High-Rate Electrochemical Energy Storage of MXene in Neutral Aqueous Electrolytes

Increasing demand for electrochemical energy storage devices with high energy density, high power density, and long lifespan has posed significant challenges for developing suitable electrodes and electrolyte systems with fast charge transport kinetics and superior cycling stability.1 As one of the promising electrode materials explored recently, MXenes (Mn+1XnT x ) are a group of two-dimensional transition metal carbides and/or nitrides, in which M is early transition metal, X presents C and/or N, T x stands for the surface groups such as =O, -OH, -Cl or -F. They exhibit excellent high-rate performance in electrochemical energy storage applications due to their unique properties: superior electronic conductivity (~15 000 S cm-1) and large interlayer spacing (~1 nm).2 The charge storage mechanism and performance of the MXenes electrode is highly dependent on the electrode and electrolyte composition. For example, Ti3C2T x has shown rapid pseudocapacitive (de)intercalation of protons and transformation between the oxygen surface groups (=O) and hydroxyl (-OH) groups in acidic aqueous electrolytes.3,4 As a result, Ti3C2T x exhibits an ultrahigh volumetric capacitance of 1500 F cm-3 in H2SO4 electrolyte.5 In less corrosive neutral aqueous electrolytes, the cation intercalation is accompanied by negligible charge transfer, leading to ultrafast charge storage.6 Consequently, a moderate capacitance with superior cycling stability is reported in neutral electrolytes compared to acidic electrolytes. In contrast to dilute aqueous electrolyte, which has a limited electrochemical stability window (ESW), the highly concentrated water-in-salt (WIS) electrolyte possesses a large ESW (∼3.0 V) owing to the depressed activity of free water. Besides, the unique solvation structure of WIS electrolyte induces a novel intercalation of fully-solvated Li+ at positive potential and increases the capacitance of MXene.7 As a result, WIS electrolyte helps overcome the drawback of the low energy density of MXene in neutral aqueous electrolytes. In our research, we explored the impact of MXene structure and a novel electrolyte system on the pseudocapacitive performance of MXene. The charge storage behavior of Ti3C2T x in a molecular crowding electrolyte was explored. Interestingly, Ti3C2T x displayed one pair of redox peaks with low polarization, differing from both dilute neutral aqueous electrolyte and WIS electrolyte, which may be attributed to the intercalation of Polyethylene glycol (PEG)-solvated Li+ cluster. The in situ X-ray diffraction are employed to better understand the charge storage process. We also constructed a MXene/polymer heterostructure with enlarged interlayer spacing. The unique heterostructure delivered a significantly enhanced rate performance in neutral aqueous electrolyte. (a high capacitance retention of 81% at 10 A g-1) Reference: Simon, P. & Gogotsi, Y. Nature Materials 7, 845–854 (2008). Anasori, B. et al. Nature Reviews Materials 2, 16098 (2017). Mu, X. et al. Advanced Functional Materials 29, 1902953 (2019). Zhan, C. et al. J. Phys. Chem. Lett. 9, 1223–1228 (2018). Lukatskaya, M. R. et al. Nat Energy 2, 1–6 (2017). Ando, Y. et al. Advanced Functional Materials 30, 2000820 (2020). Wang, X. et al. ACS Nano 15, 15274–15284 (2021).