:40 wt%) by adding acetonitrile and stirring at room temperature until a homogenous solution was obtained. Then the solution was cast on PVDF fibrous matrix to get a composite polymer electrolyte. Due to the sticky behaviour of the composite, the casting was performed on a horizontally placed Teflon sheet in fume hood until most of the solvent was evaporated. Finally, the membranes were dried at 50°C in vacuo for at least 48 hours. 2.3 Characterisations 2.3.1

Physicochemical measurements

Scanning electron microscopy (SEM). The microstructures of the composite polymer electrolytes with different Li salt concentrations were characterised by using a JEOL JSMIT300 SEM at an accelerating voltage of 5 or 10 kV. The samples were secured on stainless steel stubs individually in argon-filled glove box, to avoid material contamination or side reactions of the composite materials with atmospheric moisture and oxygen, the samples were then transferred to the SEM chamber via a purposely-designed load-lock chamber for loading air-sensitive samples. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR). The interactions in the prepared composite materials were investigated by FTIR. The environmental N2-filled box was used to reduce the effects of water and the H2O level was monitored to make sure the water level is less than 100 ppm. The analysis was carried out on a PerkinElmer IR



101820 series spectrometer (4000 cm-1 to 500 cm-1) and a scan number of 32 was selected. Before measurement, background scans were performed. Differential scanning calorimetry (DSC). A Netzsch DSC (214 polyma) was used to investigate the thermal behaviours of the composites. 5 - 10 mg of sample was sealed in an Al pan in an argon-filled glove box. The instrument was calibrated by cyclohexane before all the tests. All samples were cooled to -120 °C first and then heated to 180 oC with a cooling and heating rate of 10 oC/min. The DSC scans reported in this study were extracted from the second heating scan. The glass transition temperature (Tg) was determined as the mid-point temperature between the extrapolated onset temperature and extrapolated end temperature. Dynamic mechanical analysis (DMA). The mechanical properties of the composites were investigated by DMA 8000 (PerkinElmer) from -80 oC to 80 oC. The samples were cut into 0.5 mm×15.0 mm in a N2-filled glove box (H2O < 100 ppm). The stress-strain mode was used to determine the storage modulus and loss modules of each electrolyte. The test sample was first cooled down to -80 oC and then heated to 80 oC with heating rate of 2 oC/min. The pre-stress was applied in order to straighten the membrane and minimize noise in the data. Pulsed-field Gradient (PFG) NMR. PFG NMR experiments were performed on a Bruker Avance III 300 MHz wide-bore NMR spectrometer. A 5 mm Diff50 probe head was used to record the spectra. Different RF coils were used for each individual nuclei 1H, 19F and 7Li NMR. The maximum strength of the gradient amplifier is 29.4 T·m-1. The sample was first sealed in a 4 mm NMR rotor in a glove box filled with argon atmosphere. The sealed rotor was transferred in a 5mm glass tube, and then inserted into the diffusion probe for measurements. The pulsed-field gradient stimulated echo (PFG-STE) pulse sequence was used to obtain the diffusion coefficients. The gradient pulse duration was set to 5 ms for 7Li, and 10 ms for 19F and 1H. The typical diffusion time was 50 ms for all nuclei. Gradient strength was varied


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between 0.01 and 25 T·m-1 in 32 steps in a log scale to obtain the whole NMR signal attenuation curve. 2.3.2

Electrochemical characterisations

Ionic conductivity. The ionic conductivity of the composite electrolytes was measured by electrochemical impedance spectroscopy (EIS) using polished stainless steel electrolytes, which was performed on a MTZ-35 impedance analyzer (Bio-Logic Science Instruments, France). A temperature scan range from room temperature (23 oC) to 120 oC was selected and the temperature was controlled by a Eurotherm 2204 temperature controller. Considering the high hygroscopicity of the high Li salt concentration electrolytes. The pre-punched sample (Ø = 13mm, ~100 um thick) was assembled into 2032 coin cells to ensure a good seal. Then the cell was placed into a purpose-designed barrel cell for conductivity measurement. 25 All these steps were completed in argon-filled glove-box to eliminate moisture. During the measurements, the barrel cell was equilibrated for 20 mins at every set temperature before the EIS scan. The frequency range was set from 1 MHz to 1 Hz and the amplitude was 20 mV. Two heating scans with 10 oC intervals were conducted and the data shown in this study were extracted from the second heating scan. Li symmetric cell cycling tests. CR2032 coin-type cells were assembled for battery tests by following our procedure previously reported with solid state electrolytes.26 The electrolyte was sandwiched between two Li metal discs (Ø = 8.0 mm, 100 µm thick, Gelon LIB Co., Ltd) and assembled into a 2032-type coin cell (Hohsen Corp. Japan). Before cell assembly the Li foil was brushed with cyclohexane (>99%, Lab Supply Australia). The cyclohexane solvent was distilled under rotary evaporation and dried on molecular sieves. Post-assembly cells were placed inside an oven and stored for 24 hours (Precision Compact, Thermo Scientific, ΔT = ± 0.1°C) to ensure similar initial SEI properties.



Galvanostatic cycling tests were carried out using a Multi Potentiostat VMP3 (Bio-Logic, France) at 50°C ± 0.1°C. A fast plating slow stripping protocol was applied. The amount of charge applied was 0.3 mAh.cm-2. The current density for Li metal plating/stripping was set at 0.6 and 0.15 mA.cm-2 for 30 min and 2 hours respectively (corresponding to the same amount of charge) with cutoff voltages of + 2.0 V and - 2.0 V vs. Li+/Li0. Li transference number (tLi+). The transference number was electrochemically determined by direct current (d.c) polarisation following a previously described procedure.27 Cells were polarised at 50°C with a constant potential of 20 mV for 4 hours. EIS spectra were acquired before and after polarisation using an amplitude of 10 mV and a frequency range from 7 MHz to 0.1 Hz. High voltage positive electrode cycling tests. Two materials were used to evaluate the performance of high voltage positive electrodes in the CPEs: lithium nickel-manganese-cobalt LiNiMnCoO2 (NMC referred to as 1-1-1, i.e. 33% Ni, 33% Mn and 33% Co) and lithium nickel cobalt aluminium oxide LiNi0.8Co0.15Al0.05O2 (NCA). Li | NMC and Li | NCA cells were respectively cycled between 3.0 and 4.3 V, 2.5 and 4.5 V vs. Li+/Li0 at a 0.05C rate (1C ≡ 160 mAh.g-1 ≡ 0.58 mA for NMC and 1C ≡ 199 mAh.g-1 ≡ 0.80 mA for NCA respectively). Cells were maintained at 50°C for 24 h prior to measurement. The active material loading in NMC and NCA was estimated to be 7.2 ± 0.1 mg.cm-2 and 8.0 ± 0.1 mg.cm-2 respectively.


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3. Results and discussion 3.1 Physical properties of composite polymer electrolytes

Figure 1. (a) Schematic illustration of composite electrolyte preparation along with used materials. (b) SEM image of electrospun PVDF nanofibres. (c) SEM image of PVDF nanofibres enhanced composite polymer electrolyte (CPE). (d) - (e) Digital images of the CPED (D= double). (f) Ionic conductivity of CPEs with different Li salt content. The initial composite consists of 60/40 wt% poly(ionic liquids) (polyIL)/ionic liquid solution (3.2 mol.kg1

LiFSI in C3mpyrFSI), which is defined as composite polymer electrolyte “single” (CPE-S).

The composites “double” (CPE-D) and “triple” (CPE-T) contain an amount of LiFSI that is



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“doubled” and “tripled” respectively. The exact compositions and mole fractions for different electrolytes are summarized in Table 1. Table 1. The composition of composite polymer electrolytes (CPEs) prepared in this study. Molar fraction of each specie n(Li+)/kg

n(Li+)

“solvent”c

/[n(C3mpyr+)+n(DADMA+)]

(total=1.00)

System PDADMA NTf2

LiFSI C3mpyrFSI

ESa

3.2

1.00:1.00

0

0.50

0.50

CPE-Sb

1.55

0.55:1.00

0.30

0.35

0.35

CPE-D

1.55*2=3.1

1.44:1.00

0.18

0.59

0.23

CPE-T

1.55*3=4.65

3.15:1.00

0.11

0.76

0.13

a. Initial electrolyte solution (ES) 3.2 m LiFSI in C3mpyrFSI b. CPE-S is based on 60/40 (wt% IL/PolyIL) composition using 3.2 m LiFSI-C3mpyrFSI ionic liquid c. “Solvent”: C3mpyrFSI + PDADMA NTf2.

Previous studies have shown that the mechanical properties of the plasticized electrolyte are dependent on the mass fraction of the IL phase, which limits the further optimization towards highly conductive and flexible composite electrolytes.23, 26 From our earlier work, we have demonstrated that PVDF nanofibres can serve as a good mechanical support to prepare thin and flexible composite polymer electrolytes (CPEs).28, 29 The CPEs were prepared by solvent casting method as illustrated in Figure1a. Figure 1b shows the microstructure of the electrospun nano PVDF fibres. It can be seen the fibres are randomly distributed and the diameter of the fibres is around 500 nm. Figure 1c shows the SEM image of the prepared composite electrolyte, which is incorporated with uniformly distributed PVDF fibres, and the thickness of CPE is around 100 um. Stress-strain measurement, as shown in Figure S1, indicates that the CPE-D shows a high Young’s Modulus of 11.2 MPa and tensile strength of 3.6 MPa even under high


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relative humidity (67 ± 3%). Given the fact that the high content of hygroscopic LiFSI will easily absorb moisture and plasticize the sample, we believe the real mechanical strength is even higher than the values reported here. Benefiting from high mechanical strength of PVDF fibres and good adhesion between PVDF and bulk electyolyte (Figure S2), the proposed composite electrolyte can be prepared in large-scale as illustrated in Figure 1d and Figure S3, which is in contrast with previous polyIL-based electrolytes,23, 26. More encouragingly, a 0.5 cm × 2.0 cm piece of the composite is not only strong enough to hold a 100 g weight (Figure 1e), but also can undergo high elongation before failure (see video in the Supporting Information). All these results highlight the advantages of CPE regarding high mechanical strength and high flexibility, which are required for future flexible and wearable devices.30, 31 It is reported that, for ionic liquid electrolytes, increasing the Li salt content can significantly benefit the electrochemical performance including stable and fast charge-discharge measurements.32 In this study, the high mechanical strength provides a platform to further increase the Li content in this CPE system. Therefore, the investigation of salt effects in this polyIL-based electrolyte system is achievable up to high Li concentrations. As demonstrated in Figure 1f, due to the existence of the mechanical support, the mass fraction of salt and IL mixture can be increased to 60 wt%, which greatly enhances the ionic conductivity of the CPEs. The CPE-S shows the highest conductivity of all of the composites, which is close to 4.5×10-4 S/cm at room temperature (RT). Further increase of the Li salt content decreases the conductivity (e.g. 1.7×10-5 and 4.9×10-6 S/cm for CPE-D and CPE-T respectively at RT). Although similar conductivities have been reported even for some liquid-free polymer electrolytes,33 our approach provides flexible and relatively strong polymer electrolytes with respectable conductivity and improved transference number, which are also crucial for high energy density batteries.



Figure. 2. DSC traces of composite electrolytes with various LiFSI concetrations. The glass transition temperature (Tg) is determined as the mid-point of the start and end of glass transition. The thermal behaviours of the CPEs and the initial ionic liquid electrolytes were investigated by DSC (Figure 2). The 3.2 mol.kg-1 LiFSI in C3mpyr FSI (3 .2 m ILs) mixture, is selected due to its superior electrochemical performance, as investigated in previous studies.32, 34 For the CPEs with different PDADMA NTf2 / 3.2 m IL ratios, the higher the IL electrolyte content, the lower the Tg (Figure S2), suggesting a plasticizing effect on the polyIL when IL is added. For the effects of salt content, compared with CPE-S, increasing LiFSI salt slightly increases the Tg of the CPE, indicative of slowed segmental movements of polymer backbones when more Li+ ions are present. Interestingly, there is only one Tg observed in all these three CPEs even when very high concentration of Li salt is used, which suggests there is no significant phase separation in any of these composites.


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(a)

(b)

Figure 3. Dynamic mechanical analysis (DMA) of CPEs with different salt content. (a) Variation of storage modulus of composite electrolytes as a function of temperature. The temperature range is from -80 to 80°C. (b) The variation of storage modulus (E’, black), loss modulus (E”, blue) and Tan Delta (magenta) as a function of temperature. The applied frequency is 1 Hz and the heating rate is 2°C/min for all the DMA tests. Figures 3a and b illustrate the temperature dependence of the mechanical properties, which were investigated by dynamic mechanical analysis (DMA). As shown in Figure 3a, the storage modulus (E’) for the CPEs is almost independent of the temperature before glass transition. During the glass transition, the E’ decays around two orders of magnitude, indicative of a relaxation process. It can be found that the CPE-S shows a lower E’, which could be attributed to the higher IL content (or lower LiFSI content) compared with other compositions. In addition, the storage modulus (E’) and conductivity show similar dependence on electrolyte composition, which indicates the high correlation between mechanical relaxation and ion dynamics. Figure 3b represents the variation of storage modulus, loss modulus (E”) and Tan Delta of the CPED composite. For the whole temperature range, the E’ is higher than E’’, indicating that this composite can maintain its solid behaviour until 80 oC. Above the glass transition, the storage



modulus (E’) of CPE-D decreases with increasing of temperature, but still shows relatively high modulus around 10 MPa at room temperature.

Figure 4. The diffusion coefficient of 7Li (Li+, black), 1H (C3mpyr+, red), and 19F (NTf2-, blue) of CPE-S, CPE-D and CPE-T composite electrolytes measured by solid-state NMR. The measurements were conducted at 60 oC. The diffusion coefficient of each species in the CPEs determined from the PFG-NMR experiments are shown in Figure 4. Overall, the diffusion coefficient for 7Li, 1H and 19F are highly dependent on the salt concentration; the diffusion coefficient decreases with increasing LiFSI salt content. This agrees well with the conductivity data which also show a decrease with increasing salt content. Interestingly, the 1H diffusion coefficient is more dependent on LiFSI salt content compared with other species. Note that the 1H signals in the PFG-NMR are attributed to the C3mpyr+ only, whereas the 1H signal of the polyIL backbone are not observable in the spectra due to fast T2 relaxation. This behaviour suggests that more LiFSI salt somehow limits the C3mpyr+ mobility compared with 7Li and 19F diffusion. The 19F diffusion coefficient showed the least sensitivity. Considering the

19F

is assigned to NTf2 - anions (the diffusion

coefficient of FSI anions is not available due to the extremely short T2 relaxation time), this reduced sensitivity suggests that the added Li salt might benefit the release of more NTf2-


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anions from the polyIL backbones, which also indicates the plasticizing effects of LiFSI salt in the system. Whilst PFG-NMR is commonly used to determine the apparent transference number, we are unable to do the calculation in the present systems. In these composite electrolytes, there are two F-containing species, FSI and NTf2 anions. The signal for the D19F is only from the NTf2. Although the diffusion coefficient of NTf2 anions is relatively high, the population of NTf2 is very low (e.g. only 11% of all anions in CPE-T) (Table 1). The majority of the anions in the present system are FSI, however, we are unable to determine the diffusion coefficient since the relaxation time for the FSI anions in these materials is too rapid for the PFG method. 3.2 Fast plating/slow stripping of Li metal in composite polymer electrolytes Pulse charging is a practical technique to characterise and model Li dendrite growth inhibition and is of great interest to a wide range of industrial applications.35 From the practical point of view, the performances of the composite polymer electrolytes as a function of Li salt content are evaluated by ‘fast’ rate charging experiments combined with electrochemical impedance spectroscopy. The fast charge cycling protocol was used for similar reasons as an extensive comparative study of a pulse charge effect. The amount of Li plated/stripped was q=0.3 mAh.cm-2 (j=0.3 mA.cm-2) for each process, corresponding to a current density range that can allow better plating/stripping efficiency and homogeneity in solid electrolyte systems when the effective surface area is increased.36 In Li | Li symmetric cells a Li electrode was plated at 0.6 mA.cm-2 for 30 min and stripped at 0.15 mA.cm-2 for 2 hours (i.e., representing an equivalent amount of charge of 0.3 mAh.cm-2 for each step). Figure 5 (a)-(c) shows the voltage-time response for each composite polymer electrolyte. Figure 5 (d)-(f) shows the voltage-time response of the first two cycles. The surface morphology of each electrolyte, before and after the cycling process, was characterised by



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scanning electron microscopy (SEM) and the images are provided in ESI (Figure S5 (a) to (f)). The polarisation profiles are different for each CPE and the CPE-D system already stands out with the performance illustrated in Figure 5 (b). The charge polarisation values at different cycling stages are presented in Table 2 for each system. Interestingly the highest polarisation potentials were observed in the CPE-S system and the higher the Li+ content in the CPE the lower the potential recorded. This is confirmed by the electrochemical measurement of the Li+ transference number, tLi+. As shown in Figure 5 (h), tLi+ increases from 0.13 to 0.53 when the Li+ content in the CPE is tripled (CPE-T). Although the obtained transference numbers are respectable for this type of composite electrolyte, there are several other polymer-based electrolyte systems with higher transference numbers, which have been presented recently (e.g. fluorinated polyethers,

37

polycarbonates

38and

nanocomposite polymer electrolytes

39).

We

believe there is still room for further improvement of the transference number of our polyILbased electrolytes. For example, electrospun fibres of a different nature with functionalized surface groups, may allow modification of the polymer-anion interactions, leading to improved transference numbers. It is still quite surprising to observe an improved stability with a higher Li+ content within the solid composite. The CPE-D can sustain continuous galvanostatic deposition for longer periods before polarising. Also, based on the EIS measurements before cycling, the higher the Li+ content in the CPE the higher the overall cell resistance before cycling. These results suggest that the anodic performance of Li metal in these CPEs is not only controlled by the CPE ionic conductivity and cell resistance. For the CPE-D the charge polarisation potential continually decreased over one hundred cycles. The charge polarisation potential was initially about 0.9 V but continually dropped to around 0.3 V after one hundred cycles. For the CPE-T the charge polarisation potential was initially about 0.6 V but continually dropped to about 0.2 V with strong potential oscillations. The


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polarisation with CPE-D was higher than the CPE-T electrolyte but with very few polarisation artefacts, which in the case of the CPE-T system can reflect the formation of dendrites or dead/mossy Li deposits, as previously reported in ionic liquid-based electrolytes.40 The stable performance of Li metal in the CPE-D is confirmed by a stable cell resistance before and after cycling as shown in Figure 5 (g). The ability of the CPE-D system to withstand these overpotentials and high rate Li plating/stripping, while exhibiting minimal changes in the surface features provides another example of the stability of the highly concentrated CPE. Furthermore, inhibited Li dendrite formation at Li metal/polymer composite interface was confirmed by SEM cross-section imaging, Figure S5 (g) and (h).

Figure 5. (a)-(c) Voltage profiles during Li plating / stripping processes in Li | Li symmetric cells containing different composite polymer electrolyte. Plating was performed at a current density j = 0.6 mA.cm-2 for 30 min and stripping at j = 0.15 mA.cm-2 for 2 hours (q = 0.3



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mAh.cm-2) at 50 °C; (d)-(f) Voltage profiles of the first two cycles for each composite polymer electrolyte; (e) Cell resistance, before and after cycling, calculated from Nyquist spectra acquired by EIS on the corresponding cells; (h) Li transference number measured for each electrolyte.

Table 2. Plating/stripping polarisation potential values (± 5%) extracted from the voltage-time responses at the end of each process in each composite polymer electrolyte. The values reported are an average value from replicate cells.

Cycle 1 Cycle 2 Cycle 100

Plating Stripping Plating Stripping Plating Stripping

CPE-S 1.14 -0.09 0.73 -0.10 0.45 -0.10

CPE-D 0.91 -0.12 0.48 -0.11 0.34 -0.09

CPE-T 0.61 -0.13 0.51 -0.13 0.24 -0.08

3.3 Performance of high-voltage (HV) positive electrodes with CPEs It has been recommended that high active mass loading electrodes need to be considered for achieving favourable energy density for practical applications. Unfortunately, most of the solid-state batteries are challenging to have areal capacities higher than 1.0 mAh.cm-2.[37] In this study, we have investigated the high loading batteries performances of CPEs at 50 oC. Based on the stable performance of Li metal electrodes, the CPE-D was first chosen as electrolyte to assess the performance of the LiNiMnCoO2 electrodes. Figure 6 shows the charge/discharge voltage profiles of the Li metal coin cells with high loading of NMC active


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materials (7.2 ± 0.1 mg.cm-2, 1.4 mAh.cm-2 ± 1%). The measurements were conducted at 50oC at a current rate of 0.05C (C/20). The initial specific discharge capacity of the cell using CPED is 170 mAh.g-1. The coulombic efficiency reaches 94% at the second cycle. Both charge and discharge capacities slowly decrease over fifty cycles and the specific discharge capacity reaches 151 mAh.g-1 (areal capacity of 1.1 mAh.cm-2). The relatively low coulombic efficiency, especially during the first cycle, indicates an irreversible capacity between charge and discharge. The irreversible capacity possibly arises from active Li consumption, electrolyte components (e.g. salts) decomposition and subsequent SEI formation as reported previously.41 Post-cycling microscopic analysis gave further insight into the electrode surface morphology and helped compare the surface morphology features prior to and after cycling in the CPE-D. SEM images of the NMC composite electrode before and after cycling are provided in ESI (Figure S6). Cross-section analysis allowed us to identify and locate the NMC particles. The particle diameter on the pristine NMC electrode is up to 10 µm (Figure S6a) and we observe entanglement of the CPE fibres, most likely PVDF, with the NMC particles (Figure S6c and d), an indicative of good interfacial contact between CPE and NMC particles. The rate capability of NMC electrodes was also tested in the CPE-D and CPE-T. The current rate was gradually increased from 0.05C to 1C and capacity retention was assessed at 0.1C. Overall, as shown in Figure S7, the performance of NMC with the CPE-D is superior to CPET: higher specific capacities are obtained in the CPE-D and higher capacity retention at 0.1C, post-cycling at 1C is reached for both charge and discharge processes. Although a good capacity retention is also observed in the CPE-T, the capacities are lower. This low capacity may be attributed to the very low ionic conductivity as shown in Figure 1e.



For even higher voltage applications, the cycling performance of LiNi0.8Co0.15Al0.05O2 (NCA) positive electrode material was tested with the CPE-D system using the same C-rate and temperature conditions (0.05C and 50 °C). The active material content was 8.0 ± 0.1 mg.cm-2 (1.6 mAh.cm-2 ± 1%). The profiles are provided in Figure S6. Although a decrease of specific capacity is obvious, the initial charge capacity (197 mAh.g-1) is close to the expected practical capacity (199 mAh.g-1). The coulombic efficiency starts at 91% in the first cycle and rapidly jumps to 97.8%. After 20 cycles, a reasonable discharge capacity of 135 mAh.g-1 (areal capacity of 1.1 mAh.cm-2) can still be maintained.

Figure 6. (a) Charge-discharge profiles after 1, 2 and 10 cycles for Li | NMC cells containing CPE (D) composite electrolyte; (b) Corresponding cycling performance for 50 cycles at 0.05C (0.03 mA) and 50 °C. Cut-off voltages for charge and discharge are 4.3 V and 3.0 V (vs. Li+), respectively. The active material loading in NMC was 7.2 ± 0.1 mg.cm-2 (1.4 mAh.cm-2 ± 1%).

4. Conclusion In summary, polymerised ionic liquid-based composite polymer electrolytes (CPEs) containing different concentrations of LiFSI salts (up to 0.76 molar fraction of LiFSI) were prepared. The effects of Li salt concentration on the ionic conductivity, mechanical properties, ion dynamics


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as well as electrochemical performance were investigated. It is revealed that, although the diffusivity decreases, the addition of higher amounts of Li salt can significantly increase the Li+ transference number (tLi+) as evidenced in electrochemical tests. Considering the ionic conductivity and the transference number, the double Li content (0.59 molar fraction of LiFSI) shows long term, stable cycling performances in pulse charging tests. Li metal batteries with high-voltage, high loading cathodes (e.g. NMC and NCA, 7.2 and 8.0 mg.cm-2, respectively) were successfully assembled and high areal capacities of 1.1 mAh.cm-2 were achieved. These results indicate that the proposed high Li salt content polyIL-based composites are promising polymer electrolytes for safe, high energy density battery applications.



Acknowledgements The authors would like to acknowledge the support of the Australia-India Strategic Research Fund (AISRF 48515). Prof Maria Forsyth and Prof Douglas MacFarlane thank the Australian Research Council for their Australian Laureate Fellowship programs. The authors also thank the Battery Technology Research and Innovation Hub (BatTRI-Hub) at Deakin University for their battery prototyping facilities.

Declarations of interest There are no conflicts to declare.


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Table of content


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Electrospun Nanofiber Composite Polymer

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