1) The all-solid-state battery consists of a cathode composite layer, a sulfide solid-electrolyte layer and a carbon-free micro-silicon anode. 2) Before charging, discrete micro-scale silicon particles make up the energy-dense anode. During battery charging, positive lithium ions move from the cathode to the anode, and a stable 2D interface is formed. 3) As more lithium ions move into the anode, it reacts with the micro-silicon to form interconnected lithium-silicon alloy (Li-Si) particles. The reaction continues to propagate throughout the electrode. 4) This reaction causes expansion and densification of the micro-silicon particles, forming a dense Li-Si alloy electrode. The mechanical properties of the Li-Si alloy and the solid electrolyte play a crucial role in maintaining the integrity and contact along the 2D interfacial plane. Image: University of California San Diego.Engineers have created a new type of battery that weaves two promising battery sub-fields into a single battery. This battery uses both a solid-state electrolyte and an all-silicon anode, making it a silicon all-solid-state battery.
An initial round of tests showed that the new battery is safe, long lasting and energy dense, and thus holds promise for a wide range of applications, from grid storage to electric vehicles. Nanoengineers at the University of California (UC) San Diego led the research, in collaboration with researchers at LG Energy Solution (LGES). They report the novel battery in a paper in Science.
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 how liquid electrolytes cause them to degrade.
These challenges have kept all-silicon anodes out of commercial lithium-ion batteries, despite their tantalizing energy density. The new work offers a promising path forward for all-silicon-anodes, thanks to the right electrolyte.
"With this battery configuration, we are opening a new territory for solid-state batteries using alloy anodes such as silicon," said Darren Tan, lead author of the paper, who recently completed his chemical engineering PhD at the UC San Diego Jacobs School of Engineering.
Next-generation, solid-state batteries with high energy densities have always relied on metallic lithium as the anode. But metallic lithium places restrictions on battery charge rates and requires elevated temperatures (usually 60°C 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. This represents exciting progress for both the silicon anode and solid-state battery communities.
Silicon anodes 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 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 are paired with. The situation is complicated by the large volume expansion of silicon particles during charge and discharge, which results in severe capacity losses over time.
“As battery researchers, it's vital to address the root problems in the system. For silicon anodes, we know that one of the big issues is the liquid electrolyte interface instability," said UC San Diego nanoengineering professor Shirley Meng, corresponding author of the Science paper and director of the Institute for Materials Discovery and Design at UC San Diego. “We needed a totally different approach.”
So the UC San Diego-led team took a different approach: they eliminated the carbon and the binders that are commonly used with all-silicon anodes. In addition, they used micro-silicon, which is less processed and less expensive than the nano-silicon that is usually used.
In addition to removing all the carbon and binders from the anode, the team also removed the liquid electrolyte, replacing it with a sulfide-based solid electrolyte. Their experiments showed that this solid electrolyte is extremely stable in batteries with all-silicon anodes.
"This new work offers a promising solution to the silicon anode problem, though there is more work to do," said Meng, "I see this project as a validation of our approach to battery research here at UC San Diego. We pair the most rigorous theoretical and experimental work with creativity and outside-the-box thinking. We also know how to interact with industry partners while pursuing tough fundamental challenges."
Past efforts to commercialize silicon alloy anodes mainly focused on silicon-graphite composites, or on combining nano-structured particles with polymeric binders. But they still struggled 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 operates.
At the same time, by eliminating the carbon in the anode, the researchers significantly reduced the interfacial contact (and unwanted side reactions) with the solid electrolyte, avoiding the continuous capacity loss that typically occurs with liquid-based electrolytes. This two-part move allowed the researchers to fully reap the benefits of the low cost, high energy and environmentally benign properties of silicon.
“The solid-state silicon approach overcomes many limitations in conventional batteries,” said Tan. “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 the traditional thermodynamic interpretations used for 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.
Tan is the CEO and co-founder of a start-up, UNIGRID Battery, that has licensed the technology for these silicon all-solid-state batteries. In parallel, related fundamental work will continue at UC San Diego, including additional research collaboration with LGES.
“LG Energy Solution is delighted that the latest research on battery technology with UC San Diego made it onto the journal of Science, a meaningful acknowledgement,” said Myung-hwan Kim, president and chief procurement officer at LGES. “With the latest finding, LG Energy Solution is much closer to realizing all-solid-state battery techniques, which would greatly diversify our battery product line-up.
“As a leading battery manufacturer, LGES will continue its effort to foster state-of-the-art techniques in leading research of next-generation battery cells.”
To this end, LGES said it plans to further expand its solid-state battery research collaboration with UC San Diego.
This story is adapted from material from the University of California, San Diego, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.