Stabilizing the surface of lithium metal
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oduction Over the past decade, researchers have been extending the boundaries of Li-ion battery-based energy storage systems by utilizing new approaches that enable high capacity systems based on non-intercalation cathodes, such as sulfur, selenium, and oxygen.1–6 Although significant effort has been put into the cathode side of these systems, the success of these high capacity energy storage systems relies on the realization of the promise of Li-metal anodes. Li metal has many advantageous properties, including an extremely high theoretical specific capacity (3860 mAh g–1), the lowest electrochemical potential (–3.040 V versus standard hydrogen electrode), and low density (0.59 g cm–3), which make it a very desirable electrode for energy storage devices. However, while primary Li batteries are used for numerous commercial applications, rechargeable Limetal batteries that utilize Li-metal anodes have not been as successful, although they have been investigated in the last 40 years.7–9 This has been attributed to several underlying issues—among them the growth of high surface area Li dendritic structures formed during repeated charge/ discharge processes and the low Coulombic efficiency (CE) of repeated electrodeposition.10,11 These issues are highlighted in Figure 1a–b with a schematic diagram comparing a Li-ion battery and a Li-metal battery.12
In Li-ion batteries, the Li cation is never reduced to metal; it shuttles between two electrodes—insertion, conversion, or intercalation—that store charge via redox reactions. For negative electrodes, graphitic carbon is most widely used, because Li ions can be intercalated into its layered structure at low potentials (see the Introductory article in this issue), creating a relatively high cell voltage, depending on the cathode used. By comparison for Li-metal batteries, the source of lithium for the cell is the Li-metal anode, which operates by electrodepositing lithium onto its surface during charge and stripping lithium from its surface during discharge. This constant stripping and deposition reworks the Li-metal surface on each cycle and leads to uneven surface morphologies that eventually lead to either mossy or dendritic growth or poor CE as active lithium is lost to side reactions. The consequences of this process are shown in Figure 1c. The benefit of Li-metal batteries is that the metal anode represents a plentiful source of lithium for the cell creating the high energy density needed to support the development of new technologies, such as the Li-S battery or Li-air/oxygen battery (see the Nazar et al. and Kwabi et al. articles, respectively, in this issue). Both dendrite growth and low CE problems of Li-metal electrodes are partly derived from the attributes that make Li metal a desirable anode—its low electrochemical potential (high reactivity with its environment) and low density
J.T. Vaughey, Argonne National Laboratory, IL, USA; [email protected] Gao Liu, Lawrence Berkeley National Laboratory, CA, USA; [email protected] Ji-Guang Zhang, Pacific Northwest National Laborator
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