Muratahan Aykol

Controlling the intercalation chemistry to design high-performance dual-salt hybrid rechargeable batteries

We have conducted extensive theoretical and experimental investigations to unravel the origin of the electrochemical properties of hybrid Mg(2+)/Li(+) rechargeable batteries at the atomistic and macroscopic levels. By revealing the thermodynamics of Mg(2+) and Li(+) co-insertion into the Mo6S8 cathode host using density functional theory calculations, we show that there is a threshold Li(+) activity for the pristine Mo6S8 cathode to prefer lithiation instead of magnesiation. By precisely controlling the insertion chemistry using a dual-salt electrolyte, we have enabled ultrafast discharge of our battery by achieving 93.6% capacity retention at 20 C and 87.5% at 30 C, respectively, at room temperature.

High-throughput computational design of cathode coatings for Li-ion batteries

Cathode degradation is a key factor that limits the lifetime of Li-ion batteries. To identify functional coatings that can suppress this degradation, we present a high-throughput density functional theory based framework which consists of reaction models that describe thermodynamic and electrochemical stabilities, and acid-scavenging capabilities of materials. Screening more than 130,000 oxygen-bearing materials, we suggest physical and hydrofluoric-acid barrier coatings such as WO3, LiAl5O8 and ZrP2O7 and hydrofluoric-acid scavengers such as Sc2O3, Li2CaGeO4, LiBO2, Li3NbO4, Mg3(BO3)2 and Li2MgSiO4. Using a design strategy to find the thermodynamically optimal coatings for a cathode, we further present optimal hydrofluoric-acid scavengers such as Li2SrSiO4, Li2CaSiO4 and CaIn2O4 for the layered LiCoO2, and Li2GeO3, Li4NiTeO6 and Li2MnO3 for the spinel LiMn2O4 cathodes. These coating materials have the potential to prolong the cycle-life of Li-ion batteries and surpass the performance of common coatings based on conventional materials such as Al2O3, ZnO, MgO or ZrO2.

Interfacial Interactions and Flammability of Flame-Retarded and Short Fiber-Reinforced Polyamides

Interfacial properties, crystallinity and flammability of short fiber reinforced and flame retarded polyamide 6 and polyamide 66 compounds are investigated, emphasizing the influence of flame retardant fillers on the resistance of fiber/matrix interface to shear. Interfacial shear strengths are derived through a micromechanical approach by determining the tensile properties and residual fiber length distributions. Validated by fracture morphologies, interfacial strengths are found to be governed by filler – induced apparent crystallinities and fractional occurrence of polyamide polymorphs, obtained via peak deconvolution of X-Ray diffraction patterns. Although flame retardant additives based on Br/Sb synergism are found to impart excellent flammability reductions regarding oxygen index and UL94 classifications (V-0 rating), degree of crystallinity; thus, interfacial properties are deteriorated due to lowered thermal expansion and increased cooling rates. Red phosphorus as a flame retardant also induces a UL94 V-0 and significant reduction in flammability together with the facts that crystallinity is not altered and a strong fiber/matrix interface is maintained. Use of melamine cyanurate in an unreinforced polyamide improves the limiting oxygen index considerably; however, the UL94 rating remains unchanged as V-2 as a consequence of increased level of melt dripping. Melamine cyanurate additionally increases the degree of crystallinity through promotion of heterogeneous nucleation.

Material design of high-capacity Li-rich layered-oxide electrodes: Li2MnO3 and beyond

Lithium-ion batteries (LIBs) have been used widely in portable electronics, and hybrid-electric and all-electric vehicles for many years. However, there is a growing need to develop new cathode materials that will provide higher cell energy densities for advanced applications. Several candidates, including Li2MnO3-stabilized LiM′O2 (M′ = Mn/Ni/Co) structures, Li2Ru0.75Sn0.25O3 (i.e., 3Li2RuO3–Li2SnO3), and disordered Li2MoO3–LiCrO2 compounds can yield capacities exceeding 200 mA h g−1, alluding to the constructive role that Li2MO3 (M4+) end-member compounds play in the electrochemistry of these systems. Here, we catalog the family of Li2MO3 compounds as active cathodes or inactive stabilizing agents using high-throughput density functional theory (HT-DFT). With an exhaustive search based on design rules that include phase stability, cell potential, resistance to oxygen evolution, and metal migration, we predict a number of new Li2MIO3–Li2MIIO3 active/inactive electrode pairs, in which MI and MII are transition- or post-transition metal ions, that can be tested experimentally for high-energy-density LIBs.

Layered-Layered-Spinel Cathode Materials Prepared by a High-Energy Ball-Milling Process for Lithium-ion Batteries

In this work, we report the electrochemical properties of 0.5Li2MnO3·0.25LiNi0.5Co0.2Mn0.3O2·0.25LiNi0.5Mn1.5O4 and 0.333Li2MnO3·0.333LiNi0.5Co0.2Mn0.3O2·0.333LiNi0.5Mn1.5O4 layered-layered-spinel (L*LS) cathode materials prepared by a high-energy ball-milling process. Our L*LS cathode materials can deliver a large and stable capacity of ∼200 mAh g(-1) at high voltages up to 4.9 V, and do not show the anomalous capacity increase upon cycling observed in previously reported three-component cathode materials synthesized with different routes. Furthermore, we have performed synchrotron-based in situ X-ray diffraction measurements and found that there are no significant structural distortions during charge/discharge runs. Lastly, we carry out (opt-type) van der Waals-corrected density functional theory (DFT) calculations to explain the enhanced cycle characteristics and reduced phase transformations in our ball-milled L*LS cathode materials. Our simple synthesis method brings a new perspective on the use of the high-power L*LS cathodes in practical devices.

Interactions at fiber/matrix interface in short fiber reinforced amorphous thermoplastic composites modified with micro- and nano-fillers

This study aims at systematically extracting fiber/matrix interfacial strength in short-glass fiber-reinforced polymer composites using an experimental micromechanics approach which employs mechanical properties and residual fiber length distributions to derive the apparent interfacial shear strength. We started from neat high-impact polystyrene matrix short-glass fiber-reinforced composites (HIPS/GF) with varying fiber loading and proceeded toward HIPS/GF hybrid composites containing micro- and nano-fillers where complex fiber/matrix interfacial interactions exist. It was found that apparent interfacial shear strength does not vary with fiber content, while the presence of fillers with different length-scales alters fiber/matrix interactions depending on their influence on physical properties of the polymer matrix, particularly in the vicinity of reinforcing fiber surfaces.

Continuum Micro-mechanics of Estimating Interfacial Shear Strength in Short Fiber Composites

A new continuum approach to micro-mechanics of short fiber composites yielded two separate methods of estimating the apparent interfacial shear strength and fiber orientation efficiency. The methods exploit the compilation of the effects of fiber length distribution and interfacial shear strength on strengthening efficiency into a function of strain. The In-Built Method derives a unique combination of apparent interfacial shear strength and fiber orientation efficiency being able to reproduce the experimental stress–strain curve of a short fiber reinforced composite with a very low residual standard deviation. The Boundary Method accomplishes rapid interfacial shear strength screening in materials selection by constructing and utilizing the proposed selection chart.

A generalized polytetrahedral cluster approach to partial coordination numbers in binary metallic glasses

Partial coordination numbers (CNs) play a substantial role in description of hetero-coordinated local structure and related short-range order in metallic glasses. By defining a polytetrahedral aggregation for solute–solvent type atomic clusters, which retains cluster sphericity, high solute–solvent and solvent–solvent CNs, a preliminary model is developed to estimate the CN of a solute atom within a non-isolated cluster embedded in the glassy environment. Employing the result, a generalized quasi-hard sphere model is constructed by defining intra- and inter-cluster correlations, which can yield significantly close values to experimentally derived partial CNs in a great many metallic glass systems, such as Fe–B, Ni–B, Ni–P, Co–P, Pd–Si, Al–Y, Co–Zr, Co–Ti, Ni–Ti, Zr–Ni, Zr–Pd, Zr–Pt and Cu–Zr. The approach allows evaluation of a complete set of partial CNs in a given binary system as a function of atomic radii and composition.