Effect of Hf-doping on electrochemical performance of anatase TiO2 as an anode material for lithium storage

Part II: superconductivity observed in magnetically separated nanoscale anatase titania at ambient temperature and pressure in an aqueous environment at its point of zero charge

This is the first work to investigate if and/or how changes in the surface structure/properties affect the charge transfer resistance (R CT) of anatase titania with decreasing particle size. It was accomplished by measuring the R CT (Ω) of same weight anatase titania pellets, with particle sizes ranging from 5.31 nm to 142.61 nm. Measurements were made using Electrochemical Impedance Spectroscopy (EIS) at each material's point of zero charge (PZC). Results demonstrated two regions of R CT. Above an average primary particle diameter of 23.54 nm, R CT remained essentially constant. Below, this diameter the R CT value first increased significantly, then decreased almost linearly toward zero. The projected average primary particle diameter where the materials R CT was projected to reach zero resistance is at a diameter of approximately 4.39 nm. A simple test was then developed to determine if at a small enough particle size the material would be affected by an external magnetic field. It was found that a sample with an average particle diameter of 12.689 nm, formed fine needles/threads of particles in deionized water, perpendicular to the settled powder at the base of the potash tube. This led to the development of a simple magnetic separation method to obtain strongly diamagnetic material from a parent population with an average primary particle diameter of 5.31 nm. A pellet consisting of these magnetically separated particles was then pressed at the same weight and pressure as the prior samples. The pellet's R CT was then measured using EIS under the identical conditions as the prior samples. EIS results of the magnetically separated particles in pellet form, under multiple conditions, resulted in Nyquist plots indicating the material exhibited no detectable R CT (i.e., superconductivity). Correlation of the shift in the materials R CT with known structure/property changes for each sample with decreasing particle size allowed the development of a model explaining: (1) the significant increase in diamagnetic strength of the magnetically separated particles and (2) the mechanism controlling the material's R CT.

Revealing the Impact of Dual Site Modification on the Phase Transformation and Ion Transport Mechanism of Ni-Rich Cathode Materials

Ni-rich cathode materials have garnered significant attention attributable to the high reversible capacity and superior rate performance, particularly in the electric vehicle industry. However, the structural degradation experienced during cycling results in rapid capacity decay and deterioration of the rate performance, thereby impeding the widespread application of Ni-rich cathodes. Herein, a Mg/Ti co-doping strategy was developed to boost the structure stability and Li-ion transport kinetics of the Ni-rich cathode material LiNi0.90Co0.05Mn0.05O2 (NCM9055) under long cycle. It is demonstrated that the Mg2+ ions inserted into the lithium layer could serve as pillars, enhancing the stability of the delithiated layer structure. The introduction of robust Ti-O bonding mitigated the detrimental H2-H3 phase transition (∼4.2 V) during cycling. In addition, despite the fact that Mg/Ti co-doping slightly reduces Li+ diffusion coefficient in the modified cathode material (NCM9055-MT), it effectively stabilized the robustness of the layered structure and maintained the Li+ diffusion channel while charging and discharging, thereby improving the Li+ diffusion coefficient after a long cycle. Therefore, the Mg/Ti co-doped cathode materials exhibited an exceptional capacity retention rate of 99.9% (100 cycles, 1 C). Additionally, the Li+ diffusion coefficient of the co-doped NCM9055-MT (2.924 × 10-10 cm2 s-1) after 100 cycles was effectively enhanced compared with the case of undoped NCM9055 (4.806 × 10-11 cm2 s-1). This work demonstrates that the Mg/Ti co-doping approach effectively enhanced the stability of layered Ni-rich cathode materials.

Enhancing Lithium and Sodium Storage Properties of TiO2(B) Nanobelts by Doping with Nickel and Zinc

Nickel- and zinc-doped TiO2(B) nanobelts were synthesized using a hydrothermal technique. It was found that the incorporation of 5 at.% Ni into bronze TiO2 expanded the unit cell by 4%. Furthermore, Ni dopant induced the 3d energy levels within TiO2(B) band structure and oxygen defects, narrowing the band gap from 3.28 eV (undoped) to 2.70 eV. Oppositely, Zn entered restrictedly into TiO2(B), but nonetheless, improves its electronic properties (Eg is narrowed to 3.21 eV). The conductivity of nickel- (2.24 × 10-8 S·cm-1) and zinc-containing (3.29 × 10-9 S·cm-1) TiO2(B) exceeds that of unmodified TiO2(B) (1.05 × 10-10 S·cm-1). When tested for electrochemical storage, nickel-doped mesoporous TiO2(B) nanobelts exhibited improved electrochemical performance. For lithium batteries, a reversible capacity of 173 mAh·g-1 was reached after 100 cycles at the current load of 50 mA·g-1, whereas, for unmodified and Zn-doped samples, around 140 and 151 mAh·g-1 was obtained. Moreover, Ni doping enhanced the rate capability of TiO2(B) nanobelts (104 mAh·g-1 at a current density of 1.8 A·g-1). In terms of sodium storage, nickel-doped TiO2(B) nanobelts exhibited improved cycling with a stabilized reversible capacity of 97 mAh·g-1 over 50 cycles at the current load of 35 mA·g-1.

Stability Evaluation of Candidate Precursors for Chemical Vapor Deposition of Hafnium Diboride (HfB2)

Alternative candidate precursors to [Hf(BH₄)₄] for low-temperature chemical vapor deposition of hafnium diboride (HfB₂) films were identified using density functional theory simulations of molecules with the composition [Hf(BH₄)₂L₂], where L = -OH, -OMe, -O-t-Bu, -NH₂, -N=C=O, -N(Me)₂, and -N(CH₂)₅NH₂ (1-piperidin-2-amine referred to as Pip2A). Disassociation energies (E _D), potential energy surface (PES) scans, ionization potentials, and electron affinities were all calculated to identify the strength of the Hf-L bond and the potential reactivity of the candidate precursor. Ultimately, the low E _D (2.07 eV) of the BH₄ ligand removal from the Hf atom in [Hf(BH₄)₄] was partially attributed to an intermediate state where [Hf(BH₄)₃(H)] and BH₃ is formed. Of the candidate precursors investigated, three exhibited a similar mechanism, but only -Pip2A had a PES scan that indicated binding competitive with [Hf(BH₄)₄], making it a viable candidate for further study.

Effect of Ni Doping Content on Phase Transition and Electrochemical Performance of TiO2 Nanofibers Prepared by Electrospinning Applied for Lithium-Ion Battery Anodes

Titanium dioxide (TiO₂), as a potential anode material applied for lithium-ion batteries (LIBs), is constrained due to its poor theoretical specific capacity (335 mAh·g⁻¹) and low conductivity (10⁻⁷-10⁻⁹ S·cm⁻¹). When compared to TiO₂, NiO with a higher theoretical specific capacity (718 mAh·g⁻¹) is regarded as an alternative dopant for improving the specific capacity of TiO₂. The present investigations usually assemble TiO₂ and NiO with a simple bilayer structure and without NiO that is immersed into the inner of TiO₂, which cannot fully take advantage of NiO. Therefore, a new strategy was put forward to utilize the synergistic effect of TiO₂ and NiO, namely doping NiO into the inner of TiO₂. NiO-TiO₂ was fabricated into the nanofibers with a higher specific surface area to further improve their electrochemical performance due to the transportation path being greatly shortened. NiO-TiO₂ nanofibers are expected to replace of the commercialized anode material (graphite). In this work, a facile one-step electrospinning method, followed by annealing, was applied to synthesize the Ni-doped TiO₂ nanofibers. The Ni doping content was proven to be a crucial factor affecting phase constituents, which further determined the electrochemical performance. When the Ni doping content was less than 3 wt.%, the contents of anatase and NiO were both increased, while the rutile content was decreased in the nanofibers. When the Ni doping content exceeded 3 wt.%, the opposite changes were observed. Hence, the optimum Ni doping content was determined as 3 wt.%, at which the highest weight fractions of anatase and NiO were obtained. Correspondingly, the obtained electronic conductivity of 4.92 × 10⁻⁵ S⋅cm⁻¹ was also the highest, which was approximately 1.7 times that of pristine TiO₂. The optimal electrochemical performance was also obtained. The initial discharge and charge specific capacity was 576 and 264 mAh·g⁻¹ at a current density of 100 mA·g⁻¹. The capacity retention reached 48% after 100 cycles, and the coulombic efficiency was about 100%. The average discharge specific capacity was 48 mAh·g⁻¹ at a current density of 1000 mA·g⁻¹. Approximately 65.8% of the initial discharge specific capacity was retained when the current density was recovered to 40 mA·g⁻¹. These excellent electrochemical results revealed that Ni-doped TiO₂ nanofibers could be considered to be promising anode materials for LIBs.

Biomass-Derived Graphitic Carbon Encapsulated Fe/Fe3C Composite as an Anode Material for High-Performance Lithium Ion Batteries

Lithium ion (Li-ion) batteries have been widely applied to portable electronic devices and hybrid vehicles. In order to further enhance performance, the search for advanced anode materials to meet the growing demand for high-performance Li-ion batteries is significant. Fe3C as an anode material can contribute more capacity than its theoretical one due to the pseudocapacity on the interface. However, the traditional synthetic methods need harsh conditions, such as high temperature and hazardous and expensive chemical precursors. In this study, a graphitic carbon encapsulated Fe/Fe3C (denoted as Fe/Fe3C@GC) composite was synthesized as an anode active material for high-performance lithium ion batteries by a simple and cost-effective approach through co-pyrolysis of biomass and iron precursor. The graphitic carbon shell formed by the carbonization of sawdust can improve the electrical conductivity and accommodate volume expansion during discharging. The porous microstructure of the shell can also provide increased active sites for the redox reactions. The in-situ-formed Fe/Fe3C nanoparticles show pseudocapacitive behavior that increases the capacity. The composite exhibits a high reversible capacity and excellent rate performance. The composite delivered a high initial discharge capacity of 1027 mAh g−1 at 45 mA g−1 and maintained a reversible capacity of 302 mAh g−1 at 200 mA g−1 after 200 cycles. Even at the high current density of 5000 mA g−1, the Fe/Fe3C@GC cell also shows a stable cycling performance. Therefore, Fe/Fe3C@GC composite is considered as one of the potential anode materials for lithium ion batteries.

Synthesis of One-Dimensional Mesoporous Ag Nanoparticles-Modified TiO2 Nanofibers by Electrospinning for Lithium Ion Batteries

TiO₂ is regarded as a prospective electrode material owing to its excellent electrochemical properties such as the excellent cycling stability and the high safety. However, its low capacity and low electronic conductivity greatly restrict the further improvement in electrochemical performance. A new strategy was put forward to solve the above defects involved in TiO₂ in which the low capacity was enhanced by nanomerization and porosity of TiO₂, and the low electronic conductivity was improved by introducing Ag with a high conductivity. One-dimensional mesoporous Ag nanoparticles-embedded TiO₂ nanofibers (Ag@TiO₂ nanofibers) were successfully synthesized via a one-step electrospinning process combined with subsequent annealing treatment in this study. The microstructure and morphology of mesoporous TiO₂@Ag nanofibers were confirmed by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption–desorption. TiO₂ nanofibers mainly consisted of a large amount of anatase TiO₂, accompanied with traces of rutile TiO₂. Ag nanoparticles were uniformly distributed throughout TiO₂ nanofibers and promoted the transformation of TiO₂ from the anatase to the rutile. The corresponding electrochemical performances are measured by galvanostatic charge-discharge, cycle stability, rate performance, cycle voltammetry, and electrochemical impedance spectroscopy measurements in this research, with pristine TiO₂ nanofibers as the reference. The results indicated that the introduction of Ag nanoparticles into TiO₂ nanofibers significantly improved the diffusion coefficient of Li ions (5.42 × 10⁻⁹ cm²⋅s⁻¹ for pristine TiO₂, 1.96 × 10⁻⁸ cm²⋅s⁻¹ for Ag@TiO₂), and the electronic conductivity of TiO₂ (1.69 × 10⁻⁵ S⋅cm⁻¹ for pristine TiO₂, and 1.99 × 10⁻⁵ S⋅cm⁻¹ for Ag@TiO₂), based on which the comprehensive electrochemical performance were greatly enhanced. The coulombic efficiency of the Ag@TiO₂ nanofibers electrode at the first three cycles was about 56%, 93%, and 96%, which was higher than that without Ag (48%, 66%, and 79%). The Ag@TiO₂ nanofibers electrode exhibited a higher specific discharge capacity of about 128.23 mAh⋅g⁻¹ when compared with that without Ag (72.76 mAh·g⁻¹) after 100 cycles at 100 mA·g⁻¹. With the current density sharply increased from 40 mA·g⁻¹ to 1000 mA·g⁻¹, the higher average discharge capacity of 56.35 mAh·g⁻¹ was remained in the electrode with Ag, when compared with the electrode without Ag (average discharge capacity of about 12.14 mAh·g⁻¹). When the current density was returned to 40 mA·g⁻¹, 80.36% of the initial value was returned (about 162.25 mAh·g⁻¹) in the electrode with Ag, which was evidently superior to that without Ag (about 86.50 mAh·g⁻¹, only 55.42% of the initial value). One-dimensional mesoporous Ag@TiO₂ nanofibers can be regarded as a potential and promising candidate as anode materials for lithium ion batteries.