| Compared with traditional liquid lithium-ion batteries,all-solid-state lithium batteries have many unparalleled advantages.Liquid lithium-ion batteries cannot use lithium metal as the negative electrode,are fear of high temperature,and have a risk of explosion caused by electrolyte volatilization,while all-solid-state batteries do not have these issues,therefore have a higher safety performance.The core issue of solid-state lithium batteries is the solid electrolyte.Garnet-type solid-state electrolyte(Li7La3Zr2O12,LLZO)stands out among many solid-state electrolytes due to its high ionic conductivity,stability to lithium metal and wide electrochemical window,and is considered as one of the most commercially promising solid-state electrolytes.However,problems such as excessive interfacial impedance between LLZO and lithium metal cathode have seriously affected the commercialization of LLZO.In this paper,the following studies were done to address this problem.(1)B-doped LLZTO(B2O3-LLZTO)ceramic electrolyte sheets were synthesized by a two-step solid-phase method.B2O3 was distributed at the grain boundaries of LLZTO and an Li-B-O layer was in situ generated at the interface of cathode/electrolyte when B-LLZTO was in contact with molten lithium.The Li-B-O layer increases the interface contact area,reduces the interface impedance,improves the current distribution,and suppresses the growth of lithium dendrites during the charging and discharging process.The effect of B element doping on the characteristics of B-LLZTO electrolyte was investigated,and the full cell was assembled to study its performance.The results show that due to the low melting point of B2O3,the relative density of B-LLZTO decreases as the increase of doping amount of B element.When the doping amount of B is 1 wt.%,the interfacial impedance between Li|B-LLZTO reaches as low as 2.5?·cm-2.The first cycling discharge specific capacity of Li|B0.1-LLZTO|Li Fe PO4 full cell is133.47 m Ah·g-1 at 0.1 C and 60°C,and the initial Coulombic efficiency is above 99.5%.(2)The phosphorus-modified lithium metal anode was prepared by a simple melting method.It was found that the main component of the phosphorus-modified lithium metal anode is Li3P,which has high ionic conductivity and good wettability to LLZTO electrolyte,therefore increases the interfacial contact area and reduces the anode/electrolyte interfacial impedance.The effect of phosphorus doping on the interfacial impedance of Li-P|LLZTO was investigated,and the performance of Li-P electrode was studied by assembling a full cell.The results show that the Li-P|LLZTO interfacial impedance is as low as 68.8?·cm-2.The first-turn discharge specific capacity of Li-P0.15|LLZTO|Li Fe PO4 full cell is 164.5 m Ah·g-1 at 0.1C and 60°C,and the initial Coulomb efficiency is above 99%.At 1C,the discharge specific capacity of Li-P0.15|LLZTO|Li Fe PO4 full cell can reach 142.7 m Ah·g-1,which is 86%of that at 0.1C,demonstrating excellent high-rate performance.(3)A Fe-doped LLZO ceramic sheet was synthesized by solid phase method using Fe2O3as raw material.An ion-electron mixing conductor layer is formed in situ at the Li-metal/Fe-LLZO electrolyte interface after full contact of Fe-LLZO electrolyte with Li metal.By constructing Fe-LLZO|LLZTO bilayer ceramic electrolyte sheets,the more Li can be formed in the pores of Fe-LLZO via the reaction of Fe-LLZO and Li metal,and the ion-electron conductor layer helps to confine Li inside the pores of Fe-LLZO.The effect of Fe doping on the ceramic electrolyte sheet was investigated,and a full cell was assembled to study its performance.It is shown that the Li|Fe-LLZO|LLZTO interface impedance can reach as low as365.2?·cm-2.At a current density of 0.1 m A-cm-2,the Li|Fe0.1-LLZO|LLZTO|Fe0.1-LLZO|Li symmetric cell can maintain a stable overpotential cycle of 0.009 V for more than 450 hours.The Li|Fe0.1-LLZO|LLZTO|Li Fe PO4 full cell has a first-cycle discharge specific capacity of154.5 m Ah·g-1 at 60°C and 0.1C with the initial Coulombic efficiency of over 99%. |
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