Preetam Singh

Eldfellite, NaFe(SO4)2: an intercalation cathode host for low-cost Na-ion batteries

The mineral eldfellite, NaFe(SO4)2, is characterized as a potential cathode for a Na-ion battery that is fabricated in charged state; its 3 V discharge versus sodium for reversible Na+ intercalation is shown to have a better capacity, but lower insertion rate than Li+ intercalation. The theoretical specific capacity for Na+ insertion is 99 mA h g−1. After 80 cycles at 0.1C versus a Na anode, the specific capacity was 78 mA h g−1 with a coulomb efficiency approaching 100%.

Monoclinic Sr1–xNaxSiO3–0.5x: New Superior Oxide Ion Electrolytes

Oxide ion electrolytes determine the temperature of operation of solid oxide fuel cells, oxygen separation membranes, and oxygen sensors. There is a strong incentive to lower their operating temperatures, in a solid oxide fuel cell, for example, from Top > 800 °C to Top ≈ 500 °C. The use of low-cost Na(+) rather than K(+) as the dopant in monoclinic SrSiO3 (C12/C1) is shown to provide a larger solid solution range (0 < x ≤ 0.45) in Sr1-xNaxSiO3-0.5x and to achieve an oxide ion conductivity σo ≥ 10(-2) S·cm(-1) by 525 °C as a result of lowering the temperature of a smooth transition to full disorder of the mobile oxide ions. The Sr1-xNaxSiO3-0.5x electrolytes are much less hygroscopic than Sr1-xKxSiO3-0.5x and are stable with a nickel composite anode in 5% H2/Ar as well as with cathodes such as La1-xSrxMnO3-δ and Sr0.7Y0.3CoO3-δ in air, which makes them candidate electrolytes for intermediate-temperature solid oxide fuel cells or for other applications of oxide ion electrolytes.

Sr1−xKxSi1−yGeyO3−0.5x: a new family of superior oxide-ion conductors

Monoclinic SrMO3 (M = Si or Ge) contains (001) planes of M3O9 units of three corner shared MO4 coplanar complexes separated by close-packed planes of Sr2+ ions each coordinated, top and bottom, by three terminal oxygen on three different M3O9 units. Substitution of K+ for Sr2+ introduces terminal-oxygen vacancies that are either not accommodated by corner sharing with a neighboring M3O9 unit, apparently because of steric hindrance by the large Sr2+ and K+ ions, or are accommodated by a distortion that creates an interstitial mobile oxide ion. Stabilization of a terminal-oxygen vacancy in a tetrahedral anion complex by steric hindrance in Sr1−xKxSi1−yGeyO3−0.5x represents a new design principle for superior oxide-ion electrolytes at intermediate temperatures.

Sr3−3xNa3xSi3O9−1.5x (x = 0.45) as a superior solid oxide-ion electrolyte for intermediate temperature-solid oxide fuel cells

We here report that a newly discovered superior oxide-ion conductor Sr3−3xNa3xSi3O9−1.5x (x = 0.45) (SNS) demonstrates full potential to be a practical solid electrolyte for intermediate temperature-solid oxide fuel cells (IT-SOFCs). It exhibits the highest oxide-ion conductivity with the lowest activation energy among all the chemically stable solid oxide-ion conductors reported. The ionic conductivity is stable over a broad range of partial pressures of oxygen (10−30 to 1 atm) for an extended period of time. A SOFC based on a 294 μm thick SNS-electrolyte produces peak power densities of 431 and 213 mW cm−2 at 600 and 500 °C, respectively. Considering its competitive costs in materials and manufacturing and rare-earth free composition, SNS has great potential to become a new class of technologically and strategically important electrolytes for commercial IT-SOFCs.

Structural investigation of the oxide-ion electrolyte with SrMO3 (M = Si/Ge) structure

A neutron-diffraction study of the oxide-ion solid electrolytes Sr1−xNaxSiO3−0.5x (x = 0.2 and 0.4) and Sr0.8K0.2Ge1−ySiyO2.9 (y = 0.0 and 0.5) reveals that there are no interstitial oxygen atoms in the structures; the oxygen vacancies are more concentrated in the planar oxygen sites (O3 and O5) of corner-sharing tetrahedral units of the M3O9 (M = Si/Ge) complexes. From thermogravimetric analysis (TGA), the K-substituted samples lose weight above 100 °C and are hygroscopic at room temperature; the oxygen vacancies are concentrated at the in-plane O3 and O5 sites and those in the terminal O2 or O4 are responsible for a 2D oxide-ion conductivity between the M3O9 complexes. The Na-substituted samples lose little weight by 800 °C and are not hygroscopic; the oxygen vacancies are located at all oxygen atom positions, being more pronounced at the in-plane O3 and O5; those in the terminal oxygen sites give an excellent oxide-ion conductivity. Moreover the high temperature NPD data for Sr0.6Na0.4SiO2.8 disclose that the vacancies become more randomly dispersed above 400 °C to give a smaller activation energy above 550 °C for vacancy transfer between Si3O9−0.5x complexes.

Glass-amorphous alkali-ion solid electrolytes and their performance in symmetrical cells

Precursors of the crystalline antiperovskites A3−xHxOCl (A = Li or Na and 0 < x < 1) can be rendered glass/amorphous solid Li+ or Na+ electrolytes by the addition of water to its solvation limit with/without the addition of a small amount of an oxide or hydroxide. The solvated water is evaporated as HCl and 2(OH)− = O2− + H2O. The O2− attracts a Li+ or Na+ to form dipoles; the remaining Li+ or Na+ are mobile. The Li+ or Na+ ionic conductivities of the glass/amorphous solids have activation energies ΔHm < 0.1 eV and a room-temperature conductivity comparable to that of the best organic liquid electrolytes. Measurements of the dielectric loss tangent versus frequency show two overlapping resonances at room temperature with the Ba-doped Li-glass; they are nearly overlapping at temperatures 41 °C < T < 141 °C in the Ba-doped Na-glass. Galvanostatic charging of a symmetric Cu/Na-glass/Cu cell for 1 h showed a remarkable self-charge on switching to open circuit; charging for 15 h followed by discharging at an applied −0.1 mA of the symmetric cell showed, in the discharge mode, a replating of sodium on the anode at a positive cell current of +0.07 mA for over 15 h. A model for these behaviors is proposed. A symmetric Li/Li-glass/Li cell was cycled to demonstrate plating of Li on a current collector from the Li-glass electrolyte.

NaFe2PO4(SO4)2: a potential cathode for a Na-ion battery

Hexagonal Na3Zr2PO4(SiO4)2 is a well-known fast Na+ conductor commonly referred to as NASICON. The low cost and wide availability of sodium invites exploration of Na+-insertion cathodes with the hexagonal NASICON framework structure. Here we report a low-temperature synthesis of NaFe2PO4(SO4)2 with the same host framework as NASICON and its performance as the cathode of a Na/Na1+xFe2PO4(SO4)2 cell operating on the Fe3+/Fe2+ redox couple. The cell provides a single-phase reaction having a capacity approaching 100 mA h g−1 at 0.1C after 50 cycles over the voltage range 2 ≤ V ≤ 4 V with a coulomb efficiency approaching 100%. An increase in capacity with cycling is the result of aging of an unoptimized electrode morphology.

Conditions for TaIV–TaIV Bonding in Trirutile LixMTa2O6

Stabilization of Ta-Ta bonding in an oxide across a shared octahedral-site edge of a Ta2 dimer is not known. Investigation of Li insertion into the trirutile structure of MTa2O6 with M = Mg, Cr, Fe, Co, and Ni indicates that Ta-Ta bonding across the shared octahedral-site edge of the dimer can be stabilized by a reversible electrochemical reduction of Ta(V) to Ta(IV) for M = Cr, Fe, Co, and Ni but not for M = Mg. Chemical reduction of MTa2O6 by n-butyl lithium only reduced NiTa2O6 to any significant extent. With M = Fe, Co, or Ni, electrochemical formation of the Ta-Ta bonds is accompanied by a partial reduction of the Fe(II), Co(II), or Ni(II) to Fe(0), Co(0), or Ni(0). For M = Cr, two Li per formula unit can be inserted reversibly with no displacement of Cr(0). For M = Mg, no Mg(II) are displaced by Li insertion, but a solid-electrolyte interphase (SEI) layer is formed on the oxide with no evidence of Ta-Ta bonding. Stabilization of Ta-Ta bonding across a shared octahedral-site edge in a dimer appears to require significant hybridization of the Ta(V) 5d(0) and M 4s(0) states.