Silicon–carbon composites consisting of Si particles embedded in a dense and non‐porous carbon matrix are prepared by the pyrolysis of intimate mixtures of poly(vinyl chloride) (PVC) and Si powder at 900 °C under a flow of N2. In contrast to bare micrometer‐sized (1–10 μm) and nanometer‐sized (10–100 nm) Si powders, which show poor cycling behavior with almost no capacity remaining after 15 cycles, the texture of the composite is seen to greatly enhance the reversibility of the alloying reaction of Si with Li. For instance, a capacity of ca. 1000 mA h g–1 is achieved for 20 cycles (0–2.0 V vs. Li+/Li) for a silicon–carbon composite containing nanometer‐sized Si particles. We also demonstrate that a mild manual grinding treatment degrades the cycling performance of the composites to levels as low as the parent Si, even though free Si is not released. The electrochemical measurements in conjunction with Raman spectroscopy data indicate that a huge stress is exerted on the Si domains by the in situ formed carbon. This carbon‐induced stress is found to disappear during the milling of the composites, indicating that the carbon‐induced pressure, along with the accompanying improvement in electrical connectivity, are the key parameters for the improved cycling behavior of Si versus Li.

The possibility of using electrolyte systems based on room-temperature ionic liquids ͑RTILs͒ in lithium-battery configurations is discussed. The nonflammability and wide potential windows of RTIL-based systems are attractive potential advantages, which may ultimately lead to the development of safer, higher energy density devices than those that are currently available. An evaluation of the compatibility of these electrolyte systems with candidate electrodes is critical for further progress. A comparison of the electrochemical behavior of Li/RTIL/Li x MnO 2 and Li x Ti 0.11 Mn 0.89 O 2 cells with those containing conventional carbonate solutions is presented and discussed in terms of the physical properties of two RTIL systems and their interactions with the cathodes. Strategies to improve performance and minimize cathode dissolution are presented.The worldwide market for rechargeable lithium batteries currently exceeds U.S. $3 billion per year. 1 These devices are primarily for consumer applications, such as cell phones and laptop computers, but lithium-ion battery sales are projected to capture 5% of the hybrid and electric vehicle market by 2010, and 36% by 2015. 2 Safety requirements for vehicular applications are particularly stringent, and, in many cases, higher energy densities are needed than those that are currently available. Further improvements over the state of the art are needed to achieve these goals.Of particular interest for these applications are room-temperature ionic liquids ͑RTILs͒ because of several unique properties, including low volatility, low flammability, thermal and chemical stability, high ionic conductivity, and wide potential windows. 3 RTILs are ionic salts that are liquid over a wide temperature range, characteristically from below room temperature to above 200°C. Typical ionic liquids consist of quaternary ammonium cations and anions with low Lewis basicity. For example, the combination of 1-ethyl-3-methylimidazolium cations with various anions produces RTILs with low-enough viscosity and high-enough conductivity to be applied to different energy storage devices. 4,5 However, the practical application of imidazolium-based ionic electrolytes for lithium batteries is not possible because imidazolium cations are reduced at the surface of lithium metal at around 1 V vs Li + /Li. 6 RTILs based on tetraalkylammonium, pyrrolidinium, or piperidinium cations are somewhat more stable against reduction on lithium metal and these have shown better prospects for use in lithium batteries. 7-9 For example, it has been found that systems based on N-methyl-N-alkyl pyrrolidinium cations ͑P 1X + , where the subscript indicates the lengths of the alkyl chains coordinated to the pyrrolidinium nitrogen͒, and bis͑trifluoromethanesulfonyl͒imide ͑TFSI͒ anions ͑Fig. 1͒ allow lithium to be cycled with a high degree of reversibility such that cycling efficiencies exceeding 99% have been obtained. 10 The reversible cycling is attributed to the presence of the P 1X + cation, which is stable at negative poten...

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Partial titanium substitution of electrodes with the Na 0. 44