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Synthesis of nanostructured lithium cobalt oxide using cherry blossom leaf templates and its electrochemical performances
Accepted Manuscript Title: Synthesis of nanostructured lithium cobalt oxide using cherry blossom leaf templates and its electrochemical performances Author: M. Jeevan Kumar Reddy Sung Hun Ryu A.M. Shanmugharaj PII: DOI: Reference:
S0013-4686(15)31004-5 http://dx.doi.org/doi:10.1016/j.electacta.2015.12.079 EA 26228
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
24-8-2015 3-11-2015 11-12-2015
Please cite this article as: M.Jeevan Kumar Reddy, Sung Hun Ryu, A.M.Shanmugharaj, Synthesis of nanostructured lithium cobalt oxide using cherry blossom leaf templates and its electrochemical performances, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.12.079 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.
Synthesis of nanostructured lithium cobalt oxide using cherry blossom leaf templates and its electrochemical performances M. Jeevan Kumar Reddy, Sung Hun Ryu* [email protected], A. M. Shanmugharaj* [email protected] Department of Chemical Engineering, Kyung Hee University, Yongin-si, Gyeonggi-do 446-701, Republic of Korea *
Corresponding author: Tel.: +82-31-201-3342.
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Graphical Abstract
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Highlights
Sol-gel synthesis of LiCoO2 (LCO) in cherry leaf template is reported for first time. BET Surface area is higher for LiCoO2 prepared in cherry blossom leaf templates. Excellent charge-discharge property is observed for LiCoO2 prepared in leaf template. Enhanced cycling stability is observed for LiCoO2 synthesized using leaf template.
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Abstract Nanostructured high surface area lithium cobalt oxide (LCO-CBL) cathode materials with uniform particle sizes are prepared by template assisted sol-gel synthesis using metal precursors and urea chelating agent in cherry blossom leaf templates followed by the calcination of the prepared material at 800oC for 12 hrs. Structural and morphological characterization using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), field emission transmission electron microscopy (FE-TEM) and Raman spectroscopy of LCO-CBL shows the presence of rhombohedral (layered) structures quite similar to the pristine LCO, synthesized by simple sol-gel method. BET surface area measurements of the prepared cathode material revealed that the surface area increased about 79.7 % for LCO-CBL, when compared to pristine LCO. Electrochemical characterization of the prepared LCO-CBL delivered an initial discharge capacity of the 166 mAhg-1 with the capacity retention of 81 % in comparison to the pristine LCO that exhibit initial discharge capacity of 127 mAhg-1 and capacity retention of less than 72 %.
Keywords: Cherry blossom leaves; lithium cobalt oxide; sol-gel synthesis; specific discharge capacity; BET surface area.
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Introduction High performance lithium ion batteries (LIBs) have gained the great success in powering portable devices such as cellphones, notebooks etc., owing to their highest energy density and output voltage among the present rechargeable batteries [1]. However, it is desired to further increase their energy density, especially at high charge/discharge rates, to meet the requirements of electric vehicles and smart grids. Development of LIBs with high energy density relies on the successful development of cathode materials with high energy density. Most commonly, layered lithium metal oxides LiMO2 (M=Co, Ni) [1-3] and spinel lithium manganese oxide (LiMn2O4) have been used as cathode materials for lithium-ion batteries [4-9]. As a result of the intense search for high specific energy cathode materials for use in lithiumion rechargeable battery technology, Lithium cobalt oxide (LiCoO2) has become the first, and one of the most promising for commercial application. LiCoO2 is the most attractive cathode material owing to its best performance in terms of high specific energy density and excellent cycle life. Although theoretical capacity of LiCoO2 (0
materials [26-28]. Several morphologically controlled nanostructured materials viz. thin films [29, 30], desert rose flower like structures [31], composites with carbonaceous materials [32] and template assisted structures like spheres, nano rods [27] nano wires [33] have been reported. The surface area of the cathode material is one of the important factor for sustaining more number of Li ions in the structure that leads to better performance in cycling process [34]. Template assisted synthesis of nanostructured materials resulted in the formation of porous structures providing high surface area as well as morphological support [35]. Natural leaves such as cherry blossom leaves have demonstrated to have three dimensional (3D) elaborated architectures with high porosity, high connectivity and high surface areas for efficient mass loading including gas exchange for photosynthesis and water transportation [36]. Owing to their 3D unique hierarchical structures, cherry blossom leaves could be used as ideal template for the synthesis of electrodes [36]. In the present study, three dimensional (3D) hierarchical architectured cherry blossom leaves (CBL) have been used as the template for the preparation of LiCoO2 nanostructures. The prepared materials have been characterized using various characterization tools to understand the surface composition, morphology and surface area. Finally, the lithium storage properties of the prepared materials were determined by fabricating lithium-ion half cells and characterized for its electrochemical performances.
2. Experimental 2.1 Sol-gel synthesis of pristine LiCoO2 (LCO) and template assisted LiCoO2 (LCO-CBL) Pristine lithium cobalt oxide (LCO) and template assisted lithium cobalt oxide (LCOCBL) were synthesized via. sol-gel synthesis route using lithium acetate dihydrate (Sigma 6
Aldrich, USA) as lithium source, cobalt acetate tetra hydrate (Sigma Aldrich, USA) as cobalt source and urea (Sigma Aldrich, USA) as chelating agent. Ammonium hydroxide was used for controlling the pH of the reaction during the gel precursor synthesis. In the typical synthesis of pristine LCO, precursor solutions of lithium (1M) and cobalt (1M) were mixed and heated to 90oC under constant stirring conditions for few hours. To this desired quantity of urea chelating agent (1M) was slowly added followed by the addition of ammonium hydroxide solution so as to maintain the pH to about 8.5~9.5. The obtained gel precursor was dried under vacuum for 12 hrs to remove the solvent completely. The powdered precursors obtained by grinding the dried samples were calcined at 800oC for 12 hrs in air atmosphere to get black colored pristine LCO samples [37]. Template assisted sol-gel synthesis of Lithium cobalt oxide (LCO-CBL) was done in quite similar as that of pristine LCO using treated cherry blossom as carbon templates. Pretreatment of the cherry blossom leaves were done followed by the method proposed by Zhou et al.,[36]. In short, Fresh green leaves (cherry blossom) were cut into 1 cm х 1 cm pieces and washed with deionized water followed by immersion in 5 % HCl overnight to get rid of magnesium (Mg), potassium (K), calcium (Ca) and phosphorous (P) and other ions. Then the treated leaves were washed with deionized water and ethanol several times followed by drying of treated leaves at room temperature [38]. The typical synthesis procedure of LiCoO2-CBL is summarized as follows: Precursor solutions of lithium (1M) and cobalt (1M) were mixed in the presence of two grams of pretreated cherry blossom leaves and it was heated to 90oC under constant stirring conditions for few hours. To this desired quantity of urea chelating agent (1M) was slowly added followed by the addition of ammonium hydroxide solution so as to maintain the pH to about 8.5~9.5. The obtained gel precursor/leaf composite was dried under vacuum for 12 hrs to remove 7
the solvent completely. Then the samples were calcined at 800oC for 12 hrs under air atmosphere and ground to get ash colored powders.
2.2 Measurement of structural and electrochemical properties The synthesized LCO and LCO-CBL samples were deposited onto a carbon coated copper grid using particle dispersant (dispersed in ethanol), dried and observed by transmission electron microscope (TEM, JEM-2100F, JEOL), powder samples were analyzed using X-ray diffractometer (XRD, D8 Advance, Bruker). Surface elemental compositions of LCO and LCOCBL
were
determined
using
X-ray
photoelectron
spectroscopy
(XPS,
K-Alpha,
Thermoelectron). The samples for XPS measurements were prepared by casting the particle dispersant (dispersed in ethanol) onto silicon wafer followed by drying under vacuum at 70 °C. The synthesized LCO-CBL were also characterized by Fourier Transform Infrared Spectroscopy (FT-IR, Spectrum one, Perkin-Elmer) in KBr matrix. Thermogravimetric analysis (TGA) of LCO-CBL was characterized in the temperature range of 25–900 °C at the heating rate of 10 °C/min in air environment using TA instruments (TGA Q5000 IR/SDT Q600). The specific surface areas were determined with a surface-area analyzer (BEL Sorp-II mini, BEL Japan Co., Japan) by the Brunauer−Emmett−Teller (BET) method. The electrochemical performances of pristine LOC and LCO-CBL cathode material were measured by fabricating 2032-type coin Li-ion half cells. Electrodes were prepared by casting a slurry with a composition of 80 wt. % active material, 10 wt. % of super P (TIMCAL) as additional conductive agent and 10 wt. % of poly(vinylidene difluoride) (PVDF) (Kureha KF100) as binder onto an Aluminum foil which acts as the current collector. The slurry was prepared by grinding the mixture in the presence of N-methyl pyrrolidone (NMP) solvent using 8
mortar for 15 min. The viscous slurry coated onto Aluminum foil was dried in oven at 100 °C for 5 h. The dried coating electrode was pressed under a 7T load and then punched out with size of 16 mm in diameter. The active material mass loading was ~2.0 mg cm−2. The electrode was assembled into 2032 type coin type cell with 1 M LiPF6 solution in a 1:1 (volume) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (Merck Co.) as an electrolyte. Metallic lithium was used as the counter and reference electrode. Electrochemical properties of the coin type cells were studied using galvanostatic charge and discharge measurements in the voltage range of 2.5–4.2 V vs. Li/Li+ using Wonatech battery analyzer. Cyclic voltammetry curves were measured at 0.1 mV s−1 within the range of 2.5–4.2 V using an electrochemical work station (VersaSTAT 3 Electrochemical System). Electrochemical impedance studies of the Li-ion cells were performed using an electrochemical work station (VersaSTAT 3 Electrochemical System) by applying sine wave with amplitude of 5.0 mV in the frequency range of 100 kHz to 0.01 Hz.
3. Results and Discussion The schematic representation showing the cherry blossom leaf template assisted synthesis of lithium cobalt oxide cathode materials is depicted in Fig. 1 (a). The cherry blossom leaves were initially pretreated using 5 % hydrochloric acid following the procedure reported by Zhou et al. [36], to remove inorganic metal ions such as magnesium, potassium, phosphorus and calcium. Stoichiometric composition of the precursor solution and chelating agent was then impregnated in the pretreated cherry blossom leaves followed by calcination at 800oC resulted in the formation of the ash colored crystalline LiCoO2 (LCO-CBL) powder with trace amount of carbonaceous matter (Fig. S1; Table S1). Pristine LiCoO2 (LCO) was also synthesized by using same composition of precursor and chelating agent without cherry blossom leaf templates. 9
Phase purity and crystallographic patterns of the LCO and LCO-CBL were determined using powder X-ray diffraction (XRD) and the results are included in Fig. 1(b). As observed all the patterns exhibit metallic fingerprint peaks with orientations of (003), (101), (006), (102), (104), (015), (107), (108), (110) and (113) at 2θ angles of 18°, 37°, 38°, 39°, 45°, 49°, 59°, 65°, 66° and 69° respectively. From the XRD pattern, it can be seen that no impurity peaks were found in the LCO-CBL prepared by template assisted method and the results are quite consistent with the materials prepared by sol-gel method, revealing the formation of ultrapure nanocrystalline materials. Clear splitting of the doublet peaks at (006/102) and (108/110) both in case of LCO and LCO-CBL suggests the formation of layered structures with typical R3m space group symmetry and it could be ascribed to the ordered distribution of lithium in the 3a sites [38]. XRD pattern matches with the JCPDS data card number 50-0653 for LCO and 16-0427 for LCO-CBL respectively [8, 27]. The intensity ratio of diffraction peaks of (I(003)/I(104)) and ((I(006)/I(102))/I(101)) were calculated and the results are observed to be 2.06997 (I(003)/ (104)) and 0.37208 ((I(006)/I(102))/I(101)) for LCO, which is quite consistent with the values displayed in JCPDS file No. 50-0653. Similarly, the intensity ratio values are observed to be 2.07082 (I(003)/ (104))
and 0.39555 ((I(006)/I(102))/I(101)) for LCO-CBL and the results are consistent with the values
displayed in JCPDS file No. 16-0427. These results indicated slight increase in layered characteristics with good phase structure for LCO-CBL in comparison to pristine LCO [15, 39]. The interlayer distance i.e. d-spacing plays a vital role in the charge discharge of a lithium ion battery, The average d-spacing values calculated using Bragg’s equation of (003) reflections are observed to be 4.2673 Å (LCO) and 4.67316 Å (LCO-CBL) respectively. Slight increment (~9.5 %) in average d-spacing values for LCO-CBL could effectively improves the Li+ ion diffusion during intercalation and deintercalation process, when compared to the pristine LCO. 10
The crystallite domain sizes of the LCO and LCO-CBL were calculated using the well- known Scherrer’s formula [40, 41] and the results are included in Table 1.
D
K cos
Where D is Crystallite domain size K is Scherrer’s constant (0.9) λ is X-Ray wave length (1.541836) β is Angular width of diffracted peak at the half maximum (FWHM-Full Width Half Maximum) in radians for diffraction angle θ is the diffraction angle The experimental values calculated using (003) diffraction peak is in agreement with the calculated crystallite domain size (Table 1). Significant reduction in crystallite domain sizes (~66.7 %) for LCO-CBL in comparison to LCO is attributed to the 3D architectured structure of cherry blossom leaf that restricts the crystallite growth of LCO during the sintering process. Fig. 2 (a-c) shows a typical field emission scanning electron microscopic (FE-SEM) images of LCO and Fig. 2 (d-f) are LCO-CBL at different magnifications. While, LCO displayed more aggregated and uneven particle shapes and particle sizes ranging from 300~1200 nm (Fig. 2 (a-c)), whereas, LCO-CBL showed macro porous 3D architectured structure consisting of embedded LCO particles with uniform particle distribution ranging from 100~600 nm (Fig. 2 (df)). Few more FE-SEM images of LCO-CBL consisting of 3D architectured structures is included in supporting information, which clearly depicts the entrapped LCO formation in the cherry blossom leaf templates (Fig. S2 (a-f) and Fig. S3 (a-d)). In contrast to earlier report on the synthesis of nanostructured materials using cherry blossom leaf templates, the present work 11
doesn’t show the presence of micro-tubular nanostructures and this fact may be attributed to the preferential growth of LCO particles on the cylindrical vein structures of cherry blossom leaves rather than the growth of nanostructures in the interconnected micro-tubular structure (Fig. S2 (g, h) [36]. The preferential growth of LCO particles on cylindrical veins results in the complete degradation of interconnected micro-tubular structures of cherry blossom leaves during calcination resulting in the formation of fused LCO particles with relatively fine particle size distribution. The particle sizes and crystalline nature of LCO and LCO-CBL were further corroborated using field emission transmission electron microscopy characterization (FE-TEM) and the results are displayed in Fig. 2 (g, h) and Fig. S4. TEM images showed that LCO and LCO-CBL consists of equiaxed particles ranging from 300~500 nm as shown in Fig. 2 (g, h). Slight variation in contrast of bright field TEM image revealed a single, well-ordered α-NaFeO2 phase structure both in case LCO and LCO-CBL [42]. To understand the range of particle sizes present in LCO and LCO-CBL, TEM characterization was done at various magnification and representative results are displayed in Fig. S4. Pristine LCO has particle sizes in the range of 300~1500 nm and LCO-CBL has sizes in the range of 100 to 600 nm. The crystalline nature of LCO and LCO-CBL samples are further corroborated using selected area electron diffraction patterns (SAED) patterns as shown Fig. 2 (g, h) (inset). While, the pristine LCO exhibit SAED patterns with (hkl) values (003), (104), (101), (006) and (102), LCO-CBL showed bright spots with (hkl) values of (003), (104) and (110) planes clearly corroborating the crystalline nature of the materials [27, 43]. Absence of certain crystalline planes in LCO-CBL may be attributed to the restricted growth of crystals due to the usage of cherry blossom templates.
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The local cationic order in a close packed oxygen array of LCO and LCO-CBL was reexamined using vibrational spectroscopic measurements viz. Raman and Fourier transform Infrared spectroscopy. Vibrational modes, which correspond to vibrations involving primarily atomic motion of cations against their oxygen neighbors, are sensitive to the space group and consequently, these modes are very sensitive to the cationic local environment in the host matrix [44]. Fig. S5 (a) showed the Raman spectroscopic measurements of LCO and LCO-CBL recorded at room temperature. According to a factor group analysis, the layered phase ( 3m space group) exhibits only two Raman-active modes, i.e. the A1g and the Eg. In the present investigation, both LCO and LCO-CBL showed two Raman active modes at 590 cm-1 (A1g) and at 480 cm-1 (Eg) confirming the layered structure with 3m space group symmetry. Appearance of the broad peak at. 615 cm-1 for LCO may be attributed to polyhedral distortion of octahedral CoO6 groups. Interestingly, the peak at 615 cm-1 doesn’t appear in case of LCO-CBL corroborating that polyhedral distortion of CoO6 groups is hindered by using CBL templates for LCO synthesis [45]. Fig. S5 (b-c) showed the FT-IR results of LCO and LCO-CBL. Pristine LCO displayed two sharp and intense peaks between 450 ~ 600 cm-1 along with the small hump at around 650 cm-1, which corresponds to Li-O vibration of tetrahedral LiO4 structure (~500 cm1
), Co-O vibration of octahedral CoO6 structures (~600 cm-1) and Co-O vibration of tetrahedral
CoO4 structures (~650 cm-1) [46]. The surface chemical compositions of the LCO and LCO-CBL materials have been studied by X-ray photoelectron spectroscopy (XPS) and the results are displayed in Fig. 3 and Fig. S5. Suvey scan results of both LCO and LCO-CBL exhibit spectral peaks corresponding to Li1s, Co3p, Co3s, C1s, O1s and Co2p centered at 52, 61, 104, 285, 530 and 780 eV respectively (Fig. 3 (a)). Significant rise in peak intensity ratio (I285/I780) corresponding to C1s (285 eV) and 13
Co2p (780 eV) peaks from 1.9 (LCO) to 3.21 (LCO-CBL) revealed the rise in carbonaceous matter due to the usage of cherry blossom leaves as templates. High resolution Co2p core peaks of LCO and LCO-CBL are shown in Fig. 3 (b, c). Due to the spin-orbit coupling, Co2p spectrum of pristine LCO is split in two parts Co2p3/2 (A; 779 eV) and Co2p1/2 (D; 793.5 eV), with the intensity value almost 2.5 times higher for the former (Co2p3/2) compared to the later (Co2p1/2) (A/D = 2.5). Apart from the main peaks (A & C), each part consists of a satellite peaks at 788 eV (B) and at 803 eV (F). Similarly, Co2p spectrum of LCO-CBL also split into two parts Co2p3/2 (A; 779 eV) and Co2p1/2 (D; 794 eV), with the intensity value almost 2.5 times higher for the former (Co2p3/2) compared to the later (Co2p1/2) (A/D = 2.5). Apart from the main peaks (A & C), each part consists of a satellite peaks at 789 eV (B) and at 803 eV (F). Appearance of main peaks (A & D) together with a satellite peak (C & F) results from a ligand-to-metal charge transfer during the photoemission process and can be interpreted, at the simplest level of approximation, by a molecular orbital description. In the initial state LCO & LCO-CBL has six electrons in the cobalt 3d shell and a filled ligand shell L (oxygen 2p shell), which can be written 2p63d6L. The Co 2p photoemission process can lead to several final states after creation of the 2p core hole (2p5). The main line A is mainly characterized by the 2p53d7L-1 configuration, where one electron is transferred from the ligand shell L to the metal 3d shell (screening effect), resulting in a 3d7 configuration. The shake-up satellite B can be assigned to 2p53d6L and 2p53d8L-2 configurations [47]. Appearance of the satellite peaks both in the case of LCO and LCO-CBL clearly indicated the existence of Co3+ and Co4+ ions in the prepared materials. The Co2p3/2 peak of LCO also displayed a weak shoulder at 780.4 eV (B) with upon deconvolution showed relative area of 18.1 %. This fine structure can also be noticed on the Co2p1/2 component by the presence of a shoulder at 795 eV (E) with relative area of 12.6 % (Fig. 3 (b)). 14
Alternatively, the Co2p3/2 peak of LCO-CBL displayed a weak shoulder at 780.5 eV with upon deconvolution showed relative area of 14.2 % and Co2p1/2 component showed a weak shoulder at 795 eV with relative area of 9.1 % (Fig. 3 (c)). Appearance of this shoulder peak both in case of LCO and LCO-CBL is attributed to the final state of the photoemission process like in the case of NiO (nonlocal screening) [48]. Deconvolution of O1s peaks of LCO and LCO-CBL were carried out and the results are included in Fig. S6. The spectrum of pristine LCO and LCO-CBL displayed two characteristic peaks at 529.0 and 531.2 eV, which corresponds to O2- anions (529.0 eV) of the crystalline network and oxygen anions in subsurface (531.2 eV) with deficient coordination [48]. Decrease in peak intensity ratio (I529.5/I531.4) from 0.84 (LCO) to 0.42 (LCOCBL) revealed the rise in oxygen anions with deficient coordination in LCO-CBL, when compared to LCO. Deconvolution of high resolution C1s spectra of LCO showed peaks corresponding to C-C/C-H (284.5 eV), C-O (285.7 eV) and C=O (287.8 eV) groups, whereas LCO-CBL exhibited the peaks corresponding to Co-C (283.8 eV), C-C/C-H (284.5 eV), C-O (285.7 eV) and C=O (287.7 eV) groups corroborating the interaction between carbonaceous matter and LCO in LCO-CBL (Fig. S6). Deconvolution of high resolution spectra in the range of 51~61 eV of LCO and LCO-CBL showed two peaks at 52 eV and 59.9 eV (Fig. 3 (d & e)). While the peak at higher binding energy (59.9 eV) is attributed to Co3p, the peak at lower binding energy (52 eV) corresponds to interstitial Li ions. The deconvoluted area ratio of Li1s and Co3p peaks (Li1s/Co3p) reflects the relative concentration of Li present in LCO and LCOCBL (Fig. 3 (d & e)). Relative area (Li1s/Co3p) ratio increases to about 47.6 % for LCO-CBL in comparison to the LCO revealing higher concentration of lithium on the surface. The surface area of pristine LCO and LCO-CBL were determined using BET analysis and the results are included in Fig. 4. The N2 adsorption-desorption curves of LCO-CBL showed 15
a clear hysteresis loop in the range of 0.5~1.0 revealing type IV of the IUPAC classification, suggesting that these samples are mesoporous, when compared to the pristine LCO, which showed a small hysteresis loop in the range of 0.9~1.0. The N2 surface area of LCO-CBL determined using BET method is observed to be 1.898 m2g-1 and it is significantly higher (~79.7 %) in comparison to the pristine LCO (1.056 m2g-1). The electrochemical cycling performance of LCO and LCO-CBL were examined over the range 2.5–4.2 V vs Li+/Li and the initial charge-discharge curves are included in Fig. 5. At a low rate (0.1 C), the first discharge capacity of LCO-CBL (166 mAhg-1) is 30.7 % higher than that of pristine LCO (127 mAhg-1) at the cutoff potential of 2.5~4.2 V (vs Li+/Li). After first electrochemical cycling, specific discharge capacity of LCO decreased to about 6.3 % with capacity value of 119 mAhg-1 in second cycle (Fig. S7). Similarly, the specific discharge capacity of LCO-CBL decreased to about 6.4 % with capacity value of 156 mAhg-1 in second cycle (Fig. S7). The cyclability of the LCO and LCO-CBL electrodes is examined under longterm cycling over 50 cycles, which demonstrated a good cyclic performance and reversibility (Fig. 6). After 50th cycle, LCO-CBL electrodes still maintained specific capacity of 134 mAhg-1 (~19 % decrement), which represents much enhanced performance than that of pristine LCO (specific capacity, 92 mAhg-1, ~28 % decrement). Earlier reports revealed the electrochemical performance of LCO is mainly governed by Li+ diffusion length and diffusion rate, which are often influenced by the electrode microstructure and particle sizes [49]. LCO with larger crystallite sizes tend to lower the kinetics of diffusion and increase the polarization during charge and discharge, thus reducing cycling efficiency of cathode [50]. The higher reversible capacity of LCO-CBL is ascribed to the lower crystallite sizes (determined using XRD) and higher surface
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area (determined using BET method), in comparison to pristine LCO. The columbic efficiency was calculated using the formula given below. Columbic Efficiency(CE)=
QDischarge Qcharge
Where Qcharge is the number of charges that enter the battery during charge and QDischarge is the number of charges extracted from the battery during discharge The columbic efficiency of both pristine and LCO-CBL for 50 cycles are included in Fig. 6 (inset). Relatively better columbic efficiency of LCO-CBL revealed its beneficial effect to the reversible intercalation and de-intercalation of Li+ with good redox capacity in comparison to the pristine LCO. The electrochemical performances at high rate (1C rate) was also done in the voltage range 2.5–4.2 V vs Li+/Li and the initial and 10th charge-discharge curves are included in Fig. S8. The first discharge capacity of LCO-CBL (108 mAhg-1) is 10.2 % higher than that of pristine LCO (97 mAhg-1) at the cutoff potential of 2.5~4.2 V (vs Li+/Li). After 10th cycle, LCOCBL electrodes still maintained specific capacity of 96 mAhg-1 (~11 % decrement), which represents much enhanced performance than that of pristine LCO (specific capacity, 75 mAhg-1, ~22.7 % decrement) corroborating its enhanced electrochemical performances. To identify the electrochemical reactions in pristine LCO and LCO-CBL, cyclic voltammetry (CV) is conducted on the cell with LCO electrode at ambient temperature in the 2.5~4.2 V range and a scan rate of 1 mV/s as shown in Fig. S9. The cyclic voltammograms of both LCO and LCO-CBL displayed single broad cathodic peak (~3.6 V) with no distinct anodic peak revealing that both lithium extraction/insertion is one step process and these results are quite similar to high temperature processed LCO as reported by Gummow and Thackeray [51].
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To get more insights on the electrochemical performance, electrochemical impedance spectroscopy (EIS) of LCO and LCO-CBL was performed in the frequency range of 100 kHz~10 MHz to understand the interfacial electrochemistry and reaction mechanisms [52]. The typical Nyquist plots of AC impedance measured before and after cycling of LCO and LCO-CBL are included in Fig. 7 and Fig. S10. In general, the impedance spectrum of LCO consists of depressed arc followed by the straight line inclined at 45° angle. The equivalent circuit adopted for calculation is shown in Fig. S11. Here, Re represents the internal ohmic resistance, involving the resistance of the electrolyte and other resistive components, corresponding to the intercept of the plots with the real axis (Zre) at high frequency. Rsf and Csl corresponding to the semicircle at high frequency represent the resistance and capacitance of solid-electrolyte-interface (SEI) films. Rct and Cdl related to the semicircle at medium-to-low frequency characterize the charge transfer resistance and capacitance. The Warburg impedance noted as W referring to the sloping region at low frequency is directly associated with the lithium-ion diffusion process in the electrode [5355]. The values of Re, Rf, Rct and Rtotal are listed in Table 2. The Re values decreased progressively with increasing cycle number (2nd cycle, ~7.7 %; 50th cycle, ~15.4 %) for LCO corroborating decrease in solution resistance with electrochemical cycling. Similar trend observed in Re values of LCO-CBL (2nd cycle, ~4.5 %; 50th cycle, ~13.6 %). Rf value, which provides information on Li+ ion diffusion through SEI film and electrode, is observed to be decreased with electrochemical cycling for LCO (2nd cycle, ~59.3 %; 50th cycle, ~61.8 %) corroborating ease of diffusion. In contrast, significantly lower Rf value of LCO-CBL (1184 Ω) reveals better Li+ ion diffusion through SEI film in comparison to LCO (1861 Ω). As expected Rf value of LCO-CBL decreased (2nd cycle, ~44.1 %; 50th cycle, ~71.5 %) with electrochemical cycling though the relative decrement is less, when compared to LCO. Rct values that provide 18
information on charge-transfer resistance decreased slightly with electrochemical cycling (2nd cycle, ~35.4 %; 50th cycle, ~41.0 %) for LCO due to increase in electrical conductivity of the electrodes. Alternatively, drastic decrease in Rct value of LCO-CBL (3229 Ω) corroborating enhanced electrical conductivity in comparison to pristine LCO (4978 Ω) based electrodes. As expected Rct value of LCO-CBL decreased (2nd cycle, ~7.7 %; 50th cycle, ~49.2 %) with electrochemical cycling though the relative decrement is less, when compared to LCO corroborating significant rise in electrical conductivity with electrochemical cycling. Similar trend is observed in Rtotal values revealing faster electrode kinetics in case of LCO-CBL electrodes in comparison to pristine LCO. This fact is further corroborated from the exchange current density (i0), which has been calculated using the equation i0 = RT/nFRct, where R is the universal gas constant, T is the absolute temperature, n is the number of electrons, F is the Faraday constant and Rct is the charge-transfer resistance [55, 56]. Significant rise in exchange current density (i0) after electrochemical cycling (2nd cycle, ~8.2 %; 50th cycle, ~96.8 %) for LCO-CBL, when compared to pristine LCO (2nd cycle, ~54.9 %; 50th cycle, ~69.6 %) implying enhanced electrochemical activity of LCO-CBL relative to LCO. Finally, the specific discharge capacities of LCO and LCO-CBL based cathode materials at various C rates were determined and the results are shown in Fig. 8. As expected the discharge capacity decreases with increasing C rates both in case of LCO and LCO-CBL. However, the decrement in specific discharge capacity with increasing C rate is less for LCO-CBL in comparison to pristine LCO. For instance, the first specific discharge capacity of LCO-CBL decreased to 133 mAhg-1 (0.5 C rate), 101 mAhg-1 (1C rate), 69 mAhg-1 (2C rate) and 36 mAhg-1 (3C rate) from 171 mAhg-1 (0.1 C rate). These values are significantly higher compared to LCO electrodes revealing its enhanced electrochemical performances. However, when the C-rate 19
returns to the initial 0.1 C rate at 60th cycle, both the LCO and LCO-CBL based electrodes recovers its 83 % of its original initial specific capacity values corroborating its good reversibility and excellent cyclability of these electrodes. Conclusions Nanostructured high surface area lithium cobalt oxide (LCO) particles have been successfully synthesized by sol-gel route using cherry blossom leaves as templates. Template assisted synthesis of LiCoO2 (LCO-CBL) showed significant improvement in BET surface area, when compared to the pristine LCO. Average crystallite sizes determined using XRD is observed to be lower for LCO-CBL in comparison to the pristine LCO and this fact is further corroborated using TEM. Electrochemical measurements revealed that the LCO cathode materials prepared through template assisted synthesis (LCO-CBL) showed better cyclic performances with high reversible capacity and excellent capacity retention (81 %) relative to the pristine LCO based cathode materials.
20
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[47] L. Dahéron, R. Dedryvère, H. Martinez, M. Ménétrier, C. Denage, C. Delmas, D. Gonbeau, Electron Transfer Mechanisms upon Lithium Deintercalation from LiCoO2 to CoO2 Investigated by XPS, Chem. Mater., 20 (2008) 583-590. [48] M.A. van Veenendaal, G.A. Sawatzky, Nonlocal screening effects in 2 \textit{p} x-ray photoemission spectroscopy core-level line shapes of transition metal compounds, Phys. Rev. Lett., 70 (1993) 2459-2462. [49] S.H. Choi, J.-W. Son, Y.S. Yoon, J. Kim, Particle size effects on temperature-dependent performance of LiCoO2 in lithium batteries, J. Power Sources, 158 (2006) 1419-1424. [50] Y. Gan, L. Zhang, Y. Wen, F. Wang, H. Su, Carbon combustion synthesis of lithium cobalt oxide as cathode material for lithium ion battery, Particuology, 6 (2008) 81-84. [51] R.J. Gummow, M.M. Thackeray, W.I.F. David, S. Hull, Structure and electrochemistry of lithium cobalt oxide synthesised at 400°C, Mater. Res. Bull., 27 (1992) 327-337. [52] . Sathiyamoorthi, P. Shakkthivel, . Gangadharan, T. asudevan, ayered iCo1−x Mg x O2 (x = 0.0, 0.1, 0.2, 0.3 and 0.5) cathode materials for lithium-ion rechargeable batteries, Ionics, 13 (2007) 25-33. [53] J. Fan, G. Li, D. Luo, C. Fu, Q. Li, J. Zheng, L. Li, Hydrothermal-Assisted Synthesis of LiRich Layered Oxide Microspheres with High Capacity and Superior Rate-capability as a Cathode for Lithium-ion Batteries, Electrochim. Acta, 173 (2015) 7-16. [54] E. Meza, J. Ortiz, D. Ruíz-León, J.F. Marco, J.L. Gautier, Lithium-nickel cobalt oxides with spinel structure prepared at low temperature. XRD, XPS, and EIS measurements, MATER LETT, 70 (2012) 189-192.
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27
Figure Captions Fig. 1 (a) Schematic representation on template assisted synthesis of lithium cobalt oxide (LCOCBL); (b) X ray diffraction (XRD) patterns of LCO and LCO-CBL. Fig. 2 (a-f) Field emission scanning electron microscopy (FE-SEM) images of (a-c) LCO and LCO-CBL (d-f) at various magnifications. (g, h) Transmission electron microscopy (TEM) images of (g) LCO and (h) LCO-CBL (Inset: SAED pattern). Fig. 3 X ray photoelectron spectroscopic (XPS) results of LCO and LCO-CBL (a) Survey scan (b, c) High resolution spectra of Co2p (d, e) High resolution spectra of Li1s and Co3p. Fig. 4 BET nitrogen adsorption-desorption curves of LCO and LCO-CBL. Fig. 5 Charge-discharge profiles of LCO and LCO-CBL Fig. 6 Cycle performance of LCO and LCO-CBL (inset: Columbic efficiency of LCO and LCOCBL) Fig. 7 Electrochemical impedance spectroscopic (EIS) results of LCO and LCO-CBL before and after cycling. Fig. 8 Cycle performance of LCO and LCO-CBL at various C rates
28
Fig. 1
29
Fig. 2
30
Fig. 3 31
Fig. 4 32
Fig. 5
33
Fig. 6
34
Fig. 7
35
Fig. 8
36
Tables Table 1 Particle sizes calculated using X-ray diffraction (XRD) results Component
Particle Size(nm) Calculated
Experiment
LCO
298.3
300
LCO-CBL
100.7
100
37
Table 2 Values of Re, Rf, Rct and Rtotal for LCO and LCO-CBL Zero cycle
After 2nd cycle
After 50th cycles
LCO
LCO-CBL
LCO
LCO-CBL
LCO
LCO-CBL
Re (Ω)
26
22
24
21
22
19
Rf (Ω)
1861
1184
753
662
710
337
Rct (Ω)
4978
3229
3215
2980
2937
1640
Rtotal (Ω)
6865
4435
3992
3898
3668
1996
5.19
8.01
8.04
8.67
8.80
15.76
I0 (х1
-6
A)
38
S0013-4686(15)31004-5 http://dx.doi.org/doi:10.1016/j.electacta.2015.12.079 EA 26228
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
24-8-2015 3-11-2015 11-12-2015
Please cite this article as: M.Jeevan Kumar Reddy, Sung Hun Ryu, A.M.Shanmugharaj, Synthesis of nanostructured lithium cobalt oxide using cherry blossom leaf templates and its electrochemical performances, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.12.079 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.
Synthesis of nanostructured lithium cobalt oxide using cherry blossom leaf templates and its electrochemical performances M. Jeevan Kumar Reddy, Sung Hun Ryu* [email protected], A. M. Shanmugharaj* [email protected] Department of Chemical Engineering, Kyung Hee University, Yongin-si, Gyeonggi-do 446-701, Republic of Korea *
Corresponding author: Tel.: +82-31-201-3342.
1
Graphical Abstract
2
Highlights
Sol-gel synthesis of LiCoO2 (LCO) in cherry leaf template is reported for first time. BET Surface area is higher for LiCoO2 prepared in cherry blossom leaf templates. Excellent charge-discharge property is observed for LiCoO2 prepared in leaf template. Enhanced cycling stability is observed for LiCoO2 synthesized using leaf template.
3
Abstract Nanostructured high surface area lithium cobalt oxide (LCO-CBL) cathode materials with uniform particle sizes are prepared by template assisted sol-gel synthesis using metal precursors and urea chelating agent in cherry blossom leaf templates followed by the calcination of the prepared material at 800oC for 12 hrs. Structural and morphological characterization using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), field emission transmission electron microscopy (FE-TEM) and Raman spectroscopy of LCO-CBL shows the presence of rhombohedral (layered) structures quite similar to the pristine LCO, synthesized by simple sol-gel method. BET surface area measurements of the prepared cathode material revealed that the surface area increased about 79.7 % for LCO-CBL, when compared to pristine LCO. Electrochemical characterization of the prepared LCO-CBL delivered an initial discharge capacity of the 166 mAhg-1 with the capacity retention of 81 % in comparison to the pristine LCO that exhibit initial discharge capacity of 127 mAhg-1 and capacity retention of less than 72 %.
Keywords: Cherry blossom leaves; lithium cobalt oxide; sol-gel synthesis; specific discharge capacity; BET surface area.
4
Introduction High performance lithium ion batteries (LIBs) have gained the great success in powering portable devices such as cellphones, notebooks etc., owing to their highest energy density and output voltage among the present rechargeable batteries [1]. However, it is desired to further increase their energy density, especially at high charge/discharge rates, to meet the requirements of electric vehicles and smart grids. Development of LIBs with high energy density relies on the successful development of cathode materials with high energy density. Most commonly, layered lithium metal oxides LiMO2 (M=Co, Ni) [1-3] and spinel lithium manganese oxide (LiMn2O4) have been used as cathode materials for lithium-ion batteries [4-9]. As a result of the intense search for high specific energy cathode materials for use in lithiumion rechargeable battery technology, Lithium cobalt oxide (LiCoO2) has become the first, and one of the most promising for commercial application. LiCoO2 is the most attractive cathode material owing to its best performance in terms of high specific energy density and excellent cycle life. Although theoretical capacity of LiCoO2 (0
materials [26-28]. Several morphologically controlled nanostructured materials viz. thin films [29, 30], desert rose flower like structures [31], composites with carbonaceous materials [32] and template assisted structures like spheres, nano rods [27] nano wires [33] have been reported. The surface area of the cathode material is one of the important factor for sustaining more number of Li ions in the structure that leads to better performance in cycling process [34]. Template assisted synthesis of nanostructured materials resulted in the formation of porous structures providing high surface area as well as morphological support [35]. Natural leaves such as cherry blossom leaves have demonstrated to have three dimensional (3D) elaborated architectures with high porosity, high connectivity and high surface areas for efficient mass loading including gas exchange for photosynthesis and water transportation [36]. Owing to their 3D unique hierarchical structures, cherry blossom leaves could be used as ideal template for the synthesis of electrodes [36]. In the present study, three dimensional (3D) hierarchical architectured cherry blossom leaves (CBL) have been used as the template for the preparation of LiCoO2 nanostructures. The prepared materials have been characterized using various characterization tools to understand the surface composition, morphology and surface area. Finally, the lithium storage properties of the prepared materials were determined by fabricating lithium-ion half cells and characterized for its electrochemical performances.
2. Experimental 2.1 Sol-gel synthesis of pristine LiCoO2 (LCO) and template assisted LiCoO2 (LCO-CBL) Pristine lithium cobalt oxide (LCO) and template assisted lithium cobalt oxide (LCOCBL) were synthesized via. sol-gel synthesis route using lithium acetate dihydrate (Sigma 6
Aldrich, USA) as lithium source, cobalt acetate tetra hydrate (Sigma Aldrich, USA) as cobalt source and urea (Sigma Aldrich, USA) as chelating agent. Ammonium hydroxide was used for controlling the pH of the reaction during the gel precursor synthesis. In the typical synthesis of pristine LCO, precursor solutions of lithium (1M) and cobalt (1M) were mixed and heated to 90oC under constant stirring conditions for few hours. To this desired quantity of urea chelating agent (1M) was slowly added followed by the addition of ammonium hydroxide solution so as to maintain the pH to about 8.5~9.5. The obtained gel precursor was dried under vacuum for 12 hrs to remove the solvent completely. The powdered precursors obtained by grinding the dried samples were calcined at 800oC for 12 hrs in air atmosphere to get black colored pristine LCO samples [37]. Template assisted sol-gel synthesis of Lithium cobalt oxide (LCO-CBL) was done in quite similar as that of pristine LCO using treated cherry blossom as carbon templates. Pretreatment of the cherry blossom leaves were done followed by the method proposed by Zhou et al.,[36]. In short, Fresh green leaves (cherry blossom) were cut into 1 cm х 1 cm pieces and washed with deionized water followed by immersion in 5 % HCl overnight to get rid of magnesium (Mg), potassium (K), calcium (Ca) and phosphorous (P) and other ions. Then the treated leaves were washed with deionized water and ethanol several times followed by drying of treated leaves at room temperature [38]. The typical synthesis procedure of LiCoO2-CBL is summarized as follows: Precursor solutions of lithium (1M) and cobalt (1M) were mixed in the presence of two grams of pretreated cherry blossom leaves and it was heated to 90oC under constant stirring conditions for few hours. To this desired quantity of urea chelating agent (1M) was slowly added followed by the addition of ammonium hydroxide solution so as to maintain the pH to about 8.5~9.5. The obtained gel precursor/leaf composite was dried under vacuum for 12 hrs to remove 7
the solvent completely. Then the samples were calcined at 800oC for 12 hrs under air atmosphere and ground to get ash colored powders.
2.2 Measurement of structural and electrochemical properties The synthesized LCO and LCO-CBL samples were deposited onto a carbon coated copper grid using particle dispersant (dispersed in ethanol), dried and observed by transmission electron microscope (TEM, JEM-2100F, JEOL), powder samples were analyzed using X-ray diffractometer (XRD, D8 Advance, Bruker). Surface elemental compositions of LCO and LCOCBL
were
determined
using
X-ray
photoelectron
spectroscopy
(XPS,
K-Alpha,
Thermoelectron). The samples for XPS measurements were prepared by casting the particle dispersant (dispersed in ethanol) onto silicon wafer followed by drying under vacuum at 70 °C. The synthesized LCO-CBL were also characterized by Fourier Transform Infrared Spectroscopy (FT-IR, Spectrum one, Perkin-Elmer) in KBr matrix. Thermogravimetric analysis (TGA) of LCO-CBL was characterized in the temperature range of 25–900 °C at the heating rate of 10 °C/min in air environment using TA instruments (TGA Q5000 IR/SDT Q600). The specific surface areas were determined with a surface-area analyzer (BEL Sorp-II mini, BEL Japan Co., Japan) by the Brunauer−Emmett−Teller (BET) method. The electrochemical performances of pristine LOC and LCO-CBL cathode material were measured by fabricating 2032-type coin Li-ion half cells. Electrodes were prepared by casting a slurry with a composition of 80 wt. % active material, 10 wt. % of super P (TIMCAL) as additional conductive agent and 10 wt. % of poly(vinylidene difluoride) (PVDF) (Kureha KF100) as binder onto an Aluminum foil which acts as the current collector. The slurry was prepared by grinding the mixture in the presence of N-methyl pyrrolidone (NMP) solvent using 8
mortar for 15 min. The viscous slurry coated onto Aluminum foil was dried in oven at 100 °C for 5 h. The dried coating electrode was pressed under a 7T load and then punched out with size of 16 mm in diameter. The active material mass loading was ~2.0 mg cm−2. The electrode was assembled into 2032 type coin type cell with 1 M LiPF6 solution in a 1:1 (volume) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (Merck Co.) as an electrolyte. Metallic lithium was used as the counter and reference electrode. Electrochemical properties of the coin type cells were studied using galvanostatic charge and discharge measurements in the voltage range of 2.5–4.2 V vs. Li/Li+ using Wonatech battery analyzer. Cyclic voltammetry curves were measured at 0.1 mV s−1 within the range of 2.5–4.2 V using an electrochemical work station (VersaSTAT 3 Electrochemical System). Electrochemical impedance studies of the Li-ion cells were performed using an electrochemical work station (VersaSTAT 3 Electrochemical System) by applying sine wave with amplitude of 5.0 mV in the frequency range of 100 kHz to 0.01 Hz.
3. Results and Discussion The schematic representation showing the cherry blossom leaf template assisted synthesis of lithium cobalt oxide cathode materials is depicted in Fig. 1 (a). The cherry blossom leaves were initially pretreated using 5 % hydrochloric acid following the procedure reported by Zhou et al. [36], to remove inorganic metal ions such as magnesium, potassium, phosphorus and calcium. Stoichiometric composition of the precursor solution and chelating agent was then impregnated in the pretreated cherry blossom leaves followed by calcination at 800oC resulted in the formation of the ash colored crystalline LiCoO2 (LCO-CBL) powder with trace amount of carbonaceous matter (Fig. S1; Table S1). Pristine LiCoO2 (LCO) was also synthesized by using same composition of precursor and chelating agent without cherry blossom leaf templates. 9
Phase purity and crystallographic patterns of the LCO and LCO-CBL were determined using powder X-ray diffraction (XRD) and the results are included in Fig. 1(b). As observed all the patterns exhibit metallic fingerprint peaks with orientations of (003), (101), (006), (102), (104), (015), (107), (108), (110) and (113) at 2θ angles of 18°, 37°, 38°, 39°, 45°, 49°, 59°, 65°, 66° and 69° respectively. From the XRD pattern, it can be seen that no impurity peaks were found in the LCO-CBL prepared by template assisted method and the results are quite consistent with the materials prepared by sol-gel method, revealing the formation of ultrapure nanocrystalline materials. Clear splitting of the doublet peaks at (006/102) and (108/110) both in case of LCO and LCO-CBL suggests the formation of layered structures with typical R3m space group symmetry and it could be ascribed to the ordered distribution of lithium in the 3a sites [38]. XRD pattern matches with the JCPDS data card number 50-0653 for LCO and 16-0427 for LCO-CBL respectively [8, 27]. The intensity ratio of diffraction peaks of (I(003)/I(104)) and ((I(006)/I(102))/I(101)) were calculated and the results are observed to be 2.06997 (I(003)/ (104)) and 0.37208 ((I(006)/I(102))/I(101)) for LCO, which is quite consistent with the values displayed in JCPDS file No. 50-0653. Similarly, the intensity ratio values are observed to be 2.07082 (I(003)/ (104))
and 0.39555 ((I(006)/I(102))/I(101)) for LCO-CBL and the results are consistent with the values
displayed in JCPDS file No. 16-0427. These results indicated slight increase in layered characteristics with good phase structure for LCO-CBL in comparison to pristine LCO [15, 39]. The interlayer distance i.e. d-spacing plays a vital role in the charge discharge of a lithium ion battery, The average d-spacing values calculated using Bragg’s equation of (003) reflections are observed to be 4.2673 Å (LCO) and 4.67316 Å (LCO-CBL) respectively. Slight increment (~9.5 %) in average d-spacing values for LCO-CBL could effectively improves the Li+ ion diffusion during intercalation and deintercalation process, when compared to the pristine LCO. 10
The crystallite domain sizes of the LCO and LCO-CBL were calculated using the well- known Scherrer’s formula [40, 41] and the results are included in Table 1.
D
K cos
Where D is Crystallite domain size K is Scherrer’s constant (0.9) λ is X-Ray wave length (1.541836) β is Angular width of diffracted peak at the half maximum (FWHM-Full Width Half Maximum) in radians for diffraction angle θ is the diffraction angle The experimental values calculated using (003) diffraction peak is in agreement with the calculated crystallite domain size (Table 1). Significant reduction in crystallite domain sizes (~66.7 %) for LCO-CBL in comparison to LCO is attributed to the 3D architectured structure of cherry blossom leaf that restricts the crystallite growth of LCO during the sintering process. Fig. 2 (a-c) shows a typical field emission scanning electron microscopic (FE-SEM) images of LCO and Fig. 2 (d-f) are LCO-CBL at different magnifications. While, LCO displayed more aggregated and uneven particle shapes and particle sizes ranging from 300~1200 nm (Fig. 2 (a-c)), whereas, LCO-CBL showed macro porous 3D architectured structure consisting of embedded LCO particles with uniform particle distribution ranging from 100~600 nm (Fig. 2 (df)). Few more FE-SEM images of LCO-CBL consisting of 3D architectured structures is included in supporting information, which clearly depicts the entrapped LCO formation in the cherry blossom leaf templates (Fig. S2 (a-f) and Fig. S3 (a-d)). In contrast to earlier report on the synthesis of nanostructured materials using cherry blossom leaf templates, the present work 11
doesn’t show the presence of micro-tubular nanostructures and this fact may be attributed to the preferential growth of LCO particles on the cylindrical vein structures of cherry blossom leaves rather than the growth of nanostructures in the interconnected micro-tubular structure (Fig. S2 (g, h) [36]. The preferential growth of LCO particles on cylindrical veins results in the complete degradation of interconnected micro-tubular structures of cherry blossom leaves during calcination resulting in the formation of fused LCO particles with relatively fine particle size distribution. The particle sizes and crystalline nature of LCO and LCO-CBL were further corroborated using field emission transmission electron microscopy characterization (FE-TEM) and the results are displayed in Fig. 2 (g, h) and Fig. S4. TEM images showed that LCO and LCO-CBL consists of equiaxed particles ranging from 300~500 nm as shown in Fig. 2 (g, h). Slight variation in contrast of bright field TEM image revealed a single, well-ordered α-NaFeO2 phase structure both in case LCO and LCO-CBL [42]. To understand the range of particle sizes present in LCO and LCO-CBL, TEM characterization was done at various magnification and representative results are displayed in Fig. S4. Pristine LCO has particle sizes in the range of 300~1500 nm and LCO-CBL has sizes in the range of 100 to 600 nm. The crystalline nature of LCO and LCO-CBL samples are further corroborated using selected area electron diffraction patterns (SAED) patterns as shown Fig. 2 (g, h) (inset). While, the pristine LCO exhibit SAED patterns with (hkl) values (003), (104), (101), (006) and (102), LCO-CBL showed bright spots with (hkl) values of (003), (104) and (110) planes clearly corroborating the crystalline nature of the materials [27, 43]. Absence of certain crystalline planes in LCO-CBL may be attributed to the restricted growth of crystals due to the usage of cherry blossom templates.
12
The local cationic order in a close packed oxygen array of LCO and LCO-CBL was reexamined using vibrational spectroscopic measurements viz. Raman and Fourier transform Infrared spectroscopy. Vibrational modes, which correspond to vibrations involving primarily atomic motion of cations against their oxygen neighbors, are sensitive to the space group and consequently, these modes are very sensitive to the cationic local environment in the host matrix [44]. Fig. S5 (a) showed the Raman spectroscopic measurements of LCO and LCO-CBL recorded at room temperature. According to a factor group analysis, the layered phase ( 3m space group) exhibits only two Raman-active modes, i.e. the A1g and the Eg. In the present investigation, both LCO and LCO-CBL showed two Raman active modes at 590 cm-1 (A1g) and at 480 cm-1 (Eg) confirming the layered structure with 3m space group symmetry. Appearance of the broad peak at. 615 cm-1 for LCO may be attributed to polyhedral distortion of octahedral CoO6 groups. Interestingly, the peak at 615 cm-1 doesn’t appear in case of LCO-CBL corroborating that polyhedral distortion of CoO6 groups is hindered by using CBL templates for LCO synthesis [45]. Fig. S5 (b-c) showed the FT-IR results of LCO and LCO-CBL. Pristine LCO displayed two sharp and intense peaks between 450 ~ 600 cm-1 along with the small hump at around 650 cm-1, which corresponds to Li-O vibration of tetrahedral LiO4 structure (~500 cm1
), Co-O vibration of octahedral CoO6 structures (~600 cm-1) and Co-O vibration of tetrahedral
CoO4 structures (~650 cm-1) [46]. The surface chemical compositions of the LCO and LCO-CBL materials have been studied by X-ray photoelectron spectroscopy (XPS) and the results are displayed in Fig. 3 and Fig. S5. Suvey scan results of both LCO and LCO-CBL exhibit spectral peaks corresponding to Li1s, Co3p, Co3s, C1s, O1s and Co2p centered at 52, 61, 104, 285, 530 and 780 eV respectively (Fig. 3 (a)). Significant rise in peak intensity ratio (I285/I780) corresponding to C1s (285 eV) and 13
Co2p (780 eV) peaks from 1.9 (LCO) to 3.21 (LCO-CBL) revealed the rise in carbonaceous matter due to the usage of cherry blossom leaves as templates. High resolution Co2p core peaks of LCO and LCO-CBL are shown in Fig. 3 (b, c). Due to the spin-orbit coupling, Co2p spectrum of pristine LCO is split in two parts Co2p3/2 (A; 779 eV) and Co2p1/2 (D; 793.5 eV), with the intensity value almost 2.5 times higher for the former (Co2p3/2) compared to the later (Co2p1/2) (A/D = 2.5). Apart from the main peaks (A & C), each part consists of a satellite peaks at 788 eV (B) and at 803 eV (F). Similarly, Co2p spectrum of LCO-CBL also split into two parts Co2p3/2 (A; 779 eV) and Co2p1/2 (D; 794 eV), with the intensity value almost 2.5 times higher for the former (Co2p3/2) compared to the later (Co2p1/2) (A/D = 2.5). Apart from the main peaks (A & C), each part consists of a satellite peaks at 789 eV (B) and at 803 eV (F). Appearance of main peaks (A & D) together with a satellite peak (C & F) results from a ligand-to-metal charge transfer during the photoemission process and can be interpreted, at the simplest level of approximation, by a molecular orbital description. In the initial state LCO & LCO-CBL has six electrons in the cobalt 3d shell and a filled ligand shell L (oxygen 2p shell), which can be written 2p63d6L. The Co 2p photoemission process can lead to several final states after creation of the 2p core hole (2p5). The main line A is mainly characterized by the 2p53d7L-1 configuration, where one electron is transferred from the ligand shell L to the metal 3d shell (screening effect), resulting in a 3d7 configuration. The shake-up satellite B can be assigned to 2p53d6L and 2p53d8L-2 configurations [47]. Appearance of the satellite peaks both in the case of LCO and LCO-CBL clearly indicated the existence of Co3+ and Co4+ ions in the prepared materials. The Co2p3/2 peak of LCO also displayed a weak shoulder at 780.4 eV (B) with upon deconvolution showed relative area of 18.1 %. This fine structure can also be noticed on the Co2p1/2 component by the presence of a shoulder at 795 eV (E) with relative area of 12.6 % (Fig. 3 (b)). 14
Alternatively, the Co2p3/2 peak of LCO-CBL displayed a weak shoulder at 780.5 eV with upon deconvolution showed relative area of 14.2 % and Co2p1/2 component showed a weak shoulder at 795 eV with relative area of 9.1 % (Fig. 3 (c)). Appearance of this shoulder peak both in case of LCO and LCO-CBL is attributed to the final state of the photoemission process like in the case of NiO (nonlocal screening) [48]. Deconvolution of O1s peaks of LCO and LCO-CBL were carried out and the results are included in Fig. S6. The spectrum of pristine LCO and LCO-CBL displayed two characteristic peaks at 529.0 and 531.2 eV, which corresponds to O2- anions (529.0 eV) of the crystalline network and oxygen anions in subsurface (531.2 eV) with deficient coordination [48]. Decrease in peak intensity ratio (I529.5/I531.4) from 0.84 (LCO) to 0.42 (LCOCBL) revealed the rise in oxygen anions with deficient coordination in LCO-CBL, when compared to LCO. Deconvolution of high resolution C1s spectra of LCO showed peaks corresponding to C-C/C-H (284.5 eV), C-O (285.7 eV) and C=O (287.8 eV) groups, whereas LCO-CBL exhibited the peaks corresponding to Co-C (283.8 eV), C-C/C-H (284.5 eV), C-O (285.7 eV) and C=O (287.7 eV) groups corroborating the interaction between carbonaceous matter and LCO in LCO-CBL (Fig. S6). Deconvolution of high resolution spectra in the range of 51~61 eV of LCO and LCO-CBL showed two peaks at 52 eV and 59.9 eV (Fig. 3 (d & e)). While the peak at higher binding energy (59.9 eV) is attributed to Co3p, the peak at lower binding energy (52 eV) corresponds to interstitial Li ions. The deconvoluted area ratio of Li1s and Co3p peaks (Li1s/Co3p) reflects the relative concentration of Li present in LCO and LCOCBL (Fig. 3 (d & e)). Relative area (Li1s/Co3p) ratio increases to about 47.6 % for LCO-CBL in comparison to the LCO revealing higher concentration of lithium on the surface. The surface area of pristine LCO and LCO-CBL were determined using BET analysis and the results are included in Fig. 4. The N2 adsorption-desorption curves of LCO-CBL showed 15
a clear hysteresis loop in the range of 0.5~1.0 revealing type IV of the IUPAC classification, suggesting that these samples are mesoporous, when compared to the pristine LCO, which showed a small hysteresis loop in the range of 0.9~1.0. The N2 surface area of LCO-CBL determined using BET method is observed to be 1.898 m2g-1 and it is significantly higher (~79.7 %) in comparison to the pristine LCO (1.056 m2g-1). The electrochemical cycling performance of LCO and LCO-CBL were examined over the range 2.5–4.2 V vs Li+/Li and the initial charge-discharge curves are included in Fig. 5. At a low rate (0.1 C), the first discharge capacity of LCO-CBL (166 mAhg-1) is 30.7 % higher than that of pristine LCO (127 mAhg-1) at the cutoff potential of 2.5~4.2 V (vs Li+/Li). After first electrochemical cycling, specific discharge capacity of LCO decreased to about 6.3 % with capacity value of 119 mAhg-1 in second cycle (Fig. S7). Similarly, the specific discharge capacity of LCO-CBL decreased to about 6.4 % with capacity value of 156 mAhg-1 in second cycle (Fig. S7). The cyclability of the LCO and LCO-CBL electrodes is examined under longterm cycling over 50 cycles, which demonstrated a good cyclic performance and reversibility (Fig. 6). After 50th cycle, LCO-CBL electrodes still maintained specific capacity of 134 mAhg-1 (~19 % decrement), which represents much enhanced performance than that of pristine LCO (specific capacity, 92 mAhg-1, ~28 % decrement). Earlier reports revealed the electrochemical performance of LCO is mainly governed by Li+ diffusion length and diffusion rate, which are often influenced by the electrode microstructure and particle sizes [49]. LCO with larger crystallite sizes tend to lower the kinetics of diffusion and increase the polarization during charge and discharge, thus reducing cycling efficiency of cathode [50]. The higher reversible capacity of LCO-CBL is ascribed to the lower crystallite sizes (determined using XRD) and higher surface
16
area (determined using BET method), in comparison to pristine LCO. The columbic efficiency was calculated using the formula given below. Columbic Efficiency(CE)=
QDischarge Qcharge
Where Qcharge is the number of charges that enter the battery during charge and QDischarge is the number of charges extracted from the battery during discharge The columbic efficiency of both pristine and LCO-CBL for 50 cycles are included in Fig. 6 (inset). Relatively better columbic efficiency of LCO-CBL revealed its beneficial effect to the reversible intercalation and de-intercalation of Li+ with good redox capacity in comparison to the pristine LCO. The electrochemical performances at high rate (1C rate) was also done in the voltage range 2.5–4.2 V vs Li+/Li and the initial and 10th charge-discharge curves are included in Fig. S8. The first discharge capacity of LCO-CBL (108 mAhg-1) is 10.2 % higher than that of pristine LCO (97 mAhg-1) at the cutoff potential of 2.5~4.2 V (vs Li+/Li). After 10th cycle, LCOCBL electrodes still maintained specific capacity of 96 mAhg-1 (~11 % decrement), which represents much enhanced performance than that of pristine LCO (specific capacity, 75 mAhg-1, ~22.7 % decrement) corroborating its enhanced electrochemical performances. To identify the electrochemical reactions in pristine LCO and LCO-CBL, cyclic voltammetry (CV) is conducted on the cell with LCO electrode at ambient temperature in the 2.5~4.2 V range and a scan rate of 1 mV/s as shown in Fig. S9. The cyclic voltammograms of both LCO and LCO-CBL displayed single broad cathodic peak (~3.6 V) with no distinct anodic peak revealing that both lithium extraction/insertion is one step process and these results are quite similar to high temperature processed LCO as reported by Gummow and Thackeray [51].
17
To get more insights on the electrochemical performance, electrochemical impedance spectroscopy (EIS) of LCO and LCO-CBL was performed in the frequency range of 100 kHz~10 MHz to understand the interfacial electrochemistry and reaction mechanisms [52]. The typical Nyquist plots of AC impedance measured before and after cycling of LCO and LCO-CBL are included in Fig. 7 and Fig. S10. In general, the impedance spectrum of LCO consists of depressed arc followed by the straight line inclined at 45° angle. The equivalent circuit adopted for calculation is shown in Fig. S11. Here, Re represents the internal ohmic resistance, involving the resistance of the electrolyte and other resistive components, corresponding to the intercept of the plots with the real axis (Zre) at high frequency. Rsf and Csl corresponding to the semicircle at high frequency represent the resistance and capacitance of solid-electrolyte-interface (SEI) films. Rct and Cdl related to the semicircle at medium-to-low frequency characterize the charge transfer resistance and capacitance. The Warburg impedance noted as W referring to the sloping region at low frequency is directly associated with the lithium-ion diffusion process in the electrode [5355]. The values of Re, Rf, Rct and Rtotal are listed in Table 2. The Re values decreased progressively with increasing cycle number (2nd cycle, ~7.7 %; 50th cycle, ~15.4 %) for LCO corroborating decrease in solution resistance with electrochemical cycling. Similar trend observed in Re values of LCO-CBL (2nd cycle, ~4.5 %; 50th cycle, ~13.6 %). Rf value, which provides information on Li+ ion diffusion through SEI film and electrode, is observed to be decreased with electrochemical cycling for LCO (2nd cycle, ~59.3 %; 50th cycle, ~61.8 %) corroborating ease of diffusion. In contrast, significantly lower Rf value of LCO-CBL (1184 Ω) reveals better Li+ ion diffusion through SEI film in comparison to LCO (1861 Ω). As expected Rf value of LCO-CBL decreased (2nd cycle, ~44.1 %; 50th cycle, ~71.5 %) with electrochemical cycling though the relative decrement is less, when compared to LCO. Rct values that provide 18
information on charge-transfer resistance decreased slightly with electrochemical cycling (2nd cycle, ~35.4 %; 50th cycle, ~41.0 %) for LCO due to increase in electrical conductivity of the electrodes. Alternatively, drastic decrease in Rct value of LCO-CBL (3229 Ω) corroborating enhanced electrical conductivity in comparison to pristine LCO (4978 Ω) based electrodes. As expected Rct value of LCO-CBL decreased (2nd cycle, ~7.7 %; 50th cycle, ~49.2 %) with electrochemical cycling though the relative decrement is less, when compared to LCO corroborating significant rise in electrical conductivity with electrochemical cycling. Similar trend is observed in Rtotal values revealing faster electrode kinetics in case of LCO-CBL electrodes in comparison to pristine LCO. This fact is further corroborated from the exchange current density (i0), which has been calculated using the equation i0 = RT/nFRct, where R is the universal gas constant, T is the absolute temperature, n is the number of electrons, F is the Faraday constant and Rct is the charge-transfer resistance [55, 56]. Significant rise in exchange current density (i0) after electrochemical cycling (2nd cycle, ~8.2 %; 50th cycle, ~96.8 %) for LCO-CBL, when compared to pristine LCO (2nd cycle, ~54.9 %; 50th cycle, ~69.6 %) implying enhanced electrochemical activity of LCO-CBL relative to LCO. Finally, the specific discharge capacities of LCO and LCO-CBL based cathode materials at various C rates were determined and the results are shown in Fig. 8. As expected the discharge capacity decreases with increasing C rates both in case of LCO and LCO-CBL. However, the decrement in specific discharge capacity with increasing C rate is less for LCO-CBL in comparison to pristine LCO. For instance, the first specific discharge capacity of LCO-CBL decreased to 133 mAhg-1 (0.5 C rate), 101 mAhg-1 (1C rate), 69 mAhg-1 (2C rate) and 36 mAhg-1 (3C rate) from 171 mAhg-1 (0.1 C rate). These values are significantly higher compared to LCO electrodes revealing its enhanced electrochemical performances. However, when the C-rate 19
returns to the initial 0.1 C rate at 60th cycle, both the LCO and LCO-CBL based electrodes recovers its 83 % of its original initial specific capacity values corroborating its good reversibility and excellent cyclability of these electrodes. Conclusions Nanostructured high surface area lithium cobalt oxide (LCO) particles have been successfully synthesized by sol-gel route using cherry blossom leaves as templates. Template assisted synthesis of LiCoO2 (LCO-CBL) showed significant improvement in BET surface area, when compared to the pristine LCO. Average crystallite sizes determined using XRD is observed to be lower for LCO-CBL in comparison to the pristine LCO and this fact is further corroborated using TEM. Electrochemical measurements revealed that the LCO cathode materials prepared through template assisted synthesis (LCO-CBL) showed better cyclic performances with high reversible capacity and excellent capacity retention (81 %) relative to the pristine LCO based cathode materials.
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Figure Captions Fig. 1 (a) Schematic representation on template assisted synthesis of lithium cobalt oxide (LCOCBL); (b) X ray diffraction (XRD) patterns of LCO and LCO-CBL. Fig. 2 (a-f) Field emission scanning electron microscopy (FE-SEM) images of (a-c) LCO and LCO-CBL (d-f) at various magnifications. (g, h) Transmission electron microscopy (TEM) images of (g) LCO and (h) LCO-CBL (Inset: SAED pattern). Fig. 3 X ray photoelectron spectroscopic (XPS) results of LCO and LCO-CBL (a) Survey scan (b, c) High resolution spectra of Co2p (d, e) High resolution spectra of Li1s and Co3p. Fig. 4 BET nitrogen adsorption-desorption curves of LCO and LCO-CBL. Fig. 5 Charge-discharge profiles of LCO and LCO-CBL Fig. 6 Cycle performance of LCO and LCO-CBL (inset: Columbic efficiency of LCO and LCOCBL) Fig. 7 Electrochemical impedance spectroscopic (EIS) results of LCO and LCO-CBL before and after cycling. Fig. 8 Cycle performance of LCO and LCO-CBL at various C rates
28
Fig. 1
29
Fig. 2
30
Fig. 3 31
Fig. 4 32
Fig. 5
33
Fig. 6
34
Fig. 7
35
Fig. 8
36
Tables Table 1 Particle sizes calculated using X-ray diffraction (XRD) results Component
Particle Size(nm) Calculated
Experiment
LCO
298.3
300
LCO-CBL
100.7
100
37
Table 2 Values of Re, Rf, Rct and Rtotal for LCO and LCO-CBL Zero cycle
After 2nd cycle
After 50th cycles
LCO
LCO-CBL
LCO
LCO-CBL
LCO
LCO-CBL
Re (Ω)
26
22
24
21
22
19
Rf (Ω)
1861
1184
753
662
710
337
Rct (Ω)
4978
3229
3215
2980
2937
1640
Rtotal (Ω)
6865
4435
3992
3898
3668
1996
5.19
8.01
8.04
8.67
8.80
15.76
I0 (х1
-6
A)
38