Lithium-ion battery dispersants
Dispersants are used in various stages of the battery manufacturing process to optimise the viscosity of processing slurries, making it easier to handle and speed up processing time, and to increase the distribution of conductive carbon in electrodes. This leads to improved capacity retention and electrochemical stability.
Our dispersants under the Hypermer™ Volt brand help optimise the rheology of processing slurries.
As experts in chemical synthesis, we are now developing new dispersant materials using a fundamental based approach, with the aim to improve capacity retention and reduce cell degradation at higher charge and discharge rates.
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Why use conductive carbon black and carbon nanotube dispersants?
Benefits for manufacturing
At the electrode production stage, dispersants help to stabilise dispersions of conductive carbon in the solvent; they reduce the viscosity of slurries enabling increased solids loading and reduce solvent use.
Benefits for battery performance
In electrodes, dispersants help achieve a more uniform distribution of carbon particles, resulting in a network of carbon that improves conductivity. This reduces the internal resistance within the battery, promoting improved electron transfer. As a result, the battery experiences slower degradation of the cathode, longer service life and better capacity retention at high charge and discharge rates.
Many different carbon blacks can be used with different particle sizes, structures and porosities, but all carbon materials must be distributed evenly in the electrode slurry to optimise electron flow. The distribution of carbon black can be observed empirically using a micrograph from a scanning electron microscope (SEM). If the carbon additives are not evenly distributed in the electrode slurry, initial battery capacity, battery capacity retention and cycle life can be adversely affected in the final battery cell.
Figure 1: SEM image of a cathode cross-section, showing active material (light grey) surrounded by a conductive carbon layer (darker grey)
We examined the effect of Hypermer Volt 4000 on the rheological properties of carbon black dispersions using a broad range of shear rates. The study was carried out on five common battery grades of conductive carbon with different particle sizes and surface areas.
We used a rheometer equipped with parallel plate geometry using a sample of 1 wt. % of Hypermer Volt 4000 and 5 wt. % of carbon black in n-methyl-2-pyrrolidone (NMP). When used commercially, the viscosity of carbon black dispersions can be adjusted by using Hypermer Volt 4000.
While surface area and particle size of carbon black can vary significantly, Hypermer Volt 4000 demonstrates excellent viscosity reduction in dispersions of many commercially available battery grade conductive carbons.
Figure 2: Five battery-grade conductive carbon dispersions, dispersed with and without Hypermer Volt 4000
Viscosity reduction of carbon black slurry
We examined the effect of Hypermer Volt 4000 on:
- Discharge rate capability
- Resistance of cathodes
- Electrochemical stability.
Full experimental conditions and results are available in our whitepaper that you can login and download at the top of the page.
Discharge rate improvement
Discharge capacities of half cells with LCO, NMC622, NMC811 and LFP cathodes as a function of discharge rate are shown below.
Using Hypermer Volt 4000 in the cathode formulation results in higher discharge capacity. This effect is observed for all tested cathode chemistries. At higher discharge rates, Hypermer Volt 4000 is particularly efficient in a combination with LCO and NMC622.
Figure 3: Discharge rate capacities versus C-rate
Series and charge transfer resistance
In figure 4 below, we show impedance spectroscopy results obtained for cathodes with Hypermer Volt 4000, PVP and with no dispersant, in half-cells with lithium metal anode. The spectra contain typical features: arcs due to Faradaic impedance and straight lines attributed to lithium diffusion.
Fitting results obtained for the cathode with Hypermer Volt 4000 are shown as an example and calculated series and charge transfer resistances are available in our full test paper available as a download.
Regarding series resistance, significant improvement was achieved by using Hypermer Volt 4000. This observation implies indirectly that Hypermer Volt 4000 results in better uniformity of carbon black distribution and more effective electronically conductive network in the structure of composite cathode. Charge transfer resistance is higher compared to blank sample but reduced vs. sample with PVP.
Figure 4: Impedance spectra obtained for NMC622 cathodes containing no dispersant, Hypermer Volt 4000 and PVP
We assessed the electrochemical stability of Hypermer Volt 4000 in combination with a high-voltage cathode using cyclic voltammetry to cover potential range exceeding stability of the industrial standard PVP, which is 4.2 V vs. lithium. There is experimental evidence that electrooxidation of an organic component can result in increased resistance and accelerated degradation of cell capacity.
The results obtained for reference cathode (no dispersant) and cathodes with Hypermer Volt 4000 and PVP are shown in figure 5 below. All samples showed similar large anodic and cathodic peaks corresponding to a change of the oxidation state of nickel. Apart from these main features, a pronounced oxidation peak upon the first anodic sweep was observed for the cathode prepared using PVP. This peak results from the current generated during the irreversible oxidation process. This feature can be ascribed to the presence of PVP, which is known to be unstable above 4.2 V. Hypermer Volt 4000 showed no or much smaller oxidation current upon the first anodic sweep, implying better electrochemical stability vs. PVP.
Hypermer Volt 4000 showed no (or insignificant) oxidation current during the anodic sweep. Pronounced oxidation current was detected at ca. 4.25 V vs. Li/Li+ for the cathode containing PVP. The data suggest better electrochemical stability of Hypermer Volt 4000 in the extended range of potentials.
Figure 5: Cyclic voltammetry of cells containing dispersant additives and a blank control
Hypermer Volt 4000 is an effective carbon black dispersant for cathode slurries exhibiting:
- Viscosity reduction of the cathode slurry for easier processing
- Capacity retention at high C-rates for both charging and discharging events
- Lowered cell resistance vs cells containing no dispersant or PVP
- Improved electrochemical stability vs PVP, especially at high voltages.
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