Understanding Charge Transport At Interfaces in Tough Solid Electrolytes to Enable Lithium Metal Batteries

Solid electrolytes offer the promise of safe, energy-dense secondary lithium ion and lithium metal batteries. To date, an obvious choice for a single solid electrolyte material with sufficient ionic conductivity, compatibility with lithium metal, and mechanical robustness does not exist. Mechanical properties are a key consideration given their role in suppressing lithium dendrite formation, which has plagued lithium metal anodes to date and ultimately must be solved to enable their safe commercial implementation. Fabrication of composite materials where both phases conduct Li cations is one approach to addressing these requirements.

Both phases being Li⁺ conductive is critical and distinguishes these composites from several previous studies that demonstrated enhanced Li conductivities in nanocomposite solid polymer electrolytes where insulating nanoparticles, e.g. Al₂O₃ and SiO₂, comprise the minor phase. High loadings of the particulate phase are required for improved mechanical properties. With insulating fillers, the optimal weight loading is approximately 15%; above that, reduction of the conductive matrix phase overwhelms the enhancement from additional particles. For particulate phases that conduct Li⁺, it is anticipated that much higher filler loadings can be utilized to improve mechanical properties while maintaining or enhancing Li⁺ transport.

This poster will present my approach to understanding the transport of Li cations at interfaces between polymeric and inorganic solid electrolytes. Initially, charge transport was studied in laminated bilayers of thin films of polymer and inorganic electrolytes. A typical polymer electrolyte was poly(ethylene oxide) (PEO) or PEO-based copolymer mixed with a lithium salt, such as LiClO₄ or LiCF₃SO₃. Lithium phosphate oxynitride (Lipon) was employed as the stiff, inorganic electrolyte. It was discovered that the interfacial resistance was dominant in this system but could be controlled and essentially eliminated through fabrication techniques. The study was repeated for bulk electrolyte materials and similar conclusions were found. After understanding the charge transport at planar interfaces with well-defined areas, the study was extended to nanoparticulate composite solid electrolytes designed to optimize conductivities and mechanical properties. Initial results suggested that negligible charge transfer occurs through the higher conductivity particulate phase. Efforts to reduce the interfacial impedance in these nanocomposites will be presented.

Two additional research efforts concerning solid electrolytes will be discussed. The first involves the development of new flow battery designs that pair protected Li metal anodes with aqueous catholyte solutions. Laminating lithium metal between a water and air-impermeable solid electrolyte and current collector prevents the oxidation of Li by water, and the electrochemical cell can operate outside the standard electrochemical window of water. Low cost, renewable organic redox species with high aqueous solubilities and reduction potentials above 3V vs. Li⁺/Li for energy dense catholytes have been discovered. In the second project, plating and stripping of Li metal is being characterized by neutron reflectometry. For long-term cycling of metallic anodes, electrodeposition of Li into perfectly uniform, fully dense, conformal layers over the entire electrically active area of the current collector is required. Subsequent reoxidation must maintain the uniformity of the layer such that local islands or grains with disparate mass transfer kinetics do not form and propagate throughout the layer, resulting in the mossy or dendritic morphologies. It is important to understand how the composition and operation of electrochemical cells influence the initial electrodeposition/reoxidation processes. Neutron reflectometry was used to characterize the density, composition, and roughness of ultrathin lithium films as they were deposited and reoxidized in situ. Reflectivity profiles of Ni and Lipon-coated Ni current collectors revealed changes in the sample structure for plated Li films as thin as 10 nm.