H. Laisa

Bio: H. Laisa is an academic researcher. The author has an hindex of 1, co-authored 1 publications receiving 21 citations.

Reversible anionic redox chemistry inhigh-capacity layered-oxide electrodes

Abstract: Li-ion batteries have contributed to the commercial success of portable electronics and may soon dominate the electric transportation market provided that major scientific advances including new materials and concepts are developed. Classical positive electrodes for Li-ion technology operate mainly through an insertion–deinsertion redox process involving cationic species. However, this mechanism is insufficient to account for the high capacities exhibited by the new generation of Li-rich (Li1CxNiyCozMn(1

(Invited) Evolution of Redox Couples in Li- and Mn-Rich Cathode Materials and Mitigation of Voltage Fade by Reducing Oxygen Release

Abstract: Voltage fade is a major problem in battery applications for high-energy lithium- and manganese-rich (LMR) layered materials. As a result of the complexity of the LMR structure, the voltage fade mechanism is not well understood. Here we conduct both in situ and ex situ studies on a typical LMR material (Li1.2Ni0.15Co0.1Mn0.55O2) during charge–discharge cycling, using multi-length-scale X-ray spectroscopic and three-dimensional electron microscopic imaging techniques. Through probing from the surface to the bulk, and from individual to whole ensembles of particles, we show that the average valence state of each type of transition metal cation is continuously reduced, which is attributed to oxygen release from the LMR material. Such reductions activate the lower-voltage Mn3+/Mn4+ and Co2+/Co3+ redox couples in addition to the original redox couples including Ni2+/Ni3+, Ni3+/Ni4+ and O2−/O−, directly leading to the voltage fade. We also show that the oxygen release causes microstructural defects such as the formation of large pores within particles, which also contributes to the voltage fade. Surface coating and modification methods are suggested to be effective in suppressing the voltage fade through reducing the oxygen release.Voltage decay is a major problem in applications of high-energy Li- and Mn-rich layer-structured battery materials. Here, the authors report the evolution of redox couples as the origin of the voltage decay and discuss strategies to suppress the problem.

Requirements for Reversible Extra-Capacity in Li-Rich Layered Oxides for Li-Ionbatteries

Abstract: The structural stability and the redox mechanism of Li-rich layered oxides (LLOs) are two very important aspects for high energy density. The former is related to the irreversible loss of lattice oxygen and capacity fading during cycling, while the latter determines the overall capacity of the materials. This paper aims at clarifying the factors governing the structural stability, the extra capacity and the redox mechanism of LLOs upon Li-removal. The results show that the structural stability against oxygen vacancy formation is improved with increasing M–O covalency, while it decreases with increasing d-shell electron number and with electrochemical extraction of lithium from the lattice. The redox mechanism of Li2−xMO3 electrodes formed by 3d metals or by heavier metals with a d0 electronic configuration is related to the electron depletion from the oxygen lone-pairs (localized non-bonding O(2p) states) leading to an irreversible anionic redox ending with the reductive elimination of O2 upon cycling. For these phases, long-term cycling is predicted to be very unlikely due to the irreversible loss of lattice oxygen upon charging. For the electrodes formed by 4d and 5d metals with intermediate dn electronic configurations, reversible cationic and anionic redox activities are predicted, therefore enabling reversible extra-capacities. The very different redox mechanisms exhibited by Li2−xMO3 electrodes are then linked to the delicate balance between the Coulomb repulsions (U term) and the M–O bond covalency (Δ term) through the general description of charge-transfer vs. Mott–Hubbard insulators. The present findings will provide a uniform guideline for tuning the band structures of Li2MO3 phases and thus activating desired redox mechanisms, being beneficial for the design of high-energy density electrode materials for Li-ion battery applications.

Origin of structural degradation in Li-rich layered oxide cathode

Abstract: Li- and Mn-rich (LMR) cathode materials that utilize both cation and anion redox can yield substantial increases in battery energy density1-3. However, although voltage decay issues cause continuous energy loss and impede commercialization, the prerequisite driving force for this phenomenon remains a mystery3-6 Here, with in situ nanoscale sensitive coherent X-ray diffraction imaging techniques, we reveal that nanostrain and lattice displacement accumulate continuously during operation of the cell. Evidence shows that this effect is the driving force for both structure degradation and oxygen loss, which trigger the well-known rapid voltage decay in LMR cathodes. By carrying out micro- to macro-length characterizations that span atomic structure, the primary particle, multiparticle and electrode levels, we demonstrate that the heterogeneous nature of LMR cathodes inevitably causes pernicious phase displacement/strain, which cannot be eliminated by conventional doping or coating methods. We therefore propose mesostructural design as a strategy to mitigate lattice displacement and inhomogeneous electrochemical/structural evolutions, thereby achieving stable voltage and capacity profiles. These findings highlight the significance of lattice strain/displacement in causing voltage decay and will inspire a wave of efforts to unlock the potential of the broad-scale commercialization of LMR cathode materials.

Operando Lithium Dynamics in the Li-Rich Layered Oxide Cathode Material Via Neutron Diffraction

Abstract: Neutron diffraction under operando battery cycling is used to study the lithium and oxygen dynamics of high Li-rich Li(Lix/3Ni(3/8-3x/8)Co(1/4-x/4)Mn(3/8+7x/24)O2 (x = 0.6, HLR) and low Li-rich Li(Lix/3Ni(1/3-x/3)Co(1/3-x/3)Mn(1/3+x/3)O2 (x = 0.24, LLR) compounds that exhibit different degrees of oxygen activation at high voltage. The measured lattice parameter changes and oxygen position show largely contrasting changes for the two cathodes where the LLR exhibits larger movement of oxygen and lattice contractions in comparison to the HLR that maintains relatively constant lattice parameters and oxygen position during the high voltage plateau until the end of charge. Density functional theory calculations show the presence of oxygen vacancy during the high voltage plateau; changes in the lattice parameters and oxygen position are consistent with experimental observations. Lithium migration kinetics for the Li-rich material is observed under operando conditions for the first time to reveal the rate of lithium extraction from the lithium layer, and transition metal layer is related to the different charge and discharge characteristics. At the beginning of charging, the lithium extraction predominately occurs within the lithium layer. The lithium extraction from the lithium layer slows down and extraction from the transition metal layer evolves at a faster rate once the high voltagemore » plateau is reached.« less

Advances in manganese-oxide âcompositeâ electrodes for lithium-ion batteries

Abstract: Recent advances to develop manganese-rich electrodes derived from ‘composite’ structures in which a Li2MnO3 (layered) component is structurally integrated with either a layered LiMO2 component or a spinel LiM2O4 component, in which M is predominantly Mn and Ni, are reviewed. The electrodes, which can be represented in two-component notation as xLi2MnO3·(1 − x)LiMO2 and xLi2MnO3·(1 − x)LiM2O4, are activated by lithia (Li2O) and/or lithium removal from the Li2MnO3, LiMO2 and LiM2O4 components. The electrodes provide an initial capacity >250 mAh g−1 when discharged between 5 and 2.0 V vs. Li0 and a rechargeable capacity up to 250 mAh g−1 over the same potential window. Electrochemical charge and discharge reactions are followed on compositional phase diagrams. The data bode well for the development and exploitation of high capacity electrodes for the next generation of lithium-ion batteries.