Ni-rich layered lithium transition metal oxides LiNi_xMn_yCo_zO_(2)(1-y-z≥0.6)are promising candidates for cathode materials,but their practical applications are hindered by high-voltage instability and fast capacity...Ni-rich layered lithium transition metal oxides LiNi_xMn_yCo_zO_(2)(1-y-z≥0.6)are promising candidates for cathode materials,but their practical applications are hindered by high-voltage instability and fast capacity fading.Using density functional theory calculations,we demonstrate that Na-,F-doping,and Na/F-co-doping can stabilize the structure and result into a higher open circuit voltage than pristine LiNi_(0.6)Mn_(0.2)Co_(0.2)O_(2)(NMC622)during the charging process,which may attain greater discharge capacity.F doping may inhibit the diffusion of Li ions at the beginning and end of charging;Na doping may improve Li ion diffusion due to the increase in Li layer spacing,consistent with prior experiments.Na/F-codoping into NMC622 promotes rate performance and reduces irreversible phase transitions for two reasons:(i)a synergistic effect between Na and F can effectively restrain the Ni/Li mixing and then enhances the mobility of Li ions and(ii)Ni/Li mixing hinders the Ni ions to migrate into Li layers and thus,stabilizes the structure.This study proposes that a layer cathode material with high electrochemical performance can be achieved via rational dopant modification,which is a promising strategy for designing efficient Li ion batteries.展开更多
The short-range repulsive interactions of any force field must be modified to be applicable for high energy atomic collisions because of extremely far from equilibrium state when used in molecular dynamics(MD)simulati...The short-range repulsive interactions of any force field must be modified to be applicable for high energy atomic collisions because of extremely far from equilibrium state when used in molecular dynamics(MD)simulations.In this work,the short-range repulsive interaction of a reactive force field(ReaxFF),describing Fe-Ni-Al alloy system,is well modified by adding a tabulated function form based on Ziegler-Biersack-Littmark(ZBL)potential.The modified interaction covers three ranges,including short range,smooth range,and primordial range.The short range is totally predominated by ZBL potential.The primordial range means the interactions in this range is the as-is ReaxFF with no changes.The smooth range links the short-range ZBL and primordial-range ReaxFF potentials with a taper function.Both energies and forces are guaranteed to be continuous,and qualified to the consistent requirement in LAMMPS.This modified force field is applicable for simulations of energetic particle bombardments and reproducing point defects'booming and recombination effectively.展开更多
Hydrogen economy,as the most promising alternative energy system,relies on the hydrogen production through sustainable water splitting which in turn relies on the high efficiency electrocatalysts.PtAuCu A1-phase alloy...Hydrogen economy,as the most promising alternative energy system,relies on the hydrogen production through sustainable water splitting which in turn relies on the high efficiency electrocatalysts.PtAuCu A1-phase alloy has been predicted to be a promising electrocatalyst for the hydrogen evolution.As such preferred phase of Pt-Au-Cu is not thermodynamically favored,herein,we stabilize PtAuCu alloy by engineering the high-entropy phase in the form of nanowire.Density functional theory(DFT)calculations indicate that,in comparison with the ordered phase and segregated phases with discrete hydrogen binding energy,the high-entropy phase provides a diverse combination of site composition to continuously tune the hydrogen binding energy,and thus generate a series of highly active sites for the hydrogen evolution.Reflecting the theoretical prediction,electrochemical tests show that the A1-phase PtAuCu nanowire significantly outperforms its nanoparticle counterpart with phase segregation,toward the electrocatalysis of hydrogen evolution,offering one of the best hydrogen evolution electrocatalysts.展开更多
基金supported through the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical, Biological, and Geosciences Division under DE-SC0010379the support from an ECS Toyota Young Investigator Fellowship
文摘2050-2060年实现碳净零排放已经成为世界上很多国家的战略使命.为实现该目标,氢燃料电池将在汽车燃料方面发挥重要作用.虽然客运燃料电池电动汽车已经成功推出,但其广泛应用仍是一个挑战,主要是因为质子交换膜燃料电池的铂阴极氧还原反应动力学过于缓慢.理论上,提高氢燃料电池的运行温度有助于改善氧还原反应动力学,例如低温质子交换膜燃料电池的工作温度在80℃,如果反应活性位点和反应机制不变,将操作温度提升到120℃就可以使反应速率提高一个数量级.但是在实际应用过程中,目前最先进的高温质子交换膜燃料电池的性能仍然低于低温质子交换膜燃料电池的性能.为了进一步提高高温质子交换膜燃料电池的性能,研究人员在优化质子交换膜、电解质、催化剂结构、纳米颗粒的稳定性等方面开展了大量的研究工作并取得了显著进展,但对活性位点和反应机制仍缺乏了解.虽然已知高温质子交换膜燃料电池中的铂催化剂在温度高于100℃的无水条件下的活性位点和氧还原机制可能与低温质子交换膜燃料电池的不同,但以往研究并没有从原子尺度上获得相关的认识.本文结合平整的Pt(111)表面上以及含有(110)和(100)型台阶的Pt(332)和Pt(322)台阶表面上的氧还原反应,采用密度泛函理论计算研究了无水条件下铂基催化剂的氧还原反应的活性位点和催化机理.结合反应自由能和表面相图,发现在典型的氧还原反应电位下,Pt(111),Pt(332)和Pt(322)表面上分别集聚0.25,0.2和0.4 ML的氧原子.与有水环境下Pt(111)平台面的氧还原反应不同,由于无水条件下平台面上O*的聚集,使得氧还原中间体的吸附减弱,过电势远大于实验值.此外氧的聚集会增加O_(2)的解离势垒,这些都表明平台面在无水环境下不是氧还原反应的活性位.虽然台阶边缘位置在有水环境下不是氧还原反应的活性位,但是理论计算表明,它们是无水环境下的活性位点.这是因为台阶边缘的存在能稳定O_(2)的吸附,促进O_(2)的解离势垒.独特O聚集构型的(110)型台阶边缘尤其如此:与0.25 ML O集聚的Pt(111)表面相比,O_(2)的吸附能增强了0.36 eV,O_(2)的解离势垒降低了0.32 eV,反应过电势为0.38 V.综上,本文研究为设计具有更高性能的高温质子交换膜燃料电池的氧还原催化剂提供了理论指导.
基金the National Natural Science Foundation of China(Grant Nos.51802092 and 51771073)the Fundamental Research Funds for the Central Universities.China。
文摘Ni-rich layered lithium transition metal oxides LiNi_xMn_yCo_zO_(2)(1-y-z≥0.6)are promising candidates for cathode materials,but their practical applications are hindered by high-voltage instability and fast capacity fading.Using density functional theory calculations,we demonstrate that Na-,F-doping,and Na/F-co-doping can stabilize the structure and result into a higher open circuit voltage than pristine LiNi_(0.6)Mn_(0.2)Co_(0.2)O_(2)(NMC622)during the charging process,which may attain greater discharge capacity.F doping may inhibit the diffusion of Li ions at the beginning and end of charging;Na doping may improve Li ion diffusion due to the increase in Li layer spacing,consistent with prior experiments.Na/F-codoping into NMC622 promotes rate performance and reduces irreversible phase transitions for two reasons:(i)a synergistic effect between Na and F can effectively restrain the Ni/Li mixing and then enhances the mobility of Li ions and(ii)Ni/Li mixing hinders the Ni ions to migrate into Li layers and thus,stabilizes the structure.This study proposes that a layer cathode material with high electrochemical performance can be achieved via rational dopant modification,which is a promising strategy for designing efficient Li ion batteries.
基金Project supported by the National Magnetic Confinement Fusion Energy Research Project(Grant Nos.2019YFE03120003,2018YFE0307100,and 2017YFE0302500)the National Natural Science Foundation of China(Grant Nos.11975034,11921006,12004010,and U20B2025).
文摘The short-range repulsive interactions of any force field must be modified to be applicable for high energy atomic collisions because of extremely far from equilibrium state when used in molecular dynamics(MD)simulations.In this work,the short-range repulsive interaction of a reactive force field(ReaxFF),describing Fe-Ni-Al alloy system,is well modified by adding a tabulated function form based on Ziegler-Biersack-Littmark(ZBL)potential.The modified interaction covers three ranges,including short range,smooth range,and primordial range.The short range is totally predominated by ZBL potential.The primordial range means the interactions in this range is the as-is ReaxFF with no changes.The smooth range links the short-range ZBL and primordial-range ReaxFF potentials with a taper function.Both energies and forces are guaranteed to be continuous,and qualified to the consistent requirement in LAMMPS.This modified force field is applicable for simulations of energetic particle bombardments and reproducing point defects'booming and recombination effectively.
基金supported by the National Natural Science Foundation of China(NSF-C)(Nos.21773023 and 21972016)。
文摘Hydrogen economy,as the most promising alternative energy system,relies on the hydrogen production through sustainable water splitting which in turn relies on the high efficiency electrocatalysts.PtAuCu A1-phase alloy has been predicted to be a promising electrocatalyst for the hydrogen evolution.As such preferred phase of Pt-Au-Cu is not thermodynamically favored,herein,we stabilize PtAuCu alloy by engineering the high-entropy phase in the form of nanowire.Density functional theory(DFT)calculations indicate that,in comparison with the ordered phase and segregated phases with discrete hydrogen binding energy,the high-entropy phase provides a diverse combination of site composition to continuously tune the hydrogen binding energy,and thus generate a series of highly active sites for the hydrogen evolution.Reflecting the theoretical prediction,electrochemical tests show that the A1-phase PtAuCu nanowire significantly outperforms its nanoparticle counterpart with phase segregation,toward the electrocatalysis of hydrogen evolution,offering one of the best hydrogen evolution electrocatalysts.
