Silicon (Si) has been regarded as a promising high-capacity anode material for developing advanced lithium-ion batteries (LIBs), but the practical application of Si anodes is still unsuccessful mainly due to the insufficient cyclability. To deal with this issue, we propose a new route to construct a dual core-shell structured Si@SiO@C nanocomposite by direct pyrolysis of poly(methyl methacrylate) (PMMA) polymer on the surface of Si nanoparticles. Since the PMMA polymers can be chemically bonded on the nano-Si surface through the interaction between ester group and Si surface group, and thermally decomposed in the subsequent pyrolysis process with their alkyl chains converted to carbon and the residue oxygen recombining with Si to form SiO, the dual core-shell structure can be conveniently formed in a one-step procedure. Benefiting from the strong buffering effect of the SiO interlayer and the efficient blocking action of dense outer carbon layer in preventing electrolyte permeation, the obtained nanocomposite demonstrates a high capacity of 1972 mA h g, a stable cycling performance with a capacity retention of >1030 mA h g over 500 cycles, and particularly a superiorly high Coulombic efficiency of >99.5% upon extended cycling, exhibiting a great promise for practical uses. More importantly, the synthetic method proposed in this work is facile and low cost, making it more suitable for large-scale production of high capacity anode for advanced LIBs.

Safety issues have severely retarded the commercial applications of high-capacity and high-rate lithium ion batteries (LIBs) in electric vehicles and renewable power stations. Thermal runaway is a major cause for the hazardous behaviors of LIBs under extreme conditions. In this paper, a new thermal shutdown separator with a more reasonable shutdown temperature of $90 C is developed by coating thermoplastic ethylene-vinyl acetate copolymer (EVA) microspheres onto a conventional polyolefin membrane film and tested for thermal protection of lithium-ion batteries (LIBs). The experimental results demonstrate that owing to the melting of the EVA coating layer at a critical temperature, this separator can promptly cut off the Li + conduction between the electrodes and thus shut down the battery reactions, so as to protect the cell from thermal runaway. In addition, this type of the separator has no negative impact on the normal battery performance, therefore providing an internal and self-protecting mechanism for safety control of commercial LIBs.