Scientists revealed the formation process and failure mechanism of sodium anode SEI

On July 5, 2023, Beijing time, the team of Pan Feng and Yang Luyi of Peking University published a new study entitled “In-situ probing the origin of interfacial instability of Na metal anode” in the journal Chem.

As an ideal anode material for sodium-ion batteries, the practical application of sodium metal is subject to its unstable SEI: on the one hand, there is a more serious dissolution phenomenon of SEI in sodium electrolyte than that of lithium battery, resulting in continuous interfacial side reactions; On the other hand, SEI still lacks sufficient mechanical strength to inhibit the growth of sodium dendrites. At the same time, the instantaneous formation process of SEI makes its electrochemical growth mechanism difficult to resolve, and the test results of off-site characterization inevitably deviate from the state under real battery conditions, although a large number of studies have characterized and analyzed the chemical composition and physical properties of SEI, but the direct observation of SEI growth process is still a tricky challenge. In this work, the electrochemical growth process of sodium anode SEI was jointly revealed by combining multiple in situ interface characterization methods, and the key factors affecting the stability of SEI were systematically studied.

The first authors of this work are Yuchen Ji and Jimin Qiu, and the corresponding authors are Professor Pan Feng and Associate Professor Yang Luyi.

Through in situ 3D confocal microscopy, electrochemical quartz crystal balance, in situ atomic force microscopy, in situ Raman and electrochemical differential mass spectrometry, combined with cryo-EM and time-of-flight secondary ion mass spectrometry, the authors revealed that the formation process of SEI can be divided into two stages: passivation stage (stage 1) and growth stage (stage 2). The cause of the chemical-mechanical instability of SEI is closely related to the electrochemical growth process of SEI. During the “passivation phase” of SEI growth (~2.3 V to ~1 V), soluble organic compounds are the main components of SEI, and they dissolve so that the sodium metal surface cannot be effectively passivated and exposed to the electrolyte. When the potential is lower than 1 V, it enters the “growth stage” of SEI, and a large number of organic and inorganic substances are formed at the same time. However, due to the high solubility, the repeated formation and dissolution of organic matter leads to the separation of inorganic matter from the surface, resulting in SEI evolving into a uniformly distributed structure in which organic/inorganic components are mixed together. The structure has poor mechanical strength of SEI due to the organic matter as the main body, and the dissolution of organic matter will also lead to the loss of support and detachment of the embedded inorganic components, resulting in the overall difficulty of stable existence of SEI in the electrolyte environment.

By adding electrolyte additives and other methods, an inorganic passivation layer is pre-constructed in the passivation stage, which can effectively enhance the stability of SEI. The sodium metal surface is passivated by a stable inorganic passivation layer, insulating it from exposure in the electrolyte environment. In subsequent growth stages, SEI evolved further from this passivation layer, so that the inorganic components continued to grow to thicker, harder inorganic layers clinging to the sodium metal surface, while organic decomposition products could only grow on top of the inorganic layer. The structure’s SEI provides higher mechanical strength and chemical stability, and better resistance to interfacial side reactions and dendrite growth.

Figure 1: SEI stability in PC and PC-FEC.

Figure 2: In situ morphology and mechanical detection of SEI formation.

Figure 3: In situ detection of the formation and subsequent evolution of the passivation layer.

Figure 4: Internal spatial distribution of SEI components.

Figure 5: Schematic diagram of the SEI formation process.

By studying the formation process and dissolution model of sodium anode SEI, the research work reveals the relationship between the components, structure and stability of SEI, which provides new ideas for the optimization design of related electrolytes and interfaces in the future. (Source: Science Network)

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