Design And Preparation Of Mn Based Composites And Their Energy Storage Applications For Li/Na Ion Batteries

With the improvement of low-carbon environmental protection requirements,the demend for green energy storage devices in the society is also increasing.Lithium ion batteries?LIBs?gradually occupied the secondary energy storage market owing to their excellent properties such as environmental friendliness,high power density,long cycle life,et al.However,the LIBs are also face many peoblems,such as shortage of lithium resources and difficulty in mining,which greatly limits the application of LIBs.In addition,owing to similar electrochemical behavior as LIBs and rich reserve,low price,easy mining,sodium ion batteries are considered as the promising energy storage device.However,graphite as traditional commercial anode materials for LIBs cannot meet the demand of the high energy density application due to the low theoretical specific capacity?372 mAh/g?.Meanwhile,graphite cannot be used in SIBs because the increased stretching of the C-C bonds of graphite.Therefore,the development of newly and highly efficient alternative materials with high storage capacity of lithium and sodium has alway been the research focus.Due to their high theoretical capacity,abundant resources and low toxicity,Mn-based composites are considered as one of the most promising anode materials for LIBs.However,Mn-based composites have the following disadvantages in practical application.Firstly,the volume change caused by lithiation/delithiation results capacity decay.Secondly,poor electronic conductivity impedes electrons transport,suggesting that real specific capacity is lower than the theoretical capacity.Moreover,incomplete decomposition of reaction products brings about large capacity loss.The shortcomings mentioned above limit the application of Mn-based composites in lithium ion batteries.Therefore,in this thesis,Mn-based composites are regared as research object.The lithium/sodium storage performance,cycling stability and rate performance of the Mn-based composites are improved through morphology control,structure optimization,valence increase,elemental S doping and substitution.The results are summarized as follows:?1?Firstly,the MnCO₃ precursor was synthesized by hydrothermal method,and then MnO@rGO was obtained through introduction of reduced graphene oxide?rGO?.The lithium storage property of MnO@rGO as anode materials is studied.High electrical conductivity of graphene can improve the electron transfer efficiency.The flexible graphene effectively accommodates the drastic volume changes of MnO during charging and discharging processes,resulting in good cycling stability.The MnO@rGO hybrid shows best lithium ion storage capacity?a reversible capacity of 798.6 mAh/g was obtained at a current density of 400 mA/g after 100 cycles?,good rate performance and high coulombic efficiency as an anode material for LIBs compared with the pure MnO.?2?To further improve the cyclic stability of MnO,nitrogen doped carbon coated MnO@NC with yolk-shell structure was synthesized through dopamine situ polymerization and used as anode materials for LIBs.MnO with porous morphology was consisted of MnO sub-nanoparticles and was coated by nitrogen doped carbon shell.The porous structure of the YS-MnO@NC facilitates the penetration of electrolyte and shortens the path of lithium ions.The yolk-shell structure can alleviate the volume change of MnO core.The YS-MnO@NC delivers a stable higher reversible capacity?a reversibe capacity of 1280 mAh/g was obtained at a current density of 100 mA/g after 100 cycles with capacity retention of 83.0%?.The results of in situ XRD demonstrates that the MnO core and NC shell have the same contribution to the total capacity of YS-MnO@NC,and the process of lithiation/delithiation have good reversibility.?3?To further improve the lithium storage performance and the electronic conductivity of MnO,nitrogen doped carbon coated Mn₃O₄@NC with core-shell structure was synthesized through high temperature oxidation and used as anode materials for LIBs and SIBs.Compared with the YS-MnO@NC,the charge transfer impedance of the Mn₃O₄@NC is smaller,which results in excellent rate performance?a high reversible capacity of 452.5mAh/g was obtained at high current density of 5.0 A/g?.In addition,the Mn₃O₄@NC shows ultra-long cycles life?a stable capacity of 1020.2 mAh/g at high current density of 1.0 A/g after 1000 cycles?and excellent cycling performance?capacity retention of 95.3%after a long cycle?.The results of the CV curves at different scan rate demonstrates that the storage lithium performance of Mn₃O₄@NC is determined by both of the pseudocapacitive controlled and diffusion controlled.The DFT calculation results show that the band gap of Mn₃O₄ is narrower and the conductivity is better than that of MnO.For sodium ion battery,the reversible capacity of Mn₃O₄@NC is 249.5 mAh/g after 400 cycles at 100 mA/g,which demonstrates that Mn₃O₄@NC is a good anode material for SIBs.?4?To further improve the lithium storage performance and the electronic conductivity of MnO,N,S co-doped carbon coated MnOS@NSC with core-shell structure was synthesized through high temperature solid-sulfidation and used as anode materials for LIBs.Compared with YS-MnO@NC,MnOS@NSC have excellent lithium storage capacity,due to S doped and S half replacement of O.MnOS@NSC delivers a high capacity of 1400 mAh/g after 200cycles at a current density of 100 mA/g.The kinetic analysis indicates that the contribution of pseudocapacitive is beneficial for the long cycle life of the cell.Besides,the analysis of DFT results reveals that lithium ion diffusion rate on MnOS is fast with a diffusion barrier of only0.156 eV.