Sodium Ion Diffusion in Nasicon (Na₃Zr₂Si₂PO₁₂) Solid Electrolytes: Effects of Excess Sodium

The Na superionic conductor (aka Nasicon, Na_1+<i>x</i>Zr₂Si_<i>x</i>P_3–<i>x</i>O₁₂, where 0 ≤ <i>x</i> ≤ 3) is one of the promising solid electrolyte materials used in advanced molten Na-based secondary batteries that typically operate at high temperature (over ∼270 °C). Nasicon provides a 3D diffusion network allowing the transport of the active Na-ion species (i.e., ionic conductor) while blocking the conduction of electrons (i.e., electronic insulator) between the anode and cathode compartments of cells. In this work, the standard Nasicon (Na₃Zr₂Si₂PO₁₂, bare sample) and 10 at% Na-excess Nasicon (Na_3.3Zr₂Si₂PO₁₂, Na-excess sample) solid electrolytes were synthesized using a solid-state sintering technique to elucidate the Na diffusion mechanism (i.e., grain diffusion or grain boundary diffusion) and the impacts of adding excess Na at relatively low and high temperatures. The structural, thermal, and ionic transport characterizations were conducted using various experimental tools including X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS). In addition, an ab initio atomistic modeling study was carried out to computationally examine the detailed microstructures of Nasicon materials, as well as to support the experimental observations. Through this combination work comprising experimental and computational investigations, we show that the predominant mechanisms of Na-ion transport in the Nasicon structure are the grain boundary and the grain diffusion at low and high temperatures, respectively. Also, it was found that adding 10 at% excess Na could give rise to a substantial increase in the total conductivity (e.g., ∼1.2 × 10^–1 S/cm at 300 °C) of Nasicon electrolytes resulting from the enlargement of the bottleneck areas in the Na diffusion channels of polycrystalline grains

Supporting video files: "Characterisation of the diffusion properties of metal foam hybrid flow-fields for fuel cells using optical flow visualisation and X-ray computed tomography"

Underlying experimental data for publication titled 'Characterisation of the diffusion properties of metal foam hybrid flow-fields for fuel cells using optical flow visualisation and X-ray computed tomography'</p

Supplementary data for: "X-ray tomography and modelling study on the mechanical behaviour and performance of metal foam flow-fields for polymer electrolyte fuel cells"

Supplementary data for publication titled "X-ray tomography and modelling study on the mechanical behaviour and performance of metal foam flow-fields for polymer electrolyte fuel cells" published in the International Journal of Hydrogen Energy. https://doi.org/10.1016/j.ijhydene.2019.01.206</p

Uniform Surface Modification of Li₂ZnTi₃O₈ by Liquated Na₂MoO₄ To Boost Electrochemical Performance

Poor ionic and electronic conductivities are the key issues to affect the electrochemical performance of Li₂ZnTi₃O₈ (LZTO). In view of the water solubility, low melting point, good electrical conductivity, and wettability to LZTO, Na₂MoO₄ (NMO) was first selected to modify LZTO via simply mixing LZTO in NMO water solution followed by calcining the dried mixture at 750 °C for 5 h. The electrochemical performance of LZTO could be enhanced by adjusting the content of NMO, and the modified LZTO with 2 wt % NMO exhibited the most excellent rate capabilities (achieving lithiation capacities of 225.1, 207.2, 187.1, and 161.3 mAh g^–1 at 200, 400, 800, and 1600 mA g^–1, respectively) as well as outstanding long-term cycling stability (delivering a lithiation capacity of 229.0 mAh g^–1 for 400 cycles at 500 mA g^–1). Structure and composition characterizations together with electrochemical impedance spectra analysis demonstrate that the molten NMO at the sintering temperature of 750 °C is beneficial to diffuse into the LZTO lattices near the surface of LZTO particles to yield uniform modification layer, simultaneously ameliorating the electronic and ionic conductivities of LZTO, and thus is responsible for the enhanced electrochemical performance of LZTO. First-principles calculations further verify the substitution of Mo⁶⁺ for Zn²⁺ to realize doping in LZTO. The work provides a new route for designing uniform surface modification at low temperature, and the modification by NMO could be extended to other electrode materials to enhance the electrochemical performance