Solid polymer electrolyte – challenges and solutions

All-solid-state batteries (ASSBs) still require breakthroughs in science and engineering to achieve high energy density, stable cycle life, and large-scale low-cost manufacturing. In this paper, the ion transport mechanism and research progress of solid polymer electrolyte is introduced, as well as the challenges and corresponding solutions.

Polymer ion transport mechanism

The ion transport mechanism in polymer/composite polymer is a complex process, which is related to various parameters such as dielectric constant, ion pair, ion size, amorphous phase characteristics, etc. ITIC technology is widely used to estimate the ionic conductivity of materials.

In addition to ITIC, NMR can be used to study and determine the ionic diffusion coefficient based on free volume theory, VTF model, etc. Polymers used as solid state batteries (PEO, PAN, PPC, etc.) generally contain some polar functional groups, such as -O-, -B-, -N-, -CO, -CN, -Cs -, etc. These polar functional groups are easy to combine with lithium ions or complex reactions with lithium salts containing lithium to form polymer-salt complexes.

Generally, the transport of lithium in inorganic solid electrolyte follows Arrhenius theory, while the transport of lithium ion in solid polymer electrolyte represented by PEO is indeed consistent with VTF model. In the polymer, lithium ions can move between the amorphous phase chain segments to achieve ion transport. The higher the amorphous phase of the polymer, the higher the ionic conductivity.

The degree of the amorphous phase of the polymer is related to the glass transition temperature. Generally, the lower the glass transition temperature of the polymer, the higher the amorphous phase. The lower the molecular weight of the polymer, the lower the glass transition temperature, but too low glass transition temperature will seriously affect the mechanical stability of the polymer.

Therefore, adjusting the molecular weight of the polymer to adjust its glass transition temperature can ensure that it not only has a high ionic conductivity, but also meets the requirements of mechanical stability in processing. Taking PEO as an example, PEO can be directly bonded with lithium salts by the lone pair electrons of oxygen atoms in the ether-oxygen bond (-COC-), and through the formation and uncoupling of Li-O bonds to achieve lithium ion transport.

Polymer lithium ion mobility

The mobility of lithium ions in the polymer depends on several factors, including the amount of solvated lithium ions in the polymer body, the uniformity of solid polymer electrolyte, the dissociation energy of the Li-O bond, and the mobility of the polymer chain.

The amount of solvated lithium ions is usually related to the number of solvated sites in the polymer, and the lower active groups in the general polymerization usually show fewer solvation sites, for example, compared with poly (propylene oxide) or PTMO, PMO shows a higher amount of lithium solvation, in addition, the polymer structure also has a huge impact on the ionic conductivity, such as compared with PTMO, PPO can hold more lithium ions, although they have the same C:O ratio in the polymer chain.

Copolymerization can also effectively enhance lithium solvation sites, such as PEO-PMO solid polymer electrolyte with repeating and alternating units having more solvation sites than a single PEO or PMO unit. In acrylates or carbonate polymers, lithium ions prefer to bond with carbonyl groups, carboxyl groups avoid steric hindrance, and can be combined with lithium ions with weak binding force to facilitate the dissociation of lithium ions.

Molecular dynamics simulation studies show that solid polymer electrolyte with Lewis acid-base active groups can replace PEO solid polymer electrolyte to achieve higher lithium ion transport capacity. Reducing the crystallinity of the polymer and designing the chain with lower molecular weight can improve the transport rate of lithium ions, and can also improve the ionic conductivity by blending polymerization.

Based on these studies, a variety of polymer blends based on PEO blocks, grafts, and hyperbranched copolymers have been explored to improve polymer lithium ion transport rates, typically PEG with very high ionic conductivity (~ 10 -3S /cm) at room temperature.

Unfortunately the lower PEG chains are liquid at room temperature and cannot be used as solid polymer electrolyte, but can be grafted onto the polymer body as solid polymer electrolyte. The disadvantage of solid polymer electrolyte is that anion migration occurs during the process of ion conduction, which seriously affects the migration of lithium ions. In addition, in order to ensure the migration rate of lithium ions, the solid polymer electrolyte membrane is usually made as thin as possible.

In order to overcome the above-mentioned problems of solid polymer electrolyte, the researchers enhance bulk ion conduction by introducing inorganic ceramic fillers into the polymer body, that is, the polymer composite solid battery electrolyte. The ion transport mechanism becomes more complex in the polymer composite electrolyte. The use of inactive fillers in polymer composite electrolytes helps to absorb anions and reduce the effect of anion migration.

Solid polymer electrolyte for solid-state batteries

Polyether: mainly including PEO, PMO, PPO, of which PEO is the most studied and the earliest solid polymer electrolyte used for all-solid-state batteries, the main disadvantage is that the ionic conductivity at room temperature is very low (3.8×10-8S/cm). The interaction between lithium ions and oxygen atoms on the PEO backbone can be helped by the addition of plasticizers/ceramic fillers such as LATP, complexing additives glycoldimethyl ether or carbonate.

According to reports, PEO and LAGP combined with Li/LFP at 55°C to form a half battery, the initial discharge capacity at 0.2C rate is 137.6mAh/g, and the capacity retention rate is 96.5% after 100 cycles. Another disadvantage of PEO is that the electrochemical window is small, and PMO and PPO have higher electrochemical stability than PEO.

Polycarbonate:

Studies have shown that PC can exhibit higher lithium ion transport and better electrochemical stability compared to polyether. In polycarbonate, lithium ions are preferally combined with carbonyl oxygen, and the strength of the interaction between carboxylic groups and lithium ions is weaker than that of Li-O in polyethers.

Adding LiTFSI, LiFSI or LiBF4 lithium salts to PC can improve the ionic conductivity, and more salt load also helps to improve the ionic conductivity. Moreover, there is no phenomenon that adding lithium salt to polyether will reduce the mechanical properties of SE.

Ionic polymers:

Ionic polymers have been known as solid ionic conductors since 1998. Ionic polymers can be further upgraded to dense charging 3D networks of polyion networks (pins) during charging. Compared to conventional ionic polymers, PIN’s multiple active charge carriers have higher ionic conductivity than single-ion IP. The ionic conductivity of the reported ionic polymer can reach 3.8×10-4S/cm, and the electrochemical window can reach more than 4V.

Block copolymer (BCP) :

The ion conduction path of BCP is more complex, with the characteristics of three-dimensional ion conduction channels, and some BCP polymers have an ionic conductivity of 2.1×10-4S/cm at 28°C, with good mechanical properties and a wide electrochemical window (5.6V). This SE can be used to match LFP or NMC cathode materials.

Ionic block copolymer (ion BCP) :

As the name suggests, ionic block copolymer is the polymerization of ionic monomers or ionic components onto the chain, and studies have shown that the ion BCP of tbS-b-EP-b-MS-b-EP-b-tbS is 0.1×10-3S/cm at 28°C, and the electrochemical window performance is slightly worse.

Polyeutectic SE(PEE) :

As a new solid polymer electrolyte, they are named polyeutectic electrolytes (but they are not solid, similar to IL electrolytes), in which the alkali metal salts LiTFSI/NaTFSI or KTFSI and n-isopropylacrylamide molecules (polymer precursors) form a eutectic electrolyte during polymerization.

The synthesized PEE showed high room temperature ionic conductivity and a wide electrochemical window. These electrolytes are not volatile, have a wide voltage window, and, more importantly, they are relatively inexpensive to synthesize and involve environmentally friendly preparation routes. This type of material can be coupled with polymer bodies such as PEO and other polymers to prepare SCEs for alkali metal ASSBs.

Polymer composite electrolyte (SCE) :

SCE consists of a flexible polymer body, dissolved lithium salts, and rigid (active or inactive) inorganic fillers. This type of material is now very popular because of their high mechanical strength, as most ceramic solids combine with polymer mixtures to produce strong hybrid solid electrolytes that combine the properties of both ceramic solid materials and polymer materials. SCE has been a popular choice for various lithium metal batteries, and several solutions are being investigated for use in lithium-ion batteries with different chemical systems.

Recently, in situ polymer ceramic SCE synthesis technology has been reported. By preparing PEO/PEG-3LGPS composites, the ionic conductivity of the materials can be effectively improved, which can reach 9.83×10-4S/cm at room temperature, and the lithium ion mobility coefficient is 0.68.

Another interesting type of composite electrolytes as quasi-solid electrolytes consists of solid polymers and non-flammable liquid electrolytes, they are called gel polymer electrolytes (SGPs), they combine the advantages of solid and liquid electrolyte properties, as well as high ionic conductivity.

Stability of solid polymer electrolyte

Chemical stability
Studying and understanding the chemical stability of SE is essential to ensure the long-term healthy operation of batteries. Air, humidity, temperature and other atmospheric conditions will greatly affect the stability of SE materials. Therefore, before SE can be used in batteries on a large scale, the chemical behavior of solid electrolytes under ambient conditions needs to be studied.

When the temperature is increased, the oxide type inorganic solid electrolyte will undergo H+/Li+ ion exchange, while the sulfide-based solid electrolyte is sensitive to air, and the chemical stability of the polymer SE is second only to NASICON type SE.

However, it has been found that regional oxidation occurs as a widely used ether polymer. On the other hand, when the voltage rises from 2.95 to 3.53V (compared with Li/Li+), severe oxidative degradation reaction occurs, the chemical stability of SE can be significantly improved by adding ceramic nano-fillers, additives, and changing the chemical composition of polymers and composites.

The most complex stability is the electrode/electrolyte interface stability. The interfaces to be considered in the production of solid state batteries mainly include positive /SE, negative /SE and other interfaces. According to the research results reported so far, no effective method has been found to effectively solve the solid-solid interface.

The solid-solid interface problem is relatively easy to solve in solid polymer electrolyte all-solid-state batteries. Thanks to the polymer processability, the customized hybrid SEs made from P(DVF-HFP)/LATP/P(VDF-HFP) can be in close contact with both electrodes and increase the ionic conductivity to 0.763mS/cm, with 4.7V voltage stability and excellent thermal performance stability up to 460°C. The battery showed a reversible capacity of 145.4mAh/G in a lithium cobaltate half battery at 0.1C magnification.

Electrochemical stability
The common electrochemical potential window of polymeric SE is slightly lower, although the maximum reported is ~5V, but most of them are lower than 4V. For ceramics, it is ~9V or higher. For SCE, if ceramic packing is used, the window can be expanded to more than 5V. Recent studies have shown that dendrites can be generated in PEO all-solid-state batteries and lead to eventual battery failure. Therefore, for solid polymer electrolyte, the electrochemical window and ionic conductivity cannot be taken into account simultaneously.

Mechanical stability
Mechanical stability studies (those related to elasticity, crack resistance, brittleness, etc.) have been largely ignored for a long time. Studies have shown that mechanical property parameters are important for determining dendrite formation/crack formation inside the battery, but the mechanical property of the battery is still not the main factor determining dendrite formation or crack expansion during charging/discharging.

Poor contact between SE/ electrodes can also lead to dendrite formation. Repeated volume changes during the battery cycle can cause deformation, poor contact, and mechanical instability, ultimately leading to battery failure.

Thermal stability
Another important parameter for all-solid-state batteries is to consider thermal stability to avoid safety incidents such as explosions under overcharging or abuse conditions. A comparative study of thermal runaway between solid polymer electrolyte and liquid lithium batteries showed that both solid and liquid batteries had low thermal runaway temperatures at higher state of charge (SOC).

It is very important to study the thermal stability of polymer/polymer composite solid-state electrolyte solid state battery, usually the thermal stability of solid polymer electrolyte all-solid state battery is about 200℃, the study found that the nano fillers in SCE can effectively improve the mechanical properties, not only that, they can also improve the thermal stability of SCE.

Solid polymer electrolyte and battery manufacturing technology

Most inorganic SE (except sulfides) have poor contact with the electrode, whereas for solid polymer electrolyte, this contact problem is not as obvious due to the “soft” nature of the polymer. In general, the polymer SE will undergo chemical or electrochemical reactions between the positive electrode and solid polymer electrolyte, which may form a passivation layer to prevent ions from shuttling between the electrodes, and it is necessary to coat the passivation layer on the positive electrode to modify the surface of the positive electrode material.

However, the current high-end coating technology is not economical for large-scale production of synthetic batteries, which poses a greater challenge for large-scale low-cost production of all-solid-state batteries.

Similarly, the negative side needs to prevent dendrites from forming and causing short circuits. For solid polymer electrolyte, it is “too soft” to prevent the formation of dendrites. Thus, an artificial SEI layer can be induced, or the composition of solid polymer electrolyte can be modified to avoid dendrite growth.

However, for the negative electrode of lithium metal, it is more difficult, and no effective means have been found up to now. At present, the all-solid-state production technology of thin film deposition under vacuum is not suitable for large-scale commercialization due to its high price.

For sulfide and oxide all-solid-state batteries, especially oxide all-solid-state batteries do not have a good solution, in contrast, polymer solid-state battery production process is basically compatible with the existing lithium-ion battery production process, with a certain industrial basis.

In order to compromise on the process, a compromise measure has been taken for the oxide, that is, on the existing battery production process, a small amount of electrolyte is added to the battery to reduce the interface resistance problem caused by SE.

This technology is often called mixed solid-liquid battery, but due to the existence of liquid or polymer substances, it still fails to solve the safety of the battery, and at the same time, with the consumption of liquid electrolytes, The interface resistance will become more and more obvious, which will eventually seriously affect the battery life.

Recently, ASSBs has received a lot of attention from researchers and engineers, and to date, nearly 40 companies and startups around the world have invested heavily in the field of solid-state batteries. Although ASSBs are seen as an ideal energy storage unit for EV battery cell, there are still scientific and engineering issues mentioned above that need to be addressed.