硅中锂与位错相互作用的多尺度模拟

发布时间：2018-05-30 04:06

本文选题：锂离子电池 + Si　；参考：《哈尔滨工业大学》2014年博士论文

【摘要】：由于Si对Li有着很强的存储能力，近些年来人们为了设计出对Li离子储存能力更强、使用寿命更长的新一代Li离子电池，将Si以及Si的各种纳米结构视为新一代锂离子电池中最热门的电极材料。但Li在进入Si后，会导致Si产生将近400%的大变形，如此强烈的大变形将使Si极锂离子电池的寿命大大降低。伴随着变形的产生，Si中必然会产生各种缺陷用来释放变形所造成的应力，位错是其中最重要的一类缺陷。因此研究Si中位错与Li的相互作用就显得非常必要，这方面的研究能帮助我们深入理解Li在含位错Si体系中的动力学特性，并能帮助人们解决大变形带来的问题，早日设计出性能优异的Si极Li离子电池。本文使用耦合了第一性原理（DFT）以及分子动力学(MD)的QM/MM多尺度计算方法模拟Li在含位错Si中的稳定构型以及动力学特性，这将对日后解决Si电极大变形问题有着重要的理论参考价值。首先本文使用多尺度方法模拟了Li原子在Bulk Si中的稳定构型以及扩散运动过程。Li原子在Bulk Si中的最稳定位置处于四面体晶格的中心(Td)。它的扩散路径是周期性锯齿形（zig-zag）路径，过渡态位置位于四面体晶格的底面六边形原子环中心处。这一结果与已存在的第一性原理研究结果相吻合。并且在这部分内容中还计算了Li原子在Bulk Si中的结合能以及扩散势垒，并将这一结果和完全第一性原理(DFT)计算结果对比，证明了本文中所使用的这种多尺度方法无论是在能量数值计算精度上还是能量计算收敛性上都比较让人满意。接着模拟了Si中Li原子与glide型60°位错的相互作用。通过模拟发现，，Li原子在glide型60°位错内部的稳定位置处于每两个位错芯重构时形成的Si-Si原子对所夹的开阔区域的中心处。通过这种每两个Si-Si原子对之间的开口，Li原子可以以较小的势垒由位错芯周边扩散进入位错芯内部。Li原子在位错芯内部扩散时，过渡态在沿着位错线上的两种七边形原子环与一个Si-Si原子对构成的区域内，另外存在两种局部过渡态位置位于两种七边形原子环的中心。通过计算发现Si中glide型60°位错对于Li原子的扩散具有加速的作用，这与已存在的研究中提到的位错对于粒子扩散的加速现象（pipe diffusion）相一致。其后模拟了shuffle型60°位错对于Si中Li原子动力学特性的影响。包括：Li原子在位错芯内部以及周边的稳定构型和在位错芯内部的扩散过程。通过模拟发现，Li原子在位错芯中存在两种稳定位置，这两种位置分别处于两种由两个七边形原子环所围成的区域中。当Li原子在位错芯内部发生扩散时，过渡态位于七边形原子环的中心。通过计算发现，Li原子在shuffle型60°位错中构型更加稳定，并且扩散的加速效果也更加明显。最后还研究了30°部分位错和堆垛层错对于Li原子在Si中的动力学性质的影响。模拟了Li原子在位错芯内部以及周围的稳定构型、从周边位置扩散进入位错芯内部以及在位错芯里发生扩散的过程。在30°位错中Li原子也存在两种稳定位置Oct-A和Oct-B，分别位于位错芯在（111）面上投影的八边形中心处，处在不同的（111）面中。Li原子在这两种稳定位置的结合能也都比在Bulk Si中要低。Li原子在30°部分位错中的扩散路径有两条，分别是以两种不同的稳定位置为起点，且扩散路径互不干扰。通过计算发现，与两种60°位错不同，30°部分位错对于Li的扩散有明显的阻碍作用。通过对堆垛层错与Li原子相互作用的模拟，同样发现了堆垛层错也对Li原子的扩散有阻碍作用。在这一部分的末尾还讨论和总结了Li原子在两种60°位错、30°位错以及堆垛层错中的稳定位置和过渡态位置的规律。
[Abstract]:Since Si has a strong storage capacity for Li, in recent years, in order to design a new generation of Li ion batteries with more Li ion storage capacity and longer service life, Si and the various nanostructures of Si are considered as the most popular electrode materials in the new generation of lithium ion batteries. But Li will lead to the production of nearly 400% of Si after entering Si. Shape, such strong large deformation will greatly reduce the life of Si polar lithium ion battery. With the formation of deformation, various defects will be produced in Si to release the stress caused by deformation. Dislocation is one of the most important defects. Therefore, it is necessary to study the interaction between dislocation and Li in Si, and this research can help. It is helpful to understand the dynamic characteristics of Li in the dislocated Si system, and help people to solve the problems caused by the large deformation, and to design a Si polar Li ion battery with excellent performance at an early date. This paper uses the QM/MM multiscale calculation method coupled with the first principle (DFT) and molecular dynamics (MD) to simulate the stable configuration of Li in the dislocation containing Si. And dynamic characteristics, which will have important theoretical reference value for solving the problem of large deformation of Si electrode in the future.
First, we use the multiscale method to simulate the stable configuration of Li atoms in the Bulk Si and the diffusion motion process. The most stable position of.Li atoms in Bulk Si is in the center of the tetrahedral lattice (Td). Its diffusion path is a periodic serrated (zig-zag) path, and the transition state is located in the hexagonal hexagonal atomic ring of the tetrahedral lattice. The results coincide with the results of the existing first principles. And in this part, the binding energy and the diffusion barrier of Li atoms in the Bulk Si are calculated, and the results are compared with the results of the complete first principle (DFT) calculation, which proves that the multiscale method used in this article is in the energy. The accuracy of numerical calculation is more satisfactory than the convergence of energy calculation.
The interaction between Li atom and glide type 60 degree dislocation in Si is then simulated. Through simulation, it is found that the stable position of the Li atom in the glide type 60 degree dislocation is at the center of the open region of the Si-Si atom formed by each of the two dislocation cores. By this opening, the Li atom can be smaller by each of the two Si-Si original pairs. When the potential barrier diffuses from the periphery of the dislocation core into the internal diffusion of the.Li atom in the dislocation core, the transition state is in the region composed of two kinds of seven - sided atomic rings along the dislocation line and a Si-Si atom, and the other two local transition states are located at the center of the two kinds of seven - sided atomic rings. By calculation, the glide in Si is found. The type 60 degree dislocation has an accelerated effect on the diffusion of Li atoms, which is in accordance with the accelerated phenomenon of particle diffusion (pipe diffusion) of the dislocations mentioned in the present study.
Subsequently, the effect of shuffle type 60 degree dislocation on the dynamic characteristics of Li atom in Si is simulated, including: the stable configuration of Li atoms in the dislocation core and the diffusion process in the dislocation core. Through simulation, there are two stable positions in the Li atom in the dislocation core, and the two positions are in the two form of two seven of the seven sides, respectively. In the region enclosed by the subring, the transition state is located at the center of the seven sided atomic ring when the Li atom is diffused in the dislocation core. It is found that the Li atom is more stable in the shuffle type 60 degree dislocation, and the acceleration effect of the diffusion is more obvious.
Finally, the effect of 30 degree partial dislocations and stacking faults on the dynamic properties of Li atoms in Si is also studied. The process of simulating the stable configuration of the Li atoms in the dislocation core and around the dislocation core and the process of diffusion in the dislocation core in the dislocation core and in the dislocation core are simulated. There are also two stable positions of O in the 30 degree dislocation. Ct-A and Oct-B are located at the eight side center of the dislocation core on (111) surface respectively. In different (111) surfaces, the binding energy of the.Li atom in these two stable positions is also more than two in the 30 degree partial dislocation of the.Li atom in the Bulk Si, respectively, with two different stable positions as the starting point and the diffusion path mutual. No interference. It is found that the partial dislocation of 30 degrees has obvious hindrance to the diffusion of Li through the calculation of two 60 degrees dislocations. Through the simulation of the interaction between stacking faults and Li atoms, the stacking fault also hinders the diffusion of Li atoms. At the end of this part, the Li atom is also discussed and summed up in two 60 degrees. The position of dislocation, 30 degree dislocation and stacking fault in the stable position and transition state.
【学位授予单位】：哈尔滨工业大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TM912

【参考文献】