Phosphorus Based Functional Materials And Their Applications In Energy Conversion And Storage

Lithium-ion battery(LIB)is an indispensable energy storage technology that deeply rooted in numerous areas spanning space science to casual entertainment.With the demand of carbon neutrality and sustainable development drastically expanding,LIBs with superior performance have become a constant target of the corresponding research,which,for example,could serve as a power source for vehicles and reduce the consumption of fossil fuels,benefiting the environment.Current commercial carbon-based anode materials,however,cannot provide enough capacity.Therefore,replacement materials are in dire need.Silicon and phosphorus are classical typical alloy-type anode materials,which have a high theoretical specific capacity owe to the ability of storing considerable lithium ions.Unfortunately,it is prone to expand in volume during charge-discharge process,leading to a sever loss in performance.Silicon phosphate layered compounds(SiP)combines the large specific capacities of elemental Si and P,while its decent layer distance is beneficial to the diffusion of ions.And,the different lithium intercalation potentials of silicon and phosphorus can also partially alleviate the volume expansion of material.Synergistically,SiP is a promising candidate for anode materials.In the current work,chemical vapor transport is employed to regulate the morphology,crystallinity,and size of the silicon monophosphide,establishing the relationship between the material structure and the performance of corresponding LIBs.The main contents are as follows:The layered SiP is synthesized using stoichiometrically mixed silicon and red phosphorus,with its morphology tailored by calcination temperature,amounts of transport agent(i.e.,iodine)and additive(i.e.,sulfur).From XRD,Raman,XPS,SEM,TEM and AFM analysis it appears that SiP adopts a belt-like morphology with orthorhombic crystal lattice.The addition of I₂ significantly reduces the reaction temperature for synthesis and promotes crystallization.The crystallinity and size of SiP is further improved with higher synthesis temperature and the addition of S.Electrochemical properties of SiP vary substantially with altered size and crystallinity,showing significant differences in lithium storage performance.The SiP synthesized at high temperature with the presence of I₂ possesses an average thickness of 72 nm,which achieves a specific capacity of 2644 m Ah g^-1(at current density of 100m A g^-1)and an initial coulomb efficiency of 61%.The final reversible specific capacity maintains at 615 m Ah g^-1 after 200 cycles.Its capacity maintains at 320 m Ah g^-1 at high current density of 5000 m A g^-1,and the capacity recovers to 780 m Ah g^-1when the current density is back to 200 m A g^-1 again,suggesting the advantageous rate performance.In the contrary,the smaller and thinner SiP synthesized at low temperature demonstrates an average thickness of only 20.5 nm.High transportation impedance and low diffusivity of Li ions of this material are conducted by kinetic tests(e.g.,GITT and EIS),indicating a poor rate performance.Bulk SiP synthesized with the addition of S shows an average thickness of 8.4?m,which suffers radically from the tension of lattice expansion during Li insertion/removal,resulting in a destructive performance of LIBs.In this work,we systematically analyzed the failure mechanism of SiP in LIBs.Increase of EIS impedance after cycling implies the deteriorating of electrodes.Through surface and cross-section analysis,the expansion and pulverizing of the anode materials during the de-intercalation with Li ions are directly captured by SEM,which seriously affects the electrode performance.XPS depth analysis is adopted to observe the formation of SEI film(thickness of 50-100 nm)and a large number of Li P,generated by incomplete lithium intercalation.Those changes negatively affect the insertion/removal of Li ions,resulting in a low reversible capacity.