| The improvement of power battery property is one of a main factor of the development of electric vehicles. The performances of batteries are directly determined by electrode materials. Therefore, it's an important issue to find negative electrode materials with excellent performances in battery research. With the development of lithium batteries, the choice of electrode materials becomes more and more critical. It's well known that simple electrode materials are very difficult to meet the requirements of high-capacity and excellent cycling performance. However, the composite anode materials combining each performance of different materials are exploited the greatest degree of negative material advantage, such as improving the electrochemical performance of lithium batteries, improving the large capacity and long cycle life. Electrodeposition has more important research value because of its simple process, easy to scale, and strong combination with the collector.In this work, Sn-Ni-C composite anodes were prepared by electrodeposition. The effects of composition and structure of the electrodes on the electrochemical performances were studied.In order to obtain anode materials with different compositions, the influences on the contents of Sn-Ni-C composite anodes were obtained by researching compositions of pyrophosphate, complex dispersants, and process conditions. The results illustrated that the ratio of Sn2+/Ni2+ had significant influence on the content of Sn-Ni alloy coatings. By controlling the the ratio of Sn2+/Ni2+and current density, a series of Sn-Ni alloy electrodes containing 50.04%(mass fraction, similarly hereinafter),60.03%,73.30%, and 81.23% were obtained. Complex dispersants were composed of hexadecyl trimethyl ammonium bromide and NaCl. Graphite particles were dispersed uniformly in bath solution and the conductivity was enhanced by using complex dispersant. That's conducive to obtain composite electrodes with high carbon contents. When hexadecyl trimethyl ammonium bromide was 1.4g·L-1 and NaCl was 12g·L-1, the composite electrodes containing carbon contents below 12.99%, tin contents of 55%, and 80%, were obtained.The structures of Sn-Ni alloy electrodes and Sn-Ni-C composite anodes were studied by XRD before heat treatment. The effects on the structures of Sn-Ni alloy electrodes were examined with respect to the different temperatures. The results showed when Sn content is 50.04%,60.03%,73.30% and 81.23%, the Sn-Ni alloy electrodes is NiSn2,Ni3Sn2, Ni3Sn2, and Ni3Sn4, respectively. When heated at 200℃,300℃,and 450℃for an hour, the structures of Sn-Ni alloy electrodes were composed of NiSn2, Ni3Sn2, Ni3Sn2, and Ni3Sn4, respectively. The strcutures of Sn-Ni-C composite anodes were different although the tin contents were similar.The electrochemical performances of Sn-Ni alloy and Sn-Ni-C composite anodes were examined by battery test system and electrochemical workstation. The results showed that the influences on electrochemical performances were studied with respect to structures, composition, and temperature. Sn-Ni alloy anodes with different Sn contents heated at 200℃had the highest specific capacity and better cycle performances, and its phase structures were Ni3Sn2, Ni3Sn4, and Sn. But the comprehensive performances of alloy electrodes were worst after heat treatment at 450℃. Maybe, it was caused by passivity. The capacity and cycle performance were improved with the tin contents increasing. Sn-Ni-C composite electrodes exhibited better comprehensive performances than Sn-Ni alloy electrodes with the similar tin contents. The performances of composite electrodes become better with the carbon contents increasing in composite coatings. The reason was that graphite particles were porous clusters shape in coating, and the unit mass surface area of the electrodes increased. Tin content above 80% in the composite electrodes with carbon content of 7.82% had a good performance, and its first capacity reached 1034.4mAh-g-1. And after 50 cycles, the specific capacity retained as much as 218.3mAh-g-1. However, the first discharge capacity of Sn-Ni alloy electrodes with tin content of 80% was 723.8mAh-g-1. Then specific capacity maintained at 118.1mAh-g-1 after 50th cycles.The electrochemical performances of Sn-Ni-C composite electrodes with different carbon contents were examined by XRD, SEM, and CV curves before and after cycles. The charge-discharge mechanism was discussed. The results of XRD indicated that new phases appeared in the structure such as Li7Sn2 and Li13Sn5. The results of SEM showed that continuous phases generated between graphite particles after 50 cycles. Thus interface definition of particles was reduced, which related to carbon, tin contents in composite electrodes and new alloy phases and SEI film formed in the process of charging and discharging. The charging and discharging mechanism could be xLi+SnNiy→LixSnNiy (Discharge reaction, reversible) xLi+SnNiy→LiSnx+yNi (Discharge reaction, irreversible) LixSn→Sn+xLi++xe- (0 |
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