Structural Design And In Situ Carbon Embedding Nanosizing Of High Specific Capacity Si/Ge Lithium-ion Battery Anode Materials

The rapid development of portable electronics and electric vehicles has increased the demand for highenergy density lithium-ion batteries.Graphite has been widely used as anode material for lithium-ion batteries because of its high conductivity,long cycle life,abundant resources,and low price.The theoretical capacity of graphite is only 372 m Ah g^-1,and the limited theoretical capacity can hardly meet the requirements of anode materials for next-generation lithium-ion batteries.Si has received much attention from researchers for its ultra-high theoretical capacity(Si:4200 m Ah g^-1),and as an anode material for Li-ion batteries,Si produces a volume change of up to 300%when charge/discharge.The considerable volume deformation will cause the active material to detach from the binder and conductive agent.Under the repeated volume deformation and enormous deformation stress,the interior of the material will be shattered,and a large number of"dead zones"will be formed,which cannot participate in the electrochemical process.During the first discharge,SEI films are formed on the surface of the material particles,which will continue to fragment and grow during the repeated volume changes of the material.Ge,a homolog of Si,has also received much attention from researchers because of its high theoretical capacity(Ge:1600 m Ah g^-1).Like Si,Ge faces capacity degradation due to volume distortion when used as an anode in lithium-ion batteries.（1）The construction of nanoporous structures can provide buffer space for the volume expansion of Si anode materials when embedded in lithium,thus effectively improving the electrochemical performance of Si anodes.However,conventional preparation methods often suffer from complex processes,harsh conditions,and high costs.Si and Mg can form Mg₂Si alloy at 500°C,and Mg has a saturation vapor pressure as high as 17 k Pa at 900°C.Based on the physicochemical properties of Si and Mg,a reasonable ramp-up procedure can be designed to achieve the preparation of nanoporous Si and the recycling of Mg.The specific discharge capacity of nanoporous Si prepared by this method was maintained at 2400 m Ah g^-1 after 100charge/discharge cycles at a current density of 1 A g^-1 and 1446 m Ah g^-1 after 650 charge/discharge cycles at a current density of 2 A g^-1.（2）The precursor Mg₂Si@CTAB/Si O₂ is prepared by grafting CTAB（cetyltrimethylammonium bromide）on the surface Mg₂Si alloy particles,followed by liquid phase deposition of Si O₂.After heat treatment,the material is etched with hydrochloric acid to form a Si anode material with a core-shell gradient pore structure.The core layer of this material has lower porosity and higher strength,while the shell layer has a more significant porosity.The core layer can withstand higher deformation stresses due to its high strength,which provides the possibility of the overall stability of the material particles,while the higher porosity of the shell layer provides a more stable attachment interface for the SEI film.The high porosity of the shell layer also facilitates the rapid transport of lithium ions within the particles and reduces the concentration gradient of lithium ions.The prepared Si anode with a core-shell gradient porosity structure maintains a discharge specific capacity of 2127 m Ah g^-1 at 1 A g^-1 after 100 charge/discharge cycles,with a retention rate of 85.7%the second cycle.The prepared core-shell gradient pore structure improves the strength of the porous Si material and provides the possibility for the stable existence of SEI films.（3）Si O₂ is an important Si source for the preparation of Si anode materials.Nanoporous Si prepared directly from Si O₂ is often challenging to use as anode materials for lithium-ion batteries,and carbon cladding is usually required to obtain better electrochemical properties.Although carbon cladding can effectively improve the electrochemical properties of nanoporous Si to a certain extent,it is still difficult to effectively solve material comminution during repeated volume deformation.The Si/C composite lithium-ion battery anode material with a porous structure can be directly prepared by reacting Si O₂ as the Si source with excess Mg.The carbon in this material is embedded in the porous Si to form a three-dimensional network,which can buffer the volume deformation and increase the electrical conductivity of the material,and the presence of carbon can prevent the Si from growing again after nanosizing.（4）Like Si,coating carbon on the surface of Ge particles can also improve the electrochemical performance of Ge anode materials,but the problem of comminution inside the particles is still challenging to solve.Therefore,designing the structure of Ge anode materials to achieve more effective carbon modification is an essential direction for the research of Ge anode materials.Firstly,Ge and Mg reacted to form Mg₂Ge alloy at 500°C,CO₂ was introduced at 630°C.CO₂ reacted with Mg in Mg₂Ge alloy to form carbon and Mg O,and finally,a nanoporous carbon network was formed to realize the nanosize of Ge.In the formed carbon nanoporous network,the scale of carbon is less than 10 nm,and the actual scale of nanosized Ge is also less than 10 nm due to the in situ embedding of carbon,which prevents the bonding and re-growth of nanosized Ge.The carbon nanonetwork provides the essential buffer layer for the volume expansion of the Ge distributed in it,which provides the overall stability of the material particles during charging/discharging.Due to the pore structure with micro-and nano-gradient pore characteristics formed by the combined effect of volume deformation during Mg₂Ge alloys and processes such as Mg O etching,the interleaved distribution of this micron and nanopores promotes the rapid transport of lithium ions within the MN-GP-Ge/C material particles.Due to the in situ embedding of carbon,the actual particle size of Ge is less than 10 nm,which is the limiting scale（5-15 nm）that can be maintained during Ge charging/discharging.Due to its unique structure,the MN-GP-Ge/C electrode material exhibits excellent electrochemical performance even at low temperatures.At a current density of 4 A g^-1 and 1000 cycles,the capacity retention is 84%,and the discharge specific capacity is maintained at 1127 m Ah g^-1.At a current density of 0.5 A g^-1 and test temperatures of-20,-10,and 0°C,the discharge specific capacities are 676,972,and 1151 m Ah g^-1,respectively.