Chunying Chen,Jinjun Ren,Qing Jiao,Jingping Tang,Lili Hu
Joint doping with trivalent and monovalent ions is a widely adopted strategy to enhance the ionic conductivities of NASICON-type solid-state electrolytes (SSEs) with the general formula Na1+xZr2SixP3–xO12, but the underlying microscopic mechanism remains unclear. In this study, we synthesized a series of Na3+xLaxZr2–xSi2PO12 (NLZSPx, 0 ≤ x ≤ 0.4) ceramics and characterized their electrical properties and structures by using electrochemical impedance spectroscopy (EIS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and advanced solid-state nuclear magnetic resonance (SSNMR). Among all of the samples, the composition with x = 0.2 exhibited the highest ionic conductivity, achieving 1.05 × 10–3 S/cm at 25 °C, which is 3.65 times higher than that of the undoped sample. XRD and SSNMR results prove that La3+ ions did not integrate into the main phase lattice to replace Zr4+ ions. Instead, they formed a heterogeneous phase predominantly composed of Na3La(PO4)2 at the grain boundaries. The content of Na3La(PO4)2 and the composition of the NASICON main phase were accurately determined by SSNMR. This impurity resulted in changes in the Si/P and Na/Zr ratios within the main phase. Some comparison experiments are carried out and prove that these changes were not the principal drivers of the increased conductivity. SEM results further indicated that doping with an appropriate amount of La3+ ions significantly increased the sample densification, thus promoting Na+ ions transport between grain boundaries─this being the key factor driving the observed improvement in ionic conductivity.
Wenxi Zhao,Xiaodeng Wang,Xiaoqing Ma,Luchao Yue,Qian Liu,Yonglan Luo,Yang Liu,Abdullah M. Asiri,Xuping Sun
Pseudocapacitive electrochemical Na+-storage has been highlighted as one of the exploitable strategies for overcoming the sluggish diffusion-limited redox kinetics due to the effective structural preservation and fast ion-adsorption/desorption at the surface or quasi-surface of electrode materials. However, exploiting pseudocapacitive hosts with a micro–nano hierarchitecture and further achieving competitive pseudocapacitive contributions are still in their infancy so far. Herein, a yolk–shell NiS2/CuS hollow microspherical architecture with superb kinetically pseudocapacitive features was successfully constructed through an in situ hydrothermal sulfidation and subsequent ion-exchange route using Ni-based bimetallic (NiZn) organic frameworks (NiZn-MOFs) as a template precursor. As expected, the strongly synergistic coupling effect and hollow structural characteristic of the NiS2/CuS heterostructure enabled fast charge transfer and Na+ immigration, as well as the release of the mechanical stress/strain induced by the conversion reaction, and not unexpectedly, the NiS2/CuS electrode afforded extraordinary Na+-storage capability, including a remarkable specific capacity of 410.9 mA h g−1 after 750 cycles at 2.0 A g−1, excellent rate capability, and prolonged cyclability in terms of a remarkable 283.4 mA h g−1 even after 4200 cycles at 20.0 A g−1. More significantly, the kinetic analysis demonstrated that the electrochemical charge storage of the NiS2/CuS electrode manifested considerable pseudocapacitive contributions at all rates (90.0% to 96.9%), distinctly outperforming the previously reported NiS2-/CuS-based anodes. Furthermore, the density functional theoretical calculations suggested a fast Na+-transport kinetics and enhanced antibonding state energy level and Na2S adsorption energy due to the electronic redistribution and lattice distortion in the NiS2/CuS heterointerfaces.
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