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Open Access Publications from the University of California

Scholarly Works (18 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2020)

The tremendous success and growth of Lithium (Li)-ion based energy storage in a broad range of applications is likely to strain our natural resources. Projected growth of Li-ion production towards 1 TWh/year will require more than a million tons of Co/Ni combined, which constitutes a very sizeable fraction of the annual production of these metals. The recent development of Li-excess cation-disordered rocksalt (DRX) cathodes, which require no separate Li layer and transition metal (TM) layer in the cation sublattice, is providing an avenue for the Li-ion battery field to develop high energy density cathodes with more abundant and less expensive metals, such as Mn, Fe and Ti. Unlike the traditional layered NCM cathodes in which Li ions transport within the well-defined Li layers, the Li ions diffuse through a “3-D” Li rich pathway (‘0-TM’ channel) in DRX cathodes, i.e., the feasible Li hops require that the intermediate tetrahedral sites along the Li diffusion path are coordinated only by Li (no transition metal). The complexity of this class of DRX cathodes mainly lies in the fact that lacking in long-range order though, DRX cathodes, in most cases, present various types of cation short-range order (SRO), which controls the frequency and connectivity of Li migration channels, thus, macroscopic Li transport.In this thesis, two general strategies to engineer the cation SRO in DRX cathodes will be demonstrated through compositional design, combining electrochemical tests, advanced characterizations and computational investigations. The first part of the thesis will talk about the control of SRO in DRX cathodes through fluorine doping on anion sublattices. The strong Li-F interaction modifies the cation SRO in DRX materials by forming Li-rich domains around F ions. Four well-chosen compositions within the Li–Mn–O–F DRX chemical space with different Li content and F content were synthesized, characterized, electrochemically tested and modeled: Li1.333Mn(III)0.667O1.333F0.667 (HLF67), Li1.333Mn(III)0.5Mn(IV)0.167O1.5F0.5 (HLF50), Li1.333Mn(III)0.333Mn(IV)0.333O1.667F0.333 (HLF33), and Li1.25Mn(II)0.167Mn(III)0.583O1.333F0.667 (LLF67). While all compositions tested achieve higher than 200 mAh/g initial capacity the material with high Li-excess (1.333 per formula unit, LixMn2-xO2-yFy) and moderate fluorination (0.333 per formula unit) achieves 349 mAh/g initial capacity and 1068 Wh/kg specific energy. Higher fluorination (0.667 per formula unit) leads to a less efficient Li diffusion network, resulting in a decrease in capacity (specific energy) to be 256 mAh/g (822 Wh/kg). However, the activation of Mn2+/Mn4+ redox enabled by F substitution significantly improve the cycle life of DRX cathodes, with more than 85% retained after 30 cycles even upon charging to 5.0V. It can thus be concluded that the Li-site distribution, which can be significantly modified by fluorine substitution, plays a more important role than the metal-redox capacity in determining the initial capacity, whereas the metal-redox capacity is more closely related to the cyclability of the materials. A design map will also be presented, which shows that the macroscopic Li transport is strongly controlled by the Li-excess level and F content within a fixed DRX chemical space and can be correlated to the observed capacities of a DRX cathode, as corroborated by electrochemical tests. Similar computational evaluations will also be shown in various different DRX chemistries to demonstrate that fluorination is a generalized strategy to engineering SRO in DRX cathodes. The second part of the thesis will talk about the control of SRO in DRX cathodes through TM doping on cation sublattices. With a fixed Li and F content, TM doping can frustrate the cation SRO in DRX cathodes by increasing configurational entropy to form so-called high-entropy (HE) DRX, thus improve the Li transport, i.e. capacity and rate capability in a significant way. As a demonstration, a group of DRX cathodes containing two, four or six TM species are designed. It is shown that SRO in these DRX cathodes systematically decreases, as a consequence, energy density and rate capability systematically increase as more metal cations are mixed together, even though the total metal content are fixed. Remarkably, a HE DRX cathode with six TM species achieves 307 mAh g−1 (955 Wh kg−1) at a low rate (20 mA g−1), and retains more than 170 mAh g−1 when cycling at a high rate of 2,000 mA g−1. The compatibility between various TM ions in DRX compounds is also investigated to facilitate future experimental design and a phase-pure HE DRX compound containing twelve TM species was successfully synthesized as a proof-of-concept.

    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2024)
    Artificial intelligence (AI) has emerged as a tool for discovering and optimizing novel battery materials. However, the adoption of AI in battery cathode representation and discovery is still limited due to the complexity of optimizing multiple performance properties and the scarcity of high-fidelity data. We present a machine learning model (DRXNet) for battery informatics and demonstrate the application in the discovery and optimization of disordered rocksalt (DRX) cathode materials. We have compiled the electrochemistry data of DRX cathodes over the past 5 years, resulting in a dataset of more than 19,000 discharge voltage profiles on diverse chemistries spanning 14 different metal species. Learning from this extensive dataset, our DRXNet model can capture critical features in the cycling curves of DRX cathodes under various conditions. Our approach offers a data-driven solution to facilitate the rapid identification of novel cathode materials, accelerating the development of next-generation batteries for carbon neutralization.
      Cover page: Deep learning of experimental electrochemistry for battery cathodes across diverse compositionsCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2021)
      Cation-disordered rocksalt (DRX) materials have emerged as a class of novel high-capacity cathodes for Li-ion batteries. However, the commercialization of DRX cathodes will require reducing their capacity decay, which has been associated with oxygen loss during cycling. Recent studies show that fluorination of DRX cathodes can effectively reduce oxygen loss and improve cycling stability; however, the underlying atomic-scale mechanisms remain elusive. Herein, using a combination of electrochemical measurements, scanning transmission electron microscopy, and electron energy loss spectroscopy, the correlation between the electrochemical properties and structural evolution in Mn-redox-based DRX cathodes, Li1.2Ti0.4–xMn0.4+xO2.0-xFx (x = 0 and 0.2) is examined. It is found that fluorination strongly suppresses structural amorphization and void formation initiated from the particle surface, therefore greatly enhancing the cyclability of the cathode. A novel rocksalt-to-spinel-like structural transformation in the DRX bulk is further revealed, which surprisingly contributes to a gradual capacity increase during cycling. The results provide important insight for the design of novel DRX cathodes with high capacity and long cycle life.
        Cover page: Fluorination‐Enhanced Surface Stability of Cation‐Disordered Rocksalt Cathodes for Li‐Ion Batteries
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2020)
        Fluorine substitution is a critical enabler for improving the cycle life and energy density of disordered rocksalt (DRX) Li-ion battery cathode materials which offer prospects for high energy density cathodes, without the reliance on limited mineral resources. Due to the strong Li–F interaction, fluorine also is expected to modify the short-range cation order in these materials which is critical for Li-ion transport. In this work, density functional theory and Monte Carlo simulations are combined to investigate the impact of Li–F short-range ordering on the formation of Li percolation and diffusion in DRX materials. The modeling reveals that F substitution is always beneficial at sufficiently high concentrations and can, surprisingly, even facilitate percolation in compounds without Li excess, giving them the ability to incorporate more transition metal redox capacity and thereby higher energy density. It is found that for F levels below 15%, its effect can be beneficial or disadvantageous depending on the intrinsic short-range order in the unfluorinated oxide, while for high fluorination levels the effects are always beneficial. Using extensive simulations, a map is also presented showing the trade-off between transition-metal capacity, Li-transport, and synthetic accessibility, and two of the more extreme predictions are experimentally confirmed.
          Cover page: Effect of Fluorination on Lithium Transport and Short‐Range Order in Disordered‐Rocksalt‐Type Lithium‐Ion Battery Cathodes
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2023)
          Li-rich rocksalt oxides are promising cathode materials for lithium-ion batteries due to their large capacity and energy density, and their ability to use earth-abundant elements. The excess Li in the rocksalt, needed to achieve good Li transport, reduces the theoretical transition metal redox capacity and introduces a labile oxygen state, both of which lead to increased oxygen oxidation and concomitant capacity loss with cycling. Herein, it is demonstrated that substituting the labile oxygen in Li-rich cation-disordered rocksalt materials with a vacancy is an effective strategy to inhibit oxygen oxidation. It is found that the oxygen vacancy in cation-disordered lithium manganese oxide favors high Li coordination thereby reducing the concentration of unhybridized oxygen states, while increasing the theoretical Mn capacity. It is shown that in the vacancy-containing compound, synthesized by ball milling, the Mn valence is lowered to less than +3, providing access to more than 300 mAh g−1 capacity from the Mn2+/Mn4+ redox reservoir. The increased transition metal redox and decreased O oxidation are found to improve the capacity and voltage retention, indicating that oxygen vacancy creation to remove the most vulnerable oxygen ions and reduce transition metal valence provides a new opportunity for the design of high-performance Li-rich rocksalt cathodes.
            Cover page: Oxygen Vacancy Introduction to Increase the Capacity and Voltage Retention in Li‐Excess Cathode MaterialsCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2020)
            Li-excess cation-disordered rocksalt (DRX) oxides have shown potential as high-energy-density Li-ion cathodes. They typically exploit O redox to achieve high capacity, which can trigger oxygen loss at the surface, thereby affecting the cathode performance. Here, we elucidate the impact that the surface structural evolution has on their electrochemical properties by comparing two prototypical DRX cathodes, Li1.2Ni0.333Ti0.333Mo0.133O2 (LNTMO) and Li1.2Mn0.6Nb0.2O2 (LMNO). Both cathodes achieve high capacity, but oxygen loss leads to significant polarization for LNTMO, whereas LMNO is far less affected. We show that while metal densification at the particle surface occurs for both materials, the resulting surface structure is strikingly different. A spinel phase forms at the surface of LMNO, which effectively alleviates oxygen loss and allows fast Li transport, whereas a densified DRX forms at the LNTMO surface, which impedes Li transport and cannot mitigate oxygen loss. These findings demonstrate the importance of the surface structure of DRX cathode.
              Cover page: The Impact of Surface Structure Transformations on the Performance of Li-Excess Cation-Disordered Rocksalt Cathodes
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2020)
              Using both computations and experiments, we demonstrate that the performance of Li-excess cation-disordered rocksalt cathodes can be improved by Mg substitution. Mg reduces the amount of Li in the compound that is strongly bound to F and thereby increases the capacity. This enables the use of fluorination as a tool to improve stability of the compounds without significant loss of capacity. Mg emerged as the most optimal substitution element from a systematic computational study aimed at identifying inactive doping elements with a strong enough bonding strength to fluorine to displace Li from the F environments. Our results also show that capacity can be traded for cycle life depending on whether Mg is substituted for Li or for the redox metal. This design strategy should be considered in fluorinated cathodes, which will facilitate the design of optimized disordered rocksalt oxyfluoride cathodes.
                Cover page: Increasing Capacity in Disordered Rocksalt Cathodes by Mg Doping
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2022)
                Relating the synthesis conditions of materials to their functional performance has long been an experience-based trial-and-error process. However, this methodology is not always efficient in identifying an appropriate protocol and can lead to overlooked opportunities for the performance optimization of materials through simple modifications of the synthesis process. In this work, the authors systematically track the structural evolution in the synthesis of a representative disordered rock salt (a promising next-generation Li-ion cathode material) at the scale of both the long-range crystal structure and the short-range atomic structure using various in situ and ex situ techniques, including transmission electron microscopy, X-ray diffraction, and pair distribution function analysis. An optimization strategy is proposed for the synthesis protocol, leading to a remarkably enhanced capacity (specific energy) of 313 mAh g−1 (987 Wh kg−1) at a low rate (20 mA g−1), with a capacity of more than 140 mAh g−1 retained even at a very high cycling rate of 2000 mA g−1. This strategy is further rationalized using ab initio calculations, and important opportunities for synthetic optimization demonstrated in this study are highlighted.
                  Cover page: Thermodynamically Driven Synthetic Optimization for Cation‐Disordered Rock Salt CathodesCreative Commons 'BY-NC' version 4.0 license
                  • Article
                  • Peer Reviewed
                  LBL Publications (2023)
                  Chemical short-range-order has been widely noticed to dictate the electrochemical properties of Li-excess cation-disordered rocksalt oxides, a class of cathode based on earth abundant elements for next-generation high-energy-density batteries. Existence of short-range-order is normally evidenced by a diffused intensity pattern in reciprocal space, however, derivation of local atomic arrangements of short-range-order in real space is hardly possible. Here, by a combination of aberration-corrected scanning transmission electron microscopy, electron diffraction, and cluster-expansion Monte Carlo simulations, we reveal the short-range-order is a convolution of three basic types: tetrahedron, octahedron, and cube. We discover that short-range-order directly correlates with Li percolation channels, which correspondingly affects Li transport behavior. We further demonstrate that short-range-order can be effectively manipulated by anion doping or post-synthesis thermal treatment, creating new avenues for tailoring the electrochemical properties. Our results provide fundamental insights for decoding the complex relationship between local chemical ordering and properties of crystalline compounds.
                    Cover page: Atomic-scale probing of short-range order and its impact on electrochemical properties in cation-disordered oxide cathodes.
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2021)
                    The tremendous improvement in performance and cost of lithium-ion batteries (LIBs) have made them the technology of choice for electrical energy storage. While established battery chemistries and cell architectures for Li-ion batteries achieve good power and energy density, LIBs are unlikely to meet all the performance, cost, and scaling targets required for energy storage, in particular, in large-scale applications such as electrified transportation and grids. The demand to further reduce cost and/or increase energy density, as well as the growing concern related to natural resource needs for Li-ion have accelerated the investigation of so-called "beyond Li-ion" technologies. In this review, we will discuss the recent achievements, challenges, and opportunities of four important "beyond Li-ion" technologies: Na-ion batteries, K-ion batteries, all-solid-state batteries, and multivalent batteries. The fundamental science behind the challenges, and potential solutions toward the goals of a low-cost and/or high-energy-density future, are discussed in detail for each technology. While it is unlikely that any given new technology will fully replace Li-ion in the near future, "beyond Li-ion" technologies should be thought of as opportunities for energy storage to grow into mid/large-scale applications.
                      Cover page: Promises and Challenges of Next-Generation “Beyond Li-ion” Batteries for Electric Vehicles and Grid Decarbonization
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