To make energy sustainability a reality, batteries are being designed to replace fossil fuels in electric and hybrid-electric vehicles with fast charging, long life, and high capacity in mind. To understand how to effectively create this technology, a team at the Center for Mesoscale Transport Properties (m2M), an Energy Frontier Research Center, looked inside a battery while it was operating to discover how certain materials are able to charge quickly. Studying how certain materials, such as lithium titanate, accommodate fast charging without major degradation has the potential to expedite the use of electric vehicles by allowing batteries to fully charge as fast as it takes to pump a tank of gasoline.
Until recently, when scientists tried to understand why a battery failed, they ripped the battery apart, ground up its components, and tried to figure out what happened. This would be the equivalent of sawing your car in half only to discover you needed an oil change. These types of analysis (referred to as ex situ) are commonplace and prone to error from the deconstruction process. Therefore, scientists at m2M designed a battery with a specialized window that allows X-rays to pass through the battery during its operation (in situ analysis). These high-intensity X-rays interact with the materials inside that battery and provide fingerprints that identify the structural changes that are occurring. The fingerprints for the lithium titanate material are interesting because, unlike current battery materials, lithium titanate can cycle for longer with very little structural changes or degradation over its lifetime.
Lithium titanate contains lithium, titanium, and oxygen atoms that make up its crystal structure, as shown in the image below. This structure is defective because the atoms are not densely packed together. In lithium-ion batteries, moving lithium in a material is very important to obtain the optimal device performance, and this defective structure allows lithium to move freely throughout the material. What eluded scientists for so long was WHY the lithium moved freely in the structure with very little displacement of the titanium and oxygen atoms.
This m2M team found out how lithium traveled by monitoring the subtle movements of titanium. For example, most materials with densely packed atoms need to move atoms around to accommodate lithium; however, the movement of titanium in lithium titanate is very small. This accommodation mechanism is like squeezing a new book on an already filled shelf, you could either move current books off one shelf to another, or you could just squeeze in the additional book by only slightly moving the books closest to the new addition. In batteries, large rearrangements (or reordering from one shelf to another) can cause slow degradation over time and reduce the lifetime of the system. However, an incredible feature of lithium titanate is that lithium tends to surround the titanium and oxygen atoms and restricts large rearrangements, often observed in other materials. This observation shows that lithium stabilizes the titanium atoms during charging and discharging of the battery.
The ability of lithium to move freely in the material and restrict titanium and oxygen reordering allows lithium titanate to provide stable battery operation over many cycles. Without the battery window, the path traveled by lithium during battery operation would still be a mystery. Designing materials where lithium movement stabilizes other atoms in the structure could pave the way for quick charging and long lifetime for the future of electric vehicle battery technology.
