Preparation Of Silicon-based Composite Nanomaterials And The Optimization Of Lithium Storage Performance

Lithium ion batteries（LIB）with high energy density and long cycle life are widely used in energy storage systems of electric vehicles and portable devices.The market’s demand for lithium-ion batteries with better performance has stimulated the industry’s enthusiasm for research on new electrode materials.Silicon-based materials become promising anode materials and arouse extensive attention due to their high theoretical capacity(Li₂₂Si₅,4200 mAh?g^-1),low discharge voltage（0.5 V vs.Li/Li+）,and wide range of sources.Especially,when the battery is under low temperature or rapid charge and discharge conditions,the silicon anode electrode can better avoid the phenomenon of lithium evolution on the traditional anode surface（graphite or lithium sheet）,which makes it possible that the silicon anode electrode becomes the preferred anode materials to replace graphite for the next generation of long-lasting power lithium-ion batteries.However,the agglomeration of nano-silicon and the volume effect（>300%）during（de）lithiation will produce a series of problems that limit the application of silicon negative electrodes,such as:cracking of silicon crystals,collapse of electrode structure,deterioration of electrical contacts inside the electrode and so on.For the above problems,starting from the structural design,the commercial nano-Si material treated with"piranha solution etching method"was selected to improve its hydrophilicity while alleviating the serious problem of nano-silicon particle aggregation.Furthermore,the advances of"in-situ coating"and"conductive framework material"were taken into consideration by introducing a carbon material co-doped with boron and nitrogen.Moreover,different binders were explored in an electrochemical testing system of porous silicon dendrite anodes.The details are as follows:（1）Surface treatment of nano-Si materials by piranha solutionCommercial Si powder with a particle size of 50～120 nm was purchased as raw material,and piranha solutio（concentrated sulfuric acid:hydrogen peroxide volume ratio of 3:1）was etched in an oil bath environment at 95°C for 2 hours,and then washed to different pH to prepare nano-Si materials with various surface situations.The etching process of the piranha-treated nano silicon was investigated by XRD phase composition detection,SEM and TEM observation morphology.The results show that comparing with the raw silicon powder(the first discharge capacity is 2621.2 mAh?g^-1,the discharge capacity after 120cycles is decreased to 307.1 mAh?g^-1),the electrochemical performance of the silicon powder treated by the piranha and diluted to pH=7 is significantly improved.At the current density of 200 mA?g^-1,the initial discharge capacity of the treated-silicon is 4173.88 mAh?g^-11 along with a first coulomb efficiency of 86.29%.For that treated-silicon anode,even after 120 cycles at a current density of 500 mA?g^-1,its discharge capacity can mantian 823.79 mAh?g^-1.（2）Polymer-assisted preparation of Si@C-B,N nano-composite anodesA layer of boric acid-doped polyaniline was grown in situ on the surface of the nano-Si material prepared by surface treatment of piranha solution,and Si@C-B,N nano-composite anode materials were prepared after carbonization at 850oC.Among them,approximately 5 nm boron and nitrogen doped carbon acts as a conductive agent and a skeleton in the system.At a current density of 200 mA?g^-1,the discharge capacity of the first loop is 3634.19 mAh?g^-11 with a first coulomb efficiency of87.68%,and the discharge capacity is mantian 1298.85 mAh?g^-11 after340 cycles at a current density of 500 mA?g^-1.The doping of boron and nitrogen improveed the conductivity and electrochemical stability of the carbon layer,making the carbon layer not only avoid direct contact between Si and the electrolyte,but also stably maintain the lithium ions and electrons of the Si@C composite negative electrode rapid conduction.（3）Effects of binder composition on the electrochemical performance of porous Si dendritesThe porous Si dendrites prepared by the dealloying method were used as active materials.polyacrylic acid（PAA）,sodium alginate（SA）,and a mixture of the two with a mass ratio of 1:1 was used as binders to prepare pole pieces,and their electrochemical performance were investigated.Fourier infrared test showed that PAA and SA undergo dehydration condensation reaction under high temperature conditions,successfully generated a crosslinked network structure.The batteries assembled with different binders were activated at 200 mA?g^-11 for the first five cycles,and then circulated for more 200 cycles at a current density of500 mA?g^-1.Among those batteries,the one by using PAA+SA binder maintained a stable specific capacity of 1206 mAh?g^-1,which was better than those of PAA(378 mAh?g^-1)and SA(801 mAh?g^-1)binders.In summary,the PAA+SA binder has a significant effect on improving the cycling stability of porous Si dendrites.