THE development of low-cost, solid-state rechargeable batteries is of considerable technological importance. A key requirement of such batteries is that the density of energy stored electrochemically in the electrodes is high. In this context, the use of organic materials has attracted interest; they combine high theoretical energy storage capability with low weight and good mechanical strength. Here we report the development of a rechargeable lithium battery with a composite organic cathode based on a mixture of a dimercaptan and polyaniline. The redox behaviour of the dimercaptan, which is normally too slow for practical applications(1,2), is accelerated when coupled to that of the polyaniline(3-5) (which itself functions as an active cathode material). Intimate mixing of the two materials is achieved by casting them jointly from solution. The resulting electrode can be repeatedly charged to near its theoretical limit and discharged. The gravimetric energy density of our materials exceeds that of the oxide electrodes in commercially available lithium-ion batteries(6), a feature that is likely to prove advantageous in applications where weight, rather than volume, is a critical factor.