Strategic Cell Design for Long-Lasting Lithium-Ion Batteries with Sio/Graphite Anodes
Authors: Kim Hyoyeong , Kim Chanmyeong , Lee Sang-Min 
Published: December 2024

Lithium-ion batteries (LIBs), first commercialized by Sony in the 1990s, have become the power source for numerous electronic devices and a key driver of the electric vehicle (EV) market growth. However, the development of next-generation batteries with an energy density exceeding 250 Wh/kg remains a critical challenge. High-energy-density batteries are essential to extend the driving range of EVs, improve the runtime of portable devices. Conventional graphite anodes are limited in their ability to meet the increasing demand for higher energy densities. As a result, silicon-based anodes, which offer 5-10 times higher specific capacity, have attracted significant interest. Silicon oxide (SiO), in particular, exhibits better cycling stability and reduced volumetric expansion (~100%) than pure silicon (~400%), though it faces challenges related to mechanical instability, low conductivity, and poor initial coulombic efficiency (ICE). To overcome these challenges, research on SiO/Graphite composite electrode has increased, combining graphite’s good conductivity and SiO’s high capacity to enhance energy density and cycle performance.

Despite the great potential of SiO/graphite composite electrodes, problems such as capacity fading during long-term cycling still remain challenges. Various pre-lithiation strategies have been proposed to both compensate for irreversible lithium losses and improve cycling performance. Sacrificial cathodes are particularly attractive because they can be integrated into existing electrode fabrication processes, improving the electrochemical performance by mitigating excessive delithiation of the anode. However, over-lithiated sacrificial cathodes are prone to moisture sensitivity during fabrication and structural instability after delithiation, both of which negatively impact the battery's electrochemical performance. Additional processing steps, such as coating or composite material design, have been proposed to enhance the stability of sacrificial cathodes, but these approaches increase complexity and cost, hindering their practical application. Although various pre-lithiation strategies, including sacrificial cathodes, effectively enhance performance, they still face significant limitations when it comes to practical industrial applications.

This study proposes a simple effective cell design strategy to significantly enhance the cycle performance of SiO/Graphite anode based full cells. It was observed that bi-modal (BM) cathodes, composed of a mixture of two types of NCM811 particles, exhibited greater overpotential at the end of discharge compared to uni-modal (UM) electrodes made entirely same type of NCM811 particles. The increased overpotential in bi-modal cathodes is attributed to the differences in reaction kinetics between the particles, leading to non-uniform reaction distributions within the electrode and resulting in greater overpotential during the later stages of discharge. This bi-modal cathode design lowers the anode’s end-of-discharge voltage, enabling better control of the delithiation swing range in SiO and improving the full cell's cycling stability.

As a result, the SiO/Graphite-BM full cell retained 96% of its initial capacity after 150 cycles at a C-rate of C/3, while the SiO/Graphite-UM full cell dropped to 80% after 150 cycles, which is typically considered the end of battery life. Various electrochemical evaluations, including GITT and 3-electrode tests, confirmed that the observed performance differences were due to the overpotential variation in the cathode, which controlled the charge-discharge behavior of the anode and, in turn, influenced the cycle performance of the full cell. Furthermore, synchrotron operando XRD analysis and various post-mortem analyses revealed that the capacity fade was primarily attributed to the degradation of silicon within the anode. This approach offers a promising solution for enhancing the long-term stability and energy density of SiO/Graphite-based lithium-ion batteries.