Development of Hybrid Electrolytes for Solid-State Batteries

Abstract

Lithium-ion batteries that use solid-state electrolytes are crucial energy storage devices with widespread applications in small and large electronics, electric vehicles, electric aircrafts and grid-level energy storage. Conventional lithium-ion batteries based on liquid electrolytes lack chemical stability, have inherent safety issues, and incur a high production cost. Solid state electrolytes (SSEs) not only have the potential to correct these drawbacks but exhibit improved mechanical properties, which allows one to reduce the battery size, suppress dendrite growth, opening the possibility for metal anodes, and thus increase its energy density. However, power density requires good charge carrier mobility, which varies conversely with the factors that control mechanical properties. Thus, to simultaneously achieve high ionic conductivity and elastic moduli, we pursue a hybrid organic-inorganic composite materials design approach for creating the required SSEs. Our hybrid electrolytes consist of a nano-porous silica backbone obtained through sol-gel synthesis that provides a three-dimensional percolating mechanically rigid scaffold. Polymer is subsequently deposited in the pores of this network via solution exchange, where it establishes the conducting phase. This unique approach allows us to decouple mechanical from cation transport properties of the material and achieve both high elastic stiffness and ionic conductivity. To increase the cation transference number, and thereby the Coulomb efficiency of the devices, we tether the cation donor to the silica scaffold. Initially, we aimed to do this with polymer chains as the intermediary. While this approach did not yield the desired outcome, we discovered that the properties of the gel-cast material are strongly influenced by unexpected structural evolution during drying, to the effect that ionic conductivities can vary by up to three orders of magnitude in these hybrids, without modifying their chemical makeup. Depending on the sample shape and aspect ratio, the drying process occurs inhomogeneously, imparting various degrees of anisotropy and spatial gradients that can be affect the development of the network topology. Cylindrical disk-shaped samples dry and rigidify first on their periphery, causing tensile stresses to build towards the center as the drying front progresses inward. This causes reconditioning of the network structure at the core of the disk, resulting in a markedly higher conductivity with minimal reduction of mechanical stiffness. We successfully developed an alternative approach for immobilizing cation donors and increasing the transference number of Li+ to greater than 0.9. To this end we modified sulfonyl (trifluoromethylsulfonyl) imide (STFSI) chemistry by functionalizing the side chain of tetraethyl orthosilicate to incorporate the STFSI cation donor directly into the silica backbone. This approach significantly enhances the ionic mobility without negatively impacting the chemical or physical stability of the material. Moreover, we show that of anchoring oligo-PEO to the silica backbone, entangles with additional non-bonded short-chain PEO further enhances ionic conductivity in the solid electrolyte. To boost the ionic conductivity even more, a mixture of propylene carbonate (PC) and ethylene carbonate (EC) with various weight fractions was introduced in the silica backbone. This configuration yields the highest conductivity for the composite system, while the nano-confinement enhances the physical stability of EC and PC.PHDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/174532/1/kvazrik_1.pd