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Open Access Publications from the University of California

Scholarly Works (16 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2012)

In recent years, resistive random access memory (RRAM) has gained significant attention as one of the promising candidates for next generation memory applications. This is due to its anticipated advantages versus Flash technology with respect to high density, low power and fast read and write speed. The main operation mechanism of these devices is a resistance change induced by filament formation through metal-cations or oxygen vacancies.

In the first part of this work, a Kinetic Monte Carlo (KMC) simulator is built to study the filament formation process in an electrochemical metallization (ECM) RRAM. This simulator takes into account most important physical and chemical processes such as oxidation, reduction, metal crystallization, adsorption, desorption and ionic transportation. The characteristics of the forming voltage, forming time and switching speed are investigated. In addition, studies on filament overgrowth and on-state resistance distribution are presented. Further, filament topography, which strongly influences device properties, is studied under different device operation conditions. The simulator also predicts that depending on the strength of lateral electric field, the conductive filament can break at various locations during the RESET process. The simulation results are verified by experiments conducted on Ag/Ag2S/W and Cu/H2O/Cu systems.

In the second part of this work, RRAM memory devices based on amorphous Yttria stabilized Zirconia (YSZ) are systematically studied. The effects of different top electrodes of Au, Cu, Ni, Al and Ti are investigated. And the characteristics and the mechanisms of Ti/YSZ and Cu/YSZ are studied in details. It is found that Ti/YSZ has much better endurance, retention and reliability than Cu/YSZ. The underlining physics driving this behavior is investigated. In addition, it is found that Ti/YSZ has very smooth transition in the RESET stage and the off state resistance exponentially increases with an increase of erase voltage. Based on those properties, a multi-level programming (MLP) cell is realized that shows good endurance. The underlying physics that makes the MLP possible for Ti/YSZ is investigated. Finally, it is shown that an incremental step pulse programming (ISPP) technique can significantly increase the device endurance and reliability. Furthermore, it can optimize the tradeoff between resistance programming window and device lifetime.

    Cover page: Experimental and Simulation Study of Resistive Switches for Memory Applications
    • Article
    • Peer Reviewed
    LBL Publications (2020)
    The widely used density functional theory (DFT) has a major drawback of underestimating the band gaps of materials. Wannier–Koopmans method (WKM) was recently developed for band gap calculations with accuracy on a par with more complicated methods. WKM has been tested for main group covalent semiconductors, alkali halides, 2D materials, and organic crystals. Here we apply the WKM to another interesting type of material system: the transition metal (TM) oxides. TM oxides can be classified as either with d0 or d10 closed shell occupancy or partially occupied open shell configuration, and the latter is known to be strongly correlated Mott insulators. We found that, while WKM provides adequate band gaps for the d0 and d10 TM oxides, it fails to provide correct band gaps for the group with partially occupied d states. This issue is also found in other mean-field approaches like the GW calculations. We believe that the problem comes from a strong interaction between the occupied and unoccupied d-state Wannier functions in a partially occupied d-state system. We also found that, for pseudopotential calculations including deep core levels, it is necessary to remove the electron densities of these deep core levels in the Hartree and exchange–correlation energy functional when calculating the WKM correction parameters for the d-state Wannier functions.
      Cover page: Wannier–Koopmans method calculations for transition metal oxide band gaps
      • Article
      • Peer Reviewed
      LBL Publications (2018)
      Even though tremendous achievement has been made experimentally in the performance of lithium–sulfur (Li–S) battery, theoretical studies in this area are lagging behind due to the complexity of the Li–S systems and the effects of solvent. For this purpose, a new methodology is developed for investigating the 2D hexaaminobenzene-based coordination polymers (2D-HAB-CPs) as cathode candidate materials for Li–S batteries via density functional theory calculations in combination with an in-house developed charge polarized solvent model and a genetic algorithm structure global search code. With high ratios of transition metal atoms and two-coordinated nitrogen atoms, excellent electric conductivity, and structural porosity, the 2D-HAB-CP is able to address all of the three main challenges facing Li–S batteries: confining the lithium polysulfides from dissolution, facilitating the electron conductivity and buffering the volumetric expansion during the lithiation process. In addition, the theoretical energy density of this system is as high as 1395 Wh kg−1. These results demonstrate that the 2D-HAB-CP is a promising cathode material for Li–S batteries. The proposed computational framework not only opens a new avenue for understanding the key role played by solution and liquid electrolytes in Li–S batteries, but also can be generally applied to other processes with liquids involved.
        Cover page: Theoretical Investigation of 2D Conductive Microporous Coordination Polymers as Li–S Battery Cathode with Ultrahigh Energy Density
        • Article
        • Peer Reviewed
        LBL Publications (2019)
          Cover page: A Perspective on Studying Electronic Structure of Batteries through Soft X-ray Spectroscopy
          • Article
          • Peer Reviewed
          LBL Publications (2017)
          We performed a comparative study of the soft x-ray absorption spectroscopy of the LiMn2O4 and Li1.15Mn1.85O4 electrode materials with a quantitative analysis of Mn oxidation states. The revealed redox evolution of Mn upon electrochemical cycling clarifies the effect of excess Li in the materials, which naturally explains the different electrochemical performance. The spectral analysis perfectly agrees with different initial cycling capacities of the two materials. The results show unambiguously that Mn3+ starts to dominate the electrode surface after only one cycle. More importantly, the data show that, while LiMn2O4 electrodes follow the nominal Mn redox evolution, the formation of Mn3+ on the electrode surface is largely retarded for Li1.15Mn1.85O4 during most of the electrochemical processes. Such a different surface Mn redox behavior leads to differences in the detrimental effects of Mn2+ formation on the surface, which is observed directly after only two cycles. Our results provide strong evidence that a key effect of the (bulk) excess Li doping is actually due to processes on the electrode surfaces.
            Cover page: Effect of excess lithium in LiMn2O4 and Li1.15Mn1.85O4 electrodes revealed by quantitative analysis of soft X-ray absorption spectroscopy
            • Article
            • Peer Reviewed
            UC Santa Cruz Previously Published Works (2020)
            The evolving oxygen state plays key roles in the performance and stability of high-energy batteries involving oxygen redox reactions. Here, high-efficiency full energy range O-K mapping of resonant inelastic X-ray scattering (mRIXS) was collected from O2 (O0) and CO2 (O2- with strong covalency) molecules and compared directly with Li2O2 (O-) and the oxidized oxygen state in representative Na/Li-ion battery electrodes. Our results confirm again that the critical mRIXS feature around the 523.7 eV emission energy is from intrinsically oxidized oxygen, but not from the highly covalent oxygen state (CO2). The comparison of the mRIXS profile of the four different oxygen states, i.e., O2-, O-, On- (0 < n < 2), and O0, reveals that oxygen redox states in batteries have distinct widths and positions along the excitation energy compared with Li2O2 and O2. The nature of the oxidized oxygen state in oxide electrodes is thus beyond a simple molecular configuration of either peroxide or O2.
              Cover page: Full Energy Range Resonant Inelastic X‑ray Scattering of O2 and CO2: Direct Comparison with Oxygen Redox State in Batteries
              • Article
              • Peer Reviewed
              LBL Publications (2018)
              We report the first direct experimental evidence of the dynamic behavior of the solid-electrolyte-interphase (SEI) on copper electrodes upon electrochemical cycling. Synchrotron-based soft X-ray absorption spectroscopy (sXAS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) consistently show that both the chemical composition and the thickness of the SEI change with electrochemical potential throughout the slow formation process. In particular, sXAS results show that the nascent carbonate species in SEI show redox reversibility and decompose during the delithiation (oxidation) process, which leads to a significant shrinking of the SEI thickness as confirmed by TOF-SIMS. Meanwhile, the carbonates also matures and become more and more inactive at every lithiation (reduction) process. These experimental observations reveal unambiguously that SEI formation is much more complicated than a simple and monotonous deposition of electrolyte decomposition product; instead, it could be an oscillating process with a breathing growth.
                Cover page: Breathing and oscillating growth of solid-electrolyte-interphase upon electrochemical cycling
                • Article
                • Peer Reviewed
                LBL Publications (2018)
                The discovery of anion redox activity is promising for boosting the capacity of lithium ion battery (LIB) cathodes. However, fundamental understanding of the mechanisms that trigger the anionic redox is still lacking. Here, using hybrid density functional study combined with experimental soft X-ray absorption spectroscopy (sXAS) measurements, we unambiguously proved that Li(2- x)FeSiO4 performs sequent cationic and anionic redox activity through delithiation. Specifically, Fe2+ is oxidized to Fe3+ during the first Li ion extraction per formula unit (f.u.), while the second Li ion extraction triggered the oxygen redox exclusively. Cationic and anionic redox result in electron and hole polaron states, respectively, explaining the poor conductivity of Li(2- x)FeSiO4 noted by previous experiments. In contrast, other cathode materials in this family exhibit diversity of the redox process. Li2MnSiO4 shows double cationic redox (Mn2+-Mn4+) during the whole delithiation, while Li2CoSiO4 shows simultaneous cationic and anionic redox. The present finding not only provides new insights into the oxygen redox activity in polyanionic compounds for rechargeable batteries but also sheds light on the future design of high-capacity rechargeable batteries.
                  Cover page: Mechanism of Exact Transition between Cationic and Anionic Redox Activities in Cathode Material Li2FeSiO4
                  • Article
                  • Peer Reviewed
                  LBL Publications (2021)
                  Investigation of Li metal and ionic compounds through experimental and theoretical spectroscopy has been of tremendous interest due to their prospective applications in Li-metal and Li-ion batteries. Li K-edge soft X-ray absorption spectroscopy (sXAS) provides the most direct spectroscopic characterization; unfortunately, due to the low core-level energy and the highly reactive surface, Li-K sXAS of Li metal has been extremely challenging, as evidenced by many controversial reports. Here, through controlled and ultra-high energy resolution experiments of two kinds of in situ prepared samples, we report the intrinsic Li-K sXAS of Li-metal that displays a prominent leading peak that has not been revealed before. Furthermore, theoretical simulations show that, due to the low number of valence electrons in Li, the Li-K sXAS is strongly affected by the response of the valence electrons to the core hole. We successfully reproduce the Li-K sXAS by state-of-the-art calculations with considerations of a number of relevant parameters such as temperature, energy resolution, and, especially, contributions from transitions which are forbidden in the single-particle treatment. Such a comparative experimental and theoretical investigation is further extended to a series of Li ionic compounds, which highlight the importance of considering the total and single-particle energies for obtaining an accurate alignment of the spectra. Our work provides the first reliable Li-K sXAS of the Li metal surface with advanced theoretical calculations. The experimental and theoretical results provide a critical benchmark for studying Li chemistry in both metallic and ionic states.
                  • 1 supplemental ZIP
                  Cover page: Controlled Experiments and Optimized Theory of Absorption Spectra of Li Metal and Salts
                  • Article
                  • Peer Reviewed
                  LBL Publications (2020)
                  Lattice anionic (oxygen) redox reactions (ARR) offers opportunities for developing high-capacity batteries, however, often suffers the notoriously high hysteresis in voltages and low initial coulombic efficiency. Particularly, ARR was widely considered inherent to these kinetic issues. In this paper, unambiguous evidences of strong and reversible ARR is found in Na2/3Ni1/3Mn2/3O2 through mapping of resonant inelastic X-ray scattering (mRIXS). Strikingly, the material displays negligible voltage hysteresis (0.1 V) and high initial coulombic efficiency with a highly stable electrochemical profile. Our independent analysis of the Ni, Mn and O states consistently interpret the redox mechanism of Na2/3Ni1/3Mn2/3O2, which reveals a strong ARR system with facile kinetics and highly stable electrochemical profile that previously found only in cationic redox systems of conventional non-Alkali-rich materials.
                    Cover page: Negligible voltage hysteresis with strong anionic redox in conventional battery electrode
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