Dendrites will form on the lithium metal electrode of a battery at certain current densities [1]. Nanometric or micrometric in size, dendrites may take the form of a cone, column, or spike. They also consume lithium, reducing a battery's capacity, and may sometimes cause a short circuit.

“This is preventing their commercialization, but in spite of the many ongoing works around the world, dendrite formation remains a highly unpredictable phenomenon. What's more, we've looked into a hardly studied aspect of this issue," said CEA-Liten's Hervé Manzanarez, who is supervising the thesis on this modeling tool.

Accounting for heterogeneities in the SEI

The SEI, or Solid Electrolyte Interphase, appears between the lithium metal electrode and the solid electrolyte, and has been found to contribute to dendrite formation [2]. However, works that have simulated dendrite formation have mostly overlooked the fact that such a phenomenon is heterogenous.

​“Whether or not the SEI provides lithium ions with conduction paths facilitating their movement very much depends on their composition, thickness, surface irregularities, etc. [3]. If we wish to understand the formation of dendrites, we must take into account their heterogeneity."

The thesis has been carried out in a so-called “multi-phase field" approach. This involves integrating various types of domains [PF2] into the model: lithium metal electrode, solid electrolyte, conductive SEI zones, and insulating SEI zones. Nevertheless, other heterogeneities – chemical, structural, etc. – have not been reproduced.

The higher the current, the greater the number of dendrites

​Thanks to this streamlined approach, we can now understand previously unexplained phenomena. At lower currents, lithium is deposited slowly enough to enable the relaxation of the interfaces between domains and the homogenous rearrangement of the lithium metal. The electrode grows homogenously, and no dendrites appear.

However, at higher current densities, the lithium ions are deposited too quickly to enable the relaxation of the interface. The electrode therefore starts to grow in-homogeneously, hence the significant deformations of the SEI until fractures occur. When the lithium and the electrolyte come into contact, dendrite nucleation is triggered—the start of an irreversible aging process.

The thickness, length and deformation capacity of the SEI fields may help to delay the fractures. This means that the challenge now lies in finding a suitable way to integrate mechanical properties into the domains, in order to investigate the impact of pressure fields [PF3] on managing these topological irregularities of the interface.

According to Hervé Manzanarez, “This initial model provides new parameters for dendrite formation, but it needs to be enriched, particularly with mechanical equations, and be available in both 2D and 3D. A post-doctorate thesis looking into these matters is already underway."