Field Assisted Sintering Of Solid Electrolyte Li₇La₃Zr₂O₁₂ And Its Transportation Properties Regulation

Recently,all-solid-state lithium-ion batteries have been widely concerned because of their good thermal stability,safety,and potential high energy and power density.Especially,it can solve the safety issues existing in the current lithium ion batteries with liquid electrolytes.In the development of all-solid-state battery technology,the garnet-type Li₇La₃Zr₂O₁₂?LLZO?has become one of the most promising solid electrolytes for the advantages of high room temperature lithium ion conductivity(10^-4S cm^-1),widely electrochemical window?>5 V?and good stability with lithium anode as well as in the air.In this paper,the high performance cubic phase LLZO was prepared by field assisted sintering technology?FAST?which was a one-step sintering technology.The lithium ion transportation performance was regulated by modifying the crystal structure?carrier concentration and carrier migration channel?and the regulation mechanism was revealed.Besides,the application performance of LLZO in all solid-state batteries was studied.A high performance LLZO was prepared by FAST,which maintained cubic phase at room temperature and reached a high relatively density of⁹9.8%.The suitable process for FAST to obtain high density cubic phase LLZO is at the sintering temperature range of 1100¹180 ^oC,and sintering pressure of 5¹0 MPa with the green thickness of 6.3⁸.4mm.FAST has a very short sintering time?10 min?and a relatively low sintering temperature?1150^oC?compared to the conventional solid-state sintering method,both of which can effectively reduce the loss of Li.The entire sintering has only one step heat treatment process,which effectively avoid the introduction of impurities.In addition,the axial pressure of FAST is very favorable for the densification of LLZO solid electrolyte.The Ca²⁺,Al³⁺,Sn⁴⁺and Ta⁵⁺doping ions on the stability of cubic phase LLZO was studied,respectively.Results show that,in the low-valence ion(Ca²⁺,Sn⁴⁺)doping LLZO,cubic phase is unstable,because the Coulomb repulsion between the lithium ions was strong.While for the high-valence(Al³⁺,Ta⁵⁺)doping LLZO,cubic phase is stable in which the repulsion between Li⁺is small.²⁷Al magic angle spinning nuclear magnetic resonance?MAS NMR?results show that one Al³⁺substitute one tetrahedral Li⁺and forms two Li⁺vacancies,which reduces the coulomb repulsion between Li⁺ions and finally stabilizes the cubic phase.Thus,the mechanism of ultra-valence ion stabilized cubic phase is the vacancy stabilization.The effects of lithium ion concentration on the phase and microstructure of LLZO were investigated.The microstructure evolution tested by ⁷Li MAS NMR and Raman spectra shows that the optimal lithium ion concentration is 6.35moels(vs.7moles of nominal Li₇La₃Zr₂O₁₂).At Li⁺concentration of 6.35 moles,Li⁺has a high occupancy at octahedral site,and high degree of disorder in the tetrahedral site.The tested Li⁺ionic conductivity results show that LLZO with 6.35moles Li⁺concentration has the highest grain ionic conductivity of 9.48×10^-4S cm^-1and lowest Li⁺migration activation energy of 0.29 eV.Combined with the microstructure evolution and the tested results,it is concluded that Li⁺concentration affects Li ionic conductivity via changing Li⁺distribution in the grain and the ordering degree of Li⁺arrangements.The optimum Li⁺concentration range is 6.35±0.1 moles.Three kinds of doping ways including equivalent single doping,equivalent double doping and un-equivalent single doping were designed.Firstly,the regulation mechanism of Li⁺migration channel on the Li⁺transportation property were investigated by equivalent single doping(M⁴⁺=Ge⁴⁺,Ti⁴⁺,Sn⁴⁺,Hf⁴⁺and Te⁴⁺doping Zr⁴⁺respectively).The X-ray absorption fine structure?XAFS?and the first-principles calculation results show that,as the doping ion radius increases,the M-O bond length becomes longer,MO₆ becomes larger and the lattice constant increases.The regulation mechanism of Li⁺migration channel is that the M⁴⁺ions doping in the Zr site mainly via changing the M-O bond length by ion radius difference.The smaller the ionic radius is,the M-O bond length is shorter,and the corresponding MO₆volume becomes smaller which result in contraction of the Li⁺migration channel.Secondly,for the Ca²⁺and Ta⁵⁺equivalent double doped Li₇La_3-XCa _XZr_2-YTa_YO₁₂?X=Y=0¹moles?,the Li-ion channel size is regulated continuously,and the corresponding lattice constant is 12.94¹2.96?of the optimum Li-ion channel size.Finally,the synergistic regulation of carrier concentration and carrier channel size was achieved by Ta⁵⁺un-equivalent single-doped Li_7-XLa₃Zr_2-XTa_XO₁₂?X=0¹.0moles?.The total ionic conductivity increases to 1.01×10^-33 S cm^-11 compared with the un-doped 5.7×10^-4S cm^-1.The interfacial properties of Li_6.4La₃Zr_1.4Ta_0.6O₁₂?LLZT?/cathode and LLZT/Li anode was studied,respectively.Results show that the LLZT solid electrolyte is very stable against lithium metal anode and the electrochemical window is wide more than5V.All-solid-state lithium batteries?ASLBs?based on LLZT utilizing a composite cathode with three-dimensional ionic conductive network has been successfully obtained.The designed LiCoO₂-polyethylene oxide?LCO-PEO?composite cathode improves the Li⁺conduction between cathode active material particles as well as optimizes the interfacial contact between cathode and solid electrolytes.It has a high specific capacity of 124¹36 mAh g^-1at 0.1C,delivering approximately 90⁹8%of the theoretical discharge capacity.Besides,the internal resistance of the designed LCO-PEO/LLZT/Li battery is only 25⁴5%of the original Li/LLZO/LCO-PVDF battery.