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Owing to the flammability of dendrites and organic liquid electrolytes, the safety performance of lithium-ion batteries cannot be effectively ensured. The sodium-ion solid electrolyte is anticipated to become the leading next generation battery in commercial applications due to the high thermal stability and low price. Yet, the insufficient ionic conductivity of Na3Zr2Si2PO12 solid electrolytes, along with the large interfacial resistance between the cathode material and the solid electrolyte, substantially restricts the performance of room-temperature all-solid-state sodium rechargeable batteries. In the present study, the effects of Bi2O3 additive on the phase, microstructure and ionic conductivity of Na3.1Y0.1Zr1.9Si2PO12 ceramic electrolyte are reported. Moreover, a significant room temperature conductivity of 1.21 × 10-3 S cm-1 and the dense microstructure was attained by adding 1 wt% Bi2O3 to Na3.1Y0.1Zr1.9Si2PO12 sintered at 1100 °C for 6 h. Simultaneously, an electrochemical test exhibited that the electrochemical stability window increased to 4.8 V and revealed good cycle stability.

Inorganic ceramic as one of the most promising candidate for solid state electrolytes (SSEs) have received extensive attention due to their excellent thermal stability, good electrochemical stability and wide electrochemical window. The sodium super ionic conductor (NASICON, Na3Zr2Si2PO12) structure possesses a three-dimensional network, which helps the migration of Na+. However, because of its low Na+ ion conductivity and poor sintering performance, Na3Zr2Si2PO12 is difficult to achieve application. Therefore, it is quite urgent to solve these questions. In this study, we have reported the effects of additive antimony-tin oxide (ATO) on the phase, microstructure and ionic conductivity of Na3Zr2Si2PO12 ceramic electrolytes. With the ATO sintering additive added, the Na+ ion conductivity and density of Na3Zr2Si2PO12 ceramic electrolytes were improved. The Na3Zr2Si2PO12 ceramic with 5 wt% ATO additive sintered at 1100 °C shows a relatively high ionic conductivity of 1.43 × 10 − 3 S cm-1 at room-temperature, most densest microstructure and wide electrochemical window up to 4.6 V (vs. Na/Na+). In addition, Na/NASICON/Na symmetrical cells were assembled and exhibited good cycling stability which implied good compatibility between sodium (Na) metal and NASICON electrolyte.

Silver-coated carbon fibers successfully fabricated by a modified electroless plating process were characterized by scanning electron microscope (SEM) and X-ray diffraction analysis (XRD). The electroless plating process was modified by replacing the conventional pretreatment and sensitization steps by only using surface hydroxylation step. And the activation and electroless plating steps were merged into one step. The silver layer was successfully coated onto the surface of carbon fibers under the given coating conditions. The effects of AgNO3 solution concentration and reaction temperature on the conductivity of silver-coated carbon fibers were studied in detail. The results indicated that the optimum AgNO3 solution concentration was 0.05 mol/L, and the optimum range of reaction temperature was from 45°C to 65°C.

The effect of co-doping of Sr and Al or Fe on the microstructure, sinterability and oxide-ion conductivity of lanthanum silicate oxyapatites is investigated in detail at 300–800 °C by the electrochemical impedance spectroscopy. The oxide-ion conductivity is 1.46 × 10 −2 S cm −1 for La 9.5 Sr 0.5 Si 5.5 Fe 0.5 O 26.5 (LSSFO) and 1.34 × 10 −2 S cm −1 at 800 °C for La 9.5 Sr 0.5 Si 5.5 Al 0.5 O 26.5 (LSSAO), respectively, which is one order of magnitude higher than 6.16 × 10 −3 S cm −1 measured on La 9.67 Si 6 O 26.5 (LSO) oxyapatite under the identical test conditions. The grain bulk and grain boundary resistances of co-doped oxyapatite are significantly smaller than that of LSO oxyapatite, and decrease significantly with the increase of the sintering temperature. LSSFO and LSSAO also show significantly higher density as compared to that of LSO. The results indicate that co-doping of Sr and Al or Fe significantly improves the densification, sinterability and oxide-ion conductivity of lanthanum silicate oxyapatites.

The co-doped lanthanum silicate oxyapatites, La9.5A0.5Si5.5Fe0.5O26.5 (A = Ba, Sr, and Ca), are synthesized by the high-temperature solid state reaction process. The phase formation and structure properties of undoped lanthanum silicate oxyapatite (La9.67Si6O26.5, LSO), Fe-doped lanthanum silicate oxyapatite (La10Si5FeO26.5, LSFO) and co-doped lanthanum silicate oxyapatites (La9.5A0.5Si5.5Fe0.5O26.5, A = Ba, Sr, and Ca) are characterized by X-ray diffraction (XRD) and scanning electron spectroscopy (SEM). The effect of co-doping of A (A = Ba, Sr, Ca) and Fe on the microstructure, sinterability and oxide ion conductivity of lanthanum silicate oxyapatites is investigated in detail. The results show that, as compared to LSO and LSFO oxyapatites, co-doping of A (A = Ba, Sr, and Ca) and Fe significantly benefits the sintering and densification process, and enhances the oxide ion conductivity. For co-doped oxyapatites, the oxide ion conductivities are related to the dopant size, the best properties are obtained for the oxyapatite co-doped with Ca and Fe. The co-doped La9.5Ca0.5Si5.5Fe0.5O26.5 (LCSFO) oxyapatite is a good electrolyte for SOFCs, with an oxide ion conductivity of 1.39 × 10−2 S cm−1 at 800 °C and a low activation energy of 90.71 kJ mol−1. The bulk density and oxide ion conductivities of co-doped oxyapatite ceramics increase significantly with the increase of the sintering temperature. The grain bulk and grain boundary resistances of La9.5Ba0.5Si5.5Fe0.5O26.5 (LBSFO), La9.5Sr0.5Si5.5Fe0.5O26.5 (LSSFO) and LCSFO oxyapatite ceramics are significantly smaller than those of LSO and LSFO oxyapatite ceramics sintered under the identical conditions.

Apatite ceramics are interesting alternative solid oxide fuel cells (SOFCs) electrolytes because of their open structure for the transportation of oxide ions and their good chemical stability. This study reports the influence of barium doping on the microstructure, sinterability and oxide conductivity properties of the aluminium lanthanum oxyapatite La9.5Ba0.5Si5.5Al0.5O26.5. SEM results show that lanthanum substitution with barium improves the sinterability of apatite ceramics. The barium doping also enhances the conductivity of the aluminium lanthanum silicates. The oxygen ion conductivity of La9.5Ba0.5Si5.5Al0.5O26.5 sintered at 1600 °C is 2.21 × 10−2 S cm−1 at 800 °C, higher than 9.81 × 10−3 S cm−1 of La10Si5AlO26.5 sample prepared under the same conditions. The results in the present study demonstrate that doping Ba on the La site for aluminium lanthanum oxyapatite reduces the sintering temperature and improves the ion conductivity. The enhancement of Ba dopant is mainly on the improvement of the densification and thus substantially reduced grain boundary resistance of aluminium lanthanum oxyapatite particularly at low temperatures