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We report on a simple and fast route to prepare lithium selenide (Li 2 Se) nanoparticles and show a versatile solution-based method to form uniform nanostructured carbon (C)-Li 2 Se composites with and without additional carbon shell. We systematically compare electrochemical performance characteristics of 50–100 nm high purity Li 2 Se nanoparticles with that of the C-Li 2 Se nanocomposites for rechargeable Li battery applications. While Li 2 Se nanopowder show high initial capacity, it suffers from active material loss and shuttle of dissolved polyselenides, resulting in low cycling stability and resistance growth, additionally aggravated by mechanical cathode degradation induced by repetitive volume changes during cycling. By embedding Li 2 Se nanoparticles into a conductive carbon matrix, mechanical stability of electrodes was greatly enhanced. More importantly, the dissolution and shuttle of polyselenides was suppressed significantly even for smaller (~20 nm) and thus more reactive Li 2 Se nanoparticles. As a result, C-Li 2 Se nanocomposite cathodes showed high rate capability and promising cycling stability with carbon-shell protected C-Li 2 Se showing virtually no degradation in 100 cycles. When compared with somewhat similar lithium sulfide (Li 2 S) nanoparticles and C-Li 2 S electrodes, we observe lower over-potential at different C-rates in case of Li 2 Se and C-Li 2 Se materials, which is advantageous for battery applications. Based on the postmortem analysis, significant Li dendrite growth observed in Li 2 Se/Li cells did not take place in C-Li 2 Se/Li and C-Li 2 Se@C/Li cells, suggesting that polyselenide shuttle may affect Li plating morphology. Beyond the organic electrolyte-based Li-Se batteries, all-solid Li-Se batteries based on the produced C-Li 2 Se nanocomposite cathode were built for the first time using conventional Li 2 S-P 2 S 5 solid state electrolyte. These solid state cells showed very promising cycling stability, a single flat plateau and very small voltage hysteresis in the range of 0.1–0.4 V when tested at 60 and 80 °C. [ABSTRACT FROM AUTHOR]