<Graphical abstract>

With the growing demand for lithium sulfide (Li₂S) as a solid-state electrolyte and high-capacity cathode material, the development of scalable and byproduct-free synthesis routes is critical. We present a solvent-guided sulfidation strategy that controls the reaction pathway of lithium hydroxide (LiOH) with H₂S gas. In polar protic systems (e.g., deionized water), LiOH dissolves to form HS⁻ ions, which then undergo a solution-mediated reaction through the formation of a transient LiHS intermediate. In polar aprotic solvents such as N,N-Dimethylformamide, although LiOH is insoluble, HS⁻ ions are stabilized and remain dissolved, facilitating the same solution-based pathway. These conditions promote the formation of mesoporous Li₂S with a high surface area and a uniform particle size. In contrast, under nonpolar (e.g., o-xylene) or solvent-free conditions, both LiOH and HS⁻ remain insoluble, leading to a solid-state phase transition that initiates from the LiOH surface and progresses inward. This results in slower reaction kinetics and denser Li₂S particles with higher micropore volume. These mechanistic distinctions between solvent participation and the sulfidation mechanism offer a unified framework for tuning Li₂S morphology and porosity through solvent selection. Furthermore, the use of LiOH and H₂S, combined with efficient solvent/gas recyclability, supports the feasibility of cost-effective and scalable industrial production. This process enables direct recovery of high-purity Li₂S without high-temperature calcination or complex purification steps, offering an economically attractive route. This work provides a rational design principle for the high-performance byproduct-free synthesis of Li₂S tailored for next-generation energy storage applications.