(Student Research Award of the Battery Division) The Importance of Microstructure in Lithium Ion Batteries

While the performance of lithium ion batteries (LIBs) has steadily increased over the past two decades, improving energy density, rate capability, life, and safety are still challenges, particularly for the deployment of LIBs in electric vehicles. In this talk, we will (1) identify physical origins of processes that limit battery performance at both the materials and microstructural level, and (2) use these findings to develop design guidelines and new strategies to address these fundamental performance limitations.

Battery performance is governed by electrical, chemical, physical, and mechanical processes, which are hard to decouple. Understanding these interrelated phenomena in a quantitative way is now becoming possible due to advances in imaging technologies and numerical techniques. We perform synchrotron radiation x-ray tomographic microscopy at the TOMCAT beamline of the Swiss Light Source to obtain spatially resolved chemical and structural information of lithium ion batteries.

Novel, high energy density materials such as those undergoing conversion and/or alloying reactions typically suffer from short lifetime due to large volume expansion and contraction during battery operation leading to particle fracture. Understanding and mitigating these processes is a key area of research. The fast acquisition times on the order of minutes possible at the TOMCAT beamline enable operando imaging of LIBs during electrochemical operation. Using tin-(II)-oxide (SnO) as a model system, we observe a core-shell process, quantify volume expansion, witness particle fracture, and correlate crack initiation and growth to crystallographic plane defects in crystalline particles. The insights into electrode structure and material degradation during cell operation highlight the problems this class of materials faces and guides synthesis of novel materials and optimization of electrode preparation techniques.

LIBs with high electrode loading (i.e. thick electrodes) are favorable because the fraction of electrochemically inactive material is kept to a minimum and the energy density of the cell is maximized while the cost is minimized. However, the rate performance of LIBs depends on electrode thickness, porosity and tortuosity. Manufacturers must thus compromise between energy density and speed. Using a combination of experimental evidence based on 3D reconstructions of electrodes from tomographic data and simulations, we show that electrode tortuosity (a parameter that measures the effective path length of ion transport in the electrolyte) plays a key role in governing the rate performance of natural graphite electrodes where the platelet shape of natural graphite causes strong tortuosity anisotropy and large tortuosity values in the direction perpendicular to the current collector. To overcome these limitations, we present a method to engineer the microstructure of electrodes to control and reduce tortuosity. The improved effective lithium ion transport through the porous electrode can be leveraged to create faster batteries, or to fabricate thicker, cheaper electrodes without compromising speed.