Study On Failure Mechanism And Size Effect Of Tin Oxide Anode Materials

Lithium-ion batteries（LIBs）are one of the most widely applied and most favored energy storage solutions in the market due to their high energy density,high cell voltage,low self-discharge current,long cycle life and environmental friendliness.Nowadays,the anode materials of lithium-ion batteries are mainly graphite anodes with capacity of 372 m Ah g^-1,which can no longer meet the soaring demand for energy storage.It is of great importance to develop anode materials with high specific capacity,high rate performance and long working life in order to boost the performances of LIBs.Tin oxide（Sn O₂）is widely considered as one of the most promising anode materials for the next generation owing to its specific capacity as high as 1494 m Ah g^-1.However,Sn O₂ anodes suffer from the poor cycling reversibility and stability,which prevents the electrode from making full use of the high theoretical capacity.Previous studies have shown that the large Li_xSn/Sn crystals formed in the Sn O₂ anode during working cycles were crucial factors to reduce reaction reversibility and induce electrode failure.Nevertheless,the formation mechanism of these large crystals is still unclear,which severely hinders the development of a specific method to suppress Li_xSn/Sn crystals’formation.In terms of this goal,we successfully synthesize Sn O₂ nanoparticles with different sizes.The formation process of Li_xSn particles in Sn O₂ anodes and the influence of the Sn O₂ size on the anode’s failure behavior are studied by using in situ TEM.The main research contents and results of this thesis are as follows:（1）5 nm Sn O₂ nanoparticles and Sn O₂-graphene oxide（Sn O₂@GO）composites with single particle dispersity are prepared by a hydrothermal method.Sn O₂ nanoparticles of 50 nm and 30 nm are obtained respectively via the calcination of 5 nm Sn O₂ samples and Sn O₂@GO composites at800℃.It is found that the size of calcined products increases with extending the calcination time and reversely decreases with reducing the amount of GO in the Sn O₂@GO precursor.（2）In situ TEM investigation shows that the lithiation process of Sn O₂ nanoparticles can be divided into 3 stages.1)ESA stage.The sample’s volume remains basically unchanged and the crystalline Sn O₂ is converted into an amorphous containing a large number of dislocations.2)Conversion stage.The sample’s volume increases sharply and the further reaction of Sn O₂ and Li⁺results in the formation of amorphous Sn and Li₂O.3)Alloying stage.The sample’s volume increases gently and Sn alloy with Li⁺to form Li_xSn amorphous.（3）The structural evolution of Sn O₂ anodes under different electron beam dose rate is studied.Under the irradiation of a weak electron beam dose rate,Li_4.4Sn nanocrystals are formed in the lithiated Sn O₂ amorphous.Under the irradiation of an intense electron beam dose rate,large Li_4.4Sn crystals are formed.These Li_4.4Sn crystals further convert to aggregated Sn crystals after the delithiation.Systematically experimental results demonstrate that the formation process of large Li_4.4Sn crystals is controlled by electron beam dose rate rather than the cumulative electron beam dose.（4）The relationship between Li⁺concentrations in the sample and the formation of Li_4,4Sn crystals is studied.It is found that the high Li⁺concentration can lead to the formation of large Li_4.4Sn particles,while the low Li⁺concentration can effectively inhibit the formation of large particles.Theoretical calculation results shows that the unstable c-Li_4.4Sn/a-Li₂O interface in high Li⁺concentration environment can promote the aggregation of nanocrystals into large Li_4.4Sn crystals.The Li⁺concentration in the sample can be remarkably reduced by placing the freshly lithiated Sn O₂for enough time.Besides,the stable c-Li_4.4Sn/a-Li_xSn O_y interface in standing samples can effectively suppress the formation of aggreagted Li_4.4Sn crytals.（5）The Sn O₂’s size effect on the formation of Li_4.4Sn crystals is studied.It is found that the size and number of Li_4.4Sn crystals decrease with decreasing Sn O₂’s dimension.The formation of L_i4.4Sn crystals can be effectively inhibbited by reducing the size of Sn O₂ anode below 15 nm.The electrochemical performance of Sn O₂ anode with different size is further studied.It is found that 5nm Sn O₂ anode shows a superior cycling reversibility and stability than 50 nm Sn O₂.Moreover,the5 nm Sn O₂ anode still maintains 62.12%of its capacity after 95 working cycles.