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Open Access Publications from the University of California

Scholarly Works (11 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2024)

Energy storage technology is one of the most important technological pillars for achieving reduced carbon emissions to decelerate global warming, together with clean energy generation technologies such as solar energy. The development of a low-cost, safe, and high-energy-density energy storage technology is essential for accelerating the electrification of transportation, as well as the widespread adoption of grid energy storage systems.

The search for advanced energy storage technologies has increasingly focused on all-solid-state batteries (ASSBs) due to their potential for improved safety and higher energy density. Inorganic lithium superionic conductors are crucial to this emerging technology, offering fast ion transport that rival those of liquid-based systems. However, the challenge lies in finding materials that not only provide superionic conductivity but also meet all practical requirements, underscoring the need for the discovery of novel conductors. The development and identification of new functional materials could significantly transform energy storage technology, particularly as the limitations of current leading technologies are often tied to basic material properties.

Understanding the structure-property relationship is at the core of the field of materials science. By understanding how the bulk crystal structure, various defects and microstructures, or local structures within disordered or amorphous systems affect ionic conductivity, the rational design of inorganic lithium superionic conductors can be achieved. In particular, because fast ion transport through the bulk is often a necessary condition in energy storage devices that require a high current of lithium ions, it is essential to understand the structural and chemical features that lead to high bulk ionic conductivities of crystalline materials.

This thesis comprehensively investigates the diverse structural and chemical factors that improve ionic conductivity, and the atomistic mechanism by which each factor affects them. Chapter 1 introduces the computational and experimental background for understanding lithium-ion diffusion in inorganic materials as well as some of the chemical factors that can be used to optimize a fast conductor that already has desirable structural features. What often plays a decisive role is the structural feature; the structural features of inorganic crystals dictate whether it can be optimized to a superionic conductor or not.

In this dissertation, we decompose the crystal into the framework (i.e., non-lithium cation sites) and lithium-ion diffusion network (i.e., lithium-ion sites), with the former discussed in Chapter 2 and the latter discussed in Chapter 3. In Chapter 4, we investigate structures in which the framework is rotationally mobile and analyze whether the rotational motion of anion groups leads to faster lithium-ion diffusion, also known as the paddlewheel effect. Chapter 5 investigates a remarkable class of ultrafast lithium-ion conductors with van der Waals-bonded frameworks. This chapter demonstrates how the mechanisms of ion transport discussed in Chapters 2, 3, and 4 result in the highest ionic conductivity predicted from computational studies so far. Finally, in Chapter 6, I provide an overview of how the diverse structural and chemical mechanisms of fast lithium-ion diffusion have led to the development of various classes of state-of-the-art superionic conductors by 2024. Here, I attempt to put together a global conceptual framework of various mechanisms that may be used for superionic conductors. Finally, I end with my perspective on future research avenues that may accelerate the discovery and optimization of fast lithium-ion conductors for the accelerated deployment of safer next-generation energy storage technologies.

    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2023)
    Amorphous Li-P-S materials have been widely used as solid-state electrolytes for all-solid-state batteries because of their high ionic conductivity (10-4 to 10-3 S cm-1) as well as good synthetic accessibility and processability. Despite the potential of these materials, their amorphous structures have made it challenging to quantify the relation between the structure and conductivity. In this paper, we use ab initio molecular dynamics simulations to investigate the role of the local structure and density in determining the conductivity of amorphous Li-P-S structures with different polyanion units. We observe similar rates for Li-ion hopping regardless of the local P-S polyanion environment in these amorphous materials, indicating that the path connectivity at a larger length scale may be controlling the overall Li conductivity. This finding will serve as an important guideline in the continued development of amorphous solid electrolytes for advanced all-solid-state batteries.
      Cover page: Weak Correlation between the Polyanion Environment and Ionic Conductivity in Amorphous Li–P–S Superionic ConductorsCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2024)
      Since the 1980s, the paddlewheel effect has been suggested as a mechanism to boost lithium-ion diffusion in inorganic materials via the rotation of rotor-like anion groups. However, it remains unclear whether the paddlewheel effect, defined as large-angle anion group rotations assisting Li hopping, indeed exists; furthermore, the physical mechanism by which the anion-group dynamics affect lithium-ion diffusion has not yet been established. In this work, we differentiate various types of rotational motions of anion groups and develop quaternion-based algorithms to detect, quantify, and relate them to lithium-ion motion in ab initio molecular dynamics simulations. Our analysis demonstrates that, in fact, the paddlewheel effect, where an anion group makes a large angle rotation to assist a lithium-ion hop, does not exist and thus is not responsible for the fast lithium-ion diffusion in superionic conductors, as historically claimed. Instead, we find that materials with topologically isolated anion groups can enhance lithium-ion diffusivity via a more classic nondynamic soft-cradle mechanism, where the anion groups tilt to provide optimal coordination to a lithium ion throughout the hopping process to lower the migration barrier. This anion-group disorder is static in nature, rather than dynamic and can explain most of the experimental observations. Our work substantiates the nonexistence of the long-debated paddlewheel effect and clarifies any correlation that may exist between anion-group rotations and fast ionic diffusion in inorganic materials.
        Cover page: The nonexistence of a paddlewheel effect in superionic conductors
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2021)
        The key component in lithium solid-state batteries (SSBs) is the solid electrolyte composed of lithium superionic conductors (SICs). Lithium oxide SICs offer improved electrochemical and chemical stability compared with sulfides, and their recent advancements have largely been achieved using materials in the garnet- and NASICON (sodium superionic conductor)- structured families. In this work, using the ion-conduction mechanisms in garnet and NASICON as inspiration, a common pattern of an “activated diffusion network” and three structural features that are beneficial for superionic conduction: a 3D percolation Li diffusion network, short distances between occupied Li sites, and the “homogeneity” of the transport path are identified. A high-throughput computational screening is performed to search for new lithium oxide SICs that share these features. From this search, seven candidates are proposed exhibiting high room-temperature ionic conductivity evaluated using ab initio molecular dynamics simulations. Their structural frameworks including spinel, oxy-argyrodite, sodalite, and LiM(SeO3)2 present new opportunities for enriching the structural families of lithium oxide SICs.
          Cover page: Lithium Oxide Superionic Conductors Inspired by Garnet and NASICON Structures
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2023)
          The Li2S-P2S5 pseudo-binary system has been a valuable source of promising superionic conductors, with α-Li3PS4, β-Li3PS4, HT-Li7PS6, and Li7P3S11 having excellent room-temperature Li-ion conductivity >0.1 mS/cm. The metastability of these phases at ambient temperature motivates a study to quantify their thermodynamic accessibility. Through calculating the electronic, configurational, and vibrational sources of free energy from first principles, a phase diagram of the crystalline Li2S-P2S5 space is constructed. New ground-state orderings are proposed for α-Li3PS4, HT-Li7PS6, LT-Li7PS6, and Li7P3S11. Well-established phase stability trends from experiments are recovered, such as polymorphic phase transitions in Li7PS6 and Li3PS4, and the instability of Li7P3S11 at high temperature. At ambient temperature, it is predicted that all superionic conductors in this space are indeed metastable but thermodynamically accessible. Vibrational and configurational sources of entropy are shown to be essential toward describing the stability of superionic conductors. New details of the Li sublattices are revealed and are found to be crucial toward accurately predicting configurational entropy. All superionic conductors contain significant configurational entropy, which suggests an inherent correlation between fast Li diffusion and thermodynamic stability arising from the configurational disorder.
            Cover page: Crystal Structures and Phase Stability of the Li2S–P2S5 System from First Principles
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2023)
            Large-scale simulations with complex electron interactions remain one of the greatest challenges for atomistic modelling. Although classical force fields often fail to describe the coupling between electronic states and ionic rearrangements, the more accurate ab initio molecular dynamics suffers from computational complexity that prevents long-time and large-scale simulations, which are essential to study technologically relevant phenomena. Here we present the Crystal Hamiltonian Graph Neural Network (CHGNet), a graph neural network-based machine-learning interatomic potential (MLIP) that models the universal potential energy surface. CHGNet is pretrained on the energies, forces, stresses and magnetic moments from the Materials Project Trajectory Dataset, which consists of over 10 years of density functional theory calculations of more than 1.5 million inorganic structures. The explicit inclusion of magnetic moments enables CHGNet to learn and accurately represent the orbital occupancy of electrons, enhancing its capability to describe both atomic and electronic degrees of freedom. We demonstrate several applications of CHGNet in solid-state materials, including charge-informed molecular dynamics in LixMnO2, the finite temperature phase diagram for LixFePO4 and Li diffusion in garnet conductors. We highlight the significance of charge information for capturing appropriate chemistry and provide insights into ionic systems with additional electronic degrees of freedom that cannot be observed by previous MLIPs.
              Cover page: CHGNet as a pretrained universal neural network potential for charge-informed atomistic modellingCreative Commons 'BY' version 4.0 license
              • Article
              • Peer Reviewed
              LBL Publications (2022)
              Ion-exchange reactions are commonly used to develop novel metastable electrode materials for alkali-ion batteries that cannot be synthesized using direct chemical reactions. In this study, the electrochemical K to Na ion-exchange reaction mechanisms in a layered KxCoO2 cathode as a model system were investigated using operando and ex situ structure characterization techniques. Some level of K ions was observed to remain in the layered structure during the electrochemical ion-exchange reactions. Interestingly, the K ions are well separated from the Na-rich phases in the discharged state, and they form an intermediate phase in which K and Na ions are mixed at the top of charge. We discovered that such residual K ions prevent the collapse of the layered structure in the high-voltage regime, thereby improving the cycling stability in a Na-battery system.
                Cover page: Understanding of electrochemical K+/Na+ exchange mechanisms in layered oxides
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2023)
                The use of a sacrificial cathode additive that contains a large amount of lithium is one potential solution to compensate for the irreversible capacity loss associated with next-generation anodes such as silicon. Antifluorite-type Li6CoO4 has attracted attention as a potential cathode additive owing to its remarkably high theoretical lithium extraction capacity. However, the complex mechanism of lithium extraction as well as the oxygen loss from Li6CoO4 is not well understood. A generalizable computational thermodynamics and experimental framework is presented to understand the lithium-extraction pathway of Li6CoO4. It is found that one lithium per formula unit can be topotactically extracted from Li6CoO4, followed by an irreversible and nontopotactic phase transformation to Li2CoO3 or LiCoO2 depending on the temperature. The results show that peroxide species may form to charge-compensate for Li extraction which is undesirable as this can lead to gas release during battery operation. It is suggested that charging Li6CoO4 at an elevated temperature that the electrolyte can withstand, redirects the reaction pathway and prevents the formation of intermediate peroxide species making it an effective and stable sacrificial cathode additive.
                  Creative Commons 'BY-ND' version 4.0 license
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2024)
                  Metastable polymorphs often result from the interplay between thermodynamics and kinetics. Despite advances in predictive synthesis for solution-based techniques, there remains a lack of methods to design solid-state reactions targeting metastable materials. Here, we introduce a theoretical framework to predict and control polymorph selectivity in solid-state reactions. This framework presents reaction energy as a rarely used handle for polymorph selection, which influences the role of surface energy in promoting the nucleation of metastable phases. Through in situ characterization and density functional theory calculations on two distinct synthesis pathways targeting LiTiOPO4, we demonstrate how precursor selection and its effect on reaction energy can effectively be used to control which polymorph is obtained from solid-state synthesis. A general approach is outlined to quantify the conditions under which metastable polymorphs are experimentally accessible. With comparison to historical data, this approach suggests that using appropriate precursors could enable targeted materials synthesis across diverse chemistries through selective polymorph nucleation.
                    Cover page: Selective formation of metastable polymorphs in solid-state synthesis
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2022)
                    Superionic lithium conductivity has only been discovered in a few classes of materials, mostly found in thiophosphates and rarely in oxides. Herein, we reveal that corner-sharing connectivity of the oxide crystal structure framework promotes superionic conductivity, which we rationalize from the distorted lithium environment and reduced interaction between lithium and non-lithium cations. By performing a high-throughput search for materials with this feature, we discover ten new oxide frameworks predicted to exhibit superionic conductivity-from which we experimentally demonstrate LiGa(SeO3)2 with a bulk ionic conductivity of 0.11 mS cm-1 and an activation energy of 0.17 eV. Our findings provide insight into the factors that govern fast lithium mobility in oxide materials and will accelerate the development of new oxide electrolytes for all-solid-state batteries.
                      Cover page: Lithium superionic conductors with corner-sharing frameworks.Creative Commons 'BY-NC' version 4.0 license
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