Graphene-Encapsulated CuP2: A Promising Anode Material with High Reversible Capacity and Superior Rate-Performance for Sodium-Ion Batteries

Metal Phosphide Anodes in Sodium-Ion Batteries: Latest Applications and Progress

Sodium-ion batteries (SIBs), as the next-generation high-performance electrochemical energy storage devices, have attracted widespread attention due to their cost-effectiveness and wide geographical distribution of sodium. As a crucial component of the structure of SIBs, the anode material plays a crucial role in determining its electrochemical performance. Significantly, metal phosphide exhibits remarkable application prospects as an anode material for SIBs because of its low redox potential and high theoretical capacity. However, due to volume expansion limitations and other factors, the rate and cycling performance of metal phosphides have gradually declined. To address these challenges, various viable solutions have been explored. In this paper, the recent research progress of metal phosphide materials for SIBs is systematically reviewed, including the synthesis strategy of metal phosphide, the storage mechanism of sodium ions, and the application of metal phosphide in electrochemical aspects. In addition, future challenges and opportunities based on current developments are presented.

A Dual Organic Solvent Zn-Ion Electrolyte Enables Highly Stable Zn Metal Batteries

Aqueous zinc (Zn) batteries have been regarded as an alternative to lithium-ion batteries due to their high abundance, low cost, and higher intrinsic safety. However, the low Zn plating/stripping reversibility, Zn dendrite growth, and continuous water consumption have hindered the practical application of aqueous Zn anodes. Herein, a hydrous organic Zn-ion electrolyte based on a dual organic solvent, namely hydrated Zn(BF₄)₂ zinc salt dissolved in dimethyl carbonate (DMC) and vinyl carbonate (EC) solvents [denoted as Zn(BF₄)₂/DMC/EC], can address these problems, which not only inhibits the side reactions but also promotes uniform Zn plating/stripping through the formation of a stable solid state interface layer and Zn²⁺-EC/2DMC pairs. This electrolyte enables the Zn electrode to stably undergo >700 cycles at a rate of 1 mA cm^-2 with a Coulombic efficiency of 99.71%. Moreover, the full cell paired with V₂O₅ also demonstrates excellent cycling stability without capacity decay at 1 A g^-1 after 1600 cycles.

Phonons in Copper Diphosphide (CuP2): Raman Spectroscopy and Lattice Dynamics Calculations

Copper diphosphide (CuP₂) is an emerging binary semiconductor with promising properties for energy conversion and storage applications. While functionality and possible applications of CuP₂ have been studied, there is a curious gap in the investigation of its vibrational properties. In this work, we provide a reference Raman spectrum of CuP₂, with a complete analysis of all Raman active modes from both experimental and theoretical perspectives. Raman measurements have been performed on polycrystalline CuP₂ thin films with close to stoichiometric composition. Detailed deconvolution of the Raman spectrum with Lorentzian curves has allowed identification of all theoretically predicted Raman active modes (9A_g and 9B_g), including their positions and symmetry assignment. Furthermore, calculations of the phonon density of states (PDOS), as well as the phonon dispersions, provide a microscopic understanding of the experimentally observed phonon lines, in addition to the assignment to the specific lattice eigenmodes. We further provide the theoretically predicted positions of the infrared (IR) active modes, along with the simulated IR spectrum from density functional theory (DFT). Overall good agreement is found between the experimental and DFT-calculated Raman spectra of CuP₂, providing a reference platform for future investigations on this material.

Rational construction of yolk-shell CoP/N,P co-doped mesoporous carbon nanowires as anodes for ultralong cycle life sodium-ion batteries

Owing to the natural abundance and low-cost of sodium, sodium-ion batteries offer advantages for next-generation portable electronic devices and smart grids. However, the development of anode materials with long cycle life and high reversible capacity is still a great challenge. Herein, we report a yolk–shell structure composed of N,P co-doped carbon as the shell and CoP nanowires as the yolk (YS–CoP@NPC) for a hierarchically nanoarchitectured anode for improved sodium storage performance. Benefitting from the 1D hollow structure, the YS–CoP@NPC electrode exhibits an excellent cycling stability with a reversibly capacity of 211.5 mA h g⁻¹ at 2 A g⁻¹ after 1000 cycles for sodium storage. In-depth characterization by ex situ X-ray photoelectron spectroscopy and work function analysis revealed that the enhanced sodium storage property of YS–CoP@NPC might be attributed to the stable solid electrolyte interphase film, high electronic conductivity and better Na⁺ diffusion kinetics.

Owing to the natural abundance and low-cost of sodium, sodium-ion batteries offer advantages for next-generation portable electronic devices and smart grids.

Recent Progress on Fe-Based Single/Dual-Atom Catalysts for Zn-Air Batteries

As one of the most competitive candidates for large-scale energy storage, zinc-air batteries (ZABs) have attracted great attention due to their high theoretical specific energy density, low toxicity, high abundance, and high safety. It is highly desirable but still remains a huge challenge, however, to achieve cheap and efficient electrocatalysts to promote their commercialization. Recently, Fe-based single-atom and dual-atom catalysts (SACs and DACs, respectively) have emerged as powerful candidates for ZABs derived from their maximum utilization of atoms, excellent catalytic performance, and low price. In this review, some fundamental concepts in the field of ZABs are presented and the recent progress on the reported Fe-based SACs and DACs is summarized, mainly focusing on the relationship between structure and performance at the atomic level, with the aim of providing helpful guidelines for future rational designs of efficient electrocatalysts with atomically dispersed active sites. Finally, the great advantages and future challenges in this field of ZABs are also discussed.

Fully Active Bimetallic Phosphide Zn0.5Ge0.5P: A Novel High-Performance Anode for Na-Ion Batteries Coupled with Diglyme-Based Electrolyte

Metal phosphides are promising candidates for sodium-ion battery (SIB) anode owing to their large capacities with suitable redox potential, while the reversibility and rate performances are limited due to some electrochemically inactive transition-metal components and sluggish reaction kinetics. Here, we report a fully active bimetallic phosphide Zn_0.5Ge_0.5P anode and its composite (Zn_0.5Ge_0.5P-C) with excellent performance attributed to the Zn, Ge, and P components exerting their respective Na-storage merit in a cation-disordered structure. During Na insertion, Zn_0.5Ge_0.5P undergoes an alloying-type reaction, along with the generation of NaP, Na₃P, NaGe, and NaZn₁₃ phases, and the uniform distribution of these phases ensures the electrochemical reversibility during desodiation. Based on this reaction mechanism, excellent electrochemical properties such as a high reversible capacity of 595 mAh g^-1 and an ultrafast charge-discharge capability of 377.8 mAh g^-1 at 50C for 500 stable cycles were achieved within the Zn_0.5Ge_0.5P-C composite in a diglyme-based electrolyte. This work reveals the Na-storage reaction mechanism within Zn_0.5Ge_0.5P and offers a new perspective on designing high-performance anodes.

Crystallize It before It Diffuses: Kinetic Stabilization of Thin-Film Phosphorus-Rich Semiconductor CuP2

Numerous phosphorus-rich metal phosphides containing both P–P bonds and metal–P bonds are known from the solid-state chemistry literature. A method to grow these materials in thin-film form would be desirable, as thin films are required in many applications and they are an ideal platform for high-throughput studies. In addition, the high density and smooth surfaces achievable in thin films are a significant advantage for characterization of transport and optical properties. Despite these benefits, there is hardly any published work on even the simplest binary phosphorus-rich phosphide films. Here, we demonstrate growth of single-phase CuP₂ films by a two-step process involving reactive sputtering of amorphous CuP_2+x and rapid annealing in an inert atmosphere. At the crystallization temperature, CuP₂ is thermodynamically unstable with respect to Cu₃P and P₄. However, CuP₂ can be stabilized if the amorphous precursors are mixed on the atomic scale and are sufficiently close to the desired composition (neither too P poor nor too P rich). Fast formation of polycrystalline CuP₂, combined with a short annealing time, makes it possible to bypass the diffusion processes responsible for decomposition. We find that thin-film CuP₂ is a 1.5 eV band gap semiconductor with interesting properties, such as a high optical absorption coefficient (above 10⁵ cm^–1), low thermal conductivity (1.1 W/(K m)), and composition-insensitive electrical conductivity (around 1 S/cm). We anticipate that our processing route can be extended to other phosphorus-rich phosphides that are still awaiting thin-film synthesis and will lead to a more complete understanding of these materials and of their potential applications.

Honeycomb-like 3D carbon skeletons with embedded phosphorus-rich phosphide nanoparticles as advanced anodes for lithium-ion batteries

Phosphorus-rich iron phosphides (FeP₂) have been regarded as excellent anode candidates for lithium storage owing to their low cost, high natural abundance, high theoretical capacity, and reasonable redox potential. However, FeP₂ suffers from a few challenging problems such as low reversibility, fast capacity degradation, and big volume variation. Herein, we have designed and synthesized a 3D honeycomb-like carbon skeleton with embedded FeP₂ nanoparticles (denoted as FeP₂ NPs@CK), which can significantly promote the kinetics and maintain the structural stability during the cycling, resulting in an excellent electrochemical performance reflected by high reversibility and long-term cycling stability. FeP₂ NPs@CK shows high reversibility, delivering a reversible capacity as high as 938 mA h g^-1 at 0.5 A g^-1. It also shows excellent cycling stability, delivering a capacity of 620 mA h g^-1 after 500 cycles at 1 A g^-1. Moreover, the fast kinetics and lithium storage mechanism of FeP₂ NPs@CK are investigated by quantitative analysis and in situ X-ray diffraction. Such superior performance demonstrates that FeP₂ NPs@CK could be a promising and attractive anode candidate for lithium storage.

Disclosing the Origin of Irreversible Sodiation-Desodiation of a Cu4SnP10 Nanowire Anode by Ex Situ Transmission Electron Microscopy

Cu₄SnP₁₀, a promising phosphide material for sodium-ion battery anode applications, suffers from poor cycling stability, and its mechanism remains unclear. This is largely due to the amorphous nature of the active materials upon cycling and its possible structural change at a small length scale (e.g., nanometers), making it difficult to access the phase/structural evolution of the electrode. In the present work, we show that the phase/structural change of the Cu₄SnP₁₀ nanowire electrode can be systematically investigated using a comprehensive set of ex situ transmission electron microscopy-based techniques, which are ideal for decay mechanism analysis of electrode materials of amorphous nature and with nanoscale structural evolution. The compositional elements of Cu₄SnP₁₀ nanowires are found to be spatially redistributed at a nanometer scale upon the initial sodiation, and this is partially reversible in the following desodiation process. Damage accumulates until a critical size of phase separation/segregation is reached, when the active material loss takes place, leading to fast deterioration of the entire Cu₄SnP₁₀ nanowire structure and thus its electrochemical performance. The phase segregation driven-active material loss is found to dominate the cycle-dependent capacity decay of the Cu₄SnP₁₀ nanowire electrode.

Ether-based electrolytes for sodium ion batteries

Sodium-ion batteries (SIBs) are considered to be strong candidates for large-scale energy storage with the benefits of cost-effectiveness and sodium abundance. Reliable electrolytes, as ionic conductors that regulate the electrochemical reaction behavior and the nature of the interface and electrode, are indispensable in the development of advanced SIBs with high Coulombic efficiency, stable cycling performance and high rate capability. Conventional carbonate-based electrolytes encounter numerous obstacles for their wide application in SIBs due to the formation of a dissolvable, continuous-thickening solid electrolyte interface (SEI) layer and inferior stability with electrodes. Comparatively, ether-based electrolytes (EBEs) are emerging in the secondary battery field with fascinating properties to improve the performance of batteries, especially SIBs. Their stable solvation structure enables highly reversible solvent-co-intercalation reactions and the formation of a thin and stable SEI. However, although EBEs can provide more stable cycling and rapid sodiation kinetics in electrodes, benefitting from their favorable electrolyte/electrode interactions such as chemical compatibility and good wettability, their special chemistry is still being investigated and puzzling. In this review, we provide a thorough and comprehensive overview on the developmental history, fundamental characteristics, superiorities and mechanisms of EBEs, together with their advances in other battery systems. Notably, the relation among electrolyte science, interfacial chemistry and electrochemical performance is highlighted, which is of great significance for the in-depth understanding of battery chemistry. Finally, future perspectives and potential directions are proposed to navigate the design and optimization of electrolytes and electrolyte/electrode interfaces for advanced batteries.

In-Situ Growth of Urchin Manganese Sulfide Anchored Three-Dimensional Graphene (γ-MnS@3DG) on Carbon Cloth as a Flexible Asymmetric Supercapacitor

In energy storage-device it is highly crucial to develop durable electrode materials having high specific capacitance and superior energy density without disturbing its inherent flexibility. Herein, we demonstrate three-dimensional graphene oxide decorated monodispersed hollow urchin γ-MnS (γ-MnS@3DG) via proficient one-step solvothermal method. The designed material delivers a remarkable capacitance of 858 F g^-1 at 1 A g^-1. A flexible solid state asymmetric supercapacitor (ASCs) device assembled using surface activated carbon cloth (CC) decorated with γ-MnS@3DG as positive and three-dimension graphene on carbon cloth (3DG@CC) as negative electrode, (γ-MnS@3DG//3DG). The device delivers 26 Wh kg^-1 energy density at power density 500 W kg^-1 @ 1A g^-1 and retains favorable energy density 17.8 Wh kg^-1 at an ultrahigh power density of 1500 W kg^-1@3 A g^-1. This carbon embedded transition-metal sulfide (TMS) based ASC demonstrates eminent mechanical flexibility under rigorous bending states maintaining invariant performance.

Achieving High Pseudocapacitance Anode by An In Situ Nanocrystallization Strategy for Ultrastable Sodium-Ion Batteries

Conversion/alloying type anodes have shown great promise for sodium-ion batteries (SIBs) because of their high theoretical capacity. However, the poor structural stability derived from the large volume expansion and short lifetime impedes their further practical applications. Herein, we report a novel anode with a pomegranate-like nanostructure of SnP₂O₇ particles homogeneously dispersed in the robust N-doped carbon matrix. For the first time, we make use of in situ self-nanocrystallization to generate ultrafine SnP₂O₇ particles with a short pathway of ions and electrons to promote the reaction kinetics. Ex situ transmission electron microscope (TEM) shows that the average particle size of SnP₂O₇ decreases from 66 to 20 nm successfully based on this unique nanoscale-engineering method. Therefore, the nanoparticles together with the N-doped carbon contribute a high pseudocapacitance contribution. Moreover, the N-doped carbon matrix forms strong interaction with the self-nanocrystallization ultrafine SnP₂O₇ particles, leading to a stable nanostructure without any particle aggregation under a long-cycle operation. Benefiting from these synergistic merits, the SnP₂O₇@C anode shows a high specific capacity of 403 mAh g^-1 at 200 mA g^-1 and excellent cycling stability (185 mAh g^-1 after 4000 cycles at 1000 mA g^-1). This work presents a new route for the effective fabrication of advanced conversion/alloying anodes materials for SIBs.

Cubic MnS-FeS2 Composites Derived from a Prussian Blue Analogue as Anode Materials for Sodium-Ion Batteries with Long-Term Cycle Stability

Cubic N,S codoped carbon coating MnS-FeS₂ composites (MnS-FeS₂@NSC) with a hollow structure were prepared and used as anode materials for sodium-ion batteries. MnS-FeS₂@NSC exhibits excellent cycle performance and high rate capability and delivered a reversible capacity of 501.0 mAh g^-1 after 800 cycles at a current density of 0.1 A g^-1 with a capacity retention of 81%. More importantly, the MnS-FeS₂@NSC anode holds long-term cycle stability; the capacity can remain 134.0 mAh g^-1 after 14 500 cycles at 4 A g^-1. Kinetic analysis demonstrated that Na⁺ storage follows a pseudocapacitive dominating process, which is ascribed to the origin of the outstanding rate performance of the MnS-FeS₂@NSC material. The enhancement of electrochemical performance is attributed to the hollow structure and the N,S codoped carbon coating structure, which can reduce the diffusion distance for sodium ions and electrons, alleviate volume expansion during sodium-ion insertion/extraction, and retain the structural integrity effectively. Furthermore, a two-step sodiation processes with FeS₂ sodiation prior to MnS was demonstrated by X-ray diffraction (XRD), and the electrochemical impedance spectroscopy (EIS) spectra might indicate that the accumulation of the metallic elements in the preconversion reaction can accelerate the transfer of electrons and ions in the further conversion process.

Metal-organic framework derived amorphous VOx coated Fe3O4/C hierarchical nanospindle as anode material for superior lithium-ion batteries

Lithium-ion batteries (LIBs) are widely regarded as a promising electrochemical energy storage device, due to their high energy density and good cycling stability. To date, the development of anode materials for LIBs is still confronted with many serious problems, and much effort is required for constructing more ideal anode materials. Herein, starting with metal-organic frameworks (MOFs), an amorphous VO_x coated Fe₃O₄/C hierarchical nanospindle has been successfully synthesized. The obtained Fe₃O₄/C@VO_x nanospindle has a uniform particle size of ∼100 nm in diameter and ∼400 nm in length and consists of ultrafine Fe₃O₄ nanoparticles (∼5 nm) embedded in a porous carbon matrix as the core and an amorphous VO_x layer as the shell. Notably, as the anode material for LIBs, Fe₃O₄/C@VO_x delivers a high coulombic efficiency (74.2%) and a large capacity of 845 mA h g^-1 after 500 cycles at 1000 mA g^-1. A prominent discharge reversible capacity of 340 mA h g^-1 is also still retained at 5000 mA g^-1. More importantly, the presented facile MOF-derived route could be easily extended to other functional materials for widespread applications.

Novel Skutterudite CoP3 -Based Asymmetric Supercapacitor with Super High Energy Density

Skutterudite CoP₃ holds a unique structural formation that exhibits much better electronic properties for obtaining high energy density supercapacitors. Herein, novel skutterudite Ni-CoP₃ nanosheets are constructed by etching and coprecipitating at room temperature and subsequent low-temperature phosphorization reaction. Benefiting from the enhanced electrical conductivity and more electroactive sites brought about by adjusting the electronic structure with Ni incorporating the Ni-CoP₃ electrode with a battery-type demonstrates an ultrahigh specific capacity of 0.7 mA h cm^-2 and exceptional cycling stability. The asymmetric supercapacitor (ASC) device fabricated by employing Ni-CoP₃ and activated carbon (AC) as positive and negative electrodes, resepectively, exhibits a remarkable high energy density of 89.6 Wh kg^-1 at 796 W kg^-1 and excellent stability of 93% after 10 000 cycles, due to the skutterudite structure. The skutterudite Ni-CoP₃ shows a great potential to be an excellent next-generation electrode candidate for supercapacitors and other energy storage devices.

Core-Shell C@Sb Nanoparticles as a Nucleation Layer for High-Performance Sodium Metal Anodes

Sodium metal anode (SMA) is one of the most favored choices for the next-generation rechargeable battery technologies owing to its low cost and natural abundance. However, the poor reversibility resulted from dendrite growth and formation of unstable solid electrolyte interphase has significantly hindered the practical application of SMAs. Herein, we report that a nucleation buffer layer comprising elaborately designed core-shell C@Sb nanoparticles (NPs) enables the homogeneous electrochemical deposition of sodium metal for long-term cycling. These C@Sb NPs can increase active sites for initial sodium nucleation through Sb-Na alloy cores and keep these cores stable through carbon shells. The assembled cells with this nucleation layer can deliver continuously repeated sodium plating/stripping cycles for nearly 6000 h at a high areal capacity of 4 mA h cm^-2 with an average Coulombic efficiency 99.7%. This ingenious structure design of alloy-based nucleation agent opens up a promising avenue to stabilize sodium metal with targeted properties.

Biomass-Derived P/N-Co-Doped Carbon Nanosheets Encapsulate Cu3P Nanoparticles as High-Performance Anode Materials for Sodium-Ion Batteries

Biomass-derived approaches have been accepted as a practical way for the design of transitional metal phosphides confined by carbon matrix (TMPs@C) as energy storage materials. Herein, we successfully synthesize P/N-co-doped carbon nanosheets encapsulating Cu₃P nanoparticles (Cu₃P@P/N-C) by a feasible aqueous reaction followed by a phosphorization procedure using sodium alginate as the biomass carbon source. Cu-alginate hydrogel balls can be squeezed into two-dimensional (2D) nanosheets through a freeze–drying process. Then, Cu₃P@P/N-C was obtained after the phosphorization procedure. This rationally designed structure not only improved the kinetics of ion/electron transportation but also buffered the volume expansion of Cu₃P nanoparticles during the continuous charge and discharge processes. In addition, the 2D P/N co-doped carbon nanosheets can also serve as a conductive matrix, which can enhance the electronic conductivity of the whole electrode as well as provide rapid channels for electron/ion diffusion. Thus, when applied as anode materials for sodium-ion batteries, it exhibited remarkable cycling stability and rate performance. Prominently, Cu₃P@P/N-C demonstrated an outstanding reversible capacity of 209.3 mAh g⁻¹ at 1 A g⁻¹ after 1,000 cycles. Besides, it still maintained a superior specific capacity of 118.2 mAh g⁻¹ after 2,000 cycles, even at a high current density of 5 A g⁻¹.

Electrode Materials for High-Performance Sodium-Ion Batteries

Sodium ion batteries (SIBs) are being billed as an economical and environmental alternative to lithium ion batteries (LIBs), especially for medium and large-scale stationery and grid storage. However, SIBs suffer from lower capacities, energy density and cycle life performance. Therefore, in order to be more efficient and feasible, novel high-performance electrodes for SIBs need to be developed and researched. This review aims to provide an exhaustive discussion about the state-of-the-art in novel high-performance anodes and cathodes being currently analyzed, and the variety of advantages they demonstrate in various critically important parameters, such as electronic conductivity, structural stability, cycle life, and reversibility.