Solid Polymer Electrolytes (SPE)

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1. Basic Definition

Solid Polymer Electrolytes (SPE), also known as ion-conducting polymers or solvent-free polymer electrolytes, are a class of solid-state ionic conductors formed by using high molecular polymers as the matrix and compounding them with metal salts (such as lithium salts and sodium salts). Their core composition consists of two parts: one is the polymer matrix, which provides structural support and an ion transport environment; the other is the conductive salt, which provides migratable ions (such as lithium ions and sodium ions). SPE can replace traditional liquid electrolytes in electrochemical devices like batteries and supercapacitors to realize ion transport between electrodes.

2. Composition, Characteristics and Ion Conduction Mechanism

2.1 Main Components

Polymer Matrix: Common types include polyethylene oxide (PEO), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVDF), etc. Among them, PEO has been the most extensively studied, featuring good film-forming properties but high crystallinity; PVDF has low crystallinity and excellent interface stability; PAN exhibits strong thermal stability and a high lithium ion transference number but is brittle; PMMA has good compatibility with plasticizers and relatively high electrical conductivity yet poor mechanical properties.

Conductive Salt: Mainly includes lithium salts (e.g., LiTFSI, LiPF₆) and sodium salts (e.g., NaClO₄). Its function is to provide freely migratable ions, which affect the ionic conductivity and electrochemical stability of the electrolyte. Selecting lithium salts with low dissociation energy (such as LiTFSI) or adopting a dual-salt system is an important direction for optimizing electrolyte performance.

2.2 Core Characteristics

• Ionic Conductivity: Generally low at room temperature, usually in the range of 10⁻⁶ – 10⁻⁴ S/cm, and needs to be improved through modification methods. The conductivity increases with the rise of temperature, following the laws of the Arrhenius or VTF equation.

• Mechanical Properties: It possesses both flexibility and film-forming properties, and can be processed into ultra-thin films (with a thickness of less than 50 μm). Some systems have high mechanical strength, which can inhibit the growth of lithium dendrites and ensure battery safety.

• Electrochemical Stability: The electrochemical window of most systems can reach more than 4V (vs. Li⁺/Li), and new systems (such as HVPE) can even reach 5.1V, which is compatible with high-voltage cathode materials.

• Other Characteristics: It is light in weight, which is beneficial to reduce the weight of batteries and other equipment; it has good processability and can be made into electrolyte films of different structures and morphologies; it has no risk of liquid leakage, is impact-resistant, and has better safety than liquid electrolytes.

2.3 Ion Conduction Mechanism

Ion transport relies on the local movement of polymer segments (i.e., segment relaxation). The specific process is as follows: migratory ions first complex with polar groups on the polymer chain (such as ether oxygen bonds in PEO). Under the action of an electric field, with the thermal movement of molecular segments in the high elastic region, the ions and polar groups continuously undergo “complexation-decomplexation”, and then jump and migrate between different polar groups to realize ion conduction.

The ion conduction effect is affected by two major factors: one is crystallinity. The amorphous region of the polymer is more conducive to ion movement, and reducing the crystallinity (such as modifying PEO) can improve the conductivity; the other is temperature. An increase in temperature will accelerate the movement of polymer segments, promote ion migration, and thus improve the conductivity.

3. Classification System

3.1 Classification by Matrix Material

Type	Characteristics
PEO-based	The most widely studied, with good film-forming properties. However, it has high crystallinity at room temperature and low ionic conductivity (usually 10⁻⁸ – 10⁻⁷ S/cm), and needs to work above 60℃ to obtain good conductivity.
PAN-based	Has good thermal stability and a high lithium ion transference number, but the material is brittle and its mechanical properties need to be improved.
PMMA-based	Has excellent compatibility with plasticizers, which can improve conductivity, but its own mechanical strength is poor and it is easy to be damaged.
PVDF/PVDF-HFP-based	Has low crystallinity, relatively high ionic conductivity at room temperature, and good interface stability with electrodes, making it one of the popular research systems at present.

3.2 Classification by Morphology and Composition

• Dry Solid Polymer Electrolytes (DSPE): Contain no organic liquid solvents at all and mainly rely on polar polymer networks for ion conduction. The typical representative is PEO-based electrolytes. Its advantage is high safety, but the ionic conductivity at room temperature is low. Usually, it is necessary to improve its performance through blending, copolymerization, cross-linking modification or adding inorganic fillers.

• Gel-type Solid Polymer Electrolytes (GPE): Liquid solvents (such as carbonate-based EC/PC) are introduced into the polymer matrix to form a ternary component system of “polymer compound-metal salt-polar organic compound”. It combines the advantages of high conductivity of liquid electrolytes (up to 1.4×10⁻³ S/cm at room temperature) and the safety of solid electrolytes, and has good cycle stability. The capacity retention rate of some formulations is still over 90% after 1000 cycles, but its thermal stability is slightly lower than that of dry systems.

• Nanocomposite Conductors/Composite Polymer Electrolytes (CPE): Prepared by adding nano-sized ceramic powders (such as SiO₂, Al₂O₃), fast ion conductors (such as LLZO, LATP) and other inorganic fillers into polymer electrolytes. It can enhance both mechanical strength and interface stability. The ionic conductivity of some systems at room temperature can reach 2.21×10⁻⁵ S/cm, and the tensile strength exceeds 9.5 MPa, which is compatible with the requirements of all-solid-state batteries (SSB).

• Polymer-in-Salt Electrolytes (PISE): Have an extremely high lithium salt content (usually more than 50 wt%), significantly enhanced ion conductivity and electrochemical stability, and can reduce the growth of lithium dendrites, making them suitable for lithium-ion and sodium-ion batteries. The performance can be further optimized by adding polymer matrices such as PAN and PVDF-HFP, or introducing special fillers such as LLZTO.

• Single-Ion Conducting Polymer Electrolytes (SICPE): The polymer main chain covalently binds anions, which can selectively promote the migration of lithium ions. The lithium ion transference number can be as high as 0.92, which can alleviate the problems of ion polarization and lithium dendrites. Modern synthesis methods such as ring-opening polymerization and RAFT polymerization, or the introduction of materials such as carbon quantum dots and polysiloxane, can improve its ionic conductivity and mechanical properties.

• Sustainable and Environmentally Friendly Polymer Electrolytes: Prepared by using biopolymers such as chitosan, pectin, tapioca starch, and P(HB-HV) as the matrix and mixing with lithium salts, which have the advantages of environmental friendliness and low cost. The ionic conductivity of some systems is relatively high (for example, the conductivity of tapioca starch-based electrolytes can reach 9.54×10⁻³ S/cm after adding LiCl and Li₂SO₄), and they have good thermal stability, providing a new direction for green energy storage equipment.

4. Performance Modification Strategies

4.1 Structural Optimization Modification

• Copolymerization and Cross-Linking: Introduce random copolymers (such as PEO-PPO copolymers) or construct cross-linked networks to inhibit polymer crystallization, increase the proportion of amorphous regions, and improve ionic conductivity. At the same time, the cross-linked structure can enhance mechanical strength and improve the tensile and impact resistance of the electrolyte.

• Construction of Special Structures: Design two-phase polymer electrolytes (DPE), where one phase provides mechanical support and the other forms an efficient ion transport channel; adopt in-situ polymerization technology (such as UV-cured polyethylene glycol diacrylate) to form an electrolyte layer in close contact with the electrode surface, thereby reducing interface impedance.

4.2 Additive Regulation

• Adding Plasticizers: Introduce plasticizers such as carbonates (EC/PC) and ionic liquids to reduce the glass transition temperature (Tg) of the polymer, accelerate segment movement, and improve ionic conductivity. Some plasticizers can also enhance flame retardancy and improve the thermal stability of the electrolyte.

• Introducing Inorganic Fillers: Adding passive fillers such as nano-SiO₂ and Al₂O₃ can enhance mechanical strength and thermal stability; adding fast ion conductors (active fillers) such as LLZO and LATP can construct additional ion transport paths and significantly improve ionic conductivity. The room-temperature conductivity of some systems can reach the level of 10⁻⁴ S/cm.

4.3 Salt System Optimization

Select lithium salts with low dissociation energy (such as LiTFSI) or adopt a dual-salt system to improve the dissociation efficiency of the salt and increase the number of freely moving ions; for polymer-in-salt electrolytes, increasing the lithium salt content to more than 50 wt% can enhance ion conduction capacity and electrochemical stability.

5. Application Fields

5.1 All-Solid-State Batteries (SSB)

As a core component of all-solid-state batteries, solid polymer electrolytes can completely solve the safety problems such as leakage and combustion of traditional liquid electrolytes, and are compatible with high-energy-density cathode materials (such as NCM811, sulfur cathodes) and lithium metal anodes. For example, the combination of PEO-based composite electrolytes and lithium metal anodes can inhibit dendrite growth and prolong the cycle life of batteries; PVDF-HFP-based electrolytes can improve the high-temperature stability of batteries, making them suitable for high-power application scenarios.

5.2 Flexible Electronic Devices

With excellent mechanical flexibility, solid polymer electrolytes can be used in bendable batteries, wearable devices (such as smart bracelets, flexible watches), etc. They can adapt to the bending and folding requirements of devices, have no risk of liquid leakage, ensure the safety and stability of devices in daily use, and can also be made into a “body skin” structure to adapt to special-shaped designs and improve the space utilization of devices.

5.3 High-Temperature/Special Environment Batteries

Compared with liquid electrolytes, solid polymer electrolytes have better high-temperature resistance. Some systems (such as PVDF-based ones) can work stably at 60 – 100℃, which are suitable for energy storage equipment in high-temperature environments such as deserts and equatorial regions; through modification by adding ionic liquids or nano-fillers, their low-temperature working range can also be expanded to below -20℃, meeting the power demand in special scenarios such as polar scientific expeditions and high-altitude areas.

5.4 UAV Batteries

• Improving Endurance: Solid polymer electrolytes can increase the energy density of batteries. For example, the lithium-ion batteries using this electrolyte developed by the Moscow Aviation Institute in Russia have extended the flight time of small UAVs from 20 minutes to 35 minutes; the composite SPE solutions of domestic Heyuan Lichuang and Tailan New Energy have enabled the cell energy density to reach 350 – 600 Wh/kg, which can extend the UAV flight time from 30 minutes to 60 – 90 minutes.

• Enhancing Environmental Adaptability: It has a wide operating temperature range. Within the range of -40℃ – 120℃, it can maintain normal internal chemical reactions, and the lithium ion transport is less affected by temperature, ensuring the discharge performance of UAVs in harsh environments such as extreme cold and high temperature.

• Improving Safety Performance: There is no risk of leakage and flammability, and it can inhibit the formation of lithium dendrites, reducing the probability of accidents caused by battery problems during UAV flight. For example, the metal lithium-SPE battery packs purchased by Aerospace Science and Industry in 2024 can operate without derating in an environment of 55℃ and level 5 wind.

• Optimizing Adaptability: It has good processability and can be made into batteries of different shapes and sizes according to the design requirements of UAVs, adapting to special-shaped layouts such as wing internal placement and fuselage attachment.

6. Differences from Other Electrolytes

Compared with solid inorganic electrolytes and liquid electrolytes, the core advantages of solid polymer electrolytes are reflected in the following aspects:

• Flexibility and Processability: The polymer matrix has good flexibility and can adapt to deformations such as bending and stretching. It does not require rigid container packaging, can be made into ultra-thin and special-shaped structures, and is suitable for the needs of flexible electronics and special-shaped batteries; moreover, the processing technology is simple, which is convenient for large-scale production and can simplify the design and construction of electrochemical batteries.

• Safety: It contains no liquid components, avoiding the problems of leakage and volatilization of liquid electrolytes; it is non-flammable and non-explosive, and has good stability in the air. The flash point of some systems exceeds 200℃, and the shear modulus is higher than that of lithium metal, which can effectively inhibit the growth of lithium dendrites and reduce the risk of short circuits.

• Interface and Cycle Adaptability: It can better adapt to the volume change of electrodes during charging and discharging, reducing interface separation; it has good interface contact performance with electrodes, which can reduce interface impedance. Some systems can further improve interface stability through interface engineering (such as in-situ polymerization, buffer layers) and prolong the cycle life of batteries.

Solid inorganic electrolytes have high mechanical strength and high ionic conductivity, but they are brittle and difficult to process; liquid electrolytes have high conductivity and good interface contact, but they have safety hazards such as leakage and combustion, and it is difficult to inhibit lithium dendrites. The comprehensive advantages of solid polymer electrolytes in flexibility, safety, and processability make them an important development direction of the next-generation energy storage technology.