Lithium ion batteries are so named due to the ion that travels between the anode and cathode during charging and discharging. Lithium in particular is useful because of the ease in which it gives up it’s electron to the electrochemical reactions that occur and ultimately provide the electricity to power our devices. The main components of a lithium ion battery are current collectors, anode, cathode, separator, and electrolyte. Traditionally, the anode has been graphite (a particular form of carbon), and the lithium ions travel IN-BETWEEN the sheets of graphite during charging, and then back to the cathode during discharge. The Endurion solution addresses specifically the anode.
As advanced are made in lithium-ion technology to support electrification, many in the industry believe silicon to be the next logical step change in battery anode chemistry, owning to its theoretical 10x charge capacity over traditional graphite anodes, low electrochemical potential vs Li/L+, natural abundance, and potential integration into existing manufacturing methods.
However, such developments have been plagued by a lack of mechanical integrity, poor cycling stability, and poor conductivity. Endurion’s approach includes an innovative use of silicon nanostructures, in such a way as to intentionally manipulate resultant solid-electrolyte interphases, and inclusion of carbon to address all of these issues.
Mechanical degradation and pulverization of early silicon anodes largely occurred due to a fundamental difference in the way that graphite and silicon interact with lithium-ions upon charging and discharging. As opposed to traditional “intercalation” chemistry characteristics of graphite anodes, silicon anodes use “allying” chemistry to incorporate lithium ions into its structure, leading to up to a 3-4x expansion and contraction during typical cycling of such anodes.
These repeated mechanical forces ultimately cause the anode to break down and sever contact with the current collector, rendering them useless. Recent research has shown critical length scales below which silicon nanostructures DO NOT crack upon repeated lithiation and de-lithiation.
A solid-electrolyte-interphase (SEI) is a necessary part of any lithium-ion battery and generally forms during the first few charge-discharge cycles. Such a layer passivates the anode surface and prevents further electrolyte degradation. The particular issue with this SEI layer in silicon anodes is the continual formation and degradation of the layer as the silicon swells and contracts during normal battery cycling. (Even silicon nanostructures expand and contract while interacting with lithium – even if they do not ultimately fail by pulverization). As this occurs, more and more lithium is consumed and unavailable for charging and discharging.
As the “SEI issue” is unavoidable for any anode material, and is a SERIOUS problem for any silicon containing material, the industry MUST find solutions if they intend to commercialize and scale such materials. To date, the four main approaches in the industry have been 1) intentional structure design, 2) creation of an artificial SEI layer, 3) optimization of electrolyte via judicious selection of additives, and 4) including a “pre-lithiation” step during the construction of the battery to replace any lithium that is expected to be consumed in an SEI layer. The Coretec Group is working to demonstrate that our unique nanostructured Silicon active anode material with an engineered SEI will significantly lesson the need for specialized electrolyte additives and pre-lithiation.
The unique aspect of the Coretec solution to silicon based anodes is the use of chemistry to create nanoparticles significantly smaller than other nanoparticles currently in use in the industry, which are typically created from mechanical ball-milling of bulk silicon. This bottom-up approach is what allows us to have the precise control over the surface chemistry that will allow us to create our engineered SEI layer.
Finally, while a 10X increase in theoretical charge capacity of silicon over graphite is indicative of the ability to “hold” 10X the amount of lithium-ions, the big stumbling block is the abysmal conductivity of silicon, which is required to “move” the lithium-ions through the silicon structure(s). This has been addressed in the industry via judicious use of carbon in the manufacture of anode slurries, as well as reliance on silicon nanostructures where surface interactions between lithium and silicon are inherently orders of magnitude more pronounced; and bulk effects that depend upon conductivity are less important.
Using our expertise in silicon, The Coretec Group intends to build unique structures and active anode formulations that will incorporate nano-sized length scales, appropriate use of carbon sources, and an SEI layer that can better withstand the typical formation and degradation that occurs with current silicon anode particles.