ARTICLES https://doi.org/10.1038/s41560-019-0339-9 1 A. J. Drexel Nanomaterials Institute and Department of Materials Science and Engineering, Drexel University, Philadelphia, PA, USA. 2 Materials Science Department—CIRIMAT, Université Paul Sabatier, Toulouse, France. 3 Department of Materials Science and Engineering, Sichuan University, Chengdu, China. 4 Materials Science and Technology Division, Materials Theory Group, Oak Ridge National Laboratory, Oak Ridge, TN, USA. 5 Joint Institute for Computational Sciences, University of Tennessee, Knoxville, Oak Ridge, TN, USA. 6 Murata Manufacturing Co., Ltd, Nagaokakyo-shi, Kyoto, Japan. 7 Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA. 8 NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD, USA. 9 Department of Materials Science and Engineering, University of Maryland, College Park, MD, USA. 10 Réseau sur le Stockage Electrochimique de l’Energie (RS2E), CNRS FR3459, Amiens, France. *e-mail: gogotsi@drexel.edu G rowing demand for fast-charging electrochemical energy stor- age devices with long cycle lifetimes for portable electronics has led to a desire for alternatives to current battery systems, which store energy via slow, diffusion-limited Faradaic reactions. The devices that fit these demands most closely are electrochemi- cal double-layer capacitors (EDLCs), also called supercapacitors, which can be fully charged within minutes with almost unlimited cyclability. However, the energy that can be stored via physisorption of ions at the pore walls of the porous carbons currently used in supercapacitor electrodes is much lower than the charge stored in battery materials, making them unsuitable replacements for many applications 1 . Replacing the carbon electrodes in supercapacitors with pseudocapacitive materials results in devices with the potential for higher energy storage capabilities versus devices using EDLC- type materials 2 . Unlike the diffusion-controlled intercalation of charge-storing ions into the crystal structure of battery materials, pseudocapacitive charge storage is defined by fast surface-confined redox reactions, which enable high-rate energy storage. The most prevalent mindset for designing energy storage systems is that the choice of electrode material is the dominant factor in determining the mechanism by which charge will be stored 3,4 . Bulky materials, such as graphite, tin and silicon, are battery anode materials with diffusion-controlled intercalation 5 , where the insertion of ions into the bulk of these types of material leads to phase transformations and requires a large overpotential as the driving force to overcome the intercalation energy barrier in the material lattice. Surface- functionalized two-dimensional (2D) materials, e.g. reduced gra- phene oxide 6 , allow for the insertion of ions between their layers with relative ease, but mainly double-layer capacitive charge storage has been observed for these materials unless the electrode surface is modified by pseudocapacitive materials (metal oxide or sulfide nanoparticles, redox-active conducting polymers or organic mol- ecules) or if a redox-active electrolyte is used 7,8 . Materials with intrinsic pseudocapacitive behaviour, such as RuO 2 , MnO 2 , Nb 2 O 5 , conductive polymers and VN 9–11 , usually feature the presence of fast intercalation tunnels or surface redox reactions. However, most of these materials, with the exceptions of costly RuO 2 and VN, have limited electronic conductivity, limiting the high-power capabilities of devices made using these materials. MXenes, a class of 2D materials with values of electrical conduc- tivity up to 10,000 S cm −1 , have shown potential as pseudocapaci- tive electrode materials in acidic aqueous electrolytes 12 , where the MXene Ti 3 C 2 has demonstrated high volumetric capacitance, up to 1,500 F cm −3 (ref. 13 ). However, the small voltage window of aque- ous electrolytes limited the maximum energy storage capability of these studies, leaving organic electrolytes as a better alternative for higher-energy applications. Before this study, limited intercalation, or no intercalation, was reported for Ti 3 C 2 with organic electrolyte systems using organic cations where moderate capacitance values were reported (70–85 F g −1 at 2 mV s −1 ) 14,15 . Another study reported higher capacitance values (200 F g −1 , or 80 F cm −3 , at 0.2 mV s −1 ) for half-cells with electrodes made from multilayer Ti 3 C 2 and using a typical battery electrolyte, lithium hexafluorophosphate in ethyl- ene carbonate/dimethyl carbonate 16 . Their further demonstration of a hybrid Li-ion capacitor using a Ti 2 C negative electrode and a battery-type material for the positive electrode shows the potential Influences from solvents on charge storage in titanium carbide MXenes Xuehang Wang  1 , Tyler S. Mathis 1 , Ke Li 1 , Zifeng Lin 2,3 , Lukas Vlcek 4,5 , Takeshi Torita 6 , Naresh C. Osti  7 , Christine Hatter 1 , Patrick Urbankowski 1 , Asia Sarycheva 1 , Madhusudan Tyagi 8,9 , Eugene Mamontov 7 , Patrice Simon  2,10 and Yury Gogotsi  1 * Pseudocapacitive energy storage in supercapacitor electrodes differs significantly from the electrical double-layer mecha- nism of porous carbon materials, which requires a change from conventional thinking when choosing appropriate electrolytes. Here we show how simply changing the solvent of an electrolyte system can drastically influence the pseudocapacitive charge storage of the two-dimensional titanium carbide, Ti 3 C 2 (a representative member of the MXene family). Measurements of the charge stored by Ti 3 C 2 in lithium-containing electrolytes with nitrile-, carbonate- and sulfoxide-based solvents show that the use of a carbonate solvent doubles the charge stored by Ti 3 C 2 when compared with the other solvent systems. We find that the chemical nature of the electrolyte solvent has a profound effect on the arrangement of molecules/ions in Ti 3 C 2 , which correlates directly to the total charge being stored. Having nearly completely desolvated lithium ions in Ti 3 C 2 for the carbon- ate-based electrolyte leads to high volumetric capacitance at high charge–discharge rates, demonstrating the importance of considering all aspects of an electrochemical system during development. NATURE ENERGY | www.nature.com/natureenergy