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. Polymer electrolytes are widely studied for use in next‑generation lithium‑ion and solid‑state batteries because they combine ion transport with mechanical stability, enabling safer alternatives to conventional liquid electrolytes.

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: (1) gel polymer electrolyte, (2) solid-state polymer electrolyte, (3) plasticized polymer electrolyte, and (4) 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(ethylene oxide) Poly(vinyl alcohol) Poly(methyl methacrylate) Poly(caprolactone) Poly(chitosan) Poly(vinyl pyrrolidone) Poly(vinyl chloride) Poly(vinylidene fluoride) Poly(imide) Many of these polymers have other applications. The structures of several of these polymers are shown in the adjacent image. Showcases several of these polymers. Other types of polymers capable of ion conduction include polymeric ions, which incorporate either an oxidized (for anion transport) or reduced element of the polymer main chain through a process called chemical doping. Chemical doping makes these polymers behave as either n-type or p-type semiconductors.

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 (and swell accordingly) 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(ethylene oxide) (PEO) 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(vinylidene fluoride) (PVDF) 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.

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