Innovative Strategies for Lithium Battery Separators and Materials

Among the four main materials for lithium batteries, the separator faces particularly severe and complex market conditions. On one hand, separator manufacturers must contend with ongoing pressure from continuously weak prices; on the other, the potential for industrialization of solid-state batteries poses risks of market contraction for separators.

In this context, lithium battery separator companies must accelerate the evolution of their products towards higher performance to obtain technological premiums and buy time for transformation, seeking long-term development amidst industry challenges.

Given the demands for higher energy density, higher charge-discharge rates, longer cycle life, and greater safety in liquid lithium batteries, as well as the clear trend toward fully solid-state batteries from hybrid solid-liquid designs, the industry’s thoughts on the iteration of lithium battery separators mainly focus on the following four aspects:

Development of Thinner Base Membranes and Functional Coatings

Thinning has become one of the primary directions for the development of lithium battery separators. The wet method base membrane is advancing towards 5 micrometers and below, while the dry method base membrane is targeting 10 micrometers and below, with a consensus on improving mass production efficiency.

While achieving a thinner profile, it is also essential to meet the demands for high safety, ultra-fast charging, and long lifespan. This requires enhancing the thermal resistance of the separators, increasing ionic conductivity, and reducing impedance (achieved through high porosity, large pore sizes, and low tortuosity), as well as improving liquid absorption and retention properties.

In terms of safety, for example, a 5-micrometer base membrane's strength needs to withstand puncture strengths of 380GF, 450GF, or even higher. The closed-cell temperature should be gradually reduced from 4°C to 6°C, and the membrane rupture temperature needs to strive for withstanding high temperatures of 180°C or even 200°C, ultimately aiming to comprehensively enhance the safety performance of thin separators.

Several companies have made progress in this area. Putailai has developed a high-transparency, high-mechanical-strength ultra-thin separator base membrane at 5 micrometers, which has improved puncture strength by 40% compared to traditional 5-micrometer products in the 3C category. ZTE New Materials has launched a next-generation ultra-thin, high-strength, high-porosity, high-safety dry method base membrane product in the range of 7-10 micrometers, suitable for ultra-fast charging needs.

Regarding coatings, ceramic-coated membranes are evolving from 2.5 micrometers down to 1.5 micrometers, and even 1 micrometer. In this process, the thermal resistance needs to increase from 150°C to 180°C or even 200°C to meet higher safety requirements.

Increased Demand for Binding Coatings: PVDF, Alternatives etc..

As large-capacity battery cells evolve, the demand for enhanced cycle life and energy density has intensified, highlighting the importance of separator binding coatings.

Currently, PvDF (polyvinylidene fluoride) plays a primary role as a binding coating in separators. To achieve better impedance performance, PVDF coatings are transitioning from oil-based applications to water-based spraying. However, water-based spraying still faces issues with consistency, leading the industry to consider point coating techniques to address this challenge.

Notably, with increasing regulations on fluorinated substances both domestically and internationally, there may be future restrictions on the use of fluorinated alkyl substances. In this context, various alternatives are being explored, including water-based PMMA (polymethyl methacrylate) blending technology, ceramic hybrid solutions that combine heat resistance and adhesion benefits, and non-fluorinated large-particle coating products. For instance, Enjie has developed a non-fluorinated large-particle PMMA blended separator, optimizing the PVDF blending system without fluorine. This separator offers both heat resistance and adhesion, effectively enhancing battery safety and cycle life.

Emergence of New Coating Materials like Aramid and Nanocoatings

While ceramic-coated materials are widely used due to their cost-effectiveness, they have limitations such as heavy weight and high moisture content, which are not conducive to improving battery energy density.

Against this backdrop, various new coating materials are becoming popular choices in the industry, including polyimide, polytetrafluoroethylene, aramid, polyester, and nanofibers.

Aramid-coated separators are a typical example. Enjie has developed a high-safety aramid-coated film made by coating para-aramid material onto a PE base membrane. This film exhibits excellent thermal shrinkage properties and a very high rupture temperature; it utilizes a special low-closed-cell base membrane to reduce the closed-cell temperature and has outstanding electrolyte wettability.

These characteristics significantly enhance the safety of lithium batteries in high-temperature environments and improve both cycle and rate performance. Currently, this product has received mass production approval from overseas customers and has been successfully applied in cylindrical batteries for a specific client.

Another noteworthy example is the aramid series separator products jointly released by Xingyuan Materials and Taihe New Materials. Compared to mainstream ceramic or PVDF coatings, aramid-coated separators offer superior thermal performance, liquid absorption and retention capabilities, as well as a lighter weight.

Additionally, Xingyuan Materials has developed a 0.3-micrometer nanofiber coating that has achieved mass production. Compared to ceramic-coated materials, nanomaterials bond more closely with the base membrane and provide a larger contact area. This not only enhances thermal performance (with no shrinkage at 180°C and a rupture temperature above 180°C) but also offers better wettability (increasing battery manufacturing efficiency) and lower impedance.

Despite these new coating materials demonstrating numerous advantages, they still face challenges in scalable production, which remains a key issue for future industry development.

Solid Electrolyte Membranes as the Intersection of Solid-State Battery and Separator Development

While solid-state batteries will replace separators with solid electrolytes, solid electrolytes will still exist in film form within the battery cells. Their performance is influenced by two main factors: material structure and preparation processes.

Although the development of solid electrolytes is still at a stage where the materials and membranes have not yet been finalized, and the application methods remain undefined, this also opens up numerous possibilities for new technological applications, creating market opportunities for separators with varying performances.

Moreover, the differentiated development of dry and wet separator processes and the optimization of their preparation techniques are essentially a preparatory phase for the technologies needed for solid-state batteries. This aligns with the trend of solid battery cells favoring stacking processes and reflects the long-term significance of blade battery companies’ layouts for stacking lines and equipment.

Taking the first-generation all-solid-state battery technology from Penghui Energy, which has attracted market attention, as an example, one major highlight is its use of wet processes during the preparation of oxides to replace high-temperature sintering, ultimately achieving the goal of “only 15% higher costs compared to conventional lithium batteries.” The wet preparation of oxide solid electrolyte membranes is similar to lithium battery coating processes, and subsequent cost reductions can also leverage shared strategies, mainly addressing the cost challenges of solvent removal and improving membrane uniformity.

The similarity in preparation technologies also opens the door for solid electrolyte membranes to compete alongside coating membranes. Jiangsu Sanhe (jointly established by Shanghai Enjie, Beijing Weilan, and Liyang Tianmu) focuses on the research, production, and sales of electrolyte-coated membranes. Their LATP-coated membrane (oxide solid electrolyte membrane) has shown superior performance in electrolyte wettability, ionic conductivity, and safety compared to traditional ceramic-coated membranes made from alumina materials.

From this perspective, LATP membranes can be seen as an iteration of existing ceramic-coated membranes and can also be applied to conventional lithium batteries that require higher performance in terms of energy density, rate capability, safety, and lifespan.

However, the large-scale production of solid electrolyte membranes still faces challenges regarding production efficiency and cost control. If related processes can be validated in mature production lines, it would significantly accelerate the industrialization process of solid-state batteries.