The present invention generally relates to energy storage devices, and to metal sulfide energy storage devices in particular. Some aspects of the invention relate to energy storage devices comprising at least one flowable electrode, wherein the flowable electrode comprises an electroactive metal sulfide material suspended and/or dissolved in a carrier fluid. In some embodiments, the flowable electrode further comprises a plurality of electronically conductive particles suspended and/or dissolved in the carrier fluid, wherein the electronically conductive particles form a percolating conductive network. An energy storage device comprising a flowable electrode comprising a metal sulfide electroactive material and a percolating conductive network may advantageously exhibit, upon reversible cycling, higher energy densities and specific capacities than conventional energy storage devices.
 Research leading to various aspects of the present invention was sponsored, at least in part, by the U.S. Department Energy, Grant No. DE-AR0000065; Grant No. DOE-FOA-0000559, Energy Innovation Hub--Batteries and Energy Storage; and Subcontract No. 3F-31144, issued under DOE Prime Contract No. DE-ACO2-06CH11357 between the United States Government and UChicago Argonne, LLC representing Argonne National Laboratory. The U.S. Government has certain rights in the invention.