Research progress of polymers in high-capacity and high safety lithium based batteries

Polymers are playing increasingly critical and versatile roles in advancing high-capacity and high-safety lithium-based batteries (including 

Li-ion, Li-metal, Li-S, etc.). Their development is key to overcoming the fundamental trade-off between energy density and safety.

Here is a comprehensive overview of the research progress, categorized by the functional role of polymers:

1. As Solid/Quasi-Solid Electrolytes

This is the most prominent direction, aiming to replace flammable liquid electrolytes for intrinsic safety.

Traditional Polymer Electrolytes (PEs):

Materials: Poly(ethylene oxide) (PEO), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), polyacrylonitrile (PAN).

Progress & Challenges: Early PEO-based electrolytes suffer from low room-temperature ionic conductivity (<10⁻⁴ S/cm) and instability 

against Li metal. Recent breakthroughs involve creating composite polymer electrolytes (CPEs) by blending polymers with ceramic fillers 

(e.g., LLZO, Al₂O₃) or designing single-ion conducting polymers (where anions are tethered to the polymer backbone), which suppress Li 

dendrite growth by eliminating concentration polarization.

Novel Polymer Electrolyte Designs:

Supramolecular Polymer Electrolytes: Utilize dynamic non-covalent bonds (H-bonds, ionic interactions) to create self-healing networks 

that can repair damage from Li dendrites.

MOF/COF-based Polymer Composites: Integrate porous, ordered Metal/Covalent Organic Frameworks into polymer matrices to provide 

well-defined Li⁺ transport pathways and enhance mechanical stability.

In-situ Polymerized Electrolytes: Liquid precursor monomers are injected and then polymerized inside the cell, forming perfect 

electrode-electrolyte contact. This is particularly promising for high-volume-change electrodes like silicon anodes.

2. As Binders and Functional Coatings for High-Capacity Electrodes

High-capacity electrodes (Si anodes, S cathodes) undergo large volume changes or suffer from polysulfide shuttling.

For Silicon (Si) Anodes:

Progress: Evolved from linear polymers (e.g., polyacrylic acid - PAA, carboxymethyl cellulose - CMC) to multifunctional cross-linked polymer 

networks.

Advanced Designs:

Self-Healing Binders: Incorporate dynamic covalent bonds (disulfide, boronic ester) that can re-form, maintaining electrode integrity during 

cycles.

Conductive Binders: e.g., PEDOT:PSS, providing both adhesion and electronic conductivity, reducing inactive material.

Multifunctional Binders: Engineered with specific functional groups (e.g., catechol, carboxyl) that form strong bonds with Si particles and 

accommodate volume stress.

For Lithium-Sulfur (Li-S) Batteries:

Cathode Side: Polymers like polymeric sulfur composites (e.g., sulfurized polyacrylonitrile) or polar polymer hosts (rich in carbonyl or amine 

groups) are used to confine sulfur/polysulfides, mitigating the "shuttle effect."

Anode Side (Li Metal Protection): Constructing artificial Solid Electrolyte Interphase (SEI) layers using flexible polymers (e.g., PVDF-HFP) 

blended with inorganic particles to guide uniform Li deposition and prevent dendrite penetration.

3. As Functional Separator Coatings or Novel Separator Matrices

Thermally Stable Separators: Coating commercial polyolefin (PP/PE) separators with high-temperature-resistant polymers like polyimide 

(PI) or aramid (Nomex) significantly improves their thermal shutdown temperature (>200°C), preventing thermal runaway.

All-Polymer Separators: Electrospun nanofiber membranes made of PI or poly(m-phenylene isophthalamide) (PMIA) offer high porosity, 

excellent wettability, and superior heat resistance.

"Smart" Functional Separators:

Shutdown Separators: Using thermally responsive polymers (e.g., PEO/PMMA blends) that melt and close pores at a specific temperature, 

acting as an internal fuse.

Polysulfide-Blocking Separators: Coating with positively charged or polar polymers (e.g., polyethylenimine) to electrostatically repel or 

adsorb polysulfide anions.

4. As Key Components for Advanced Cell Designs & Safety

Gel Polymer Electrolytes (GPEs): A mainstream compromise, where a polymer matrix (e.g., PVDF-HFP) is swollen with liquid electrolyte. 

GPEs offer higher ionic conductivity than solid PEs while significantly reducing leakage and improving safety.

Interlayers: Functional polymer-based membranes (e.g., PI nanofilms, polymer-garnet composites) placed between the separator and 

Li-metal anode serve as physical barriers to block dendrites.

In summary, polymers have evolved from passive components (binders, separators) to active, multifunctional enablers for next-generation 

batteries. The research trend is moving towards rational molecular design, multifunctional integration, and precise interface control to 

simultaneously unlock high capacity and guarantee high safety.