Shaohua Fang

Improving the electrochemical performance of layered Li-rich transition-metal oxides by alleviating the blockade effect of surface lithium

As promising cathodes for high-energy Li-ion batteries, the commercial application of layered Li-rich transition-metal oxides (LROs) is significantly prevented by their non-ideal cycling stability and rate capability. In this work, Li1.23Mn0.46Ni0.15Co0.16O2 samples are prepared by combining a solvothermal process and a Li2CO3 molten-salt method. When the molar ratio of Li2CO3 to transition-metal ions is 4:1, the obtained sample has the best electrochemical performance. Its initial discharge capacities at 20 and 300 mA g−1 are larger than 307.8 and 232.2 mA h g−1, respectively. After 200 cycles, the capacity retention ratio at 300 mA g−1 is still as large as 85.3%. In addition, we discovered that the relative content of lithium on the particle surface of the samples is more than that in the particle interior, and this distribution behavior of lithium is adjustable. In particular, we demonstrate for the first time that the overmuch enrichment of lithium on the particle surface will hinder the diffusion of Li-ions. This is the blockade effect of surface lithium, and this blockade effect can be alleviated by the Li2CO3 molten-salt method. This is the reason that the electrochemical performance of LROs can be greatly improved by the Li2CO3 molten-salt method.

Low-viscosity ether-functionalized pyrazolium ionic liquids based on dicyanamide anions: properties and application as electrolytes for lithium metal batteries

Four new ether-functionalized pyrazolium ionic liquids (ILs) based on dicyanamide (DCA) anions are synthesized and characterized. The physical and electrochemical properties of these ILs, including melting point, thermal stability, viscosity, conductivity and electrochemical stability are investigated. All these ILs are liquids and their viscosities are lower than 40 mPa s at 25 °C. Though the four IL electrolytes with 0.6 mol kg−1 LiDCA have good chemical stability against lithium metals, only 1-(2-methoxyethyl)-2-methylpyrazolium dicyanamide (PZ2o1-1-DCA) and 1-(2-methoxyethyl)-2-ethylpyrazolium dicyanamide (PZ2o1-2-DCA) electrolytes can be used as electrolytes for lithium metal batteries due to the formation of a beneficial SEI film on the lithium metal during the cycling test of a symmetric lithium cell. At room temperature, Li/LiFePO4 coin cells using the two electrolytes show good cycling performance at 0.1C, and the cell using PZ2o1-2-DCA electrolyte shows better rate performance.

A novel mixture of diethylene glycol diethylether and non-flammable methyl-nonafluorobutyl ether as a safe electrolyte for lithium ion batteries

A mixture of diethylene glycol diethylether and non-flammable methyl-nonafluorobutyl ether was introduced as a new electrolyte for lithium ion batteries. And a fluoroethylene carbonate additive was used in this electrolyte to improve the compatibility with the graphite anode. The non-flammability and high flash point of this electrolyte indicated its high safety. This electrolyte had low viscosity and its conductivity reached 3.8 mS cm−1 at 25 °C. Graphite/LiFePO4 coin cells with commercial electrodes were utilized to evaluate the performances of the electrolyte. The rate, cycle and low-temperature performances of the graphite/LiFePO4 cells using this electrolyte were close to those of the cells using the conventional electrolytes with and without the fluoroethylene carbonate additive. These results indicated that this kind of safe electrolyte based on non-flammable hydrofluoroethers and solvents with a high flash point has great potential for practical application.

Countering the Segregation of Transition-Metal Ions in LiMn1/3Co1/3Ni1/3O2 Cathode for Ultralong Life and High-Energy Li-Ion Batteries

High-voltage layered lithium transition-metal oxides are very promising cathodes for high-energy Li-ion batteries. However, these materials often suffer from a fast degradation of cycling stability due to structural evolutions. It seriously impedes the large-scale application of layered lithium transition-metal oxides. In this work, an ultralong life LiMn1/3 Co1/3 Ni1/3 O2 microspherical cathode is prepared by constructing an Mn-rich surface. Its capacity retention ratio at 700 mA g(-1) is as large as 92.9% after 600 cycles. The energy dispersive X-ray maps of electrodes after numerous cycles demonstrate that the ultralong life of the as-prepared cathode is attributed to the mitigation of TM-ions segregation. Additionally, it is discovered that layered lithium transition-metal oxide cathodes with an Mn-rich surface can mitigate the segregation of TM ions and the corrosion of active materials. This study provides a new strategy to counter the segregation of TM ions in layered lithium transition-metal oxides and will help to the design and development of high-energy cathodes with ultralong life.

Uniform LiMO2 assembled microspheres as superior cycle stability cathode materials for high energy and power Li-ion batteries

A novel solvothermal-precursor method is designed to prepare uniform assembled microspherical LiMO2 (layered Li transition metal oxide) cathodes for high energy and power Li-ion batteries. The LiMO2 assembled microspheres exhibit superior long-term cycle stability and rate capability.

Physicochemical properties of functionalized 1,3-dialkylimidazolium ionic liquids based on the bis(fluorosulfonyl)imide anion

A new series of ether- or alkenyl-functionalized 1,3-dialkylimdazolium ILs based on the FSI anion were prepared and their physicochemical properties (melting point, thermal stability, viscosity, conductivity and electrochemical stability) were studied in detail and compared with the corresponding TFSI-based ILs. It was confirmed that introduction of ether or alkenyl groups and FSI anions jointly could reduce viscosity and enhance conductivity. These FSI-based ILs owned viscosities lower than 30 mPa s and conductivities higher than 7 mS cm−1. AEI-FSI had the lowest viscosity (17.4 mPa s) among all the reported FSI-based ILs and it had relatively higher conductivity (12.8 mS cm−1) as well. The electrochemical windows of most ILs were wider than 3.7 V, indicating their promising application for electrochemical devices.

New ether-functionalized pyrazolium ionic liquid electrolytes based on the bis(fluorosulfonyl)imide anion for lithium-ion batteries

Four new ionic liquids (ILs) composed of ether-functionalized pyrazolium cations and the bis(fluorosulfonyl)imide (FSI) anion are prepared and characterized in this report. The physicochemical properties of these ILs, such as melting point, thermal stability, viscosity, conductivity and electrochemical stability, are systematically investigated. These four FSI-based ILs are all in the liquid state and their viscosities are lower than 40 mPa s at room temperature. The charge–discharge performances of Li/LiFePO4 cells containing these IL electrolytes with 0.8 mol L−1 LiFSI are also examined. Three electrolytes exhibit nice cycling stability at 0.1C, and the PZ2o2-2-FSI electrolyte shows excellent rate performance.

Li1.17Mn0.50Ni0.16Co0.17O2 assembled microspheres as a high-rate and long-life cathode of Li-ion batteries

Assembled microspherical cathodes have attracted great attention thanks to their high tap density, good rate capability and cycling stability. However, for layered Li-rich transition-metal oxides (LROs), the preparation of uniformly assembled microspheres still faces many challenges due to harsh synthetic conditions and the nature of multiple metal elements. In this work, Li1.17Mn0.50Ni0.16Co0.17O2 assembled microspheres have been prepared by a new route tactfully combining a solvothermal process and a molten-salt method. The use of a solvothermal process is helpful for the preparation of precursors with assembled microspherical morphology, and the addition of complex salts (NaCl and KCl), can increase the uniformity of cation distribution. The product obtained at 800 °C delivers the best electrochemical performances among all samples. At a current density of 300 mA g−1, its initial discharge capacity is larger than 228 mA h g−1, corresponding to a capacity retention ratio of 86.8% after 200 cycles. Even if the current density increases to 2000 mA g−1, its discharge capacity is still as large as 156 mA h g−1. Whats more, we discover the moving rate of Li-ions during the sintering process will affect the uniformity of Li2MnO3-like and LiMO2 components in LRO assembled microspheres. This discovery is helpful for the preparation of LRO assembled microspheres with excellent electrochemical performances.

In-situ preparation of novel layered-spinel-microsphere/reduced-graphene-oxide heterostructured cathode for ultrafast charge-discharge Li-ion batteries

Although Li-rich layered oxides (LLOs) have the highest capacity of any cathodes used, the rate capability of LLOs falls short of meeting the requirements of electric vehicles and smart grids. Herein, a layered-spinel microsphere/reduced graphene oxide heterostructured cathode (LS@rGO) is prepared in situ. This cathode is composed of a spinel phase, two layered structures, and a small amount of reduced graphene oxide (1.08 wt % of carbon). The assembly delivers a considerable charge capacity (145 mA h g-1 ) at an ultrahigh charge- discharge rate of 60 C (12 A g-1 ). The rate capability of LS@rGO is influenced by the introduced spinel phase and rGO. X-ray absorption and X-ray photoelectron spectroscopy data indicate that Cr ions move from octahedral lattice sites to tetrahedral lattice sites, and that Mn ions do not participate in the oxidation reaction during the initial charge process.

A novel mixture of lithium bis(oxalato)borate, gamma-butyrolactone and non-flammable hydrofluoroether as a safe electrolyte for advanced lithium ion batteries

In this work, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (F-EPE), a kind of non-flammable hydrofluoroether, was initially mixed with lithium bis(oxalato)borate (LiBOB) and gamma-butyrolactone (GBL) to formulate a safe electrolyte for lithium-ion batteries. It was found that the addition of F-EPE endowed this novel electrolyte with a high safety level, low surface tension and good wettability to the separator and electrodes. More importantly, this safe electrolyte supported the graphite/LiCo1/3Mn1/3Ni1/3O2 full cell in achieving excellent electrochemical performances. During the prolonged cycle test at room temperature, its capacity retention after 500 cycles could reach 80.6%, which was comparable to that of a commercial electrolyte. Its rate performance at room temperature and cycle performance at elevated temperature (60 °C) surpassed those of the commercial electrolyte. In particular, its low-temperature performance was remarkable, and the cell could deliver a capacity as high as 74.2 mA h g−1 at −40 °C. In view of these results, such a safe electrolyte showed tremendous potential for practical application.