Sideris Research Group
Undergraduate Research Students
- Hui Zhu
- Sindy Ferreiras
- Krystal Lee
- Yueli Chen
- Heera Choe
- Nicole Yu
Our group focuses on the synthesis and characterization of phospho-olivine cathode materials, LiMPO4 (M= Fe2+, Mn2+, Co2+, Ni2+), and garnet-like oxides with the nominal chemical formula Li7-xLa3Zr2-xTaxO12. We primarily use powder X-ray diffraction (PXRD), nuclear magnetic resonance (NMR) spectroscopy, and scanning electron microscopy (SEM) to investigate the structure of these materials. Our group collaborates extensively with the Greenbaum Laboratory at CUNY Hunter College, and has access to several CUNY research facilities, such as the New York Structural Biology Center (NYSBC) and the Advanced Science Research Center (ASRC).
Scroll down for brief project summaries and some promising, preliminary results obtained by our team!
"Low Temperature" Synthesis of Phospho-olivines
Lithium phospho-olivines containing Fe2+ and Mn2+, LiFe1-xMnxPO4 (0.0≤x≤1.0), are attractive cathode materials for rechargeable lithium ion batteries. LiFePO4 has a relatively high theoretical capacity (~170 mAhg-1), a charge/discharge potential of ~3.4 V (relative to Li/Li+), good thermal stability, relatively low cost, and is more environmentally friendly than the commercially available cathode material, LiCoO2. Substitution of Mn2+ for Fe2+ in the crystal structure can produce materials with operating voltages between 3.5-4.1 V.
Hydrothermal methods, utilizing surfactants as structure-directing agents, can be used to synthesize these phospho-olivines at much lower temperatures (relative to solid state reactions) with greater control over the particle size and morphology. Both the particle size and morphology have a profound effect on the electrochemical performance of the cathode materials. Our group performs these low temperature reactions using various solvents, additives, and surfactants to determine their effect on the crystallites.
Ionothermal syntheses, where room temperature ionic liquids are used as a solvent, can also be used to synthesize phospho-olivine cathode materials from salt precursors. Ionic liquids can be chemically modified to tune their melting point, viscosity, hydrophobicity, solvating power, and decomposition temperature. Our group is interested in determining how the structure and chemical nature of an ionic liquid used in an ionothermal reaction affects the resulting phospho-olivine products. We prepare various asymmetric imidazolium-based ionic liquids containing the bis(trifluoromethylsulfonyl)amide anion, where the structure of the imidazolium cation is modified in a systematic manner. Currently, the effect of the length of hydroxyl (-OH) terminated alkyl chains on the imidazolium cation in the ionothermal synthesis of LiFePO4 is being explored.
Li7-xLa3Zr2-xTaxO12 Solid Electrolytes
Some desirable properties of an electrolyte are: (i) the ability to make a stable interface with the electrodes, (ii) chemical inertness to metallic lithium, (iii) high ionic (Li+) conductivity, (iv) low electrical conductivity, (v) low flammability, and (vi) structural integrity over repeated cycling of the electrochemical cell. Li-ion solid electrolytes, particularly those in the Li-rich garnet-like family containing d0 metal cations (such as Zr4+, Ta5+, Nb5+, etc.), have stimulated interest as potential electrolyte materials for the reasons outlined above.
Li7-xLa3Zr2-xTaxO12 (0≤x≤1) has been synthesized in our laboratory using standard high-temperature reaction methods and investigated using powder X-ray diffraction (PXRD) and variable temperature solid state 6,7Li nuclear magnetic resonance (NMR) spectroscopy. PXRD is routinely used for phase identification and structure validation. 6,7Li NMR spectroscopy probes the local structure of the material and elucidates the lithium ion (Li+) dynamics.
Surfactant-Assisted Hydrothermal Reactions
LiFePO4 crystallizes in the Pnma spacegroup (No. 62) with a~10.329 Å, b~6.007 Å, c~4.691 Å. The top figure depicts a typical PXRD pattern we obtain of LiFePO4 that is prepared by a hydrothermal synthetic route. Select reflections are indexed. Below the diffraction pattern are SEM images of LiFePO4 particles synthesized without any surfactant (left), and in the presence of sodium dodecylbenzenesulfonate (right) - an anionic surfactant.
New Ionic Liquid for Ionothermal Synthesis
The figure depicts the 1H-decoupled 13C NMR spectrum of 1-(9-hydroxynonyl)-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide (abbreviated as C9OHmim-NTf2), a new ionic liquid for our ionothermal reaction study. The structure of the cation and anion, along with the estimated 13C shifts, are shown above the spectrum. A series of ionic liquids can be readily made in high yield using a two-step, microwave-assisted synthesis.
Evidence of Fe-O-P Bonds In Ionothermal Reactions
Ionothermal syntheses of LiFePO4 from salt precursors were attempted in three ionic liquids: 1-(3-hydroxypropyl)-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide, 1-(6-hydroxyhexyl)-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide, and 1-(9-hydroxynonyl)-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide, or C3OHmim-NTf2, C6OHmim-NTf2, and C9OHmim-NTf2 respectively. The 31P magic angle spinning (MAS) NMR spectra of the solids obtained after the reactions are depicted above. Spectra were collected at 7 T using a Hahn echo sequence and a sample spinning rate of ~35 kHz. Asterisks (*) denote spinning sidebands.
The phosphorus local environment in LiFePO4 is shown on the top-left of the spectrum. Phosphorus (P) atoms are in blue, oxygen (O) atoms are in red, and iron (Fe) atoms are in orange. The Fermi Contact Interaction, which describes the through-bond transfer of electron density from a paramagnetic center (the Fe atom) to a probe nucleus (the P atom), is primarily responsible for the very large shifts (~3928 ppm) observed. These shifts are consistent with the formation of Fe-O-P bonds.
Lithium Ion Dynamics in Solid Electrolytes
The powder X-ray diffraction pattern of Li6.4La3Zr1.4Ta0.6O12 prepared by a high temperature solid state reaction is shown in the top figure. The relatively intense and sharp reflections indicate a highly crystalline, well-ordered sample.
A polyhedral representation of the framework of the garnet-like material is depicted on the top-right; the lithium (Li) atoms are omitted for clarity. Lanthanum (La) atoms are shown in green, Zirconium (Zr) and Tantalum (Ta) atoms are in blue, and oxygen (O) atoms are in red.
Variable temperature solid state 7Li NMR spectra of Li6.4La3Zr1.4Ta0.6O12 are shown in the bottom figure. The measurements were made at 7 T using a static probe. Motional narrowing of the lithium signal is observed as the sample is heated to 200 °C due to Li+ conduction through the garnet-like framework.