Revealing the Doping Effect in Stabilizing Layered Cathode Materials for Sodium-Ion Battery
Authors: Yan Pengfei 
Published: February 2020

Energy storage devices allow us to use energy in a flexible, high efficient and eco-benign way, which has profoundly shaped our everyday life. The most successful energy storage device is lithium-ion battery (LIB), which has dominated the portable electronic devices and are now penetrating deep into vehicle markets. However, the limited reserves of lithium and cobalt in the earth crust cannot fulfill the huge demand gap of electrochemical energy storage, and the fast expansion of LIB industry has already led to accelerated cost rising. Therefore, worldwide research programs strategically encourage the development of energy storage devices beyond LIB, among of which, sodium ion battery (SIB) is a very attractive one. Sodium is much more natural abundance and more importantly SIB industry can share facilities and technologies from LIB because of the similarities of physical and chemical properties between Li and Na. To achieve superior performance of SIB, high performance cathode material plays a critical role. Among of variety of cathode candidates, layered NaxTMO2 (TM refers to transition metal) oxides are promising and widely studied, which can be grouped into two families, P2-type and O3-type structures. However, one critical drawback for both P2-type and O3-type layered oxides is the multi-phase-transition reactions during charge/discharge, which leads to the annoying multi-voltage-plateaus in voltage-capacity profiles. Various ordering processes (TM ordering, charge ordering and Na-vacancy ordering) and structure transitions (P2-O2, O3-P3, et al) have been identified previously. The structural ordering will result in sluggish Na-ion transportation kinetics and it can further couple with the structural transformation to collectively cause structural instability, significantly plaguing the cycling performance. Therefore, many efforts have been taken to suppress the structural ordering and transformation, namely eliminating the multi-voltage-plateaus in the charge/discharge profile. Particularly for P2-Na2/3Ni1/3Mn2/3O2 (NNM), doping electrochemically inactive elements (Li, Mg, Cu or Zn) is proven to be an effective approach to suppress the P2-O2 phase transition and Na-vacancy ordering to achieve improved electrochemical performance.
In this work, by virtue of variety of analytical tools, such as X-ray diffraction (XRD), focused ion beam/scanning electron microscopy (FIB/SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) imaging, we conduct systematic investigations on the degradation mechanisms of P2-NNM and find that P2-O2 phase transition induced grain cracking is the main cause of performance decay. Our further investigations on the role of dopant on P2-Na0.67Ni0.33-xMn0.67MgxO2 (x=0, 0.05, 0.1) (P2-NMMx) and find that doping electrochemically inactive elements can suppress intragranular cracking to improve cycle stability of P2-NM. we show that dopants segregation is more effectively to suppress intragranular crack and improve cycling stability than dopant uniform distribution in TM layer. By virtue of variety of analytical tools and simulation, dopant segregation have been characterized comprehensively and come up a new strategy, precipitation strengthening mechanism, to alleviate electrochemomechanical degradation and enhance stability of layered cathode operating at high voltage.