Low-electrolyte lithium-sulfur battery designs represent a critical advancement in achieving practical energy densities for next-generation energy storage systems. The lithium-sulfur chemistry offers a theoretical energy density of 2600 Wh/kg, significantly higher than conventional lithium-ion batteries. However, realizing this potential requires addressing the electrolyte-to-sulfur ratio (E/S), a key parameter influencing both energy density and electrochemical performance.

The electrolyte/sulfur ratio directly impacts the gravimetric and volumetric energy density of lithium-sulfur batteries. Traditional designs use excessive electrolyte quantities (E/S > 10 µL/mg) to facilitate ion transport and mitigate polysulfide dissolution. This approach severely limits practical energy density, as the electrolyte contributes dead weight without enhancing capacity. Reducing the E/S ratio below 5 µL/mg is essential for achieving energy densities exceeding 500 Wh/kg at the cell level. However, lean electrolyte conditions introduce challenges such as sluggish reaction kinetics, incomplete sulfur utilization, and accelerated capacity fade due to increased polysulfide concentration gradients.

Lean electrolyte formulations are a primary strategy for optimizing E/S ratios. These formulations employ high-concentration electrolytes or localized high-concentration electrolytes (LHCEs) to maintain sufficient ionic conductivity while minimizing total electrolyte volume. For example, electrolytes with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in ether solvents at concentrations above 3 M have demonstrated stable cycling at E/S ratios as low as 3 µL/mg. The high salt concentration reduces polysulfide solubility, mitigating the shuttle effect while preserving electrochemical activity.

Absorbent separators play a complementary role in lean-electrolyte systems by immobilizing free electrolyte and preventing dry-out. Functionalized separators with polar groups or porous carbon coatings can absorb and retain electrolyte, ensuring uniform distribution across the cathode. Materials such as metal-organic frameworks (MOFs) or graphene oxide-modified separators have shown promise in reducing electrolyte consumption while maintaining effective lithium-ion transport. These separators also act as polysulfide barriers, further enhancing cycle life under low-E/S conditions.

Cathode prelithiation is another critical strategy for compensating lithium loss in electrolyte-starved designs. During cycling, lithium is irreversibly consumed through side reactions with polysulfides and electrolyte decomposition. Prelithiation methods, such as incorporating lithium-rich additives (Li2S or stabilized lithium metal powder) into the cathode, provide an internal lithium reservoir. This approach offsets active lithium loss, enabling higher sulfur loading and prolonged cycle life. Studies indicate that prelithiated cathodes can achieve >80% sulfur utilization at E/S ratios of 4 µL/mg, compared to <60% utilization in non-prelithiated systems.

Maintaining reaction kinetics under lean electrolyte conditions requires careful engineering of the cathode architecture. High-surface-area carbon hosts, such as hierarchical porous carbon or carbon nanotubes, enhance electrolyte accessibility and sulfur distribution. These structures facilitate rapid charge transfer even with limited electrolyte volume. Additionally, catalytic materials like metal sulfides (CoS2, NiS2) or single-atom catalysts embedded in the cathode can accelerate polysulfide conversion kinetics, reducing polarization and improving rate capability.

Comparative studies of different low-electrolyte approaches reveal trade-offs between energy density, cycle life, and manufacturability. For instance, LHCE-based systems exhibit superior stability but face challenges in viscosity and cost. Absorbent separators offer a scalable solution but may add parasitic weight. Prelithiation improves initial coulombic efficiency but complicates electrode processing. An optimal design often combines multiple strategies, such as a lean LHCE with a catalytic carbon host and prelithiated cathode, to balance performance metrics.

Practical implementation of low-electrolyte lithium-sulfur batteries also demands attention to cell engineering. Thin, dense electrodes with high sulfur loading (>4 mg/cm²) are necessary to minimize inactive material fractions. Stack pressure management is critical to maintain electrode-electrolyte contact in starved conditions. Furthermore, advanced battery management systems must account for the unique voltage profiles and degradation mechanisms of lean-electrolyte cells.

In summary, low-electrolyte lithium-sulfur battery designs are pivotal for unlocking the technology's high-energy potential. By optimizing the E/S ratio through lean electrolytes, absorbent separators, and cathode prelithiation, researchers can overcome the trade-offs between energy density and cycle life. Continued innovation in materials and cell engineering will be essential to transition these advancements from laboratory-scale demonstrations to commercial viability. The progress in this field underscores the importance of holistic design approaches that address both electrochemical and practical manufacturing constraints.