Polymer Structural Framework to Enable Embedded High Performance Silicon
Authors: Riley Leah A , Sakshaug Avery J , Kron Ben E , Geramita Katharine 
Published: February 2020

The silicon problem

Silicon is the next generation commercial material for lithium ion batteries. With a high capacity and promise to revolutionize both the automotive and consumer electronic industries, silicon-based anodes continue to be a focus for the field. However, silicon remains elusive for substantial market penetration due to high cost, difficult-to-manufacture nanostructures, or poor performing, low cost materials.

Bulk silicon is an extremely cheap commodity. Below $10/kg, the price is on par with if not better than typical commercial graphite. The cost of silicon-based anodes tend to increase exponentially as specialized 3D silicon nanostructures are needed to achieve enhanced stability. The question remains, how do we create a step change capacity, high stability silicon-based anode that is commercially viable?

Carbon as the solution

Regarded as a surface passivation technique or a conductivity enhancer, carbon coatings act as a secondary, supporting role in silicon-carbon anodes. Largely ignored, we have found the carbon to be the critical component within high volume expansion anodes. The carbon helps retain needed electrical contact and prevent capacity fade while keeping high energy density. At EnerG2, we have developed a unique, capacity enhancing carbon backbone that both enables the use of cost effective silicon and is electrochemically active.

The methods employ our state-of-the-art Carbon Technology Platform, based on polymer structural frameworks. This multifaceted development method allows for the synthesis and structural modification of carbons at the polymer level, leading to extremely high purity and application driven properties. EnerG2 uses basic polymer building blocks to encapsulate silicon protecting against capacity fade without significant costs.

A basic demonstration of the anode performance is shown in Figure 1. By embedding micron-sized, commercially available solar-grade silicon into EnerG2 polymer-based amorphous carbon, the stable efficiency pushes the boundaries of 100%. Electrochemically active signals from both the silicon and the carbon are present in the sample, with improved stability beyond pure silicon. The demonstrated capacity is simply controlled through silicon content dispersed within the polymer matrix. The added capacity of the electrochemically active carbon is best seen from the composite performances. Stable capacities of 1500 mAh/g (with respect to both Si and C mass) are seen for composites of 50% silicon loading, significantly higher than 50% of the bare silicon capacity. The capacity boost is attributed to both the advanced amorphous carbon scaffold that alone stores close to 600 mAh/g and to the increased mechanical stability afforded by the polymer-derived framework.

In addition to highlighting work on cost effective, commercially available silicon, EnerG2 has developed advanced silicon-carbon structures using our novel mesoporous and macroporous carbons. Synthesized without the need for complex templates, the pore distribution of the carbon is critical in determining the performance of silicon-carbon anodes. The silicon is either coated or embedded into the pores, creating next generation, scalable, 3D structures.

EnerG2 continues to push the boundaries of polymer-derived structures both at manufacturing scale and in the laboratory for energy storage. We will discuss both the recent advancements in materials development as well as its profound impact on energy storage and electrochemistry.