Rigid TiO2−x coated mesoporous hollow Si nanospheres with high structure stability for lithium-ion battery anodes

Rigid oxygen-deficient TiO2−x coated mesoporous hollow Si nanospheres with a mechanically and electrically robust structure have been constructed through a facile method for high-performance Li-ion battery anodes. The mesoporous hollow structure provides enough inner void space for the expansion of Si. The oxygen-deficient TiO2−x coating has functions in three aspects: (1) avoiding direct contact between Si and the electrolyte; (2) suppressing the outward expansion of the mesoporous hollow Si nanospheres; (3) improving the conductivity of the composite. The combined effect leads to high interfacial stability and structural integrity of both the material nanoparticles and the whole electrode. By virtue of the rational design, the composite yields a high reversible specific capacity of 1750.4 mA h g−1 at 0.2 A g−1, an excellent cycling stability of 1303.1 mA h g−1 at 2 A g−1 with 84.5% capacity retention after 500 cycles, and a high rate capability of 907.6 mA h g−1 even at 4 A g−1.

Fig. S2a and b show the SEM images of the TiO 2-x nanoparticles prepared by calcination of the hydrolyzatein of TBOT in Ar flow at 800 o C. The particle size is approximately 300 nm, which is same with that of TiO 2 nanoparticles.However, the TiO 2-x shows black color.According to the literature (J.Am.Chem. Soc., 2012, 134, 7600-7603), black TiO 2 usually arises from the oxygen loss in TiO 2 , resulting in nonstoichiometric TiO 2-x .SEM images and (o) digital camera image of MHSi@TiO 2-x -900.Fig. S3 shows the SEM images of MHSi@TiO 2-x prepared by calcination of the products after the hydrolysis of TBOT on the surface of MHSi in Ar flow at different temperatures.All the samples are spherical, and the hollow structures, observed from the broken spheres, are still maintained.The surfaces of the samples are smooth except MHSi@TiO 2-x -900 (Fig. S3n), which is attributed to the particle aggregation of TiO 2-x under high-temperature calcination of 900 o C.Many reports have discussed coloration of TiO 2 in relation with the electronic structure (Nat. Chem., 2011, 3, 296-300;J. Am. Chem. Soc., 2012, 134, 7600-7603;J. Phys. Soc. Jpn., 2004, 73, 703-710).As shown in Fig. S3c, f, i, l, and o, the coloration of these samples gets gradually dark-green, implying the presence of nonstoichiometric TiO 2-x in our samples.

Sample
Initial reversible specific capacity (mAh g -1 ) Reversible specific capacity after 100 cycles (mAh g -1 ) Capacity retention MHSi 1317.7 521.9 39.6% MHSi@TiO 2-x -500 As shown in Fig. S4, the electrochemical cyclic performances of MHSi, MHSi@TiO 2-x -500, MHSi@TiO 2-x -600, MHSi@TiO 2-x -700, MHSi@TiO 2-x -800, and MHSi@TiO 2-x -900 as anode materials for LIBs were preliminarily evaluated.MHSi shows a higher initial discharge specific capacity, but the initial coulombic efficiency is only 40.1%.While, the initial coulombic efficiency of MHSi@TiO 2-x -500, MHSi@TiO 2-x -600, MHSi@TiO 2-x -700, MHSi@TiO 2-x -800, and MHSi@TiO 2-x -900 are 64.1%,64.3%, 65.6%, 66.2% and 66.1%, respectively, indicating that TiO 2-x coating is benefit for the improvement of the initial coulombic efficiency.The initial reversible specific capacities, the reversible specific capacities after 100 cycles, and the corresponding capacity retentions of MHSi and the samples calcined at different temperatures are summarized in Table S1.MHSi@TiO 2-x -800 exhibits the highest initial reversible specific capacity, but it is basically the same with the initial reversible specific capacities of the samples calcined at other temperatures.This is because the contents of Si in these samples are almost the same.Besides, the chemical components of TiO 2-x in these samples may be different, but the reversible specific capacity of TiO 2-x is only 140 mA h g −1 (Fig. S4), and the contents of TiO 2-x in these samples are not high (See Fig. S6 and the corresponding discussion), so the contribution of TiO 2-x to the total specific capacity of the composite is very little.Therefore, there are little differences in the initial reversible specific capacities of these samples.After 100 cycles, the capacity retentions of these samples gradually increase with the increase of the calcination temperature, except MHSi@TiO 2-x -900.According to Fig. S3, these samples show different colors, which are in relation with the electronic structure of TiO 2-x , implying different electrical conductivities.As shown in Fig. S5, the electrical conductivities of MHSi@TiO 2-x -500, MHSi@TiO 2-x -600, MHSi@TiO 2-x -700, MHSi@TiO 2-x -800, and MHSi@TiO 2-x -900 were evaluated.The semicircles in the Nyquist plots are in relation with the charge transfer resistance.The diameters of the semicircles of these samples (MHSi@TiO 2-x -500, MHSi@TiO 2-x -600, MHSi@TiO 2-x -700, MHSi@TiO 2-x -800) decrease with the increase of the calcination temperature, indicating that the electrical conductivities of these samples increase.The diameter of the semicircle of MHSi@TiO 2-x -800 is very close to that of MHSi@TiO 2- x -900, indicating that little increase in the electrical conductivity is obtained.The improvement of the electrical conductivity facilitates the formation of a stable conductive network inside the samples during cycling, promoting the utilization rate of Si.Additionally, with the increase of the calcination temperature, the strength of TiO 2-x may increase (Although the strength of TiO 2-x in the composite is not easily measured), which is benefit for the improvement of structural stability.
However, as shown in Fig. S3n, particle aggregation of TiO 2-x exists when the calcination temperature increases to 900 o C, leading to the inhomogeneous coating of TiO 2-x .The inhomogeneous coating layer of TiO 2-x may fracture during cycling, resulting in bad structural stability of the composite.Therefore, MHSi@TiO 2-x -800 shows the highest capacity retention.

EDS elemental analysis of MHSi@TiO 2-x -800
Fig. S6 (a) SEM image and (b) the corresponding EDS spectrum of MHSi@TiO 2-x -800 (the embedded table shows the elemental contents of Si, Ti and O).
As shown in Fig. S6, the EDS spectrum of MHSi@TiO 2-x -800 shows the characteristic peaks of Si, Ti and O.The embedded table shows that the elemental content of Ti is 16.74%, and that of O is 13.12%.According to the chemical components of TiO 1.939 , which is obtained from the analysis of the X-ray photoelectron spectroscopy of MHSi@TiO 2-x -800, the corresponding elemental content of O should be 10.82%.The difference between the above two elemental contents of O is ascribed to the partial oxidation of Si.So the content of TiO 2-x in MHSi@TiO 2-x -800 is 27.6%.As shown in Fig. S7a and c, nitrogen sorption isotherms of both MHSi and MHSi@TiO 2-x -800 exhibit typical IV curves with distinct H1 hysteresis loops.The Brunauer−Emmett−Teller (BET) surface area of the MHSi is 209 m 2 g −1 .After TiO 2-x coating, the BET surface area of the MHSi@TiO 2-x -800 gets smaller to 100 m 2 g −1 , which is ascribed to the decrease of some mesopores with pore diameter of 10 nm (Fig. S7b and d).The reduction of the specific surface area is beneficial to reduce the growth of SEI film.The mesopores with pore diameter of 1~2 nm are still maintained, which will not prevent the infiltration of electrolyte.Yolk-shell structured Si@TiO 2 composite 1562 at 0.4 A g -1 990 at 2 A g -1 63.4% after 1500 cycles [41] Core-shell Si@TiO 2-x /C microfiber 1000 at 0.2 A g -1 939 at 12 A g -1 90% after 50 cycles [42] Ref. * refers to the reference number in the main paper.

Fig
Fig. S1 (a,b) SEM images and (c) digital camera image of TiO 2 prepared by calcination of the

Fig.
Fig. S1a and b show the SEM images of the TiO 2 nanoparticles prepared by calcination of the

2.
Fig. S2 (a,b) SEM images and (c) digital camera image of TiO 2-x prepared by calcination of the Fig. S3 (a,b) SEM images and (c) digital camera image of MHSi@TiO 2-x -500; (d,e) SEM images Fig. S7 (a) Nitrogen sorption isotherms and (b) pore size distribution of MHSi; (c) Nitrogen

2-x -800 in this work and other Si/TiO 2 composites reported in previous studiesTable S2 .
Performance comparison between MHSi@TiO 2-x -800 in this work and other Si/TiO 2 composites reported in literatures.