Preperation And Performances Of Porous Materials As Li-air Battery Cathodes

Li-air batteries have a large theory capacity of3860mAh.g^-1and huge theoryenergy of11,400Wh.kg^-1, which is far more than that of any other chemical cells. Andalso Li-air batteries are friendly to environment. In recent years, researchers have gotsome achievements in relation with electrolytes and cathode catalysts. Some resultsindicated that the porosity of cathode materials had big influences on the diffusion ofoxygen inside the batteries, which determined the electrochemical performances of Li-air batteries, however, specific interactions between micro-structures of cathodematerials （BET surface area, pore volume and pore size） and performances of Li-airbatteries （capacity of charging-discharging, rate performance and cycle performance）has not been investigated widely. In this paper, our investigation was concentrated tothe preparation of positive porous catalysts with good performances, and at the sametime we studied the influences on performances of batteries caused by micro structuresof catalysts.We adopted a method of in-situ synthesis on the surface of MWNTs to synthesizeNiO/MWNTs and Co₃O₄/MWNTs firstly. We got some results by comparing these twomaterials and pure MWNTs relating to structures and performances. Of these threematerials, NiO/MWNTs had the lowest value of V of1.05V and MWNTs had thehighest value of V of1.50V. The lower value of V was, the better catalyticperformance was. Catalytic performances of catalysts were determined by their ownproperties and BET surface areas. The larger BET surface area was, the better catalyticperformance was. NiO/MWNTs had a BET surface area of177.08m2·g^-1and MWNTshad a BET surface area of97.86m2·g^-1. Co₃O₄/MWNTs had the largest pore volume of0.63cm³Â·g^-1and the biggest discharging-capacity of2682mAh·g^-1, however, MWNTshad the smallest pore volume of0.3984cm³Â·g^-1and the lowest discharging-capacity of2054mAh·g^-1. The BJH average pore size of Co₃O₄/MWNTs was10.46nm, and in thecondition of charging-discharging with a current of0.05C, Co₃O₄/MWNTs showeddischarging-capacity of1163mAh·g^-1and charging-capacity of1092mAh·g^-1, andafter12cycles retentions of capacity of discharging and charging were14.5%and12.1%, respectively. The BJH average pore size of NiO/MWNTs was7.68nm, and inthe condition of0.05C, NiO/MWNTs showed discharging-capacity of878mAh·g^-1andcharging-capacity of736mAh·g^-1, and after10cycles retentions of capacity ofdischarging and charging were5.5%and4.5%, respectively. And what’s more,Co₃O₄/MWNTs had better performances in larger currents than NiO/MWNTs. In thecondition of0.2C, Co₃O₄/MWNTs showed discharging-capacity of340mAh·g^-1andcharging-capacity of292mAh·g^-1. As a result, we came up with a standpoint on the relationship between structures of materials and their electrochemical performances.Materials with larger BET surface areas could display higher catalytic performances.Also, materials with larger pore volumes would exhibit higher capacities of chargingand discharging. What’s more, materials with larger pore sizes would show betterperformances of cycling and charging and discharging in larger currents.In order to confirm the standpoint above, we used a method of chemicaldeposition to synthesize one type of materials of Co₃O₄with the BJH average pore sizeof20.51nm and the outer pore size in the macro-pore range of100²00nm, followedby a method of in-situ electro-deposition on the surface of Ni mesh to prepare one typeof materials of Co₃O₄/Ni with the average pore size of27.88nm and the outer poresize in the macro-pore range of200³00nm. In the condition of0.05C, Co₃O₄showeddischarging-capacity of1598mAh·g^-1and charging-capacity of1491mAh·g^-1, andafter15cycles Co₃O₄showed discharging-capacity of565mAh·g^-1and charging-capacity of459mAh·g^-1, and retentions of capacity of charging and discharging were35.4%and30.8%, respectively. And in the condition of0.2C, Co₃O₄showeddischarging-capacity of409mAh·g^-1and charging-capacity of399mAh·g^-1.Theperformances of Co₃O₄were better than Co₃O₄/MWNTs obviously. And Co₃O₄/Nidisplayed far more excellent performances compared with other materials. In thecondition of0.05C, Co₃O₄/Ni showed discharging-capacity of1917mAh·g^-1andcharging-capacity of1793mAh·g^-1, and after20cycles Co₃O₄/Ni showed discharging-capacity of650mAh·g^-1and charging-capacity of630mAh·g^-1, and retentions ofcapacity of charging and discharging were33.9%and35.1%, respectively. And in thecondition of0.2C, Co₃O₄/Ni showed discharging-capacity of601mAh·g^-1andcharging-capacity of565mAh·g^-1. By the comparison between these materialssynthesized by us relating to structures and electrochemical performances, we hadvalidated our standpoint in former. Materials with larger BET surface areas coulddisplay higher catalytic performances. Also, materials with larger pore volumes wouldexhibit higher capacities of charging and discharging. What’s more, materials withlarger pore sizes would show better performances of cycling and charging anddischarging in larger currents.