| All-solid-state sodium batteries have potential advantages of low cost,high safety and high energy density,which has become research hotpot in the field of energy storage.Sodium ion sulfide solid electrolytes show high room-temperature ionic conductivity of 10-4～10-2 S cm-1 and good interface contact and ion transfer with electrode material.In addition,sulfide electrolytes have soft texture and can be densified by cold pressing,simplifying the fabrication of all-solid-state sodium batteries.Therefore,the research and development of high-performance sodium ion sulfide solid electrolytes are of great significance to the development of practical all-solid-state sodium batteries.Sodium ion sulfide solid electrolytes are mainly divided into two types,ternary（Na3Pn S4）and quaternary(Na11Sn2Pn S12)sulfide solid electrolytes,where Pn=P or Sb.In this thesis,we focus on the Na11Sn2PS12,Na11Sn2Sb S12 and Na3Sb S4 solid electrolytes,investigating their preparation methods,aliovalent ions doping and electrochemical performances in all-solid-state sodium batteries.The main results are as follows:1.Preparation methods for Na11Sn2PS12 solid electrolyteFirstly,Na11Sn2PS12 solid electrolyte is successfully synthesized by high-energy ball milling method(BM-Na11Sn2PS12).The raw materials including Na2S,P2S5 and Sn S2 are transformed into amorphous phase during high-energy ball milling process,and the amorphous precursor is further sintered at 460 oC to obtain the Na11Sn2PS12,which exhibits room-temperature ionic conductivity of 2.16×10-4 S cm-1.The electronic conductivity,activation energy and electrochemical stability window of BM-Na11Sn2PS12 are 2.83×10-8 S cm-1,0.35 e V and 0.8～2.8 V,respectively.In addition,severe interfacial reaction between BM-Na11Sn2PS12 solid electrolyte and Na metal would occur,resulting in an increased interfacial impedance.Moreover,the influence of Br doping on the structure and ionic conductivity of Na11Sn2PS12 is investigated.The improvement of Br doping on ionic conductivity of Na11Sn2PS12 is limited(Na10.94Sn2PS11.94Br0.06,2.57×10-4 S cm-1).And the Br doping is not conducive to the acquisition of pure Na11Sn2PS12 phase when doping content is>=0.12,which can further decrease the ionic conductivity.Secondly,nanosized（～200 nm）Na11Sn2PS12 solid electrolyte is successfully synthesized by liquid phase method using 1,2-dimethoxyethane as solvent(LP-Na11Sn2PS12).After the liquid phase reaction of Na2S,P2S5 and Sn S2,the liquid-phase precursor powders are further sintered at 460 oC to obtain the Na11Sn2PS12,which exhibits room-temperature ionic conductivity of 1.73×10-4 S cm-1.The electronic conductivity,activation energy and electrochemical stability window of LP-Na11Sn2PS12 are 3.36×10-8 S cm-1,0.34 e V and 0.8～2.8 V,respectively.Nanosized LP-Na11Sn2PS12 is used as ionic conductive agents in composite cathode and solid electrolyte layer in the Ti S2/LP-Na11Sn2PS12/Na all-solid-state sodium battery.Compared with Ti S2/BM-Na11Sn2PS12/Na battery,the smaller particle size of LP-Na11Sn2PS12 solid electrolyte is beneficial to increase the interface contact between solid electrolyte and active material,and further reduce the interface impedance and improve the utilization of active materials.The Ti S2/LP-Na11Sn2PS12/Na battery shows an initial discharge specific capacity of 231.0m Ah g-1 with initial Coulombic efficiency of 67.9%at 20 m A g-1 and 25 oC.After 10cycles,the capacity retention is 61.6%.Finally,Na11Sn2PS12 solid electrolyte is successfully synthesized by high-temperature quenching method(HQ-Na11Sn2PS12).The crystal Na11Sn2PS12 can be directly obtained after quenching the raw materials of Na2S,P2S5 and Sn S2 from high temperature,but there are plenty of impurities in the product,demonstrating low ionic conductivity of 0.34×10-4 S cm-1.After annealing the quenched precursor at430 oC,higher purity of Na11Sn2PS12 is obtained,showing improved ionic conductivity of 6.26×10-4 S cm-1.The ionic conductivity of HQ-Na11Sn2PS12 can be further enhanced to 7.09×10-4 S cm-1 by hot pressing.The electronic conductivity,activation energy and electrochemical stability window of HQ-Na11Sn2PS12 are 2.25×10-8 S cm-1,0.27 e V and 0.8～2.8 V,respectively.The all-solid-state sodium battery Ti S2/HQ-Na11Sn2PS12/Na3Sb S4/Na is assembled by using HQ-Na11Sn2PS12 as ionic conductive agents in composite cathode and solid electrolyte layer.Na3Sb S4 as buffer layer towards Na metal is used.The Ti S2/HQ-Na11Sn2PS12/Na3Sb S4/Na battery shows good electrochemical performance,demonstrating an initial discharge specific capacity of 181.3 m Ah g-1 with initial Coulombic efficiency of 63.0%at 0.1 C and 25oC.After 30 cycles,the capacity retention is 50.7%.2.Preparation of Ti-doped Na11Sn2Sb S12 solid electrolyte by high-temperature quenching methodTi-doped Na11Sn2Sb S12 solid electrolyte is successfully synthesized by high-temperature quenching method.The crystal Na11Sn2Sb S12 can be directly obtained after quenching the raw materials of Na2S,Sb2S3,Sn S2 and S from high temperature,which indicates that the Na11Sn2Sb S12 is the thermodynamic preferred phase,but its ionic conductivity is only 5.33×10-5 S cm-1.When the quenched precursor is further annealed at 550 oC,the ionic conductivity of Na11Sn2Sb S12increases to 3.12×10-4 S cm-1.However,the increase of ionic conductivity is accompanied by the formation of Na Sb S2 impurity,which is originated from the solubility limit of Sb in Na11Sn2Sb S12 lattice.Furthermore,the formation of Na Sb S2can be effectively inhibited by partial substitution of Sb by Ti,and the pure phase Na11.5Sn2Sb0.5Ti0.5S12 is obtained,which shows high ionic conductivity of 1.01×10-3S cm-1.The electronic conductivity,activation energy and electrochemical stability window of Na11.5Sn2Sb0.5Ti0.5S12 are 6.64×10-9 S cm-1,0.27 e V and 1.0～2.5 V,respectively.Meanwhile,the Na11.5Sn2Sb0.5Ti0.5S12 exhibits good air stability with limited H2S release.Finally,all-solid-state sodium battery is assembled by using Na11.5Sn2Sb0.5Ti0.5S12 as ionic conductive agents in composite cathode and solid electrolyte layer.The Ti S2/Na11.5Sn2Sb0.5Ti0.5S12/Na3Sb S4/Na battery shows high reversible capacity and stable cyclic performances,demonstrating an initial discharge specific capacity of 177.6 m Ah g-1 with initial Coulombic efficiency of 74.4%at 0.1C and 25 oC.After 50 cycles,the capacity retention is 63.8%.3.Preparation of W and O co-doped Na3Sb S4 solid electrolyte by high-temperature quenching methodW and O co-doped Na3Sb S4 solid electrolyte is successfully synthesized by high-temperature quenching method.The raw materials including Na2S,Sb2S3 and S are transformed into a mixing phase of Na Sb S2 and Sb2S3 during quenching process from high temperature.The tetragonal Na3Sb S4 was obtained after sintering the quenched precursor at 280 oC with addition of 5wt%excess S and achieves an optimal ionic conductivity of 0.98×10-3 S cm-1.After introducing sodium vacancy in Na3Sb S4 by W doping,the Na2.95Sb0.95W0.05S4 shows significantly improved ionic conductivity of 10.37×10-3 S cm-1.However,the introduced W in Na3Sb S4 can seriously deteriorate the interface between solid electrolyte and Na metal,which is originated from the reduction product of W metal by Na metal.The existence of W metal would accelerate the deterioration rate of solid electrolyte/Na metal interface,further resulting in rapidly failed all-solid-state sodium battery.The introduction of O element through W and O co-doping can effectively passivate the deteriorative interface induced by W substitution with low sacrifice of ionic conductivity(8.49×10-3 S cm-1)for Na2.95Sb0.95W0.05S3.9O0.1,which is due to the lattice contraction by partial substitution of S by O element.Using Na2.95Sb0.95W0.05S3.9O0.1 as ionic conductive agents in composite cathode and solid electrolyte layer,the Ti S2/Na2.95Sb0.95W0.05S3.9O0.1/Na batteries show high reversible capacity,good cyclic stability and rate capabilities.The Ti S2/Na2.95Sb0.95W0.05S3.9O0.1/Na battery shows an initial discharge specific capacity of 230.0 m Ah g-1 with initial Coulombic efficiency of 63.2%at 0.1 C and 25 oC,and the capacity retention is 50.3%after 50 cycles.The Ti S2/Na2.95Sb0.95W0.05S3.9O0.1/Na battery shows reversible capacities of 148.2,128.3,110.9 and 82.6 m Ah g-1 at 0.1,0.2,0.3 and 0.5 C,respectively. |
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