(39b) Localized States-of-Charge in Battery Electrodes: Using Microscopic X-Ray Diffraction Data to Solve Current Distributions

Batteries, like many electrochemical devices, are primarily controlled using readings of their current (in A) or voltage (in V). Current and potential are macroscopic quantities, with a single value read for each electrode by leads connected to the current collectors. However, in reality, there are localized values of current and potential within electrochemical systems. Sometimes these localized values are uniform, meaning the macroscopically-read values are representative of the entire system. But in many cases, current and potential are heterogeneous, meaning there is a current distribution. "Current distribution" is the term in electrochemistry for a non-uniform reaction rate on or in an electrode. Localized current and potential values are microscopic, and thus require microscopic techniques to quantify them. This talk will address the practice of coupling macroscopic current and voltage data to microscopic materials characterization data. In this way, heterogeneity of the electrochemical reaction can be understood. It will provide an overview of solid-state batteries, X-ray diffraction, and the theory of current distributions.

This topic of reaction heterogeneity is highly relevant for batteries. With batteries, thickening electrodes is beneficial because it increases the battery specific energy density (Wh/kg). However, increasing an electrode length scale also increases the chance of a current distribution. In a battery this could have negative consequences, because if the electrochemical reaction is heavily localized, it means the active material experienced a higher rate of use than one would assume from the macroscopic current. This causes a stress on the material and can lead to early failure. The cathode active material of Li-ion batteries is typically a particulate intercalation host, such as NMC111 which has the electrochemical reaction below:

Li1Ni1/3Mn1/3Co1/3O2 ⇌ Li1-xNi1/3Mn1/3Co1/3O2 + xLi+ + xe-

NMC111 is a layered transition metal oxide, in which crystalline slabs based on MO6 (M = Ni, Mn, and Co) are separated by an interlayer space. This interlayer space is where Li+ is stored or intercalated during battery discharge. During battery charging, the reaction proceeds to the right as the NMC111 is delithiated. As Li+ is deintercalated, it moves out of the interlayer, and this affects the crystal structure. Since Li+ no longer screens the charge of the negative oxygen atoms on the slabs, the slabs are forced apart from each other. If the lattice parameters of the NMC111 are known, the value of c/a is correlated to the Li content (1−𝑥), and this is essentially a localized state-of-charge value. This makes it possible to know the local extent of reaction x, which in a non-uniform system will be different than the global extent of reaction calculated from the Ah of capacity charged or discharged.

Acknowledgments

This work was supported by the National Science Foundation under Award Number CBET-ES-1924534 and IUCRC Phase I: Northeastern University: Center for Solid-State Electric Power Storage (CEPS), Award number 2052796.