Understanding the role of the solid-electrolyte interphase for Li and Na rechargeable batteries by operando liquid-cell transmission electron microscopy

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EMAG - Energy Materials
Mr Chen Gong (1), Mr Shengda Pu (1), Dr Alex Robertson (2)
1. University of Oxford, Department of Materials
2. University of Warwick, Department of Physics

in-situ, TEM, liquid-cell, batteries, electrochemistry

Abstract text

Rechargeable batteries will be essential for the sustainability transition society must undergo. Looking beyond current Li-ion batteries, the next-generation of batteries may employ a ’metal anode’, where we cycle the active ion directly to and from its metallic state, rather than intercalating it into a graphite host. This would allow for far better battery performance. However, sodium and lithium metals are highly reactive, so rapidly form a solid-electrolyte interphase (SEI) layer across their surface due to reacting with the battery electrolyte. This sub-micron SEI layer is the rate limiting step for the transmission of ions from the electrolyte to the anode, and thus its properties are crucial for governing the stability and performance of the metallic anode.

The SEI’s structure and composition can be tailored by modifying the electrolyte, as this leads to different electrolyte decomposition products forming the SEI. If we can understand the link between the nature of the SEI and the metal anode cycling behaviour we can then tailor the electrolyte such that it optimises the SEI that is formed. This idea of a designed SEI via electrolyte engineering, leading to a more stable and durable anode robust to dendrite formation and other sources of battery capacity fade, is one of the most promising approaches for realising practical high performance metal anodes.

Unfortunately, diagnosing the link between SEI properties and anode cycling behaviour is highly challenging; the SEI is a ’buried’ solid-liquid interface that is not easily accessible; it is a highly reactive chemical environment, so cannot be exposed to ambient atmosphere; and the interface is fragile, so cannot be easily extracted. And on top of these concerns, the electrode cycling itself is a dynamic process that is difficult to map accurately through traditional post-mortem characterisations. Operando imaging of the interface and electrode in-situ are necessary.

In our recent work we have used operando liquid-cell TEM [1] to understand the relationship between electrolyte, SEI, and electrode cycling performance for the cases of Li and Na metal anodes [2][3]. For the Li metal anode, we explored how a fluoride-rich interphase layer can encourage the uniform dissolution of lithium during discharge, allowing for more reliable repeated cycling. In the fluoride-poor condition, the formation and detachment of Li dendrites is clearly observed. With the Na metal anode, we identified how the choice of electrolyte solvent can enable high cycling performance, with ether solvents suppressing gas evolution localised at the SEI on discharge. These gas bubbles block ionic cycling, preventing Na dissolution back into the electrolyte and thus inhibit good cycling performance.


[1] Liquid cell transmission electron microscopy and its applications. Shengda Pu, Chen Gong, Alex W Robertson. Royal Society Open Science. (2020), 7, 191204. DOI: 10.1098/rsos.191204

[2] Revealing the role of fluoride-rich battery electrode interphases by operando transmission electron microscopy. C Gong, S Pu, X Gao, S Yang, J Liu, Z Ning, G Rees, I Capone, L Pi, B Liu, G Hartley, J Fawdon, M Pasta, C Grovenor, P Bruce, A Robertson. Adv. Energy Mat. (2021), 11, 2003118. DOI:10.1002/aenm.202003118

[3] The role of an elastic interphase in suppressing gas evolution and promoting uniform electroplating in sodium metal anodes. Chen Gong, Shengda D Pu, Shengming Zhang, Yi Yuan, Ziyang Ning, Sixie Yang, Xiangwen Gao, Chloe Chau, Zixuan Li, Junliang Liu, Liquan Pi, Boyang Liu, Isaac Capone, Bingkun Hu, Dominic LR Melvin, Mauro Pasta, Peter G Bruce, Alex W Robertson. Energy & Environ. Sci. (2023) DOI:10.1039/D2EE02606F