Design,Preparation And Electrochemical Properties Of High-Capacity Li-Rich Mn-Based Cathode And Li Metal Anode Materials

With continuous social progress and technological development,the current lithium-ion batteries(LIBs)will not meet ever-increasing high-energy-density demands.It is urgent to develop next-generation LIBs with higher energy density.In fact,the energy density of LIBs is mostly devided by the electrode materials.Therefore,it is necessary to exploit high-performance cathode and anode materials,which could replace current commercial eleoctrode materials for higher energy density or be used to develop new battery systems with much higher energy density.Among those cathode and anode candidates,Li-rich Mn-based oxide cathode and lithium metal anode materials are ideal choices.Li-rich Mn-based oxide cathode materials(LLOs)possess high energy density over 1000 Wh kg-1 and high specific capacity more than 300 mAh g-1,which are superior to commercial LiCoO2,LiFePO4,LiMn2O4 and NCA/NCM cathode materials.The employment of LLOs enables higher energy density exceeding 400 Wh kg-1 in LIBs.However,many issues,such as the low Coulombic efficiency,serious capacity/voltage attenuation and poor rate performance,hinder the large-scale application of LLOs.Fundamentally,the above issues are mainly caused by the irreversible release of lattice oxygen,deterioration of layered structure and slow reaction dynamics.In this thesis,the strategies of surface Ni-rich engineering and electronic structure manipulation are used to improve the formation energy of oxygen vacancy and decrease the activation energy of Li+ diffusion.Thus,the lattice oxygen is stabilized for preventing the layered-to-spinel transformation and the processes of lithium intercalation and deintercalation are accelerated.Finally,excellent electrochemical performances are achieved.In addition,lithium metal anode possesses ultra-high theoretical specific capacity(3860 mAh g-1)and low electrochemical reaction potential(-3.04 V vs.SHE),which is the best choice for next-generation lithium battery.However,the problems of serious Li-dendrite growth,unstable interface and large volume change would lead to short circuit,reduction of Coulombic efficiency and short cycle life.In order to solve these problems,the 3D lithiophilic-lithiophobic-lithiophilic porous current collector is designed in this thesis,contributing to realizing the uniform plating and stripping of lithium metal.Thus,the electrochemical stability of lithium metal anode is significantly improved.Research contents and main conclusions in this thesis are summaried as follows:(1)Through the surface Ni-rich engineering,a composite Li-rich Mn-based cathode material of Li1.2Mn0.54Ni0.13Co0.13O2-Li1.2Mn0.44Ni0.32Co0.04O2,abbreviated as LLO-111@111/811,was constructed.Experimental and theoretical results show that the formation energy of oxygen vacancy is improved after surface Ni-rich modification,which is beneficial for stabilizing the lattice oxygen,suppressing the layered-to-spinel transformation and enhancing the interface stability.Therefore,LLO-111@111/811 cathode exhibits improved electrochemical performance.It shows high specific capacity,high discharge voltage and stable cycling performance at different rates.When tested at 0.5 C,the specific capacity,discharge voltage and energy density are 249 mAh g-1,3.57 V and 887 Wh kg-1,respectively,and corresponding retention rates are 89.3%,83%and 74%after 350 cycles.The simple and effective strategy of surface Ni-rich engineering exhibits huge potential for commercialization and can also be used to synthesize other cathode materials.(2)The Li-rich Mn-based cathode oxide of Li(1.2-y)NayMn0.56Ni0.16Co0.08O(2-z/2)Fz(LLO-NaF)is prepared with the characteristics of the Na+/F-co-doping,Li+/Ni2+mixing and increased Li-O-Li configuration.According to experimental data and theoretical calculations,the Na+/F-co-doping and Li+/Ni2+ mixing would change the local coordination environment around O and TM and additional Na-O,Ni-O and TM-F bonds form.The electronic structure can be regulated successfully.The TM 3d-O 2p bands and O-2p non-bonding bands shift towards lower energy level,contributing to stablizing the lattice oxygen and improving the discharge voltage.The theoretical calculation results also show that the formation energy of oxygen vacancy gets improved and the activation energy of Li+diffusion is decreased.Thus,the oxygen release can be inhibited,the layered structure can be stabilized and the Li+migration can be accelerated,which are beneficial for achieving high rate performance and stable long-term cycle.In addition,increasd Li-O-Li configuration can activate more lattice oxygen and additional capacity can be provided,which compensates the capacity loss caused by inactive Na+/F-co-doping and Li+/Ni2+mixed effectively and maintains the high-capacity advantage.Therefore,LLO-NaF shows high intial Coulombic efficiency,excellent high-rate and long-term performance as well as good voltage stability.At the high rate of 5 C,the specific capacity and energy density are 222 mAh g-1 and 752 Wh kg-1,respectively,and their retention rates are 85.7%and 75%after 500 cycles.This work explains the deep mechanism of performance improvement via electronic structure manipulation.The proposed strategy of electronic structure manipulation offers a new possibility to achieve long-term cycle,high energy density and high power density in LLOs simultaneously.(3)Combining with electrospinning with magnetron sputtering,a 3D lithiophilic-lithiophobic-lithiophilic porous Cu-Au-ZnO-PAN-ZnO(CAZPZ)current collector is prepared successfully.According to experimental data and theoretical calculations,the unique lithiophilic-lithiophobic-lithiophilic design can reduce the lithium nucleation overpotential as well as realize uniform distribution of electric field and lithium ion flux;and the 3D porous structure is beneficial for alleviating the lithium volume expansion.Therefore,the lithium dendrite is inhibited effectively and uniform lithium plating and stripping can be realized.Finally,the stable electrochemical performances of lithium metal anode are achieved.When CAZPZ is tested under the conditions of 0.5 mA cm-2 and 1 mAh cm-2,the average Coulombic efficiency reaches 99%.When Li ‖CAZPZ-Li symmetrical battery is tested under the conditions of 1 mA cn-2 and 1 mAh cm-2,the overpotential is only 18 mV and can maintain stability for 600 h.The design type of 3D lithiophilic-lithiophobic-lithiophilic porous current collector broadens the design ideas of lithium metal anode current collector,and can be also applied to construct the sodium metal anode current collector and zinc metal ande current collector.