Are lithium-sulfur battery separators the same as lithium-ion battery separators? The rapid rise of modern electric vehicles has placed increasingly higher demands on battery energy density. Currently commercially available lithium-ion batteries no longer meet the requirements of some electronic devices. Lithium-sulfur batteries, which have seen significant research interest in recent years, boast high energy density, with a theoretical energy density reaching 2600 Wh·kg⁻¹, five times that of commercially available lithium-ion batteries. This meets the requirements of electric vehicles and the "light, small, and thin" requirements of portable electronic devices. So, are lithium-sulfur battery separators the same as lithium-ion battery separators? What are their similarities and differences?
Lithium-sulfur battery separator
A typical lithium-sulfur battery uses elemental sulfur as the positive electrode and lithium metal as the negative electrode. Its reaction mechanism is different from the ion insertion/extraction mechanism of lithium-ion batteries; it is an electrochemical mechanism.
Lithium-sulfur batteries use sulfur as the positive electrode reactant and lithium as the negative electrode. During discharge, the negative electrode reaction involves lithium losing electrons to become lithium ions, while the positive electrode reaction involves sulfur reacting with lithium ions and electrons to form sulfides. The potential difference between the positive and negative electrode reactions is the discharge voltage supplied by the lithium-sulfur battery. Under applied voltage, the positive and negative electrode reactions of the lithium-sulfur battery proceed in reverse, which is the charging process. Based on the amount of electricity supplied by a unit mass of elemental sulfur completely converting to S2-, the theoretical discharge specific capacity of sulfur is 1675 mAh/g. Similarly, the theoretical discharge specific capacity of elemental lithium is 3860 mAh/g. The theoretical discharge voltage of a lithium-sulfur battery is 2.287V when sulfur and lithium completely react to form lithium sulfide (Li2S). The corresponding theoretical discharge specific energy of the lithium-sulfur battery is 2600 Wh/kg.
Recently, a team of professors from Nanjing University reported the design and fabrication of a highly flexible metal-organic framework membrane (MOF@PVDF-HFP) using HKUST-1 nanoparticles as assembly units and PVDF-HFP as a binder. The prepared MOF@PVDF-HFP battery separator exhibits excellent flexibility and can be considered a good physical barrier to effectively confine polysulfide intermediates on the cathode side. Furthermore, even at ultra-high current densities (10 mA cm⁻²), the MOF@PVDF-HFP separator can achieve stable lithium deposition/stripping. Introducing the MOF@PVDF-HFP separator into Li-S coin cells resulted in ultra-long cycle life and ultra-low capacity decay. More importantly, flexible Li-S pouch cells with high sulfur loading assembled using the MOF@PVDF-HFP separator exhibited excellent stability under different bending shapes, further demonstrating the potential of the MOF@PVDF-HFP membrane in practical applications.
Lithium-ion battery separator
The primary function of the separator in lithium-ion batteries is to separate the positive and negative electrodes, preventing short circuits caused by contact between them. It also allows electrolyte ions to pass through. The separator material is non-conductive, and its physicochemical properties significantly influence battery performance. Different types of batteries use different separators. For lithium-ion batteries, since the electrolyte is an organic solvent system, separator materials resistant to organic solvents are required; generally, high-strength, thin-film polyolefin porous membranes are used.
Important lithium-ion battery separator materials include single-layer PP, single-layer PE, PP+ceramic coating, PE+ceramic coating, double-layer PP/PE, double-layer PP/PP, and triple-layer PP/PE/PP. The first two types are primarily used in small 3C batteries, while the latter are mainly used in power lithium-ion batteries. Among power lithium-ion battery separator materials, double-layer PP/PP separators are primarily processed by Chinese companies and used in mainland my country. This is mainly because currently, no Chinese company possesses the technology and capability to fabricate a double-layer composite film from PP and PE. Globally, automotive power lithium-ion batteries primarily use triple-layer PP/PE/PP, double-layer PP/PE, and PP+ceramic coating, PE+ceramic coating, and other separator materials.
What are the applications of lithium-ion battery separator materials?
I. Ensuring Battery Supply Security
The separator material must first have good insulation to prevent short circuits caused by contact between the positive and negative electrodes or by punctures from burrs, particles, or dendrites. Therefore, the separator must have certain tensile and puncture strength, be not easily torn, and maintain dimensional stability under sudden high temperature conditions so as not to melt and shrink, which would lead to large-area short circuits and thermal runaway of the battery.
II. Microporous channels for supplying lithium-ion batteries to achieve charge/discharge functionality and rate performance.
Therefore, the separator must be a thin film with high porosity and evenly distributed micropores. The inherent properties of the material and the porosity characteristics after film formation restrict the migration of lithium ions in the battery, which is currently reflected in the performance parameter of ionic conductivity.
As a key material in lithium-ion batteries, the battery separator plays a crucial role in electron isolation, preventing direct contact between the positive and negative electrodes while allowing lithium ions to pass freely through the electrolyte. Simultaneously, the separator is vital for ensuring the safe operation of lithium-ion batteries. High-performance separators are essential for improving the overall performance of lithium-ion batteries. The lithium-ion battery separator industry is projected to maintain a compound annual growth rate of around 30% until 2020.
