Extreme ultraviolet (XUV) light sources based on high
harmonic generation are enabling the development of novel
spectroscopic methods to help advance the frontiers of ultrafast
science and technology. In this account we discuss
the development of XUV-RA spectroscopy at near grazing
incident reflection geometry and highlight recent applications
of this method to study ultrafast electron dynamics at
surfaces. Measuring core-to-valence transitions with broadband,
femtosecond pulses of XUV light extends the benefits
of x-ray absorption spectroscopy to a laboratory tabletop by
providing a chemical fingerprint of materials, including the
ability to resolve individual elements with sensitivity to oxidation
state, spin state, carrier polarity, and coordination
geometry. Combining this chemical state sensitivity with
femtosecond time resolution provides new insight into the
material properties that govern charge carrier dynamics in
complex materials. It is well known that surface dynamics
differ significantly from equivalent processes in bulk materials,
and that charge separation, trapping, transport, and
recombination occurring uniquely at surfaces governs the efficiency
of numerous technologically relevant processes spanning
photocatalysis, photovoltaics, and information storage
and processing. Importantly, XUV-RA spectroscopy at near
grazing angle is also surface sensitive with a probe depth of
3 nm, providing a new window into electronic and structural
dynamics at surfaces and interfaces. Here we highlight
the unique capabilities and recent applications of XUVRA
spectroscopy to study photo-induced surface dynamics
in metal oxide semiconductors, including photocatalytic oxides
(Fe2O3, Co3O4 NiO, and CuFeO2) as well as photoswitchable
magnetic oxide (CoFe2O4). We first compare the
ultrafast electron self-trapping rates via small polaron formation
at the surface and bulk of Fe2O3 where we note that
the energetics and kinetics of this process differ significantly
at the surface. Additionally, we demonstrate the ability to
systematically tune this kinetics by molecular functionalization,
thereby, providing a route to control carrier transport
at surfaces. We also measure the spectral signatures
of charge transfer excitons with site specific localization of
both electrons and holes in a series of transition metal oxide
semiconductors (Fe2O3, NiO, Co3O4). The presence of
valence band holes probed at the oxygen L1-edge confirms
a direct relationship between the metal-oxygen bond covalency
and water oxidation efficiency. For a mixed metal oxide
CuFeO2 in the layered delafossite structure, XUV-RA
reveals that the sub-picosecond hole thermalization from O
2p to Cu 3d states of CuFeO2 leads to the spatial separation
of electrons and holes, resulting in exceptional photocatalytic
performance for H2 evolution and CO2 reduction
of this material. Finally, we provide an example to show the
ability of XUV-RA to probe spin state specific dynamics in a
the photo-switchable ferrimagnet, cobalt ferrite (CoFe2O4).
This study provides a detailed understating of ultrafast spin
switching in a complex magnetic material with site-specific
resolution. In summary, the applications of XUV-RA spectroscopy
demonstrated here illustrate the current abilities
and future promise of this method to extend molecule-level
understanding from well-defined photochemical complexes
to complex materials so that charge and spin dynamics at
surfaces can be tuned with the precision of molecular photochemistry.
more »
« less
Surface Reduction Stabilizes the Single-Crystalline Ni-Rich Layered Cathode for Li-Ion Batteries
The surface of the layered transition metal oxide cathode plays an important role in its function and degradation. Modification of the surface structure and chemistry is often necessary to overcome the debilitating effect of the native surface. Here, we employ a chemical reduction method using CaI2 to modify the native surface of single-crystalline layered transition metal oxide cathode particles. High-resolution transmission electron microscopy shows the formation of a conformal cubic phase at the particle surface, where the outmost layer is enriched with Ca. The modified surface significantly improves the long-term capacity retention at low rates of cycling, yet the rate capability is compromised by the impeded interfacial kinetics at high voltages. The lack of oxygen vacancy generation in the chemically induced surface phase transformation likely results in a dense surface layer that accounts for the improved electrochemical stability and impeded Li-ion diffusion. This work highlights the strong dependence of the electrode’s (electro)chemical stability and intercalation kinetics on the surface structure and chemistry, which can be further tailored by the chemical reduction method.
more »
« less
- NSF-PAR ID:
- 10352785
- Editor(s):
- Yiying Wu
- Date Published:
- Journal Name:
- ACS Applied Materials & Interfaces
- ISSN:
- 1944-8244
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Conventional lithium-ion batteries are unable to meet the increasing demands for high-energy storage systems, because of their limited theoretical capacity. 1 In recent years, intensive attention has been paid to enhancing battery energy storage capability to satisfy the increasing energy demand in modern society and reduce the average energy capacity cost. Among the candidates for next generation high energy storage systems, the lithium sulfur battery is especially attractive because of its high theoretical specific energy (around 2600 W h kg-1) and potential cost reduction. In addition, sulfur is a cost effective and environmentally friendly material due to its abundance and low-toxicity. 2 Despite all of these advantages, the practical application of lithium sulfur batteries to date has been hindered by a series of obstacles, including low active material loading, poor cycle life, and sluggish sulfur conversion kinetics. 3 Achieving high mass loading cathode in the traditional 2D planar thick electrode has been challenged. The high distorsion of the traditional planar thick electrodes for ion/electron transfer leads to the limited utilization of active materials and high resistance, which eventually results in restricted energy density and accelerated electrode failure. 4 Furthermore, of the electrolyte to pores in the cathode and utilization ratio of active materials. Catalysts such as MnO 2 and Co dopants were employed to accelerate the sulfur conversion reaction during the charge and discharge process. 5 However, catalysts based on transition metals suffer from poor electronic conductivity. Other catalysts such as transition metal dopants are also limited due to the increased process complexities. . In addition, the severe shuttle effects in Li-S batteries may lead to fast failures of the battery. Constructing a protection layer on the separator for limiting the transmission of soluble polysulfides is considered an effective way to eliminate the shuttle phenomenon. However, the soluble sulfides still can largely dissolve around the cathode side causing the sluggish reaction condition for sulfur conversion. 5 To mitigate the issues above, herein we demonstrate a novel sulfur electrode design strategy enabled by additive manufacturing and oxidative vapor deposition (oCVD). Specifically, the electrode is strategically designed into a hierarchal hollow structure via stereolithography technique to increase sulfur usage. The active material concentration loaded to the battery cathode is controlled precisely during 3D printing by adjusting the number of printed layers. Owing to its freedom in geometry and structure, the suggested design is expected to improve the Li ions and electron transport rate considerably, and hence, the battery power density. The printed cathode is sintered at 700 °C at N 2 atmosphere to achieve carbonization of the cathode during which intrinsic carbon defects (e.g., pentagon carbon) as catalytic defect sites are in-situ generated on the cathode. The intrinsic carbon defects equipped with adequate electronic conductivity. The sintered 3D cathode is then transferred to the oCVD chamber for depositing a thin PEDOT layer as a protection layer to restrict dissolutions of sulfur compounds in the cathode. Density functional theory calculation reveals the electronic state variance between the structures with and without defects, the structure with defects demonstrates the higher kinetic condition for sulfur conversion. To further identify the favorable reaction dynamic process, the in-situ XRD is used to characterize the transformation between soluble and insoluble polysulfides, which is the main barrier in the charge and discharge process of Li-S batteries. The results show the oCVD coated 3D printed sulfur cathode exhibits a much higher kinetic process for sulfur conversion, which benefits from the highly tailored hierarchal hollow structure and the defects engineering on the cathode. Further, the oCVD coated 3D printed sulfur cathode also demonstrates higher stability during long cycling enabled by the oCVD PEDOT protection layer, which is verified by an absorption energy calculation of polysulfides at PEDOT. Such modeling and analysis help to elucidate the fundamental mechanisms that govern cathode performance and degradation in Li-S batteries. The current study also provides design strategies for the sulfur cathode as well as selection approaches to novel battery systems. References: Bhargav, A., (2020). Lithium-Sulfur Batteries: Attaining the Critical Metrics. Joule 4 , 285-291. Chung, S.-H., (2018). Progress on the Critical Parameters for Lithium–Sulfur Batteries to be Practically Viable. Advanced Functional Materials 28 , 1801188. Peng, H.-J.,(2017). Review on High-Loading and High-Energy Lithium–Sulfur Batteries. Advanced Energy Materials 7 , 1700260. Chu, T., (2021). 3D printing‐enabled advanced electrode architecture design. Carbon Energy 3 , 424-439. Shi, Z., (2021). Defect Engineering for Expediting Li–S Chemistry: Strategies, Mechanisms, and Perspectives. Advanced Energy Materials 11 . Figure 1more » « less
-
A New Type of Li‐Rich Rock‐Salt Oxide Li 2 Ni 1/3 Ru 2/3 O 3 with Reversible Anionic Redox Chemistrymore » « less
Abstract Li‐rich oxide cathodes are of prime importance for the development of high‐energy lithium‐ion batteries (LIBs). Li‐rich layered oxides, however, always undergo irreversible structural evolution, leading to inevitable capacity and voltage decay during cycling. Meanwhile, Li‐rich cation‐disordered rock‐salt oxides usually exhibit sluggish kinetics and inferior cycling stability, despite their firm structure and stable voltage output. Herein, a new Li‐rich rock‐salt oxide Li2Ni1/3Ru2/3O3with
Fd ‐3m space group, where partial cation‐ordering arrangement exists in cationic sites, is reported. Results demonstrate that a cathode fabricated from Li2Ni1/3Ru2/3O3delivers a large capacity, outstanding rate capability as well as good cycling performance with negligible voltage decay, in contrast to the common cations disordered oxides with space groupFm ‐3m . First principle calculations also indicate that rock‐salt oxide with space groupFd ‐3m possesses oxygen activity potential at the state of delithiation, and good kinetics with more 0‐TM (TM = transition metals) percolation networks. In situ Raman results confirm the reversible anionic redox chemistry, confirming O2−/O−evolution during cycles in Li‐rich rock‐salt cathode for the first time. These findings open up the opportunity to design high‐performance oxide cathodes and promote the development of high‐energy LIBs. -
more » « less
The ever‐growing demand for high capacity cathode materials is on the rise since the futuristic applications are knocking on the door. Conventional approach to developing such cathode relies on the lithium‐excess materials to operate the cathode at high voltage and extract more lithium‐ion. Yet, they fail to satiate the needs because of their unresolved issues upon cycling such as, for lithium manganese‐rich layered oxides—their voltage fading, and for as nickel‐based layered oxides—the structural transition. Here, in contrast, lithium‐deficient ratio is demonstrated as a new approach to attain high capacity at high voltage for layered oxide cathodes. Rapid and cost effective lithiation of a porous hydroxide precursor with lithium deficient ratio is acted as a driving force to partially convert the layered material to spinel phase yielding in a multiphase structure (MPS) cathode material. Upon cycling, MPS reveals structural stability at high voltage and high temperature and results in fast lithium‐ion diffusion by providing a distinctive solid electrolyte interface (SEI) chemistry—MPS displays minimum lithium loss in SEI and forms a thinner SEI. MPS thus offers high energy and high power applications and provides a new perspective compared to the conventional layered cathode materials denying the focus for lithium excess material.
-
Access to well-defined, model-like, non-noble metal intermetallic compound nanomaterials (<10 nm) with phase pure bulk, bulk-like 1st-atomic-layer surface composition, and unique electronic and surface chemical properties is critical for the fields of catalysis, electronics, and sensor development. Non-noble metal intermetallic compounds are compositionally ordered solid compounds composed of transition metals and semimetals or post-transition metals. Their synthesis as model-like high-surface-area supported nanoparticles is challenging due to the elevated reactivity of the constituent elements and their interaction with the support material. In this study, we have developed a systematic understanding of the fundamental phenomena that control the synthesis of these materials such that phase pure bulk nanoparticles (<10 nm) may be produced with bulk-like surface terminations. The effects of the precursor and support choice, chemical potential of H 2 , reduction temperature, and annealing procedures were investigated to understand the fundamental kinetics of particle formation and interactions that dictate phase purity and stability and 1st-atomic-layer surface composition. The understanding developed may serve as a foundation for further developing advanced synthesis procedures for well-defined nanoparticles with increasing compositional complexity.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acsami.2c09937
