Lithium ion (Li-ion) batteries have been revolutionary in the advancement of portable electronic devices due to their unparalleled energy density.  However, even with the energy densities afforded by existing lithium-ion battery technologies, requirements of sustained and long-range operation of electrified vehicles and devices have prompted research on new materials and strategies for improving performance and safety of current battery systems.

One of the most tantalizing strategies for improving energy density is to use lithium metal as the anode.  Lithium’s low redox potential, high gravimetric/volumetric specific capacities, and fast reaction kinetics all contribute towards both high energy and high power density.  However, the implementation of lithium metal as the anode has largely been prevented due to safety concerns resulting from lithium dendrite growth – causing short-circuits, leading to thermal runaway reactions, and fires/explosions [1].

Many models have been proposed to describe dendrite morphologies during initiation and propagation, with conflicting theories as to the preferential location of Li electro-deposition at the early stages of dendrite formation [2-5].  Admittedly, the complexity of dendrite morphology is exacerbated by numerous factors influencing the initiation and propagation including, but not limited to, temperature, electrolyte composition, current density and over-potentials [6].

Herein, we employ in situ optical microscopy and x-ray nano-CT (computed tomography) to establish relationships between electrochemical parameters and the resultant dendrite morphologies during initiation (early stage) and propagation (late stage) of Li electro-deposits on copper wires.  X-ray nano-CT is employed as a unique, non-destructive technique capable of 3-dimensional imaging of the surface and internal structures of opaque materials, with ~50 nm resolution.  This allows for 3D morphological characterization during the early stages of Li dendrite initiation.  The subsequent propagation morphologies will be characterized and tracked via in situ optical microscopy. 

Results and discussions

Single projection, large field-of-view, phase contrast mosaic radiographs of lithium electro-deposited onto 80 micron diameter copper (Cu) wire are shown in Figure 1.  The dark contrasts within the radiographs reflect highly x-ray attenuating elements (i.e. Cu) and the lighter contrast features are due to the electro-deposited lithium.  At low current densities, lithium deposits uniformly along the surface of the Cu wire as seen in Figure 1a.  At high current densities, non-homogeneous (~10 – 50 mm) features are evident in Figure 1b.  These non-homogeneous features appear to exhibit complex architectures with regions containing both continuous and isolated porous networks, depicted with the 3-dimensional reconstructed rendering in Figure 1c. 

References

[1] Love, C. T., et al.  ECS Electrochem. Lett. 4(2) (2015) A24-A27.

[2] Barton, J. L. et al. Proc. R. Soc. Lond. A. 268 (1962) 485-505.

[3] Voss, R. F. et al. J. Electrochem. Soc. 132 (1985) 371-375.

[4] Chazalviel, J.-N. Phys. Rev. A. 42 (1990) 7355-7367.

[5] Yamaki, J. J. Power Sources. 74 (1998) 219-227.

[6] Li, Z. et al. J. Power Sources. 254 (2014) 168-182.