A single-ion conducting covalent organic framework for aqueous rechargeable Zn-ion batteries

Despite their potential as promising alternatives to current state-of-the-art lithium-ion batteries, aqueous rechargeable Zn-ion batteries are still far away from practical applications. Here, we present a new class of single-ion conducting electrolytes based on a zinc sulfonated covalent organic framework (TpPa-SO3Zn0.5) to address this challenging issue. TpPa-SO3Zn0.5 is synthesised to exhibit single Zn2+ conduction behaviour via its delocalised sulfonates that are covalently tethered to directional pores and achieve structural robustness by its beta-ketoenamine linkages. Driven by these structural and physicochemical features, TpPa-SO3Zn0.5 improves the redox reliability of the Zn metal anode and acts as an ionomeric buffer layer for stabilising the MnO2 cathode. Such improvements in the TpPa-SO3Zn0.5-electrode interfaces, along with the ion transport phenomena, enable aqueous Zn-MnO2 batteries to exhibit long-term cyclability, demonstrating the viability of COF-mediated electrolytes for Zn-ion batteries

Theoretical Study on Interfacial Phenomena and Ion Transport in Energy Materials and Nanoparticles

School of Energy and Chemical Engineering (Chemical Engineering)The increasing demands on clean renewable energy resources to mitigate climate change is causing a huge shift towards the usage of electricity as the main power source. This drives the development and advancement of energy storage systems. Nanomaterials provide various possibilities in addressing this issue since materials at nanoscale exhibit distinct and tunable phyico-chemical properties compared to its bulk properties. Despite significant advancement in experimental techniques and apparatus, it is still difficult to have a holistic understanding on atomistic level phenomena occurring at the surface or interfaces. To this end, theoretical study based on multiscale simulation comes into play. The fast-growing computational power adds on to the staggering opportunity to accurately predict and virtually observe ion transport mechanism, interfacial phenomena, and numerous physical and chemical properties. Particularly in this dissertation, we will discuss the interplay of ion transport and interfacial phenomena in current collector design for Li-ion battery, solid electrolyte for Zn-ion battery, electrolyte additives design for Li-ion battery, and nucleation phenomena in Ag nanoparticles (nanocrystals) by the mean of multiscale molecular simulation. In this dissertation, we first underlined the motivation behind our studies on energy materials and nanoparticles. The scope of the interfacial phenomena and ion transport in nanomaterials covered are mainly focused on thermodynamics parameters which can be used as guiding principle in energy applications. We further discussed on the multiscale molecular simulation method which we used as our experimental tools, namely density functional theory (DFT), Monte Carlo (MC), and molecular dynamics (MD) simulations. These tools allow us to probe quantum level up to molecular level phenomena and dynamics. In the second chapter, we discussed the thermodynamic parameters involved in current collector design for anode-less Li-ion battery. To achieve 2D uniform Li deposition, it is necessary that the current collector possess a low nucleation overpotential. The Li adsorption energy on the current collector surface signifies the thermodynamic nucleation overpotential and the interfacial energy describes the thermodynamic stability of the Li interface during plating. The hBN/Cu Janus current collector proposed in this chapter showed superior properties as compared to graphene/Cu current collector. The hBN/Cu shows low thermodynamic nucleation overpotential while also demonstrates negative interfacial energy that enables the 2D uniform Li deposition. Furthermore, we also could confirm the capability of hBN/Cu current collector to suppress the galvanic corrosion by limiting the charge transfer during Li plating. In Chapter 3, we investigated interfacial phenomena and ion transport related with silver nanocrystals nucleation mechanism. The interfacial energetics that describes thermodynamic stability of the interface could be used to elucidate growth mechanism in uniform Ag nanocubes synthesis. The heterogenous nucleation of Ag nanocubes through AgCl particles was achieved through strong reduction agent (i.e., DMF) that allows Ag+ dissociation from AgCl (100) surface. The stability of AgCl (100) induced the formation of Ag (100) surface, this was indicated by the low interface formation energy, negative interfacial energy, and low strain energy. The presence of PVP surface directing agent further stabilized the Ag (100) to allow Ag (100) surface to the dominant surface, thus formed uniform Ag nanocubes. We also further investigated early nucleation phenomena of Ag nanocrystals by investigating shear rate and solvent environment reduction strength by developing new MD simulation protocol to model Ag+ dynamics and reduction leading to the formation of Ag clusters. We found that the stronger the shear rate and solvent reduction strength the larger the clusters produced. Interestingly, the strong shear rate also induced the formation of flatter and more rounded clusters. In the last chapter (Chapter 4), we presented in-depth studies on ion transport and dynamics in single-ion conduction covalent organic framework (COF) for aqueous Zn-ion battery and in electrolyte additives design for high-performance Li-ion battery. We demonstrated that the high stability of the aqueous Zn-ion battery with COF solid electrolyte was originated from the superior ion conduction behavior of the TpPa-SO3Zn0.5 as compared to the conventional liquid electrolyte (LE). The migration of Zn2+ ions under electric field clearly shows that in the TpPa-SO3Zn0.5 the Zn-ions were transported in relatively uniform distribution in z-direction, while in the LE irregularity was observed. This can be further confirmed through the fraction of Zn-O coordination, in the TpPa-SO3Zn0.5 coordination with O from H2O was far more dominant as compared to that of LE (i.e., mostly coordinated with O (SO4 2-). In the following section, we discussed on how the dynamics of additives (Li-salts) can be used to rationalize the SEI formation. We showed that the interesting behavior of DFBP- and NO3- anion to move towards the anode in the direction of electric field with orderly distribution was important to ensure the dual-layer SEI formation (i.e., LiF and Li3N) could occur at the anode surface. The low LUMO energy level of DFBP- anion further confirmed that once it reached the anode it can act as F-source which enables the formation of LiF layer. The same goes for the NO3- anion since it has the second lowest LUMO energy level. Hence, through the MD simulation on ion transport dynamics and DFT calculation, we could provide theoretical evidence on the dual-layer SEI formation mechanism.clos

2D HS Free Energy Calculation through Free Volume Approach

For the free energy calculation, statistical geometry approach has been developed and explored extensively to obtain simple yet accurate free energy value of hard-sphere system. Previous study has shown that free volume combined with Molecular Dynamics (MD) simulation can be used to estimate the free energy of 3D hard-sphere system accurately. The idea was extended to calculate the free energy of 2D hard-sphere system unlike 2D hard-disk system. Because of its close resemblance of realistic 2D material (e.g., graphene), it cannot be treated simply as the conventional disk-like 2D system, rather its 3D characteristics coming from spherical shape of atom, wrinkles, and corrugation must be taken into consideration. Thus, we employed unconfined 2D hard-sphere model. Our result shows that the free energy of 2D hard-sphere system was comparable to 2D hard-disk system only when the system height is below a dimensional transition critical point. The discrepancy can be attributed to the 3D characteristics, which could be quantified into a mathematical formulation

Molecular Dynamics Study on 2D Hard-sphere Dimensional Transition

Hard-core particles (i.e. hard-rod, hard-disc, hard-sphere) have been extensively studied as a fundamental model to describe numerous physical phenomena occurring in systems of purely repulsive nature. 3D hard-sphere has been used to explain colloidal system behavior including melting and freezing phenomena. 2D hard-disc was mainly investigated on its correlation with theoretical equations. In this study, we are interested to extend hard-sphere model to describe two-dimensional systems (e.g. graphene, h-BN) behavior through molecular dynamics study. As opposed to common approach in investigating 2D hard-sphere, where monolayer hard-spheres are confined between rigid walls, we employed periodic boundary condition across all directions. We examined thermodynamic properties of the system in 3D tensor form. The distribution of transversal and lateral pressure can be used to observe the dimensional transition of the system. We found that the dimensional transition occurs when the height of the model system is about 10% of the hard-sphere diameter. Below this point, the system behaves comparably to 2D hard-disc system, while higher height allows the system to behave as 3D system

Molecular Dynamics Study on Transition of Hard-Sphere System from 2D to 3D

Hard-sphere system has been studied extensively in the past decades as a fundamental model to study solid and fluid systems behavior. In this study we utilize molecular dynamics to observe 2D hard-sphere system behavior. Instead of employing a common approach in 2D hard-sphere system study, where a monolayer of hard-sphere confined between rigid walls, we introduced a unique approach to observe the dimensional characteristic changes on hard-sphere system. We examined system thermodynamic properties in tensor form, distribution changes in transversal and lateral pressure indicates the transitions of the system. We found that the characteristics changes happens when the system height is around 10% of the sphere diameter, beyond this point the system behaves as a 3D hard-sphere system

Computational Study on Sodium Metal Platting on Pre-patterned Current Collector for Highly Rechargeable Seawater Battery

Clean energy resources and storage have received immense attention to address various issues related with global warming and climate change. Li-ion battery has been widely used as the prime clean energy storage. Despite extensive research and development on Li-ion battery technology, tremendous increase in Li metal demand has become another concern. Seawater battery has emerged as a promising energy storage to address this concern owing to the abundance supply of sodium ions. However, dendrite growth during charge-discharge cycles poses a significant challenge on Na battery performance and safety. To mitigate this issue, we presented an in-depth study to determine Na metal growth on the current collector. We used Density Functional Theory (DFT) calculation to elucidate Na plating preference on different metals (i.e., Au, Ag, Cu, Al, and Ni) of the pre-patterned current collector by examining interfacial stability of Na with each metal. Interfacial stability was assessed based on the work of adhesion and binding energy of each interfaces. We found that Cu/Al patterned current collector has decent interfacial stability while keeping the cost minimum