PNNL team reports growth of dendrite-free lithium films with self-aligned and compact nanorod structure

2 December 2014

Suppressing lithium (Li) dendrite growth is one of the most critical challenges for the development of Li-metal batteries—i.e., high-energy density batteries using a Li-metal anode such as Li-sulfur or Li-air. (Earlier post.) Researchers at Pacific Northwest National Laboratory (PNNL) report for the first time the growth of dendrite-free lithium films with a self-aligned and highly compacted nanorod structure. Their paper appears in the ACS journal Nano Letters.

Lithium metal is a very promising anode material for high-capacity rechargeable batteries due to its theoretical high capacity of 3,860 mAh g⁻¹ (~10x that of the 372 mAh g⁻¹ of graphite anodes in Li-ion batteries), but it fails to meet cycle life and safety requirements due to electrolyte decomposition and dendrite formation on the surfaces of the lithium metal anodes during cycling. Thus, numerous efforts have been and are being made to develop a safe, extended cycling lithium-metal electrode and/or supporting electrolyte (e.g., earlier post, earlier post, earlier post, earlier post.)

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Credit: ACS, Zhang et al. Click to enlarge.

Most of the work in suppression of dendrite formation can be divided into three categories, the team noted in their paper:

• Improve the stability and uniformity of the solid electrolyte interphase (SEI) on the Li electrode surface by optimization of solvents salts, and electrolyte
  additives. However, the SEI alone cannot completely eliminate dendrite formation/growth because of its weak mechanical strength.

• Form alloys of Li and non-Li metal during electrodeposition, which is achieved by adding inorganic compounds or a second salt to the electrolytes. However, most of these metal cation additives will be consumed by forming alloys during Li deposition, so the suppression of Li dendrite formation may not be sustainable.

• Use mechanical barriers to block dendrite growth. However, while such a protective layer acts as an effective physical barrier to block dendrite penetration, it does not alter the growth mechanism of Li dendrites on a fundamental level. As a result, porous Li may still be generated beneath the physical barrier and lead to a rapid increase in the impedance of the Li anode and battery failure even without a dendrite-related short.

The PNNL team itself had earlier developed a cesium hexafluorophosphate (CsPF₆) as an electrolyte additive which was shown to suppress Li dendrite formation.

In this work, we further investigated the evolution of the cross-sectional morphologies of such Li films during deposition/stripping cycles. The effects of Cs⁺ and other components in electrolytes on the voltage profiles of electrochemical deposition and the chemistry of the SEI formed on copper (Cu) substrates prior to Li deposition are systematically investigated. It is revealed for the first time that the apparently dense Li films grown in the presence of CsPF₆ additive actually consist of self-aligned, highly compacted nanorods. We also found that, before the reductive decomposition of carbonate solvents at about 0.9−1.2 V, a thin SEI has already formed on the Cu surface at about 2.05 V vs Li/Li⁺. The quality of this underlying SEI predecessor would essentially dictate the morphologies of the Li to be deposited.

The new understanding on the internal microstructure of the dendrite-free Li films deposited electrochemically, their structural evolution during stripping/deposition processes, and the synergistic effect of Cs⁺ additive and preformed underlying SEI on Li deposition will help researchers in this field design new approaches to enable a metallic Li anode, the “Holy Grail” of Li-based battery chemistries.

Schematic of Li deposition and stripping processes when the Cs additive is used. (a) The initial distribution of cations/anions before Li deposition; (b) redistribution of cations/anions after an electric field is applied and the formation of the initial SEI layer (represented by a blue line) before Li deposition; (c) the initial growth of small Li (represented by gray area) tips; (d) the dendrite−free Li films with self-aligned and highly compact nanorods; (e) Li film after partial stripping; and (f) Li film after redeposition. The blue lines covering at the surface of deposited Li (gray) represent the SEI layer formed during the deposition and stripping processes. Credit: ACS, Zhang et al. Click to enlarge.

Resources

• Yaohui Zhang, Jiangfeng Qian, Wu Xu, Selena M. Russell, Xilin Chen, Eduard Nasybulin, Priyanka Bhattacharya, Mark H. Engelhard, Donghai Mei, Ruiguo Cao, Fei Ding, Arthur V. Cresce, Kang Xu, and Ji-Guang Zhang (2014) “Dendrite-Free Lithium Deposition with Self-Aligned Nanorod Structure” Nano Letters doi: 10.1021/nl5039117