Tin dioxide-based nanomaterials as anodes for lithium-ion batteries

Abstract

The development of new electrode materials for lithium-ion batteries (LIBs) has attracted significant attention because commercial anode materials in LIBs, like graphite, may not be able to meet the increasing energy demand of new electronic devices. Tin dioxide (SnO₂) is considered as a promising alternative to graphite due to its high specific capacity. However, the large volume changes of SnO₂ during the lithiation/delithiation process lead to capacity fading and poor cycling performance. In this review, we have summarized the synthesis of SnO₂-based nanomaterials with various structures and chemical compositions, and their electrochemical performance as LIB anodes. This review addresses pure SnO₂ nanomaterials, the composites of SnO₂ and carbonaceous materials, the composites of SnO₂ and transition metal oxides, and other hybrid SnO₂-based materials. By providing a discussion on the synthesis methods and electrochemistry of some representative SnO₂-based nanomaterials, we aim to demonstrate that electrochemical properties can be significantly improved by modifying chemical composition and morphology. By analyzing and summarizing the recent progress in SnO₂ anode materials, we hope to show that there is still a long way to go for SnO₂ to become a commercial LIB electrode and more research has to be focused on how to enhance the cycling stability.

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1. Introduction

Lithium-ion batteries (LIBs) have been widely used in modern electronic equipment, such as laptops, mobile phones, and mechanical devices for their high energy densities, long cycle life, and environmental friendliness.^1–6 As an important component of LIBs, electrode materials are responsible for energy storage and cycling life of LIBs.^7–11 Currently, graphite is the main anode in commercial LIBs owing to its good stability.^12–14 However, it cannot fully meet the increasing energy demand for batteries due to its low theoretical specific capacity (372 mA h g⁻¹).^15–18 In order to solve this problem, much work has been focused on exploring alternative materials with high specific capacity.^19–26

Potential candidates like metal oxides have been widely researched due to their high theoretical specific capacities, such as MnO,^27–30 MnO₂,^31–36 Mn₃O₄,^37–40 Fe₂O₃,^41–45 Fe₃O₄,^46–50 Co₃O₄,^51–54 and SnO₂.^55–57 Among these materials, SnO₂ has attracted significant attention due to its low cost, natural abundance, and high theoretical specific capacity (782 mA h g⁻¹ of bulk SnO₂). SnO₂ is superior to other metal oxides as it has a low charge and discharge potential, i.e., an average charge and discharge potential of 0.5 V and 0.3 V vs. Li/Li⁺, respectively,⁵⁸ resulting in LIBs with higher energy density. However, the commercial use of SnO₂ as an anode material is still hindered by poor cycling stability and inferior rate performance, which is attributed to the electrochemical reaction mechanism of SnO₂ during lithiation/delithiation. For SnO₂-based anode material for LIBs, the electrochemistry includes two steps, shown as follows:^59–61

SnO₂ + 4Li⁺ + 4e⁻ → Sn + 2Li₂O (1)

Sn + xLi⁺ + xe⁻ ↔ Li_xSn (0 ≤ x ≤ 4.4) (2)

In the first reaction, SnO₂ reacts with Li⁺ and electrons to generate Sn and Li₂O. It is believed as an irreversible process, and this is the main reason why SnO₂ suffers severe capacity deterioration in the initial lithiation process.⁶² In the second reaction, Sn obtained from the first step reacts with Li⁺ and electrons to reversibly generate Li_xSn alloys. The alloying and dealloying processes represent discharging and charging processes of SnO₂-based anode material, respectively.⁶³ However, Li-alloying anode materials like Li_xSn and Li_xSi, possess the disadvantages of limited cycle life and severe capacity loss because of large volume changes, pulverization, and continuous formation of solid electrolyte interphase (SEI) during the alloying/dealloying process. Therefore, owing to this irreversible phase transformation process during lithiation/delithiation, the commercial use of SnO₂ is largely hampered.

In order to solve these problems and to improve the electrochemical performance of SnO₂, researchers have synthesized many SnO₂-based anode materials with various well-designed architectures. These SnO₂-based anode materials can be classified into four types, according to their chemical composition. The first category includes pure nanostructured SnO₂ materials, such as one-dimensional (1D) nanorods (NDs), nanotubes (NTs)^64–68 and nanowires (NWs),^69–74 two-dimensional nanobelts,^75,76 nanosheets^77–82 and nanoplates,^83–85 three-dimensional (3D) hollow nanostructures,^86–92 and hierarchical nanostructures.⁹³ Nanosized materials shorten transmission distance for electrons and Li⁺ and also help to reduce the extent of volume changes during the electrochemical process. In addition, it has been reported that the reduction reaction of SnO₂ nanomaterials becomes reversible or partly reversible, which improves the lithium storage capacity and reduces the capacity loss during the charge/discharge process.⁹⁴ So the theoretic capacity of SnO₂ nanomaterials can be increased up to 1495 mA h g⁻¹. The second category includes the composites of SnO₂ and carbonaceous materials,^95–97 such as SnO₂/carbon nanotubes (CNTs),^98–101 SnO₂/hollow carbon spheres,^102–104 SnO₂/graphene^105–110 and SnO₂/amorphous carbon. Carbonaceous materials improve the conductivity of the composites and also provide abundant nanosized voids as buffers to decrease the effect of large volume changes during the charge/discharge process.^111–113 The third category includes the composites of SnO₂, transition metal oxides, and carbonaceous materials (SnO₂/TMOs/C). Various composites of SnO₂ and transition metal oxides (SnO₂/TMOs) have been synthesized in the past 20 years, such as SnO₂/Fe₂O₃,^114–117 SnO₂/Co₃O₄,^118,119 SnO₂/TiO₂,^120–122 SnO₂/ZnO¹²³ and SnO₂/MoO₃ (ref. 124) and they showed enhanced lithium storage capacity compared to pure SnO₂ anode material.¹²⁵ It has been reported that the introduction of TMOs, such as Fe₂O₃ (ref. 126) and Co₃O₄,¹²⁷ can effectively enhance the capacity because the transition metal nanoparticles in the composite can reversibly convert the extra Li₂O into Li⁺; thus, influencing the charge/discharge processes. However, cycling stability and rate performance of the composites still need to be further improved. Therefore, based on SnO₂/TMOs materials, much work has been done to synthesize the composites of SnO₂, TMOs, and carbonaceous materials (SnO₂/TMOs/C),^128–131 like Fe₃O₄/SnO₂/rGO,¹³² SnO₂@C@Fe₃O₄ (ref. 133) and SnO₂/MoO₃/C.^134,135 SnO₂/TMOs/C materials more effectively alleviate the impact of volume changes and improve conductivity, leading to better electrochemical performance as an anode, compared to the SnO₂/TMOs materials.^136,137 The fourth category includes some other tin dioxide-based compounds, such as heteroatom-doped SnO₂ (ref. 138 and 139) (Fe-doped SnO₂,¹⁴⁰ Zn-doped SnO₂ (ref. 141)), Li₄Ti₅O₁₂/SnO₂ (ref. 142) and SnO₂/C₃N₄.¹⁴³

In this article, we provide the recent progress in the research of these four major types of SnO₂-based anode material, as mentioned above (Scheme 1). For the following sections, we will introduce the various SnO₂-based nanomaterials as well as their corresponding synthesis methods and electrochemical performance. We hope this review article will serve as a good reference for further research.

Scheme 1 Schematic presentation of different SnO₂-based anode materials for LIBs.

2. Pure nanostructured SnO₂ materials

2.1 1D nanomaterials

Many 1D nanostructured SnO₂ materials, such as nanorods,^144–146 nanotubes^147–150 and nanowires^151–153 have been synthesized in recent years. 1D SnO₂ nanomaterials usually exhibit superior specific discharge capacity since they can offer extra channels and pathways for electron transmission compared to bulk SnO₂ and SnO₂ powders.^154–156

2.1.1 SnO₂ nanorods. Pure SnO₂ nanorods can be synthesized via many methods such as chemical vapor deposition (CVD), vapor liquid solid,¹⁵⁷ hydrothermal treatment¹⁵⁸ and spray pyrolysis approach.¹⁵⁹ Among these reported methods, the hydrothermal synthesis of SnO₂ nanorods is the most used routine as it can be conveniently controlled due to its facile operation. The formation mechanism in a hydrothermal system can be summarized as follows:

Sn⁴⁺ + 4OH⁻ → Sn(OH)₄ → SnO₂ + 2H₂O (3)

First, Sn(OH)₄ forms by the hydrolysis of Sn-based salts in aqueous medium. During the hydrothermal treatment, Sn(OH)₄ tends to convert into SnO₂ and subsequently grows along the [001] direction.^160,161 Early in 2003, Zhang et al. fabricated uniform SnO₂ nanorods with diameters of about 8–15 nm and lengths of about 150–200 nm by a one-step procedure under mild conditions.¹⁴⁸ They dissolved sodium dodecyl sulfate and Sn(OH)₆²⁻ salt in a solution consisting of heptane and hexanol by stirring. Then, the homogeneously dispersed solution was transferred into a Teflon-lined autoclave and heated to 200 °C for 18 h. The as-prepared SnO₂ nanorods displayed a crystalline rutile structure. Zhang et al. also discovered that the concentration of Sn(OH)₆²⁻ ions and the ratio of NaOH and SnCl₄ determined the shape of the SnO₂ nanorods. It was found that on increasing the concentration of Sn(OH)₆²⁻ from 0.2 M to 0.3 M, the number of nanorods significantly decreased; additionally, on increasing the molar ratio of NaOH to SnCl₄ from 10 : 1 to 30 : 1, the aspect ratio of SnO₂ nanorods increased.¹⁶²

In 2004, Cheng et al. investigated a large-scale hydrothermal method to synthesize single-crystalline SnO₂ nanorods with lengths of 15–20 nm and diameters of 2.5–5 nm (Fig. 1A). Sn⁴⁺ precursor was dissolved in a mixture of water and alcohol, and pH was adjusted 12 and the solution was then heated at 150 °C for 24 h.¹⁶³ Based on Zhang and Cheng's work, many groups have synthesized SnO₂ nanorods via hydrothermal methods in other different systems. Guo et al. synthesized SnO₂ nanorods with diameter in the range of 120–260 nm and length up to 2–3 μm by using hexadecyltrimethylammonium bromide as a template (Fig. 1B).⁵⁸ Chen et al. synthesized single crystalline SnO₂ nanorods with diameters of 4–15 nm and lengths of 100–200 nm.¹⁴⁵ Xi et al. investigated a new synthesis method of ultrathin SnO₂ nanorods with an average diameter of 2 ± 0.5 nm.¹⁶⁴ Therefore, various SnO₂ nanorods with distinct morphology can be synthesized by controlling hydrothermal conditions.

Fig. 1 (A) Transmission electron microscopy (TEM) images of the as-prepared SnO₂ nanorods (adapted with permission from ref. 163 copyright 2004 American Chemical Society). (B) Scanning electron microscopy (SEM) images of long SnO₂ nanorods (adapted with permission from ref. 58 copyright 2004 Elsevier). (C) SEM images, (D) high-resolution TEM (HRTEM) images, (E) FFT patterns of the HRTEM images, and (F) cycling performance of SnO₂ nanorods (adapted with permission from ref. 154 copyright 2009 Royal Society of Chemistry).

SnO₂ nanorods can be applied as anode materials in LIBs.¹⁶⁵ Liu et al. synthesized SnO₂ nanorods arrays on a flexible alloy substrate via hydrothermal method. The nanorods arrays have average diameter and length of 60 and 670 nm, respectively (Fig. 1C). Observed by HRTEM images (Fig. 1D) and FFT pattern of HRTEM images (Fig. 1E), SnO₂ nanorod array was growing on substrate along [001] direction, since (001) plane is more loosely packed and has a relatively high surface energy compared to {110} planes.¹⁶⁶ As-collected hierarchical array structure can directly be used as a binder-free electrode for LIBs, which shows good energy performance, including high discharge capacity (the first discharge is 1918 mA h g⁻¹) and good cycling stability (580 mA h g⁻¹ after 100 cycles at 0.1C, with coulombic efficiency of nearly 100%) (Fig. 1F).¹⁵⁴

2.1.2 SnO₂ nanotubes. SnO₂ nanotubes have been widely used as anode materials in lithium-ion batteries. Compared to SnO₂ nanorods, SnO₂ nanotubes usually possess superior lithium storage capacity and cycling stability due to their hollow characteristics.¹⁶⁷ Sacrificial template-based approaches are commonly used in synthesizing this tubular structure. Anodic aluminum anode (AAO), polycarbonate (PC) membrane, SiO₂ and ZnO can be served as sacrificial templates in synthesizing metal oxides nanostructures. The synthesis process of SnO₂ nanotubes usually includes three steps: (1) infiltration of reactants into the templates; (2) growth of SnO₂ with designed shapes and morphology; (3) removal of the template.

Early in 2006, Lai et al. reported the preparation of SnO₂ nanotubes with a thickness of 10 nm and a length of about 0.4–1.4 μm via the electrodeposition method (Fig. 2A). They first electroplated the SnO₂ nanoparticles on a gold electrode which was modified with a porous PC membrane. Then, the SnO₂ particles were annealed at 650 °C in ambient conditions. The shape and size of the as-prepared nanotubes could be easily controlled by monitoring the charge passed.⁶⁶ Meanwhile, Lai et al. also reported that the SnO₂ nanotubes possessed better crystallinity and uniformity in terms of length and width.

Fig. 2 (A) TEM image of SnO₂ nanotubes (adapted with permission from ref. 66 copyright 2006 Royal Society of Chemistry). (B) HRTEM image of SnO₂ nanotubes for measuring thickness, (C) FESEM image of the side view, and (D) FESEM images of the top view of SnO₂ nanotubes (adapted with permission from ref. 68 copyright 2005 American Chemical Society). (E) TEM image, (F) HRTEM image, and (G) discharge capacity vs. cycle number of SnO₂ nanotube arrays on Ti substrates (adapted with permission from ref. 64 copyright 2011 American Chemical Society).

The AAO and PC membrane-template methods have been widely used in the synthesis of metal oxide nanotubes. Wang et al. fabricated uniform polycrystalline SnO₂ nanotubes via this template.⁶⁸ They found that the diameter, thickness, length, and texture of nanotubes can be controlled with template structure, pristine particle size, and heating-rate temperature. As shown in Fig. 2B–D, nanotube walls, with a thickness of about 10–25 nm, were composed of abundant nanocrystallites of 6–15 nm. Furthermore, the SnO₂ nanotube electrode showed superior charge/discharge performance compared to SnO₂ nanoparticles. The specific capacity of the SnO₂ nanotube electrode can be 525 mA h g⁻¹ after 80 cycles.⁶⁸ They summarized that the improved cycling performance resulted from the following factors, (1) cavities in the nanotubes which provide space and reduce the effect of volume change during lithiation/delithiation process; (2) SnO₂ nanotubes provide more active sites for Li⁺ intercalation and deintercalation; and (3) compared to SnO₂ nanoparticles, the SnO₂ nanotubes are less movable and result in less agglomeration.⁶⁸ Si and SiO₂ have been commonly used in the synthesis of hollow nanostructures, including nanotubes. Morphology and shapes of SnO₂ nanotubes can be well designed by controlling the shape of the Si templates and the hydrothermal conditions.¹⁶⁸ Ye et al. also discovered that the shorter SnO₂ nanotubes showed superior electrochemical performance, since the hollow structure in the short nanotubes can alleviate the volume changes.¹⁴⁷ ZnO is another promising sacrificial template used for synthesizing SnO₂ nanotube arrays as ZnO can be conveniently synthesized and removed. The as-prepared nanotubes exhibited a diameter and thickness of about 100–300 nm and about 10–20 nm, respectively, composed of nanoparticles with diameters of about 2–5 nm (Fig. 2E and F). The material also showed high capacity and improved cycling performance, i.e., 750–800 mA h g⁻¹ after 20 cycles at 0.1C, as shown in Fig. 2G.⁶⁴

2.1.3 SnO₂ nanowires. Template-based methods have also been widely used to synthesize SnO₂ nanowires. These sacrificial templates contribute to the development of mesoporous and hollow SnO₂ nanowires.^63,169 Kim et al. fabricated a SnO₂ nanowire-based anode material for LIBs by using KIT-6 and SBA-15 SiO₂ as hard templates. They first dissolved SnCl₄·5H₂O and the hard templates in DI water and stirred the mixture at room temperature until Sn⁴⁺ was adsorbed by templates. Then, the resulting composites were annealed, and the templates were removed from the composites using a NaOH solution. The as-obtained SnO₂ nanowires had a diameter of 6 nm and a length greater than 3 μm, and the size of mesoporous SnO₂ on the surface of the nanowire was 3.8 nm with a BET surface area of 160 m² g⁻¹. Such a porous and interconnected SnO₂ nanowire structure showed better lithium storage capacity. The initial discharge and charge capacities of the nanowire anode were 1595 and 800 mA h g⁻¹ at 0.2C, respectively, and the discharge capacity remained at 773 mA h g⁻¹ after 50 cycles. These observations can be attributed to the mesopores which act as a buffer zone against SnO₂ volume expansion.⁷¹

Researchers have synthesized SnO₂ nanowires via many methods. For example, Ko et al. synthesized SnO₂ nanowires on the current collector via thermal evaporation at (600 °C). The as-prepared SnO₂ nanowires exhibited a highly-ordered single-crystalline phase with a thin diameter of 40–50 nm and length of more than 1 μm (Fig. 3A and B). The SnO₂ nanowire-based anode exhibited high specific discharge capacity and good cycling performance, i.e., 2140 mA h g⁻¹ at the first cycle and 510 mA h g⁻¹ at the 50^th cycle at 1C (Fig. 3C).⁷² Ding et al. reported a facile strategy for the synthesis of SnO₂ nanowire arrays using SBA-15 nanorods as a template by infiltrating molten SnCl₂ into the channels of the SBA-15 nanorods followed by calcination and removal of the template.⁶⁹ Han et al. investigated the synthesis of porous SnO₂ nanowire bundles with a high yield via solution-based approaches. This hierarchical nanostructure is made up of SnO₂ nanowires with an overall diameter of 80–120 nm and an average length of 6 μm (Fig. 3D). Moreover, the as-prepared nanowires have a highly porous structure composed of numerous nanocrystals.¹⁷⁰ Ren et al. synthesized 3D hierarchical SnO₂ nanowire arrays on carbon cloth by first, a CVD method for SnO₂ nanowires, followed by a Plasma Enhanced-CVD (PECVD) method for Si thin film coating. The SnO₂@Si nanowire arrays can directly serve as a flexible and binder-free anode for LIBs. In this unique structure, SnO₂ nanowires act as a lithium storage material and a conductive matrix to support Si; in addition, the thin Si layer acts as a buffer for SnO₂ to reduce the effect of volume changes (Fig. 3F). Anodes based on such novel hierarchical structures showed excellent electrochemical performance with discharge capacity of 2.13 mA h cm⁻² in the first cycle and 1.386 mA h cm⁻² after 50 cycles, i.e., 65% of the first cycle (Fig. 3G).⁷³ Thus, in this section, different synthesis methods for SnO₂ nanowires, corresponding morphologies, and electrochemical properties have been presented, demonstrating that SnO₂ nanowires are promising anode materials for LIBs.

Fig. 3 (A) Cross-sectional FESEM image, (B) TEM image, and (C) comparative cycling performance of the SnO₂ nanowires (adapted with permission from ref. 72 copyright 2009 IOP Publishing). (D and E) SEM images of SnO₂ porous nanowire bundles (adapted with permission from ref. 170 copyright 2011 Royal Society of Chemistry). (F) HRTEM image, and (G) cycling performance of the SnO₂@Si nanowires (adapted with permission from ref. 73 copyright 2013 Royal Society of Chemistry).

2.2 2D nanomaterials

Due to the difficulty of synthesizing well-defined 2D SnO₂ nanomaterials, obtaining a large variety of 2D SnO₂ nanostructures is significantly harder than 1D and 3D nanomaterials. The anisotropic crystal growth of SnO₂, in the formation process, limits the synthesis of 2D nanomaterials.⁸² In order to overcome this problem, in situ growth of SnO₂ nanosheets on physical or chemical substrates has been widely investigated.^171,172 Among these fabrication processes, SnO₂ nanosheets grown on physical substrates, such as metal foam,^49,173–175 foil,^{77,154,176–178} carbon cloth^78,179 and graphite paper^180–183 can be directly used as binder-free and current-collector free electrode in LIBs due to excellent conductive and supportive properties of such substrates. Meanwhile, some organic macromolecules can be used as chemical templates to fabricate SnO₂ nanosheets. These substrates could provide a growth plane for SnO₂ nanosheets and inhibit self-aggregation during the lithiation/delithiation processes.¹⁸⁴ Moreover, SnO₂ nanosheets have a tendency to assemble into complex structures, such as nanosheet arrays, clusters, and spheres, which usually exhibit superior lithium storage capacities due to their highly interconnected and hierarchical nanostructures.^184–187

Zhao et al. fabricated SnO₂ nanosheet arrays on Ni foam with a thickness of 20 nm and a length of 500 nm (Fig. 4A) via a hydrothermal method. Nickel foam, i.e., a 3D macroporous conductive network, was used as a supportive substrate to fabricate SnO₂ nanosheet arrays, which can be directly used as a binder-free anode in LIBs. Due to the high electroactive surface area, ultrathin sheets, and shorter electron transport pathways, the nanosheets exhibited excellent electrochemical properties. The novel anode showed an initial discharge capacity of approximately 1800 mA h g⁻¹ and remained at 674.9 mA h g⁻¹ after 50 cycles at 0.5C (Fig. 4B).⁷⁹ Moreover, interconnected single and double layer SnO₂ nanosheets were also fabricated on three different conductive substrates, i.e., Ti, Cu foil, and flexible graphite paper, as integrated binder-free electrodes for LIBs. The nanosheets were interconnected with each other to form a hierarchical network, and the thickness of a single SnO₂ nanosheet layer was about 250 nm (Fig. 4C and D). The electrodes delivered high specific capacity, excellent cycling stability, and good rate capability.^77,79 Zhu et al. reported an ultra-rapid, low-cost, and simple microwave-assisted synthesis of ultrathin SnO₂ nanosheets with a thickness of less than 5 nm (Fig. 4E). The ultrathin SnO₂ nanosheets exhibited significantly enhanced electrochemical lithium storage properties with a high reversible capacity of 757.6 mA h g⁻¹ at a current density of 200 mA g⁻¹ up to 40 cycles. The ultrathin 2D nanosheets can significantly reduce the ion diffusion paths and allow faster phase transitions; furthermore, the sufficient external surface interspace and porous configuration successfully accommodate the large volume changes.^77,78,80

Fig. 4 (A) SEM image, and (B) cycling performance of hierarchical SnO₂ nanosheet arrays on nickel foam (adapted with permission from ref. 79 copyright 2014 Wiley). (C) Top view, and (D) cross-sectional FESEM images of SnO₂ nanosheets double layer on Ti foil (adapted with permission from ref. 77 copyright 2014 Royal Society of Chemistry). (E) HRTEM image of ultrathin SnO₂ nanosheets of less than 5 nm (adapted the permission from ref. 80 copyright 2015 American Chemical Society).

Many complex hierarchical SnO₂ nanostructures were also assembled by 2D SnO₂ nanosheets. Flower-like SnO₂ assembled by many SnO₂ nanosheets, with an average thickness of 100 nm, have been synthesized on a flexible carbon cloth using CVD at 750 °C.⁷⁸ SEM images (Fig. 5A) show that as-prepared flower-like SnO₂ nanostructures were formed by an assembly of numerous nanosheets with a thickness of 100 nm. Ding et al. synthesized hollow spheres assembled from SnO₂ nanosheets (Fig. 5B), using sulfonated polystyrene hollow spheres (sPSHSs) as a template; the surface of the sPSHS is covered with –SO₃– functional groups. Therefore, the Sn²⁺ ions can easily interact with these templates via electrostatic forces and subsequently grow into SnO₂ nanosheets, assisted by mercaptoacetic acid. Moreover, sPSHSs templates are beneficial for minimizing the gas outflux during the template removal process, which helps the retention of the final structure.^188–191 The as-prepared samples showed a superior cycling capacity retention compared to other SnO₂ nanoflowers assembled by SnO₂ nanosheets as well as SnO₂ nanoparticles, indicating the positive effect of the unique nanostructure (Fig. 5C).⁸⁷ Wei et al. fabricated nanoporous SnO₂ nanosheets via a simple one-step ultrasonic-assisted chemical precipitation strategy with polyvinylpyrrolidone (PVP) as a soft template. Due to the porous nanosheet structure, it exhibited high capacity, i.e., an initial discharge capacity of 2231 mA h g⁻¹ at 0.2 A g⁻¹, and excellent cycling performance, i.e., an initial discharge capacity of 710 mA h g⁻¹ and 606 mA g⁻¹ at 1.6 A g⁻¹ and 4 A g⁻¹, respectively, which remained at 497 mA h g⁻¹ and 280 mA h g⁻¹ after 60 cycles, respectively (Fig. 5D).⁸¹

Fig. 5 (A) SEM images of flower-like SnO₂ nanosheets (adapted with permission from ref. 78 copyright 2014 Elsevier). (B) TEM image, and (C) cycling performance of SnO₂ nanosheets hollow spheres (adapted the permission from ref. 87 copyright 2011 Royal Society of Chemistry). (D) Cycling performance of SnO₂ nanosheets at 1.6 A g⁻¹ and 4.0 A g⁻¹ (adapted the permission from ref. 81 copyright 2017 Elsevier).

2.3 3D nanomaterials

In recent years, 3D SnO₂ structures have attracted significant attention.⁸⁶ Various 3D nanostructures have been synthesized, such as hollow spheres^91,192–194 and microboxes.^195–197 Furthermore, many new synthetic methods to prepare SnO₂ hollow structures, including Ostwald ripening,^198–200 Kirkendall effect,^201–204 removable templates¹⁹⁵ and chemically induced self-assembly^192,205 have been reported.

In 2006, Lou et al. investigated a one-pot template-free synthesis of hollow SnO₂ nanostructures, based on an unusual inside-out Ostwald ripening mechanism. The synthesis process was performed in an ethanol–water (EtOH–H₂O) mixed solvent with K₂SnO₃ as the precursor, followed by a hydrothermal treatment. Lou et al. discovered that the concentration of the precursor and the ratio of EtOH in the mixed solvent determined both the particle size and morphology of the product. Moreover, the addition of urea or thiourea in the synthetic mixtures was also found to increase the product yield, morphological yield to nearly 100%, and a well-dispersed monodispersity (Fig. 6A–C). Such hollow SnO₂ nanosphere-based anode material exhibited high discharge capacity and good cycling performance, i.e., an initial discharge capacity of 1140 mA h g⁻¹, which is comparable to the theoretical capacity of graphite after more than 40 cycles.⁸⁶

Fig. 6 (A) FESEM images of SnO₂ hollow nanosphere prepared in the solvent of EtOH : H₂O ratio of 1 : 1. (B) TEM images of SnO₂ hollow nanosphere prepared with the value of EtOH in the solvent of EtOH–H₂O = 37.5%. (C) TEM images of SnO₂ hollow nanosphere prepared with the value of EtOH in the solvent EtOH–H₂O = 37.5% and 0.1 M urea (adapted the permission from ref. 86 copyright 2006 WILEY). (D) TEM images of SnO₂ microboxes (adapted the permission from ref. 195 copyright 2016 Royal Society of Chemistry). (E) SEM images of SnO₂ hierarchical hollow microspheres (adapted the permission from ref. 205 copyright 2017 Royal Society of Chemistry). (F) TEM images, and (G) rate performance of SnO₂ hollow spheres (adapted with permission from ref. 91 copyright 2012 Elsevier). (H) TEM image of the hierarchical SnO₂ (adapted with permission from ref. 210 copyright 2010 American Chemical Society).

Porous hollow SnO₂ micro-boxes have been synthesized via a selective leaching strategy using ZnSn(OH)₆ as the precursor. ZnSn(OH)₆ micro-boxes were first formed through a modified one-pot co-precipitation method and subsequently, the Zn species were removed via a selective leaching strategy. The TEM image (Fig. 6D) shows that the thickness of the shell was about 100 nm.¹⁹⁵ Hu et al. used a template-and additive-free hydrothermal route to prepare a uniquely shaped SnO₂ material that comprised of a hollow spherical morphology with uniform diameter and very thin petal-like nanosheets grown perpendicularly on the surface of the spheres, resembling a “chestnut cupule” (Fig. 6E). In contrast to conventional SnO₂ materials, this unique morphology significantly improved the storage capacity and cycling performance of SnO₂ as an anode material for lithium and sodium ion batteries.²⁰⁶

In addition, 3D hierarchical SnO₂ nanomaterials can be assembled by 0D, 1D, and 2D SnO₂ nanomaterials. For example, 0D crystalline SnO₂ nanoparticles were successfully assembled into a high-order nanostructure of hollow core–shell. First, SnCl₄ was dissolved in a mixture of DI water and ethanol to form a homogeneous solution which was heated to 180 °C for 24 h. Then, the collected sediments were calcined in air to remove carbon by oxidation. FESEM showed that the meso-spheres had an overall dimension of 1–3 μm, and the surface of the spheres was formed by small aggregated SnO₂ nanoparticles (11 nm). The shell was estimated to be approximately 200 nm in thickness. This unique SnO₂ nanostructure could store an exceedingly large amount of Li⁺ and cycled well for a pure phase SnO₂ anode.^207–209 Hollow urchin-like SnO₂ nanospheres have been fabricated using ultrathin nanorods via a solvothermal route. The diameters of urchin-like nanospheres and nanorods are about 300 and 100 nm, respectively. The as-obtained hollow urchin-like SnO₂ nanospheres with ultrathin 1D nanorods exhibited high capacity and excellent rate discharging performance. The 1^st, 2^nd, 20^th, and 50^th discharge capacities were 1881 mA h g⁻¹, 1090 mA h g⁻¹, 781 mA h g⁻¹, and 719 mA h g⁻¹, respectively, at a current density of 100 mA g⁻¹. Upon changing the discharge–charge rates to 0.2, 0.4, 0.8, 1, 2, and 0.4C, the capacities of urchin-like SnO₂ were maintained at 815, 687, 601, 446, 282, and 520 mA h g⁻¹, respectively, while for commercial SnO₂, the capacities were only 828, 677, 512, 386, 246, 133, and 375 mA h g⁻¹, respectively. The retention of the reversible capacity of the hollow nanosphere electrodes was better than that of commercial SnO₂ samples.²¹⁰ Furthermore, SnO₂ hollow nanospheres can also be synthesized by sol–gel methods.⁹¹ The size of the hollow spheres is controlled by using different-sized templates. As-prepared SnO₂ shells are almost amorphous and exhibit a rutile phase after annealing at 600 °C. The size of the SnO₂ hollow spheres ranges from 25 to 100 nm (Fig. 6F), and the thickness of the shell is constantly 5 nm despite the size of the hollow spheres. Due to the nanosized hollow sphere and thin shell thickness, SnO₂ hollow spheres show excellent electrochemical performance. The smallest hollow sphere of SnO₂ (25 nm) exhibited a high reversible capacity of 750 mA h g⁻¹ as well as good rate performance, i.e., 700 mA h g⁻¹ at 0.2 A g⁻¹ and 530 mA h g⁻¹ at 0.5 A g⁻¹ (Fig. 6G).⁹¹ Hierarchical SnO₂ hollow nanostructures can also be assembled by 2D nanosheets. As shown in Fig. 6H, the hierarchical SnO₂ shows high capacities and excellent cycle performance as an anode material for LIBs. The improved electrochemical properties could be ascribed to the large surface area, enhanced structure stability, and short diffusion length for both lithium ions and electrons.²¹¹

3. Composites of SnO₂ and carbonaceous materials

3.1 SnO₂ with carbon nanotubes (CNTs)

Nanocomposites of SnO₂ and CNTs reportedly exhibit improved lithium storage capacities compared to pure SnO₂ materials.²¹² This can be attributed to the flexible nature and superior conductivity of the CNTs, which alleviates the internal stress caused during the charge/discharge process^213–215 and enhances the electron transportation. With increased conductivity and surface area, such nanocomposites show enhanced lithium storage capability.^216,217

Wen et al. reported an in situ synthesis of mesoporous SnO₂ on the surface of multi-walled CNTs (MWCNTs) through a hydrothermal method utilizing CTAB as the structure-directing agent. The MWCNTs/SnO₂ hybrid electrodes showed great electrochemical performance and cycling stability. This can be attributed to the synergistic effects of the unique combination of properties including their one-dimensional hollow structure, high-strength with flexibility, excellent electric conductivity, and large surface area, which helped alleviate the effect of volume expansion, shorten the distance of Li⁺ diffusion, and contribute to the transmission of electrons.^{100,218–221}

In another hydrothermal system, Du et al. synthesized SnO₂/MWCNT composites by a simple solvothermal method and subsequent heat treatment at 360 °C with SnCl₂ and CNTs as reactants. The distribution of SnO₂ nanocrystals can be controlled by changing the molar ratio of Sn²⁺ and CNTs in the precursor. For SnO₂/MWCNTs composites prepared with a molar ratio of Sn : C = 0.3 : 1, a uniform layer of SnO₂ nanocrystals, with a crystal size of about 5 nm, was deposited on the surface of the MWCNTs (Fig. 7A).²²² These composites showed very stable cycling retention, of up to 100 cycles, because of nanosized materials and the introduction of CNTs.^201,223 Jin et al. prepared SnO₂/MWCNTs electrodes via a hydrothermal method at 150 °C for 24 h. The SnO₂ nanoparticles with diameters of less than 3 nm were uniformly loaded onto the surface of MWCNTs. The MWCNT/SnO₂ nanocomposites exhibited a high reversible capacity of 420 mA h g⁻¹ even after 100 cycles.^137,224

Fig. 7 (A) FESEM image of SnO₂/CNT composites prepared with MSn : C = 0.3 : 1 (adapted with permission from ref. 221 copyright 2009 Elsevier). (B) SEM image, and (C) rate performance of SnO₂@C/MWCNTs-LiF (adapted with permission from ref. 224 copyright 2019 Elsevier). (D) TEM image and (E) cycling performance of CNT@void@SnO₂@C composite (adapted the permission from ref. 225 copyright 2015 Elsevier). (F) SEM image, and (G) rate performance of SnO₂/CNTH composites (adapted with permission from ref. 226 copyright 2019 American Chemical Society).

In order to further improve electrochemical properties of the SnO₂/CNTs hybrid nanomaterials, many groups have synthesized various SnO₂/CNTs-based anode materials. For example, Liang et al. synthesized SnO₂@C/MWCNTs-lithium fluoride (LiF) composite nanomaterials (Fig. 7B). Carbon-coated SnO₂ (SnO₂@C) was prepared by a spray drying method, with water-soluble asphalt as the carbon source and MWCNTs as the conductive agent. The conductivity was significantly enhanced, and the extent of volume expansion of the SnO₂ was reduced. Compared to SnO₂/CNTs, the presence of LiF enhanced the stability of the SEI film and improved the coulombic efficiency and capacity retention rate of the electrode. After 200 cycles, the SnO₂@C/MWCNTs-LiF anode still maintained 70.1% of the capacity retention rate and the specific capacity was held at 274 mA h g⁻¹ at 2400 mA g⁻¹, compared with 136 mA h g⁻¹ for the SnO₂@C/MWCNTs anode (Fig. 7C).²²⁵ Tian et al. fabricated a tube-in-tube nanostructure, denoted as CNT@void@SnO₂@C, by a facile hydrothermal method and subsequent carbonation with polysaccharide as the carbon source, SiO₂ as the sacrificial template, and NH₄F as the etchant. The CNT@void@SnO₂@C exhibited one-dimensional nanostructures with average diameters of about 100–150 nm and hollow structures filled with abundant voids (Fig. 7D). Tian et al. interpreted the formation mechanism as follows: (1) the crystallization and deposition of SnO₂ could occur prior to the polycondensation of glucose; (2) the formation rate of SnO₂@polysaccharide was faster than the etching rate of SiO₂ by NH₄F due to the slow etching process of SiO₂ by NH₄F solution under hydrothermal condition; and (3) the large void-space between the SnO₂@polysaccharide and CNT, formed after the SiO₂ coating layer, was etched away completely. The CNT@void@SnO₂@C exhibited good electrochemical properties, delivering a reversible capacity of 702.5 mA h g⁻¹ at 200 mA g⁻¹ even after 350 cycles (Fig. 7E). This indicated that the unique tube-in-tube nanostructure, as well as the one-dimensional void space, which formed between the inner CNT and outer SnO₂@C nanotubes, contributed significantly to the electrochemical performance.²²⁶ Liu et al. synthesized ultrafine SnO₂ (6–7 nm)/carbon nanotube hairball (SnO₂/CNTH) composites with a 3D hierarchical structure (Fig. 7F), which was prepared by spray drying and a solvothermal method. Fig. 7G shows that SnO₂/CNTH exhibited superior electrochemical performance and improved the lithium storage capacity compared to conventional SnO₂/CNT. The improved electrochemical performance can be attributed to the increased conductivity and enhanced electrode reactivity due to the 3D hierarchical cross-linked structure. This structure can also address the large volume changes upon cycling.²²⁷

3.2 SnO₂ with amorphous carbon

Hydrothermal treatment followed by carbonization is the most common method for synthesizing SnO₂/amorphous carbon nanomaterials.^228–230 There are mainly two synthesis routes of SnO₂/amorphous carbon; one is to treat Sn²⁺ or Sn⁴⁺ salt with the precursor of carbon,^231–236 and the other is to deposit a carbon layer on the as-prepared SnO₂ nanostructures.^237,238 Conventional amorphous carbon can be derived from glucose,^239,240 sucrose^241,242 and many organic compounds.²⁴³ The morphology and shapes of the nanostructures can be adjusted by the hydrothermal and annealing conditions.

Various SnO₂/C nanomaterials have been synthesized in recent years, such as carbon-coated SnO₂ NPs,^244–247 carbon-coated SnO₂ nanorods,^248,249 carbon-coated SnO₂ nanowires^151,217 and carbon-coated SnO₂ nanotubes.⁵⁷ Different structures and morphologies of SnO₂ are known to lead to different electrochemical performance. In 2008, Lou et al. prepared SnO₂/C composite hollow spheres. The mesoporous SnO₂ hollow spheres were embedded in 3D carbon networks (Fig. 8A). The carbon networks act as a physical buffering cushion for the intrinsic large volume change and electronically conducting pathways. Compared to SnO₂ hollow spheres and graphite, these SnO₂/carbon hollow spheres were able to deliver a reversible lithium storage capacity of 473 mA h g⁻¹ after 50 cycles (Fig. 8B).⁹⁹ In 2009, Lou et al. also synthesized a thin layer of carbon-coated SnO₂ nano-colloids (Fig. 8C) and coaxial SnO₂@carbon hollow nanospheres (Fig. 8D) by a simple hydrothermal method followed by carbonization; both exhibited improved electrochemical performance.^57,250 Courtel et al. reported an in situ synthesis of SnO₂ nanoparticles (5–10 nm)/carbon composite materials using the polyol method by oxidizing SnCl₂·2H₂O in the presence of a carbon matrix. The TEM image (Fig. 8E) show that the SnO₂ nanoparticles were uniformly embedded in the carbon matrix. Based on the nanostructure, the as-obtained composites showed an improved lithium storage capacity of 370 mA h g⁻¹ at 200 mA g⁻¹ and a lower capacity fading compared to commercial SnO₂ (50 nm).²⁵¹

Fig. 8 (A) TEM image, and (B) cycling performance of SnO₂/carbon composites hollow spheres (adapted with permission from ref. 99 copyright 2008 American Chemistry Society). (C) TEM image of carbon-coated SnO₂ nanocolloids obtained after carbonization at 550 °C (adapted with permission from ref. 239 copyright 2009 American Chemical Society). (D) TEM image of SnO₂@carbon hollow nanospheres (adapted with permission from ref. 240 copyright 2009 Wiley). (E) TEM image of nanosized SnO₂/carbon composites (adapted with permission from ref. 248 copyright 2009 Elsevier). (F) TEM image, and (G) HRETM image of SnO₂/RHPC composite materials (adapted with permission from ref. 231 copyright 2019 Elsevier). (H) The fabrication schematic of SnO₂@non-smooth carbon. (I) TEM image of SnO₂ quasi-nanocubes@non-smooth carbon (adapted with permission from ref. 255 copyright 2019 Elsevier).

While the morphology and structure of SnO₂ influence the electrochemical properties of the SnO₂/C composites, different carbonaceous materials also influence the performance of the composite. Xu et al. prepared composites of SnO₂/ordered mesoporous carbon (SnO₂/OMC) through a hydrolysis process. OMC, a novel kind of carbon material, has been widely used in LIBs due to its large surface area, high conductivity, and highly porous structure; it promotes the diffusion of lithium ions and electrolyte. The SnO₂/OMC composites delivered a good cycling performance with a reversible capacity of 395.6 mA h g⁻¹ for up to 50 cycles.²⁵² Shi et al. synthesized SnO₂/rice-husk-based porous carbon composites (RHPC) via a simple melt-impregnation method with a heat treatment route. RHPC can be easily and cheaply obtained by convenient carbonization and activation of rice husk. The TEM images (Fig. 8F and G) show that the SnO₂ nanoparticles with the average size of 4 nm can be loaded on the RHPC matrix. The cycling measurements showed that the discharge capacity of the RHPC/SnO₂ anode at the current density of 100 mA g⁻¹, at the 50^th cycle, was 550 mA h g⁻¹, which demonstrated that in contrast to pure SnO₂ anodes, the cycling performance of the RHPC/SnO₂ anode was remarkably enhanced by the introduction of the RHPC matrix. It also demonstrates that biomass-sourced carbonaceous materials like RHPC^253–255 have promising applications in LIBs due to their low cost and porous structure.²³³ Tian et al. fabricated non-smooth carbon-coated SnO₂ quasi-nanocubes. Generally, in SnO₂/C nanocomposites, nanostructured SnO₂ is usually coated with smooth carbon, which is easily fabricated from organically sourced carbon via hydrothermal or CVD methods.^90,256 However, it is believed that SnO₂-coated 3D non-smooth carbon usually shows better lithium storage properties owing to the substantial free space and larger surface area of the 3D structure.²⁵⁷ In this work, Tian et al. synthesized the hybrid nanostructures via multiple hydrothermal and calcination methods (Fig. 8H). As shown by the TEM images (Fig. 8I), the porous SnO₂ quasi-nanocubes were coated by a carbon layer with a rough surface. The introduction of 3D non-smooth carbon can reduce the transmission length of electrons and Li⁺ and increase the electrochemical reaction sites. Based on such 3D porous structures, the SnO₂@C anode displayed extraordinary cycling performance and outstanding rate capability, maintaining a capacity of 1089.5 mA h g⁻¹ at 200 mA g⁻¹, even after 400 cycles, as well as 479.2 mA h g⁻¹ at 3000 mA g⁻¹.²⁵⁷

It is worth mentioning that even though the amorphous carbon layer can enhance the cycling stability and conductivity of the composites, it has a relatively low lithium storage capacity compared to SnO₂. Therefore, the ratio of the carbon layer and the SnO₂ nanomaterial determines the capacity and cycle life of the composites.

3.3 SnO₂/graphene

As an important 2D carbon material, graphene has quickly gained importance in material science in recent years. Due to its excellent mechanical properties and superior conductivity, graphene has been widely studied in LIBs.^258–260 Compared to CNTs and amorphous carbon, graphene exhibits higher specific discharge capacity, high surface area, good mechanical properties, and high chemical stability;^261–263 thus, SnO₂/graphene, as an anode material in LIBs, shows better electrochemical performance.^264–268

Graphene can be used as a supporting substrate for the synthesis of hierarchical SnO₂ nanostructures. For example, Ding et al. fabricated 2D SnO₂ nanosheets grown on a graphene oxide (GO) support via a facile hydrothermal method. The SnO₂ nanosheets were uniformly embedded in the GO support, approximately 100 nm in length and 5–10 nm in thickness (Fig. 9A). This unique SnO₂/GO hybrid structure exhibited enhanced lithium storage properties with high reversible capacities, i.e., an initial discharge capacity of 1666 mA h g⁻¹, and good cycling performance, i.e., 518 mA h g⁻¹ after 50 cycles at 400 mA h g⁻¹.¹⁷¹ Additionally, due to its mechanical properties and electronic conductivity, graphene can serve as the carbon matrix to accommodate the SnO₂ nanoparticles. In 2017, Shi et al. investigated a facile microwave-assisted hydrothermal method to synthesize a composite of SnO₂ and graphene, which took only 30 min and did not require any chelating agents. As shown by TEM images (Fig. 9B) and the nitrogen adsorption–desorption isotherms (Fig. 9C), Shi et al. found that ultra-small SnO₂ nanoparticles were well dispersed on the surface of the graphene, with an average particle size of about 3–8 nm. It also showed superior lithium storage capability. The charge/discharge capacity of this material was 969.4/978.6 mA h g⁻¹ after 100 cycles at 200 mA g⁻¹.²⁶⁹ Binder-free multilayered SnO₂/graphene on Ni foam was fabricated via a dip-coating method. SnO₂ nanoparticles and grapheme were alternatively coated on to the Ni foam to obtain a sandwich-like structure. As shown in SEM images in Fig. 9D and E, SnO₂ nanoparticles were uniformly distributed on the surface of the Ni foam and wrapped tightly in GN. Such a multilayered nanostructure showed superior electrochemical performance due to the following factors: (1) 3D porous Ni foam serves as a conductive network and binder-free current which is beneficial for electron and ion diffusion; (2) the graphene layer improves the conductivity of anode material and buffers the SnO₂ volume changes; and (3) the extent of volume change of the SnO₂ nanoparticles is lower than that of bulk SnO₂. Owing to such porous Ni foam frameworks and sandwich-like structures, the SnO₂/graphene composites exhibited good rate performance and excellent cycling stability. High capacities, i.e., 708 and 609 mA h g⁻¹ were achieved at current densities 1 and 2 A g⁻¹, respectively (Fig. 9F). Furthermore, the SnO₂/GN electrode delivered a high capacity of 757 mA h g⁻¹ after 500 cycles at 1 A g⁻¹.¹⁷⁴

Fig. 9 (A) SEM image of SnO₂ nanosheets/graphene (adapted with permission from ref. 170 copyright 2011 Royal Society of Chemistry). (B) TEM image and (C) nitrogen adsorption–desorption isotherms of SnO₂ nanoparticles/graphene (adapted with permission from ref. 169 copyright 2017 Elsevier). (D and E) SEM images and (F) rate performance of SnO₂/GN on Ni foam (adapted with permission from ref. 267 copyright 2018 Elsevier). (G) HRTEM image and (H) rate performance of SnO₂NC@N-RGO (adapted with permission from ref. 173 copyright 2013 Wiley). (I) SEM images of SnO_2−x/N-rGO (adapted with permission from ref. 280 copyright 2018 Royal Society of Chemistry). (J) Cycling stability of SnO₂QDs@S-rGO (adapted with permission from ref. 277 copyright 2018 Elsevier). (K) Cycling performance and (L) rate performance of FTO/rGO (adapted with permission from ref. 109 copyright 2015 American Chemical Society).

Moreover, it is worth mentioning that atom-doped graphene can also be used as a carbon matrix for SnO₂, which effectively enhances the electrochemical performance of the composites. Heteroatoms in graphene can act as anchor sites that prevent aggregation and exfoliation of the SnO₂ anchored on graphene; this helps improve the cycling stability of such anode materials.^270–274 Liu et al. synthesized SnO₂ nanoparticles anchored on chlorinated graphene (SnO₂@rGO-Cl) as binder-free electrodes that exhibit a long cycling life, of up to 400 cycles, with a discharge capacity of 1008 mA h g⁻¹ via a facile strategy using a one-step heat treatment at low temperature.²⁷⁵ Liu et al. found that Cl-doping can enhance the electrical conductivity of graphene and the Cl–Sn bonds can prevent the exfoliation of SnO₂ nanoparticles during the charge/discharge process; thus, improving the electrochemical properties of SnO₂-based hybrid nanomaterials. SnO₂/nitrogen-doped graphene (N-rGO) was also applied as an anode in LIBs. It is believed that N-doped carbonaceous materials enhance the electronic conductivity and SEI film stability.^276–278 Zhou et al. fabricated SnO₂@N-rGO via a hydrazine monohydrate vapor reduction approach for anchoring SnO₂ nanocrystals uniformly into N-rGO (Fig. 9G). Due to the bond formed between SnO₂ and graphene, and the void pores dispersed in N-rGO, the as-prepared hybrid materials displayed superior mechanical properties and lithium storage capacity. SnO₂@N-rGO anode showed a reversible charge capacity of 1346 mA h g⁻¹ after 500 cycles; furthermore, as the current density increased from 0.5 to 1, 2, 5, 10, and 20 A g⁻¹, their discharge capacity varied from 1074 to 994, 915, 782, 631, and 417 mA h g⁻¹, respectively (Fig. 9H).²⁷⁹ Wu et al. synthesized the SnO_2−x/N-rGO hybrid material through electrostatic adsorption-induced self-assembly together with a thermal reduction process. This treatment induced the generation of the oxygen vacancies on the surface of SnO₂ hollow nanospheres; thus, building up a long-range and bi-continuous transfer channel for rapid electron and ion transport. Meanwhile, SnO_2−x hollow spheres are well-wrapped by graphene sheets; thus, enhancing the conductivity of the anode material (Fig. 9I). Due to these structural advantages, the as-obtained SnO_2−x/N-rGO electrode exhibited excellent robust cycling stability, i.e., about 912 mA h g⁻¹ after 500 cycles at 0.5 A g⁻¹ and 652 mA h g⁻¹ after 200 cycles at 1 A g⁻¹, and superior rate capability, i.e., 309 mA h g⁻¹ at 10 A g⁻¹.²⁸⁰ Sulfur-doped graphene (S-rGO) also proved to be a feasible anode material for LIBs.²⁸¹ Compared to the C atom, the S atom has a larger volume and lower electronegativity, which is beneficial for the diffusion of Li⁺ and electrons. For instance, Wu et al. successfully loaded SnO₂ quantum dots (QDs) on sulfur-doped reduced graphene oxide (S-rGO), and it exhibited excellent lithium storage with a high specific capacity of 897 mA h g⁻¹ and a long cycling stability with 88% capacity retention after 500 cycles (Fig. 9J).²⁸² The abovementioned results demonstrate that heteroatoms can tailor the electronic structure of carbon and create topological defects in the carbon lattice.^281,283

Apart from doped-graphene, atom-doped SnO₂, such as fluorine-doped tin oxide¹³⁸ and antimony-doped tin oxide¹²² can also be used with pristine graphene as an anode material.^284–287 For example, Xu et al. has successfully fabricated composites of fluorine-doped tin oxide (FTO) and rGO from a colloidal solution containing FTO nanocrystals and rGO by a hydrothermal treatment; the FTO nanocrystals were tightly embedded in the RGO nanosheets. As an anode material, the FTO/RGO composite showed high structural stability during the lithiation and delithiation processes. The conductive FTO nanocrystals help form stable and thin SEI films. Moreover, the FTO/RGO composite retains a discharge capacity as high as 1439 mA h g⁻¹ after 200 cycles at 100 mA g⁻¹ (Fig. 9J), and 1148 mA h g⁻¹ at 1000 mA g⁻¹ (Fig. 9K).¹⁰⁹

3.4 Complex SnO₂/C nanostructure

Compared to 1D or 2D carbonaceous materials, complex 3D carbonaceous matrices usually show better electrochemical properties due to their high specific surface area and porous structure, which is beneficial in suppressing volume changes.²⁸⁸ In recent years, various SnO₂/C complex hybrid nanomaterials have been reported, such as the combination of CNTs and amorphous carbon,²⁸⁸ and CNTs and graphene.^283,289,290 These complex hierarchical SnO₂/C nanostructures often show superior electrochemical properties, making them an important research subject.

A sandwich structure with carbon nanofiber, SnO₂, and nanofiber bundle for carbon-coating (C@SnO₂@C) has been fabricated by using collagen fiber (CF), which is a typical fibrous protein extracted from cattle skin and is used as a bio-template as well as the carbon source. FESEM image (Fig. 10A) and TEM image (Fig. 10B) show that the average diameter of the SnO₂ nanofiber bundle was about 5–10 μm and a layer of SnO₂ was sandwiched between the carbon nanofiber and the carbon coating layer. Such hierarchical architectures of the C@SnO₂@C nanofiber bundle guaranteed a good balance between electron transport and Li⁺ diffusion kinetics. Thus, efficient ambipolar diffusion and reduced volume changes of SnO₂ were obtained to ensure structural integrity with high cycling stability.¹⁶²

Fig. 10 (A) FESEM image, and (B) TEM image of C@SnO₂@C nanofiber bundle (adapted with permission from ref. 162 copyright 2016 Royal Society of Chemistry). (C) FESEM image, (D) TEM image, (E) cycling performance, and (F) rate performance of CNT@SnO₂@G (adapted with permission from ref. 263 copyright 2017 Elsevier). (G) SEM image, (H) cycling performance, and (I) rate performance of SnO₂@G-SWCNT materials (adapted with permission from ref. 289 copyright 2017 American Chemical Society).

A porous 3D core–shell structured CNT@SnO₂ composite, with a graphene coating (CNT@SnO₂@G), has been synthesized via a two-step hydrothermal method. The first step consists of the synthesis of CNT@SnO₂, and then, CNT@SnO₂@G is formed in the subsequent step. FESEM image (Fig. 10C) and TEM image (Fig. 10D) show that CNT@SnO₂ (about 20–40 nm in width) particles was distributed across the graphene sheets and was encased in the graphene coating, which suppressed the formation of SEI layers on the surface of the CNT@SnO₂@G. The as-prepared CNT@SnO₂@G electrode exhibited outstanding lithium storage capability, including a large specific capacity, remarkable cycling stability, and excellent rate capability (Fig. 10E and F).²⁶⁵

SnO₂ nanoparticles anchored on an aerogel based on 3D graphene-single walled carbon nanotube (SnO₂@G-SWCNT), as shown in Fig. 10G, has been fabricated by a hydrothermal self-assembly process.²⁹¹ The 3D G-SWCNT matrix provides a flexible conductive matrix and a more porous network to support SnO₂. This is beneficial for facilitating electronic and ionic transportation and mitigating the volume changes of the SnO₂ during lithiation/delithiation; thus, leading to enhanced electrochemical performance of the SnO₂ anodes for LIBs. The discharge capacity remained 758 mA h g⁻¹ at 100 mA g⁻¹ after 200 cycles (Fig. 10H) and 537 mA h g⁻¹ at 1 A g⁻¹ after 300 cycles (Fig. 10I).²⁹¹

4. SnO₂/TMOs/C

The composites of SnO₂ and transition metal oxides (TMOs) have been considered as promising anode materials for LIBs.^292–294 By the introduction of metal oxides, the reversible decomposition of Li₂O can be increased because TMOs can convert extra Li₂O into Li⁺;^295–297 thus, enhancing their cycling stability, rate performance, and rate capability. However, the electrochemical performance of SnO₂/TMOs is unsatisfactory because of low conductivity and large volume expansion, leading to poor cycling stability.¹³⁶ In order to overcome these problems, much work has been focused on the fabrication of SnO₂/TMOs/C materials in recent years. Supported and coated by carbonaceous material and TMOs, the structural stability and conductivity of SnO₂ can be significantly improved. This section will cover synthesis methods, morphology, and electrochemical performance of SnO₂/TMOs/C.^298–302

4.1 SnO₂/Fe₂O₃/C

Fe₂O₃ has been considered as another promising anode material for LIBs due to its large theoretical capacity (1007 mA h g⁻¹) and low cost.^303,304 However, the short carrier diffusion length inhibits lithiation/delithiation processes. It has been reported that SnO₂/Fe₂O₃ showed an improvement in photocatalysis, energy storage, and gas sensing.^{303,305–307} Furthermore, the SnO₂/Fe₂O₃/C electrode showed better lithium storage capability.³⁰⁸ In this composite, SnO₂ possesses a high intrinsic conductivity and a shortened charge diffusion distance, while Fe₂O₃ facilitates the reversible decomposition of Li₂O and prevents Sn aggregation during charging and discharging.³⁰⁹

In 2014, Wu et al. proposed a facile hydrothermal method to synthesize a ternary phased SnO₂/Fe₂O₃/SWCNTs composite. As shown in Fig. 11A, the composites of SnO₂ and Fe₂O₃ nanoparticles were well distributed and firmly anchored on to SWCNTs, which serve as a buffer and conductive matrix. Nanosized Fe₂O₃/SnO₂ composites can suppress the effect of volume changes and particle agglomeration. The SnO₂/Fe₂O₃/SWCNTs electrode showed a high reversible capacity, superior cycle performance, and high rate capability. It delivered a capacity of 692 mA h g⁻¹ at 200 mA g⁻¹ after 50 cycles. Even at a rate as high as 2000 mA g⁻¹, this composite could still maintain its capacity at 656 mA h g⁻¹ (Fig. 11B).³¹⁰

Fig. 11 (A) TEM image, and (B) rate performance of SnO₂/Fe₂O₃/SWCNTs (adapted with permission from ref. 308 copyright 2014 Elsevier). (C) TEM images, and (D) schematic illustration of the synthesis of γ-Fe₂O₃@SnO₂@C core–shell nanorods (adapted with permission from ref. 115 copyright 2013 Royal Society of Chemistry).

The growth of SnO₂ on Fe₂O₃ and subsequent carbon coating on SnO₂/Fe₂O₃ is another common synthesis route for SnO₂/Fe₂O₃/C. Du et al. synthesized γ-Fe₂O₃@SnO₂@C porous core–shell nanorods. They first formed FeOOH nanorods via a hydrothermal process, which served as a template for the subsequent SnO₂ deposition process in another hydrothermal system. After the deposition of SnO₂ on the FeOOH nanorods, the as-synthesized FeOOH@SnO₂ nanorods were coated with a carbon layer via another hydrothermal process and carbonized under N₂ at 500 °C for 2 h (Fig. 11C). The TEM image (Fig. 11D) shows that SnO₂ can successfully grow on the Fe₂O₃ nanostructure with well-defined interfaces. The thickness of the SnO₂ layer is 5–10 nm, which is composed of SnO₂ nanoparticles with a diameter of about 3–5 nm, and the core nanorod is highly porous. Such a porous core–shell hybrid nanorod-based electrode showed good cycling and rate performance due to the improvement of conductivity and structural stability by the introduction of carbon and Fe₂O₃.¹¹⁵

4.2 SnO₂/Co₃O₄/C

Many studies have concentrated on improving electrochemical properties of SnO₂ and Co₃O₄ by designing and synthesizing unique nanostructures. Like most transition metals, the presence of Co can improve the reversibility of the reduction reaction of Li₂O and further enhance the reversible capacity.^311–315 As shown in Fig. 12A, Co₃O₄@SnO₂@C core–shell nanorods have been synthesized via a hydrothermal method followed by carbonization. Fig. 12B and C show that SnO₂ nanoparticles uniformly coated the surface of Co₃O₄ nanorods and that the coating was smooth and uniform, with a thickness of 5–10 nm. Such materials exhibited improved cycling performance and higher specific capability as an anode material for LIBs (860 mA h g⁻¹ after 50 cycles at 0.2 A g⁻¹). This result demonstrated that the combination of SnO₂ and Co₃O₄, into an integrated core–shell nanorod structure, exhibited a better and more elegant synergistic effect during the charge/discharge processes.

Fig. 12 (A) Schematic illustration of the fabrication of Co₃O₄@SnO₂@C core–shell nanorods. (B) TEM image, and (C) HRTEM images of Co₃O₄@SnO₂@C core–shell nanorods (adapted with permission from ref. 313 copyright 2012 Royal Society of Chemistry). (D) HRTEM image and, (E) cycling performance of SnO₂/Co₃O₄@NC-1 nanoflakes (adapted with permission from ref. 119 copyright 2019 Elsevier).

SnO₂/Co₃O₄@N-doped carbon (NC) usually shows better lithium storage properties compared to most SnO₂/Co₃O₄/C materials.³¹⁶ Wang et al. successfully designed and fabricated SnO₂/Co₃O₄/NC nanoflakes via a combined strategy of CVD and template synthesis.^119,317 HRTEM images (Fig. 12D) show that SnO₂/Co₃O₄ nanoparticles were uniformly distributed on the NC nanoflakes. By adjusting the ratio of the precursor and reaction conditions, they found that the SnO₂/Co₃O₄@NC (R_Sn/Co = 1 : 1) nanoflakes-based electrode demonstrated excellent lithium storage capability, i.e., a discharge capacity of 1450.3 mA h g⁻¹ after 300 cycles at 200 mA h g⁻¹ (Fig. 12E). The superior lithium storage of such materials may result from the synergistic effect between the combination of SnO₂ and Co₃O₄ and better conductivity caused by the N-doped carbon matrix.¹¹⁹

Therefore, in SnO₂/Co₃O₄/C composites, SnO₂ and Co₃O₄ display a synergistic enhancement effect. It can provide additional active sites for lithium storage and shorten the lithium diffusion distance. Additionally, the carbon layer can greatly improve the electrode conductivity and restrain volume changes during the lithiation/delithiation processes.

4.3 SnO₂/TiO₂/C

SnO₂, as one of the most extensively investigated anode material, which offers a high specific capacity. However, poor cycling stability, due to large volume expansion and subsequent pulverization during Li⁺ insertion/extraction processes, greatly restricts its application as an anode material.^{130,318–320} In contrast, TiO₂ exhibits negligible volume change (less than 4%) and stable electrochemical properties, but its application is limited due to its low specific capacity.^321–325 Furthermore, SnO₂ and TiO₂ have complementary characteristics in LIBs, i.e., Sn⁴⁺ and Ti⁴⁺ possess similar radii and the lattice matching of SnO₂ and TiO₂ is good.^326–328 Considering the facts mentioned above, in order to promote cycling stability of SnO₂, various SnO₂/TiO₂ based hybrid materials have been synthesized.^329–334

Mesoporous SnO₂@C@TiO₂ nanochains have been synthesized by first fabricating SnO₂@C core–shell nanochains via a hydrothermal method and subsequent continuous mechanical stirring of the solution of SnO₂@C and tetrabutyl titanate (C₁₆H₃₆O₄Ti). It is noticeable that the SnO₂ core is composed of SnO₂ nanoparticles with a diameter of 2–6 nm and is coated by a thin carbon layer (2–6 nm) as well as a TiO₂ layer (about 8 nm) (Fig. 13A). In this 3D hierarchical structure, the carbon layer and the TiO₂ layer can effectively improve cycling stability and discharge capacity, i.e., the initial discharge capacity of 807 mA h g⁻¹ and 369 mA h g⁻¹ after 100 cycles at 100 mA h g⁻¹.²⁹⁸

Fig. 13 (A) TEM images of SnO₂@C@TiO₂ nanochains (adapted with permission from ref. 296 copyright 2015 Elsevier). (B) Schematic illustration of the synthesis, (C) STEM image, and (D) cycling performance of HTSO/GC-HSs (adapted with permission from ref. 299 copyright 2017 Wiley).

Xie et al. reported the synthesis of hierarchical TiO₂/SnO₂ hollow spheres coated with graphitized carbon (HTSO/GC-HSs) by a multi-step approach. As shown in Fig. 13B, titanate-silicone gel particles were first reacted with SnCl₂ via a sol–gel process to obtain core–shell hybrid structures, which were further coated with a polysaccharide via a hydrothermal process and subsequent carbonization in Ar atmosphere. As shown in Fig. 13C, the as-prepared mesoporous HTSO/GC-HSs had a yolk–shell structure and elements Sn, Ti, O and C were found to be uniformly distributed in this hierarchical hollow nanostructure. Additionally, they also found that due to the uniform distribution of SnO₂, TiO₂, and the carbon layer, a solid solution was formed which could effectively suppress the effect of volume changes during the charge/discharge process. The specific discharge capacity remained at about 680 mA h g⁻¹ at 1 A g⁻¹ after 500 cycles, which demonstrates the excellent cycling performance of HTSO/GC-HSs at high current densities (Fig. 13D).³⁰¹

SnO₂/TiO₂/C combines the high capacity of SnO₂ with the long cycle life and high rate capability of TiO₂.^335,336 Furthermore, carbonaceous materials can inhibit agglomeration and pulverization of SnO₂ and enhance the conductivity of TiO₂.^337,338 Therefore, SnO₂/TiO₂/C present an important area of research for the future.

4.4 SnO₂/TMOs/graphene

In the previous sections, we have discussed the synthesis methods, morphologies, and electrochemical performance of some representative SnO₂/TMOs/C anode materials. We classified them into different categories according to metal oxides because different TMOs greatly influence the physical and chemical properties of such complex materials due to different reaction mechanisms. In addition, different carbonaceous materials used in SnO₂/TMOs/C also influence the electrode performance in LIBs. Among these complex materials, much work has been focused on the fabrication of SnO₂/TMOs/graphene materials in recent years. Compared to amorphous carbon and CNTs, graphene shows higher lithium storage capacity.^339–342 Additionally, graphene, as a supportive matrix and conductive network, displays excellent mechanical properties and electronic conductivity, which is conducive to improvement in cycling stability and rate performance of SnO₂. Therefore, effectively combining SnO₂, TMOs, and graphene, to synthesize high-performance and practical anode material, has been a popular research subject in recent years.^{46,275,343–345}

Wang et al. synthesized a Fe₃O₄/SnO₂/rGO (FSG) composite via a facile hydrothermal method. SEM images (Fig. 14A) and elemental mapping show that FSG consists of SnO₂ and Fe₃O₄ nanoparticles with diameters of about 10–100 nm, uniformly distributed on the surface of the rGO. In this composite, rGO served as a conductive and robust matrix to prevent aggregation of Fe₃O₄ and SnO₂ nanoparticles on rGO. As shown in Fig. 14B, the nanocrystallites of Fe₃O₄ and SnO₂ tend to link with each other, which is beneficial for suppressing both, i.e., the formation of the SEI film and the volume changes. Such a novel nanostructure can effectively shorten the transport path of Li⁺ and electrons and helps improve electrolyte penetration. The FSG nanocomposite exhibited a reversible capacity of 947 mA h g⁻¹ at a current density of 200 mA g⁻¹ in the first cycle and maintained a capacity of 831 mA h g⁻¹ after 200 cycles (Fig. 14C).¹³²

Fig. 14 (A) SEM image, (B) HRTEM image, and (C) cycling performance of FSG (adapted with permission from ref. 132 copyright 2016 Elsevier). (D) SEM image, (E) TEM image from top and side view of CNT/S, and (F) cycling performance of FNT/S/RGO (adapted with permission from ref. 136 copyright 2017 Elsevier).

Lee et al. have synthesized hollow nanostructured α-Fe₂O₃ nanotubes/SnO₂/rGO (FNT/S/G) via a microwave-assisted hydrothermal method. As shown in Fig. 14D, SnO₂ nanorods grow on the surface of FNT, and FNT/SnO₂ are uniformly anchored on rGO sheets. The diameter and length (Fig. 14E) of FNT/S are about 230 nm and 660 nm, respectively. Lee et al. also compared the electrochemical performance of FNT, FNT/S, and FNT/S/G. As shown in Fig. 14F, FNT/S/G exhibited the highest discharge capacity under all tested current densities. FNT/S was the second best, demonstrating that the introduction of SnO₂ and rGO can effectively enhance the lithium storage capability and cycling stability of the anode material. The specific discharge capacity of FNT/S/RGO remained at 629 mA h g⁻¹ at 1 A g⁻¹ after 1000 cycles.^136,346

Other SnO₂/TMOs/graphene composites like SnO₂/TiO₂/GN,³³⁸ SnO₂/CuO/GN³⁴⁷ and SnO₂/In₂O₃/GN³⁴⁸ also exhibit improved electrochemical performance. The introduction of TMOs and graphene effectively improve the lithium storage capacity of SnO₂. Graphene serves as a supportive and conductive matrix that inhibits agglomeration and pulverization of SnO₂ and TMOs during the charge/discharge process.

5. Other SnO₂-based compounds

Li₄Ti₅O₁₂ (LTO) has been considered as a promising anode material for LIBs due to its excellent cycling stability during lithiation/delithiation.^349,350 However, like TiO₂, LTO possesses a lower theoretical capacity than other anode materials (175 mA h g⁻¹) and shows poor rate performance.^351,352 Therefore, in order to improve lithium storage of LTO and cycling performance of SnO₂, an effective strategy is to synthesize composites of SnO₂ and LTO nanostructures. Ding et al. synthesized hierarchical yolk–shell LTO–SnO₂ structures via a two-step hydrothermal method. They first synthesized SnO₂ microspheres at 180 °C for 12 h in a hydrothermal system and then, coated LTO on the surface of the SnO₂ in another hydrothermal system, followed by calcination at 600 °C for 3 h. SEM images (Fig. 15A) and TEM images (Fig. 15B) clearly show that the yolks are SnO₂ microspheres composed of SnO₂ nanoparticles, while the shells consist of LTO nanosheets. Hence, this hierarchical coating structure can effectively improve structural stability and suppress the volume changes of SnO₂ microspheres; thus, improving the cycling performance and showing high rate capacity.¹⁴²

Fig. 15 5(A) SEM image, and (B) TEM image of LTO–SnO₂ composites (adapted with permission from ref. 142 copyright 2018 Elsevier). (C) HRTEM image of carbon-coated Fe-doped SnO₂ nanoparticles (adapted with permission from ref. 351 copyright 2015 Elsevier). (D and E) SEM images, and (F) cycling performance of Zn-doped SnO₂ nanomaterials (adapted with permission from ref. 355 copyright 2014 Elsevier). (G) SEM image, and (H) cycling performance of Cu/N-doped SnO₂ (adapted with permission from ref. 139 copyright 2014 Elsevier).

Heteroatom-doped SnO₂ reveals better electrochemical performance compared to pure SnO₂. The presence of a dopant may favor the reversible formation of Li₂O and improve the conductivity of SnO₂; thus, enhancing the specific capacity of the anode material.³⁵³ Various heteroatom-doped SnO₂ nanomaterials have been synthesized and tested as anode materials in LIBs.^354–356 For example, Mueller et al. have synthesized carbon-coated Fe-doped SnO₂ nanoparticles (Fig. 15C). The discharge capacity of such materials is about 1726 mA h g⁻¹ after 10 cycles at 50 mA g⁻¹, which is around twice the theoretical capacity of pure SnO₂.³⁵³ Zn-doped SnO₂ hierarchical cube-like nanomaterials were fabricated via hydrothermal method. As shown in Fig. 15D and E, the cube is about 2 μm in length and composed of assembled nanorods which are 20–40 nm in diameter. The discharge capacity of Zn-doped SnO₂ composites is 488.3 mA h g⁻¹ after 50 cycles at 10 mA g⁻¹ (Fig. 15F).³⁵⁷ Wan et al. fabricated Cu/N-doped SnO₂ nanocomposites, as shown in Fig. 15G. They found that with the introduction of Cu/N, the average diameter of SnO₂ became smaller, and the surface became rough, which is beneficial for ion diffusion and suppressing volume expansion. Cu/N-doped SnO₂ electrode materials are known to deliver a discharge capacity of 1939 mA h g⁻¹ in the first cycle and remain at 664 mA h g⁻¹ after 50 cycles at 0.1C.¹³⁹

6. Conclusions

In this review, we summarized various SnO₂-based nanomaterials as anode materials for lithium-ion batteries (Table 1). By optimizing the structure and composition of the materials, i.e., by synthesizing nanostructured SnO₂ and making composites with carbonaceous materials or transition metal oxides (TMOs), the surface area and reaction sites of the anode materials can be effectively increased, improving the electrochemical properties of SnO₂-based anode materials. The introduction of carbonaceous materials or TMOs can effectively buffer the internal stresses caused by large volume changes, reduce the irreversible capacity loss, and improve the cycle performance. These aspects lay the foundation for the development and commercialization of high-performance LIBs in the future. Since lithium storage in SnO₂ is accompanied by repeatedly inserting and removing lithium ions, volume changes and irreversible capacity loss of SnO₂ electrode cannot be completely avoided. Significant research on nanosized SnO₂-based anode materials has been carried out, and while these materials display superior lithium storage capacity and cycling stability, critical issues like volume expansion and pulverization of SnO₂ remain to be solved. Further extensive research is needed before new, non-carbonaceous anode materials, such as SnO₂-based nanomaterials, can be commercialized.

Table 1 Electrochemical property comparison table of some typical SnO₂-based anode materials for LIBs

Materials	Morphology		Preparation approach	Voltage window (V)	Current density (A g^−1)	Cycle number	Specific capacity (mA h g^−1)	Reference
SnO2 nanorods	Diameter of 60 nm and length of 670 nm	Hydrothermal		0.005–2.5	0.078	100	580	154
SnO2 nanotube arrays	Diameter of 100–300 nm and thickness of 10–20 nm	Solvothermal and annealing		0.005–2	0.078	20	750–800	64
SnO2 nanowires	Diameter of 6 nm and length of >3 μm	Solvothermal and annealing		0–1.2	0.156	50	773	71
SnO2 nanosheet arrays	Thickness of 20 nm and length of 500 nm	Hydrothermal		0.01–1.2	0.391	50	674.9	79
Hollow SnO2 nanospheres	Size of 100–250 nm	Solvothermal and hydrothermal		0.01–2	0.391	40	1140	86
Urchin-like SnO2 nanospheres	Diameter of 300 nm	Solvothermal		0.01–3	0.1	50	719	210
SnO2 nanocrystals/MWCNT composites	Crystal size of 5 nm	Solvothermal and heat treatment		0.01–3	0.1	100	402	223
CNT@void@SnO2@C	Tube in tube diameter of 100–150 nm	Spray drying		0.01–3	0.2	350	702.5	226
SnO2/C composite hollow spheres	Size of 150–400 nm	Hydrothermal and carbonation		0–2.5	0.16	50	473	99
SnO2 nanoparticles/carbon composite	Nanoparticles of 5–10 nm	Polyol method		0.005–1.5	0.2	100	370	251
SnO2 nanosheets/GO	Length of 100 nm and thickness of 5–10 nm	Hydrothermal		0.01–1.2	0.4	50	518	171
SnO2 nanoparticles/graphene	Particle size of 3–8 nm	Microwave-assisted hydrothermal		0.01–3	0.2	100	978.6	269
SnO2@rGO-Cl	Nanoparticle size of 5 nm	Heat treatment		0.01–3	0.2	400	1008	275
CNT@SnO2@G	Width of 20–40 nm	Hydrothermal		0.01–3	0.1	100	947	265
SnO2@G-SWCNT	Diameter of 3–5 nm and particle size of 6–8 nm	Hydrothermal		0.01–3	1	300	537	291
SnO2–Fe2O3/SWCNTs nanocomposite	Particle size of 10–50 nm	Hydrothermal		0.01–3	0.2	50	692	310
Co3O4@SnO2@C core–shell nanorods	Thickness of 5–10 nm	Hydrothermal		0.01–2.5	0.2	50	860	313
SnO2@C@TiO2 nanochains	SnO2 for 2–6 nm; thickness of 2–6 nm and 8 nm for carbon and TiO2	Hydrothermal and subsequent mechanical stirring		0.01–3	0.1	100	369	298
Fe2O3 nanotubes/SnO2/rGO	Diameter of 230 nm and length of 660 nm	Microwave-assisted hydrothermal		0.01–3	1	1000	629	136
Yolk–shell LTO–SnO2	Diameter of 1.0–1.5 μm	Hydrothermal and calcination		0.01–3	0.175	200	253.2	142
Fe-doped SnO2 nanoparticles	Diameter of 15 nm	Hydrothermal and calcination		0.01–2	0.05	10	1726	351

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Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the National Key Research and Development Project (Grant No. 2020YFB1506001), Major Project of Education Department in Sichuan (Grant No. 18ZB0229), Sichuan Province Science and Technology Support Program (Grant No. 18ZDYF2973).

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