Study On The Compatibility Between Sodium Ion Solid-state Electrolyte And Electrode Materials

The key materials of solid-state sodium ion battery mainly includes anodes,solidstate electrolytes(SSEs),and cathodes.However,there is an interface incompatibility between solid-state electrolyte and electrode material,which limits the development of solid-state sodium ion battery.The solid-state sodium ion battery can be operating only when the solid-state electrolyte and electrode material can be compatible with each other.For the anodes,when sodium metal is directly used as the anode,there will be serious interface reaction between sodium metal and electrolyte,which is mainly because the chemical properties of sodium metal are relatively active and easy to react with electrolyte.The volume expansion of the conversion-type anodes will occur during the process of sodiation/desodiation,which will lead to irreversible attenuation of the battery capacity.However,the intercalated anodes with layered structure have little volume change during the charge-discharge cycle,which are the most potential anodes for realizing the commercialization of solid-state sodium ion batteries.For solid electrolyte,it is required to have ionic conductivity comparable to that of liquid electrolyte and to have good compatibility with electrode material.One of the important prerequisites for the operation of solid-state battery is the high ionic conductivity of solid-state electrolyte,among which the sulphide solid-state electrolyte is the ideal research objects.The cathodes need to satisfy voltage compatibility with both anode and sulphide solid-state electrolyte(with a narrow voltage window),so the cathode with the appropriate voltage range should be selected.Based on the purpose of preparing security sodium ion batteries,the compatibility between electrolyte and cathode,electrolyte and anode were investigated respectively in this paper.The sulfide Na11Sn2PS12(NSPS)with high ionic conductivity was firstly selected as solid-state electrolyte.The optimal synthesis condition was obtained by exploring different sintering temperatures,and the compatibility of NSPS and Na was also explored.Finally,the compatibility of Na and NSPS was improved by loading Na on copper-mesh(Na-Cu).Furthermore,the NASICON-type cathode of Na3MnTi(PO4)3@C(NMTP@C)was prepared,and the compatibility between NSPS and NMTP@C was also investigated,it found that they are compatible thermodynamically.The solid-state battery of Na-Cu‖NSPS‖NMTP@C possess excellent electrochemical performance.In order to solve the compatibility of the anode and NSPS,the two-dimensional layered material of MoS2@RGO was selected to replace Na-Cu as the anode in the solid-state battery.The application of MoS2@RGO in the liquid sodium ion battery was explored firstly,and then the solid-state battery was assembled with the electrolyte of NSPS.It found that MoS2 was compatible with NSPS electrochemically.Moreover,based on the defects of low capacity of MoS2 with maintaining the structural integrity,and the interface compatibility between metallic sodium and electrolyte,this paper designed layered MXene phase anode through the theoretical calculation.It found that the layered MXene phase anode can store large amounts of sodium and maintain the stability of layered structure,it may be a new type of anode for solid-state batteries.The specific research contents are as follows:(1)The liquid electrolyte and separator were replaced by solid-state electrolyte in this paper.Sulfide solid electrolyte of NSPS was prepared by solid-phase sintering,and the optimum condition of ionic conductivity was obtained by changing sintering temperature.The ionic conductivity of NSPS obtained at room temperature is 1.19 mS cm-1 when the sintering temperature is 500℃ and keeping for 8 h.The result from experiment and calculation found that NSPS has a very narrow voltage window(2.28～2.65 V)and unstable extremely with metal sodium.In view of its instability to metal sodium,this paper adopted the method of loading sodium on copper-mesh to effectively reduce abundant of detrimental reactions.From the charge-discharge tests of symmetrical battery Na‖NSPS‖Na and Na-Cu‖NSPS‖Na-Cu under different current density found that short-circuit current of Na-Cu‖NSPS‖Na-Cu(more than 1 mA cm-2)has a lot of improvement than Na‖NSPS‖Na(0.2 mA cm-2).Therefore,the introduction of copper-mesh provides a new strategy for the assembly of solid-state sodium ion battery with metal Na as anode.(2)As the voltage window of solid-state electrolyte NSPS is relatively narrow,the matching cathode has not been found at present.In this paper,Na3MnTi(PO4)3@C(NMTP@C)cathode was prepared by sol-gel method and sintered at 650℃ for 4 h in argon.The theoretical calculation found that NMTP has not only the high voltage plateau of 3.98 V(provide high capacity),but also the low voltage plateau of 2.4 V(compatible with the oxidation voltage of NSPS).The GITT test indicated NMTP has the ability of super-fast ion transport.Finally,using NMTP as cathode,NSPS as solid electrolyte,Na-Cu as anode,assembly into solid-state battery NaCu‖NSPS‖NMTP@C.This solid-state battery shows excellent rate performance and cycle performance,the capacity is 128.7mAh g-1 with the rate of 0.05 C,and the capacity retention ratio is 82%after 160 cycles with the rate of 0.1 C.The results show that NSPS and NMTP@C are compatible thermodynamically and electrochemically,but there is a weak detrimental reaction between NSPS and Na-Cu.(3)In view of the poor compatibility between metal sodium and NSPS,this paper considered using layered material MoS2 to replace Na-Cu.Because of the complexity of the interface problem in solid-state batteries,the electrochemical properties of MoS2 in liquid sodium ion batteries were firstly investigated.GO was prepared by the improved Hummers’ method,then sulfur and molybdenum sources were added,and the MoS2@RGO composites were obtained by one-step hydrothermal method.MoS2@RGO electrode has the good rate performance(513.8 mAh g-1 at 0.1 A g-1 and 171.5 mAh g-1 at high current density of 20 A g-1).In addition,the reaction mechanism of sodiation/desodiation in MoS2 was studied by theoretical calculation.The long cycle stability of more than 1000 cycles was achieved in the experiment under the guidance of theory(the discharge cut-off voltage can be controlled above 0.4 V).We further explore the feasibility of MoS2@RGO in solid-state battery NaCu‖NSPS‖MoS2@RGO by controlling the 0.4-3 V voltage window,which can guarantee the stability of layered structure of MoS2,as well as buffer the decomposition of NSPS under low voltage.Finally,the assembled solid-state battery MoS2@RGO‖NSPS‖NMTP@C shows an excellent cycle performance(the capacity retention ratio is 83%after 100 cycles with the rate of 0.1 C),indicating there has a good compatibility between MoS2@RGO and NSPS.(4)In view of the compatibility of metal sodium and NSPS,and the structural stability of MoS2 below 0.4 V,a new anode was designed by inserting Na+into the standard layer of Ti4C2O4 MXene phase using the material genomics method based on density functional theory(DFT).It is fascinating that a series of MXene compounds of NaxTi4C2O4(0≤x≤12)maintain the integrity of layered structure of Ti4C2O4 after a fairly large extent of Na+ intercalation.The total capacity for Na12Ti4C2O4 exceeds 579 mAh g-1.The electrochemical potential vs.Na/Na+ is correlated to the Na content,decreasing from 2.96 V to 2.40 V for NaxTi4C2O4 when 1≤x≤2,finally down to 0.05 V for Na12Ti4C2O4 when x=12.The minimum operating voltage calculated in this system is 0.05 V(slightly higher than the voltage of Na/Na+),which avoided problem of structural stability caused by charge-discharge with low voltage.Besides,the Oterminated Ti4C2O4 layers are hydrophobic and oxygen-resistant,so that the intercalated Na would be protected by Ti4C2O4 in ambient condition.Therefore,it can be predicted that NaxTi4C2O4 may have a good compatibility with electrolyte NSPS,and it not only has the properties of sodium metal,but also has a stable structure as a layered material,which provides a reliable theoretical basis for the design of anode of security solid-state sodium ion battery.