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

Polymers play a crucial role in high-capacity and high safety lithium based batteries, mainly including lithium-ion batteries and future 

lithium metal batteries. Its research progress mainly focuses on two directions: as a solid electrolyte and as a key component of 

electrodes (such as binders and conductive agents).

The following will elaborate on the latest research progress of polymers in these two directions.

Polymer as solid electrolyte

This is the core area where polymers contribute to high safety. Replacing traditional flammable organic liquid electrolytes with solid 

polymer electrolytes can fundamentally solve the problems of battery combustion and explosion.

1. Tradition and Foundation: Polyethylene Oxide System

- Material : Composite of PEO and lithium salt (such as LiTFSI).

-Working principle: The ether oxygen bond (- C-O-C -) on the PEO chain can coordinate with Li ⁺, and the transport of Li ⁺ is achieved 

through local movement of the chain segment.

-Advantages: Good stability to lithium metal, simple preparation, and good adhesion.

-Challenge :

-Low ion conductivity: Usually<10 ⁻⁵ S/cm at room temperature, it needs to be heated to above 60 ° C to work, which limits its application.

-Low lithium ion migration number: Anion migration contributes most of the current, which can easily lead to concentration polarization and 

increased internal resistance of the battery.

- Research Progress:

- Co polymerization: Introducing other monomers (such as polypropylene oxide) to disrupt the crystallinity of PEO and enhance its chain 

segment mobility.

-Crosslinking: forms a three-dimensional network structure, inhibits crystallization, and enhances mechanical strength.

-Add inorganic fillers such as SiO ₂, Al ₂ O3, LLZO, etc. to form a composite polymer electrolyte. Fillers can not only further inhibit PEO 

crystallization, but also provide additional Li ⁺ fast transport channels (especially at the filler interface), significantly improving ion conductivity 

and migration number.

2. Emerging polymer systems (research hotspots)

To overcome the limitations of PEO, researchers have developed various novel polymer matrices:

-Polycarbonate esters:

-represents : poly (propylene carbonate), poly (ethylene carbonate).

-Advantages: Lower glass transition temperature, higher dielectric constant (better dissociation of lithium salts), and better interface stability 

with high-voltage positive materials (such as NMC811) compared to PEO.

-Progress: Through copolymerization, cross-linking, and composite with inorganic fillers, its room temperature ionic conductivity can be 

increased to the order of 10 ⁻⁴ S/cm.

-Polysiloxane :

- represents: Side chain functionalized polydimethylsiloxane.

-Advantages: Extremely flexible main chain (extremely low Tg, up to -100 ° C or below), conducive to ion transport. Structure can be 

designed.

Overall, the research on polymers in high-capacity and high safety lithium based batteries is shifting from an "auxiliary role" to a "core 

material". The future development directions include:

Molecular level design: Guided by computational simulations, accurately design polymer chain structures and functional groups to achieve 

high ionic conductivity, high migration number, wide electrochemical window, and excellent mechanical properties simultaneously.

Multi dimensional composite: Developing multiphase composite electrolytes such as polymer/inorganic/liquid crystal, complementing each 

other's strengths and weaknesses, and achieving the effect of "1+1>2".

Interface Engineering: In depth study and regulation of the interface chemistry between polymer electrolytes and lithium metal negative 

electrodes and high-voltage positive electrodes, to construct stable solid-state electrolyte interface phases.

Green and Sustainable: Developing biodegradable or biomass derived polymers to reduce the environmental footprint of batteries.

The continuous innovation of polymer materials is one of the key driving forces for lithium based batteries to move towards higher energy 

density and intrinsic safety.