The Case for Silicon in a Fully Dry Battery System – Jumping into the Future of Battery Technology 

The Case for Silicon in a Fully Dry Battery System – Jumping into the Future of Battery Technology

Silicon (Si) is the last stop on the periodic table to achieve higher battery energy density. And the best way to deploy Si is in a fully dry elastic composite electrolyte chemistry system.  The lithium-ion battery (LIB) at the heart of every EV and mobile device has reached a point of diminishing returns with energy density and performance.  Since the LIB was first commercialized by Sony in 1991 its energy density has increased more than threefold [Figure 1] while costs have dropped by over 97 percent [Figure 2]. But now the innovation curve has flattened, and energy density has become a blocking item for the next generation of mobile devices and EVs.  For example, Apple’s recently announced VR-Pro runs for only two hours using an external battery compared with a full day or more for laptops and wearables.  And Tesla’s electric semi-truck travels only 500 miles compared with diesel trucks that can drive 1500 miles or halfway across the US, on a single refill.   

 

Figure 1:  Lithium-ion cell energy density over time, Royal Society of Chemistry, 2021 [6]

 

Figure 2:  IEEE Spectrum:  Behind the Three-Decade Collapse of Lithium-Ion Battery Costs [5]

 

Transformational change happens with breakthroughs in the use of materials.  Consider the shift from vacuum tubes to transistors in the 1950s and from spinning magnetic hard disk drives to solid-state disk (SSD) memory in the 2000s that sparked the modern computing and mobility revolutions.  Both were the result of innovation with Si and its unique semiconductor properties.  Spinning magnetic disk drives followed a similar flattening maturity cycle to LIB batteries.  Now, many in the battery industry believe that Si will enable the next leap forward in energy storage.  Si is the second most abundant material, after oxygen, at 30% of the earth’s crust, and it has remarkable storage properties enabling thin electrodes required for high energy density.   

But Si is not without its challenges in batteries.  In 2022, Elon Musk stated that “(for)…high energy…you need to change the anode to silicon…”, but he also mentioned that “our highest energy density cells will use…maybe 10% silicon…It’s a small percentage….”[1]  The reason for this small percentage is that Si degrades when cycled using liquid electrolytes.  Like all anodes, a protective solid electrolyte interface (SEI) shell surrounds Si particles, but expansion and contraction of the particles leads to break-up of the SEI shell leading to capacity-fade in the battery.  The industry is working to create engineered (and expensive) silicon-carbon composite materials, but the compromise between high capacity and cycle life remains to be quantified – in addition to high material costs.  

There is growing interest in deploying Si into a fully dry solid-state electrolyte where SEI formation is minimized.  The first research we’re aware of in this area is from Notten in 2007 and it states that “The SEI layer is completely absent and, consequently, the cycle life of the Si electrode is not negatively affected at all.” [2].  More recently findings in 2021 by Tan show that one can deploy large loading of silicon with sulfide electrolytes, “we enabled the stable operation of a 99.9 weight % micro silicon anode by using the interface passivating properties of sulfide solid electrolytes.” [3] With SEI formation controlled there is still the challenge of needing pressure to contain the Si as it expands in an all-solid battery.  This is noted in 2022 by Huo and Janek as follows, “Silicon is one of the most promising anode materials due to its very high specific capacity….  The effects of external pressure on structure evolution and electrochemical performance of composite silicon anodes remain elusive….” [4] 

Blue Current has been working since 2018 to address both challenges through a silicon first approach.  We do this with a fully dry Si elastic composite chemistry. In addition to being dry, thus minimizing issues with SEI formation, it is also adhesive and compliant, enabling the Si to expand and contract at low pressure without losing electrical contact.  At the time of writing, we are testing cells with 50-90% Si active material content in the anode. This is 5-10x higher than state of the art LIB with 5-10% Si, while still enabling the cells to cycle at pressure down to roughly 150 PSI (1 MPa).  We’ve also built cells that cycle hundreds of times at 75 PSI (.5 PSI).  This is higher than LIB pouch cells that require virtually no external pressure.  But over time we plan to continue reducing the need for external pressure to the point where it becomes irrelevant for overall system design.  We’re also working towards the use of raw Si that has the highest energy capacity and lowest cost.  We’re encouraged by the gains that we’ve seen and expect this approach will provide the jump forward in energy storage required for the new phase of electrification and sustainability. 

 

1 – Elon Musk on the Early Days of Tesla, THE KILOWATTS, 2022 

2 – Notten, P. H. et al., 3‐D integrated all‐solid‐state rechargeable batteries. Advanced Materials, 2007; 19(24), 4564-4567. 

3 – Tan, D. et al., Carbon-free high-loading silicon anodes enabled by sulfide solid electrolytes. Science. 2021; 373(6562):1494-1499. 

4 – Huo, Hanyu and Jurgen Janek. Silicon as an Emerging Anode in Solid State Batteries, ACS Energy Letters, 2022; 7, 4005-4016 

5 – Rao, Rahul. Chart: Behind the Three-Decade Collapse of Lithium-Ion Battery Costs. IEEE Spectrum, 2022.

6 – Ziegler, Micah S. and Jessica E. Trancik. Re-examining rates of lithium-ion battery technology improvement and cost decline. Royal Society of Chemistry, 2021; 14, 1635

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