Alternative lithium secondary batteries(Here is a company to introduce to you – TYCORUN ENERGY) on the market mainly use liquid electrolytes and use 10-20 m thick separators to promote the movement of lithium ions. When polymer electrolytes are used instead of liquid electrolytes, it is easier to make compact batteries because metal packaging is not necessary. Polymer electrolytes mainly include solid polymer electrolytes, gel polymer electrolytes and polyelectrolytes. The migration of lithium ions in polymer electrolytes depends on the segmental motion of polymer chains, while gel polymer electrolytes are affected by the liquid electrolyte mixed into the polymer and their ionic conductivity. Among polymer electrolytes, gel polymer electrolytes have high ionic conductivity and mechanical strength at room temperature. The properties, applications and development of different polymer electrolytes are described below.
1. gel polymer electrolyte
Gel polymer electrolyte consists of polymer, organic solvent and lithium salt. Gel polymer electrolytes are prepared by mixing an organic electrolyte with a solid polymer matrix. Although in the form of solid-state membranes, the ionic conductivity of gel polymer electrolytes can reach 10-3 S/cm because the electrolytes are confined in the polymer chains. Gel polymer electrolytes combine the advantages of solid electrolytes and liquid electrolytes and are being actively studied for use in lithium secondary batteries65.60. As shown in Figure 1, the representative polymers of the gel polymer electrolyte matrix are polyacrylonitrile (polyacrlonitrile), polyvinylidene fluoride (poly(vinylidene fluoride), PVdF), poly(methyl methacrylate) ), PMMA) and polyethylene oxide PEO.
2. Polyacrylonitrile system
The highly polar CN bond side chains in PAN attract Li ions and solvents, making it suitable as a matrix for gel polymer electrolyte polymers. Generally speaking, PAN is impregnated in LiPF, prepared in electrolytes based on organic solvents such as EC and PC, and the ionic conductivity is as high as 10-3 S/cm at room temperature. The PAN-based gel polymer electrolyte has a wide potential window, Strong mechanical properties and weak chemical reactivity with cathode materials. Electrolyte preparation, described later, involves dissolving PAN into EC or PC at up to 100 °C. After the PAN and lithium salt were completely dissolved, the solution was cast into shape and then cooled at room temperature.
3. Polyvinylidene fluoride system
Gel polymer electrolytes composed of PVdF polymers can be dissolved at high temperature and can undergo phase separation and crystallization during cooling, resulting in physical gelation. Since the PVdF-based gel polymer electrolyte membrane forms micropores during phase separation, the liquid electrolyte in the micropores enables it to have high ionic conductivity. P(VdF-co-HFP), a copolymer of vinylidene fluoride and hexachloropropylene, was used as the matrix for the gel polymer electrolyte.
These electrolytes are obtained by preparing porous membranes, which are prepared by extracting plasticizers from polymer membranes. In this case, the electrolyte membrane was prepared under air and activated only in the last step.
4. Polymethyl methacrylate system
PMMA gel polymer electrolyte is prepared by the polymerization of MMA monomer and dimethacrylic acid difunctional monomer. A PMMA-based gel polymer electrolyte sandwiched in a transparent film can be used in electrochromic elements. The chemically cross-linked gel polymer electrolyte prepared from a non-aqueous electrolyte composed of LiClO4 and EC/PC has an ionic conductivity of 10-3 S/cm at room temperature and an electrochemical window of 4.5 V to lithium, among other things , the interfacial impedance of lithium metal remains constant even after a period of time.
5. Polyoxyethylene system
Polyethylene oxide gel electrolyte is composed of PEO polymer matrix with EO structure on the main chain or side chain. For polymer matrices with EO structures on the main chain, the hydroxyl groups at the end of PEO are chemically cross-linked using isocyanates. Methacrylates and acrylates are common derivatives for gel polymer electrolytes consisting of a polymer matrix with ethylene oxide groups on the side chains. The side chains can also contain polyurethane, polyphosphonitrile or grafted PEO groups with a molecular weight of 2000.
In gel polymer electrolytes, ions migrate in the liquid medium, and the polymer matrix is used to maintain the mechanical strength of the film and store the liquid. Gel polymer electrolytes exhibit different mechanical strengths depending on the liquid electrolyte content. As mentioned before, the ionic conductivity of gel polymer electrolytes is close to that of liquid electrolytes. Gel polymer electrolytes can be divided into two categories according to whether the cross-linking is physical or chemical. When the polymer segments are intertwined with each other or part of the polymer segments are cross-linked in the molecular orientation, the physically cross-linked gel polymer electrolyte exhibits a physically cross-linked structure. This electrolyte acquires the ability to migrate due to the unwinding of polymer chains upon heating and the transformation into a gel state upon cooling. Taking advantage of this property, the liquid electrolyte can be injected into the battery before being cooled into a gel. However, gel polymer electrolytes flow at high temperatures and are prone to leaks. Among the physically crosslinked gel polymer electrolytes, gel polymer electrolytes composed of PVdF-HFP copolymers of vinylidene fluoride and hexafluoropropylene have been widely studied.
To overcome the disadvantage of poor physical cross-linking mechanical properties, Li-ion polymer batteries are fabricated by coating polyolefin separators or electrodes with gel polymer electrolytes. This lithium-ion polymer battery has the same structure as a lithium-ion battery. The polymers used for physical gelation are PEO, PAN, PVdF and PMMA. The performance of lithium-ion polymer batteries depends on the thickness of the polyolefin separator coating. The gel coating layer on the porous film makes up for the defect of poor mechanical strength of the electrolyte, improves the bonding performance of the electrode, and improves the safety of the battery. Due to the good electrode-electrolyte interface contact, the cell can be encapsulated with an aluminum encapsulation bag. Meanwhile, the cylindrical battery maintains good interfacial contact through the use of winding pressure and metal casing. Recently, low-melting PEs have been dispersed into gel polymer electrolytes as fuses. These particles melt at around 100 °C, increasing the impedance and greatly improving the battery performance. On the other hand, structural changes in chemically crosslinked gel polymer electrolytes are more difficult because the network structure is based on chemical bonds rather than van der Waals forces. Battery performance may also be affected by non-reactive monomers or crosslinkers. In order to manufacture a gel polymer lithium-ion battery using chemical crosslinking, it is necessary to dissolve a polymer precursor capable of chemical crosslinking into the electrolyte and inject it into the battery. Uniform gelation of the electrolyte is then achieved by thermal polymerization. The battery fabrication process is shown in Figure 2. Polymer batteries fabricated by this method show similar performance to lithium-ion batteries and are used in a variety of mobile devices. Thanks to the complete use of gel polymer electrolytes, there is no risk of leakage in such a battery, even when aluminum foil is used as the packaging material.
6. Polyelectrolytes: Single Ion Conductors
Polyelectrolytes are conductive substances obtained by dissociating cations and anions in polymers. Because cations and anions can migrate independently, they are also known as single-ion conductors. In polymer electrolytes containing lithium salts, dissociated ions do not migrate by reacting with polymer segments. This indicates that the cation migration number for cation conductivity is generally below 0.5. When used in a secondary battery, both lithium ions and opposite anions migrate during charge and discharge. Anions will accumulate on the surface of the electrode active material, and lithium ions will flow between the two electrodes. This leads to concentration polarization between the poles and an increase in electrolyte impedance with time. In polyelectrolytes, the lithium cation migration number is close to 1.0 due to the lack of motion of anions. When the polyelectrolyte is applied to a secondary battery, a stable discharge current can be obtained because concentration polarization does not occur and the impedance remains constant over time.
For polyelectrolytes containing lithium cations, the anions are immobilized in the polymer segments. Molecular design should promote the degree of dissociation and provide a migration path for lithium ions. To improve the ionic conductivity of polyelectrolytes containing anions covalently linked to the polymer chains, weakening of ion pair formation promotes dissociation. This can be achieved by reducing the charge density of cations or by using substitutes to limit the use of anions. PEO segments containing repeating EO units or ethylene oxide and other aldehyde chains are used as migration pathways for lithium ions. In addition to incorporating polymers into the alkaloid chain, polyelectrolytes can be mixed with polyethers. Representative polyelectrolytes developed so far are shown in Figure 3.
Among the networked EO-PO copolymers, the polymers (1 and 2) containing the fluoroalkanesulfonamide structure have a high degree of dissociation and thus high ionic conductivity. Here, the ionic conductivity is affected by the spacing between the anions. Polyelectrolyte 1 has a small ionic conductivity due to the short distance between anions, while polyelectrolyte 2 has an ionic conductivity of 10-6 S/cm at room temperature. The polyelectrolyte 3 containing benzene sulfonate was not sufficiently decomposed, resulting in low ionic conductivity at room temperature. The ionic conductivities of polyelectrolytes 4, 5 and 6 containing halothanesulfonamide and aldehyde chains were higher than 10-6 S/cm. According to percolation theory calculations, we found that the polyelectrolyte should contain an ionic group and an oligomeric aldehyde chain on the side chain as an ionic conduction path. In other words, structures containing oligoethers and relatively unstable cations on the side chains are suitable polyelectrolytes. The polyelectrolyte 7 in which the desired salt group was introduced to replace the butyl group at 2 and 6 had an ionic conductivity of 10-6~10-5 S/cm at room temperature. However, most polyelectrolytes have low ionic conductivity because of the strong interaction between cations and anions and the low ion concentration. Ionic conductivity can be improved by adding highly polar plasticizers or small amounts of lithium salts
