In-Situ Probing the Origin of Interfacial Instability of Na Metal Anode
Authors: Ji Yuchen , Yang Luyi , Pan Feng 
Published: February 2024

The chemical-mechanical stability of solid–electrolyte interphase (SEI) is probably the most critical factor determining the performance of alkali metal anode (Li, Na, etc.) in secondary batteries. Although extensive advanced characterization methods have been carried out to study SEI layers of Na metal anode, including solid state nuclear magnetic resonance1, 2, cryogenic transmission electron microscopy3, etc., the structural/componential evolution of SEI is still an uncharted territory due to its transient formation process and complicated components. In this work, we systematically analyze the SEI formation and dissolution processes via jointly combining multiple in-situ characterization technologies. By revealing spatial-temporal resolved information of SEI evolution, the buried origin of chemical-mechanical instability of SEI in Na anode is further clarified, which provides valuable guidelines for SEI engineering. A dynamic SEI formation/dissolution model of Na metal anode is demonstrated as follow:
Quantitative evaluation methods for the chemical instability (i.e., solubility) and mechanical instability (i.e., modulus) are designed. According to the mass variation in EQCM and the modulus measurement in in-situ AFM, we firstly quantitatively observe the chemical and mechanical stability evolution during SEI formation process.
The dynamic evolution picture of SEI formation has been explicitly established. We discover the instantaneous electrochemical formation process of SEI is obviously divided into two stages based on the potential. It is revealed that the formation of efficient passivation layer anchored on Na surface during the 1st (passivating) stage (2.3 – 1 V vs Na/Na+) (Scheme 1
a-b) is the critical factor to construct stable SEI. In absence of passivation layer, the Na mental surface will trigger unrestricted electrolyte decomposition and homogenous components distribution during the subsequent (growing) stage.
The dissolution model of SEI was revealed related to its spatial distribution of organics and inorganics. SEI with layered structure evolved from a compact passivation layer is found to have higher stability than that with homogenously distributed components. The inorganic species in the latter structure tend to detach from the SEI with the dissolution of organics, resulting in poor SEI chemical stability (Scheme 1
c and e). By contrast, SEIs with hierarchical structure growing based on the top of a passivation layer exhibits lower dissolution tendency (Scheme 1 d and f).
The dynamic analysis of SEI evolution of Na anode presented in this work not only sheds light on how to construct a stable SEI, but also provides guiding significance in unveiling the seemingly complicated interfacial chemistry in batteries via a concerted characterization approach.
References

Gao, L.N., Chen, J.E., Chen, Q.L. et al. The chemical evolution of solid electrolyte interface in sodium metal batteries. Science Advances
8, 4606 (2022).

Xiang, Y., Zheng, G., Liang, Z. et al. Visualizing the growth process of sodium microstructures in sodium batteries by in-situ 23Na MRI and NMR spectroscopy. Nat. Nanotechnol.
15, 883–890 (2020).

Han, B., Zou, Y., Zhang, Z. et al. Probing the Na metal solid electrolyte interphase via cryo-transmission electron microscopy. Nat Commun
12, 3066 (2021).

Figure 1