Polymer electrolyte is a kind of functional polymer material with ionic conductivity in the solid state, which is formed by complexing strong polar polymer and metal salt through Lewis acid-base reaction mode with polymer as the matrix. The polymer matrix should contain functional groups that have a solvation effect on the electrolyte salt, and have an association with the polar organic molecules used for plasticization to ensure that the polymer electrolyte exhibits solid characteristics. Compared with inorganic solid electrolyte materials, polymer electrolytes have the flexibility of polymer materials, good film-forming properties, viscoelasticity, stability, light weight, low cost, and good electrochemical stability. The polymer electrolyte membrane is the core material of polymer lithium ion batteries, which has the dual role of isolating the positive and negative electrodes from the ion transport medium in the battery. The key technology of polymer lithium ion battery lies in the composition of polymer electrolyte membrane, condensed structure and membrane preparation method. The polymer electrolyte membrane used as a polymer lithium ion battery not only requires high ionic conductivity, but also requires appropriate mechanical strength, flexibility, condensed structure favorable for ion transport, and chemical and electrochemical stability. Since there is no free-flowing electrolyte in the polymer electrolyte, the polymer lithium ion battery completely eliminates the risk of battery leakage.
Before the 1970s, the research on solid electrolytes mainly focused on inorganic substances. In 1973, Fenton et al. first discovered that polyethylene oxide (PEO) "dissolved" part of the alkali metal salt to form a polymer salt complex. In 1975, Wright et al. first reported that the complex system had good ionic conductivity. In 1979, Armand et al. reported that the alkali metal salt complex system of PEO has good film-forming properties, and the ion conductivity can reach 10-5S/cm at 40~60℃, which can be used as an electrolyte for lithium batteries.
As a polymer electrolyte, it should have the following advantages.
(1) Inhibit dendrite growth
Traditional battery separators can also be used as ion conductive media in rechargeable lithium batteries. Unfortunately, this kind of separator contains a large number of interconnected pores filled with liquid electrolyte, which can form a sufficiently large channel between the positive electrode and the negative electrode, thus promoting the formation and growth of lithium dendrites during the charging process. These lithium dendrites reduce the cycle efficiency of the battery and eventually cause an internal short circuit of the battery. The use of continuous or non-porous polymer membranes that can only provide little or no continuous electrolyte free channels is an effective method to suppress the problem of dendrite growth.
(2) Enhance the battery's ability to withstand changes in electrode volume during cycling. Polymer electrolytes are more flexible than traditional inorganic glass or ceramic electrolytes, and can easily adapt to the volume changes of positive and negative electrodes during charging and discharging.
(3) Reduce the reactivity of the electrode material and the liquid electrolyte. It is generally believed that any solvent is thermodynamically unstable for metal lithium and even carbon anodes. Due to its solid-like properties and low liquid content, the reactivity of polymer electrolytes is lower than that of liquid electrolytes.
(4) Higher security
The solid structure of the polymer electrolyte battery is more resistant to impact, vibration and mechanical deformation. In addition, because there is no or very little liquid component in the polymer electrolyte, the polymer battery can be packaged in a evacuated flat plastic bag instead of a rigid metal container that is susceptible to corrosion.
(5) Higher shape flexibility and production integration
Due to the need for smaller and lighter batteries, the shape of the battery is becoming an important factor that must be considered in battery design. In this respect, thin-film polymer electrolyte batteries have a very large market. Another feature related to polymer electrolyte batteries is the integration of production: all components of the battery, including the electrolyte and the positive and negative electrodes, can be automatically pressed into a thin sheet through the well-developed coating technology. In an inorganic solid electrolyte, the transport of ions is usually achieved by jumping between fixed positions in the solid electrolyte, and these fixed positions generally do not change significantly over time. In polymer electrolytes, the polymer bulk material for ion conduction is not as rigid as the defect crystals of traditional inorganic solid electrolytes, and ion transport actually occurs through the movement and rearrangement of the polymer backbone. Therefore, the ion conductive polymer is actually a special electrolyte between the liquid (and melt) electrolyte and the solid (defect crystal) electrolyte. In fact, the transport of ions in the polymer electrolyte is more like in a liquid medium.
The polymer electrolyte used in rechargeable lithium batteries must meet some basic requirements, including the following points.
① Ionic conductivity. Generally, the ionic conductivity of the liquid electrolyte used for lithium batteries or lithium ion batteries used at room temperature is 10-3~10-2S/cm. In order to achieve the conductivity level of a liquid electrolyte system capable of discharging at a current density of several milliamperes/cm², the room temperature conductivity of the polymer electrolyte must be close to or exceed 10-3S/cm.
②Number of migrations. The migration number of lithium ions in the electrolyte system is preferably close to 1. Some electrolyte systems, whether liquid or polymer, have an ion migration number less than 0.5. The charge conducted by the movement of lithium ions is less than one-half. Anions and ion pairs are important charge transport tools. A large ion migration number can reduce the concentration polarization of the electrolyte during charging and discharging, and thus can provide a greater power density.
③Chemical, thermal and electrochemical stability. The electrolyte membrane is used between the positive electrode and the negative electrode. The requirements for its chemical stability are: it must be ensured that no side reactions occur when the electrode material is in direct contact with the electrolyte. In order to have a proper temperature operating range, the polymer electrolyte must also have good thermal stability. The electrochemical stability range of the polymer electrolyte must be able to coexist stably with lithium and cathode materials such as TiS2, V6O13, LiCoO2, LiNiO2 and LiMn2O4 in the range from 0V to 4.5V (relative to metallic lithium).
④ Mechanical strength. When the battery shifts from the laboratory to the pilot test and real production, processability is the most important factor among all the issues that must be considered. Although many electrolyte systems can be made into self-supporting films and achieve various good electrochemical properties, their mechanical strength still needs to be further improved to meet the processing requirements of traditional large-scale membrane production processes.
There are many polymer electrolyte systems, such as PEO, PAN, PMMA, PVC and PVdF, but they can be roughly divided into two categories, namely, pure solid polymer electrolytes and plasticized or gelled polymer electrolyte systems. Pure solid polymer electrolytes are made by dissolving lithium salts such as LiClO4, LiBF4, LiPF6, LiAsF6, LiCF3SO3, LiN(CF3SO2)2 or LiC(CF3SO2)3 in solid solvents such as PEO and PPO. These polymer electrolyte systems are usually made into thin-film electrolytes through coating and solvent volatilization, including grafted polyether, polysiloxane and polyphosphazene skeleton copolymers. The ion conduction mechanism of this type of polymer electrolyte is closely related to the movement of polymer chain segments. Compared with pure solid polymer electrolytes, gelled (type) electrolytes have the characteristics of higher room temperature ionic conductivity and poor mechanical properties. Gel-type polymer electrolytes are usually formed by mixing a larger amount of liquid plasticizer/solvent with a stable polymer capable of forming a polymer bulk structure. In order to improve the mechanical properties of the gel polymer electrolyte, components that can be crosslinked or cured by heat are usually added to the gel polymer electrolyte formulation. The gel state is a special state, which is neither liquid nor solid. Describing a gel is much easier than defining a gel, because an accurate definition of a gel must involve the problem of molecular structure and connectivity methods. Generally, a polymer gel is defined as a system composed of a polymer network that is swollen by a solvent. We must be clear that the solvent is dissolved in the polymer and not in other ways. Due to the unique hybrid network structure, the gel always has both solid viscoelasticity and liquid diffusion and transport properties. This dual nature makes the gel have many important uses including polymer electrolytes.
Gels can be obtained through chemical or physical cross-linking processes. When gelation occurs, a dilute or more viscous polymer solution is transformed into a system with infinite viscosity, that is, a gel.
According to the composition and morphology of polymer electrolytes, we can roughly divide them into pure solid polymer electrolytes without plasticizers, gel polymer electrolytes with plasticizers, and polymer electrolytes with nano ceramic powder additives(With or without plasticizer) and porous gel polymer "electrolyte". According to the relative content of salt and polymer in the polymer electrolyte, polymer electrolytes are divided into two types of polymer electrolytes, "polymer-in-salt" and "salt-in-polymer".