Visualizing the Carbon Binder Phase of Battery Electrodes in Three Dimensions

Sohrab R Daemi,Tobias Volkenandt,Dan J L Brett,James Cookson,Samuel J Cooper,Anna Palacios-Padros,Chun Tan,Paul R Shearing

doi:10.1021/acsaem.8b00501

Abstract

This study presents a technique to directly characterize the carbon and binder domain (CBD) in lithium-ion (Li-ion) battery electrodes in three dimensions and use it to determine the effective transport properties of a LiNi0.33Mn0.33Co0.33O2 (NMC) electrode. X-ray nanocomputed tomography (nano-CT) is used to image an electrode composed solely of carbon and binder, whereas focused ion beam–scanning electron microscopy is used to analyze cross-sections of a NMC electrode to gain morphological information regarding the electrode and CBD porosity. Combining the information gathered from these techniques reduces the uncertainty inherent in segmenting the nano-CT CBD data set and enables effective diffusivity of its porous network to be determined. X-ray microcomputed tomography (micro-CT) is then used to collect a NMC data set that is subsequently segmented into three phases, comprised of active material, pore, and CBD. The effective diffusivity calculated for the nano-CT data set is incorporated for the CBD present in the micro-CT data set to estimate the ensemble tortuosity factor for the NMC electrode. The tortuosity factor greatly increases when compared to the same data set segmented without considering the CBD. The porous network of the NMC electrode is studied with a continuous pore size distribution approach that highlights median radii of 180 nm and 1 μm for the CBD and NMC pores, respectively, and with a pore throat size distribution calculation that highlights median equivalent radii of 350 and 700 nm.

Highlights

Li-ion batteries have emerged as the preferred choice for energy storage applications that require high energy and power densities, such as consumer electronics and electric vehicles (EVs).[1]
The finer carbon and binder domain (CBD) porosity is thought to be formed by gaps between carbon clusters as well as finer micro- and mesopores within the carbon particles that cannot be detected with the FIB-scanning electron microscopy (SEM) resolution
The nano-CT data set was used to provide a representative carbon and binder volume containing a characteristic porous network, whereas the focused ion beam−SEM (FIB-SEM) data were used to determine the porosity of the CBD and its morphology with regard to the overall electrode structure

Summary

Introduction

Li-ion batteries have emerged as the preferred choice for energy storage applications that require high energy and power densities, such as consumer electronics and electric vehicles (EVs).[1]. The performance of a Li-ion battery is intrinsically linked to the microstructure of its electrodes. The electrochemical reactions within these devices occur in porous electrodes comprised of active material particles, typically a transition metal oxide, mixed with conductive carbon and binder and coated on a metallic current collector.[2] The conductive carbon and binder forms a porous network around the active material particles, known collectively as the CBD. The main function of the CBD is to aid electrical conduction from the current collector to the active material particles, while the binder ensures structural integrity and good electrical contact of the electrode through adhesion with the current collector.[2] At the same time, ionic conduction occurs through the electrolyte-filled pore space and within each of the active particles.[3]

Methods

Results

Conclusion