Si/Carbon Nanotube Anode Material for High-Performance Lithium Ion Battery: Effects of Adhesive Anchor on the Cyclic Performance
Authors: Park Byung Hoon , Lee Geon-Woo , Kim Young Hwan , Kim Kwang-Bum 
Published: May 2020

Lithium-ion batteries (LIBs) have emerged over the past decade as one of the most promising energy storage and delivery devices because of their high energy densities and high energy efficiencies. Among anode materials investigated for LIBs, silicon has great potential because of its high theoretical capacity (3600 mAh g−1 for fully lithiated Li15Si4), which is ten times greater than that of the conventional graphite anode (372 mAh g−1). Nevertheless, the application of Si anode to LIBs is still challenging because silicon undergoes severe volume changes (ca. 400%) during the electrochemical reactions, which leads to fracture of silicon particles, resulting in electrical contact loss and capacity fading.
In order to address the shortcomings of silicon as an anode, a number of nanoscale structural modifications of silicon itself have been proposed, including nanoparticles, nanowires, nanotubes, hollow and porous structure. These approaches have exhibited the idea that stress/strain in such nanostructures can be relaxed without mechanical fracture so that significant plastic deformation and macroscale failure could be avoided. However, the high surface area of nanostructures promotes excessive SEI formation and other parasitic reactions with the electrolyte that consume Li ions and give unstable electrochemical behaviors.
Another way to improve the electrochemical performance of silicon anode is to introduce carbonaceous materials as an electric conductive buffer to accommodate the severe volume changes of silicon. Among the carbonaceous materials, nanocarbons such as graphene and carbon nanotube (CNT) have been widely used to improve the silicon anode materials since their superior high electrical conductivity and ductility characteristics were beneficial for accommodating the severe volume changes of silicon in the composites. Although it is revealed that the introduction of nanocarbon can improve electrochemical performances of silicon anodes to a certain degree, however, compositing with silicon alone does not guarantee good electrochemical performance completely. Rather, the better electrochemical performance is observed when the additional carbon is employed as binding material forming a robust electrical contact between the nanocarbon and silicon. In practice, a number of previous studies have focused on the introduction of additional carbon as a function of improving electrical conductivity and blocking direct contact with electrolytes. However, it is doubtful whether the electric conductive pathways are well maintained during the repeated volume change of silicon in the composite. To date, diverse designs of silicon-based anodes have geared towards releasing the lithiation-induced stress but failed to retain reversible capacity and durability in real battery systems. For the effect of carbonaceous materials introduction, robust contact between silicon and carbon is essential, and the contact should be maintained stable during repeated volume change of silicon in order to be widely adopted as a main anode material for LIBs.
Herein, we report on rational design and synthesis of Si/CNT microsphere composite with structural reinforcement using triethoxysilane-derived SiOx (denoted as SiOx-reinforced Si/CNT microsphere) as an adhesive anchor to bind the Si NPs to the CNTs and link the neighboring CNTs together. Triethoxysilane is adopted since the silanol group of triethoxysilane reacts with the hydroxyl groups of ACNT and silicon surfaces and forms a covalent bond. Considering the benefits of SiOx, the combination with carbonaceous materials would be effective for simultaneously boosting electrochemical performance and securing dimensional stability of silicon based materials since the in situ generated Li2O and/or Li4SiO4 in the lithiation could be act as stable buffer matrix accommodating the large volume change of silicon. Carbon nanotube microsphere provide not only mechanical support but also efficient electrical pathways during the silicon nanoparticles stores lithium ions due to the CNT’s superior mechanical and electrical properties. Consequently, the Si/CNT/SiOx microsphere composite exhibit better cyclability, and negligible changes of electrodes due to buffer matrix for volume changes, provision of a fast ion conduction pathway, and electrically conductive pathway. For example, the Si/CNT/SiOx microsphere electrode exhibited an initial capacity of 1112 mA h g−1 at 0.5 A g−1 and maintained ∼88% retention of its initial capacity after 100 cycles. Moreover, Si/CNT/SiOx microsphere showed negligible volume change at the electrode level since there is an interconnected pore structure to accommodate the large volume expansion. The very stable electrochemical behavior of the Si/CNT/SiOx microsphere could be attributable to the irreversibly formed lithium silicates buffer layer which are not only tolerate the large volume change of the Si but also act as an adhesive anchor enabling maintain the electrical pathways to silicon. This study provides new insights regarding the introducing of nanocarbon into silicon based anode to apply LIB anode materials with high capacity and cycling stability. More details will be discussed at the meeting.