Electropolymerisation Technologies for Lithium–Sulphur Batteries

Electropolymerisation Technologies for Lithium–Sulphur Batteries: Comparison

Please note this is a comparison between Version 1 by Soochan Kim and Version 2 by Sirius Huang.

Lithium–sulphur batteries (LiSBs) have garnered significant attention as the next-generation energy storage device because of their high theoretical energy density, low cost, and environmental friendliness. However, the undesirable “shuttle effect” by lithium polysulphides (LPSs) severely inhibits their practical application. To alleviate the shuttle effect, conductive polymers have been used to fabricate LiSBs owing to their improved electrically conducting pathways, flexible mechanical properties, and high affinity to LPSs, which allow the shuttle effect to be controlled.

electropolymerisation
conductive polymer
battery components
lithium–sulphur batteries

1. Introduction

Polymers are ubiquitous and associated closely to human life, such as in food, housing, transportation, and electric applications. In general, polymers are large molecules composed of repeated organic building blocks, and they were regarded as electrically insulating materials prior to the introduction of conducting polymers [1]. Conductive polymers exhibit unique electrical and optical properties, which are similar to those of inorganic semiconductors [2]. Polyacetylene, as a representative electrically conductive polymer with the simplest linear conjugated structures, was discovered by Shirakawa, Heeger, and MacDiarmid and has been widely investigated to understand its application in microelectronics such as organic semiconductors [3]. The discoverers of polyacetylene were awarded the Nobel Prize in Chemistry (2000) [4]. In addition to polyacetylene, various conductive polymers such as polyaniline (PANI), polypyrrole (PPy), polythiophene, poly(3,4-ethylene dioxythiophene) (PEDOT), poly(para-phenylene), poly(phenylenevinylene), and polyfuran have been introduced into the core of next-generation energy, biological, and environmental applications ^[5][6][5,6].

Various approaches have been introduced to synthesise conductive polymers, such as chemical, electrochemical, photochemical, metathesis, concentrated emulsion, inclusion, solid-state, plasma polymerisation, and pyrolysis [7]. Among them, electropolymerisation (EP) is a cost-effective, facile, and rapid technique that can provide unique and sophisticated morphological structures depending on the applied polymerisation conditions (current density, potential, time, temperature, additives, etc.) [8].

EP is considered a promising technique for the fabrication of high-performance lithium–sulphur batteries (LiSBs) owing to their facile and controllable processes. LiSBs have been recognised as promising high-energy-density rechargeable batteries since their introduction in the 1960s ^[9][12]. Sulphur, as a cathode material, offers various advantages such as its low cost, abundance in nature, and high theoretical energy density (~2600 Wh Kg⁻¹) ^[10][11][12][13,14,15]. However, the practical applications of LiSBs are impeded by challenges such as the low electrical conductivity of sulphur (~5 × 10⁻²⁸ S m⁻¹), the significant volumetric change (~80%) in sulphur upon lithiation, and lithium polysulphide (LPS) shuttle effects caused by the dissolution of intermediate LPS species during cycling. When the solid S₈ accepts Li⁺ ions from the liquid electrolyte by electrochemical reaction, S₈ will convert into long-chain LPSs. Long-chain LPS can dissolve into liquid electrolyte and diffuse to the anode side, losing the active material and increasing the viscosity of the electrolyte. Diffused LPS will react with the Li metal anode and form a solid-state short-chain LPS (Li₂S or Li₂S₂) layer on the surface of the anode as a passivation layer ^[10][11][12][13,14,15]. Moreover, LPS shuttles, as the main issues hindering the practical application, are accelerated under practical conditions of LSBs such as high loading content of sulphur, low electrolyte–sulphur ratio (E/S ratio, μL mg⁻¹) and low capacity ratio between the anode and cathode (N/P ratio). To suppress LPS shuttles, various strategies have been proposed based on the modification of the materials used for the cathode, electrolyte, separator, and anode ^{[13][14][15][16]}[16,17,18,19]. For example, sulphur host design is based on nanostructured organic/inorganics, interlayers, electrolyte additives, a Li-alloy anode, and so on ^[15][16][18,19]. In particular, methods for trapping or obstructing LPSs during battery cycling have been extensively investigated, ^{[12][13][14][15][16]}[15,16,17,18,19] and EP technologies have been considered promising for effectively controlling LPSs in the cathode and separator in a simple and sophisticated manner.

2. Application in Cathode

In terms of the optimisation of sulphur cathodes, EP technologies have been reported as an effective strategy for designing cathode materials, sulphur hosts, and coating layers on the cathode. In the various conductive polymers, thiol-containing polymers are promising candidates for sulphur cathodes ^[17][24]. At temperatures above 180 °C, linear polysulfane can be obtained along the thiol surfaces through the ring-opening of cyclic S₈ ^[11][17][18][14,20,24]. Thiol-based monomers cannot polymerise, but they can easily form copolymers with molten sulphur by connecting thiol groups and thiyl radicals. Consequently, sulphur is homogeneously distributed within the polymer matrix ^[17][24].

Ning et al. introduced a poly(4-(thiophene-3-yl) benzenethiol) (PTBT)-based organosulphur cathode, which was constructed via EP and vulcanisation without a binder or carbon additives ^[19][25]. A 4-(thiophene-3-yl)benzenethiol monomer was polymerised onto a nickel foam current collector via cyclic voltammetry to form porous and conducting PTBT frameworks, which served as chemical binding sites for LPSs and sites for sulphur hosts through vulcanisation. This strategy not only exploits the advantage of the chemical confinement of the conductive polymer for improving the performance of cathodes but it also simplifies the fabrication process of electrode materials for LiSBs. The S/PTBT cathode showed remarkable effects in suppressing LPS shuttles via covalent bonds between polymers and LPSs, and the assembled LiSBs showed stable long-term battery cycling performance over 200 cycles, with a low capacity fading rate of 0.178% per cycle (E/S ratio = 18.7; S-loading = 1.6 mg cm⁻²) ^[19][25].

Schütze et al. investigated the effect of the regiochemistry of an electropolymerised PTBT as an organosulphur cathode on its aggregation behaviour and charge transport ^[20][26]. The morphology of a polymer can significantly affect its conductive behaviour. Generally, polymers with more ordered and aligned structures provide more efficient charge transport, thus yielding higher conductivities compared with polymers with disordered structures ^[20][21][26,27]. Schütze et al. discovered that polymer chains can form crystalline phases only for a regioregular head-to-tail/head-to-tail regularity, whereas for a head-to-head/tail-to-tail regularity, the system did not show any long-range order after annealing. Furthermore, electronic structure simulations were performed to observe the mechanism by which the structural disorder in the head-to-head/tail-to-tail phases resulted in an energetic disorder that can potentially limit charge transport. Based upon the simulation results and the structural understanding obtained, their study provided new insights into the importance of polymer regularity and morphology modifications in the design of high-mobility crystalline phases, which allows the electrochemical performance of cathode materials for LiSBs to be optimised. In addition, this model can be used for more realistic cathode structures and geometries, including those that consider the effects of cross-linking sulphur between polymer chains as well as the degrees of freedom of the solvent and electrolyte ^[20][26]. As a sulphur host that can be prepared via EP, polycarbazole (PCBZ) is considered a promising material, owing to its electrical conductivity of 10⁻²–10⁻³ S cm⁻¹, low monomer cost, high chemical stability, and porous structure ^[21][22][23][27,28,29]. The carbazole monomer can undergo EP from either the 3– and 6– or 2– and 7– positions, rendering it extremely versatile ^[21][24][27,30]. In addition, electropolymerised PCBZ (a p-type conducting polymer) maintains positively charged sites on its backbone because of the oxidative EP, which facilitates LPS adsorption, thus resulting in highly stable LiSB cycling. The approach of coating the cathode surface and active materials via EP has been utilised in many studies on other conversion-type electrodes (metal chalcogenides, Si, metal fluorides, etc.) as well as sulphur cathodes. This approach allows the electrode surface to be controlled precisely based on the polymerisation conditions. Kaiser et al. fabricated a flexible freestanding PPy film via EP, which was then employed in a self-supported slurry-free sulphur-containing flexible electrode ^[25][31]. In the EP process, a freestanding PPy film was prepared by applying different current densities at a fixed charge density of 14.4 C cm⁻² onto a stainless-steel plate (the operating electrode). Subsequently, the freestanding PPy film was peeled off from the stainless-steel plate and then attached to a powder-pressed sulphur cathode through mechanical pressing, which resulted in a sandwich structure. This sandwich electrode not only facilitated the activation of sulphur materials but also served as a barrier to minimise LPS shuttling and provided design flexibility and high areal capacity. In addition, the PPy film was a protective film that maintained the structural stability after many rounds of cycling and addressed the structural instability of the cathode caused by the significant volume expansion of sulphur. Thus, the fabricated sandwich electrode exhibited an initial discharge capacity of 804 mAh g⁻¹ and stable cycling behaviour after 400 cycles, with a Coulombic efficiency of 93% (E/S ratio = 33; S-loading = 3 mg cm⁻²) ^[25][31].

3. Application in Anode

Li is well known as one of the most promising materials for anodes, owing to its ultrahigh capacity (3860 mAh g⁻¹) and extremely low standard negative electrochemical potential ^[26][27][34,35]. However, LPS shuttles can corrode the surface of Li via the formation of a Li₂S layer on the Li surface, thus resulting in low Coulombic efficiency and Li−S batteries with short cycle lives. In addition, the formation of an unstable solid–electrolyte interphase (SEI) by organic electrolytes and LPS shuttles can result in irregular Li plating/stripping and Li dendrite growth during repeated cycling ^{[28][29][30][31]}[36,37,38,39]. Therefore, the formation of a stable SEI layer on the surface of Li that is corrosion-resistant is necessitated for realising high-performance LiSBs. As a representative strategy, a protective layer is formed on the surface of Li, which should exhibit the following characteristics: chemical/electrochemical stability to electrolytes, high Li-ion diffusion rate, stability to LPSs, and flexible mechanical properties ^{[30][32][33][34][35]}[38,40,41,42,43]. However, even if an ideal protective layer exists, the uniform coating on the surface of Li for constructing a stable interface between the protective layer and Li remains an issue to be addressed. In this regard, EP technologies are considered a promising strategy that can achieve uniform coverage on the surface of Li, depending on the EP conditions. Liu et al. demonstrated the formation of a protective polymeric layer on the surface of Li by electrochemically regulating the polymerisation of allyl disulphides ^[30][38]. An allyl disulphide monomer was mixed with an ether-based electrolyte (1,3-dioxolane; DOL and 1,2-dimethoxyethane; DME) and LiTFSI salt; subsequently, this electrolyte was injected into a Li symmetric cell, which was discharged at a current of 0.5 mA cm⁻² for 3, 10, and 20 h. Lithium isopropyl sulphide macromolecules, which formed a protective layer during EP and indicated different mechanical properties, ionic conductivities, and electrochemical stabilities. The protective layer deposited for 10 h presented the best balance. The ideal protective layer not only afforded sufficient mechanical strength to resist dendrite growth but also weakened the diffusion of LPS anions toward the Li metal, thus inhibiting their reduction to insulated Li₂S on Li. Therefore, the resulting Li symmetric cell with the protective layer presented a voltage plateau and a low overpotential (~28.2 mV) at 2.0 mA cm⁻² for 700 cycles. At a high current density (8.0 mA cm⁻²), the symmetric cell maintained its stable long-term Li plating/stripping behaviour. When the anode was paired with a sulphur cathode, the LiSBs (E/S ratio = 10; S-loading = 2.5 mg cm⁻²) showed an ultralong cycle life and high-rate capacity, thus demonstrating their promising application in LiSBs. Moreover, although they have not been applied to LiSBs, the protective layers on the surface of Li formed via the EP of PANI and polyacrylonitrile, as reported by Xiong et al. ^[36][44] and Zhang et al. ^[37][45], are promising candidates.

4. Application in Separator

In conventional Li-ion batteries, the separators do not participate in electrochemical reactions; they provide physical separation between the cathode and anode and thus battery safety ^[29][38][37,46]. In the case of LiSBs, which exhibit severe issues, the separator must provide additional functions, such as suppression of the LPS shuttle via physical or electrochemical/chemical methods, improved mechanical properties, and enhanced electrolyte wettability ^[39][40][47,48]. Since the introduction of the concept of an interlayer between the electrodes and separator by Manthiram et al. in 2012, researchers have proposed inserting an interlayer and coating functional materials onto the separator, which has been shown to be a cost-effective and facile strategy for improving the cycling performance of LiSBs ^[41][42][49,50]. Inserting interlayers composed of organic or inorganic materials enhances the regulation of LPS shuttles and Li dendrite growth, thus resulting in stable long-term cycling performance ^[43][51]. However, loading/inserting organic or inorganic materials as an interlayer increases the thickness/weight, obstructs ion-transport pathways, and requires a significant amount of the electrolyte ^[29][44][37,52]. Therefore, the development of a thin functional interlayer with facile ion transport, similar to that of conventional separators, remains challenging.

EP technologies that can form thin and uniform interfaces on the separator based on EP conditions are regarded as a feasible approach for developing next-generation functional separators, and the electrical conductivity of CPs can expand the electrochemically active area for the conversion of diffused LPSs ^[45][46][54,55]. Guo et al. introduced a new EP strategy to develop an in situ PCBZ-type interlayer ^[47][53]. Using a carbon nanotube-coated commercial polypropylene separator and a TCB monomer in an electrolyte mixture (dichloromethane/acetonitrile, 3/2, v/v), they grew a PCBZ interlayer via cyclic voltammetry scanning at 50 mV s⁻¹ between –0.8 and 1.03 V (vs. Ag/AgCl). The developed interlayer showed a porous morphology with uniform 0.82 nm nanochannels and an ultrathin thickness of 60 nm, which afforded facile ion transport and a high redox-active area as an interacting site for the LPSs. The LiSBs developed with this interlayer (E/S ratio = 10; S-loading = 2.1 mg cm⁻²) exhibited high sulphur utilisation and stable battery cycling, with a reversible capacity of 920 mAh g⁻¹ after 600 cycles at 0.2 C. Even at high sulphur loading conditions, the Li–S cell showed remarkable battery cycling of 10 mAh cm⁻² ^[47][53].

5. Application in Electrolyte

In LiSBs, ethers, including DOL and DME, are considered suitable solvents owing to their compatibility with LPSs ^[48][49][50][56,57,58]. Binary mixtures of electrolyte (1 M LiTFSI in DME/DOL, 1/1, v/v) are typically utilised to achieve LiSBs of low viscosity, high ionic conductivity, and favourable electrochemical performance ^[45][48][54,56]. However, LPS shuttles remain during battery cycling because of the high solubility of highly ordered Li₂S_x (x

\geq

4), resulting from the Lewis basicity of the oxygen lone pair electrons of ethers and the Lewis acidity of the Li atoms of Li₂S_x ^[48][56]. Therefore, a few methods have been proposed to control LPSs in electrolytes, such as the development of new Li salts and new mixtures of electrolyte solvents, the use of organic/inorganic additives, and the design of solid/quasi-solid-state electrolytes ^{[49][51][52][53]}[57,59,60,61]. The use of new additives in electrolytes can form stable interfaces on the cathode and anode surfaces and inhibit LPS shuttles. Recently, the development of next-generation batteries to improve the overall battery safety and energy density through the use of solid/semi-solid electrolytes has received considerable attention ^[54][55][56][62,63,64]. For next-generation electrolytes, EP technologies have enabled the formation of stable interfaces between electrodes through the polymerisation of additives in the electrolytes and the development of polymer electrolytes.