Modification Of Silicon Suboxide-based Anode Materials For Lithium Ion Batteries

Silicon oxide（SiO_x）exhibits high theoretical specific capacity(2680m Ah g^-1)and low lithium-intercalation potential（～0.5 V vs.Li/Li⁺）as an anode material for lithium-ion batteries,showing its promising potential as a new anode material for high-performance lithium-ion batteries.However,its low electrical conductivity and large volume expansion（～160%）during charge/discharge lead to poor cycling and rate performance,which severely limits its commercial application.In this dissertation,aiming to obtain high-performance SiO_x-based electrode materials,strategies such as carbon coating,establishing porous structure,combing with highly conductive materials and optimizing the electrode structure,are used to improve the electrochemical lithium storage performance of SiO_xelectrodes.The main research results are as follows:（1）Through a simple rotary evaporation method,micron-scale SiO_xparticles are coated by sulfonated graphene（SGe）layers to prepare SiO_x@sulfonated graphene composites.The sulfonic acid groups on the surface of the graphene sheet are electronegative and have a certain electron-withdrawing ability,which can promote the rapid transport of lithium ions on the surface of SiO_x;the graphene sheets have high electrical conductivity and can form a highly conductive network with fast electron transport channels around SiO_x,significantly improving the conductivity of the material;the SGe uniformly coated on the SiO_xcan also effectively buffer the volumetric change during the alloying process of SiO_x.Based on the above advantages,the as-prepared SiO_x@SGe exhibits a reversible capacity of 971.9 m Ah g^-1at 0.1 A g^-1in the initial cycle as anode material for lithium ion batteries,and the capacity retention after 100 cycles reaches 96.6%.When the current density increases to 3.2 A g^-1,a capacity of 619.9 m Ah g^-1is retained;after 250 cycles at 0.5 A g^-1,the capacity remains 744.4 m Ah g^-1,indicating excellent rate performance and cycle stability.（2）SiO_xand reduced graphene oxide（rGO）are combined by a liquid phase dispersion-vacuum filtration-self-propagating reduction method,forming an integrated free-standing porous SiO_x@rGO film electrode.In the film electrode,the SiO_xblocks are embedded in the highly conductive network formed by the rGO sheets.During the self-propagation process,a large amount of gas is instantly released,endowing the electrode with a fluffy and porous structure.It is conducive to the rapid ion transport and electrolyte penetration,in favor of the rate performance of the electrode.The loose structure also provides space for the volume expansion of SiO_x,which is beneficial for the improvement of cycle stability.In addition,the self-propagating reduction method is convenient and fast,which can significantly limit the disproportionation of SiO_xand reduce the SiO₂content in the electrode,in favor of the specific capacity of the electrode.Based on the above advantages,the prepared SiO_x@rGO film electrode exhibits excellent lithium storage performance.It has a reversible specific capacity of 1098.4 m Ah g^-1in the first cycle at 0.1 A g^-1,which increases to 1189.7 m Ah g^-1after 5 cycles.The capacity retention is 81%after 100 cycles（comparing with the capacity in the first cycle）.At a current density of 3.2 A g^-1,it still exhibits a capacity of349.2 m Ah g^-1.（3）The SiO_xmaterial and Ti₃C₂T_xMXene are mixed in liquid phase and then freeze-dried to prepare the SiO_x@MXene composite material.In the composite,MXene nanosheets can buffer the volume expansion of SiO_xduring the alloying process,and improve the structural stability of the composite.MXene nanosheets with metallic conductivity form a conductive network and promote the rapid charge transfer,improving the rate performance of the composite.Moreover,the freeze-drying process makes the material form a fluffy structure,which is conducive to the full exposure of active sites,facilitates the electrolyte penetration and ion transport,and buffers the volume expansion of SiO_x.Therefore,the SiO_x@MXene composite（8:2mass ratio of SiO_xto MXene）has a reversible specific capacity of 1033.5m Ah g^-1in the first cycle at a current density of 0.5 A g^-1,and the capacity retention is 84.2%after 100 cycles.When the current density increases to 3.2A g^-1,it still remains a capacity of 624.1 m Ah g^-1.These results show the electrode has high specific capacity,good cycle stability and rate performance.（4）Using low-density polyethylene（LDPE）as the carbon source,SiO_x@C composite is prepared by uniformly coating SiO_xparticles with a carbon layer.The carbon coating improves the conductivity of SiO_x,and a strong Si-C bond is formed between the carbon layer and SiO_x,which is beneficial to suppress the volume expansion of SiO_x.Furthermore,the electrodes were prepared with the prepared SiO_x@C as the active material and Ti₃C₂T_xMXene as the conductive binder.The SiO_x@C particles in the electrode are uniformly embedded between the MXene layers,and the MXene sheets can further buffer the volume expansion of SiO_x@C,thus improving the cycling stability of the electrode.The MXene sheets also construct a highly conductive network,which can facilitate the rapid electron transport and significantly improve the rate capability of the electrode.The MXene-SiO_x@C electrode has a porous structure,which is conducive to the full exposure of active sites,promotes the rapid ion diffusion,buffers the volume expansion of SiO_x@C,so the specific capacity and cycle performance of the electrode are improved.As a result,the MXene-SiO_x@C electrode constructed with MXene as conductive binder exhibits excellent electrochemical performance.It shows a reversible specific capacity of 1363.3 m Ah g^-1in the first cycle at a current density of 0.1 A g^-1,and remains 1336.2 m Ah g^-1after 50 cycles,indicating a capacity retention as high as 98%.When the current density increases to 6.4 A g^-1,it still shows a high specific capacity of 604.5 m Ah g^-1.Additionally,a capacity of 961 m Ah g^-1is remained after 300 cycles at 0.8 A g^-1.