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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP An additional charge storage mechanism that has been investigated is known as pseudocapacitance, which broadly describes rapid and reversible Faradaic reactions that feature charge/discharge profiles that mimic those of electrical double layer capacitors.42 In this sense, pseudocapacitors represent a clear example of materials exhibiting multiple charge storage mechanisms. Pseudocapacitive reactions are often near-surface reactions that occur with fast kinetics, and they can feature higher capacitance than pure double-layer charge storage.43 Nanostructured materials that are thin enough can exhibit primarily pseudocapacitive behavior, which can result in increased power performance.44 A related reaction mechanism, intercalation pseudocapacitance, represents a novel type of reaction that combines advantageous features of both high-power capacitors and high-energy batteries.38 The intercalation pseudocapacitance mechanism involves electron transfer reactions and ion insertion within the bulk with fast, pseudocapacitance-like kinetics. This is enabled by crystal structures that have open transport pathways and negligible structural changes upon intercalation. T-Nb2O5 nanocrystals were the first nanomaterials demonstrated to exhibit intercalation pseudocapacitance (Figure 3.1.4), and this list has expanded to include other oxides, nitrides, carbides and metal phosphates.45-47 Electrode materials that exhibit intercalation pseudocapacitance hold the promise of achieving relatively high capacity (although not as high as alloying/conversion materials) combined with the cycle life and power density of electrical double layer capacitors. Intercalation pseudocapacitance offers combined high energy and power, but there are a limited number of materials that exhibit this mechanism. A way to expand the number of active materials fundamentally capable of exhibiting high energy and power is to discover and develop individual or heterostructured materials that are capable of exhibiting multiple charge storage mechanisms. These materials would be useful for applications that require different rate capabilities at different times, and their use in a single system would likely result in decreased cost compared to the alternative of separate batteries and supercapacitors. The rational design of heterostructured nanomaterials is a promising strategy to achieve multiple charge mechanisms in single structures.48,49 One major advantage of designing and using heterostructured materials is the synergistic improvement of the intrinsic properties of each component for better electronic conductivity, faster ionic transport, greater electrochemical reversibility, and overall cycling stability. Owing to these advantages, pseudocapacitive heterostructured materials are currently considered to be potential candidates for electrodes that exhibit simultaneous high power and high energy.45 Beyond pseudocapacitance, there is also growing interest in 2D materials for electrochemical energy storage, because they offer fascinating physical and chemical properties that are particularly appropriate for ion storage.50,51 2D materials show high redox activity during intercalation processes,52 but they can suffer from irreversible restacking/agglomeration, which can lead to capacity degradation with cycling and sluggish ion transport. 2D heterostructures may be a possible solution to overcome this major limitation, and pursuit of such materials could open up new opportunities for creating electrodes with simultaneous high energy and power capability. By combining the metallic electrical conductivity of graphene and MXenes (a class of materials that includes 2D transition metal carbides, carbonitrides, and nitrides53) with the high redox activity of transition metal oxides or chalcogenides, stacked 2D heterostructures with attractive energy storage characteristics can be realized. Stacked 2D heterostructures containing different active materials have been shown to demonstrate various energy storage mechanisms, including surface adsorption/desorption, intercalation pseudocapacitance, 86 PANEL 1 REPORT a c a b 200 180 160 140 120 100 80 60 40 20 0 Li4Ti5O12 1 10 C rate Nb2O5 100 1000 Figure 3.1.4. (a) Crystal structure of T-Nb2O5 stacked along the c-axis showing the layered arrangement of O (red) and Nb (inside polyhedra) atoms along the a-b plane. The empty octahedral sites between (001) planes provide natural tunnels for rapid Li+ ion transport throughout the a-b plane. (b) Comparison of the rate capability of T-Nb2O5 with a high-rate lithium-ion anode, Li4Ti5O12, at various C-rates. From Ref. 38. Capacity (mAh g-1)PDF Image | Next Generation Electrical Energy Storage
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