NASICON type ordered mesoporous lithium-aluminum-titanium-phosphate as electrode materials for lithium-ion batteries

Accepted Manuscript NASICON type ordered mesoporous lithium-aluminum-titanium-phosphate as electrode materials for lithium-ion batteries Piyali Bhanja, Chenrayan Senthil, Astam Kumar Patra, Manickam Sasidharan, Asim Bhaumik PII:

S1387-1811(16)30525-X

DOI:

10.1016/j.micromeso.2016.11.005

Reference:

MICMAT 7997

To appear in:

Microporous and Mesoporous Materials

Received Date: 24 August 2016 Revised Date:

28 October 2016

Accepted Date: 4 November 2016

Please cite this article as: P. Bhanja, C. Senthil, A.K. Patra, M. Sasidharan, A. Bhaumik, NASICON type ordered mesoporous lithium-aluminum-titanium-phosphate as electrode materials for lithium-ion batteries, Microporous and Mesoporous Materials (2016), doi: 10.1016/j.micromeso.2016.11.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Table of contents NASICON type ordered mesoporous lithium-aluminum-titanium-phosphate

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as electrode materials for lithium-ion batteries Piyali Bhanja, Senthil Chenrayan, Astam Kumar Patra, Mankicam Sasidharan and

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Asim Bhaumik*

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An ordered 2D-hexagonal mesoporous lithium-aluminum-titanium-phosphate material has been synthesized for the first time, which exhibits stable discharge-charge performances with delievered capacity of 121 - 117 mAh.g-1, with a high coulombic efficiency as an anode material

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in a Li-ion battery.

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NASICON type ordered mesoporous lithium-aluminum-titaniumphosphate as electrode materials for lithium-ion batteries

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Piyali Bhanja,a Chenrayan Senthil,b Astam Kumar Patra,a Manickam Sasidharanb and Asim Bhaumik*,a a

Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur,

b

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Kolkata–700 032, India.

SRM Research Institute, SRM University, Kattankulathur, Chennai 603203, India

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Abstract

An ordered 2D-hexagonal mesoporous Lithium-Aluminum-Titanium-Phosphate (LATP-1) material has been synthesized by using cetyltrimethylammonium bromide (CTAB) as the structure directing agent (SDA) thorough hydrothermal route. After acid-ethanol extraction of

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the as-synthesized LATP-1 material, the extracted white solid product has been characterized thoroughly using small and wide angle powder X-ray diffraction (XRD), nitrogen adsorption/desorption analysis, X-ray photoelectron spectroscopy (XPS), Fourier transform

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infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FESEM), Ultra-high resolution transmission electron microscopy (UHR-TEM) and thermogravimetric analysis

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(TGA). Elemental analysis revealed 2.53% Li, 2.02% Al, 4.84% Ti, 28.16% P, 62.44% O in the mesoporous LATP-1 material. The template-extracted LATP-1 material has been employed as anode and cathode materials for lithium ion batteries: as an anode material, the LATP-1 electrode exhibits stable discharge-charge performances with delivered capacity of 121 mAh.g-1 and 117 mAh.g-1, respectively and a high coulombic efficiency. Keywords. Mesoporous materials; Lithium-Aluminum-Titanium-Phosphate; lithium ion battery; anode material; discharge-charge performances. 1

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Introduction

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The invention of ordered mesoporous silicas like M41S [1,2], FSM-16 [3,4] and their versatile application potentials made a major milestone in the material chemistry research and opened a new window to gain total control over tuning of the pore size of a material through the

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introduction of a generalized supramolecular templating approach. Surfactant assisted synthesis of mesoporous materials usually comprised of three steps involving micelle formation in the

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aqueous phase, polymerization of an inorganic source in the presence of surfactant micelles and removal of surfactant from porous architecture [5]. These mesoporous materials have attracted immense attention due to their tunable porosity in the nanoscale regime and these are explored is several frontline applications like gas storage [6], catalysis [7-9], light harvesting [10-12], biosensing [13] and so on. Although, there are numerous reports of silica or metal oxide based

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mesoporous materials using the surfactant templating pathways, synthesis of mesoporous binary or ternary oxides or phosphate based systems involve the complex process of simultaneous aggregation and condensation of multiple inorganic components. Thus, there are only few reports

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of mesoporous mixed metal phosphates and synthesis of such ternary materials using soft-

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templating strategy is still challenging [14,15]. In this context Tian et al [16] have demonstrated self-adjustable synthetic pathways for various porous transition metal oxides and phosphates with tunable pore size. In the field of electrocatalysis [17,18], optoelectronics [19,20], ion exchange [21,22], conductivity [23-25], and Li-ion batteries [26-29] metal phosphates play crucial role in comparison to other porous materials. For the last two decades, researchers have devoted considerable efforts in search of an alternative, environment friendly, and efficient energy storage device due to increasing use of 2

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fossil fuels and gradual depletion of environmental eco-system with an ever widening gap between demand and supply [30-32]. In recent years researchers have developed several energy storage devices but among them only lithium batteries are front-runners due to its long cycle life,

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excellent safety performance, high energy density, which are widely used in electronic gadgets like laptop, cell phone, digital camera and so on [33,34]. As the power density of lithium batteries mainly depends on the Li+ transportation kinetics during insertion and extraction

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processes, diffusion of lithium and electron within electrodes and electrolytes, charge transfer reaction at the electrolyte interface and diffusion of Li+ ion within electrolyte play crucial role.

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As phosphate based crystalline compounds are much more structurally stable than the metal oxide counterparts, it has been widely investigated as electrode materials. Recently, Li1+xAlxTi2x(PO4)3

has been investigated as an efficient solid electrolyte materials for all types of solid state

lithium ion batteries due to their good ionic conductivities [35-39]. NASICON (Na super-ionic

x(PO4)3

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conductor with space group R_3c) type materials having chemical composition Li1+xAlxTi2are interesting due to their three dimensional diffusion framework [40]. In 1976 Hong et

al have innovated NASICON, as an isostructural Na+-conducting material, named by Na1+xZr2P3[41,42]. These Na super-ionic conductors have the advantage of good thermal and

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xSixO12

chemical stability together with high ion-diffusion property. But the electronic conductivity of

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these conductors still limits its application towards a better electrode material for non-aqueous lithium-ion battery [43]. Till date these materials are employed as solid electrolytes in solid state batteries, and in aqueous lithium-ion batteries [44]. Here we have explored the electrochemical property of NASICON based material LATP-1 bearing periodic pores of nanoscale range as the electrodes for lithium-ion batteries. Herein, we report the synthesis of a new 2D-hexagonal mesoporous lithium-aluminumtitanium-phosphate material LATP-1 by using cetyltrimethylammonium bromide (CTAB) as the 3

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structure directing agent (SDA). Mesoporous structure, surface area, framework-bonding, composition, and stability of the LATP-1 material have been investigated thoroughly through small and wide angle powder X-ray diffraction studies, N2 adsorption/desorption, FTIR, UHR-

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TEM, FE-SEM, XPS, TGA/DTA analysis. The LATP-1 has been employed as electrode material in lithium ion batteries using CR-2032 coin type cells with pure lithium as either an anode or a cathode in half-cell configurations.

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EXPERIMENTAL SECTION:

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Material

Cetyltrimethylammonium bromide (M=364.45 g/mol), titanium isopropoxide (M=284.22 g/mol), phosphoric acid 85% aqueous (M=98 g/mol), Lithium hydroxide monohydrate (M=41.96 g/mol) and aluminum chloride hexahydrate (M=241.43 g/mol) were purchased from the Sigma Aldrich. The organic solvents were used as received without further purification. Lithium foils

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(battery grade) and celgard separator were obtained from Sigma-Aldrich and electrolyte LiPF6: EC:DMC (1:1) and copper foils were obtained from Eager corporation, Japan.

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Synthesis procedure of mesoporous LATP-1

In a typical synthesis of LATP-1, 1.82 g (0.0049 mol) of cetyltrimethylammonium

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bromide (CTAB) was dissolved in 10 mL deionized water taken in a 100 mL clean glass beaker. Then 1.14 g (0.0117 mol) of 85% aqueous phosphoric acid (H3PO4) was added dropwise to the former solution followed by addition of 0.24 g (0.0009 mol) aluminum chloride hexahydrate (AlCl3, 6H2O) under constant stirring for 30 min. The 1.608 g (0.0056 mol) of titanium isopropoxide (Ti(OiPr)4) was taken in 5 mL isopropanol solvent and this solution was added to the former solution drop by drop. The resulting white gel was allowed to stir for 45 min followed by the addition of aqueous solution of lithium hydroxide (0.182 g) to the final white gel. The 4

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final gel was stirred for another 1 h and hydrothermally treated under static condition at 348 K for 3 days after transferring the content into a polypropylene bottle. The final pH of the white gel was 5.0. After hydrothermal treatment, polypropylene bottle was cooled down to room

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temperature and the white colored precipitate was filtered through Whatman filter paper using simple filtration technique followed by washing with deionized water for 4-5 times to get rid of unreacted CTAB and other metal sources. About 0.5 g of white solid product thus obtained was

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subjected for acid-ethanol extraction (1N 0.5 g of hydrochloric acid and 20 mL absolute ethanol) to remove template from as synthesized materials. This extraction procedure was repeated for 2

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times to remove most of the organic residues and finally the extracted solid product was washed thoroughly with deionized water for several times. Then the final white solid was dried in air overnight, named as LATP-1, characterized through various techniques. It is seen that with the increase in pH level of the reaction mixture beyond 5.0 the mesophase of the solid LATP-1

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material has been disappeared. Similarly, at lower pH value (ca. 3.0) of the reaction mixture no mesophase was observed from the small angle powder XRD analysis. Thus the optimum pH for obtaining LATP-1 mesophase is ca. 5.0. Without changing any stoichiometric ratio of metal

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sources we have also carried out the synthesis at 273 K where pH of reaction medium was 5.0 and the resulting white solid product has been designated as LATP-2.

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Instrumentation

The powder X-ray diffraction analysis of LATP-1 was carried out on a Bruker D8

Advance SWAX diffractometer operated at voltage of 40 kV and current 40 mA. This instrument was calibrated with a standard silicon sample, using Ni-filtered Cu Kα (λ=0.15406 nm) radiation. Nitrogen adsorption/desorption isotherms were obtained using a Quantachrome Autosorb 1-C surface area analyzer at 77 K. Prior to gas adsorption, sample was degassed for 12 h at 423 K

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under high vacuum conditions. The NLDFT (Non-local density functional theory) was employed to measure the pore size distributions from nitrogen adsorption/desorption isotherm using the cylindrical pore model for oxide based materials as reference. FT-IR spectra of as-synthesized

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and extracted samples were recorded using a Perkin-Elmer spectrum 100 spectrophotometer. For TEM analysis, 15 mg material was dispersed into dry ethanol for 5 min. under sonication. Then one drop of the dispersed solution was dropped onto the carbon coated side of copper grid and

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dried before TEM analysis in vacuum. To analyze the morphology and particle size of the samples JEOL JEM 6700 field emission scanning electron microscope (FE SEM) was used.

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Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the sample were carried out in a TGA instrument thermal analyzer TA-SDTQ-600 under air flow. TGA analysis has been carried out at temperature ramp of 10 °C per minute. The X-ray photoelectron spectroscopy (XPS) analysis was performed using an Omicron Nanotechnology XPS serial no:

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0571 spectrometer using Mg X-ray source. The electrochemical study was carried out utilizing VSP 300 electrochemical workstation (Bio-Logic SAS) at room temperature. Bulk Li, Al and Ti contents

in

LATP-1

were

by

Shimadzu

AA-6300

atomic

absorption

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spectrophotometer (AAS).

determined

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Fabrication of LATP-1 electrode:

The fabrication of working electrode for lithium-ion battery constitute of 80 wt% of

active material methodically mixed with 15 wt % of conductive super P block carbon (SpC) and 5 wt % of binder polyvinylidenedifluoride (PvdF) dissolved in N-methyl pyrolidone as a solvent to obtain a fine slurry. The slurry was transferred and coated over an aluminum foil (thickness 9 µm) for the cathode and copper foil (thickness 17 µm) for the anode, the films was dried in a

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vacuum oven at 80 °C for overnight. The dried film was cut into round disks, the active material loading of the disks were nearly 5 mg/cm2.

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Lithium-ion battery assembly and testing: The lithium-ion battery assembly of LATP-1 electrode was made with CR2032 coin type cells constructed in an argon filled glove box (MBraun) of > 0.5 ppm of O2 and moisture level.

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The counter electrode was used lithium disk of 0.5 cm2. Lithium cut disks (0.5 cm2) served as counter electrode for investigation as both anode and cathode materials. The working and

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counter electrodes were separated by a separator (Glass fiber GF/D), which is saturated with an electrolyte solution. A mixture of 1M of LiPF6 dissolved in 1:1 ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC) was used as the electrolyte solution. Finally the electrolyte solutions were added into the assembly to immerse the electrodes. Galvanostatic chargedischarge studies of the fabricated lithium-ion cells were carried out in the potential range of

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0.05 − 3 V for anode material, and 2.5 − 3.5 V for cathode material at a current rate of 0.1 C. Cyclic voltammogram studies were performed in the potential range of 0 − 4.5 V at a scan rate of

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0.1 mV.s−1. All the electrochemical measurements were carried out using VSP 300 electrochemical workstation (Bio-Logic SAS) at room temperature, unless specified.

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Results and Discussion

Nanostructure analysis

The small angle powder XRD patterns of as-synthesized and extracted LATP-1 materials

are shown in Figure 1. It is noticed from the figure that two samples exhibit strong peaks at 2θ value of 2.4° and 2.8°, corresponding to the 100 (strong) planes and other two characteristic peaks for the 110 (weak) and 200 (weak) for the distinct two dimensional hexagonal mesoporous structure of the material [45]. The small shift of peak position towards higher 2θ value and also 7

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the decrease in d-spacing value of 0.53 nm (from 3.68 nm to 3.15 nm) suggested that after removal of cationic template, the pore has been shrunk in the extracted LATP-1 sample. But LATP-2 material does not show any peak in small angle region, suggesting the absence of

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ordered mesopores in this material. On the other hand the wide angle XRD pattern of LATP-1 (electronic supporting information Figure S1a) suggested the presence of broad peak corresponding to a semi-crystalline nature of the pore wall. Moreover, the wide angle XRD

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pattern of LATP-2 as shown in Figure S1b, suggested the amorphous nature of material. These results suggested that elevated synthesis temperature (348 K) is needed for the synthesis of

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mesoporous lithium-aluminum-titanium-phosphate material. Surface area analysis

The nitrogen adsorption/desorption isotherms for LATP-1 are shown in Figure 2, which displayed the type IV isotherm without any hysteresis loop [46,47]. The BET (Brunauer, Emmett

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and Teller) surface area of LATP-1 material was found to be 76 m2g-1 with a pore volume of 0.0704 ccg-1. By employing Non Local Density Functional Theory (NLDFT), the pore size distribution has been calculated to be 3.0 nm as shown in the inset of Figure 2. The De Boer

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statistical thickness (t-plot) represents that the surface area of LATP-1 material is mainly due to

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the mesopores as the contribution of micropores to the surface area is negligible. The physicochemical properties of LATP-1 material have been calculated and listed in Table S1 (supporting information).

Spectroscopic analysis

The FTIR spectra have been recorded for both the as-synthesized and template-extracted LATP-1 in the range of 4000-400 cm-1. The representative FTIR spectra for the as-synthesized and extracted LATP-1 samples are shown in Figure S2 (supporting information). Figure S2a and 8

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S2b represent the FTIR spectra of as-synthesized and extracted materials, respectively. As seen from the Figure S2a, the characteristic peaks appeared at 1638, 2922 and 2849 cm-1 are due to the presence of adsorbed water and –C-H group in the organic template (CTAB) of as-

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synthesized material. Almost disappearance of these two characteristic peaks in extracted sample confirms the almost complete removal of the CTAB molecules from the as-synthesized material [48]. Broad peaks observed at 3419 cm-1 in Figure S2a/2b could be ascribed due to the presence

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of defect –O−H groups present at the surface of mesoporous LATP-1 material.

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Electron microscopic analysis

The scanning electron microscopic images for LATP-1 material are shown in Figure 3. As seen from the figure, the material has flower like of morphology with a particle dimension of ca. 700 nm. Figure 3a and 3b displayed the FESEM images for the LATP-1 material at low and high two different magnifications, respectively. The transmission electron microscopic analysis

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has been performed for extracted LATP-1 material. Figure 4a displayed the high resolution TEM image of LATP-1 material. As seen from this image that pores are arranged in hexagonally

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ordered honeycomb like fashion corresponding to the other 2D-hexagonal mesoporous materials synthesized in the presence of CTAB template [46,47]. The pore width estimated from this TEM

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image is of ca. 2.7 nm. Thermal analysis

The quantitative analysis on the stability of the porous framework upon heat treatment of

the template free and as-synthesized mesoporous LATP-1 material is estimated from the thermogravimetric and differential thermal analysis (TG/DTA). Figures 5a and 5c displayed the TGA pattern of both LATP-1 materials in temperature range of 25 to 500 °C, where the first weight loss in Figure 5a corresponds to the evaporation of surface adsorbed water molecules and 9

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second weight loss up to 500 °C temperature is attributed to the decomposition of organic template molecule. Figure 5c represents the TGA pattern of the template-free LATP-1 material, where the first weight loss up to 95 °C could be attributed to the adsorbed water molecule from

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the material surface and there is no other significant weight loss up to 500 °C, suggesting the material has high thermal stability.

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X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy is one of the most important characterization tools to

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understand the elemental composition of a material is solid state. The narrow and full range XPS spectrum of as-synthesized LATP-1 material is shown in Figures 6 and 7. Figure 6a, 6b, 6c, 6d, 6e and 6f represent the narrow XPS spectrum of Li1s, Al2p, Ti2p3/2, Ti2p1/2, P2s, P2p and O1s with binding energy of 59.1 eV, 74.2 eV, 457.3 eV, 463.1 eV, 190.7 eV, 134.2 eV and 531.9 eV, respectively [49,50]. Further, the full range XPS spectrum of LATP-1 material is shown in

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Figure 7. By employing CASA software, the elemental analysis has been performed. XPS analysis revealed the elemental composition of LATP-1 framework as 2.53% Li, 2.02% Al, 4.84% Ti, 28.16% P, 62.44% O. Thus, the chemical formula of the LATP-1 material based on

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this surface chemical analysis data is Li2.53Al2.02Ti4.84P28.16O62.44. Further, bulk chemical

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composition of LATP-1 has been estimated through AAS analysis and it revealed 2.44% Li, 1.98% Al and 4.96% Ti in LATP-1. This result agrees well with the surface compositional analysis performed through XPS analysis within the limit of experimental errors. Electrochemical studies

Cyclic voltammograms (CV) of the LATP-1 electrode are shown in Figure 8. The CV studies were carried in the entire potential range of 0 – 4.5 V, at a scan rate of 0.1 mV.s−1 and for clarity only few cycles are shown. The NASICON structured electrode show the redox peaks 10

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located at 1.63 and 2.03 V corresponding to the weak peak of TiO2. Additionally a weak redox couple located at 2.33 V and 2.55 V and a broad peak at 2.67 V to 2.90 V corresponds to the oxidation-reduction27 couple of Ti4+/Ti3+. During the subsequent redox process, broad peak is

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visible originating at regions of 2.65 to 2.95 V reflecting the polarization of the electrode and is apparent to trace the very sharp redox behavior of titanium in the subsequent cycles.

Galvanostatic charge–discharge profile for the LATP-1 electrode as cathode material has

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been studied in the voltage window 2.5-3.5 V (vs Li+/Li) at current rate 0.1 C (1 C = 0.515

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A.g−1) is shown in Figure 9. Charge-discharge studies for the electrode are performed up to 50 cycles and for representation only few cycles are shown. Considering LATP-1 as cathode, the charge capacity observed in the first cycle is 1123 mAh.g-1, while the subsequent discharge capacity is only 103 mAh.g-1; this high irreversible capacity loss in the first charge-discharge cycle is presumably due to the electrolyte decomposition leading to an irreversible formation of a

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stable solid electrolyte interphase (SEI) [51] or could be associated to any irreversible reaction by the residual surfactant causing lithium losses at the electrodes, which supports the results observed from TG/DTA and CV analysis. Figure 10 showed the charge-discharge cycling

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performance of the LATP-1 constructed as a cathode electrode. It is clear from the 2nd to 10th cycle; the electrode experiences a less irreversible capacities loss, i.e. 220 mAh.g-1 to 99 mAh.g-1

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in the charge cycles and 98 mAh.g-1 to 78 mAh.g-1 in the discharge cycles. From the successive 20th, 30th, 40th and 50th charge-discharge cycles; a stable charge-discharge capacities of 79, 76, 74, 73 mAh.g-1 for charging and 77, 75, 72, and 71 mAh.g-1 for the corresponding discharge capacities are observed for the LATP-1 as cathode material with coulombic efficiency of 97%. This high coloumbic efficiency of NASICON based LATP-1 electrode is quite appreciable, despite of its decreased charge/discharge capacities.

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Alternatively, we have also investigated LATP-1 as anode material and discharge-charge studies are performed in the voltage window 0.05-3V (Figure 11). LATP-1 electrode delivered discharge capacity 976 mAh.g-1 and charge capacity 159 mAh.g-1 in the first cycle. This initial

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capacity loss is due to SEI formation as discussed earlier [52]. Figure 12 show the dischargecharge performance of LATP-1 as anode material cycled for 50 cycles; the discharge and charge capacity slowly fades and attains 137 mAh.g−1 and 126 mAh.g−1, respectively at end of 10th cycle

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and thereafter the electrode attains the stability. After 50 consecutive cycles, the dischargecharge capacity observed is 122 and 117 mAh.g-1, which is very similar to the capacity observed

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during the 15th cycle. It is noteworthy that the capacity of LATP−1 negative electrode (anode) retains its capacity after 15th to 50th cycle without any loss when compared to its counterpart as cathode (positive electrode). We believe that the reason for high reversible capacity of the LATP-1 electrode may be due to the presence of Al ions in the host facilitating for better

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electronic conductivity and in storing lithium ions over the surface or at sites [53]. The rate profile and rate capabilities of LATP-1 material as anode are shown in Figures 13 and 14, respectively. As the current rate increases from 0.1 C to 5 C, (1 C = 0.515 A.g-1) specific

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capacity of the electrode decreases as with other electrode materials reported. The dischargecapacities of the electrode cycled for five cycles at different rates of 0.1 C, 0.5 C, 1 C, 3 C and 5

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C is 113, 105, 92, 71, and 52 mAh.g−1, respectively; whereas the charge capacities are found to be 108, 99, 85, 63, and 49 mAh.g−1 respectively for 0.1 C, 0.5 C, 1 C, 3 C and 5 C. After subjecting the electrode to high current rate of 5 C, the electrode again subjected to a current rate of 0.5 C. Then the electrode regained the discharge-charge capacity of 96 and 93 mAh.g-1, which is comparable to the capacity observed at current rate 0.1 C after the 10th cycle. The substantial rate performance of the LATP-1 electrode as anodes at a different current rate indicates the structural stability of the electrode, which confirms the effective contribution of the 2D12

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hexagonal mesoporous structure in facilitating the volume changes during electrochemical reaction [54,55] and also the role of aluminum ions. Furthermore, we are also investigating composite electrodes comprising of LATP-1 and graphene oxide. The preliminary results exhibit

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improved charge/discharge capacity as shown in Figure S3. Thus, Na super ionic conductors with three dimensional diffusion frameworks, needs better understanding to make use of the 3D

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diffusion frameworks as suitably applicable for lithium-ion batteries. Conclusion

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Our experimental results revealed that an ordered 2D-hexagonal mesoporous lithiumaluminum-titanium-phosphate material LATP-1 can be synthesized by using CTAB as a template under hydrothermal conditions. Characterization results revealed that LATP-1 has 2Dhexagonal mesophase, which is useful for the electrochemical application. The electrochemical

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measurement of this material has been performed both as anode and cathode material using cyclic voltammogram studies and galvanostatic charge-discharge cycles. As an anode material, the LATP-1 electrode exhibits stable discharge-charge performances after 50 cycles with

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delievered capacity of 121 mAh.g-1 and 117 mAh.g-1, respectively together with a high coulombic efficiency of 94% compared to its ability as cathode materials. Thus, the synthesis of

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mesoporous lithium-aluminum-titanium-phosphate material and its electrochemical results reported herein may open a fresh outlook to explore it as an anode material in Li-ion batteries. Acknowledgement

PB thanks to CSIR, New Delhi for the junior research fellowship. AB wishes to thank DST, New Delhi for instrumental facilities through DST Unit on Nanoscience, DST-SERB and DSTUKIERI project grants. MS thanks DST Nano Mission (Ref. SR/NM/NS-1099/2012G), New Delhi for a project grant. 13

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Scheme 1. Schematic representation for the formation of ordered 2D hexagonal mesoporous

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mixed metal phosphate material (LATP-1).

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Figure 1. Small angle powder XRD pattern of the as-synthesized (a) and template-extracted (b)

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LATP-1 material. Respective planes are indexed in Figure 1a.

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Figure 2. N2 adsorption/desorption isotherm of LATP-1 material. The filled and empty circle

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represents the adsorption and desorption of nitrogen gas, respectively. The pore size distribution

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plot estimated by employing NLDFT theory is shown in the inset of the figure.

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Figure 3. Scanning electron microscopy images of LATP-1 material at two different

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magnification low (a) and high (b).

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Figure 4. TEM image of LATP-1 material.

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Figure 5. TGA/DTA plot for LATP material where (a), (c) and (b), (d) represents TGA and DTA curve of as-synthesized and extracted LATP material.

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Figure 6. X-ray photoelectron spectra of three metals present in LATP-1 material (a) Li1S, (b) Al2P, (c) Ti2P, (d) P2S, (e) P2p and (f) O1S.

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Figure 7. The X-ray photoelectron spectrum in full range for LATP material.

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Figure 8. Cyclic Voltammogram of NASICON type LATP-1 electrode at a scan rate of 0.1

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mV.s−1 in the potential range of 0 – 4.5 V vs Li+/Li.

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Figure 9. Galvanostatic charge-discharge profile of NASICON type LATP-1 electrode cycled between 2.5 V to 3.5 V at a rate of 0.1 C.

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Figure 10. Capacity retention plot of NASICON type LATP-1 electrode cycled between 2.5 V to

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3.5 V at a rate of 0.1C.

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Figure 11. Galvanostatic charge-discharge profile of NASICON type LATP-1 electrode cycled between 0.05 – 3 V at a rate of 0.1C.

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Figure 12. Capacity retention plot of NASICON type LATP-1 electrode cycled between 0.05 to

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Figure 13. Rate performance profile of NASICON type LATP-1 electrode cycled between 0.05

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Figure 14. Rate Capability Studies of NASICON type LATP-1 electrode cycled between 0.05 to

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Research Highlights •

2D-hexagonal mesoporous lithium-aluminum-titanium-phosphate (LATP-1) has been

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synthesized for the first time.

Cationic surfactant CTAB was used as template for the synthesis of mesoporous LATP-1.

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LATP-1 has been employed as anode material for lithium ion batteries.

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LATP-1 electrode exhibits stable discharge-charge performances with delievery capacity of

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121 mAh.g-1.