Elsevier

Ceramics International

Volume 45, Issue 11, 1 August 2019, Pages 14327-14337
Ceramics International

Synergistic effects from super-small sized TiO2 and SiOx nanoparticles within TiO2/SiOx/carbon nanohybrid lithium-ion battery anode

https://doi.org/10.1016/j.ceramint.2019.04.147Get rights and content

Abstract

TiO2/SiOx/carbon nanohybrid consisting of super-small sized TiO2 and SiOx nanoparticles is in situ formed within the in situ formed carbon matrix using difunctional methacrylate monomers as solvent and carbon source. The relative composition of TiO2 and SiOx is systematically tuned by varying the feeding mass ratio of the precursors (TEOS/TTIP) and HF etching treatment after the carbonization process. The super-small sized TiO2 and SiOx nanoparticles are well intermixed with each other and homogeneously dispersed throughout the carbon matrix, where a good control over crystallinity, morphology, and microstructure of the nanohybrids is achieved. Synergistic effects are demonstrated from both TiO2 and SiOx regarding the increased reversible capacity from the SiOx phase and maintenance of good cyclic stability by the TiO2 component when being used in lithium-ion batteries. A good balance is achieved in terms of reversible capacity, capacity retention, and rate performance in the TiO2/SiO2/C nanohybrid prepared with the TEOS/TTIP mass ratio of 1.0. The reversible capacities are improved by around two times compared to the SiOx free sample with a good capacity retention of 78 %, which exhibits good rate capability at the current densities up to 3350 mA g−1 as well.

Graphical abstract

TiO2/SiOx/C nanohybrids with super-small sized TiO2 and SiOx nanoparticles well intermixed with each other and homogeneously dispersed in the carbon matrix are prepared, where a good balance is achieved in terms of reversible capacity, capacity retention, and rate capability with the TiO2/SiO2/C nanohybrid prepared with the TEOS/TTIP mass ratio of 1.0.

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Introduction

Last thirty years have witnessed a great success of the lithium-ion battery (LIB) technology due to its distinct advantages over other rival rechargeable battery types such as higher energy density, longer cycling life, and limited memory effect [[1], [2], [3]]. However, the LIB technology is shadowed by its huge safety risk originating from excessive lithium plating on graphite anode [4,5]. TiO2 is regarded as a promising anode alternative to graphite because of its excellent operation safety [6,7]. However, so far limited capacity of graphite is of concern. Silicon base anodes (silicon, silicon sub-oxide) have attracted considerable attention due to their exceptional capacities [8,9]. However, the silicon sub-oxide anode (SiOx) is featuring a poor cyclic performance originating from series of adverse effects associated with huge volume change upon lithiation [[10], [11], [12], [13], [14], [15], [16]]. TiO2 nanoparticles with the average size down to 5 nm, carbon coating and doping have been proved to be an effective strategy to improve the electrochemical performance of TiO2 [[17], [18], [19], [20]]. Nevertheless, TiO2 suffers from a moderate theoretical capacity [21]. Numerous research work have been carried out to improve the electrochemical performance of SiOx based anodes including tuning particle morphology, coating with conductive buffer materials, and reducing particle sizes [9,[22], [23], [24], [25], [26], [27], [28], [29]]. Especially, carbon coating on the SiOx particles is attractive because of its dual role as electron conductor and mechanical buffer medium [27,30]. The integration of both TiO2 and SiOx into the carbon matrix enables a good synergistic effect towards an improved electrochemical performance in terms of an increased reversible capacity and extended cycling life [8,22,[31], [32], [33]]. On the one hand, TiO2 acts as a buffer medium to mitigate mechanical stress generated by the volume expansion of SiOx upon lithiation. On the other hand, the SiOx phase effectively lifts actual capacities of the composite. Besides the mutual enhancement by TiO2 and SiOx, the morphologies of TiO2 and SiOx are crucial for the electrochemical performance. It is well accepted that size reduction to a few nanometers can effectively enhance the electrochemical kinetics and alleviate mechanical stress as well [8,34,35]. Thus, it appears essential to construct TiO2/SiOx nanocomposites with super-small sized TiO2 and SiOx nanoparticles being well intermixed with each other and homogeneously dispersed in carbon matrix.

Inspired by our previous work, here we report a facile scalable strategy to synthesize TiO2/SiOx/C nanohybrids as demonstrated in Scheme 1 [[17], [18], [19],[36], [37], [38], [39], [40], [41]]. Super-small sized TiO2 and SiOx nanoparticles are in situ formed, well mixed with each other, and homogenously embedded in the in situ formed carbon matrix. Difunctional methacrylate monomers of bisphenol A glycidyl dimethacrylate (Bis-GMA) and triethylene glycidyl dimethacrylate (TEGDMA) are used as solvent and carbon source simultaneously. Titanium tetraisopropoxide (TTIP) and tetraethyl orthosilicate (TEOS) are applied as the precursors for the TiO2 and SiOx respectively, which are homogeneously mixed together with the resin monomer solvent at a molecular level. Both TTIP and TEOS are linked with the Bis-GMA resin monomer through alcoholysis, which are further integrated with the cross-linking methacrylate network after thermal polymerization. The calcination process at high temperatures generates TiO2 and carbon in situ simultaneously, where the silicate species is in situ reduced to silicon sub-oxide as well through carbothermal reduction. During the high temperature carbonization process, the thermosetting methacrylate polymers undergo very limited melting process. As a result, the nucleation, particle growth, and aggregation of both the TiO2 and SiOx are significantly suppressed, leading to the in situ formation of super-small TiO2 and SiOx nanoparticles well dispersed in the carbon matrix. Further HF treatment creates mesoporous structures within the TiO2/SiOx/C nanohybrid and tunes the composition of the SiOx within the nanohybrid as well. The plenty of mesoporous structure formed after HF etching can not only stabilize the electrode by alleviating the mechanical stress generated by the lithiated SiOx, but also enhance the electrochemical kinetics [10].

Section snippets

Materials

All chemicals were used as received without further purification. Titanium tetraisopropoxide (TTIP), bisphenol A glycidyl dimethacrylate (Bis-GMA), triethylene glycol dimethacrylate (TEGDMA) were purchased from Aladdin Reagent Co., Ltd., China. Tetraethyl orthosilicate (TEOS and hydrofluoric acid were bought from Sinopharm Group Co., Ltd., China. Tert-butyl peroxy benzoate (TBPB) was received from Sigma-Aldrich. Conductive carbon (Super P, more than 99%) was acquired from SCM Chem. Co., Ltd.

Results and discussions

The crystallization behaviour of the TiO2/SiOx/C nanohybrids with different TEOS contents is shown in Fig. 1. All of the weak peaks in the XRD diffraction patterns of all samples can be well-indexed to a poorly crystallized pure anatase phase of TiO2 (JCPDS 21–1272). The incorporation of the TEOS component does not modify the XRD diffraction patterns significantly. The average particle sizes of TiO2 derived by the Scherrer equation on the anatase (101) diffraction peaks are 3.6 nm, 4.5 nm,

Conclusions

In summary, TiO2/SiOx/C nanohybrids are prepared using difunctional methacrylate monomers as solvent and carbon source. The in situ formed super-small sized TiO2 nanoparticles are composited with the in situ formed SiOx phase within the in situ formed carbon matrix through a facile scalable thermal polymerization process and subsequent calcination under inert atmosphere and HF etching treatment. The relative mass composition of the SiOx phase and mesoporous structures of the TiO2/SiOx/C

Acknowledgement

This research is funded by the National Key R&D Program of China (Grant No. 2016YFB0100100), the National Natural Science Foundation of China (51702335, 21773279), the Zhejiang Non-profit Technology Applied Research Program (LGG19B010001), Ningbo Municipal Natural Science Foundation (2018A610084), CAS-EU S&T cooperation partner program (174433KYSB20150013), and Key Laboratory of Bio-based Polymeric Materials of Zhejiang Province. S.Y. and S. Xia acknowledge the China Scholarship Council (CSC)

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