Unveiling the Role of Surface Films on Carbon Anodes in Lithium-Ion Batteries

In the quest for improved performance in lithium-ion batteries (LIBs), understanding the characteristics and behavior of carbon anodes is crucial. One of the pivotal elements in this context is the formation of a protective surface film during the battery's initial charging phase. This film, known as the solid electrolyte interface (SEI), plays a dual role: it facilitates lithium ion conduction while simultaneously acting as an electronic insulator. However, the creation of this film comes with its own set of challenges, including the irreversible consumption of capacity.

The SEI is formed through the reductive decomposition of the electrolyte used in LIBs, typically nonaqueous solutions that contain lithium salts. Mixtures of ethylene carbonate with less viscous alkyl carbonates, such as dimethyl carbonate and diethyl carbonate, are commonly employed. While these solutions provide the necessary environment for lithium intercalation, they must also be stable under the extreme negative potentials encountered during charging.

Research into the composition and stability of the SEI has gained momentum over the past decade. Various analytical techniques, such as in situ Fourier-transform infrared spectroscopy and impedance spectroscopy, have shed light on the chemical makeup of the SEI. Notably, studies have indicated that lithium alkyl carbonates emerge as major constituents when using ethylene carbonate-based solutions, highlighting the complex chemistry occurring at the anode during cycling.

In addition to lithium alkyl carbonates, researchers have identified other components within the SEI. Through techniques like gas chromatography and transmission electron microscopy, compounds such as lithium hydroxide and oligomers resembling poly(ethylene oxide) have been detected. This diverse composition suggests that the SEI is not merely a passive barrier but an intricate system that influences the overall performance of lithium-ion batteries.

Furthermore, studies have revealed that the properties of the SEI can differ significantly based on the structure of the carbon anode. For instance, the SEI formed on the edge planes of highly oriented pyrolytic graphite is rich in inorganic compounds, while the basal planes tend to generate a film dominated by organic materials, primarily polymers. This disparity further emphasizes the need for tailored approaches when developing battery materials and understanding their electrochemical behavior.

As advancements in lithium-ion battery technology continue, the emphasis on optimizing the SEI and its interactions with carbon anodes will be pivotal. Ongoing research aims to identify solvent systems that minimize charge loss while enhancing SEI stability, ultimately leading to safer and more efficient battery solutions.