Rational Design of Bismuth Metal Anodes for Sodium-/Potassium-Ion Batteries: Recent Advances and Perspectives

Abstract

Sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) have drawn widespread attention for application in large-scale accumulation energy because of their plentiful resources and lower cost. However, the lack of anodes with high energy density and long cycle lifetimes has hampered the progress of SIBs and PIBs. Bismuth (Bi), an alloying-type anode, on account of its high volumetric capacity and cost advantage, has become the most potential candidate for SIBs and PIBs. Nevertheless, Bi anodes undergo significant volume strain during the insertion and extraction of ions, resulting in the crushing of structures and a volatile solid electrolyte interface (SEI). As a result, the tactics to boost the electrochemical properties of Bi metal anodes in recent years are summarized in this study. Recent advances in designing nanostructure Bi-based materials are reviewed, and the reasonable effects of architectural design and compound strategy on the combination property are discussed. Some reasonable strategies and potential challenges for the design of Bi-based materials are also summarized. This review aims to provide practical guidance for the development of alloying-type anode materials for next-generation SIBs and KIBs.

1. Introduction

Daily increasing global warming and environmental pollution have attracted worldwide attention to exploring eco-friendly renewable energy to replace traditional fossil fuels [1]. With the rise of the new energy industry and the continuous demand for energy transformation, rechargeable batteries as a key energy storage technology will usher in great development opportunities. For many years, lithium-ion batteries (LIBs) have had an overwhelming role in the electrochemical energy storage field due to their high energy density and long cycle life [2,3]. However, the restricted lithium salt in the earth’s crust and unevenly distributed resources impede the wide application of LIBs [4,5]. Therefore, researchers have turned their attention to exploring alternative energy storage technologies to partly replace LIBs.

Na, K, and Li are part of the same main class and have semblable equilibrium potential and physicochemical characteristics. Furthermore, the luxuriant reserves of Na/K in the earth and lower cost make SIBs and PIBs an ideal substitute for LIBs [6,7]. However, due to the larger ionic radii of Na⁺ (1.02 Å) and K⁺ (1.38 Å) than that of Li⁺ 0.76 Å), the electrode materials undergo severe volumetric alteration during the ion insertion process, thereby resulting in serious pulverization [8]. Thus, it is difficult to use existing mature LIB anodes directly in SIBs/PIBs [9]. Graphite, a traditional anode material, has been commercialized in LIBs. However, Na⁺ cannot form the corresponding intercalation compound, and K⁺ can only form KC₈ at a capacity of 280 mA h g⁻¹, seriously limiting its application in SIBs and PIBs. Oxides, sulfide, and selenides with a conversion-reaction mechanism could provide a high capacity of over 400 mA h g⁻¹. However, these transition-metal compounds have a high voltage terrace of around 1.5V, which will dramatically decrease the output voltage and energy density in full cells. Therefore, the design of the materials’ structure is more demanding when exploring anode materials for SIBs/PIBs. Bismuth (Bi) has a suitable redox potential (Bi³⁺/Bi = 0.308 V vs. SHE), insignificant thermoconductivity, low fusion point, and a high theoretical specific capacity (384 mAh g⁻¹) and volume energy density (3800 mAh cm⁻³) [10,11]. Bi metal belongs to the hexagonal structure, and the large layer spacing along the c-axis (d (003) = 0.395 nm) is beneficial to the diffusion of Na⁺/K⁺ ions, which has attracted considerable attention to SIBs/PIBs [12,13]. However, there are obvious structural changes in Bi during charging/discharging (352% volume expansion for complete form Na₃Bi; 506% volume expansion for K₃Bi), which lead to rupturing of the structure and comminution of the material [14]. The newly exposed surface will continuously form new SEI films, bringing about less coulomb efficiency and faster capacity attenuation [15]. To settle the issue of volume expansion and promote the electrochemical property of Bi, various design strategies have been reported such as, for instance, reducing granules to nano size, adopting ether-based electrolytes, compounding with conductive carbon materials [16,17], and making Bi alloys [18], etc. Bi-based anode materials for SIBs and PIBs develop rapidly, so it is essential to clarify the status of the most recent research by retrospecting the impact of Bi-based anode morphology on cell performance. This article expounds on the development of Bi-based materials in recent years and mainly summarizes the impact of the adopted rational structural design strategies on battery performance. The hope is that this will guide the designing of nanostructured alloying-type materials with boosting reversible capacity and stable cycling stability for SIBs and PIBs.

2. Bismuth Metal

To moderate the huge volume expansion of Bi metal anodes, researchers have paid attention to hollow/porous structures, which own distinct preponderances such as large specific areas and kinetically favorable open structures. Recently, hollow and porous bismuth anode structures can be designed by using templates. In this context, the rate capability and specific capacity of Bi metal anodes are effectively improved to a certain extent.

2.1. Bulk Bismuth

Decreasing the bulk to the nanometer size shortens the Na⁺/Li⁺ diffusion distance and suppresses the structural fragmentation of the electrode [19]. Bulk Bi finally forms a three-dimensional (3D) porous network structure after repeated cycles in ether electrolyte. Kim et al. [20] use commercial bulk bismuth without any pretreatment as an anode for SIBs. As shown in Figure 1a–h, during the cycle based on glycol dimethyl ether (DME) electrolytes, bulk Bi eventually forms a nano-porous structure. DFT results (Figure 1i,j) reveal that Bi stores Na⁺ ions by forming an intermediate (NaBi and Na₃Bi) phase with high Na⁺ diffusivity. When applied as an anode for SIBs, it has 379 mAh g⁻¹ after 3500 cycles at 7.7 A g⁻¹ (Figure 1h). DFT calculations proved that the porous structure formed by the stacking of Bi NPs not only improved the structural stability and provided a sodium-ion transport path but also achieved fast reaction kinetics and long-term cycle performance.

Wang et al. [21] reported a bulk Bi anode, observing the structural transformation by SEM before and after several cycles (Figure 2a,b). Figure 2c shows the structural evolution of the Bi anode. This nano-porous structure effectively improved the rate performance and cycle stability of Bi metal. Lei et al. [22] prepared bulk Bi anodes for PIBs, which can provide long-term cycle stability and about 400 mAh g⁻¹ in DME-based electrolytes. Enhanced half-cell performance can be ascribed to the formation of porous structures in DME. Therefore, the use of DME electrolytes can improve the performance of Na/K-ion batteries.

2.2. Nanostructured Bismuth

Designing efficient nanostructures enhances the reversible kinetics of alloying/dealloying reactions, shorten diffusion distances, and alleviate huge volume expansion. Generally, the recent structural design of bismuth metal anodes in scale can be divided into zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) structures.

One of the efficient strategies to improve the performance of Bi anodes is to construct hollow nanostructures. The interstitial space can accommodate the destructive volume contraction/expansion during the repeated insertion/extraction of Na⁺/K⁺, enhancing cycling stability. Pu et al. [23] synthesized hollow Bi nanotubes (Bi NTs) via a simple iodide-assisted galvanic replacement method (as shown in Figure 3a). In Figure 3b,c, the TEM and SEM images reveal that the precursor Cu has a nanotube structure. SEM and TEM images (Figure 3d–f) manifest that the galvanic replacement sample preserved the nanotube structure well, and the Bi and Cu elements distribute on the shell. Figure 3h–j displays that Bi NTs achieved a superior rate (319 mAh g⁻¹ at an extremely high density of 150 A g⁻¹) and high capacity retention (90% after 15,000 cycles at 20 A g⁻¹). It is effective for hollow nanotube structures to alleviate structural strain and provide ultra-fast Na⁺ transport. The Bi NT anode exhibits extraordinary rate capability and excellent cycle stability. The constructed Bi NTs/DME-based electrolytes/Na₃(VOPO₄)₂F (NVOPF) sodium-ion full battery has a high capacity of 181 mAh g⁻¹ after 550 cycles at 10 A g⁻¹ (based on anode mass calculation).

Cheng et al. [24] prepared 3D nano-porous Bi (3DPBi) by a convenient liquid phase reduction reaction (Figure 4a). As shown in Figure 4b–d, TEM characterizations proved the 3D porous structure and the lattice fringe corresponding to the (012) crystal plane of Bi. When used as an anode for SIBs (Figure 4e,f), 3DPBi has an outstandingly long cycle life and rate capacity. Figure 4g illustrates the unique structure that can self-repair volume changes and form a stable SEI during cycling. Three-dimensional flower-like Na₃V₂(PO₄)₃ coated with N and B co-doped carbon (NVP@BN-C) was used as the cathode. The full cell exhibited stable cycle performance at 10 A g⁻¹ (maintained at a reversible discharge specific capacity of 273 mA h g⁻¹ after 150 cycles) and a high weight energy density (116 Wh kg⁻¹).

3. Bi/C Composite

Constructing Bi nanoparticles combined with carbon materials can provide more reaction sites and internal space to improve battery performance. However, too high a carbon content will reduce the specific capacity of the composites. Many approaches have been devised to achieve high-efficiency Bi anodes, such as coating Bi with various nanostructured carbons, encapsulating Bi into 2D/3D carbon frameworks [25], etc. The carbon skeleton or carbon shell can effectively mitigate volume changes and preserve the structure from collapsing during the sodiation process.

3.1. Zero-Dimensional Bi/C Composite

A zero-dimensional Bi/C composite means that the size is from 1 nm to 100 nm. The carbon material is a typical soft material to restrain the volumetric strain and control the aggregation of Bi nanoparticles. Recently, yolk–shell [26], hollow, and core–shell structures have been designed as anodes for SIBs/PIBs.

3.1.1. Hollow Structure

For example, Zhu et al. [27] prepared pomegranate-like Bi@C nanospheres (PBCNSs) by a solvothermal method combined with carbothermal reduction treatment (Figure 5a). Figure 5b–g reveals that this composite material completely wrapped ultrasmall Bi nanoparticles (~7 nm) into carbon spheres with a size of ~50nm, which achieved fast kinetics, effectively relieved volume strain, and prevented accumulation and crushing of Bi particles. Figure 5h,i exhibits that PBCNS anodes achieve excellent sodium storage and rate performance (400.3 mA h g⁻¹ at 0.2 A g⁻¹ and 372.8 mAh g⁻¹ at 25 A g⁻¹) and long-term cycling stability (340 mA h g⁻¹ after 16 000 cycles at 20 A g⁻¹). Coupled with the Na₃V₂ (PO₄)₃/C(NVP@C) cathode, the PBCNSs could maintain a high specific capacity of 370 mA h g⁻¹ at 1 A g⁻¹.

Li et al. [28] prepared N-doped carbon-coated Bi nanoparticles (Bi@C) by one-step pyrolysis of organic bismuth salt and attained stable Na⁺ storage performance. Nam et al. [29] designed two different types of Bi/carbon composites to investigate the Na⁺ storage performance. Yang et al. [30] successfully prepared a hierarchical Bi@C by uniformly encapsulating Bi nanoparticles in a carbon microsphere by an aerosol spray pyrolysis technique. Ma et al. [31] synthesized nitrogen-doped wrapped Bi nanospheres (Bi@NC) that exhibited splendid rate capability and satisfactory cycle stability that can be ascribed to the unique Bi nanocrystal and nitrogen-doped carbon shell’s effective magnitude and the volume change during the cycling process. Zhang et al. [32] combined Bi@C nanospheres with graphene (GR) to form Bi@C@GR composites, which effectively improved electrical conductivity, prevented particle aggregation, and enhanced cyclic stability and rate capability. The above design strategy shows that the nanostructure can provide added active sites and reduce the diffusion distances, while the carbon layer can moderate the volume change during the Na⁺/K⁺ embedding process, avoiding the anode structure breaking and thus promoting the cycle performance.

3.1.2. Yolk–Shell/Core–Shell Structure

The yolk–shell/core–shell structure (the core is Bi and the shell is conductive carbon) has extra void space to better accommodate volume changes [33,34,35,36]. Yao et al. [37] synthesized BiOCl nanosheet precursors via a solvothermal method; subsequently, dopamine coating and annealing prepared the Bi@C (Figure 6a). The TEM results confirm that BiOCl nanospheres decompose into lesser nanoparticles, resulting in a looser nanosphere structure, and the gaps formed between the conductive carbon layers can relieve volume changes and expose more active sites (Figure 6b–d). Therefore, Bi@C displays better K⁺ storage performance. Yang et al. [38] encapsulated Bi nanoparticles in N-doped carbon shells by dopamine coating and annealing and prepared core–shell carbon nanospheres (denoted as Bi@N-C). A series of characterization shows that a N-doped carbon shell can improve electrochemical activity, nanoscale Bi particles can lessen the diffusion distance of electrons/ions, and the unique core–shell structure makes Bi@N-C have an excellent rate performance in SIBs and PIBs and a longer cycle life.

The close connection of active materials and conductive materials in the yolk–shell/core–shell Bi/C composites may be unable to fully release the strain stress caused by Na⁺/K⁺ ion insertion. Partial comminution of active materials may still occur at the electrodes. These structures can be further optimized by modifying the structures with some void space [39,40,41]. Gao et al. [42] constructed a multi-core shell heterostructure (Bi@Void@TiO₂⊂CNF) using Bi, hollow TiO₂, and 1D carbon nanofibers (CNFs). The cavity between the TiO₂ shell and the Bi core can relieve the volume expansion of Bi and inhibit the agglomeration of Bi particles, and the one-dimensional nanofiber structure can shorten the diffusion distance. To explore the effect of voids on the volume expansion of Bi anodes, Yang et al. [43] prepared a series of samples with different void sizes Bi@Void@C-x (x = 1,2,3) by using the template method (Figure 7a). Figure 7b–d clearly exhibits that the Bi@SiO₂@C-2 preserved a core–shell structure. Figure 7e displays that the Bi@Void@C-2 inherits the core–shell structure well and endows more void space. Figure 7h–g depicts the TEM results of Bi@SiO₂@C-2, revealing that the element distribution directly proves a core–shell structure. The electrochemical performance of these Bi@Void@C materials (Figure 7j–l) provides visual proof that appropriate gaps can effectively improve the volume change in Bi and increase battery capacity. However, voids that are too small will lead to the rupture of the carbon shell after circulation, and voids that are too large will increase the diffusion length of Na ions and reduce the volume energy density. Suitable void space in a core–shell structure can reduce the pulverization of the anode and suppress the volume change, reduce the formation of SEI films on the bismuth particles, and improve the battery performance.

3.2. One-Dimensional Bi/C Composite

A one-dimensional Bi/C composite means that the structure has a high length-to-diameter aspect ratio including tubes [44], rods [45,46], wires [47], and other morphologies [48,49] which have been proved to efficiently moderate the material volume expansion effect, improve the structural stability, and enhance the electrochemical performance. After combining 1D carbon nanostructures with Bi metal, the carbon can resist the accumulation of Bi nanoparticles, and the 1D structure can dramatically shorten the ion diffusion distance, thus significantly improving the electrochemical performance.

For example, Hu et al. [50] prepared a Bi/C nanotubes (Bi/CNTs) composite via a one-step electro-deoxidation method and applied it as an anode for SIBs. The incorporation of heteroatoms (N, S, etc.) improves the electrochemical activity and electronic conductivity and further improves the storage performance of Na⁺/K⁺. [51,52,53] Furthermore, Li et al. [54] prepared N-doped carbon-coated Bi nanotubes (Bi@N-CT) with a hollow structure by annealing Bi₂S₃ nanorod precursors, which achieved excellent long-term cycle performance and surprising rate capability. Xiang et al. [55] encapsulated Bi nanoparticles in N-doped carbon nanotubes by treating PDA-coated Bi₂S₃ nanorods (Bi@C). Liu et al. [56] prepared a bamboo-shaped N-doped carbon nanotube embedded with Bi nanoparticles, and this 1D nanostructure’s design can offer ion transport aisles and can buffer the volume strain during ion insertion.

The direct pyrolysis of a metal–organic framework (MOF) can yield uniformly dispersed metal nanoparticles on carbon materials [57]. Liang et al. [58] synthesized the Bi@C composite (denoted as Bi@C⊂CFs) assembled from carbon nanoribbons by direct pyrolysis of 1D Bi-based metal–organic framework (Bi-MOF) nanorods. Figure 8b–d shows that the carbon-coated Bi particles are uniformly distributed in the carbon nanoribbon. Figure 8e–h shows the SEM after different cycles, demonstrating that the materials undergo a continuous porous process in the ether electrolytes. Figure 8i vividly illustrates the structural changes in the composite anode. Figure 8j shows that the cycling performance of the Bi@C⊂CFs anode exhibited excellent Na⁺ storage stability. Selecting perylene 3,4,9,10-tetracarboxylic dianhydride (PTCDA) as the organic cathode material to assemble the full battery (Bi@C⊂CFs/PTCDA) can provide a reversible capacity of 272.5 mAh g⁻¹ at a current of 1 A g⁻¹. Su et al. [59] also pyrolyzed Bi-MOF rods and obtained new ultra-thin carbon@Bi nanoparticle (UCF@CNs@BiN) materials for achieving a long cycle life for PIBs.

The carbon skeleton generated by the MOF has a larger specific surface area and higher porosity, which is beneficial for improving the utilization rate of active substances. As displayed in Figure 9a, Liu et al. [59] designed a unique egg-box-shaped N-doped carbon matrix with Bi nanoparticles inserted into it (Bi NPs/NPC) via an MOF-derived method (Figure 9b,c). The SEM image reveals that Bi nanoparticles are uniformly distributed on the nanoribbons. In Figure 9d–f, the TEM results exhibit that Bi (~20 nm) intersperses on the carbon matrix. Figure 9g displays the element mapping signals verifying that the Bi, N, and C are well distributed and emerge as a nanoribbons structure. The electrochemical results suggest that the unique porous structure of Bi NPs/NPC is sufficient to narrow the ion/charge transfer path and form good anode/electrolyte contact. Using PTCDA as the cathode, the PIB full cell achieved excellent energy density (130.7 Wh kg⁻¹) and power density (829.8 W kg⁻¹).

3.3. Two-Dimensional Bi/C Composite

Two-dimensional (2D) structural materials have many advantages, such as a large specific surface area, increasing the contact area between active material and electrolytes, shortening the diffusion path of Na ions, and so on [60,61,62]. Shen et al. [63] proposed an ultrasound-assisted electrochemical stripping method for the preparation of an ultra-thin multilayer Bi nanosheet (FBN). Xiang et al. [64] prepared a 2D carbon-coated Bi nanosheet (Bi@C) using (BiO)₂CO₃ nanosheets as templates by in situ conversion reaction. The carbon layer effectively prevented the accumulation of Bi and moderated the volumetric strain during the charge/discharge process. Xu et al. [65] prepared 2D Bi@N-doped carbon sheets (2DBi@NOC) with Bi-O-C bonds and internal voids (Figure 10a). From Figure 10b–d, it can be found that after annealing, the 2DBi@NOC compound still maintains a flaky structure, and the Bi nanosheet is located at the edge of the carbon, with a large number of voids filled in the middle. In Figure 10e,f, the TEM results reveal the size of Bi flakes at the micron level and the coated carbon shell with a thickness of ~21 nm. This characteristic sheet structure with Bi-O-C bonds and voids can effectively adapt to the expansion strain in the process of alloying/dealloying and displayed remarkable cycling stability and rate capability (Figure 10g,h).

Designing Bi nanoparticles combined with carbon nanosheets to form a unique 2D structure could provide abundant active sites for the diffusion of Na⁺/K⁺ ions. Wang et al. [66] anchored 0D Bi nanoparticles on 2D N-doped carbon nanosheets (NCSs) from the ML-Bi@NCS composites (Figure 11a). As depicted in Figure 11b–d, the NCSs with Bi nanospheres are stacked layer by layer to form multi-layer structures (denoted as ML-Bi@NCSs). This layered structure has enough voids to provide abundant active sites and ion migration paths. When used as the anode of SIBs (Figure 11e,f), ML-Bi@NCSs can provide an outstanding capacity of 378 mA h g⁻¹ at 200 m A g⁻¹. The SEM results (Figure 11g–i) after cycles verified that the anode forms porous frameworks under repeated discharging/charging.

Hu et al. [67] prepared a C_PVP+C2H2/Bi/rGO anode with a sandwich structure via a simple subsequent chemical vapor deposition (CVD) and solvothermal method strategy. The surface of Bi nanoparticles is coated with a double carbon material (uniform thin layer of graphene and carbon). The graphene layer can prevent the accumulation of Bi particles, and the carbon layer can restrain the volume strain of Bi and improve the conductivity. Zhao et al. [68] prepared a multistage Bi nanoparticles/graphene (BiND/G) composite formed in situ from two-dimensional sodium bismuth (2D-SB), achieving ultra-high rate capability for PIBs. Zhu et al. [69] induced solid chemical bonds between graphene and Bi particles by modulating laser irradiation conditions (Figure 12a). Figure 12b–d exhibits Bi nanoparticles with homogeneous size distributed on a graphene nanosheet (Bi@LIG). The cross-section images (Figure 12e) verify the good distribution of Bi, C, Cr, Fe, and O, and the thickness of Bi@LIG is about 20 μm. This Bi@LIG (Figure 12f) displays a highly stable long cycling life over 9500 cycles. This structure allows the Bi to form good contact with electrolytes, effectively mitigating volume changes for ultra-long-life water-based sodium storage.

3.4. Three-Dimensional Bi/C Composite

A three-dimensional porous network structure can offer a short ion spread distance and abundant transmission channels for Na⁺ ions, can augment the contact with the electrolytes, and can relieve volumetric strain, thus improving the anode’s structural stability and the ion migration ability of materials [70,71,72,73,74]. Qiu et al. [75] prepared uniformly distributed Bi nanospheres (Bi-NS) of 20~30 nm and embedded them in a spongy 3D porous conductive framework (Bi-NS@C). This structure can form a conductive network to prevent particles from gathering. Cheng et al. [76] encapsulated Bi nanoparticles in porous conductive graphene frameworks (Bi@3DGFs), which can achieve an extremely long cycle life and rate performance of SIBs. Designing 3D interconnected porous nanostructures can connect active Bi particles, buffer volume expansion, prevent Bi anode collapsing, and generate a steady SEI.

Nitrogen-doped carbon could supply added active sites and promote the transport of Na⁺/K⁺ ions. Wang et al. [77] prepared N-doped carbon/porous Bi nanocomposites (Bi/N-Cs) by simple solution combustion synthesis (SCS). The porous structure can promote the diffusion of Na and electrolytes through the electrode. Sun et al. [78] directly annealed the 2DBi-MOF precursor and successfully encapsulated Bi nanoparticles in N-doped carbon nanocages (Bi@N-CNCs) with considerable gaps. The K⁺ storage mechanism of Bi@N-CNCs was studied by in situ TEM, SAED, and XRD characterization. Glycol-dimethyl-ether-based (DME) electrolytes and porous nano-networks benefit from achieving rapid kinetics during cycling. Cui et al. [79] prepared a mixture of spherical cactus-like Bi nanospheres as the anode of a potassium-ion battery. Figure 13a displays that Bi nanospheres are embedded in N-rich carbon nano-networks (BiNSs/NCNs) by simple electrospinning. Figure 13b–d displays that BiNSs/NCNs attained an excellent rate (489.3 mAh g⁻¹ at 50 A g⁻¹) and long cycle performance (457.8 mAh g⁻¹ after 2000 cycles at 10 A g⁻¹). TEM results before and after cycles (Figure 13e–j) confirm that this composite with unique spherical cactus-like structures effectively improved the utilization of active materials, and the structure gradually turns into 3D porous nano-networks during the cycling process.

Related studies have shown that the alloyed product of Bi for SIBs and PIBs is Na₃Bi and K₃Bi, respectively [26]. In addition, through various in situ/ex situ characterization methods, the alloying reaction mechanism is clarified: Bi follows a two-step reversible alloying reaction (Bi↔NaBi↔Na₃Bi) in the process of sodiation/desodiation [50]. As the discharge process proceeds, from 1.5 V to 0.64 V, Bi and Na become NaBi due to the alloying reaction and finally completely transfer to Na₃Bi at 0.01 V. Subsequently, during the charging process, Na₃Bi gradually transfers to Na, NaBi begins to appear around 0.78 V, and Na₃Bi completely disappears to form Bi when the charge reaches 1.5 V [57]:

$$N{a}^{+}+e^{-}+Bi\leftrightarrow NaBi$$

(1)

$$2N{a}^{+}+e^{-}+NaBi\leftrightarrow N{a}_{3}Bi$$

(2)

As for PIBs, one view holds that the reversible alloying reaction of Bi and K includes three steps (Bi↔KBi₂↔K₃Bi₂↔K₃Bi) [59]. During the discharge process, the elemental bismuth decreases continuously and gradually transforms into KBi₂ in the cubic phase (~0.96 V), K₃Bi₂ in the monoclinic phase (~0.62 V), and finally K₃Bi in the hexagonal phase (0.01 V) [64]:

$$x{K}^{+}+x{e}^{-}+Bi\to K_{x}Bi \left(0<x<3\right)$$

(3)

$$K_{x }Bi\to x{K}^{+}+x{e}^{-}+Bi \left(0\le x\le 3\right)$$

(4)

There is also a view that the reaction is divided into two steps (Bi↔KBi↔K₃Bi) [80] The K-Bi intermediate of Bi in PIBs is still controversial, but all studies have proved that the final alloying product is K₃Bi. So, more sensitive and accurate characterization methods are needed to analyze the multiphase transformation during the reaction.

4. Bismuth Alloys

To solve the material crushing problem caused by the increase in the volume of the anodes’ alloy type (Bi, Sb, Si, Pb), another effective method is to make binary [81,82] or ternary alloys [83], the other metals acting synergistically as a buffer substrate, which could provide abundant active sites, relieve the volumetric strain, and accelerate the diffusion kinetics. Nevertheless, a simple alloying strategy cannot meet the requirements of electrode material properties, and further modifications are needed to enhance the performance of Bi alloys, such as nanostructure design [84], a coupled 3D conducting carbon matrix, etc. [85,86].

Sb and Bi are the same main group and have similar structures. Alloying Bi and Sb as an anode for PIBs and SIBs can enhance material stability. Among binary alloys, the BiSb alloy is one of the most promising candidates [87]. Xiong et al. [88] obtained a Bi-Sb alloy composite nanosheet (Figure 14a) in which Bi-Sb alloy nanoparticles were evenly dispersed in a porous carbon matrix (BiSb@C). Figure 14b,c revealed that this composite possesses a layered nanosheet structure. Due to the introduction of carbon in composites (Figure 14d–g), BiSb@C effectively inhibits the volume variation during cycles. As an anode for PIBs (Figure 14h,i), this BiSb@C delivered excellent electrochemical performance and provided a highly reversible capacity of 320 mA h g⁻¹ after 600 cycles at 500 mA g⁻¹. By matching the K₄Fe(CN)₆ Prussian blue cathode with the composite bismuth–antimony alloy anode, the full PIB can still deliver a high discharge capacity of 396 mA h g⁻¹ after 70 cycles at 200 mA g⁻¹. Li et al. [89] developed a distinct design in which BiSbS_x was uniformly inserted into sulfide polyacrylonitrile (SPAN) fibers by electrospinning and annealing techniques. BiSb nanoparticles provide multiple active sites and a short diffusion distance.

Huang et al. [90] prepared tremella-shaped carbon microspheres (BiSb@TCS) by using KCl as an inorganic template and subsequent pyrolysis reduction processes (Figure 15a). Figure 15b,c verifies that this BiSb@TCS has a porous microspherical structure. Figure 15d reveals that the BiSb alloy has a size of ~10 nm. In Figure 15e–i, the TEM element mapping signals demonstrate the well-distributed and overlapped Bi, Sb, C, and O elements with a microspherical shape. Figure 15j suggests that the prepared BiSb@TCS has excellent K⁺ storage properties and obtains a capacity of 181 mAh g⁻¹ after 5700 cycles at 2.0 A g⁻¹. BiSb nanoparticles are restricted in the carbon layer and assembled into hollow microspheres, which can effectually moderate volumetric change and decrease the diffusion paths. Wu et al. [91] synthesized BiSb particles wrapped on graphene-like 3D carbon structures (BiSb@C) via a NaCl template-assisted method. This 3D structure provides rich channels for K⁺ diffusion, facilitating the contact between electrolytes and the anode.

Although there are few reports on ternary alloys, they can also effectively enhance the performance of Bi anodes. Xie et al. [92] designed a series of three ternary Sn-Bi-Sb alloys with different proportions and investigated their structural stability, cycle performance, and rate capability. Adding metal elements to construct alloy anode materials is an effective way to improve capacity retention and rate capability. Compared with the pure Bi, Sb, and Bi anodes, the capacity retention rate of ternary Sn₁₀Bi₁₀Sb₈₀ alloy is significantly improved.

5. Conclusions and Prospects

In summary, this review recommended the latest developments in Bi-based anode materials for PIBs and SIBs, including bulk bismuth metal, nanostructured carbon materials and bismuth metal composites, bismuth alloys, and other materials. The preparation methods of Bi-based anodes for PIBs and SIBs, their structural characteristics, and their electrochemical properties were investigated.

Table 1. Electrochemical comparison of Bi-based anode materials for SIBs/PIBs.

	Anode	Morphology	Current Density	Cycles	Capacity	Ref
			mA g−1		mA h g−1
Bismuth metal	Bismuth (Na+)	Bulk	7700	3500	379	[20]
	Bismuth (Na+)	Bulk	400	2000	400	[21]
	Bismuth (K+)	Bulk	371	300	800	[22]
	Bi NTs (Na+)	Nanotubes	1000	5000	351.7	[23]
Bi/C composite material	3DPBi (Na+)	Nano-porous	1000	200	380	[24]
	PBCNS (Na+)	Nanospheres	500	2500	373.4	[27]
	Bi@C (Na+)	Nanospheres	1000	1500	344	[28]
	Bi@a-C (Na+)	Nanospheres	50	100	202.1	[29]
	p-Bi@C (Na+)	Bi-MOF	50	100	301.9	[29]
	Bi@C (Na+)	Microsphere	100	100	123	[30]
	Bi@NC (Na+)	Nanospheres	200	100	371	[31]
	Bi@C@GR-2 (Na+)	Nanospheres	100	80	300	[32]
	Bi@C@GR-2 (K+)	Nanospheres	100	70	200	[32]
	Bi@C (K+)	Nanospheres	100	100	389	[37]
	Bi@N-C (Na+)	Core–shell	1000	400	300	[38]
	Bi@N-C (K+)	Core–shell	1000	100	268	[38]
	Bi@Void@TiO2 (K+)	Core–shell	2000	3000	171	[42]
	Bi@Void@C (Na+)	Core–shell	1000	500	253	[43]
	Bi/CNTs (Na+)	Nanotubes	1000	200	350	[50]
	Bi@N-CT (K+)	Nanorods	3850	1000	266	[54]
	Bi@C (K+)	Nanorods	50	300	353	[55]
	NBCNTs (K+)	Nanotubes	500	1000	204	[56]
	Bi@C⊂CFs (Na+)	Nanorods	1000	1000	325	[57]
	UCF@CNs@BiN (K+)	Nanorods	100	600	425	[58]
	Bi/CFC (Na+)	Nanosheet	50	300	350	[65]
	FBN (K+)	Nanosheet	7500	100	318	[66]
	2D Bi@NOC (K+)	Nanosheet	1000	1000	341.7	[67]
	ML-Bi@NCSs (Na+)	Nanosheet	5000	3000	343	[68]
	C PVP + C2H2/Bi/rGO (Na+)	Nanosheet	5000	1200	327.6	[69]
	BiND/G (K+)	Nanosheet	5000	500	213	[70]
	Bi@LIG (Na+)	Nanosheet	250	2500	481.4	[71]
	Bi/PPy/CNT (K+)	3D porous	100	200	302	[72]
	Bi-NS@C (Na+)	Nano-porous	200	1000	106	[76]
	Bi@3DGF (Na+)	Nano-porous	100	95	208	[77]
	Bi/N–C (Na+)	3D porous	5000	1600	203	[78]
	Bi@N-CNCs (K+)	Nanocages	1000	300	327.5	[79]
	Bi NSs/NCNs (K+)	Nano network	1000	100	525.1	[80]
Bismuth Alloy	BiSb@C (K+)	Nano sheet	500	600	320	[88]
	BiSbSx@SPAN (K+)	Nanocrystals	100	50	790	[89]
	BiSb@TCS (K+)	Nanospheres	500	100	265	[90]
	Sn10Bi10Sb80 (Na+)	Alloy film	200	100	621	[92]

summarizes a comparison of the performance of Bi-based anode materials for SIBs and PIBs.

Figure 16 shows the structural design of bismuth metal anodes and future challenges. The bulk bismuth undergoes huge volume expansion during the discharge/charge process. Recent investigations reveal that bulk bismuth cycling in glycolic dimethyl ether (DME) electrolytes spontaneously forms a nano-porous structure, which can reduce diffusion paths and improve electrode performance. In addition, the void space inside the hollow nanostructured material can mitigate volumetric strain and enhance cycle stability. However, the structural evolution mechanism of the porous process is still unclear, and the relationship between the resulting unstable solid electrolyte interface (SEI) and cycling stability also needs to be revealed.

Bi/carbon composites have proven to be the most effective strategy to improve structural stability and mitigate volume changes by embedding bismuth metal into carbon skeletons. The designs of the above unique composite structures show that: (1) the nanometer size can shorten the ion diffusion distance; (2) the hollow structure can accommodate the volumetric change; (3) the introduction of heteroatoms can enhance the conductivity and active sites; (4) the integration of 0D nano-Bi into 1D nanorods, 2D nanosheets, or 3D porous/core–shell structures can provide more active sites; (5) the carbon layer can alleviate the volume expansion, inhibit the decomposition and aggregation of the SEI film during charge and discharge, and improve the conductivity. However, the performance of Bi coupled with carbon materials largely depends on various factors, such as surface properties, morphology, structure, etc. For example, enough porosity of carbon can relieve the volumetric strain of Bi, but this may result in a lower volumetric energy capacity. In terms of practical applications, more efforts should be focused on the balance between unique structure and electrochemical performance. Furthermore, alloying strategy is considered a promising route to mitigate the volume deformation and preserve the high theoretical capacity of Bi anodes for SIBs and PIBs. However, the crushing phenomenon of alloy materials is still serious, and it is necessary to design suitable alloy compositions or special structures, such as core–shell structures and nanofiber combinations.

SIBs and PIBs undergo a period of rapid development in the laboratory, but the commercial application is still full of changes. Nevertheless, Bi-based anodes have made great progress in structural design and capacity improvement, and the existence of poor cycle stability and low initial coulomb efficiency seriously influence the full cell performance. Moreover, the matching of anode and cathode materials and electrolytes is still challenging. Further development of cathode materials to match Bi-based anode materials is also required, such as Na₃V₂(PO₄)₃, Prussian blue [K₄Fe(CN)₆], and perylene 3,4,9,10-tetracarboxylic dianhydride (PTCDA). Most of the current works report Bi anodes cycled in ether electrolytes, and whether the electrolytes are suitable for cathode materials needs investigation. So, the full cell performance may be an important indicator to evaluate the performance of Bi-based anodes.

To develop Bi anodes for practical application, a large-scale preparation procedure is also needed. Complex composite methods have difficulty achieving large-scale mass production. Moreover, safety problems such as high- and low-temperature performance must be further solved before use in commercial applications. It is worth developing scalable preparation technologies. The relationship between preparation technology, material structure, and battery performance is worth studying and investigating. It is believed that reasonable structural design and process optimization are expected to solve the existing issues of Bi anodes and broaden the way for the application of alloy-type anodes in large-scale energy storage. In this review, we provided a comprehensive summary of the synthesis methods and performance of existing Bismuth-based anode materials, aiming to stimulate more interest. Overall, the future development prospects and application potential of Bismuth anode materials are promising. By conducting in-depth research on the electrochemical performance and improvement strategies of Bismuth anode materials, we can promote further development of sodium-/potassium-ion battery technology, achieve more efficient, reliable, sustainable energy storage and utilization, and make outstanding contributions to energy transformation and sustainable development.