| In traditional lithium/sodium ion batteries employing liquid organic electrolyte,the side reactions of solvent and lithium/sodium salt will lead to capacity decay and potential safety problems of the batteries.All-solid-state batteries can well address the safety problem and simultaneously improve the energy density.Among the cathode materials used in all-solid-state battery,metal sulfides and element sulfur possess high specific capacity and good interfacial compatibility with sulfide solid electrolytes.However,the slow electrochemical kinetics and reduced triple-phase boundaries caused by large volume change of the active material during charge/discharge process are the main challenges restricting the electrochemical performance of all-solid-state batteries employing these cathode materials.Therefore,in this thesis,rational triple-phase interface construction and enhanced electrochemical kinetics of the rate-determining step are designed to obtain high-performance all-solid-state batteries.The main results are shown below:1.Rational triple-phase interface design in the cathode for all-solid-state lithium batteriesCu2Zn Sn S4 was selected as model active cathode material,and Cu2Zn Sn S4@graphene composite with electronic conduction pathway is further synthesized.The presence of electronic conduction pathway in the cathode material can effectively reduce the charge transfer resistance.However,due to the alloying reaction of Zn in Cu2Zn Sn S4begins below 0.5 V which is beyond the range of the setting cut-off voltage of 0.5-3 V for Cu2Zn Sn S4 cathodes,the capacity of Cu2Zn Sn S4can not be fully utilized.Thus,Cu2Sn S3@graphene-Li7P3S11 nanocomposite with both electronic and ionic conduction pathways was synthesized to enhance interfacial contact and structure integrity.Further by miantaining the electronic/ionic conduction pathways in the cathode,a metal-organic frameworks derived carbon nanotubes threaded core-shell Co3S4@Li7P3S11 hollow nanoboxes are designed using ZIF-67 as template.This unique structure can improve the electrochemical kinetics,maintain the structure integrity of the cathode,realize intimate triple-phase contacts between active material/electrolyte/conductive additive and reduce the strain/stress accumulation,leading to the all-solid-state lithium batteries based on Co3S4-CNT@Li7P3S11nanocomposite exhibit long high rate cycling stability with capacity of 414.4 m Ah g-1after cycling at current density of 500 m A g-1 for 150 cycles.2.Rational triple-phase interface design in the cathode for all-solid-state sodium batteriesAll-solid-state sodium battery possesses the same battery structure and electrochemical reaction mechanism to that of all-solid-state lithium battery.Therefore,constructing nano-scale distributed and fast electronic/ionic conduction pathway is also of the same importance to achieve high-performance all-solid-state sodium battery.Solvothermal method and in-situ liquid-phase reaction were employed to synthesize core-shell structured Fe1-xS/Na2.9PS3.95Se0.05 composite with intimate active material/electrolyte interfacial contacts,and the resultant Fe1-xS@Na2.9PS3.95Se0.05/Na2.9PS3.95Se0.05/Na all-solid-state sodium battery demonstrates excellent electrochemical performance.Besides,the reaction mechanism of Fe1-xS is confirmed by means of ex-situ X-ray diffraction techniques,showing that partially reversible reaction occurs in the Fe1-xS electrode.Apart from the interfacial contacts between active material and electrolyte,the electrochemical reaction kinetics is also related to the ionic conduction of the solid electrolyte.Therefore,a nanoscaled Na3Sb S3.75Se0.25 solid electrolyte with less grain-boundary resistance was synthesized using a liquid/solid fusion technology.The Na3Sb S3.75Se0.25 solid electrolyte shows a high ionic conductivity of 4.03×10-3 S cm-1at room temperature due to the significantly decreased amorphous phase in the electrolyte.Moreover,the small particle size of the solid electrolytes enhances the contact between solid electrolyte and electrode,reducing the interfacial contact resistance.As a result,Fe S2/LS-Na3Sb S3.75Se0.25/Na all-solid-state sodium batteries achieve a high specific capacity of 140.6 m Ah g-1 for 300 cycles at 500 m A g-1.Then,to further improving the capacity and cycling stability of all-solid-state sodium battery,an S-Na3Sb S4-C cathode with distributed micro-scaled primary electronic/ionic highways along with nano-scaled secondary local-roads is fabricated by combining of the liquid-phase reaction and mechanical milling.The resultant S-Na3Sb S4-C nanocomposite cathode provides a high initial discharge capacity of1504.3 m Ah g-1 at 50 m A g-1.Meanwhile,S-Na3Sb S4-C/Na cells also demonstrate an excellent high rate cycling stability with reversible capacity of 468.1 m Ah g-1 after cycling at current density of 1000 m A g-1 for 100 cycles at room temperature.Even at ultrahigh cathode loading of 6.34 and 12.74 mg cm-2,the batteries can still deliver reversible discharge specific capacities of 742.9 and 465.6 m Ah g-1 at 100 m A g-1,respectively.This work opens up a facile method for construction of high-performance cathode material in room-temperature all-solid-state sodium-sulfur battery.3.Investigation of the impact of electrochemical reaction kinetics on the performance of all-solid-state lithium/sodium batteryImproving the electrochemical reaction kinetics of the rate-determining step is also vital to achieve high-performance all-solid-state battery.Using Fe S2 as a model cathode material and through experiments and density functional theory(DFT)calculations,we find that the introduction of catalytic cobalt can effectively improve the electrochemical activity of Fe S2 without scarifying the energy density of the battery.The optimized loose structured Co0.1Fe0.9S2 shows high specific surface area,which can ameliorate the volume change of the active material and reduce the diffusion distance of Li+during charge/discharge process.Electrochemical tests reveal that doped cobalt can effectively improve the reaction kinetics of rate determining step Li2-xFe S2(?)Fe Sy+(2-y)S+(2-x)Li++(2-x)e-.The resultant all-solid-state lithium battery shows improved reversible capacity of 543.5 m Ah g-1 after cycling at a current density of 500 m A g-1 for 100 cycles.This optimized Co0.1Fe0.9S2 cathode can also be employed in all-solid-state sodium battery,and the resultant battery employing Na11Sn2Sb S11.5Se0.5-Na3PS4 bilayer electrolyte exhibits reversible capacity of 159.8m Ah g-1 after cycling at a current density of 100 m A g-1 for 100 cycles.By combining improved electrochemical reaction kinetics and enhanced active material/electrolyte interfacial contacts,Fe3S4@S@0.9Na3Sb S4×0.1Na I composite with self-formed ionic conduction pathway is synthesized through one-step wet-mechanochemical milling procedure.In this composite,the introduction of Fe3S4can increase the electronic conductivity of the composite cathode by one order of magnitude and nearly double enhance the ionic conductivities,thus improving the electrochemical kinetics of sulfur.Therefore,the resultant room-temperature all-solid-state sodium-sulfur battery employing Fe3S4@S@0.9Na3Sb S4×0.1Na I composite cathode shows excellent cycling stability with a reversible capacity of 410m Ah g-1 at 500 m A g-1 for 50 cycles.This facile strategy for sulfur-based composite cathode is attractive for achieving room-temperature sodium-sulfur batteries with superior electrochemical performance. |
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