Preparation And Performance Improvement Of Silicon-based Anode Materials For Lithium-ion Batteries

The widespread application of lithium-ion batteries in the field of electric vehicles and energy storage has raised higher demands on the energy density of batteries.Silicion?Si?is considered to be the most promising anode material due to its extremely high theoretical specific capacity,moderate operating voltage,and abundant resources.However,the large volume change during alloying/dealloying process and the poor conductivity of Si limit its commercial application.In this thesis,the stability of Si-based materials,electrode and interfaces was improved from three aspects: structure design of Si-based anode materials,binders and electrolyte additives.Meanwhile,the effects of several modification methods on lithiation/delithiation characteristics,cycling stability and rate performance of Sibased negative electrodes were characterized by constant current charge-discharge,differential capacity curve,cyclic voltammetry,and electrochemical impedance spectroscopy.Scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,X-ray diffractometer?XRD?,and X-ray photoelectron spectroscopy?XPS?were also used to characterize the morphology,structure,and composition of the Si-based negative electrodes,and the reasons for the improvement of the performance of Si-based negative electrodes were revealed.Using simple solution reaction followed by magnesiothermic reduction method,MCMB/Si/C?MSC?composite with "sandwich structure" was prepared,and its electrochemical performance was studied.The optimized MSC-2 composite delievers a high initial capacity of 646 mAh g^-1 at 50 m Ag^-1,and a satisfactory capacity retention ratio of 90.3% after 100 cycles at 100 m Ag^-1.Additionally,even at a high current density of 1500 mAh g^-1,the reversible specific capacity of 210 mAh g^-1 can be maintained.The performance improvement of MSC composite can be mainly attributed to its unique structural design: the micron-scale spherical mesocarbon microbeads?MCMB?core can act as an ideal material supporting for nano-Si and conductive network with high electrochemical lithiation stability;the nano-Si can provide high lithium storage capacity;the outer layer of amorphous carbon can further enhance the binding force between nano-Si and MCMB particles,effectively alleviate the volume expansion of nano-Si and improve the structural stability of the material.Moreover,the amorphous carbon coating layer also facilitates the formation of a stable solid electrolyte interface?SEI?layer on the electrode surface.Therefore,the obtained MSC composite exhibits high mass specific capacity,high volume specific capacity,long cycle stability and high rate performance.Two-dimensional sheet-like nano-Si/N-doped carbon?Si NC-2D?and threedimensional porous nano-Si/N-doped carbon?Si NC-3D?composites were designed and prepared,and the influence of the structure of the composites on their electrochemical performance was studied.Compared with Si NC-2D composite,the Si NC-3D composite exhibits more excellent performance.It can deliver a high specific capacity of 1396 mAh g^-1 and capacity retention of 84.3% after 100 cycles at 0.2 Ag^-1,along with an enhanced rate capability of 825 mAh g^-1 at a high current density of 10 Ag^-1.The excellent electrochemical performance is mainly attributed to the three-dimensional N-doped carbon network obtained by a facile approach with NaCl,which is more conducive to the penetration of the electrolyte and the transport of lithium ions,alleviates the volume effect and improves the structural stability of the Si material.Meanwhile,the N-doped carbon material with good electrical conductivity can improve the electrical conductivity of the composite,reduce the contact between Si and electrolyte,and stabilize the SEI film on the surface.The effects of LiPO₂F₂ additive on the electrochemical performance of MCMB,graphite and graphite/Si/C?GSC?composite were studied.The results show that LiPO₂F₂ can significantly improve the cycling and rate performance of MCMB,graphite and GSC composite.The study on the performance of MCMB/Li cells using electrolytes with different contents of LiPO₂F₂ shows that,the MCMB/Li cell with 1.0 wt% LiPO₂F₂-containing electrolyte can achieve a capacity retention of 96.9% after 100 cycles and an enhanced rate capability of 223 mAh g^-1 at 1.0 C.In the presence of 1 wt% LiPO₂F₂ in the electrolyte,the GSC anode can obtain an initial reversible specific capacity of 632 mAh g^-1 and capacity retention of 86.1% after 100 cycles,along with an enhanced rate capability of 527 mAh g^-1 at 1.0 C,significantly higher than that of the base electrolyte.XPS and SEM analysis shows that the addition of LiPO₂F₂ is conducive to form a more stable and dense SEI film rich in Lix PFy Oz and Li F on the negative electrode surface,thereby reducing the impedance of the electrode,improving the lithiation/ delithiation rate of Li+ and improving the performance of the negative electrode.The effects of a new binder amorphophallus konjac gum?APK?and sodium carboxymethyl cellulose?CMC?binder on the performance of nano-Si anodes were studied.The reversible specific capacity of the nano-Si anode with APK binder is 1115 mAh g^-1 after 202 cycles,which is much higher than the 657 mAh g^-1 of CMC binder.In addition,at a current density of 5 Ag^-1,the average reversible specific capacity of the nano-Si anode with APK binder is 1359 mAh g^-1,which is also higher than that of CMC binder(855 mAh g^-1).Research results show that when APK is used as a binder,it can strengthen the interaction between nano-Si material and current collector,improve the stability of electrode structure and the electrode/electrolyte interface,and thus the electrochemical performance of the obtained nano-Si anode is significantly better than that of the CMC binder.