Revolutionizing High-Energy Batteries: XPS and Cryo-TEM Uncover Breakthrough in Li Metal Battery Technology

In a new study published in the journal Chemistry of Materials, a team of researchers, led by Ping Liu at the University of California San Diego, has provided a breakthrough in the development of high-energy-density Li metal batteries. By employing cutting-edge techniques, including X-ray photoelectron spectroscopy (XPS) and cryogenic transmission electron microscopy (cryo-TEM), they have unlocked the potential of Li-rich, Mn-rich (LMR) layered oxide cathodes, paving the way for future energy storage solutions with remarkable performance (1).

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The researchers acknowledge the current limitations of Li-ion batteries, with energy densities below 250 Wh/kg, and the increasing demand for high-energy-density power sources for portable electronic devices and electric vehicles. To meet these demands, Li metal batteries, which can potentially reach energy densities of up to 500 Wh/kg, have emerged as a promising solution. However, challenges such as electrode and electrolyte loading, inactive material content, and Li metal anode reversibility must be addressed for their widespread adoption.

The unit “Wh/kg” represents energy density, specifically the amount of energy stored or delivered per unit mass. It combines "Wh," which stands for watt-hour, measuring energy, and "1/kg," indicating mass in kilograms. In practical terms, it allows us to assess how much energy a substance or device can hold or provide for each kilogram of its weight, making it a crucial metric for evaluating the efficiency and capacity of various energy storage technologies like batteries, where higher energy density is often desirable for longer-lasting and more powerful energy sources.

One particularly promising avenue involves LMR cathodes, characterized by their high Mn content and potential to offer superior capacity output while reducing reliance on costly transition metals like Ni and Co. These cathode materials hold the key to achieving significantly higher energy densities and cost-effective energy storage solutions.

What sets this research apart is the innovative use of a localized-high-concentration electrolyte (LHCE) based on ether solvents. The team discovered that a well-designed LHCE can provide remarkable reversible performance for Li||LMR cells. This surprising result challenges previous assumptions and opens the door to new possibilities for Li metal batteries.

The critical role of XPS and cryo-TEM cannot be overstated. XPS analysis allows researchers to gain insights into the cathode-electrolyte interphase (CEI) chemistry, providing a deeper understanding of the battery's performance and stability. Cryo-TEM, on the other hand, enables researchers to visualize the microstructure of the battery components at extremely low temperatures, offering valuable insights into the battery's behavior under various conditions.

By leveraging these advanced analytical techniques, the research team achieved a remarkable 95.8% capacity retention after 100 cycles, surpassing the performance of a carbonate-based control electrolyte. Moreover, they demonstrated the feasibility of 4 mAh/cm² LMR||2× Li full cells, which retained 87% capacity after 80 cycles using the LHCE electrolyte, while the control electrolyte led to rapid failure.

The term "mAh/cm²" in the context of LMR||2× Li full cells refers to the measurement of energy or capacity per unit area. Specifically, it quantifies the amount of charge (measured in milliampere-hours, mAh) that a battery can store or deliver per square centimeter (cm²) of its electrode surface area. In this case, "LMR||2× Li" indicates the configuration of the battery, with LMR represents the cathode material, and "2× Li" suggests two lithium metal anodes. Therefore, "mAh/cm²" in LMR||2× Li full cells helps assess the energy storage capability of the battery on a per-unit-area basis, which is crucial for optimizing energy density and performance in high-energy battery systems.

This research not only uncovers the potential of LMR cathodes but also emphasizes the critical role of advanced analytical techniques like XPS and cryo-TEM in optimizing high-energy-density Li metal batteries. This breakthrough promises to drive the development of cost-effective, high-performance energy storage solutions, making a significant impact on the future of portable electronics and electric vehicles.

Ping Liu notes that advanced characterization techniques are revolutionizing the field of battery technology, addressing longstanding questions and driving progress in next-generation lithium battery development. Researchers, like Ping Liu at the University of California, San Diego, are using synchrotron X-ray diffraction to conduct in operando experiments, enabling real-time observation during charging and discharging. He notes, “cryo-electron microscopy (cryoEM) has also emerged as a game-changer, particularly in understanding the solid electrolyte interface (SEI) passivation layer's formation on lithium metal anodes (2).” Yuzhuang Li from the University of California in Los Angeles, highlights, “that unlocking the solid electrolyte interface (SEI) fundamental mechanisms could lead to 99.9% efficiency, reducing lithium losses and extending battery longevity (2).”

The Solid Electrolyte Interface (SEI) plays a crucial role in lithium-ion batteries, forming due to reactions between the electrode materials and electrolyte. Its fundamental mechanisms involve a combination of electrochemical and chemical processes. Initially, the SEI forms as the electrolyte undergoes reduction at the lithium electrode, creating a protective layer that prevents further reactions. Comprising various components, including inorganic lithium salts, organic compounds, and lithium oxide/hydroxide species, the SEI's composition is diverse. This layer is dynamic and evolves during charge and discharge cycles, influencing battery performance and lifespan. Researchers aim to comprehend these mechanisms to enhance SEI stability, mitigate battery degradation, and improve overall lithium-ion battery efficiency and safety.

References

(1) Liu, P. et al. Locally Saturated Ether-Based Electrolytes With Oxidative Stability For Li Metal Batteries Based on Li-Rich Cathodes. ACS Applied Materials & Interfaces 2023, acsami.3c07224. DOI: 10.1021/acsami.3c07224

(2) Mitchell, J. Building better batteries. Chemistry News 2023, April 24, 2023. https://www.chemistryworld.com/features/building-better-batteries/4017313.article (accessed 2023-10-02).

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