How Does the Rheology of Electrode Suspension Impact the Performance of Li-Ion Batteries?

By Samudrapom DamReviewed by Susha Cheriyedath, M.Sc.Aug 29 2022

A paper recently published in the journal Electrochemistry Communications demonstrated the connection between slurry rheology and the electrochemical performance of graphite anodes used in lithium-ion batteries (LIBs).

Study: On the Connection Between Slurry Rheology and Electrochemical Performance of Graphite Anodes in Lithium-ion Batteries. Image Credit: petrmalinak/Shutterstock.com

Background

Rechargeable lithium-ion batteries (LIBs) are used extensively in different mobile energy storage applications owing to their high energy density compared to alternative energy storage devices. Graphite is typically utilized as an anode material in LIBs as lithium ions can reversibly intercalate with the stoichiometry of stage-1 graphite intercalation compound/LiC₆.

Carbon black (CB) is added as an electron conductive agent to LIB electrodes to promote electron conductivity. In an electrode fabrication process, a slurry containing an active material, solvent, binder, and conductive CB is deposited on a current collector using a slot-die coating or doctor blade coating technique.

Subsequently, the coated slurry with t₁ thickness is oven-dried to remove the solvent, and the obtained coated electrode with t₂ thickness is assembled into cells after calendaring and sizing.

Typically, slurry rheology is defined by measuring the viscosity and loss and storage moduli as a function of shear rate and frequency, respectively. 100 – 500 s^-1 shear rates are often used in the slurry deposition processes on the current collector. The CB morphology significantly influences the slurry rheology.

Currently, LIB anodes are fabricated from a slurry containing small amounts of conductive CB, graphite, n-methyl-2-pyrrolidone (NMP), and polyvinylidene fluoride (PVDF). The rheology of this slurry is a crucial indicator of its microstructure that affects the dried electrode structure, which influences the anode electrochemical performance.

The Study

In this study, researchers modulated the electrode/anode slurry rheology by selecting three different conductive CBs used in LIB electrodes and investigated the connection between the LIB anode electrochemical performance and slurry rheology.

PVDF, NMP, graphite, and three different commercially available conductive CBs, including LITX-50, Timcal Super C 65, and Timcal Super P, were used for the preparation of anode slurries. The specific surface area of CBs used in this study varied significantly.

The Brunauer, Emmett, and Teller (BET) specific surface areas of the Super P, Super C 65, and LITX-50 were 63.4 m²/g, 58.9 m²/g, and 54.4 m²/g, respectively. Additionally, the oil absorption numbers of the Super P, Super C 65, and LITX-50 were 1.65 ml/g, 6.40 ml/g, and 6.40 ml/g, respectively.

Initially, PVDF was mixed with NMP at a ratio of 1:9, and the mixture was vortexed for 30 min at 3000 rpm in a closed vial. The as-obtained mixture was left for 12 h to dissolve the PVDF, and the CB and graphite were then added to the solution. The final composition of the prepared suspension was 50 wt.% NMP, 3.5 wt.% PVDF, 1 wt.% CB, and 45.5 wt.% graphite.

This composition was similar to the composition of the slurry used to commercially fabricate LIB anodes. Subsequently, these suspensions were vortexed for 20 min at 3000 rpm as the final step in sample preparation.

The rheological response of the prepared slurries was measured using TA Instruments HR20. The shear stress in the synthesized suspensions was determined over 3 s^-1 to 2000 s^-1 shear rates.

A 0.1 rad/s pre-shear was used over 30 s for oscillatory measurements. Researchers obtained the linear viscoelastic region (LVR) at one s^-1 frequency over the 0.05 Pa -150 Pa stress range. All oscillatory experiments were performed within the LVR for every sample.

The oscillatory response was obtained with a three s per point minimum measurement time and a three s equilibration time. Loss modulus (G”) and storage modulus (G’) was determined over the 0.01 – 100 rad/s frequency range. All measurements were obtained at 25 ^oC.

150 µm thick slurries were coated on a copper foil at 310 s^-1 shear rate using a doctor blade coating technique on an automatic thick film coater. A vacuum oven was used to dry the coated slurries at 90 ^oC.

A digital multimeter and a digital micrometer were utilized to determine the electrical resistance and thickness of the dried anodes, respectively. Researchers later performed electrochemical characterization of the fabricated electrodes using CR 2032 coin cells with a lithium metal cathode.

Observations

LIB anodes were fabricated successfully using slurries prepared using three different CB formulations, graphite, NMP, and PVDF. The rheological characterization of the slurries effectively predicted the electrochemical performance of the dried electrodes/anodes.

Among the different rheological properties of the slurries, the storage modulus was identified as the most crucial factor that directly affected the electrical properties of the dried electrode/graphite anode and its electrochemical performance.

A considerable reduction in the G’ of LITX-50 containing samples under coating shear resulted in disconnected and dispersed CB clusters in the slurry, which significantly increased the thickness and resistance of the fabricated dry electrode. A low discharge capacity was observed in the battery cell using the LITX-50 containing anode/electrode.

No significant increase in the G’ of Super C 65 containing samples were observed after coating them above the yield stress, and dried electrodes/anodes fabricated using this slurry displayed a lower resistance and thickness compared to the electrodes prepared using LITX-50 containing slurries. Moreover, a substantial loss of discharge capacity was observed in cells using these anodes during cycling.

A significant increase in the G’ of the Super P-containing slurries was observed during coating, which indicated the strengthening of the CB network, and electrodes fabricated using this slurry displayed a low resistance. Cells containing these electrodes demonstrated a superior discharge capacity for 10 cycles.

To summarize, the findings of this study demonstrated that the anode electrochemical performance could be effectively predicted by the rheological modeling of slurry coating conditions. Slurries with a high G’ during coating formed compact and thinner electrodes with superior discharge capacity and lower resistance.

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