Developments toward higher capacity, longer lasting rechargeable batteries have been ongoing well before the march of the Energizer® Bunny. Indeed, the iconic notion of a battery that "keeps going and going and going" is increasingly being realized, especially with continual improvements in lithium-ion battery technologies.
At the Center for Electrical Energy Storage (CEES), researchers discovered a route to develop a new generation of lithium-ion battery materials. These batteries could essentially store more charge and retain it after extended charge/discharge cycles.
Traditionally, lithium-ion battery electrodes are built from intercalation materials, which are often layered crystal structures into which lithium ions can be reversibly inserted and extracted. These electrodes exhibit structural reversibility, because they maintain their layered structures throughout lithium-ion insertion (lithiation) and extraction (delithiation) processes. However, electrodes built from intercalation materials have low lithium-ion capacities, meaning the batteries have low charge capacities.
On the other hand, electrode frameworks built from compounds that alloy with lithium (such as silicon) provide batteries with much higher charge capacities. However, purely silicon-based electrodes lack the structural reversibility of intercalation-based electrodes. The silicon-based electrodes undergo dramatic volume and structural changes during lithiation and delithiation, which consequently deteriorate the electrode and reduce its charge capacity after repeated uses.
To achieve the best of both worlds, the team at CEES, led by Tim Fister, constructed a multilayer electrode composed of alternating silicon (Si) thin films and chromium silicide (Cr3Si) layers. The result was a Si/Cr3Si multilayer that combined the structural reversibility of an intercalation material with the high charge capacity provided by silicon.
"These multilayers allow us to control the direction of the expansion, which can help prevent issues—such as cracking and separation of the layers—that have previously plagued silicon electrodes," said Fister.
The team conducted real-time X-ray reflectivity studies to measure the volume and structure changes during lithiation and delithiation. Data from these studies revealed that the Si/Cr3Si multilayer expanded and contracted 3.3-fold vertically, but maintained its layered structure despite repeated volume changes. "We were surprised by how uniform and reversible the changes were," said Fister.
Additionally, electrochemical studies demonstrated the stability of the Si/Cr3Si multilayer throughout repeated charging and discharging cycles. Compared to a silicon film of identical thickness, the Si/Cr3Si multilayer retained a high charge capacity even after 1,000 cycles.
"This research could head in a variety of directions. For example, we're interested in using this structural reversibility to develop new design rules for the battery electrodes. The layered architecture could also be applied to other current collector morphologies, such as wires or spheres," explained Fister.
