Stefan Adams | National University of Singapore (original) (raw)

Papers by Stefan Adams

Physical chemistry chemical physics : PCCP, Jan 14, 2015

Bond-valence site energy modelling, classical molecular dynamics and DFT simulations were employe... more Bond-valence site energy modelling, classical molecular dynamics and DFT simulations were employed to clarify Na(+) ion migration in monoclinic Na2+δFe2-δ/2(SO4)3, the recently reported first representative of a new promising class of alluaudite-type high voltage cathode materials for sodium-ion batteries. Empirical potential parameters derived from our softBV bond valence parameter set reproduce experimental unit-cell parameters. Migration energy barrier calculations based on both these empirical and on ab initio approaches consistently show a strongly anisotropic and fairly fast Na(+) ion mobility along partially occupied Na(3) channels in the c-direction. Nominally fully occupied Na(1) sites are attached to these paths with a moderate activation energy as sources of mobile ions. At elevated temperatures separate parallel Na(2) channels contribute to the ionic conductivity. As such one-dimensional pathways are highly vulnerable to blocking by structural defects, the experimentally...

State-of-the-art oxides and sulfides with high Liion conductivity and good electrochemical stabil... more State-of-the-art oxides and sulfides with high Liion conductivity and good electrochemical stability are among the most promising candidates for solid-state electrolytes in secondary batteries. Yet emerging halides offer promising alternatives because of their intrinsic low Li + migration energy barriers, high electrochemical oxidative stability, and beneficial mechanical properties. Mechanochemical synthesis has enabled the characterization of LiAlX 4 compounds to be extended and the iodide, LiAlI 4 , to be synthesized for the first time (monoclinic P2 1 /c, Z = 4; a = 8.0846(1) Å; b = 7.4369(1) Å; c = 14.8890(2) Å; β = 93.0457(8)°). Of the tetrahaloaluminates, LiAlBr 4 exhibited the highest ionic conductivity at room temperature (0.033 mS cm −1), while LiAlCl 4 showed a conductivity of 0.17 mS cm −1 at 333 K, coupled with the highest thermal and oxidative stability. Modeling of the diffusion pathways suggests that the Li-ion transport mechanism in each tetrahaloaluminate is closely related and mediated by both halide polarizability and concerted complex anion motions.

Scientific Data, 2020

The combination of a materials database with high-throughput ion-transport calculations is an eff... more The combination of a materials database with high-throughput ion-transport calculations is an effective approach to screen for promising solid electrolytes. However, automating the complicated preprocessing involved in currently widely used ion-transport characterization algorithms, such as the first-principles nudged elastic band (FP-NEB) method, remains challenging. Here, we report on high-throughput screening platform for solid electrolytes (SPSE) that integrates a materials database with hierarchical ion-transport calculations realized by implementing empirical algorithms to assist in FP-NEB completing automatic calculation. We first preliminarily screen candidates and determine the approximate ion-transport paths using empirical both geometric analysis and the bond valence site energy method. A chain of images are then automatically generated along these paths for accurate FP-NEB calculation. In addition, an open web interface is actualized to enable access to the SPSE database...

Physica Status Solidi a-Applications and Materials Science, 2011

All-solid-state batteries (SSBs) are attracting widespread attention as next-generation energy st... more All-solid-state batteries (SSBs) are attracting widespread attention as next-generation energy storage devices, potentially offering increased power and energy densities and better safety than liquid electrolyte-based Li-ion batteries. Significant research efforts are currently underway to develop stable and high-performance bulk-type SSB cells by optimizing the cathode microstructure and composition, among others. Electronically conductive additives in the positive electrode may have a positive or negative impact on cyclability. Herein, it is shown that for high-loading (pelletized) SSB cells using both a size-and surface-tailored Ni-rich layered oxide cathode material and a lithium thiophosphate solid electrolyte, the cycling performance is best when low-surface-area carbon black is introduced. Materials and methods Materials Small particle size NCM622 [Li 1+x (Ni 0.6 Co 0.2 Mn 0.2) 1Àx O 2 ] (d 50 ¼ 2.9 mm, d 90 ¼ 6.0 mm) was supplied by BASF SE. 10,17 Prior to use, a 1 wt% LiNbO 3 coating was applied to the cathode material. 4,5 Super C65 carbon black (Timcal), Ketjenblack EC-600JD (Akzo-Nobel), conical carbon nanobers (100 nm Â 20-200 mm; Sigma

ACS Omega, 2020

NASICON-type Li 1+x Al x Ge 2−x (PO 4) 3 (LAGP) is a promising electrolyte with high ionic conduc... more NASICON-type Li 1+x Al x Ge 2−x (PO 4) 3 (LAGP) is a promising electrolyte with high ionic conductivity (>10 −4 S cm −1), excellent oxidation stability, and moderate sintering temperature. However, preparing dense LAGP pellets with high ionic conductivity is still challenging because of the hazards of dopant loss and partial decomposition on conventional sintering. Here, spark plasma sintering (SPS) of LAGP membranes is explored as a promising ultrarapid manufacturing technique, yielding dense electrolyte membranes. Optimizing the SPS temperature is important to achieve desirable density and hence ionic conductance. Our results show that LAGP samples spark plasma-sintered at 750°C exhibit the highest total ionic conductivity of 3.9 × 10 −4 S cm −1 with a compactness of 97% and nearly single-crystalline particles. Our solid-state NMR results, X-ray diffraction studies, and scanning electron microscopy micrographs confirm that the achievable ionic conductivity is controlled by the retention of the Al dopant within the LAGP phase, necking between particles, and the minimization of grain boundaries between crystallites within a particle. To benchmark the performance of our spark plasma-sintered solid electrolyte membranes over conventionally prepared LAGP, we demonstrate their favorable performance in hybrid Li−air batteries. The highest energy efficiency is achieved for the fastest ion-conducting membrane sintered at 750°C.

Journal of Materials Chemistry A, 2015

High-performance all-solid-state lithium/sulfur batteries stably running over 700 high-rate cycle... more High-performance all-solid-state lithium/sulfur batteries stably running over 700 high-rate cycles are demonstrated by combining a MoS2 precursor cathode with argyrodite-type solid electrolyte.

Solid State Ionics, Jul 1, 2016

Here, we report the valence of transition metals at surface, magnetic ordering and oxide-ion tran... more Here, we report the valence of transition metals at surface, magnetic ordering and oxide-ion transport pathway of the double perovskite-type Ba 2 Ca 0.67 M 0.33 NbO 6 − δ (M_Mn, Fe, Co). In-situ X-ray photoelectron spectroscopy (XPS) reveals that the surface valences for Mn and Co-doped Ba 2 Ca 0.67 M 0.33 NbO 6 − δ are 2.5 + and 2 +, respectively at 700°C. All the Ba 2 Ca 0.67 M 0.33 NbO 6 − δ samples are paramagnetic with no long-range cooperative interaction between individual spins. Bond valence sum (BVS) map and molecular dynamics (MD) reveal that the oxide ion migration pathways are isotropic in all investigated Ba 2 Ca 0.67 M 0.33 NbO 6 − δ. In particular, MD simulations (≥ 800 K) show a long-range oxide ion transport with an activation energy of 0.69 eV for Ba 2 Ca 0.67 Co 0.33 NbO 6 − δ .

arXiv (Cornell University), Sep 28, 2021

The state of health (SOH) estimation plays an essential role in battery-powered applications to a... more The state of health (SOH) estimation plays an essential role in battery-powered applications to avoid unexpected breakdowns due to battery capacity fading. However, few studies have paid attention to the problem of uneven length of degrading cycles, simply employing manual operation or leaving to the automatic processing mechanism of advanced machine learning models, like long short-term memory (LSTM). As a result, this causes information loss and caps the full capability of the data-driven SOH estimation models. To address this challenge, this paper proposes an innovative cycle synchronization way to change the existing coordinate system using dynamic time warping (DTW), not only enabling the equal length inputs of the estimation model but also preserving all information. By exploiting the time information of the time series, the proposed method embeds the time index and the original measurements into a novel indicator to reflect the battery degradation status, which could have the same length over cycles. Adopting the LSTM as the basic estimation model, the cycle-synchronizationbased SOH model could significantly improve the prediction accuracy by more than 30% compared to the traditional LSTM.

Elements, Jun 3, 2020

* constrained to be equal to B iso of Fe atom,** constrained to equal SOF(V(4)), *** constrained ... more * constrained to be equal to B iso of Fe atom,** constrained to equal SOF(V(4)), *** constrained to equal B iso of C(1).

Physical Review Letters, 2000

Reverse Monte Carlo produced structural models of silver ion conducting glasses and crystals have... more Reverse Monte Carlo produced structural models of silver ion conducting glasses and crystals have been investigated by the bond-valence technique. Both absolute ionic conductivity and activation energy can be determined directly from the "pathway volume" of the structural models, i.e., from the volume fraction of the percolating pathway cluster. This pathway volume-conductivity relation was found to hold for glassy and crystalline systems with silver ion conductivities differing by more than 11 orders of magnitude. Earlier, less universal rules were rationalized and unified by means of this approach.

International Journal of Computational Materials Science and Engineering, 2012

A simple first principle model was developed based on extended Hückel-type orbital calculation, M... more A simple first principle model was developed based on extended Hückel-type orbital calculation, Marcus electron transport theory and two-dimensional-electron-gas model for the treatment of charge transport in conjugated polymers. Though simple and easy to compute, the effect of the applied electric-field is factored in. Based on this, a complete one-dimensional device model with a single layer of conjugated polymer sandwiched between two electrodes was developed with poly(3-hexylthiophene) (P3HT) as a case study. Simulated J-V curves show that π-π charge transport is much more pronounced than intra-chain transport, hence agree with previous findings. Using the same framework, we also calculated the absorption spectra of P3HT by considering the electronic energy barrier for electronic transitions that would satisfy Franck–Condon principle. Absorption spectra closely harmonize to experimental UV-Vis result. The model also reveals intra-chain electronic transitions to be the dominant a...

Solid State Ionics, Mar 31, 2004

Abstract A method for the automatized localization of proton sites in a crystal structure and of ... more Abstract A method for the automatized localization of proton sites in a crystal structure and of probable proton transport pathways is developed from a slightly modified bond valence (BV) concept that in contrast to conventional parameter sets accounts for the polarizabilities of the interacting particles. The method is tested for several recent structure determinations of inorganic solids with mobile protons. Peculiarities of the application to mixed conductors are discussed for the hydrogen molybdenum bronze phase H 0.95 MoO 3 .

ACS Sustainable Chemistry & Engineering, Dec 8, 2020

The superior energy density renders hybrid Li-air batteries (HLABs) promising candidate energy st... more The superior energy density renders hybrid Li-air batteries (HLABs) promising candidate energy storage systems to enhance the sustainability of power grids. Nevertheless, HLABs operated at ambient temperature struggle to meet power and cycle life performance requirements for commercial application. At the same time, low-grade heat is abundantly available from industrial processes as well as from solar−thermal or geothermal sources, but there is a blatant lack of technologies to efficiently convert lowgrade waste heat into valuable electrical energy. We find that cells operated with an anolyte of tetraethylene glycol dimethyl ether and 1 M aqueous lithium hydroxide as the catholyte achieve a marked decrease in cell polarization with an increasing operation temperature of up to 80°C. Therefore, the energy efficiency, η, can be increased significantly. While the increase from η RT = 90% at room temperature to η 353K = 98% efficiency at a reference current density 0.03 mA cm −2 may appear gradual, the increase in efficiency becomes rapidly more prominent with increasing current density (e.g., from η RT = 59% to η 353K = 84% at 0.5 mA cm −2). The additional electrical energy that can be drawn from a HLAB heated by low-grade waste heat leads to a highly attractive heat-to-power conversion efficiency.

Journal of materials chemistry. A, Materials for energy and sustainability, 2015

Indonesian Journal of Materials Science, Dec 1, 2010

Applied Materials Today, Dec 1, 2022

Journal of Solid State Electrochemistry, Sep 20, 2017

In this project, we synthesized TiO 2 compounds through the molten salt method (MSM) using Ti(IV)... more In this project, we synthesized TiO 2 compounds through the molten salt method (MSM) using Ti(IV) oxysulfate, as the Ti source. Molten salts in the ratio of 0.375 M LiNO 3 :0.180 M NaNO 3 :0.445 M KNO 3 were added and heated at temperatures of 145, 280, 380, and 480°C for 2 h in air, respectively. A part of the sample prepared at 145°C was further reheated to 850°C for 2 h in air. X-ray diffraction studies showed that the amorphous phase was obtained when the sample was prepared at 145°C, and polycrystalline to crystalline anatase phase was formed when heated from 280 to 850°C, which is complementary to the results of selected area electron diffraction studies. Electrochemical properties were studied using galvanostatic cycling, cyclic voltammetry, and electrochemical impedance spectroscopy at a current density of 33 mA g −1 (0.1 C rate) and a scan rate of 0.058 mV s −1 , in the voltage range 1.0-2.8 V vs. Li. Electrochemical cycling profiles for the amorphous TiO 2 samples prepared at 145°C showed single-phase reaction with a low reversible capacity of 65 mAh g −1 , whereas compounds prepared at 280°C and above showed a two-phase reaction of Li-poor and Li-rich regions with a reversible capacity of 200 mAh g −1. TiO 2 produced at 280°C showed the lowest capacity fading and the lowest impedance value among the investigated samples.

Chemistry of Materials, Aug 27, 2021

Chemistry of Materials, Dec 2, 2021