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Scalable in situ Synthesis of Li4Ti5O12/Carbon Nanohybrid with SuperSmall Li4Ti5O12 Nanoparticles Homogeneously Embedded in Carbon Matrix Luyao Zheng, Xiaoyan Wang, Yonggao Xia, Senlin Xia, Ezzeldin Metwalli, Bao Qiu, Qing Ji, Shanshan Yin, Shuang Xie, Kai Fang, Suzhe Liang, Meimei Wang, Xiuxia Zuo, Ying Xiao, Zhaoping Liu, Jin Zhu, Peter Muller-Buschbaum, and Ya-Jun Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16578 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Applied Materials & Interfaces

Cheng, Ya-Jun; Ningbo Institute of Materials Technology and Engineering Chinese Academy of Sciences; University of Oxford, Department of Materials

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Scalable in situ Synthesis of Li4Ti5O12/Carbon Nanohybrid

with

Super-Small

Li4Ti5O12

Nanoparticles Homogeneously Embedded in Carbon Matrix Luyao Zheng1,2, Xiaoyan Wang1,2, Yonggao Xia1, Senlin Xia3, Ezzeldin Metwalli3, Bao Qiu1, Qing Ji1,4, Shanshan Yin1,5, Shuang Xie1,2, Kai Fang1,6, Suzhe Liang1,5, Meimei Wang1, Xiuxia Zuo1,2, Ying Xiao1, Zhaoping Liu1, Jin Zhu1, Peter Müller-Buschbaum3, and Ya-Jun Cheng1,7* 1.

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of

Sciences, 1219 Zhongguan West Road, Zhenhai District, Ningbo, Zhejiang Province 315201, People’s Republic of China 2.

University of Chinese Academy of Sciences, 19A Yuquan Rd, Shijingshan

District, Beijing, 100049, People’s Republic of China 3.

Physik-Department,

Lehrstuhl

für

Funktionelle

Materialien,

Technische

Universität München, James-Franck-Str. 1, 85748 Garching, Germany 4.

The University of Nottingham Ningbo China, 199 Taikang East Road, Ningbo,

Zhejiang Province 315100, People’s Republic of China 5.

North University of China, Shanglan Rd, Taiyuan, Shanxi Province 030051,

People’s Republic of China 6.

Nano Science and Technology Institute, University of Science and Technology of

China, 166 Renai Rd, Suzhou, Jiangsu Province 215123, People’s Republic of China 7.

Department of Materials, University of Oxford, Parks Rd, OX1 3PH, Oxford, 1


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United Kingdom E-mail address: [email protected]

Abstract Li4Ti5O12 (LTO) is regarded as a promising lithium-ion battery anode due to its stable cyclic performance and reliable operation safety. The moderate rate performance originated from the poor intrinsic electron and lithium-ion conductivities of the LTO has significantly limited its wide applications. A facile scalable synthesis of hierarchical Li4Ti5O12/C nanohybrids with super-small LTO nanoparticles (ca. 17 nm in diameter) homogeneously embedded in the continuous sub-micrometer sized carbon matrix is developed. Difunctional methacrylate monomers are used as solvent and carbon source to generate TiO2/C nanohybrid, which is in situ converted to LTO/C via a solid-state reaction procedure. The structure, morphology, crystallinity, composition, tap density, and electrochemical performance of the LTO/C nanohybrid are systematically investigated. Comparing to the control sample of the commercial LTO composited with carbon, the reversible specific capacity after 1000 cycles at 175 mA g-1 and rate performance at high current densities (875 mA g-1, 1750 mA g-1, and 3500 mA g-1) of the Li4Ti5O12/C nanohybrid have been significantly improved. The enhanced electrochemical performance is due to the unique structure feature, where the super-small sized LTO nanoparticles are homogeneously embedded in the continuous carbon matrix. Good tap density is also achieved with the LTO/C nanohybrid due to its hierarchical micro-/nanohybrid structure, which is even higher than that of the commercial LTO powder. 2



Keywords:

Lithium-Ion

Batteries;

Anode;

Lithium

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Titanate;

Super-small

nanoparticles; Photo Polymerization; Dental Methacrylate Resin

3


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Introduction Spinel Li4Ti5O12 (LTO) is considered as one of the most promising anode materials for lithium-ion batteries because of its superior cycling stability1-9. It is featured with negligible volume change and good operation safety due to its relatively higher potential of about 1.55 V (vs. Li/Li+)10-15. However, its moderate electronic and ionic conductivities lead to its poor rate performance, which significantly limits its wide use16-17. In order to solve these detrimental drawbacks, several strategies have been proposed to enhance the electrochemical performance such as doping with ions, surface coating, and size reduction18-20. Among these strategies, decreasing the Li4Ti5O12 particle size is an efficient way to improve the rate capability, because the transportation lengths of both lithium ions and electrons can be significantly shortened21-25. Several methods have been proposed to synthesize Li4Ti5O12, such as solid-state reaction26, microwave processing27, molten-salt synthesis28, sol–gel synthesis29, and hydrothermal synthesis30. However, a few major drawbacks remain with the conventional synthetic methods. Firstly, it is very difficult to synthesize nanoscale LTO particles with a small diameter of around 10 nm, where the particle size could possibly effectively improve the rate performance31. Only a few methods have been developed to synthesize small-sized LTO particles. For example, glycothermal synthesis generated well-crystallized LTO nanoparticles with the primary size of 4 nm – 8 nm 6. Super fluidic methods under different conditions have been developed to synthesize LTO nanoparticles with different morphologies and size ranges32-36. Microwave-assisted synthesis was also used to synthesize LTO with the 4



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size of 6 nm37. Secondly, even though some methods may be applicable to synthesize nano-sized LTO particles, the methods are still not immediately ready to enable an up-scaling of the synthesis. They are not fully adaptable to the current widely used materials processing technology of the battery industry. Therefore, it will still need time and efforts to fully realize practical applications of these methods38. Thirdly, the as-synthesized LTO particles tend to aggregate to form large sized secondary particles, which could partially deteriorate the rate performance enhancement39. Fourthly, the tap density of the LTO nanoparticles may be low, which is not good for practical applications where volumetric specific capacity is crucial40. Fifthly, homogeneous surface coating of the LTO nanoparticles is normally mandatory to avoid undesirable side reactions and improve electrochemical performance. Nevertheless, the homogeneous surface coating of the nanoparticles could be very tricky41. Among the various synthetic methods reported so far, solid-state reaction is particularly attractive because it is practical relevant and scalable42. However, it often suffers from a series of problems, such as inhomogeneity, irregular morphology, poorly controlled particle growth and agglomeration38. It is difficult to synthesize small sized LTO particles using the solid-state reaction method. Difunctional methacrylates monomers have been widely used as dental resins in aesthetic dentistry43. Typical dental resin monomers utilized are bisphenol A glycidyl dimethacrylate (Bis-GMA) and triethylene glycol dimethacrylate (TEGDMA)44, which are composited with inorganic fillers to improve the mechanical properties43. Curing process illuminated by blue light is applied in clinic to form cross-linking 5


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network with decent mechanical properties suitable for tooth repair45. Inspired by the research on the polymeric dental restorative materials, a new strategy is developed by our group to synthesize TiO2/carbon or SiOC/carbon nanohybrids43,

46-48

. The

precursor of the TiO2 or SiOC is dissolved in the thermosetting difunctional methacrylate monomer solution, followed by photo polymerization to convert the liquid solution into solid phase. The titania or SiOC species are integrated into the cross-linking network of the methacrylate polymer by coordination bonds and/or covalent bonds at molecular level. By calcination in an argon atmosphere, the thermosetting methacrylate polymers are converted to carbon matrix, where the TiO2 or SiOC nanoparticles are in situ formed within the carbon matrix. Because of the cross-linking network structure, the methacrylate polymers have a very limited melting process during calcination. As a result, the nucleation, growth, and agglomeration of the TiO2 or SiOC nanoparticles are significantly inhibited. Super-small TiO2 or SiOC nanoparticle are in situ formed and homogeneously embedded in the micrometer sized continuous carbon matrix.

6



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Scheme 1. In situ solid-state reaction synthesis of the Li4Ti5O12/C nanohybrids from the TiO2/C using difunctional methacrylate monomers as solvent and carbon source via photo polymerization. Inspired by our previous work, synthesis of hierarchical Li4Ti5O12/carbon nanohybrid based on difunctional methacrylate monomer is reported here. The TiO2/C nanohybrid obtained in our previous work is used as a starting material for the synthesis of Li4Ti5O12/C nanohybrid through a facile scalable solid-reaction process (Scheme 1). By reacting with lithium carbonate, the super-small TiO2 nanoparticles embedded in the carbon matrix are in situ converted to Li4Ti5O12 nanoparticles, which are still homogeneously embedded in the carbon matrix. As a result, hierarchical Li4Ti5O12/C micro-/nanohybrid are synthesized. The typical merits of the presented 7


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strategy are as following: Firstly, Good control over morphology and dispersion of the super-small sized LTO particles within the carbon matrix is possible. The precursor of the TiO2 (titanium tetraisopropoxide) is dissolved in the monomer solution of the difunctional methacrylate. The fast photo polymerization process allows for an efficient integration of the titania species into the cross-linking methacrylate network46. The synthesis of the TiO2 nanoparticles is simultaneously accompanied with the in situ formation of carbon matrix through the high temperature calcination process in an inert atmosphere. As a result, non-agglomerated super-small TiO2 nanoparticles are homogeneously embedded in the carbon matrix49. Therefore, the presented method is significantly much better than the conventional mechanical mixing methods to prepare the TiO2/C composite, where the agglomeration of the TiO2 nanoparticles and inhomogeneous dispersion of the TiO2 nanoparticles in the carbon matrix is very difficult to avoid50. It is also better than the normal carbon coating method because the TiO2 nanoparticles are in situ formed in the carbon matrix, where the TiO2 nanoparticles are not agglomerated51. Secondly, because the non-agglomerated super-small TiO2 nanoparticles are homogeneously embedded in the carbon matrix, further solid-state reactions with lithium carbonate at high temperature will not cause severe particle growth and agglomeration of the lithium titanate. The super-small size feature of the LTO nanoparticles and homogeneous embedding in the carbon matrix facilitates electron and lithium-ion transportation leading to enhanced electrochemical kinetics and improved rate performance. The 8



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continuous micrometer sized carbon matrix helps to circumvent typical problems associated with the nanoparticles. It inhibits possible side reactions between electrolyte and the super-small sized LTO nanoparticles by reducing the surface area of the LTO nanoparticles exposed to the electrolyte. The tap density is increased with the hierarchical micro-/nanohybrid structure feature, which is normally very low for the super-small sized particles. Furthermore, the synthetic method reported here is facile and scalable because it is based on conventional processing technologies of the thermosetting polymer and typical solid-state reaction method. The difunctional methacrylate monomers used in this work are fairly cheap and huge amount of the resin monomers are produced worldwide every year. Only tiny amount of photo initiator of I-819 is used. The photo polymerization illuminated by visible light is fast, energy effective, environment friendly, technologically mature, and easy to scale up. The precursor of TTIP is widely used in the production of LTO with a reasonable cost. Further solid-state reaction process is typically employed by industry to produce LTO. Finally, no tedious, costly, and environment unfriendly solvent disposal is needed, which is almost inevitable for typical synthetic methods of the Li4Ti5O12.

EXPERIMENT Materials All chemicals were used as received without further purification. Titanium tetraisopropoxide (TTIP, 95 %), Bisphenol A glycidyl dimethacrylate (Bis-GMA), and triethylene glycol dimethacrylate (TEGDMA, 95 %) were purchased from Aladdin Reagent Co., Ltd., China. Phenyl bis (2, 4, 6-trimethyl benzoyl)-phosphine oxide 9


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(Irgacure I-819, 95 %) was bought from Sigma-Aldrich. Lithium carbonate (LCO, 98 %) was acquired from Sinopharm Chemical Reagent Co., Ltd. Lithium Titanate was purchased from MTI Corporation. Conductive carbon (Super P) was obtained from SCM Chem. Shanghai, China. Poly (vinylidene fluoride) (PVDF) was donated by Solvay. Sample Preparation and Characterization Synthesis of TiO2/C nanohybrid and bare carbon (TiO2 free sample): Photo initiator (I-819) was firstly dissolved in a resin mixture of Bis-GMA/TEGDMA (mass ratio: 2:3) with a composition of 2 % to obtain photoactive B/T solution. TTIP of 0 g (synthesis of bare carbon) or 4 g (synthesis of TiO2/C) was added to 4 g of the photoactive B/T solution and stirred for 5 min. The solution mixture was thereafter poured into a rectangular silicon rubber mould (1 cm × 4.5 cm × 0.4 cm), which was then clamped between two pieces of glass slides. Photo polymerization was carried out in a visible light curing unit (Huge G01 D05, blue light, 9 W, emission wavelength range: 30 nm − 510 nm) for four minutes each side. The obtained yellow solid was released from the mould and cut into small pieces with a mixer. Calcination under argon atmosphere was performed in a tube furnace at 800

°C for 4 hours, with a ramp rate of 5 °C/min starting from room temperature. After calcination, the samples were cooled down to room temperature with the same ramp rate. Synthesis of Li4Ti5O12/C nanohybrids via solid-state reaction (LTO/C-SSR): The as-synthesized TiO2/C powder was mixed with Li2CO3 by a Li/Ti molar 10



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ratio of 0.86. Ball milling was carried out for 5 hours at 1000 rpm with a planetary ball miller (XQM-0.2, Tecan Powder, Changsha, China) using a similar protocol applied in our previous work47. The obtained mixture was then calcined under argon atmosphere for 12 hours at 800 °C with a ramp rate of 5 °C/min starting from room temperature, followed by natural cooling down to room temperature with the same ramp rate. The obtained Li4Ti5O12/C nanohybrids were further ball-milled for 1 hour at 1000 rpm with the same protocol mentioned previously47. Synthesis of Li4Ti5O12/C control sample (LTO/C-CS): The carbon obtained by pyrolyzing the pure B/T resin was mixed and ball-milled with commercial Li4Ti5O12 powder with a mass ratio equal to that of the LTO/C-SSR. The ball-milling protocol was the same as the one applied in the preparation of the LTO/C-SSR sample except a longer ball-milling time of 5 hours was used. Field emission scanning electron microscopy (FESEM) images were obtained with a Hitachi S4800 scanning electron microscope (Tokyo, Japan) at an accelerating voltage of 20 kV. The sample powder was dropped on a conductive tape and sputtered with gold before imaging. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) data were collected with a JEOL JEM-2100F TEM operated at 200 kV. The sample was prepared by casting a drop of aqueous suspension onto a copper grid supported by carbon. The inter-planar space was measured based on the lattice fringes displayed in the HRTEM images. The crystallographic phases of the samples were investigated by x-ray diffraction (XRD) (Bruker AXS D8 Advance, λ = 1.541 Å, 2.2 kW) with a 2θ ranging from 5 ° to 11


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90 °. The tap densities of the nanohybrids were measured according to GB/T 5162-2006. Similar amount of the powders were put in graduates of 1 mL separately. The graduates were vibrated manually for more than 3000 times until no volume change was observed by naked eyes. The final volume was measured and the corresponding densities of the powders were calculated. A particle size distribution (PSD) test was performed with a laser particle size and Zeta potential analyzer (Malvern, Nano ZS). Ethanol was used as the dispersion medium. The contents of carbon were studied by a thermo gravimetric analyzer (TGA, Mettler Toledo, Switzerland) with a temperature range from 50 °C to 800 °C and a ramp rate of 20

°C/min in air. The samples were dried at 120 °C for 4 hours in an oven before the TGA test. The Raman data was collected on a Renishaw (inVia reflex) with a wavelength of 532 nm. The pore size distribution of these nanohybrids were obtained by the Barrett-Joyner-Halenda (BJH) method. Specific surface areas were measured with the Brunauer-Emmett-Teller (BET) method. The vacuum point was set at 100 mm Hg. The desorption temperature range was set from 120 °C to 200 °C and held for 400 min at 200 °C. The isothermal curves were collected by 60 data points each. The porosity was calculated by BJH absorption pore volume. Small-angle x-ray scattering (SAXS) measurements

were performed

on a Ganesha

300XL

SAXS-WAXS system (SAXS LAB ApS, Copenhagen/Denmark). The x-ray radiation was generated at 50 kV/0.6 mA from a copper anode with a wavelength of 0.1541 nm. The sample-to-detector distance was set as 1056 mm. The sample powder was placed within a glass capillary suitable for x-ray transmission experiments. 12



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Electrochemical performance test 2032-type coin cells were fabricated to test the electrochemical performance of the samples. A slurry mixture was prepared by dispersing active material, Super P, and PVDF (mass ratio: 8:1:1) in N-methyl pyrrolidone (NMP). The electrodes were prepared by spreading the slurry mixture on a piece of copper foil, followed by drying at 80 °C in vacuum for 12 h. The copper foil was cut into a circle disk with a diameter of 13 mm. The mass loading densities were controlled to be in the range between 1.0 mg cm-2 and 1.5 mg cm-2. The disks were then pressed and dried in an oven at 80 °C for 4 h. Commercial electrolyte (Zhangjiagang Guotaihuarong battery material Co., Ltd) was used. The electrolyte solution was prepared by dissolving 1.0 M LiPF6 in an electrolyte solvent mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:2 v/v). Celgard separator with a thickness of 16 µm was used. The coin cells were assembled in an argon-filled glove box using lithium foil as a counter electrode. The rate performance and cyclic performance of the coin cells were tested using a multichannel Neware Testing System and Land Battery Testing System. The rate performance was measured at the current density sequence of 17.5 mA g-1, 35 mA g-1, 87.5 mA g-1, 175 mA g-1, 350 mA g-1, 875 mA g-1, 1750 mA g-1, 3500 mA g-1, and 17.5 mA g-1 in the voltage range between 3.0 V and 0.01 V (vs. Li/Li+) (five cycles each current density). The cyclic measurement was carried out by cycling at a current density of 35 mA g-1 for initial five rounds, followed by cycles at 175 mA g-1 for further cycles. The specific capacity was calculated against the active material only. The discharge and charge processes were referred to the lithiation and delithiation 13


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processes respectively. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) experiments were carried out on CHI 660 (Shanghai Chenhua instrument Co., Ltd.) Autolab electrochemical work station. The voltage range of the CV test was set between 0.01 V and 3.0 V, with a scanning rate of 0.2 mV/s. The electrochemical impedance spectroscopy (EIS) measurement was performed after 3 cycles at 175 mA g-1 with a frequency range of 105 Hz - 0.01 Hz. The open circuit voltage was set as the testing voltage. The EIS data fitting was conducted with a Zsimdemo software.

Results and Discussions

14



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Figure 1. TEM (a, c) and HRTEM (b, d) images of the TiO2/C (a, b) and LTO/C-SSR (c, d) nanohybrids. Insets in image 1a and 1c: SAED patterns; Insets in image 1b and 1d: local crystal planes in the nanoparticles. The TEM images of the TiO2/C and LTO/C-SSR are shown in Error! Reference source not found.. Error! Reference source not found.(a) shows that the TiO2 nanoparticles are homogeneously embedded in the carbon matrix. Uniform size distribution of 5 nm ± 1.6 nm is recorded. The SAED pattern in Figure 1a indicates that crystalline TiO2 is formed. The HRTEM image of the TiO2/C sample (Figure 1(b)) 15


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displays the (101) crystalline planes of both the anatase and rutile phases, which is consistent with our previously reported results47. Error! Reference source not found.(c) shows that the average size of the lithium titanate nanoparticles is 17 nm ± 5.6 nm. The SAED pattern suggests the existence of crystalline LTO phase in the carbon matrix. Compared to the pristine TiO2 nanoparticles, the size of the LTO nanoparticles is enlarged during the solid-state reaction process52. It seems that the reaction between TiO2 and lithium carbonate combines the local neighboring TiO2 nanoparticles together to form a new LTO nanoparticle. In average, from two to four TiO2 nanoparticles are fused together to form one lithium titanate nanoparticle. The HRTEM image of the LTO/C-SSR sample (Error! Reference source not found.(d)) displays the (111) crystalline plane of the spinel LTO phase. It suggests that crystalline LTO phase is successfully obtained through the in situ solid-state reaction between the TiO2/C nanohybrid and lithium carbonate.

16



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Figure 2. SEM images of the bare carbon (a), TiO2/C (b), LTO/C –SSR (c), LTO/C-CS (d), and commercial LTO (e). The SEM images of the bare carbon, TiO2/C, LTO/C–SSR, LTO/C-CS and commercial LTO are presented in Error! Reference source not found.. Error! Reference source not found.(a) shows that the bare carbon sample is of irregular shape with a size in the sub-micrometer range. Error! Reference source not found.(b) and Error! Reference source not found.(c) displays the morphology change before and after solid-state reaction. Before reaction, agglomerated particles 17


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with a broad size distribution exist in the pristine TiO2/C nanohybrid. However, after the solid-state reaction, the small particles disappear and only large particles are present in the LTO/C-SSR sample. The shape of the large particles is more regular than the TiO2/C powder because it reflects the shape of the crystalline Li4Ti5O12 nanocrystals. The results indicate that crystallization process takes place during the reaction53. The small TiO2/C particles are consumed to form large sized Li4Ti5O12/C particles induced by the reaction between TiO2 and lithium carbonate. The image of the LTO/C-CS is exhibited in Error! Reference source not found.(d). The average particle size is comparable to that of the bare carbon. It indicates that the LTO particles are simply composited with carbon instead of an encapsulation by the carbon matrix. Error! Reference source not found.(e) shows the morphology of the commercial Li4Ti5O12. The Li4Ti5O12 primary particles with regular shape are observed and the average size is of around 800 nm. Further agglomeration of the primary particles forms secondary particles with the size of a few micrometers. It can be seen that the morphology of the pristine commercial LTO particles is quite different from that of the LTO/C-CS sample. It suggests that the surface of the LTO particles is effectively coated by the carbon component. The morphologies of the samples are further characterized by particle size distribution (PSD) measurements. According to the Figure S1, the PSD profiles indicate that the observed sizes of all samples are mainly in the range of between 100 nm and around 1 µm. The bare carbon and TiO2/C samples exhibit very similar size distributions. Thus, the sub-micrometer size feature of the particles is mainly 18



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determined by the carbon matrix. With the conversion from TiO2 to the LTO phase, the particle size distribution of the LTO/C-SSR sample is shifted to a larger size range, but still below 1 µm, which indicates that the in situ formation of the LTO particles and reduced carbon amount enlarge the particle size during the solid-state reaction process. Moreover, it can be seen that the particle size range of the LTO/C-SSR sample is approaching the size range of the commercial pure LTO sample. However, unlike the commercial LTO sample, the LTO/C-SSR sample is composed of tiny LTO nanoparticles, which are embedded in the sub-micrometer sized carbon matrix. The size of the LTO/C-CS sample is decreased as compared to the bare commercial LTO sample due to the ball milling effect. Consequently, the simple mechanical mixing between LTO and carbon does not help to protect the structure of the LTO particles because the interface between the LTO and carbon matrix is poorly controlled.

19


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Figure 3. (a) XRD patterns of the LTO/C-SSR (black), LTO/C-CS (red), and TiO2/C (blue) samples (residual phase in the LTO/C-SSR sample, square: anatase, circle: rutile); (b) TGA curves of the LTO/C-SSR (black), LTO/C-CS (red) and TiO2/C (blue) samples; (c) Raman spectroscopy of the TiO2/C (blue) and LTO/C-SSR (black); (d) Nitrogen adsorption/desorption isotherm and pore size distribution (inset) curves of the LTO/C-SSR (black), LTO/C-CS (red) and TiO2/C (blue) samples. The XRD pattern of the control sample (LTO/C-CS) shown in Figure 3(a) is in good accordance with standard Li4Ti5O12 pattern (PDF No. 49-0207). It suggests that the composition with carbon matrix and further ball-milling processes do not significantly change the crystallinity of the LTO particles. The XRD pattern of the LTO/C-SSR nanohybrid is almost identical to that of the LTO/C-CS sample. It indicates that crystalline Li4Ti5O12 in spinel phase has been successfully synthesized through the reaction between the TiO2/C and lithium carbonate. Besides the major phase of the Li4Ti5O12 within the LTO/C-SSR sample, tiny amounts of residual anatase and rutile phases still exist, as indicated by the weak diffraction peaks. The pristine TiO2 nanoparticle is well embedded in the carbon matrix. As a result, the diffusion process of the lithium carbonate molten salt to approach the TiO2 nanoparticle surface is a critical step governing the synthesis of the lithium titanate. The conversion from the TiO2 to Li4Ti5O12 is slightly suppressed because of the continuous carbon matrix. However, it is still reasonable to say that the reaction is almost fully complete because the residual TiO2 phase is minimum compared to the as-synthesized lithium titanate phase from the XRD results. 20



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The contents of the TiO2 in the TiO2/C nanohybrid, LTO within the LTO/C-SSR and LTO/C-CS samples are revealed by TGA (Error! Reference source not found.(b)). It is found that the residual mass ratios of the TiO2/C, LTO/C-SSR, and LTO/C-CS are 60 %, 71 %, 68 % respectively. The theoretical content of LTO within the LTO/C-SSR sample can be estimated by combining the mass content of TiO2 in the TiO2/C nanohybrid and the stoichiometric chemical reaction equation54. Consequently, the theoretical mass ratio of LTO is expected to be 67.9 %, which is almost identical to the experimental result (71 %) within the resolution limit of the TGA measurement. It suggests that the original TiO2 within the carbon matrix is nearly fully converted to Li4Ti5O12, which is consistent with the XRD results. Furthermore, the starting mass loss temperature of the LTO/C-CS sample is higher than those of the TiO2/C and LTO/C-SSR samples. The reason for the different starting mass loss temperature originates from the specific surface areas of the samples, which will be addressed in following sections. The structure of the carbon matrix is investigated by the Raman spectroscopy. Error! Reference source not found.(c) shows that both D band (1345 cm-1) and G band (1601 cm-1) exist in the LTO/C-SSR and TiO2/C samples. It indicates the existence of both ordered and disordered carbon in the carbon matrix55. The integrative intensity ratio of the D band over the G band (ID/IG) of the LTO/C-SSR sample is 1.52, which is lower than that of the TiO2/C nanohybrid (2.40). It indicates that the existence of graphitized carbon is enhanced during solid-state reaction47. A carbon dioxide activation process may take place at the solid-state reaction 21


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temperature of 800 °C. During the solid-state reaction process, CO2 is generated in situ by the decomposition of lithium carbonate at 800 °C. The decent amount of carbon dioxide further reacts with the carbon matrix. The amorphous carbon is more favorable to react with CO2 compared to the graphitized carbon domain because of its relatively high specific surface area56. To prove the effect of CO2 activation, an additional thermal treatment at 800 °C for 12 hours under argon atmosphere was carried out for the TiO2/C sample. It is found that the integrative intensity ratio of the ID/IG does not change significantly (before 2.40; after calcination 2.38) based on the Raman spectroscopy results (Figure S2 in supporting information). Thus the additional high temperature treatment under argon atmosphere does not modify the structure of the carbon matrix significantly. For the LTO/C-SSR sample, additional three peaks at 228 cm−1, 414 cm−1, and 676 cm−1 are also displayed. These three peaks are in good agreement with the typical Raman features of the spinel LTO phase. Specifically, the peak at 228 cm−1 is ascribed to the bending vibration of the O-Ti-O bond. The peak at 676 cm−1 originates from the vibration of the Ti-O bond in TiO6 octahedral. And the peaks located at around 414 cm−1 are assigned to the stretching bending vibration of the Li-O bond57-59. The porosity of the samples is revealed by the Brunauer Emmett Teller (BET) test. Figure 3(d) shows the nitrogen adsorption/desorption curves and pore size distribution profiles (inset) of the LTO/C-SSR, LTO/C-CS and TiO2/C samples. It can be seen that all the three samples possess type IV absorption-desorption isotherm with a H3 hysteresis loop at the range of 0.8 P/P0 - 1.0 P/P0.60. The specific area of the 22



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LTO/C-SSR (257.6 m2/g) is higher than that of the TiO2/C (209.9 m2/g), while the LTO/C-CS has the lowest specific area of 7.4 m2/g. Broad pore size distribution is presented in all of the three samples. The LTO/C-CS has the lowest volume fraction (1.3 %). Compared to the TiO2/C, the LTO/C-SSR has a higher pore volume fraction (18.4 % vs. 3.7 %). The increase of porosity is likely due to the carbon dioxide activation process during the solid-state reaction. The in situ generated CO2 reacts with carbon to form gaseous carbon monoxide, where pores are formed within the carbon matrix, leading to increased specific surface area and pore volume fraction. Because of the lowest specific surface and pore volume fraction, the LTO/C-CS sample displays the highest starting mass loss temperature in the TGA profile. Compared to the TiO2/C sample, the starting mass loss temperature of the LTO/C-SSR sample is slightly decreased because of the increased specific surface area and pore volume fraction from the solid-state reaction process. The oxidation of carbon is a heterogeneous reaction that happens at the solid-gas interface between carbon and oxygen. Along with increasing temperature, the interaction between carbon and air is enhanced. It is further promoted by the increased contact area between carbon and oxygen due to high specific surface area and pore volume fraction. The initial starting mass loss temperature is defined by the temperature where a detectable mass loss is observed. As a result, more carbon loss is observed at a relatively lower temperature in the sample with a high specific surface area and pore volume fraction, leading to a decreased starting mass loss temperature.

23


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Table 1. Tap densities of the TiO2/C, LTO/C-SSR, LTO/C-CS, and Commercial LTO. TiO2/C Tap density (g/cm3)

1.08±0.02

LTO/C-SSR

LTO/C-CS

Commercial LTO

1.78±0.08

0.98±0.12

1.23±0.03

Tap density is one of the critical factors for practical application of the Li4Ti5O12 anode. Here the tap densities of the TiO2/C, LTO/C-SSR, LTO/C-CS, commercial LTO are measured. As shown in Table 1, the tap density of the starting material TiO2/C is 1.08 g/cm3. It is relatively high because the tiny TiO2 nanoparticles are homogeneously embedded in the continuous carbon matrix. After the solid-state reaction, the tap density is further increased to 1.78 g/cm3 with the Li4Ti5O12/C nanohybrid. As shown by the SEM images in Figure 1, every two to four TiO2 nanoparticles are fused together to form one Li4Ti5O12 nanoparticle. Furthermore, the number of the small TiO2/C nanohybrid particles is also significantly reduced. Both these two reasons make contributions to lift the tap density. It is noticing that the tap density is even higher than that of the commerical Li4Ti5O12 powder (1.23 g/cm3). The high tap density is ascribed to the hierarchical micro-/nanohybrid structure feature, where the LTO nanoparticles are well embedded in the micrometer sized continuous carbon matrix. Compared to the LTO/C-SSR sample, the commercial LTO powder has a less dense structure, leading to inferior tap density. The LTO/C-CS has a tap density of 0.98 g/cm3, which is sightly lower than that of the commerical LTO. The carbon component has a smaller density than that of the LTO. Besides, the composition of the pristine LTO particles with the carbon matrix generates less dense structure with the LTO/C-CS sample. The size reduction of the LTO/C-CS due to ball milling may also 24



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contribute to the slightly reduced tap density.

Figure 4. Small angle x-ray scattering (SAXS) profiles of the TiO2/C nanohybrid (red) and LTO/C-SSR (black). Characteristic scattering features are present in the SAXS data (Figure 4). To get more details, the obtained cuts are fitted using a sphere form factor with a Gaussian distribution function. For the TiO2/C system, a pronounced peak is observed, which implies a homogeneous dispersion of TiO2 nanoparticles inside the carbon matrix. With incorporation of the lithium compound, the scattering peak shifts to a lower qy position, meaning the nanoparticles’ size increases from 5 ± 2 nm to 18 ± 7 nm. Meanwhile, the peak of the LTO system becomes broader and weaker, which suggests a larger size distribution compared with the pristine TiO2/C nanohybrid.

25


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Figure 5. Representative discharge/charge profiles of the LTO/C-SSR (a) and LTO/C-CS (b) samples of different cycles with the current density of 35 mA g-1 for the first 5 cycles and 175 mA g-1 for the remaining 1000 cycles. Black: 1st cycle, blue: 2th cycle, red: 100th cycle, and green: 500th cycle. The discharge and charge curves of the LTO/C-SSR and LTO/C-CS are shown in Error! Reference source not found.(a) and Error! Reference source not found.(b) with the current density of 35 mA g-1 for the first 5 cycles and 175 mA g-1 for the remaining 1000 cycles. Both samples show moderate initial columbic efficiency, 26



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which is due to the formation of SEI and irreversible lithiation process in the carbon matrix. The LTO/C-SSR shows an initial capacity of 440 mA h

g-1, which is 140 mA

h g-1 higher than that of the LTO/C-CS. At the second cycle, the capacity of the LTO/C-SSR decreases to 300 mA h g-1. while the capacity of the LTO/C-CS declines to 200 mA h g-1. From the 100th to the 500th cycle, the LTO/C-SSR sample always shows higher capacity than that of the LTO/C-CS sample (LTO/C-SSR: 100th : 200 mA h g-1, 500th : 185 mA h g-1, LTO/C-CS: 100th : 130 mA h g-1, 500th : 130 mA h g-1). Voltage plateaus from the lithiation/delithiation processes of the crystalline LTO are observed in both curves at around 1.55 V, indicating a typical two phase reaction mechanism between Ti3+ and Ti4+61. Plateau fading is observed in both samples with increasing cycles. The plateau of the LTO/C-SSR is shortened compared with the bare commercial LTO anode (Figure S3 in supporting information). The reason is ascribed to the amorphous structure nature of the LTO in the LTO/C-SSR sample and the existence of carbon matrix62. The plateau of the LTO/C-CS is also narrowed with respect to that of the commercial LTO. As shown in Figure S4 in the supporting information, the decreased carbon content elongates the plateau, which confirms the assumption that the carbon content modifies the feature of the plateau. It is well recognized that the relatively high lithiation/delithiation potentials of the conventional LTO reduces the overall voltage and energy density output63. The shortened plateau achieved in this work differs from typical voltage profile of the bare crystalline LTO. However, the inclined feature of the discharge profile may be able to contribute to improve the overall energy output when used in full lithium-ion batteries. 27


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Figure 6. Cyclic voltammetry curves of the LTO/C-SSR (a) and LTO/C-CS (b) samples. Black: first cycle, red: second cycle, and blue: third cycle. The cyclic voltammetry (CV) curves of the first three cycles of the LTO/C-SSR and LTO/C-CS are shown in Error! Reference source not found.(a) and Error! Reference source not found.(b). Both of the samples show the formation of solid electrolyte interface (SEI) layer. The patterns of the second cycle and the third cycle are almost identical. It indicates a good stability of the SEI layer. An oxidation peak at 1.55 V (1.53 V) and reduction peak at 1.61 V (1.65 V) are shown in Figure 6(a) and 28



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Figure 6(b) respectively, which are ascribed to the oxidation/reduction reactions of the Ti3+/Ti4+ couple in the cubic structure58. Figure 6(b) shows that the LTO/C-CS has a weak peak at 0.6 V, which is attributed to the transition from Li7Ti5O12 to Li8.5Ti5O1264. The peaks are consistent with the discharge/charge curves. The CV curves of the LTO/C-SSR show a weak peak at around 2.1 V, which is due to the lithium intercalation of the residual TiO2 phase65.

Figure 7. (a) Cyclic performance of the LTO/C-SSR (discharge: blue, charge: magenta) and LTO/C-CS (discharge: red, charge: black), coulombic efficiency of the LTO/C-SSR (purple) and LTO/C-CS (green), current density: 35 mA g-1 for the first 5 29


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cycles and175 mA g-1 for the remaining 1000 cycles; (b) Rate performance of the LTO/C-SSR (discharge: blue, charge: magenta) and LTO/C-CS (discharge: red, charge: black). The cyclic performance is shown in Error! Reference source not found.(a) with the current density of 35 mA g-1) for the first 5 cycles and 175 mA g-1 for the remaining 1000 cycles. The LTO/C-SSR shows a better cyclic performance compared to the LTO/C-CS. The LTO/C-SSR possesses a reversible capacity of 203 mA h g-1 after the initial five cycles at 35 mA g-1, which is around 20 % higher than that of the LTO/CS (173 mA h g-1). Regarding the remaining cycles at 1 C, the LTO/C-SSR nanohybrid shows an average capacity advantage of 48 mA h g-1 over the LTO/C-CS sample (178 mA h g-1 vs. 130 mA h g-1). After 1000 cycles, the reversible capacity of the LTO/C-SSR remains at 185 mA h g-1 with a capacity retention of 91 %. On the contrary, the reversible capacity of the LTO/C-CS is 135 mA h g-1, corresponding to a capacity retention of 78 %. Excellent coulombic efficiencies of over 99.1 % are achieved with the LTO/C-SSR sample. It is comparable to that of the LTO/C-CS sample containing the commercial Li4Ti5O12 particles. Besides the good cyclic performance, the LTO/C-SSR also shows outstanding performance at all rates (Figure 7(b)). At a current density of 35 mA g-1, the LTO/C-SSR shows an average capacity of 325 mA h g-1, while the LTO/C-CS shows a relatively low capacity of 220 mA h g-1. The capacity decreases with the initial cycles at 35 mA g-1 because the structure is still not fully stabilized upon lithiation/delithiation. However, after cycling at 35 mA g-1 for five cycles, the capacity is stablized significantly. The charge capacities of the 30



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LTO/C-CS are 244 mA h g-1, 195 mA h g-1, 166 mA h g-1, 147 mA h g-1, 128 mA h g-1, 67 mA h g-1, 40 mA h g-1, and 18 mA h g-1 at 17.5 mA g-1, 35 mA g-1, 87.5 mA g-1, 175 mA g-1, 350 mA g-1, 875 mA g-1, 1750 mA g-1 and 3500 mA g-1 respectively. The charge capacities of the LTO/C-SSR are 316 mA h g-1, 245 mA h g-1, 213 mA h g-1, 185 mA h g-1, 160 mA h g-1, 117 mA h g-1, 100 mA h g-1 and 80 mA h g-1 at the same above-mentioned rates respectively, which are 30 %, 26 %, 28 %, 26 %, 25 %, 75 %, 150 % and 344 % higher than those of the LTO/C-CS. With increasing current densities, the gap between the two samples becomes larger. At the highest rate of 20 C, the difference becomes biggest, which indicates that the LTO/C-SSR possesses a better performance at high rates. The superior performance at high current densities is attributed to the structure feature of the LTO/C-SSR sample, where super-small sized LTO particles (around 17 nm in diameter) are homogeneously embedded in the continuous carbon matrix. It effectively shortens the lithium ion transportation path compared to the LTO/C-CS sample, where the average size of the LTO particles is in the sub-micrometer range. The electrochemical performance of the pure commercial LTO is also measured (Figure S3, S5), and compared with both the LTO/C-SSR and LTO/C-CS. It is as expected that the pure commercial LTO exhibit a better initial coulombic efficiency than the LTO/C-CS sample due to the absence of the hard carbon component. Besides, the cyclic performance of the pure commercial LTO is also better than that of the LTO/C-CS. Nevertheless, the cyclic performance of the pure commercial LTO is still worse than that of the LTO/C-SSR. In details, the LTO/C-SSR exhibits a higher 31


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reversible capacity than that of the pure commercial LTO in the voltage range between 0.01 V and 3.0 V (185 mA h g-1 vs. 160 mA h g-1). The improved cyclic performance may come from the following aspects: Firstly, the super-small size feature of the LTO particles in the LTO/C-SSR sample enhances the performance compared to the large sized LTO particles in the pure commercial LTO sample. Secondly, a good interface control between the super-small sized LTO particles and carbon matrix is achieved with the LTO/C-SSR because both the LTO and carbon matrix are both in situ formed. Thirdly, the presence of the carbon component also contributes to the capacity. Nevertheless, considering that the LTO/C-SSR has similar carbon content to the LTO/C-CS, the impact of the capacity contribution from the carbon component may be not so essential. Regarding the rate performance, the LTO/C-SSR shows a better performance than the commercial pure LTO at the current densities no higher than 350 mA g-1. However, with further increased current densities, the commercial LTO exhibits a better performance. The reason may originate from the carbon component, which does not possess a good rate capability at high current densities. Further reduction carbon content may be able to improve the rate performance of the LTO/C-SSR.

32



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Figure 8. Nyquist plots (circle) and fitted results (line) of the LTO/C-SSR (black) and LTO/C-CS (red) samples. Inset: circuit model used for EIS data fitting. The electrochemical behavior is further studied by electrochemical impedance spectroscopy (EIS). Error! Reference source not found. shows the Nyquist plots of both the LTO/C-SSR and LTO/C-CS samples. It suggests that the charge transfer rate of the LTO/C-SSR nanohybrid is higher than that of the LTO/C-CS sample, which is reflected by the diameter of a semicircle in the medium frequency region57. According to the equivalent circuit (inset), the total resistance (Rcell) of the lithium-ion cell is mainly composed of the bulk resistance of lithium ion diffusion (Rs), solid electrolyte interface resistance (Rsei) and charge-transfer resistance (Rct)66. The fitting data of the impedance spectra is listed in Table 2. Particularly, the Rct value of the LTO/C-SSR is lower than that of the LTO/C-CS, which is attributed to the fact that the small-sized LTO nanoparticles are homogeneously embedded in the carbon matrix67. The sum of 33


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the total resistance of the LTO/C-SSR is also lower than that of the LTO/C-CS. Both the experimental and fitting results of the EIS profiles prove that the LTO/C-SSR has an enhanced electrochemical kinetics and better rate performance than that of the LTO/C-CS sample. Table 2. Representative EIS fitting data of the LTO/C-SSR and LTO/C-CS samples. SAMPLE

Rs

Rsei

Rct

Rcell (Ω)

LTO/C-SSR

5.332

24.01

22.45

51.79

LTO/C-CS

9.856

36.86

41.52

88.24

Conclusions In summary, hierarchical LTO/C nanohybrid materials with super-small LTO nanoparticles (around 17 nm in diameter) homogeneously embedded in the continuous micrometer-sized carbon matrix has been successfully synthesized in a scalable way. Difunctional methacrylate resin monomers are used as solvent and carbon source for the synthesis of the TiO2/C, which is further used as a starting material to prepare the LTO/C nanohybrid via an in situ solid-state reaction with lithium carbonate. Because of the unique structure feature of the LTO/C nanohybrid, the electrochemical performance including cyclic stability and rate performance is significantly improved compared to the control sample of the LTO/C composite containing the commercial LTO particles. After 1000 cycles at 1 C, the reversible capacities of the LTO/C sample is still maintained at 185 mA h g-1 with a capacity retention of 91 %, where the control sample only exhibits a reversible capacity of 135 mA h g-1 with a capacity retention of 78 %. The as-synthesized LTO/C nanohybrid 34



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exhibits a better rate performance than that of the control sample, particularly at large current densities such as 875 mA g-1, 1750 mA g-1, and 3500 mA g-1. The LTO/C nanohybrid also shows a good tap density of 1.78 g/cm3 because of its unique hierarchical structure. The improved performance is ascribed to the super-small size feature of the LTO nanoparticles and the homogeneous embedding of the LTO nanoparticles within the continuous carbon matrix. The work presented here provides a new way to improve the electrochemical performance of the LTO anode. Further work on optimization of the structure and performance of the LTO/C nanohybrid is in progress and will be published in the near future.

Acknowledgement This research is funded by the National Key R&D Program of China (Grant No. 2016YFB0100100), open project of the Beijing National Laboratory for Molecular Science

(20140138),

the

CAS-EU

S&T

cooperation

partner

program

(174433KYSB20150013) and Key Laboratory of Bio-based Polymeric Materials of Zhejiang Province. S.X. acknowledges the China Scholarship Council (CSC) and P.M-B acknowledges funding by the International Research Training Group 2022 Alberta/Technical University of Munich International Graduate School for Environmentally Responsible Functional Hybrid Materials (ATUMS).

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Particle size distribution (PSD) profiles of the LTO/C-SSR, LTO/C-CS, TiO2/C, bare carbon, and commercial LTO, Raman spectroscopy of the pristine 35


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TiO2/C and TiO2/C with an additional thermal treatment at 800 °C for 12 hours under argon atmosphere, representative discharge/charge profiles of the pure commercial LTO and LTO/C nanohybrid with a lower carbon content of 15 %, cyclic and rate performance, and coulombic efficiency of the pure commercial LTO sample.

Notes The authors declare no competing financial interest.

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Carbon Nanohybrid with

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