Current collector design plays a critical role in the performance of lithium-sulfur battery cathodes. The collector must facilitate efficient electron transfer, accommodate volume changes during cycling, and provide a stable interface for sulfur species. The choice of material, morphology, and surface engineering directly influences sulfur utilization, rate capability, and overall energy density.
Aluminum foil is the conventional current collector for lithium-sulfur cathodes due to its conductivity, mechanical stability, and compatibility with existing manufacturing processes. However, its smooth surface limits sulfur loading and restricts polysulfide adsorption, leading to rapid capacity fade. Sulfur utilization often remains below 80% with planar aluminum collectors due to poor interfacial contact and polysulfide shuttling. Modifications such as surface roughening or chemical treatments can enhance adhesion but do not fully address the challenges of low conductivity in sulfur cathodes.
Carbon-coated current collectors offer improved performance by enhancing electrical conductivity and polysulfide retention. Coatings of carbon nanotubes, graphene, or porous carbon layers create a conductive network that improves sulfur distribution and redox kinetics. These coatings increase the active material's accessibility to electrons, boosting sulfur utilization to 85-90% in some cases. However, the additional carbon mass reduces gravimetric energy density, and long-term cycling stability remains a challenge due to gradual polysulfide dissolution.
Three-dimensional porous collectors represent a significant advancement in lithium-sulfur cathode design. These collectors feature interconnected pore structures that provide high surface area for sulfur loading and accommodate volume expansion during cycling. Materials such as nickel foam, carbon foam, or 3D-printed conductive scaffolds enable sulfur loadings exceeding 5 mg/cm² while maintaining mechanical integrity. The porous architecture also traps polysulfides, improving cycle life. Sulfur utilization in 3D collectors can exceed 90%, with rate capabilities enhanced by reduced charge transfer resistance. However, the increased mass and thickness of these collectors partially offset the gains in energy density.
Lightweight alternatives to traditional metal collectors are being explored to maximize energy density. Carbon fiber papers, ultrathin graphene films, and metal meshes with sub-micron thicknesses reduce inactive mass while maintaining conductivity. For instance, freestanding graphene collectors with a density below 0.1 g/cm³ have demonstrated sulfur loadings of 3-4 mg/cm² without binders or additional substrates. These lightweight designs improve specific energy but often require careful optimization to balance mechanical strength and electrochemical performance.
Recent innovations in surface engineering aim to further enhance collector performance. Atomic layer deposition (ALD) of metal oxides such as Al₂O₃ or TiO₂ creates ultrathin protective layers that suppress polysulfide migration while maintaining conductivity. Plasma treatments introduce functional groups that strengthen sulfur adhesion and promote uniform deposition. Another approach involves patterning microstructures onto collector surfaces using laser ablation or chemical etching, which increases active material anchoring sites without adding significant weight.
The impact of collector design on energy density is substantial. A conventional aluminum foil collector contributes approximately 5-10% of the total cathode mass, whereas advanced lightweight or porous collectors can reduce this to 3-5%. For a lithium-sulfur battery targeting 500 Wh/kg at the cell level, optimizing the current collector can contribute 10-20 Wh/kg of improvement. However, tradeoffs exist between sulfur loading, rate capability, and cycle life, requiring careful system-level design.
In summary, current collector design is a key factor in lithium-sulfur battery performance. Planar aluminum collectors are limited by poor sulfur utilization, while carbon-coated and 3D porous collectors improve electrochemical performance at the cost of added mass. Lightweight alternatives and advanced surface engineering techniques offer pathways to higher energy density and longer cycle life. Future developments will likely focus on multifunctional collectors that integrate conductivity, polysulfide trapping, and mechanical resilience without compromising gravimetric or volumetric energy density.