Lilong Xiong

Graphene oxide sheets-induced growth of nanostructured Fe3O4 for a high-performance anode material of lithium ion batteries

Nanostructured Fe3O4 is intrinsically prone to aggregation, which hinders insertion and extraction of lithium ions. To overcome this problem, we adopt graphene oxide sheets (GOS) and induce growth of nanostructured Fe3O4 on the GOS in the presence of hexamethylenetetramine (HMT). During the synthetic process, GOS provides a template to regulate the growth of nanostructured Fe3O4 and the HMT is able to coordinate with Fe3+ and control its hydrolysis rate. After the annealing process, GOS is reduced to GS (graphene sheets), and Fe3O4/GS composite is obtained. In this hierarchical structure, GS is capable of enhancing the electronic transport of Fe3O4, and the Fe3O4/GS composite has superior electronic conductivity (106 S m−1). Benefitting from the uniform dispersion of the nanosized Fe3O4 on GS and the superior electronic conductivity, the obtained Fe3O4/GS exhibits prolonged cycling stability (1002 mA h g−1 after 175 cycles at a current density of 0.50 A g−1) and excellent rate capability (715, 647 and 535 mA h g−1 at 1.6, 3.2 and 5.0 A g−1, respectively).

Double roles of aluminium ion on surface-modified spinel LiMn1.97Ti0.03O4

Spinel LiMn2O4 is one of the cathode active materials for the rechargeable lithium-ion batteries with the most potential, but suffers from a fast capacity fade when cycling at elevated temperatures (55 °C). Spinel LiMn1.97Ti0.03O4 coated with aluminum nitrate is heat-treated at various temperatures. The sample treated at low temperature (500 °C) exhibits porous Al2O3 layer coated on the surface, and little Al ion diffusing into the spinel structure. As for the sample treated at mid-temperature (700 °C), it exhibits double layers coated on the surface: the inner layer is LiAlxMn1.97−xTi0.03O4 solid solution and the outer layer is compact Al2O3 metal oxides. When the heat-treatment temperature increases to a high level (850 °C), nearly all the Al ion diffusing into the structure to form Al doped solid solution layer coated on the spinel. The relationship between the structure of the modified samples and their electrochemical properties is investigated. The galvanostatic charge–discharge results show that those surface-modified samples exhibit improved cycling performance compared to LiMn1.97Ti0.03O4; and the sample treated at mid-temperature delivers the best capacity retention, especially at 55 °C. The improved cycling performance is ascribed to the coated layers of Al2O3 and LiAlxMn1.97−xTi0.03O4 on the spinel particles. The Al2O3 layer could reduce the HF damage by scavenging of HF. While the LiAlxMn1.97−xTi0.03O4 layer could enhance the stability of the spinel structure due to the stronger Al–O bond; moreover, this coating layer having the same spinel structure with the LiMn1.97Ti0.03O4 core material would not hinder the lithium ion diffusion. The sample treated at mid-temperature has the double layers coated on the surface, and its Al2O3 coating becomes compact and thin which could effectively block the direct contact between the electrolyte and spinel particles as well as reduce the lithium ion diffusion distance in the inactive layer, leading to greatly improved capacity retention.

Low-Cost Al2O3 Coating Layer As a Preformed SEI on Natural Graphite Powder To Improve Coulombic Efficiency and High-Rate Cycling Stability of Lithium-Ion Batteries.

Coulombic efficiency especially in the first cycle, cycling stability, and high-rate performance are crucial factors for commercial Li-ion batteries (LIBs). To improve them, in this work, Al2O3-coated natural graphite powder was obtained through a low-cost and facile sol-gel method. Based on a comparison of various coated amounts, 0.5 mol % Al(NO3)3 (vs mole of graphite) could bring about a smooth Al2O3 coating layer with proper thickness, which could act as a preformed solid electrolyte interface (SEI) to reduce the regeneration of SEI and lithium-ions consumption during subsequent cycling. Furthermore, we examined the advantages of Al2O3 coating by relating energy levels in LIBs using density functional theory calculations. Owing to its proper bandgap and lithium-ion conduction ability, the coating layer performs the same function as a SEI does, preventing an electron from getting to the outer electrode surface and allowing lithium-ion transport. Therefore, as a preformed SEI, the Al2O3 coating layer reduces extra cathode consumption observed in commercial LIBs.

Towards low-cost, high energy density Li2MnO3 cathode materials

The extremely sluggish Li-ion diffusion rate at the activation plateau (∼4.5 V) in the initial charging is an activation bottleneck of Li2MnO3, which seriously restricts its discharge capacity (energy density) and rate performance. Herein, a targeted strategy is proposed where a few fluorines are substituted for oxygen (Li2MnO3−xFx, x = 0.00, 0.015, 0.03, 0.045). F-substitution reduces the deintercalation barrier of Li+ from the crystal structure by weakening the Li–O bond, and improves the electronic transmission performance by inducing more Mn3+ and oxygen vacancies. Therefore, a major breakthrough is made for the activation bottleneck as the lithium-ion diffusion coefficient at 4.5 V is increased by more than two orders of magnitude. Meanwhile, F-ions inhibit the grain growth along the (001) direction and decrease the primary particle sizes. In the narrow electrochemical window of 2.0–4.6 V, a first discharge capacity of 299 mA h g−1 with a high energy density of 934 W h kg−1 at 0.05 C, and a capacity of 200 mA h g−1 at 1 C with a retention ratio of 92% after 100 cycles are achieved.

Study on capacitance evolving mechanism of polypyrrole during prolonged cycling.

A simple model on the evolution mechanism of PPy capacitance during prolonged cycling offers a reasonably description on the rapid increase and decay of PPy capacitance in 1 M 1-ethyl-3-methylimidazolium tetrafluoroborate/propylene carbonate (EtMeImBF4/PC). The capacitance of PPy films reached a very high specific capacitance of 420 F·g(-1) after 15 cycles when they worked in 1 M MeEt3ImBF4/PC. However, the capacitance rapidly decreased to 5% after only 400 cycles. The electronic conductivity and protonation level on the nitrogen site of PPy films rapidly decreased with the increase of cyclic number. The salt of EtMeImBF4 was monitored in PPy matrix by FTIR spectra after 400 cycles. The EQCM results indicated that a lot of 1-ethyl-3-methylimidazolium cations (EtMeIm(+)) were inserted during reduction process and retained in PPy matrix. The detained EtMeIm(+) cations bonded with doped p-toluenesulfonate anions (PTS(-)) in PPy matrix or BF4(-) anions from electrolyte and formed salts. Small amount of salts in PPy matrix can open more channels of ion insertion and resulted in a very high capacitance after 15 cycles. The continuous combination of detained EtMeIm(+) cations with doping anions of PTS(-) resulted in the rapid decrease of PPy protonation level on the nitrogen site and formation of compensate semiconductor state in PPy matrix. This should be responsible for the rapid decay of PPy conductivity and capacitance. The continuous accumulation of salts resulted in the great increase of PPy internal resistance.

Simple thermal decomposition method to synthesize LiTi2(PO4)3/C core–shell composite for lithium ion batteries

Polyanionic LiTi2(PO4)3 material with 3D framework structure is intensively investigated to be used in lithium ion batteries. However, the LiTi2(PO4)3-based materials suffer from poor electronic conductivity hindering the application as electrode active materials. This work describes an effective and simple strategy to synthesize LiTi2(PO4)3/C core–shell structure without the addition of external carbon sources. This approach is achieved by a simple one-step solid state reaction using organometallic salt as raw material. The as-prepared LiTi2(PO4)3 exhibits uniform and thin carbon coating on the particle surface. The electrochemical properties of the LiTi2(PO4)3/C composite are investigated, and the results demonstrate that the as-prepared LiTi2(PO4)3/C shows good cycling performance and rate capability.

The effect of K-Ion on the electrochemical performance of spinel LiMn2O4

Spinel LiMn2O4 is regarded as one of the cathode active materials for rechargeable lithium-ion batteries with the most potential. K+-ion modified LiMn2O4 samples are synthesized by solid-state reaction. The SEM analysis shows that the modified samples exhibit uniform particle size distribution and much better crystallinity. The modified sample K1 exhibits ionic diffusion coefficient of 1.43 × 10-10 cm-2 s-1 and 2.04 × 10-10 cm-2 s-1, which is much higher than that of the un-modified spinel LiMn2O4 sample. The electrochemical measurements show that K+-ion modification could effectively reduce the charge transfer resistance, improve the capacity retention and rate capacity of the spinel materials.

The effect of Na0.44MnO2 formation in Na+-modified spinel LiMn2O4

Na0.44MnO2 impure phase is formed during the synthetic process of Na+-modified spinel LiMn2O4 by solid state reaction which is confirmed by x-ray diffraction analysis. Scanning electron microscopy, transmission electron microscopy and galvanostatic charge-discharge measurements are carried out to investigate the effect of the formation of Na0.44MnO2 impurity on the morphology and electrochemical properties of the spinel material. The results show that the spinel material with impure phase exhibits improved cyclability compared to that of the pristine LiMn2O4. The improved electrochemical performance is mainly ascribed to the improved crystallinity of the spinel particles, enhanced stability of the spinel structure and good electronic conductivity of the composite.