Probing the Mechanistics of a Molecularly Tailored Solid/Solid Interface

Our research team comprises a synergistic experimental and theoretical collaboration of complementary expertise, and access to crucial resources. Prof. Whittaker-Brooks (LWB) is an expert in nanomaterial synthesis and structural and electronic characterization.9,10 In addition to in-house synthetic and characterization facilities, LWB has allocated beam-time to perform in operando X-ray absorption fine structure (XAFS) spectroscopy, and access to in operando X-ray diffraction (XRD) facilities. LWB will perform the material synthesis, and electrochemical and impedance characterization of the macroscopic devices. Prof. Guiton (BSG) is an expert in in situ transmission electron microscopy (TEM) of inorganic nanomaterial systems,11–13 and has access to ultra-high resolution aberration-corrected scanning TEMs, equipped with in situ heating and biasing capabilities at her home institution and at Oak Ridge National Laboratory (at which part of her research group is based full-time). BSG will perform TEM sample fabrication and in situ analyses. Prof. Mukherjee (PPM) is an expert in mesoscale computational physics and stochastics of materials-transport-interface interactions in electrochemical storage systems.14,15 PPM will perform computational modeling and theoretical analysis. Specific Aim 1: Synthesize and fabricate solid/solid interfaces with a controllable interlayer. Our motivation is to enable systematic interrogation of a controllable solid-solid interface for sodium intercalation. Towards this aim we have chosen: (1) a good electronic conductor known to intercalate sodium without large volume expansion as cathode material (TiS2); (2) a mediating molecular interlayer of perylene diimides (PDIs) tethered to the TiS2 surface; (3) sodium-ion conducting solid polymer electrolyte with high ionic conductivity (Na[(FSO2)(n-C4F9SO2)N], (NaFNFSI) and poly(ethylene oxide) (PEO)); and (4) a sodium metal anode TiS2 nanowires will be grown in a synthesis previously developed by LWB, and functionalized with PDIs tethered via a thiol terminating group, for which we have chosen five functional groups with varying polarities to modify the interlayer energetics (Fig. 2). NaFNFSI-PEO will be prepared using a solid-state literature preparation.16 As-synthesized heterostructures will be fabricated into coin cells for impedance measurement, and into samples for in situ TEM characterization using focused ion beam (FIB) milling, and lift out, with deposition onto an electrical-biasing TEM substrate, using a technique previously optimized by BSG. Specific Aim 2: Determine the key morphological and structural characteristics dictating electrochemical-transport mechanisms at the interface. Using in situ microscopy characterization, coupled with impedance and in operando measurements, we will correlate local dynamics with mesoscale characteristics related to transport properties. In situ devices (Fig. 3) will be biased in the TEM, while performing simultaneous atomic resolution imaging of the interface. Low accelerating voltages will be used to minimize electron-beam dose to the solid polymer electrolyte. Electron energy loss spectroscopy mapping will be employed in real-time, to correlate the atomic structure with the presence and oxidation states of Na, S, and Ti atoms in the vicinity of the interface. Selected area electron diffraction and Fourier transform analysis will aid in correlative mapping of local crystalline features to the structure/composition information. Macroscopic half-cells will be characterized using impedance measurements and in operando XAFS and XRD. Specific Aim 3: Mesoscale modeling and analysis of electrochemicaltransport mechanisms at the solid/solid interface. The mesoscale interactions due to morphological and interfacial heterogeneities on short-range (charge transfer) and long-range (transport – diffusion and migration) electrochemical interactions will be modeled. Specifically, stochastics due to these interfacial heterogeneities will be interrogated, and their influence on macroscopic properties (impedance) and performance will be elucidated. This collaboration represents the first example of a comprehensive multimodal study, including in situ characterization coupled with mesoscale modeling, of a systematically controlled solid/solid sodium-ion battery interface. At the successful completion of these studies we expect to have set the stage for a systematic pathway toward understanding and designing heterogeneous, active solid-state interfaces. This will have a significant positive impact on tailored interface design for large scale energy storage technologies,17 leading to a vertical advancement of the field.