Holey Ti3C2 MXene-Derived Anode Enables Boosted Kinetics in Lithium-Ion Capacitors.

Lithium-ion capacitors (LICs) attract enormous attention because of the urgent demands for high power and energy density devices. However, the intrinsic imbalance between anodes and cathodes with different charge-storage mechanisms blocks the further improvement in energy and power density. MXenes, novel two-dimensional materials with metallic conductivity, accordion-like structure, and regulable interlayer spacing, are widely employed in electrochemical energy storage devices. Herein, we propose a holey Ti3C2 MXene-derived composite (pTi3C2/C) with enhanced kinetics for LICs. This strategy effectively decreases the surface groups (-F and -O) and generates expanded interplanar spacing. The in-plane pores of Ti3C2Tx lead to increased active sites and accelerated lithium-ion diffusion kinetics. Benefiting from the expanded interplanar spacing and accelerated lithium-ion diffusion, the pTi3C2/C as an anode implements excellent electrochemical property (capacity retention about 80% after 2000 cycles). Furthermore, the LIC fabricated with a pTi3C2/C anode and an activated carbon cathode displays a maximum energy density of 110 Wh kg-1 and a considerable energy density of 71 Wh kg-1 at 4673 W kg-1. This work provides an effective strategy to achieve high antioxidant capability and boosted electrochemical properties, which represents a new exploration of structural design and tuneable surface chemistry for MXene in LICs.

Investigating the Electrocatalysis of a Ti3C2/Carbon Hybrid in Polysulfide Conversion of Lithium-Sulfur Batteries.

Despite the fact that lithium-sulfur batteries are regarded as promising next-generation rechargeable battery systems owning to high theoretical specific capacity (1675 mA h g-1) and energy density (2600 W h kg-1), several issues such as poor electrical conductivity, sluggish redox kinetics, and severe "shuttle effect" in electrodes still hinder their practical application. MXenes, novel two-dimensional materials with high conductivity, regulable interlayer spacing, and abundant functional groups, are widely applied in energy storage and conversion fields. In this work, a Ti3C2/carbon hybrid with expanded interlayer spacing is synthesized by one-step heat treatment in molten potassium hydroxide. The subsequent experiments indicate that the as-prepared Ti3C2/carbon hybrid can effectively regulate polysulfide redox conversion and has strong chemisorption interaction to polysulfides. Consequently, the Ti3C2/carbon-based sulfur cathode boosts the performance in working lithium-sulfur batteries, in terms of an ultrahigh initial discharge capacity (1668 mA h g-1 at 0.1 C), an excellent rate performance (520 mA h g-1 at 5 C), and an outstanding capacity retention of 530 mA h g-1 after 500 cycles at 1 C with a low capacity fade rate of 0.05% per cycle and stable Coulombic efficiency (nearly 99%). The above results indicate that this composite with high catalytic activity is a potential host material for further high-performance lithium-sulfur batteries.