| The specific capacity and life of electrode materials for lithium ion batteries is important to its performance indicators, so it becomes an important topic for scholars to improve the capacity and cyclic stability of electrode material of lithium ion batteries both at home and abroad, More and more attentions are attached on tin-based alloys due to their high theoretical specific capacity. But tin-based alloy is unstable because of its pulverization during charge-discharge reaction process, which is still a difficulty in this field. Recently, alloying of some inert metal or nonmetal materials with tin have been used to decrease pulverization effect and then to improve their cycle stability. Though a lot of efforts have been made by scholars both at home and abroad, the cycle stability of tin-based alloys as electrode materials for lithium ion batteries is still an important factor to restrict its application. Therefore, porous processing was carried on the tin-based alloys in this paper. Then the cycle stability of materials can be improved by increasing the sports space of tin atom, enhancing the utilization rate and the superficial area of active materials and reducing the degree of pulverization of tin-base alloys.Porous Sn-Ni alloy was prepared by two different methods respectively:one is electrodeposition on porous substrate and the other is selective corrosion of three-element alloy Sn-Ni-Zn. The effect of electrodeposition preparation, coating composition, morphology and structure on electrochemical properties were studied in this paper.The porous Sn-Ni alloy was deposited on porous nickel cloth, porous copper cloth, porous foams copper and porous foamed nickel using the method of pyrophosphate electrodeposition in this paper. The relationship between the composition of plating solution and coating composition, structure and aperture was analyzed by EDS, XRD and SEM. Cycle performance curve, capacity-voltage curves, cyclic voltammetry curve and AC impedance spectroscopy were tested by button cell battery test system and the electrochemical workstation. The different porosity, different compositions, the electrochemical properties of the different structures of the Sn-Ni alloy were researched. The total molarity of [Sn2+]+[Ni2+] was remained as constant of0.4mol/L, changing the value of [Sn2+]/[Ni2+] in the plating solution from0.58to2.5. The content of coating was analyzed by EDS, which shows that the tin content (mass fraction) in nickel alloy coating changed in the range of53.42%～94.43%. The structure of coating is comprised of Ni3Sn, NiSn, Ni3Sn2, Ni3Sn4, Sn, Ni, etc, respectively. Morphology of porous coating material was analyzed by SEM. The hole diameter in copper and nickel cloth base was3.42～41.67μm. The void between the two fibers was110～450nm. Cycle performance curve, capacity-voltage curves and cyclic voltammetry curve, AC impedance spectroscopy of button cell indicate that the specific capacity and cycle stability of the button cell with porous electrode materials are better than the button cell without porous electrode materials contained porous Sn-Ni when the contents of Sn changed from50%to60%. After50times of charge and discharge cycle, the number and size of crack of3D porous materials reduced greatly, and degree of pulverization is obviously improved. This may be attributed to the fact that porous material provides more space for the moving of atoms in tin-based alloys during the process of charging and discharging. The first discharge capacity of electrode materials based on porous nickel cloth, porous copper cloth, porous foamed nickel, porous foams copper were189.3mAh/g,509.5mAh/g,240.5mAh/g,354mAh/g and the50th cycles discharge capacity were186.3mAh/g,109mAh/g,124.2mAh/g, and211.5mAh/g. The specific capacity of the battery was increased to950.1mAh/g by improving the content of tin. What’s more, charge and discharge efficiency still remained about100%.The porous Sn-Ni-Zn alloy with zinc content between0%and65.97%was obtained on copper foil by citrate electrodeposition. The relationship of conductive salts, complexing agents, main salt and the coating composition was researched. According to the difference of three-element electrode potential, we employed appropriate corrosive medium to corrode zinc in Zn-Ni-Sn alloy selectively and then3D poriferous alloy plating was prepared. Morphology of Sn-Ni-Zn ternary alloy coating before and after corrosion and charge-discharge was analyzed by SEM. The coating before corrosion consisted of spherical particles. Some spherical particles tend to grape shape after corrosion, and there are some trenches. After charge and discharge cycles, coating particles have a minute crack, and the original spherical particles were broken forming flower type particles. Cycle performance curve, capacity voltage curves and cyclic voltammetry curve were tested by button cell battery test system and the electrochemical workstation. The first specific capacity of the porous Sn-Ni-Zn ternary alloy plating obtained by Corrosion was463.5mAh/g and the50th specific cycles discharge capacity was143.5mAh/g. The charge-discharge efficiency basically maintained at95%to110%. Its performance is much higher than imporous alloy electrode materials.The charge and discharge mechanism of the porous electrode materials is discussed through the electrode materials of electrochemical process and the structure of coating was analyzed by XRD. There are many factors that affect the coating structure, for instance, the choice of the substrate before electroplating, the composition of plating, the temperature and the pH of the plating solution, electrodeposition time, etc. The structure of Sn-Ni alloy is Ni3Sn2before charging and discharging. After the cycle of charging and discharging, the structure of Sn-Ni alloy plating changes to Ni3Sn2、Ni、Li7Sn3、LiSn. And the discharge platform potential is1.1V,0.4V. The reaction mechanism of lithium ions embedded alloy may be:2Li+Ni3Sn2'2LiSn+3Ni14Li+3Ni3Sn2'2Li7Sn3+9NiAfter selective corrosion by corrosion medium, the Sn-Ni-zinc ternary alloy plating structure composed of Ni3Sn4, Ni4Sn and Ni3Sn, NiZn. After200times charging and discharging cycles, the structure of Sn-Ni-zinc ternary alloy plating changes to Ni3Sn4、 Ni4Sn、Li2SnZn. And the discharge platform potential is1.1V,0.4V. The reaction mechanism of lithium ions embedded metal alloy may be: Ni3Sn4+xLi'LixNi3Sn4Ni4Sn+xLi'LixNi4Sn Ni3Sn+NiZn+2Li'Li2SnZn+4Ni... |
PDF Full Text Request