Research Development on Sodium-Ion Batteries

Room-temperature rechargeable sodium-ion batteries are considered as a promising alternative technology for grid and other storage applications due to their competitive cost benefit and sustainable resource supply, triumphing other battery systems on the market. To facilitate the practical realization of the sodium-ion technology, the energy density of sodium-ion batteries needs to be boosted to the level of current commercial Li-ion batteries. An effective approach would be to elevate the operating voltage of the battery, which requires the use of electrochemically stable cathode materials with high voltage versus Na+/Na. This review summarizes the recent progress with the emerging high-voltage cathode materials for room-temperature sodium-ion batteries, which include layered transitional-metal oxides, Na-rich materials, and polyanion compounds. The key challenges and corresponding strategies for these materials are also discussed, with an emphasis placed on the intrinsic structural properties, Na storage electrochemistry, and the voltage variation tendency with respect to the redox reactions. The insights presented in this article can serve as a guide for improving the energy densities of room-temperature Na-ion batteries.

Progress in High-Voltage Cathode Materials for Rechargeable Sodium-Ion Batteries

Sodium-ion batteries, representative members of the post-lithium-battery club, are very attractive and promising for large-scale energy storage applications. The increasing technological improvements in sodium-ion batteries (Na-ion batteries) are being driven by the demand for Na-based electrode materials that are resource-abundant, cost-effective, and long lasting. Polyanion-type compounds are among the most promising electrode materials for Na-ion batteries due to their stability, safety, and suitable operating voltages. The most representative polyanion-type electrode materials are Na3V2(PO4)3 and NaTi2(PO4)3 for Na-based cathode and anode materials, respectively. Both show superior electrochemical properties and attractive prospects in terms of their development and application in Na-ion batteries. Carbonophosphate Na3MnCO3PO4 and amorphous FePO4 have also recently emerged and are contributing to further developing the research scope of polyanion-type Na-ion batteries. However, the typical low conductivity and relatively low capacity performance of such materials still restrict their development. This paper presents a brief review of the research progress of polyanion-type electrode materials for Na-ion batteries, summarizing recent accomplishments, highlighting emerging strategies, and discussing the remaining challenges of such systems.

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Polyanion-Type Electrode Materials for Sodium-Ion Batteries.

The development of efficient and stable water oxidation catalysts is necessary for the realization of practically viable water-splitting systems. Although extensive studies have focused on the metal-oxide catalysts, the effect of metal coordination on the catalytic ability remains still elusive. Here we select four cobalt-based phosphate catalysts with various cobalt- and phosphate-group coordination as a platform to better understand the catalytic activity of cobalt-based materials. Although they exhibit various catalytic activities and stabilities during water oxidation, Na2CoP2O7 with distorted cobalt tetrahedral geometry shows high activity comparable to that of amorphous cobalt phosphate under neutral conditions, along with high structural stability. First-principles calculations suggest that the surface reorganization by the pyrophosphate ligand induces a highly distorted tetrahedral geometry, where water molecules can favourably bind, resulting in a low overpotential (∼0.42 eV). Our findings emphasize the importance of local cobalt coordination in the catalysis and suggest the possible effect of polyanions on the water oxidation chemistry.

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Coordination tuning of cobalt phosphates towards efficient water oxidation catalyst

Room-temperature sodium-ion batteries (SIBs) are regarded as promising candidates for smart grids and large-scale energy storage systems (EESs) due to their significant benefits of abundant and low-cost sodium resource. Among the previously reported cathode materials for SIBs, layered transition-metal oxides and polyanion-type materials are considered to be the most attractive options. Although many layered transition-metal oxides can provide high capacity due to their small molecular weight, their further application is hindered by low output voltage (mostly lower than 3.5 V), irreversible phase transition as well as storage instability. Comparatively, polyanion-type materials exhibit higher operating potentials due to the inductive effect of polyanion groups. Their robust 3D framework significantly decreases the structural variations during sodium ion de/intercalation. Moreover, the effect of strong X-O (X = S, P, Si, etc.) covalent bonds can effectively inhibit oxygen evolution. These advantages contribute to the superior cycle stability and high safety of polyanion-type materials. However, low electronic conductivity and limited capacity still restrict their further application. This review summarizes the recent progress of polyanion-type materials for SIBs, which include phosphates, fluorophosphates, pyrophosphates, mixed phosphates, sulfates, and silicates. We also discuss the remaining challenges and corresponding strategies for polyanion-type materials. We hope this review can provide some insights into the development of polyanionic materials.

Polyanion-type cathode materials for sodium-ion batteries

Synthesis and crystal structure of Na2CaP2O7

σ-Dibarium diphosphate, Ba 2 P 2 O 7 , crystallizes in space group P62m with the Ba atoms, separated by 4.703 A, arranged in columns. Ba atoms are in sites of eleven- and tenfold coordination with Ba(1)-O and Ba(2)-O distances in the ranges 2.776(8)-2.869(6) and 2.878(9)-3.080(9) A, respectively. One Ba atom has three interactions with bridging O atoms. P 2 O 7 groups have threefold axes passing through the P atoms. The bridging O atom is disordered about these axes, with P-O-P angles of 130 (1) and 134 (4)°.

σ-Dibarium Pyrophosphate

Structural order in complexes of the AIIBIIP2O7 series: BaPbP2O7, PbCoP2O7, PbMnP2O7 and PbCuP2O7

New palladium phosphate complexes: K2PdP2O7 and K3.5Pd2.25(P2O7)2 synthesis, single crystal structure and conductivity

Crystalline K2NiP2O7 (I): monoclinic, P21,a=9.230(2),b=17.540(8),c=8.319(9)A, β=91.44(2)°,Z=8,dcalc=3.067 g cm−3, λMoKα=0.71069A,R/Rw=6.5/9.4%, is characterized by the existence of face sharing NiO6 octahedra with Ni−Ni separation of 2.827 A (Ni−O; 1.93(2)–2.17(2)A). K+ is seen in sites of seven, six, and fivefold coordination (K−O, averages; 2.83(2), 2.81(2), and 2.77(2)A, respectively) P2O74− groups are observed in semieclipsed conformation. K6Sr2Ni5(P2O7)5 (II) crystallizes in monoclinic space group P21/c;a=11.038(7),b=9.533(13),c=7.438(2)A, β=100.13(4)°,Dcalc=3.309 g cm−3,Z=2,R/Rw=6.4/8.1%. Nickel atoms are distributed on planes parallel to the crystallographic (100) plane at χ=0 and χ=0.5. On the planes, χ=0, Ni2+ ions form successive linked trimers of edge sharing NiO6 octahedra (Ni−Ni, 3.018(1), 3.008(1)A; Ni−O, av 2.07(2)A). The intermediate planes (χ=0.5) contain isolated and six coordinate Ni2+ ions (Ni−O av. 2.09(2)A). Seven-coordinate potassium ions (K−O, av. 2.74(2)A) are located on intermediate planes at χ=0.25 and 0.75. P2O74− groups are found in eclipsed conformation. Strontium atoms are located between nickel and potassium planes and are surrounded by seven oxygen atoms (Sr−O, av. 2.586(2)A).

Ali Boukhari

Papers

Synthesis and crystal structure of Na2CaP2O7

σ-Dibarium Pyrophosphate

Structural order in complexes of the AIIBIIP2O7 series: BaPbP2O7, PbCoP2O7, PbMnP2O7 and PbCuP2O7

New palladium phosphate complexes: K2PdP2O7 and K3.5Pd2.25(P2O7)2 synthesis, single crystal structure and conductivity

Crystal structures of the new diphosphates, K2NiP2O7 and K6Sr2Ni5(P2O7)5