Semi-solid-state batteries are driving demand for composite separators; an analysis of high-temperature-resistant separator technologies.

Preface 

The new energy sector has entered a phase of large-scale commercial deployment for semi-solid-state batteries, with numerous automakers and leading battery manufacturers successively integrating these batteries into production vehicles, thereby driving structural growth opportunities in the upstream separator market. Compared with conventional liquid lithium-ion batteries, semi-solid-state batteries feature high-nickel cathodes. / Silicon-based anodes, a significant reduction in electrolyte content, and a wider operating temperature range for the cell—standard. PE/PP Single-layer separators can no longer meet the demanding operating conditions of high temperature, high voltage, and lithium dendrite suppression. High-Temperature-Resistant Composite Separator It has become a standard, mandatory material. 

Traditional single-layer diaphragm 130℃ Significant thermal shrinkage occurs almost immediately, readily leading to internal short circuits in the cell. Meanwhile, fast‑charging, nail‑penetration, and thermal‑box tests for semi‑solid batteries impose entirely new requirements on separator thermal stability, liquid retention, interfacial compatibility, and puncture resistance, driving a rapid shift in industry demand from conventional coated separators to multilayer high‑temperature‑resistant composite separators. 

This paper integrates current mass-production technology roadmaps, key performance metrics, mainstream composite separator solutions, and selection criteria to comprehensively dissect the technical rationale behind high-temperature‑resistant composite separators tailored for semi-solid-state batteries, providing industry‑wide, practical insights for membrane manufacturers, battery procurement teams, and process engineers. 

I. Why Must Semi-Solid-State Batteries Use High-Temperature-Resistant Composite Separators? 

Semi-solid-state batteries represent an intermediate pathway in the transition from liquid‑to‑all‑solid‑state chemistries, retaining a small amount of liquid electrolyte while incorporating a solid electrolyte layer. They offer significantly enhanced operating conditions and safety performance, whereas conventional single‑layer polyolefin separators suffer from four critical drawbacks. 

1. The risk of thermal runaway at high temperatures is escalating. 

The energy density of semi-solid-state batteries has been surpassed. 300 Wh/kg , during fast charging, the local temperature of the battery cell can reach 150℃ The above, ordinary. PE The diaphragm’s melting point is only 135℃ , high-temperature shrinkage causes the separator to rupture, leading to direct contact between the positive and negative electrodes, which triggers thermal runaway and ignition. High-temperature‑resistant composite separators can… 150℃ Constant temperature 1 hour Heat shrinkage is controlled within 5% Within, significantly raise the safety threshold. 

2. Lithium-metal anodes readily pierce conventional separators. 

High‑end semi‑solid‑state batteries employ a thin lithium‑metal anode, and during charge–discharge cycling, lithium dendrites continue to grow. The single‑layer separator lacks sufficient puncture resistance; in contrast, the composite separator incorporates inorganic ceramic and aramid coatings to form a rigid protective layer that effectively prevents dendrite penetration. 

3. Electrolyte retention and interfacial compatibility pose more stringent requirements. 

In semi-solid systems, the electrolyte dosage is low, requiring the separator to exhibit strong liquid absorption and retention capabilities. At the same time, the separator surface must achieve stable adhesion with the solid electrolyte layer; however, conventional base separators often suffer from relatively high interfacial impedance, which degrades cycling and rate performance. 

4. Under long-cycle operating conditions, the coating is prone to delamination. 

Conventional single-sided ceramic separators experience coating pulverization and particle shedding after prolonged charge–discharge cycling, contaminating the cell. In contrast, multilayer composite high‑temperature‑resistant separators employ a crosslinked bonding system, providing superior coating adhesion and meeting the demanding cycle life requirements of semi‑solid-state batteries for thousands of cycles. 

II. Key Performance Metrics of the Semi-Solid High-Temperature-Resistant Composite Separator 

For composite separators intended for mass production of power batteries, the industry‑wide unified acceptance criteria are as follows and serve as the primary basis for selection: 

1. Heat resistance: 150℃ Constant temperature 1 hour Longitudinal and transverse thermal shrinkage rates ≤5% , membrane rupture temperature ≥200℃ ；

2. Mechanical Properties: Puncture Strength ≥550gf , tensile strength MD/TD Balance to prevent winding breakage; 

3. Electrochemical parameters: interfacial impedance < 5Ω ・ cm² ， 3C High-rate capacity retention > 85%；

4. Coating stability: After thermal cycling, there is no powder loss or delamination, and the liquid retention rate is improved. 30% The above; 

5. Compatibility: Compatible with oxides LATP Solid-state coating and gel electrolyte, with no side reactions. 

III. Analysis of the Three Major Mainstream High-Temperature-Resistant Composite Separator Technology Paths 

Currently, mass‑produced semi‑solid‑state separators fall into three categories: ceramic‑based composites, aramid‑based composites, and solid‑electrolyte‑modified composites, each exhibiting distinct differences in performance, cost, and application‑specific suitability. 

(1) Double-sided ceramic-coated composite separator (mass‑production mainstream, cost‑effective solution) 

1. Structure: 5–7 μm Wet-process high-strength PE Basement membrane +  Double-sided boehmite /  Aluminum oxide ceramic coating 

2. High-temperature resistance principle: Inorganic ceramic particles have a melting point exceeding 2000℃ Under high temperatures, it forms a rigid insulating framework that prevents the base film from shrinking; when combined with a crosslinked water-based binder, it enhances coating stability. 

3. Core Advantages: Mature technology, low investment in production-line upgrades; significantly superior puncture resistance and heat resistance compared to single-layer films; excellent liquid‑retention capability; compatible with lithium‑iron‑phosphate semi‑solid‑state batteries. 

4. Weaknesses: Moderate compatibility with lithium-metal anodes, and inferior oxidative stability at high voltages compared to aramid separators. 

(2) Aramid high-temperature-resistant composite separator (high-end, high-nickel) / For lithium metal use only) 

1. Ultra-thin wet-process support membrane + Double-sided aramid polymer coating 

2. High-temperature resistance principle: The thermal decomposition temperature of aramid polymers exceeds 500℃ , it exhibits no melt‑induced shrinkage, forming a continuous high‑temperature‑resistant protective layer with outstanding oxidation resistance and excellent resistance to dendrite formation. 

3. Core Advantage: Delamination Temperature 200℃ In summary, the needle‑penetration test demonstrates an exceptionally high pass rate, making it suitable for high‑nickel ternary and lithium‑metal semi‑solid‑state cells, while also delivering longer cycle life under fast‑charging conditions. 

4. Weakness: Raw material and coating costs remain relatively high, placing significant procurement pressure on small and medium-sized battery manufacturers. 

(3) Solid-Electrolyte Composite Separator (Next-Generation Iterative Direction) 

1. Structure: Basement membrane + Ceramic transition layer + LATP Oxide solid electrolyte coating 

2. High-temperature resistance mechanism: The inorganic solid electrolyte simultaneously exhibits both thermal stability and ionic conductivity, while the separator fulfills three critical functions—electrolyte isolation, lithium-ion conduction, and liquid storage. 

3. Core advantages: lowest interfacial impedance, direct formation of high‑speed lithium‑ion pathways, optimal suppression of dendrite growth, and broad compatibility. 400 Wh/kg The above are high-energy-density semi-solid-state batteries. 

4. Current situation: Leading materials companies are supplying in small volumes, with production capacity still ramping up; looking ahead, 2–3 Gradually becoming widespread year by year. 

IV. Key Process Parameters for the Production of High-Temperature-Resistant Composite Separators 

To consistently produce separators that meet semi-solid-state standards, the three critical stages—coating, base‑film selection, and slurry formulation—are all indispensable. 

1. The separator membrane is preferably an ultra-thin, high-strength wet-process separator. 

Dry-process separators suffer from uneven porosity and relatively low mechanical strength, so they have largely been phased out; the mainstream choice is… 5 μm 、 7 μm Biaxially stretched wet-process base film, with the longitudinal–transverse tensile strength difference controlled within 15% Within this range, reduce winding wrinkles. 

2. Gradient-layer coating process enhances heat resistance. 

Adopts an inner dense ceramic layer. + The outer layer features a porous gradient coating, while the inner layer prevents high‑temperature shrinkage; the outer layer enhances electrolyte absorption, addressing the limitations of a single‑layer coating. “ Heat resistance and liquid retention cannot be achieved simultaneously. ”  Pain point. 

3. Crosslinked binders are the high-temperature-resistant core. 
   

Common PVDF Binder 120℃ It softens and flakes off easily; high‑temperature‑resistant diaphragms must employ polyurea or modified polyacrylonitrile crosslinked binders to ensure that the coating structure remains intact at elevated temperatures, thereby eliminating powder‑falling defects. 

4. End-to-end low-dust production control 

The semi-solid separator coating features fine particles, and the workshop maintains a Class 100,000 cleanroom environment to prevent dust from causing self-discharge or short-circuit defects in the battery cells. 

V. Four Common Misconceptions in Industry Selection

1. Misconception: The thicker the coating, the better its heat resistance. 
   Incorrect. An excessively thick coating can significantly increase the separator’s impedance, thereby reducing the battery’s rate capability; for semi-solid-state separators, the thickness of the ceramic coating on a single side should be carefully controlled. 1–2 μm , aramid 0.8–1.5 μm This represents the optimal range, balancing thermal stability and electrochemical performance. 

2. Misconception: A standard single-layer ceramic coating can replace a composite high-temperature-resistant separator. 
   Incorrect. A single-sided coating provides thermal insulation only on one side; the uncoated side rapidly shrinks at high temperatures and fails to pass the rigorous semi‑solid heat‑box and needle‑puncture safety tests. 

3. Misconception: Aramid separators are compatible with all semi-solid-state systems. 
   Incorrect. Aramid is expensive, whereas lithium iron phosphate with its lower energy density can meet the required standards by using a double-sided ceramic separator; blindly opting for aramid would significantly increase cell costs. 

4. Misconception: As long as thermal stability meets the requirements, it is compatible with solid-state electrolytes. 
   Wrong. Some ceramic coatings have a high impurity content and will react with… LATP Side reactions occur in solid-state electrolytes, so interface compatibility testing must be conducted concurrently during material selection. 

VI. Industry Market Trend Forecast

1. Demand continues to grow strongly: 2026 The market size of the one-and-a-half-year solid-state high-temperature-resistant composite separator remains… 65% At the aforementioned growth rates, power batteries remain the core demand driver, while energy storage and specialized power‑supply systems are simultaneously scaling up in volume. 

2. Dual technological pathways: In the short term, bifacial ceramic‑coated separators will dominate the market; in the medium to long term… LATP Solid-state electrolyte composite separators are gradually gaining traction in high-end vehicle models. 

3. Thinning has become standard: separator thickness has decreased from 9 μm 、 7 μm To 5 μm Iteration: ultra-thin, high-temperature‑resistant composite films have become a mandatory requirement for leading battery manufacturers. 

4. Domestic substitution is accelerating: Leading domestic separator manufacturers are steadily commissioning wide‑format, high‑temperature‑resistant coating lines, gradually reducing their reliance on imports of high‑end aramid separators from overseas. 

Conclusion 

Amid the wave of semi-solid-state battery commercialization, the separator industry is moving beyond single-layer, low‑cost price wars, with high‑temperature‑resistant multilayer composite separators emerging as a new growth driver. Conventional coated separators can no longer meet the demands of high‑energy, high‑safety semi‑solid‑state cells; to gain access to leading battery‑pack supply chains, companies must thoroughly master the technical distinctions and selection criteria for three types of composite separators: ceramic, aramid, and solid‑electrolyte. 

For membrane manufacturers, proactively establishing production lines for ultra-thin wet‑process base films, gradient high‑temperature‑resistant coatings, and solid‑state composite coatings is essential to capture the incremental growth opportunities brought by the rapid expansion of semi‑solid‑state batteries. Meanwhile, at the battery procurement and process levels, aligning the composite separator roadmap with the cathode chemistry and energy‑density requirements enables an optimal trade‑off among safety, cycle life, and cost. As the volume of semi‑solid‑state batteries deployed in vehicles continues to grow, technical standards for high‑temperature‑resistant composite separators will keep evolving, necessitating ongoing advancements in material formulations and coating processes across the industry.