The intermittent nature of renewable energy sources, such as solar and wind, calls for sustainable electrical energy storage (EES) technologies for stationary applications. Li will be simply too rare for Li-ion batteries (LIBs) to be used for large-scale storage purposes. In contrast, Na-ion batteries (NIBs) are highly promising to meet the demand of grid-level storage because Na is truly earth abundant and ubiquitous around the globe. Furthermore, NIBs share a similar rocking-chair operation mechanism with LIBs, which potentially provides high reversibility and long cycling life. It would be most efficient to transfer knowledge learned on LIBs during the last three decades to the development of NIBs. Following this logic, rapid progress has been made in NIB cathode materials, where layered metal oxides and polyanionic compounds exhibit encouraging results. On the anode side, pure graphite as the standard anode for LIBs can only form NaC64 in NIBs if solvent co-intercalation does not occur due to the unfavorable thermodynamics. In fact, it was the utilization of a carbon anode in LIBs that enabled the commercial successes. Anodes of metal-ion batteries determine key characteristics, such as safety and cycling life; thus, it is indispensable to identify suitable anode materials for NIBs. In this Account, we review recent development on anode materials for NIBs. Due to the limited space, we will mainly discuss carbon-based and alloy-based anodes and highlight progress made in our groups in this field. We first present what is known about the failure mechanism of graphite anode in NIBs. We then go on to discuss studies on hard carbon anodes, alloy-type anodes, and organic anodes. Especially, the multiple functions of natural cellulose that is used as a low-cost carbon precursor for mass production and as a soft substrate for tin anodes are highlighted. The strategies of minimizing the surface area of carbon anodes for improving the first-cycle Coulombic efficiency are also outlined, where graphene oxide was employed as dehydration agent and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was used to unzip wood fiber. Furthermore, surface modification by atomic layer deposition technology is introduced, where we discover that a thin layer of Al2O3 can function to encapsulate Sn nanoparticles, leading to a much enhanced cycling performance. We also highlight recent work about the phosphorene/graphene anode, which outperformed other anodes in terms of capacity. The aromatic organic anode is also studied as anode with very high initial sodiation capacity. Furthermore, electrochemical intercalation of Na ions into reduced graphene oxide is applied for fabricating transparent conductors, demonstrating the great feasibility of Na ion intercalation for optical applications.

mechanism. The large impedance associated with transport for ions and electrons limits the thickness of the electrode usually less than 100 µm. To overcome the issue of charge transport, 1D and 2D nanomaterials (i.e., carbon nanotubes and graphene) have been used to provide fast percolative pathways for electron transport. [16][17][18][19] It is also found that low-tortuosity electrodes can provide fast ion transport, which indeed lead to much-improved rate performance. [ 20,21 ] Wood has a unique anisotropic structure, where there are open channels along the growth direction to help pump water, ions, and other ingredients. Herein, we design a 3D carbon electrode through directly carbonizing wood that is cut perpendicularly to the growth direction. The carbonized wood has the perfect open channels, which lead to a low tortuosity for ion transport. The well-connected carbon also provides excellent path for electron transport with small impedance. In this work, we demonstrated for the fi rst time that ultra-thick, mesoporous carbon with a thickness up to 850 µm and an areal mass of 55 mg cm −2 . The mesoporous carbon not only shows a high specifi c capacity of 270 mA h g −1 but also a high areal capacity of 13.6 mA h cm −2 evaluated as anode for SIB in half cells. The thickness, areal mass, and areal capacity are signifi cantly higher than the values from the state-of-the-art batteries. The ultra-thick wood-derived carbon is also a binder-free, current collector-free electrode that also signifi cant increases the weight percentage of the active mass in SIBs. Excellent cycling performance was demonstrated in full cells based on Na 3 V 2 (PO 4 ) 3 cathode and wood carbon anode. The woodbased freestanding, mesoporous carbon with a unique anisotropic structure is a promising anode in the emerging SIB technology. Note that other biomass has been investigated as the precursors for high-performance SIB anodes, [22][23][24] but the biomass-derived carbons follow traditional battery design with binders coated on current collectors and the areal capacity is much smaller than the reported value in this study. The lowtortuosity wood with the open, ordered channels can also open a range of other energy and environmental-related applications, such as membrane for gas separation, water fi ltration, and fl ow batteries.Wood is one of the most abundant biomass on Earth and has a heterogeneous and anisotropic structure ( Figure 1 a). There are a large number of straight multichannels in the up-growing direction of the tree, leading to a lowest tortuosity, close to one, along the growth direction. Such a unique structure inspires us a new insight for fabricating low-tortuosity carbon based on wood, which can be dramatically different from any other types of macroscopic carbon. As shown in Figure 1 b, a 3D structured carbon electrode is Grid-scale energy storage is critical in the renewable energy landscape due to the intermittent nature of the renewable energy sources such as wind, solar, and others. Compared with many other grid-sca...

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Graphene has attracted a lot of attention for ultracapacitor electrodes because of its high electrical conductivity, high surface area, and superb chemical stability. However, poor volumetric capacitive performance of typical graphene-based electrodes has hindered their practical applications because of the extremely low density. Herein we report a scalable synthesis method of holey graphene (h-Graphene) in a single step without using any catalysts or special chemicals. The film made of the as-synthesized h-Graphene exhibited relatively strong mechanical strength, 2D hole morphology, high density, and facile processability. This scalable one-step synthesis method for h-Graphene is time-efficient, cost-efficient, environmentally friendly, and generally applicable to other two-dimensional materials. The ultracapacitor electrodes based on the h-Graphene show a remarkably improved volumetric capacitance with about 700% increase compared to that of regular graphene electrodes. Modeling on individual h-Graphene was carried out to understand the excellent processability and improved ultracapacitor performance.