The Electrochemical Properties Of CoB Alloys And Co-Graphene Composites Prepared By High Energy Ball Milling

With the rapid development of economy all over world and the enhancement of theenvironmental protection consciousness, the requirement for batteries with highhigh-performance and environmental friendly is more exigent. Ni-MH battery with lightweight, small size, cost-effective, environmental friendly and a series of advantages satisfiedthe demand for the secondary battery. Through the joint efforts of scientists worldwide, aseries of negative electrode materials with high hydrogen storage capacity have beendeveloped. Cobalt-based alloy and Cobalt-based composite materials have attracted growingattention of many researchers and renewable-energy enterprises. In this paper, we report theelectrochemical hydrogen storage behaviors of the Co-B alloys and Co-graphene compositematerials synthesized by high energy ball mill and describe the charge-discharge properties ofthese materials used as a hydrogen storage electrode. The hydrogen storage mechanism wasalso discussed.（1） Generally, the energy density is low as the ball milling speed is below500rpm. In thispaper, we take the high ball milling speeds （600,800,900rpm） to prepare a series of Co-Balloys. The power of Co-B alloys become alloying or amorphization with the time ofhigh-energy ball milling increases under the ball milling speeds of600,800or900rpm. XRDpattern shows that the Co-B alloy sample transformed into amorphous phase after millingmore than5h under600,800rpm. After milled1h at a speed of900rpm, no peaks of Co orB can be obviously observed. With the milling time extended, more and more orthorhombicCoB phase formed. The best effect of ball milling is obtained when the ball milling speed is800rpm; It’s helpful to improve the energy states and reduce the activation energy of Co-Balloys. The diameter of alloy particle first decreased with the ball-milling time increasing thenincreased.（2）As Co-B alloys have good activation property and show the maximum specificdischarge capacity in the first two charge-discharge cycles. CoB, Co2B and Co3B alloys havehigh discharge capacity of603.3mAh/g、638.4mAh/g、379.7mAh/g, respectively. High ratedischarge capability have shown: CoB and Co2B alloy milled for15and20h have the bestelectrochemical performance at500mA/g charge-discharge current densities, and for Co2Balloy （BM20h） electrode at a discharge current density of100mA/g, the maximum discharge capacity can reach771mAh/g during the first cycle. After100cycles, the discharge capacitycan still remain at136mAh/g.（3） A series of Co-graphene composites are prepared by ball milling at the speed of200,400,600and800rpm. It shows that the compound effect at the ball milling speed of400rpmis best. The compound effect at the ball milling speed of200rpm is unsatisfactory; the crystalphase Co₃C is appeared when the ball milling speed turns up to600and800rpm, which is outof scope of composites. The XRD pattern of Co-graphene milled under400rpm for10hgives several peaks of Co, which indicate that some Co still co-exist with the main phase.SEM images show that Co-graphene composite materials have sandwich structure.（4） Co-graphene composite materials have good activation property and show themaximum specific discharge capacity in the first4charge-discharge cycles. The compositematerials have high specific discharge capacity and show a bad-good-bad trend with theincrease of Co content, reaching a maximum （900mAh/g） at the ratio between Co andgrapheme is6:1. The discharge capacity reaches583mAh/g after20cycles,69.8%of theinitial capacity remained. The cycle stability is much better than Co-B alloys. High ratedischarge capability of Co-graphene composite materials decreased with the increasedcharge-discharge current density. When the ratio of Co and graphene is6:1, the high ratedischarge capability is best.