Design Of Functional Interface On Porous Silicon Anode For Enhanced Electrochemical Performance

Silicon（Si）is considered as a promising anode material for high energy density lithium-ion batteries（LIBs）due to its higher theoretical specific capacity(4200 m Ah g^-1).However,compared to conventional graphite anodes,the large volume expansion（>300%）,unstable solid electrolyte interphase（SEI）and poor Li ion transport capability during cycling hinder the commercial application for Si anodes.Therefore,the design of electrode/electrolyte interface with ion/electron conduction capability and mechanical properties is crucial to improve the electrochemical performance of Si anodes.In this thesis,three artificial SEI interface layers are designed on the surface of ant-like porous Si through reasonable interface design and simple and effective preparation methods,which can be in situ converted into mixed ion/electron conducting layer and highly mechanically stable inert phase during the electrochemical process to obtain high strength and fast ion/electron transfer at the Si/solid electrolyte interface,significantly improving the Li storage performance of ant-like porous Si.The main research work and innovations are as follows:（1）To address the problem of insufficient ion/electron transport capability at the interface of Si and SEI,a modified porous silicon anode material（p-Si@Mg F2）with a Li F/Li–Mg alloy hybrid ion/electron conduction layer（MIECI）derived in situ from surface-coated Mg F2 is designed.Among MIECI,Li F,the lithium-ion conductor(～10^-9S cm^-1),is able to increase the ion transport capacity at the electrode/electrolyte interface,while the presence of Li–Mg alloy compensates for the lack of electron conductivity and toughness of Li F,enhancing the electron conductivity of the interface,which can withstand the stress changes during lithiation/delithiation and maintain the overall stability of the electrode.The p-Si@Mg F2 anode has a capacity retention of 90%(over 1403 m Ah g^-1)after 200 cycles at 1.0 A g^-1 and a capacity of 942 m Ah g^-1 even at 5.0 A g^-1.（2）To further enhance the Li ion conductivity of the SEI,Li₃N(～10^-4 S cm^-1)is in situ introduced on the surface of porous Si,a hybrid artificial interface layer with higher Li ion conductivity of Li3N and high mechanical strength ceramic phase Si3N4 is designed.A multifunctional Si N nano coating layer consisting of inactive Si3N4 and active Si Nx of 3-5 nm is constructed on the surface of the porous Si（p-Si@Si N）by direct nitriding of fresh porous Si at a low temperature of 860°C.The active Si Nx is able to generate the fast ion conduction Li3N and high mechanical strength’s inactive ceramic phase Si3N4 during lithiation,whcih not only increases the ionic conductivity of the SEI,but also improves its mechanical strength and stability.Then,the p-Si@Si N anode has a capacity of 2180 m Ah g^-1 at 0.5 A g^-1,and capacity retention of 84%after 200 cycles at 0.5 A g^-1,even delivers a capacity of 1125 m Ah g^-1 at 5.0 A g^-1 and the capacity of721 m Ah g^-1 after 500 cycles.（3）Combining the advantage of the above two design strategies,the high Li ion transport medium of Li3N and the high electron transport medium of Li–Mg alloy are combined to form a multifunctional interface layer.The Mg Si N2-coated porous Si composite（p-Si@Mg Si N2）is obtained in N2 atmosphere at 880°C using Mg3N2 phase formed during the synthesis of porous Si.Mg Si N2 can form a hybrid ion/electron interfacial layer with fast Li ion conduction Li3N and fast electron conduction Li–Mg alloy during lithiation and construct a uniform ion/electron flow channel between the electrode/electrolyte,enhancing the electrochemical performance.The p-Si@Mg Si N2anode has a high ICE of 88.4%and reversible capacity of 1981 m Ah g^-1 after 150 cycles at 1.0 A g^-1.The p-Si@Mg Si N₂ anode shows an high capacity of 883 m Ah g^-1 at 5.0 A g^-1 after 500 cycles,which has been further improved at 5.0 A g^-1 compared to the previous two chapters.