Nanoscale Solid-State Separator/Electrolyte for3D All-Solid-State Batteries
Authors: Sassin Megan Bourg , Long Jeffrey W. , Rolison Debra R. 
Published: February 2020

Conventional batteries comprise two opposing electrodes assembled in a 2D, coplanar configuration, and are separated by an electronically insulating layer that is infused with an ion-conducting liquid electrolyte. While providing for facile cell fabrication, the use of liquid electrolytes presents challenges in terms of safe operation; for example, the nonaqueous electrolytes used for state-of-the-art Li-ion batteries are toxic and flammable. In addition to contributing to safety concerns, electrolyte volatility restricts the temperature range over which batteries can be effectively operated. Such concerns have driven the development of a wide range of solid-state electrolytes over the past 40 years, with compositions that are broadly based on organic (polymeric) or inorganic (ceramic) components, or in some cases, organic–inorganic hybrids.[i],[ii] Despite advancements in solid-state electrolytes, their principal drawback remains modest ionic conductivity that limits power performance in the ultimate battery. The brittle nature of ceramic-type solid-state electrolytes may also prevent their use in particular types of batteries, such as wound-cell configurations.[iii]

Solid-state electrolytes will have their greatest impact when transitioning from the simple 2D cell designs that dominate conventional batteries to next-generation designs in which the three critical battery components (anode, cathode, and separator/electrolyte) are reconstructed in 3D as interpenetrating networks.[iv] In a 3D battery configuration, the interface between the cathode and anode is maximized, and the anode/cathode separation distance is minimized, thereby mitigating power limitations that would otherwise be imposed by modest ionic conductivity in the solid-state electrolyte. Although substantial progress has been made in 3D battery design and fabrication, a common roadblock to achieving a fully functioning 3D battery is the separator/solid-state electrolyte component, which must be: (i) conformal to the supporting electrode architecture; (ii) ultrathin, typically tens of nanometers to a few micrometers; (iii) pinhole-free; (iv) electronically insulating; (v) ionically conducting; and (vi) chemically and electrochemically stable.[iv],[v],[vi] These requirements, coupled with the complex geometry of 3D battery architectures, render most solid-state electrolyte fabrication methods incompatible, as they require line-of-sight.
Electrodeposition can circumvent the aforementioned issue with regard to the complex geometry because it is inherently a non-line-of-sight fabrication method, and under self-limiting conditions, can produce nanoscale, conformal to the surface, pinhole-free, electronically insulating polymer coatings. Previously, we have demonstrated the electrodeposition of ultrathin poly-(phenylene oxide)-based polymer coatings via electro-oxidation of phenol and substituted phenol monomers on ultraporous nanoarchitectures.[vii],[viii],[ix] The self-limiting conditions generate conformal poly(phenylene oxide)-based films that are tens of nanometers thick and highly electronically insulating, with dielectric strengths comparable to those measured for the corresponding bulk polymer. Ionic conductivity is imparted by impregnation of the polymer film with electrolyte salts or by using monomers with pendant ionic functionalities.
Polymer formation via electro-oxidation may be incompatible with base electrode architectures that are designed to serve as the negative electrode of the ultimate 3D battery. As an alternative, we are currently exploring polymer deposition via electro-reduction of monomers with pendant vinyl groups. As a preliminary example, we have electrodeposited siloxane-based polymer films from 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane (D3V3) on 2D planar carbon substrates. Related siloxane-based polymers have been previously developed as battery electrolytes, and demonstrate such desirable attributes as low electronic conductivity, resistance to oxidation, and high dielectric strength, but must typically be modified to improve ionic conductivity.[x] In the present case, the ether groups intrinsic to the D3V3 monomer promote Li+ solvation and transport in the corresponding electrodeposited polymer, imparting sufficient ionic conductivity for function as a solid-state electrolyte.
The chemical, electrical, and morphological properties of the poly(Li+–D3V3) films are characterized by X-ray photoelectron and vibrational spectroscopy, solid-state DC and AC methods (i-V and impedance spectroscopy), and atomic force microscopy. After the initial characteristics of this new separator/electrolyte candidate are validated on a 2D substrate, we will transition to more complex 3D architectures, such as fiber-supported carbon nanofoam papers.[xi]

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