Design And Preparation Of High-Performance Mn-based Anode Composites And Their Performance In Li/Na-Ion Batteries

The 21^th century is the era of rapid economic development and high-speed technological progress,accompanied with the consumption of a large number of traditional fossil energy.The fossil fuels will run out in this century if we don't develop green and renewable clean energy,which would lead to serious environmental pollution and Greenhouse Effect.Therefore,it is urgent to develop renewable clean energy.We need to develop advanced energy storage devices?ESD?due to the discontinuous and intermittent nature of these renewable energy sources,the electricity can be outputed stably at any time by ESD.Among various ESD,lithium ion battery stands out because of its safety,high energy/power density,light weight,environmentally friendly and high energy storage efficiency.Currently,it has been successfully applied to various fields,including various electronic products,power vehicles and energy storage stations,etc.The energy storage properties of lithium ion batteries depend on the cathode and anode materials,the current commercial anode materials are mainly graphite or carbon-based composites containing a small amount of tin?or silicon?.In order to further meet the demand of battery with high energy density,the strategy of anode material with high theoretical specific capacity?S_c?instead of the graphite anode(S_c=372 mA h g^-1)play a key role on improving the energy density of batteries.Among various anode materials,Mn-based anode materials based on the conversion mechanism have been widely studied due to its high theoretical specific capacity?>750 mAh g-1,which is twice of graphite anode?,high abundance,eco-friendly,multi-electron transfer,highly reversible conversion reaction,excellent energy storage properties and other advantages,therefore it is a very promising anode materials.However,the practical application of Mn-based anode materials have suffered great challenge due to low electrical conductivity,sluggish ion diffusion dynamics and large volume change during cycle,which resulted in poor rate and cycling performance and seriously hindered the expression of its electrochemical performance.Compared with lithium-ion batteries,sodium-ion batteries have also attracted much attention because of their low cost and wide distribution.Herein,we have also better explored the sodium-storage properties of Mn-based anode nanocomposites.In order to develop superior Mn-based anode materials for advanced lithium/sodium-ion batteries,the great efforts are devoted to investigate its reaction mechanism and construct excellent nanocomposites.Herein,the electrochemical performance of Mn-based anode material was improved by using different carbon-base materials with different dimension and shape to construct optimal conductive network.The low conductivity,sluggish transport dynamics,poor interface stability,large volume change and short cycle life can be improved through nano-functionalization?e.g.incorporating highly conductive doped carbon,constructing special nanoarchitecture and synthesizing porous materials with different nano-dimensions?and optimizing test parameters,thus achieving excellent energy storage performance and long-term cycling stability.The specific research content and methods include the following five aspects:?1?Among the transition metal oxides as anode for lithium ion batteries?LIBs?,MnO material should be the most promising one due to its many merits mainly relatively low voltage hysteresis.However,it still suffers from inferior rate capabilities and poor cycle life arising from kinetic limitations,drastic volume changes and severe agglomeration of active MnO particulates during cycling.In this paper,by integrating the typical strategies of improving the electrochemical properties of transition metal oxides,we had rationally designed and successfully prepared one superior MnO based nanohybrid?MnO@C/RGO?,in which carbon-coated MnO nanoparticles?MnO@C NPs?were electrically connected by the three-dimensional conductive networks composed of flexible graphene nanosheets.Electrochemical tests demonstrated that,the MnO@C/RGO nanohybrid not only showed the best Li storage performance in comparison with the commercial MnO material,MnO@C NPs and carbon nanotubes enhanced MnO@C NPs,but also exhibited much improved electrochemical properties compared with most of the previously reported MnO-based materials.The superior electrochemical properties of the MnO@C/RGO nanohybrid included high specific capacity(up to 847 mAh g^-1 at 80 mA g^-1),excellent high-rate capabilities(for example,delivering 451 mAh g^-1 at a very high current density of7.6 A g^-1)and long cycle life?800 cycles without capacity decay?.As is well known to all,the gradually increased capacity during cycling is a common phenomenon in previously reported oxide-based anode for lithium ion batteries.However,this may not only be superfluous for practical application but also imply the presence of some electrochemical instabilities and side reactions.To achieve the ultrastable Li-storage without such gradual increase of capacity,the increase mechanism of one MnO/graphene-based nanohybrid?MnO@C/RGO?as example has been comprehensively explored in advance.And then,the gradual increase behaviour of specific capacity is effectively restrained by rationally optimizing the cut-off voltage,making the MnO@C/RGO electrode keep nearly constant capacity during cycling at different current densities.Taking the high current density of 2 A g^-11 as an example,there is no obvious capacity change?increase/attenuation?even over 2000 cycles with stable coulombic efficiencies of around 99.7%.The ultrastable Li-storage capability should mainly benefit from the rational testing parameters and the optimal 3D conductive network in it.More importantly and interestingly,full cells have also been assembled and tested via coupling the MnO@C/RGO and commercial LiFePO₄ as anode and cathode materials respectively.The full cells impressively exhibit superior rate performance and excellent cyclic stability.?2?In order to develop superior electrode meterials for advanced energy storage devices,a new strategy is proposed and then verified firstly using the?Si@MnO?@C/RGO anode material for lithium ion battery.The core idea of this strategy is to use the positive cycling trend?gradually increasing Li-storage capacities of MnO-based constituent during cycling?to compensate the negative one?gradually decreasing capacities of Si anode?to achieve the ultralong cycling stability.As demonstrated in both half and full cells,the as-prepared?Si@MnO?@C/RGO nanocomposite exhibits superior Li-storage properties in terms of ultralong cycling stability(no obvious increase or decrease of capacity cycled at 3A g^-1 after 1500 cycles)and excellent high-rate capabilities(delivering a capacity of ca.540mA h g^-1 at a high current density of 8 A g^-1)as well as a good full-cell performance.In addition,the structure of the electrodes is stable after 200 cycles.Such strategy provides a new idea to develop superior electrode materials for the next-generation energy storage devices with ultralong cycling stability.?3?Incorporation of amorphous N,S-codoped carbon nanotubes?N,S-CNTs?can endow electrode composite with superior electrochemical properties owing to the impoved kinetics and unique nanoarchitecture.Herein,the?-MnS nanoparticles are in situ encapsulated in N,S-CNTs,preparing an andvanced anode material??-MnS@N,S-CNTs?for LIBs/NIBs.It is for the first time revealed that electrochemical???phase transition of?-MnS nanoparticles during the 1^st cycle effectively promotes Li-storage capacity(1415 mA h g^-1 at 50 mA g^-1),excellent rate capability(430 mA h g^-1 at 10 A g^-1)and long-term cycling stability(no obvious capacity decay over 5000 cycles at 1 A g^-1)with retained morphology.In addition,the N,S-CNTs-based encapsulation plays the key roles on enhancing the electrochemical properties owing to its high conductivity(1.31 S m^-1)and unique 1D nanoarchitecture with excellent protective effects to active MnS nanoparticles.Furthermore,?-MnS@N,S-CNTs also delivers high Na-storage capacity(536 mA h g^-1 at 50mA g^-1)without the occurrence of such???phase transition,and excellent full-cell performances as coupling with commercial LiFePO₄ and LiNi_0.6Co_0.2Mn_0.2O₂ cathodes in LIBs as well as Na₃V₂?PO₄?₂O₂F cathode in SIBs.?4?Coaxial nanotubes are a significant class of nanoscale building blocks for advanced electrodes of secondary batteries.Herein,one-dimensional?1D?coaxial double nanotubes?DNTs?consisted of?-MnSe inner tubes and N-doped carbon?N-C?outer tubes?abbreviated as?-MnSe@N-C DNTs?are successfully prepared and demonstrated to be promising anode material for Li-ion and Na-ion batteries?NIBs?.When used for LIBs,it is revealed by the studies of ex situ XRD/HRTEM and electrode kinetics that a new electrochemical???phase transition plays a crucial role on improving the cycling stability.As a result,the?-MnSe@N-C DNTs electrode delivers a high Li-storage capacity(800 mA h g^-1 at 50 mA g^-1),excellent rate capability(405 mA h g^-1 at 14 A g^-1)and ultralong cycling stabiligy(a high capacity retention of 87.2%even after 9000 cycles at 2 A g^-1)with retained1D morphology.In addition,the outer N-C nanotube can effectively protect the active?-MnSe inner nanotube to realize such outstanding electrochemical properties owing to its high electrical conductivity(4.02 S m^-1)and particular 1D coaxial nanoarchitecture.Moreover,?-MnSe@N-C DNTs also exhibit excellent Na-storage properties and full-cell performances as coupling with commercial LiFePO₄ and LiNi_0.6Co_0.2Mn_0.2O₂ cathodes in LIBs as well as Na₃V₂?PO₄?₂O₂F cathode in NIBs.