Crystal growth upon firing of hydrous transition metal oxide gels can be effectively inhibited by replacing the surface hydroxyl group before firing with another functional group that does not condense and that can produce small, secondary-phase particles that restrict advancing of grain boundaries at elevated temperatures. Accordingly, fully crystallized SnO(2), TiO(2), and ZrO(2) materials with mean crystallite sizes of approximately 20, 50, and 15 angstroms, respectively, were synthesized by replacing the hydroxyl group with methyl siloxyl before firing at 500 degrees C. An ultrasensitive SnO(2)-based chemical sensor resulting from the microstructural miniaturization was demonstrated.

Pseudocapacitive charge-storage reaction of MnO 2 •nH 2 O in several aqueous alkali and alkaline salts solutions, including LiCl, NaCl, KCl, CsCl, and CaCl 2 , has been studied on fine-grained MnO 2 •nH 2 O thin films and particles which possess the-MnO 2-type crystal structure. In situ synchrotron X-ray diffraction analysis shows that charge transfer at Mn sites upon reduction/ oxidation of MnO 2 •nH 2 O is balanced by bulk insertion/extraction of the solution cations into/from the oxide structure, which causes reversible expansion and shrinkage in lattice spacing of the oxide during charge/discharge cycles. Electrochemical quartz-crystal microbalance and X-ray photoelectron spectroscopy data further indicate that H 3 O + plays the predominant ͑Ͼ60%͒ role in all cases, while the extent of participation of alkali cations first decreases and then increases with ionic size. The charge-storage reaction can be summarized as: Mn͑IV͒O 2 •nH 2 O + ␦e − + ␦͑1 − f͒H 3 O + + ␦fM + ͑H 3 O͒ ␦͑1−f͒ M ␦f ͓Mn͑III͒ ␦ Mn͑IV͒ 1−␦ ͔O 2 •nH 2 O, where M + is alkali cation.

The solid-electrolyte-interphase ͑SEI͒ layers formed on the electrodes of pristine Si and carbon-coated Si ͑C-Si͒ particles in Li cells have been studied. The counter electrode is Li, and the electrolyte is LiPF 6 in the mixture of ethylene carbonate and ethyl methyl carbonate. Other than those, such as Li carbonates and fluoride, already known to the SEI of graphite electrode, there were detected significant amounts of SEI species unique to each of the Si electrodes. On the pristine Si electrode, there was concurrence of abundance of C and Si fluorides after long cycles. Coating the Si particles with a graphitized carbon layer has significant effects on the SEI formation. It helps to keep the Si particles remaining integrated after cycling, resulting in a smooth superficial SEI layer. It removes the native oxide layer not only to reduce humidity contamination but also to significantly change the SEI compositions. The SEI of the C-Si electrode shows the absence of Si and C fluorides but the presence of siloxane species. Reaction mechanisms leading to the formation of the fluoride and siloxane species have been proposed, elucidating an important role played by the native Si oxide layer. Si possesses a maximum capacity exceeding 3000 mAh/g for being a negative electrode for Li-ion batteries.1,2 Two issues are considered critical to realize this application. The first critical issue is the dramatic volume expansion and shrinkage of the Si particles during lithiation and delithiation, [3][4][5] respectively. Such cyclic volumetric variations tend to cause fast mechanical failure of the electrode structure, resulting in a very poor cycle life. A fairly large amount of literature adopting different approaches to enhance the structural robustness of the electrode has been dedicated to tackle this crucial problem. In the case of the conventional thick-film electrode made of particulate materials, for example, studies 3-8 have coated the Si particles with different conducting materials, which may serve either to enhance the conductivity of the electrode or to act as a buffer to partially accommodate the volumetric variations during cycling. In particular, coating with a carbon/graphite surface layer 3-6 has shown a significant beneficial effect on enhancing cycle life.The second critical issue is the properties of the surface layer on Si in contact with the Li + -containing electrolyte, also known as the solid-electrolyte-interphase ͑SEI͒. The SEI properties, on either cathode or anode, have been well recognized to play an important role in, among others, the safety, power capability, and cycle life of Li-ion-based batteries. It is believed to be equally important to the electrochemical performance of Si anode. Unfortunately, study on this critical issue is scarce. Choi et al.9,10 once reported preliminary results on the effects of certain electrolyte additive and salt on the SEI compositions of Si thin-film electrodes.In this work, the morphology and composition of the SEI formed on the electrodes containing either pris...

No abstract

A mechanically robust and ion-conductive polymeric coating containing two polymers, polyethylene glycol tert-octylphenyl ether and poly(allyl amine), with four tailored functional groups is developed for graphite and graphite-Si composite anodes. The coating, acting as an artificial solid electrolyte interphase, leads to remarkable enhancement in capacity reversibility and cycling stability, as well as a high-rate performance of the studied anodes.