Benchmark Mineral Intelligence provides advisory services for lithium-ion battery, electric vehicle and energy storage supply chains. The electric vehicle and battery cell supply chain is its sole focus and specialty. It also publishes an informative well-written quarterly review.
The Q2 2019 issue features an article titled “Silicon’s Rally,” by Paul Harris. The introduction reads, “Battery anode materials are made predominantly from natural graphite or synthetic graphite. But battery technology rarely stands still, and pure silicon anodes promises to offer greater energy density in batteries. Paul Harris explores the advances in silicon anode technology, the companies developing them, and the obstacles to adoption.”
You can read the complete article here. Following are selected excerpts.
MARKET SIZE AND GROWTH
Projections from Persistence Market Research see lithium-silicon batteries growing in dominance through 2024 and beyond. It said the global silicon anode battery market was valued at US$90 million in 2015 and is anticipated to register a compound annual growth rate of 21.5% in the 2016–2024 period.
Portable consumer devices like smartwatches, smartphones and tablets will be the first products to benefit from the commercialisation of silicon anodes, as they are less price sensitive. “We are focused on wearables, smartphones and notebooks where we can have a significant impact on performance and establish ourselves and work on bringing our production costs down. These are less price sensitive markets compared to EVs,” said Bruce Pharr, senior director of marketing at Enovix.
Instead of looking at the materials per se, Enovix has looked at reworking battery architecture to overcome the mechanical issues of silicon that way. Murali Ramasubramanian, Enovix co-founder and vice president of research and development, and other company co-founders had previously used 3D architecture to increase density for read-write heads of high-density disc drives and high-density semiconductor test products and thought it could be applied to batteries to increase energy density. It has developed products that it is now bringing to market in consumer electronics.
“We solved the issue by adding an integrated constraint to the cell. We add a stainless-steel constraint that prevents silicon in the anode from cracking. This provides pressure which limits expansion and maintains the silicon’s contact with the collector, which stops over discharge and breakage,” he told Benchmark Minerals.
The constraint applies pressure to inhibit silicon swelling. A square millimetre of silicon needs about 1 kilogram-force (kgf) to push back the expansion and maintain connection during discharge.
This created another issue. “A 20x20mm conventional lithium-ion battery needs 400 kilogram-force and a cell phone battery needs over 2,000 kilogram-force,” he said. To apply force without overly adding to the weight and size of a device, Enovix again looked at the architecture. “Instead of building batteries with electrode and separator sheets layered bottom to top we structure them left to right. The 90-degree change in orientation allows this to work as the anode has a smaller surface area. It is an elegant solution that looks really obvious in hindsight,” said Ramasubramanian.
While the constraint takes up some volume, high-capacity silicon provides an anode with 97% active material (3% is binder and conductive aids), which more than makes up for the constraint volume.
Enovix says its EX1 wearable cell gets a 46% increase in energy density and 38% for its smartphone cell over a conventional lithium-ion cell and is achieving around 600 cycles for its wearable watch cell and is confident it can get above 1,000 in the future. “We see less than 2% swelling after 500 cycles. Even graphite swells 6-7% at the end of its life, so silicon is much better than graphite cells because of the constraint. We have done enough testing to know that our EX2 and EX3 cells [due for production in 2020 and 2021, respectively] will get 74-105% more energy density,” said Ramasubramanian.
“To put this in context, since 1992, there has been about a 5% per year increase in energy density in lithium-ion batteries. With Enovix technology we can get 30 to 50 to 100% increases over the next few years,” said Enovix’s Pharr.
ELECTRIC VEHICLE MARKET
For Enovix, the auto industry still needs to provide guidance of where it wants to be and what it wants to achieve. “There is a lot of excitement about the EV market and it is going to be the biggest. But, unlike with mobile devices, the battery manufacturers and auto manufacturers have not yet worked out exactly where they want to be in terms of packaging, size, weight, cost, duration and other factors. This will need to be jointly determined and developed. There is a tipping point in 2024-2025 but what that is going to look like is still not clear to the industry,” said Enovix’s Pharr. The target to drive mass-market adoption could be something like an average, compact SUV costing US$28,000 which gets 350-400 miles on a charge and can fully recharge in 1-2 hours.