Engineers created a new type of battery that weaves two promising
battery sub-fields into a single battery. The battery uses both a solid state electrolyte
and an all-silicon anode, making it a silicon all-solid-state battery. The initial rounds of
tests show that the new battery is safe, long lasting, and energy dense. It holds
promise for a wide range of applications from grid storage to electric vehicles.
The battery technology is described in the journal Science. University of California San Diego
nanoengineers led the research, in collaboration with researchers at LG Energy Solution.
Silicon anodes are famous for their energy density, which is 10 times greater than
the graphite anodes most often used in today's commercial lithium ion batteries.
On the other hand, silicon anodes are infamous for how they expand and contract as the battery
charges and discharges, and for how they degrade with liquid electrolytes.
These challenges have kept all-silicon anodes out of commercial lithium ion batteries despite the
tantalizing energy density. The new work published in the journal Science provides a promising path
forward for all-silicon-anodes, thanks to the right electrolyte.
Next-generation, solid-state batteries with high energy densities have always
relied on metallic lithium as an anode. But that places restrictions on battery charge
rates and the need for elevated temperature (usually 60 degrees Celsius or higher) during
charging. The silicon anode overcomes these limitations, allowing much faster
charge rates at room to low temperatures, while maintaining high energy densities.
The team demonstrated a laboratory scale full cell
that delivers 500 charge and discharge cycles with 80% capacity retention at room temperature,
which represents exciting progress for both the silicon anode and solid state battery communities.
Silicon anodes, of course, are not new. For decades, scientists
and battery manufacturers have looked to silicon as an energy-dense material to mix into,
or completely replace, conventional graphite anodes in lithium-ion batteries.
Theoretically, silicon offers approximately 10 times the storage capacity of graphite.
In practice however, lithium-ion batteries with silicon added to the anode to increase
energy density typically suffer from real-world performance issues: in particular, the number of
times the battery can be charged and discharged while maintaining performance is not high enough.
Much of the problem is caused by the interaction between silicon anodes and the liquid electrolytes
they have been paired with. The situation is complicated by large volume expansion
of silicon particles during charge and discharge. This results in severe capacity losses over time.
Indeed, the UC San Diego led team took a different approach: they eliminated the carbon
and the binders that went with all-silicon anodes. In addition, the researchers used micro-silicon,
which is less processed and less expensive than nano-silicon that is more often used.
In addition to removing all carbon and binders from the anode, the team also removed
the liquid electrolyte. Instead, they used a sulfide-based solid electrolyte. Their experiments
showed this solid electrolyte is extremely stable in batteries with all-silicon anodes.
Past efforts to commercialize silicon alloy anodes mainly focus on silicon-graphite composites,
or on combining nano-structured particles with polymeric binders.
But they still struggle with poor stability.
By swapping out the liquid electrolyte for a solid electrolyte,
and at the same time removing the carbon and binders from the silicon anode,
the researchers avoided a series of related challenges that arise
when anodes become soaked in the organic liquid electrolyte as the battery functions.
At the same time, by eliminating the carbon in the anode, the team significantly reduced the
interfacial contact (and unwanted side reactions) with the solid electrolyte,
avoiding continuous capacity loss that typically occurs with liquid-based electrolytes.
This two-part move allowed the researchers to fully reap the benefits of low cost,
high energy and environmentally benign properties of silicon.
The researcher says that the solid-state silicon approach overcomes many limitations
in conventional batteries. It presents exciting opportunities for us to meet market demands for
higher volumetric energy, lowered costs, and safer batteries especially for grid energy storage.
Sulfide-based solid electrolytes were often believed to be highly unstable. However, this was
based on traditional thermodynamic interpretations used in liquid electrolyte systems,
which did not account for the excellent kinetic stability of solid electrolytes. The team saw an
opportunity to utilize this counterintuitive property to create a highly stable anode.