【作者】 李静；

【导师】 慈立杰；

【作者基本信息】 山东大学 ， 材料加工工程， 2022， 硕士

【摘要】 高比容的负极材料是提高锂离子电池(Lithium-ion batteries,LIBs)能量密度的重要突破口,硅(Si)基材料凭借丰富的自然储备量、较低的放电电位以及高的理论比容量被认为是最有前景的下一代锂离子电池负极材料。然而单质Si在循环过程中会造成超过300%的体积变化,导致活性颗粒的破碎以及固态电解质膜(Solid electrolyte interphase,SEI)的重复生长,严重限制了单质Si的循环稳定性。氧化亚硅材料(SiOx)由于锂化时会生成硅酸锂/氧化锂,减小了整体体积变化,并作为缓冲层减轻体积变化对结构的影响,被认为是Si基家族中目前商业实用化程度较高的负极材料。尽管硅酸锂等不可逆相抑制了体积膨胀,但造成了活性锂的损失,因此SiOx负极的首圈库伦效率(Initial coulombic efficiency,ICE)较低。本论文以提升库伦效率为出发点,通过固定微米SiOx颗粒中氧组分为对锂惰性的石英晶相以及对SiOx极片化学预锂化,获得了具有优异的综合电化学性能的氧化亚硅负极材料,主要研究内容如下:(1)利用碱金属在SiOx歧化过程的助熔作用以及氟离子对硅氧网络结构的破坏作用,采用LiF为掺杂物,900℃下短时间歧化获得具有较高结晶度的SiO2石英晶相与Si晶相共存的两晶相氧化亚硅材料。短时间的歧化获得了大小合适的两种晶畴,既释放了更多的活性Si,提高了可逆容量;又固定了部分氧,提高了首圈库伦效率。改性后的样品ICE提高到80%,首圈可逆容量为1521 mAh g-1,循环100圈的平均库伦效率为99.14%,在200圈循环中库伦效率超过99.5%的占比比未加LiF的样品更高。此外研究发现,虽然延长保温时间或者提高歧化温度可以固定更多氧组分为石英晶体而提高首效,但损失了大量可逆容量。同时,通过原位XRD研究了石英晶相与硅晶相在脱嵌锂过程中的变化。(2)利用1-甲基萘-锂为有机锂源对SiO@C负极片进行化学预锂化。研究发现1-甲基萘-锂能够部分还原SiO表层的SiO2,进一步提升了SiO@C的可逆比容量,浸泡十小时的极片组装半电池的首圈可逆比容量达到了 2341.4 mAh g-1,有助于提升电池的能量密度。预锂化阶段预生成的硅酸锂相提高了Li+扩散速率,有利于高容量的发挥与高倍率性能的实现。应用预处理的极片与磷酸铁锂组装的全电池的首圈库伦效率从65%提升到了 80%,实现了可观的补锂效果。更多还原

【Abstract】 The anode materials with high specific capacity play a prominent role in increasing the energy density of lithium-ion batteries.Silicon-based anode materials are constituted as the most promising next-generation lithium-ion battery anode materials owing to their abundant natural reserves,low discharge potential,and high theoretical specific capacity.However,elemental silicon will bring about stupendous volume change more than 300%during cycling,leading to the pulverization of active particles,short of contact and the trapping of active lithium due to the repeated growth of solid electrolyte interface(SEI),which severely limit the cycling stability of silicon anode.The most commercially practical candidate in the silicon-based family-silicon monoxide anode material,displays the reduced overall volume change due to the formation of lithium silicate and lithium oxide during the initial lithiation,which act as a buffer to reduce the impact of volume change on the structure.Although irreversible phases such as lithium silicate suppress the volume change,they accordingly cause the loss of active lithium,and the initial coulombic efficiency of the silicon monoxide anode is low.Herein,starting from the improvement of initial coulombic efficiency,micron-sized silicon monoxide anode with excellent comprehensive electrochemical performance is obtained via fixing the oxygen component as a crystalline quartz phase that is inert to lithium,and chemical pre-lithiation on the electrode.The main research contents are depicted as follows:(1)Due to the effect of the alkali metal element and fluorion on promoting atomic rearrangement in the disproportionation process of silicon oxide,a two-phase silicon monoxide material,in which quartz SiO2 crystal phase and Si crystal phase with high crystallinity coexist,can be produced in a short-time disproportionation at 900℃ by using LiF as a dopant.This process not only release more active silicon,improving the reversible capacity,but also fix part of the oxygen,increasing the initial coulombic efficiency.The initial coulombic efficiency is up to 80%and the initial reversible capacity is 1521 mAh g-1 with the higher average coulombic efficiency.In addition,it is found that extending the holding time or increasing the disproportionation temperature can fix more oxygen components into quartz crystals and improve the initial coulombic efficiency,but a large amount of reversible capacity is sacrificed.The conversion of quartz crystal phase and silicon crystal phase in the process of lithium cyclingare investigated by in-situ XRD.(2)1-methylnaphthalene-lithium is selected as the organolithium source for chemical prelithiation of SiO@C anode electrode.It is found that 1-methylnaphthalene-lithium can partially reduce SiO2 on the surface of silicon monoxide,which improves the reversible specific capacity of SiO@C.The reversible specific capacity of the first cycle reaches 2341.4 mAh g-1,which helps to improve the energy density of the battery.The pre-generated SEI in the prelithiation process improves the diffusion rate of lithium ions,which is conducive to the development of high capacity.The initial coulombic efficiency of the full cell deploying the pre-treated electrode matched with LiFePO4 increases from 65%to 80%,achieving a considerable lithium replenishment efficacy.更多还原