Green Strategy to Single Crystalline Anatase TiO2 Nanosheets with

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Green Strategy to Single Crystalline Anatase TiO2 Nanosheets with Dominant (001) Facets and Its Lithiation Study towards Sustainable Cobalt-Free Lithium Ion Full Battery Jun Ming, Junwei Zheng, Yang-Kook Sun, Lain-Jong Li, Wenjing Yang, Hai Ming, Pushpendra Kumar, Won-Jin Kwak, and Yu Fu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b00553 • Publication Date (Web): 03 Nov 2015 Downloaded from http://pubs.acs.org on November 6, 2015

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ACS Sustainable Chemistry & Engineering

Green Strategy to Single Crystalline Anatase TiO2 Nanosheets with Dominant (001) Facets and Its Lithiation Study towards Sustainable Cobalt-Free Lithium Ion Full Battery Hai Ming,a Pushpendra Kumar,b Wenjing Yang,c Yu Fu,a Jun Ming,b,* Won-Jin Kwak,d Lain-Jong Li,b Yang-kook Sund,* and Junwei Zhenga,*

a

College of Chemistry, Chemical Engineering and Materials Science, Soochow

University, Suzhou 215123, P. R. China. E-mail: [email protected]. b

Physical Sciences and Engineering, King Abdullah University of Science and

Technology, Thuwal, Saudi Arabia. E-mail: [email protected]. c

Reliability Research and Analysis Center, CEPREI (East China) Laboratories, The

Fifth Research Institute of MIIT East China, P. R. China. d

Department of Energy Engineering, Hanyang University, Seoul, 133-791, Republic

of Korea. E-mail: [email protected].

1

ACS Paragon Plus Environment


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Abstract A green hydrothermal strategy starting from the Ti powders was developed to synthesis a new kind of well dispersed anatase TiO2 nanosheets (TNSTs) with dominant (001) facets, successfully avoiding using the HF by choosing the safe substitutes of LiF powder. In contrast to traditional approaches targeting TiO2 with dominant crystal facets, the strategy presented herein is more convenient, environment friendly and available for industrial production. As a unique structured anode applied in lithium ion battery, the TNSTs could exhibit an extremely high capacity around 215 mAh g-1 at the current density of 100 mA g-1 and preserved capacity over 140 mAh g-1 enduring 200 cycles at 400 mA g-1. As a further step towards commercialization, a model of lithiating TiO2 was built for the first time and analyzed by the electrochemical characterizations, and full batteries employing lithiated TNSTs as carbon-free anode versus spinel LiNixMn2-xO4 (x = 0, 0.5) cathode were configured. The full batteries of TNSTs/LiMn2O4 and TNSTs/LiNi0.5Mn1.5O4 have the sustainable advantage of cost-effective and cobalt-free characteristics, and particularly they demonstrated high energy densities of 497 and 580 Wh kganode-1 (i.e., 276 and 341 Wh kgcathode-1) with stable capacity retentions of 95% and 99% respectively over 100 cycles. Besides the intriguing performance in batteries, the versatile synthetic strategy and unique characteristics of TNSTs may promise other attracting

applications

in

the

fields

of

photo-reaction,

electro-catalyst,

electrochemistry, interfacial adsorption photovoltaic devices etc. Keywords: Titanium dioxide, Hydrothermal, Anode, Cathode, Lithiation, Battery. 2


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Introduction Morphology-controlled synthesis of metal (oxide) micro- and nanoparticles, such as Pt,1 TiO2,2 Fe2O3,3 and CuO4 with exposed high-energy or active facets, are of great interest, because they typically exhibit fascinating surface-dependent properties and cover widespread applications in the fields of sensor,4 photoreaction,5 catalysis,6 electrochemistry,7 energy storage and conversion.8 Among these versatile metal (oxides), TiO2 has attracted great attention and particularly was considered to be one of the most promising anode in rechargeable batteries owing to its low cost, robust capability and superior safety.9 To date, numerous anatase TiO2 with highly exposed (001) facets, include micro-/nanoflakes10 and its hierarchical structured materials,11 have been investigated for its higher surface energy (i.e., 0.90 J m-2 for (001) > 0.53 J m-2 for (100) > 0.44 J m-2 for (101)),12 and indeed series of extraordinary performances were attained in photovoltic solar cell,10 electro-catalytic reaction,13 photo-catalysis,14 interfacial adsorption15 and energy storage.16 For example, a high coulomb efficiency (CE) and capacity retention in lithium ion battery were always demonstrated due to the fast diffusion of lithium ions along the good crystalline channels.17 However, their synthetic process always suffered from the complex conditions, high cost of precursors and limited production. It may be mainly determined by the high ionic charge to radius ratio of Ti4+ which is too sensitive to control its hydrolization.18 Although variable Ti-based inorganic/organic precursors (e.g., TiCl4,19 TiF4,20 Ti(SO4)2,21 or titanium isopropoxide22), with capping agents (e.g., 3



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HF,23 NH4F,11 NH4HF2,24 oleic acid/oleylamine,25 or disodium ethylene diamine tetraacetate22), in different solvents (e.g., H2O-ethanol,11 H2O,24 ethanol,25 or isopropyl alcohol22) have been well reported for the synthesis, the easy-hydrolysis of precursors, volatile HF and metastable NH4+ based compounds may bring troubles in control and are dangerous in scalable production. To facilitate the synthesis of TiO2 nanosheets and its derived hierarchical structures (e.g., spheres,11 flowers23) with highly exposed (001) facets, one typical review have summarized the existing synthetic strategies well,26 such as the fluoride-mediated hydrothermal synthesis,2, 19, 27-28

fluoride-mediated solvothermal synthesis29-31 and amine-mediated nonaqueous

synthesis,32-33 which can give rise to different kind of products using different chemicals. Herein in this work, a green and efficient strategy was developed to synthesize a new structured TiO2 nanosheets (TNSTs) with highly exposed (001) facets starting from the Ti powders. Although the Ti powders were utilized as precursor in few reports,23 the success to avoid using hydrofluoric acid (HF) was achieved by choosing the safe substitutes of LiF powder for the first time. The presented approach could well satisfy an industrial production for sustainability, and particularly the TNSTs products exhibited excellent performance as an anode in lithium ion battery. An extremely high capacity around 215 mAh g-1 at the current density of 100 mA g-1 and a well-persevered capacity over 140 mAh g-1 even after 200 cycles at 400 mA g-1 were demonstrated. Unlike from the researches on preparing TiO2-based composite via carbon modification/doping34-35 or surface nitridation/hydronation,36-37 the pristine 4


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TNCBs synthesized herein could also promise amazing performances in energy storage, or even better. To date, most TiO2 and TiO2-based composites, include the TiO2 ribbon,34 TiO2/CoO,38 TiO2/Graphene,39 TiO2@CNT,40 and TiO2@Li4Ti5O1241, were intensively investigated in lithium battery (half battery vs. lithium metal). However in these cases, the mass of lithium (e.g., several tens milligram at least for the diameter of 10-16 mm) was always excess compare to that of TiO2 with similar sized electrode (i.e., several milligrams), because the needed amount of lithium metal was only 4.4% mass percent of TiO2 (i.e., which is calculated by the equation of 0.5Li+ + TiO2 + 0.5e- → Li0.5TiO242) without considering the irreversibility in initial cycles. Practically, the mass of lithium should be quantitively controlled because the source of lithium could not be excess considering the high cost of cathode for the sustainability. To evaluate the performance of TNSTs in practical application and minimize the amount of lithium for safety, attempts for assembling full batteries of TNSTs versus LiNixMn2-xO4 (x = 0, 0.5) cathode has been undertaken. The reason of choosing spinel structured LiNixMn2-xO4 (x = 0, 0.5) as cathode was stimulated by the advantage of high voltage over 4.1 V and low cost without using toxic/rare-earth element of cobalt. To overcome the large irreversible capacities of oxide-based anode,43-44 an original electrochemical analysis of lithiating TNSTs and its effect on the performance of full battery were studied for the first time. Via optimizing the working voltage window and

the

lithiation

conditions,

the

full

batteries

of

TiO2/LiMn2O4

and

TiO2/LiNi0.5Mn1.5O4 using the carbon-free TNSTs could deliver high energy densities 5



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of 497 and 580 Wh kganode-1 (i.e., 276 and 341 Wh kgcathode-1) with superior capacity retention of 95% and 99% over 100 cycles. As per our knowledge, this is the first example on assembling full battery using the TiO2 nanosheets with dominant (001) facets versus cobalt-free spinel structured cathode. Besides the excellent performances of TNSTs in lithium ion batteries, the green synthetic strategy and unique characteristics of TNSTs could be further explored in the areas of photo-catalysis, solar battery, electrochemistry, environmental and materials science etc.

Experimental Materials All chemicals were analytical grade and used as received without further purification. The LiF, polyacrylic acid (PAA) and carboxymethyl cellulose (CMC) were purchased from the Sigma-Aldrich Co. Ltd.; while the Ti powders, citric acid, urea, Li(CH3COO)·2H2O, Ni(CH3COO)2·4H2O and Mn(CH3COO)2·4H2O were purchased from Alfa Aesar Co. Ltd.. The aqueous solution of HCl (37 wt%) and NH3·H2O (25%-28%) solution were purchased from Sinopharm Co. Ltd.. The relative materials used in electrode preparation, including graphite, Super P and polyvinylidene difluoride (PVDF), were provided by the Hanwha Company in Korea with the industrial grade. Synthesis of TNSTs In the typically synthesis, 0.75 g LiF and 0.5 g Ti powders were dissolved into 70 mL 4.0 M HCl solution with stirring for 1 hour. And then, the green solution was 6


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transferred into Teflon lined stainless steel autoclaves and kept for heating at 180 oC for 12-32 h. After cooling down the autoclave, the powders were collected by centrifugation of the blue-white suspension and washed by ethanol and water several times. Note that the autoclave was opened in the hood and the hydrothermal solution after the reaction was treated by CaO for consuming any possible HF in the solution. Calcination of the powders at 500 oC for 5 h under a steady air gives rise to the product of TNSTs. Synthesis of LiNixMn2-xO4 (x =0, 0.5) The LiMn2O4 was prepared by sol-gel method using citric acid as chelating agent. The precursors of Li(CH3COO)·2H2O and Mn(CH3COO)2·4H2O with stoichiometric Li/Mn molar ratio of 1.05/2.00 were dissolved in distilled water under stirring. After the dissolution, aqueous solution of citric acid and urea was added to the system. The pH value of the solution was adjusted to 6-7 using the buffer solution of aqueous ammonia (NH3·H2O, 25%-28%). The mixture was dried at 80 oC and then preheated at 400 oC for 5 h under the O2 atmosphere to remove the organic components. Finally, ball milling the solid powders for 3 h and calcination at 800 oC in air for 12 h give rise to the spinel LiMn2O4 product. Alternatively for the preparation of LiNi0.5Mn1.5O4, the raw materials of 2.142 g Li(CH3COO)·2H2O, 2.489 g Ni(CH3COO)2·4H2O, 12 g urea and 7.352 g Mn(CH3COO)2·4H2O were mixed well and calcined at 400 oC for 4 h. Thus obtained powders were ball-milled for 3 h and further calcined at 900 oC for 12 h, and finally the spinel structured LiNi0.5Mn1.5O4 was obtained. Electrode Preparation 7



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The active materials of TNSTs, conductive carbon (Super P), and binders of PAA-CMC45 with the mass ratio of 7.5 : 1.5 : 0.5 : 0.5 were homogeneously mixed in the water to form slurry, which was then casted on the copper foil by doctor blade. The foil was punched into circular electrode with a diameter of 14 mm (alternatively Ø16 for full battery) after a vacuum drying process at 80 oC overnight. The mass density of active materials was about 2 mg cm-2. The preparation of cathode electrode composing of LiNixMn2-xO4, Super P, graphite and PVDF with the mass ratio of 8.5 : 0.5 : 0.5 : 0.5 was similar as that of anode, in which the PVDF was used as the binder and additives of graphite was applied. The resultant slurry was casted on the alumina foil and dried at 120 oC for 12 h before punching. The mass density of LiMn2O4 and LiNi0.5Mn1.5O4 were about 3.6 and 3.4 mg cm-2, respectively. Characterizations The crystal information was acquired by X-ray powder diffraction (XRD) using a X‘Pert-ProMPD (Holand) D/max-γA X-ray diffractometer with Cu Kα radiation (λ = 0.154178 nm). Scanning electron microscopy (SEM) and EDX were obtained on a FEI-quanta 200F scanning electron microscope with acceleration voltage of 30 kV. The distribution and crystalline structure of TNSTs were analyzed by the transmission electron micrograph (TEM) with using the FEI-Tecnai F20 (200 kV) transmission electron microscope (FEI). Nitrogen adsorption-desorption isotherms were obtained using the instrument of ASAP2050 (Micromeritics Instrument Corporation) surface area & porosity Analyzer at 77 K. STXM was carried out on the BL08U beamline of the Shanghai Synchrotron Radiation Facilities (SSRF). The electrochemical tests were 8


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carried out with using the 2032-type coin cell, and were assembled in the glove box filled with pure argon, in which the moisture and oxygen were strictly controlled to less than 0.1 ppm. The half cell is configured with Li metal (−) | Microporous polypropylene separator | electrode (+) filled with the electrolyte of 1.0 mol L-1 LiPF6 in a mixture of Diethyl carbonate (DEC)/ethylene carbonate (EC) (w/w, 1/1). In the full battery, the electrodes of LiMn2O4 or LiNi0.5Mn1.5O4 were applied as cathode versus the anode of pre-lithiated TNSTs. The principles of assembling full battery have been well described in our recent work.46 Galvanostatic cycling was conducted by the TOSCAT-3100 unit at different current densities, and cyclic voltammetry was collected by the instrument of Biologic VMP3 under the scan rate of 0.1 mV s-1.

Results and Discussion Characterizations of TNSTs Figure 1 represents the structural and morphological features of the TNSTs synthesized by the green hydrothermal technique. The crystalline structure of TNSTs was ascribed to the anatase TiO2 (Figure 1a, JCPDS Card No. 73-1764) with a high purity and crystallization, as confirmed by the strong intensity of XRD pattern (Figure 1a). Well distributed TNSTs with uniform cubic morphology having average diameter of about 415 nm and smooth surface is observed under the SEM investigations (Figure 1b). TEM characterizations clearly reveal the cubic morphology of TNSTs. The brighter border of (101) facets indicated the ultralow thickness (Figure 1c), which is distinct from the previous one.47 The single-crystalline characteristic of TNST in a 9



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regular crystal structure was characterized by the HRTEM (Figure 1d). The lattice fringe with the space of 0.235 nm belongs to the (001) planes, as observed on the surface of TNSTs. Independent and inerratic spots in the Fast Fourier Transform (FFT) pattern (left inset of Figure 1c, marked by the white circle) also confirmed the single-crystalline nature of the obtained TNSTs with high percentage of exposed (001) facets.2, 27, 48 The proportion of the (001) facets ratio was calculated by the surface area and it was about 78.1% (Figure S1). Comparing to most TiO2 nanosheets prepared in previous literatures (Table S1), this value is not low and seems still attractive. Varying the reaction time, the proportion of (001) facet and thickness of (101) facets could be changed (Figure S2). Further, according to the evolution of anatase TiO2 shapes, the exposed surface also belongs to the (001) facet.47 The presence of (001) facets could also be judged from the variation of (004) peak in XRD (Figure 1a) because of the extinction of (001) facet in X-ray crystallography (Figure S3). In this reaction system, the LiF powders act as the fluorine source and also the anions of fluorine could ions-exchange with HCl. Thus, the solution can dissolve the Ti powers well; meanwhile the presence of fluorine anions is important for the forming of (001) facets as reported by many researchers. Varying the concentration of fluorine ions, pH value of the solution, or adding certain amount of organic solvent (e.g., ethanol) and inorganic salt may further affect the morphology and proportion of exposed (001) facets, which deserve to be further investigated. The structure of TNSTs was further studied with the help of Raman Spectum (λex = 633 nm) (Figure 2a). The typical peaks located at 148 cm-1 (Eg), 387 cm-1 (B1g) and 10


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TNSTs

20

30

40

50

(204)

(200)

(004)

10

(105) (211)

(101)

a Intensity / a. u.

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60

b

(116) (220) (215)

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70

1µ µm

80

2θ / Degree (101)

d

(001) (101)

(001)

200 nm (200)

(020)

d(001) = 0.235 nm 1 µm

5 nm

Figure 1 (a) XRD pattern, (b) SEM images, (c) TEM and (d) HRTEM images of TNSTs. Left insert of (c) are FTR pattern of the spot of white circle on TNSTs as marked.

590 cm-1 (A1g) were assigned to the symmetric stretching, symmetric bending and anti-symmetric bending vibration of O-Ti-O respectively.49 Particularly the relative high E1g peak at 641 cm-1 confirmed again the nature of anatase phase.50 The Ti L2, 3-edge spectrum of TNSTs showed that the cubic structure with highly exposed (001) facets does not change the anatase crystal structure and Ti-O chemical bonds of TNSTs (Figure 2b), which are similar as we reported before50 and in accordance with the results in Raman (Figure 2a). Note that the TNSTs have a band gap absorption onsets around 400 nm in UV-vis diffuse reflectance absorption spectrum (Figure 2c), implying its superior potential photo-catalytic activity under ultraviolet light,51 which deserve to be further explored. Besides, the characteristics of porosity and external 11



structure were measured by BET analysis, in which the N2 adsorption-desorption isotherm could be categorized as type III isotherm considering the absence of the typical distinct hysteresis loop.52 This result is quite conceivable and consistent with the observation in the TEM/SEM images. The porosity of TNSTs was little limited

a

b

B1g A1g 200

400

E1g

600

7

Adsorption (a. u.)

Intensity (a. u.)

Eg

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800

1000

3

1 450

Raman shift (cm )

d 120

1.0

0.6 0.4 0.2 0.0

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Adsorbed (cm g )

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Wavelength (nm)

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455

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470

Photon Energy (ev) Adsorption Desorption

60 40 20 0 0.0

750

6

4 5

-1

Absorbance (a. u.)

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0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

Figure 2 (a) Raman spectrum, (b) Ti L2, 3-edge spectrum, (c) UV-Vis diffuse reflectance absorption spectrum, (d) Nitrogen adsorption-desorption isotherm of the TNSTs. (e.g., porosity volume, 0.16 cm3 g-1) due to the ultrathin thickness and smooth surface, but the integrity of structure could well promise its continuity of single crystalline structure (Figure 1d). But the relative high surface area of 22.1 m2 g-1 with dominant (001) facets of nanosheets having ultrathin thickness should bring diverse properties in many fields such as in lithium ion battery.

12


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Electrochemical Performance of TNSTs versus Lithium Metal The lithium storage ability of TNSTs in lithium ion battery was evaluated firstly in half cell versus lithium metal. An extremely high stability of TNSTs was directly observed in the cycled CV curves, in which the intensities of anodic/cathodic peaks associated Ti4+/Ti3+ redox couple around 1.72 V vs. 2.0V remains almost constant in initial 25 cycles, demonstrating the highly reversible reaction of lithium metal and TiO2 (Figure 3a). Note that the location difference of the anodic and cathodic peaks is about 0.28 V in the case of using the PAA/CMC as binder, which is much lower than the value of 0.4 V using the PVDF.53 a 1.0

b

th

Current (mA)

+

Voltage (V,vs. Li/Li )

1

th

25

0.5 0.0 -0.5 -1.0 1.0

1.5

2.0

2.5

1.5

0

50

400 800 1600

100

Unit: mA g

-1

3200

Discharge Charge

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-1

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Discharge Charge

150 +

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180 Voltage (V, vs. Li/Li )

50 -1

200

d 100

-1

Capacity (mAh g )

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50 mA g

2.0

Voltage (V, vs. Li/Li )

c 250

-1

2.5

1.0

3.0

3200 mA g

3.0

+

Capacity (mAh g )

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120 90 60 30

3.0 2.5 2.0 1.5 1.0

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Cycle number

Capacity (mAh g )

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Figure 3 (a) Cyclic voltammetry, (b) Voltage vs. capacity profiles, (c) rate capability under the current densities of 50-3200 mA g-1 and (d) cycle performance at 400 mA g-1 of TNSTs in lithium ion half battery. Insert of (d) is the detail voltage vs. capacity profiles. 13



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In the voltage versus capacity profiles, the discharge curves could be typically divided into three main stages: (i) a fast decrease in potential starting from the open-circuit voltage (OCV) to 1.7 V resulted from the surface reactions; (ii) a quite long plateau region around 1.7 V, corresponding to the lithium insertion into the crystalline channel of TiO2; (iii) a gradual decayed tail after the voltage plateau region, resulting from the storage of lithium into the surface structure (Figure 3b). Starting from the low current density of 50 mA g-1 to the hash rate of 400 mA g-1 (~2.5 C), the voltage plateau could be well maintained; demonstrating its excellent rate capability. Further increasing the current density to 3200 mA g-1 (cal. ~20 C), the polarization increased

and

the

capacity

decreased

successively

due

to

the

limited

insertion/de-insertion of lithium ions at a high rate, as confirmed directly by the reduced platform in the stage (ii) (Figure 3b). However, the performance of TNSTs was still largely improved comparing to most previous results.34-40 With a further search of different structured TiO2 in previous literatures, including other kind of TiO2 nanosheets, hierarchical TNSTs, hollow spheres, particles, fibers, nanotube arrays, nanoweb and porous TiO2-carbon composite (e.g., carbon nanotubes, graphene), as well as heteroatom-doped TiO2, we found that such TNSTs showed an equal or even better performances as listed in Table S2. The reason should be ascribed to the single crystalline structure and ultrathin thickness of TiO2 nanosheets, in which the lithium ions could be conveniently inserted into the crystalline channels along the [100] and [010] directions (e.g., xLi+ + TiO2 + xe- → LixTiO2, x < 0.5) (Figure S4); another factor should be ascribed to the highly active spots on the dominant (001) 14


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facets which could react with lithium ions (e.g., Li+ + e- → Li) and then contribute the lithium storage capability. And also, the regular morphology and stable structures are very important for getting a good reversible capacity in cycles. Higher capacities over 228, 213, 197, 175, 150, 120, 88 mAh g-1 were obtained at the current densities of 50, 100, 200, 400, 800, 1600, 3200 mA g-1, respectively. Finally, the capacity could recover to 223 mAh g-1 at 50 mA g-1 (Figure 3c). Even further cycled under a hash current density of 400 mA g-1, the capacity of TNSTs could still be well persevered as high as 140 mAh g-1 over 200 cycles with a capacity retentions 84% (Figure 3d), particularly there is no further increased polarization in the cycling (inset of Figure 3d).

Characteristics of Cobalt-Free Cathode To get more insights and a green battery with a high energy density, the cobalt-free high voltage cathodes of LiNixMn2-xO4 (x=0, 0.5) were prepared and characterized. As confirmed by the XRD pattern and SEM image (Figure S5a-b), the LiMn2O4 powder has an octahedron structure with a size of 300-500 nm, possessing pure phase with spinel structure (Fd-3m, JCPDS No: 70-3120). The electrochemical characterizations depicted that the (dis-)charge curves of LiMn2O4 has two voltage plateaus at 4.05/3.94 and 4.17/4.08 V which could be assignable to the well-known two-phase transition of λ-MnO2/Li0.5Mn2O4 and Li0.5Mn2O4/LiMn2O4, respectively.54 The capacity retention of LiMn2O4 was about 94.3% over 100 cycles at 50 mA g-1 with the capacity nearly 118 mAh g-1, confirm the high stability and cycle ability of electrode (Figure 4a). On the other hand, the LiNi0.5Mn1.5O4 powder with the averaged grain size of 500 nm, 15



a

94.3%

120

-1

Capacity (mAh g )

150

4.5 Voltage (V)

90 60 Charge Discharge

30

4.0 3.5 3.0 2.5

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20 40 60 80 100 120 140 -1 Capacity (mAh g )

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95.4%

120

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90

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-1

Capacity (mAh g )

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60 Charge Dischagre

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20

40

20 40 60 80 100 120 140 -1 Capacity (mAh g )

60

80

100

Cycle number

Figure 4 Capacity and cycling performance of (a) LiMn2O4 and (b) LiNi0.5Mn1.5O4 in half battery at the current density of 50 mA g-1. Insert are the corresponding charge-discharge curves.

synthesized from the molten-salt synthesis method, could clearly be indexed to standard cubic spinel structure (P4332, JCPDS No: 32-0581) (Figure S5c-d). Typical charge-discharge curves were shown insert in Figure 3b, in which pair of charge/discharge plateaus associated with the Ni2+/Ni3+ and Ni3+/Ni4+ couples can be observed in the high-voltage region of 4.66-4.75 V. A pair of minor plateaus due to the Mn3+/Mn4+ couple can also be observed at about 4 V. The LiNi0.5Mn1.5O4 electrode demonstrated a high capacity nearly 130 mAh g-1 with capacity retention of 95.4% 16


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over 100 cycles at 50 mA g-1 (Figure 4b). All these results indicated that the as-prepared LiMn2O4 and LiNi0.5Mn1.5O4 have a superior cycling stability, which may be a suitably greener candidate than LiCoO2 as intercalation host to provide lithium sources for TNSTs. Electrochemical Performance of TNSTs versus Cobalt-Free Cathode A comparative voltage vs. capacity profiles of TNSTs, LiMn2O4 and LiNi0.5Mn1.5O4 were investigated and the results were described in Figure 5. The expected work voltage of the TNSTs/LiMn2O4 and TNSTs/Li0.5Mn1.5O4 should be about 2.0 and 2.7 V within the work voltage window of 1.5~4.2 V and 2.0~4.8 V, respectively. Note that an appropriate mass ratio of anode and cathode is compulsory in which any changes of mass ratio may lead to inferior performance. In this study, the mass loading of the anode to cathode of LiMn2O4 and Li0.5Mn1.5O4 were optimized to be 1:1.8 and 1:1.7 accordingly.55 The irreversible capacity of 60 mAh g-1 in the initial cycles (Figure 5a) was compensated by the lithiation strategy to enhance the utilization of cathode and coulombic efficiency. Particularly, a lithiation model of TiO2 was built and discussed herein for the first time. In the lithiation, a lithiated layer of LixTiO2 could be speculated, start forming on the surface of electrode and its thickness increased gradually with increasing the time, as illustrated in the inset of Figure 5b. This speculation was confirmed and discussed in lithium half battery (Figure S6). As an example, when the lithiation time is about 3 min, one part of lithium ions extracted from LiMn2O4 during the charge (i.e., 120 mAh gLiMn2O4-1) could not move back in the following discharge (i.e., 91 mAhg LiMn2O4-1), as further confirmed by the 17



a

5

TNSTs LiMn2O4

~4.66 V

LiNi0.5Mn1.5O4

Voltage (V)

4

~3.95 V Working voltage

3

-1

130 mAh g

~2.0V -1

~2.7V

119 mAh g

~1.89 V

2

-1

215 mAh g

1 0

50

100

150

200

250

-1

Capacity (mAh g ) -1

Capacity (mAh g , vs. cathode) 0

b

20

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4.5 4.0

Voltage (V)

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

Page 18 of 36

c 3.5

3min

5min

7min

3.0 Electrode of TiO2Super P-PAA/CMC Lithiated layer of electrode

Copper foil

2.5 3 min 5 min 7 min

2.0 1.5 0

36

72

108

144

180

216

-1

Capacity (mAh g , vs. anode)

Figure 5 Comparative voltage vs. capacity profiles of TNSTs at the current density of 100 mA g-1, LiMn2O4 and LiNi0.5Mn1.5O4 at 50 mA g-1; (b) The first voltage vs. capacity profiles of the TNSTs/LiMn2O4 full battery at 100 mA g-1 with different lithiation time; insert is the schematic illustration of pre-lithiation process.

irreversible capacity of 53 mAh ganode-1 (black line, Figure 5b). Inversely, if the lithiation time was prolonged to 7 min, one part of lithium ions in the cathode could 18


Page 19 of 36

not extracted to the anode under a low cut-off voltage of 4.2 V, because there is excess lithium in the anode of TNSTs which behaviors like a lithium metal, as proved by the low capacity of 90 mAh gLiMn2O4-1 (green line, Figure 5b). Finely controlling the lithiation time around 5 min, a capacity of 118 mAh gLiMn2O4-1 was obtained in the first charge process and most of lithium ions could also move back to the cathode (i.e.,

Voltage (V)

3.5 3.0

1st cycle 2-100th cycle

2.5 2.0 1.5 0

30

60

90 120 150 180 210 240

270

150

b

216

120

162

90

108

60 Charge Chage Charge

54 0 0

20

-1

Capacity (mAh g , vs. anode) 4.0

c -1

200 mA g -1 400 mAg -1 800 mAg -1 1600 mAg -1 100 mAg

3.5 3.0 2.5 2.0 1.5 0

30

60

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Cycle number

Capacity (vs. anode)

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Discharge 3min Discharge 5 min Discharge 7 min

Capacity (vs. cathode)

a

4.0

90 120 150 180 210 240 -1

270 216

150

d

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200 100

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90

800 1600

108 54 0 0

-1

Unit: mA g 10

60

Charge Discharge

20

30

40

30

0 50


4.5


102 mAh gLiMn2O4-1) with a coulombic efficiency of 86.4% (Figure 4b, blue curve).

Voltage (V)

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


Cycle number


Figure 6 TNSTs/LiMn2O4 full battery: (a) Voltage vs. capacity profiles at 100 mA ganode-1 in which the electrode of TNSTs was lithiated for 5 min; (b) Comparative trend of capacity and its cycling performance under different lithiation time; (c) Rate capability and capacities of TNSTs/LiMn2O4 (i.e., lithiation for 5 min) ranging from 100 mA g-1 to 1600 mA ganode-1. The unit of capacity in the Y-axis of (b) and (d) are mAh g-1.

19



Cycled charge-discharge curves and rate performance of TNSTs/LiMn2O4 battery were presented in Figure 6. The oxidation of Mn3+ to Mn4+ in cathode and the reduction of Ti4+ to Ti3+ in anode take place during charging via the reaction of LiMn2O4 + TiO2 → 2λ-MnO2 + 2Li0.5TiO2. During discharge, the long plateau at 2.10 V was ascribed to the extraction of lithiutm ions from the TNSTs (~1.89 V) and the successive insertion into the λ-MnO2 lattice (~3.94 and 4.08 V). This process was confirmed more clearly by the curve of dQ/dv-V (Figure 7), in which the pair of redox peaks located at 2.10/2.21 V and 2.19/2.12 V are well consistence with the voltage flats of LiMn2O4 (i.e., 4.05/3.94 V and 4.17/4.08 V) comparing to the voltage flat of TNSTs around 1.89 V (Figure 5a).

3.0

2.19 V 2.21 V

1.5

dQ/dV

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

Page 20 of 36

0.0 -1.5

2.12 V -3.0

2.10 V -4.5 1.6

2.0

2.4

2.8

3.2

3.6

4.0

4.4

Voltage (V) Figure 7 Typical dQ/dV-V curve of TNSTs/LiMn2O4 within the work voltage of 1.5-4.2V.

20


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More importantly, the TNSTs/LiMn2O4 full cell could deliver a reversible capacity of 190 mA h ganode-1 at 100 mA g-1 with a capacity retention of 95.6% after 100 cycles (vs. the 10th cycle). The energy density could achieve as high as 497 Wh kganode-1 (i.e., 276 Wh kgcathode-1) with an average coulombic efficiency of 98% (Figure 6a, b). Note that the crucial effects of lithiation time to the performance of battery are much clear with cycling. As shown in Figure 6b, the capacity of battery decayed when the lithiation time was 3 min, which mainly resulted from the irreversible reaction of lithium ions trapped in anode. While an increased trend of capacity existed in the case of lithiation for 7 min due to the redundant of lithium which could continually compensate the loss of lithium ions in the cycling. An appropriate lithiation time of 5 min is the ideal choice ensuring high capacity, better stability and robust cycle ability. High capacities of 100, 87, 72, and 58 mAh gcathode-1 were obtained with varying the current densities from 200, 400, 800 to 1600 mA ganode-1, which particularly recover to 90 mAh gcathode-1 at 100 mA ganode-1 during rate test, that fully demonstrates its superior rate capability and better stability (Figure 6c, d). On the other hand, the working voltage platform for TNSTs/LiNi0.5Mn1.5O4 is around 2.78 V (Figure 8a-b), which is 0.68 V higher due to the high Ni4+/Ni3+ redox couple around 4.66 V in LiNi0.5Mn1.5O4. The curve of dQ/dV-V exhibiting pair of redox peak located at 2.78/2.92 V (insert in Figure 8b), which is in well accordance with the high working voltage profile of LiNi0.5Mn1.5O4 (Figure 4b). Although the coulombic efficiency of 77.5% in the first cycle was not that high due to a dynamic balance between anode and cathode, which retains very fast to the value of 97.4% 21



during cycling. An average capacity of 113 mAh gcathode-1 with the capacity retention of 99% (vs. the 10th cycle) over 100 cycles (Figure 8b). It is rather stable and the energy density could reach to 580 Wh kganode-1 (i.e., 341 Wh kgcathode-1). The delivered capacity versus TNSTs was about 193 mAh ganode-1, which is slightly less than 200

3.5

st

1 cycle th 2-100 cycle

3.0 2.5 2.0 0

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0

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-1

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4.0 3.5 3.0 2.5 2.0 0

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-10

Charge Discharge

-1

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a

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mAh g-1 of graphite vs. LiNi0.4Mn1.6O4,56 but its safety was significantly improved.

Voltage (V)

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

Page 22 of 36

Cycle number


Figure 8 (a) TNSTs/Li0.5Mn1.5O4 full battery: (a) Voltage vs. capacity profiles and (b) cycling performance at 100 mA ganode-1; (c) Rate capability and (d) capacities at different current densities of100-1600 mA ganode-1.

During rate test, the capacities of TNSTs/LiNi0.5Mn1.5O4 battery can be achieved to 102, 86, 67 and 33 mAh gcathode-1 for the applied current densities of 200, 400, 800 and 1600 mA ganode-1, and recover to 100 mAh gcathode-1 at 100 mA ganode-1 (Figure 8c-d). The rate capability of TNSTs/LiNi0.5Mn1.5O4 battery was not as good as that of 22


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TNSTs/LiMn2O4 that may be due to the decomposition of electrolyte under the high charge potential, but its capacity retention is better benefiting from the suppressive dissolution of Mn in the presence of Ni in the structure.57-58 Finally, the full batteries of TNSTs/LiMn2O4 and TNSTs/LiNi0.5Mn1.5O4 possessing high energy density nearly 497 and 580 Wh kganode-1 (i.e., 276 and 341 Wh kgcathode-1) with superior rate capability were successfully assembled using novel structured TNSTs as anode for the first time. Benefiting from the single crystalline structure of TiO2 nanosheets with highly active (001) facets, the full battery versus LiNi0.5Mn1.5O4 demonstrated higher performances comparing to previous result.59-60 The working potential of 2.1 and 2.78 V in the presented configuration are comparable or even much higher than commercial battery systems of Pb–Acid (2 V), Ni–Cd (1.5 V), Ni–MH (0.9 V), and most aqueous battery system (< 1.5 V). More importantly, the concept of using nonflammable/carbon-free TNSTs versus low-cost/cobalt-free cathode towards green battery could well satisfy the safety and sustainability issues in rechargeable battery.

Conclusion A green and convenient hydrothermal strategy starting from the Ti powders was developed to synthesis novel structured anatase TiO2 nanosheets (TNSTs) with dominant (001) facets, successfully avoiding using the HF by choosing the safe substitute of LiF powder. The presented approach herein could well satisfy the industrial production sustainably, and the TNSTs products exhibited an extremely high capacity of 215 mAh g-1 at the current density of 100 mA g-1 and it can further 23



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Page 24 of 36

retains the capacity over 140 mAh g-1 even after 200 cycles at higher current density of 400 mA g-1. Furthermore, full green batteries of TNSTs/LiMn2O4 and TNSTs/LiNi0.5Mn1.5O4

was

introduced

with

the

concept

of

using

carbon-free/nonflammable TNSTs versus low-cost/cobalt-free cathode for the first time. Particularly, a new model of lithiating TNSTs was built originally and analyzed by electrochemical characterizations to enhance coulombic efficiency and pursue good stability. Promisingly, the TNSTs/LiMn2O4 and TNSTs/LiNi0.5Mn1.5O4 green battery demonstrated high and as well as average reversible capacity of 105 mAh gcathode-1 and 113 mAh gcathode-1 at the current density of 100 mA ganode-1 with stable capacity retentions of 95% and 99% over 100 cycles, in which their energy densities were achieved as high as 497 and 580 Wh kganode-1 respectively which could well satisfy different market requirement according to the specific behaviors. Further, the adaptable green synthetic strategy and novel characteristics of TNSTs could promise other prospect applications in the fields of photo-catalysis, electrochemistry, sensor, energy storage and environmental science etc.

AUTHOR INFORMATION Corresponding Author a

College of Chemistry, Chemical Engineering and Materials Science, Soochow

University, Suzhou 215123, P. R. China. E-mail: [email protected]. b

Physical Sciences and Engineering, King Abdullah University of Science and

Technology, Thuwal, Saudi Arabia. E-mail: [email protected]. 24


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c

Department of Energy Engineering, Hanyang University, Seoul,133-791, Republic of

Korea. E-mail: [email protected].

ACKNOWLEDGMENTS Financial supports from the Nature Science Foundation of China (Nos. 20873089, 20975073), Nature Science Foundation of Jiangsu Province (Nos. BK2011272), Industry-Academia Cooperation Innovation Fund Projects of Jiangsu Province (Nos. BY2011130), Project of Scientific and Technologic Infrastructure of Suzhou (SZS201207), Graduate Research and Innovation Projects in Jiangsu Province (CXZZ13_0802) and Hunan Provincial Innovation Foundation for Postgraduate (CX2012B206) are gratefully acknowledged. This work was also supported by the Human Resources Development program (No. 20124010203310) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy and also supported by the Global Frontier R&D Program (2013M3A6B1078875) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning. J. M., P. K. and L. J. L. thank the great support from King Abdullah University of Science and Technology, Thuwal, Saudi Arabia.

Electronic Supplementary Information (ESI) available: SEM images of TNSTs and the morphology variation relative to the reaction time. Comparative XRD pattern of NTSTs and previous results. Crystalline structure of TiO2 visualized from (100) and (010) directions. Comparative proportion of (001) facets and electrochemical 25



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performances of TNSTs in lithium battery and other kind of TiO2 nanosheets in previous literatures. XRD and SEM images of LiMn2O4 and LiNi0.5Mn1.5O4. Cross-sectional images of lithiated TNSTs and their electrochemical performances in lithium half battery. These materials are available free of charge via the Internet at http://pubs.acs.org.

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{001}

Facets

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Activity.

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59. Brutti, S.; Gentili, V.; Menard, H.; Scrosati, B.; Bruce, P. G., TiO2-(B) Nanotubes as Anodes for Lithium Batteries: Origin and Mitigation of Irreversible Capacity. Adv. Energy Mater. 2012, 2, 322-327. 60. Ming, H.; Ming, J.; Kwak, W. J.; Yang, W. J.; Zhou, Q.; Zheng, J. W.; Sun, Y. K., Fluorine-doped

porous

carbon-decorated

Fe3O4-FeF2

composite

versus

LiNi0.5Mn1.5O4 towards a full battery with robust capability. Electrochim Acta 2015, 169, 291-299.

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For Table of Contents Use Only

Li + 5

Voltage (V)

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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4 3 2

0

100 nm

TNSTs/LiMn2O4, 2.1 V

1

TNSTs/LiNi0.5Mn1.5O4, 2.78 V

0

50

100

-1

150

200


A green strategy to synthesis TiO2 nanosheets with dominant (001) facets and its lithiation study towards sustainable lithium ion battery.

36


Green Strategy to Single Crystalline Anatase TiO2 Nanosheets with

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