The ubiquitous use of Li-ion batteries is hindered in part by limitations to achievable specific and volumetric energy. In addition to the physical constraints these properties address, they lead to higher cost per Wh. Replacing the intercalation or alloying based negative electrode leads to significant increases in both of the aforementioned energy related properties. In-situ formed or “anodeless” Li metal batteries, where Li is plated directly on the current collector, enable significant cost savings and improvement of energy of Li metal batteries relative to traditional Li metal which contain Li metal upon initial fabrication [1].

Using anodeless configurations, we imparted on an effort to investigate the deleterious capacity consuming phenomena of the plating process by electrochemically isolating the key degradation and performance limiting phenomena. Using ultra low-capacity Li metal plating (0.08mAh/cm²) in Li-metal anodeless half cells, the formation and quality of the solid electrolyte interphase (SEI) can be investigated with amplification. The solid electrolyte interphase (SEI) is a protective and passivating layer formed during the initial reduction of electrolyte. A robust SEI protects against excess electrolyte consumption and allows for subsequent stable, high efficiency cycling while being electronically resistive and ionically conductive [2]. The composition and mechanical stability of this dynamic layer influences efficiency and cycle lifetime [2]. At higher lithium plating capacities however, the influence of the SEI is not easily observed. Instead, capacity fade attributed to dendrite formation and mechanical damage to the SEI can be more easily investigated. Using higher capacity lithium plating (2.5mAh/cm²) in anodeless cell configurations, dendrite formation and capacity fade over cycle lifetime can be observed. Dendrite formation, as the result of irregular lithium deposition, can hinder the amount of active lithium available as well as lead to battery failure through short circuiting and thermal runaway [3].

In this work we have isolated the study SEI and dendrite formation electrochemically, using Li-metal and LiCoO₂ counter electrode anodeless cell configurations, respectively. As a result, we developed novel non-aqueous, non-ionic liquid electrolytes that have achieved efficiency of 85% at 0.08mAh/cm² where most of the capacity is associated with the formation of the SEI and 96% at 2.5mAh/cm² at the more challenging initial cycles. In contrast, a standard electrolyte composition such as 1M LiPF6 EC/DMC shows poor initial efficiency of 52% at 0.08mAh/cm² and 66% at 2.5mAh/cm². High efficiencies were also achieved with optimized ionic liquid electrolytes. This work may provide insight into the initial stages of SEI formation and provide a systematic methodology for electrolyte optimization through SEI capacity loss and dendritic capacity fade observation.

[1] S. H. Park, D. Jun, G. H. Lee, S. G. Lee, and Y. J. Lee, J. Materials Chemistry A 9 (2021) 14656-14681.

[2] S. J. Park, J. Li, C. Daniel, D. Mohanty, S. Nagpure, and D. L. Wood, Carbon 105 (2016) 52-76.

[3] J. B. Goodenough, J. Solid State Electrochem. 16 (2012) 2019-2029.