NOVELTY - An electrode composition, comprises: sulfur or metal sulfide electro-active material suspended and/or dissolved in a fluid; and electronically conductive particles suspended in the fluid. The electronically conductive particles form a percolating conductive network.    USE - As electrode composition used in an electrode for an energy storage device (all claimed) such as battery.    ADVANTAGE - The electrode composition has a viscosity of less than or equal to 100 Pa-s, or 100 Pa-s to 1 mPa-s, at a shear rate of 100 s-1; yield point of less than or equal to 1000 Pa, preferably 1-1000 Pa; and yield stress of less than or equal to 1000 Pa; preferably 1-1000 Pa. The electronic conductivity of the electrode is at least 0.1 mS/cm; preferably at least 5 mS/cm; especially at least 10 mS/cm; particularly 2-10 mS/cm. The energy storage device exhibits, upon reversible cycling, higher energy densities and specific capacities than conventional energy storage devices. The device comprising a percolating conductive network experiences lower charge-transfer resistance than conventional energy storage devices comprising electronically-insulating redox flow compositions and stationary current collectors. In the precipitation regime, the percolating conductive network provides greater surface area over which to deposit insoluble polysulfides, allowing an equivalent volume of polysulfide precipitate to be more thinly deposited. This provides higher specific capacities and energy densities than prior art devices. The percolating conductive network renders the electrode composition both ionically and electronically conductive. The percolating network allows the electrode to function as a "liquid wire" and allows charge transfer to occur throughout the volume of the electrode composition. This increases the charge transfer rate of the electrode composition when used in an electrode in an energy storage device. By contrast, in conventional flow batteries, the electrode compositions typically comprise electronically-insulating redox fluids that only undergo charge transfer reactions upon contact with a stationary current collector. The use of intermittent flow (as opposed to continuous flow) can reduce the amount of energy required to transport the fluid through the electrode compartment to achieve a given state of charge. Substantially all of the electrode composition can be charged and/or discharged to a given state of charge by intermittently transporting the electrode composition once through the electrode compartment.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) an electrode comprising an electro-active material containing sulfur or a metal sulfide; and a percolating electronically conductive network comprising several electronically conductive particles, where the electrode is fluid prior to first use;    (2) an energy storage device (100), comprising: a first electrode (102) including the electrode composition containing a first electro-active material comprising sulfur or a metal sulfide, where the first material is suspended and/or dissolved in a fluid, and electronically conductive particles suspended in the fluid, where the electronically conductive particles form a percolating conductive network; and a second electrode (104) in electrochemical communication with the first electrode; and    (3) method of operating the energy storage device, involving: reversibly cycling the energy storage device over a voltage range of 0.45-2.7 V.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of an energy storage device.    Energy storage device (100)    First electrode compartment (102)    Second electrode compartment (104)    First storage tank (108)    Second storage tank (110)    Conduit (112, 114)    Current collectors (116, 118)    External circuit (120)