Ionics DOI 10.1007/s11581-017-2147-1
ORIGINAL PAPER
Nanocrystalline Li2TiO3 electrodes for supercapattery application A. Lakshmi Narayana 1 & M. Dhananjaya 1 & N. Guru Prakash 1 & O. M. Hussain 1 & C. M. Julien 2
Received: 11 March 2017 / Revised: 2 May 2017 / Accepted: 8 May 2017 # Springer-Verlag Berlin Heidelberg 2017
Abstract Nanocrystalline Li2TiO3 was successfully synthesized using solid-state reaction method. The microstructural and electrochemical properties of the prepared material are systematically characterized. The X-ray diffraction pattern of the prepared material exhibits predominant (002) orientation related to the monoclinic structure with C2/c space group. HRTEM images and SAED analysis reveal the welldeveloped nanostructured particles with average size of ∼40 nm. The electrochemical properties of the prepared sample are carried out using cyclic voltammetry (CV) and chronopotentiometry (CP) using Pt//Li 2 TiO 3 cell in 1 mol L−1 Li2SO4 aqueous electrolyte. The Li2TiO3 electrode exhibits a specific discharge capacity of 122 mAh g−1; it can be used as anode in Li battery within the potential window 0.0–1.0 V, while investigated as a supercapacitor electrode, it delivers a specific capacitance of 317 F g−1 at a current density of 1 mA g−1 within the potential range −0.4 to +0.4 V. The demonstration of both anodic and supercapacitor behavior concludes that the nanocrystalline Li2TiO3 is a suitable electrode material for supercapattery application.
Keywords Nanostructured material . Li batteries . Supercapacitor . Solid-state reaction synthesis . Cyclic voltammetry . Chronopotentiometry
* O. M. Hussain
[email protected]
1
Thin Films Laboratory, Department of Physics, Sri Venkateswara University, Tirupati 517502, India
2
Université Pierre et Marie Curie, Physicochimie des Electrolytes et des Nanosystèmes Interfaciaux (PHENIX), Sorbonne Universités, UMR 8234, 75005 Paris, France
Introduction With growing population and increasing energy demand, countries all around the world are suffering from shortage in electric power [1]. One of the suitable solutions to overcome these problems is to develop the electrochemical energy storage devices such as Li-ion batteries and supercapacitors which store charges rapidly and efficiently. However, Li-ion batteries have highest energy density ranging from 120 to 200 Wh kg−1 but very often suffer from lower power density 0.2– 1.8 kW kg−1 [2, 3], whereas the supercapacitors exhibit high power density and excellent cycleability. Supercapacitors are classified into three categories: (1) electric double layer capacitors (EDLCs) depend on carbon materials, which provide large surface area and high electrical conductivity; (2) pseudocapacitors based on redox reaction of materials (RuO2, MnO2 and Ni(OH)2 etc.); and (3) hybrid capacitors that are the combination of EDLC and pseudocapacitor [4]. EDLCs perform high power density 90 kW kg−1, but have poor energy density 2–8 Wh kg−1 [5]. However, most of the electrodes of the Li+-ion battery perform faradic charge storage mechanism, which exhibit peak-shaped cyclic voltammetry and non–linear galvanostatic charge-discharge profiles representing non-capacitance nature [6]. The redox-active electrode material that perform either capacitive charge storage (non-faradic) or non-capacitive charge storage (faradic) or both is termed as Bsupercapattery^ (supercapacitor + battery) material [7, 8]. Supercapattery has been introduced as a new term to present characteristics of both supercapacitor (EDLC+ pseudocapacitor) and energystorage battery that aims to achieve high energy and high power density. Several Ti-based oxides can react with lithium to form insertion compounds. The maximum of Li insertion into the tetragonal anatase TiO2 polymorph (I41/amd space group)
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was x = 0.47 in LixTiO2, which is converted to the orthorhombic structure (Imma space group) [9–11]. TiO2 (B) crystallizing with a monoclinic structure (C2/m space group) has an open channel parallel to the b-axis and is able to accommodate 1Li+ per Ti giving a specific capacity of 335 mAh g−1 for nanoparticles [12]. In its nanosized form, TiO2 rutile with a structure (P42/mnm space group) [13, 14] is able to reversibly accommodate 0.5Li/Ti (168 mAh g−1) at 1–3 V vs. Li+/Li0 with excellent capacity retention, while micrometer TiO2 is poorly electroactive. The polymorph TiO2(R) ramsdellite with an orthorhombic structure (Pbnm space group) can uptake 0.85 Li/Ti that provides a specific capacity of 285 mAh g−1 between 2.3 and 1.3 V vs. Li anode [15]. Both spinels LiTi2O4 and Li4/3Ti5/3O4 crystallizes with a cubic structure and react with one further lithium atom to form Li 2 Ti 2 O 4 and Li7/3Ti5/3O4 phases, respectively. These materials are known to be zero-strain insertion material. LiTi2O4 delivers a specific capacity of about 100 mAh g−1 with a discharge-charge voltage plateau ranging from 1.3 to 1.7 V vs. Li+/Li0 [16, 17], while the specific capacity delivered by Li4/3Ti5/3O4 is about 175 mAh g−1 with a voltage plateau at 1.5 V vs. Li+/Li0 [18]. The ramdellite Li2Ti3O7 displays an orthorhombic structure (space group Pbnm), in which Li ions occupied both tetrahedral channel and octahedral lattice sites. A maximum insertion ratio of 1.25 mol of Li for Li2Ti3O7 has been determined that provides a specific capacity of 160 mAh g−1 [19]. In the present investigation, the Li2TiO3 nanoparticles prepared by solid-state reaction were obtained after annealing the powders at 800 °C for 5 h. The microstructural and electrochemical properties were studied thoroughly. To the best of our knowledge there were meager reports on electrochemical properties of Li2TiO3. Moreover, Li2TiO3 electrode has the trait of anodic properties as well as pseudocapacitive behavior.
Fig. 1 Representation of the elementary unit cell of the monoclinic β-Li2TiO3. The lamellar structure consists of alternating Li pure layers and LiTi2 layers
Therefore, we have made an attempt to carry out a detailed study on the electrochemical properties of synthesized nanocrystalline Li2TiO3 as a Bsupercapattery^ material.
Survey of lithium titanate Li2TiO3 Lithium titanate Li2TiO3 has been described in three structural modifications α, β and γ [20]. The cubic α-Li2TiO3 (space group Fm-3 m) is an unstable phase, β-Li2TiO3 (space group C2/c) is a thermodynamically stable polymorph, while the cubic γ-Li2TiO3 is the high-temperature stable phase. The monoclinic β-Li2TiO3 phase is a layered structured material, which crystallizes in the Li2SnO3-type structure with an interlamellar (002) d-spacing value of 0.48 nm [9]. The layered lattice consists of alternating Li pure layers and LiTi2 layers. Li layer is occupied by only Li atoms, while LiTi2 layer consists of 1/3 Li and 2/3 Ti. TiO6 octahedra construct a hexagonal cavity forming the [TiO3]−2 layers. Lithium atoms occupy three inequivalent positions, namely, Li(1), Li(2) and Li(3) (8f, 4d and 4e Wyckoff sites, respectively). Li+-ion at 4e position is in the hexagonal cavity and others are in the interlayer space as shown in Fig. 1 [21, 22]. Li2TiO3 has a 3-dimensional pathway for Li+-ion migration that occurs in the (003) plane along c-direction [23]. Moreover, Li2TiO3 exhibits a moderate ionic conductivity and does not change its volume during charging-discharging hence it is termed as Bzero-strain material^ [24]. However, conventional Li2 TiO 3 has poor electronic conductivity (10−11 S cm−1), which seriously limits its power performance and rate capability [23]. In order to improve ionic conductivity, many methods have been developed such as surface modification with conductive metals, doping with aliovalent metal
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ions and reduction of particles size [6]. The major advantages of the material preparation at nanoscale are to shorten the Li+ion diffusion path and enlarge the electrode/electrolyte contact area involving excellent anodic properties [25] and supercapacitor performances [26]. Therefore, several synthesis methods have been proposed to prepare Li2TiO3 nanoparticles such as hydrothermal technique [27, 28], sol–gel method [29], modified indirect wetchemistry route [30], microwave induced combustion method [31], organic–inorganic solution route [32] and flux method [33]. Among them, the conventional solid-state reaction method is suitable for the synthesis of single phase, ultrafine particles of β-Li2TiO3 by using TiO2 and LiCO3 as raw materials and can produce mass-scale material for industries [33, 34]. The Li2TiO3 can be considered as a promising supercapattery material for electrochemical energy-storage systems because of its unique properties including excellent structural stability over cycling, high coulombic efficiency, minimum solidelectrolyte interphase, low cost, and non-toxic nature [23]. In the Li2+xTiO3 lattice, there are three possible lithium-ion occupations [35]. These are as follows: (1) The excess of Li+ occupies the tetrahedral interstices, (2) Li+ occupies the Ti4+ site as antisite defect, i.e., 2Li″Ti = 3Vo••, (3) Li+ occupies itself site and Ti4+ and O2− site vacancies. The first two mechanisms imply an expansion of the elementary cell due to larger ionic radius of Li+ (0.72 Å) than Ti4+ (0.605 Å). Prior work related to the electrochemical reaction of Li2TiO3 against lithium was reported by Chauvaut and Cassir [36], who believed that Li+ insertion is a diffusive reaction corroborated with the Ti nonstoichiometry in Li2+xTiIV1-xTiIIIxO3. Wang et al. [26] have checked the galvanostatic discharge-charge profiles of Li2TiO3 agglomerated nanoparticles (∼0.5 μm) and reported a specific discharge capacity of 30 mAh g−1 in the first cycle at a current rate of C/2. Early work by Tabuchi et al. [37] indicated a value of 10 mAh g−1 for cubic α- Li2TiO3 particles (>200 nm), while Morales et al. [38] have also mentioned a small electroactivity of 47 mAh g−1 for 100 nm-sized particles of α-Li2TiO3 cycled at C/10 rate in the voltage range 1.5– 2.5 V vs. Li+/Li0. The discrepancy could be due to different particles sizes with different morphologies. Several attempts on Li2TiO3-based composites as cathode or anode materials were reported considering the merits of lithium titanate, viz. excellent structural stability, improved electrochemical performance of the electrode for Li-ion batteries [17, 39–41]. Zhang et al. [40] attempted to understand the electrochemical features of the LiNiO2•Li2TiO3 compounds studied as cathodes after charging to 4.6 V. Considering that Ni4+ and Ti4+ are generally impossible to be oxidized to a higher valence, it was inferred that oxygen ions in these materials were responsible for charge compensation. The Si/ Li2TiO3 nanocomposite synthesized by sol-gel process as an anode material exhibited a specific capacity of 471 mAh g−1 even after 50 cycles. In this nanocomposite, the structural
stability is due to Li2TiO3, which acts as excellent buffer to the Si powders and partly compensate the capacity loss, providing reversible Li specific capacity of about 100 mAh g−1 [24].
Experimental Synthesis Li2TiO3 powders were synthesized by solid-state reaction using TiO2 (anatase A.R) and Li2CO3 as raw materials in stoichiometry ratio Li/Ti = 2:1. Then ethanol was added to the mixture and subjected for ball-planetary milling at 400 rpm speed. The ball feed ratio was fixed at 5:1. Subsequently, the slurry was collected then transferred into a ceramic crucible and dried at 80 °C. The product was then sintered at temperature of 800 °C for 5 h. This synthesis process can be described by the following chemical reaction Li2 CO3 þ TiO2 →Li2 TiO3 þ CO2 ↑
ð1Þ
Sample characterizations X-ray diffraction (XRD) measurements of powders were performed with an X-ray diffractometer (Siefert, model 3003 TT) using a CuKα radiation (λ = 0.15406 nm). Data were collected in the 2θ range 10–70°. RAYFLEX Analyze software was used to determine the Bragg peak positions and calculate the lattice parameters. The surface morphology was characterized by field emission scanning electron microscopy (SEM) using a FESEM, SIRION 200 microscope. High-resolution transmission electron microscope (HRTEM) images were collected with a HRTEM-FEI microscope (TECHNAI G2-30 S-twin D905). The elemental composition of the sample was analyzed by energy dispersive spectroscopy (EDS) using an EDAX spectrometer (model SEM ZEISS 40w). Raman scattering (RS) spectrum was collected with a Raman spectrometer (Model Lab Ram HR800) at the excitation wavelength of 632.8 nm of a He-Ne laser. Each spectrum was recorded in the spectral range of 100–1000 cm−1 with a spectral resolution of 1 cm−1. Electrochemical tests Electrochemical tests were carried out using an aqueous threeelectrode glass cell (Pt//Li2TiO3). The Li2TiO3 electrode with an active area of 0.5 cm2, a platinum metal strip, and Ag/AgCl reference electrode were used as the working, counter and reference electrodes. The electrode was prepared by thoroughly mixing lithium titanate active material (annealed at 800 °C for 5 h), acetylene black as conductive agent and
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polyvinylidine fluoride (PVDF) binder at a weight ratio of 80:10:10. The N-methyl-pyrrolidinone (NMP) was added to the mixture to form slurry, spread on the Ni substrate and then dried at 100 °C for 3 h. The electrode loading was 1– 2 mg cm−2. The electrolyte was a saturated lithium sulfate aqueous solution, i.e., 3 mol L−1 Li2SO4. The electrochemical measurements were conducted with an electrochemical workstation model CHI 608C (CH Instruments Inc., USA) in the cutoff voltage between 0.0 and 1.0 V for galvanostatic tests and in the range from −0.4 to +0.4 V for capacitance tests. Cyclic voltammetry (CV) data were collected at different scan rates in the range 1–100 mV s−1. Fig. 3 X-ray diffraction pattern of Li2TiO3 nanoparticles synthesized by solid-state reaction with sintering at 800 °C for 5 h
Material characterization Elemental analysis The energy-dispersive X-ray spectroscopy (EDS) spectrum of the Li2TiO3 powders sintered at 800 °C for 5 h (Fig. 2) shows the existence of elements Ti and O. As usual, lithium cannot detected by EDS because of its light weight [42]. The phase purity is displayed by the unique signature of Ti and O elements without any impurity. The elemental analysis provides the composition is 74.5 at.% of oxygen and 25.5 at.% of titanium (accuracy of ±0.5%). Structural characterization Figure 3 shows the XRD spectrum of the Li2TiO3 powders synthesized by solid-state method at a sintering temperature of 800 °C. The XRD pattern exhibits eight diffraction peaks at around 18.48°, 20.5°, 35.6°, 43.3°, 47.5°, 57.3°, and 62.9°,
67.0° corresponding to the (002), (110), (1 31), (1 33), (2 04), (006), (212), and (062) Bragg reflections of the monoclinic structure, respectively. The XRD peak positions match well with standard data of the monoclinic Li2TiO3 (C2/c space group) (JCPDS: 33–0831) and can be assigned to the thermodynamically stable low-temperature β-phase. This phase was previously identified by several authors [20, 33, 34, 43]. No other impurity phases such as TiO2, for example were detected in the XRD measurements. The predominant high orientation (002) indicates that the crystallites have grown on (001) direction. The lattice parameters were calculated using (002) Bragg reflection and the average crystallite size (Lc) is calculated using the Scherrer’s formulae considering the full width half maximum (FWHM) values of six Bragg lines: Lc ¼
Kλ β . cosθ 1
ð2Þ
2
where K is the Scherrer’s constant (0.89), λ is the wave length of X-ray radiation (0.15406 nm), and β1/2 the half intensity width expressed in radians and θ the Bragg angle. The estimated lattice parameters are a = 5.06 Å, b = 8.79 Å, c = 9.75 Å, β = 100.2°, thus the elementary unit volume (V = abc sinβ) is 427.1 Å3. These values are in good agreement with the reported values [32]: a = 5.0623 Å, b = 8.7876 Å, c = 9.7533 Å, β = 100.2°. The average crystallite size is 32 nm with a lattice strain 0.0105 in the rock-salt Li2TiO3 powders sintered at 800 °C for 5 h. Raman spectroscopy
Fig. 2 EDS analysis of Li2TiO3 powders sintered at 800 °C for 5 h
The Raman scattering (RS) spectrum of the Li2TiO3 shown in Fig. 2 displays the vibrational modes in the frequency range 100–1000 cm−1. This spectrum is dominated by three strong bands at 666, 409/429, and 358 cm−1, which are the fingerprints of Li2TiO3 [34, 44, 45]. The RS spectrum of the monoclinic Li2TiO3 phase usually displays specific bands near 355
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and 425 cm−1 due to the existence of Li+ ions in different coordinations. The vibrational mode at 666 cm−1 band can be attributed to Ti–O stretching vibration in TiO6 octahedra. In the Li2TiO3 structure, the lithium is located in both octahedral and tetrahedral sites; consequently, the Raman characteristic peaks corresponding to Li-O stretching vibrations are observed at 429 and 358 cm−1. The O-Ti-O and O-Li-O bending vibrations appear in the low-frequency spectral range. The band at 283 cm−1 is assigned to Li-O bending mode, while the O-Ti-O bending vibrations appear in the region 190– 220 cm−1. These data match well with those reported by Denisova et al. [44]. The independent phases corresponding to anatase or rutile TiO2 which generally appears in Raman spectrum at 144 cm−1 (Eg mode) or at 610 cm−1, respectively [45] are not observed indicating the phase purity of Li2TiO3 (Fig. 4).
Fig. 5 SEM images of Li2TiO3 powders sintered at 800 °C for 5 h.
results, we conclude that the Li2TiO3 nanoparticles sintered at 800 °C for 5 h have high purity and high crystallinity. Particle morphology Figure 5 displays the SEM images of Li2TiO3 powders sintered at 800 °C for 5 h. The material morphology displays spherical shaped small particles distributed homogeneously with narrow size distribution (30–80 nm). Moreover, the obtained particles are in nanometer scale with an estimated average size of 50 nm. Figure 6 presents the low magnification TEM bright field image of synthesized Li2TiO3 sample. These results are consistent with the crystallite size calculated from the Scherrer equation. Figure 6b shows the corresponding selected area electron diffraction (SAED) pattern of Li2TiO3. The SAED pattern of Li2TiO3 exhibits sharp spots and clear diffraction rings suggesting that spherical particles have a polycrystalline structure. The high-resolution TEM image presented in Fig. 6c exhibits clearly lattice fringes with spacing of 0.48 nm, which matches well with the d-spacing of the (002) planes of the monoclinic Li2TiO3 structure [41]. From these
Fig. 4 Raman spectrum of Li2TiO3 nanoparticles synthesized by solidstate reaction with sintering at 800 °C for 5 h
Li2TiO3 as electrode for electrochemical cell Voltammetry and galvanostatic cycling The electrochemical tests of the Li2TiO3 electrodes were conducted in a three-electrode configuration by cyclic voltammetric and galvanostatic measurements. All electrochemical tests were done at constant temperature of 25 °C. Figure 7 shows typical cyclic voltammogram of Li2TiO3 recorded at a sweep rate of 0.5 mV s−1 in the potential range 0.0–1.0 V. One set of current peaks with broad shape are observed in this diagram, which correspond to the process of extraction and insertion of Li+ ions from/into the monoclinic structure. In the aqueous cell, the reduction (cathodic) peak occurs at 0.338 V that corresponds to the insertion state of Li+ ions into the monoclinic framework Li2+xTiO3 (x ≈ 0.5). The anodic peak of Li2TiO3 is located at 0.643 V corresponding to the potential of Li extraction state to form the pristine structure. Note that this pair of peaks is characteristic of β-Li2TiO3 and yields upon averaging potential of ca. 0.49 V vs. Li+/Li0. The potential difference between cathodic and anodic peak dV = 305 mV, means slow kinetics of Li+ ions. This value is larger than that reported for Li4Ti5O12 and LiTi2O4 lithium titanates [16, 46] due to the low electronic conductivity of the β-Li2TiO3 phase although it was demonstrated that the conductivity of nanosized particles is enhanced compared to that of bulk material [47]. Furthermore, from XRD patterns (not shown here), we observed that the discharged Li2TiO3 phase was not modified during cathodic/anodic cycles. However, we have also reported CV curves of nanocrystalline Li2TiO3 (pseudocapacitor electrode) even at high rates. The CV curves at 100 mV/s scan rate also showed similar behavior.
Ionics Fig. 6 TEM images of Li2TiO3. a Assembly of particles. b SAED pattern. c HRTEM image showing lattice fringes
Figure 8 shows the galvanostatic charge-discharge curves of the Pt//Li2TiO3 aqueous cell cycled at C/50 in the potential cutoff 0–1 Vat 25 °C. These electrochemical features obtained by partial reduction and oxidation of Ti ions are compatible with CV measurements. The discharge curve of Li2TiO3 displays an S-shape voltage due to the Ti4+/Ti3+ reduction that contrasts with the typical long voltage plateau at 1.5 V of Li 4 Ti 5 O 12 spinel. The specific discharge capacity is 122 mAh g − 1 for the first cycle that retains at ca. 116 mAh g−1 after 30 cycles. This value corresponds to the concentration of Li+ ions inserted of x ≈ 0.5, since the theoretical specific capacity is 244 mAh g−1 for the transfer of 1 mol of electrons. The charge profile displays an abrupt increase of the potential up to ca. 0.5 V with almost no capacity. Thus, the insertion-deinsertion process can be written by the relation: Li2 TiO3 þ xLiþ þ xe− ↔Li2þx TiO3
ð3Þ
As evoked in Section 2, the electrochemical insertion of Li into Li2TiO3 was reported showing various specific discharge capacity leading the Li2+xTiIV1-xTiIIIxO3 compound in which Ti4+ ions are reduced to Ti3+ [26, 37, 38]. From comparison of
these results, it appears that the particles size and morphology strongly affect the electrochemical behavior. As far as we know, the better electroactivity of 47 mAh g−1 was obtained for 100-nm-sized particles cycled at C/10 rate in the voltage range 1.5–2.5 V vs. Li+/Li0. Similarly, Bian and Dong [35] synthesized lithium excessive Li2+xTiO3 (0 ≤ x ≤ 0.2) by conventional solid-state reaction. As no obvious cell expansion has been detected from the XRD and Raman profile data, it was concluded that the lithium excess occupy itself site and Ti4+ and O2− site vacancies formed simultaneously. According to Hao et al. [48], during discharge reaction, Li+ ions are inserted into LiTi2 interlayer then in-plane Li(1), Li(2) and forming Ti+4, O−2 site vacancies with charge compensation of Ti+4/Ti+3, which is a reversible process. The lithium diffusion mechanism in β-Li2TiO3 was investigated using DFT methods and showed that Li+ migrates along the ab plane with activation energy in the range 0.44–0.54 eV [14]. This indicates that the Li-diffusion energy barrier is lower than that of Li4Ti5O12 spinel (i.e., 0.86–0.94 eV). The experimental verification was investigated by neutron diffraction experiments [49]. It should be noted that the electrochemical behavior of the β-Li2TiO3 particle (300 nm) and the Li4Ti5O12/Li2TiO3
1st 30th cycle
Fig. 7 Cyclic voltamogram of monoclinic Li2TiO3 nanoparticles recorded at scan rate 0.5 mV s−1
Fig. 8 Galvanostatic charge-discharge curves of the Pt//Li2TiO3 aqueous cell cycled at C/50 in the potential cutoff 0–1 V
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composite (400 nm) prepared by sol-gel method were examined in the potential range 1–3 V [41]. The first discharge is characterized by a fast polarization with a negligible capacity lower than 20 mAh g−1 at 0.2C rate that is attributed to the lack of electro-activity as high energy needed to oxidize Ti4+. EIS of Li2TiO3 electrode Figure 9 presents the Nyquist plot (Z^ vs. Z’) of nanocrystalline Li2TiO3. The Nyquist plot consists of a depressed semicircle in the high-frequency region and inclined line in lowfrequency region. The depressed semicircle in middle to high frequency region is originated to Li+-ion migration through the SEI film and charge transfer resistance. The inclined line at low-frequency region is originated to Li+-ion diffusion in the bulk electrode. The equivalent circuit of the EIS is simulated by Z-VIEW software as shown in inset of Fig. 9. CPE is a constant phase element instead of double layer capacitance and Zw the Warburg impedance [50]. Results indicate that the electrolyte resistance is Rs = 38 Ω, and charge transfer resistance at electrode/electrolyte interface Rct is 250 Ω. With 10% acethylene black, this active electrode possesses a high rate performance and structural stability. The Rct value suggests that the reduction of particle size leads to reduce in the polarization of the electrode and enhances the electrical conductivity and Li+-ion diffusion [48]. Note that the Warburg impedance angle of 45° indicates that the electrode is strongly controlled by an ion diffusion process. Cycling performance Figure 10 shows the specific discharge capacity vs. cycle number of Li2TiO3 electrode over 30 discharge-charge cycles in the voltage range of 0.0–1.0 V. The Li2TiO3 anode exhibited initial discharge capacity of about 122 mAh g−1, the sample retained
Fig. 9 Nyquist plot of Pt//Li2TiO3 anode
Fig. 10 Cycling performance of the Li2TiO3 anode
discharge capacity of about 116 mAh g−1 even after 30 cycles, and the capacity retention is about 94%. As a result, the decay of the specific capacity is 6%. Consequently, such a lithium battery is able to deliver 90% of its initial capacity even after 100 cycles that is acceptable for many applications [40].
Li2TiO3 as electrode of pseudocapacitor Li2TiO3 nanoparticles were investigated as electrode materials for asymmetric supercapacitor. Figure 11 shows the cyclic voltammogram of Li2TiO3 electrode at differential scan rates (i.e., 1, 5, 10, 50, 100 mV s−1) in the potential range −0.4 to +0.4 V. The cyclic voltammograms, which appear almost with a rectangular shape without obvious redox peaks, indicate a capacitive behavior of the electrode (pseudocapacitance) with non-faradic contribution from the double layer effect. When
Fig. 11 Cyclic voltammograms of Li2TiO3 electrode collected at different scan rates
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the particle size decreased to nanoscale dimensions the potential is changing from high potential to low potential, both types of capacitive effects, pseudocapacitive and double layer process can substantially occur due to the high surface area to volume ratio as suggested by Wang et al. [26]. Further increase of current density with increasing scan rates from 1 to 100 mV s−1 represents the good rate capability of the Li2TiO3 electrode. When the scan rate reaches to 100 mV s−1 the cyclic voltammogram keeps the same shape but the potential window is minimized. It happens that for high scan rate the Liions intercalation/deintercalation process should be very fast at the surface of the electrode and the surface is not completely adsorbed/desorbed with ions due to high-potential rate. So, charge on the surface was not established well comparative to low-scan rates [51]. Figure 12 shows the galvanostatic charge–discharge curves of Li2TiO3 at current density of 1, 2, 5, 10 and 50 mA cm−2 in the potential range −0.4 to 0.4 V. The linear and symmetric shape of the charge and discharge curves at different current densities indicates a good reversibility of forward and backward reaction, which is another typical trait of the ideal capacitive behavior. The specific capacitance can be calculated as follows: I C sp ¼ dV m dt
ð4Þ
where Csp is the specific capacitance (in F g −1), I is the charge/ discharge current, dt is the time of discharge, dV is the voltage difference between the upper and lower potential limits, and m is the mass of the active material [52]. Values of capacitance for charge/discharge curves cycled at the above current densities were 317, 198, 165, 141, and 120 F g−1, respectively. Li2TiO3 acts as negatrode (negative electrode) in supercapacitor in the potential window −0.4 to +0.4 V. From cyclic voltammograms, we observe mirror like rectangle shape characteristic of capacitive behavior [53], which is attributed to the complete
Fig. 12 Galvanostatic charge–discharge curves of Li2TiO3 at current density of 1, 5, 10, 20, and 50 mA g−1 in the potential range −0.4 to 0.4 V
reversibility of electroactive atoms from electrode surface to solution and solution to surface. From the kinetics view point, the Li2TiO3 negatrode used minimum current (io) during intercalation/deintercalation process. EIS of negatrode Figure 13 shows the Nyquist plot of Li2TiO3 electrode in the frequency range of 1 Hz–1 MHz using 3 mol dm−3 Li2SO4 aqueous solution as electrolyte. For this pseudocapacitor, the ohmic resistance of the electrode Rs is 18 Ω and the charge transfer resistance Rct is about 180 Ω. Nearly vertical impedance line in the low-frequency region indicates the good capacitive behavior. The appearance of pronounced vertical line leaning to imaginary axis is because of low particle size and hence large surface area of the Li2TiO3 powders [42, 54] leading to more interfacial electrolyte diffusion. These results indicate the pseudocapacitor property of Li2TiO3 electrode material. Cycling performance of negatrode The cycle stability indicating performance of the electrode is shown in Fig. 14. One observes an excellent electrochemical stability of the Li2TiO3 electrode material over 500 cycles with a 50 mV s−1 scan rate. The Li2TiO3 negatrode exhibited an initial specific capacitance of about 317 F g−1 and retained 312 F g−1 even after 500 cycles. The capacity retention of the sample was 95%, with good structural stability.
Conclusions Nanocrystalline Li2TiO3 was successfully synthesized using solid-state reaction method. The powders have a monoclinic
Fig. 13 Nyquist plot of nanocrystalline Li 2TiO 3 as electrode of pseudocapacitor
Ionics Acknowledgements One of the authors, A. Lakshmi Narayana, would like to thank CSIR, New Delhi, for providing fellowship.
References 1.
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3.
4. Fig. 14 Cycling performance of the Li2TiO3 negatrode
5. 6.
structure with C2/c space group and the particle size is in the range 20–50 nm. The Raman vibrational modes and HRTEM and SAED patterns confirm the existence of single phase monoclinic Li2TiO3 structure. While Li2TiO3 macroparticles are electrochemically inert materials, nanoparticles are active in a wide voltage range and they exhibit structural stability in aqueous electrolyte. In the present study, we have conducted electrochemical experiments on nanocrystalline Li2TiO3 using three electrode aqueous cell with 3 mol L−1 Li2SO4 saturated electrolyte. The nanocrystalline Li2TiO3 exhibited perfect redox couple in the potential window 0.0–1.0 V. we observed peak shaped CVs and plateau shaped charge- discharge curves to nanocrystalline Li2TiO3, which signifies the anodic (battery) property of the electrode. The nanocrystalline Li2TiO3 electrode showed rectangular cyclic voltammetry and linear plots of galvanostatic charge and discharge curves (GCDs) in −0.4 to +0.4 V potential region, which indicates that the electrode also exhibited pseudocapacitive nature. These results conclude that the nanocrystalline Li2TiO3 electrode display anodic as well as pseudocapacitive properties. Hence, the synthesized nanocrystalline Li2TiO3 is a suitable electrode for supercapattery applications. The electrochemical tests revealed that nanocrystalline Li 2 TiO 3 delivered a specific discharge capacity of ca. 122 mAh g−1, in potential window 0.0–1.0 V. Moreover, this material exhibits pseudocapacitance properties in the potential window −0.4 to +0.4 V with a specific capacitance of 317 F g−1 and shows good cycling stability as well. Hence, we concluded that the synthesized nanocrystalline Li2TiO3 is a potential energy storage material such as anode element of lithium rechargeable batteries and negatrode of supercapacitors as well.
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