TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The backbone comprises amorphous carbon and does not comprise a hollow core running along the length of the nanofibers. The nanoparticles comprise silicon and silicon dioxide, and a surface that is at least 95% coated with carbon. The nanoparticles have an average diameter of less than 100 nm, preferably 10-80 nm. The nanoparticles are non-aggregated and dispersed along the entire length of the nanofiber. The discrete domains comprise nanoparticles comprising at least one silicon material. The nanocomposite nanofibers do not comprise a concentration of domains 20 times higher along a 500 nm long segment along the length of the nanofiber than an adjacent 500 nm length of the nanofiber. The discrete domains comprise at least 50 wt.% of silicon having an oxidation state of zero. The second material comprises carbon, and the nanocomposite nanofiber comprises less than 25 wt.% of carbon. The weight-to-weight ratio of the silicon component to organic polymer is at least 1:10, preferably at least 1:2 or at least 1:1. The silicon component is a silicon-containing nanoparticle or a silicon precursor. The silicon precursor comprises silicon acetate, silicon carboxylate, silicon halide and/or silicon alkoxide. At least one non-silicon metal precursor comprises molybdenum precursor, niobium precursor, tantalum precursor, tungsten precursor, iron precursor, nickel precursor, copper precursor, cobalt precursor, manganese precursor, titanium precursor, vanadium precursor, chromium precursor, zirconium precursor and/or yttrium precursor. The metal concentration in the fluid stock is at least 200 mM. The second material comprising no silicon comprises a second material containing carbon without silicon.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The nanocomposite nanofibers further comprise a second material comprising ceramic, metal, organic polymer (water-soluble organic polymer), or carbon. The nanocomposite nanofibers comprise X-ray diffraction toe theta peaks as given in the specification and comprise X-ray diffraction as given in the figure 6 of the specification. The nanocomposite nanofibers have a specific energy capacity of at least 1500 (preferably at least 2000) mAh/g on a first cycle at 0.1C and a specific energy capacity of at least 250 (preferably at least 400) mAh/g on a 98th cycle at 0.1C. The nanofibers comprise, on average, less than 25 (preferably 10-20) wt.% of carbon and at least 50 (preferably at least 75) wt.% of silicon. The nanocomposite nanofibers comprise discrete domains of at least one silicon material and a continuous matrix material (e.g. carbon, polymer, metal, metal oxide or ceramic, more particularly carbon or polymer). The nanocomposite nanofibers are coaxially layered nanofibers, where the nanofibers comprise a core and a sheath that at least partially surrounds the core. The nanofibers comprise a continuous matrix core of a second material and the discrete domains are embedded in and at least partially coated by the second material. The nanofibers comprises at least 25 wt.% of the silicon containing material or silicon. The nanofibers have an average diameter of less than 1 micron, and an average aspect ratio of at least 100. The nanofibers are cross-linked. The polymer is polyvinyl alcohol, polyvinyl acetate, polyethylene oxide, polyvinyl ether, polyvinyl pyrrolidone, polyglycolic acid, hydroxyethylcellulose, ethylcellulose, cellulose ethers, polyacrylic acid, olyisocyanate and/or polyacrylonitrile. Preferred Process: The process further comprises: thermally treating the as-spun nanofiber; oxidizing the as-spun nanofiber (e.g. concurrently with thermal treatment) to remove the polymer; and reducing the as-spun nanofiber (e.g. concurrently with thermal treatment) to minimize oxidation of metal components. The step of electrospinning the fluid stock is electrospun coaxially with a gas, where the fluid stock is gas assisted. The fluid is aqueous. The thermal treatment occurs under inert conditions to carbonize the polymer. The step of preparation of the fluid stock further comprises combining, in any order, at least one non-silicon metal precursor.