Study On Surface/Interface Engineering And Lithium Storage Mechanism Of Silicon Anode For Li-Ion Batteries

As a key link in the transformation of energy structure,energy storage technology has become the focus of attention in recent years.Due to its unique advantages,lithium-ion batteries(LIBs)start to dominate in the field of energy storage,such as consumer electronics,electric vehicles,large-scale energy storage,etc.Higher requirements for the electrochemical performance and safety of the LIBs are put forward.Theoretical calculation shows that increasing the capacity of anode makes a great contribution to improving energy density of batteries if the capacity of anode is below 1200 mAh g-1.Silicon anode has been considered as one of the promising candidates in advanced LIBs due to its higher theoretical specific capacity(4200 mAh g-1)than that of graphite(372 mAh g-1).Moreover,low intercalated Li potential(～0.2 V),high content in the earth,environmentally friendly of Si makes it one of the promising candidates in advanced LIBs.However,tremendous volume expansion(～300%)of silicon during cycling results in particle pulverization and unstable solid electrolyte interphase(SEI),which causes rapid capacity fading.The electrode reaction near the electrode/electrolyte interface is the most critical step in the whole process of the electrode reaction.The structure and properties of the electrode/electrolyte interface(including the impedance characteristics of the electric field distribution,mechanical properties,Li+ diffusion ability,the adjustment ability of charge transfer process and the ability to prevent side effects)directly determine the lithiation/delithiation process and evolution of the Si/electrolyte interface,which determines the performance of the battery(such as volume density,temperature characteristic,self-discharge rate,cycle life,ratio characteristics,etc.).Consequently,four key scientific problems(inhibiting the electronic isolation in Si electrode,releasing the Si anode internal mechanical stress,building Si anode with high stability and high security interface)are focused on in this thesis.Researches on surface/interface engineering and lithium storage mechanism of silicon anode for Li-ion batteries are systematically carried out.The innovative results in the thesis are as follows.1.Firstly,the multi-point contact interface design of porous Si-CNT was carried out to solve the problems of poor conductivity and deactivation of conductive network.Cu-Si alloy,as the connection between CNT and MSi,is chemically inert and can always keep stable contact without falling off during the volume change of Si.The flexible CNT conductive network can adapt to the drastic changes in the volume of Si anode.Even if the porous Si particle is crushed,each crushed Si particle can still maintain electrical contact.The multi-point contact network structure effectively improves the electron transfer efficiency of the MSi electrode and enhances the rate performance of the LIBs.As a result,the reversible specific capacity is 2533 mAh g-1 with an initial Coulomb efficiency of 89.07%,and the reversible specific capacity is more than 840 mAh g-1 at 2 A g-1 after 1000 cycles.Even at the rate of 20 A g-1,the capacity of the electrode retains 463 mAh g-1.2.To further enhance the mechanical properties of Si anode so that it can bear huge stress from internal or external electrodes,flexible carbon chains consisting of stubby carbon nanotubes(SCNTs)and long carbon nanotubes(LCNTs)are designed.The SCNTs and LCNTs serve as chain joints and separate chain unit,separately.As a result,the well-ordered Si nanowire array are weaved by carbon chains and forming a robust and integrated configuration.The flexible and stretchable silicon array anode shows excellent electrochemical performance at dynamic status.A high initial specific capacity of 2856 mAh g-1 is achieved,even after 1000 cycles with capacity retention of 60%(1602 mAh g-1)is maintained.In addition,the capacity attenuation is less than 1%after 100 times of bending.Furthermore,this excellent cycling stability is obtained with a high Si loading of 6.92 mg cm-2.The rationality of the structure is proved by finite element analysis.This new approach offers great promise for applications in high-loading flexible energy-storage devices.3.In order to further reduce the carbon content in the Si/C composite anode and increase interface stability of the Si anode,a novel silicon nano-ribbon(SiNR)with(110)crystal plane is proposed as anode for the LIBs.The SiNRs with(110)crystal plane have been synthesized by a simple electrochemical micromachining method.Because of the anisotropy of Si,both invasion direction of the lithium-ions and expansion directions of the lithiated silicon are limited to the<110>crystal direction.Recrystallization of SiNR induced by the retention of silicon atomic chains after delithiation is verified experimentally and theoretically.Such SiNR,without necessary surface coating treatment,exhibits high ionic conductivity,high stable solid electrolyte interphase(SEI)and long cycling stability,retaining a specific capacity of 1721.3 mAh g-1(～80%capacity retention)after 2000 cycles with an initial coulombic efficiency(CE)of 83%.The rational design of nanostructured battery materials and electrodes in this work also opens a new dimension in material design for other batteries.4.Using solid state electrolyte(SPE)instead of flammable liquid electrolyte can improve the safety of LIBs.However,the compatibility of SPE/electrode interface is poor due to the rigid characteristics of SPE.In this work,the solid interface of Si/PEO was designed,and the mechanism of the interface evolution during the lithiation/delithiation was studied.As a result,the Si-Si symmetric battery maintained stable for 2000 h at 0.1 mA g-1.Our study shows that Si anode with PEO-based SPE has good interfacial compatibility.On the one hand,Si acts as a container to store Li+,which avoids the formation of Li dendrite in the interface.On the other hand,C-C band of PEO turns into C=C band at the Si/SPE interface to form a stable SEI film with a thickness of 1 nm.The continuous formation of SEI films is inhibited due to the lack of organic solvents required by SEI.