Low volumetric performance is a common bottleneck of carbon-based electrode materials for practical applications, owing to the low density of porous carbons caused by the intrinsic void space [1]. Specifically, lithium-sulfur (Li-S) batteries as a hot topic of next-generation energy storage devices face the same dilemma that we have to balance their intrinsically electrochemical performance and volumetric performance [2][3][4]. The use of conductive porous carbon materials in the cathode of Li-S batteries, such as mesoporous carbon, carbon nanotube and graphene-derived carbons, can effectively accelerate the reaction kinetics, improve the electrochemical performance of sulfur cathode and promote the practical application of insulting sulfur [3,5]. However, addition of these materials results in massive void space in the electrode, which cannot meet the requirement of compact structure in real applications [6]. Such electrodes usually deliver relatively low volumetric performance even though their gravimetric performance can reach a high value.The significance of volumetric performance for electrochemical energy storage (EES) devices has been addressed by our group previously, especially for carbon-based electrodes in supercapacitors, Li-S batteries and so on [1]. It is noted that a very effective approach for improving the volumetric performance of carbon-based electrodes and EES devices is densifying the electrode materials. The density change of carbon-based electrode materials thus will alter the microstructure. In Li-S batteries, the cathode consists of porous carbon and sulfur. Generally, the density of porous carbon is in a very low level (0.3-0.6 g cm −3 ) [7,8], while the density of bulk sulfur is above 2.0 g cm −3 [9]. Thus, the integration of carbon and sulfur is actually the combination of a low-density component and a high-density one. The precondition is that the density of sulfur is not changeable while the density of porous carbon materials can be tuned by controlling their microstructure, such as the pore volume, surface area, etc. [8,[10][11][12]. It is of great interest that a density ceiling of porous carbon materials is of about 2.2 g cm −3 , which is the density of graphite with the most compact structure. Thus, sulfur is always the heavier phase in carbon/sulfur cathode, while carbon is always the lighter one.Based on the understanding of the role of carbon and sulfur in the hybrid electrode, the remedy for improving the density of carbon/sulfur cathode can be classified into two approaches as shown in Fig. 1: improving the sulfur fraction and densifying the carbon matrix. Zhang et al. [13] pioneered the research on the volumetric performance of carbon/sulfur cathode. They fabricated a series of carbon nanotube (CNT)/sulfur hybrids with different sulfur contents to tailor the evolution of the density of the hybrids and their corresponding volumetric performance. A high sulfur fraction up to 90 wt.% was realized with a density of 1.98 g cm −3 for the hybrid material, and it shows an ultrahig...