Full paperSeeding lithium seeds towards uniform lithium deposition for stable lithium metal anodes
Graphical abstract
Introduction
Lithium (Li) metal anode (LMA), with ultrahigh specific capacity (3860 mAh g−1), the lowest negative reduction potential (−3.040 V) vs. standard hydrogen electrode and low density (0.536 g cm−3), is the most attractive anode for high energy density batteries such as lithium-sulfur and lithium-oxygen batteries [[1], [2], [3]]. However, the practical use of LMA has been hindered by several challenges. The uncontrolled Li dendrite growth during plating/stripping caused by non-uniform Li nucleation and deposition not only induces severe safety issue but also leads to capacity fading by continuously consuming electrolyte and Li ions [[4], [5], [6], [7]]. Furthermore, the solid electrolyte interface (SEI) film on the LMA surface suffers the repeated breaking and reformation during cycling because of the large volume change, resulting in a low Coulombic efficiency [[8], [9], [10], [11], [12], [13]]. Constructing three-dimensional (3D) host structures, such as carbon-based materials and 3D metallic materials, has attracted considerable interest because they can simultaneously accommodate the volume change as well as lower the local current density [[14], [15], [16], [17], [18], [19], [20], [21], [22]], effectively suppressing the Li dendrite growth. However, most of these materials are lithiophobic [16,17], which induces a large nucleation overpotential during Li nucleation and the large surface area of these structures leads to the high activity and thus the increase of the irreversible reaction.
The electrodeposition involves two steps: the nucleation and the following growth, and the deposited structure is determined by the number of initial nuclei and affected by the nuclei structure [11,[23], [24], [25], [26]]. In order to achieve a uniform deposition layer with fine grains, a high density and uniformly distributed nuclei layer is required. However, due to the lithiophobic nature of many LMA hosts, the large nucleation barrier leads to the non-uniform formation of nuclei, and thus, heterogeneous seeds including Au and Ag are dispersed on the substrate to decrease the nucleation barrier and guide the uniform Li nucleation, and then, the dendrite-free deposition [[26], [27], [28], [29], [30], [31], [32]]. However, these seeds are hard to densely and uniformly distributed on the electrode surface because of the complicated preparation process. Even worse, on Li metal surface, they are improper species causing much larger nucleation barrier than that on Li, leading to the following non-uniform Li deposition. Thus, planting the densely and uniformly distributed Li seed layer should be a versatile way to guide the Li deposition on different surfaces. However, how to seed this nuclei layer and its influence on the following Li deposition have not been well demonstrated.
In this work, we developed a simple but effective way to guide the Li deposition by pre-seeding a dense Li seed layer on the surface of electrode. The seeds with precisely controlled size can be easily planted by a fast nucleation process under a high current density in an assembled cell, avoiding the use of complicated surface modification methods, especially for the Li metal surface which is hard to manipulate in most situations. The uniformly pre-deposited ultrafine Li seeds in this layer greatly decrease the barrier and overpotential for the following Li deposition and thus, leads to formation of homogeneous Li layer and suppresses the dendrite growth. With these advantages, the Li‖Li symmetric cells with the Li seed layer demonstrate a long cycling performance of 350 h with a low overpotential at high current density of 3 mA cm−2. Moreover, full cells with a NCA cathode exhibit an improved cycling performance, revealing its potential for practical applications.
Section snippets
Material characterization
The surface morphologies of Li seeds and the deposited Li after cycling were characterized with a field emission scanning electron microscope (FE-SEM, ZEISS SUPRA 55). The cells were disassembled in the air-filled glove box after planting Li seeds or cycling. The surface chemistry of Li seeds and bare Li were investigated by X-ray photoelectron spectroscopy (XPS) on a PHI 5000 VersaProbe II spectrometer using monochromatic Al Kα X-ray source. The electrodes were washed with dimethyl ether (DME)
Results and discussion
Fig. 1 shows the schematics for different Li plating morphologies without and with Li seeds. As shown in Fig. 1a, taking Cu foil as a typical substrate, lithiophobic Cu surface induces a non-uniform Li deposition due to the large nucleation barrier and the uncontrolled Li deposition. In contrast, the introduction of Li seeds makes the Cu surface lithiophilic and provides abundant nucleation and growth sites, and thus achieving controllable Li deposition. Li ions tend to deposit on these nuclei
Conclusion
In summary, we used a pre-planted thin Li seed layer to regulate the following Li deposition, which effectively suppresses the dendrite growth and realizes the stable cyclic ability. The dense and uniform Li seed layer with small nuclei lowers the following Li deposition energy barrier and overpotential, restraining the non-uniform distribution. In addition, the Li ion flux and the electric field on electrode surface are homogenized and thus suppress the Li dendrite growth. As a result, a
Acknowledgment
We appreciate support from National Science Fund for Distinguished Young Scholars, China (No. 51525204), National Natural Science Foundation of China (Nos. 51772164 and U1601206), Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01N111), Guangdong Natural Science Funds for Distinguished Young Scholars (2017B030306006), Guangdong Special Support Program (2017TQ04C664), Shenzhen Graphene Manufacturing Innovation Center (201901161513) and the Shenzhen
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These authors are equal main contributors to this work.