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Open Access Publications from the University of California

Scholarly Works (14 results)

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

A major limit of electric vehicle performance is the energy density of available mobile energy storage systems. Lithium ion (Li-ion) batteries are now well-known and have enabled many portable electronics as well as electronic vehicles. However, for greater market penetration and range competitiveness, a battery with greater practical energy density must be designed. This dissertation focuses on three methods of improving energy density in lithium based batteries: lithium oxygen (Li-O2) batteries, Li-rich cathodes for Li-ion batteries, and high voltage operation of simple transition metal oxide cathodes for Li-ion batteries. To evaluate each of these technologies, the reactivity of each system is monitored.

Li-O2 batteries, a “beyond Li-ion" battery technology, have a very high theoretical energy density that is difficult to realize. After initial excitement surrounding the novel chemistry, many challenges associated with Li-O2 batteries have been highlighted in the past decade. Among these challenges, the reactivity of oxygen in the system is one of the most pressing. In this dissertation, the possible stability of LiO2, an advantageous discharge product to the typical Li2O2, is examined alongside binder degradation in the system.

As the workings of a Li-ion battery require the removal and intercalation of Li, the theoretical capacity is determined by the amount of Li available for extraction from the cathode. Consequently, a method of increasing energy density in Li-ion batteries is to increase the stoichiometric ratio of Li in the cathode material. These "Li-rich" cathode materials demonstrate large capacities, but must compensate the additional Li with cation double redox or anion redox. The electrochemical cells must be operated to > 4.5 V vs Li/Li+ to reach these additional capacities, and this operation results in greater reactivity and instabilities. This dissertation examines the trends of high voltage instability, especially as it relates to oxygen redox, in these Li-rich cathode materials.

Typical Li-ion batteries utilize only a fraction of the theoretical capacity available, only extracting around half of the available lithium from the layered transition metal oxide cathode material during charge. This is due to enhanced degradation mechanisms and reduced cyclability when additional Li is extracted at the necessitated higher voltages. Enabling operation of layered transition metal oxides at high voltages would result in increased capacity without the need for “beyond Li-ion" technologies. But first, the instabilities associated with Li extraction at voltages beyond the typical cut-off must be well-studied. Currently, the stability and degradation mechanisms of cathode materials even as common as LiCoO2 remain unclear. In studies presented in this dissertation, high voltage reactivity for layered transition metal oxide cathode materials is investigated.

The main conclusions of the studies presented here are drawn from measurements monitoring the reactivity of each of the aforementioned technologies. Among the measurements presented in these studies, outgassing and gas evolution measurements by differential electrochemical mass spectrometry have proved paramount in utility. As cell reactivity and instability at high voltages is often accompanied by outgassing, these measurements have assisted in the elucidation of instability origins. Practical application of Li-O2 batteries remains elusive as additional instabilities are discovered, Li-rich cathode materials show promise as various methods of mitigating high voltage instabilities are discussed, and the major sources of high voltage reactivity of layered transition metal oxide cathode materials are evaluated here.

    Cover page: High Voltage Reactivity in Lithium Batteries
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2020)
    Owing to the capacity boost from oxygen redox activities, Li-rich cation-disordered rocksalts (LRCDRS) represent a new class of promising high-energy Li-ion battery cathode materials. Redox-inactive transition-metal (TM) cations, typically d0 TM, are essential in the formation of rocksalt phases, however, their role in electrochemical performance and cathode stability is largely unknown. In the present study, the effect of two d0 TM (Nb5+ and Ti4+ ) is systematically compared on the redox chemistry of Mn-based model LRCDRS cathodes, namely Li1.3 Nb0.3 Mn0.4 O2 (LNMO), Li1.25 Nb0.15 Ti0.2 Mn0.4 O2 (LNTMO), and Li1.2 Ti0.4 Mn0.4 O2 (LTMO). Although electrochemically inactive, d0 TM serves as a modulator for oxygen redox, with Nb5+ significantly enhancing initial charge storage contribution from oxygen redox. Further studies using differential electrochemical mass spectroscopy and resonant inelastic X-ray scattering reveal that Ti4+ is better in stabilizing the oxidized oxygen anions (On - , 0 < n < 2), leading to a more reversible O redox process with less oxygen gas release. As a result, much improved chemical, structural and cycling stabilities are achieved on LTMO. Detailed evaluation on the effect of d0 TM on degradation mechanism further suggests that proper design of redox-inactive TM cations provides an important avenue to balanced capacity and stability in this newer class of cathode materials.
      Cover page: Role of Redox‐Inactive Transition‐Metals in the Behavior of Cation‐Disordered Rocksalt Cathodes
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2020)
      Current trend of high-capacity Ni-rich layered cathode is to develop high-Ni and low-Co or Co-free oxides. The high Ni content layered oxides provide great advantages in capacity, cost and environmental benignity, similar to LiNiO2 parent compound. But they also inherit drawbacks in structural and chemical instability at high voltages upon cycling. Elemental substitution plays a profound role in addressing these challenges. This work elucidates the roles of Al doping in cationic redox and anionic oxygen activity in LiAlxNi1-xO2 using combined X-ray absorption spectroscopy (XAS), resonant inelastic X-ray scattering (RIXS) as well as operando differential electrochemical mass spectrometry (DEMS). Using synchrotron wide-angle X-ray scattering (WAXS), we extrapolate a general principle of phase transition and its coupling with lattice anionic oxygen redox. These findings shed light on the advancement of high-capacity, stable-cycling and safe-operating Ni-rich cathode materials for next generation Li-ion batteries.
        Cover page: Correlating the phase evolution and anionic redox in Co-Free Ni-Rich layered oxide cathodes
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2019)
        Increasing interest in high-energy lithium-ion batteries has triggered the demand to clarify the reaction mechanism in battery cathodes during high-potential operation. However, the reaction mechanism often involves both transition-metal and oxygen activities that remain elusive. Here we report a comprehensive study of both cationic and anionic redox mechanisms of LiNiO2 nearly full delithiation. Selection of pure LiNiO2 removes the complication of multiple transition metals. Using combined X-ray absorption spectroscopy, resonant inelastic X-ray scattering, and operando differential electrochemical mass spectrometry, we are able to clarify the redox reactions of transition metals in the bulk and at the surface, reversible lattice oxygen redox, and irreversible oxygen release associated with surface reactions. Many findings presented here bring attention to different types of oxygen activities and metal-oxygen interactions in layered oxides, which are of crucial importance to the advancement of a Ni-rich layered oxide cathode for high capacity and long cycling performance.
          Cover page: Unraveling the Cationic and Anionic Redox Reactions in a Conventional Layered Oxide Cathode
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2021)
          Thermal energy storage (TES) based on phase-change materials (PCMs) has many current and potential applications, such as climate control in buildings, thermal management for batteries and electronics, thermal textiles, and transportation of pharmaceuticals. Despite its promise, the adoption of TES has been limited, in part due to limited tunability of the transition temperature, which hinders TES performance for varying use temperatures. Transition temperature tuning of a material using an external stimulus, such as pressure or an electric field, typically requires very large stimuli. To circumvent this problem, here, we report on the dynamic transition temperature tunability of a PCM using ions. We achieve a transition temperature tunability up to 6°C in polyethylene glycol (PEG) by using the salt lithium oxalatodifluoroborate at a low voltage of 2.5 V, which may enable simpler and safer devices/system designs. We also explain the thermal properties of the salt/PCM solution using the Flory-Huggins theory.
            Cover page: Dynamic tunability of phase-change material transition temperatures using ions for thermal energy storage
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2018)
            Li-excess cathodes comprise one of the most promising avenues for increasing the energy density of current Li-ion technology. However, the first-cycle surface oxygen release in these materials causes cation densification and structural reconstruction of the surface region, leading to encumbered ionic transport and increased impedance. In this work, we use the first principles Density Functional Theory to systematically screen for optimal cation dopants to improve oxygen-retention at the surface. The initial dopant set includes all transition metal, post-transition metal, and metalloid elements. Our screening identifies Os, Sb, Ru, Ir, or Ta as high-ranking dopants considering the combined criteria, and rationalization based on the electronic structure of the top candidates are presented. To validate the theoretical screening, a Ta-doped Li1.3Nb0.3Mn0.4O2 cathode was synthesized and shown to present initial improved electrochemical performance as well as significantly reduced oxygen evolution, as compared with the pristine, un-doped, system.
              Cover page: Alleviating oxygen evolution from Li-excess oxide materials through theory-guided surface protection
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2018)
              Recent reports on high capacities delivered by Li-excess transition-metal oxide cathodes have triggered intense interest in utilizing reversible oxygen redox for high-energy battery applications. To control oxygen electrochemical activities, fundamental understanding of redox chemistry is essential yet has so far proven challenging. In the present study, micrometer-sized Li1.3Nb0.3Mn0.4O2 single crystals were synthesized for the first time and used as a platform to understand the charge compensation mechanism during Li extraction and insertion. We explicitly demonstrate that the oxidation of O2- to On- (0 < n < 2) and O2 loss from the lattice dominates at 4.5 and 4.7 V, respectively. While both processes occur in the first cycle, only the redox of O2-/On- participates in the following cycles. The lattice anion redox process triggers irreversible changes in Mn redox, which likely causes the voltage and capacity fade observed on this oxide. Two drastically different redox activity regions, a single-phase behavior involving only Mn3+/4+ and a two-phase behavior involving O2-/On- (0 ≤ n < 2), were found in LixNb0.3Mn0.4O2 (0 < x < 1.3). Morphological damage with particle cracking and fracturing was broadly observed when O redox is active, revealing additional challenges in utilizing O redox for high-energy cathode development.
                Cover page: Unravelling Solid-State Redox Chemistry in Li1.3Nb0.3Mn0.4O2 Single-Crystal Cathode Material
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2017)
                Recent progress in the understanding of percolation theory points to cation-disordered lithium-excess transition metal oxides as high-capacity lithium-ion cathode materials. Nevertheless, the oxygen redox processes required for these materials to deliver high capacity can trigger oxygen loss, which leads to the formation of resistive surface layers on the cathode particles. We demonstrate here that, somewhat surprisingly, fluorine can be incorporated into the bulk of disordered lithium nickel titanium molybdenum oxides using a standard solid-state method to increase the nickel content, and that this compositional modification is very effective in reducing oxygen loss, improving energy density, average voltage, and rate performance. We argue that the valence reduction on the anion site, offered by fluorine incorporation, opens up significant opportunities for the design of high-capacity cation-disordered cathode materials.The performance of lithium-excess cation-disordered oxides as cathode materials relies on the extent to which the oxygen loss during cycling is mitigated. Here, the authors show that incorporating fluorine is an effective strategy which substantially improves the cycling stability of such a material.
                  Cover page: Mitigating oxygen loss to improve the cycling performance of high capacity cation-disordered cathode materials
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2018)
                  K-ion batteries are promising alternative energy storage systems for large-scale applications because of the globally abundant K reserves. K-ion batteries benefit from the lower standard redox potential of K/K+ than that of Na/Na+ and even Li/Li+, which can translate into a higher working voltage. Stable KC8 can also be formed via K intercalation into a graphite anode, which contrasts with the thermodynamically unfavorable Na intercalation into graphite, making graphite a readily available anode for K-ion battery technology. However, to construct practical rocking-chair K-ion batteries, an appropriate cathode material that can accommodate reversible K release and storage is still needed. We show that stoichiometric KCrO2 with a layered O3-type structure can function as a cathode for K-ion batteries and demonstrate a practical rocking-chair K-ion battery. In situ X-ray diffraction and electrochemical titration demonstrate that KxCrO2 is stable for a wide K content, allowing for topotactic K extraction and reinsertion. We further explain why stoichiometric KCrO2 is unique in forming the layered structure unlike other stoichiometric K-transition metal oxide compounds, which form nonlayered structures; this fundamental understanding provides insight for the future design of other layered cathodes for K-ion batteries.
                    Cover page: Stoichiometric Layered Potassium Transition Metal Oxide for Rechargeable Potassium Batteries
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2017)
                    We show that a common Li-O2 battery cathode binder, poly(vinylidene fluoride) (PVDF), degrades in the presence of reduced oxygen species during Li-O2 discharge when adventitious impurities are present. This degradation process forms products that exhibit Raman shifts (∼1133 and 1525 cm-1) nearly identical to those reported to belong to lithium superoxide (LiO2), complicating the identification of LiO2 in Li-O2 batteries. We show that these peaks are not observed when characterizing extracted discharged cathodes that employ poly(tetrafluoroethylene) (PTFE) as a binder, even when used to bind iridium-decorated reduced graphene oxide (Ir-rGO)-based cathodes similar to those that reportedly stabilize bulk LiO2 formation. We confirm that for all extracted discharged cathodes on which the 1133 and 1525 cm-1 Raman shifts are observed, only a 2.0 e-/O2 process is identified during the discharge, and lithium peroxide (Li2O2) is predominantly formed (along with typical parasitic side product formation). Our results strongly suggest that bulk, stable LiO2 formation via the 1 e-/O2 process is not an active discharge reaction in Li-O2 batteries.
                      Cover page: Poly(vinylidene fluoride) (PVDF) Binder Degradation in Li–O2 Batteries: A Consideration for the Characterization of Lithium Superoxide
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