Revolutionizing sodium-ion batteries with lithium salt additives

Sodium-ion batteries (SIBs) are emerging as a promising alternative to lithium-ion batteries, thanks to the abundance of sodium resources and their potential for cost-effective, large-scale energy storage. However, the commercialization of SIBs is hindered by challenges such as poor cycle stability and significant capacity fade, primarily due to the weak solid electrolyte interphase (SEI) and interfacial degradation. A recent study published in Nano-Micro Letters by Professor Ji-Sang Yu and Professor Hyun-seung Kim from the Korea Electronics Technology Institute (KETI) and Kangwon National University, South Korea, has demonstrated a transformative approach to mitigate these issues by incorporating lithium salt (LiPF₆) into the electrolyte.

Why Lithium Salt Matters

• Enhanced SEI Formation: The addition of LiPF₆ to the electrolyte significantly improves the formation of a robust SEI layer on the hard carbon anode. This SEI layer is less soluble and more stable compared to the Na-based SEI, effectively suppressing sodium-ion and electron leakage, and mitigating electrolyte decomposition.
• Surface Stabilization of O3-Type Cathode: The incorporation of LiPF₆ also leads to marginal surface doping of the O3-type cathode, forming a Li-ion pillar that reduces oxygen release and electrolyte degradation. This results in improved capacity retention and cycle stability.

Innovative Design and Mechanisms

• Controlled Solvation Structure: The introduction of LiPF₆ alters the local solvation structure of the electrolyte, forming a more reducible Li-solvated cluster compared to the Na-solvated cluster. This facilitates the formation of a highly passivating SEI layer on the hard carbon anode.
• Formation of Li-ion Pillars: The slight surface doping of the O3-type cathode with Li ions creates a structural reinforcement that serves as a pillar, preventing the collapse of the layered structure and reducing gas evolution during cycling.

Results and Discussion

• Improved Cycle Performance: The addition of LiPF₆ in the electrolyte led to a significant improvement in the cycle performance of SIBs. The capacity retention reached 92.7% after 400 cycles, surpassing the typical 80% retention reported for SIBs.
• Reduced Gas Evolution: Differential electrochemical mass spectrometry (DEMS) analysis showed a significant reduction in CO₂ gas evolution from the O3-type cathode surface, indicating effective suppression of oxygen release and electrolyte degradation.
• Enhanced Interfacial Stability: Post-mortem analysis using X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HR-TEM) revealed a well-preserved layered structure of the O3-type cathode and a stable SEI layer on the hard carbon anode.

Future Outlook

• Scalability and Practical Applications: The scalable synthesis of the LiPF₆-added electrolyte highlights its potential for practical SIB applications. The insights gained from this study can guide the development of more efficient and cost-effective sodium-ion battery technologies.
• Further Research: Future work may focus on exploring other additives and electrolyte compositions to further enhance the performance and stability of SIBs. Additionally, the integration of advanced characterization techniques can provide deeper insights into the interfacial phenomena.

Conclusion: This study demonstrates a transformative approach to enhancing the performance and stability of sodium-ion batteries by incorporating lithium salt into the electrolyte. The formation of a robust SEI layer and the stabilization of the O3-type cathode surface significantly improve cycleability and capacity retention. This work paves the way for the development of high-performance, cost-effective sodium-ion batteries, contributing to a more sustainable energy future.

Stay tuned for more groundbreaking advancements from Professor Ji-Sang Yu and Professor Hyun-seung Kim as they continue to push the boundaries of sodium-ion battery technology!