Functional Materials Letters

Solid-state polymer electrolyte is a promising alternative to the flammable liquid electrolyte due to its improved safety. In this work, we develop a novel polymer electrolyte with fire-retardant property and excellent electrochemical performance. This fire-proof polymer electrolyte (FPE) demonstrates an outstanding ionic conductivity of 6.7 × ${10}^{-4}$S ${\mathrm{\text{cm}}}^{-1}$ at ${25}^{\circ }$C. The Li//FPE//Li symmetric cell using such FPE could stably charge/discharge for 500 h at a current density of 0.5 mA ${\mathrm{\text{cm}}}^{-2}$ and an areal capacity of 0.5 mAh ${\mathrm{\text{cm}}}^{-2}$. The Li//FPE//LiFePO₄ lithium-metal cell also presents a good cycling performance with a discharge capacity of 127.8 mAh ${\mathrm{\text{g}}}^{-1}$ after 450 cycles at 0.5 C.

All-solid-state battery (ASSB) has attracted much attention since it is a potential candidate to solve the safety issue of batteries. However, the rigid contact between the electrode and electrolyte limits the electrochemical performance of ASSB. It is therefore necessary to modify the electrode–electrolyte interface to decrease the interfacial resistance. Herein, we use the aerosol deposition (AD) method to directly deposit the electrode, Na₃V₂(PO₄)₃ (NVP), on the Na₃Zr₂Si₂PO₁₂ (NZSP) electrolyte at room temperature. The electrolyte pellet coated with NVP demonstrates a negligible interfacial resistance and low total resistivity of about 1831.64 $\mathrm{\Omega }$ cm${}^{-1}$. Furthermore, the electrochemical performance in terms of cyclic voltammetry and charge/discharge of the ASSB consisting of NVP ∣ NZSP ∣ sodium (Na) metal is also investigated.

Nitrogen-doped graphene (N-Graphene) has been extensively researched as the anode material for lithium-ion batteries, as the nitrogen doping provides massive active sites and improves the electrical conductivity and the ion diffusion kinetics, thus, significantly enhancing the lithium storage performance with reversible capacity and cycling stability. However, the high-rate performance and cyclability of the N-Graphene-based anodes are still to be achieved for fast-charging applications. Here, a new N-Graphene was successfully fabricated by annealing the commercial few-layer graphene with dicyandiamide. Along with the plenty of defects in the pristine graphene sheets, the doping of nitrogen effectively reduces the resistance and increases pore volume and the diffusion coefficient. Consequently, the N-Graphene anode exhibits outstanding lithium storage performance with superior high-rate performance and remarkable long-term capability (up to 10,000 cycles at 15 A g${}^{-1}$ with a reversible capacity of 133 mAh g${}^{-1}$). Further kinetic analysis reveals that this excellent electrochemical behavior during the rapid discharge/charge operations can be attributed to the enhanced diffusion-controlled and surface capacitive storages. Our process may lead to an alternative way for producing competitive N-Graphene anode materials for efficient lithium ion storage.

Electrolytic manganese dioxide (EMD) is the critical component of the cathode material in modern alkaline, lithium, and sodium batteries. Aimed at the remediation of EMD crystalline structures, this research explored the effect of external power-supplying models on EMD structure regulation. The chaos electro circuit was found to impact the characteristics of EMD such as the crystalline structure, also the nanostructure morphology, and the hysteresis behavior.

Two-dimensional (2D) titanium carbide (MXene) is considered as a potential anode material for lithium-ion batteries (LIBs) by virtue of its unique structural and electronic properties. However, its performance for actual energy storage is seriously affected by the loss of transition metal elements during the preparation process of MXene. Herein, using a simple hydrothermal method, vanadium was successfully doped into the delaminated Ti₃C₂T${}_x$ (named as V-doped Ti₃C₂T${}_x$). As-prepared V-doped Ti₃C₂T${}_x$ MXenes provide more active sites and lower Li${}^{+}$ diffusion resistance. As a result, their rate performance is significantly improved compared to that of pristine Ti₃C₂T${}_x$. Additionally, its discharge capacity retains a value of 63.6 mAh g${}^{-1}$ after 10,000 cycles at 10 A g${}^{-1}$.

Although the lithium-ion battery has dominated the sustainable energy field for many years, environmentally friendly and inexpensive sodium (Na) is a great contender to reduce the use of limited lithium resources. Sodium-ion batteries (SIBs) have a big catalog of cathode materials grouped into four main types: the Prussian blue analogs, organic-based materials, layered metal oxides, and polyanions, whereas the latter is characterized by sodium superionic conductors (NASICON) with its chemical formula such as Na₃V₂(PO₄)₂F₃ (NVPF) gaining prevalence. Therefore, this review focuses on the most recent studies to promote the long-term stability, capacity, and energy density of NASICON’s high-potential NVPF.

Metal-organic frameworks (MOFs) have broad prospects as functional nanoparticles in lithium metal batteries (LMBs) due to their long-distance arrangement, adjustable size and chemical custom channels. Many MOFs and their derivatives are being studied as substrates or filling components of electrode and electrolyte materials. The application of metal-organic framework materials in this field is expected to further improve the safety, stability, energy density and cycle performance of solid-state LMBs. The controllable synthesis and predictable physical and chemical properties facilitate structure building and exploration of the reaction mechanism of LMBs at the molecular or atomic level. The composition of the electrolyte often has a complex impact on the performance of the battery, especially in flexible solid electrolytes, so the reasonable and controllable design of the three-dimensional electrolyte network is an important topic. In this review, the applications of MOFs and their derivatives in solid-state polymer electrolytes for LMBs are summarized.

Si@polyaniline-based activated-carbon (Si@P-AC) composites, consisting of porous polyaniline-based carbon layers, were synthesized via in-situ polymerization, high-temperature sintering, and steam activation utilizing silicon and aniline. Our experimental results demonstrate that Si@P-AC60 composites, prepared via steam activation at 700${}^{\circ }$C for 60 min, exhibit superior rate and cycling performance. At current densities of 100, 200, 300, 500, and 1000 mA⋅g${}^{-1}$, the discharge-specific capacities of Si@P-AC60 are 1617, 1133, 1005, 855, and 679 mAh⋅g${}^{-1}$, respectively. Additionally, the specific capacity retention rate reaches 75.5% when the current density returns to 100 mA⋅g${}^{-1}$, with the specific capacity recovering to 1221 mAh⋅g${}^{-1}$. This extraordinary electrochemical performance can be attributed to the improved electron and ion transport channels provided by the polyaniline-based activated-carbon (P-AC) coating on the surface of silicon particles.

Thermal runaway (TR) is one of the challenging problems in the safety of lithium-ion batteries (LIBs). The monitoring and early warning of TR events, the analysis and modeling of TR mechanisms, and the control of TR are crucial in battery safety research. This review first analyzes the three abuse factors. The identification and analysis of the characteristic temperatures in TR, including the onset temperature of self-heating, the initiation temperature of TR, and the maximum temperature of the TR are reviewed and analyzed. The heat of internal side reactions (ISRs) comes from the separate decomposition, oxidation or mutual reactions of solid electrolyte interphase, positive electrolyte interphase, positive and negative active materials, and electrolyte, which induce the gradual development of the battery towards TR. The ISR mechanism, thermodynamic and reaction kinetic characteristics are reviewed in detail.

Alkali metal-ion batteries (Li/Na/K) are important energy storage systems due to their ample elemental reserves and suitable working voltage. Many electrode materials had been developed and utilized to seek higher capacity and cycle stability. Single atom materials (SAMs) have high active atom dispersion and are very promising. The active metal sites can promote the diffusion kinetics and adsorption of lithium, sodium and potassium ions, which plays a key role in improving the capacity and cycling-stability. However, the experimental investigation of SAMs in Li/Na/K is still in its infancy stage. In this mini review, the structural design and electrochemical performance of single atom electrode materials in Li/Na/K are systematically summarized. The current challenges and potential research directions of designing SAMs for high-performance electrodes are also discussed.

In recent years, there has been a gradual increase in the development and research of photocapacitors as integrated devices, which hold the potential to address the intermittent output problem of solar cells. However, the overall efficiency of existing photocapacitors is much lower than that of the photovoltaic devices they employ, greatly limiting their usability. As photocapacitors are at the intersection of photovoltaic devices and supercapacitors, the development of these devices is still in its initial stage, and the related research is faced with numerous challenges. This review provides a constructive introduction to the classification of photocapacitors and briefly outlines the development process. Finally, the problems of photocapacitors that need to be solved are pointed out.

Solid-state sodium-ion batteries (SSIBs) will be the next generation batteries due to the high safety, low price, and availability of sodium resources. Composite polymer electrolytes (CPEs) are promising electrolytes because of easy fabrication, good stability, and high flexibility facilitating a good electrolyte-electrode interface, although their ionic conductivities are still unsatisfied. In this work, a CPE made of polyethylene oxide (PEO), Na₃Zr₂Si₂PO₁₂, and NaClO₄ has been fabricated. To further enhance the ionic conductivity and a good electrolyte–electrode interface, polyethylene glycol (PEG) is added as a plasticizer. The electrochemical impedance spectroscopy (EIS) demonstrates effect on conductivity by introducing PEG.

Temperature is closely related to the life activities of the human body, especially at the cellular scale. A large number of biological reactions often take place in cells, including gene expression, enzyme interaction, and cell metabolism, accompanied by the release or absorption of heat. Therefore, it is necessary to monitor the intracellular temperature in real time. Due to the excellent thermal stimulus-response ability and good biocompatibility of thermosensitive polymers, thermometers based on polymers have successfully achieved temperature detection at the cellular scale with high spatial resolution, which has attracted tremendous attention from researchers. In this paper, the design, synthesis strategy, and the ability of temperature imaging in cells of polymer-based thermometers are introduced in detail, which provides a good foundation for the future design of polymer-based thermometers and their biomedical applications.

Lithium-metal batteries with high-energy density have been widely studied in the field of electric vehicles in recent years. However, the hazard of the conventional flammable liquid electrolyte causes serious safety concern and impedes their wide applications. New liquid electrolytes with improved safety-level, in terms of the ability to restrict the Li dendrite growth and the nonflammability are strongly desired. In this work, a new trimethyl phosphate (TMP)-based liquid electrolyte with lithium difluoro(oxalato)borate (LiDFOB) as additives is developed. The results show that this electrolyte can effectively restrict the Li dendrite growth and avoid catching fire. The Li//Li symmetric cell using such an electrolyte can be stably cycled over 200 h with a low overpotential as small as 0.02 V at a current density of 0.5 mA ${\mathrm{\text{cm}}}^{-2}$ and an aerial capacity of 0.5 mAh ${\mathrm{\text{cm}}}^{-2}$. The capacity retention of the LiFePO₄//Li cell could reach $\sim$100% after 160 cycles at 0.5 C.

Responsive hydrogels based on Poly(N-isopropylacrylamide) (PNIPAM) are known to exhibit distinctive thermosensitive properties. However, isotropic PNIPAM hydrogels with weak mechanical properties and low deformation rates tend to exhibit only regular volume expansion/contraction, which limits them to promising applications such as intelligent actuators. In order to prepare programmable hydrogel actuators with satisfactory mechanical properties and fast deformation capability, a polyvinyl alcohol/PNIPAM (PVA/PNIPAM) bilayer hydrogel with anisotropic structure is proposed by combining preprepared PVA polymer film with outstanding mechanical properties as the passive layer and thermosensitive PNIPAM hydrogel as the active layer, forming a semi-interpenetrating network structure at the interface via the hydrogen-bond interaction between PNIPAM and PVA. The microstructures, mechanical properties and actuation behaviors of bilayer hydrogel were studied by scanning electron microscope, Fourier transform infrared spectrograph, mechanical testing machine and actuation test device. Results show that the introduction of PVA can improve the tensile stress of the bilayer hydrogel from 23.6 kPa to 62.6 kPa, and favor the hydrogel actuator excellent actuation preformation with a maximum bending amplitude of 500° and a maximum bending velocity of 13°/s within first 40 s. The bilayer hydrogel is further designed to work as a fluidic system valve that can recognize various temperature solutions and control solution flow rate. This design provides a simple and practical strategy to construct responsive hydrogels with anisotropic structure for further development in the field of intelligent actuators and flexible microfluidic systems.

In this study, the post-annealing process of garnet-type electrolyte films produced by the powder aerosol deposition method was optimized by utilizing high-power LEDs. The post-annealing time was greatly reduced from hours to several seconds. The film conductivity at room temperature could be increased by three orders of magnitude from 5.0 ⋅ ${10}^{-8}$ S ${\mathrm{\text{cm}}}^{-1}$ to 5 ⋅ ${10}^{-5}$ S ${\mathrm{\text{cm}}}^{-1}$. For the first time, the required energy for this almost complete conductivity restoration could be quantitatively estimated.

Novel catalysts of Rh/ZrSnO₄ were synthesized, and their catalytic activities for the direct decomposition of nitrous oxide (N₂O) were investigated. ZrSnO₄ with an $\alpha$-PbO₂-type structure was used as a promoter, since the oxygen supply from its lattice was expected to facilitate N₂O decomposition. Among the prepared catalysts, 1.0 wt.% Rh/ZrSnO₄ exhibited the highest catalytic activity: N₂O was completely decomposed at ${450}^{\circ }$C. This activity was higher than those for 1.0 wt.% Rh/ZrO₂ and 1.0 wt.% Rh/SnO₂, indicating that the ZrSnO₄ solid promoted N₂O decomposition. In addition, the 1.0 wt.% Rh/ZrSnO₄ catalyst featured high durability in the presence of O₂, CO₂, and H₂O vapors.

Recently, visible light-driven overall water splitting without any sacrificial reagents has served as a promising technology for utilizing solar energy. Herein, CoOOH@Zn${}_x$Co${}_{1-x}$S heterostructure was successfully prepared by in-situ creating CoOOH on the surface of Zn${}_x$Co${}_{1-x}$S nanospheres via an electrochemical oxidation route. Such CoOOH@Zn${}_x$Co${}_{1-x}$S heterostructure shows an enhanced capacity for overall water splitting under visible light without any sacrificial reagents. Besides, based on the results of linear sweep photovoltammetry (LSV) and open-circuit photovoltage decay (OCVD), the unique cascade charge transport channel between the in-situ creating CoOOH and Zn${}_x$Co${}_{1-}$${}_x$S will significantly enhance the transfer behaviors and prolongs the lifetime of photo-generated charge. It is hoped that this work could provide new insight into reasonably designing efficient photocatalytic materials for converting solar energy to chemical energy.

The electronic, the thermal, and the optical properties of hexagonal MgX monolayers (where $X$ = $C$, $N$, and $O$) are investigated via first principles studies. Ab-initio molecular dynamic, AIMD, simulations using NVT ensembles are performed to check the thermodynamic stability of the monolayers. We find that an MgO monolayer has semiconductor properties with a good thermodynamic stability, while the MgC and the MgN monolayers have metallic characters. The calculated phonon band structures of all the three considered monolayers show no imaginary nonphysical frequencies, thus indicating that they all have excellent dynamic stability. The MgO monolayer has a larger heat capacity then the MgC and the MgN monolayers. The metallic monolayers demonstrate optical response in the IR as a consequence of the metal properties, whereas the semiconducting MgO monolayer demonstrates an active optical response in the near-UV region. The optical response in the near-UV is beneficial for nanoelectronics and photoelectric applications. A semiconducting monolayer is a great choice for thermal management applications since its thermal properties are more attractive than those of the metallic monolayer in terms of heat capacity, which is related to the change in the internal energy of the system.

The exploration of highly efficient and low-cost catalysts for photocatalytic hydrogen evolution is of great increasing interest under the dual pressure of energy shortage and environment pollution. In this work, flake-like SrCO₃ was in-situ self-assembled at the surface of SrTiO₃ nanosheet to fabricate a novel 2D/2D heterojunction nanocomposite catalyst with strong interaction inside these two semiconductors by a simple hydrothermal method. An accurate loading amount of SCO was responsible for the high photoactivity of 10SCO-STO-Pt, which leads to a 12.9-fold improvement of photocatalytic hydrogen evolution rate (from 0.22 to 2.83 mmol ${\mathrm{\text{h}}}^{-1}$${\mathrm{\text{g}}}^{-1}$) compared to that of pure STO-Pt. Meanwhile, the apparent quantum efficiency (AQE) of 10SCO-STO-Pt composite with the optimum SCO loading (10 wt.%) reached up to 16.9% under the irradiation of 313 nm light. The introduction of SCO can form a II-type heterojunction in close contact with STO, which can significantly improve the photogenerated electron transfer and reduce the recombination of photogenerated carrier in STO. As a result, the charge separation efficiency and photocatalytic activity of STO were obviously enhanced.

Photoelectrochemical reduction of nitrate (PEC NRR) to ammonia (NH₃) is a promising means with water as hydrogen source under room temperature. The construction of photoelectrode with efficient light response and reaction kinetics is the core point to achieve high NH₃ yield efficiency. Herein, a cerium-based metal organic framework (Ce-MOF) was successfully decorated on the copper phthalocyanine (CuPc). The CuPc/Ce-MOF heterostructure exhibits the ideal nitrate catalytic reduction activity, and the NH₃ yield (−0.6 V versus RHE) increases to 1.58 $\mu$g ${\mathrm{\text{h}}}^{-1}$ ${\mathrm{\text{cm}}}^{-2}$ from 0.67 $\mu$g ${\mathrm{\text{h}}}^{-1}$ ${\mathrm{\text{cm}}}^{-2}$(CuPc). Besides, the cycle experiments reveal that the CuPc/Ce-MOF heterostructure has maintained good catalytic stability. This work explores the PEC NRR process mechanism of CuPc/Ce-MOF samples and provides a new idea for green NH₃ production.

Li₄Ti₅O₁₂(LTO) is one of the most popular ${\mathrm{\text{Li}}}^{+}$-storage anode materials. However, the influences of different operating temperatures on the electrochemical performance of LTO and the underlying mechanisms are still unclear. Herein, we systematically investigate its temperature-dependent electrochemical performance, electrochemical kinetics, and crystal-structural evolution at −10${}^{\circ }$C, $5^{\circ }$C, ${25}^{\circ }$C, ${45}^{\circ }$C, and ${60}^{\circ }$C. When the operation temperature increases from −10${}^{\circ }$C to ${45}^{\circ }$C, more intensive electrolyte decomposition increases the irreversible capacity in the first cycle, which decreases the initial Coulombic efficiency. Meanwhile, the electrochemical kinetics becomes faster, leading to reduced electrode polarization, faster ${\mathrm{\text{Li}}}^{+}$Transport, and higher rate capability. Finally, the maximum unit-cell-volume shrinkage enlarges, resulting in the decay of the cyclic stability. However, when the temperature further rises to 60°C, the rate capacity and cyclic stability rapidly decay due to the severe electrolyte decomposition catalyzed by ${\mathrm{\text{Ti}}}^{4+}$ and the formation of thick solid electrolyte interface (SEI) films.

The development of non-precious metal electrocatalysts is an effective approach to achieving environmentally friendly electrolytic water for hydrogen production. In this paper, we report a method for the rapid preparation of excellent HER activity and durable WC thin-film electrodes using one-pot electrolysis with K₂CO₃ − Na₂CO₃ molten salts. During the electrolysis, soluble carbonate anions reduced and in situ formed WC nanoparticles on a tungsten cathode, which become firmly immobilized on the surface of the tungsten cathode, resulting in the formation of the WC/W electrode. The WC/W electrode exhibited efficient electrocatalytic HER activity, good reaction kinetics and long-term stability. In particular, the WC/W-3.1 V catalysts exhibited low overpotentials (127 mV and 221 mV) at current densities of 10 mA ${\mathrm{\text{cm}}}^{-2}$ and 50 mA ${\mathrm{\text{cm}}}^{-2}$, along with a small Tafel slopes (129.00 mV dec${}^{-1}$). The experimental results indicate that the WC/W electrode possesses more electrochemical active site, faster electron transfer rate and strong interfacial interaction and stability. This binder-free WC thin film electrode rapid preparation method via molten salts electrolysis provides a new approach for the development of highly active electrocatalysts.

Hierarchical flower-like In₂S₃ nanostructures were synthesized using a solvothermal process. The device based on In₂S₃ nanostructure was fabricated and the photoresponse properties were investigated. The flower-like In₂S₃ nanostructures display a strong quantum confinement with a wide band response in range from ultraviolet (UV) to the visible region. The results indicated that the photodetector based on hierarchical flower-like $\beta$-In₂S₃ nanostructures will have important applications in multiple band detection devices.