Article

Exploring Aluminum Anodes for Li-S Batteries Using XPS

Surface Analysis Spotlight: XPS

by Malli Komarneni XPS Scientist

The demand for efficient and high-capacity energy storage systems has been escalating in recent years, driven by the rapid growth of renewable energy sources and the need for grid stabilization. However, the widespread adoption of advanced energy storage technologies, such as solid-state batteries and lithium metal batteries, has been hindered by several challenges, including electrolyte compatibility, electrode stability, and cycling performance.

Here we shed light on one promising approach that addresses challenges in battery technologies, published in Science Advances. In particular, we emphasize the vital role played by PHI XPS instruments. This study focuses on the development of all-solid-state lithium-sulfur (Li-S) batteries, which have a sulfur-based cathode. Li-S batteries offer a high specific energy, making them a promising solution for reducing carbon emissions and achieving a sustainable society. However, the “shuttle effect” caused by the dissolution of polysulfides in liquid electrolytes has hindered their practical application.

This study introduces a carbon-free and binder-free anode comprised of a Li_0.8Al alloy. The Li_0.8Al alloy demonstrates excellent compatibility with the Li₁₀GeP₂S₁₂ (LGPS) electrolyte because its operating potential falls within the practical stability window of the electrolyte, which prevents the reductive decomposition of the electrolyte. Additionally, in situ X-ray diffraction (XRD) analysis revealed that the Li-Al alloy undergoes a two-phase reaction with a modest volume change, further enhancing its stability with LGPS. The LGPS electrolyte is significant due to its high ionic conductivity and moderate Young's modulus. It offers a high rechargeability and low internal resistance for batteries. Furthermore, as a solid-state electrolyte, LGPS addresses safety concerns associated with liquid electrolytes, such as leakage, fire, and explosion. Its compatibility with various anode materials and ability to prevent the shuttle effect make LGPS a promising electrolyte for advanced energy storage devices. The current study presents extensive investigations and electrochemical tests, highlighting the remarkable stability and cycling performance exhibited by all-solid-state Li-S batteries.

X-ray photoelectron spectroscopy (XPS) analysis was performed using a PHI 5000 VersaProbe II instrument to investigate the composition and surface properties of the LGPS solid electrolyte after disassembling the symmetric cells. The XPS results (Figure 1) revealed that the LGPS electrolyte underwent strong reduction by Li, resulting in the formation of Li₂S and reduced Ge. This reduction led to the continuous decomposition of LGPS due to the mixed ionic-electronic conducting property of the reduction products. Consequently, the anode-electrolyte interface deteriorated, resulting in poor Li plating/stripping stability and a low coulombic charge-discharge efficiency (CCD) in the Li-LGPS-Li symmetric cell. However, the XPS tests confirmed that the substitution of Li metal with Li_0.8Al alloy effectively inhibited the decomposition of the LGPS electrolyte. Moreover, the cathode design included continuous electronic and Li-ion pathways. The sulfur (S) was uniformly loaded onto the surface of carbon nanotubes (CNTs) through a heat treatment method and then homogeneously mixed with the LGPS electrolyte, ensuring efficient electron and Li-ion transport. Therefore, XPS analysis provided crucial evidence for understanding the degradation mechanisms and the effectiveness of the Li_0.8Al alloy anode in improving the stability of the all-solid-state LSB system studied in this research.

Figure 1. (A) S 2p and (B) Ge 3d XPS spectra of the LGPS surface. Sample state sequence (Top to bottom): Pristine state, Li_0.8Al contact (8 hours), Li_0.8Al-LGPS-Li_0.8Al cycling (100 hours), and Li contact (8 hours).

In conclusion, the Li_0.8Al-LGPS-S battery system demonstrated excellent performance. The cathode design enabled efficient electron and Li-ion transport, with S uniformly loaded on CNTs and homogeneously mixed with LGPS electrolyte. The battery achieved a reversible specific capacity of 1237 mAh gS^-1 at 0.2C, maintained stable operation for 200 cycles with 93.29% capacity retention, and exhibited a reversible areal capacity of 3.458 mAh cm^-2 over 100 cycles. By minimizing the Li_0.8Al anode and achieving a low N/P ratio of 1.125, a high specific energy of 541 Wh kg^-1 was obtained. This work addresses the anode-SSE instability issue in solid-state batteries and presents a viable anode selection scheme for all-solid-state LSBs. The Li_0.8Al alloy anode has potential for broader application in solid- state battery systems due to its appropriate operating potential, substantial capacity, processability, and cost-effectiveness. This work marks a significant step towards the practical realization of safe and high-performance Li-S batteries, contributing to the pursuit of a carbon-neutral future.

For more information on how surface analysis techniques can be used to study battery materials, please attend the upcoming CQLMNS conference, where Sarah Zaccarine, Ph.D., Staff Scientist is giving a talk entitled "Advances in XPS Analysis of Battery Materials".