With population growth and environmental degradation, freshwater resources are becoming scarce, and desalination is an effective method to resolve this problem [[1], [2], [3]]. The major water treatment technologies currently are membrane treatment [4,5], thermal treatment [6], microbial treatment [7], and capacitive deionization (CDI). Membrane technology has been well-researched and is extensively used in water treatment [[8], [9], [10]]. However, compared with CDI, CDI has the advantages of lower energy consumption, more energy storage, and more flexibility; therefore, it has attracted much attention [[11], [12], [13]]. Similar to supercapacitors, the electrode material is a critical factor that affects capacitor deionization [[14], [15], [16]]. Biomass materials are commonly used for water treatment due to their excellent properties [17,18]. Sodium lignosulfonate (LS) is a well-stocked biomass feedstock that is easily degradable, has a benzene ring and several oxygen-containing functional groups and contains numerous reactive sites for the adsorption of additional ions. In recent years, research on the application of sodium as an electrode material has gradually increased [19]. For instance, Zhang [20] carbonized sodium lignosulfonate and prepared an electrode with a specific capacitance of 170 F g−1 at 0.5 A g−1 current density. The porous carbon spheres synthesized by Liu et al. using LS had a high specific capacitance of 236.2 A g−1 at 0.2 F g−1 [21]. This proved that LS has significant potential for electrochemical development; however, they predominantly applied the carbonization method, and few were directly applied.

The electrical insulating property of LS limits its application; therefore, an effective combination with conductive polymers is a favorable solution [22,23]. Graphene oxide (GO) is a carbon material commonly used as a water-treatment material due to its excellent electrical conductivity, large specific surface area, and good stability [24]. It can be hydrothermally synthesized as a graphene hydrogel (GH) and converted to reduced graphene oxide (rGO) owing to the absence of oxygen-containing functional groups. Since its carbon six-membered ring structure and oxygen-containing functional groups are identical to those of LS, it can be bonded together by π-π conjugation and hydrogen bonding [25,26]. LS incorporation suppressed the lamellar stacking in GO. However, the double electric layer (EDL) overlap effect of its micropores as a carbon material leads to low electrosorption capacity [27]. Micropores have slow ion transport and high capacitance loss in the double layer, and increasing the pore size improves the electrochemical accessibility.

This problem can be resolved by introducing Faraday materials and increasing the voltage. Molybdenum sulfide is often investigated as a capacitive deionization electrode material, whereas cobalt sulfide, also a transition metal sulfide, has rarely been reported [28]. In our previous study, cobalt sulfides exhibited excellent pseudocapacitance in supercapacitors [29]. It has been found that Na+ can be embedded in CoS, but it has not been tested on CDI [30].

Herein, sodium lignosulfonate/reduced graphene oxide/cobalt sulfide (LGC) composite electrodes were prepared by a hydrothermal method. These are typical pseudocapacitive materials as cobalt sulfide can be embedded in Na+ and the pore size is in mesopores, which can overcome the disadvantages of conventional CDI carbon-based electrodes (double layer overlap effect). A composite of LGC electrodes with diverse CoS qualities is studied and discussed. Additionally, the desalination performance mechanism was explored by evaluating the contributions of diffusion capacitance and pseudocapacitance, which are important guidelines for improving desalination performance.