Microstructure Control And Lithium-storage Performances Of MnO/C Anode Materials

As an important component of Lithium ion battery,anode material plays a key role on the performance of the battery.Transition metal oxides are considered as promising anode materials for the next-generation Li-ion batteries due to their high theoretical specific capacity?2³ times of commercial graphite?,low cost and high density.However,their commercialization is hindered by their short cycle life and poor rate performance,which are due to the drastic volume change during the lithiation/delithiation process and their inferior electrical conductivity.Herein,Aiming at MnO/C anode with high performance,MnO nanocrystals were embeeded in carbon nanosheets firstly,so that the rate performance was improved due to the optimized conductivity.Then,yolk-shell MnO@C nanostructures were synthesized through a carbothermal reduction process to further improve the structure stability.A high cycling performance was obtained for yolk-shell MnO@C nanoparticles because the problem of volume variations was solved.Finally,we designed and synthesized MnO@C nanosheets with ball-in-pore structure by rationally combing the advantages of the above two structures,so as to achieve ultra-high rate capacity and long cycle life simultaneously.Through high-temperature annealing of ammonium manganese citrate,MnO/C nanosheets with multicore-shell structures were obtained.The in-situ embedding and N doping in carbon nanosheets were achieved during annealing.The pore structure,degree of graphitization,the particles size of the embedded MnO could be adjusted by changing the temperature of annealing.MnO/C nanosheets obtained after annealing at 800 ^oC exhibited a relative highest specific capacity(951 mAh g^-1@0.1 A g^-1)and cycling performance because of the largest specific surface area(126.28 m² g^-1)and pore volume(0.201 cm³ g^-1).More importantly,all the MnO/C nanosheets show enhanced rate performance than bare MnO particles.Additionally,MnO/C nanosheets exhibit self-compensated reversible capacity during cycling,which is originated from the gradual refining of MnO particles,leading to the enhancement of the reaction kinetics,brings out the rapid increasing of Mn^3+/4+,contributing extra capacity to compensate the capacity fading originated from the volume change of MnO.The self-compensation mechanism enabled MnO/C nanosheets with high cycling performance,which could maintain a reversible capacity of 701 mAh g^-1?with capacity retention of 110%?after 500cycles at 1 A g^-1.Yolk-shell MnO@C nanodiscs were synthesized through a carbothermal reduction method which contains Zn evaporation.Specifically,Zn_0.5Mn_0.5CO₃nanodiscs with Zn rich at the peripheries were prepared through a modified precipitate progress.Then,after PDA coating,Zn_0.5Mn_0.5CO₃@PDA nanodiscs were annealed at 700°C in Ar atmosphere,during which polydopamine-derived amorphous carbon was utilized to reduce Zn²⁺to Zn⁰,leading to the evaporation of Zn,successfully forming yolk-shell MnO@C nanodiscs.Benefited from the improved electronic conductivity by the uniform carbon shells,as well as the enhanced structural stability ensured by large void space,the yolk-shell MnO@C nanodiscs electrode exhibited much improved rate and cycling performance than bare MnO particles,and delivered no capacity fading related to Mn²⁺during 600cycles at 1 A g^-1.Furthermore,the internal voids between MnO cores and carbon shells were optimized by adjusting the Zn content of the precursors,leading to a much enhanced rate performance while maintaining the good cycling performance.MnO@C nanosheets with ball-in-pore structure were desgined and synthesized.Specifically,by annealing polydopamine?PDA?coated porous ZnMnO₃ nanosheets in a reducing atmosphere,MnO nanoparticles and surrounding in-situ formed pores could be encapsulated in the PDA-derived carbon nanosheets synchronously,forming MnO@C nanosheets with ball-in-pore structure.The internal voids originating from Zn evaporation could not only accommodate the volume expansion of MnO nanoparticles,but also significantly enlarge the specific surface area?330m²/g?,leading to an enhanced pseudocapacitive Li-storage behavior,and significantly enhancing the rate performance.Moreover,the as-prepared MnO@C nanosheets with thickness of about 30 nm could provide a short path for fast Li-ion diffusion,while the PDA-derived N doped continuous carbon framework promotes the charge transfer of the MnO@C nanosheets.Because of these merits,the MnO@C nanosheets maintained a reversible specific capacity of 383 mAh g^-1 at 15A g^-1,and exhibited outstanding cycling performance(1212 mAh g^-1 after 1000cycles at 2 A g^-1).