Abstract

Carbon nanosprings (CNSs) with spring diameter of ∼140 nm, carbon ring diameter of ∼100 nm and pitch distance of ∼150 nm, synthesized by using a catalytic chemical vapor deposition technology, have been investigated for potential applicability in lithium batteries as anode materials. The electrochemical results demonstrate that the present CNSs are superior anode materials for rechargeable lithium-ion batteries with high-rate capabilities, as well as long-term cycling life. At a current density as high as 3 A g⁻¹, CNSs can still deliver a reversible capacity of 160 mA h g⁻¹, which is about six times larger than that of graphite and three times larger than that of multi-wall carbon nanotubes under the same current density. After hundreds of cycles, there is no significant capacity loss for CNSs at both low and high current densities. The much improved electrochemical performances could be attributed to the nanometer-sized building blocks as well as the unusual spring-like morphology.

SnO₂ hollow spheres have been synthesized via a facile hydrothermal method using sulfonated polystyrene beads as a template followed by a calcination process in air. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy show that the as-obtained SnO₂ hollow spheres have a wall thickness of about 50 nm, and consist of nanosized SnO₂ particles with a mean diameter of about 15 nm. Electrochemical measurements indicate that the SnO₂ hollow spheres exhibit improved electrochemical performance in terms of specific capacity and rate capability in comparison with commercial SnO₂ when used as anode materials for lithium-ion batteries. The enhanced performance may be attributed to the spherical and hollow structure, as well as the building blocks of SnO₂ nanoparticles.

Hierarchical structures, in which structure is generated and controlled simultaneously at different size scales, have attracted increasing attention due to their potentials in both theoretical research and practical applications. In this review, a “non-classical crystallization” mechanism is discussed for their possibilities in morphology control of hierarchically-structured materials. Differently, this crystallization route is not based on the attaching and detaching of monomers as happened in the classical case, but through the self-organization of preformed building blocks as nanosized subunits, whose oriented attachment leads to mesocrystals with favorable morphology and texture. Representative materials including both inorganic and organic crystals are reported with possible mechanisms proposed. Synthetic protocols based on this mechanism provide unique inspirations for materials design and could be applied to morphological and structural control of new materials with optimized functions.

Abstract

LiFePO₄ thin films were deposited on Ti substrates by pulsed laser deposition (PLD). The apparent chemical diffusion coefficients of lithium in the films, , were measured by cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT), and electrochemical impedance spectroscopy (EIS). The average values calculated from CV results were in the order of 10⁻¹⁴ cm² s^–1. The values obtained by GITT, and EIS techniques were in the range of 10^–14–10^–18 cm² s^–1, 10^–14–10^–18 cm² s^–1, respectively. The values obtained by the two methods show a minimum point at x ∼ 0.5 for Li_1−xFePO₄. However, the overpotential values of the LiFePO₄ thin film electrodes obtained from the GITT results and the diffusion impedance deduced from the impedance spectra also show the minimum values at x ∼ 0.5 for Li_1–xFePO₄. This contradict could be caused by the improper use of GITT and EIS techniques for measuring the chemical diffusion coefficient of Li in Li_1–xFePO₄ which constitutes two phase, i.e., LiFePO₄ and FePO₄ in this region.

Highlights

? The average chemical diffusion coefficients of Li are 1.8 × 10⁻¹⁴–2.1 × 10⁻¹⁴ cm² s^–1 measured from CV. ? The minimum values obtained from GITT and EIS are in contradiction with other experimental data. ? The deduction of is highly dependent on the dE/dx values for GITT and EIS methods.

Abstract

Olivine structured LiFe_1/4Mn_1/4Co_1/4Ni_1/4PO₄ in pure solid solution single phase was obtained using solid-state reaction. For the carbon-coated material, in a voltage range of 3.0–5.1 V (vs. Li/Li⁺), three voltage plateaus were observed clearly in both galvanostatic discharging and galvanostatic intermittent titration technique (GITT) curves. These three plateaus are corresponding to Co³⁺/Co²⁺, Mn³⁺/Mn²⁺ and Fe³⁺/Fe²⁺ redox couples. No definite plateau can be assigned to Ni³⁺/Ni²⁺ redox couple in this voltage range. The overpotentials show the maximum when the redox reaction shifts from one redox couple to another couple. The apparent chemical diffusion coefficients of lithium () are in the same order of magnitude 10⁻¹⁵ cm² s⁻¹ for all transition reactions.

Abstract

The low-temperature performance of LiFePO₄/C cathode in a quaternary carbonate-based electrolyte (1.0 M LiPF₆/EC+DMC+DEC+EMC (1:1:1:3, v/v)) was studied. The discharge capacities of the LiFePO₄/C cathode were about 134.5 mAh/g (20 °C), 114 mAh/g (0 °C), 90 mAh/g (−20 °C) and 69 mAh/g (−40 °C) using a 1C charge–discharge rate. Cyclic voltammetry measurements show obviously sluggish of the lithium insertion–extraction process of the LiFePO₄/C cathode as the operation temperature falls below −20 °C. Electrochemical impedance analyses demonstrate that the sluggish of charge-transfer reaction on the electrolyte/LiFePO₄/C interface and the decrease of lithium diffusion capability in the bulk LiFePO₄ was the main performance limiting factors at low-temperature.