For a mass commercialization of Li-S chemistry the gravimetric energy density must be clearly above that of state-of-theart lithium-ion cells (with the Panasonic NCR18650B as current energy density champion) to compensate for the much lower cycle stability. The number 18650 describes the cell's shape with a diameter of ≈18 mm and a height of ≈65 mm. The NCR18650B provides a capacity of ≈3.3Ah with a nominal voltage of 3.6 V resulting in a gravimetric energy density of ≈240 Wh kg −1 and a volumetric energy density of ≈670 Wh L −1 . Additionally, the corresponding cell type can achieve several hundred cycles until 80% of the initial capacity is reached. By contrast, although high cycle numbers are reported for Li-S cells in the literature, the fl aw is that these high cycle numbers are only obtained because of an excess of lithium, an excess of electrolyte and low sulfur areal loads, [ 7 ] resulting in very poor potential gravimetric energy density. Figure 1 shows the gravimetric and volumetric energy density of various electrochemical energy storage systems. The Li-S cell manufacturers Sion Power and Oxis Energy expect that future Li-S cells will have a volumetric energy density comparable to that of state-of-the-art Li-ion cells (≈700 Wh L −1 ) but more than twice the gravimetric energy density with values of 400-600 Wh kg −1 .The scope of this article can be summarized as follows:• A Li-S review will be provided focusing on statistical information like sulfur load and sulfur electrode fraction which determine the energy density and discussing the state-of-theart of the worldwide Li-S research.• By opening an NCR18650B we obtained information about the passive weight distribution of state-of-the-art high energy 18650 cells. With this information we were able to calculate the possible energy densities and prices of future Li-S cells for various sulfur loads, sulfur utilizations, and electrolyte/ sulfur (E/S) ratios. Keeping in mind that a Li-S cell must have a superior gravimetric energy density to the NCR18650B these results provide insights into which electrode properties and electrochemical results must be obtained. Additionally, they allow an evaluation of the state of the art of international scientifi c Li-S research.• Finally, an electrode that meets important demands for high gravimetric energy densities is introduced. Li-S cells

Sodium-ion hybrid capacitors are known for their high power densities and superior cycle life compared to Na-ion batteries. However, low energy densities (<100 Wh kg–1) due to the lack of high-capacity (>150 mAh g–1) anodes capable of fast charging are delaying their practical implementation. Herein, we report a high-performance Na-ion hybrid capacitor based on an interface-engineered hierarchical TiO2 nanosheet anode consisting of bronze (∼15%) and anatase (∼85%) crystallites (∼10 nm). This pseudocapacitive dual-phase anode demonstrated exceptional specific capacity of 289 mAh g–1 at 0.025 A g–1 and excellent rate capability (110 mAh g–1 at 1.0 A g–1). The Na-ion hybrid capacitor integrating a dual-phase hierarchical TiO2 nanosheet anode and an activated carbon cathode exhibited a high energy density of 200 Wh kg–1 (based on the total mass of active materials in both electrodes) and power density of 6191 W kg–1. These values are in the energy and power density range of Li-ion batteries (100–300 Wh kg–1) and supercapacitors (5000–15 000 W kg–1), respectively. Furthermore, exceptional capacity retention of 80% is observed after 5000 charge–discharge cycles. Outstanding electrochemical performance of the demonstrated Na-ion hybrid capacitor is credited to the enhanced pseudocapacitive Na-ion intercalation of the two-dimensional TiO2 anode resulting from nanointerfaces between bronze and anatase crystallites. Mechanistic investigations evidenced Na-ion storage through intercalation pseudocapacitance with minimal structural changes. This approach of nanointerface-induced pseudocapacitance presents great opportunities toward developing advanced electrode materials for next-generation Na-ion hybrid capacitors.

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