| With the updating of science and technology,people have put forward a higher energy density demand for lithium ion batteries.As far as the cathode materials are concerned,the capacity of the high-nickel ternary cathode materials has reached the extreme and it is difficult to change significantly again.However,the theoretical specific capacity of the commercial graphite is only 372 mAh g-1.Compared with other new anode materials,there is still much room for improvement.For example,the theoretical specific capacity of silicon(Si)is 4200 mAh g-1,which is ten times that of graphite.In addition,Si,as an anode material,is not only excellent in safety performance,but also has abundant reserves and low price.Therefore,Si is expected to become the next-generation high-capacity anode material.However,Si,as a semiconductor,is accompanied by the low electrical conductivity and dramatic volume swing during the alloying/de-alloying processes.Therefore,its application has been greatly hindered.Based on these problems,a study of the surface oxidation modification of crystalline Si as anode materials is carried out in two parts.In the first part,the particles with different SiOx thicknesses are successfully prepared via wet oxidation under high-pressure conditions.We mainly study the effect of SiOx thickness on the cycle stability performance of the cells,and determine the SiOx thickness with the best electrochemical performance.In the second part,the Si particles are annealed at different temperatures in a tube furnace by dry oxidation,so that the Si particles have reasonable SiOx thickness,but the SiOx crystallinity is different.Besides,we study the effect of SiOx crystallinity on the electrochemical performance of the cells,and determine the crystallinity that is most favorable for lithium ion transport.Finally,the research has obtained the following results:(1)The SiOx thickness makes a significant difference in the electrochemical performance of the core-shell Si@SiOx particles.Properly increasing the SiOx thickness is beneficial to the cycle stability of the cells.Among them,3.3 nm oxide layer is formed on the surface of the ball-milled Si particles with a median diameter of 0.78 μm,and the system is considered to have the reasonable thickness.Therefore,the discharge specific capacity of the Si@SiOx-2.37%sample(oxidized sample with oxygen mass fraction of 2.37%)is up to 1701 mAh g-1 after 100 cycles at 0.1 C.which is 1068 mAh g-1 higher than the Si@SiOx-0.98%sample(pristine Si with oxygen mass fraction of 0.98%).However,as the SiOx thickness continues to increase,the discharge specific capacity gradually fades due to the irreversible reactions between the oxide layer and lithium ions.(2)The SiOx coatings with different crystallinity are successfully prepared on the surface of Si particles by dry oxidation at different temperature.The SiOx coatings with low crystallinity create the more open framework to promote charge transfer,so the 550℃ sample shows the best rate performance.For example,the specific discharge capacity of the 550℃ sample is 1072 mAh g-1 at 2 C,but that of the 1150℃ sample is only 278 mAh g-1.When the rate is restored to 0.1 C,the specific discharge capacity of the 550℃ sample is 2492 mAh g-1,but that of the 1150℃sample is only 1141 mAh g-1.Therefore,the 550℃ sample has better rate performance than the sample with a high crystallinity oxide layer.In addition,the SiOx coatings with low crystallinity are mechanically isotropic,so the specific discharge capacities of the 550℃ and 1150℃ samples after 100 cycles at 0.5 C rate are 1108 mAh g-1 and 143 mAh g-1.The 550℃ shows more excellent cycle stability performance.(3)The dry oxidation method is more likely to produce the continuous and uniform oxide layers than the wet oxidation.The two results show that the oxide layer should have a reasonable thickness,and the continuous,uniform and low-crystallinity oxide layer is more conducive to showing excellent comprehensive electrochemical performance for Si@SiOx anode materials. |
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