Two-dimensional (2D) materials, such as graphene, are attractive candidates for energy storage devices due to their large areas of electrochemically-active surfaces. Recently, MXenes, a new family of 2D carbides discovered at Drexel University in 2011, have shown promise for electrodes in Lithium-ion batteries and supercapacitors. The goal of this study was to determine if a combination of MXenes, which have high metallic conductivity but moderate capacity, and transition metal oxides (TMOs) with high lithium storage capacity but poor conductivity may result in improved performance.

Three different methods, including alternating filtration, spray coating, and in-situ growth, were employed to achieve the hybridization of Ti₃C₂ MXenes and TMOs. Flexible and free-standing Ti₃C₂/TMO papers were obtained. In these composites, the 1-nm thin flakes of Ti₃C₂-MXene provide superior conductivity, ensure mechanical integrity and flexibility, as well as some Li-ion storage capacity; the TMOs (e.g. Co₃O₄, NiCo₂O₄) nanosheets/nanorods serve as spacers between MXene flakes to improve the accessibility of electrolyte ions and provide additional capacity. The synergetic effect of the two materials leads to much improved performance compared to pure Ti₃C₂ or TMOs. The Ti₃C₂/TMO paper electrodes containing alternation layers of carbide and oxide achieved high reversible capacities of 1200-1400 mAh/g at 0.1C (10-hrs discharge), 4 times higher than commercial graphite anodes. These paper electrodes also exhibited excellent rate performance and superior cycling stability. A highly stable capacity around 500 mAh/g was retained for >1000 cycles at 1C rate (1-hr charge/discharge), with no obvious decay. This work provides a simple, scalable, and effective strategy for the fabrication of advanced electrode materials that can be used in wearable or structural electrochemical energy storage and conversion systems.