Elsevier

Journal of Energy Chemistry

Volume 54, March 2021, Pages 587-594
Journal of Energy Chemistry

2D polyaniline with exchangeable interlayer fluid for fast and stable volumetric dual ion storage

https://doi.org/10.1016/j.jechem.2020.06.015Get rights and content

Abstract

Two-dimensional (2D) layered materials are widely applied in energy devices including lithium-ion battery and supercapacitor due to their unique properties, such as tunable interlayer structure, numerous active sites, large aspect ratio versatile interlayer chemistry. In this work, 2D layered tungstate acid-linked polyaniline (TALP) presented a fluid-in-solid structure, which allowed facile exchange of the interlayer fluid from moisture to conventional Li+ containing electrolyte. With fast and stable dual ion storage (Li+ and PF6), TALP demonstrates high-rate volumetric capacity (39 mAh cm−3 at 2000 mA g−1) and good stability (2000 cycles at 200 mA g−1) within the working potential window of 1.5–4.5 V versus Li+/Li.

Graphical abstract

The interlayer of tungsten acid-linked polyaniline is modified via solvent exchange method to generate an organic electrolyte-filled ion diffusion channel, which enables the intercalation/de-intercalation of both cation and anion.

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Introduction

Lithium-ion capacitor (LIC) has high energy density, high power output and long lifespan because of its integration of a supercapacitor-type electrode and a lithium-ion battery type electrode. LIC is widely regarded as one competitive player for next generation electrochemical energy systems (EESs). Dual-ion intercalation is a common electrode mechanism in LIC, which could greatly enhance the capacity [1]. There are few dual-ion hosts. The well-studied examples include graphite, metal organic frameworks, polycyclic aromatic hydrocarbon and diamino-rubicene [2]. However, the lifespan and power performance of the reported materials are very limited, which is because of the narrow interlayer spacing that impedes ion movement [3]. Hence, porous carbon-based materials, which permit more surface-like anion diffusion, are studied recently for dual-ion host with rapid charge/discharge capability and long life span, e.g. activated carbon, N-doped porous carbon foam and porous graphitic carbon [4], [5], [6]. However, given the low density of porous carbon-based materials, their application in compact devices may be limited [7]. A novel dual ion host with large interlayer spacing, high volumetric performance and long lifespan is highly demanded for advanced LICs.

2D layered material is widely studied for LIC applications because of the strong in-plane chemical bond and the weak interlayer van der Waals force. The unique 2D characteristics could provide more active surface sites and bulk channels for easy ion adsorption/intercalation and redox reactions [8]. To enhance the electrochemical performance, pillared 2D structures using organic interlayer spacers have attracted growing interest [9], [10]. Notably, it is reported that the layered structure could also be expanded by the interlayer fluids, e.g. moisture. These 2D fluid-in-solid structures could render fast ion dynamics and less volume variation during charging/discharging, which is essentially attributable to the interlayer fluid [11]. The interlayer fluid could behave like ultrathin layer of electrolyte that can effectively reduce the charge transfer resistance by relaxing the ion desolvation. The interlayer water in MXene (Ti3C2) has shown its role in improving the rate capability for capacitive charge storage [12]. Similar phenomena were also observed in other 2D carbon-based structures [13], [14] and layered hydrated oxides [15], [16]. Despite the arising interest in the 2D structures, little has been devoted to understanding the functions of fluid-in-solid 2D electrodes for dual ion storage.

In this work, we report the interlayer fluid exchange and the dual-ion storage of 2D tungstic acid-linked polyaniline (TALP), which is made through the synchronous supramolecular self-assembly and conjugated polymerization of tungstate acid and aniline [17], [18]. The original TALP was made in aqueous solution and its hydrophilic surfaces retained hydrogen-bonded water within the layered gallery, giving rise to a 2D fluid-in-solid structure. By stepwise reducing the solvent polarity in a two-step solvent exchange treatment, the H-bonded water was replaced with ester electrolyte. The solvent exchange relaxed the lamellar spacing by softening the interlayer hydrogen bonding and made the modified TALP more lithophilic to the ester electrolyte. The fluid-exchanged TALP exhibited high-rate volumetric capacity of 39 mAh cm−3 at 2000 mA g−1 and long lifespan of 2000 cycles at 200 mA g−1. This charge storage mechanism was validated to be the (de)intercalation of dual ions (Li+ and PF6) by ion switching that was regulated by the electrode potentials.

Section snippets

Synthesis of TALP

Solution A of 0.15 M H2SO4 and 0.1 M aniline mixing solution was prepared by diluting 7.5 mL 2 M H2SO4 into 100 mL in a jacketed reaction beaker and adding 0.93 mL aniline solution into the beaker subsequently. Circulating water bath (John Morris Scientific) was used to control the reacting temperature around 5 °C and solution A was stirring by magnetic stirrer continuously.

Solution B of 0.2 M ammonium persulfate (APS, Sigma-Aldrich) and 0.05 M ammonium metatunagstate (AMT, Sigma-Aldrich, ≥85%

Results and discussion

Both the physical and chemical structures of TALP are different from polyaniline (PANi) as shown in Fig. 1. From the XRD pattern presented in Fig. 1(a), a notable peak at low degree (2θ = 7.47°) is detected in the TALP powder, while the other peaks relating to the crystalline structure of PANi decrease or disappear, which are unlike the twisted molecular structure of PANi [22]. TALP owns a 2D layered structure with an interlayer distance of 11.8 Å as illustrated in Fig. S1 [17], [18]. Besides,

Conclusions

In summary, 2D layered structure tungsten acid-linked polyaniline (TALP) was synthesized and modified by a facile method and studied as cathode active material for lithium-ion capacitors. The interlayer spacing of TALP was tuned by solvent exchange with various nanoconfined fluids from moisture to electrolyte solvent, enabling TALP stores charge ion exchange mechanism. with the benefit of 2D layered structure and solvated electrolyte, TALP cathode exhibits superior rate performance (39 mAh cm−3

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.

Acknowledgments

This work was financially supported by the Australian Research Council Discovery Projects Discovery Project (DP190101008), Future Fellowship (FT190100058), ARC ITRP (IH180100020), the UNSW Scientia Program, and the UNSW-SJTU joint grant. This research was partially supported by funding from the UNSW Digital Grid Futures Institute, UNSW, Sydney, under a cross disciplinary fund scheme; the views expressed herein are those of the authors and are not necessarily those of the institute. The authors

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