Ahmed Shamail, Pokle Anuj, Schweidler Simon, Beyer Andreas, Bianchini Matteo, Walther Felix, Mazilkin Andrey, Hartmann Pascal, Brezesinski Torsten, Janek Jürgen, Volz Kerstin
Materials Science Centre and Faculty of Physics , Philipps University Marburg , Hans-Meerwein-Strasse 6 , 35043 Marburg , Germany.
Battery and Electrochemistry Laboratory, Institute of Nanotechnology , Karlsruhe Institute of Technology (KIT) , Hermann-von-Helmholtz-Platz 1 , 76344 Eggenstein-Leopoldshafen , Germany.
ACS Nano. 2019 Sep 24;13(9):10694-10704. doi: 10.1021/acsnano.9b05047. Epub 2019 Sep 10.
Ni-rich layered LiNiCoMnO (NCM, + ≤ 0.2) is an intensively studied class of cathode active materials for lithium-ion batteries, offering the advantage of high specific capacities. However, their reactivity is also one of the major issues limiting the lifetime of the batteries. NCM degradation, in literature, is mostly explained both by disintegration of secondary particles (large anisotropic volume changes during lithiation/delithiation) and by formation of rock-salt like phases at the grain surfaces at high potential with related oxygen loss. Here, we report the presence of intragranular nanopores in Li(NiCoMn)O (NCM851005) and track their morphological evolution from pristine to cycled material (200 and 500 cycles) using aberration-corrected scanning transmission electron microscopy (STEM), electron energy loss spectroscopy, energy dispersive X-ray spectroscopy, and time-of-flight secondary ion mass spectrometry. Pores are already found in the primary particles of pristine material. Any potential effect of TEM sample preparation on the formation of nanopores is ruled out by performing thickness series measurements on the lamellae produced by focused ion beam milling. The presence of nanopores in pristine NCM851005 is in sharp contrast to previously observed pore formation during electrochemical cycling or heating. The intragranular pores have a diameter in the range between 10 and 50 nm with a distinct morphology that changes during cycling operation. A rock-salt like region is observed at the pore boundaries even in pristine material, and these regions grow with prolonged cycling. It is suggested that the presence of nanopores strongly affects the degradation of high-Ni NCM, as the pore surfaces apparently increase (i) oxygen loss, (ii) formation of rock-salt regions, and (iii) strain-induced effects within the primary grains. High-resolution STEM demonstrates that nanopores are a source of intragranular cracking during cycling.
富镍层状LiNiCoMnO（NCM，+≤0.2）是一类被深入研究的锂离子电池正极活性材料，具有高比容量的优势。然而，它们的反应活性也是限制电池寿命的主要问题之一。在文献中，NCM的降解主要通过二次颗粒的分解（锂化/脱锂过程中巨大的各向异性体积变化）以及在高电位下晶粒表面形成类岩盐相并伴随相关的氧损失来解释。在此，我们报告了Li(NiCoMn)O（NCM851005）中晶内纳米孔的存在，并使用像差校正扫描透射电子显微镜（STEM）、电子能量损失谱、能量色散X射线光谱和飞行时间二次离子质谱追踪了它们从原始材料到循环材料（200次和500次循环）的形态演变。在原始材料的一次颗粒中就已发现纳米孔。通过对聚焦离子束铣削产生的薄片进行厚度系列测量，排除了TEM样品制备对纳米孔形成的任何潜在影响。原始NCM851005中纳米孔的存在与先前在电化学循环或加热过程中观察到的孔形成形成鲜明对比。晶内孔的直径在10至50纳米之间，具有独特的形态，在循环操作过程中会发生变化。即使在原始材料中，在孔边界也观察到一个类岩盐区域，并且这些区域随着循环时间的延长而生长。有人认为，纳米孔的存在强烈影响高镍NCM的降解，因为孔表面明显增加了（i）氧损失，（ii）岩盐区域的形成，以及（iii）一次晶粒内的应变诱导效应。高分辨率STEM表明，纳米孔是循环过程中晶内开裂的一个来源。
