High-power Na-ion batteries have tremendous potential in various large-scale applications. However, conventional charge storage through ion intercalation or double-layer formation cannot satisfy the requirements of such applications owing to the slow kinetics of ion intercalation and the small capacitance of the double layer. The present work demonstrates that the pseudocapacitance of the nanosheet compound MXene Ti2C achieves a higher specific capacity relative to double-layer capacitor electrodes and a higher rate capability relative to ion intercalation electrodes. By utilizing the pseudocapacitance as a negative electrode, the prototype Na-ion full cell consisting of an alluaudite Na2Fe2(SO4)3 positive electrode and an MXene Ti2C negative electrode operates at a relatively high voltage of 2.4 V and delivers 90 and 40 mAh g−1 at 1.0 and 5.0 A g−1 (based on the weight of the negative electrode), respectively, which are not attainable by conventional electrochemical energy storage systems.

MXene, a family of layered compounds consisting of nanosheets, is emerging as an electrode material for various electrochemical energy storage devices including supercapacitors, lithium-ion batteries, and sodium-ion batteries. However, the mechanism of its electrochemical reaction is not yet fully understood. Herein, using solid-state (23)Na magic angle spinning NMR and density functional theory calculation, we reveal that MXene Ti3C2Tx in a nonaqueous Na(+) electrolyte exhibits reversible Na(+) intercalation/deintercalation into the interlayer space. Detailed analyses demonstrate that Ti3C2Tx undergoes expansion of the interlayer distance during the first sodiation, whereby desolvated Na(+) is intercalated/deintercalated reversibly. The interlayer distance is maintained during the whole sodiation/desodiation process due to the pillaring effect of trapped Na(+) and the swelling effect of penetrated solvent molecules between the Ti3C2Tx sheets. Since Na(+) intercalation/deintercalation during the electrochemical reaction is not accompanied by any substantial structural change, Ti3C2Tx shows good capacity retention over 100 cycles as well as excellent rate capability.

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...

A new class of battery electrode material using a rapid charge accumulation at the surface, a pseudocapacitor electrode, is a key for advanced batteries. In article number https://doi.org/10.1002/aenm.201601873, Atsuo Yamada and co‐workers demonstrate that the capacity of a titanium carbide (MXene) pseudocapacitor electrode increases drastically by surface state modification, which also accelerates Li‐ion transport in the electrode.

An analytical method to determine the electrochemical energy efficiencies of electrolytes with partial electronic conduction has been developed previously and reported in the literature. However, this analytical method does not address the effects of differing ionic species in electrolytes, i.e., the oxide‐ions or protons. Therefore, we aimed to modify this analytical method to account for the effects of differing ionic species, and applied it to compare the energy efficiencies of oxide‐ion conducting solid electrolytes such as yttria‐stabilized zirconia (YSZ) and gadolinia‐doped ceria (GDC) to proton‐conducting solid electrolytes, such as yttria‐doped barium zirconate (BZY). With the modification, difference in the influence of the fuel consumption between the oxide‐ion conducting electrolyte and the proton‐conducting electrolyte has been successfully taken into account. The energy efficiency of the BZY electrolyte relatively increased against those of YSZ or GDC electrolytes by the modification. Additionally, partial oxide‐ion conduction in the proton‐conducting electrolyte was successfully estimated using the modified analytical method.