N-doped porous hard-carbon derived from recycled separators for efficient lithium-ion and sodium-ion batteries

Minimizing life-cycle environmental impacts of rechargeable batteries has been attracting tremendous interest recently. Of a number of key components for lithium-ion and sodium-ion batteries, hard carbon represents a promising material for battery anodes due to its advantages such as high and fast rate capability, no intercalation-induced exfoliation, and ease of forming molecularly doped anodes. While there are several methods to synthesize hard carbon, here we demonstrate that waste polyimide separators could serve as an ideal precursor to synthesize hard carbon with in situ nitrogen doping. By inheriting the unique net-like structure of the polyimide fiber separator, the resultant material exhibits a three-dimensional macro-porous structure consisting of an interconnected network of nitrogen-doped carbon fibres. The porous architecture of the material achieved at a carbonization temperature of 900 °C yields a high capacity anode with a long cycling life for rechargeable Li-ion and Na-ion batteries.