The development of high-performance anode materials for potassium-based energy storage devices with long-term cyclability requires combined innovations from rational material design to electrolyte optimization. A three-dimensional K+ -pre-intercalated Ti3 C2 Tx MXene with enlarged interlayer distance was constructed for efficient electrochemical potassium-ion storage. We found that the optimized solvation structure of the concentrated ether-based electrolyte leads to the formation of a thin and inorganic-rich solid electrolyte interphase (SEI) on the K+ -pre-intercalated Ti3 C2 Tx electrode, which is beneficial for interfacial stability and reaction kinetics. As a proof of concept, 3D K+ -Ti3 C2 Tx //activated carbon (AC) potassium-ion hybrid capacitors (PIHCs) were assembled, which exhibited promising electrochemical performances. These results highlight the significant roles of both rational structure design and electrolyte optimization for highly reactive MXene-based anode materials in energy storage devices.

Achieving High-Performance 3D K+-Pre-intercalated Ti3C2Tx MXene for Potassium-Ion Hybrid Capacitors via Regulating Electrolyte Solvation Structure

Sodium-ion storage technologies are promising candidates for large-scale grid systems due to the abundance and low cost of sodium. However, compared to well-understood lithium-ion storage mechanisms, sodium-ion storage remains relatively unexplored. Herein, we systematically determine the sodium-ion storage properties of anatase titanium dioxide (TiO2(A)). During the initial sodiation process, a thin surface layer (~3 to 5 nm) of crystalline TiO2(A) becomes amorphous but still undergoes Ti4+/Ti3+ redox reactions. A model explaining the role of the amorphous layer and the dependence of the specific capacity on the size of TiO2(A) nanoparticles is proposed. Amorphous nanoparticles of ~10 nm seem to be optimum in terms of achieving high specific capacity, on the order of 200 mAh g-1, at high charge/discharge rates. Kinetic studies of TiO2(A) nanoparticles indicate that sodium-ion storage is due to a surface-redox mechanism that is not dependent on nanoparticle size in contrast to the lithiation of TiO2(A) which is a diffusion-limited intercalation process. The surface-redox properties of TiO2(A) result in excellent rate capability, cycling stability and low overpotentials. Moreover, tailoring the surface-redox mechanism enables thick electrodes of TiO2(A) to retain high rate properties, and represents a promising direction for high-power sodium-ion storage. 

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Surface-redox sodium-ion storage in anatase titanium oxide

It has long been the goal to develop rechargeable batteries with low cost and long cycling life. Polyanionic compounds offer attractive advantages of robust frameworks, long-term stability, and cost-effectiveness, making them ideal candidates as electrode materials for grid-scale energy storage systems. In the past few years, various polyanionic electrodes have been synthesized and developed for sodium storage. Specifically, doping regulation including cation and anion doping has shown a great effect in tailoring the structures of polyanionic electrodes to achieve extraordinary electrochemical performance. In this review, recent progress in doping regulation in polyanionic compounds as electrode materials for sodium-ion batteries (SIBs) is summarized, and their underlying mechanisms in improving electrochemical properties are discussed. Moreover, challenges and prospects for the design of advanced polyanionic compounds for SIBs are put forward. It is anticipated that further versatile strategies in developing high-performance electrode materials for advanced energy storage devices can be inspired.

Doping Regulation in Polyanionic Compounds for Advanced Sodium-Ion Batteries.

Lithium-ion capacitors (LICs) have become one of the most popular energy storage devices because of the combination of high energy densities and power densities. However, the kinetic imbalance of anode and cathode restricts the specific capacities and voltage windows of LICs. Herein, an in-situ nitrogen-doped activated porous carbon (ANMPC) material with high specific surface area (1894.9 m2/g) is proposed to act as both cathode and anode for the preparation of the 4.5 V “all carbon” LICs. The hierarchically porous ANMPC obtained by KOH activation and carbonization of polypyrrole (PPy) has plenty of mesopores created by surfactant (added during the PPy process) as well as micropores generated by KOH activation, thus can provide abundant active sites for ion intercalation and large area for electrostatic adsorption simultaneously, satisfying the critical requirements of high-performance anode and cathode, respectively. After coupling the pre-lithiated ANMPC anode and fresh ANMPC cathode, the complete LIC device delivers large energy density of 167.5 Wh/kg at a power density of 269.0 W/kg, and still remains 88.9 Wh/kg at an ultrahigh power density of 13,198.5 W/kg, exhibiting enormous potential for applications in high performance lithium-ion capacitors. 

Nitrogen-doped activated porous carbon for 4.5 V lithium-ion capacitor with high energy and power density

Pseudocapacitive Anode Materials toward High-Power Sodium-Ion Capacitors

With the increasing popularity of smart wearable devices, flexible pressure sensors are highly desired in various complex application scenarios. A great challenge for existing flexible pressure sensors is to maintain high sensitivity over a wide temperature range, which is critical for their applications in harsh environments. Herein, a flexible piezoresistive sensor made of polyetherimide (PEI) fibrous network evenly covered with MXene nanosheets is reported to construct conductive pathways, showing ultrahigh sensitivity over a wide temperature range from −5 °C (sensitivity of 80 kPa−1) to 150 °C (20 kPa−1), low detection limit of 9 Pa, fast response time of 163 ms, outstanding durability over 10 000 cycles at room temperature, 2000 cycles at 100 °C and 500 cycles at −5 °C. The pressure sensor can monitor various human activities in real‐time, apply to human–machine interaction, and measure pressure distribution. It also can sensitively respond to external mechanical stimuli at 150 °C and extremely low temperature (in liquid nitrogen). Moreover, the fibrous network exhibits an excellent Joule heating performance, which can reach 78 °C at an applied voltage of 12 V. Thus, the piezoresistive sensor has considerable potential for wearable garments and personal heating applications in harsh temperature conditions.

3D MXene‐Based Flexible Network for High‐Performance Pressure Sensor with a Wide Temperature Range

In recent years, the integration of flexible printed electronics and electrochemical sensors has emerged as a new approach for developing wearable biochemical detecting devices. Among the materials utilized in flexible printed electronics, carbon-based conductive inks are considered to be crucial. In this study, we propose a cost-effective, highly conductive, and environmentally friendly ink formulation utilizing graphite and carbon black (CB) as conductive fillers, resulting in a very low sheet resistance of 15.99 Ω sq−1 (conductivity of 2.5 × 103 S m−1) and a printed film thickness of 25 μm. The unique “sandwich” structure of the working electrode (WE) printed with this ink enhances its electrical conductivity, leading to high sensitivity, selectivity, and stability, with almost no water film generated between the WE and the ion-selective membrane (ISM), strong ion selectivity, long-term stability, and anti-interference. The lower detection limit of the sensor for Na+ is 0.16 mM with a slope of 75.72 mV per decade. To validate the sensor's usability, we analyzed three sweat samples collected during physical activity, with Na+ concentrations within the typical range for human sweat (51 ± 4 mM, 39 ± 5 mM, and 46 ± 2 mM).

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Jian Wang

Papers

Pseudocapacitive Anode Materials toward High-Power Sodium-Ion Capacitors

3D MXene‐Based Flexible Network for High‐Performance Pressure Sensor with a Wide Temperature Range

A high-performance, all-solid-state Na+ selective sensor printed with eco-friendly conductive ink