Cobalt-Free Core-Shell Structure with High Capacity and Long Cycle Life As an Alternative to NMC811
Authors: Liu Yulong , Wu Haohan , Dahn Jeff R. 
Published: December 2020

Layered transition metal oxides, such as lithium nickel manganese cobalt oxide (NMC) and lithium nickel cobalt aluminum oxide (NCA), have been an area of active research to further improve their capacity, cycle life and lower their cost of production. Over the years, researchers and scientists have come to realize that improving the capacity of these oxides by increasing their nickel content will inevitably compromise their cycle life, which hinder their application in commercial lithium-ion cells. Limited cycle life of layered nickel-rich transition metal oxides, on one hand, is due to the large anisotropic unit cell volume change that causes active material loss and impedance growth due to microcracking of polycrystalline particles during charge-discharge cycling, which universally occurs in all nickel-rich layered oxides1,2. On the other hand, at the top of charge, the presence of highly oxidizing Ni4+ has been shown by many reports to be responsible for parasitic reactions like electrolyte oxidation that create harmful products and damage the surface of active particles3.
The use of surface coatings, which act as a barrier to avoid the direct contact of the active materials with the electrolyte, is a common method to stabilize the interface between nickel-rich electrodes and electrolyte especially at high voltage. However, commonly adopted coating materials such as Al2O3, TiO2, etc4,5. have low Li+ and electron conductivity and do not contribute to any specific capacity in a lithium-ion cell. Moreover, coating these “non-active” materials onto lithiated layered transition metal oxides is an extra step in a large-scale industrial synthesis process that will inevitably increase the cost of production. Therefore, a more cost-effective approach is required to solve the problems of nickel-rich materials.
In a core-shell structure, a nickel-rich core with high capacity and a low nickel content shell with high structural stability are utilized. A low nickel content shell prevents direct contact of the nickel-rich core with the electrolyte, therefore enabling improved cycle life over the nickel-rich core alone. In contrast to the commonly adopted coatings, which contribute no capacity to the coated material and require an extra coating process, the low nickel shell not only minimizes the loss of material specific capacity due to “non-active” coatings, but also can be easily synthesized by co-precipitation method without an extra step. Based on these merits, the core-shell structure with a nickel-rich core and a low nickel shell possesses great potential as a high capacity and long cycle life positive electrode materials.
It has been demonstrated in the Dahn group that interdiffusion of transition metal occurs between core and shell6. Mn was shown to have a lower interdiffusion coefficient than Mg and Al. Therefore, Mn would be a better element to use in the shell than Mg and Al without compromising the overall core-shell structure during heat treatment. Co is expensive and less abundant than Ni and Mn. Minimizing or complete elimination of Co has been an area of active research. Li et al7 have demonstrated that the presence of Co in layered transition metal oxides brings no value to NCA-type materials with high nickel content.
In this presentation, a core-shell structure precursor with a Ni(OH)2 core and a Ni0.8Mn0.2(OH)2 shell was heated with LiOH·H2O at 750oC and 800oC. The cross-sectional EDS mapping shows a well-defined core-shell structure when lithiated at 750oC (CS-750) and a diminished core-shell structure at 800oC (CS-800). Compared to single crystal and polycrystalline NMC811 (SC811 and PC811, respectively), CS-750 shows higher specific capacity and comparable capacity retention without any Co, which makes it a promising positive electrode material as an alternative to NMC811.

References

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Figure 1