Research

Tackling Solid-State Electrochemical Interfaces from Density Functional Theory (DFT) and Machine Learning (ML) Techniques

Objective:
• Gain fundamental understanding of current materials limits and identify key materials parameters for optimizing the performance of solid state batteries (SSBs);
• Determine structure-property relationships of the heterostructural solid-state interfaces (SSIs) in SSBs systems by corroborating atomic scale theory with experiment

Theoretical Study of Spinel Electrode Materials in Lithium-Ion Batterie

The high energy efficiency of lithium-ion batteries (LIBs) allows various applications in portable electronics and electric vehicles. Extensive research has been devoted to electrode materials aiming to design the electrodes with high power and capacity. The improvement of electrode materials in terms of cost and performance is hindered by the lack of fundamental understanding on discharge/charge mechanisms.
Here, density functional theory (DFT) calculations are performed to tackle the challenges. Our research focuses on a mechanistic study of spinel anode materials.
Objective:
• Gain better understanding on structures, mechanisms and optimal sites for rate performance, which were synthesized and characterized experimentally;
• Identify key descriptors that control the rate performance, capacity and durability;
• Optimize material at a theoretical level based on key descriptors.

Accomplishments:
1. Investigated lithiation mechanisms of spinel AFe2O4 (A = Zn, Mg, Cu) at atomic level and identified the key descriptors that control the rate performance and cyclability based on the bulk and surface/interface calculations
(Chem. Mater., Phys. Chem. Chem. Phys., ACS Appli. Mater. Interfaces, Adv. Mater. Interfaces, J. Electrochem. Soc.)
2. Conducted STEM image simulation and spectroscopy simulation to explain experimental observations
3. Investigated doping effects on electronic structures and electrochemical properties of Li4Ti5O12 (Ca, Nb, F-doped)
(Chem. Mater.)
4. Investigated doping effects on surface stability and morphology of hollandite α-MnO2.

Spinel ferrites, such as ZnFe2O4 and MgFe2O4, are prospective anode materials in LIBs with relatively high theoretical capacity and abundant reserve. Nevertheless, these ferrites materials suffer from capacity fading upon cycling.
To solve the problem, we conducted a detailed mechanistic study with DFT method. The initial lithiation process of ferrites was systematically studied. The key intermediates were determined based on bulk calculations, which were confirmed by experimental techniques, such as X-ray diffraction (XRD), X-ray absorption near-edge structure (XANES) measurements, and cyclic voltammetry (CV) profile.
However, a divergence was observed in DFT estimated average cell voltages with experiments in early lithiation process, which was likely due to the effect of surface reactions. To verify our assumption, a detailed surfaces calculation was conducted. The phase diagrams of ferrites materials were plotted to identify the stable exposed facets, which were further implemented for Li ions adsorption and/or diffusion. With surfaces modeling, the experimental lithiation voltages were well reproduced, confirming the essential role of surfaces in early lithiation stage.
Based on the key descriptors identified from bulk and surfaces calculations, we are currently working on the screening of spinel ferrites AFe2O4 (A = +2 cation) as LIBs electrode materials.

Computational Modeling of DNA-Protein Interaction

DNA oxidization is one of the deleterious mutations in organisms, and 8-oxoguanine (8-oxoG) is the most common DNA lesions product. Since the mutation is detrimental, most organisms have enzymes that can target and repair it. The formamidopyrimidine-DNA glycosylase (Fpg) protein is one of the 8-oxoG repair protein. The crystal structures of Fpg-DNA complex have been extensively studied in previous research. However, the mechanisms of recognition 8-oxoG by Fpg protein still remains unclear due to the complex dynamics processes.
Hereby, we conducted molecular dynamics (MD) simulations on Fpg protein, implementing TIP3P explicit solvent model. During the simulations, a conformational variation of active sites in Fpg protein was observed; meanwhile, the hydrogen bonds were broken between the protein and lesion DNA, indicating that there are a series of conformation changes of Fpg protein in the recognition of 8-oxoG, and eventually the Fpg-DNA complex was formed in an energetic preferred pathway. (Thesis)

Nano Wave-Absorption Compound Materials

1. White‐fungus‐like NiSx microspheres were synthesized on a large scale implementing a simple hydrothermal method. The influence of the reaction time and the surfactant on the final products was investigated, with the formation mechanism discussed. The synthesized white‐fungus‐like NiSx microspheres were used firstly as fillers in the fabrication of NiSx/polyvinylidene fluoride (PVDF) composites. Relationships between the loadings of the NiSx and wave‐absorption properties of the composites were analyzed, with the mechanism demonstrated. (ChemPlusChem)
2. Bi2S3 nanorods were synthesized on a large scale through a simple hydrothermal method at low temperature. To investigate the wave‐absorption properties of Bi2S3 nanorods, the homogeneous nanocomposites consisting of the nanorods and paraffin or PVDF matrix were fabricated and characterized. The results indicated that the introduction of Bi2S3 nanorods to PVDF significantly enhanced the wave‐absorption properties of the absorber. The loss mechanism of Bi2S3/PVDF nanocomposites with different loadings was discussed and theories were employed to explain the mechanism of the enhancement based on the experimental results. (ChemPlusChem)
3. CoxSy nanorods were synthesized via a simple hydrothermal method, utilizing ethanediamine (EDTA). By mixing CoxSy porous nanorod with PVDF, a composite material was prepared. The wave absorbing, dielectric and mechanical properties of the composite material were measured and analyzed. The composite material showed outstanding wave absorption properties under the frequency range of 2– 18 GHz. In a loading of as low as 5 wt%, the reflection loss of the composite material can reach – 45 dB, and the band is relatively wide. The mechanism of the wave absorption enhancement properties was discussed. (Graduation Thesis)

The Impact of Eucalyptus Planting on Soil Properties

Since the year of 2010, a rumor has become popularized that the large-scale planting of eucalyptus caused serious damage to the nutritions storage properties of soil and led to the drought in Yunnan province, China. To investigate the issue, we carried out a series of field studies in Malong County, Qujing, Yunnan and conducted soil test experiments in analytical chemistry laboratory. During the field research, the density of plants including trees and shrubs, were carefully counted. However, little differences were observed between the eucalyptus forests and the forests with birch, pines, and firs. Furthermore, to better qualify and quantify the effect of eucalyptus on the soil properties, analytical chemistry experiments were conducted via referring to the Environmental Quality Standard for Soils (GB). A series of soil samples were collected in the range of depths from 20 cm to 40 cm in various regions of different eucalyptus forests; while the blank samples were collected from the forests with pines. The analysis on the concentration of organic substances indicated that the nutritions in the soil were mainly retained in the upper layer and were not degraded significantly in eucalyptus forests compared with the blank group. (Thesis)

Interests

Density Functional Theory
Solid-State Batteries
Electrode Materials
Sulfide Electrolytes
Surfaces and Interfaces

Publications

HaoYue Guo

haoyue1619@gmail.com

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