Study On Property Manipulation And Corresponding Mechanisms Of Silicon And Iron Oxides-based High-capacity Anode Materials For Lithium Ion Batteries

Si and transition metal oxides?TMOs?have been extensively studied as high-capacity anode materials of lithium ion batteries.Si has high theoretical capacity of4200 mAh g^-1,low working potential,natural abundance and environmental friendliness.Also,the theoretical capacity of iron oxides is as high as 745-1007 mAh g^-1,with natural abundance and low cost.However,both of Si and iron oxide electrodes suffer from large volume expansion during the discharging/charging process,which leads to the crack,fracture and pulverization of particles,consequently losing the electrical contacts and forming the unstable SEIs at fresh surface.In addition,Si and iron oxides have low intrinsic conductivity,which limits their rate performance.Nanostructuring is regarded as one of most important modification methods for the improvement of the cyclic stability of Si and iron oxide materials.However,the low tapped density of nano-sized particles with large specific surface area severely limits volumetric energy density.Moreover,the large contact area between the material and the electrolyte leads to increased side reactions with the electrolyte and further reduces initial coulombic efficiency?ICE?.To solve the above problems,in this study,micron size Si and iron oxides were modified through porositization,surface coating,binder modification and prelithiation.The structural characteristics and electrochemical performances of the prepared composite materials were studied systematically,and mechanisms related to the improved performances were elucidated.High ICE,stable cyclability and superior rate performance are expected.In order to improve the cycling performance of micron-Si,the SiO_x coated porous Si materials with N doping were prepared by heating reaction of Mg₂Si and NH₄Cl and its morphology,structure and electrochemical properties were studied systematically.The specific surface area of porous Si is about 112.901 m² g^-1,more than 20-fold higher than that of pristine Si(6.501 m² g^-1).Electrochemical testing indicated that the first charging capacity of porous Si material was 973.9 mAh g^-1,and the reversible capacity remained at 926.1 mAh g^-1 after 50 cycles.The capacity retention rate was as high as95.1%,much higher than that of the pristine Si?8.4%?.The strengthened multicomponent layer protected micron-Si composite was prepared by heating of Si particles with LiBH₄ followed by a mechanochemical reaction with CO₂.The ultrafine nanocrystals of SiC and Li₂SiO₃ and amorphous B and B₂O₃species were existed in thin outer layer consisting of amorphous SiO_x/C.These multicomponent nanocrystals and B species reinforce effectively the mechanical properties of the microparticles.The elastic modulus and hardness were 40.3±2.2 GPa and 1.77±0.11 GPa for the samples prepared from Si-0.3LiBH₄.After 200 cycles,the strengthened multicomponent layer protected micron-Si composite has a capacity of1080 mAh g^-1,and the corresponding capacity retention rate is 78.0%.Long-term performance shows that the capacity was still 975 mAh g^-1 after 600 cycles,and the capacity retention rate is 70.4%,which is 2.6 times of the capacity of commercial graphite.On the basis of above studies,the amorphous Li₂CO₃ coated N-doped micron-Si composites were prepared by heating of Si particles with LiNH₂ followed by a mechanochemical reaction with CO₂.The coating layer is composed of amorphous Li₂CO₃,SiO_x and dispersed Si₃N₄ nanocrystals,and the presence of Si₃N₄ nanocrystals should greatly enhance the mechanical properties of surface layer,the elastic moduli and hardness values of which were estimated to be 26.6 and 0.73 GPa,so as to improve the cycling performance.Meanwhile,N doping improves the electronic conductivity of the Si composite.The Si–0.3LiNH₂ sample showed a specifc capacity of 1020 mAh g^-1 after 600 cycles at 100 mA g^-11 and a good rate capability of 1117 mA h g^-1 at 2000 mA g^-1.Inspired by the above studies,the metal hydride was introduced into prelithiation of transition metal oxide.A staghorn coral-like 3D porous Fe?II?-rich LiFeO_2-x-x comprising many small particles of 40-100 nm was successfully prepared by heating the mixtures of LiH and Fe₂O₃.The presence of Fe?II?which makes electron transportation much easier,improve electrochemical activity.As a result,the prepared Fe?II?-rich LiFeO_2-x anode exhibited a high ICE of 90.2%,much higher than B-LiFeO₂?78.5%?at a current density of 100 mA g^-1.After 100 cycles,the reversible specific capacity is 853 mAh g^-1 twice as much as that of B-LiFeO₂(429 mAh g^-1)with high rate capability of 556 mAh g^-1 at 2000 mA g^-1.Finally,in order to improve the cycling performance of TMO,a novel organic/inorganic hybrid binder comprises of NaBO₂·2H₂O and SA or CMC.Taking advantage of tetrahedral-configuration polar hydroxyl groups,sodium metaborate hydrates can physically crosslink linear chains of sodium alginate?SA?or carboxymethylcellulose?CMC?upon vacuum drying,resulting in the in situ construction of a three-dimensional?3D?polymeric network with superior adhesion properties.Moreover,such organic/inorganic hybrid binders also contribute a fast transfer of lithium ions,thanks to high Li⁺conductivity of NaBO₂·H₂O.The Fe₂O₃anode fabricated from the commercial micron-Fe₂O₃ materials with CMC-NaBO₂·H₂O hybrid binder delivers a specific capacity as high as 1182 mAh g^-1 at 100 mA g^-1 after200 cycles,corresponding to 81.2%of capacity retention.While the Fe₂O₃ anode fabricated from the commercial micron-Fe₂O₃ materials with SA-NaBO₂·H₂O hybrid binder delivers a specific capacity as high as 1142 mAh g^-1 at 100 mA g^-1 after 200cycles.The specific capacities of the Fe₂O₃ electrodes stayed at 507 and 595 mAh g^-1for SA-NaBO₂·H₂O and CMC-NaBO₂·H₂O binder systems,respectively,largely higher than those of the pristine SA and CMC binders(below 250 mAh g^-1).