Fu-Ming Wang

Using Poly(4-Styrene Sulfonic Acid) to Improve the Dispersion Homogeneity of Aqueous-Processed LiFePO4 Cathodes

The dispersion properties of aqueous lithium iron phosphate (LiFePO 4 ) slurries and the electrochemical properties of the corresponding as-prepared cathodes with the addition of a dispersant, poly(4-styrene sulfonic acid) (PSSA), have been investigated. The influence of PSSA on the dispersion behaviors of LiFePO 4 and conductive agents, graphite and carbon black, in aqueous suspensions are explored by the analyses of zeta-potential and rheology. The results show that the PSSA adsorbs onto the LiFePO 4 and conductive agents so that they can be electrosterically stabilized in aqueous suspensions. Through the observation using a scanning electron microscope, it is shown that the homogeneity and dispersion of the composition distributed in the LiFePO 4 electrode sheet are significantly improved with increasing content of PSSA. However, an excess addition of PSSA is detrimental to the electronic conduction and the electrical capacity of the cathode. To obtain a homogeneous electrode with a good cell performance, the optima content of PSSA is suggested to be 2.0 wt % based on the LiFePO 4 weight.

Effects of Aromatic Esters as Propylene Carbonate-Based Electrolyte Additives in Lithium-Ion Batteries

The role of aromatic esters as additives in propylene carbonate (PC) based electrolytes used in lithium-ion batteries has been investigated. The addition of aromatic esters into the 1.0 M LiPF 6 -PC:DEC (3:2 in volume) suppresses the co-intercalation of PC and solvated Li + ions and inhibits the further decomposition of electrolytes during the first lithium intercalation process. A graphitic anode (MCMB-2528, mesocarbon microbeads) in a PC-based electrolyte with the aromatic esters exhibits a high reversible capacity. Scanning electron microscopy and the first discharge curve results show that the aromatic esters decompose and form a proper solid electrolyte interphase (SEI) film on the MCMB surface to not only prevent exfoliation of the graphite electrode but also stabilize the electrolyte. Both cyclic voltammogram and the lowest unoccupied molecular orbital energy level show that the aromatic esters have higher reduction potentials than that of the electrolyte solvents. An increase in reverse capacity may be achieved by increasing the addition. However, when the addition exceeds a certain critical value, the SEI film inhibits the intercalation of lithium ions and lowers the capacity. Furthermore, the percentage of the aromatic esters in electrolytes directly influences the low-temperature performance and the rate capability of the cells. An optimal result may be obtained when the addition is approximately 2-4%.

Structural Evolution and Electrochemical Performances of Oxygen Plasma-Treated LiMn2O4 Thin-Film Cathodes

Polycrystalline LiMn 2 O 4 thin films were deposited by radio-frequency (rf) magnetron sputtering followed by annealing at 600°C in air. The films were then treated with an rf-driven oxygen plasma. The crystallization and surface morphology of LiMn 2 O 4 thin films were correlated with rf powers. The treated samples were tested under harsh conditions such as deep discharge to 1.5 V and cycling at an elevated temperature of 60°C to verify the electrochemical performances of the LiMn 2 O 4 cathodes. The oxygen plasma treatments significantly improved the electrochemical properties of LiMn 2 O 4 thin films. The results showed that the plasma treatments not only resulted in surface densification but also induced finer grains in the subsurface region (10-100 nm), which were believed to enhance the stability of LiMn 2 O 4 cathodes.

Gel Polymer Electrolytes Prepared by In Situ Atom Transfer Radical Polymerization at Ambient Temperature

The preparation of gel polymer electrolytes using atom-transfer radical polymerization (ATRP) has been investigated. A liquid electrolyte with poly(ethylene glycol) methyl ether methacrylate and poly(ethylene glycol) dimethacrylate was successfully polymerized by ATRP at ambient temperature to form a gel polymer electrolyte. Infrared spectroscopy confirms that the polymerization of gel polymer electrolytes can take place at room temperature. The concentration of salt and the percentage of either polymer in electrolytes directly influence the ionic conductivity. An optimal ionic conductivity of the gel polymer electrolyte is obtained to be 2.1 X 10 -3 S cm -1 at 30°C. Furthermore, the lithium transference number is found to be about 0.32. Cyclic voltammogram and cell performance show that the gel polymer electrolyte has good electrochemical stability and good cyclability.

Combinational Effects of Oxygen Plasma Irradiation and Annealing on LiMn2O4 Thin Film Cathodes

Polycrystalline LiMn 2 O 4 thin films are deposited by radio frequency (rf) magnetron sputtering followed by air annealing at 600 and 400°C. The films are then treated with an rf driven oxygen plasma. The crystallographic structures and surface morphologies of the LiMn 2 O 4 thin films are studied. Plasma treatments induce different trends of microstructural evolution in 600 and 400°C annealed films. The microstructural evolutions are induced by the combination effects of plasma treatments and air annealing. The results can be interpreted by the atomic relaxation and resputtering models. For both high and low temperature annealed films, a well-tuned plasma treatment can result in films with a dense subsurface microstructures and a grain size of ∼ 10 nm, which shows a greatly enhanced electrochemical performances. Under optimal treatments conditions, the electrochemical performances of the LiMn 2 O 4 films can be greatly improved. The treated samples are tested under harsh conditions, including deep discharge to 1.5 V and cycling at elevated temperature of 60°C, and such subsurface structure is found to suppress the development of Li 1+x Mn 2 O 4 rock-salt phase.

Combined effects of ceramic filler size and ethylene oxide length on the ionic transport properties of solid polymer electrolyte derivatives of PEGMEMA

In this work, the synthesis of a series of solid polymer electrolyte (SPE) derivatives of poly(ethylene glycol) methyl ether methacrylate (PEGMEMA), homogenously dispersed with TiO2 ceramic nano-filler, has been reported. The interactions between filler size and the length of the ethylene oxide (EO) polymer backbone are discussed, and transport properties such as ionic conductivity and cation transference number are determined. Results show that the improved performance of the SPE is due to an interaction between the ceramic filler and the entwining behavior of the PEGMEMA backbone. An optimal ceramic filler size and an appropriate length of EO have been suggested for the enhanced performance of SPE derivatives of PEGMEMA for a next-generation polymer battery.

In situ electrochemical synchrotron radiation for Li-ion batteries

Observing the electronic structure, compositional change and morphological evolution of the surface and interface of a battery during operation provides essential information for developing new electrode materials for Li-ion batteries (LIBs); this is because such observations demonstrate the fundamental reactions occurring inside the electrode materials. Moreover, obtaining detailed data on chemical phase changes and distributions by analyzing an operating LIB is the most effective method for exploring the intercalation/de-intercalation process, kinetics and the relationship between phase change or phase distribution and battery performance, as well as for further optimizing the material synthesis routes for advanced battery materials. However, most conventional in situ electrochemical techniques (other than by using synchrotron radiation) cannot clearly or precisely demonstrate structural change, electron valence change and chemical mapping information. In situ electrochemical-synchrotron radiation techniques such as X-ray absorption spectroscopy, X-ray diffraction spectroscopy and transmission X-ray microscopy can deliver accurate information regarding LIBs. This paper reviews studies regarding various applications of in situ electrochemical-synchrotron radiation such as crystallographic transformation, oxidation-state changes, characterization of the solid electrolyte interphase and Li-dendrite growth mechanism during the intercalation/de-intercalation process. The paper also presents the findings of previous review articles and the future direction of these methods.