Study Of Mg-Mn Oxides Cathode Material And Fe-V Oxides Anode Material For Aquesou Rechargeable Magnesium Ions Batteries

Aqueous magnesium ion battery,as new large-scale energy storage,owns the advantage of the simple manufacturing process,environmental friendiness,high security,low cost and so on.This system utilizes the aqueous solution of abundant inorganic magnesium salt as electrolyte,which can reduce the cost greatly and improve the safety.The ionic conductivity in aqueous electrolyte is two orders of magnitude higher than that in organic electrolyte.Therefore,aqueous battery possesses the better rate performance.In this paper,the cathode and anode materials for aqueous magnesium ion battery were explored.We prepared the Mg-Mn oxides as cathode and Fe-V oxide as anodes in aqueous rechargeable magnesium ion battery and checked the electrochemical performance in different magnesium salt electrolyte.The electrochemical reaction mechanism was researched for cathode and anode materials and the aqueous magnesium ion full battery was assembled,of which the electrochemical performance was also measured.1×1 tunnel magnesium manganese oxides（Mg-OMS-7）were prepared by one-step hydrothermal method.Based on results of scanning electron microscopy（SEM）,the Mg-OMS-7 with controllable morphology was obtained by changing the concentration of reactants simply.When the H₂SO₄ is 0.2 mol dm^-3,the morphology of 0.2-Mg-OMS-7 was micro-rugby and it was usually³5μm in length and⁵μm in diameter.The nano-club 1.6-Mg-OMS-7 which obtained by the 1.6 mol dm^-3H₂SO₄ owned a smaller size,which was about 200 nm in diameter and 2-4μm in length.In comparison with the electrochemical performance of two materials as cathodes in three different magnesium salt electrolytes by cyclic voltammetry and the galvanostatic charge and discharge technology,the nano-club 1.6-Mg-OMS-7 with smaller size showed the better electrochemical performance.When the current density was 10 mA g^-1,the initial discharge capacities of 1.6-Mg-OMS-7 electrodes in 0.5mol dm^-3 MgCl₂,Mg（NO₃）₂ and MgSO₄ electrolytes were 194.6,205.9 and 150.8mAh g^-1.2×2 tunnel magnesium manganese oxides（Mg-OMS-2）was prepared by hydrothermal method and further modified by in-situ synthesis,which used the graphene as substrate to obtain the Mg-OMS-2/Graphene composite.Based on results of SEM and TEM,the nanowires Mg-OMS-2 was observed,which were generally0.2²μm in length and 10¹5 nm in diameter.For Mg-OMS-2/Graphene composite,the nanowires attached to the graphene sheet and they bonded closely together.In comparison with the electrochemical performance of two materials as cathodes in three different magnesium salt electrolytes by cyclic voltammetry and the galvanostatic charge and discharge technology,the Mg-OMS-2/Graphene composite showed the better electrochemical performance.Meanwhile,the aqueous magnesium ion battery capacitor was assembled by Mg-OMS-2/Graphene as cathode and AC as anode in 0.5 mol dm^-3 Mg（NO₃）₂ electrolyte.This Mg-OMS-2/Graphene//AC system can remain the discharge capacity of 44.1 mAh g^-1 at a current density of 100mA g^-1 after 500 cycles and the capacity retention rate is 95.8%,and can also reach a energy density of 46.9 Wh kg^-1.The electrochemical reaction mechanism was researched by X-ray diffraction and X-ray photoelectron spectroscopy and the Mg²⁺ions insert/deinsert into/from the lattice of Mg-OMS-2/Graphene accompanied with the changed valence of manganese Mn⁴⁺/Mn³⁺、Mn³⁺/Mn²⁺,which was in good accordance with the two pairs of redox peaks showed in CVs.3×3 tunnel magnesium manganese oxides（Mg-OMS-1）with two different morphologies were prepared by sol-gel method and hydrothermal method.The phases of them were confirmed by XRD.The nanobelt Mg-OMS-1 obtained by sol-gel method was 50²00 nm in diameter and 10μm in length and the nanosheet Mg-OMS-1 obtained by hydrothermal method was 200 nm in thickness and 2μm in diameter,which were determined by SEM and TEM.The electrochemical performance of two materials as cathodes in three different magnesium salt and different concentration electrolytes were achieved by cyclic voltammetry and the galvanostatic charge and discharge technology.The nanobelt Mg-OMS-1 electrode can obtain the discharge capacity of 99.5 mAh g^-1,93.5 mAh g^-1 and 98.9 mAh g^-1 in0.5 mol dm^-3 MgCl₂,Mg（NO₃）₂ and MgSO₄ electrolytes at a current density of 100mA g^-1 and the capacity retention rates were 88.9%,90.4%and 43.8%after 200cycles.The nanosheet Mg-OMS-1 electrode can obtain the discharge capacity of106.0 mAh g^-1,105.5 mAh g^-1 and 109.7 mAh g^-1 in these three electrolytes at a current density of 100 mA g^-1 and the capacity retention rates were 83.7%,64.5%and62.0%after 300 cycles.The insertion/deinsertion of Mg²⁺in three magnesium salt electrolytes were analyzed by electrochemical impedance technique.The magnesium ion diffusion coefficients in these electrolytes were 9.65E-15,8.88E-15 and 4.05E-15cm² s^-1,respectively.The electrochemical reaction mechanism of nanosheet Mg-OMS-1 electrode was studied by XRD and XPS and the Mg²⁺ions insert/deinsert into/from the lattice of Mg-OMS-2/Graphene accompanied with the changed valence of manganese.The possible theoretical specific capacity of nanosheet Mg-OMS-1electrode was 356 mAh g^-1 obtained by TG and ICP technology.The FeVO₄·0.9H₂O and FeVO₄ were prepared by one-step hydrothermal method and calcination method.And they were further modified by in-situ synthesis method andcarboncoatedmethod,whichobtainedthecompositesof FeVO₄·0.9H₂O/Graphene and FeVO₄/C,respectively.The nanoneedle FeVO₄·0.9H₂O and porous FeVO₄ were identified by SEM and TEM.In comparison with the electrochemical performance of these materials as anodes in 1.0 mol dm^-3 MgSO₄electrolyte by cyclic voltammetry and the galvanostatic charge and discharge technology,the FeVO₄·0.9H₂O/Graphene and FeVO₄/C composites showed the better electrochemical performance.When the current density was 50 mA g^-1,the initial discharge capacity of FeVO₄·0.9H₂O/Graphene electrode in 1.0 mol dm^-3 MgSO₄electrolytes were 183.8 mAh g^-1,the discharge capacity of FeVO₄·0.9H₂O was 150.3mAh g^-1.And the discharge capacities of FeVO₄/C and FeVO₄ were 184.2 mAh g^-1and 150.3 mAh g^-1.The magnesium ion diffusion coefficients of FeVO₄/C and FeVO₄in this electrolyte were 4.61E-11 and 1.53E-10 cm² s^-1,which was analyzed by electrochemical impedance technique.The electrochemical reaction mechanism of FeVO₄/C electrode was studied by XRD and XPS.The intermediate of MgxFeVO₄/C was found after first discharged process.And the Mg²⁺ions insert/deinsert into/from the lattice of MgxFeVO₄/C accompanied with the changed valence of iron and vanadium.Finally,the full aqueous magnesium ion batteries were assembled by nanosheet Mg-OMS-1 as cathode,1.0 mol dm^-3 MgSO₄ as electrolyte and FeVO₄/C as anodes.The energy density of Mg-OMS-1//FeVO₄/C was 70.4 Wh kg^-1,which also compared with other aqueous battery system.