Synthesis of conversion reaction based anode materials and their electrochemical properties for Na-ion batteries

Abstract

i) mechanical destruction of the active materials and failures in electrical contacts due to the volume expansion during the charging and discharging process, thereby reducing the cycle stability, ii) large polarization and low rate capability from sluggish kinetic properties. Consequently, the purpose of this work is to demonstrate feasibility of new conversion reaction based materials synthesized by electrodeposition, mechanical ball-milling and electrophoretic deposition for Na-ion battery anode, and to improve their cycle performance and rate capability.

1. An Sb/Sb₂O₃ composite is synthesized by a one-step electrodeposition process from an aqueous electrolytic bath containing a potassium antimony tartrate complex. The synthesis process involves the electrodeposition of Sb simultaneously with the chemical deposition of Sb₂O₃, which allows for the direct deposition of morula-like Sb/Sb₂O₃ particles on the current collector without using a binder. Structural characterization confirms that the Sb/Sb₂O₃ composite is composed of approximately 90 mol.% metallic Sb and 10 mol.% crystalline Sb₂O₃. The composite exhibits a high reversible capacity $(670 mAh g^{-1})$ that is higher than the theoretical capacity of Sb $(660 mAh g^{-1})$. The high reversible capacity results from the conversion reaction between Na₂O and Sb₂O₃ that occurs additionally to the alloying/dealloying reaction of Sb with Na ion. Moreover, the $Sb/Sb₂O₃ composite shows excellent cycle performance with 91.8 % capacity retention over 100 cycles, and a superior rate capability of $212 mAh g^{-1}$ at a high current density of $3300 mA g^{-1}$. The outstanding cycle performance is attributed to an amorphous Na₂O phase generated by the conversion reaction, which inhibits agglomeration of Sb particles and acts as effective buffer against volume change of Sb during cycling.

2. $SnP_x@(P-C)$ composites are successfully synthesized by a simple mechanical high-energy ball-milling process. The structural characterization confirms that the $SnP_x@(P-C)$ composites have a structure in which nanosized SnP₃ and Sn₄P₃ particles are embedded in a matrix consisting of phosphorus-doped carbon and excess phosphorus. This unique structural feature provides the following beneficial effects on the electrochemical performance

Recently, as interest in exhaustion of fossil fuels and environmental pollution have increased, introduction and demonstration of the renewable energy to replace fossil fuels are ongoing. However, the renewable energy using a solar power, wind power, hydropower and tidal power has limitations because of the temporal and geographical imbalance between energy production and demand. Therefore, it is essential to develop large-scale energy storage systems (ESSs). Among the various energy storage systems, Li-ion battery is now commercially available as power sources for large-scale energy storage systems due to its high energy density. However, because of the limited amount of Li resources present in the crust and the high price, it will be difficult to cope the surging demands when energy storage systems are popularized in the future. Accordingly, Na-ion battery is attracting attention as an alternative candidate to Li-ion batteries owing to abundant Na resources, excellent price competitiveness and similar chemical properties of Na and Li.

Among the various anode materials for Na-ion batteries, conversion reaction based materials have great potential as a high-capacity anode material because they can utilize the alloying reaction and conversion reaction at the same time when Na-alloying metals are used. However, there are many researches on conversion reaction based materials using inactive metals for Na-ion batteries whereas few studies of conversion reaction based materials using active metals for Na-ion batteries have been conducted. Therefore, it is necessary to study the conversion reaction based materials using various active metals. Furthermore, a way to overcome following problems should also be studied

i) amorphous (P-C) matrix can act as an effective buffer against volume changes during the cycling, thereby improving cycle stability, and ii) heteroatom doping of carbon can allow improvement in electrical conductivity of the composites, resulting in enhanced rate capability. As a consequent, the $SnP_x@(P-C)$ electrode delivers a very high reversible capacity of $920.4 mAh g^{-1}$, which is a remarkable value in the anodic Na storage materials reported to date. Furthermore, the $SnP_x@(P-C)$ composites exhibits an excellent cycle performance (80.7 % capacity retention over 100 cycles) and superior rate capability (a charge capacity of $348.6 mAh g^{-1}$ at a high current density of $7500 mA g^{-1}$). This facile synthesis method provides an effective approach to preparing highly stable metal phosphides anodes for Na-ion batteries and other energy storage applications.

3. GeP₃/rGO nanocomposites in which GeP₃ particles are encapsulated in interconnected rGO sheets are successfully synthesized by the electrophoretic deposition and electrochemical reduction method. The electrochemical synthetic approach has the advantage of controlling the composition of the nanocomposites by adjusting the composition ratio of the colloidal suspension and of selectively reducing the GO without affecting the active material in the composites. The GeP₃/rGO nanocomposites deliver a very high reversible capacity and exhibit a stable cycle performance and a superior rate capability. These excellent electrochemical performances are attributed to the graphene sheets which act not only as a binder for holding GeP₃ particles, but also as a conductive network for connecting the GeP₃ particles to each other and transferring electrons. This novel approach can provide an effective way to design various composites between metal phosphides and any materials that can be electrochemically synthesized, such as metals, conducting polymers, etc.