Graphene-based Covalent Encapsulation For Highly Stable Silicon-Oxide Lithium Battery Anodes

With the rapid development of portable electronic devices and electric vehicles,the capacity requirements of lithium-ion batteries are increasing day by day,which puts forward higher requirements for the anode and cathode materials of lithium-ion batteries.Not only should the capacity be high enough,the circulation performance is excellent,but also the safety should be guaranteed.Among them,lithium-ion battery anode materials are mainly divided into three categories:plug-in storage lithium-ion anode materials,conversion anode materials,alloy anode materials.Currently,graphite anode is the most used anode material on the market.However,its theoretical capacity is low(372 m A h g^-1),which cannot meet the rising demand for energy storage.Therefore,extensive research has been conducted to look for the best anode replacement materials.As a representative of alloy-type anode materials,silicon has been widely concerned because of its high theoretical capacity and abundant reserves and has been considered as a promising anode material for the next generation of lithium-ion batteries.However,due to the formation of Si-Li alloy in the process of silicon charge and discharge reaction,there will be a huge volume change（>300%）,resulting in electrode material broken,separated from electrical contact,solid electrolyte interface film repeatedly generated and broken,continuous consumption of electrolyte,low coulomb efficiency.Many scholars have put forward various solutions,such as nanostructure design,electrolyte additive design,binder design,silicon matrix composite material design,etc.,to solve the problem of large volume expansion of silicon negative electrode and try to promote the commercialization of silicon negative electrode.However,most of these methods are complicated,expensive and difficult to produce on a large scale,which greatly hinders the application of silicon anode.At the same time,as one of the silicon-based materials,silicon monoxide（SiO）has attracted the attention of many scholars because of its abundant reserves,low price,higher theoretical capacity than graphite,and smaller volume expansion（200%）compared with silicon anode.However,there are still some problems such as high energy consumption in the preparation process,high cost,poor conductivity,low first turn coulomb efficiency,200%volume expansion,poor magnification performance,etc.Various strategies,including porous structures and surface engineering,have been tried to address the interface instability caused by large volume changes during the cycle.However,due to the weak inherent adhesion between the electrode and the adjacent conductive medium or the low physical bonding efficiency,the repeated expansion and contraction of the electrode in the cycle process leads to the separation of the electrode from the conductive medium,as well as the breaking and shedding of the conductive medium,and other problems,and the final cycle stability has not been fundamentally improved.With the separation of the conductive medium,the conductivity of the electrode decreases,and the magnification performance also deteriorates.Therefore,to realize the comprehensive commercialization of silicon-based anode,two problems must be solved first:1.The high energy consumption and high cost of SiO preparation process.2.The contact interface between SiO and the conductive medium.To solve the problem of high energy consumption and high cost of SiO preparation,a simple high-energy ball milling method was proposed to synthesize SiO.Using H₂O and Sias raw materials,SiO_x can be prepared by high-energy ball milling at ambient temperature and pressure by adjusting the ratio of raw materials.This method can significantly reduce energy consumption and achieve mass production,which provides a new idea for the synthesis of SiO.For interface contact problems,we introduce a structural design based on the covalent encapsulation strategy of graphene.Using methane as a carbon source,we grew the graphene cage on SiO nanoparticles at high temperatures.This design provides sufficient mechanical strength and flexible cushioning during the cycle to achieve electrical connection at the electrode level.More importantly,with repeated regulation of the growth conditions,Si-O-C covalent bonds can be formed between SiO and graphene while growing the graphene cage.The covalent binding based on Si-O-C significantly improves the interfacial adhesion between SiO and graphene and can maintain a stable interfacial electrical contact during the cycle of repeated expansion and contraction,which is conducive to stable and rapid electron and ion transport.Our prepared graphene-encapsulated SiO material（SiO@G）has a 97%capacity retention rate after 500 cycles,even at a 0.5 C magnification.At a high rate of 2 C,the SiO@G anode still shows a high reversible capacity of 700 m A h g^-1.We successfully increased the first-cycle coulomb efficiency of SiO material from its intrinsic 50%to 71%,and quickly increased it to 99.5%within 5 cycles.We compared the SiO,SiO@C and the flat electrodes after the cycle.It was found that the first two electrodes had obvious phenomena of electrode expansion and fracture and shedding of active substances,while the latter did not have large volume expansion and fragmentation.The circulated SiO@G particles still adhered to the surface of SiO under transmission electron microscopy,without obvious disconnection and large amount of graphene loss.This is evidenced by the excellent cycle performance of the battery.Therefore,the covalently bonded SiO@G negative electrode has remarkable reversibility and magnification properties.We anticipate that this graphene-covalent package design will be beneficial not only for silicon-based anode applications,but also for a variety of battery material systems where interface behavior needs to be adjusted.