Study of Electrochemical Characteristics of Differently Produced Nickel-Rich Cathodes
Authors: Grenda Tim , Kwade Arno 
Published: December 2024

Lithium-ion batteries stand as the forefront technology in energy storage. Significant endeavors have been dedicated to enhancing the energy density of lithium-ion batteries, primarily by incorporating high-capacity nickel-rich cathode active materials like NCM831205 or even higher nickel contents [1]. Nevertheless, an elevated nickel content result in reduced structural and thermal stabilities, increased reactivity with the electrolyte, and heightened vulnerability to environmental influences due to the formation of surface films [2–4]. To mitigate these effects, specialized processing conditions are necessary, such as treatment exclusively in a dry atmosphere or the incorporation of specific electrolyte additives [5,6]. However, the processing method and its diverse parameters play a critical role in battery performance. Particularly, the mixing process of the slurry holds pivotal importance in determining electrode quality, as it is during this phase that the intricate structures and internal networks of the electrode are established. These structures significantly influence the diffusion of lithium ions, thereby impacting cell performance profoundly. The mixing conditions, along with the selection of the mixing device and geometry, exert a substantial impact on material dispersion, aimed at reducing the size of carbon black (CB) agglomerates to smaller aggregates.

For optimal quality in electrode production, an efficient dispersion step is crucial, often performed using a dissolver in a dry room environment to mitigate any impact from residual moisture and potential chemical reactions with the electrolyte. The efficacy of the dispersion process hinges significantly on the selected disk diameter and height concerning the liquid level. Since both geometric parameters influence the slurry's flow properties during dispersion, discerning their exact impact on the formation of the conductive network poses a challenge. Hence, this study employs variations in these parameters to produce diverse electrode structures. The dispersion index (DICB), as introduced by Weber et al., serves as a quantitative metric to assess the progress of dispersion [7].

The electrodes produced undergo impedance measurements to assess contact and ionic resistances. For this evaluation, the model proposed by Landesfeind et al. and its associated circuitry are employed [8]. Moreover, 4-point measurements are conducted to ascertain the volume resistance or electrical conductivity within the electrode. Subsequently, the electrodes are subjected to measurements in a 3-electrode setup within the full cell configuration, with a capacity of 4 mAh, to analyze their performance and fast-charging capability. Moreover, the utilization of differential capacity plots enables the derivation of direct correlations between CB decomposition, the resultant structure, and the kinetic limitations induced by these structures. These assessments reveal three distinct ranges of electrode properties within the DICB. Notably, these ranges exhibit direct correlations with contact, ionic and volume resistances, as well as cell performance.

The observed ranges indicate that insufficient CB decomposition results in low ionic resistance but also leads to elevated contact resistance. Additionally, the presence of excess binder, acting as inert material, contributes to kinetic limitations. Conversely, excessive DICB’s lead to a notable surge in ionic resistance and to the occupation of intercalation sites on the active material by unbound CB particles, thereby restricting the number of phase transitions of nickel and subsequently diminishing the capacity. For this reason, a defined range can be established within which the electrochemical properties of the electrode must lie in order to achieve maximum capacity and optimal fast-charging capability.

References

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