Infilling of highly ion-conducting gel polymer electrolytes into electrodes with high mass loading for high-performance energy storage

https://doi.org/10.1016/j.jiec.2020.03.039Get rights and content

Highlights

  • Infilling of gel polymer electrolytes into thick electrodes with high mass loadings was performed.

  • Via the infilling, large extent of electrical double layers was formed.

  • The process led to excellent rate performance and energy density.

  • The flexible device provided reliable power output under electrochemical and mechanical stresses.

Abstract

Full utilization of electrodes toward high-performance energy storage is challenging in cases where electrode/electrolyte interface is significant. From a practical perspective, this is particularly important in cases where a thick electrode or one with a high mass loading is needed. Here, we report an approach to increase the electrode performance by the infilling of a highly ion-conductive organic gel polymer electrolyte (EI-GPE, ionic conductivity ∼9.2 mS cm−1) into a multi-walled carbon nanotube (MWCNT) electrode with high mass loadings of up to 26 mg cm−2 (or significant thicknesses of up to 443 μm). Typical GPE (t-GPE) with a film-forming property but moderate ionic conductivity (1.2 mS cm−1) is then placed over the EI-GPE-filled electrode surface, resulted in flexible supercapacitor. Infilling of EI-GPE into MWCNT electrode provides a large-ion accessible interface that affords the increase in volumetric capacitance and energy density, about sixfold greater than that of the typical supercapacitors configured by sandwiching t-GPE as both electrolyte and the separator between a pair of electrodes. Importantly, this method enables scaling of the areal capacitance with electrode thickness (or mass loading of active material). A pouch type EI-SC provides stable performance after bending, suggesting it holds the promise of flexible energy storage.

Introduction

Advances in electrode materials drive considerable improvement of energy storage technologies [1], [2]. Most of studies for energy storage have been concentrated on electrode materials accordingly. Now there is certainly a desire of a way to maximize electrode efficiency, for example, the development of high-performance electrolyte which is the other key component of energy storage or a method of using the electrolytes developed. Polymer-based electrolytes including dry solid polymer electrolytes (SPEs) and gel polymer electrolytes (GPEs) have received great interest due to their form factor flexibility, which offers a significant advantage over liquid electrolytes for their target applications, typically as power sources to flexible electronics [3], [4], [5]. GPEs are attracting tremendous attention, since they could potentially provide intermediate properties between those of dry SPEs and liquid electrolytes such as high ionic conductivity (10−3 to 10−5 S cm−1) and good electrochemical stability while maintaining a solid-state form [6], [7]. They can also solve the intrinsic shortcomings of liquid electrolytes including leakage of harmful liquid, volatility, flammability, and poor chemical and electrochemical properties [8], [9].

Energy storage is creating a promising opportunity for GPE applications, wherein their large modulus and ionic conductivity give them a particular advantage over liquid electrolytes such as flexible supercapacitors (SCs) and secondary batteries. SCs, also called electrochemical capacitors, are attracting interest, driven by the promise of addressing requirements of high power, high round-trip efficiency, and approximately infinite cycle life [10], [11], [12], [13], [14], [15]. However, they still suffer from poor energy density (5–8 Wh L−1) compared to those of Li-ion batteries (50–90 Wh kg−1) [16], [17]. SCs commonly store energy via reversible ion adsorption at the surface of the electrode by creating so-called electrical double layers (EDLs) formed at the electrode/electrolyte interface [18], [19], [20]. The energy density of SCs thus could be increased by increasing the extent of EDLs. It is known that liquid electrolytes could be fully accessible to the electrode surface due to their good flowability, resulting in good formation of EDLs. However, it is challenging to provide large energy density using GPEs, since moderate EDLs, particularly in thick electrodes, form owing to their limited flow into the pores [21], [22], [23].

The development of porous carbon nanomaterials as electrode materials constitutes the most successful route to improving the energy density for SCs because they are good for wetting by electrolyte ions. There have been lots of advances in the fields of SCs, which can be mostly attributed to well-developed pores in the carbon electrodes. However, these porous structures exhibit very low active mass loading per electrodes (typically <10 mg cm−2) [24], [25]. As such, appropriate thickness and active mass loading of carbon electrodes, at least 100–200 μm and >10 mg cm−2, respectively, are the mainstay of practical applications [26]. Otherwise, mass-based SC performances are likely overestimated.

Here, we propose an approach of improving electrochemical performances of SCs that can be adapted for thin to thick electrodes with mass loadings as high as 26 mg cm−2 (with the electrodes being as thick as 443 μm in thickness). Flexible, high-performance SCs were prepared by the infilling of highly ion-conductive GPE into electrodes. A continuous GPE membrane containing PEO-based polymers as a matrix and an organic liquid electrolyte was formed throughout the electrode, leading to a substantial electrode/electrolyte interface. The SCs showed an exceptional volumetric/areal capacitance and energy density, rate-retention capability, and long cycle lives even under mechanical deformation.

Section snippets

Preparation of electrode-infilling gel polymer electrolyte (EI-GPE) and typical gel polymer electrolyte (t-GPE)

A precursor solution for EI-GPE was prepared by mixing poly(ethylene glycol) methyl ether acrylate (4 g, PEGMA, Mn = 500, Aldrich) and trimethylolpropane ethoxylate triacrylate (0.38 g, ETPTA, Mn = 428, Aldrich) for 60 min at a molar ratio of 9:1 and then 2-hydroxyl-2-methyl-1-phenyl-1-propanone (0.14 g, HMPP, Aldrich) as a photoinitiator (3 wt% against the precursor) was added to the solution. A mixture of the precursor solution and 1.0 M lithium hexafluorophosphate (LiPF6) in an ethylene

Result and discussion

Gel polymer electrolytes (GPEs) were made from photo-curable monomers including PEGMA and ETPTA with a photo-initiator by irradiating UV in the presence of an organic liquid electrolyte, 1 M LiPF6 EC/DMC (Fig. 1a). In this study, the mechanical properties and ionic conductivities of GPEs, both of which are exceptionally significant for solid polymer electrolytes, were tuned by simply changing the wt% of the mixture of PEGMA and ETPTA to organic liquid electrolyte. The composition of the polymer

Conclusions

In summary, fabrication of high-performance flexible SCs via the infilling of largely ion-conducting GPE (EI-GPE) into thick electrodes was successfully demonstrated. Combining photo-curable monomers and an organic liquid electrolyte enabled the complete wetting of the electrode by the electrolyte. Following polymerization through UV irradiation, a substantial electrical double layer was formed at the electrode/electrolyte interface. It afforded improved SC performances in terms of the

Conflict of interest

None declared.

Acknowledgements

This work was also supported by the Materials and Components Technology Development Program (no. 10062226) funded by the Ministry of Trade, Industry & Energy (MOTIE/KEIT, Korea) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2018R1A4A1025528).

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