Polymer electrolytes
A polymer electrolyte is a polymer matrix capable of ion conduction. Much like other types of electrolyte—liquid and solid-state—polymer electrolytes aid in movement of charge between the anode and cathode of a cell. The use of polymers as an electrolyte was first demonstrated using dye-sensitized solar cells. The field has expanded since and is now primarily focused on the development of polymer electrolytes with applications in batteries, fuel cells, and membranes.
Molecular design of polymer electrolytes
Generally, polymer electrolytes comprise a polymer which incorporates a highly polar motif capable of electron donation. Performance parameters impact selection of homo- or heterogenous electrolyte. There exist four major types of polymer electrolyte: gel polymer electrolyte, solid-state polymer electrolyte, plasticized polymer electrolyte, and composite polymer electrolyte. The degree of crystallinity of a polymer electrolyte matrix impacts ion mobility and the transport rate. Amorphous regions promote greater percolation of charge in gel and plasticized polymer electrolytes. Crystal defects promote weaker chain-ion interactions.Another key parameter of transport is the temperature dependence of polymer morphology on transport mechanisms by the glass transition temperature. These electrolytes differ from one another in their processing methods and applications where they are to be used. Their properties and morphology can be tuned to that desired of the application they are intended for. A shared structural feature of these polymers is the presence of a heteroatom, namely nitrogen or oxygen, although sulfur has also been demonstrated.
Common polymers
- Poly
- Poly
- Poly
- Poly
- Poly
- Poly
- Poly
- Poly
- Poly
Mechanical properties
The mechanical strength of a polymer electrolyte is an important parameter for its dendrite suppression capabilities. It is theorized that a polymer electrolyte with a shear modulus twice that of metallic lithium should be able to physically suppress dendrite formation. High elastic moduli or yield strengths can similarly decrease the uneven lithium deposition that leads to dendrite formation. Higher shear moduli polymer electrolytes have lower ionic conductivity due to their increased stiffness impeding polymer chain mobility and ion movement. The contrasting relationship between tensile strength and ionic conductivity inspires research into plasticized and composite polymer electrolytes.Types
Gel polymer electrolyte
Gel polymer electrolytes capture solvent constituents and aid in ion transport across the polymer matrix. The gel supports the polymer scaffold. It is noted that amorphous domains of these polymers absorb larger amounts of solvent than do crystalline domains. As a result, ion conduction, which is primarily a diffusion-controlled process, is typically greater across regions of amorphous character than through crystalline domains. The adjacent image illustrates this process. An important aspect of gel electrolytes is the choice of solvent primarily based on their dielectric constants which is noted to impact ion conductivity. Percolation of charge does occur in highly ordered polymer electrolyte, but the number and proximity of amorphous domains is correlated with increased percolation of charge.Gel polymer electrolytes using poly are the most studied due to its compatibility with lithium electrodes. However, the plasticizing of PEO decreases the mechanical strength of these electrolytes. Gel polymer electrolytes that combine PEO with mechanically strong polymers such as poly can benefit from improved mechanical strength while maintaining the good electrochemical properties of PEO. A typical tensile strength for a gel polymer electrolyte is around 0.5 MPa, while typical yield strength and shear strength measurements are around 1 MPa. A typical elastic modulus for a gel polymer electrolyte is 10 MPa, which is two orders of magnitude below that of a typical liquid electrolyte.
Gel polymer electrolytes also shown specific applications for lithium-ion batteries to replace current organic liquid electrolytes. This type of electrolyte has also been shown to be able to be prepared from renewable and degradable polymers while remaining capable of mitigating current issues at the cathode-electrolyte interface.
Solid-state polymer electrolyte
Solid-state polymer electrolyte arises from coordination of an inorganic salt to the polymer matrix. Application of a potential results in ion exchange through coordination, decoordination, and recoordination along the polymer. Performance of the electrochemical cell is influenced by the activity of the salt. The potential between the phases and charge transport through the electrolyte is impacted. Solid-state polymer electrolytes have also been employed in processing of gallium nitride wafers by providing a liquid- and radiation-free method of oxidizing the surface of the gallium nitride wafer to enable easier polishing of the wafer than previous methods.Recent research has focused on characterizing the dynamics of solid polymer electrolytes, including transference number, coordination strength, and conductivity. In SPEs, cations migrate through the electrolyte medium, driven by the electric field between the positive and negative electrodes. This migration is associated with the formation of polymer–salt complexes and is followed by localized motion of polymer segments, as well as inter- and intra-chain ion hopping between coordinating sites. Specifically, ion transport in SPEs can be described as a ligand exchange process within the coordination structure of the cations. Consequently, the coordination structure has a significant impact on the cation's contribution to the total conductivity.
Plasticized polymer electrolyte
Plasticized polymer electrolyte is a polymer matrix with incorporated plasticizers that enhance their ion conductivity by weakening intra- and interchain interactions that compete with ion-polymer interactions. A similar phenomenon to that previously discussed with polymer gel electrolytes is observed with plasticized polymer electrolytes. The addition of plasticizer lowers the glass transition temperature of the polymer and effectively enhances salt dissociation into the polymer matrix which increases the ability of the polymer electrolyte to transport ions. One limitation of plasticizer incorporation is the alteration of the polymer's mechanical properties. Reduction in the crystallinity of the polymer weakens its mechanical strength at room temperature. Plasticizers also modulate properties of polymer electrolytes other than conductivity such as affecting charge/discharge times and enhanced capacity.Composite polymer electrolyte
polymer electrolyte is a polymer matrix that incorporates inorganic fillers that are chemically inert, but with a high dielectric constant to enhance ion conductivity by inhibiting the formation of ion pairs in the polymer matrix. It has been demonstrated that the blending of polymer electrolytes with an inorganic filler affords a composite material with properties exceeding the sum of those of the individual components. In particular, ion conduction in polymer electrolytes is low, but blending with inorganic materials has been shown to enhance the ion mobility and conductivity of the polymer electrolyte. The additional benefit is that the desirable properties of the polymer are maintained, particularly its mechanical strength.Ceramic materials such as SiO2, Al2O3, and TiO2 are popular filler materials that will improve the mechanical properties of the composite electrolyte, increase the lithium-ion transference number, and improve ionic conductivity. The improved conductivity comes from the decreased crystallinity of the material. On their own, these ceramic fillers are brittle and of low dielectric permittivity. Metal-organic framework particles can also be used as a filler material with high surface area and high chemical and thermal stability. 2D boron nitride is a potential filler material due to its high mechanical strength arising from modulation of the electrolyte membrane.