Research On Cobalt Nickel Phosphide Positive Electrode Materials For Zinc-Based Batteries

With the increasing demand for safety,stability and electrochemical performance of energy storage devices,the research on flexible batteries has been intensifying in recent years.Quasi-solid-state zinc-based batteries not only have excellent safety,but also exhibit the advantages of high reversible capacity and low redox potential.The current researches on zinc-based battery positive electrode materials were mainly based on transition metal oxides or hydroxides,which are prone to irreversible redox reactions during long time charging and discharging,and their surface structures are prone to instability such as deformation and collapse.In this paper,we propose to grow bimetallic cobalt nickel phosphide active material on nickel foam and encapsulate it with quasi-solid hydrogel to form quasi-solid flexible zinc-based battery as positive electrode material.And the electrodes were encapsulated with quasi-solid hydrogel for quasi-solid flexible batteries.Cobalt phosphide（NF-CoP）,nickel phosphide（NF-NiP）and cobalt nickel phosphide（NF-NiCoP）were grown on nickel foam（NF）by a one-step hydrothermal method.In this paper,each phosphide electrode was studied by a series of characterizations such as field emission scanning electron microscopy（SEM）,transmission electron microscope（TEM）,and specific surface area testing（BET）.After systematic characterization measurements and electrochemical tests on the electrodes,it was proved that the quasi-solid-state phosphide nickel-cobalt battery not only has excellent electrical conductivity of phosphide,but also has excellent electrochemical performance due to the synergistic effect of bimetallic ions.The battery exhibits excellent stability and capacity retention in a long time charge/discharge test.In addition,it is found that the surface morphology of the cobalt nickel phosphide electrode could be changed by modulating the preparation method of the electrode.In order to further improve the performance of cobalt nickel phosphide,cobalt nickel phosphide electrodes with different microscopic morphologies are also investigated in this paper.Cobalt phosphide（NF-CoP）,nickel phosphide（NF-NiP）and cobalt nickel phosphide（NF-NiCoP）were grown on nickel foam（NF）by the one-step hydrothermal method,and the differences of the electrode surfaces of each phosphide are investigated by a series of characterizations in this paper.This study found that the bimetallic phosphide electrode surface has larger specific surface area,more low-valent ions,and more difficult to be oxidized.The charge transfer resistance of the bimetallic phosphide was found to be lower than that of the monometallic phosphide by electrochemical tests such as cyclic voltammetry test（CV）and galvanostatic charge-discharge test（GCD）,and the NF-NiCoP electrode exhibits better reversibility and stability.To study the applicability of phosphide electrodes,the above phosphide electrodes were packaged into a quasi-solid-state zinc-based batteries in this paper.By analyzing series of electrochemical test results of the quasi-solid-state zinc-based batteries,it was found that the nickel-cobalt phosphide zinc-based battery（NF-NiCoP//Zn）has excellent electrochemical performance at the high current density.And NF-NiCoP//Zn can light up the bulb normally even when it is bent at various angles or even penetrated by a needle,which confirms that the quasi-solid-state NF-NiCoP//Zn battery has excellent performance and safety.For further investigation of bimetallic phosphide electrodes,this paper obtained cobalt nickel phosphide electrodes with completely different surface morphologies by modulating the hydrothermal solution.And the systematic characterization confirmed the successful growth of cobalt nickel phosphide on NF with different surface microscopic morphologies,which are micrometer rod structure（NF-NiCoP1）,micrometer flower structure（NF-NiCoP2）and micrometer sphere structure（NF-NiCoP3）.After series of electrochemical experiments,NF-NiCoP2 electrode was found to have excellent performance in electrochemical reversibility,electrochemical capacity,and long-time cycling stability.The unique micron flower structure provides larger reaction area and more active sites In addition,the NF-NiCoP2 electrode has the lower impedance and the fast diffusion of ions in the reaction providing the main electrochemical reaction.It was believed that the surface structure of this electrode provides the shorter channel for ion transport.Finally,the quasi-solid-state Zn-ion batteries were assembled with different surface morphologies of cobalt-nickel phosphide electrodes using a hydrogel electrolyte,and electrochemical tests revealed that the NF-NiCoP2//Zn batteries have high discharge voltages between 1.616 V and1.82 V in the GCD tests with current densities ranging from 15·m A·cm^-2～50·m A·cm^-2.After 2000 charges and discharges,the capacity retention rate is about 93%,and the area specific capacity of this battery is 0.61 m Ah·cm^-2 at high current density(80m A·cm^-2).The maximum energy density of NF-NiCoP2//Zn is calculated to be 6.95m Wh·cm^-2 and the maximum power density is 29.8·m W·cm^-2,which is comparable to the published high performance zinc-based batteries in recent years.Comparing with the high performance zinc-based batteries published in recent years,the electrochemical performance of quasi-solid-state phosphated cobalt-nickel zinc-based batteries are relatively excellent,indicating that this kind of batteries have great potential to become members of future green energy storage devices.