The development of efficient electrochemical energy storage (EES) devices is an important sustainability issue to realize green electrical grids. Charge storage mechanisms in present EES devices, such as ion (de)intercalation in lithium-ion batteries and electric double layer formation in capacitors, provide insufficient efficiency and performance for grid use. Intercalation pseudocapacitance (or redox capacitance) has emerged as an alternative chemistry for advanced EES devices. Intercalation pseudocapacitance occurs through bulk redox reactions with ultrafast ion diffusion. In particular, the metal carbide/nitride nanosheets termed MXene discovered in 2011 are a promising class of intercalation pseudocapacitor electrode materials because of their compositional versatility for materials exploration (e.g., TiCT , TiCT , VCT , and NbCT , where T is a surface termination group such as F, Cl, O, or OH), high electrical conductivity for high current charge, and a layered structure of stacked nanosheets for ultrafast ion intercalation. Various MXene electrodes have been reported to exhibit complementary battery performance, such as large specific capacity at high charge/discharge rates. However, general design strategies of MXenes for EES applications have not been established because of the limited understanding of the electrochemical mechanisms of MXenes. This Account describes current knowledge of the fundamental electrochemical properties of MXenes and attempts to clarify where intercalation capacitance ends and intercalation pseudocapacitance begins. MXene electrodes in aqueous electrolytes exhibit intercalation of hydrated cations. The hydrated cations form an electric double layer in the interlayer space to give a conventional capacitance within the narrow potential window of aqueous electrolytes. When nonaqueous electrolytes are used, although solvated cations are intercalated into the interlayer space during the initial stage of charging, the confined solvation shell should gradually collapse because of the large inner potential difference in the interlayer space. Upon further charging, desolvated ions solely intercalate, and the atomic orbitals of the desolvated cations overlap with the orbitals of MXene to form a donor band. The formation of the donor band induces the reduction of MXene, giving rise to an intercalation pseudocapacitance through charge transfer from the ions to MXene sheets. Differences in the electrochemical reaction mechanisms lead to variation of the electrochemical responses of MXenes (e.g., cyclic voltammetry curves, specific capacitance), highlighting the importance of establishing a comprehensive grasp of the electrochemical reactions of MXenes at an atomic level. Because of their better charge storage kinetics compared with those of typical materials used in present EES devices, aqueous/nonaqueous asymmetric capacitors using titanium carbide MXene electrodes are capable of efficient operation at high charge/discharge rates. Therefore, the further development of novel MXene electrodes...

high power and high energy. Although state-of-the-art Li-ion batteries efficiently store energy by Li-ion (de)intercalation into the host electrode materials, their power is limited in part by slow ion transfer. [1,2] Furthermore, carbonaceous compounds, which are the most used negative electrode materials in Li-ion batteries, exhibit Li-ion (de)intercalation near the Li metal plating potential, hindering the charging of batteries at a high rate. [3,4] Accordingly, the development of negative electrode materials that are capable of more charge at a faster rate remains a major challenge.Electrode materials for electrochemical capacitors store the charge by surface ion adsorption, which intrinsically achieves a high power density. [5][6][7] The modest energy density of conventional double-layer capacitors owing to their insufficient capacitance can be enhanced by accumulating pseudocapacitance by surface ion adsorption accompanied with surface redox reactions. [8][9][10][11][12] However, the use of electrochemical cells composed of pseudocapacitive electrodes do not avoid the compromise between the high power and high energy densities, and a practical technical solution has been Li-ion hybrid capacitor, in which intercalation-type compounds are employed either in the cathode or anode. [13,14] One option is a Li-ion hybrid capacitor with a pseudocapacitive porous carbon cathode and an intercalation-type anode (e.g., Li 4 Ti 5 O 12 ). [15][16][17] Another example is a combination of an intercalation-type cathode (e.g., LiMn 2 O 4 ) and a pseudocapacitive anode such as a MnO 2 /carbon nanotube composite. [18,19] However, the energy and power densities of the Li-ion hybrid capacitors are not yet satisfactory for commercialization. Hence, tremendous efforts have been devoted to the development of superior pseudocapacitive electrode materials, such as nitrogendoped carbon, [20] RuO 2 ⋅nH 2 O, [21] or T-Nb 2 O 5 . [22] In particular, nanosheet compounds are of potential interest because (1) the stacked nanosheets enable a high packing density for the high volumetric capacitance;(2) open interlayer space between the nanosheets offers fast ion accessibility to the redox center, what we call "intercalation pseudocapacitance"; [22] and (3) electrically conductive nanosheets permit high power operation. [23][24][25] Among various nanosheet compounds, MXene (M n+1 X n T x ; M: Ti, V, Cr, Nb, etc.; X: C, N; n = 1-3; T: surface termination groups) is an important emerging class of electrode materials for both supercapacitors and batteries. [26][27][28][29][30][31][32] One of the advantages of MXene is its very high electronic conductivity which Pseudocapacitance is a key charge storage mechanism to advanced electrochemical energy storage devices distinguished by the simultaneous achievement of high capacitance and a high charge/discharge rate by using surface redox chemistries. MXene, a family of layered compounds, is a pseudocapacitor-like electrode material which exhibits charge storage through exceptionally fast ion accessibil...

Electric double-layer capacitors are efficient energy storage devices that have the potential to account for uneven power demand in sustainable energy systems. Earlier attempts to improve an unsatisfactory capacitance of electric double-layer capacitors have focused on meso- or nanostructuring to increase the accessible surface area and minimize the distance between the adsorbed ions and the electrode. However, the dielectric constant of the electrolyte solvent embedded between adsorbed ions and the electrode surface, which also governs the capacitance, has not been previously exploited to manipulate the capacitance. Here we show that the capacitance of electric double-layer capacitor electrodes can be enlarged when the water molecules are strongly confined into the two-dimensional slits of titanium carbide MXene nanosheets. Using electrochemical methods and theoretical modeling, we find that dipolar polarization of strongly confined water resonantly overscreens an external electric field and enhances capacitance with a characteristically negative dielectric constant of a water molecule.

Electrochemical double-layer (EDL) capacitors operating at high charge/ discharge rates are an important class of electrochemical energy storage devices. Aqueous EDL capacitors show great potential for use as inexpensive devices with much higher power; however, their energy density is severely limited by the narrow electrochemical window of water (1.23 V) and the small specific capacity of the electrodes. Here, we develop a highvoltage aqueous supercapacitor based on a highly concentrated Li + aqueous electrolyte (hydrate melt) and a two-dimensional titanium carbide MXene electrode. Experimental and theoretical analyses reveal the existence of dense hydrated Li + in the interlayer space of the deeply charged MXene, which is realized by the wide electrochemical window of a hydrate-melt electrolyte. The hydrate-melt electrolyte together with the largecapacitance MXene Ti 2 CT x improves the performance of an aqueous lithium-ion supercapacitor, offering a promising strategy for advanced aqueous capacitors.

A spin transition between high-spin (HS) and low-spin (LS) states in a solid can occur when the energies of two spin configurations intersect, which is usually caused by external perturbations such as temperature, pressure, and magnetic fields, with substantial influence to its physical and chemical properties. Here, we discover the electrochemical "redox reaction" as a new driving force to induce reversible HS−LS spin transition. Although reversible solid-state redox reaction has been thoroughly investigated as the fundamental process in battery electrode materials, coupling between redox reactions and spin transitions has not been explored. Using density functional theory calculations, we predicted the existence of redox-driven spin transition occurring exclusively for the Co 3+ /Co 2+ redox couple in layered transition-metal oxides, leading to a colossal potential hysteresis (>1 V) between the cathodic (LS Co 3+ to LS Co 2+ ) and anodic (HS Co 2+ to HS Co 3+ ) reactions. The predicted potential hysteresis associated with the spin transition of Co was experimentally verified for Na x Ti 0.5 Co 0.5 O 2 by monitoring the electrochemical potential, local coordination structure, electronic structure, and magnetic moment.