Design,Preparation And Performance Studies Of Layered Ni-rich Oxide Cathode For Sulfide-based All-solid-state Lithium Batteries

With the rapid development of various electronic products and electric vehicles,electrochemical energy storage devices are required to have higher safety and energy density.Traditional lithium-ion batteries are difficult to simultaneously meet the requirements of high safety and high energy density due to using liquid electrolyte with volatile,flammable and explosive.All-solid-state lithium batteries（ASSLBs）using nonflammable solid electrolyte have attracted more and more attention,because it is possible to simultaneously achieve high safety and high energy density.In order to obtain high capacity and high energy density ASSLBs,Ni-rich layered oxide cathodes LiNi_xCo_yM_1-x-yO₂（M=Mn or Al,x≥0.6）have been widely studied in sulfide-based ASSLBs due to their high specific capacity and high voltage.However,its electrochemical performance is still poor.In particular,the reported layered oxide cathodes can only charge-discharge under low current density and low loading in sulfide-based ASSLBs.Physical contact failure,chemical/electrochemical side reactions and space charge layer between Ni-rich oxide cathode and sulfide electrolyte are considered to be the causes of performance degradation.Therefore,in order to obtain high performance ASSLBs,it is very important to reasonably design Ni-rich layered oxide cathode for solving these interfacial problems and improving the interfacial stability.Based on this,the following work is done in this paper.（1）Deterioration mechanism of the interface between LiNi_0.85-xCo_0.15Al_xO₂（Li-Ni-Co-Al-O）and Li₁₀Ge P₂S₁₂（LGPS）is investigated in detail by combining in/ex-situ Raman spectra and Electrochemical Impedance Spectroscopy.It can be determined that chemical side reaction between Li-Ni-Co-Al-O and LGPS will occur immediately once contacted,and the interfacial deterioration becomes more serious after the charging-discharging process under the dual effects of chemical and electrochemical side reactions.Moreover,this research reveals that the interfacial stability and the cycle performance of ASSLBs can be greatly enhanced by Al-substitution for Ni in Li-Ni-Co-Al-O.In particular,the capacity retention of LiNi_0.6Co_0.15Al_0.25O₂ cathode after 200 cycles can reach 81.9%,much higher than that of NCA cathode（12.5%@200 cycles）.This work provides a foundation for the design of layered Ni-rich oxide cathode possessing stable interface with sulfide electrolyte.（2）To boost the interfacial stability and further improve the electrochemical performance of LiNi_0.8Co_0.15Al_0.05O₂（NCA）cathode for sulfide-based ASSLBs,a Ni-rich cathode material is prepared by core-shell structure and surface coating strategy in this chapter.Firstly,NCA is designed as a core-shelled structure,in which Ni-rich core LiNi_0.85Co_0.15O₂can provide high capacity,and Al-rich shell LiNi_0.6Co_0.15Al_0.25O₂ layer can provide the complete coating on the Ni-rich core.Secondly,a thin LiNbO₃（LNO）layer is coated on the core-shelled NCA（CS-NCA）particles to increase the interfacial stability.As expected,LNO-coated CS-NCA（CS-NCA@LNO）cathode displays high discharge capacity(184.1 m Ah g^-1 at 0.06 C),excellent initial coulombic efficiency（90.2%,0.06 C）,outstanding rate performance(130 m Ah g^-1 at 4.2 C)and cycle performance（capacity retention of 89.4%after 400cycles at 0.3 C）at 60℃.（3）To effectively improve the interfacial stability and electrochemical performances of LiNi_0.8Co_0.1Mn_0.1O₂（NCM）cathode for sulfide-based ASSLBs,NCM precursor is for the first time coated with nano-thickness Li Co O₂（LCO）precursor to form core-shelled NCM@LCO precursor,which is then calcined to form NCM@LCO material with Ni-poor surface.Subsequently,a small amount of LNO is further coated on the surface of NCM@LCO particles to form NCM@LCO@LNO material.As expected,NCM@LCO@LNO cathode displays the outstanding electrochemical performance,especially cycle stability(capacity retention of 80%after 585 cycles at 0.612 m A cm^-2),indicating that this strategy is very effective to construct high performance Ni-rich LiNi_xCo_yMn_1-x-yO₂ cathodes for sulfide-based ASSLBs.To our knowledge,this excellent result is at the leading level in the reported electrochemical performance of sulfide-based ASSLBs.Moreover,the performance improvement mechanism is also investigated by Raman spectra in combination with cyclic voltammetry.This work reveals that this strategy can effectively suppress the irreversible phase transformation and interfacial side reaction for Ni-rich layered oxide.（4）Based on the above research foundation,core-shelled Ni-rich Li(Ni_0.9Mn_0.05Co_0.05)_0.8Co_0.2O₂ is constructed and further modified with Al source.In this strategy,the core-shelled structure design of Ni-rich oxide material can decrease the surface nickel content and reduce the volume change,and the surface modification process of core-shelled Ni-rich oxide material with cheap aluminum isopropoxide can generate Al-modifying layer（Al₂O₃ and Li Al O₂ layer）,which will not only remove the lithium compounds but also form the coating layer.Under high mass loading of36.94 mg cm^-2,Al-modified Li(Ni_0.9Mn_0.05Co_0.05)_0.8Co_0.2O₂（CS-NCM@Al）cathode displays the discharge capacity of 158.6 m Ah g^-1 at 0.2 C with high initial coulombic efficiency of 88.3%.More significantly,it shows a superhigh capacity retention of96.3%after 500 cycles at 1 C(7.39 m A cm^-2).This work provides an effective method to design high energy density sulfide-based ASSLBs.