Solid electrolytes have many advantages over liquid electrolytes. For example, they can mitigate electrode deformation during charging and discharging, improving safety. They also have excellent stability, are easy to process, and the growth of lithium dendrites can be minimized in solvent-free solid polymer electrolytes.
Research on polymer electrolytes began as early as 1973, when Fenton et al. discovered that polyethylene oxide (PEO) complexes with alkali metals could conduct ions. Since then, polymer electrolytes have attracted considerable attention.
In 1978, Dr. Armand predicted that PEO-based solid-state polymer electrolytes might be used as electrolytes for batteries.
Over the next two decades, researchers devoted tremendous efforts to studying the mechanism of ion conduction and the physicochemical properties of the electrolyte-electrode boundary in the battery, and made good progress.
Lithium-ion batteries using solid polymer electrolytes can prevent leakage problems associated with liquid electrolytes.
Polymers are easy to process and can be miniaturized. Due to their high plasticity, polymers can also be used to create thin-film batteries. Different battery structures can be fabricated using polymer electrolytes to meet various application requirements. Furthermore, polymer electrolytes offer higher chemical, electrochemical, and thermal stability compared to liquid electrolytes, with fewer side reactions with the electrodes and a wider operating temperature range. The flexibility of polymer electrolytes can buffer volume changes in the electrodes during charge and discharge, stabilizing the battery structure. Therefore, after the commercialization of liquid-ion batteries, lithium-ion battery technology based on polymer electrolytes will rapidly develop and achieve successful commercialization.
There are many methods for classifying polymer electrolytes, and the standards vary. Currently, solid polymer electrolytes are mainly distinguished according to the type of polymer used, such as the most famous polyether-based polyethylene oxide (PEO), as well as polymethyl methacrylate (PMMA) and polyacrylonitrile (PAN). Generally speaking, polymer electrolytes need to meet the following conditions to be practically used in lithium-ion batteries.
High ionic conductivity
Considerable lithium-ion transference number
Good mechanical strength
Wide electrochemical window
Excellent chemical and thermal stability
In current polymer electrolyte systems, polymers exhibit significant crystallinity at room temperature, which explains why the conductivity of solid polymer electrolytes at room temperature is much lower than that of liquid electrolytes. Most crystals in polymers are spherulites, with amorphous regions between them. It is generally believed that lithium-ion conduction primarily occurs in these amorphous regions.
Therefore, understanding the phase structure of polymers is helpful for studying the lithium-ion conduction mechanism.
For binary polymer electrolyte systems, the phase structure mainly consists of two types: crystalline regions and amorphous regions. The formation of crystalline regions is kinetically driven and directly related to specific preparation conditions and time. Strictly speaking, due to the presence of crystalline regions in the polymer system, and the significant variation of these regions with different conditions, comparing the conductivity of different types of polymer electrolytes is not very scientific. However, under certain conditions, if the growth of crystalline regions is slow and the deviation in ionic conductivity is within an acceptable range, comparing conductivity is acceptable. This is why we often compare different results.
Since the growth of spherulites in the polymer is time-dependent, the ionic conductivity at temperatures below the polymer's melting point is also time-dependent. Furthermore, the lithium-ion conductivity of polymer electrolytes is related to the heating rate, cooling rate, and relaxation time. For example, a longer relaxation time results in a more complete polymer crystal structure and higher crystallinity, leading to a gradual decrease in ionic conductivity to a minimum with increasing relaxation time. Similarly, a slower cooling rate results in more complete crystallization, and the corresponding ionic conductivity will also gradually decrease to a minimum.
Taking the binary solid polymer electrolyte of PEO and LiCIO4 as an example, this structure contains multiple phase structures. First, LiClO4 and PEO can form various complexes, including PEO6-LiCIO4, PEO3-LiCIO4, PEO2-LiCIO4, and PEO-LiClO4. Among them, when O:Li = 10:1, PEO6-LiCIO4 can form a eutectic with PEO, with a melting point of 50℃. In addition, when the temperature is raised to 160℃, a large eutectic can be formed. During the cooling process, the large eutectic will produce three different types of spherulites: the first type melts above 120℃ and has a high salt content; the second type melts between 45 and 60℃, has a low salt content, and forms slowly; the third type has a melting point slightly lower than the host polymer and forms more rapidly. Research and analysis suggest that: the first type of spherulite is likely PEO3-LiCIO4; the second type may be a mixture of PEO-LiCIO4 and PEO3-LiCIO4 complexes; and the third type corresponds to PEO itself. Furthermore, the lithium salt content and the heat treatment process can both lead to structural changes.
Polymer electrolytes are a class of functional polymer materials with high ionic conductivity, formed by complexation reactions between polymers and metal salts using polymers as the matrix. Depending on the polymer matrix, common polymer electrolytes include PEO-based polymer electrolytes, PVDE-based polymer electrolytes, PMMA-based polymer electrolytes, and others. Unlike inorganic solid-state electrolytes, polymer electrolytes are lightweight, elastic, and stable. Like inorganic solid-state electrolytes, polymer electrolytes not only conduct ions in lithium-ion batteries but also act as battery separators. Polymer electrolytes mainly have the following advantages:
It can effectively solve the problem of lithium dendrite formation in lithium-ion batteries
It can adapt well to the deformation during the charging and discharging process of lithium-ion batteries
It can reduce or even eliminate the chemical reaction between the electrolyte and electrode materials in lithium-ion batteries
It has high safety performance
The complexes formed by different lithium salts (including LBF4, LIPF6, LiCFSO4, and LiASF6) with PEO are basically similar to those formed by LiCIO4, meaning that the type of lithium salt has no direct impact on the type of complex formed with PEO. Specifically, LiBF can form two complexes with PEO: PEO4-LIBF and PEO,S-LiBF. When the O/Li ratio is between 16 and 20, PEO2.5-LIBF4 can form a eutectic with PEO. LPF6 can also form two complexes with PEO: PEO6-LiPF6 and PEO:-LiPF6. The two complexes formed by LiASF6 with PEO are similar to those of LiPF6, but with relatively higher melting points. Large anion lithium salts can also form complexes with PEO, but the kinetics are much slower. Furthermore, the pressure also affects crystal growth to some extent. Higher pressure promotes spherulite growth, reduces the amorphous region, and correspondingly decreases lithium-ion conductivity.