Lithium-Ion Storage Properties Of Metal Phosphides Nanomaterials As Anodes

Lithium-ion batteries have been widely used in portable electronic equipment and electric vehicles.However,with the continuously increasing demand for electronic products,the current commercially available graphite anodes,with a limited theoretical specific capacity of 372 mAh g-1,can not meet users’ requirements.Therefore,it is urgent to develop high-performance anode materials for lithium-ion batteries.Metal phosphides with high theoretical specific capacities exhibit low polarization and low voltage range.Therefore,metal phosphide is one of the ideal choices as anode materials for lithium-ion batteries.However,the electronic conductivity of metal phosphides is poor.Besides,the metal phosphides will experience inevitable volume expansion during the lithiation process,which may lead to aggregation,pulverization,and final loss of electrical contact between the active materials and electric collector.To solve these problems,two types of strategies are developed in this research,namely "the nanometerization of active materials" and "the construction of carbon composites".Effects of some key parameters(such as nanomaterials,carbon composite structure,and carbon content)on the lithium-ion storage properties are investigated.In addition,the lithiation mechanisms of metal phosphides have not been well understood by the existing literature,and the available studies are only conducted via experimental investigation.There is a lack of theoretical analysis to further understand the lithiation mechanisms of metal phosphides.Therefore,the physical comprehension of the lithiation process remains to be clarified by more in-depth investigations including experimental and theoretical analysis.Therefore,another main aim of this research is to clarify the lithiation mechanism of metal phosphides based on experimental investigations and theoretical calculations.The main research work includes the following three aspects:(1)Progressive lithium-ion storage properties of two-dimensional Sn4P3 nanosheets.Based on the strategy of "nanometerization of active materials",Sn4P3 nanosheets are prepared by direct current(DC)arc plasma and solid-phase phosphating reaction.Effect of nanomaterials on the lithium-ion storage properties of anodes is investigated.As the anodes for lithium-ion batteries,Sn4P3 electrodes provide more reaction sites for Li+storage,leading to a higher specific capacity.Moreover,Sn4P3 nanosheets are beneficial for the diffusion of Li+between adjacent layers,thus alleviating the volume expansion.Therefore,the Sn4P3 anodes exhibit excellent cycle stability and rate performance:At 0.1 A g-1,the discharge specific capacity of Sn4P3 anodes is 603 and 567 mAh g-1 after 50 and 500 cycles,respectively,and the capacity retention is 94%.In addition,the experimental research and the theoretical calculations reveal that Sn4P3 experiences a progressive lithiation process,i.e.,Li+is firstly inserted into Sn4P3 to form the main intermediate products of amorphous LixSn4P3(a-LixSn4P3),followed by a conversion to the amorphous Li3P(a-Li3P)and the crystalline Li4.4Sn(c-Li4.4Sn).The formation of a-Li3P is earlier than c-Li4.4Sn.(2)Long-term lithium-ion storage properties of FeP2 nanoparticles constrained inside the carbon shell.Based on the good solid solution of Fe and C,the pure FeP2,core-shell FeP2@C(5.74%),and FeP2@C(21.16%)nanocomposites are prepared by direct current(DC)arc plasma and solid-phase phosphating reaction.Effects of carbon composite nanoparticles and carbon content on the lithium-ion storage properties of the anodes are investigated.As the anodes for lithium-ion batteries,the pure FeP2 electrodes exhibit the worst cycle stability and rate performance among three kinds of samples,which is ascribed to the poor conductivity,serious aggregation,and pulverization of FeP2 nanoparticles.For FeP2@C(5.74%)electrodes,the cycle stability and rate performance are better than that of the pure FeP2.However,the volume expansion of FeP2 still leads to the destruction of core-shell structure owing to the lower carbon content(5.74%),thus frustrating the binding role after cycling.Regarding the case with a higher carbon content(21.16 wt.%),carbon materials have a stronger constraint on FeP2.During the long-term cycle process,the volume expansion could be effectively relieved by the mechanical confinement of carbon materials.Moreover,carbon materials could enhance the conductivity of active materials.As a result,the structural integrity and electrical contact of electrodes are maintained.Therefore,compared to FeP2 and FeP2@C(5.74 wt.%),the FeP2@C(21.16 wt.%)electrodes exhibit the best cycle stability and rate performance:At 0.3 A g-1,the discharge specific capacity of FeP2,FeP2@C(5.74 wt.%),and FeP2@C(21.16 wt.%)electrodes is 78,234,and 1827 mAh g-1 after 500 cycles,respectively.In addition,the cyclic voltammetry analysis and the first-principles calculations reveal that the lithium ions are gradually inserted into the lattice of FeP2 to form LinFeP2 and then transformed to Li3P and Fe during the discharge process:FeP2+nLi++ne-→LinFeP2(0<n<6);FeP2+6Li++6e-→Fe+2Li3P(n=6).(3)Inverse capacity growth and progressive lithium-ion storage properties of SnP-semifilled carbon nanotubes anodes.Based on the catalytic effect of Sn on growth of CNTs and the difference of thermal expansion coefficient between them,the composites of SnP-semifilled carbon nanotubes(SnP@CNTs)are prepared by direct current(DC)arc plasma and solid-phase phosphating reaction.Effect of semifilled carbon composite nanoparticles on the lithium-ion storage properties of the anodes is investigated.The unfilled space in CNTs provides sufficient buffer space for the volume expansion of SnP due to the semifilled structure of SnP@CNTs.Moreover,CNTs have good electronic conductivity,which is conducive to the rapid conduction of electrons between the active material and the current collector,thus maintaining the excellent structural integrity and electrical contact of the electrodes.Under the effective constraints of CNTs,as SnP is gradually pulverized into smaller particles,the reaction sites increase and the Li+diffusion distance is shortened.Therefore,more Li+can be stored on the newly created surface,resulting in an inverse increase of the specific capacity:At 0.5 A g-1,the discharge specific capacity of SnP@CNTs electrodes is 699 and 1232 mAh g-1 after 150 and 790 cycles,respectively.In addition,the experimental results and the theoretical calculations reveal that the lithiation mechanism of SnP is similar to that of Sn4P3.During the discharge process,SnP experiences a progressive lithiation process:SnP+xLi+→a-LixSnP(0<x<7.4);SnP+7.4Li+→c-Li4.4Sn+(a-Li3P)(x=7.4).The formation of a-Li3P is earlier than c-Li4.4Sn.