Zhengliang Gong

Nanostructured Li2FeSiO4 electrode material synthesized through hydrothermal-assisted sol-gel process

A carbon-coated Li 2 FeSiO 4 material with uniform nanoparticles (approximately 40-80 nm in diameter) is synthesized by a synthesis route, i.e., a hydrothermal-assisted sol-gel process. As an electrode material for rechargeable lithium batteries, the Li 2 FeSiO 4 sample shows high rate capability and excellent capacity retention. The Li 2 FeSiO 4 electrode delivers a discharge capacity of 160 mAh g -1 at C/16 rate, corresponding to 96% of the theoretical value. A discharge capacity of about 91 and 78 mAh g -1 can be obtained at 5 and 10 C rate, respectively. No capacity loss can be observed up to 50 cycles.

Origin of Deterioration for LiNiO2 Cathode Material during Storage in Air

Lithium nickel oxide, a potential candidate for cathode material for lithium-ion batteries, showed a distinct deterioration after storage in air for a time. The origin of this deterioration was explored by investigating surface structure, surface species, and ionic oxidation state of fresh and stored LiNiO 2 materials. Rietveld analysis of X-ray diffraction patterns showed not only the formation of Li 2 CO 3 on the surface, but also a weakening of ordered layered structure for the stored materials. X-ray photoelectron spectroscopy revealed that Ni 3 + transforms to Ni 2 + and active oxygen species exist on the surface of stored materials. Temperature programmed desorption-mass spectroscopy measurements gave evidence that active oxygen species (O-, O - 2) occur on the surface of LiNiO 2 after storage. A surface reaction mechanism based on the transformation of Ni 3 + /Ni 2 + and the evolution of active oxygen species is proposed.

Sol–gel synthesis and electrochemical properties of fluorophosphates Na2Fe1−xMnxPO4F/C (x = 0, 0.1, 0.3, 0.7, 1) composite as cathode materials for lithium ion battery

Fluorophosphates Na2Fe1−xMnxPO4F/C (x = 0, 0.1, 0.3, 0.7, 1) composite were successfully synthesized via a sol–gel method. The structure, morphology and electrochemical performance of the as prepared materials were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and charge/discharge measurements. XRD results show that, consistent with Na2FePO4F, Na2Fe0.9Mn0.1PO4F (x = 0.1) crystallize in a two-dimensional (2D) layered structure with space groupPbcn. However, increasing the content of Mn to x ≥ 0.3 results in a structure transition of Na2Fe1−xMnxPO4F from the 2D layered structure of Na2FePO4F to the three-dimensional (3D) tunnel structure of Na2MnPO4F. SEM and TEM analysis indicates nanostructured primary particles (about tens of nanometres in diameter) are obtained for all samples due to uniform carbon distribution and low calcining temperature used. Na2FePO4F is able to deliver a reversible capacity of up to 182 mA h g−1 (about 1.46 electrons exchanged per unit formula) with good cycling stability. Compared with Na2FePO4F, partial replacement of Fe by Mn in Na2Fe1−xMnxPO4F increases the discharge voltage plateau. Similar to Na2FePO4F, iron-manganese mixed solid solution Na2Fe1−xMnxPO4F (x = 0.1, 0.3, 0.7) also show good cycling performance. Furthermore, Na2MnPO4F with high electrochemical activity was successfully prepared for the first time, which is able to deliver a discharge capacity of 98 mA h g−1. The good electrochemical performance of Na2Fe1−xMnxPO4F materials can be attributed to the distinctive improvement of ionic/electronic conduction of the materials by formation of nanostructure composite with carbon.

Nanostructured 0.8Li2FeSiO4/0.4Li2SiO3/C composite cathode material with enhanced electrochemical performance for lithium-ion batteries

A strategy is proposed and developed to promote Li+ diffusion in polyanion cathode materials such as 0.8Li2FeSiO4/0.4Li2SiO3/C with the incorporation of Li2SiO3 as a lithium ionic conductive matrix. It is shown that the presence of Li2SiO3 separates the Li2FeSiO4 particles into small domains of a few nanometres and provides a fast Li+ diffusion channel, thus effectively enhancing Li+ diffusion in the 0.8Li2FeSiO4/0.4Li2SiO3/C composite. As a result, the composite material shows enhanced electrochemical performance and delivers a capacity as high as 240 mA h g−1 (corresponding to 1.44 electrons exchange per active Li2FeSiO4 formula unit) with good cyclic stability at 30 °C. The XRD and FTIR results indicate that the Li2SiO3 component exists in an amorphous phase. SEM and TEM analyses show an aggregate structure consisting of primary nanocrystallites (about tens of nanometres in diameter). The primary particles consist of a crystal Li2FeSiO4 phase and an amorphous Li2SiO3 and C, and a nanocrystalline Li2FeSiO4 surrounded by amorphous Li2SiO3 and C which are well known as a lithium ion conductor and electron conductor. The smaller nanoparticles of Li2FeSiO4 and the presence of lithium ionic and electronic conducting amorphous Li2SiO3 and carbon matrix both contributed to the enhanced electrochemical performance of the composite.

Comparison of Electrochemical and Surface Properties of Bare and TiO2-Coated LiNi0.8Co0.2 O 2 Electrodes

The electrochemical behaviors of native and TiO 2 -coated LiNi 0.8 Co 0.2 O 2 electrodes which were cycled in different potential regions (i.e., 3.0-4.3 and 3.0-4.6 V) were investigated by the powder microelectrode technique. The surface species on the electrodes were also studied by Fourier transform infrared and temperature-programmed desorption-mass spectroscopy (TPD-MS) techniques. Different oxidation products were formed on the bare and coated electrodes, respectively. The results indicated that the improved cyclic stability of the coated electrodes may be attributed to formation of different surface oxide films through an electro-oxidation mechanism of the solvents on the electrodes. A possible electro-oxidation mechanism has been proposed based on the results.

Zero-Strain Na2FeSiO4 as Novel Cathode Material for Sodium-Ion Batteries

A new cubic polymorph of sodium iron silicate, Na2FeSiO4, is reported for the first time as a cathode material for Na-ion batteries. It adopts an unprecedented cubic rigid tetrahedral open framework structure, i.e., F4̅3m, leading to a polyanion cathode material without apparent cell volume change during the charge/discharge processes. This cathode shows a reversible capacity of 106 mAh g(-1) and a capacity retention of 96% at 5 mA g(-1) after 20 cycles.

Insights into the Effects of Zinc Doping on Structural Phase Transition of P2-Type Sodium Nickel Manganese Oxide Cathodes for High-Energy Sodium Ion Batteries

P2-type sodium nickel manganese oxide-based cathode materials with higher energy densities are prime candidates for applications in rechargeable sodium ion batteries. A systematic study combining in situ high energy X-ray diffraction (HEXRD), ex situ X-ray absorption fine spectroscopy (XAFS), transmission electron microscopy (TEM), and solid-state nuclear magnetic resonance (SS-NMR) techniques was carried out to gain a deep insight into the structural evolution of P2-Na0.66Ni0.33-xZnxMn0.67O2 (x = 0, 0.07) during cycling. In situ HEXRD and ex situ TEM measurements indicate that an irreversible phase transition occurs upon sodium insertion-extraction of Na0.66Ni0.33Mn0.67O2. Zinc doping of this system results in a high structural reversibility. XAFS measurements indicate that both materials are almost completely dependent on the Ni(4+)/Ni(3+)/Ni(2+) redox couple to provide charge/discharge capacity. SS-NMR measurements indicate that both reversible and irreversible migration of transition metal ions into the sodium layer occurs in the material at the fully charged state. The irreversible migration of transition metal ions triggers a structural distortion, leading to the observed capacity and voltage fading. Our results allow a new understanding of the importance of improving the stability of transition metal layers.

Economical Synthesis and Promotion of the Electrochemical Performance of Silicon Nanowires as Anode Material in Li-Ion Batteries

Silicon is considered as one of the most promising anodes alternative, with a low voltage and a high theoretical specific capacity of ~4200 mAh/g, for graphite in lithium-ion batteries. However, the large volume change and resulting interfacial changes of the silicon during cycling cause unsatisfactory cycle performance and hinder its commercialization. In this study, electrochemical performance and interfacial properties of silicon nanowires (SiNWs) which are prepared by the Cu-catalyzed chemical vapor deposition method, with 1 M LiPF6/EC + DMC (1:1 v/v) containing 2 wt % or no vinylene carbonate (VC) electrolyte, are investigated by using different electrochemical and spectroscopic techniques, i.e., cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) techniques. It is shown that the addition of VC has greatly enhanced the cycling performance and rate capability of SiNWs and should have an impact on the wide utilization of silicon anode materials in Li-ion batteries.

Promoting long-term cycling performance of high-voltage Li2CoPO4F by the stabilization of electrode/electrolyte interface

High-voltage Li2CoPO4F (∼5 V vs. Li/Li+) with double-layer surface coating has been successfully prepared for the first time. The Li3PO4-coated Li2CoPO4F shows a high reversible capacity of 154 mA h g−1 (energy density up to 700 W h kg−1) at 1 C current rate, and excellent rate capability (141 mA h g−1 at 20 C). XRD and MAS NMR results show that Li2CoPO4F can be indexed as an orthorhombic structure with space group Pnma and coexists with Li3PO4. The XPS depth profiles and TEM analysis reveal that the as-prepared material has a double-layer surface coating, with a carbon outer layer and a Li3PO4 inner layer, which greatly enhances the transfer kinetics of the lithium ions and electrons in the material and stabilizes the electrode/electrolyte interface. Using LiBOB as an electrolyte additive is another way to further stabilize the electrode/electrolyte interface, and the LiBOB has a synergistic effect with the Li3PO4 coating layer. In this way, the Li2CoPO4F cathode material exhibits excellent long-term cycling stability, with 83.8% capacity retention after 150 cycles. The excellent cycling performance is attributed to the LiBOB electrolyte additive and the Li3PO4 coating layer, both of which play an important role in stabilizing the charge transfer resistance of Li2CoPO4F upon cycling.

Exploring the working mechanism of Li+ in O3-type NaLi0.1Ni0.35Mn0.55O2 cathode materials for rechargeable Na-ion batteries

Na-ion batteries (NIBs) have recently attracted much attention, due to their low cost and the abundance of sodium resources. In this work, NaLi0.1Ni0.35Mn0.55O2 as a promising new kind of cathode material for Na-ion batteries was synthesized by a co-precipitation method. Powder XRD patterns show that the sample has a primary O3-type structure after Li+ substitution. The material delivers excellent electrochemical performance, with an initial discharge specific capacity of 128 mA h g−1 and a capacity retention of 85% after 100 cycles at a rate of 12 mA g−1 in the voltage range of 2.0–4.2 V. In a widened voltage range of 1.5–4.3 V, the specific capacity can reach up to 160 mA h g−1. The structural stability of the material is substantially improved compared with lithium-free NaNi0.5Mn0.5O2, which can be attributed to the formation of an O′3 phase caused by Li-substitution, as proven by in situ XRD and solid state NMR (ss-NMR) measurements.