University of California San Diego nanoengineers, in collaboration with researchers at LG Energy Solution, have developed a battery that combines a solid-state electrolyte and an all-silicon anode. 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.

From left:
(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 micro-Silicon to form interconnected Lithium-Silicon alloy (Li-Si) particles. The reaction continues to propagate throughout the electrode.
(4) The 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 have a crucial role in maintaining the integrity and contact along the 2D interfacial plane.

Silicon anodes are notable for their energy —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 promise of energy density.

With this battery configuration, we are opening a new territory for solid-state batteries using alloy anodes such as silicon.

—Darren H. S. Tan, lead author

Tan recently completed his chemical engineering PhD at the UC San Diego Jacobs School of Engineering and co-founded startup UNIGRID Battery that has licensed this technology.

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—exciting progress for both the silicon-anode and solid-state battery communities.

The UCSD-led team eliminated the carbon and the binders that usually go 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, using a sulfide-based solid electrolyte instead. 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.

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.

Resources

• Tan Darren H. S., Chen Yu-Ting, Yang Hedi, Bao Wurigumula, Sreenarayanan Bhagath, Doux Jean-Marie, Li Weikang, Lu Bingyu, Ham So-Yeon, Sayahpour Baharak, Scharf Jonathan, Wu Erik A., Deysher Grayson, Han Hyea Eun, Hah Hoe Jin, Jeong Hyeri, Lee Jeong Beom, Chen Zheng, Meng Ying Shirley (2021) “Carbon-free high-loading silicon anodes enabled by sulfide solid electrolytes” Science doi: 10.1126/science.abg7217