On the Use of Ti(3)C(2)T(x) MXene as a Negative Electrode Material for Lithium-Ion Batteries

[Image: see text] The pursuit of new and better battery materials has given rise to numerous studies of the possibilities to use two-dimensional negative electrode materials, such as MXenes, in lithium-ion batteries. Nevertheless, both the origin of the capacity and the reasons for significant variations in the capacity seen for different MXene electrodes still remain unclear, even for the most studied MXene: Ti(3)C(2)T(x). Herein, freestanding Ti(3)C(2)T(x) MXene films, composed only of Ti(3)C(2)T(x) MXene flakes, are studied as additive-free negative lithium-ion battery electrodes, employing lithium metal half-cells and a combination of chronopotentiometry, cyclic voltammetry, X-ray photoelectron spectroscopy, hard X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy experiments. The aim of this study is to identify the redox reactions responsible for the observed reversible and irreversible capacities of Ti(3)C(2)T(x)-based lithium-ion batteries as well as the reasons for the significant capacity variation seen in the literature. The results demonstrate that the reversible capacity mainly stems from redox reactions involving the T(x)–Ti–C titanium species situated on the surfaces of the MXene flakes, whereas the Ti–C titanium present in the core of the flakes remains electro-inactive. While a relatively low reversible capacity is obtained for electrodes composed of pristine Ti(3)C(2)T(x) MXene flakes, significantly higher capacities are seen after having exposed the flakes to water and air prior to the manufacturing of the electrodes. This is ascribed to a change in the titanium oxidation state at the surfaces of the MXene flakes, resulting in increased concentrations of Ti(II), Ti(III), and Ti(IV) in the T(x)–Ti–C surface species. The significant irreversible capacity seen in the first cycles is mainly attributed to the presence of residual water in the Ti(3)C(2)T(x) electrodes. As the capacities of Ti(3)C(2)T(x) MXene negative electrodes depend on the concentration of Ti(II), Ti(III), and Ti(IV) in the T(x)–Ti–C surface species and the water content, different capacities can be expected when using different manufacturing, pretreatment, and drying procedures.