Synthesis Of NH₄Co_0.4Ni_0.6PO₄·H₂O Nanosheets For Energy Storage

With the development of electric vehicles,the research on energy storage devices has become a hotspot in many research institutes and companies.Among the energy storage devices,supercapacitors have many advantages,such as high power density,long cycle life of the device,rapid charge and discharge,clean energy,and no damage to the environment.Meanwhile,supercapacitors have also been widely used in aerospaces,industries,mobile phones,mobile powers,buses,electric vehicles and medical devices.The electrode materials play a major role in the performance of energy storage devices and thus become the focus of research and hotspots.In particular,the metal phosphate NH₄Co_0.4Ni_0.6PO₄·H₂O belongs to the orthorhombic system,with a typical layered structure.The layered structure provides more channels for the charge migrating and transportation.At the same time,the NH₄Co_0.4Ni_0.6PO₄·H₂O crystal possesses both Co²⁺and Ni²⁺ elements,and can produce cobalt phosphate and nickel phosphate used for energy storage in the redox reaction.This work reported the synthesis of NH₄Co_0.4Ni_0.6PO₄·H₂O by solvent thermal method,and subsequent synthesis of layered two-dimensional material by using the formamide as the exfoliating solvent.The morphologies and structural changes of the materials before and after exfoliation were characterized by various means.The properties of the materials were tested in the three-electrode system,and the samples of exfoliated 0 day?0D?and 4days?4D?were further assembled into NH₄Co_0.4Ni_0.6PO₄·H₂O//AC?activated carbon?devices to test the performance of energy storage,showing good performance.?1?Co?NO₃?₂·6H₂O,Ni?NO₃?₂·6H₂O and?NH₄?₃PO₄·3H₂O were used as cobalt,nickel,ammonium and phosphorus sources,respectively.Ethylene glycol was used as the solvent and no any stabilizers and surfactants were added during the syntheses.The multilayers NH₄Co_0.4Ni_0.6PO₄·H₂O nanomaterials were successfully prepared by one-step hydrothermal method.More importantly,the products had uniform particle sizes,high purities and good crystallinities.?2?According to the obtained SEM images,it was found that the materials prepared by solvothermal method are 10¹6 ?m long and 4⁶ ?m wide with a multi-layer sheet structure,and thus resulted in the poor conductivity and small specific surface area.Therefore,this work mainly use the liquid exfoliation method to improve the performance of the material.We finally choosed the formamide as the exfoliatingsolvent after carefully analysing several solvents.The effects of exfoliating time on the morphologies and sizes of the materials were studied.In particular,we compared the samples after exfoliating 0,2,4,8 and 14 days,which denoted 0D,2D,4D,8D and 14 D,respectively.The morphologies and structures of the samples were characterized by SEM,TEM,XPS,AFM,BET,ICP,XRD and Raman before and after exfoliation.The sizes and thickness of the sample are significantly reduced and the crystallinity is also intensified after exfoliation.According to the BET test results,the specific surface area increased from 8.680 to 10.827 m² g^-1,and the specific surface area increased significantly.?3?We test the energy storage performance of the material in the three-electrode system.The specific capacities are calculated from the CV plots at the scanning rate of10 mV s-1 and the specific capacities are 1086?0D?,1233?2D?,1381?4D?,1348?8D?and 1345 F g^-1?14D?,respectively.The quantitative results show that the performance of the 4D sample is the best.We measure the rate capacities of the materials when the current densities is between 1 and 20 A g^-1.In particular,the specific capacitance of the4 D sample is 79.7% when the current density reaches 20 A g^-1.These results indicate that the materials can still maintain a good rate performance under large charge and discharge current conditions.In addition,the 0D and 4D samples are assembled into NH₄Co_0.4Ni_0.6PO₄·H₂O//AC devices,respectively.It should be noted that the devices composed of 4D samples show a power density of 703.1 W kg^-1 and an energy density of 10.8 Wh kg^-1.In contrast,the 0D samples have a power density of 693.6 W kg^-1 and an energy density of 8.4 Wh kg^-1.These results demonstrate that liquid exfoliation can indeed improve the power density and energy density of energy storage materials.We further measure the cycle stabilities of NH₄Co_0.4Ni_0.6PO₄·H₂O//AC devices.The storage capacities of the energy storage devices assembled by 0D and 4D samples still maintain at 117.1% and 102.3%,respectively,after 10000 cycles.These results show that the NH₄Co_0.4Ni_0.6PO₄·H₂O//AC devices have good stability and the materials after exfoliation have high crystallinity.