For the first time, cryo-electron microscopy (cryo-EM) has been used to study the most challenging material in lithium batteries – the lithium metal anode. Previously, it was impossible to study lithium and its interfaces with high-resolution electron microscopy because of the severe beam damage resulting from high electron doses. By using cryogenic conditions to prevent such damage, this international investigation across the United States, China and Germany reveals how lithium dendrites that cause battery failure form. They also show how tailoring electrolyte composition affects the structure of the solid-electrolyte interface.
Lithium-ion batteries have proven to be the most effective and commercially relevant power sources in electric vehicles and portable electronic devices. Current technology utilizes a metal oxide cathode, and an organic (carbonate-based) electrolyte and graphitic carbon anode. Researchers have focused on stabilizing the Li metal anode to improve the capacity from that of carbon (372 mAh/g) to that of lithium (Li) metal (3862 mAh/g), an order of magnitude increase. However, utilizing lithium metal as anodes comes with safety and practical issues.
During cycling, lithium does not deposit flat on the anode surface, rather it deposits long dendrites that can puncture the separator, causing short-circuits between the anode and cathode. These short-circuits generate heat, triggering device failure or possibly fires. In a recent article published in Science, this international team led by Stanford University scientist Yi Cui used cryo-EM to try and understand the formation of these Li dendrites. They gained new insights into the crystallographic planes Li dendrites will form along, and their interface in carbonate-based electrolyte compositions.
Taking a snapshot of lithium metal in a battery
Batteries are dynamic devices, constantly changing and undergoing multiple chemical reactions simultaneously. To preserve the chemical state of the lithium metal anode, Cui and his team had to borrow some strategies from biology. First, Li metal was deposited on a Cu metal electron microscopy grid in a battery system comprised of Li ions in the electrolyte, which form the dendrites. That grid was removed from the device and flash-frozen in liquid nitrogen, which preserved the relevant structural and chemical information. Transfer and flash-freezing was difficult due to the chemical reactivity of Li metal. However, by maintaining an internal temperature of ~–170 ºC in the cryo-EM vessel, the Li metal remained stable.
How dendrites grow
The exact structure of the Li dendrites has been difficult to determine since only low-resolution transmission electron microscopy (TEM), indirect-imaging and surface-sensitive techniques could be used to study the dendrites. The detailed nanostructure evaluated by the cryo-EM has exposed that the dendrites grow along the <111>, <211> and <110> crystallographic facets, with approximately 50% of those dendrites growing along the <111> direction. The major growth direction is rationalized from the low surface energy of that particular facet. The other two facets, <211> and <110>, also have a low surface energy. They switch at “kinks” in the dendrites and change their growth direction while maintaining crystallinity throughout the structure. These “kinks” could be heavily influenced by the solid-electrolyte interface (SEI).
Electrolyte composition tailors anode interface
The surface–electrolyte interface (SEI) on the Li anode is caused by electrolyte decomposition at the anode surface arising from the low potentials reached during the battery’s operation. This SEI consists of organic and inorganic (Li2CO3 and Li2O) components, which can prevent Li ion’s diffusion from the anode surface to the electrolyte. Excessive SEI build-up will create an insulating layer around the electrode, degrading battery capacity.
The nanostructure of the Li-ion SEI was explored for the first time with cryo-EM, showing that in a standard electrolyte (ethylene carbonate and diethyl carbonate) mixture, the SEI forms with a randomized distribution of organic and inorganic composition. However, with a common fluorinated additive (flouroethylene carbonate) in the electrolyte, the SEI formed is structured with the organic layer on the Li metal crystallographic surface and the inorganic SEI only formed on the organic surface. The fluorinated additive in the electrolyte has been studied extensively in battery literature and has proven to form a more stable surface, which elevates device operation throughout the battery’s lifetime. The work by Cui and colleagues is the first study to show that this surface could be the result of the ordered multilayer formation unique to this additive.
The future of the Li metal anode
The Li metal anode suffers greatly from the dendrite and SEI formation, but its theoretical capacity is an order of magnitude higher than current technology, making it an extremely desirable battery material. This information on nanostructure only available from cryo-EM is critical to the progress of state-of-the-art battery technology and uncovers a new opportunity to directly tune crystallographic facets of Li metal or SEI heterostructure to overcome anode degradation and potentially make Li metal a viable option for future energy storage devices.
Full details are reported in Science.
