Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS) researchers have created an improved lithium metal battery that offers faster charging. This battery can be charged and discharged at least 6,000 times, surpassing other pouch battery cells, and can be recharged within minutes.
The study, featured in Nature Materials, not only introduces a new method for producing solid-state batteries with a lithium metal anode but also enhances our understanding of the materials used in these potentially groundbreaking fast batteries.
According to Xin Li, Associate Professor of Materials Science at SEAS and senior author of the paper, “Lithium metal anode batteries are considered the holy grail of batteries due to their tenfold capacity compared to commercial graphite anodes, potentially leading to a significant increase in the driving range of electric vehicles. Our research represents a crucial step towards more practical solid-state batteries for industrial and commercial applications.”
One of the main hurdles in designing these batteries is the formation of dendrites on the anode’s surface. These structures grow into the electrolyte like roots, penetrating the barrier between the anode and cathode, leading to short circuits or even fires.
Dendrites form as lithium ions move from the cathode to the anode during charging, attaching to the anode’s surface in a process called plating. This plating creates an uneven, non-uniform surface, akin to plaque on teeth, allowing dendrites to take hold. When discharged, this plaque-like coating needs to be removed from the anode. Uneven plating during the stripping process can be slow and result in potholes, leading to further uneven plating during the next charge.
In 2021, Li and his team proposed a solution to tackle dendrites by designing a multilayer battery sandwiching different materials of varying stabilities between the anode and cathode. This design prevented the penetration of lithium dendrites by controlling and containing them.
In this new research, Li and his team prevent dendrite formation by using micron-sized silicon particles in the anode to restrict the lithiation reaction and enable homogeneous plating of a thick layer of lithium metal.
In this design, when lithium ions move from the cathode to the anode during charging, the lithiation reaction is restricted at the shallow surface, with the ions attaching to the surface of the silicon particle without penetrating further. This differs significantly from the chemistry of liquid lithium-ion batteries, in which the lithium ions penetrate through a deep lithiation reaction, ultimately destroying silicon particles in the anode.
In a solid-state battery, the ions on the surface of the silicon are constrained and undergo the dynamic process of lithiation to form lithium metal plating around the core of silicon.
“In our design, lithium metal envelops the silicon particle, similar to a hard chocolate shell around a hazelnut core in a chocolate truffle,” explained Li. These coated particles create a uniform surface across which the current density is evenly distributed, preventing dendrite growth. Moreover, because plating and stripping can occur quickly on an even surface, the battery can recharge in approximately 10 minutes.
The researchers developed a postage stamp-sized pouch cell version of the battery, which is 10 to 20 times larger than the coin cell typically made in university labs. The battery retained 80% of its capacity after 6,000 cycles, outperforming other pouch cell batteries on the market today. The technology has been licensed through the Harvard Office of Technology Development to Adden Energy, a Harvard spinoff company cofounded by Li and three Harvard alumni. The company has scaled up the technology to produce a smartphone-sized pouch cell battery.
Li and his team also characterized the properties that allow silicon to restrict the diffusion of lithium, facilitating the dynamic process favoring homogeneous plating of thick lithium. They then defined a unique property descriptor to describe such a process and computed it for all known inorganic materials. In doing so, the team identified dozens of other materials that could potentially yield similar performance.
“Previous research had found that other materials, including silver, could serve as good materials at the anode for solid-state batteries,” said Li. “Our research explains one possible underlying mechanism of the process and provides a pathway to identify new materials for battery design.”
