Synthesis And Electrochemical Energy Storage Properties Of Two-dimensional Layered Silicates

The storage plays an equally important role as the development in electric energy,a type of green new energy while lithium ion batteries（LIBs）are one of the important energy storage devices.According to the evaluation indicators E=Vd Q and P=E/t（E:energy density;V:voltage;Q:specific capacity;P:power density;t:charging and discharging time）,increasing specific capacity Q and shortening charging and discharging time t are effective means to improve energy/power density.The cathode and anode materials greatly determine these two key indicators of LIBs.Among many new candidate anode materials,metal oxides and silicon-based materials have been widely studied due to their outstanding lithium storage capacity,low cost,and environmental friendliness.However,as three-dimensional materials,the phase transition and significant volume change caused by a large number of lithium ions entering the bulk phase can lead to electrode gradual pulverization,low coulombic efficiency,and even failure during cycling.Therefore,the development of new high-performance LIBs anode materials requires a balance between high capacity and low volume strain.When two-dimensional materials store lithium,lithium ions can be stored between their layers,so only the cell parameter c increases slightly and the volume expansion is small.Therefore,the development of two-dimensional silicon-based metal oxides has potential research value.In terms of high-capacity cathode materials,polyanionic spinel Li₂Fe Si O₄materials have ultra-high theoretical specific capacity(330 m Ah g^–1)due to double electron transfer.However,in practical research,due to the rearrangement of Fe³⁺and Li⁺,poor conductivity,slow diffusion of Li⁺,and difficulty in generating Fe⁴⁺,their actual capacity is often only half of the theoretical value.Currently,existing modification methods such as coating,doping,and structural design can only partially alleviate the above difficulties.The use of large interlayer spacing two-dimensional silicates may fundamentally avoid drawbacks such as rearrangement and slow diffusion.As a natural cationic clay,two-dimensional layered silicate has large interlayer spacing and variable valence metal centers,which are conducive to the storage of a large number of ions.However,there are few reports on the direct application of layered silicates as electrode materials.This paper synthesized various layered silicates and applied them as electrode materials in LIBs.Their lithium storage performance and energy storage mechanism were studied,providing reference for the development of efficient lithium storage materials and new layered silicate electrode materials,and expanding the application fields of layered silicates.The main research content and results of this paper are as follows:1.Layered silicates as new type of LIBs anode materialsSodium ion intercalated nickel-iron saponite（NF-SAP,a trioctahedral layered silicate）and magnesium-aluminum saponite（MA-SAP）were synthesized using hydrothermal method,and their performance as anode for LIBs was studied.During the charging and discharging process,Si changes between Si²⁺and Si⁴⁺,while Fe and Ni change between near metallic states（M⁰）and Fe^x+and Ni^y+（0<x<3,0<y<2）,while hydroxyl groups also provide capacity（Li OH and Li H convert each other）.The maximum capacity of NF-SAP is 815 m Ah g^-1and maintains a capacity of 646 m Ah g^-1(500 m A g^-1)after 1000 cycles;MA-SAP exhibits a maximum capacity of up to 536m Ah g^-1and a capacity retention rate of 88%（400 cycles）.The high capacity indicates that the silicon,transition metals,and hydroxyl redox pairs in layered silicates have high electrochemical activity.In addition,the assembled Li Mn₂O₄/NF-SAP full cell exhibits an initial specific capacity of 104 m Ah g^-1(100 m A g^-1)and a retention rate of 68%（50 cycles）,indicating that NF-SAP layered silicates have certain application prospects as anode materials.2.Construct layered silicate composite materials to improve lithium ion storage performanceBased on iron beidellite（Fe-BEI,a kind of dioctahedral layered silicate）,a general method was developed to easily prepare layered silicate/carbon superlattice（silicate layer and graphene layer alternately stacked）materials through glucose intercalation-carbonization.Prepared Fe-BEI@C superlattice material has good rate performance.When be tested at a high current density of 5 A g^-1,a high specific capacity of 403 m Ah g^-1can be obtained,which is significantly improved（338%）compared with that of NF-SAP,proving the effectiveness of improving conductivity by superlattice strategy.In addition,in order to further improve the performance of layered silicates,this work constructed a layered silicates heterostructure（PPy@C/SAP）based on NF-SAP,and improved its capacity and stability by introducing additional reversible reactions in solid state electrolyte interface（SEI）,and applied it for lithium,sodium,and potassium storage.In the PPy@C/SAP anode materials,the synergistic effect of metal Ni,Fe doping,carbon superlattice,polypyrrole（PPy）coating and oxygen vacancy leads to good electronic conductivity.The lithium,sodium,and potassium half cells of PPy@C/SAP have the following performance:Li(659/550 m Ah g^-1;2.0/5.0 A g^-1;1000 cycles),Na(327 m Ah g^-1;100 m A g^-1;50 cycles),and K(236 m Ah g^-1;100 m A g^-1;100 cycles).In addition to the valence change of transition metals and silicon,Ni⁰catalyzed conversion reactions between Li OH/Li H and Li₂CO₃/Li₂C₂also occur in the SEI.This mixing mechanism allows the material to have a volume expansion of as low as 9%,and the layered structure is maintained during electrochemical cycling.Assembled Li Ni_0.85Co_0.10Al_0.05O₂|PPy@C/SAP full battery retains a capacity of 104 m Ah g^-1(1 A g^-1)after 200 cycles,indicating that this anode material has good application value.3.New layered silicate LIBs cathode materials with extremely high diffusion coefficientsIn this work,sodium ion pillared ferro-saponite（Na⁺-FSAP）with mixed valence states was synthesized in the presence of reducing agents utilizing hydrothermal method,and used as a layered Li₂Fe Si O₄cathode material.During the charging and discharging process,Na⁺-FSAP follows the（de-）intercalation mechanism of lithium ions.With the migration of lithium ions,the average valence state of Fe changes between Fe^1.86+and Fe^2.71+,while Si remains unchanged,indicating that a stable skeleton structure.In terms of performance,Na⁺-FSAP showed an initial specific capacity of 125 m Ah g^-1at a current density of 50 m A g^-1,with a retention rate of 80.8%after 75 cycles.The large interlayer spacing and unique"sandwich"structure of the laminates prevent the rearrangement of lithium and iron.The Na⁺pillaring effect and abundant structural water in interlayer channels promote the rapid migration of Li⁺,resulting in a very high diffusion coefficient of Li⁺,up to 10^-6.5～10^-7.5cm²s^-1,which is 6-10 orders of magnitude larger than that of traditional polyanionic silicate materials.It belongs to the high level in silicate cathode materials,and a high diffusion coefficient is conducive to rapid charging and discharging,thereby improving power density.