Reports: ND754945-ND7: Crystal Engineering for High Performance Solid Polymer Electrolytes

Christopher Y. Li, Drexel University

Lithium-ion batteries are the systems of choice for portable electronic devices and they dominate our consumer market today. The advantages of a lithium-ion battery include high energy density, flexible and light weight design and long lifespan. However, if lithium metal is used as the anode instead of lithium interaction materials to form a lithium metal battery, even higher power density can be achieved, and this high powder density is particularly critical for applications such as electric cars. However, lithium metal batteries are not viable because that the formation of Li dendrites at the Li/liquid electrolytes interface during charge-discharge processes could lead to explosion hazards. In order to circumvent this problem, one approach is to use SPEs to form lithium polymer batteries (LPBs): it has been found that, due to the high shear modulus of SPE, lithium dendrite formation, and the explosion hazard associated with it, can be avoided., clearly understanding the ion transport mechanism is key to fabricating next generation lithium battery devices.

One “general rule” for designing SPE is to reduce polymer crystallinity by using short PEO molecules whose melting point is much lower than ambient temperature. Since these short PEO molecules are essentially liquid with poor mechanical properties, many studies have been done to improve the latter, including using short PEO molecules as the pendant groups on an amorphous polymer backbone and creating a short PEO-containing, crosslinked polymer network. Nanoparticles have been used to reinforce SPE. Another promising approach is using block copolymers (BCP). Upon BCP phase separates, if we confine ions in one phase while leaving the other phase mechanically robust, we can decouple the mechanical and ion conducting properties in BCP; materials with higher shear modulus and ionic conductivity may be obtained.

In the first year of the project, a graphene oxide (GO)/PEO nanocomposite SPE has been prepared by solution casting a homogeneous mixture of PEO, GO and LiClO4. GO was highly aligned with the nanoplatelet surface parallel to the film surface during a slow solvent evaporation process, which further confined PEO crystallization, resulting in the polymer chain perpendicular to the film surface (PEO crystalline lamellae surfaces parallel to the film surfaces). The presence of GO and Li+ ions had a synergistic effect of confining PEO crystal orientation and retarding PEO crystallization. The ion transport is guided by GO nanoplatelets and PEO lamellae, leading to highly anisotropic ionic conductivity in both SPEs. In particular, conductivity anisotropy factors as high as ~ 70 have been achieved in the nanocomposite SPE. This study demonstrated that PEO crystallization can be tuned and controlled using 2D templates, furthering our understanding of the complex interactions during ion transport at the fundamental level, which can help guide engineering new and improved PEO-incorporated batteries.

In the second year of the project, we introduced a new 2D materials into our study: MXenes. MXenes is a new family of 2D transition metal carbides and/or nitrides, which is best described as Mn+1XnTx, where M is an early transition metal, X is carbon and/or nitrogen, T is a terminating group (O, OH or F), x is the number of T, and n is the number of X (vary from 1, 2, to 3). Different from graphene, MXenes are hydrophilic due to their terminal groups. This hydrophilicity is critical in applications such as capacitors, LIB anodes, electromagnetic interference (EMI) shielding etc. For composite polymer electrolytes (CPEs), this hydrophilic surface can enhance the interaction between MXene and the polymer chain, leading to reduced PEO crystallinity and enhanced ionic conductivity. In our study, we first prepare control samples of poly(ethylene oxide) (PEO)/MXene nanocomposites. Ti3AlC2 was selected as the MAX precursor to synthesize Ti3C2Tx MXene because its exfoliation and delamination are reasonably well understood. This selective dissolution of the ‘A’ element has been realized by immersing fine powders of certain MAX phases in fluoride-containing aqueous etchants such as hydrofluoric acid or hydrochloric acid with dissolved lithium fluoride. Ti3C2Tx/PEO nanocomposites were fabricated, and the polymer's crystallization behavior was systematically characterized. Both non-isothermal and isothermal crystallization behaviors were systematically studied. We then hypothesized that because of its large surface area and hydrophilic surface with rich functional groups, MXene could be excellent nanofiller for CPE. We demonstrated that in MXene-containing CPE (MCPE), MXene inhibits the PEO crystallization, enhances the ionic conductivities and accelerates polymer chain dynamics. MCPE-based LMBs have also been fabricated. Our tests demonstrated state-of-the-art rate capabilities and stability are achieved at a much lower nanofiller content compared with other CPE systems. We therefore envisage that MCPEs could be a new class of materials for all-solid-state LMBs.