Supported Ionic Liquid Gel Membrane Electrolytes for a Safe and

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Supported Ionic Liquid Gel Membrane Electrolytes for a Safe and Flexible Sodium Metal Battery Tiago Mendes, Xiaomin Zhang, Youting Wu, Patrick C. Howlett, Maria Forsyth, and Douglas R. Macfarlane ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06212 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 27, 2019

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ACS Sustainable Chemistry & Engineering

Supported Ionic Liquid Gel Membrane Electrolytes for a Safe and Flexible Sodium Metal Battery Tiago C. Mendes, a Xiaomin Zhang, ab* Youting Wu, b Patrick C. Howlett,c Maria Forsyth,c and Douglas R. MacFarlane a*

Dr. T. C. Mendes, Dr. X. M. Zhang, Prof. D. R. MacFarlane a

School of Chemistry, Monash University, Wellington Road, Clayton, Victoria 3800, Australia.

Dr. X. M. Zhang, Prof. Dr. Y. T. Wu b

Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical

Engineering, Nanjing University, 163 Xianlin Road, Qiaxia District, Nanjing, Jiangsu Province, 210023. P.R. China. Prof. Patrick C. Howllet. Prof. Maria Forsyth c

ARC Centre of Excellence for Electromaterials Science (ACES), Institute for Frontier Materials

(IFM), Deakin University, 221 Burwood Highway, Burwood, Victoria 3125, Australia.

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Corresponding Authors Email: [email protected]; [email protected]; [email protected]

Abstract: Concerns about the sustainability of lithium supplies has stimulated interest in alternative energy storage chemistries including in sodium-metal and sodium-ion batteries. Gel ionic liquid electrolytes are investigated here as an important option for secondary sodium batteries due to their leakage-free and superior safety when compared to standard flammable electrolytes. Supported ionic liquid gel membranes (SILGMs) were prepared as both electrolyte and separator for a sodium metal battery using a carboncoated sodium vanadium phosphate material (Na3V2(PO4)3@C or NVP@C) as cathode. SILGM-based coin cells exhibit a specific capacity retention of 92% after 150 chargedischarge cycles with a coulombic efficiency of 99.9%. We also demonstrate the operation of SILGMs in a laminated ﬂexible sodium battery. The SILGM-based flexible battery exhibits a good flexibility and shows a remarkably stable operation even when opening the device or cut into pieces (Supporting Videos). It is expected that SILGMs will


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become promising separator/electrolyte materials in practical application and thus will promote the development of non-flammable and flexible sodium batteries.

Keywords: Ionogel, gel polymer electrolyte, sodium metal battery, ionic liquid, flexible sodium battery

INTRODUCTION The soaring demand for energy storage in portable electronics, electric vehicles and large-scale power grids has triggered intense concerns about the sustainability of lithium



supplies for lithium batteries. When compared to lithium, sodium emerges as an appealing alternative because sodium is significantly cheaper, highly abundant, less toxic, and can achieve comparable energy densities; these features certainly label sodium metal batteries as an excellent, sustainable option to current day lithium technologies.1 Electrolytes play a critical role in the performance of batteries. Organic-based liquid electrolytes have been widely reported in sodium batteries.2 However, they suffer from toxicity, flammability and potential leakage. These issues may become even more serious in large-scale energy storage systems. Moreover, sodium metal is highly reactive with organic liquid electrolytes and suffers from severe sodium dendrite formation, which is an issue that potentially results in explosion.3 Therefore, solid electrolytes have been considered as a promising alternative to liquids because of their most appealing feature of free standing consistency, contributing to easy handling and cell design, modularity and reliability. More importantly, gel or solid electrolytes can suppress the formation of dendrites.4 Ionic liquid gels (ionogels) are typically a kind of semi-solid electrolytes that show a promising future for electrochemical energy storage devices. They have the solid-liquid 4 ACS Paragon Plus Environment

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duality with good mechanical flexibility and high ionic conductivity.5-6 Nonetheless, there are only few research groups that have investigated ionogel electrolytes for sodium batteries. Hashmi et al.3 reported a gel electrolyte consisting of NaCF3SO3 (sodium triflate) in a 1-ethyl-3-methylimidazolium trifluoromethane sulfonate (EMIM-triflate) immobilized in poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) for a sodium ion battery with a high ionic diffusivity, a sufficiently wide electrochemical potential window and excellent thermal stability. In 2016, Hashmi et al.7 proceeded with the same system, but introducing a passive filler Al2O3 and active filler NaAlO2 particles in order to form conductive free-standing films of the composite gel polymer electrolytes (GPEs); the freestanding films showed impressive ionic conductivity at room temperature (6.3-6.8x10-3 S.cm-1 and 5.5-6.5x10-3 S.cm-1 for Al2O3 and NaAlO2, respectively), displayed good electrochemical stability and remarkable thermal stability up to 340 C, however, the authors did not provide evidence of applicability of their electrolytes in a real sodium battery. It is worth noting that in literature, several sodium ion conducting GPEs can be found,8-12 but they are based on flammable organic solvents. In this contribution, we emphasize the use of ionic liquids to form intrinsically safer gel electrolytes (ionogels). In 5 ACS Paragon Plus Environment


our previous work, we have developed a new methodology for incorporating ionogels into several different commercial supports to prepare SILGMs for superior stability in flexible supercapacitors.13 In this present manuscript, we use commercial glass fiber separator due to its low cost and excellent conductivity of the impregnated ionic liquid gel (SILGM - data provided in table S1). It

has

been

reported

that

the

ionic

liquid

N-propyl-N-methylpyrrolidinium

bis(fluorosulfonyl)imide (C3mpyrFSI) has excellent thermal stability and electrochemical performance in sodium-based electrochemical devices.14-16 Therefore, the ionogel electrolyte based on NaFSI-C3mpyrFSI is expected to have favorable properties as an electrolyte for rechargeable sodium batteries. Furthermore, we also demonstrate the application of this supported ionic liquid gel membrane in a flexible sodium battery based on a carbon-coated sodium vanadium phosphate cathode material (Na3V2(PO4)3@C, abbreviated thoroughly as NVP@C), highlighting the superior safety of the system (refer video S1). Preparation and additional characterization of the ionogel can be found in Supporting Information.


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RESULTS AND DISCUSSION In order to confirm that NVP@C material was successfully synthesized, the powder Xray diffraction (XRD) was carried out and is displayed in Figure 1(a). According to the XRD pattern, all peaks are indexed to the Na superionic conductor (NASICON)-type 3Dframework with R-3c space group (rhombohedral unit cell, ICSDS data card no-248410), which is in good agreement with previous reports.17-19 Peaks position between 15-30° are under a broad shape feature ascribed to the presence of an amorphous carbon coating, suggesting that the NVP@C was successfully obtained.20



Figure 1. XRD pattern (a) and Raman spectrum (b) of the as synthesized NVP@C. TEM images: (c) low magnification, (d) HR-TEM.

The Raman spectrum in Figure 1(b) supports the XRD results and reveals the carbon coating of the NVP particles by the presence of the D- and G-bands, which represent the out of plane vibration of carbon atoms due to some disorder and the vibration of sp2-hybridized


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carbon atoms, respectively. The ID/IG ratio, which is equal to 0.86 shows that the carbon coating has an amorphous nature, which is a characteristic when glucose is used as a carbon source.21 To provide further information related to morphological and crystalline structure, the NVP@C material was analyzed by TEM and HR-TEM. The TEM in Figure 1(c) shows NVP particles completely coated by a carbon layer, the particles also seem to be interconnected by the amorphous carbon. This carbon coating is crucial for a better conductivity of the NVP material and enhance the particle-to-particle electronic contact.22 The HR-TEM in Figure 1(d) confirms the amorphous carbon coating of a few nanometers on NVP particles. Also, the lattice fringes of the NVP can be clearly observed and have a d-spacing of 0.62 nm, which corresponds to the (012) plane of the NASICON structure.2324

Detailed characterization of the Ionogels and the SILGMs in terms of phase behavior is provided in the supplementary information (Fig. S2) and conductivity (Table S1). The electrochemical performance of the SILGMs was evaluated by assembling coin cells (CR2032) using the NVP@C as cathode and sodium metal foil as counter/reference 9 ACS Paragon Plus Environment


electrode. A typical cyclic voltammogram (CV) of the SILGMs-based sodium metal battery is displayed in Figure 2(a) over a voltage range of 2.5–3.8 V at a scan rate of 0.01 mV/s. The CV shows a prominent oxidation peak at 3.46 V as well as a reduction peak at 3.31 V vs. Na+/Na, which corresponds to the reversible redox couple of V3+/V4+ during desodiation/sodiation process and is in good agreement with literature.25 Figure 2(b) shows the charge-discharge curves of a Na/SILGMs/NVP@C battery tested in a cell voltage range of 2.5-3.8 V at different current densities at room temperature. Flat charge and discharge plateaus can be observed at 3.4 V and 3.3 V respectively, following the typical behavior of the NVP material. The observed plateaus are consistent with the deinsertion/insertion of sodium ions and reflect the reversible reaction of NVP reported in the literature. The maximum discharge capacity (at 0.1C) was 83 mAh.g-1, which is slightly below the theoretical capacity (117 mAh.g-1). This is attributed to the higher charge transfer resistance values of ionic liquid-based electrolytes against sodium metal when compared to traditional organic liquid electrolytes, as already discussed elsewhere.25-26


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Figure 2- (a) Cyclic voltammogram of SILGMs-based sodium cell; (b) Voltage profiles at various C-rates; (c) Charge–discharge cycling performance of sodium metal battery at different rates; (d) Long term cycling performance of a fresh cell at 1C-rate for 150 cycles.

The rate capability of the sodium-metal battery was tested at various charge/discharge current densities from 0.1 to 2 C (Figure 2(c)). The discharge specific capacities of the



battery with the SILMGs delivered reversible capacities of 81.7, 80.1, 76.8, 71.5, and 60.1 mAh g-1 at 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C, respectively. The cycling performance was investigated by measuring the discharge capacities under a constant current density of 1C (1C=117 mA.g-1) for 150 cycles (Figure 2(d)). It can be seen that the cell exhibited excellent charge–discharge cycling performance, retaining 92% of the initial capacity and close to 100% coulombic efficiency over 150 cycles. Notably the discharge capacity appears to becoming more constant towards the end of the cycle testing. This can be ascribed to the high stability of SILMGs and NVP materials. High stability of supported ionic liquid gel membrane and interfacial stability are also beneficial to improve the cycling performance. In order to explore the dynamics of the interfacial resistance scenario of our gel polymer electrolyte in the battery, electrochemical impedance spectroscopy (EIS) was performed on both a fresh cell and after 150 deep cycles at the 1C-rate (frequency ranging from 106-10-2 Hz). As shown in figure 3, the Nyquist plot for the fresh cell collected at OCP shows a semi-circle at mid-high frequencies and a sloping line at low frequencies. The semi-circle is ascribed to the charge-transfer resistance of the electrode-electrolyte interfaces (Rct) and the sloping line reflects the diffusion of Na ions into the bulk of the electrode material (Warburg diffusion). It is interesting to note that after 150 cycles, the Nyquist plot reveals an extra semi-circle at mid frequencies. To facilitate


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interpretation, both plots were fitted according to equivalent circuit models (fitting parameters are provided in table S2, where CPE stands for constant phase element and W for the Warburg diffusion element). The changes observed for the cycled battery is explained by two factors. First is the contribution of the SEI layer, which is formed on the first cycles due to the decomposition of electrolyte; the SEI layer of this particular electrolyte is well known to form a homogeneous and compact layer, resulting in superior plating/stripping efficiency of sodium on the anode side.1415

The extra semi-circle at high frequency is hence due to the SEI layer formation during cycling.

The second component arises from the charge transfer resistance (Rct), which after 150 cycles increases due to the aging mechanism led by successive de/intercalation of Na ions into the NASICON structure of the cathode. It is important to note that the bulk resistance (the intersection point of the semi-circle to the x-axis, Rb), which is assigned to the gel-electrolyte resistivity does not clearly change after 150 cycles, suggesting that the impedance response of the cell is governed by the SEI layer formation and the charge transfer occurring at the interphase electrode/electrolyte.



Figure 3. Nyquist plot of the Na battery before and after 150 cycles measured at OCP (left) and respective equivalent circuits (right). Left figure Inset: Maximization of EIS spectrum at high frequencies. To verify the safety and flexibility of the SILGMs, a 6 cm x 4 cm laminated cell was assembled in the argon glove box, by sandwiching the SILGM NVP@C coated on aluminium foil and a sodium metal foil anode between standard laminating plastic film. The SILGMs-based sodium battery was capable of powering a LED light under both nonbending and bending conditions, indicating the SILGM provided a strongly adhering core to the device producing good flexibility and recoverable properties (Video S1). Moreover, the safety of our SILGMs was also demonstrated through cutting sodium battery into long strips in open air (Refer to Supporting information for safety procedures). It can be seen from the supplementary video that the flexible battery could still power the LED after this treatment, indicating safety of the SILGM based device towards potential damage in operation (Video S2).

CONCLUSIONS


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In summary, SILGMs were prepared as both electrolyte and separator for the sodium ion battery. The SILGM-based sodium metal battery exhibits a specific capacity retention of 92% at the current density of 1C after 150 charge-discharge cycles with a coulombic efficiency of 99.9%. Furthermore, the SILGMs-based sodium battery exhibits a high flexibility and safe performance. It is conceived that the SILGMs may be an attractive alternative as a solid electrolyte as both electrolyte and separator for sodium batteries that would also be suitable for roll to roll manufacturing. ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website.

Experimental Section, photograph of the Na-based ionogel, Differential scanning calorimetry

traces,

ionic

conductivity

of

samples,

spectroscopy - fitting parameters, safety procedures.


electrochemical

impedance


AUTHOR INFORMATION

Corresponding Authors Email: [email protected]; [email protected]; [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

D. R. MacFarlane is grateful to the Australian Research Council for his Australian Laureate Fellowship. X. M. Zhang acknowledges China Scholarship Council for partial financial support (CSC. 201606190114) and the program B for outstanding PhD candidate of Nanjing University (201702B054).

REFERENCES


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TOC Graphic



A

Na-based

Supported

Ionic

Liquid

Gel

Membrane

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ensures

electrolyte/separator with superior safety for flexible sodium metal batteries.


a

non-flammable

Supported Ionic Liquid Gel Membrane Electrolytes for a Safe and

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