Polymer coating on silicon anodes improves lithium battery capacity

Although silicon anodes can greatly increase the capacity of lithium-ion batteries, the battery performance degrades rapidly. Polymer coatings can solve this problem, but few studies have explored the underlying mechanism. Scientists at JAIST have studied how polymer (borosiloxane) coatings greatly stabilize the capacity of silicon anodes, which could help invent better and more durable lithium-ion batteries that can be used in electric vehicles and renewable energy storage.

Lithium-ion batteries (LIBs) have been continuously improved and adapted since their introduction, making them suitable for applications such as mobile devices, electric vehicles , and renewable energy collection and storage units. In large-scale applications (such as the latter two), LIB research focuses on increasing capacity and voltage limits without changing the overall size. Therefore, battery components and materials must be replaced.

(Image source: ACS Applied Energy Materials)

Many researchers believe that silicon anodes have more potential than traditional graphite anodes. Battery anodes store lithium ions when they are charged, and when the battery is used, the lithium ions move to the cathode through the electrolyte. Silicon is currently a very promising anode material that can increase LIB capacity by nearly ten times, but it also brings a series of problems. Therefore, these problems must be solved before silicon anodes can be commercialized.

A new study published in the journal ACS Applied Energy Materials shows that a JAIST research team solved this problem by using a polymer coating: poly(borosiloxane) (PBS). The research was led by Professor Noriyoshi Matsumi and conducted by JAIST doctoral students Sai Gourang Patnaik and Tejkiran Pindi Jayakumar.

The polymer coating solves one of the biggest problems with silicon anodes, which is the formation of an excessively large solid electrolyte interphase (SEI). The spontaneous formation of SEI between the electrolyte and the anode is critical to the long-term performance of the battery. However, the use of materials such as silicon often leads to battery volume expansion and continuous formation of SEI, which depletes the available electrolyte. This inevitably weakens battery performance, and the battery capacity will gradually drop significantly.

This is where polymer coatings come into play. They prevent excessive SEI formation on silicon and form an artificial and stable SEI. The researchers pointed out that PBS has great potential as a coating for silicon anodes, but previous studies have not provided a clear explanation of its mechanism of action. As Professor Matsumi said: "There are few reports on well-defined polymers based on PBS, which cannot provide sufficient mechanistic basis for their application and reaction. Therefore, we hope to evaluate and clarify the role of polymer coatings on silicon anodes: polymer coatings can not only serve as self-healing artificial interfaces, but also prevent harmful volume expansion."

The team compared the short-term and long-term performance of silicon anodes with and without polymer coatings in terms of stability, capacity, and interface properties. Through a series of electrochemical measurements and theoretical calculations, they understood how PBS helps stabilize the capacity of silicon anodes.

The self-healing properties of PBS and its reversible lithium-ion accommodation significantly improved battery stability compared to bare silicon anodes and anodes coated with polyvinylidene fluoride (a commercial coating used in LIBs), partly due to the ability of PBS to fill SEI cracks during operation. Unlike the other two anodes, the capacity of the battery coated with PBS silicon anode remained almost unchanged after more than 300 cycles of charging.

By solving the main problems associated with silicon anodes, this research paves the way for a new generation of LIBs with higher capacity and durability. Professor Matsumi is very satisfied with the results of the research and said: "The widespread application of large-capacity LIBs will increase the range of electric vehicles, support larger drones, and improve the storage efficiency of renewable energy." He also added that within ten years, LIBs can even be used as secondary energy in large vehicles such as trains, ships and airplanes.