The Potential of Silicon-based Anodes in Lithium Ion Batteries
New, innovative materials play a crucial role in improving key properties of battery cells such as energy density, performance, lifetime, safety, cost-effectiveness and sustainability. Lithium ion batteries (LIB) with silicon anodes are currently prominent in research and development. They appear to feature fast-charge capability and can store more energy per volume and weight than state-of-the-art graphite anodes. In addition, the material is highly abundant. On the occasion of the National Battery Day on February 18, Dr Johannes Kasnatscheew, Head of the Research Division “Materials” at MEET Battery Research Center at the University of Münster, and Dr Markus Börner, Head of the MEET Research Division “Cell System”, give an overview of the benefits and drawbacks of this energy storage system.
How is silicon used in batteries?
Markus Börner: Silicon (Si) can be used as an active material in LIB anodes. Currently, it is added partially to existing anode materials, in so-called composite anodes. In these anodes, graphite still remains the main component of the anode. Only a small fraction of two to five percent by weight of the active material of silicon is added. This is usually not pure silicon, but silicon oxide (SiOx). It contains active Si nanodomains which enable the specific capacity to be tripled compared to conventional graphite anodes.
Johannes Kasnatscheew: Such composite anodes are also being studied using, for example, silicon nitride (SiNx) instead of silicon oxide. The material provides a high storage capacity as well as a stable and safe operation. Although anodes made of 100 percent silicon have enormous potential, as they have around ten times the specific capacity of graphite, research still needs to be carried out.
What other challenges does the use of silicon anodes pose?
Johannes Kasnatscheew: Lithiation during charging and delithiation during discharging cause volume changes of up to 300 percent in silicon which, thus, drastically increase the material stress. By comparison, the volume expansion of graphite upon lithiation ranges at around ten percent. One of the biggest challenges is the reduced cycle life as the extreme volume expansion ruptures the solid electrolyte interphase (SEI) on the silicon surface. The SEI is essential to prevent parasitic reactions of the charged and thus reactive silicon surface with the electrolyte and to avoid loss of active lithium and thus capacity. However, in order to reconstruct the cracked SEI, active lithium is required, which is therefore no longer available for the capacity. This process causes the reduced lifetime of these battery systems.
Markus Börner: Silicon oxide can accommodate the volume changes of the active silicon during charging and discharging to a certain extent, as silicon is surrounded by a quartz matrix (SiO2). This in turn has the disadvantage that it is highly insulating, which makes a carbon coating necessary. This is one of the reasons why silicon oxide is currently only used in very small quantities.
What approaches are being pursued to solve these challenges?
Johannes Kasnatscheew: Present research is working intensively on electrolytes to indirectly design a suitable, ideally flexible, SEI that can better withstand the volume changes of the silicon. In addition, special coatings and binder systems are being investigated to better withstand material stress of composite anode. The modification of the silicon particle itself is also promising, especially scaling towards nano dimensions.
At MEET Battery Research Center, we are also researching interesting concepts based on composite materials. The aim is to prevent contact between the silicon and the electrolyte in order to counteract the continuous SEI reformation, thus lithium and capacity losses.
Markus Börner: Another important research approach for us is pre-lithiation. This strategy aims to compensate the unavoidable lithium and capacity losses caused by the volume changes in the LIB cell. Our approaches for pre-lithiation of composite anodes with silicon are based, for example, on known electrochemical processes, but also on novel chemical processes as well as thermal vaporization of the Si anodes with lithium metal.
Despite all the challenges, are silicon anodes already being used?
Markus Börner: Anodes with low fractions of silicon oxide have already been used commercially in high-energy applications such as electric vehicles for several years, although the reduced cycle life is still a major challenge, especially with higher contents beyond ten percent by weight of the active material.