High-capacity 2D layered materials for Mg-ion batteries

The main aim of the project was to develop a new cathode material for use in rechargeable Mg batteries. Rechargeable batteries based on the exchange of Mg ions have the potential for high energy density batteries based on cheap, abundant, and sustainable materials. Hence, become an important alternative to the current Li-ion battery technology, both in transport and stationary applications. In this project we have chosen to start with cathode materials based on two-dimensional MXenes, these compositions have their origin in layered MAX phases. The MAX phases are carbides and nitrides of transition metals, where M represents a transition metal, A is usually aluminum, and X is carbon and/or nitrogen. By selectively removing the A element from the structure atomic flakes are formed by the MX components which provide open passages where different ions can move both in and out (so-called intercalation). There are many different elements that can be used at the M site in the structure and based on literature we have chosen to focus on Ti and V, corresponding to the MAX-phase stoichiometries Ti3AlC2 and V2AlC. Al in these stoichiometries was removed by etching in hydrofluoric acid (HF) and formation of the MXene stoichiometries Ti3C2Tx and V2CTx were documented, where T symbolizes surface termination. It was verified that the surface termination was dominated by fluorine. Simulations (DFT calculations) showed that it was necessary to replace fluorine with oxygen termination to achieve a significant increase in the reversible capacity of Mg intercalation. Based on an assessment of possible reaction products combined with thermodynamic simulations, gas hydrolysis was considered to be an appropriate method. The hydrolysis experiments showed that it was possible to reduce the F-termination between 70 and 80 % for V2CTx and Ti3C2Tx, which is one of the best results reported for similar materials so far. The method has potential as a general method for changing and controlling termination in MXenes. Electrochemical testing of hydrolyzed cathodes showed an increase in the capacity for exchange of Li ions, but for the exchange of Mg ions the capacity was still moderate. The project concludes that hydrolyzed MXenes based on V2CTx is a potential material for use as cathodes in rechargeable batteries based on magnesium, but that a more precise control of the type of termination is necessary.

Virkninger: Prosjektets deltagere: En generell økt forståelse av hvilke muligheter et nytt materialsystem (MXener) kan tilføre utviklingen innen batteriteknologi. Spesielt har prosjektet vist at DFT beregninger er et meget nyttig verktøy knyttet til utvikling av materialer med nye egenskaper. Økt kompetanse i beregningsmetoder med særlig vekt på hvilken betydning fononer, defekter og dopanter har for ionemobilitet i faste stoffer. Den tette kontakten mellom SINTEF og NTNU gjennom hele prosjektperioden har styrket kompetansen hos begge parter og vil realisere framtidige samarbeidsprosjekter knyttet til utvikling av batteriteknologi. Målgruppe, brukere: Identifisering av nye og bærekraftige materialer som kan benyttes som elektroder (MXener) og faststoff elektrolytter (LiSiNO) i oppladbare batterier. Resultatene viser viktigheten av å kontrollere kjemisk sammensetning særlig knyttet til terminering og demonstrerer metoder får å oppnå dette. Effekter: Gjennom formidling og publikasjoner er det framkommet forslag til endring av de eksisterende batteriteknologier, men at ytterligere utviklingsarbeid er nødvendig. På lengre sikt kan dette fremme utviklingen av nye batteriteknologier som dreier seg mot anvendelse av billigere og mer bærekraftige materialer. Konkret er det foreslått at Mg-ion batterier kan realiseres, men at dette krever ytterligere raffinering av de metoder som ble utviklet i prosjektet. En slik endring i batteriteknologi vil få stor samfunnsmessig betydning og potensielt bli en hjørnesten i realiseringen av det grønne skiftet.

This project will focus on developing Mg-ion batteries as an alternative to Li-ion battery technology. And the main aim is to investigate and develop a new family of cathode materials based on MXenes, which can outperform current state of the art cathodes. These materials are two-dimensional transition metal carbides that are able to reversibly host a variety of intercalation ions. The proposed project will take on an interdisciplinary approach covering DFT modelling, innovative material synthesis routes and advanced experimental characterization techniques, which provides a powerful combination of tools for the development of new and low cost Mg-ion batteries. The MXenes have through modelling been suggested as promising new candidates as electrodes in Li-ion as well as Mg-ion batteries. There are a few very recent studies on using Nb4C3 and Ti2C as anodes in Li-ion batteries. Due to their relatively low potential vs Li the MXenes are more suitable as anode material in Li-ion batteries. However, in Mg-ion batteries where the anode is pure Mg metal or a Mg alloy, the MXenes are potential candidates as cathodes. In this project we will syntesize a selection of compositions based on materials with the general formula M2C, where M = Zr, Nb, Sc, Ti, V or Cr or a combination of two of these elements. The three compositions Zr2C, Ti2C and Nb2C will be the starting point due to easy availability of precursors and higher abundance than i.e. Mo. DFT calculations will be used to assess the structural and thermodynamic stability of bulk materials as well as calculating point defect energetics and surface relaxation. Extensive experimental studies on materials and electrochemical properties will be performed. A combination of in operando, in situ, ex situ and post mortem characterization techniques and DFT modelling will be used to characterize and understand charge/discharge and degradation mechanisms upon electrochemical cycling.