Exploring the Molecular Weight Effects of Poly(isobutylene) (PIB) in Slurry-Cast NMC811 Cathode for the Development of Sulfide Solid-State Batteries
Authors: Li Yuanshun , Williams Ella , Su Yi-Feng , Nanda Jagjit , Yang Guang 
Published: December 2024

Sulfide-based solid-state battery emerges as the next generation of traditional lithium-ion battery with liquid electrolyte due to high room temperature ionic conductivity of sulfide electrolyte (eg. Halide substituted argyrodite, LPSCl at 1 mS/cm) and inherent safety. However, the production of high-performance thin film cathodes continues to be a significant hurdle. Such cathodes are typically processed by mixing cathode active material (CAM) and solid electrolyte, with an optimal ratio of 50 vol% to ensure the ionic percolation1, followed by tape casting. To fulfill the manufacture purpose, binder was often introduced in the cathode to further improve its mechanical integrity for large scale battery’s production. However, binder in cathode designs impede cell performance due to their non-conductive nature, blocking the pathways for electron and ion flow, as evidenced in many reports2-4.

Herein, we assess the impact of Poly(isobutylene) (PIB), a non-polar binder, with different molecular weights (low, medium, and high kg/mol values, denoted as L_PIB, M_PIB, H_PIB) at 2 wt% loading on the mechanical strength and electrochemical performance of slurry-cast LiNi0.82Mn0.07Co0.11O2 (NMC811) cathodes in full cells. Prior study indicated molecular weight of binder affects the structural cohesion of the electrodes and cycling performance5. In this study, we use slurry-casting to process our cathodes instead of pellet-type traditional dry pressing because its scalability and its ability to ensure a uniform distribution of active materials. Our full cell testing reveals that the cell with H_PIB-cathode exhibits ~44% of discharge capacity in average higher than L_PIB-cathode at discharging rate of 1.6 mAh cm-2. This possibly indicates H_PIB-cathode possesses harden-straining property which allows strong adhesion between NMC 811 and LPSCl particles. H_PIB-cathode outperforms L_PIB-cathode in terms of capacity retention (88% over 70 cycles vs. 77% over 60 cycles), which is potentially attributed to higher molecular weight PIB in cathode protecting the crack formation over cycling.

These electrochemical results are further elucidated by: 1) Galvanostatic intermittent titration technique (GITT) to quantify the apparent diffusion coefficient in cathode; 2) Scanning electron microscopy (SEM) to examine the particles distribution and morphology; 3) Raman spectroscopy (RS) to detect the change of chemical composition in cathode interphase.

Acknowledgement

This research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) and is sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) in the Vehicle Technologies Office (VTO) through the Advanced Battery Materials Research (BMR) Program, managed by Drs. Simon Thompson and Tien Duong. EW would acknowledge the support from the Advanced Materials and Manufacturing Technologies Office (AMMTO) of the EERE through its summer internship program. SEM and AFM research was conducted as part of a user project at the Center for Nanophase Materials Sciences (CNMS), which is a US Department of Energy, Office of Science User Facility at Oak Ridge National Laboratory. This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. We thank Dr. Thomas A. Zawodzinski, Dr. Anna Mills and Dr. Daniel Hallinan for fruitful discussions.

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