Computational Study on Polymorphism and Charge Storage Mechanism of Battery Materials via Multiscale Simulation

Abstract

Rechargeable batteries have attracted a lot of attention owing to their wide applicability, such as portable/consumer electronics, electric vehicles, and grid-scale applications. Over the past two decades, significant advances have been made in battery technologies. However, advancement in various technologies necessitates batteries that are more efficient because the current levels of performance are inadequate. This has encouraged researchers to design and discover new battery materials to meet future demands. In this context, a fundamental understanding of the polymorphism and charge storage mechanism of battery materials can provide design principles and promote the discovery of novel materials. To achieve this, the multiscale simulation method has been used to study physicochemical phenomena or properties of different time and space scales. In this dissertation, we introduced theoretical studies on polymorphism and charge storage mechanism of battery materials. Specifically, we discussed three newly designed electrode materials, a conventional binder material, and a separator material. In Chapter 1, we provide an overview and the challenges of rechargeable batteries. We then present a general background of the charge storage mechanism and polymorphism phenomenon and their importance in the study and design of rechargeable battery materials. Finally, we describe the modern multiscale computational techniques for rechargeable battery materials such as the density functional theory calculation, density functional tight binding calculation, molecular dynamics simulation, and Monte Carlo simulation.

In Chapter 2, we present a theoretical study on the polymorphism and charge storage mechanism of contorted hexabenzocoronene (c-HBC) as a new type of anode material for Li-ion batteries. In this study, the packing polymorphism was demonstrated by disclosing the crystal structure of polymorph Ⅱ’, which is the metastable R-3 crystal phase, using computational polymorph prediction. It was also revealed that polymorph Ⅱ was not a polymorph of c-HBC; instead, it is the P31 (or P32) crystal phase of c-HBC with Pd atoms. Moreover, our investigation on the lithium storage mechanism showed that the c-HBC anode exhibited a single-stage Li-ion insertion behavior without voltage penalty, which was attributed to the 3D-ordered empty pores originating from the contorted structure of c-HBC.

In Chapter 3, we present a theoretical study on the polymorphism and charge storage mechanism of fluorinated-contorted hexabenzocoronene (F-cHBC) as a potential electrochemical organic electrode material. Based on Monte Carlo computational study, it was revealed that the crystal structure of polymorph I was the energetically stable P21/c crystal phase. Furthermore, theoretical investigation on lithium/sodium storage mechanism showed that Li- and Na-ions could be stored in two distinct sites surrounded by electronegative fluorine atoms and a negatively charged bent edge aromatic ring.

In Chapter 4, we present a theoretical study on the polymorphism and charge storage mechanism of the redox-active covalent triazine framework (rCTF) as a promising organic anode material for Li-ion batteries. The potential energy analysis suggested that the rCTF can potentially exhibit packing polymorphism for two energy-minimum packing modes, namely, AB and slipped-parallel packing modes. The most stable was the slipped-packing mode. Furthermore, we revealed that the rCTF provided a theoretical capacity of up to 1200 mAh g−1 using quinone, triazine, and benzene rings as the redox-active sites. The structural deformation of rCTF during activation allowed more redox-active sites to be accessible, especially the benzene rings.

In Chapter 5, we present a theoretical study on poly(vinylidene fluoride) (PVDF), which is a conventional polymeric binder material for rechargeable batteries. Although it is rarely considered in the battery field, PVDF is a semicrystalline polymer with various polymorphs that have different polarization characteristics. In this study, the effect of the crystal phases of PVDF, specifically α- and β-PVDFs, on battery performance was investigated. We showed that compared to negligible polarization of the paraelectric α-PVDF, the strong polarization generated by the ferroelectric β-PVDF can effectively transport electrons and Li-ions, leading to reduction in the charge transfer resistance and mitigation of the concentration polarization in the Li-ion battery system.

In Chapter 6, we present a theoretical study on polymorphism of chitin separator material and its interaction with electrolyte. As a semicrystalline biopolymer, chitin can exist in two polymorphs, α- and β-phase. These crystals have different molecular conformation and arrangement, resulting in different polarization characteristics. Based on density functional theory calculations and molecular dynamics simulations, we revealed that both polymorphs of chitin had excellent electrolyte-uptaking property and high physicochemical affinity to Li-ions with binding reversibility.