Transition metal (TM) cations exhibit distinct electronic properties, compared to alkali and alkaline earth metal cations. In this work, we aim to explore the electronic coupling between different redox-active TMs guest and conductive MXene host. Specifically, within the confined environment of MXene layers, different inserted TM cations demonstrate varying electronic structures and exert distinct influences on the MXene. Unlike Cu which undergoes unusual reduction upon insertion within MXenes, our Bader charge analysis and X-ray absorption spectroscopy (XAS) results indicate that the oxidation states of inserted Ni, Co, Mn, and Zn ions remain unchanged. Despite the higher degree of Ti oxidation led to higher resistivity of intercalated MXenes, the density of states analysis of Ni and Co highlighted the important contributions of these TM cation guests on the overall electrical conductivity of intercalated-MXenes. Moreover, the TM-intercalated MXenes displayed varying redox contributions to the charge storage, where the redox properties of transition metal can be suppressed. This study highlights the unique influence of each transition metal cation under MXene confinement and lays the groundwork for utilizing transition metal guests in MXenes for various electrochemical applications.

Water-in-salt electrolytes (WISEs) offer extended voltage stability, enabling the use of high-voltage materials in aqueous energy storage systems. The solid-electrolyte interphase (SEI) formed in some WISEs plays a key role in enabling this enhanced stability by passivating the electrode surface. However, non-SEI-forming WISEs also exhibit similar stability window, despite the lack of an SEI, though the mechanism behind remains insufficiently understood. In this work, we investigate the origins of the hydrogen evolution reaction (HER) suppression across a range of electrolyte concentrations of the non-SEI-forming electrolytes: from dilute solutions to WISE, using electrochemical techniques, molecular dynamics (MD) simulations, and density functional theory (DFT) calculations. We disentangle the contributions of water activity, local pH changes induced by HER in non-buffering neutral electrolytes, and kinetic and mass transport effects. We establish a correlation between HER activity and active surface coverage by water molecules: in WISEs sluggish HER kinetics is observed, as indicated by a sevenfold decrease in exchange current density. Additionally, we reveal that in WISE HER is significantly limited by the sluggish water transport in the double layer, further contributing to the extended experimental voltage stability window.

Controlling solid electrolyte interphase (SEI) in batteries is crucial for their efficient cycling. Herein, we demonstrate an approach to enable robust battery performance that does not rely on high fractions of fluorinated species in electrolytes, thus substantially decreasing the environmental footprint and cost of high-energy batteries. In this approach, we use very low fractions of readily reducible fluorinated cations in electrolyte (~0.1 wt.%) and employ electrostatic attraction to generate a substantial population of these cations at the anode surface. As a result, we can form a robust fluorine-rich SEI that allows for dendrite-free deposition of dense Li and stable cycling of Li metal full cells with high-voltage cathodes. Our approach represents a general strategy for delivering desired chemical species to battery anodes through electrostatic attraction while using minute amounts of additive.

MXenes are 2D transition metal carbides, nitrides, and/or carbonitrides that can be intercalated with cations through chemical or electrochemical pathways. While the insertion of alkali and alkaline earth cations into Ti3C2Tx MXenes is well studied, understanding of the intercalation of redox-active transition metal ions into MXenes and its impact on their electronic and electrochemical properties is lacking. In this work, we investigate the intercalation of Cu ions into Ti3C2Tx MXene and its effect on its electronic and electrochemical properties. Using X-ray absorption spectroscopy (XAS) and ab initio molecular dynamics (AIMD), we observe an unusual phenomenon whereby Cu2+ ions undergo partial reduction upon intercalation from the solution into the MXene. Furthermore, using in situ XAS, we reveal changes in the oxidation states of intercalated Cu ions and Ti atoms during charging. We show that the pseudocapacitive response of Cu-MXene originates from the redox of both the Cu intercalant and Ti3C2Tx host. Despite highly reducing potentials, Cu ions inside the MXene show an excellent stability against full reduction upon charging. Our findings demonstrate how electronic coupling between Cu ions and Ti3C2Tx modifies electrochemical and electronic properties of the latter, providing the framework for the rational design and utilization of transition metal intercalants for tuning the properties of MXenes for various electrochemical systems.

Synchrotron X-ray absorption spectroscopy (XAS) serves as a robust and powerful technique for probing the oxidation state and coordination surrounding of specific elements. In particular, operando XAS has been employed to elucidate charge storage mechanisms by monitoring changes in the oxidation state through the absorption edge energy of metals in electrodes such as MnO 2 and RuO 2 during charging/discharging [1] [2] . Herein, we utilized in-situ XAS technique to uncover the charge storge mechanisms of transition metal (TM)-intercalated MXenes (Ti 3 C 2 T x ). We specifically probed the K-edges of intercalated TM and Ti MXene to reveal their oxidation states changes, shedding light on their respective contributions to capacitance. Aiming to gain further insights into the confined environment between MXene layers, we also employed ab-initio molecular dynamics (AIMD) to further interpret and validate the coordination environment obtained through in-situ extended X-ray absorption fine structure (EXAFS). This investigation lays the groundwork for tailoring the electronic and electrochemical properties of MXenes by the intercalation of various transition metal cations across diverse electrochemical systems. [1] J.-K. Chang, M.-T. Lee, W.-T. Tsai, Journal of Power Sources 2007 , 166 , 590-594. [2] Y. Mo, M. R. Antonio, D. A. Scherson, The Journal of Physical Chemistry B 2000 , 104 , 9777-9779.

MXenes, are a family of two-dimensional (2D) transition metal carbides, nitrides and/ or carbonitrides. [1] The intercalation of different metal cations and organic molecules into MXene for tuning MXenes’ electronic and electrochemical properties have been widely reported. [2] [3] [4] However, transition metals (TMs) which have distinct electrochemical behaviors [5] [6] have not been explored in the literature of MXenes’ intercalation. Therefore, we herein compared the intercalation effects of different TM cations on Ti 3 C 2 T x , for offering a general insight on the influences of multi-valent, redox-active TMs on MXenes. In particular, the changes on the physiochemical and electrochemical properties of MXenes were investigated using different in-situ and ex-situ methods, for a better understanding on the interfacial interactions of TM cations and MXenes in the confined environment of MXenes’ multilayer structures. This work serves as a foundation for the fine-tuning of MXenes’ properties by utilizing a variety of TM cations intercalation for different electrochemical systems. [1] B. Anasori, M. R. Lukatskaya, Y. Gogotsi, Nature Reviews Materials 2017 , 2 , 16098. [2] M. R. Lukatskaya, O. Mashtalir, E. Ren Chang, Y. Dall’Agnese, P. Rozier, L. Taberna Pierre, M. Naguib, P. Simon, M.W. Barsoum, Y. Gogotsi, Science 2013 , 341 , 1502-1505. [3] J. Li, H. Wang, X. Xiao, Energy & Environmental Materials 2020 , 3 , 306-322 [4] M. Ghidiu, S. Kota, J. Halim, A. W. Sherwood, N. Nedfors, J. Rosen, V. N. Mochalin, M. W. Barsoum, Chemistry of Materials 2017 , 29 , 1099-1106. [5] D. Göhl, H. Rueß, M. Pander, A. R. Zeradjanin, K. J. J. Mayrhofer, J. M. Schneider, A. Erbe, M. Ledendecker, Journal of The Electrochemical Society 2020 , 167 , 021501. [6] M. Esmaeilirad, A. Baskin, A. Kondori, A. Sanz-Matias, J. Qian, B. Song, M. Tamadoni Saray, K. Kucuk, A. R. Belmonte, P. N. M. Delgado, J. Park, R. Azari, C. U. Segre, R. Shahbazian-Yassar, D. Prendergast, M. Asadi, Nature Communications 2021 , 12 , 5067.

Conventional Li-ion electrolytes consist of carbonate solvents and Li-ion salts that were made for carbonaceous negative electrodes. When the electrolytes are used with metallic lithium anodes, carbonate electrolytes lead to low charge-discharge cycle stability due to adverse reactive species at the anode surface. 1-4 This reactive species are called the solid electrolyte interface (SEI). 1 SEI instabilities/inhomogeneities result in its continuous thickening and formation of electronically non-active dead lithium. 2 SEI composition and structure are affected by both solvent and electrolyte chemistry in bulk solution and, more importantly, at the electrode-electrolyte interface. 4 Current research studies suggest that X-rich (where X is a halogen) SEI yields superior performance compared to halogen-free SEI. 1,5 Electrolytes that have large volume fractions of halogenated species have statistically higher probability to be reduced at the electrode surface and yield X-rich SEI layers. 6 Therefore, the use of halogenated solvent and/or high concentrated (> 1 M) lithium salt with halogenated anions has been actively explored for LiMBs. However, the high cost of Li salts and high viscosity of the electrolyte at high salt concentration make the concentrated electrolytes unrealistic for commercial battery applications. In search of a cost-effective solution for high performance LiMBs, halogenated electrolyte additives could serve as an ideal approach since only a small amount of the additive would be potentially required to induce similar X-rich interface that can be seen in the case of the halogenated ether electrolyte. In this meeting abstract, we present a new concept that leverages favorable electrostatic interactions with the electrolyte additive to drive the formation of robust SEI layers even at low additive content. Specifically, custom-designed additives can be electrostatically attracted to the negatively charged electrode, creating a high population of halogenated species at the anode surface even at a dilute Li salt concentration. Effective SEI formation with a low-concentration additive circumvents the challenges associated with the current state-of-the-art approach of using halogenated species at high concentration (i.e., unfavorably high solution viscosity and high cost). Reference 1 Louli, A. J. et al. Diagnosing and correcting anode-free cell failure via electrolyte and morphological analysis. Nature Energy 5 , 693-702, (2020). 2 Chen, K.-H. et al. Dead lithium: mass transport effects on voltage, capacity, and failure of lithium metal anodes. Journal of Materials Chemistry A 5 , 11671-11681, (2017). 3 Lin, D. et al. Three-dimensional stable lithium metal anode with nanoscale lithium islands embedded in ionically conductive solid matrix. Proc Natl Acad Sci U S A 114 , 4613-4618, (2017). 4 Lin, D., Liu, Y. & Cui, Y. Reviving the lithium metal anode for high-energy batteries. Nat Nanotechnol 12 , 194-206, (2017). 5 Yu, Z. et al. Molecular design for electrolyte solvents enabling energy-dense and long-cycling lithium metal batteries. Nature Energy 5 , 526-533, (2020). 6 Yamada, Y., Wang, J., Ko, S., Watanabe, E. & Yamada, A. Advances and issues in developing salt-concentrated battery electrolytes. Nature Energy 4 , 269-280, (2019).

This approach utilizes electrostatic attraction of very low fractions (~0.1 wt%) of readily reducible fluorinated cations in electrolyte to form a robust fluorine-rich SEI and enable stable Li-metal batteries.

Rechargeable aqueous Zn metal batteries are promising systems for grid storage because of their high energy density, low cost, and non-flammability. However, Zn metal anodes have major limitations due to dendrite formation and concurrent water splitting during charge-discharge cycling. Both processes negatively affect coulombic efficiency (CE) and long-term cycling stability. Water-in-salt (WIS) electrolytes were previously proposed to address these challenges, yielding improvements in the cycling stability of Zn metal anodes. While WIS electrolytes help increase CE, they require high amounts of salt (often toxic) and have dramatically increased viscosity, which in turn limit their transport properties, charge-discharge rates, and usability in advanced Zn batteries. In this manuscript, we propose a strategy for simultaneously achieving high CE (>99.99%), high rate, low cost and reduced environmental footprint. Specifically, we show that by using coordinating anions like acetate a WIS-like Zn coordination environment can be achieved even in relatively dilute conditions, enabling prolonged cycling of Zn metal anodes. Such electrolytes have an order of magnitude higher conductivity and lower viscosity than traditional WIS electrolytes, thus enabling lower overpotentials and higher rate of Zn plating/stripping.

MXenes are 2D transition metal carbides, nitrides, and/or carbonitrides, capable of intercalation by various cations through chemical or electrochemical means. Previous research has primarily focused on intercalating alkaline and alkaline earth cations, such as Li+, K+, Na+, Mg2+ or alkylammonium cations, into Ti3C2Tx MXenes. However, the impact of intercalated transition metal (TM) ions on the electronic and electrochemical properties of MXenes remains largely unexplored. In this study, we investigated the effects of pre-intercalated Cu ions on Ti3C2Tx MXenes and vice versa to gain a comprehensive understanding of how the electronic and electrochemical properties of both intercalated TM ion and MXene host are altered. Using in-situ X-ray absorption spectroscopy (XAS), we reveal changes in the oxidation states of intercalated Cu ions and Ti atoms during charging and their corresponding role in charge storage mechanisms. Our findings show that electronic coupling between Ti3C2Tx and Cu ions results in modified electrochemical and electronic properties compared to pristine Ti3C2Tx. These insights lay the foundation for the rational design and utilization of TM ion intercalants to tailor the properties of MXenes for various electrochemical systems and beyond.