Amorphization of Pseudocapacitive T−Nb₂O₅ Accelerates Lithium Diffusivity as Revealed Using Tunable Isomorphic Architectures

Abstract

Intercalation pseudocapacitance can combine capacitor‐like power densities with battery‐like energy densities. Such surface‐limited behavior requires rapid diffusion where amorphization can increase solid‐state diffusivity. Here intercalation pseudocapacitive materials with tailored extents of amorphization in T‐Nb2O5 are first reported. Amorphization was characterized with WAXS, XPS, XAFS, and EPR which suggested a peroxide‐rich (O22−) surface that was consistent with DFT predictions. A series of tunable isomorphic architectures enabled comparisons while independently varying transport parameters. Through process of elimination, solid‐state lithium diffusion was identified as the dominant diffusive‐constraint dictating the maximum voltage sweep rate for surface‐limited kinetics (vSLT), termed the Surface‐Limited Threshold (SLT). The vSLT increased with amorphization however stable cycling required crystalline T‐Nb2O5. A current‐response model using series‐impedances well‐matched these observations. This perspective revealed that amorphization of T‐Nb2O5 enhanced solid‐state diffusion by 12.2 % and increased surface‐limitations by 17.0 % (stable samples). This approach enabled retaining 95 % lithiation capacity at ∼800 mV s−1 (1,600 C‐rate equivalent).

Faster Intercalation Pseudocapacitance Enabled by Adjustable Amorphous Titania Where Tunable Isomorphic Architectures Reveal Accelerated Lithium Diffusivity

“…This XPS trend indicates the progressive removal of O which is similar to that seen with sol-gel derived niobia. [78] Here we term the samples with the lowest calcination temperatures as thus having the greatest degree of amorphization for both the amorphous and anatase phases. Thus, a set of tailored isomorphic architectures were prepared with controlled pore size, titania wall thickness, and extent of amorphization.…”

Understanding Rapid Intercalation Materials One Parameter at a Time

“…[59,60,63,77] Another perspective is that intercalation pseudocapacitance operates by charge storage via normal intercalation with additional rate-controlling processes such as a surface-limited faradaic reaction or ohmic drop in the pores. [63,[78][79][80] As more rapid intercalation processes are discovered it is expected that other processes may increasingly become rate limiting. [39] This perspective is equivalent to a series circuit with two (or more) rate-dependent processes which all impede the overall rate as a collective (Figure 3).…”

“…[39] This perspective is equivalent to a series circuit with two (or more) rate-dependent processes which all impede the overall rate as a collective (Figure 3). [78] A corresponding i(ν) equation for the "series model" is explained later in detail. This series perspective predicts that the intercalation length scale contributes to the rate-dependence of the current response.…”

“…Persistent micelle templates (PMTs) [78][79][80][128][129][130][131] are a synthesis platform that uniquely enables the production of series of porous nanomaterials that vary by a single spatial parameter at time and follow model predictions. PMTs rely on self-assembly, yield macroscopically homogeneous materials, and are scalable with simple solution processing.…”

“…[131,136,145] The PMT model is best-suited to micelle templated samples due to the assumed constant S structure factor term. [145] PMT-derived samples can be either bulk solids [135,146] or deposited as thin films, [78][79][80] for example, upon current collectors. When the precursors correspond to active materials only, then the resulting porous materials are free from the binders and carbon additives associated with slurry-based electrodes, thereby avoiding electrochemical contributions from side reactions [147,148] while enabling the measurement of fundamental descriptors of the active material behaviors.…”