End-Group Assisted Electrodeposition of Ultrathin Solid Polymer Electrolyte Films on 3D-Micro Architected Electrodes

Solid state batteries with three-dimensional micro-architected cell geometry are promising candidates for next-generation batteries with high power density, high energy density, and low safety risks because they take advantage of short ion diffusion distance and high active material loading in 3D interdigitated thin-film structures.<br/>A major challenge in producing 3D micro-architected batteries is the fabrication of a thin and uniform solid electrolyte layer that separates the two interdigitated electrodes. Electrodeposition as a non-line-of-sight fabrication method is readily adapted to modify conductive 3D substrates such as architected electrodes with a polymer film as a thin electrolyte layer. While previous studies reported the electrodeposition of polymer electrolytes on micro structured materials, the resulting thin films exhibited limited functionality, heterogeneous topography, or were only applicable to a single substrate material. To address those issues, we developed the electrodeposition of a polymer electrolyte utilizing a two-component electrocoupling reaction to form a step-growth polymer network instead of the conventional electrochemically initiated chain-growth polymerization that causes uncontrolled polymer formation and precipitation in solution.<br/>Specifically, poly(ethylene oxide) (PEO), known for high dielectric constant and Li-ion conductivity, is selected as the backbone of the polymer electrolyte network. The PEO chain ends are functionalized with electrochemically labile groups that undergo direct coupling reactions with a small molecule crosslinker. This electrochemically mediated coupling reaction of polymeric extender and polyfunctional crosslinker generates a polymer network close to the 3D electrode surface that leads to film deposition. This method is applied to coat a layer of solid polymer electrolyte on a low-tortuosity 3D Carbon electrode. As a result, the entire surface of the 3D Carbon electrode is effectively enveloped by a sub-micron polymer film. The deposited polymer film exhibits high electronic resistance and is able to serve as solid electrolytes. Our approach decouples the polymer functionality from its electrodeposition chemistry with electrochemically active end groups that make up only a small fraction of the resulting polymer thin film and can be combined with arbitrary functional polymer chains. Therefore, this method can be a universal method to coat various conductive micro-architected 3D substrates with tailorable functional polymers.