Recent Progress of Boron-based Materials in Lithium-sulfur Battery

Author: LI Gaoran, LI Hongyang, ZENG Haibo

MIIT Key Laboratory of Advanced Display Materials and Devices, Institute of Nano Optoelectronic Materials, School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094

Abstract

Lithium-sulfur (Li-S) batteries play a crucial role in the development of next-generation electrochemical energy storage technology due to its high energy density and low cost. However, their practical application is still hindered by the sluggish kinetics and low reversibility of the conversion reactions, which contribute to relatively low practical capacity, Coulombic inefficiency, and cycling instability. In this regard, the rational design of conductive, adsorptive and catalytic functional materials presents a critical pathway to stabilize and promote sulfur electrochemistry. Benefiting from the unique atomic and electronic structures of boron, boron-based materials exhibit multifarious and tunable physical, chemical and electrochemical properties, and have received extensive research attentions in Li-S batteries. This paper reviews the recent research progress of boron-based materials, including borophene, boron atom-doped carbon, metal borides and non-metal borides in Li-S batteries, concludes the remaining problems and proposes the future developing perspective.

Keywords: lithium-sulfur battery, boride, chemical doping, borophene, shuttle effect, review

Developing green renewable energy, developing advanced energy conversion and storage methods, and establishing an efficient and clean energy system are inevitable choices to deal with the energy crisis and climate change in today's world. Electrochemical energy storage technology, represented by batteries, can convert and store new clean energy and utilize it in a more efficient and convenient form, playing an important role in promoting green energy economy and sustainable development [1,2]. Among many battery technologies, lithium-ion batteries have the advantages of high energy density and no memory effect. It has achieved rapid development since its commercialization in 1991, and has been widely used in electric vehicles, portable electronic devices, national defense and other fields [3,4]. However, with the continuous development of electrical equipment, traditional lithium-ion batteries have been unable to meet the growing energy demand. Against this background, lithium-sulfur batteries have attracted widespread attention due to their high theoretical specific capacity (1675 mAh·g-1) and energy density (2600 Wh∙kg-1). At the same time, sulfur resources are abundant, widely distributed, low-priced, and environmentally friendly, making lithium-sulfur batteries a research hotspot in the field of new secondary batteries in recent years [5,6].

1 Working principle and existing problems of lithium-sulfur batteries

Lithium-sulfur batteries usually use elemental sulfur as the positive electrode and metallic lithium as the negative electrode. The basic battery structure is shown in Figure 1(a). The electrochemical reaction is a multi-step conversion reaction process involving multiple electron transfers, accompanied by solid-liquid phase transition and a series of lithium polysulfide intermediates (Figure 1(b)) [7,8]. Among them, elemental sulfur and short-chain Li2S2/Li2S located at both ends of the reaction chain are insoluble in the electrolyte and exist in the form of precipitation on the electrode surface. Long-chain lithium polysulfide (Li2Sx, 4≤x≤8) has higher solubility and migration ability in the electrolyte. Based on the intrinsic properties of electrode materials and their solid-liquid phase transformation reaction mechanism, lithium-sulfur batteries have energy and cost advantages, but they also face many problems and challenges [9,10,11,12]:

Fig. 1   Schematic diagram of (a) lithium-sulfur battery configuration and (b) corresponding charge-discharge process[7]

1) Solid-phase elemental sulfur and Li2S accumulate on the electrode surface, and their intrinsic electron and ion inertia lead to difficulty in charge transmission and slow reaction kinetics, thereby reducing the utilization rate of active materials and the actual capacity of the battery.

2) There is a large density difference between sulfur and Li2S at both ends of the reaction chain (2.07 vs 1.66 g∙cm-3). The material experiences a volume change of up to 80% during the reaction process, and the mechanical structural stability of the electrode faces huge challenges.

3) The dissolution and migration behavior of lithium polysulfide in the electrolyte causes a severe "shuttle effect", resulting in severe active material loss and Coulomb loss. In addition, lithium polysulfide participates in chemical/electrochemical side reactions on the anode surface, which not only causes further loss of active materials, but also passivates and corrodes the anode surface, aggravates the formation and growth of lithium dendrites, and increases safety risks.

These problems are interrelated and influence each other, which greatly increases the complexity of the battery system, making it difficult for current lithium-sulfur batteries to meet the needs of practical applications in terms of active material utilization, actual energy density, cycle stability and safety. From the analysis of the above problems, it can be seen that reasonable control of the sulfur electrochemical reaction process is the only way to improve the performance of lithium-sulfur batteries. How to achieve effective management and improvement of sulfur electrochemistry depends on the targeted design, development and application of advanced functional materials. Among them, the most representative strategy is to develop functional materials with conductive, adsorption and catalytic properties as sulfur cathode hosts or modified separators. Through its physical and chemical interaction with lithium polysulfide, the active material is confined to the positive electrode area, inhibiting dissolution and diffusion, and promoting its electrochemical conversion. Thereby alleviating the shuttle effect and improving the energy efficiency and cycle stability of the battery [13,14]. Based on this idea, researchers have developed various types of functional materials in a targeted manner, including carbon materials, conductive polymers, metal organic frameworks, metal oxides/sulfides/nitrides, etc. Good results have been achieved [15,16,17,18,19].

2 Application of boron-based materials in lithium-sulfur batteries

Boron is the smallest metalloid element. Its small atomic radius and large electronegativity make it easy to form metallic covalent compounds. Boron atoms have a typical electron-deficient structure, and their valence electron configuration is 2s22p1. They can share one or more electrons with other atoms through various hybridization forms to form multi-center bonds [20,21]. These characteristics make the boride structure highly tunable, showing unique and rich chemical and physical properties, and can be widely used in many fields such as light industry, building materials, national defense, energy, etc. [22,23]. In comparison, the research on boron-based materials in lithium-sulfur batteries is still in its infancy. In recent years, nanotechnology and characterization methods have continued to advance, and the structural characteristics of boron-based materials have been continuously explored and developed, making their targeted research and application in lithium-sulfur systems also beginning to emerge. In view of this, this article focuses on typical boron-based materials such as borophene, boron atom-doped carbon, metal borides, and non-metal borides. This article reviews the latest research progress in lithium-sulfur batteries, summarizes existing problems and looks forward to future development directions.

2.1 Borene

As a very representative allotrope among boron elements, borophene has a single-atom-thick two-dimensional structure similar to graphene. Compared with bulk boron element, it shows superior electrical, mechanical and thermal properties and is a rising star in two-dimensional materials [24]. Based on topological differences in the arrangement of boron atoms, borophene has rich crystal structures and electronic properties, as well as anisotropic conductive properties. As can be seen from Figure 2(a, b), electrons in borophene tend to be concentrated on the top of boron atoms, and these electron polarization regions have higher bonding activity. It is expected to provide good chemical adsorption sites for polysulfides in lithium-sulfur battery systems [25]. At the same time, the borophene film has good electrical conductivity and physical and chemical stability, so it has good application potential in lithium-sulfur batteries.

Fig. 2   (a) Structural models of different borophenes and their corresponding charge density distributions, (b) adsorption energies of polysulfides on different borophenes[25]

Jiang et al. [26] found through theoretical calculations that borophene shows strong adsorption capacity for lithium polysulfide. However, this strong interaction can also easily trigger the decomposition of Li-S clusters, resulting in the loss of sulfur, the active material. In comparison, the surface of borophene with an intrinsic defect structure adsorbs lithium polysulfide more gently [27], which allows it to limit the shuttle behavior while avoiding the decomposition and destruction of the ring structure. It is expected to become a more suitable lithium polysulfide adsorption material. At the same time, the energy band analysis results of the borophene-lithium polysulfide adsorption structure show that the adsorption clusters are metallic, which is mainly due to the intrinsic metallic characteristics of boron and its strong electroacoustic coupling strength. It is expected to help the electrochemical conversion process of sulfur to obtain better reaction kinetics [28]. In addition, Grixti et al. [29] simulated the diffusion process of lithium polysulfide molecules on the surface of β12-borene. It was found that β12-borene showed strong adsorption to a series of lithium polysulfides. The lowest diffusion energy barriers of Li2S6 and Li2S4 molecules in the armchair direction are 0.99 and 0.61 eV respectively, which is easier than the diffusion in the zigzag direction. Thanks to its good adsorption capacity and moderate diffusion energy barrier, β12-borene is considered an excellent lithium polysulfide adsorption material, which is expected to suppress the shuttle effect in lithium-sulfur batteries and improve the reversibility of sulfur electrochemical reactions.

However, most of the current research on boron dilution in lithium-sulfur batteries still remains at the theoretical prediction stage, and experimental confirmations are rarely reported. This is mainly due to the difficulty in preparing boron dilute. The existence of boron was predicted in the 1990s, but it was not actually prepared until 2015 [30]. Part of the reason may be that boron has only three valence electrons and needs to form a framework structure to compensate for the missing electrons, making it easier to form a 3D rather than a 2D structure. At present, the preparation of boron usually relies on technologies such as molecular beam epitaxy and high vacuum, high temperature and other conditions, and the synthesis threshold is high [31]. Therefore, it is necessary to develop a simpler and more efficient boron dilute synthesis method, and further experimentally explore and demonstrate its effect and related mechanisms in lithium-sulfur batteries.

2.2 Boron atoms doped carbon

Chemically doped carbon materials are hot materials in the field of new energy research. Appropriate element doping can retain the advantages of carbon materials such as lightweight and high conductivity, while giving them additional physical and chemical properties to adapt to different application scenarios [32,33]. Chemically doped carbon materials have been widely studied in lithium-sulfur batteries [34,35], among which doping with highly electronegative atoms such as nitrogen atoms is more common. In contrast, boron has an electron-deficient structure and is less electronegative than carbon. It becomes electropositive after being incorporated into the carbon lattice. It is expected to form a good adsorption effect on negatively charged polysulfide anions, thereby alleviating the shuttle effect [36,37].

Yang et al. [38] used boron-doped porous carbon as a sulfur cathode host material and found that boron doping not only improved the electronic conductivity of the carbon material, but also induced positive polarization of the carbon matrix. Negatively charged polysulfide ions are effectively adsorbed and anchored through electrostatic adsorption and Lewis interaction, thereby inhibiting their dissolution and diffusion (Figure 3(a, b)). Therefore, the sulfur cathode based on boron-doped porous carbon exhibits higher initial capacity and more stable cycling performance than pure carbon and nitrogen-doped samples. Xu et al. [39] obtained boron atom-doped carbon nanotube/sulfur composite cathode material (BUCNTs/S) through a hydrothermal one-pot method. Liquid-phase in-situ synthesis makes sulfur more uniformly distributed in the composite, while boron doping gives the carbon-based host material higher electrical conductivity and stronger sulfur-fixing ability. The resulting BUCNTs/S electrode obtained an initial capacity of 1251 mAh∙g-1 at 0.2C, and could still maintain a capacity of 750 mAh∙g-1 after 400 cycles. In addition to sulfur cathode hosts, boron-doped carbon materials also play an important role in the design of battery functional separators. Han et al. [40] coated lightweight boron-doped graphene on a traditional separator to construct a functional modification layer, using its adsorption and reuse of polysulfides to effectively alleviate the shuttle effect and improve the utilization rate of active materials.

Fig. 3   (a) Scheme of B-doped carbon backbone, (b) S2p XPS spectra of sulfur composites based on different element-doped porous carbon; and (c) scheme of charge-discharge process of NBCGN/S composite, (d) cycling at 0.2C and (e) rate performances of sulfur electrodes based on different element-doped curved graphene nanoribbons[44]

In view of the basic properties of different doping elements and their different modes of action in the carbon lattice structure, multi-element co-doping is one of the important strategies to regulate the surface chemistry of carbon materials and improve sulfur electrochemical reactions [41, 42, 43 ]. In this regard, Kuang's research group [44] synthesized nitrogen and boron co-doped graphene nanoribbons (NBCGNs) for the first time through a hydrothermal method as the host material for the sulfur cathode, as shown in Figure 3(c). The study found that the synergistic effect of nitrogen and boron co-doping not only induces NBCGNs to obtain larger specific surface area, pore volume and higher conductivity, but also helps to uniformly distribute sulfur in the cathode. More importantly, boron and nitrogen act as electron-deficient and electron-rich centers in the co-doped system. It can be bonded with Sx2- and Li+ respectively through Lewis interactions, thereby adsorbing lithium polysulfide more efficiently and significantly improving the cycle and rate performance of the battery (Figure 3(d, e)). Based on similar doping strategies of high and low electronegativity elements. Jin et al. [45] prepared boron and oxygen co-doped multi-walled carbon nanotube host materials using boric acid as a dopant. The resulting battery still maintains a specific capacity of 937 mAh∙g-1 after 100 cycles, which is significantly better than the battery performance based on ordinary carbon tubes (428 mAh∙g-1). In addition, researchers have also tried other co-doping forms. Including borosilicate co-doped graphene [46], cobalt metal and boron nitrogen co-doped graphene [47], etc., have effectively improved battery performance. The synergistic effect of the co-doped components plays a crucial role in improving the sulfur electrochemical reaction.

Boron element doping can effectively improve the intrinsic conductivity and surface chemical polarity of carbon materials, strengthen chemical adsorption and inhibit the shuttling behavior of lithium polysulfide, thereby improving sulfur electrochemical reaction kinetics and stability, and improving battery performance. Despite this, there are still many problems in the research of boron-doped carbon materials in lithium-sulfur batteries, which need to be further explored and analyzed. For example, the influence of boron doping amount and doping configuration on the conductivity, surface charge distribution and adsorption behavior of lithium polysulfide of carbon materials. At the same time, how to obtain carbon materials with high boron doping levels and how to precisely control the doping configuration all depend on the development of advanced preparation methods and technologies. In addition, for multi-element co-doped systems, more suitable doping element combinations still need to be further explored. Establish a systematic structure-activity relationship to clarify the synergistic effect mechanism of the co-doped structure and its impact on the mode and intensity of host-guest interactions in sulfur electrochemistry.

2.3 Metal borides

Metal compounds have always been a research hotspot for functional materials in lithium-sulfur batteries due to their intrinsic chemical polarity characteristics and good morphological and structural plasticity. It is different from common metal oxides, sulfides, nitrides and other ionic compounds. Metal borides are usually composed of boron and metal elements based on covalent bonds, and their filled structure inherits part of the metallicity. It exhibits much higher conductivity than other metal compounds (Figure 4) [48, 49, 50, 51, 52, 53, 54, 55, 56], and can provide a rapid supply of electrons for electrochemical reactions [57]. At the same time, there is a local limited ionic bond polar structure between metal and boron, which can provide good adsorption sites for polysulfides [58,59]. In addition, the stability of highly electronegative boron is weakened after alloying with transition metals, and it is easier to participate in redox reactions. This makes it possible for metal borides to participate in lithium-sulfur electrochemical reactions through surface reactions as a mediator [60].

Fig. 4   Conductivity comparison with several categories of metal compounds[48,49,50,51,52,53,54,55,56]

Guan et al. [61] prepared a host material for sulfur cathodes by loading amorphous Co2B nanoparticles on graphene using a liquid phase reduction method. Studies have found that both boron and cobalt can serve as adsorption sites to chemically anchor lithium polysulfide, thereby inhibiting its dissolution and migration. Coupled with the excellent long-range conductivity of graphene, the battery still has a discharge specific capacity of 758 mAh·g-1 after 450 cycles at 1C rate, and the capacity decay rate per cycle is 0.029%, showing excellent cycle performance. Based on a similar synergistic adsorption effect, the Co2B@CNT composite material, used as a functional separator for lithium-sulfur batteries, has an adsorption capacity of Li2S6 as high as 11.67 mg∙m-2 [62], which can effectively block the diffusion and penetration of polysulfides and achieve the purpose of inhibiting the shuttle effect. On this basis, Guan et al. [63] further used two-dimensional metal carbide (MXene) as a carrier to prepare a Co2B@MXene heterojunction composite material (Figure 5(a~d)). Through theoretical calculations, it was found that the electronic interaction at the heterojunction interface leads to the transfer of electrons from Co2B to MXene. This effect improves the adsorption and catalytic ability of Co2B for polysulfides (Figure 5(a, b)). Therefore, the capacity fading rate of the battery based on Co2B@MXene functionally modified separator during 2000 cycles is only 0.0088% per cycle. And at a sulfur loading of 5.1 mg∙cm-2, the specific capacity is still as high as 5.2 mAh∙cm-2 (Figure 5(c, d)). It should be noted that compared with crystalline phase structures, this type of amorphous phase metal boride materials is gentler and simpler in material preparation. However, the controllability and stability of its atomic and molecular structure are relatively poor, which poses a great obstacle to clarifying its components and microstructure, and exploring its influence mechanism on the sulfur electrochemical reaction process.

Fig. 5   (a) Li2S4 adsorption configurations on Co2B and Co2B@MXene surfaces, (b) scheme of the electron redistribution at the interfaces between Co2B and MXene, (c) cycling performances of cells based on Co2B@MXene and other separators, (d) long-term cycling performance of the Co2B@MXene cell[63]; (e) schematic illustration of surface-chemical entrapment of polysulfides on TiB2, (f) adsorption configurations and (g) energies of sulfur species on (001) and (111) surfaces of TiB2, (h) high-loading performance and (i) long-term cycling of TiB2-based sulfur electrode[63,65]

TiB2 is a classic metal boride with excellent electrical conductivity (~106 S∙cm-1) and is widely used in fields such as conductive ceramics, precision machining, and electrochemical devices. TiB2 has a typical hexagonal structure and has high hardness and structural elasticity, which helps adapt to the volume change of sulfur reaction. At the same time, the large number of unsaturated structures on its surface is expected to form a strong interfacial chemical interaction with lithium polysulfide [64], thereby achieving good adsorption and confinement effects. Li et al. [65] first reported that TiB2 was used as a host material for sulfur cathodes. As shown in Figure 5(e~g), during the thermal compounding process with S, the surface of TiB2 is partially sulfurized. The lithium polysulfide produced during the reaction is effectively adsorbed through van der Waals forces and Lewis acid-base interactions, and the effect of this mechanism is more significant on the (001) surface. The obtained sulfur cathode obtained a stable cycle of 500 cycles at 1C rate, and at the same time, the specific capacity still retained 3.3 mAh∙cm-2 after 100 cycles at a sulfur loading of 3.9 mg∙cm-2. showed good electrochemical performance (Figure 5(h, i)). Based on the results of XPS analysis and theoretical calculations, the excellent lithium polysulfide adsorption effect of TiB2 should be attributed to its surface "passivation" mechanism. In addition, Lu's research group [66] compared the adsorption effects of TiB2, TiC and TiO2 on lithium polysulfide and explored the competition mechanism between the corresponding chemical adsorption and solvation desorption. The results show that boron with lower electronegativity makes TiB2 have stronger adsorption capacity, and combined with ether electrolyte with weak solvation capacity, it can effectively improve sulfur utilization and enhance the reversibility of electrochemical reactions. In view of this, TiB2 has also been used to construct multifunctional separators [67], which efficiently adsorbs, anchors and reuses active materials, significantly improving the battery cycle stability. The capacity can maintain 85% of the initial value after 300 cycles at 0.5C.

Similar to TiB2, MoB has good conductivity, and its intrinsic two-dimensional structure is conducive to fully exposing the adsorption sites, and is expected to become a good sulfur cathode catalyst [68]. The Manthiram research group at the University of Texas at Austin [69] used Sn as a reducing agent and synthesized MoB nanoparticles through a solid-phase method, which showed good adsorption and catalytic capabilities for lithium polysulfide. MoB has a high electronic conductivity (1.7×105 S∙m-1), which can provide a rapid supply of electrons for sulfur reactions; at the same time, the hydrophilic surface properties of MoB are conducive to electrolyte wetting and help the rapid transport of lithium ions . This ensures the utilization of active materials under lean electrolyte conditions; in addition, nanosized MoB can fully expose the catalytic active sites induced by electron-deficient boron atoms, allowing the material to have both excellent intrinsic and apparent catalytic activity. Based on these advantages, even if MoB is added in a small amount, it can significantly improve the electrochemical performance and show considerable practicality. The resulting battery has a capacity attenuation of only 0.03% per cycle after 1,000 cycles at a 1C rate. And at a sulfur loading of 3.5 mg∙cm-2 and an electrolyte/sulfur ratio (E/S) of 4.5 mL∙g-1, excellent soft-package battery cycle performance was achieved. In addition, the Nazar research group [70] used lightweight MgB2 as the electrochemical conversion medium for lithium polysulfide. It was found that both B and Mg can serve as adsorption sites for polysulfide anions, strengthen electron transfer, and achieve better cycling stability at high sulfur loading (9.3 mg∙cm-2).

These works fully illustrate the effectiveness and superiority of metal borides in improving sulfur electrochemical reactions. However, compared with systems such as metal oxides and sulfides, there are still relatively few research reports on metal borides in lithium-sulfur batteries, and research on materials and related mechanisms also needs to be expanded and deepened. In addition, crystalline metal borides usually have high structural strength, and the preparation process requires crossing high energy barriers and involving high temperature, high pressure and other harsh conditions, which limits their research and application. Therefore, the development of simple, mild, and efficient metal boride synthesis methods is also an important direction in metal boride research.

2.4 Non-metal borides

Compared with metal borides, non-metal borides are usually less dense and lighter, which is beneficial to the development of high-energy-density batteries; however, their lower conductivity creates resistance to the efficiency and kinetics of sulfur electrochemical reactions. At present, researchers have made certain progress in constructing sulfur-fixing materials for lithium-sulfur batteries based on non-metal borides including boron nitride, boron carbide, boron phosphide, and boron sulfide [71, 72, 73].

Boron nitride (BN) and boron carbide (BC) are the two most representative and widely studied non-metal borides. BN is composed of nitrogen atoms and boron atoms alternately connected, and mainly includes four crystal forms: hexagonal, trigonal, cubic and leurite [74]. Among them, hexagonal boron nitride (h-BN) exhibits characteristics such as wide bandgap, high thermal conductivity, and good thermal and chemical stability due to its graphite-like two-dimensional structure and localized electronic polarization characteristics [75,76] . The B-N structure has obvious polar characteristics and has strong chemical adsorption capacity for lithium polysulfide. At the same time, the surface chemical characteristics can be controlled through element doping and topological defect construction to ensure the stability of the polysulfide molecular structure while improving its adsorption strength [77]. Based on this idea, Yi et al. [78] reported a nitrogen-poor few-layer boron nitride (v-BN) as a host material for sulfur cathodes (Figure 6(a)). Studies have found that the electropositive vacancies in v-BN not only help to fix and transform polysulfides, but also accelerate the diffusion and migration of lithium ions. Compared with original BN, the v-BN-based cathode has a higher initial capacity at 0.1C (1262 vs 775 mAh∙g-1), and the capacity decay rate after 500 cycles at 1C is only 0.084% per cycle. Demonstrates good cycling stability. In addition, He et al. [79] found that O doping can further improve the chemical polarity of BN surface, induce the material to form a larger specific surface area, and simultaneously improve the intrinsic and apparent adsorption properties.

Fig. 6   (a) TEM image and schematic atomic structure of v-BN[78]; (b) Scheme of g-C3N4/BN/graphene composite ion-sieve and (c) the corresponding Li-S cell cycling performance[80]; (d) Schematic and optical image of BN/Celgard/carbon trilayer separator, and (e) the corresponding cell cycling performance[83]; (f) Scheme and (g) SEM image of B4C@CNF and the model of B4C nanowire, (h) Li2S4 adsorption energies on different facets of B4C[87]

Although BN material has good chemical adsorption properties, its own poor conductivity is not conducive to reactive charge transfer. Therefore, the design of composite structures with conductive materials is an important way to further improve their comprehensive adsorption and catalytic performance. In view of this, Deng et al. [80] designed a composite ion sieve based on graphite-like carbon nitride (g-C3N4), BN and graphene as a multifunctional intermediate layer for lithium-sulfur batteries (Figure 6(b)). Among them, the 0.3 nm-sized ordered ion channels in the g-C3N4 structure can effectively block polysulfides and allow lithium ions to pass through. BN serves as a reaction catalyst to promote the conversion of polysulfides, and graphene serves as a built-in current collector to provide excellent long-range conductivity. . Thanks to the synergistic effect of these three two-dimensional components, the resulting battery can stably cycle for more than 500 cycles at a high sulfur loading of 6 mg∙cm-2 and a rate of 1C (Figure 6(c)). In addition, researchers have tried to apply a thin layer of BN nanosheet/graphene composite film on the surface of the cathode as a protective layer in a simpler and more direct form [81,82]. It effectively inhibits the dissolution and diffusion of lithium polysulfide and significantly improves the specific capacity and cycle stability of the sulfur cathode. During 1000 cycles at 3C, the capacity attenuation rate is only 0.0037% per cycle. Interestingly, the Ungyu Paik research group at Hanyang University [83] adopted another combination of ideas to construct a multifunctional separator with a BN/Celgard/carbon sandwich structure. As shown in Figure 6(d), the carbonaceous layer and the BN layer are respectively coated on the positive and negative electrode sides of the ordinary separator. Among them, the carbon layer and the BN layer can jointly block the shuttle of lithium polysulfide and limit its diffusion to the surface of the negative electrode. At the same time, the BN layer on the negative electrode side also limits the growth of lithium dendrites. Thanks to this cooperative protection mechanism, the battery has a high capacity retention rate (76.6%) and specific capacity (780.7 mAh∙g-1) after 250 cycles at 0.5C. Significantly better than ordinary separators and pure carbon modified separators (Figure 6(e)).

Compared with N, C has a lower electronegativity, so the electronegativity difference between B and C is small, resulting in a weaker chemical polarity of the B-C structure compared to N-C. But at the same time, the electron delocalization in the B-C structure is enhanced and the conductivity is better [84,85]. Therefore, BC generally shows relatively complementary physical and chemical properties to BN. It has low density, relatively good conductivity, and good catalytic properties, and has promising application prospects in the energy field [86]. Luo et al. [87] grew boron carbide nanowires (B4C@CNF) in situ on carbon fibers as the cathode host material (Figure 6(f~h)). Among them, B4C efficiently adsorbs and confines polysulfides through B-S bonding. At the same time, its carbon fiber conductive network helps the adsorbed sulfur to be quickly converted and improves reaction kinetics. The obtained sulfur cathode has a capacity retention of 80% after 500 cycles, and can achieve stable cycling under high sulfur content (mass fraction 70%) and loading capacity (10.3 mg∙cm-2). Song et al. [88] constructed a super-confined sulfur host structure around B4C. The structure uses activated porous cotton fabric carbon as the flexible matrix, B4C nanofibers as the active skeleton, and reduced graphene oxide for further coating. Efficiently combines physical and chemical confinement, alleviates the loss of active substances, and achieves excellent cycle stability. In view of the good adsorption and catalytic properties of B4C, Zhao's research group [89] uniformly distributed B4C nanoparticles in carbon fiber cloth through an in-situ catalytic-assisted growth method to efficiently disperse and expose active sites. The obtained sulfur cathode has an initial capacity of up to 1415 mAh∙g-1 (0.1C) at a loading of 3.0 mg∙cm-2 and an ultra-long life of 3000 cycles at 1C, showing good application prospects.

It can be seen from the above that non-metal boride has a good adsorption and catalytic effect on lithium polysulfide, but its conductivity is relatively low, and a conductive carrier is still needed to assist the sulfur electrochemical reaction. Among them, the difference in the electronic structure of adjacent N and C atoms makes BN and BC materials have their own advantages and disadvantages in terms of conductivity and interaction with lithium polysulfide. In view of this, combined with boron sulfide, boron phosphide, boron oxide, etc., this type of non-metal boride can be used as a good carrier and platform to study the structure-activity relationship between local chemical polar structure and adsorption catalytic ability. It is expected that further systematic correlation and analysis will help understand the relevant microscopic reaction processes, regulate the fine structure of materials, and improve the electrochemical performance of batteries. In addition, the further application and development of non-metal borides in lithium-sulfur batteries still needs to rely on the improvement and optimization of their preparation. Develop simple and mild preparation technologies, while developing material structures with higher intrinsic conductivity and designing more efficient composite materials to balance and take into account conductivity, adsorption and catalytic effects.

3 Conclusion

In summary, lithium-sulfur batteries have high theoretical energy density due to their multi-electron transfer reactions. However, their conversion reaction mechanism and the intrinsic weak conductivity of the active materials hinder the realization of the advantages. Boron-based materials have unique physical and chemical characteristics and electrochemical properties. Their targeted design and rational application are effective ways to alleviate the shuttle effect of lithium-sulfur batteries and improve reaction kinetics and reversibility. They have developed rapidly in recent years. However, the research and application of boron-based materials in lithium-sulfur batteries is still in its infancy, and the material structure design and its mechanism of action on the battery electrochemical reaction process need to be further developed and explored. Combining the material characteristics and the above research progress, the author believes that the future development of boron-based materials in lithium-sulfur batteries should pay more attention to the following directions:

1) Material synthesis. Synthetic preparation is a common problem faced by the above-mentioned boron-based materials. There is an urgent need to develop simpler, milder and more efficient material preparation methods to provide a material basis for mechanism research and application promotion. Among them, the preparation of amorphous metal borides by liquid phase reduction method is a promising development direction. At the same time, drawing on its advantages and experience, exploring and developing synthetic routes based on solvothermal or molten salt methods may also provide new ideas for the preparation of boron-based materials. In addition, during the preparation process of boride, special attention needs to be paid to the control and design of nanostructure and its stability to meet the needs of the interface reaction characteristics of lithium-sulfur batteries.

2) Mechanism exploration. Boron-based materials have unique and rich surface chemical characteristics. In-situ characterization methods should be used to further study the host-guest interactions between boron-based materials and polysulfides. Special attention should be paid to surface irreversible sulfation, self-electrochemical oxidation and reduction, etc., to reveal the decisive structural factors of its adsorption and catalytic capabilities, and to provide theoretical guidance and basis for targeted design and development of materials. In addition, for the representative amorphous metal borides, it is necessary to pay special attention to the differences in microstructure and related physical and chemical properties between amorphous and crystalline borides, and cooperate with the development of corresponding structural analysis and property characterization analysis technologies. Avoid inferring the interaction between amorphous materials, lithium polysulfide and its reaction process based solely on the crystalline structure.

3) Performance evaluation. To optimize the material and battery evaluation system, while increasing the sulfur surface loading, more attention should be paid to regulating key parameters such as the thickness and porosity of the electrode to simultaneously improve the quality and volumetric energy density of the electrode. In addition, the electrochemical properties under conditions of low electrolyte dosage (E/S<5 mL∙g-1S) and low negative/positive electrode capacity ratio (N/P<2) were further investigated. At the same time, we explore the amplification effect and related scientific and engineering issues from laboratory button cells to actual production of cylindrical or flexible packaging batteries, and make a reasonable and comprehensive assessment of the performance competitiveness of the battery level. Provide guidance and reference for the commercial development of lithium-sulfur batteries.

In summary, this article focuses on boron-based materials and reviews the latest research progress of borophene, boron atom-doped carbon, metal borides and non-metal borides in lithium-sulfur battery systems. I hope it can provide reference and inspiration to colleagues, expand the development and application of boron-based materials in the field of new energy, and promote the practical development of lithium-sulfur batteries.

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