Nano-Sn doped carbon-coated rutile TiO₂ spheres as a high capacity anode for Li-ion battery

Abstract

Nano-Sn doped carbon-coated rutile TiO₂ spheres (C-NS/TiO₂-1 and C-NS/TiO₂-2) as an improved TiO₂-based anode for Li-ion batteries were in situ fabricated. The physical characteristics of the C-NS/TiO₂ spheres were tested by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Their electrochemical properties were characterized by cyclic voltammograms (CVs), electrochemical impedance spectra (EIS) and galvanostatic charge–discharge cycles. The results indicated that the as-prepared samples had particle sizes ranging from 1 to 5 μm and were composed of several small inner rutile TiO₂ spheres, an outer mesoporous carbon coating layer and nano-Sn inhabited in the outer carbon layer. Their electronic conductivity was significantly enhanced owing to high electronic conductivity of the carbon layer and nano-Sn. Moreover, their reversibility capacity was also significantly improved due to the high specific capacity of nano-Sn. The C-NS/TiO₂-1 and C-NS/TiO₂-2 delivered a reversible capacity of 143.1 and 219.0 mA h g⁻¹, respectively, after 200 cycles at a high current density of 500 mA g⁻¹, which was increased by 52.0% and 132.6% compared to carbon-coated rutile TiO₂ (C-TiO₂).

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Li-ion batteries with high power and high energy density are promising power sources for portable electric devices, especially for electric vehicles.¹ Graphite, as a commercial anode material, has a theoretical capacity of 372 mA h g⁻¹. However, its safety issue and poor rate capability make it difficult to meet the demands of high energy density in energy storage systems.^2,3 With the advantages, such as high safety and negligible volume change (<4%) for Li⁺ intercalation and de-intercalation, the TiO₂ anode has been the subject of significant study in recent years.^4,5 Compared to anatase or brookite TiO₂, rutile TiO₂ experiences less volume change during the charge–discharge process, which is beneficial for cells' life.^6,7 However, the disadvantages of rutile TiO₂ are its low theoretical capacity (168 mA h g⁻¹) and electronic conductivity (3.65 × 10⁻¹⁵ cm S⁻¹ at 25 °C).^8,9

The most common strategy to improve the capacity of rutile TiO₂-based anode is doping by some high capacity materials, such as SnO₂,^10,11 Sb,¹² Sn,^13,14 whose theoretical capacities reach 782, 660 and 991 mA h g⁻¹, respectively. Due to the higher specific capacity of Sn, different Sn/TiO₂ composites have been studied.^15–17 Unfortunately, these composites undergo significant capacity decay and experience poor stability during the charge–discharge process, which prohibits their use in large-scale applications. The capacity decay is caused by the major drawback of Sn, which is that its large volume change (up to 360%) when reacting with Li⁺,¹⁸ causing cracking in the particles with a subsequent decrease in electronic isolation and electrochemical activity,¹⁹ thus leading to poor cycling stability. To address such problems, the formation of a carbon layer on the surface of the composite has been proven to be effective.^20–22 In fact, a carbon layer can absorb the force exerted by the volume expansion of Sn during the charging–discharging process. Another advantage for coating a carbon layer on the surface is that it can increase the electronic conductivity of the TiO₂-based anode. More importantly, the carbon layer can prevent the aggregation between TiO₂ particles and shorten the Li⁺ diffusion path dramatically because a large number of micropores can be created on the surface.

In this study, novel C-NS/TiO₂ spheres were designed, prepared and characterized. TiO₂ spheres were coated by a nano-Sn doped carbon layer. The carbon layer acts as a volume expansion-shrinkage buffer of Sn during the charge–discharge process and nano-Sn delivers a high discharge capacity during cycling. More importantly, TiO₂ can act as a hard core for the nano-Sn and carbon layer adhesion to maintain the structural integrity of the electrode. Furthermore, the carbon layer and nano-Sn are also excellent electronic conductors, which can decrease the electrode polarization. Compared to C-TiO₂, the discharge capacity and electronic conductivity of the C-NS/TiO₂ composites were obviously increased.

Experimental section

Fig. 1 illustrates the process for the synthesis of spherical C-NS/TiO₂. TiO₂@SnO₂ spheres were synthesized first according to a reported sol–gel method.²³ First, 0.7000 g (corresponding to C-NS/TiO₂-1) or 1.2000 g (corresponding to C-NS/TiO₂-2) tin chloride pentahydrate was added to 42 mL isopropyl alcohol and stirred for 1 min to obtain a clear solution and then 1.5 mL titanium isopropoxide was dropped into the solution and 0.030 mL diethylenetriamine was added subsequently under strongly stirring to obtain a white gel. The obtained gel was transferred into a 100 mL Teflon-lined stainless steel autoclave and placed in an oven at 200 °C for 12 h. After cooling to room temperature, the yellow precipitation was collected by centrifugation and washed with ethanol several times then dried at 60 °C. Second, 1.0000 g of as-prepared TiO₂@SnO₂ was dispersed in 0.5 M glucose aqueous solution in a 100 mL Teflon-lined autoclave before heating at 175 °C for 5 h. After being rinsed with deionized water and ethanol several times and fully dried, a brown powder was obtained. Finally, the powder was heat-treated under H₂/Ar (volume ratio was 1/19) at 800 °C for 4 h to obtain the final product. For comparison, C-TiO₂ as a counterpart was synthesized by reported methods with minor modifications.^24,25

Fig. 1 Schematic of the formation process of C-NS/TiO₂ spheres.

For materials characterization, XRD patterns were collected on a CHIMADZU-XRD-7000S with Cu Kα radiation (λ = 0.154056 nm) at a voltage of 40 kV and a current of 40 mA. SEM images were obtained on a S4800 microscope. TEM images were taken on an FEI Tecnai F20 microscope.

To evaluate the electrochemical performances, samples (80 wt%), conductive acetylene black (10 wt%) and polyvinylidene fluoride (10 wt%) binder were mixed into a homogeneous slurry using N-methyl pyrrolidone as the solvent and then the slurry was pasted uniformly onto a copper foil and dried to obtain the electrodes. Before 2032 coin cells assembled in an argon-filled dry glove box, the electrodes were punched into discs with 14 mm diameters. The dry paste had thicknesses of ∼40 μm and the prepared samples were loaded ∼2.3 mg per disc. The electrolyte was a 1 M LiPF₆ solution in the mixture of ethylene carbonate, dimethyl carbonate and ethylene methyl carbonate (1/1/1 by volume). A polypropylene membrane (Celgard 2400) was used as the separator. The galvanostatic charge–discharge tests were performed on a LAND battery tester with a cut-off voltage between 0.01 and 3.0 V and the capacity was calculated based on the mass of C-TiO₂, C-NS/TiO₂-1 and C-NS/TiO₂-2 (1C = 168 mA h g⁻¹). CVs were investigated by Autolab PGSTAT 128N at a scan rate of 0.2 mV s⁻¹. EIS was revealed by an Autolab PGSTAT 128N in the frequency range of 10⁻² to 10⁵ Hz. All the tests were carried out at room temperature.

Results and discussion

The crystal structures of all samples were directly confirmed by XRD. The results are shown in Fig. 2. It can be observed that the patterns of C-NS/TiO₂-2, C-NS/TiO₂-1 and C-TiO₂ can be indexed to the rutile TiO₂ (JCPDS no. 21-1276) without other TiO₂ impurities. In addition to the rutile TiO₂ peaks, other diffraction peaks in C-NS/TiO₂-2 and C-NS/TiO₂-1 can be unambiguously ascribed to tetragonal Sn (JCPDS no. 04-0673), which reveals that SnO₂ was completely reduced to metal Sn. In addition to the diffraction peaks of rutile TiO₂ and tetragonal Sn in the samples, no other peaks can be found, suggesting that the carbon in the samples may be low in graphitization degree.

Fig. 2 XRD patterns of C-NS/TiO₂-2 (a), C-NS/TiO₂-1 (b) and C-TiO₂ (c). The red and blue bars are standard rutile TiO₂ and tetragonal Sn diffractions, respectively.

To understand the morphology and components of the samples, SEM and TEM measurements were carried out. Fig. 3 shows the results of C-NS/TiO₂-1 and C-NS/TiO₂-2. Fig. 3a and b clearly shows that the morphology of C-NS/TiO₂-1 and C-NS/TiO₂-2 was similar and the diameters of the spherical particles range from ∼1 μm to ∼5 μm, but a small amount of TiO₂ spheres with sizes from ∼200 nm to ∼800 nm were exposed. It also can be observed that there were more fragmentary pieces in the C-NS/TiO₂-2 than those of C-NS/TiO₂-1. These small pieces may be caused by some SnO₂ not depositing on the surface of the TiO₂ microspheres when the Sn/Ti ratio was further increased. Unfortunately, a small amount of glucose was hydrolyzed in addition to the TiO₂@SnO₂ surface or some particles of the precursor were broken during heat treatment. In Fig. 3c, it can be observed that the carbon layer was constructed by nano-carbon particles with a size of about 50 nm. It is worth mentioning that many nano holes lie among nano-carbon particles, which can act as fast Li⁺ transfer channels to improve the rate performance. In Fig. 3d, the TEM image also reveals that a large number of nano-sized pores exist and extend from the surface of the carbon layer to its deep center. The fact that the TiO₂ spheres did not aggregate can be attributed to the volume shrinkage when the SnO₂ layer was changed to nano-Sn, despite some TiO₂@SnO₂ spheres possibly making contact with each other before H₂/Ar treatment. The trace of Sn particles can be identified in Fig. 3e. It is noteworthy that the particle size of Sn was less than 20 nm (thus, the Sn particles can be labeled as nano-Sn). From Fig. 3f, the lattice spacing of TiO₂ and nano-Sn was estimated to be 0.32 nm and 0.20 nm, respectively.

Fig. 3 SEM images of C-NS/TiO₂-1 (a, c), C-NS/TiO₂-2 (b) and TEM (d–f) images of C-NS/TiO₂-1.

The Li-ion storage performance of the as-prepared samples was studied by CV and the results are recorded in Fig. 4. The curves of CVs were taken by 1^st, 2^nd, 5^th and 10^th cycles. It can be observed that all the first cycle curves were coarse and different from other cycles. The coarse curves and the irreversible capacity loss in the first cycle should correspond to the decomposition of the electrolyte, the formation of an solid electrolyte interphase (SEI) film and the irreversible side reactions.²⁶ In Fig. 4a, a couple of broad redox peaks in the range of 0 V to ∼1.5 V and an ambiguous couple of redox peaks between 1.2 V and 2.5 V can be found after the 1^st irreversible cycle, which was consistent with the reported results,²⁷ and the reaction can be presented as follows:^28,29 TiO₂ + xLi⁺ + xe ↔ Li_xTiO₂, with x = 0.5021 for rutile. Despite the coarse 1^st and 2^nd cycles, the redox peaks of C-NS/TiO₂-1 and C-NS/TiO₂-2 became more stable starting from the 5^th cycle and the two couple of reversible redox peaks from 0.2 to 0.8 V can be separated and labeled as a/a′ and b/b′. Peaks a/a′ and b/b′ should be attributed to the reversible process of alloying and de-alloying between Sn and Li and the reactions are as follows:

Peaks a/a′: Li⁺ + Sn + e⁻ ↔ LiSn (1)

Peaks b/b′: 3.4Li⁺ + LiSn + 3.4e⁻ ↔ Li_4.4Sn (2)

Fig. 4 CVs of C-TiO₂ (a), C-NS/TiO₂-1 (b) and C-NS/TiO₂-2 (c).

The reversible capacity from (1) and (2) can be calculated as 225 and 766 mA h g⁻¹, respectively, and the total capacity is 991 mA h g⁻¹,³⁰ which is 5.9 times that of the theoretical capacity of rutile TiO₂. It is clear that compared to Fig. 4b, the redox peaks of b/b′ in Fig. 4c were more sharp and faded down more slowly, which exhibited more Sn content in C-NS/TiO₂-2.

Fig. 5 displays long cycle performance of samples at different current densities. It is clear that the discharge process becomes stable after the first 15 cycles activated, suggesting that the side reactions become weak. The C-TiO₂ delivered a reversible discharge capacity of 136.4, 129.2 and 120.3 mA h g⁻¹ at the 50^th cycle under 50, 100 and 200 mA g⁻¹, respectively. The lower degradation of the discharge capacity indicates good rate capability of C-TiO₂ when the current density is increased. For C-NS/TiO₂-1 and C-NS/TiO₂-2, their cycle performances were similar, but their discharge capacity was much higher than that of C-TiO₂. In the 50^th cycle, the discharge capacity of C-NS/TiO₂-1 was 265.5, 222.8 and 187.6 mA h g⁻¹ under 50, 100 and 200 mA g⁻¹, whereas that of C-NS/TiO₂-2 was 371.9, 300.0 and 250.7 mA h g⁻¹ under same conditions, respectively. The higher discharge capacity can be attributed to the intimate contact of the outer carbon layer and inner TiO₂ spheres as mono dispersed nano-Sn matrix, which was effective for showing capacity of nano-Sn as high capacity doping material. To understand the high rate cycle performance, cells were tested under 500 mA g⁻¹ and through 250 cycles. Significantly, stable cycle performance was retained. The discharge capacity of C-NS/TiO₂-1 and C-NS/TiO₂-2 at the 200^th cycle was 143.1 and 219.0 mA h g⁻¹, respectively, and the capacity was increased by 52.1% and 132.6% compared to C-TiO₂. Some results of previous studies are listed in Table 1. As shown in the table, these capacity values were calculated whether gravimetric capacity or areal capacity. It is clear that the discharge capacity of C-NS/TiO₂-2 was higher than those results in the table at the same current density. The capacity on ref. 16, 32 and 33 was close to or slightly higher than the capacity of C-NS/TiO₂-2, but the cycle current in those references was much lower than that of C-NS/TiO₂-2. The improvement of the discharge capacity confirms that C-NS/TiO₂ is a type of useful anode material due to the high capacity of nano-Sn and amorphous carbon as its volume expansion-shrinkage buffer layer and electronic conductor. In addition to the initial several cycles, the coulombic efficiency of the as-prepared samples during the following cycles was close to 100% (the initial coulombic efficiency of C-TiO₂, C-NS/TiO₂-1 and C-NS/TiO₂-2 was 96.5%, 83.9% and 79.8%, the second cycle efficiency of C-NS/TiO₂-1 and C-NS/TiO₂-2 was 94.6% and 93.2%, respectively), which also shows their good cycle stability.

Fig. 5 Cycle performance of C-TiO₂ (a), C-NS/TiO₂-1 (b), C-NS/TiO₂-2 (c) at different current densities, (d) shows long cycle property and relative coulombic efficiency at 500 mA g⁻¹.

Table 1 Discharge capacities, electrode state and capacity retention of different C-Sn/TiO₂ electrodes

Ref.	Initial discharge capacity		Electrode state	Capacity retention and cycle current density
	(mA h g^−1)	(μA h cm^−2)		(mA h g^−1)	(mA g^−1)	(μA h cm^−2)	(μA cm^−2)
a Means the values from the images in the references.
Our works C-NS/TiO2-2	757.5	562.1	Carbon coated nano-Sn/TiO2 spheres	307.2 (10^th), 234.1 (100^th), 219.0 (200^th)	500	227.9 (10^th), 173.7 (100^th), 162.5 (200^th)	750
16	1180^a	—	Sn-Doped rutile TiO2 nanotubes	235 (200^th)	250	—	—
32	402.5	—	Sn-Doped TiO2 nanotube	241.6 (100^th)	35	—	—
33	384.0	—	Sn-Doped TiO2 nanotube	266.0 (100^th)	35	—	—
15	250.0^a	—	Sn-Doped TiO2 nanoparticles	220.0^a (100^th)	382	—	—
34	215.0^a	—	Nano Sn coated TiO2 powder	180.0^a (10^th)	336	—	—
1	325.0^a	—	(Sn–Ti)O2 solid solution nanoparticles	220.0^a (50^th)	30	—	—
6	318.6	—	(Ti, Sn)O2 nanorods	218.2 (50^th)	30	—	—
8	—	170.0^a	Sn-Doping TiO2 nanotubes array	—	—	78.0^a (50^th)	70
35	—	182.0	Sn-Doped TiO2 nanotubes thin films	—	—	111.8 (50^th)	70
36	—	371.0^a	Sn/TiO2 nanowire array composites	—	—	160.0^a (200^th)	240
37	—	276.2	Ti1/2Sn1/2O2 nanotubes	—	—	62.0 (50^th)	70
38	—	42.0	TiO2-Core/Sn-shell nanotube arrays	—	—	40.0 (50^th)	28

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For understanding the performance when cells are being abused, the cells were charged and discharged progressively at current rates from 50 to 1000 mA g⁻¹, and the results ae shown in Fig. 6. As shown in the figure, the discharge capacity decreased greatly within the first 10 cycles, indicating some irreversible reactions occurred and the results were also consistent with the results of the CVs. From the 11^th to 50^th cycles, the discharge capacity also decreased but became less obvious despite the current density increasing at the same time, illustrating the fact that the anode became stable and the side reactions were fading away. In every stage, the discharge capacity of C-NS/TiO₂-1 and C-NS/TiO₂-2 was much higher than that of C-TiO₂, showing that nano-Sn is a useful doping material, which has been demonstrated by good cycle performance, as shown in Fig. 5. For C-NS/TiO₂ electrodes, it is worth noting that in addition to the first 10 cycles, the discharge capacity barely decreases from 51 to 70 cycles, which further demonstrates the reversibility of the as-prepared samples. The reversibility may be attributed to the small size of the nano-Sn, which results in less absolute volume change, thus less electrode pulverization during cycling. More importantly, the nano-Sn was embedded in the carbon matrix and the carbon shell can act as a barrier for hindering nano-Sn agglomeration.

Fig. 6 Progressive charge–discharge performance of as-prepared samples.

To further understand the reaction mechanisms and cycle performance of the cells, EIS of C-TiO₂, C-NS/TiO₂-1 and C-NS/TiO₂-2 electrodes was performed and the results are presented in Fig. 7. The equivalent circuit is made of R_e (electrolyte resistance), R_f (SEI film resistance), R_ct (charge-transfer resistance), C_f (SEI film capacitance), C_ct (charge-transfer capacitance), and Z_w (Warburg impedance). The R_f of C-TiO₂, C-NS/TiO₂-1 and C-NS/TiO₂-2 can be calculated as 92.0, 10.3 and 7.4 Ω, respectively. The lower R_f of the electrode is indicative of a higher electronic conductivity. According to the formula σ = d/AR_f (where σ is the conductivity, d is the thickness and A is the specific surface area of the electrodes),³¹ the electronic conductivities of C-TiO₂, C-NS/TiO₂-1 and C-NS/TiO₂-2 were 3.53 × 10⁻⁵, 3.09 × 10⁻⁴ and 4.40 × 10⁻⁴ cm S⁻¹, respectively. The higher electronic conductivities in the C-NS/TiO₂-1 and C-NS/TiO₂-2 electrodes are mainly due to the excellent electronic conductivity of nano-Sn which was as high as 9.17 × 10⁸ cm S⁻¹ at room temperature, about 10⁷ times that of a bare carbon layer.

Fig. 7 EIS of C-TiO₂, C-NS/TiO₂-1 and C-NS/TiO₂-2, the inset one is relative equivalent circuit.

Conclusions

Nano-Sn doped carbon-coated rutile TiO₂ spheres were in situ synthesized and tested. In the composites, TiO₂ spheres were coated by a carbon layer and nano-Sn particles were dispersed in the carbon shell. The carbon coated layer can hinder the volume change caused by nano-Sn, nano-Sn acts as high capacity material, and the inner TiO₂ spheres act as hard cores to maintain the overall structure. Moreover, both the carbon layer and the nano-Sn also act as electronic conductors. The results demonstrate that the C-NS/TiO₂ composites have higher discharge capacity and electronic conductivity, which is suitable for a Li-ion battery anode material. When the current density was high as 500 mA g⁻¹, the discharge capacity of C-NS/TiO₂-1 and C-NS/TiO₂-2 reached 143.1 and 219.0 after 200 cycles and the electronic conductivities were about one order of magnitude increased compared to C-TiO₂. These results indicate a high capacity anode material can be fabricated using nano-Sn as doping material and carbon as the coated layer.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (No. 21206203), Program for Chongqing Science & Technology Innovation Talents (cstc2013kjrc-qnrc50006) and the Scientific Research Innovation Team of Chongqing University of Technology (No. cqut2015srim).

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