LiNi0．5Mn1.5O4正极材料表面的双电子能量损失谱谱学成像

Dual Electron EnergyL oss Spectrum Imaging of the Surfacesof LiNi_(0.5)Mn_(1.5)O_4 Cathode Material

研究锂离子电池电极材料中的化学结构、尤其是Li元素的分布和过渡金属元素的价态分布对理解锂离子电池的电池性能具有重要的意义。尽管电子能量损失谱(EELS)具有对轻元素敏感的特点,但利用EELS观察锂离子电池正极材料中Li这一周期表中最轻的固体元素一直是个挑战。这不仅是由于EELS谱中锂K边与过渡金属M边存在部分重叠,还由于锂离子电池材料的尺寸普遍较大使得EELS分析中复散射的影响变大,影响了Li定量分析的准确性。本文以LiNi_(0.5)Mn_(1.5)O_4(LNMO)正极材料为例,利用扫描透射电子显微镜(STEM)下的双电子能量损失谱仪(Dual EELS)谱学成像技术,获取了LNMO中较为精确的Li、Mn及Ni分布图,并进一步获得了Mn/Ni的价态分布图。结合STEM原子序数衬度像表明,LNMO表面1–2nm深度范围内具有富Mn/Ni而缺Li的特征,且表面Mn(+2)相对于体相Mn(+4)的价态偏低。由于低价态的Mn^(2+)在电解液中的溶解是造成电池容量衰减的重要原因,我们的结果表明在LNMO材料合成中消除材料表面富集的低价态Mn^(2+)可能是将来减小其容量衰减的可行途径。

Acquiring the spatial distribution of Li and the valence state of transition metals （TMs） in lithium ion battery （LIB） electrode materials is critical for understanding their electrochemical performances. Electron energy loss spectrum （EELS） is in principle optimum for analyzing light elements; however, quantitative analysis of Li, the lightest solid element in the periodic table, using EELS remains challenging. This is not only because of the overlap of the Li-K edge and the M23 edge of TMs but also due to the normally large particle sizes of LiB electrode materials （hundreds of nm）, leading to significant plural scattering effect in the EELS spectra. Using LiNi0.5Mn1.5O4 （LNMO） as the cathode material, we obtained the spatial distribution of Li, Ni, and Mn by dual EELS spectra/imaging, which allows us to simultaneously acquire the zero loss and core loss spectra, thus eliminating both the energy drift and plural scattering effects. Our results reveal that the as-prepared LNMO particles have a Mn/Ni-enriched and Li-poor surface layer of thickness 1-2 nm, and the valence of Mn gradually changed from ＋4 in the bulk to ＋2 in the surface layer. Given that the Iow-valent Mn2＋ dissolution is a critical reason for structure damage and capacity degradation of LNMO, our results indicate that rational synthesis of LNMO with decreased Iow-valent Mn2＋ content could be a previously neglected approach to enhance their electrochemical performance.