Mechanics-guided design and optimization of Si-based electrode for high-performance lithium-ion battery

Abstract

The large volume change (LVC) of Si during lithiation and delithiation processes has long been a problem impeding its application as one of the most promising anode materials for lithium-ion batteries (LIBs).To shed light on issue of significant volume variation associated with new electrode materials, a number of research efforts have been made. However, they involve sophisticated chemical processes and expensive fabrication facilities, which greatly limits the scalability of these techniques in the existing LIB industry. To develop a facile, cost-effective, and scalable solution to the LVC problem of Si, we focused on the interface between the electrode film and current collector. In this thesis, bioinspired and biomimetic designs were applied to solve the practical problems of the Si-based material, which is deemed as a typical example of new electrode materials, and novel fabrication approaches of these electrode films were developed cost-effectively. Inspired by the functionally graded materials (FGMs) of biomaterials, we propose applying gradient reallocation of the Si in the electrode along thickness direction to solve the interfacial delamination problem of the Si-based LIBs. In this method, a thin Si-free layer is inserted between the current collector and the Si-rich electrode film in the traditional homogeneous electrode, resulting in a stepwise graded electrode. Surprisingly, the electrochemical performance of the graded electrode was significantly improved, which was the result of inferior charge transfer resistance and improved surface kinetics of the electrode. The rate capability and lifespan of the graded electrodes are also significantly improved compared to those of the traditional homogeneous electrodes. In addition to improving the electrochemical performance, the graded electrode also helps to enhance the effective mass loading of the electrode. This indicates that the utilization efficiency of Si can be greatly improved simply by redistributing the Si in the electrode film. Specifically, the stepwise graded electrode presents a slightly higher initial charge capacity (2,930 mAh g-1) and a comparatively higher Coulombic efficiency (76%) than the initial charge capacity and Coulombic efficiency of the Si anode with a homogeneous Si distribution (2,589 mAhg-1 and 66%, respectively), even though the mass loading of the electrode material is the same. The prepared graded electrodes are found to be quite successful in alleviating interfacial delamination. The success of the graded electrode in improving the performance of LIB prompts us to explore the optimal design of the gradient Si concentration that can promote the applicability and utility of this strategy. As the enhancement in capacity is related to the thickness of the transition layer, the optimal thickness of the translation layer, which was identified as approximately 25 μm, was characterized. According to the success of the graded electrodes in improving LIB performance, we believe that LIB performance can be further improved if a smoother or continuous variation of Si concentration is implemented in the gradient design. To quantitatively study the effect of the Si concentration gradient on the electrochemical performance of the electrode, we divided the Si-rich layer of the two-layer graded electrodes into finer sublayers. The prepared graded electrodes after optimization successfully elevated the performance, resulting in higher capacity and capacity retention, higher Coulombic efficiency, and higher effective mass loading in comparison with the unoptimized ones. Specifically, the optimal graded electrode exhibited a charge capacity of 1,299 mAh g­1 after 50 cycles, which is higher than that of the homogeneous electrode (66 mAh g­1). This graded electrode can be easily implemented by existing manufacturing techniques and synergized with other strategies for solving the LVC problem of Si. Our work provides a guideline for the design and manufacture of the graded Si-based electrodes for LIBs. The use of the biomimetic design of FGM to enhance the performance of Si-based electrodes provides an equivalent strategy for suppressing interfacial stress and strain concentration, which is beneficial for solving the LVC problem of Si. In this thesis, we also propose the use of an electrode film with gradient thickness as another strategy for homogenizing interfacial stress. In comparison to traditional counterparts with uniform thickness as well as those with graded Si concentration, the resulting thickness-gradient electrode exhibited considerable enhancement in electrochemical performances, including capacity, capacity retention, Coulombic efficiency, and rate capability. To optimize this strategy, we propose the optimal design of the gradient thickness based on the theory of stress homogenization, followed by an experimental verification. In particular, the average charge capacity of the optimized electrodes is 1,019 mAh g-1 after 100 cycles and 472 mAh g-1 after 300 cycles. The enhanced electrochemical performance can be attributed to lessening the stress concentration on the interface between the electrode film and the current collector upon the volume change of Si taking place during lithiation and delithiation processes. This work presents a facile, cost-effective, and scalable method for enhancing the performance of Si-based anodes for LIBs. This gradient thickness strategy can be further extended to other anode materials suffering from a similar lithiation-induced volume change problem.