Bréger Julien, Dupré Nicolas, Chupas Peter J, Lee Peter L, Proffen Thomas, Parise John B, Grey Clare P
Department of Chemistry, State University of New York at Stony Brook, 11794, USA.
J Am Chem Soc. 2005 May 25;127(20):7529-37. doi: 10.1021/ja050697u.
The local environments and short-range ordering of LiNi(0.5)Mn(0.5)O(2), a potential Li-ion battery positive electrode material, were investigated by using a combination of X-ray and neutron diffraction and isotopic substitution (NDIS) techniques, (6)Li Magic Angle Spinning (MAS) NMR spectroscopy, and for the first time, X-ray and neutron Pair Distribution Function (PDF) analysis, associated with Reverse Monte Carlo (RMC) calculations. Three samples were studied: (6)Li(NiMn)(0.5)O(2), (7)Li(NiMn)(0.5)O(2), and (7)Li(NiMn)(0.5)O(2) enriched with (62)Ni (denoted as (7)Li(ZERO)Ni(0.5)Mn(0.5)O(2)), so that the resulting scattering length of Ni atoms is null. LiNi(0.5)Mn(0.5)O(2) adopts the LiCoO(2) structure (space group Rm) and comprises separate lithium layers, transition metal layers (Ni, Mn), and oxygen layers. NMR experiments and Rietveld refinements show that there is approximately 10% of Ni/Li site exchange between the Li and transition metal layers. PDF analysis of the neutron data revealed considerable local distortions in the layers that were not captured in the Rietveld refinements performed using the Bragg diffraction data and the LiCoO(2) structure, resulting in different M-O bond lengths of 1.93 and 2.07 Angstroms for Mn-O and Ni/Li-O, respectively. Large clusters of 2400-3456 atoms were built to investigate cation ordering. The RMC method was then used to improve the fit between the calculated model and experimental PDF data. Both NMR and RMC results were consistent with a nonrandom distribution of Ni, Mn, and Li cations in the transition metal layers; both the Ni and Li atoms are, on average, close to more Mn ions than predicted based on a random distribution of these ions in the transition metal layers. Constraints from both experimental methods showed the presence of short-range order in the transition metal layers comprising LiMn(6) and LiMn(5)Ni clusters combined with Ni and Mn contacts resembling those found in the so-called "flower structure" or structures derived from ordered honeycomb arrays.
采用X射线和中子衍射以及同位素取代（NDIS）技术、（6）Li魔角旋转（MAS）核磁共振光谱法，并首次结合与反向蒙特卡罗（RMC）计算相关的X射线和中子对分布函数（PDF）分析，对潜在的锂离子电池正极材料LiNi(0.5)Mn(0.5)O(2)的局部环境和短程有序性进行了研究。研究了三个样品：（6）Li(NiMn)(0.5)O(2)、（7）Li(NiMn)(0.5)O(2)以及富含（62）Ni的（7）Li(NiMn)(0.5)O(2)（记为（7）Li(ZERO)Ni(0.5)Mn(0.5)O(2)），以使Ni原子的散射长度为零。LiNi(0.5)Mn(0.5)O(2)采用LiCoO(2)结构（空间群Rm），由单独的锂层、过渡金属层（Ni、Mn）和氧层组成。核磁共振实验和Rietveld精修表明，锂层和过渡金属层之间存在约10%的Ni/Li位点交换。对中子数据的PDF分析揭示了在使用布拉格衍射数据和LiCoO(2)结构进行的Rietveld精修中未捕捉到的层内相当大的局部畸变，导致Mn-O和Ni/Li-O的M-O键长分别为1.93和2.07埃。构建了由2400 - 3456个原子组成的大团簇来研究阳离子有序性。然后使用RMC方法来改善计算模型与实验PDF数据之间的拟合度。核磁共振和RMC结果均与过渡金属层中Ni、Mn和Li阳离子的非随机分布一致；平均而言，Ni和Li原子比基于这些离子在过渡金属层中的随机分布所预测的更靠近更多的Mn离子。两种实验方法的约束都表明，在包含LiMn(6)和LiMn(5)Ni团簇以及类似于在所谓“花结构”或源自有序蜂窝阵列的结构中发现的Ni和Mn接触的过渡金属层中存在短程有序。
