Research On Layered Phosphides As Anode For Lithium And Sodium Ion Batteries

As the acceleration of global development pace and ever-increasing global population,the demands for renewable clean energy in human society have been greatly increasing.According to international energy agency(IEA)key world energy statistics,we must double our present rate of energy production by 2050 to cope with the energy demands.Energy storage technologies can help alleviate energy crisis in large degree,and rechargeable batteries have played a critical role in the renewable clean energy grid-level storage and application.In order to further broaden their large-scale commercial applications,new kinds of electrodes with high capacity are generally involved,the reaction mechanism has been deeply investigated as well,and the battery parameters have been optimized,at the same time,by the researchers.Layered phosphide is one member of new emerging two-dimensional semiconductors,which has grabbed great attentions in electronic and optoelectronic applications.This dissertation focuses on deeply understanding the reaction mechanism and improving the electrochemical performance of novel layered phosphides,correspondingly modifying their morphologies and optimizing the battery testing parameters.The dynamic reaction mechanism and fundamental science of binary layered phosphides,which show highly chemical reactivity with both lithium and sodium ions,have been explored by combination of advanced in-situ transmission electron microscopy(TEM),ex-situ microstructure characterization and performance testing techniques.Based on the investigation of the reaction and failure mechanism for binary layered phosphides,strategies for improving their electrochemical performances have been proposed,including size and thickness reduction and potential window control.Similar strategies are implemented into ternary phosphide electrode,which delivers an ideal reversible capacity,excellent rate capability and cycling stability in both half and full cell,due to its more stable layered structure and better mechanical properties.The study details and achievements are listed below:(1)The electrochemical reaction mechanism investigated by in-situ transmission electron microscopy.Binary phosphides(Si/GeP)show typical black phosphorous-like layered structure.The individual nanoflake has a single crystal structure.Each metal atom was surrounded by three phosphorus atoms,and the layers stacked with Van der Walls force.The shortest interlayer spacing is even larger than lithium and sodium ions'diameter.Therefore,it provides fast diffusion channels for alkali ions with high carrier mobility and alleviates the volume expansion in discharge process,accompanying by effective improvement of cycling stability.Operando TEM technique contributes to reveal the detailed underlying reaction and failure mechanisms of an individual nanoflake anode by tracing dynamic morphological and structural evolutions during lithiation or sodiation.It was found that binary phosphide SiP only displays electrochemical reactivity to lithium ions with?233%area expansion after complete lithiation,but no reactivity with sodium ions.Interestingly,GeP presents high electrochemical reactivity with both lithium and sodium ions,and the corresponding area expansion reaches to 160%(full lithiaiton)and 248%(full sodiation),respectively.It indicates that more lithium ions involved in the lithiation of GeP than that of SiP,in addition,and GeP flakes collapse much severer in sodiation than in lithiation,accompanying with worse sodiation cycling stability.Notably,phase transition of GeP in sodiation is markedly different from lithiation.It undergoes multi-step reactions in its first sodiation by sequentially forming Na_xGeP(0<x<1/3),intermediate phase layered Na Ge₃P₃,and amophous alloys of Na Geand Na_yP(0<y?3).It fails to form stable crystal intermediate phase in lithiation due to the rapid diffusion and the instantaneous embedding,generates the final phases of amorphous Li_4.4Geand Li₃P in lithiation,and all the amorphous phases are not able return to reversibly crystal GeP phase after delithiation.However,SiP nanoflakes show reversible phase transformation in LIBs because the larger layer spacing provides more space for extraction of ions in delithiation,so after full delithiation,all the amorphous phases reversibly return to polycrystal SiP phases.(2)Size and morphology design of layered binary phosphides(Si/GeP)for better electrochemical performances.Layered binary phosphides(Si/GeP)have high theoretical specifical capacity as anodes in the rechargeable batteries.However,poor stability and capacity fading for bulk binary phosphides(Si/GeP)seriously hinders their practical application in commercial batteries.The ultrasonic exfoliation can effectively break van der Waals force between layers of the layered binary phosphides(Si/GeP)to form nanoflakes with thickness of decades of nanometer,even nanometer.Meanwhile,reducing size and thickness facilitate to increase the specific surface and shorten the diffusion length of charge carriers for alkali metal ions.The exfoliated SiP delivers an initial specific capacity of 1650/1326 m A h g^-1.In addition,exfoliated GeP also display the initial specific capacity of 1701/1525 m A h g^-1 in LIBs and 1237/852 m A h g^-1 in SIBs,respectively.Few-layered SiP and GeP exhibit better electrical conductivity,faster charge transfer kinetics and,meanwhile,lower resistance than those of bulk counterpart.The charge transfer resistances(Rct)of exfoliation SiP is reduced nearly by half from 171.5?to 97?in LIBs,compared to the initial bulk one.And the value of GeP nanoflakes is also lower than bulk one,due to optimizing of physical properties,such as bandgap,and electrical conductivity,etc.,induced by the thickness reduction.Additionally,small size help to internal stress'releasing during the insertion and extraction of alkaline metal ions,which is benefit for the structural stability.Accordingly,the exfoliated SiP delivers a much higher coulombic efficiency(ICE)of87%and capacity retention of 80%in LIBs.While 89%coulomb efficiency and capacity retention of 90%can be obtained for exfoliated GeP anodes in LIBs.(3)The influence of potential windows on the cycling performance of the binary phosphide electrode.According to the in-situ TEM study of the reaction mechanism of the binary phosphide and the analysis of the cyclic voltammetry curve,it is concluded that the different phase transition stages in the reaction process lead to different changes in the morphology and structure of the material.Therefore,we optimize the electrochemical testing parameters through narrowing the working voltage to control the reaction stage according to the phase transitions potentials,and further improve the electrochemical performance of the layered binary phosphide.SiP,as anode in LIBs,delivers a considerable capacity retention of 53%and reversible capacity of 460 m A h g-1 after 100 cycles within a potential range of 0.1-0.85 V by preventing the proceeding of alloying of Li_xSi(x?3.75).GeP anode shows a reversible capacity of460 m A h g^-1 and the capacity retention of?50%after 350 cycles in LIBs when the voltage window narrows to 0.001-0.85 V,which efficiently limits the dealloying reaction of Li₃P,facilitating to reduce the volume change of the electrode in cycles.Moreover,GeP anode also exhibits excellent rate performance and good cycle stability with the capacity retention of?65%and the capacity of 330 m A h g^-1 after 100 cycles in SIBs when applying a relative smaller potential window of 0.15-1.5 V,which cuts down the insertion/extraction of sodium ions by limiting the alloying of Na₃P and dealloying of Na Ge,(4)The investigation of the electrochemical properties of ternary phosphide.According to the analysis study of binary layered phosphides,we get that large layer spacing and good stability of the layered structure is highly beneficial to insertion/extraction of alkali metal ions.Therefore,we explored a novel ternary has a large layer spacing and contains no reactive element Mn,facilitating to buffer volume change during the insertion/extraction of ions.In-situ TEM observation reveals that the MnPS₃ nanoflake remain highly integrity without any cracks or pulverization,even though a total area expansion of?256%after the first lithiation.MnPS₃ exhibits excellent electrochemical performances superior to the layered materials with similar structures in both half-cell and full-cell testing.When supplied as the anode of LIBs in half-cell,a high reversible capacity of 490 m A h g^-1 is maintained for the MnPS₃ within the potential window of 0.005-3 V vs.Li⁺/Li after 400 cycles at a rate of 0.1 A g^-1.While further improving the current density to 4 A g^-1,a specific capacity of 380 m A h g^-1 can be still held after 3000 cycles.Moreover,the MnPS₃ anode,shows a reversible capacity of 450 m A h g^-1 after 200 cycles in full-cell testing.The superior lithium ion diffusion kinetic,stable structure and high pseudocapacitive contribute to the high capacity,rapid charge-discharge,and long cycle of the layered MnPS₃ anode.In this dissertation,we explored the high-capacity layered phosphide anodes for rechargeable batteries,and figured out the key affecting factors on the electrochemical performance through detailed investigations of their reaction mechanisms investigation.The cyclic stability of the layered phosphide with high capability has been greatly improved by the combination of designing the morphology and structure,and optimizing the working parameters in batteries This paper plays an important guiding role in designing high-capacity layered materials,and promoting the application in rechargeable batteries.