High-rate characteristics of novel anode Li4Ti5O12/polyacene materials for Li-ion secondary batteries

In recent years, spinel lithium titanate (Li₄Ti₅O₁₂) as a superior anode material for energy storage battery has attracted a great deal of attention because of the excellent Li-ion insertion and extraction reversibility. However, the high-rate characteristics of this material should be improved if it is used as an active material in large batteries. One effective way to achieve this is to prepare electrode materials coated with carbon. A Li₄Ti₅O₁₂/polyacene (PAS) composite were first prepared via an in situ carbonization of phenol-formaldehyde (PF) resin route to form carbon-based composite. The SEM showed that the Li₄Ti₅O₁₂ particles in the composite were more rounded and smaller than the pristine one. The PAS was uniformly dispersed between the Li₄Ti₅O₁₂ particles, which improved the electrical contact between the corresponding Li₄Ti₅O₁₂ particles, and hence the electronic conductivity of composite material. The electronic conductivity of Li₄Ti₅O₁₂/PAS composite is 10⁻¹ S cm⁻¹, which is much higher than 10⁻⁹ S cm⁻¹ of the pristine Li₄Ti₅O₁₂. High specific capacity, especially better high-rate performance was achieved with this Li₄Ti₅O₁₂/PAS electrode material. The initial specific capacity of the sample is 144 mAh/g at 3 C, and it is still 126.2 mAh/g after 200 cycles. By increasing the current density, the sample still maintains excellent cycle performance.     

Effect of particle dispersion on high rate performance of nano-sized Li4Ti5O12 anode

Nano-sized Li₄Ti₅O₁₂ powders with high dispersivity were fabricated by a sol-gel process using P123 as surfactant, which exhibited much better high rate performance towards Li⁺ insertion/extraction as compared to the densely aggregated Li₄Ti₅O₁₂ particles although the primary grain sizes of both samples were almost the same. The Li₄Ti₅O₁₂ electrode prepared from the well-dispersed nanopowders can preserve 88.6% of the capacity at 0.1 A g⁻¹ when being cycled at 1 A g⁻¹, which is obviously higher than that of the densely aggregated sample, in which only 30% capacity can be retained. By improving the dispersivity, the specific surface area of the Li₄Ti₅O₁₂ nanoparticles, hence the electrode-electrolyte contact area was increased; meanwhile, more homogeneous mixing of the active materials with carbon black was achieved. All these factors might have resulted in the better high rate performance.     

The preparation and characterization of Li4Ti5O12/carbon nano-tubes for lithium ion battery

Li₄Ti₅O₁₂/carbon nano-tubes (CNTs) composite was prepared by sol-gel method while Ti(OC₄H₉)₄, LiCH₃COO·2H₂O and the n-heptane containing CNTs were used as raw materials. The characters of Li₄Ti₅O₁₂/CNTs composite were determined by XRD, SEM, and TG methods. Its electrochemical properties were measured by charge-discharge cycling and impedance tests. It was found that the prepared Li₄Ti₅O₁₂/CNTs presented an excellent rate capability and capacity retention. At the charge-discharge rate of 5C and 10C, its discharge capacities were 145 and 135 mAh g⁻¹, respectively. After 500 cycles at 5C, the discharge capacity retained as 142 mAh g⁻¹. It even could be cycled at the rate of 20C. The excellent electrochemical performance of Li₄Ti₅O₁₂/CNTs electrode could be attributed to the improvement of electronic conductivity by adding conducting CNTs and the nano-size of Li₄Ti₅O₁₂ particles in the Li₄Ti₅O₁₂/CNTs composite.     

All-solid-state lithium battery with a three-dimensionally ordered Li1.5Al0.5Ti1.5(PO4)3 electrode

To fabricate all-solid-state Li batteries using three-dimensionally ordered macroporous Li_1.5Al_0.5Ti_1.5(PO₄)₃ (3DOM LATP) electrodes, the compatibilities of two anode materials (Li₄Mn₅O₁₂ and Li₄Ti₅O₁₂) with a LATP solid electrolyte were tested. Pure Li₄Ti₅O₁₂ with high crystallinity was not obtained because of the formation of a TiO₂ impurity phase. Li₄Mn₅O₁₂ with high crystallinity was produced without an impurity phase, suggesting that Li₄Mn₅O₁₂ is a better anode material for the LATP system. A Li₄Mn₅O₁₂/3DOM LATP composite anode was fabricated by the colloidal crystal templating method and a sol-gel process. Reversible Li insertion into the fabricated Li₄Mn₅O₁₂/3DOM LATP anode was observed, and its discharge capacity was measured to be 27 mA h g⁻¹. An all-solid-state battery composed of LiMn₂O₄/3DOM LATP cathode, Li₄Mn₅O₁₂/3DOM LATP anode, and a polymer electrolyte was fabricated and shown to operate successfully. It had a potential plateau that corresponds to the potential difference expected from the intrinsic redox potentials of LiMn₂O₄ and Li₄Mn₅O₁₂. The discharge capacity of the all-solid-state battery was 480 μA h cm⁻².     

Preparation and electrochemical performance of Li4Ti5O12/carbon/carbon nano-tubes for lithium ion battery

A Li₄Ti₅O₁₂/carbon/carbon nano-tubes (Li₄Ti₅O₁₂/C/CNTs) composite was synthesized by using a solid-state method. For comparison, a Li₄Ti₅O₁₂/carbon (Li₄Ti₅O₁₂/C) composite and a pristine Li₄Ti₅O₁₂ were also synthesized in the present study. The microstructure and morphology of the prepared samples are characterized by XRD and SEM. Electrochemical properties of the samples are evaluated by using galvanostatic discharge/charge tests and AC impedance spectroscopy. The results reveal that the Li₄Ti₅O₁₂/C/CNTs composite exhibits the best rate capability and cycling stability among the samples of Li₄Ti₅O₁₂, Li₄Ti₅O₁₂/C and Li₄Ti₅O₁₂/C/CNTs. At the charge-discharge rate of 0.5 C, 5.0 C and 10.0 C, its discharge capacities were 163 mAh/g, 148 mAh/g and 143 mAh/g, respectively. After 100 cycles at 5.0 C, it remained at 146 mAh/g.     

Cycling-induced stress in lithium ion negative electrodes: LiAl/LiFePO4 and Li4Ti5O12/LiFePO4 cells

In this work, we examined the electrochemical behaviour of lithium ion batteries containing lithium iron phosphate as the positive electrode and systems based on Li-Al or Li-Ti-O as the negative electrode. These two systems differ in their potential versus the redox couple Li⁺/Li and in their morphological changes upon lithium insertion/deinsertion. Under relatively slow charge/discharge regimes, the lithium-aluminium alloys were found to deliver energies as high as 438 Wh kg⁻¹ but could withstand only a few cycles before crumbling, which precludes their use as negative electrodes. Negative electrodes consisting solely of aluminium performed even worse. However, an electrode made from a material with zero-strain associated to lithium introduction/removal such as a lithium titanate spinel exhibited good performance that was slightly dependent on the current rate used. The Li₄Ti₅O₁₂/LiFePO₄ cell provided capacities as high as 150 mAh g⁻¹ under C-rate in the 100th cycle.     

Preparation and characterization of novel spinel Li4Ti5O12−xBrx anode materials

Br-doped Li₄Ti₅O₁₂ in the form of Li₄Ti₅O_12−xBr_x (0 ≤ x ≤ 0.3) compounds were successfully synthesized via solid state reaction. The structure and electrochemical properties of the spinel Li₄Ti₅O_12−xBr_x (0 ≤ x ≤ 0.3) materials were investigated. The Li₄Ti₅O_12−xBr_x (x = 0.2) presents the best discharge capacity among all the samples, and shows better reversibility and higher cyclic stability compared with pristine Li₄Ti₅O₁₂, especially at high current rates. When the discharge rate was 0.5 C, the Li₄Ti₅O_12−xBr_x (x = 0.2) sample presented the excellent discharge capacity of 172 mAh g⁻¹, which was very close to its theoretical capacity (175 mAh g⁻¹), while that of the pristine Li₄Ti₅O₁₂ was 123.2 mAh g⁻¹ only.     

Structural and electrochemical characteristics of Li4−xAlxTi5O12 as anode material for lithium-ion batteries

Al-doped Li₄Ti₅O₁₂ in the form of Li_4−xAl_xTi₅O₁₂ (x = 0, 0.05, 0.1 and 0.2) was synthesized via solid state reaction in an Ar-flowing atmosphere. Al-doping does not change the phase composition and particle morphology, but easily results in the lattice distortion and thus the poor crystallinity of Li₄Ti₅O₁₂. Al-doping decreases the specific capacity of Li₄Ti₅O₁₂, while improves remarkably its cycling stability at high charge/discharge rate. The substitution of Al for Li site can enhance the electronic conductivity of Li₄Ti₅O₁₂ via the generation of mixing Ti⁴⁺/Ti³⁺, whereas impede the Li-ion diffusion in the lattice. Excessive Al causes large electrode polarization due to the lower Li-ion conductivity, and thus leads to low specific capacity at high current densities. Li_3.9Al_0.1Ti₅O₁₂ exhibits a relatively high specific capacity and an excellent cycling stability.     

Preparation and cycling performance of Aland F co-substituted compounds Li4AlxTi5−xFyO12−y

Li₄Al_xTi_5−xF_yO_12−y compounds were prepared by a solid-state reaction method. Phase analyses demonstrated that both Al³⁺ and F⁻ ions entered the structure of spinel-type Li₄Ti₅O₁₂. Charge-discharge cycling results at a constant current density of 0.15 mA cm⁻² between the cut-off voltages of 2.5 and 0.5 V showed that the Al³⁺ and F⁻ substitutions improved the first total discharge capacity of Li₄Ti₅O₁₂. However, Al³⁺ substitution greatly increased the reversible capacity and cycling stability of Li₄Ti₅O₁₂ while F⁻ substitution decreased its reversible capacity and cycling stability slightly. The electrochemical performance of the Al³⁺-F⁻-co-substituted specimen was better than the F⁻-substituted one but worse than the Al³⁺-substituted one.     

Aging characteristics of high-power lithium-ion cells with LiNi0.8Co0.15Al0.05O2 and Li4/3Ti5/3O4 electrodes

The impedance rise that results from the accelerated aging of high-power lithium-ion cells containing LiNi_0.8Co_0.15Al_0.05O₂-based positive and graphite-based negative electrodes is dominated by contributions from the positive electrode. Data from various diagnostic experiments have indicated that a general degradation of the ionic pathway, apparently caused by surface film formation on the oxide particles, produces the positive electrode interface rise. One mechanistic hypothesis postulates that these surface films are components of the negative electrode solid electrolyte interphase (SEI) layer that migrate through the electrolyte and separator and subsequently coat the positive electrode. This hypothesis is examined in this article by subjecting cells with LiNi_0.8Co_0.15Al_0.05O₂-based positive and Li_4/3Ti_5/3O₄-based negative electrodes to accelerated aging. The impedance rise in these cells was observed to be almost entirely from the positive electrode. Because reduction products are not expected on the 1.55 V Li_4/3Ti_5/3O₄ electrode, the positive electrode impedance cannot be attributed to the migration of SEI-type fragments from the negative electrode. It follows then that the impedance rise results from mechanisms that are “intrinsic” to the positive electrode.     

Preparation and characterization of three dimensionally ordered macroporous Li4Ti5O12 anode for lithium batteries

Three dimensionally ordered macroporous (3DOM) Li₄Ti₅O₁₂ membrane (80 μm thick) was prepared by a colloidal crystal templating process. Colloidal crystal consisting of monodisperse polystyrene particles (1 μm diameter) was used as the template for the preparation of macroporous Li₄Ti₅O₁₂. A precursor sol consisting of titanium isopropoxide and lithium acetate was impregnated into the void space of template, and it was calcined at various temperatures. A macroporous membrane of Li₄Ti₅O₁₂ with inverse-opal structure was successfully prepared at 800 °C. The interconnected pores with uniform size (0.8 μm) were clearly observed on the entire part of membrane. The electrochemical properties of the three dimensionally ordered Li₄Ti₅O₁₂ were characterized with cyclic voltammetry and galvanostatic charge and discharge in an organic electrolyte containing a lithium salt. The 3DOM Li₄Ti₅O₁₂ exhibited a discharge capacity of 160 mA h g⁻¹ at the electrode potential of 1.55 V versus Li/Li⁺ due to the solid state redox of Ti^3+/4+ accompanying with Li⁺ ion insertion and extraction. The discharge capacity was close to the theoretical capacity (167 mA h g⁻¹), which suggested that the Li⁺ ion insertion and extraction took place at the entire part of 3DOM Li₄Ti₅O₁₂ membrane. The 3DOM Li₄Ti₅O₁₂ electrode showed good cycle stability.     

Hydrothermal synthesis of Li4Ti5O12 microsphere with high capacity as anode material for lithium ion batteries

Lithium titanate (Li₄Ti₅O₁₂) microsphere has been successfully synthesized by a hydrothermal method. X-ray diffraction (XRD) and scanning electron microscope (SEM) are used to characterize the structure and morphology of the prepared Li₄Ti₅O₁₂ crystallites. The results show that the as-synthesized powders exhibit outstanding rate capacities and excellent cycling performance. The first discharge capacity at 0.1 C is 172.5 mAh g⁻¹, which is close to the theoretical capacity of 175 mAh/g. After 50 cycles, the efficiency of the synthesized Li₄Ti₅O₁₂ still retains up to 92.8% at 0.1 C and 95.2% at 0.5 C of its initial value, which present a promising applications as anode materials for lithium ion batteries in hybrid and plug-in hybrid electric vehicles.     

Influence of composite LiCl-KCl molten salt on microstructure and electrochemical performance of spinel Li4Ti5O12

A series of spinel Li₄Ti₅O₁₂ samples were synthesized via a composite molten-salt method (CMSM) using the mixtures of LiCl and KCl with different L values (L is defined as the molar ratio of LiCl:KCl) as the reaction media. It is found that the melting point of the composite molten salt can effectively influence the formation of particles, and leads to different electrochemical performances of the as-prepare Li₄Ti₅O₁₂. The investigations of X-ray diffraction (XRD), particle size distribution (PSD), Brunauer-Emmet-Teller (BET) surface area, and scanning electron microscopy (SEM) indicate that the as-prepared Li₄Ti₅O₁₂ with L = 1.5 is a pure phase, and has uniform homogeneous octahedral shape particles, rather narrow PSD, and high BET surface area. Electrochemical tests show that the optimized Li₄Ti₅O₁₂ with L = 1.5 has an initial discharge capacity of 169 mAh g⁻¹ and an initial charge-discharge efficiency of 94% at 0.2 C rate, and achieves good rate performances from 0.2 C to 5 C.     

Preparation and characterization of ramsdellite Li2Ti3O7 as an anode material for asymmetric supercapacitors

Ramsdellite Li₂Ti₃O₇ was first synthesized via sol-gel process with good crystallity of an average particle size of 0.175 μm. The product was thoroughly investigated as a lithium intercalation compound, and as an active anode material in asymmetric supercapacitors coupling with activated carbon as cathode. Lithium intercalation reactions were found occurring at 1.32 and 1.62 V versus Li/Li⁺, respectively. A reversible specific capacity of 150 mA h g⁻¹ at 1C was obtained on Li₂Ti₃O₇ electrode in a nonaqueous electrolyte. The charge current was found to strongly influence the anodic discharge capacity in the asymmetric cell. The capacity retention at 10C charge-discharge rate was found to be 75.9% in comparison with that at 1C.     

Niobium doped lithium titanate as a high rate anode material for Li-ion batteries

Niobium doped lithium titanate with the composition of Li₄Ti_4.95Nb_0.05O₁₂ has been prepared by a sol-gel method. X-ray diffraction (XRD) and scanning electron microscope (SEM) are employed to characterize the structure and morphology of Li₄Ti_4.95Nb_0.05O₁₂. The Li₄Ti_4.95Nb_0.05O₁₂ electrode presents a higher specific capacity and better cycling performance than the Li₄Ti₅O₁₂ electrode prepared by the similar process. The Li₄Ti_4.95Nb_0.05O₁₂ exhibits an excellent rate capability with a reversible capacity of 135 mAh g⁻¹ at 10 C, 127 mAh g⁻¹ at 20 C and even 80 mAh g⁻¹ at 40 C. Electrical resistance measurement and electrochemical impedance spectra (EIS) reveal that the Li₄Ti_4.95Nb_0.05O₁₂ exhibits a higher electronic conductivity and faster lithium-ion diffusivity than the Li₄Ti₅O₁₂, which indicates that niobium doped lithium titanate (Li₄Ti_4.95Nb_0.05O₁₂) is promising as a high rate anode for the lithium-ion batteries.     

Improving the electrochemical performance of Li4Ti5O12/Ag composite by an electroless deposition method

Nano-sized silver particle (<20 nm) was highly dispersed on the surface of Li₄Ti₅O₁₂ particles by an electroless deposition method. The Ag additive played a positive role in improving the electrical contact between Li₄Ti₅O₁₂ particles and the current collector and therefore improved the high rate capacity of Li₄Ti₅O₁₂, but it did not take part in the electrochemical reactions with Li⁺ in Li₄Ti₅O₁₂/Ag composite during the cycling. The experimental results showed that the smaller the silver particles and the more homogeneous dispersion of silver particles in the Li₄Ti₅O₁₂ matrix, the better the cycling performance we obtained.     

Microstructure effect on the electrochemical property of Li4Ti5O12 as an anode material for lithium-ion batteries

Porous (P-) and dense (D-) lithium titanate (Li₄Ti₅O₁₂) powders as an anode material for lithium-ion batteries have been synthesized by spray drying followed by solid-state calcination. Electrochemical testing results showed that the discharge capacities of P-Li₄Ti₅O₁₂ are 144 mAh/g, 128 mAh/g and 73 mAh/g at the discharging rate of 2C, 5C and 20C, respectively (cut-off voltages: 0.5-2.5 V). The corresponding values for D-Li₄Ti₅O₁₂ are 108 mAh/g, 25 mAh/g and 17 mAh/g. The higher capacity of the P-Li₄Ti₅O₁₂ at high charge/discharge rates was attributed to the shorter transport path of Li ions and higher electronic conductivity in the P-Li₄Ti₅O₁₂ as a result of its smaller primary particle size and higher surface area compared with those of the D-Li₄Ti₅O₁₂.     

Electrochemical performance of the carbon coated Li3V2(PO4)3 cathode material synthesized by a sol-gel method

A carbon coated Li₃V₂(PO₄)₃ cathode material for lithium ion batteries was synthesized by a sol-gel method using V₂O₅, H₂O₂, NH₄H₂PO₄, LiOH and citric acid as starting materials, and its physicochemical properties were investigated using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) spectroscopy, scanning electron microscopy (SEM), energy dispersive analysis of X-ray (EDAX), transmission electron microscope (TEM), and electrochemical methods. The sample prepared displays a monoclinic structure with a space group of P2₁/n, and its surface is covered with a rough and porous carbon layer. In the voltage range of 3.0-4.3 V, the Li₃V₂(PO₄)₃ electrode displays a large reversible capacity, good rate capability and excellent cyclic stability at both 25 and 55 °C. The largest reversible capacity of 130 mAh g⁻¹ was obtained at 0.1C and 55 °C, nearly equivalent to the reversible cycling of two lithium ions per Li₃V₂(PO₄)₃ formula unit (133 mAh g⁻¹). It was found that the increase in total carbon content can improve the discharge performance of the Li₃V₂(PO₄)₃ electrode. In the voltage range of 3.0-4.8 V, the extraction and reinsertion of the third lithium ion in the carbon coated Li₃V₂(PO₄)₃ host are almost reversible, exhibiting a reversible capacity of 177 mAh g⁻¹ and good cyclic performance. The reasons for the excellent electrochemical performance of the carbon coated Li₃V₂(PO₄)₃ cathode material were also discussed.     

Effects of the starting materials and mechanochemical activation on the properties of solid-state reacted Li4Ti5O12 for lithium ion batteries

Li₄Ti₅O₁₂ was synthesized by a solid-state reaction between Li₂CO₃ and TiO₂ for applications in lithium ion batteries. The effects of the TiO₂ phase and mechanochemical activation on the Li₄Ti₅O₁₂ particles as well as the corresponding electrochemical properties were investigated. Rutile TiO₂ was more desirable in acquiring high purity Li₄Ti₅O₁₂ than anatase due to the anatase to rutile phase transformation, which was found to be more rigid in the solid-state reaction than the intact rutile phase. Mechanochemical activation of the starting materials was effective in decreasing the reaction temperature and particle size as well as increasing the Li₄Ti₅O₁₂ content. The specific capacity depended significantly on the Li₄Ti₅O₁₂ content, whereas the rate capability improved with decreasing particle size due to the enhanced contact area and reduced diffusion path. Overall, a 200 nm-sized Li₄Ti₅O₁₂ powder with a specific capacity of 165 mAh/g could be synthesized by optimizing the milling method and starting materials.     

An investigation of cell-impedance-controlled lithium transport through LiCoO2/Li1−δMn2O4 bilayer film electrode prepared by rf magnetron sputtering

Lithium transport through LiCoO₂/Li_1−δMn₂O₄ bilayer film electrode prepared by radio-frequency (rf) magnetron sputtering was investigated in a 1 M solution of LiClO₄ in propylene carbonate. From the analyses of the AC-impedance spectra experimentally measured from the Li_1−δMn₂O₄ single-layer and LiCoO₂/Li_1−δMn₂O₄ bilayer film specimens, the internal cell resistance of the LiCoO₂/Li_1−δMn₂O₄ bilayer film electrode was determined to be smaller in value than that of the Li_1−δMn₂O₄ single-layer film electrode over the whole potential range, which can be accounted for by the kinetic facility for the interfacial charge-transfer reaction in the presence of the more conductive LiCoO₂ surface film. Moreover, from the analyses of the anodic current transients measured from both the film specimens, it was suggested that the cell-impedance-controlled constraint at the electrode surface is changed to the diffusion-controlled constraint simultaneously characterised by the large potential step and the small amount of lithium transferred during lithium transport. In addition, in the case of the LiCoO₂/Li_1−δMn₂O₄ bilayer film electrode, it was found that the critical value of the applied potential step needed for the mechanism transition is reduced, which strongly indicates that the internal cell resistance plays a significant role in determining the cell-impedance-controlled lithium transport. Furthermore, from the comparison of the cathodic current transients measured on the Li_1−δMn₂O₄ single-layer film specimens with various thicknesses, it was experimentally verified that the diffusion resistance is explicitly distinguished from the internal cell resistance.