A Facile Supercritical Alcohol Route for Synthesizing Carbon Coated

Article pubs.acs.org/JPCC

A Facile Supercritical Alcohol Route for Synthesizing Carbon Coated Hierarchically Mesoporous Li4Ti5O12 Microspheres Agung Nugroho,† Kyung Yoon Chung,‡ and Jaehoon Kim*,§,∥ †

Clean Energy Research Center, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea ‡ Center for Energy Convergence, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea § School of Mechanical Engineering, Sungkyunkwan University, 2066, Seobu-Ro, Jangan-Gu, Suwon, Gyeong Gi-Do 440-746, Republic of Korea ∥ SKKU Advanced Institute of Nano Technology (SAINT), 2066, Seobu-Ro, Jangan-Gu, Suwon, Gyeong Gi-Do, 440-746, Republic of Korea S Supporting Information *

ABSTRACT: Hierarchically mesoporous Li4Ti5O12 (LTO) microspheres with a conductive layer coating are considered one of most promising structures to enhance high-rate performance as well as to retain high volumetric energy density. Herein, hierarchically mesoporous LTO microspheres with carbon coating are synthesized through a simple, supercritical alcohol route. The influence of varying synthesis conditions including concentration, solvent, reaction time, and calcination on the physicochemical and electrochemical properties of the LTO microspheres is carefully examined. Mesoporous LTO are synthesized at a short reaction time of 15 min in supercritical alcohols without using any structure-directing chemicals or templates. The use of supercritical methanol (scMeOH) results in a higher degree of surface modification, which retards the crystal growth more effectively when compared to supercritical ethanol (scEtOH) and supercritical isopropanol (scIPA). During heat treatment under a 5% H2/Ar condition, carbonization of the organic groups attached to the surface of LTO effectively restricts particle growth and reduces the surface Ti4+ to Ti3+. At rapid charge−discharge rates of >8 C, or at long cycles of >50, the discharge capacities of the carbon-coated LTO are ordered scMeOH > scEtOH > scIPA. The higher degree of surface modification from scMeOH results in LTO with higher carbon content, higher Ti3+ content, larger BET surface area, smaller average pore size, and larger porosity when compared to scEtOH and scIPA, which resulted in better electrochemical performance. The formation mechanism of the unique, hierarchically mesoporous structure in the supercritical alcohols is also discussed.

1. INTRODUCTION

However, LTO has extremely low electronic conductivity (∼10−13 S cm−1)8 and low Li+ diffusion coefficient (10−9−10−13 cm2 s−1).9 As a result, coarse LTO particles synthesized using a conventional solid-state method often exhibit poor rate capability,10,11 which limits wider large-scale Li-ion battery applications. In the past decade, the rate capability of LTO has improved significantly through the employment of nanostructured morphology and/or conductive layer coating.12−20 Among the various types of nanostructures (including nanotubes, nanowires, nanoparticles, and nanosheets), hierarchical porous structures, in which primary nanosized particles loosely aggregates and form secondary microshpere porous materials, are considered among the most promising in the enhancement

Recently, Li-ion batteries have received great attention as power sources for large-scale applications such as hybrid electric vehicles and energy storage system.1,2 Commercial use of largescale Li-ion batteries is still limited due to major issues in safety, cost, cyclability, and rate performance. As a potential alternative anode material to carbon, spinel lithium titanium oxide (Li4Ti5O12, LTO) has been widely studied and in some area, LTO is commercially used.3−5 Owing to its high charge/ discharge voltage of around 1.55 V versus Li/Li+, the formation of lithium dendrite can be avoided, and degradation of carbonate-based electrolytes on the surface of the LTO electrode would not take place during the charge/discharge processes. In addition, volume changes during Li+ insertion/ deinsertion are extremely low (less than 1%).6,7 As a result, LTO can entirely eliminate potential safety issues and exhibit excellent cycling performance.7 LTO also has the advantage of low fabrication cost due to the low cost of starting materials. © 2013 American Chemical Society

Received: October 6, 2013 Revised: November 21, 2013 Published: December 23, 2013 183

dx.doi.org/10.1021/jp4099182 | J. Phys. Chem. C 2014, 118, 183−193

The Journal of Physical Chemistry C

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Table 1. LTO Synthesis Conditions, BET Surface Areas, Lattice Parameters, and Crystallites Sizes

a

sample code

TTIP conc (M)a

LiOH conc (M)

reaction time

scMeOH-0.3M-5m scMeOH-0.3M-15m scMeOH-0.3M-1h scMeOH-0.3M-2h scMeOH-0.1M-15m scMeOH-0.2M-15m scMeOH-0.3M-15m scMeOH-0.5M-15m scMeOH-1M-15m scEtOH-0.3M-15m scIPA-0.3M-15m scMeOH-0.3M-15m-H2-600C

0.3 0.3 0.3 0.3 0.1 0.2 0.3 0.5 1.0 0.3 0.3 0.3

0.24 0.24 0.24 0.24 0.08 0.16 0.24 0.24 0.80 0.24 0.24 0.24

5 min 15 min 1h 2h 15 min 15 min 15 min 15 min 15 min 15 min 15 min 15 min

scEtOH-0.3M-15m-H2-600C

0.3

0.24

15 min

scIPA-0.3M-15m-H2-600C

0.3

0.24

15 min

scMeOH-0.3M-15m-Air-600C scEtOH-0.3M-15m-Air-600C scIPA-0.3M-15m-Air-600C

0.3 0.3 0.3

0.24 0.24 0.24

15 min 15 min 15 min

BET surface area (m2 g−1)

lattice parameter (Å)

crystallite size (nm)b

h in

117.8 119.2 109.2 74.4 117.4 115.2 119.2 102.3 80.4 110.2 105.7 60.2

8.3419 8.3435 8.3487 8.3491 8.3439 8.3438 8.3435 8.3431 8.3539 8.3446 8.3461 8.3686

6.4 7.4 9.3 10.6 6.5 6.8 7.4 8.7 9.6 7.5 8.5 32.2

h in

55.1

8.3676

44.7

h in

41.2

8.3621

49.6

h in air h in air h in air

47.8 43.2 40.1

8.3558 8.3554 8.3552

69.4 83.5 96.0

calcination condition no no no no no no no no no no no 600 °C, 12 Ar/H2 600 °C, 12 Ar/H2 600 °C, 12 Ar/H2 600 °C, 12 600 °C, 12 600 °C, 12

[LiOH]/[TTIP] = 0.8. bEstimated using Scherrer’s equation and (111)/(400) diffraction patterns.

metal oxide active materials, including LiCoO2,43,44 LiFePO4,45−50 LiMnPO4,48 LiMn2O4,51 LiNi1/3Co1/3Mn1/3O2,52 Li2MSiO4 (M = Fe, Mn),53 Zn2SnO4,54 and Li4Ti5O1218,55−57 have been synthesized in supercritical fluids for use in Li ion batteries. In 2010, the first commercial-scale production of LiFePO4 in supercritical water was implemented in Korea.37 This work describes a template-free, simple, and green approach to the synthesis of hierarchical mesoporous LTO microspheres with carbon coating in supercritical alcohols. Instead of using structure-directing chemicals or external carbon sources for the synthesis of carbon-coated mesoporous LTO, we used supercritical alcohols with varied carbon lengths as reaction media to form mesoporous structures, and as carbon sources. The effects of three supercritical alcohols (supercritical methanol, scMeOH, Pc = 8.1 MPa, Tc = 239 °C; supercritical ethanol, scEtOH, Pc = 6.1 MPa, Tc = 241 °C; and supercritical isopropanol, scIPA, Pc = 4.8 MPa, Tc = 235 °C), reaction time, and concentration on the properties of LTO were studied in detail. In addition, the effects of calcination conditions (temperature, air, Ar/H2), carbon content, carbon structure, and Ti3+ content on the LTO structure are described. Mesoporous LTO particles with the carbon coating synthesized under different conditions were investigated in a half cell. The electrochemical properties in high current rate up to 50 C and excellent cycling performance (1C) up to 200 cycles of LTO sample synthesized under various supercritical alcohol and calcination conditions are presented. Lastly, the mesoporous formation mechanism in supercritical alcohol is discussed.

of rate capability and in maintaining high volumetric energy density.12,14,16,19,21−30 The mesoporous morphology of hierarchical porous structures offers the advantages of both nanostructure morphology, such as fast Li+ and e− diffusion into the LTO structure and large contact surface area between electrode and electrolyte, as well as microsphere structure, such as high volumetric density, ease of processing, and stability. In addition, the porous structure offers good penetration of the electrolytes, which can lead to good contact with the active material. The conductive coating, such as carbon, surface nitration, and Ti3+ layer have been also shown to enhance the electrochemical properties of LTO.12,13,30−34 Recently, conductive layer-coated LTO nanoparticles were achieved simultaneously through a process in which both “nanosize” and “double surface conductive modification based on Ti3+ and carbon” from polyaniline33 or pitch carbon sources.2934 The carbonization on the particle surface can effectively restrict LTO particle growth during calcination, which allows the LTO to remain small even after extended high-temperature heat treatment. The carbonization also reduced some of the surface Ti4+ to Ti3+ via a carbothermal reduction mechanism. For large scale application, the synthesis method should be easy to scaleup, have low production and material costs, and require less toxic chemical intensive and less complicated procedures. Given these limitations, the development of more reliable, simpler, faster, and ecofriendly process of preparing mesoporous LTO with carbon coating remains a great challenge. Nanomaterial synthesis in supercritical fluids (SCFs) has been widely studied for the beneficial properties of using SCFs as the reaction media.35−41 The unique properties of SCFs such as high diffusivity of reactants in the supercritical medium, fast reaction rate, and high supersaturation ratio of reaction intermediates and nontoxicity make them promising alternatives for conventional solid-state and solution-based process. In addition, the nanomaterial synthesis in SCFs is ecofriendly, simple, fast, and easily scalable.37 Various types of metal and metal oxide nanoparticles have been prepared in SCFs.35,36,38,41,42 More recently, nanosized, well-crystalline

2. EXPERIMENTAL SECTION 2.1. Materials. Titanium tetraisopropoxide (TTIP, purity ≥ 97 wt %) and lithium hydroxide monohydrate (LiOH·H2O, purity ≥98 wt %) were purchased from Sigma-Aldrich (St. Louis, MO). Distilled and deionized (DDI) water were prepared using a Milli-Q ultrapure water purification system equipped with a 0.22 μm filter (Billerica, MA). Methanol, ethanol, and isopropyl alcohol (HPLC grade) were purchased from J. T. Baker (Phillipsburg, NJ). Poly(vinylidene difluoride) 184



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polypropylene membrane with a thickness of 25 μm (Celgard 2500, Celgard LLC., Charlotte, NC) was used as the separator. Li metal was used as the counter electrode. All the cells were fabricated in a glovebox filled with high purity argon gas. The charge and discharge behavior was monitored using a model WBCS 3000 battery test system (WonATech Corp., Korea) at the voltage range of 1.0−2.5 V (vs Li/Li+) at room temperature. The cyclability was performed at a rate of 1 C (1 C = 175 mA g−1) for up to 200 cycles. The C-rate was varied from 0.1 to 50 C for rate performance measurements.

(PVDF, Kureha Chemical Industry Co., Tokyo, Japan), acetylene black (DENKA Co. Ltd., Tokyo, Japan), and 1methyl-2-pyrrolidinone (NMP, purity ≥98 wt %, Alfa-Aesar, Ward Hill, MA) were used as received. 2.2. Synthesis of LTO. A known amount of LiOH·H2O was dissolved in 50 mL of each alcohol (methanol, ethanol, or IPA), and a known amount TTIP was added to the alcohol solution. After that, the entire solution was stirred for 2 h, and 3.6 mL of the feed solution was transferred to a high-pressure tube reactor. The inner volume of the reactor was 11 mL. The reactor was then soaked in a molten salt bath set at 400 °C. Approximately 1 min was required to reach a reaction temperature of 400 °C. After the reaction was complete, the reactor was quenched in a cold water bath. Finally the mixture was washed and purified in three cycles of decantation and centrifugation. The powder was then dried in a vacuum oven at 80 °C for 6 h. The LTO powder was calcinated at temperatures ranging from 500 to 700 °C for 12 h in, in an air flow or an Ar/ 5% H2 flow, at 100 mL/min. The synthetic conditions using different alcohols, temperatures, concentrations, and calcination are listed in Table 1. The as-synthesized LTO samples are designated according to solvent, concentration, reaction time, and calcination temperature. For example, scMeOH-0.1M-5m indicates LTO synthesized in supercritical methanol at a TTIP concentration of 0.1 M and a reaction time of 5 min, and scEtOH-0.3M-15m-H2-600C indicates LTO synthesized in supercritical ethanol at a TTIP concentration of 0.3 M and a reaction time of 15 min followed by calcination at 600 °C in an Ar/H2 flow. 2.3. Particle Characterization. The crystal structures of the samples were characterized using an X-ray diffractometer (XRD, Rigaku RINT2000, Tokyo, Japan) with Cu Kα radiation at 40 kV and 50 mA. The morphology of the synthesized materials was analyzed by using a Hitachi S-4100 field emission scanning electron microscope (FE-SEM, Tokyo, Japan) and a Tecnai-G2 high-resolution transmission electron microscope (HR-TEM, FEI Co. Ltd., Hillsboro, OR). The organic functional groups present on the LTO surfaces were characterized using a Fourier transform infrared (FT-IR) spectrometer (NICOLET 380, Thermo Electron Co., Somerset, NJ). The thermal properties of the samples were measured using a DuPont Instruments TGA 2950 thermal gravimetric analyzer (TGA). The carbon content was obtained by elemental analysis using a FLASH 2000 Series CHNS-O analyzer (Thermo Scientific, Waltham, MA, USA). Nitrogen adsorption/desorption measurements were performed with a Belsorp-mini II apparatus (BEL Inc., Osaka, Japan) to determine Brunauer−Emmett−Teller (BET) surface areas. Xray Photoelectron spectroscopy (XPS) was performed using a PHI 5000 VersaProbe (ULVAC-PHI, Chanhassen, MN) spectrometer. The Raman spectra were obtained using a Nicolet Almega XR Dispersive Raman spectrometer (Thermo Scientific, Waltham, MA). 2.4. Electrochemical Characterization. The working electrodes of the Li4Ti5O12 samples were prepared by first mixing LTO sample, acetylene black and PVDF dispersed in NMP in a weight ratio of 87: 7: 6. The slurry was then cast on a Cu foil and dried at 80 °C in vacuum for 24 h to remove the solvent. The electrode film was punched into 15 mm-diameter discs (area of 1.77 cm2) and weighed. The electrolyte used was 1 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethylmethyl carbonate (EMC) solvent (volume ratio of EC/DMC/EMC= 1:1:1). A microporous

3. RESULTS AND DISCUSSION 3.1. Effect of Reaction Time. The XRD patterns of the LTO prepared in scMeOH at reaction times ranging from 5 min to 2 h are shown in Figure 1. TTIP concentration was fixed

Figure 1. XRD patterns of the LTO particles synthesized in scMeOH at different reaction times from 5 min to 2 h.

at 0.3 M. The peaks at 18.4°, 35.6°, 43.3°, 57.2°, 62.8°, and 66.1° can be assigned to (111), (311), (400), (511), (440), and (531) diffraction peaks, respectively, that are associated with spinel-type Li4Ti5O12 (JCPDS No. 49-0207). A phase-pure LTO diffraction pattern was obtained at the reaction time of 5 min. This indicates that the formation of LTO in scMeOH is fast when compared to other synthetic approaches such as sol− gels, solvothermal and hydrothermal methods, which typically require 6−12 h reaction times.19,22,24,26,27,58,59 Impurity phases associated with anatase TiO2 or Li2TiO3, which are often observed in solid-state synthesized LTO60−62 were not detected in the as-synthesized LTO samples in scMeOH. Decrease in the full width at half-maximum (fwhm) of the associated peaks with an increase in reaction time was observed, indicating an increase in crystallite size with increasing reaction time. The crystallite size, estimated using Scherrer’s equation and (111)/ (400) diffraction patterns, increase from 6.4 to 10.6 nm with an increase in reaction time from 5 min to 2 h (Table 1). The morphology of the as-synthesized LTO at various reaction times is shown in Figure 2. The as-synthesized LTO samples in scMeOH were slightly agglomerated, and formed micrometer-sized particles in interparticle interactions. At a 5 min reaction time, the secondary particle was irregularly shaped, while at longer reaction times of 15 min to 1 h the shape of secondary particle tends toward spherical. The morphologies of the LTO samples did not change significantly 185



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particles in scMeOH, at various precursor concentrations of 0.1−1.0 M. The reaction time was fixed to 15 min. At the low concentration range of 0.1−0.3 M, phase-pure LTO was obtained, while at the high concentration range of 0.5−1.0 M, an anatase-type TiO2 impurity phase was observed. At the high concentration regime, TTIP and LiOH may not be mixed well enough to produce phase-pure LTO. The portion of TTIP that was not mixed with LiOH may result in the TiO2 phase. The higher TiO2 peak intensity at higher concentration suggests that a larger amount of TiO2 phase was formed. As shown in Figure S1 (Supporting Information), the morphology of the samples synthesized under various concentrations exhibited similar hierarchically mesoporous structures, indicating that feed concentration played a minor role in determining morphology. 3.3. Effect of Supercritical Alcohols. The XRD patterns of the LTO prepared in different supercritical alcohols at a fixed reaction time of 15 min and a fixed TTIP concentration of 0.3 M are shown in Figure 4a. Under this condition, the pressures in the reactor, estimated using the Peng−Robinson equation of state with the Mathias−Copeman expansion,63 were 36.5 MPa (scMeOH), 25.2 MPa (scEtOH), and 19.7 MPa (scIPA), indicating that the pressures and temperatures were above the critical point of each supercritical alcohol. Phase-pure LTO was obtained under different supercritical alcohol media. The crystallite size of LTO was in the order of scIPA-0.3M-15m (8.5 nm) > scEtOH-0.3M-15m (7.5 nm) > scMeOH-0.3M15m (7.4 nm), indicating an increase in crystallite size with the usage of longer-chain alcohol. Slostowski et al. also observed an increase in the crystallite size of CeO2 with an increase in the chain length of carbon in supercritical alcohols.40 It is not clear what is causing the larger crystallite size with the usage of longer-chain alcohol, but less effective surface modification from longer-chain alcohol may be responsible for the larger crystallite size. The surface functional groups of the LTO particles were examined using FT-IR, with results shown in Figure 4b. The presence of a −OH stretching peak at 3000−3750 cm−1, −CH3 stretching peak at 2926 cm−1, and −CO− stretching peak at 1050 cm−1 suggests that supercritical alcohol acts as a hydroxylation, methylation, and methoxylation agent.38,64 Similar surface modification of CeO2 and ZnO synthesized in supercritical alcohols can be found elsewhere.38,40,65 The presence of these functional groups on the surface of the particle plays a role in the inhibition of particle growth. The peak at 1637 cm−1 is due to adsorbed water on the surface of LTO.66 Additional peaks appear at 1534 and 1427 cm−1. These peaks were not completely removed by heating to 600 °C in air or Ar/H2 condition, as shown in Figure S2. These peaks can be assigned to carbonate ions originated from Li2CO3,67 which may have formed during the reaction of residual LiOH with CO2.66 As shown in Figure 4c, the weight loss of the as-synthesized samples is attributed to organic species burnoff in air flow, indicating that the surfaces of the LTO samples were covered by an organic species. The order of weight loss is scMeOH0.3M-15m (6.1%) > scEtOH-0.3M-15m (5.7%) > scIPA0.3M15m (4.9%). The degree of weight loss corresponds to the surface coverage of the surface modifiers. The LTO sample synthesis in supercritical methanol exhibited the largest surface coverage, thus more effectively inhibited particle growth. This can result in the smallest crystallite size, as measured by XRD.

Figure 2. SEM images of the LTO particles synthesized in scMeOH at different reaction times.

at reaction times of 15 min to 1h; microspheres with diameters of 0.2−0.5 μm are composed of nanosized primary particles with diameters of 20−40 nm. The surface of the sphere is rough, and an irregularly shaped mesopore structure is clearly visible between the interconnected primary nanoparticles. When the reaction time increased to 2 h, the microsphere structure was not as uniform, possibly due to the growth of the primary particle at the longer reaction time. As listed in Table 1, a slight decrease in BET surface area was observed when the reaction time varied up to 1 h, while a significant decrease in BET surface area from 109.2 to 74.4 m2 g−1 was observed when the reaction time increased to 2 h. As indicated by an arrow in the Figure 2d, the formation of well-faceted particles at the long reaction time of 2 h, along with smaller sized particles, is clearly visible. The formation of unique mesoporous structures in scMeOH can be due to the presence of organic functional groups attached to the surface of particles, which can retard further particle growth. Detailed discussion on the formation mechanism of hierarchically mesoporous LTO microspheres in scMeOH is presented in section 3.5. 3.2. Effect of Concentration. Figure 3 shows the effect of concentration on the XRD patterns of as-synthesized LTO

Figure 3. XRD patterns of the LTO particles synthesized in scMeOH at different feed concentrations. 186



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Figure 5. SEM and corresponding TEM images of LTO particle synthesized in scMeOH, scEtOH, and scIPA.

3.4. Effect of Calcination. The effect of calcination temperature on the morphology was examined, with results shown in Figure S3 (calcination under the Ar/H2 flow condition) and Figure S4 (calcination under the air flow condition). The hierarchical structure was well retained at calcination temperatures of 500−600 °C, with the exception of samples synthesized in scIPA and subsequently calcianted at 600 °C, which showed particle aggregation (Figures S3k and S4k). When calcinated at 700 °C, the mesoporous structure collapse due to the interparticle aggregation between neighboring particles and lead to formation of larger particles. As shown in Figure S5, the Ar/H2-calcined samples were black in color, while the air-calcined samples were white, indicating the formation of a carbon layer on the surface of the Ar/H2calcined LTO samples. The organic functional groups attached on the surface of the LTO particles synthesized in the supercritical alcohols thus acted as the carbon sources. The disappearance of the organic functional groups after the air and Ar/H2 calcination was confirmed by the FT-IR spectra (Figure S2). The carbon contents in the Ar/H2-calcined LTO samples, measured via elemental analysis, were 2.5% (scMeOH-0.3M15m-H2-600C), 1.7% (scEtOH-0.3M-15m-H2-600C), and 1.2% (scIPA-0.3M-15m-H2-600C). The low carbon content of LTO synthesized in scIPA is due to the low degree of surface modification. The structure of the carbon on the surface of the Ar/H2calcined LTO samples was further analyzed using Raman spectroscopy, with results shown in Figure 6. Previous works suggested that peak fitting of Raman spectra with two bands, D and G, may not give satisfactory results.30,68−70 Thus, we used four bands to deconvolute the Raman spectra. The band at

Figure 4. (a) XRD patterns, (b) FT-IR spectra, and (c) TGA profiles of the LTO particles synthesized in scMeOH, scEtOH, and scIPA.

Figure 5 shows the hierarchical mesoporous morphologies of the samples synthesized under various supercritical alcohol conditions. A closer inspection of the SEM images revealed an increase in primary particle size, and the surfaces of the spheres became rougher as the length of the carbon chain in the alcohols increased. The average primary particle size, estimated using the TEM images, was ordered 11.3 nm (scMeOH-0.3M15m) < 13.2 nm (scEtOH-0.3M-15m) < 21.1 nm (scIPA-0.3M15m). This observation agrees well with the BET surface area measurements (Table 1). Again, the higher degree of surface modification resulting from the use of scMeOH probably led to the smaller-sized primary particles in scMeOH-0.3M-15m. 187



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0.3M-15m-H2-600C (0.96), indicating that increasing the carbon chain length of alcohol from methanol to IPA results in an increase in the average sp2 domains. A high ID/IG ratio suggests that the higher amount of defects in the carbon structure can facilitate the diffusion of Li+ through the disordered region, while low ID/IG ratio suggests high electron conductivity.73 To obtain information on the formation of Ti3+ in the LTO samples synthesized in supercritical alcohols, the uncalcined samples and the Ar/H2-calcined samples were comparatively examined using XPS, and the corresponding Ti 2p spectra are shown in Figure 7. The LTO samples showed two peaks at 458.5 and 464.2 eV, which correspond to Ti 2p3/2 and Ti 2p1/2 core-level binding energies of Ti4+, respectively. A more detailed analysis of Ti 2p3/2 revealed two peaks with binding energies at 457.3 and 458.4 eV, corresponding to Ti4+ and Ti3+, respectively.74 The presence of the Ti3+ ion indicates that supercritical alcohols can reduce some Ti4+ to Ti3+. As discussed in previous papers, supercritical alcohols can act as reducing agents to produce metallic nanoparticles such as Cu, Ag, and Ni,75,76 and to reduce graphene oxide.77 The reducing characteristics of supercritical alcohol are associated with the hydrogen donation ability in the form of molecular hydrogen, hydride, or protons.78,79 Following calcination in the Ar/H2 environment, the ratio of the [Ti3+]/[Ti4+] peak area increased from 0.08 to 0.22 for the scMeOH-synthesized LTO, from 0.07 to 0.18 for the scEtOH-synthesized LTO, and from 0.04 to 0.08 for the scIPA-synthesized LTO samples. The partial transformation of the surface Ti4+ to Ti3+ during the calcination in the Ar/H2 condition probably resulted from carbothermal reduction. The increase in Ti3+ content can contribute to increased electronic conductivity,33,80 which is expected to enhance the electrochemical performance of the active material. Figure S6 shows the XRD patterns of the LTO samples prepared in the different supercritical alcohols and subsequent calcination at 600 °C under the Ar/H2 flow, or in the air flow condition. A decrease in fwhm was observed following

Figure 6. Raman spectra of the Ar/H2-calcined LTO samples.

1591 cm−1 is one of the E2g modes (or G band, sp2), which is associated with a graphene sheet. The band at 1338 cm−1 is one of the D bands (sp3), which is an amorphous carbon band. All the Raman spectra of the LTO samples consisted of intense D and G bands. The bands located at 1490 and 1190 cm−1 can be attributed to the short-range vibrations of sp3 structured carbon atoms,30,69 which were also observed in the Raman spectra of carbon-coated LiFePO4.71 The relative intensity of the D band vs the G band (ID/IG) can be used to evaluate the degree of disorder or defects in the carbon network.72 Low ID/IG value indicates a high degree of graphitization. ScMeOH-0.3M-15mH2-600C exhibited a higher ID/IG ratio of 1.03 than those obtained from scEtOH-0.3M-15m-H2-600C (1.01) and scIPA-

Figure 7. XPS spectra of the uncalcined and Ar/H2-calcined LTO samples. 188



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samples is consistent with spinel LTO structure with strong ring patterns due to (111), (311), (400), (511), (440), and (444) planes (inset Figure 8a). The HR-TEM image in Figure 8b reveals that the lattice spacing is 0.48 nm for (111) lattice plane, confirming the high crystalline structure of the calcined samples. As shown in Figure S8, scEtOH-0.3M-15m-H2-600C also exhibited mesoporous morphology, while scIPA-0.3M15m-H2-600C showed a much less distinct hysteresis at P/Po of 0.6−1.0. The scEtOH-0.3M-15m-H2-600C and scIPA-0.3M15m-H2-600C samples exhibited a smaller BET surface area, larger average pore size, and smaller porosity when compared to scMeOH-0.3M-15m-H2-600C (Table 2), which is probably due to a smaller degree of surface modification in the uncalcined samples prepared in scEtOH and scIPA. 3.5. Formation Mechanism. This work allows for the development of a plausible formation mechanism of the unique, hierarchically mesoporous structure in the supercritical alcohols as schematically presented in Figure 9. Originally, the LTO phase was formed by the reaction between TTIP and LiOH in the supercritical alcohol condition. At the early stage of nucleation, the organic species from the supercritical alcohol attach to the surface of the growing LTO particles, inhibiting further growth. The primary nanosized particles are then loosely aggregated, and form secondary micrometer-sized particles in an interparticle interaction, e.g. van der Waals interaction, with the mesopores between the primary particles. Due to the surface protection provided by the organic modification, the primary particles grow only slightly during the extended period of reaction time up to 1 h. The unique, mesoporous microsphere morphology was maintained at calcination temperatures of up to 600 °C in the Ar/H2 condition, with the exception of the low degree of surface modification using scIPA. During the calcinations in the reducing environment, the carbonization not only inhibits the growth of primary LTO particles, but also reduces the surface Ti4+ to Ti3+. Thus, carbon-coated, hierarchical mesoporous LTO microspheres can be simply prepared without using templates, structure-directing chemicals or external carbon sources. 3.6. Electrochemical Properties. The electrochemical performance of the carbon-coated, hierarchical mesoporous LTO microspheres synthesized in various supercritical alcohols was investigated. Figure 10 shows the galvanostatic charge and discharge curves and cycling performance up to 200 cycles in the voltage range of 1.0−2.5 V versus Li/Li+ at a constant 1 C rate. A flat plateau located between 1.5 and 1.6 V were observed for all electrode. These are typical characteristics of the twophase lithium insertion and extraction processes associated with LTO, suggesting the well-developed crystalline structure. As shown in Figure 10d, the discharge capacities of the three samples are quite similar up to 50 cycles, but above 50 cycles the discharge capacity was in order of scMeOH > scEtOH > scIPA. Table 2 summarizes the physicochemical and electrochemical properties of the LTO samples. With a change in the carbon chain length of alcohol from IPA to methanol, carbon content, Ti3+ content, BET surface area, and porosity values increased. The larger contents of the carbon and Ti3+ in scMeOH-0.3M-15m-H2-600C may lead to an increase in the single-phase Li insertion/extraction region out of the two-phase region,33 as shown in Figure S9. The larger surface area lead to an increase in the electrode−electrolyte interphase area, while higher porosity lead to better penetration of the electrolyte to the inside of the mesoporous structure. As a result, improved

calcination, indicating an increase in the crystallite size after calcination. The XRD pattern of the air-calcined samples exhibited narrower fwhm when compared to the Ar/H2calcinaed samples. As listed in Table 1, the crystallite sizes of the Ar/H2-calcined samples are much smaller than those of the air-calcined samples, suggesting that the carbon layers formed on the LTO surface during heat treatment in the Ar/H2 condition inhibited particle growth to some extent. The lattice parameter of the LTO samples determined from the XRD patterns are listed in Table 1. The lattice parameters of the aircalcined samples were in the range of 8.3552−8.3558 Å, which are close to the values for the high-temperature samples.7,33,34,81 The Ar/H2-calcined samples exhibited lattice parameter values of 8.3621−8.3686 Å, larger than those of the air-calcined samples.This is expected to occur if some of the Ti4+ is transformed to Ti3+, because Ti3+ retains a larger ionic radius (0.670 Å) than Ti4+ (0.605 Å).82 Further structure properties of the Ar/H2-calcined samples were examined using HR-TEM and BET measurements. As shown in Figure S7a and b, scMeOH-0.3M-15m exhibited primary particles with sizes of 12−30 nm, and pores between interconnected particles with diameters of 6−10 nm. The N2 adsorption−desorption isotherm of scMeOH-0.3M-15m in Figure S7c shows a distinct hysteresis at P/P0 of 0.6−1.0, indicating the presence of a mesoporous structure. The poresize distribution, estimated using the Barrett−Joyner−Halenda (BJH) method, indicates that the pore size of this material is not uniform, and ranges between 2 and 10 nm (inset in Figure S7c), with an average pore size of 7.5 nm. The calcination at 600 °C in the Ar/H2 condition led to an increase in primary particle sizes to 30−60 nm, without changing the hierarchical mesoporous morphology (Figure 8a−c). The increase in crystallite size led to a decrease in BET surface area from 119.2 to 60.2 m2 g−1 following calcination. The pore size of scMeOH-0.3M-15m-H2-600C was in the range of 2−20 nm, with an average pore diameter of 8.3 nm (Figure 8d). The selected area electron diffraction pattern (SAED) of the

Figure 8. TEM images and BET analysis of scMeOH-0.3M-15m-H2600C. Circles in panel (c) indicate mesopores in the sample. 189



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Table 2. Physicochemical and Electrochemical Properties of the LTO Samples Synthesized with Different Alcohols and Subsequent Calcination in the Ar/H2 Condition sample code

carbon content (%)a

[Ti3+]/[Ti4+] peak area ratiob

ID/IG

scMeOH-0.3M-600-H2-600C scEtOH-0.3M-600-H2-600C scIPA-0.3M-600-H2-600C

2.5 1.7 1.2

0.22 0.18 0.08

1.03 1.01 0.96

c

BET surface area (m2 g−1)

average pore size diameter (nm)

total pore volume (cm3 g−1)

porosity (%)d

1st discharge capacity (mAh g−1)e

200th discharge capacity (mAh g−1)e

60.2 55.1 41.2

8.3 9.1 9.0

0.1832 0.1262 0.1152

38.9 30.5 28.6

160.5 151.5 153.0

131.6 107.2 104.2

a

Analyzed with CHNS/O analyzer. bAnalyzed with XPS. cAnalyzed with Raman. dEstimated using the total pore volume and the true density of LTO (3.48 g cm−3). eMeasured in the voltage range of 1.0 to 2.5 V versus Li/Li+ at a constant 1 C rate.

Figure 9. Formation mechanism of the hierarchically mesoporous LTO in supercritical alcohols and carbon coating.

mAg−1 (8 C), 1750 mAg−1 (10 C), 3500 mAg−1 (20 C), 5250 mAg−1 (30 C), and 8750 mAg−1 (50 C) for 10 cycles, and then again at 1 and 0.1 C for 10 cycles, as shown in Figure 11. The

Figure 10. Capacity−voltage profiles of (a) scMeOH-0.3M-15m-H2600C, (b) scEtOH-0.3M-15m-H2-600C, (c) scIPA-0.3M-15m-H2600C, and (d) cycle performance of scMeOH-0.3M-15m-H2-600C, scEtOH-0.3M-15m-H2-600C, and scIPA-0.3M-15m-H2-600C at a 1 C rate. Open symbols, discharge; closed symbols, charge. Figure 11. Rate performance of scMeOH-0.3M-15m-H2-600C, scEtOH-0.3M-15m-H2-600C, and scIPA-0.3M-15m-H2-600C at Crates of 0.1 C (1st to 10th cycles), 1 C (11−20th), 2 C (21−30th), 4 C (31−40th), 8 C (41−50th), 10 C (51−60th), 20 C (61−70th), 30 C (71−80th), 50 C (81−90th), 1 C (91−100th), and 0.1 C (101− 110th). Open symbols: discharge; closed symbols: charge.

electrochemical performance of the carbon-coated hierarchical mesoporous LTO microsphere would be expected. To demonstrate the rate performance, the samples were progressively charged and discharged at 17.5 mAg−1 (0.1 C), 175 mAg−1 (1 C), 350 mAg−1 (2 C), 700 mAg−1 (4 C), 1400 190



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path, both of which are beneficial for electron and Li-ion mobility. The Ar/H2-calcined LTO from scMeOH therefore exhibited superior high-rate performance and long-term cyclability when compared to the LTO from scEtOH and scIPA. The anode of the scMeOH sample exhibited a discharge capacity of 93.5 mAh g−1 at 20 C, and a marginal capacity fading from the initial discharge capacity after the high-rate test. The results presented in this study suggest that the supercritical alcohol route is a promising approach for synthesis of mesoporous LTO with carbon coating for large-scale Li-ion battery applications.

same rate was applied for both the charge−discharge processes of each cycle. Stable discharge capacities of 173.2, 159.1, 153.2, 145.2, 130.7, 116.9, 93.5, 60.8, and 40.2 mAh g−1 were achieved for scMeOH-0.3M-15m-H2-600C at C rates of 0.1, 1, 2, 4, 8, 10, 20, 30, and 50 C, respectively. These values are comparable to those of LTO with the double surface modification of Ti3+, and carbon synthesized via the solid-state method.33,34 However, less carbon material at a lower carbon content (2.5 wt %) was used for fabricating the composite electrode in this study (conductor + binder = 13 wt %) when compared to those previous reported of LTO with double surface modification of Ti3+ and carbon (carbon content = 3−5.2 wt %; conductor + binder = 20 wt %). This suggests that a higher volumetric capacity can be achieved in the current composite electrode. At relatively slow discharge rates of 0.5−4 C, the discharge capacities are in the order of scMeOH ≈ scEtOH > scIPA, while at rapid discharge−charge rates of 8−50 C, the discharge capacities are in the order of scMeOH > scEtOH > scIPA. The difference in discharge capacities increases at higher C rates. When returning to the 0.1 C rates after 100 charge−discharge cycles, the discharge capacity of scMeOH-0.3M-15m-H2-600C was 165.6 mAh g−1, indicating a mere 4.7% capacity loss from the initial discharge capacity. The other two samples also showed marginal capacity loss from the initial discharge capacity (5.0%, scEtOH-0.3M-15m-H2-600C; 4.8%, scIPA0.3M-15m-H2-600C). This indicates that the highly crystalline spinel phase can retain its structural integrity even during the high Li ion intercalation and deintercalation process. The rate performance of the Ar/H2-calcined sample was compared with the air-calcinaed sample, and the results are shown in Figure S10. Similar discharge capacities resulted at the lower C rates of 0.1−4 C, but the Ar/H2-calcined sample exhibited higher discharge capacities at above 8 C. The improved rate performance of the Ar/H2-calcined sample can be explained by inspecting several aspects. The nanosized primary particles significantly reduce the lithium ion diffusion distance; while the mesoporous structure ensures deep penetration of the electrolyte into each individual nanosized particle and enables high availability for the Li+ storage. The use of different supercritical alcohols resulted in varied carbon and Ti3+ contents. Both the conductive carbon layer and Ti3+ present around each nanosized particle can enhance electronic conductivity to promote better charge transfer reactions, especially at high charge−discharge rates. In addition, the high porosity offers better penetration of the electrolytes, which can lead to larger contact area between electrode−electrolyte and high utilization of the active material.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed of SEM, TEM images, LTO photograph, FT-IR, XRD, BET analysis, and electrochemical properties results (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: + 82-31-299-4843. Fax: + 82-31-290-5889. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea Grant funded by the Ministry of Science, ICT & Future Planning (2013R1A1A2061020). Additional support from Korea Institute of Energy Technology Evaluation and Planning funded by Ministry of Trade, Industry and Energy (20122020100280) and the Ministry of Science, ICT & Future Planning (2009-0083540) are also appreciated.

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4. CONCLUSION In this study, mesoporous LTO microspheres with carbon coating were synthesized in supercritical alcohols and subsequent calcination under the Ar/5% H2 condition. Various process parameters including reaction time, solvent, concentration, and calcination temperature were carefully examined to gain an insight into the formation mechanism of the carbon conductive layers coated mesoporous LTO microspheres. The choice of supercritical alcohol has a significant effect on the morphological, textural, and electrochemical properties. The higher degree of organic surface modification using scMeOH resulted in hierarchically mesoporous LTO with smaller crystallite size, larger BET surface area, higher porosity, and higher carbon and Ti3+ contents. These properties can improve the electronic conductivity and shorten the Li-ion diffusion 191



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A Facile Supercritical Alcohol Route for Synthesizing Carbon Coated

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