Robust electrochemistry of black TiO2 as stable and high-rate negative electrode for lithium-ion batteries
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Electrochemically stable black TiO2 composed of Ti3+ ions and oxygen vacancies is successfully synthesized by a facile and economic sol–gel method followed by calcination in nitrogen atmosphere at 400 °C for 2 h. Several physicochemical techniques are probed to validate the desired state of the obtained material. The material is formed in a pure state with an average size of 10 nm. The electrochemical studies are conducted for its use as negative electrode for Li-ion batteries. At high current rate of 5 C, the electrodes deliver a high discharge capacity of 226 mA h g−1 even after 150 cycles. Similarly, the electrodes also deliver discharge capacities of 197 and 153 mA h g−1 at current rates of 7 C and 10 C, respectively. The robust electrochemical properties of black TiO2 including large specific capacities at high current rates and high stability are ascribed to the formation of defective structure, conductive Ti3+ ions and oxygen vacancies.
KeywordsBlack TiO2 High-rate negative electrode Lithium-ion battery Ti3+ ions
Low cost and high safety along with large specific capacity are the critical factors for choosing materials as an electrode (cathode or anode) for lithium-ion batteries (LIBs). Hence, it is important to develop a high specific capacity material along with low cost and high safety. It is often observed that materials with high theoretical capacity suffer from low cycling stability [1, 2, 3, 4]. TiO2 (white) is well established as a low-cost material with high safety. The remarkable electrochemical cycling stability of TiO2 makes it more attractive for LIBs. TiO2 is also documented as high-rate anode which operates within the potential window of the most common electrolytes, thus inhibiting the formation of solid–electrolyte interface (SEI) and related gas emission associated with electrolyte decomposition [5, 6].
A limiting factor to its implementation and application would be the high operating potential that makes the energy density of Li-ion battery based on TiO2 negative electrode lower than that featuring graphite anode. Apart from low theoretical specific capacity, it also suffers from high intrinsic resistance (evident from its wide bandgap of 3.2 eV), structural distortion (during Li+ de-/intercalation) and most importantly, formation of passive oxidation layer on the surface when it is annealed in air atmosphere which hinders the kinetics of the Li+ ions [7, 8, 9].
Many efforts have been made to tackle these shortcomings. Different polymorphs like anatase , brookite , rutile , extensive amorphous TiO2  have been introduced to enhance the electrochemical performance. The research is also focused on synthesizing TiO2 with different morphologies such as nanoparticles (NPs), nanorods (NRs), etc., to improve its electrochemical performance [6, 13, 14]. Recently, black TiO2 nanoparticles (B-TiO2 NPs) have been explored as a solution [8, 9]. B-TiO2 demonstrates narrowed bandgap with unaltered tetravalent Ti. With a decrease in the bandgap, intrinsic resistance is found to decrease [15, 16]. Moreover, B-TiO2 is accompanied by conductive Ti3+ and oxygen vacancies. Oxygen vacancies enhance the interlayer spacing, leading to faster Li+ ions kinetics, and facilitate the reinstitution of host structure during de-/intercalation . Thus, these unique factors make B-TiO2 an attractive contender as negative electrode for LIBs.
Though it has many salient features, surprisingly it has been a less explored negative electrode for LIBs. Several methods including metal reduction, plasma treatment, high-/low-pressure hydrogen treatment have been proposed to synthesize B-TiO2 NPs [18, 19, 20, 21]. However, these methods demand rigorous reaction conditions. Therefore, a facile and economic synthesis method is highly desirable. Herein, we report the synthesis of B-TiO2 NPs by simple and scalable sol–gel method followed by calcination at 400 °C in the nitrogen (N2) atmosphere for 2 h. The obtained B-TiO2 NPs were characterized with several physicochemical techniques and probed as negative electrode for LIBs.
Titanium (IV) butoxide and thiourea were procured from Sigma-Aldrich. Lauryl lactyl lactate (Koplactylate) surfactant was procured from Kumar Organic Products Pvt. Ltd., India. The chemical reagents were used as received without any purification.
Sol–gel synthesis of B-TiO2 NPs
B-TiO2 NPs were synthesized by a simple modified sol–gel method as reported in our earlier work on white TiO2 . A solution composed of 4 mL of ethanol and 0.9 mL of double-distilled (DD) water was named as A. Another solution B was prepared by mixing 0.5 mL of lauryl lactyl lactate, 8 mL of ethanol, 2 mL of titanium butoxide and 0.1 mL of hydrochloric acid. Solution A was then added to solution B and agitated for 5–8 min until gel was developed. It was allowed to age for 12 h at ambient temperature and then dried at 80 °C for 3 h. The solid gel was transferred and subjected to calcination at 400 °C for 2 h in the tubular furnace under nitrogen (N2) atmosphere. The obtained compound was black in color.
Powder X-ray diffraction (PXRD) pattern of B-TiO2 was analyzed using Rigaku SmartLab X-ray diffractometer with monochromatized Cu Kα radiation. The chemical nature of the B-TiO2 was investigated by X-ray photoelectron spectroscopy (XPS) using Al Kα radiation (1.486 keV, Axis Ultra DLD, Kratos Analytical). Raman spectrum of the sample was recorded using 514.5-nm Ar+ laser in HORIBA LabRam HR800. Morphological and structural investigations were carried out using scanning electron microscopy (SEM) [FESEM Carl Zeiss (Oxford instrument)] and transmission electron microscopy (TEM) (FEI Tecnai T20 S-TWIN TEM). Quanta Chrome Nova-1000 surface analyzer was used to record surface area and pore size distribution of the sample.
Coin cells (CR2032, Hohsen Corporation, Japan) were assembled to investigate the electrochemical properties of B-TiO2 as a negative electrode for LIBs. Lithium (Aldrich) metal was used as both reference and counter electrode. Active material (B-TiO2), carbon black (Super P, Aldrich) and sodium carboxymethyl cellulose (CMC, Aldrich) were mixed in the weight ratio 85:10:5, and a slurry was prepared in a few drops of N-methylpyrrolidinone (NMP, Aldrich). Although CMC is water-soluble, NMP was used to follow the standard practice of electrode formation which tends to avoid water as even PPM level of trapped moisture which can be detrimental to cell performance. The slurry was uniformly coated on a pretreated copper foil before drying it at 110 °C for overnight in a vacuum oven. 1 M LiPF6 dissolved in ethylene carbonate, diethyl carbonate and dimethyl carbonate (2:1:2 v/v) (Chameleon, China) was used as electrolyte, and a porous polypropylene membrane (Celgard 2400) as a separator. The coin cells were fabricated in argon-filled glove box with ~ 1.0 mg cm−2 of active material. Cyclic voltammetry, galvanostatic charge–discharge cycling and rate capability measurements were conducted using Biologic Science BCS-8xx series at 22 ± 1 °C. Electrochemical ac impedance spectroscopy was carried out on the cells in the frequency range of 10 mHz to 100 kHz with an amplitude of 10 mV in three-electrode mode.
Results and discussion
Formation of Ti interstitials and expulsion of oxygen from anion sublattice leaving oxygen vacancies are the chief processes accountable for the formation of high oxygen vacancies in the TiO2−x. Generally, the latter one is extensive than Ti interstitials. The density of carriers, which are regarded as electron donors for TiO2, is enhanced by the generation of oxygen vacancies. With an increase in donor density in the TiO2−x structure, electrical conductivity increases, whereas bandgap energy decreases, yielding visible active property to the material [23, 24, 25]. We can also observe a tiny peak at 400 eV which corresponds to 1s level of nitrogen (Fig. S1). This can be attributed to the presence of chemisorbed nitrogen. Many authors have exposed this contribution and attributed it to molecularly chemisorbed γ-nitrogen [29, 30, 31, 32, 33, 34]. Figure 2c shows the XPS spectrum of oxygen. The sample displayed a peak at 529.9 eV which is ascribed to the characteristic peak of Ti–O–Ti [23, 24, 25, 26, 27, 28]. The peak is broad and slightly asymmetric, and it may be attributed to Ti–O–N and Ti–O–H. Formation of TiN or TiO2–TiN mixed phase can be ruled out as XRD pattern does not show any significant impurity peak. Furthermore, no nitrogen precursor has been used in the synthesis. Therefore, asymmetry of oxygen peak is due to bonding of oxygen with chemisorbed nitrogen and hydrogen [29, 30, 31, 32, 33, 34]. Thus, XPS spectra reveal the formation Ti3+ ions and oxygen vacancies, causing structural defects in TiO2.
Furthermore, Reddy et al. exclusively carried out electrochemical impedance measurements on TiO2 during charge and discharge processes at different potentials. Reddy group demonstrated that the resistance will decrease as we move from potential 3.0 to 0.05 V (lithiation). However, resistance in the potential window (1.6–2.1 V) where typical redox reaction takes place will be low compared to impedance at different potentials [67, 68, 69, 70, 71]. An irreversible capacity loss is generally observed in the first cycle (here 40%) when TiO2 is cycled in the wider potential window and is due to (1) sluggish delithiation kinetics for first few cycles, which is due to excess of Li+ ions trapped in TiO2 matrix; (2) irreversible intercalation/insertion of Li+ ions into the matrix; (3) electrolyte decomposition; and (4) formation of SEI [39, 56, 61, 62, 63, 64, 65, 66].
The lithium intercalation mechanism can be disintegrated into three parts. Part-1 corresponds to 3.0–1.75 V potential window where voltage drops from OCV to 1.75 V. This range corresponds to intercalation of Li+ ions into tetragonal TiO2 matrix without the nucleation of other phases [10, 12, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60]. Part-2 corresponds to plateau at ≈ 1.75 V. In the literature, this plateau is ascribed to formation and coexistence of orthorhombic phase along with tetragonal phase [10, 12, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60]. Part-3 extends from 1.75 to 0.1 V. Earlier studies have proven that Li+ insertion in this range depends on the size, morphology and defects of host material [10, 12, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60]. Furthermore, it is also illustrated the formation of reversible interfacial lithium storage which holds responsible for excess lithium insertion by means of charge separation . The delithiation curve shows a broad hump at 2.10 V corresponding to removal of Li+ ions from the B-TiO2 matrix. The potential profile is in good agreement with CV results. All the results are coherent with earlier reports [9, 35, 36, 72].
The high capacity and excellent stability of the electrode are attributed to the presence of Ti3+ ions and oxygen vacancies. As Ti3+ ions are highly conductive in nature, TiO2 with Ti3+ ions is promoted as a strong contender for negative electrode in LIBs. Meanwhile, oxygen vacancies formed during the generation of Ti3+ ions also favor the lithium storage by providing enhanced interlayer spacing. Thus, Ti3+ ions and oxygen vacancies play a significant role in enhanced Li+ ion storage and excellent stability. It is also worth noting the use of CMC as a binder. The excessive polar functional groups are exploited as excellent chemical bonding agents to bind the active material and current collector together, thereby curtailing the structural destruction during the Li+ ions de-/intercalation. The excellent electrochemical performance of B-TiO2 is also benefited from its nanosize, high surface area, mesoporosity and spongy nature of the electrode material.
Black TiO2 NPs were synthesized by a simple surfactant-assisted sol–gel method. The formation of Ti3+ ions and oxygen vacancies were ascertained by XPS studies and were found to be responsible for the formation of black-colored defective TiO2. The synthesized defective B-TiO2 NPs were probed as negative electrode for LIBs. It exhibited discharge capacities of 275 and 220 mA h g−1 at very high current rates of 1 C and 5 C, respectively, even after 100 cycles of charge and discharge. The high specific capacity and excellent stability of B-TiO2 NPs are attributed to the formation of Ti3+ ions and oxygen vacancies. As Ti3+ ions are highly conductive in nature, they enhance the lithium kinetics. The oxygen vacancies which are formed during the generation of Ti3+ ions also favor the lithium storage by providing enhanced interlayer spacing. These are the driving forces for us to implement and investigate B-TiO2 as negative electrode for LIBs. Thus, it is reckoned that defective B-TiO2 can serve as a potential negative electrode for LIBs.
SBP and GN greatly thank BRNS-BARC, DAE (No. 37(2)/14/25/2015/BRNS), Bombay, Govt. of India, for financial sponsorship. Thanks are due to CoE—TEQIP, Director and Principal, Siddaganga Institute of Technology (SIT), Tumakuru, for constant support and encouragement. Thanks to Prof. N. Munichandraiah, Dept. of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, for providing glove box facility to assemble the cells.
Compliance with ethical standards
Conflict of interest
The authors declare no competing financial interest.
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