Sodium ion batteries (SIBs) and potassium ion batteries (PIBs) are promising as ideal alternatives to Lithium ion batteries (LIBs), because of their rich abundance (Na, K), low cost, high theoretical capacities, and similar redox potentials. However, it usually lead to low capacities and/or fast capacity fading, due to the huge volume expansion and sluggish kinetics in most electrodes (e.g., metal oxide, porous carbon) induced by large Na⁺ or K⁺ ions. Therefore, urgent development of nanostructural electrode materials with enhanced charge storage mechanism is highly required to meet the coming era of next-generation batteries (SIBs, PIBs).

Ti-based layered materials, such as sodium/potassium titanate and Ti-based MXene (two dimensional transit carbide (nitride), e.g., Ti₃C₂, Ti₂C), have been intensively explored in SIBs and PIBs. They exhibited suitable interlayer spacing for accommodating Na⁺/K⁺ ions, low working potentials, exceptional chemical durability, and environmental benignity. Notably, electrochemical performances of these materials are strongly dependent on their nanostructures.

Two-dimensional (2D) MXene nanosheets (e.g., Ti₃C₂) have gained enormous attention, because of their high conductivity, flexible interlayer space, tailored surface chemistry and promising applications in LIBs, SIBs, and PIBs. The hydrofluoric acid etched multilayer MXene nanosheets are terminated by oxygen and fluorine-containing groups. It usually shows severe structural defects, leading to large initial irreversible capacities and limited reversible capacities. Therefore, it is of great significance for improving sodium and potassium storage kinetics of SIBs and PIBs, by designing well-defined layered Ti-based nanostructures with suitable interlayer spacing and stable structure.

Ti₃C₂ MXene derived sodium/potassium titanate nanoribbons synthesized by a simultaneous oxidation and alkalization process (Image by DONG Yanfeng and ZHAO Xuejun)

To address this, scientists WU Zhongshuai and BAO Xinhe et al at Dalian Institute of Chemical Physics, collaborated with WANG Xiaohui from Institute of Metal Research, developed ultrathin nanoribbons of sodium titanate (M-NTO) and potassium titanate (MKTO) for high-performance SIBs and PIBs.

M-NTO and M-KTO were successfully synthesized by a simultaneous oxidation and alkalization process of Ti₃C₂ MXene. M-NTO has the suitable interlayer spacing (0.90 nm for M-NTO, 0.93 nm for M-KTO), ultrathin thickness (<11 nm), narrow widths of nanoribbons (<60 nm), and open macroporous structures for enhanced ion insertion/extraction kinetics. As a result, it exhibited a large reversible capacity of 191 mAh g^-1 at 200 mA g^-1 for sodium storage, higher than those of pristine Ti₃C₂ (178 mAh g^-1) and commercial TiC derivatives (86 mAh g^-1).

Notably, M-KTO displayed long-term stable cyclability over 900 times, which outperforms other Ti-based layered materials reported to date. Moreover, this strategy can be extended for preparing a large number of MXene-derived materials, from the 60+ group of MAX phases, for various applications. This work has been published online in ACS Nano (DOI: 10.1021/acsnano.7b01165).