The MGX/UBC partnership is developing a highly efficient, long-lasting silicon anode that will aide in the development of next generation lithium-ion batteries capable of increasing energy density from the current standard of ~ 200 Wh/kg to 400 Wh/kg for use in long-range electric vehicles and grid-scale energy storage. The project utilizes low-cost metallurgical-grade silicon as a feedstock to fabricate nanostructured silicon. The project has entered its second year and has completed successful 1,000 cycle testing of its Li-Si battery.
Table 1. Schedule of program milestones and tasks
Si has been regarded as the most promising new anode material, due to its 10-fold higher theoretical capacity (4200 mAh g-1) than current commercial graphite anodes (372 mAh g-1), with low working voltage, and large abundance (second-most abundant element on earth). However, the commercial large-scale application of Si materials is being substantially impeded by the complicated and high-cost approaches in producing Si with desired nanostructures. Hence, we reported a scalable and cost-effective method to obtain nano-porous Si particles by scalable metal-assisted chemical etching procedure starting with cheap metallurgical silicon as feedback. Thereafter, an ultrathin coating layer was deposited on surface of the as-prepared nano-structured Si particles to further stabilize their cycling performance. As a result, the extraordinary storage capacity can be well retained in prolonged electrochemical cycles. Specifically, a reversible capacity of 607.5 mAh g-1 is delivered after over 1000 cycles at a current density of 1C (2000 mA g-1). The superior electrochemical performance should be ascribed from the synthetic effects of the nano-porous structure and ultrathin coating, which facilitates the electrolyte penetration, constrains the huge volume expansion/ shrinkage, suppresses the undesirable side-reactions and stabilize the electrode/electrolyte interface during repeated electrochemical (de)lithiation process. The developed scalable metal-assisted chemical etching coupled with coating will pave the way for the revolution of the Si anode materials for high-energy density Li-ion batteries.
Figure 1. Proposed methodologies for fabricating and evaluating Si-based anode for Li-ion batteries
Advancement of Si Anode Li-ion Battery
Lithium ion batteries (LIBs), are one of the most important energy storage technologies, having dominated the world’s energy storage market ranging from the consumer electronics (CEs) to electric vehicles (EVs) or distributed energy storage systems (ESSs). The increasing demands from end users have stimulated the development of LIBs with higher energy and power density, better rate capacity, and longer cycling life. Silicon (Si) has been long considered as the most promising anode alternative due to its high theoretical capacity, moderate working voltage and large abundance on the earth.
Despite its advantages, Si has an intrinsic hurdle, i.e. the huge volume change of up to 400% that occurs while (de)alloying with Li during cycling, which poses great challenges towards practical applications. Firstly, the repeated volume change eventually results in mechanical fracture and pulverization of the Si anode, leading to electronic isolation of active Si materials from current collectors and considerable loss of active Si. Secondly, the solid electrolyte interphase (SEI), a protecting layer formed on the surface of Si anode as a result of electrolyte decomposition, is unstable. The dynamic volume change not only causes a break-down of existing SEI due to repeated expansion and shrinkage, but also exposes fresh Si surface where new SEI layers continuously grow upon cycling. This persistent side reaction between Si and the electrolyte consumes Li ions and the electrolyte solvent parasitically and increases the charge transfer resistance by accumulating thick SEI layers. Thirdly, on the cell level, the volume change might cause swelling of Si electrode over time, decreasing the volumetric energy density (Wh L-1) of Si anode Li-ion batteries. It has been shown that Si electrode with 50% or more swelling exhibits lower volumetric energy density than graphite electrode, thereby negating the higher theoretical capacity advantage that Si has over graphite.
Great progress has been made to address the aforementioned challenges in Si anode over the past ten years. Several strategies, including Si size control, surface coating and compositing, have been demonstrated to be effective. (1) Si size control: Nanostructured Si can better mitigate the stress and volume change, as well as provide faster ion diffusion and electron transfer, compared to their bulk counterpart. As a result, various Si nanostructures, such as nanowires, nanotubes, nanoparticles, and porous networks, have been extensively investigated and shown promising potential in Li-ion batteries. (2) Surface coating: Surface coating is indispensable to create stable SEI on Si anode in order to reduce side reactions, increase the fracture resistance, and improve electrical conductivity. In this regard, many coating materials, such as carbon, metal oxides, polymers, have been incorporated into nanostructured Si to boost its long-cycle stability. (3) Compositing Si with conductive matrix (such as graphene, graphite): This is a widely used approach to combine the advantages from both Si and conductive matrix. Especially, graphite/Si composite is considered as a viable anode alternative. The composite takes advantages of the good stability and conductivity of graphite and the high capacity of Si, while minimizing the problems associated with Si anode.
Despite significant advances, Si-based anodes still face great problems towards practical applications. One problem is the lack of a scalable and low-cost fabrication method for nanostructured Si. Current fabrication methods, either bottom-up or top-down, do not address a cost-effective way to reduce the cost of Si anode, due to the process complexity and/or high-cost starting materials. For example, chemical vapor deposition is able to deposit Si nanoparticles with less than 100 nm, but requires high temperature and expensive precursors (such as Si2H6, SiH4). On the other hand, top-down approaches are mostly based on high-cost electronic-grade Si (n-type, p-type, boron-doped, purity > 99.99999%) as feedstock, and involve the use of template or lithography steps that increases process complexity. Another problem is that most Si anodes reported so far are based on half cells (Li metal as counter electrode), and the use of excess Li metal in half cells “shield” the efficiency and cycling problems of Si anode. It is challenging to assess their feasibility in full cells which are required in practical applications.
The team has developed a cost-effective route to obtain nano-porous Si particles by scalable metal-assisted chemical etching (MCE) procedure using low cost metallurgical silicon as feedback and further introduction of an ultrathin coating to stabilize the Si anode surface. The combination of the nano-porous structure and nanoscale coating layer can not only benefit the electrolyte filtration but also accommodate large mechanical strains during the lithium insertion and extraction processes, thus leading to a significant improvement in electrochemical performance.