Design,Synthesis And Electrochemical Performance Of Iodine-based Argyrodite-type Solid Electrolytes

The conventional lithium-ion batteries using organic liquid electrolytes are limited in energy density and pose safety risks such as leakage and thermal runaway,which severely restrict their applications.All-solid-state batteries composed of solid-state electrolytes with high thermal stability are regarded as the next-generation battery system with high safety.Sulfide inorganic solid-state electrolytes with high ionic conductivity and suitable mechanical properties have received a lot of attention,especially the low-cost precursors and high room temperature ionic conductivity of Argyrodite electrolytes.However,the relatively low ion conductivity at room temperature and limited oxidation stability of Li₆PS₅I hinder its application in solid-state batteries.This study addresses these issues by focusing on process optimization,doping modification,and structural design to enhance the ion conductivity of Li₆PS₅I electrolyte and improve its stability in battery systems.Various characterization techniques and analysis methods are employed to reveal the relevant mechanisms.The specific research contents are as follows:Due to the unclear synthesis details of Li₆PS₅I and its application in solid-state batteries,The synthesis of Li₆PS₅I phase was systematically investigated using the high-energy mechanical ball milling method.The Argyrodite phase of Li₆PS₅I appeared after 12 hours of ball milling at 500 rpm.After 20 hours of ball milling,pure Li₆PS₅I phase was obtained,achieving the optimal ion conductivity(0.21 m S cm^-1).The compatibility of Li₆PS₅I with different Li Ni_0.7Co_0.1Mn_0.2O₂ cathodes treated by different methods was studied,elucidating the differences in their cycling performance.The optimally processed Li₆PS₅I combined with coated Li Ni_0.7Co_0.1Mn_0.2O₂ even demonstrated good potential for application at low temperatures（-20°C）.The impedances of different components in the solid-state battery were evaluated using in situ electrochemical impedance spectroscopy at different temperatures and charge/discharge states.It was found that the electrolyte is the main factor influencing the lithium ion interface migration kinetics and the primary bottleneck affecting the performance of the solid-state battery.In response to the low ion conductivity of Li₆PS₅I mentioned above,this work enhances the ion transport performance of Li₆PS₅I by using iodine-rich design.Li_5.5PS_4.5I_1.5 synthesized using the high-energy mechanical ball milling method exhibited an ion conductivity of 0.31 m S cm^-1 at room temperature.Symmetric cell and half-cell testing revealed that Li_5.5PS_4.5I_1.5 exhibited side reactions at the cathode/electrolyte interface in high voltage cathode systems,leading to a larger ion diffusion barrier at the interface.Solid-state batteries constructed with Li_5.5PS_4.5I_1.5 in combination with Se_0.05S_0.95@p PAN cathode achieved a first discharge specific capacity of 1580 m Ah g^-1and exhibited 64%capacity retention after 30 cycles.The high reversible specific capacity and improved cycling stability indicate that Li_5.5PS_4.5I_1.5 electrolyte is suitable for application in solid-state Li-S battery systems.Enrichment with iodine only led to a marginal improvement in the ion conductivity of Li₆PS₅I.Subsequently,the 4b site of the Li₆PS₅I electrolyte was substituted with tetravalent elements（Si,Ge,Sn）using ball milling and annealing methods.By optimizing the doping content to control the conductivity and activation energy,we obtained the optimal composition of Li_6.7P_0.3Ge_0.7S₅I.Under room temperature cold pressing conditions,it exhibited an ultrahigh ion conductivity of 13.1 m S cm^-1 and a low activation energy of0.19 e V.All-solid-state batteries were constructed using Li_6.7P_0.3Ge_0.7S₅I electrolyte and Li Ni_0.8Co_0.1Mn_0.1O₂.Structual characterizations revealed that the capacity decay was influenced by by-products originating from the unstable interface between the oxide cathode and sulfide electrolyte.Scanning electron microscopy results indicated possible volume changes during cycling,leading to interface cracking after cycling.By introducing a Li₂Zr O₃ coating layer,the reversible capacity and cycling performance of the battery were effectively enhanced（retention increased from 62%to 84%after 300 cycles）.Excellent reversible capacity and cycling performance were also demonstrated at higher current densities（1 C,2 C,5 C）.At a high cathode loading of 26.8 mg cm^-2,a high areal capacity of 5.1 m Ah cm^-2 was achieved.Benefiting from the high conductivity and low activation energy of Li_6.7P_0.3Ge_0.7S₅I,the battery exhibited good cycling performance even at-20°C.Although the introduction of the coating layer reduced the extent of interface side reactions,sulfide electrolytes still undergo oxidative decomposition in high-voltage systems,leading to performance degradation of the batteries.This is attributed to the intrinsic instability of the Li_6.7P_0.3Ge_0.7S₅I electrolyte.Therefore,a composite cathode and a bilayer electrolyte design were developed.By introducing Li₃In Cl₆（3.55 V vs.Li-In）,which possesses higher electrochemical stability,to separate Li_6.7P_0.3Ge_0.7S₅I from direct contact with the oxide cathode.The decomposition of the electrolyte was effectively alleviated,resulting in higher initial efficiency(87%at 0.5 C,compared to only 67%for Li_6.7P_0.3Ge_0.7S₅I at 0.2 C).Various characterization techniques were employed to investigate the capacity decay that still occurred during cycling.In the presence of conductive additives,electrochemical decomposition of electrolyte was observed in the composite cathode.In the absence of conductive additives,the issues in the composite cathode were less pronounced,but prominent interface problems between Li₃In Cl₆ and Li_6.7P_0.3Ge_0.7S₅I were identified,leading to side reactions and the formation of cracks at the interface after battery cycling.Therefore,the bilayer strategy represents both opportunities and challenges for solid-state batteries.