Prelithiated carbon nanotubes-containing silicon/graphite negative electrodes for high-performance lithium-ion batteries

RWTH Publications (RWTH Aachen)

Abstract

To meet the growing demand for high-capacity lithium-ion batteries (LIBs), electrode materials with higher specific capacities are required. Silicon (Si), an alloy-type negative electrode material, is very attractive due to its theoretical capacity of 3590 mAh/g (for Li₁₅Si₄ at room temperature), which is nearly ten times greater than that of commercially used Graphite (Gr) electrode material (~372 mAh/g). However, the widespread practical application of Si-based negative electrodes is hindered by several critical challenges, e.g., large volume changes of Si during (de-)lithiation, low electronic conductivity and high irreversible Li-ion consumption caused by solid electrolyte interphase (SEI) formation. To mitigate some of these drawbacks, conductive additives such as Carbon Nanotubes (CNTs) have been introduced as they enhance electronic conductivity and mechanical stability and have already been adopted in commercial electrode formulations. Nevertheless, in Si-rich electrodes, CNTs alone are insufficient. Their large specific surface area worsens irreversible Li-ion losses, leading to an obvious increase in initial capacity loss. Consequently, the combination of Si and CNTs results in considerably higher irreversible Li-ion consumption, specifically at high CNTs contents. In this context, this thesis focuses on electrochemical prelithiation that can be adopted to Si/CNTs-based negative electrodes to compensate the high Li-ion losses during initial cycles. Prelithiation means that a specific amount of lithium is introduced to negative electrode prior battery cell assembly. This enables enhanced cycling performance and improved cycle life (charge/discharge counts). In order to highlight the beneficial role of CNTs in combination with prelithiation in Si-based negative electrodes, Carbon black (Super P) was used as a reference conductive additive material. In the first part of the thesis, CNTs materials (Multi-Walled Carbon Nanotubes, MWCNTs) are characterized with respect to their structural, morphological and electrochemical properties using scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR). The impact of MWCNTs at low-content (2 wt.%) and high-contents (10, 20 and 30 wt.%) was investigated on their electrochemical performance across various Si/Gr negative electrode formulations and compared with Super P. The electrodes were further characterized by SEM, ATR-FTIR and Raman spectroscopy, while aging mechanisms were analyzed via post-mortem studies to elucidate the effects of the two conductive additives in different electrode systems. In the second part of the thesis, the electrochemical prelithiation was applied on pure MWCNTs-based negative electrodes to get insights first on MWCNTs material itself. Afterwards, prelithiation was applied to low-content and high-content additive containing Si-based negative electrodes. Finally, surface chemistry analysis via X-ray photoelectron spectroscopy (XPS) was performed on high-content prelithiated and prelithiated-cycled MWCNTs and Super P containing Si/Gr electrodes in order to elucidate and compare the SEI composition. The findings reveal that MWCNTs materials show a high defect denisty and presence of surface functional groups (e.g., –C=O, –COO–, –COOH). As negative electrode material, they exhibit a high irreversible capacity and an average specific capacity of ~200 mAh/g (vs. Li/Li⁺), which is in line with numerous research reports. When used as conductive additive at low-content (2 wt.%), MWCNTs proved highly effective in Si rich Si/Gr and pure Si negative electrodes, outperforming Super P counterpart in cycling performance and cycle life. In full cell configurations paired with NMC622, MWCNTs further improve cycling stability, increase Coulombic Efficiency (CE) and enhance rate capability across different C rates. However, at higher-contents, MWCNTs cause increased Li-ion consumption, which represents a critical limitation for full-cell applications and requires mitigation strategies. Electrochemical prelithiation addressed this issue and improved the performance of full-cells with pure MWCNTs and Si rich Si/Gr negative electrodes paired with NMC622. The process improved cycling performance, capacity retention, CE and rate capability across all systems tested. XPS analysis revealed significant differences in the SEI composition between MWCNTs-based electrodes and those using Super P, revealing that the choice of conductive additive can greatly impact SEI formation, even in similar negative electrode formulations. Overall, this thesis underscores the importance of MWCNTs as conductive additives in Si-rich electrodes in combination with electrochemical prelithiation, demonstrating a strong impact on cycling performance and cycle life.

Authors 1

1. Leyla Ünal corresponding Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen