Electrochemical Energy Storage Characteristics Of Cobalt Terephthalate As Organic Anode Materials

In view of the abundance,retrievability and structural designability,organic materials are considered as the most promising alternative electrode sources.So based on the latest research of carboxylates,this work aims at studying the energy storage characteristics of cobalt terephthalate,including electrochemical properties,lithium storage mechanism,ball-mill modification and also performance when cobalt terephthalate is used to store sodium or potassium ions.Firstly,pure cobalt terephthalate dihydrate?CoC8H8O6,CoTP·2H₂O?and anhydrous cobalt terephthalate?CoC8H4O4,Co TP?were easily prepared by cation exchange reaction.Then,lithium storage properties of the two materials were addressed.The discharge plateau of CoTP·2H₂O and CoTP are around 0.77 V and 0.7 V vs Li+/Li respectively.Amorphous CoTP is proved to be more beneficial to Li+ transportation than crystal CoTP·2H₂O through comparing their crystalline structure,thermostability,and micromorphology.Consequently,better cycle performance and rate capability are achieved with CoTP anode.Besides,the stable specific capacity,remaining 95.8% after 30 cycles,reaches 700 mAh/g.However,the first discharge capacity of CoTP·2H₂O presents more than 1900 mAh/g.In addition,its specific capacity degradation lasts nearly 10 cycles.Furthermore,researches on the intercalation and deintercalation mechanism show that both organic and inorganic groups in cobalt terephthalate contribute to the high specific capacity.Not only carboxylates in cobalt terephthalate can react with Li+,but Co2+ and H+ on benzene ring are able to be replaced by Li+ reversibly.All the three factors make cobalt terephthalate a much higher specific capacity than traditional terephthalates.To improve the conductivity of cobalt terephthalate,CoTP·2H₂O/Super P and CoTP/Super P with different ratios were prepared by ball milling.Either CoTP·2H₂O/Super P or CoTP/Super P expresses the best performances when the weight proportion of Super P is 10%.After ball milling,their particle size decreases,and particle aggregation is suppressed by Super P.Thus,Li+ transportation distance in solid cobalt terephthalate gets shorter,leading to the increase of electronic conductivity,the decrease of polarization,and the promotion of rate performance.For example,the reversible specific capacity of CoTP is as high as 1000 mAh/g under current density of 150 mA/g.Since lithium resourses are now in short,the electrochemical properties of CoTP toward sodium or potassium storage were studied here.Surprisingly,CoTP can store Na+/K+ reversibly and the reduction potential is around 0.3 V?vs Na+/Na?or 0.25 V?vs K+/K?.Although being electrochemically active to sodium and potassium,CoTP shows significant polarization and poor first-cycle coulombic efficiency,even less than 25%.Luckily,modified effect is still obvious for CoTP 10% in sodium ion batteries?NIBs?or potassium ion batteries?KIBs?.Their reversible specific capacities get much higher,about 400 mAh/g and 200 mAh/g,respectively,because the lower charge transfer resistance and faster ionic mobility of CoTP 10% make the polarization depressed and rate property improved.