Size/Shape-Controllable Carbonized Wood Electrodes Enabled by an MXene Shell with Spatial Confinement and a Traction Effect on the Wood Cell Wall for Shape-Customizable Energy Storage Devices.

The shrinkage and collapse of wood cell walls during carbonization make it challenging to control the size and shape of carbonized wood (CW) through pre- or postprocessing (e.g., sawing, cutting, and milling). Herein, a shape-adaptive MXene shell (MS) is created on the surface of the wood cell walls. The MS limits the deformation of wood cell walls by spatial confinement and traction effects, which is supported by the inherent dimensional stability of the MS and the formation of new C-O-Ti covalent bonds between the wood cell wall and MS. Consequently, the volumetric shrinkage ratio of CW encapsulated by the MS (CW-MS) is significantly reduced from 54.8% for CW to 2.6% for CW-MS even at 800 °C. The harnessing of this collapse enables the production of CW-MS with prolonged stability and high electric conductivity (384 S m-1). These properties make CW-MS suitable for energy storage devices with various designed shapes, matching the increasingly compact and complex structures of electronic devices.

Designing Dense, Robust, and Ion-Diffusion-Effective Electrodes from Natural Wood Material toward High-Volumetric-Performance Supercapacitors.

Assembling active materials into dense electrodes is a promising way to obtain high-volumetric-capacitance supercapacitors, but insufficient ion channels in the dense structure lead to a low rate capability. Herein, a dense and robust wood electrode with a large MXene volumetric mass loading (1.25 g cm-3) and abundant ion diffusion channels is designed via a facile capillary-force-driven self-densification strategy. Specifically, MXene is assembled onto a wood cell wall, endowing the wood electrode with good electrical conductivity (86 S cm-1) and high electrochemical activity (5.9 F cm-2 at 1 mA cm-2). Notably, the oriented channels along with spaces between adjacent microfibrils recast after densification ensure efficient ion transport for the wood electrode, achieving an excellent rate capability with a high capacitance retention of 77% from 1 to 20 mA cm-2. Meanwhile, the capillary force induces self-densification on the softened wood cell wall, resulting in a highly compact and robust structure for the wood electrode.

Dynamic Regulation of the Interfacial pH for Highly Reversible Aqueous Zinc Ion Batteries.

An electrolyte additive, with convenient operation and remarkable functions, has been regarded as an effective strategy for prolonging the cycle life of aqueous zinc ion batteries. However, it is still difficult to dynamically regulate the unstable Zn interface during long-term cycling. Herein, tricine was introduced as an efficient regulator to achieve a pH-stable and byproduct-free interface. The functional zwitterion of tricine not only inhibits interfacial pH perturbation and parasitic reactions by the trapping effect of an anionic group (-COO-) but also simultaneously creates a uniform electric field by the electrostatic shielding effect of a cationic group (-NH2+). Such synergy accordingly eliminates dendrite formation and creates a chemical equilibrium in the electrolyte, endowing the Zn||Zn cell with long-term Zn plating/stripping for 2060 h at 5 mA cm-2 and 720 h at 10 mA cm-2. As a result, the Zn||VS2 full cell under a high cathodic loading mass (8.6 mg cm-2) exhibits exceptional capacity retention of 93% after 1000 cycles.

Electrochemical Hydrophobic Tri-layer Interface Rendered Mechanically Graded Solid Electrolyte Interface for Stable Zinc Metal Anode.

The aqueous zinc-ion battery is promising as grid scale energy storage device, but hindered by the instable electrode/electrolyte interface. Herein, we report the lean-water ionic liquid electrolyte for aqueous zinc metal batteries. The lean-water ionic liquid electrolyte creates the hydrophobic tri-layer interface assembled by first two layers of hydrophobic OTF- and EMIM+ and third layer of loosely attached water, beyond the classical Gouy-Chapman-Stern theory based electrochemical double layer. By taking advantage of the hydrophobic tri-layer interface, the lean-water ionic liquid electrolyte enables a wide electrochemical working window (2.93 V) with relatively high zinc ion conductivity (17.3 mS/cm). Furthermore, the anion crowding interface facilitates the OTF- decomposition chemistry to create the mechanically graded solid electrolyte interface layer to simultaneously suppress the dendrite formation and maintain the mechanical stability. In this way, the lean-water based ionic liquid electrolyte realizes the ultralong cyclability of over 10000 cycles at 20 A/g and at practical condition of N/P ratio of 1.5, the cumulated areal capacity reach 1.8 Ah/cm2 , which outperforms the state-of-the-art zinc metal battery performance. Our work highlights the importance of the stable electrode/electrolyte interface stability, which would be practical for building high energy grid scale zinc-ion battery.

MXene film electrodes with high mechanical strength, graded ion channels and high pseudocapacitive activity enabled by lignin-containing cellulose fibers.

Cellulose nanofiber (CNF) has been widely used in MXene film electrodes to improve its mechanical properties and rate capability for supercapacitors. However, all the above enhancements are obtained with inevitably sacrificing the capacitance, because of the non-electrochemically-active characteristic of CNF. Herein, to address this issue, lignin-containing cellulose fibers (LCNF) is innovatively used to substitute CNF. Specifically, LCNF play a role as a bridge to significantly reinforce mechanical strength of LCNF/MXene film electrode (LM) by binding the adjacent MXene nanosheets, reaching a tensile strength of 34.2 MPa. Lignin in LCNF contributes to pseudocapacitance through the reversible conversion of its quinone/hydro-quinone (Q/QH2), thus yielding an excellent capacitance of 364.4 F g-1 at 1 A g-1. Meanwhile, LCNF has different diameters in which microfibers form a loose structure for LM, nanofibers enlarge d-spacing between adjacent MXene nanosheets, and fibers self-crosslinking creates abundant pores, thus constructing graded channels to achieve an outstanding rate capability of 87 % at 15 A g-1. The fabricated supercapacitor demonstrates a large energy density of 1.8 Wh g-1 at 71.3 W g-1. This work provides a promising approach to decouple the trade-off between electrochemical performance and mechanical properties of MXene film electrodes caused by using CNF, thus obtaining high-performance supercapacitors.

Low-temperature carbonized MXene/protein-based eggshell membrane composite as free-standing electrode for flexible supercapacitors.

The demerits of the carbonized eggshell membrane (EM), such as high cost, high brittleness, immutable shape and size, greatly limit its application in demanding supercapacitors as free-standing electrode. Herein, the reconstituted EM (REM) with good flexibility and excellent size-customizability is developed, which is due to their fibrous structure and abundant surface polar groups. Ti3C2 nanosheet (a typical MXene) with ultra-high electrical conductivity and good electrochemical activity is then coated on REM surface, and undergoes a low-temperature carbonization (350 °C) to prepare CREM/T. Multi-functions of Ti3C2 are exhibited: (1) constructing a conductive network on REM surface by randomly stacking to yield a high electrical conductivity of 78.1 S cm-1, (2) being as a protective mold to remain the inherent flexibility and porosity of REM during carbonization, (3) creating nanopores by inducing self-activation, and (4) yielding a large capacitance of 1729 mF cm-2 at 0.5 mA cm-2 and a high rate capability of 82 % after increasing the current density by 50 folds. Furthermore, an all-EM-based supercapacitor is fabricated with REM as the separator and CREM/T as the electrode. It delivers a high energy density of 16.1 µW h cm-2 at 1301 µW cm-2, and shows stable capacitive behaviors during bending.

Shape and Stiffness Switchable Hydroplastic Wood with Programmability and Reproducibility.

Stiffness switchable materials (e.g., supramolecular polymers, metals) that alter their shape and mechanical properties in response to specific stimuli are potentially utilized in the structural engineering field but still limited due to the use of petroleum-based synthetic monomers and large energy consumption. Herein, a sustainable and facile solvent casting strategy is proposed to fabricate the "hydroplastic wood" with shape and stiffness switchable properties via cell wall wetting, cell wall softening and subsequent moisture evaporation. Therein, a wetting agent with low surface tension and low viscosity is utilized for covering the rough surface of solid wood to form a liquid lubricating layer, thereby increasing the interfacial wettability and achieving uniform softening of the cell walls. This interface wetting treatment can easily break through the hydro-plasticization process for thick wood (Balsa wood, Ochroma lagopus Swartz, density: 0.25 g/cm3; Pinewood, Pinus armandii, density: 0.38 g/cm3). Additionally, the capillary force arising from moisture evaporation induces the self-densification of oriented cellulose nanofibrils and achieves moisture-mediated shape design capabilities through periodic saturation-dehydration. This work makes hydroplastic wood a promising candidate for engineering materials because of its combined advantages of strong durability, formability, and load-carrying capacity.

A Flexible Vertical-Section Wood/MXene Electrode with Excellent Performance Fabricated by Building a Highly Accessible Bonding Interface.

Cross-section wood (CW) is generally used as a host for free-standing electrodes, as the abundant opened pores can provide large space for loading guest materials with high electrical conductivity and electrochemical activity. However, there is still a challenge for CW to be used in flexible supercapacitors (SCs) because of its low mechanical strength. Herein, as an alternative to CW, vertical-section wood (VW) with excellent mechanical strength and good flexibility is developed and used as a free-standing and flexible electrode by using Ti3C2Tx (MXene) with ultrahigh conductivity and good electrochemical activity as a guest material. In particular, the highly accessible bonding interface for Ti3C2Tx is first built by delignification on VW to generate abundant pores for continuously absorbing Ti3C2Tx and to expose cellulose with abundant oxygen-containing groups for stable combination with Ti3C2Tx. Then, cyclic pressing is used to form negative pressure to pump the Ti3C2Tx suspension into VW, combining with a preheating process to trigger layer-by-layer self-assembly of Ti3C2Tx nanosheets onto a wood cell wall by evaporating water in the suspension. As a result, the free-standing electrode has a large Ti3C2Tx loading mass proportion of 33 wt %, a high conductivity of 3.14 S cm-1, and good flexibility with much higher mechanical strength of 15.1 MPa than 0.4 MPa of CW. The symmetric SC delivers a good specific capacitance of 805 mF cm-2 at 0.5 mA cm-2, a remarkably high rate capability of 84% to 10 mA cm-2, and an energy density of 13.85 µW h cm-2 at 87.5 µW cm-2. Additionally, this SC shows a long lifespan of 90.5% after 10,000th charge and discharge cycles even at a constant bending angle of 90°, suggesting promising potential in flexible devices.

Lignocellulose-based free-standing hybrid electrode with natural vessels-retained, hierarchically pores-constructed and active materials-loaded for high-performance hybrid oxide supercapacitor.

Lignocellulose including cellulose, lignin, and hemicellulose could be extracted from wood, and has been used to prepare carbon electrode. However, complicated extraction greatly increases preparation cost. To achieve maximum utilization of lignocellulose and avoid complicated extraction, wood with porous structure and good mechanical strength is used as carbon precursor. Additionally, chemical activation is commonly used to create micropores to provide high capacitance, but it brings in natural structure destruction, and generation of wastewater during pickling. Moreover, to achieve desirable energy density, multi-step strategy with long duration is required for loading active materials on carbonized lignocellulose (CL). Herein, a one-step method is developed to prepare a free-standing hybrid CL electrode (CLE) by using Lewis acid in three aspects: (1) as structure protection agent, (2) as activating agent, (3) as active materials donor, which bypasses pickling and further avoids the generation of wastewater. Additionally, natural vessels in wood can not only provide large space for active materials loading, but also act as rapid ions diffusion way, simultaneously confining active materials detachment. Benefiting from the synergistic effect of porous structure and Lewis acid, this work not only makes full utilization of lignocellulose, but also makes CLE exhibit excellent performance in hybrid oxide supercapacitor.