Enhanced pseudocapacitive energy storage properties of Nb 2 O 5 /C core‐shell structures with the surface modification

Stefik

2022

Adv Funct Materials

Demand for fast, energy‐dense storage drives the research into nanoscale intercalation materials. Nanomaterials accelerate kinetics and can modify reaction path thermodynamics, intercalant solubility, and reversibility. The discovery of intercalation pseudocapacitance has opened questions about their fundamental operating principles. For example, are their capacitor‐like current responses caused by storing energy in special near‐surface regions or rather is this response due to normal intercalation limited by a slower faradaic surface‐reaction? This review highlights emerging methods combining tailored nanomaterials with the process of elimination to disambiguate cause‐and‐effect at the nanoscale. This method is applied to multiple intercalation pseudocapacitive materials showing that the timescales exhibiting surface‐limited kinetics depended on the total intercalation length scale. These trends are inconsistent with the near‐surface perspective. A revised current‐model without assuming special near‐surface storage fits experimental data better across wide timescales. This model, combined with tailored nanomaterials and the process of elimination, can isolate material‐specific effects such as how amorphization/defect‐tailoring modifies both insertion and diffusion kinetics. Avenues for both faster intercalation pseudocapacitance and increased energy density are discussed. A relaxation time argument is suggested to explain the continuum between battery‐like and pseudocapacitive behaviors. Future directions include synthetic methods emphasizing tailored defects and analytical methods that minimize assumptions.

Section: Ambiguity Challenge With Convolved Processesmentioning

confidence: 99%

Understanding Rapid Intercalation Materials One Parameter at a Time

Stefik

2022

Adv Funct Materials

“…Surface‐limited kinetics are possible when the overall rate is not limited by diffusive processes and there is an absence of a crystallographic phase changes upon intercalation [22–26] . Numerous nanoscale niobia structures have been reported with an emphasis on individual performance metrics [27–74] . A few investigations have approached the relationship between nanostructure and T‐Nb 2 O 5 performance using computational models, [75] advanced electrochemical techniques, [76] tunable nanotubes, [77] and core‐shell particles [78,79] without experimentally isolating the rate‐limiting process(es).…”

Section: Introductionmentioning

confidence: 99%

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

Wechsler

Lokupitiya

et al. 2022

Batteries & Supercaps

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).

“…Materials architectures investigated to date include nanoparticles, [8,[22][23][24][25] nanotubes, [12,26,27] nanofibers, [28] nanorods, [29,30] nanowires, [6,31] nanosheets, [9,10,[32][33][34][35][36][37] nanocomposites, [38][39][40][41][42][43][44] and related nanostructures. [7,37,[45][46][47][48][49][50][51][52][53][54] Only few of the above works attempted a rational performance comparison of different nanostructures. [23,[52][53][54] These studies relied on either the simultaneous variation of multiple spatial parameters or were based on single parameter architectures, thus obfuscating the study of nanostructure-property relationships.…”

Section: Introductionmentioning

confidence: 99%

Nanostructure Dependence of T‐Nb₂O₅ Intercalation Pseudocapacitance Probed Using Tunable Isomorphic Architectures

Lokupitiya

Vest

et al. 2020

Adv Funct Materials

Intercalation pseudocapacitance has emerged as a promising energy storage mechanism that combines the energy density of intercalation materials with the power density of capacitors. Niobium pentoxide was the first material described as exhibiting intercalation pseudocapacitance. The electrochemical kinetics for charging/discharging this material are surface‐limited for a wide range of conditions despite intercalation via diffusion. Investigations of niobium pentoxide nanostructures are diverse and numerous; however, none have yet compared performance while adjusting a single architectural parameter at a time. Such a comparative approach reduces the reliance on models and the associated assumptions when seeking nanostructure–property relationships. Here, a tailored isomorphic series of niobium pentoxide nanostructures with constant pore size and precision tailored wall thickness is examined. The sweep rate at which niobium pentoxide transitions from being surface‐limited to being diffusion‐limited is shown to depend sensitively upon the nanoscale dimensions of the niobium pentoxide architecture. Subsequent experiments probing the independent effects of electrolyte concentration and film thickness unambiguously identify solid‐state lithium diffusion as the dominant diffusion constraint even in samples with just 48.5–67.0 nm thick walls. The resulting architectural dependencies from this type of investigation are critical to enable energy‐dense nanostructures that are tailored to deliver a specific power density.