Influence of Binders on the Structure and Performance of Anodes in Lithium-Ion Batteries

Summary:

Binders and their behaviour as inactive components in anode suspensions, electrodes and their electrochemical performance are still unpredictable, requiring detailed characterization along the process chain (from material to cell performance). In this study two different thickening binders were used: Carboxymethylcellulose (CMC) and Alginate (ALG). While CMC varies in molar mass, ALG varies in M/G ratio and thus in molecular structure. Both binder systems were processed and analyzed rheologically to evaluate their effect in anode suspensions and correlate their behavior to the performance of electrodes.
All solutions show a shear thinning effect. While CMC has different viscosities, ALG shows hardly any deviations. Regarding the Mark-Houwink relationship the molar masses of CMC are different while ALG indicates similar molar masses. To understand the polymer behaviour in application of Lithium-Ion Batteries the polymer solutions consisting of 93 wt.-% artificial graphite, 1.4 wt.-% carbon black (CB), 2.8 wt.-% styrene-butadiene rubber (SBR) and 2.8 wt.-% of the variating binder are manufactured discontinuously to electrodes in industry-related processes. Regarding comparability, parameters like recipe and manufacturing processes of the coating are kept constant. After the dispersion process the suspensions where rheologically characterized with a rotation and oscillation test and are compared to their rheological behaviour in pure binder solutions.
While CMC shows a homogeneous distribution and no interaction with SBR, ALG exhibits more pronounced interactions with CB particles and cross-linking effects with SBR. This behaviour can be seen in the resulting electrodes as well. ALG interacts with CB to form binder-CB-clusters which can be observed in SEM-images. The ALG electrodes exhibit low adhesion, high plastic deformation behaviour and a high electrical conductivity and high residual moisture due to the inhomogeneous distributed clusters. Regarding mass loading, the electrochemical performance of ALG is quite good (152 mAh/gNCM622 @ 0.1C) but has a high quote of cell failure due to low adhesion and high plastic deformation. CMC shows a homogeneous distribution via SEM-images less significant binder-CB-clusters than ALG. With an increase in molar mass the resulting electrodes display higher adhesion, higher elasticity and a higher residual moisture. Although the electrochemical performance of CMC with low molar mass is the best (152 mAh/gNCM622 @ 0.1C; SOH80 ≈ 830 Cycles) but the high adhesion and good elasticity of CMC with high molar mass are beneficial for the usage in C/Si-anodes. Linking the rheological behaviour of binders in suspensions to the performance of electrodes is a possible method to evaluate system-characteristics at the beginning of the process chain.