Tin-based nanomaterials: colloidal synthesis and battery applications

Abstract

Tin-based nanomaterials have been of increasing interest in many fields such as alkali-ion batteries, gas sensing, thermoelectric devices, and solar cells. Finely controllable structures and compositions of tin-based nanomaterials are crucial to improve their performances. The solution-based colloidal synthesis of these compounds offers a promising path toward controlling their structures and components. This feature article summarizes the progress in recent studies on the colloidal synthesis of tin-based nanomaterials (such as metallic tin, alloys, oxides, chalcogenides, and phosphides) and their applications in alkali-ion batteries including our own recent contributions to this subject. The challenges and future outlook of the controllable synthesis and practical development of tin-based anode materials are also addressed.

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Xixia Zhao

Xixia Zhao received his master's degree in chemical engineering and technology from China University of Petroleum (East China) in 2016 under the supervision of Prof. Changhua An. He continues to pursue a PhD over there under the supervision of Prof. Jun Zhang, and he has also been a visiting PhD student from 2016 in Prof. Zewei Quan's group at Southern University of Science and Technology. His current research topics focus on the controllable synthesis of inorganic nanomaterials for application in energy storage.

Qi Yang

Qi Yang is pursuing his BS degree in chemistry at Southern University of Science and Technology, where he has been conducting independent research projects under the supervision of Prof. Zewei Quan since 2016. His current research topics focus on the synthesis of colloidal tin-based alloy nanocrystals and their application in electrochemistry.

Zewei Quan

Zewei Quan is currently a professor in the Department of Chemistry at Southern University of Science and Technology in Shenzhen, China. He obtained his PhD in inorganic chemistry from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (with Prof. Jun Lin) in 2009. After that, he worked as a postdoctoral fellow and later as a research scientist at the State University of New York at Binghamton with Prof. Jiye Fang (2009–2012). He then joined Los Alamos National Laboratory as Oppenheimer Fellow (2012–2015). His current research interests include solution-phase synthesis, self-assembly, and high-pressure behavior of nanoparticles and their emerging applications.

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Introduction

Tin-based materials, including tin metal, alloys, oxides, chalcogenides, phosphides, and perovskites, are an important class of functional materials due to their earth-abundance, non-toxic nature, and intriguing physicochemical properties.^1–5 As such, they have received widespread attention in the field of alkali-ion batteries, catalysis, gas sensors, solar cells, and solders.^6–13 For example, tin possesses multiple valence states, good electrical conductivity, high mechanical and thermal stability, and the ability to form alloys with various alkali metals. Thus, tin and its various compounds are very promising host materials for the storage of different alkali-ions via the alloying/dealloying reactions between tin and alkali metals.⁶ As a promising anode in lithium-ion batteries, tin possesses a theoretical capacity of 994 mA h g⁻¹, which is two times higher than that of the commercial graphite anode.¹⁴ In addition, tin dioxide is the most used semiconductor base material for toxic gas sensors, and tin-based selenides possess high chemical stability, narrow band gap (1.0–1.5 eV), and high absorption coefficient (∼105 cm⁻¹) with p-type conductivity, which are essential for the efficient collection of solar radiation.⁷ Recently, tin-based perovskites have shown strong potential for use as absorbers in high-performance solar cells because of their electronic and optical characteristics which are comparable to those of well-developed lead-based perovskites.^15,16

Tin-based nanomaterials with well-designed structures have attracted researchers’ attention because of their enhanced performance for various applications. Compared with bulk materials, tin-based nanomaterials including metals and semiconductors usually exhibit distinctive properties.^17–24 For example, the density of electronic states at Fermi surface in tin nanoparticles is increased compared with bulk tin, showing a larger heat capacity at low temperature.¹⁷ In addition, the melting point of metallic tin can be dramatically decreased when the particle size is reduced to nanoscale due to the high surface area to volume ratio for small tin particles.^18,19 When the size of SnO₂ nanocrystals approaches their exciton Bohr radius (2.7 nm), the charge carriers become spatially confined (electrons and holes move only in a potential well) and the coulomb shielding decreases. In this case, the exciton binding energy increases, the exciton effect becomes stronger, and more exciton absorption peak may appear, resulting in blueshift of the absorption edge relative to the bulk exciton absorption.²⁰

To date, a variety of methods have been explored to prepare tin-based nanomaterials, including top-down and bottom-up approaches.^25–28 The former aims to reduce the particle size by various mechanical forces, whereas the latter consists of the initial heterogeneous/homogeneous nucleation, followed by continuous growth. The top-down approach is versatile in production capability, but shows poor control of the structure/shape features of the final product. In contrast, bottom-up approaches, including using gas and liquid phase, exhibit good control of the nucleation/growth parameters.^29,30 Among various bottom-up methods, colloidal synthesis has considerable advantages in the controlled synthesis of various tin-based nanoparticles by using long-chain organic compounds as capping ligands and/or solvents.^5,31,32 The capping ligands play an important role in tuning their properties such as shape, crystalline structure, and electronic/optical properties, providing a possibility for the construction of complex structures. At the same time, as-prepared nanoparticles can also be engineered by ligand exchange to possess desirable properties, such as solubility and electrical conductivity. Colloidal synthesis has been proven to be a robust synthetic strategy for synthesizing tin-based nanomaterials with well-controlled size, shape, composition, and structure.

Applications of tin-based materials in energy storage can be dated back to the 1990s by Idota,³³ who first reported the electrochemical performance of amorphous tin-based oxide as an anode for lithium-ion batteries. Subsequent studies indicate that tin can serve as a promising host for lithium storage through the formation of Li_4.4Sn alloy with a much higher specific capacity than graphite.⁶ In addition, tin-based materials possess lower potential hysteresis than transition metal oxides, such as Fe₂O₃, Co₃O₄, and TiO₂, which are also promising anode materials for lithium-ion batteries.^34,35 Therefore, a variety of tin-based electrodes with well-designed structures have been developed to replace the low-capacity graphite anodes. Furthermore, since tin-based materials can electrochemically react with different alkali ions, they can also be used as versatile anode materials for some emerging battery systems, such as Na-, K-, and Mg-ion batteries.^32,36–40

In this feature article, we summarize the recent advances in the colloidal synthesis of tin-based nanomaterials and their applications in alkali-ion (Li⁺, Na⁺, and K⁺) batteries, including our own recent contributions in this field. The first part is mainly focused on advances in the controlled synthesis of tin nanoparticles and various tin-based nanomaterials with specific compositions and delicate structures by the colloidal method. Then, the electrochemical properties of tin-based nanomaterials as anodes for different alkali-ion batteries are reviewed in the second part.

Colloidal synthesis of tin-based nanomaterials

Colloidal synthesis

In the past two decades, enormous developments of colloidal synthetic chemistry have made it possible to produce nanoparticles with well-controlled size, shape, composition and structure, offering researchers a new way to obtain nanomaterials with fascinating electrical, optical, and magnetic properties.^41–51 Although the colloidal synthesis of nanoparticles can be performed in both aqueous and organic phase solutions, the organic phase synthesis is suitable to prepare many more types of high quality colloidal nanoparticles, especially those materials (including many tin-based nanoparticles) sensitive to O₂ and H₂O.^52–54 During the organic phase synthesis of nanoparticles, organic reagents such as solvents and capping ligands are critical to control the size, shape and structure of nanoparticles. The synthesized colloidal nanoparticle usually presents an inorganic core with a monolayer of organic ligands.

Considering the sensitivity of tin-based nanomaterials to O₂/H₂O during their preparation,^27,32 organic phase synthesis is employed in our recent works to prepare various tin-based nanomaterials.^5,12,13,55 Based on Fang's pioneering work on the synthesis of Pt and Pt-based binary alloys using tungsten hexacarbonyl (W(CO)₆) as a reducing agent,^56–58 we recently developed a generalized colloidal method to synthesize different types of uniform metal nanoparticles by reducing metal precursors in organic solution assisted by W(CO)₆.⁵⁹ On the basis of this method, we have synthesized uniform tin nanoparticles with tunable sizes. And then, a series of well-designed tin-based nanoparticles, including hollow tin oxide, hollow/yolk–shell tin chalcogenide, and binary alloy nanoparticles, have also been successfully synthesized via the template method. These developments will be discussed in detail in the following sections.

Colloidal synthesis of tin nanoparticles

Since tin has a low melting point (∼232 °C) and strong oxygen affinity, as well as due to its low affinity towards most commonly used surfactants, it is challenging to controllably synthesize high-quality tin nanoparticles, which always needs rigorous reaction conditions, including low temperature, strong reducing agents, and effective surfactants.^32,60–66

In the early 1990s, Bönnemann et al. first demonstrated the possibility of obtaining tin particles by reducing SnCl₂ with LiBEt₃H in tetrahydrofuran (THF) at room temperature.⁶⁷ Later, Nayral et al. investigated the synthesis of tin nanoparticles (<100 nm) by the decomposition of (Sn(NMe₂)₂)₂ in dry anisole.⁶⁸ Then, Schlecht et al. tried to tune the size of tin nanoparticles (around 40–50 nm) by changing the concentration of the tin precursor (SnCl₄).⁶⁹ In contrast, Wang et al. reported the synthesis of tin nanoparticles with small sizes (3–19 nm) through the chemical reduction of a phenanthroline/tin chloride complex with NaBH₄.^70,71 Kwon et al. and Chee et al. further investigated the influence of capping agents and metallic precursors on the morphology and size of tin nanoparticles, respectively.⁷² Recently, Kravchyk et al. demonstrated the synthesis of tin nanoparticles with size control, in which monodisperse tin particles with different sizes varying from 9 to 23 nm were prepared using diisobutylaluminium hydride as a reducing agent in organic media.³²

Our new method for synthesizing uniform tin nanoparticles was inspired by the recent success in using tungsten hexacarbonyl as a reducing agent for monodisperse metal nanoparticles.⁵⁷ Monodisperse tin nanoparticles with three different sizes were readily synthesized by using tungsten hexacarbonyl as the reducing agent (Fig. 1).⁵ By changing the reaction temperature and time, uniform tin nanoparticles with average diameters of 11 nm, 13.5 nm, and 16 nm can be readily obtained. In addition, unlike traditional organometallic precursors used in previous reports, inexpensive and low-toxic SnCl₂ was employed in this method, which is necessary to achieve large-scale preparation. The rapid decomposition of tungsten hexacarbonyl at elevated temperature (>140 °C) facilitates the quick nucleation of tin, and the presence of oleylamine plays a key role in both the crystallinity and uniformity of as-prepared tin nanoparticles.

Fig. 1 TEM images of 11 nm (a and b), 13.5 nm (c and d) and 16 nm Sn nanoparticles (e and f). (g) HRTEM image of an individual Sn nanoparticle and (h) XRD patterns of Sn nanoparticles with different diameters.⁵

Template synthesis of hollow tin-based oxide/chalcogenide nanoparticles

Tin-based compounds have broad application prospects in the fields of lithium-ion batteries, solar cells, and photocatalysis,^1,73–75 some of which are highly dependent on their shapes. For example, the shape of nanocrystals with different exposed surfaces determines the electrical response and selectivity of SnO₂ chemical sensors.⁷⁶ Compared to their bulk counterparts, tin-based nanomaterials with well-defined shapes, such as 1D SnO₂ nanowires⁷⁷ and 2D SnO nanosheets,⁷⁸ possess diminutive size and high surface-to-volume ratio, which can therefore effectively alleviate the detrimental effect of volume change and shorten the diffusion length of lithium ions upon lithiation/delithiation. Due to these attractive properties of tin-based nanomaterials, the delicate preparation of tin-based nanomaterials has become one of the hot topics in materials research. Among different architectures, hollow nanoparticles with controllable hollow interior and shell thickness are an important kind of nanostructured materials because of their large surface area and low material density. Since the first synthesis of CoS and CoO hollow nanoparticles through the sulfidation and oxidation of Co nanoparticles in 2004,⁷⁹ several different types of hollow nanoparticles have been prepared by similar methods based on the nanoscale Kirkendall effect.⁸⁰

The imbalance of diffusion rates between two components is a feature of the Kirkendall effect. Typically, a diffusion couple would be generated when metal nanoparticles are exposed to precursors such as sulfur, selenium, oxygen, and phosphorus at high temperatures. If the outward diffusion of metal cations is much faster than the inward diffusion of anions, the vacancies can be formed and flow inward along with the outward flux of metal cations to balance this difference. When these vacancies become oversaturated, they would coalesce into voids and finally result in the formation of hollow nanoparticles containing two elements.⁸¹ In addition, the Ostwald ripening process is also a common strategy to fabricate hollow structures, which involves the growth of larger crystals at the expense of smaller ones.⁸²

Our group first reported the in situ conversion of uniform Sn nanoparticles to hollow tin oxide nanospheres and yolk–shell/hollow tin chalcogenides (SnS, SnSe, and SnTe) based on the nanoscale Kirkendall effect (Fig. 2).⁵ We proposed reasonable formation mechanisms of these hollow structures. As shown in Fig. 2b, in the initial stage of in situ oxidation, the surface of the Sn nanoparticle first reacts with oxygen to produce a thin tin-oxide shell, and further oxidation reaction occurs through the diffusion of atoms at the interface. Since the outward diffusion of Sn atoms (J_Sn) at the interface is faster than the inward diffusion of O atoms (J_O), small vacancies are formed at the interface. Subsequently, these small vacancies further coalesce to form hollow SnO_x nanospheres.

Fig. 2 (a) TEM images of hollow SnO_x nanospheres and (b) schematic illustration of the synthesis of hollow SnO_x nanospheres. TEM images (c–e) and XRD patterns (f) of hollow SnS, yolk–shell SnSe and hollow SnTe nanoparticles. (g) Scheme of the synthesis of hollow SnS, yolk–shell SnSe and hollow SnTe nanoparticles.⁵

In contrast, the formation process of tin-based chalcogenide (SnS, SnSe, and SnTe) nanoparticles is slightly different from the oxidation process. As shown in Fig. 2g, a two-step conversion process including the Kirkendall effect and asymmetrical Ostwald ripening process occurs when Sn reacts with S, Se, and Te. Solid tin-based chalcogenide nanoparticles are first formed because of the faster inward diffusion of chalcogen atoms compared with the outward diffusion of Sn atoms at the interface. With the extension of reaction time, an asymmetric Ostwald ripening process, including core dissolution and shell recrystallization, takes place in regions consisting of smaller or looser grains, resulting in the formation of yolk–shell/hollow structures (Fig. 2c–e).

Template synthesis of tin-based binary alloy nanoparticles

Compared with single metals, alloys consisting of two or more metals usually have improved properties, such as higher hardness, lower melting point, and corrosion resistance.⁸³ Since tin can alloy with many metals to form different tin-based alloys with distinctive properties,⁸⁴ the synthesis of tin-based alloy nanoparticles with tunable composition is intensively explored. It has been proved that the catalytic, electrical, and optical properties of some metal nanoparticles can be significantly changed by combining them with tin, as in the case of SnGe and PtSn alloys.^85–87 In addition, the fabrication of tin-based alloy anodes (e.g., SnSb, SnCo) for lithium-ion batteries can effectively alleviate their volume expansion during charge and discharge processes.^88–90

Due to the narrow bandgap and good near-infrared-emitting property, SnGe alloys are a kind of ideal candidates for infrared optoelectronics and optical devices, such as infrared lasers, photodetectors, or light-emitting diodes.^91–95 In addition, the addition of Sn to the Ge matrix also leads to enhanced electron and hole mobility, making SnGe alloys attractive for high-speed electronic devices.⁹⁶ However, the synthesis of uniform SnGe alloy nanoparticles is quite difficult due to the high crystallization temperature of Ge, their low solubility (<1%), as well as a large lattice mismatch (∼14%) between Sn and Ge.⁹⁷

Our group recently demonstrated a novel colloidal method to prepare uniform Sn_1−xGe_x alloy nanoparticles through the reaction between the Sn nanoparticle template and the Ge precursor under reducing conditions (Fig. 3).⁶¹ By fine-tuning the reaction temperature and time, uniform Sn_1−xGe_x alloy nanoparticles with a tunable bandgap from 0.51 to 0.72 eV were successfully synthesized. Notably, compared with previously reported methods (above 300 °C), this method can occur at a much lower temperature (60–180 °C) ascribed to the catalytic effect of Sn seeds on reducing the reaction energy barriers.⁹⁸ Through ex situ characterization of samples collected at different reaction stages, an evolution mechanism from Sn nanoparticles to Sn_1−xGe_x alloy nanoparticles is proposed based on the galvanic replacement and the solid diffusion mechanism, as shown in Fig. 3j. Sn nanoparticles first react with the Ge precursor under a reducing condition to form an amorphous Sn_1−xGe_x shell on the surface. Then, the amorphous Sn_1−xGe_x shell gradually transforms to the cubic phase Sn_1−xGe_x alloy with increasing temperature. Further solid diffusion process results in the final formation of homogeneous Sn_1−xGe_x alloy nanoparticles.

Fig. 3 (a–c) TEM and HRTEM images of as-prepared Sn_0.18Ge_0.82 alloy nanoparticles prepared at 180 °C for 5 h. (d–f) EDS elemental mapping results of Sn_0.18Ge_0.82 nanoparticles. The scale bars in (d–f) represent 10 nm. HRTEM images, STEM images, and the corresponding EDS elemental line scan profiles of samples collected at different reaction stages: (g) Sn-amorphous Sn_1−xGe_x heterostructure collected at 90 °C, (h) Sn-crystalline Sn_1−xGe_x heterostructure collected at 120 °C, and (i) Sn_0.18Ge_0.82 alloy nanoparticles prepared at 180 °C for 5 h. The scale bars represent 10 nm. (j) Schematic illustration of the transformation process from Sn nanoparticles to Sn_1−xGe_x alloy nanoparticles. Reprinted with permission from ref. 61. Copyright 2019 American Chemical Society.

This robust method is being extended to prepare other novel tin-based alloy materials by our group, such as Sn_1−xSb_x and Sn_1−xBi_x alloys. Similarly, Jiao's group also reported the synthesis of AgSn alloy nanoparticles, including the formation of Sn seeds and a galvanic replacement between Sn and Ag.⁷ The composition of AgSn alloys can be tuned accordingly by changing the molar ratio of Sn and Ag precursors.

Applications of tin-based nanomaterials in batteries

Tin-based nanomaterials have stimulated a growing interest in many fields owing to their useful physicochemical properties. For example, SnO₂ is investigated as a gas sensor material due to its high gas sensitivity.⁹⁹ Moreover, SnSe is considered as a novel type of promising thermoelectric material with ultralow thermal conductivity and high thermoelectric figure of merit.¹⁰⁰ In addition, the tin-based perovskite is an alternative low-toxic photovoltaic material with an ideal band gap (∼1.3 V) compared with the lead-based perovskite.² Moreover, tin-based nanomaterials possess huge potential as versatile high-performance anodes for alkali-ion (Li⁺, Na⁺, K⁺) batteries, because tin can act as a host material for the storage of all these alkali-ions via electrochemical alloying/de-alloying with them.⁶

As is known, large volume change of tin-based anode materials during the charge/discharge process would result in their pulverization and delamination from current collectors, leading to rapid capacity fading and poor rate performance. It has been proved that the size, structure, phase, and composition of tin-based nanomaterials play a great role in improving their performances.^1,6 For example, reducing the particle size and introducing hollow/porous structures can effectively accommodate the volume changes of tin-based anode materials by relieving mechanical stress and providing extra space.³² Moreover, fabrication of amorphous tin-based structures or mixing tin with other metals to form alloys is also an effective way to achieve improved electrochemical performances.^88–90

In this section, we focus on several types of tin-based nanomaterials (e.g., metallic tin, oxides, selenides, and phosphides) that are delicately designed for alkali-ion battery anodes including our recent works, and discuss the influence of size, structure, and composition on their electrochemical performance.

Lithium-ion battery

Tin-based materials can alloy with lithium and deliver higher capacities than graphite. However, their huge volume change during charging and discharging can lead to pulverization of the electrode and result in the failure of the whole battery. Previous studies have shown that reducing the size of tin-based materials is beneficial to adapt the mechanical stress caused by the alloy/de-alloy process, thus significantly improving their electrochemical properties. For example, Kravchyk et al.³² reported that tin nanoparticles of ∼10 nm can show a reversible capacity of 300 mA h g⁻¹ for 100 cycles at a current density of 1000 mA g⁻¹, which is significantly better than that of commercial Sn nanoparticles (50–150 nm). Furthermore, Cabana et al. studied the effect of particle size on the volume change of tin during lithiation using ex situ transmission electron microscopy (TEM).¹⁰¹ They found that reducing the particle size can effectively alleviate the cracking of tin nanoparticles, although slight particle pulverization still occurs.

Recently, our group demonstrated the key role of small size and an oxide layer in improving the cycle stability of tin nanoparticles (Fig. 4).¹² As-prepared Sn nanoparticle electrodes delivered high reversible capacities during long cycling. TEM images after cycling (Fig. 4c and d) revealed that the morphology of Sn nanoparticles after cycling is the same as that of the pristine one, indicating that small size plays a crucial role in alleviating the detrimental effect of volume expansion during repeated charge/discharge processes, as the mechanical stress induced by the volume change during the alloying/dealloying process can be accommodated more easily in these tiny nanoparticles,^32,101 consistent with their good cycle stability. Meanwhile, in the initial irreversible discharge reaction, the oxide layer on tin nanoparticles reacted with lithium to form a Li₂O matrix, which is conducive to maintaining the structural stability of the electrode material and further improving its cycle performance.

Fig. 4 (a) TEM image of the as-prepared Sn nanoparticles and (b) the STEM image and EDX elemental mapping of a single Sn nanoparticle. TEM images of as-prepared Sn nanoparticles (c) after one cycle and (d) after ten cycles at a current density of 500 mA g⁻¹. (e) Selected galvanostatic charge/discharge voltage profiles at 200 mA g⁻¹. Cycling performance and the corresponding coulombic efficiency (f) at 200 mA g⁻¹ and (h) at 500 mA g⁻¹. (g) Rate capability from 100 to 1000 mA g⁻¹.¹²

Apart from metallic tin, tin oxides have also been considered as a kind of promising anode candidates for high-performance lithium-ion batteries due to their considerable theoretical capacities (SnO₂, 782 mA h g⁻¹). However, similar to the tin anode, the large volume change during lithiation/de-lithiation cycles is also a significant problem of the tin oxide anode. Introduction of a hollow or porous structure into tin oxide anode materials is an effective way to improve their electrochemical properties. This is because the hollow or porous structure can not only accommodate large volume changes, but also shorten the diffusion path of lithium ions.^102–104 In addition, the effect of the crystallinity of SnO₂ on the lithium-storage performance has also been studied. The results show that compared with the crystalline structure, amorphous SnO₂ is widely believed to more effectively overcome the huge volume change of the electrode due to its isotropic volume expansion during the charge/discharge processes, thus delaying the capacity fading.^105,106

Based on the above considerations, we expect that the rational design of tin oxide nanoparticles with a hollow/porous structure and amorphous properties tends to achieve improved electrochemical performance. Therefore, our group elaborately designed and synthesized a kind of amorphous SnO_x hollow nanospheres by a template method based on the nanoscale Kirkendall effect (Fig. 5).¹² As expected, the amorphous SnO_x hollow nanospheres exhibited high reversible capacities and excellent rate properties as an anode for lithium-ion batteries, which can be attributed to the small size, hollow/amorphous structure, and surface formation of the Li₂O matrix. These structural features not only accommodate the huge volume change of the material during the charge/discharge processes, but can also help to shorten the diffusion distance of lithium ions.

Fig. 5 (a) STEM image of the as-prepared hollow SnO_x nanospheres and (b) the STEM image and EDX elemental mapping of a single hollow SnO_x nanosphere. TEM images of as-prepared hollow SnO_x nanospheres (c) after one cycle and (d) after ten cycles at a current density of 500 mA g⁻¹. (e) Selected galvanostatic charge/discharge voltage profiles at 200 mA g⁻¹. Cycling performance and the corresponding coulombic efficiency (f) at 200 mA g⁻¹ and (h) at 500 mA g⁻¹. (g) Rate capability from 100 to 1000 mA g⁻¹.¹²

Sodium-ion battery

As a promising alternative to lithium-ion batteries, sodium-ion batteries are considered to have broad application prospects in the field of large-scale energy storage, mainly due to their low-cost, abundant reserves and even distribution of sodium resources.^107–109 However, their practical application is greatly limited due to lack of reliable anodes, and thus it is urgent to develop sodium-ion battery anode materials with high capacities and long cycle life. So far, tin, antimony, phosphorus and other materials have been investigated as the anode of sodium-ion batteries, and have exhibited potential in electrochemical sodium storage.^110,111

Due to its high theoretical capacity (∼780 mA h g⁻¹), good chemical stability, and abundant resources, tin selenide (SnSe) has become a potential anode material for sodium-ion batteries in recent years.^112,113 However, similar to other tin-based materials, SnSe as anode material for sodium-ion batteries also has similar problems such as the volume change and agglomeration of SnSe particles, further leading to rapid capacity fading and poor rate performance. To solve these problems, the regulation of the shape/size of SnSe particles to mitigate volume expansion and agglomeration or the addition of conductive substrates (such as carbon fiber and graphene) into SnSe particles has been widely adopted, achieving significantly improved electrochemical sodium storage performance.^114,115

To improve the electrochemical sodium storage performance of SnSe, we first designed and synthesized monodisperse SnSe nanoparticles with a unique yolk–shell structure by a template method (Fig. 6).¹³ The unique yolk–shell structure and ultra-small particle size can not only shorten the diffusion distance of sodium ions, but can also alleviate the volume change and the aggregation of SnSe nanoparticles in the charge and discharge processes, thus leading to enhanced electrochemical performance. When such SnSe nanoparticles were used as the anode for sodium-ion batteries, they can deliver an initial capacity of 355.8 mA h g⁻¹ and a capacity retention rate of 72.5% after 150 cycles at 500 mA g⁻¹. In addition, they also showed excellent rate performance: the capacity retention rate is as high as 85.3% at a current density of 1000 mA g⁻¹, compared with the capacity at 100 mA g⁻¹. Quantitative analysis demonstrated that the capacitive contribution dominates the total sodium storage performance of our SnSe electrodes, which also explains their superior rate performance.

Fig. 6 (a) TEM images, (b) STEM image and the corresponding EDX elemental mapping of Sn and Se and their overlapped image. The inset of (a) shows the high resolution TEM image of a single SnSe nanoparticle. (c) Schematic illustration to show the advantages of yolk–shell structured SnSe nanoparticles during the sodiation/desodiation process. (d) Capacitive (green) and diffusion-controlled (yellow) contributions to charge storage of the as-prepared SnSe nanoparticles at 0.8 mV s⁻¹. (e) Normalized contribution ratio of capacitive (green) and diffusion-controlled (yellow) capacities at different scan rates. (f) Typical CV curves recorded at a scan rate of 0.1 mV s⁻¹. (g) Selected galvanostatic charge/discharge voltage profiles at 200 mA g⁻¹. (h) Cycling performance and the corresponding coulombic efficiency at 500 mA g⁻¹. (i) Rate capability and (j) capacity retention from 100 to 1000 mA g⁻¹.¹³

Potassium-ion battery

Due to the abundance and low-cost of potassium resources compared to lithium resources, potassium-ion batteries as a possible energy system have gradually attracted researchers' attention, among which the potassium ion is considered as the working medium to drive this new “rocking chair” battery.^116,117 In addition, due to the lower standard redox potential of potassium in non-aqueous electrolytes compared to that of sodium, potassium-ion batteries are more likely to construct low-cost, high-energy density and high-voltage storage systems.¹¹⁸

Although graphite is considered as a promising anode material for potassium-ion batteries, construction of high-energy density potassium-ion batteries based on the graphite anode is challenging because of its relatively low capacity. Therefore, it is necessary to explore other anode materials with higher capacities.¹¹ Tin phosphide materials are a member of burgeoning anode candidates for potassium-ion batteries due to their high theoretical capacities and much higher electronic conductivity compared to pure phosphorus.^119,120 Meanwhile, a synergistic potassium storage mechanism may occur in tin phosphide anodes, where Sn and P elements can react with K to form K–Sn and K–P alloys, respectively. The resulting K–Sn alloy can act as a conducting pathway to activate the reversible K-storage/release of nonconductive K–P phases, and the well-dispersed K–P phase would serve as a matrix to prevent the agglomeration of K–Sn/Sn.¹²¹ These characteristics together render tin phosphide to be a promising anode for potassium-ion batteries. Zhang et al.¹²² reported that Sn₄P₃–C composite material can be used as a high-capacity anode material for potassium-ion batteries, showing obvious advantages over pure phosphorus. Later, they further proposed a synergistic K-storage mechanism of Sn₄P₃ including a sequential conversion from P to K₃P₁₁/K₃P and the alloying reaction between Sn and K to form KSn.¹¹⁹ However, current investigations are limited to Sn₄P₃, and there are few reports about phosphorus-richer tin phosphides as anodes for potassium-ion batteries, which possess much higher theoretical capacities.

Our group recently developed a simple colloidal method to prepare the phosphorus-richer SnP_0.94 nanoplates. To further improve their stability, the SnP_0.94@GO composite was fabricated by combining SnP_0.94 nanoplates with graphene oxide (GO) sheets (Fig. 7).¹²³ As a new type of anode material for potassium-ion batteries, SnP_0.94@GO composite material prepared by us has shown good electrochemical properties. At a current density of 25 mA g⁻¹, the SnP_0.94@GO electrode showed an initial depotassiation capacity of 294 mA h g⁻¹, and a high-capacity retention rate of 83% after 50 cycles. Even at 200 mA g⁻¹, the composite could deliver a depotassiation capacity of 106 mA h g⁻¹, and maintain 93% of capacity after 100 cycles. The good potassium storage performance of the SnP_0.94@GO electrode is attributed to the synergetic effects of the high theoretical capacity of SnP_0.94 and GO sheet matrix that could buffer volume change, accelerate charge transfer, and reduce contact between active materials and electrolytes.

Fig. 7 (a) TEM image of as-synthesized SnP_0.94 nanoplates. (b) HRTEM images and (c) schematic diagram of a typical SnP_0.94 nanoplate viewed from top and side. (d) STEM image and EDS mapping images of the as-prepared SnP_0.94@GO composite. (e) Schematic depicting the formation of the SnP_0.94@GO composite. (f) Typical CV curves recorded with a scan rate of 0.1 mV s⁻¹. (g) Selected galvanostatic charge/discharge voltage profiles at 25 mA g⁻¹. (h) Rate capability from 25 to 1000 mA g⁻¹. (i) Cycle performances at different current densities. (j) Cycle performances and the corresponding coulombic efficiencies of the SnP_0.94 nanoplates and SnP_0.94@GO composite at a current density of 200 mA g⁻¹. Reprinted with permission from ref. 123. Copyright 2019 Elsevier.

Table 1 summarizes the recent advances in the performance of some typical tin-based nanomaterials as anodes for Li-, Na-, and K-ion batteries including our own recent contributions to this subject. Compared with traditional graphite anode materials, tin-based anode materials have some advantages such as higher mass specific capacity and higher lithiation/delithiation potential. At the same time, these tin-based anode materials usually suffer from poor cycling stability due to their larger volume changes. This stability issue can be improved by delicately designing their size/structure as discussed herein or combining them with other highly stable materials, such as carbon and graphene. It is expected that well-designed tin-based nanomaterials can deliver superior performance as anodes for different types of alkali-ion batteries.

Table 1 Initial charge capacities, initial coulombic efficiencies and cycling performances of some typical tin-based materials in Li-, Na-, and K-ion batteries

Battery type	Material	Initial cycle performance			Cycling performances			Ref.
		Initial charge capacity (mA h g^−1)	Initial coulombic efficiency (%)	Current density (mA g^−1)	Capacity retention (mA h g^−1)	Number of cycles	Current density (mA g^−1)
Li-Ion battery	3D porous graphene networks anchored with Sn nanoparticles	1245	69	200	682	1000	2000	124
	Nano-Sn/C composite	710	69	200	710	130	200	125
	Yolk–shell SnO2@C nanoparticles	1236	56.4	100	630	100	100	126
	Coaxial SnO2@C hollow nanospheres	730	49.6	200	460	100	500	127
	SnSe/C nano composite	412.4	55.1	500	324.6	200	500	114
	Monodisperse Sn nanoparticles	657.2	51	200	416.4	400	500	12
	Hollow/amorphous tin oxide nanospheres	503.0	48	200	402.5	300	200	12
Na-Ion battery	1–3 nm thick SnO on carbon cloth	848	79.1	100	452	1000	1000	128
	Carbon covered SnS/SnO2 heterostructure anchored to graphene	729	74.6	30	409	500	810	129
	SnS/3D N-doped graphene hybrid	890.5	80.1	100	509.9	1000	2000	130
	SnS2 nanocrystals on amino-functionalized graphene	749	73	200	480	1000	1000	131
	Single-crystalline SnSe nanosheet clusters	738	∼95	25	271	100	200	112
	Ultrathin layered SnSe nanoplates	738	79.3	50	312.7	300	2000	113
	Yolk–shell structured SnSe nanoparticles	406.0	76.5	200	307.1	150	500	12
K-Ion battery	3D carbon/Sn	384.5	45.3	50	276.4	100	50	132
	SnSb–graphene–carbon nanofibers	320.6	52.5	100	275.1	100	100	133
	Carbon confined SnO2	195	28	50	122	200	100	134
	SnO2–graphene–carbon nanofibers	229.2	44.1	100	202	100	100	135
	SnS2–rGO	355	56.3	25	250	30	25	136
	Sn4P3/C	∼350	59.5	50	307.2	50	50	122
	Sn4P3@carbon fiber	∼510	64.1	50	160.7	1000	500	119
	SnP0.94 nanoplates/graphene oxide composite	294	42	25	162	50	100	123

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Conclusions and outlook

Tin-based nanomaterials have great application prospects in many fields, and their performance is greatly defined by their structure and composition. Colloidal synthesis is a powerful synthetic strategy and has been successfully applied for controllably synthesizing tin-based nanomaterials. In this feature article, we have focused on the developments from our group in colloidal synthesis and application in batteries of tin-based materials. We have first summarized recent advances in the colloidal synthesis of tin nanoparticles and other tin-based nanomaterials such as oxides, chalcogenides, and alloys. These materials exhibit delicate and well-designed structures and compositions, which play crucial roles in determining their applications. We have also discussed the electrochemical performance of several types of tin-based nanomaterials, including metallic tin, oxides, selenides, and phosphides, in different alkali-ion (Li⁺, Na⁺, and K⁺) batteries.

Despite these exciting developments, some critical issues still need to be addressed for the further development of tin-based nanomaterials. In our opinion, several main concerns are listed as below:

(1) Controlled synthesis provides an effective strategy to optimize size- and shape-dependent properties of metals. Nevertheless, rigorous shape control of tin nanoparticles in colloidal synthesis still remains a great challenge due to the lack of ligands that can selectively adsorb on specific crystal surfaces of tin particles to control their growth in certain directions. To further advance this field, looking for appropriate ligands to facilitate the anisotropic growth of tin nanoparticles would be greatly desired.

(2) Organic capping ligands play a crucial role in the size- and shape-control of tin-based nanomaterials and would finally absorb on the particle surface, enabling their excellent solubility in nonpolar solvents. However, these organic capping ligands usually possess poor electrical conductivity due to their long organic chains, greatly affecting the electrochemical performance of tin-based nanomaterials. Therefore, removing them or replacing them with shorter ones is very necessary to improve their electrical conductivity. However, these post-treatment processes inevitably complicate the preparation of tin-based nanomaterials. It would be also meaningful if these ligands on the particle surface can be in situ carbonized into a conductive carbon shell or matrix with the promise that the as-prepared tin-based nanomaterials are not destroyed.

(3) To date, the cycle performance and rate capacity of tin-based materials have been significantly improved by controlling their size, structure, and composition. However, some key electrochemical parameters are still not up to commercial standards. For example, due to the large specific surface area, tin-based nanomaterials usually consume plenty of alkali ions in the formation process of a thick SEI layer, resulting in a large loss of initial capacity. Therefore, more work needs to be done to exploit modification methods for achieving higher initial coulombic efficiency. In addition, the larger radii of sodium and potassium would cause larger volume changes and slower kinetics of tin-based anode materials during the charge/discharge processes, leading to great challenges in the structural design and morphological control of tin-based anode materials.

In summary, the electrochemical performance of tin-based materials can be significantly improved by regulating their size, structure, and composition during colloidal synthesis. Meanwhile, more efforts are required to design tin-based materials with targeted properties that could overcome current limitations.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC) (No. 51772142), the Development and Reform Commission of Shenzhen Municipality (Novel Nanomaterial Discipline Construction Plan), and the Shenzhen Science and Technology Innovation Committee (JCYJ20170412152528921, KQJSCX20170328155428476, KQTD2016053019134356).

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