Yet-Ming Chiang

Identification of cathode materials for lithium batteries guided by first-principles calculations

Lithium batteries have the highest energy density of all rechargeable batteries and are favoured in applications where low weight or small volume are desired — for example, laptop computers, cellular telephones and electric vehicles. One of the limitations of present commercial lithium batteries is the high cost of the LiCoO2 cathode material. Searches for a replacement material that, like LiCoO2, intercalates lithium ions reversibly have covered most of the known lithium/transition-metal oxides, but the number of possible mixtures of these is almost limitless, making an empirical search labourious and expensive. Here we show that first-principles calculations can instead direct the search for possible cathode materials. Through such calculations we identify a large class of new candidate materials in which non-transition metals are substituted for transition metals. The replacement with non-transition metals is driven by the realization that oxygen, rather than transition-metal ions, function as the electron acceptor upon insertion of Li. For one such material, Li(Co,Al)O2, we predict and verify experimentally that aluminium substitution raises the cell voltage while decreasing both the density of the material and its cost.

TEM Study of Electrochemical Cycling‐Induced Damage and Disorder in LiCoO2 Cathodes for Rechargeable Lithium Batteries

Among lithium transition metal oxides used as intercalation electrodes for rechargeable lithium batteries, LiCoO{sub 2} is considered to be the most stable in the {alpha}-NaFeO{sub 2} structure type. It has previously been believed that cation ordering is unaffected by repeated electrochemical removal and insertion. The authors have conducted direct observations, at the particle scale, of damage and cation disorder induced in LiCoO{sub 2} cathodes by electrochemical cycling. Using transmission electron microscopy imaging and electron diffraction, it was found that (1) individual LiCoO{sub 2} particles in a cathode cycled from 1.5 to 4.35 V against a Li anode are subject to widely varying degrees of damage; (2) cycling induces severe strain, high defect densities, and occasional fracture of particles; and (3) severely strained particles exhibit two types of cation disorder, defects on octahedral site layers (including cation substitutions and vacancies) as well as a partial transformation to spinel tetrahedral site ordering. The damage and cation disorder are localized and have not been detected by conventional bulk characterization techniques such as X-ray or neutron diffraction. Cumulative damage of this nature may be responsible for property degradation during overcharging or in long-term cycling of LiCoO{sub 2}-based rechargeable lithium batteries.

The synergetic effect of lithium polysulfide and lithium nitrate to prevent lithium dendrite growth

Lithium metal has shown great promise as an anode material for high-energy storage systems, owing to its high theoretical specific capacity and low negative electrochemical potential. Unfortunately, uncontrolled dendritic and mossy lithium growth, as well as electrolyte decomposition inherent in lithium metal-based batteries, cause safety issues and low Coulombic efficiency. Here we demonstrate that the growth of lithium dendrites can be suppressed by exploiting the reaction between lithium and lithium polysulfide, which has long been considered as a critical flaw in lithium-sulfur batteries. We show that a stable and uniform solid electrolyte interphase layer is formed due to a synergetic effect of both lithium polysulfide and lithium nitrate as additives in ether-based electrolyte, preventing dendrite growth and minimizing electrolyte decomposition. Our findings allow for re-evaluation of the reactions regarding lithium polysulfide, lithium nitrate and lithium metal, and provide insights into solving the problems associated with lithium metal anodes.

Lead-free high-strain single-crystal piezoelectrics in the alkaline–bismuth–titanate perovskite family

Doped alkaline–bismuth–titanate perovskite single crystals have been grown in ferroelectric phases with high piezoelectric actuation. Rhombohedral-phase Na1/2Bi1/2TiO3–BaTiO3 crystals exhibit up to 0.25% free strain with low hysteresis along the cubic 〈001〉 direction (d33∼450 pC/N). Tetragonal phase crystals exhibit free strains as high as 0.85% with greater hysteresis characteristic of domain switching; low field d33 exceeds 500 pC/N. Strain energy densities exceed those of optimized polycrystalline lead perovskites, and actuation capability is retained at compressive stresses >100 MPa.

Electrochemically-Driven Solid-State Amorphization in Lithium-Silicon Alloys and Implications for Lithium Storage

Lithiated metal alloys such as Li-Si are of great interest as high energy density anodes for future rechargeable battery technology. We show that the mechanism of electrochemical alloying is electrochemically-driven solid-state amorphization, a process closely analogous to the diffusive solid-state amorphization of thin films. X-ray diffraction and HREM experiments reveal that the crystallization of equilibrium intermetallic compounds is circumvented during lithiation at room temperature, and that formation of highly lithiated Li-Si glass instead occurs. This glass is shown to be metastable with respect to the equilibrium crystalline phases. Similar behavior is observed in the diffusive reaction of Li and Si bilayer films, suggesting that lithium-metal alloys in general are likely candidates for solid-state amorphization.

Size-Dependent Lithium Miscibility Gap in Nanoscale Li1 − x FePO4

Olivine compounds have emerged as important and enabling positive electrode materials for high-power, safe, long-life lithium rechargeable batteries. In this work, the miscibility gap in undoped Li 1-x FePO 4 is shown to contract systematically with decreasing particle size in the nanoscale regime and with increasing temperature at a constant particle size. These effects suggest that the miscibility gap completely disappears below a critical size. In the size-dependent regime, the kinetic response of nanoscale olivines should deviate from the simple size-scaling implicit in Fickian diffusion.

Building a Better Battery

Controlling the charge-induced morphological changes of electrode materials may provide a route to improved battery performance. Innovations in the battery field are infrequent and hard-won. New electrochemical systems (a new positive or negative electrode, electrolyte, or combination thereof) reach the marketplace only once every few years, and the energy density of lithium-ion batteries as a class has increased on average by only 8 to 9% per year since the early 1990s. Thus, in the burgeoning field of nanoscale electrode materials, skepticism regarding new claims is perhaps not surprising because of the many requirements that any battery electrode must simultaneously meet to be commercialized. One route by which battery performance can be compromised is by mechanical failure due to the large volume changes associated with the charge-discharge cycle. On page 1515 of this issue, Huang et al. (1) report an ingenious in situ transmission electron microscope (TEM) experiment that uses a low–vapor pressure ionic liquid electrolyte to allow imaging of a SnO2 nanowire electrode in an “open” electrochemical cell. They observe a reaction mechanism in the SnO2 nanowires that progresses sequentially along the nanowire from end to end, allowing them to accommodate a ∼250% volume change without fracturing and at practical charging rates. These intriguing results raise the question of whether such one-dimensional phase transformations can be induced in other materials.

Rubbery Block Copolymer Electrolytes for Solid‐State Rechargeable Lithium Batteries

For nearly 20 years, poly(ethylene oxide)-based materials have been researched for use as electrolytes in solid-state rechargeab le lithium batteries. Technical obstacles to commercialization derive from the inability to satisfy simultaneously the electrical and mechanical performance requirements: high ionic conductivity along with resistance to flow. Herein, the synthesis and characterization of a series of poly(lauryl methacrylate)- b-poly[oligo(oxyethylene) methacrylate]-based block copolymer electrolytes (BCEs) are reported. With both blocks in the rubbery state (i.e., having glass transition temperatures well below room temperatu re) these materials exhibit improved conductivities over those of glassy-rubbery block copolymer systems. Dynamic rheological testing verifies that these materials are dimensionally stable, whereas cyclic voltammetry shows them to be electrochemically stable over a wide potential window, i.e., up to 5 V at 55 8C. A solid-state rechargeable lithium battery was constructed by laminating lithium metal, BCE, and a composite cathode composed of particles of LiAl0.25Mn0.75O2 (monoclinic), carbon black, and graphite in a BCE binder. Cycle testing showed the Li/BCE/LiAl0.25Mn0.75O2 battery to have a high reversible capacity and good capacity

Microstructural Modeling and Design of Rechargeable Lithium-Ion Batteries

The properties of rechargeable lithium-ion batteries are determined by the electrochemical and kinetic properties of their constituent materials as well as by their underlying microstructure. In this paper a method is developed that uses microscopic information and constitutive material properties to calculate the response of rechargeable batteries. The method is implemented in OOF ,a public domain finite element code, so it can be applied to arbitrary two-dimensional microstructures with crystallographic anisotropy. This methodology can be used as a design tool for creating improved electrode microstructures. Several geometrical two-dimensional arrangements of particles of active material are explored to improve electrode utilization, power density, and reliability of the Li yC6uLixMn2O4 battery system. The analysis suggests battery performance could be improved by controlling the transport paths to the back of the positive porous electrode, maximizing the surface area for intercalating lithium ions, and

“Electrochemical Shock” of Intercalation Electrodes: A Fracture Mechanics Analysis

Fracture of electrode particles due to diffusion-induced stress has been implicated as a possible mechanism for capacity fade and impedance growth in lithium-ion batteries. In brittle materials, including many lithium intercalation materials, knowledge of the stress profile is necessary but insufficient to predict fracture events. We derive a fracture mechanics failure criterion for individual electrode particles and demonstrate its utility with a model system, galvanostatic charging of Li x Mn 2 O 4 . Fracture mechanics predicts a critical C-rate above which active particles fracture; this critical C-rate decreases with increasing particle size. We produce an electrochemical shock map, a graphical tool that shows regimes of failure depending on C-rate, particle size, and the materials inherent fracture toughness K Ic . Fracture dynamics are sensitive to the gradient of diffusion-induced stresses at the crack tip; as a consequence, small initial flaws grow unstably and are therefore potentially more damaging than larger initial flaws, which grow stably.