Modification And Electrochemical Performance Of Manganese Carbonate-based Anode

The rapid development of the electric vehicles and portable electronic devices has prompted the demand for lithium-ion batteries?LIBs?with higher energy/power densities.Compared with the transition metal oxides,transition metal carbonates have attracted considerable attention due to their higher theoretical capacity,natural abundance,and environment friendliness.However,their practical application as LIB anodes suffers from the intrinsic poor electrical conductivity and structural degradation during charging/discharging processes.A popular strategy for addressing these issues is to hybridize carbonates with carbonaceous materials.Graphene,a two-dimensional?2D?material with sp²-bonded carbon,has been widely utilized as a promising composite because of its excellent conductivity,ultrahigh surface area and robust mechanical strength.Unfortunately,for the carbonates/graphene composites,the subpar rate performance and the structure instability still remain the bottleneck problems.Herein,by constructing the structures and constituents,the morphology and component of the metal carbonates are modified in order to improve their lithium storage properties.The single-phase mixed transition metal carbonate/graphene composites were synthesized by cobalt-doping of MnCO₃ and combining with reduced graphene oxide?RGO?.By adjusting the molar ratio of Mn/Co in the raw materials,the Mn_xCo_1-xCO₃ microparticles are obtained,which are tightly encapsulated by graphene.The chemical composition and structure features of Mn_x Co_1-xCO₃/RGO,as well as the formation mechanism of single-phase Mn_x Co_1-x-x CO₃ are investigated thoroughly.By analyzing the electrochemical performance,EIS spectra and DFT results,it is found that the improved conductivity and large surface area are benefical for rapid electron/ions transportation and providing sufficient active sites.Benefitting from the structural and componential advantages induced by the synergistic effect of Mn and Co,Mn_x Co_1-x-x CO₃/RGO possesses superior electrochemical performance than CoCO₃/RGO and MnCO₃/RGO.Especially,Mn_0.7Co_0.3CO₃/RGO exhibits excellent reversible capacity as high as 981 mAh g^-1at 2000 mA g^-11 after 1500 cycles,and a comparable capacity with commercial graphite at 5000 mA g^-11 after 1500 cycles.Taking the advantages of hierarchical structure and bimetal synergy,the RGO wrapped core-shell MnCO₃@NiCO₃?MnCO₃@NiCO₃/RGO?composites were rationally designed and synthesized to further improve the electrochemical performance of metal carbonates.Through a simple hydrothermal method,the core-shell structure carbonates are formed with Ni-doped MnCO₃ core coated by Mn-doped NiCO₃ shell.This is the first time for the fabrication of carbonates with core/shell structure as LIBs anode.We systematically investigated the evolution process of the products,as well as the effects of graphene and element-doping on the morphology of MnCO₃@NiCO₃/RGO.Due to the lower solubility and formation energy,the MnCO₃ is precipitated preliminarily with Ni elements doped and then Mn-doped Ni₂CO₃ nucleates and grows on the surface of MnCO₃ to form core-shell hierarchical structure.In virtue of the structural and component advantages,the obtained MnCO₃@NiCO₃/RGO composites manifest excellent Li-ion storage performance in view of high rate performance(300 mAh g^-1 at 10 A g^-1and 200 mAh g^-1 at 20 A g^-1)and remarkable cycling stability(capacity retention of75%after 1300 cycles at 2 A g^-1).In order to promote the practical application of manganese carbonate as LIBs anodes,a facile and efficient method is developed to synthesize the Mn-based compounds/RGO composites.Through adding different types of alkali liquor Na₂CO₃ or KOH)to GO suspension(GO/Mn suspension,including GO,Mn²⁺,SO₄^2-and H⁺)and controlling the pH values of the final solution,the MnCO₃/RGO,Mn₃O₄/RGO,and MnCO₃/Mn₃O₄/RGO composites are obtained.In this synthetic route,the residual acid in GO suspension is neutralized,and the in-situ precipitation of Mn-based compounds can dramatically improve the utilization of raw materials and simplify the operating procedure.When evaluated as LIBs anode,the ternary MnCO₃/Mn₃O₄/RGO exhibite high capacity with 980 mAh g^-1 at100 mA g^-1 after 200 cycle,and enhanced cyclability(532 mAh g^-1 at 1000 mA g^-1after 800 cycles).Moreover,cyclic voltammetry,charge/discharge curves,and corresponding dQ/dV profiles demonstrate that the synergistic effect of MnCO₃ and Mn₃O₄ as well as the decreased potential difference during cycling induced the better electrochemical activities and reaction kinetics of MnCO₃/Mn₃O₄/RGO than those of the binary composites.