Solution-processed flexible paper-electrode for lithium-ion batteries based on MoS2 nanosheets exfoliated with cellulose nanofibrils

Organized mineralized cellulose nanostructures for biomedical applications

Cellulose is the most abundant naturally-occurring polymer, and possesses a one-dimensional (1D) anisotropic crystalline nanostructure with outstanding mechanical robustness, biocompatibility, renewability and rich surface chemistry in the form of nanocellulose in nature. Such features make cellulose an ideal bio-template for directing the bio-inspired mineralization of inorganic components into hierarchical nanostructures that are promising in biomedical applications. In this review, we will summarize the chemistry and nanostructure characteristics of cellulose and discuss how these favorable characteristics regulate the bio-inspired mineralization process for manufacturing the desired nanostructured bio-composites. We will focus on uncovering the design and manipulation principles of local chemical compositions/constituents and structural arrangement, distribution, dimensions, nanoconfinement and alignment of bio-inspired mineralization over multiple length-scales. In the end, we will underline how these cellulose biomineralized composites benefit biomedical applications. It is expected that this deep understanding of design and fabrication principles will enable construction of outstanding structural and functional cellulose/inorganic composites for more challenging biomedical applications.

Nanoscale structural and electronic properties of cellulose/graphene interfaces

The development of electronic devices based on the functionalization of (nano)cellulose platforms relies upon an atomistic understanding of the structural and electronic properties of a combined system, cellulose/functional element. In this work, we present a theoretical study of the nanocellulose/graphene interfaces (nCL/G) based on first-principles calculations. We find that the binding energies of both hydrophobic/G (nCL^phob/G) and hydrophilic/G (nCL^phil/G) interfaces are primarily dictated by the van der Waals interactions, and are comparable with those of their 2D interface counterparts. We verify that the energetic preference of nCL^phob/G has been reinforced by the inclusion of an aqueous medium via an implicit solvation model. Further structural characterization was carried out using a set of simulations of the carbon K-edge X-ray absorption spectra to identify and distinguish the key absorption features of the nCL^phob/G and nCL^phil/G interfaces. The electronic structure calculations reveal that the linear energy bands of graphene lie in the band gap of the nCL sheet, while depletion/accumulation charge density regions are observed. We show that external agents, i.e., electric field and mechanical strain, allow for tunability of the Dirac cone and charge density at the interface. The control/maintenance of the Dirac cone states in nCL/G is an important feature for the development of electronic devices based on cellulosic platforms.

TEMPO-oxidized nanofibrillated cellulose assisted exfoliation of MoS2/graphene composites for flexible paper-anodes

TEMPO-oxidized nanofibrillated cellulose (ONFC) with charged carboxyl groups is introduced for the efficient exfoliation of two-dimensional (2D) MoS2/graphene composites. As an effective dispersant agent, ONFC can be easily absorbed between the adjacent layers, so as to prevent the accumulation of the exfoliated nanosheets. With the assistance of charged ONFC, the exfoliated MoS2/graphene is gradually increased in the aqueous dispersions with the elongated sonication time. After dewatering, self-standing MoS2/Graphene/ONFC/CNTs composite films are rationally constructed using ONFC as flexible fibrous skeleton, and CNTs/graphene as 1D/2D interpenetrating electrical networks. Ultrathin MoS2 nanosheets anchored on the 1D/2D heterogeneous networks is directly acted as an ideal paper-anode for lithium-ion batteries (LIBs) without using traditional metallic current collector. The self-standing flexible electrode materials based on natural cellulose will promote the future green electronics with high flexibility, miniaturization, and increased portability.

Flexible and Freestanding MoS2 Nanosheet/Carbon Nanotube/Cellulose Nanofibril Hybrid Aerogel Film for High-Performance All-Solid-State Supercapacitors

Flexible supercapacitors assembled with two-dimensional materials have become a research hotspot in recent years. Here, we have prepared two-dimensional nanomaterial MoS₂ and SWCNT, CNF aerogel composite electrode, and its flexible all-solid-state supercapacitor. SWCNT can inhibit the accumulation of MoS₂ nanosheets and enhance the conductivity of the composite electrode. CNF can improve the dispersion uniformity of MoS₂ and SWCNT, and endow the composite electrode with a high specific surface area (328.86 m² g^-1) and excellent flexibility. MoS₂-SWCNT/CNF supercapacitor has a good rectangular CV curve and symmetrical triangular GCD curve. The CV curve of the MoSCF3 supercapacitor with the highest MoS₂-SWCNT content remains rectangular even at the scanning rate of 2000 mV s^-1. Its voltage window can reach 1.5 V. MoS₂-SWCNT/CNF supercapacitor has a specific capacity of 605.32 mF cm^-2 (scanning rate of 2 mV s^-1) and 30.34 F g^-1 (0.01 A g^-1), an area specific energy of 35.61 mWh cm^-2 (area specific power of 0.03 mW cm^-2), and extremely high cycle stability (91.01% specific capacity retention rate after 10 000 cycles) and good flexibility. The fine nanocomposite structure gives MoS₂-SWCNT/CNF supercapacitor impressive electrochemical performance and excellent flexibility, which can be used in the field of portable electronic devices and flexible devices.

Advanced Nanocellulose-Based Composites for Flexible Functional Energy Storage Devices

With the increasing demand for wearable electronics (such as smartwatch equipment, wearable health monitoring systems, and human-robot interface units), flexible energy storage systems with eco-friendly, low-cost, multifunctional characteristics, and high electrochemical performances are imperative to be constructed. Nanocellulose with sustainable natural abundance, superb properties, and unique structures has emerged as a promising nanomaterial, which shows significant potential for fabricating functional energy storage systems. This review is intended to provide novel perspectives on the combination of nanocellulose with other electrochemical materials to design and fabricate nanocellulose-based flexible composites for advanced energy storage devices. First, the unique structural characteristics and properties of nanocellulose are briefly introduced. Second, the structure-property-application relationships of these composites are addressed to optimize their performances from the perspective of processing technologies and micro/nano-interface structure. Next, the recent specific applications of nanocellulose-based composites, ranging from flexible lithium-ion batteries and electrochemical supercapacitors to emerging electrochemical energy storage devices, such as lithium-sulfur batteries, sodium-ion batteries, and zinc-ion batteries, are comprehensively discussed. Finally, the current challenges and future developments in nanocellulose-based composites for the next generation of flexible energy storage systems are proposed.

3D tremella-like nitrogen-doped carbon encapsulated few-layer MoS2 for lithium-ion batteries

MoS₂ is regarded as an attractive anode material for lithium-ion batteries due to its layered structure and high theoretical specific capacity. Its unsatisfied conductivity and the considerable volume change during the charge and discharge process, however, limits its rate performance and cycling stability. Herein, 3D tremella-like nitrogen-doped carbon encapsulated few-layer MoS₂ (MoS₂@NC) hybrid is obtained via a unique strategy with simultaneously poly-dopamine carbonization, and molybdenum oxide specifies sulfurization. The three-dimensional porous nitrogen-doped carbon served both as a mechanical supporting structure for stabilization of few-layers MoS₂ and a good electron conductor. The MoS₂@NC exhibits enhanced high rate performance with a specific capacity of 208.7 mAh g^-1 at a current density of 10 A g^-1 and stable cycling performance with a capacity retention rate of 85.7% after 1000 cycles at 2 A g^-1.

Nano-GeTe Embedded in a Three-Dimensional Carbon Sponge for Flexible Li-Ion and Na-Ion Battery Anodes

Among the germanium-based compounds, GeTe is a promising anode candidate that exhibits high theoretical capacity (856 mAh g^-1 vs Li⁺/Li and 401 mAh g^-1 vs Na⁺/Na) and low volume expansion during an ion intercalation/deintercalation process. Nevertheless, achieving good dispersion of metal-like GeTe in anode materials remains a significant challenge. Herein, hybrid GeTe/graphene (GeTe/G) is proposed as a highly efficient anode for LiBs and SiBs by facile ball milling. Pulverized GeTe is effectively anchored on peeled graphene sheets that can accelerate Li⁺ transport in electrodes as predicted by theoretical calculations and thus result in improved overall electrochemical performance. For instance, GeTe/G possesses a high reversible capacity of 478 mAh g^-1 under 0.1 A g^-1 in the 300th cycle. Moreover, by further cross-linking the GeTe/G using carbon nanotube (CNT) and carbon nanofiber pyrolysis from cotton cellulose, the as-prepared three-dimensional (3D) flexible anode possesses macropores that acted as positive channels favorably for ion transport. Remarkably, the as-prepared flexible 3D GeTe/G/CNT electrode with a thickness of 1050 μm exhibits a high reversible capacity of 451.4 mAh g^-1 (4.38 mAh cm^-2) vs Li⁺/Li and 372.5 mAh g^-1 (2.08 mAh cm^-2) vs Na⁺/Na, respectively, in the second cycle under 0.1 A g^-1. These results shed some light on the direct application of 3D flexible carbon sponge electrodes in high-performance LiBs/SiBs.

MoS2/carbon composites prepared by ball-milling and pyrolysis for the high-rate and stable anode of lithium ion capacitors

Lithium ion capacitors (LICs), bridging the advantages of batteries and electrochemical capacitors, are regarded as one of the most promising energy storage devices. Nevertheless, it is always limited by the anodes that accompany with low capacity and poor rate performance. Here, we develop a versatile and scalable method including ball-milling and pyrolysis to synthesize exfoliated MoS₂ supported by N-doped carbon matrix derived from chitosan, which is encapsulated by pitch-derived carbon shells (MoS₂/CP). Because the carbon matrix with high nitrogen content can improve the electron conductivity, the robust carbon shells can suppress the volume expansion during cycles, and the sufficient exfoliation of lamellar MoS₂ can reduce the ions transfer paths, the MoS₂/CP electrode delivers high specific capacity (530 mA h g^-1 at 100 mA g^-1), remarkable rate capability (230 mA h g^-1 at 10 A g^-1) and superior cycle performance (73% retention after 250 cycles). Thereby, the LICs, composed of MoS₂/CP as the anode and commercial activated carbon (21 KS) as the cathode, exhibit high power density of 35.81 kW kg^-1 at 19.86 W h kg^-1 and high energy density of 87.74 W h kg^-1 at 0.253 kW kg^-1.

Scalable Solution Processing MoS2 Powders with Liquid Crystalline Graphene Oxide for Flexible Freestanding Films with High Areal Lithium Storage Capacity

Freestanding flexible electrodes with high areal mass loading are required for the development of flexible high-performance lithium-ion batteries (LIBs). Currently they face the challenge of low mass loading due to the limited concentrations attainable in processable dispersions. Here, we report a simple low-temperature hydrothermal route to fabricate flexible layered molybdenum disulfide (MoS₂)/reduced graphene oxide (MSG) films offering high areal capacity and good lithium storage performance. This is achieved using a self-assembly process facilitated by the use of liquid crystalline graphene oxide (LCGO) and commercial MoS₂ powders at a low temperature of 70 °C. The amphiphilic properties of ultralarge LCGO nanosheets facilitates the processability of large-size MoS₂ powders, which is otherwise nondispersible in water. The resultant film with an areal mass of 8.2 mg cm^-2 delivers a high areal capacity of 5.80 mAh cm^-2 (706 mAh g^-1) at 0.1 A g^-1. This simple method can be adapted to similar nondispersible commercial battery materials for films fabrication or production of more complicated constructs via advanced fabrication technologies.

S-Doped Carbon Fibers Uniformly Embedded with Ultrasmall TiO2 for Na+ /Li+ Storage with High Capacity and Long-Time Stability

Building a rechargeable battery with high capacity, high energy density, and long lifetime contributes to the development of novel energy storage devices in the future. Although carbon materials are very attractive anode materials for lithium-ion batteries (LIBs), they present several deficiencies when used in sodium-ion batteries (SIBs). The choice of an appropriate structural design and heteroatom doping are critical steps to improve the capacity and stability. Here, carbon-based nanofibers are produced by sulfur doping and via the introduction of ultrasmall TiO₂ nanoparticles into the carbon fibers (CNF-S@TiO₂ ). It is discovered that the introduction of TiO₂ into carbon nanofibers can significantly improve the specific surface area and microporous volume for carbon materials. The TiO₂ content is controlled to obtain CNF-S@TiO₂ -5 to use as the anode material for SIBs/LIBs with enhanced electrochemical performance in Na⁺ /Li⁺ storage. During the charge/discharge process, the S-doping and the incorporation of TiO₂ nanoparticles into carbon fibers promote the insertion/extraction of the ions and enhance the capacity and cycle life. The capacity of CNF-S@TiO₂ -5 can be maintained at ≈300 mAh g^-1 over 600 cycles at 2 A g^-1 in SIBs. Moreover, the capacity retention of such devices is 94%, showing high capacity and good stability.