Introduction to polymer electrolytes

Introduction to polymer electrolytes

At present, commercial liquid lithium ion batteries generally use organic carbonate solvents (ethylene carbonate, propylene carbonate, dimethyl carbonate, etc.) to dissolve lithium salt (lithium hexafluorophosphate, lithium bisfluorooxalate, lithium trifluoromethanesulfonimide, etc.) to form a liquid electrolyte. Therefore, there are potential safety hazards such as easy leakage, inflammability, and explosion, which greatly limits the further large-scale application of this type of liquid electrolyte.

Polymer electrolyte is a kind of polymer film that can transmit lithium ions and can effectively isolate the short circuit between the positive and negative electrodes. Polymer electrolytes can be divided into two categories according to their composition and morphology: solid polymer electrolytes (SPE) and gel polymer electrolytes (GPE). The solid polymer electrolyte does not contain any organic solvents, so it is extremely safe when used in lithium batteries, and is especially suitable for high energy density and high safety power batteries. In 1973, Wright et al. discovered that polyethylene oxide (PEO) has ion conductivity when doped with alkali metal salts. Later, Armand et al. found that the polyethylene oxide/lithium salt system can be used in electrochemical devices such as batteries. This major discovery opened up a new direction for the development of high-performance lithium batteries. Solid polymer electrolytes, according to their polymer matrix, mainly include the following three categories: polyethylene oxide, aliphatic polycarbonate and silicon-based polymers.

Polyethylene oxide solid polymer electrolyte is the earliest and most studied solid polymer electrolyte system. Its advantages are: good compatibility with lithium negative electrodes and high chemical stability. The disadvantage is: high crystallinity, resulting in low room temperature lithium ion conductivity, so it needs to operate at a relatively high temperature (60~80℃); the electrochemical stability window is narrow (≤4V), and high-voltage cathode materials cannot be used. The overall mass and energy density of the assembled solid-state battery is low; the dimensional thermal stability is not good (melting point is 55~64℃); the mechanical strength is low (≤10MPa).

The study of aliphatic polycarbonate solid polymer electrolyte is relatively late, but it is currently the most popular solid polymer electrolyte system. Aliphatic polycarbonate solid polymer electrolytes mainly include four categories: polytrimethylene carbonate, polyvinyl carbonate, polypropylene carbonate and polyvinylene carbonate. Compared with the PEO system with high crystallinity, its advantages are: the aliphatic polycarbonate has an amorphous structure, the polymer chain has high flexibility, is more conducive to the transmission of lithium ions, and the room temperature ion conductivity is higher; in addition, the electrochemical stability window is higher (up to 4.45V); the dimensional thermal stability is good (≥150°C). The main problem to be solved is: compatibility and stability with alkaline electrode materials.

The silicon-based polymer solid polymer electrolyte has good dimensional thermal stability and a high electrochemical stability window. However, since the main chain is a polysiloxane chain, the room temperature ionic conductivity is low. If polysiloxane is inserted in the side chain position, its room temperature ionic conductivity will be greatly improved due to the influence of steric hindrance and chain flexibility.

The room temperature ionic conductivity of the solid polymer electrolyte is still very low compared to that of the liquid electrolyte, and the high interfacial impedance and poor compatibility between the solid electrolyte and the positive and negative electrodes are also problems that need to be solved urgently. Gel polymer electrolyte, taking into account the high room temperature ionic conductivity of liquid electrolyte and the high safety of solid polymer electrolyte, is a very promising polymer electrolyte system. Gel polymer electrolyte is mainly composed of three parts: polymer matrix, lithium salt and plasticizer (mainly organic carbonate solvent, a small part is ionic liquid). The matrix of common gel polymer electrolytes is polyacrylate, polyvinylidene fluoride polymer, cyano polymer (polyacrylonitrile, containing cyanoacrylate), polyvinyl alcohol, maleic anhydride-based polymer, etc. Although gel polymer electrolytes have been developed for a long time, the continuous development of power lithium batteries and lithium batteries for smart devices has placed higher and higher performance requirements on gel polymer electrolytes. For example, further develop new polymer matrix materials (biomass materials) to enrich the types and sustainability of polymer electrolytes; develop high-voltage resistant polymer electrolytes, improve the oxidation resistance of polymer electrolytes, and match high-voltage cathode materials; develop multifunctional polymer electrolytes (high electrochemical stability window, good compatibility with lithium negative electrodes), prepare high-voltage lithium metal batteries, and improve battery energy density; develop flame-retardant gel polymer electrolytes to improve their flame retardancy; develop thermal shock-resistant gel polymer electrolytes to improve their safety; develop smart multi-performance polymer electrolytes, such as self-repairing solid electrolytes, to improve their intelligence in special environments and weather; develop flexible and stretchable polymer electrolytes to meet the challenges of wearable smart devices.

In view of the advantages of solid polymer electrolytes and gel polymer electrolytes, a polymer electrolyte system composed of a polymer matrix and a lithium salt is expected to become the mainstream material system for high-energy lithium secondary batteries.