# Supersonically Sprayed Carbon-Coated Bimetallic Zn2SnO4 with Reduced Graphene Oxide for Rapid-Charging, Binder-Free, and Long-Term-Stable Lithium-Ion-Battery Anodes

Authors: Chanwoo Park; Bhavana Joshi; Edmund Samuel; Kwangjin Jang; Hae-Seok Lee; Sam S. Yoon
Journal: International Journal of Energy Research
 (2023)

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## Abstract

High capacity, rapid-charging, and long-term recyclability are the essential characteristics of high-performance lithium-ion battery (LIB) electrodes, which are particularly important in the electric vehicle industry. Bimetallic Zn2SnO4 nanoparticles wrapped in carbon shells were supersonically sprayed with reduced graphene oxide (rGO) nanosheets to fabricate rapid-charging, binder-free, and long-term-stable LIB anodes. The fabricated Zn2SnO4/C@rGO composite exhibited high capacity retention and specific capacity over a wide range of current densities. Zn2SnO4 was first reduced to metal oxides (ZnO and SnO2) and then to metals (Zn and Sn) during the lithiation process. The reduced metals were successively alloyed with Li+ ions, which in turn formed LiZn and LiSn. These cyclic conversions and alloying processes enhanced the performance of the LIBs, resulting in a high capacity of 1126 mAh⋅g-1 after 400 cycles at a high current density of 1000 mA·g-1; such high performance indicates the rapid-charging capability of the fabricated LIB anodes. The agglomeration of identical oxide particles was reduced during the long-term cycling process by introducing bimetallic Zn2SnO4, which was reduced to ZnO and SnO2 in the first cycle. Metal formation through the conversion and subsequent alloying processes caused the pulverization of ZnO and SnO2, whose mixture favorably alleviated agglomeration of identical oxides owing to the distinct grain boundaries of ZnO and SnO2. In addition, ZnO and SnO2 particles were confined within a carbon shell, which originated from the introduction of polyvinylpyrrolidone. The carbon shell further alleviated the particle agglomeration for Zn2SnO4/C@rGO, which is identified as the optimal case, yielding a capacity retention of 106% for the first reversible capacity at a current density of 1000 mA⋅g-1 after 400 cycles.


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## Body

## 1. Introduction

Metal oxides, such as ZnO, SnO2, MnO2, Co2O3, and sulfide such as Sb2S3, have been extensively investigated as anode materials for rechargeable lithium-ion batteries (LIBs) [1]. The theoretical specific capacities of these metal oxides (i.e., ZnO (987 mAh·g-1), [2] SnO2 (782–1494 mAh·g-1), [3] MnO2 (1232 mAh·g-1), [4] and Co3O4 (890 mAh·g-1) [5]) are much higher than those of graphite (370 mAh·g-1) and lithium titanate oxides (175 mAh·g-1) [6, 7]. However, these metal oxides undergo a significant volume change during the lithiation and delithiation processes; the corresponding volume changes are 200% for SnO2, [8] 163% for ZnO, [9] and 170% for MnO2 [10]. Consequently, metal oxides are pulverized and, thus, readily agglomerate, which reduces their specific capacity by increasing the number of charge–discharge cycles. In contrast, carbon-based materials are relatively more resilient to the agglomeration caused by pulverization during cyclic expansion and contraction processes. Thus, carbon-based materials offer superior cyclic stability and electrical conductivity. Therefore, a hybrid combination of metal oxides and carbon-based materials has been suggested as an alternative to alleviate the shortcomings of metal oxides [11–13].Particle pulverization can be reduced, for instance using SnO2 materials manufactured in the form of nanowires, nanosheets, or hollow nanoparticles improve electrochemical stability to a certain extent [3, 14]. Lan et al. [15] observed that SnO2 nanofibers, mesoporous SnO2/C, and SnO2/CNT composites retained 36, 45, and 42% of their respective initial capacities at 50, 100, and 100 cycles, respectively. Yin et al. [16] introduced macromesoporous carbon materials decorated with ZnO nanoparticles, which exhibited an initial specific capacity of 1966 mAh·g-1 at a low current density of 97.8 mA·g-1. The corresponding capacity retention was 973 mAh·g-1 after 100 charge–discharge cycles. However, their specific capacity was quickly reduced to 404 mAh·g-1 when the current density increased from 97.8 to 978 mA·g-1 because of insufficient diffusion and the low intake capacity of Li+ ions by a single-metal oxide. As such, hybrid materials comprising either SnO2 or ZnO, together with carbon-based materials, are proposed; however, they have their own limitations, such as poor cyclability and current rate capability [17].Bimetallic materials have been introduced to overcome the shortcomings of single-metal oxides [2, 18, 19]. The introduction of bimetallic oxides containing iron, manganese, nickel, cobalt, tin, or zinc can significantly increase the electrochemical performance for the following three reasons [20]. First, bimetallic oxides are prone to rich redox and conversion processes, facilitating the rapid diffusion of the organic electrolyte. Second, bimetallic oxides are reduced to two single-metal oxides during the first lithiation/delithiation cycle [21]. In the subsequent cycles, the distinctive grain boundaries of the two newly formed metal oxides alleviate the agglomeration of the identical metal oxides [22–24]. Third, the electrical network of bimetallic oxides is superior to that of a single-metal oxide, thereby providing superior electrochemical activity [25]. Consequently, the energy storage capability can be increased by facilitating redox reactions with two different metal oxides [26, 27].In particular, Zn2SnO4 is reduced to ZnO and SnO2 during the cyclic processes. These single-metal oxides are further reduced to Sn and Zn metals, which subsequently alloy with Li+ ions to form LiZn and LiSn. The metal formation and alloying capability of Zn2SnO4 enhance its overall electrochemical performance [2, 22, 24, 28]. Consequently, Zn2SnO4 is capable of storing up to 14.4 Li+ ions, among which 8 and 6.4 Li+ ions are consumed by the conversion and alloying processes, respectively, thereby securing the maximum number of 14.4 Li+ ions [28]. In contrast, bimetallic oxides, such as MnFe2O4 and CoFe2O4, are capable of storing Li+ ions only through the conversion process. Thus, their Li-ion storage capability is relatively lower than that of Zn2SnO4 without the subsequent alloying process. The resulting theoretical capacity of Zn2SnO4 is 1231 mAh·g-1, [28] whereas the corresponding values for CoFe2O4 and MnFe2O4 are 914 and 930 mAh·g-1, respectively [29, 30].Three-dimensional octahedron nanostructures of Zn2SnO4 were introduced as LIB anodes, which exhibited a retention capacity of 402 mAh·g-1 at a current density of 1600 mA·g-1 after 300 cycles [23]. Zn2SnO4 nanoparticles were prepared using ball milling and deposited using supersonic spraying to fabricate LIB anodes, which exhibited a retention capacity of 1316 mAh·g-1 at a current density of 100 mA·g-1 after 100 cycles [31]. Zn2SnO4/SnO2@Mn2O3 microboxes were synthesized using a hydrothermal method with a retention capacity of 597.2 mAh·g-1 at a current density of 100 mA·g-1 after 100 cycles [26]. Zn2SnO4/reduced graphene oxide (rGO) nanoflowers were synthesized using a hydrothermal method, and the corresponding LIB anodes exhibited a retention capacity of 540 mAh·g-1 at a current density of 300 mA·g-1 after 50 cycles [32]. Zn2SnO4/CNT composites prepared using a hydrothermal process showed a retention capacity of 273.4 mAh·g-1 at a current density of 100 mA·g-1 after 30 cycles [33]. Thus, Zn2SnO4-based materials have considerable benefits, and their electrochemical performance can be improved by improving their synthesis and deposition methods.Pulverization is inevitable even for bimetallic Zn2SnO4 materials, which eventually leads to particle agglomeration, in all LIB active materials. Hence, this study is aimed at alleviating the agglomeration phenomenon by introducing carbon shells around the bimetallic oxide such that the converted single-metal oxides do not readily coagulate. The introduction of sodium dodecylbenzene sulfonate, polyvinylpyrrolidone (PVP), or polyethylene glycol during precursor synthesis can control the particle size, thereby optimizing the Li-ion diffusion pathway. PVP can be converted into carbon shells through annealing in an inert environment. The resulting carbon shell can protect the single-metal oxides from agglomeration [34, 35]. The surface morphology can also be tuned by the introduction of PVP [36]. In addition, the supersonic spray deposition method is expected to exfoliate rGO nanosheets, between which particles are uniformly distributed, further alleviating particle agglomeration [37, 38]. Supersonic spraying does not require the use of a binder, which increases the precursor purity and the electrochemical performance. In addition, rGO nanosheets were supersonically sprayed with PVP-coated Zn2SnO4 particles to improve the electrical network of the deposited materials.


## 2. Experimental

### 2.1. Materials

Zinc acetate dihydrate (Zn(CH3COO)2·2H2O, 219.51 g·mol-1), tin chloride (SnCl4·5H2O, 350.60 g·mol-1), sodium hydroxide (NaOH, 40.00 g·mol-1), and PVP (Mw=40kDa) were purchased from Sigma-Aldrich. They were used for the synthesis of Zn2SnO4/PVP (hereafter denoted as ZSO/PVP). rGO and polyacrylonitrile (PAN, Mw=150kDa) were purchased from Angstrom Materials and Sigma-Aldrich, respectively, and added to the precursor solution. Deionized (DI) water and N,N-dimethylformamide (HCON(CH3)2, 73.09 g·mol-1) were used as the solvents. In addition, all the parts (i.e., cell case, wave spring, gasket, and separator) necessary to manufacture the coin cell were purchased from Wellcos Corporation, Republic of Korea, and 1 M LiPF6 (in Ec/DMC/DEC=1:1:1 (v/v/v)) was used as the electrolyte solution.


### 2.2. Nanoparticle Synthesis

A brief illustration demonstrating the synthesis of the ZSO/PVP powder is shown in Figure1(a). In detail, zinc acetate (0.459 g), tin chloride (0.281 g), sodium hydroxide (0.18 g), and PVP (0.7 g) required for synthesizing the ZSO(Zn2SnO4)/PVP powder were added to 70 mL of DI water, and then, a magnetic stirrer was used to effectively dissolve the mixture for 30 min. The stirred solution was poured into a Teflon-lined autoclave to facilitate the hydrothermal reaction in an oven at 180°C for 12 h. Then, 35 mL of the hydrothermally treated solution was poured into conical tubes (Falcon conical centrifuge tube, 50 mL), and DI water was added to fill the remaining volume of 15 mL. A centrifuge (ROTOFIX 32A, Hettich Corp., Germany) with conical tubes was operated at 4000 rpm for 7 min. After one centrifugation cycle, the upper DI water mixed with impurities was removed. After refilling with clean DI water, the centrifugation was repeated three times. Subsequently, the moist ZSO/PVP powder was collected, and the remaining moisture was removed by drying in an oven at 90°C for 10 h. ZnO/PVP nanoparticles were synthesized in similar way but without the tin chloride salt, while ZSO(Zn2SnO4) particles were prepared by excluding PVP in the hydrothermal solution. Other conditions of the hydrothermal process, including the solutions and washing, were similar to that of the ZSO(Zn2SnO4)/PVP powder synthesis. Because the yield of synthesized SnO2 was negligible, the purchased SnO2 was deposited by adding PVP to the deposition precursor solution and stirring it overnight. Thereafter, SnO2/PVP particles were centrifuged only once and dried using the oven at 90°C for 10 h.Figure 1
(a) Synthesis process of ZSO/PVP powder, (b) deposition of ZSO/PVP+rGO on Cu foil through cold spraying, and (c) layout showing the components of the LIB and electron transport in the annealed ZSO/C@rGO layer.


### 2.3. Supersonic Spray Coating

First, 0.5 g of the ZSO/PVP obtained through the hydrothermal treatment (Figure1(a)) and rGO (0.1 g) were added to dimethylformamide (50 mL). Then, 0.3 mL of 8 wt% PAN was added for particle dispersion. The mixed solution was vigorously stirred for 30 min, followed by tip sonication for 2 min to obtain a well-dispersed mixture. A supersonic cold-spraying (CS) system consisting of a supersonic nozzle, x-y stage, atomizer, compressor, and inline heater was employed to deposit the well-dispersed mixture onto Cu foils. The syringe-pump module was filled with the well-dispersed mixture and supplied the mixed solution to the atomizer at a constant flow rate (1.5 mL·min-1). The CS coating was conducted after setting the additional operating conditions (i.e., nozzle-to-substrate distance, scanning speed, carrier gas, gas pressure, and temperature). The fluid sprayed by the atomizer was deposited on a Cu foil using CS at 4 bar and 250°C, as shown in Figure 1(b). The operating conditions used in this study are listed in Table 1. Table 2 summarizes the materials used in the case studies (Cases 1–4) before and after annealing. These samples (SnO2/PVP+rGO, ZnO/PVP+rGO, ZSO+rGO, and ZSO/PVP+rGO) were annealed in an Ar environment. The annealing temperature was increased to 700°C at a rate of 3°C·min-1. The temperature was maintained at 700°C for 30 min. After annealing in an Ar environment, PVP was converted to carbon. Accordingly, the samples before annealing (i.e., SnO2/PVP+rGO, ZnO/PVP+rGO, ZSO+rGO, and ZSO/PVP+rGO) were transformed into SnO2/C@rGO, ZnO/C@rGO, ZSO@rGO, and ZSO/C@rGO, respectively, after annealing (Table 2). The annealed sample served as an electrode during the fabrication of an LIB coin cell, as shown in Figure 1(c). Figure 1(c) shows the cross-section of the internal components of the coin cell and the fast electron transport pathway owing to the ZSO/C@rGO layer structures, which have an excellent electrical conductivity.Table 1
Operating conditions of supersonic cold spraying.
Experimental parameterValueCarrier gasAirGas pressure4 bar (0.2 MPa)Gas temperature250°CNozzle-to-substrate distance13 cmScanning speed1 cm·s-1Flow rate1.5 ml·min-1Table 2
Summary of cases and coating materials.
CaseBefore annealingAfter annealingCase 1SnO2/PVP+rGOSnO2/C@rGOCase 2ZnO/PVP+rGOZnO/C@rGOCase 3ZSO+rGOZSO@rGOCase 4ZSO/PVP+rGOZSO/C@rGO


### 2.4. Materials Characterization

The crystallographic structures of ZSO/C@rGO and the other materials used in this study were analyzed using a SmartLab (Rigaku) X-ray diffraction (XRD) instrument over the 2θ range of 5–80°. A Raman spectrometer (NRS-3100, Jasco), featuring a high-performance 532 nm laser and a holographic grating with 1800 grooves·nm-1, was used to analyze the graphitic and disordered C contents of the samples. The structural morphologies of all the samples were observed using field-emission scanning electron microscopy (FE-SEM, Quanta 250 FEG, FEI Inc.) and transmission electron microscopy (TEM, JEM 2100F, JEOL Inc.). The TEM sample was prepared by drop-casting the ZSO/C@rGO solution (0.01 g of ZSO/C@rGO dispersed in 10 mL of ethanol) onto the Cu grid (HC200-Cu, Electron Microscopy Sci.). Energy-dispersive X-ray spectrometers configured with the SEM and TEM devices were used for the elemental mapping analysis. The presence of elements and their chemical states were confirmed using X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Co.) theta probe system.


### 2.5. Electrochemical Characterization

The films formed through the deposition of various materials onto Cu foils were punched into 1.54 cm2 circular sheets with an areal density of 0.3 mg·cm-2. Circular sheets coated with various electrode materials were used as working electrodes with a diameter of 16 mm, and Li foils with the same diameter as the counter electrodes in CR2032-type coin cells were used. The working and counter electrodes were separated from each other by using Celgard 2400 polymer membranes (Celgard, Chungbuk) as separators. A solution of 1 M LiPF6 dissolved in ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate (1 : 1 : 1 by volume) (PuriEL, Soulbrain) was used as the electrolyte in the coin cells. Galvanostatic charge–discharge (GCD) tests of the coin cells were performed in the voltage range of 0.01–3 V at 25°C using a WBCS3000 battery cycler system for longer cycles at 1000 mA·g-1. The half-cells were tested at the current rates of 100, 200, 500, and 1000 mA·g-1. Electrochemical impedance spectroscopy (EIS, VersaSTAT 3, Princeton Applied Research, USA) was performed over the frequency range of 100 kHz to 0.01 Hz with a perturbation voltage of 10 mV to analyze the resistance components and charge-transfer kinetics.


## 2.1. Materials

Zinc acetate dihydrate (Zn(CH3COO)2·2H2O, 219.51 g·mol-1), tin chloride (SnCl4·5H2O, 350.60 g·mol-1), sodium hydroxide (NaOH, 40.00 g·mol-1), and PVP (Mw=40kDa) were purchased from Sigma-Aldrich. They were used for the synthesis of Zn2SnO4/PVP (hereafter denoted as ZSO/PVP). rGO and polyacrylonitrile (PAN, Mw=150kDa) were purchased from Angstrom Materials and Sigma-Aldrich, respectively, and added to the precursor solution. Deionized (DI) water and N,N-dimethylformamide (HCON(CH3)2, 73.09 g·mol-1) were used as the solvents. In addition, all the parts (i.e., cell case, wave spring, gasket, and separator) necessary to manufacture the coin cell were purchased from Wellcos Corporation, Republic of Korea, and 1 M LiPF6 (in Ec/DMC/DEC=1:1:1 (v/v/v)) was used as the electrolyte solution.


## 2.2. Nanoparticle Synthesis

A brief illustration demonstrating the synthesis of the ZSO/PVP powder is shown in Figure1(a). In detail, zinc acetate (0.459 g), tin chloride (0.281 g), sodium hydroxide (0.18 g), and PVP (0.7 g) required for synthesizing the ZSO(Zn2SnO4)/PVP powder were added to 70 mL of DI water, and then, a magnetic stirrer was used to effectively dissolve the mixture for 30 min. The stirred solution was poured into a Teflon-lined autoclave to facilitate the hydrothermal reaction in an oven at 180°C for 12 h. Then, 35 mL of the hydrothermally treated solution was poured into conical tubes (Falcon conical centrifuge tube, 50 mL), and DI water was added to fill the remaining volume of 15 mL. A centrifuge (ROTOFIX 32A, Hettich Corp., Germany) with conical tubes was operated at 4000 rpm for 7 min. After one centrifugation cycle, the upper DI water mixed with impurities was removed. After refilling with clean DI water, the centrifugation was repeated three times. Subsequently, the moist ZSO/PVP powder was collected, and the remaining moisture was removed by drying in an oven at 90°C for 10 h. ZnO/PVP nanoparticles were synthesized in similar way but without the tin chloride salt, while ZSO(Zn2SnO4) particles were prepared by excluding PVP in the hydrothermal solution. Other conditions of the hydrothermal process, including the solutions and washing, were similar to that of the ZSO(Zn2SnO4)/PVP powder synthesis. Because the yield of synthesized SnO2 was negligible, the purchased SnO2 was deposited by adding PVP to the deposition precursor solution and stirring it overnight. Thereafter, SnO2/PVP particles were centrifuged only once and dried using the oven at 90°C for 10 h.Figure 1
(a) Synthesis process of ZSO/PVP powder, (b) deposition of ZSO/PVP+rGO on Cu foil through cold spraying, and (c) layout showing the components of the LIB and electron transport in the annealed ZSO/C@rGO layer.


## 2.3. Supersonic Spray Coating

First, 0.5 g of the ZSO/PVP obtained through the hydrothermal treatment (Figure1(a)) and rGO (0.1 g) were added to dimethylformamide (50 mL). Then, 0.3 mL of 8 wt% PAN was added for particle dispersion. The mixed solution was vigorously stirred for 30 min, followed by tip sonication for 2 min to obtain a well-dispersed mixture. A supersonic cold-spraying (CS) system consisting of a supersonic nozzle, x-y stage, atomizer, compressor, and inline heater was employed to deposit the well-dispersed mixture onto Cu foils. The syringe-pump module was filled with the well-dispersed mixture and supplied the mixed solution to the atomizer at a constant flow rate (1.5 mL·min-1). The CS coating was conducted after setting the additional operating conditions (i.e., nozzle-to-substrate distance, scanning speed, carrier gas, gas pressure, and temperature). The fluid sprayed by the atomizer was deposited on a Cu foil using CS at 4 bar and 250°C, as shown in Figure 1(b). The operating conditions used in this study are listed in Table 1. Table 2 summarizes the materials used in the case studies (Cases 1–4) before and after annealing. These samples (SnO2/PVP+rGO, ZnO/PVP+rGO, ZSO+rGO, and ZSO/PVP+rGO) were annealed in an Ar environment. The annealing temperature was increased to 700°C at a rate of 3°C·min-1. The temperature was maintained at 700°C for 30 min. After annealing in an Ar environment, PVP was converted to carbon. Accordingly, the samples before annealing (i.e., SnO2/PVP+rGO, ZnO/PVP+rGO, ZSO+rGO, and ZSO/PVP+rGO) were transformed into SnO2/C@rGO, ZnO/C@rGO, ZSO@rGO, and ZSO/C@rGO, respectively, after annealing (Table 2). The annealed sample served as an electrode during the fabrication of an LIB coin cell, as shown in Figure 1(c). Figure 1(c) shows the cross-section of the internal components of the coin cell and the fast electron transport pathway owing to the ZSO/C@rGO layer structures, which have an excellent electrical conductivity.Table 1
Operating conditions of supersonic cold spraying.
Experimental parameterValueCarrier gasAirGas pressure4 bar (0.2 MPa)Gas temperature250°CNozzle-to-substrate distance13 cmScanning speed1 cm·s-1Flow rate1.5 ml·min-1Table 2
Summary of cases and coating materials.
CaseBefore annealingAfter annealingCase 1SnO2/PVP+rGOSnO2/C@rGOCase 2ZnO/PVP+rGOZnO/C@rGOCase 3ZSO+rGOZSO@rGOCase 4ZSO/PVP+rGOZSO/C@rGO


## 2.4. Materials Characterization

The crystallographic structures of ZSO/C@rGO and the other materials used in this study were analyzed using a SmartLab (Rigaku) X-ray diffraction (XRD) instrument over the 2θ range of 5–80°. A Raman spectrometer (NRS-3100, Jasco), featuring a high-performance 532 nm laser and a holographic grating with 1800 grooves·nm-1, was used to analyze the graphitic and disordered C contents of the samples. The structural morphologies of all the samples were observed using field-emission scanning electron microscopy (FE-SEM, Quanta 250 FEG, FEI Inc.) and transmission electron microscopy (TEM, JEM 2100F, JEOL Inc.). The TEM sample was prepared by drop-casting the ZSO/C@rGO solution (0.01 g of ZSO/C@rGO dispersed in 10 mL of ethanol) onto the Cu grid (HC200-Cu, Electron Microscopy Sci.). Energy-dispersive X-ray spectrometers configured with the SEM and TEM devices were used for the elemental mapping analysis. The presence of elements and their chemical states were confirmed using X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Co.) theta probe system.


## 2.5. Electrochemical Characterization

The films formed through the deposition of various materials onto Cu foils were punched into 1.54 cm2 circular sheets with an areal density of 0.3 mg·cm-2. Circular sheets coated with various electrode materials were used as working electrodes with a diameter of 16 mm, and Li foils with the same diameter as the counter electrodes in CR2032-type coin cells were used. The working and counter electrodes were separated from each other by using Celgard 2400 polymer membranes (Celgard, Chungbuk) as separators. A solution of 1 M LiPF6 dissolved in ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate (1 : 1 : 1 by volume) (PuriEL, Soulbrain) was used as the electrolyte in the coin cells. Galvanostatic charge–discharge (GCD) tests of the coin cells were performed in the voltage range of 0.01–3 V at 25°C using a WBCS3000 battery cycler system for longer cycles at 1000 mA·g-1. The half-cells were tested at the current rates of 100, 200, 500, and 1000 mA·g-1. Electrochemical impedance spectroscopy (EIS, VersaSTAT 3, Princeton Applied Research, USA) was performed over the frequency range of 100 kHz to 0.01 Hz with a perturbation voltage of 10 mV to analyze the resistance components and charge-transfer kinetics.


## 3. Results and Discussion

### 3.1. Materials Analysis

Figure2 shows the SEM images of all the cases at different magnifications. All the samples contained rGO, which was well intermingled with oxide particles and formed a 3D network of composites with excellent electrical conductivity and shorter Li-ion diffusion pathways. Figure 2(a) indicates that the SnO2 particles are not agglomerated, and the low-magnification image shows excellent exfoliation of the rGO nanosheets, since the SnO2 particles underneath are visible. The sample SEM images for Case 2 (ZnO/C@rGO) shown in Figures 2(d)–2(f) suggest that the particles synthesized using PVP alleviated particle agglomeration owing to the elimination of Van der Waals forces during supersonic CS. [39, 40] However, the sample for Case 3 (ZSO@rGO) shows significant agglomeration of particles (Figures 2(g)–2(i)) owing to the absence of PVP during the particle synthesis. Agglomeration is not good for electrons or Li+ ions transfer, as the pathway for Li+ ions increases while the electrons experience greater resistance. Thus, to eliminate agglomeration, the sample for Case 4 (ZSO/C@rGO), which included PVP during the particle synthesis, was characterized by observing its morphology. The SEM images indicate that PVP was well coated on the particles, and hence, the agglomeration of Zn2SnO4 was controlled (Figures 2(j)–2(l)). Notably, PVP was converted to carbon after carbonization. [41, 42] The average size of 50 particles for Case 1, Case 2, Case 3, and Case 4 was 590, 95, 87, and 71 nm, respectively. Their histograms are compared in Figure 2. While comparing the particles sizes of Case 3 and 4, it was found that the PVP inclusion in the precursor solution decreased the average particle size.Figure 2
SEM images sorted from low (left) to high (right) magnification and particle size histogram of Case 1 (a–d), Case 2 (e–h), Case 3 (i–l), and Case 4 (m–p).
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)(m)(n)(o)(p)XRD analysis was also performed to analyze the crystal structures of the Zn- and Sn-based composite materials (Figure3(a)). All the samples used for the XRD analysis showed a main Cu peak with a strong intensity for all the XRD peaks owing to the working electrode, which consisted of a Cu foil as the substrate. Peaks confirming Cu were observed at 2θ=43.3° and 50.5°, corresponding to (111) and (200), respectively. [43, 44] For Case 1, SnO2 particles purchased from Sigma-Aldrich were mixed with 0.7 g of PVP. Accordingly, Case 1 shows the SnO2 peak with relatively weak intensity at 2θ=26.6°, which corresponds to the (110) plane. The SnO peaks were observed at 2θ=30.6, 32.1, and 45°, which correspond to the (020), (004), and (024) planes, respectively (JCPDS card 04-0673). [45–47] It should be noted that SnO in Case 1 was formed due to the annealing at 700°C in Ar.Figure 3
(a) XRD patterns. Raman spectra corresponding to (b) oxide material and (c) carbon.
(a)(b)(c)Case 2 showed strong peaks of ZnO at2θ=31.6, 34.3, 36.3, 47.5, 56.6, 62.9, and 67.9°. These peaks corresponded to the (100), (002), (101), (102), (110), (103), and (112) planes of the ZnO phase structure, respectively (JCPDS card 36-1451). [48, 49] Samples for Cases 3 and 4 prepared using the ZSO powder produced by mixing ZnO and SnO2 and performing hydrothermal treatment showed ZnO, SnO, and ZSO peaks. In particular, the ZSO peaks were stronger in Case 3, in which PVP was not added, at 2θ=29.1, 34.2, 36.1, 41.7, 55.1, and 63.1°, corresponding to (220), (311), (222), (400), (511), and (531), respectively. [31, 50] Case 4 also showed relatively weak ZSO peaks at 2θ=34.5, 36.1, 63.1, and 68.1° because of the ZSO enclosed by the PVP composite structure (particle–PVP shell morphology), which caused weaker reactivity than in Case 3 (the pure ZSO sample). However, it should be noted that the Zn2SnO4 peaks were dominant in Case 3 and 4 with traces of ZnO and SnO. The crystallite size of all phases for each case was determined using the Debye Scherrer formula. The crystallite size was 46.4, 33.1, 48.0, and 45.0 nm for Case 1, Case 2, Case 3, and Case 4, respectively.Raman spectroscopy was employed to differentiate the type of materials used in each sample: defective (D) and graphitic (G) carbon derived from the organic framework and rGO sheet (Figures3(b) and 3(c)). Some of the detected peaks were useful for describing the possible vibration modes in the material as a reflection of its purity and phase. [51] As shown in Figure 3(b), in Case 1, which contained SnO2, peaks were observed at 378, 419, 480, 567, 629, and 678 cm-1 owing to the effect of SnO2. [51] In Case 2, which contained ZnO particles, ZnO peaks were observed at 328, 432, 669, and 701 cm-1. [11] In Cases 3 and 4, which contained both materials, two types of peaks, ZnO and SnO2, were observed with both characteristics. In particular, the ZnO (328, 432, 669, and 701 cm-1) and SnO2 (384 and 475 cm-1) peaks identified in Case 4 indicate information about unique characteristics. The E2H mode at 328 cm-1 contributes to the vibration mode of hexagonal ZnO. [52] In addition, the significant E2 high mode associated with the vibrations of O2 atoms that appear around 432 cm-1 and the low mode associated with the vibration of the Zn sublattice (199 cm-1) are absent in Figure 3(b). Instead, a peak was observed near 700 cm-1 (699, 701 cm-1), indicating that the samples were polycrystalline. [53, 54] The carbon network formed by the presence of rGO and partially by the carbonization of PVP formed D and G bands around 1344 and 1583 cm-1, respectively, and the intensity ratio (ID/IG) was determined based on the material constituting the sample. The relative ID/IG values were 1.35, 1.38, 1.37, and 1.30, respectively, for Cases 1–4 (Figure 3(c)). The increase in carbon defects and disorder indicated an increase in the intensity ratio (ID/IG). The intensity ratio for all the cases was above 1.30, because a defect occurred owing to the linked nanoparticle–rGO sheet structure (Figure 2) [55]. Defects caused by the linked structure increase the intercalation sites for Li-ion storage, allow the diffusion of Li+ ions, and improve the electrochemical performance [18]. In particular, Case 3 had densely distributed ZSO particles (cf. Figures 2(g)–2(i)), and the ID/IGratio was 1.37; therefore, a stable electrochemical reaction could be expected during the charge–discharge cycle. In particular, Case 4 showed a large intensity ratio (1.30) and well-dispersed ZSO/C particles (cf. Figures 2(j)–2(l)) over the rGO sheets, indicating that the carbon coating of ZSO particles is beneficial for reducing the defects of the rGO sheets.The chemical states of the elements comprising ZSO@C/rGO were identified using XPS analysis. The survey spectrum of ZSO/PVP@rGO in FigureS1 confirms the presence of Zn, Sn, C, and O in ZSO/PVP@rGO. The core-level XPS spectra of Zn, Sn, C, and O are shown in Figure 4. The deconvoluted Zn 2p spectrum contained a doublet (Zn 2p3/2 and Zn 2p1/2) with a slight shift in the peak positions owing to the formation of Zn–O–Sn bonds in the bimetallic oxide by the metal ions. Thus, the electronic interactions of the two different atoms resulted in chemical shifts in the binding energies. The binding energy difference of approximately 23.5 eV between the Zn 2p3/2 (1021.8 eV) and Zn 2p1/2 (1045.3 eV) core levels with spin–orbit splitting indicated the presence of Zn in the Zn2+ oxidation state (Figure 4(a)) [50]. Figure 4(b) shows typical Sn 3d XPS spectra of the ZSO composites. Two sharp characteristic peaks centered at 485.8 and 494.4 eV, corresponding to Sn 3d5/2 and Sn 3d3/2, respectively, showing the spin–orbit splitting of 8.6 eV. These two peaks indicate the presence of Sn4+ in Case 4 [56]. A broad peak appeared at approximately 497 eV and is attributed to the energy loss caused by the interaction between photoelectrons and conduction electrons on the surface. Similar features are reported by Du et al. [56] for Sn 3d in Zn2SnO4 and also by Moulder et al. [57]. The deconvoluted C 1s core spectrum (Figure 4(c)) exhibited three peaks at 284.2, 285.6, and 289.3 eV, which are attribute to carbon atoms, that is, delocalized sp2-hybridized C–C/C=C, C–O, and O–C=O groups, respectively [58]. The intensity of the C–O peak was lower than that of the C–C peak, indicating that oxygen functional groups were removed during the annealing process [13]. The O 1s core spectrum of ZSO@C/rGO was fitted to three Gaussian peaks, denoted as OL, Ov, and Oc, as shown in Figure 4(d). The OL peak related to lattice oxygen with a lower binding energy (531 eV) in the O 1s spectrum corresponds to O2− ions bonded to Zn2+/Sn2+ ions. The oxygen-deficient regions of the metal-oxide lattice caused an increase in the intensity of the Ov peak at 531.9 eV (standard 531 eV). The adsorbed oxygen on the surface, in compounds such as CO2 and H2O, is assigned to chemisorbed oxygen (Oc) at a higher binding energy (533 eV) [50, 56].Figure 4
XPS high-resolution spectra of deconvoluted (a) Zn 2p, (b) Sn 3d, (c) C 1s, and (d) O 1s region for the ZSO@C/rGO sample.
(a)(b)(c)(d)The annealed Zn2SnO4 with PVP produced Zn2SnO4@C, which was essentially the Case 4 sample. Figure 5(a) shows the TEM image of the Zn2SnO4-C particles on a Cu grid. Figure 5(b) shows the HRTEM images of the blue dotted region in Figure 5(a). The lattice fringes indicated by numbers were further processed using Gatan software to obtain fast Fourier transforms (FFT) and inverse FFT selective areas from the FFT images. The processed micrographs showed the crystallinity of ZSO (Zn2SnO4) and SnO (Figure 5(c)). Figure 5(c) shows three lattice fringes of ZSO and one lattice fringe of SnO, with 0.250, 0.263, 0.131, and 0.26 nm corresponding to the (222), (311), (622), and (110) planes, respectively [31, 50, 59]. In addition, the SAED pattern image in Figure 5(d) shows the ZSO crystal planes (620), (311), (111), and (400), corresponding to JCPDS card No. 24–1470 [50, 60]. The elemental mapping images in Figures 5(e)–5(h) show the presence of Zn, Sn, C, and O in the sample. Figure 5(i) shows an overlapped image of C and O (Zn and Sn are avoided for better clarity), which confirms that the carbonization of PVP forms a thin coating around the oxide particles.Figure 5
(a) TEM image, (b) high-resolution TEM (HRTEM) images of the blue area in Figure5(a), and (c) processed HRTEM images using Gatan software showing lattice fringes for Zn2SnO4 and SnO. (d) Selected-area electron diffraction (SAED) pattern of ZSO@C/rGO. (e–h) The elemental mapping images of Zn, Sn, C, and O. (i) Overlap image of (g) and (h).
(a)(b)(c)(d)(e)(f)(g)(h)(i)


### 3.2. Electrochemical Performance

Figure6(a) shows the GCD profiles of Case 3 (ZSO@rGO) at a current rate of 1000 mA·g-1 in the voltage range of 0.01–3 V (vs. Li/Li+). The first two and 400th charge–discharge profiles are presented in Figure 6(a), which indicate that the Coulombic efficiency is almost 100% from the second cycle itself. The high Coulombic efficiency for the 2nd GCD cycle suggests the complete formation of a solid electrolyte interface (SEI) layer during the 1st cycle. However, the capacity decreased from 1145 to 699 mAh·g-1, which is much lower than the theoretical capacities of ZnO and SnO2, indicating the significant pulverization of the particles owing to lithiation. Nevertheless, the presence of rGO ensured that the ZSO@rGO particles maintained a capacity retention of 649 mAh·g-1, i.e., 0.13 mAh·g-1 per cycle from N=2 to N=400. The differential capacity plot is presented in Figure 6(b) for the first two discharge cycles to analyze the lithiation depth. The dominant peak for N=1 at 0.51 V corresponds to the formation of the SEI layer, which subsequently vanishes at N= 2, suggesting that the SEI layer was formed during the first GCD cycle itself. The minor peaks at ~1.07 and 0.8 V indicate the intercalation of the Li+ ions into the Zn2SnO4 particles, whereas the shoulder peak at 0.46 V suggests the conversion of Sn and Zn to metallic states.Figure 6
GCD profiles (N=1, 2, and 400) and differential capacity curves (N=1 and 2) with an illustration of the effect of PVP during the charge–discharge cycle: (a, b) Case 3 and (c, d) Case 4.
(a)(b)(c)(d)Furthermore, a sharp peak at 0.18 V indicates the alloying of metallic Sn and Zn during the first discharge cycle. Owing to pulverization, the peaks at 0.46 and ~ 0.18 V become less intense during the second discharge cycle. In contrast, the peaks around 1.1 and 0.8 V appear to become merged and broadened. The peak associated with intercalation broadening could be due to the presence of ZnO and SnO2 in the pulverized form.Figure6(c) shows the GCD profiles, and Figure 6(d) shows the differential capacity plot for Case 4 (ZSO/C@rGO). Notably, the discharge capacity for the 2nd cycle is almost three times that of graphite. For the subsequent 399 GCD cycles, the capacity per cycle change is insignificant (0.17 mAh·g-1), which could be attributed to the continuous pulverization of ZnO and SnO2 particles and the formation of a reversible polymer gel layer. Thus, ZSO/C@rGO exhibited a superior electrochemical performance to that of Zn2SnO4/rGO, owing to the carbon coating around Zn2SnO4. The differential capacity plots shown in Figure 6(d) for Case 4 (ZSO/C@rGO) display two peaks at 0.89 and 0.66 V during the first discharge cycle. The peak at 0.89 V corresponds to the intercalation of Li+ ions, and the peak at 0.66 V corresponds to the decomposition of Zn2SnO4 to Li2O and metallic Zn and Sn. The peak associated with the formation of the SEI layer may have merged with the low-intensity, broad peak around 0.85 V. Interestingly, both peaks appear to vanish during the second discharge cycle; however, a minor peak is observed at 0.14 V. The smooth differential capacity curves during the second discharge cycle can be attributed to the carbon coating and the formation of the SEI layer.The schematics for Case 3 (ZSO@rGO) and Case 4 (ZSO/C@rGO) are depicted in Figure6 to illustrate the impact of the carbon coating on the bimetallic oxides. For Case 3, the schematics suggest that, after the first GCD cycle, the SEI layer was formed prior to the decomposition of Zn2SnO4 into a couple of ZnO and a single SnO2 molecule. As the number of cycles increased, the SEI layer formed over the individual Zn and Sn oxide particles. However, owing to the continuous expansion and contraction of the ZnO and SnO2 particles, the strain experienced by the SEI layer was extreme. Because of the excessive strain, the SEI layers broke, and subsequently, the reformation of the SEI layer caused electrolyte decomposition and the agglomeration of the pulverized metal-oxide particles.In contrast, in Case 4 (ZSO/C@rGO), the SEI layer was formed over the carbon-coated layer. After the first cycle of discharging and charging, Zn2SnO4 decomposed into Zn and Sn oxides. However, owing to the carbon coating, these individual oxides remained isolated from other particles, thus alleviating their agglomeration. In addition, the voids created within the carbon shell after cycling and the presence of the carbon shell buffered the volume expansion of ZnO and SnO2 during the lithiation and alloying processes. Consequently, the SEI layers experienced either no strain or very small strain during lithiation and remained intact even after hundreds of cycles. Consequently, the electrochemical stability of Case 4 (ZSO/C@rGO) improved.The ZSO/C@rGO anode showed a capacity retention of 106% after 400 cycles compared with the capacity forN=2, which indicates a 13% increase compared with that of the ZSO@rGO anode. The improved capacity retention can be attributed to the carbon coating around Zn2SnO4.
(1)8Li++Zn2SnO4+8e−⟶2Zn+Sn+4Li2O,(2)SnO2+2Li++2e−⟷Sn+Li2O,(3)Sn+2Li++2e−⟷SnO+Li2O,(4)2ZnO+4Li++4e−⟷2Zn+2Li2O,(5)yLi++Zn+ye−⟷LiyZn,0≤y≤2,(6)xLi++Sn+xe‐⟷LixSn,0≤x≤4.4.Equations (2)–(6) indicate the two possible mechanisms, which involve a maximum storage of 14.4 Li+ ions: 8 Li+ ions from the conversion mechanism and 6.4 Li+ ions owing to the alloying process. In addition, the bimetallic oxides and connected rGO improved the storage of Li+ ions, increasing the specific capacity [55, 61].The capacities of the electrodes were significantly influenced by the applied current rate during charging and discharging. Therefore, the electrochemical performance of the electrode materials needs to be observed under different applied current densities (100, 200, 500, and 1000 mA·g-1). The current density increased from 100 to 1000 mA·g-1 and then decreased to similar current densities. Ten GCD cycles were carried out for each current density. As expected for each case, the first discharge capacity was higher owing to the formation of the SEI layer. The capacity was observed to be the same for the remaining cycles at each current density, as shown in Figure 7(a). The first reversible capacities (N=2) of Cases 1, 2, 3, and 4 are 239, 1085, 956, and 1287 mAh·g-1, respectively, at a current density of 100 mA·g-1. The discharge capacity of Case 1 drastically decreased owing to the enormous volume expansion (200 to 360%) of SnO2 and SnO during lithiation. [13] The results of increasing the current density (Figure 7(a)) show that the bimetallic (Zn2SnO4) composite samples (Cases 3 and 4) exhibit higher electrochemical stability compared with the single-metal oxide/carbon composites (Cases 1 and 2, SnO2, and ZnO particles) [31]. Comparing Cases 3 and 4, it was observed that Case 4 exhibited higher capacity values and better electrochemical stability owing to the carbon coating. The presence of the carbon coating ensures that the SEI layer is not in direct contact with the metal oxide; thus, Li-ion diffusion directly occurs through the electrolyte. [62] The current rate capability of the discharge capacity at different current densities for all the cases is shown in Figure 7(b), based on Figure 7(a). The average capacity values of 10 cycles after N=10, 20, 30, and 40 cycles with increasing current density are shown as solid lines/solid symbols for the current densities of 100, 200, 500, and 1000 mA·g-1. After 40 cycles, the capacity was measured by decreasing the current density (dotted lines/hollow symbols), as shown in Figure 7(b). The current rate increased from 100 to 1000 mA·g-1, corresponding to the capacities of 1274 and 795 mAh·g-1, which indicated a capability retention of 62% (Figure 7(b)). In Cases 1 and 2, the capacity retention could be attributed to the presence of the carbon coating and rGO and partly to the reformation of the SEI layer. In contrast, the capacity retention of Case 3 can be attributed to the presence of the two metals, which supports the control of pulverization. Nevertheless, the contribution of rGO to the high-capacity value and capacity retention for Case 3 cannot be ignored [18, 63]. Case 4 showed high capacity values and excellent capacity retention of 97% (10th and 70th cycle capacity) compared with the other cases because of the presence of the C layer surrounding the ZSO particle surface and the optimal amount of rGO sheets (Figures 2(j)–2(l)).Figure 7
Electrochemical performance of all samples: (a) rate capability forN=70with 10 cycle intervals, (b) rate performance in terms of average discharge capacity at different current densities, and (c) long-term cycling at a current density of 1000 mA·g-1; here, CE is the Coulombic efficiency, and (d) capacity retentions calculated using 1st/last capacity (patterned) and 2nd/last capacity (solid).
(a)(b)(c)(d)The long-term cycling performance and Coulombic efficiency of all the cases at a current density of 1000 mA·g-1 are shown in Figure 7(c). Cases 1 (SnO2/C@rGO) and 2 (ZnO/C@rGO), which involve a single material, showed a low capacity throughout the cycling test. However, Cases 3 and 4, which involve bimetallic materials, showed relatively high average capacity values. In particular, as evident from the results of Case 4, the effects of the carbon-coated bimetallic nanoparticles and the presence of the rGO sheets resulted in a highly stable electrochemical performance for 400 cycles. Cases 1 and 2 exhibited significantly lower performances (capacities of 123 and 285 mAh·g-1, respectively) than the initial capacity after 400 cycles, which could be due to significant pulverization during the alloying and de-alloying of Zn and Sn.It is noteworthy that the long-term cycling shows a sharp fall in capacity for single metal-based composite electrodes (i.e., Case 1 and Case 2). However, for Case 3 and Case 4, the capacity does not deteriorate significantly. The capacity deterioration for the second cycle could be attributed to the completion of the SEI layer. FigureS2 shows that all samples were covered by the SEI layer completely (also see Figure 2). Furthermore, the SEI layer appears to get thicker after 100 charging-and-discharging cycles for Case 1, Case 2, and Case 3. Severe agglomeration is observed in Case 1 and Case 2 unlike Case 3. The SnO2, ZnO, and Zn2SnO4 (corresponding to Case 1, Case 2, and Case 3) particles were pulverized significantly within 100 cycles, resulting in severe agglomeration. In contrast, in Case 4, the decomposition of Zn2SnO4 into a couple of ZnO and a single SnO2 molecule within the carbon shell results in little agglomeration. The core-shell structure alleviates the impact of volume expansion, forming a relatively thin SEI layer (see Figure S2 and Figure 2). The formation of ZnO, SnO, and SnO2 owing to the decomposition of Zn2SnO4 after the first discharging-and-charging cycle is confirmed by the XRD data in Figure S3. The pulverization of ZnO and SnO follows, which increases the anode capacity by the increased surface area of pulverized ZnO and SnO particles, as shown in Figure 7(c). Simultaneously, the pulverized ZnO and SnO2 maintained a large interfacial area between the electrode and the electrolyte within the carbon shell in the presence of rGO [64, 65]. After 400 cycles, Case 1 and Case 2 show significant agglomeration owing to the intensified pulverization and the repetitive SEI layer formation around the agglomerated particles (Figure S2). The particle agglomeration was magnified after 400 cycles in comparison with that at 100 cycles for Case 3. Figure S2 shows that relatively little agglomeration was observed for Case 4 after 400 cycles because the carbon shell on the bimetallic particles minimized the impact of volume expansion.The quantitative analysis of the long-term cycling measurement for each case, considering the capacity retention after 400 cycles with respect to the 1st (patterned) and 2nd (solid) discharge capacities, is shown in Figure 7(d). The capacity retention values of Case 4 were observed to be 61 and 106%, which are superior to those in the other cases concerning the first and second discharge capacities, respectively. It is well-known that the SEI layer formation is inevitable in LIB. The SEI passivation layer comprises organic and inorganic complexes, such as Li2CO3, LiF, and LiOR, where R is an alkyl group [66]. The reactions involved with these complexes are irreversible, which in turn produces significant amount of irreversible capacity. It should be noted that the capacity retention with respect to the first cycle can be misleadingly high because of irreversible capacity contribution by the SEI layer formation. In contrast, the capacity retention with respect to the second cycle (or first reversible capacity) is based on the active anode materials only (i.e., ZSO/C/rGO) without the contribution by the SEI layer formation. The electrochemical performance of the bimetallic composite reported in similar studies is compared with the results of this study in Table 3. This comparison indicates that Zn2SnO4@C/rGO is a suitable anode material for LIBs.Table 3
Summary of cases and coating materials.
MaterialFirst discharge capacity (mAh·g-1)Capacity retention (%)Current density (mA·g-1)Cycle numberBinder-freeReferencesSn/ZnO/N-C149764100500N[67]ZnSnO3/rGAs160549100200N[68]ZnO/ZnSnO3 QDs1527361000100Y[69]ZnSnO3/rGO201336100100N[70]Zn2SnO4@NiO@rGO NFs11844310001600N[71]Zn2SnO4@C153060100100N[72]3D-Zn2SnO4154148100100N[23]Zn2SnO4/C/rGO1830611000400NPresentEIS is useful for examining the electrochemical properties and kinetics of anode materials. The Nyquist plots of ZSO/C@rGO, at various half-cell potentials, are presented in (Figure8(a)), and the Randles equivalent circuit is shown in Figure 8(b). Each Nyquist plot is composed of a high-frequency semicircle and Warburg tail region, followed by a steep sloping line in the low-frequency region. R1 and Cdl are the ohmic resistance between the electrolyte and the surface of the electrode and the double-layer capacitance, respectively. The high-frequency semicircle indicates the charge-transfer resistance (Rct) relevant to the interfacial Li-ion transfer. In impedance spectroscopy, the high-frequency semicircle is attributed to the charge-transfer impedance at the electrode/electrolyte interface, and the inclined line at an angle of approximately 45° to the real axis corresponds to the lithium diffusion process within the electrodes. The diameter of the semicircle in the high-frequency region of the Zn2SnO4/C@rGO electrode was smaller than that of the Zn2SnO4 electrode, revealing lower charge-transfer impedances. The low Rct value indicates that the electronic conductivity of the Zn2SnO4/C@rGO electrode improved after coating with a thin layer of carbon. Zw is the Warburg impedance, which is related to Li-ion diffusion into the structure of the active materials and corresponds to the tail at a low frequency.Figure 8
EIS analysis: (a) Nyquist plot for ZSO/C@rGO at different half-cell potentials and (b) raw data fitting of the Nyquist plot for ZSO/C@rGO at OCV (inset shows the Randles equivalent circuit). (c) Schematic depicting intercalation and deintercalaction processes of Li-ions during lithiation (left) and delithiation (right) in ZSO/C@rGO.
(a)(b)(c)Figure8(c) shows the intercalation process in rGO; initially, some Li+ ions were intercalated in the nanoparticles. Li+ ions were transferred from the separator and entered the active materials composed of Zn2SnO4 and rGO. rGO was necessary to enhance the electrical conductivity of the active material, and the adhered carbon-coated Zn2SnO4 particles ensured that the Li-ion diffusion pathway was shortened. The mesoporosity is beneficial to achieve shorter diffusion pathways for Li-ion transport. To investigate the pores and specific surface area of Case 4 (ZSO/C@rGO), N2 adsorption/desorption isotherm (Tristar 3000, Micromeritics) and Brunauer-Emmett-Teller (BET) analysis were performed. The N2 adsorption curve in Figure S4 shows the hysteresis in the relative pressure range of 0.45≤P/Po≤1.0, which corresponds to the type-IV isotherm. This hysteresis data confirms the mesoporosity of the rGO composite, whose surface area was found to be 208 m2·g–1, according to the BET data. The exfoliation of the rGO sheets and its mesoporous structure are crucial to facilitate the intercalation of Li+ ions between the rGO sheets. The carbon coating and rGO buffer the volume expansion of the particles during the lithiation process and improve the current rate capability owing to enhanced electrical conductivity, as shown in Figures 7(a)–7(b). In addition, the presence of the rGO sheets and carbon coating stabilizes the formation of the thin SEI layer and keeps it intact during current rate measurements and long-term cycling for 400 cycles.


## 3.1. Materials Analysis

Figure2 shows the SEM images of all the cases at different magnifications. All the samples contained rGO, which was well intermingled with oxide particles and formed a 3D network of composites with excellent electrical conductivity and shorter Li-ion diffusion pathways. Figure 2(a) indicates that the SnO2 particles are not agglomerated, and the low-magnification image shows excellent exfoliation of the rGO nanosheets, since the SnO2 particles underneath are visible. The sample SEM images for Case 2 (ZnO/C@rGO) shown in Figures 2(d)–2(f) suggest that the particles synthesized using PVP alleviated particle agglomeration owing to the elimination of Van der Waals forces during supersonic CS. [39, 40] However, the sample for Case 3 (ZSO@rGO) shows significant agglomeration of particles (Figures 2(g)–2(i)) owing to the absence of PVP during the particle synthesis. Agglomeration is not good for electrons or Li+ ions transfer, as the pathway for Li+ ions increases while the electrons experience greater resistance. Thus, to eliminate agglomeration, the sample for Case 4 (ZSO/C@rGO), which included PVP during the particle synthesis, was characterized by observing its morphology. The SEM images indicate that PVP was well coated on the particles, and hence, the agglomeration of Zn2SnO4 was controlled (Figures 2(j)–2(l)). Notably, PVP was converted to carbon after carbonization. [41, 42] The average size of 50 particles for Case 1, Case 2, Case 3, and Case 4 was 590, 95, 87, and 71 nm, respectively. Their histograms are compared in Figure 2. While comparing the particles sizes of Case 3 and 4, it was found that the PVP inclusion in the precursor solution decreased the average particle size.Figure 2
SEM images sorted from low (left) to high (right) magnification and particle size histogram of Case 1 (a–d), Case 2 (e–h), Case 3 (i–l), and Case 4 (m–p).
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)(m)(n)(o)(p)XRD analysis was also performed to analyze the crystal structures of the Zn- and Sn-based composite materials (Figure3(a)). All the samples used for the XRD analysis showed a main Cu peak with a strong intensity for all the XRD peaks owing to the working electrode, which consisted of a Cu foil as the substrate. Peaks confirming Cu were observed at 2θ=43.3° and 50.5°, corresponding to (111) and (200), respectively. [43, 44] For Case 1, SnO2 particles purchased from Sigma-Aldrich were mixed with 0.7 g of PVP. Accordingly, Case 1 shows the SnO2 peak with relatively weak intensity at 2θ=26.6°, which corresponds to the (110) plane. The SnO peaks were observed at 2θ=30.6, 32.1, and 45°, which correspond to the (020), (004), and (024) planes, respectively (JCPDS card 04-0673). [45–47] It should be noted that SnO in Case 1 was formed due to the annealing at 700°C in Ar.Figure 3
(a) XRD patterns. Raman spectra corresponding to (b) oxide material and (c) carbon.
(a)(b)(c)Case 2 showed strong peaks of ZnO at2θ=31.6, 34.3, 36.3, 47.5, 56.6, 62.9, and 67.9°. These peaks corresponded to the (100), (002), (101), (102), (110), (103), and (112) planes of the ZnO phase structure, respectively (JCPDS card 36-1451). [48, 49] Samples for Cases 3 and 4 prepared using the ZSO powder produced by mixing ZnO and SnO2 and performing hydrothermal treatment showed ZnO, SnO, and ZSO peaks. In particular, the ZSO peaks were stronger in Case 3, in which PVP was not added, at 2θ=29.1, 34.2, 36.1, 41.7, 55.1, and 63.1°, corresponding to (220), (311), (222), (400), (511), and (531), respectively. [31, 50] Case 4 also showed relatively weak ZSO peaks at 2θ=34.5, 36.1, 63.1, and 68.1° because of the ZSO enclosed by the PVP composite structure (particle–PVP shell morphology), which caused weaker reactivity than in Case 3 (the pure ZSO sample). However, it should be noted that the Zn2SnO4 peaks were dominant in Case 3 and 4 with traces of ZnO and SnO. The crystallite size of all phases for each case was determined using the Debye Scherrer formula. The crystallite size was 46.4, 33.1, 48.0, and 45.0 nm for Case 1, Case 2, Case 3, and Case 4, respectively.Raman spectroscopy was employed to differentiate the type of materials used in each sample: defective (D) and graphitic (G) carbon derived from the organic framework and rGO sheet (Figures3(b) and 3(c)). Some of the detected peaks were useful for describing the possible vibration modes in the material as a reflection of its purity and phase. [51] As shown in Figure 3(b), in Case 1, which contained SnO2, peaks were observed at 378, 419, 480, 567, 629, and 678 cm-1 owing to the effect of SnO2. [51] In Case 2, which contained ZnO particles, ZnO peaks were observed at 328, 432, 669, and 701 cm-1. [11] In Cases 3 and 4, which contained both materials, two types of peaks, ZnO and SnO2, were observed with both characteristics. In particular, the ZnO (328, 432, 669, and 701 cm-1) and SnO2 (384 and 475 cm-1) peaks identified in Case 4 indicate information about unique characteristics. The E2H mode at 328 cm-1 contributes to the vibration mode of hexagonal ZnO. [52] In addition, the significant E2 high mode associated with the vibrations of O2 atoms that appear around 432 cm-1 and the low mode associated with the vibration of the Zn sublattice (199 cm-1) are absent in Figure 3(b). Instead, a peak was observed near 700 cm-1 (699, 701 cm-1), indicating that the samples were polycrystalline. [53, 54] The carbon network formed by the presence of rGO and partially by the carbonization of PVP formed D and G bands around 1344 and 1583 cm-1, respectively, and the intensity ratio (ID/IG) was determined based on the material constituting the sample. The relative ID/IG values were 1.35, 1.38, 1.37, and 1.30, respectively, for Cases 1–4 (Figure 3(c)). The increase in carbon defects and disorder indicated an increase in the intensity ratio (ID/IG). The intensity ratio for all the cases was above 1.30, because a defect occurred owing to the linked nanoparticle–rGO sheet structure (Figure 2) [55]. Defects caused by the linked structure increase the intercalation sites for Li-ion storage, allow the diffusion of Li+ ions, and improve the electrochemical performance [18]. In particular, Case 3 had densely distributed ZSO particles (cf. Figures 2(g)–2(i)), and the ID/IGratio was 1.37; therefore, a stable electrochemical reaction could be expected during the charge–discharge cycle. In particular, Case 4 showed a large intensity ratio (1.30) and well-dispersed ZSO/C particles (cf. Figures 2(j)–2(l)) over the rGO sheets, indicating that the carbon coating of ZSO particles is beneficial for reducing the defects of the rGO sheets.The chemical states of the elements comprising ZSO@C/rGO were identified using XPS analysis. The survey spectrum of ZSO/PVP@rGO in FigureS1 confirms the presence of Zn, Sn, C, and O in ZSO/PVP@rGO. The core-level XPS spectra of Zn, Sn, C, and O are shown in Figure 4. The deconvoluted Zn 2p spectrum contained a doublet (Zn 2p3/2 and Zn 2p1/2) with a slight shift in the peak positions owing to the formation of Zn–O–Sn bonds in the bimetallic oxide by the metal ions. Thus, the electronic interactions of the two different atoms resulted in chemical shifts in the binding energies. The binding energy difference of approximately 23.5 eV between the Zn 2p3/2 (1021.8 eV) and Zn 2p1/2 (1045.3 eV) core levels with spin–orbit splitting indicated the presence of Zn in the Zn2+ oxidation state (Figure 4(a)) [50]. Figure 4(b) shows typical Sn 3d XPS spectra of the ZSO composites. Two sharp characteristic peaks centered at 485.8 and 494.4 eV, corresponding to Sn 3d5/2 and Sn 3d3/2, respectively, showing the spin–orbit splitting of 8.6 eV. These two peaks indicate the presence of Sn4+ in Case 4 [56]. A broad peak appeared at approximately 497 eV and is attributed to the energy loss caused by the interaction between photoelectrons and conduction electrons on the surface. Similar features are reported by Du et al. [56] for Sn 3d in Zn2SnO4 and also by Moulder et al. [57]. The deconvoluted C 1s core spectrum (Figure 4(c)) exhibited three peaks at 284.2, 285.6, and 289.3 eV, which are attribute to carbon atoms, that is, delocalized sp2-hybridized C–C/C=C, C–O, and O–C=O groups, respectively [58]. The intensity of the C–O peak was lower than that of the C–C peak, indicating that oxygen functional groups were removed during the annealing process [13]. The O 1s core spectrum of ZSO@C/rGO was fitted to three Gaussian peaks, denoted as OL, Ov, and Oc, as shown in Figure 4(d). The OL peak related to lattice oxygen with a lower binding energy (531 eV) in the O 1s spectrum corresponds to O2− ions bonded to Zn2+/Sn2+ ions. The oxygen-deficient regions of the metal-oxide lattice caused an increase in the intensity of the Ov peak at 531.9 eV (standard 531 eV). The adsorbed oxygen on the surface, in compounds such as CO2 and H2O, is assigned to chemisorbed oxygen (Oc) at a higher binding energy (533 eV) [50, 56].Figure 4
XPS high-resolution spectra of deconvoluted (a) Zn 2p, (b) Sn 3d, (c) C 1s, and (d) O 1s region for the ZSO@C/rGO sample.
(a)(b)(c)(d)The annealed Zn2SnO4 with PVP produced Zn2SnO4@C, which was essentially the Case 4 sample. Figure 5(a) shows the TEM image of the Zn2SnO4-C particles on a Cu grid. Figure 5(b) shows the HRTEM images of the blue dotted region in Figure 5(a). The lattice fringes indicated by numbers were further processed using Gatan software to obtain fast Fourier transforms (FFT) and inverse FFT selective areas from the FFT images. The processed micrographs showed the crystallinity of ZSO (Zn2SnO4) and SnO (Figure 5(c)). Figure 5(c) shows three lattice fringes of ZSO and one lattice fringe of SnO, with 0.250, 0.263, 0.131, and 0.26 nm corresponding to the (222), (311), (622), and (110) planes, respectively [31, 50, 59]. In addition, the SAED pattern image in Figure 5(d) shows the ZSO crystal planes (620), (311), (111), and (400), corresponding to JCPDS card No. 24–1470 [50, 60]. The elemental mapping images in Figures 5(e)–5(h) show the presence of Zn, Sn, C, and O in the sample. Figure 5(i) shows an overlapped image of C and O (Zn and Sn are avoided for better clarity), which confirms that the carbonization of PVP forms a thin coating around the oxide particles.Figure 5
(a) TEM image, (b) high-resolution TEM (HRTEM) images of the blue area in Figure5(a), and (c) processed HRTEM images using Gatan software showing lattice fringes for Zn2SnO4 and SnO. (d) Selected-area electron diffraction (SAED) pattern of ZSO@C/rGO. (e–h) The elemental mapping images of Zn, Sn, C, and O. (i) Overlap image of (g) and (h).
(a)(b)(c)(d)(e)(f)(g)(h)(i)


## 3.2. Electrochemical Performance

Figure6(a) shows the GCD profiles of Case 3 (ZSO@rGO) at a current rate of 1000 mA·g-1 in the voltage range of 0.01–3 V (vs. Li/Li+). The first two and 400th charge–discharge profiles are presented in Figure 6(a), which indicate that the Coulombic efficiency is almost 100% from the second cycle itself. The high Coulombic efficiency for the 2nd GCD cycle suggests the complete formation of a solid electrolyte interface (SEI) layer during the 1st cycle. However, the capacity decreased from 1145 to 699 mAh·g-1, which is much lower than the theoretical capacities of ZnO and SnO2, indicating the significant pulverization of the particles owing to lithiation. Nevertheless, the presence of rGO ensured that the ZSO@rGO particles maintained a capacity retention of 649 mAh·g-1, i.e., 0.13 mAh·g-1 per cycle from N=2 to N=400. The differential capacity plot is presented in Figure 6(b) for the first two discharge cycles to analyze the lithiation depth. The dominant peak for N=1 at 0.51 V corresponds to the formation of the SEI layer, which subsequently vanishes at N= 2, suggesting that the SEI layer was formed during the first GCD cycle itself. The minor peaks at ~1.07 and 0.8 V indicate the intercalation of the Li+ ions into the Zn2SnO4 particles, whereas the shoulder peak at 0.46 V suggests the conversion of Sn and Zn to metallic states.Figure 6
GCD profiles (N=1, 2, and 400) and differential capacity curves (N=1 and 2) with an illustration of the effect of PVP during the charge–discharge cycle: (a, b) Case 3 and (c, d) Case 4.
(a)(b)(c)(d)Furthermore, a sharp peak at 0.18 V indicates the alloying of metallic Sn and Zn during the first discharge cycle. Owing to pulverization, the peaks at 0.46 and ~ 0.18 V become less intense during the second discharge cycle. In contrast, the peaks around 1.1 and 0.8 V appear to become merged and broadened. The peak associated with intercalation broadening could be due to the presence of ZnO and SnO2 in the pulverized form.Figure6(c) shows the GCD profiles, and Figure 6(d) shows the differential capacity plot for Case 4 (ZSO/C@rGO). Notably, the discharge capacity for the 2nd cycle is almost three times that of graphite. For the subsequent 399 GCD cycles, the capacity per cycle change is insignificant (0.17 mAh·g-1), which could be attributed to the continuous pulverization of ZnO and SnO2 particles and the formation of a reversible polymer gel layer. Thus, ZSO/C@rGO exhibited a superior electrochemical performance to that of Zn2SnO4/rGO, owing to the carbon coating around Zn2SnO4. The differential capacity plots shown in Figure 6(d) for Case 4 (ZSO/C@rGO) display two peaks at 0.89 and 0.66 V during the first discharge cycle. The peak at 0.89 V corresponds to the intercalation of Li+ ions, and the peak at 0.66 V corresponds to the decomposition of Zn2SnO4 to Li2O and metallic Zn and Sn. The peak associated with the formation of the SEI layer may have merged with the low-intensity, broad peak around 0.85 V. Interestingly, both peaks appear to vanish during the second discharge cycle; however, a minor peak is observed at 0.14 V. The smooth differential capacity curves during the second discharge cycle can be attributed to the carbon coating and the formation of the SEI layer.The schematics for Case 3 (ZSO@rGO) and Case 4 (ZSO/C@rGO) are depicted in Figure6 to illustrate the impact of the carbon coating on the bimetallic oxides. For Case 3, the schematics suggest that, after the first GCD cycle, the SEI layer was formed prior to the decomposition of Zn2SnO4 into a couple of ZnO and a single SnO2 molecule. As the number of cycles increased, the SEI layer formed over the individual Zn and Sn oxide particles. However, owing to the continuous expansion and contraction of the ZnO and SnO2 particles, the strain experienced by the SEI layer was extreme. Because of the excessive strain, the SEI layers broke, and subsequently, the reformation of the SEI layer caused electrolyte decomposition and the agglomeration of the pulverized metal-oxide particles.In contrast, in Case 4 (ZSO/C@rGO), the SEI layer was formed over the carbon-coated layer. After the first cycle of discharging and charging, Zn2SnO4 decomposed into Zn and Sn oxides. However, owing to the carbon coating, these individual oxides remained isolated from other particles, thus alleviating their agglomeration. In addition, the voids created within the carbon shell after cycling and the presence of the carbon shell buffered the volume expansion of ZnO and SnO2 during the lithiation and alloying processes. Consequently, the SEI layers experienced either no strain or very small strain during lithiation and remained intact even after hundreds of cycles. Consequently, the electrochemical stability of Case 4 (ZSO/C@rGO) improved.The ZSO/C@rGO anode showed a capacity retention of 106% after 400 cycles compared with the capacity forN=2, which indicates a 13% increase compared with that of the ZSO@rGO anode. The improved capacity retention can be attributed to the carbon coating around Zn2SnO4.
(1)8Li++Zn2SnO4+8e−⟶2Zn+Sn+4Li2O,(2)SnO2+2Li++2e−⟷Sn+Li2O,(3)Sn+2Li++2e−⟷SnO+Li2O,(4)2ZnO+4Li++4e−⟷2Zn+2Li2O,(5)yLi++Zn+ye−⟷LiyZn,0≤y≤2,(6)xLi++Sn+xe‐⟷LixSn,0≤x≤4.4.Equations (2)–(6) indicate the two possible mechanisms, which involve a maximum storage of 14.4 Li+ ions: 8 Li+ ions from the conversion mechanism and 6.4 Li+ ions owing to the alloying process. In addition, the bimetallic oxides and connected rGO improved the storage of Li+ ions, increasing the specific capacity [55, 61].The capacities of the electrodes were significantly influenced by the applied current rate during charging and discharging. Therefore, the electrochemical performance of the electrode materials needs to be observed under different applied current densities (100, 200, 500, and 1000 mA·g-1). The current density increased from 100 to 1000 mA·g-1 and then decreased to similar current densities. Ten GCD cycles were carried out for each current density. As expected for each case, the first discharge capacity was higher owing to the formation of the SEI layer. The capacity was observed to be the same for the remaining cycles at each current density, as shown in Figure 7(a). The first reversible capacities (N=2) of Cases 1, 2, 3, and 4 are 239, 1085, 956, and 1287 mAh·g-1, respectively, at a current density of 100 mA·g-1. The discharge capacity of Case 1 drastically decreased owing to the enormous volume expansion (200 to 360%) of SnO2 and SnO during lithiation. [13] The results of increasing the current density (Figure 7(a)) show that the bimetallic (Zn2SnO4) composite samples (Cases 3 and 4) exhibit higher electrochemical stability compared with the single-metal oxide/carbon composites (Cases 1 and 2, SnO2, and ZnO particles) [31]. Comparing Cases 3 and 4, it was observed that Case 4 exhibited higher capacity values and better electrochemical stability owing to the carbon coating. The presence of the carbon coating ensures that the SEI layer is not in direct contact with the metal oxide; thus, Li-ion diffusion directly occurs through the electrolyte. [62] The current rate capability of the discharge capacity at different current densities for all the cases is shown in Figure 7(b), based on Figure 7(a). The average capacity values of 10 cycles after N=10, 20, 30, and 40 cycles with increasing current density are shown as solid lines/solid symbols for the current densities of 100, 200, 500, and 1000 mA·g-1. After 40 cycles, the capacity was measured by decreasing the current density (dotted lines/hollow symbols), as shown in Figure 7(b). The current rate increased from 100 to 1000 mA·g-1, corresponding to the capacities of 1274 and 795 mAh·g-1, which indicated a capability retention of 62% (Figure 7(b)). In Cases 1 and 2, the capacity retention could be attributed to the presence of the carbon coating and rGO and partly to the reformation of the SEI layer. In contrast, the capacity retention of Case 3 can be attributed to the presence of the two metals, which supports the control of pulverization. Nevertheless, the contribution of rGO to the high-capacity value and capacity retention for Case 3 cannot be ignored [18, 63]. Case 4 showed high capacity values and excellent capacity retention of 97% (10th and 70th cycle capacity) compared with the other cases because of the presence of the C layer surrounding the ZSO particle surface and the optimal amount of rGO sheets (Figures 2(j)–2(l)).Figure 7
Electrochemical performance of all samples: (a) rate capability forN=70with 10 cycle intervals, (b) rate performance in terms of average discharge capacity at different current densities, and (c) long-term cycling at a current density of 1000 mA·g-1; here, CE is the Coulombic efficiency, and (d) capacity retentions calculated using 1st/last capacity (patterned) and 2nd/last capacity (solid).
(a)(b)(c)(d)The long-term cycling performance and Coulombic efficiency of all the cases at a current density of 1000 mA·g-1 are shown in Figure 7(c). Cases 1 (SnO2/C@rGO) and 2 (ZnO/C@rGO), which involve a single material, showed a low capacity throughout the cycling test. However, Cases 3 and 4, which involve bimetallic materials, showed relatively high average capacity values. In particular, as evident from the results of Case 4, the effects of the carbon-coated bimetallic nanoparticles and the presence of the rGO sheets resulted in a highly stable electrochemical performance for 400 cycles. Cases 1 and 2 exhibited significantly lower performances (capacities of 123 and 285 mAh·g-1, respectively) than the initial capacity after 400 cycles, which could be due to significant pulverization during the alloying and de-alloying of Zn and Sn.It is noteworthy that the long-term cycling shows a sharp fall in capacity for single metal-based composite electrodes (i.e., Case 1 and Case 2). However, for Case 3 and Case 4, the capacity does not deteriorate significantly. The capacity deterioration for the second cycle could be attributed to the completion of the SEI layer. FigureS2 shows that all samples were covered by the SEI layer completely (also see Figure 2). Furthermore, the SEI layer appears to get thicker after 100 charging-and-discharging cycles for Case 1, Case 2, and Case 3. Severe agglomeration is observed in Case 1 and Case 2 unlike Case 3. The SnO2, ZnO, and Zn2SnO4 (corresponding to Case 1, Case 2, and Case 3) particles were pulverized significantly within 100 cycles, resulting in severe agglomeration. In contrast, in Case 4, the decomposition of Zn2SnO4 into a couple of ZnO and a single SnO2 molecule within the carbon shell results in little agglomeration. The core-shell structure alleviates the impact of volume expansion, forming a relatively thin SEI layer (see Figure S2 and Figure 2). The formation of ZnO, SnO, and SnO2 owing to the decomposition of Zn2SnO4 after the first discharging-and-charging cycle is confirmed by the XRD data in Figure S3. The pulverization of ZnO and SnO follows, which increases the anode capacity by the increased surface area of pulverized ZnO and SnO particles, as shown in Figure 7(c). Simultaneously, the pulverized ZnO and SnO2 maintained a large interfacial area between the electrode and the electrolyte within the carbon shell in the presence of rGO [64, 65]. After 400 cycles, Case 1 and Case 2 show significant agglomeration owing to the intensified pulverization and the repetitive SEI layer formation around the agglomerated particles (Figure S2). The particle agglomeration was magnified after 400 cycles in comparison with that at 100 cycles for Case 3. Figure S2 shows that relatively little agglomeration was observed for Case 4 after 400 cycles because the carbon shell on the bimetallic particles minimized the impact of volume expansion.The quantitative analysis of the long-term cycling measurement for each case, considering the capacity retention after 400 cycles with respect to the 1st (patterned) and 2nd (solid) discharge capacities, is shown in Figure 7(d). The capacity retention values of Case 4 were observed to be 61 and 106%, which are superior to those in the other cases concerning the first and second discharge capacities, respectively. It is well-known that the SEI layer formation is inevitable in LIB. The SEI passivation layer comprises organic and inorganic complexes, such as Li2CO3, LiF, and LiOR, where R is an alkyl group [66]. The reactions involved with these complexes are irreversible, which in turn produces significant amount of irreversible capacity. It should be noted that the capacity retention with respect to the first cycle can be misleadingly high because of irreversible capacity contribution by the SEI layer formation. In contrast, the capacity retention with respect to the second cycle (or first reversible capacity) is based on the active anode materials only (i.e., ZSO/C/rGO) without the contribution by the SEI layer formation. The electrochemical performance of the bimetallic composite reported in similar studies is compared with the results of this study in Table 3. This comparison indicates that Zn2SnO4@C/rGO is a suitable anode material for LIBs.Table 3
Summary of cases and coating materials.
MaterialFirst discharge capacity (mAh·g-1)Capacity retention (%)Current density (mA·g-1)Cycle numberBinder-freeReferencesSn/ZnO/N-C149764100500N[67]ZnSnO3/rGAs160549100200N[68]ZnO/ZnSnO3 QDs1527361000100Y[69]ZnSnO3/rGO201336100100N[70]Zn2SnO4@NiO@rGO NFs11844310001600N[71]Zn2SnO4@C153060100100N[72]3D-Zn2SnO4154148100100N[23]Zn2SnO4/C/rGO1830611000400NPresentEIS is useful for examining the electrochemical properties and kinetics of anode materials. The Nyquist plots of ZSO/C@rGO, at various half-cell potentials, are presented in (Figure8(a)), and the Randles equivalent circuit is shown in Figure 8(b). Each Nyquist plot is composed of a high-frequency semicircle and Warburg tail region, followed by a steep sloping line in the low-frequency region. R1 and Cdl are the ohmic resistance between the electrolyte and the surface of the electrode and the double-layer capacitance, respectively. The high-frequency semicircle indicates the charge-transfer resistance (Rct) relevant to the interfacial Li-ion transfer. In impedance spectroscopy, the high-frequency semicircle is attributed to the charge-transfer impedance at the electrode/electrolyte interface, and the inclined line at an angle of approximately 45° to the real axis corresponds to the lithium diffusion process within the electrodes. The diameter of the semicircle in the high-frequency region of the Zn2SnO4/C@rGO electrode was smaller than that of the Zn2SnO4 electrode, revealing lower charge-transfer impedances. The low Rct value indicates that the electronic conductivity of the Zn2SnO4/C@rGO electrode improved after coating with a thin layer of carbon. Zw is the Warburg impedance, which is related to Li-ion diffusion into the structure of the active materials and corresponds to the tail at a low frequency.Figure 8
EIS analysis: (a) Nyquist plot for ZSO/C@rGO at different half-cell potentials and (b) raw data fitting of the Nyquist plot for ZSO/C@rGO at OCV (inset shows the Randles equivalent circuit). (c) Schematic depicting intercalation and deintercalaction processes of Li-ions during lithiation (left) and delithiation (right) in ZSO/C@rGO.
(a)(b)(c)Figure8(c) shows the intercalation process in rGO; initially, some Li+ ions were intercalated in the nanoparticles. Li+ ions were transferred from the separator and entered the active materials composed of Zn2SnO4 and rGO. rGO was necessary to enhance the electrical conductivity of the active material, and the adhered carbon-coated Zn2SnO4 particles ensured that the Li-ion diffusion pathway was shortened. The mesoporosity is beneficial to achieve shorter diffusion pathways for Li-ion transport. To investigate the pores and specific surface area of Case 4 (ZSO/C@rGO), N2 adsorption/desorption isotherm (Tristar 3000, Micromeritics) and Brunauer-Emmett-Teller (BET) analysis were performed. The N2 adsorption curve in Figure S4 shows the hysteresis in the relative pressure range of 0.45≤P/Po≤1.0, which corresponds to the type-IV isotherm. This hysteresis data confirms the mesoporosity of the rGO composite, whose surface area was found to be 208 m2·g–1, according to the BET data. The exfoliation of the rGO sheets and its mesoporous structure are crucial to facilitate the intercalation of Li+ ions between the rGO sheets. The carbon coating and rGO buffer the volume expansion of the particles during the lithiation process and improve the current rate capability owing to enhanced electrical conductivity, as shown in Figures 7(a)–7(b). In addition, the presence of the rGO sheets and carbon coating stabilizes the formation of the thin SEI layer and keeps it intact during current rate measurements and long-term cycling for 400 cycles.


## 4. Conclusion

Bimetallic Zn2SnO4 (ZSO) nanoparticles were deposited along with rGO nanosheets through supersonic spraying to fabricate LIB anodes. The advantages of introducing bimetallic ZSO and PVP were elucidated by comparing the electrochemical performances of SnO2/C@rGO, ZnO/C@rGO, ZSO@rGO, and ZSO/C@rGO. Notably, all the samples contained rGO nanosheets. The case with SnO2/C@rGO had the lowest specific capacity, whereas the case with ZSO/C@rGO was identified as the optimal combination, which produced a specific capacity of 1126 mAh⋅g-1 at a current rate of 1000 mA⋅g-1 after 400 cycles. The introduction of PVP facilitated the formation of a carbon shell around the Zn2SnO4 nanoparticles, which in turn alleviated their agglomeration and contributed to the enhancement of the electrochemical performance. The presence of a carbon shell was confirmed by elemental mapping. In contrast, the ZSO anode without PVP did not form a carbon shell, and thus, the agglomeration worsened, as evidenced by the SEM images. The decomposition of Zn2SnO4 after the first cycle to a mixture of SnO and ZnO nanoparticles (confirmed by XRD) also alleviated the agglomeration of identical metal oxides. Consequently, the ZSO anode without carbon coating exhibited a relatively poorer electrochemical performance than the optimal case. This result demonstrates that carbon coating using PVP for fabricating a ZSO anode results in a high LIB performance with long-term stability.



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*Source: 3553521-2023-02-11.xml*