Hung-Chun Wu

Enhanced Cycle Life of Si Anode for Li-Ion Batteries by Using Modified Elastomeric Binder

Department of Chemical Engineering, Tung Hai University, Taichung, Taiwan 407Cycle-life of the particulate electrode of Si, either with or without carbon coating, for Li-ion battery has significantly beenimproved by using a modified elastomeric binder containing styrene-butadiene-rubber~SBR! and sodium-carboxyl-methyl-cellulose ~SCMC!. Compared with poly-vinylidene-fluoride~PVdF!, the (SBR 1 SCMC) mixture binder shows smaller moduli,a larger maximum elongation, a stronger adhesion strength on Cu current collector, and much smaller solvent-absorption inorganic carbonate. There were demonstrated cycle lives of .50 cycles for bare Si at 600 mAh/g or carbon-coated Si at 1000mAh/g, as contrast to <8 cycles for PVdF-bound electrode in all cases.© 2004 The Electrochemical Society. @DOI: 10.1149/1.1847685# All rights reserved.Manuscript submitted July 28, 2004; revised manuscript received September 27, 2004. Available electronically December 16,2004.

Electrochemical Characterizations on Si and C-Coated Si Particle Electrodes for Lithium-Ion Batteries

The understanding of cycling and electrochemical characteristics of Si particle anodes for Li-ion batteries has previously been hindered by very fast capacity fading. Optimizing the electrode architecture to significantlyimprove its stability up to the 1000 mAh/g charge-discharge level has made it possible to investigate these properties to a greater depth than before. The capacity fading and lithiation mechanisms of Si and C-coated Si particles have been studied in this paper by cycling test and electrochemical impedance spectroscopy (EIS) analysis. The capacity vs cycle number plot exhibits two regions of different fading rates, including an initial region of slow fading followed by accelerated decay. The latter may be associated with large-scale failure of the electrode structure. EIS revealed a core-shell lithiation mechanism of Si. C-coating not only exerts remarkable favorable effects against capacity fading, but also serves as a conduit for Li ions to the reaction with Si particles.

Synthesis and Characterization of Nanoporous NiSi-Si Composite Anode for Lithium-Ion Batteries

Porous NiSi-Si composite particles having homogeneously distributed intraparticle pores with the size distribution peaked at 200 nm and a porosity of ∼40% have been synthesized by a novel method, which comprises steps of ballmilling induced reaction to form Ni/NiSi/Si preform particles and subsequent dissolution of unreacted Ni. Upon lithiation/delithiation cycling, the composite particle electrode exhibits much reduced thickness expansion and capacity fading rate, as compared with the pure Si particle electrode. The improvements have been attributed to the success in introducing the preset voids to partially accommodate volume expansion arising from Si lithiation. In situ synchrotron XRD further indicates that NiSi of the composite is active toward Li alloying, and it undergoes reversible transformation to/from Ni 2 Si and Li y Si. The reversible transformation between the silicides involves volume change in opposite to lithiation of Si, and is beneficial to stabilizing the composite electrode upon charge/ discharge cycling.

Enhanced High-Temperature Cycle Life of LiFePO4-Based Li-Ion Batteries by Vinylene Carbonate as Electrolyte Additive

Addition of vinylene carbonate (VC) in electrolyte solution has been found to greatly improve the high-temperature (55°C) cycling performance of LiFePO 4 -based Li-ion batteries. It has been established that the VC additive remarkably suppresses Fe dissolution from LiFePO 4 cathode and hence, subsequent Fe deposition on the anode side. Furthermore, the VC additive also significantly reduces formation of solid-electrolyte interface layers on both LiFePO 4 cathodes and mesocarbon microbead (MCMB) anodes. With VC addition, a 18650-type LiFePO 4 /MCMB cell has been shown to retain ∼80% capacity after 980 cycles at 55°C under 1-3 C charge-discharge rates. This is in contrast with more than 25% capacity loss after merely 100 cycles when no VC is added.

Enhanced High-Rate Cycling Stability of LiMn2O4 Cathode by ZrO2 Coating for Li-Ion Battery

The effect of a sol-gel derived amorphous zirconium oxide surface coating on the high charge-discharge (CD) rate performance of LiMn 2 O 4 was studied. When cycled between 4.5 and 2.9 V (vs. Li/Li + ) at room temperature, the coated spinel electrode, containing 5 wt %-ZrO 2 , shows tremendous enhancement in cycling stability at CD rates up to 10 C. Concurrently, the coated spinel electrode exhibits a lower cubic-tetragonal transition potential, a smaller charge-transfer impedance by 4-5-fold, and it profoundly reduces, by 66%, lattice contraction upon charge (delithiation). The enhancement in the high-rate cycling stability has been attributed to the combination of these favorable effects.

Characterization and Electrochemical Behavior of Graphene-Based Anode for Li-Ion Batteries

In this study, we investigate the characteristics and electrochemical properties of graphene nanosheets derived from chemical-thermal exfoliation processes of SFG44 synthetic graphite (SFG44-GNS). The characterizations and electrochemical measurements were carried out by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, cyclic voltammetry, BET, Raman, rate capability as well as cycling tests and AC impedance. The as-synthesized SFG44-GNS with larger d-spacing of 0.3407 nm exhibits reversible capacity of 626 mAh/g and good rate capability of ~300 mAh/g at 2C rate, which are superior to those of graphite anode. The enhanced electrochemical performance of GNS anode was resulted from larger d-spacing, lower impedance in the interface and enhanced pore volume. The results indicate that graphene-based material is a good candidate for HEV/EV application.

Study on the synthesis–microstructure-performance relationship of layered Li-excess nickel–manganese oxide as a Li-ion battery cathode prepared by high-temperature calcination

Spherical composite oxide Li1.5Ni0.25Mn0.75O2.5 (or 0.5Li2MnO3·0.5LiNi0.5Mn0.5O2) powder as a Li-ion battery cathode has been synthesized by high-temperature calcination of a mixture containing Li2CO3 and (Ni0.25Mn0.75)CO3, which is synthesized by a continuous co-precipitation method. The evolution in the microstructure of the oxide during calcination is studied mainly by transmission X-ray microscopy (TXM), complemented by X-ray diffraction and thermogravimetry, and it can be divided into three major stages. The first stage takes place below 400 °C, and is characterized by the decomposition of the transition metal carbonates to form amorphous oxides, leading to the formation of internal cracks due to extensive densification. The second stage occurs between 400 and 800 °C, and involves complete Li1.5Ni0.25Mn0.75O2.5 formation and the development of a unique radially-distributed pore structure. The third stage takes place above 800 °C and during prolonged heating at 900 °C, and is characterized by grain growth and change into a randomly distributed tortuous pore structure. TXM 3D-elemental analysis gives statistical evidence showing heterogeneity in the distributions of Ni and Mn, which causes capacity loss and might be a common problem encountered in the two-step precipitation–calcination process. The electrochemical performance of the resulting Li1.5Ni0.25Mn0.75O2.5 powder exhibits complex dependence on the microstructure. The radially distributed pore pattern and small grain size produced by moderate heating favor the rate performance of the composite oxide cathode by reducing charge-transfer resistance and enhancing apparent Li ion solid-state diffusivity. A large grain size resulting from prolonged heating, on the other hand, reduces the formation of the spinel MnO2 domain upon de-lithiation of the Li2MnO3 component of the composite oxide.

Development and characterizations of PVdF-PEMA gel polymer electrolytes

A new class of gel polymer electrolytes comprising the blend of poly(ethyl methacrylate) (PEMA) and poly(vinylidene fluoride), the mixture of ethylene carbonate and propylene carbonate as a plasticizer, and lithium perchlorate (LiClO4) as a salt was prepared using solvent casting technique. The formation of polymer–salt complexes has been confirmed by XRD analysis. Morphological and thermal studies have been performed using SEM and DMA analyses. A comparative look between PEMA and poly(methyl methacrylate) (PMMA) electrolytes has showed that PEMA electrolytes exhibited better electrochemical performances than PMMA electrolytes, despites its lower conductivity.

Towards an understanding of the role of hyper-branched oligomers coated on cathodes, in the safety mechanism of lithium-ion batteries

Self-terminated hyper-branched oligomers (STOBA) were coated and then melted on a Li(Ni0.4Co0.2Mn0.4)O2 cathode to form a dense polymer film at high temperatures. The physical and structural changes of the polymer layer at different temperatures and charge conditions were investigated by nitrogen adsorption–desorption, X-ray photoelectron spectroscopy, resistance measurements, scanning electron microscopy, and solid-state 7Li-NMR and 13C-NMR spectroscopy in order to improve the understanding of the role of the STOBA layer in the enhancement of the safety mechanism of lithium ion batteries. The morphological change of the STOBA layer from the porous to nonporous state at the temperature of a thermal runaway of a battery was demonstrated. The change in the resistance values at high temperatures revealed that the STOBA coating is helpful for the prevention of internal short-circuiting and thermal runaway. Most importantly, the 7Li-NMR results acquired at a very high spinning speed (50 kHz) allow the monitoring of the subtle changes in the local environments of the Li+ ions and their interaction and mobility in the STOBA–cathode interface as functions of temperature and charge states. The combined characterization results improve the understanding of how the STOBA layer can contribute to the safety features of lithium ion batteries.