Numerous studies have established that coating high voltage Li battery cathode materials with a protective film can provide enhanced cycling stability and even enhance rate performance. While the exact mechanisms by which these coatings lead to enhanced performance is still not clear, it is likely that many coatings require some ability to intercalate and diffuse Li. We have therefore investigated the Li solubility and transport properties of several high performing coating materials (bulk phases), such as Al₂O₃, AlF₃, MgO, ZrO₂, SiO₂, and related those properties to coating overpotential ¹. Furthermore, as many coatings are just a few nanometers or fewer thick, we have also the studied the influence of interfaces between very different voltage materials on their Li intercalation properties.

In our studies of coating properties and their relation to overpotential, we use ab initio density functional theory to show that the above phases, when crystalline, are very poor Li conductors except for specific directions in SiO₂. We also propose an Ohmic continuum model for understanding the connection between the coating overpotential and Li solubility, diffusivity, coating thickness, and current density. We use this model and previous studies of Li in amorphous Al₂O₃ and AlF₃ ² to predict that that Atomic Layer Deposited (ALD) Al₂O₃ coatings have a resistivity of 1789 MOhmm (10⁶Ohmm), which value is qualitatively consistent with that extracted from multiple ALD experiments (ranges from 7.8 MOhmm to 913 MOhmm). The results of our Ohmic continuum model are summarized in the attached figure, which shows our predicted coating overpotential in terms of fundamental coating properties of Li solubility and diffusivity for key systems (amorphous Al₂O₃ and AlF₃ and lithiated Li_3.5Al₂O₃) under the condition of coating thickness L=1nm, current density J_active = 0.046 mA/cm²(approximately a 1C rate) at room temperature.

In order to model the effects of interfaces on the Li energetics we explore the Li intercalation energy profile across an interface of two materials with very different bulk intercalation energies. The olivine-structured FePO₄-MPO₄ (M=Co, Ti, Mn) and layered-structured LiNiO₂-TiO₂interfaces provide model cases to understand the physics governing Li intercalation energetics across material interfaces. We find that across the interface from a high to low voltage material (i.e. low to high Li intercalation energy), the Li site energy remains constant in the high voltage material and decays approximately linearly in the low voltage region, approaching the Li site energy of the low voltage material. This effect indicates that the existence of a high intercalation voltage material at an interface can significantly enhance the Li intercalation voltage in a low voltage region over a 1-2 nm scale. We explore possible implications of this interfacial voltage enhancement for the design of novel cathode superlattice structures. 

Figure Caption:

Fig. 1 Solubility and diffusivity dependence of potential drop across a room temperature thin coating film of L = 1 nm for a current of density J_active = 0.046 mA/cm²(approximately a 1C rate) through the active coating surface.

References

1          Xu, S. et al. Lithium transport through lithium-ion battery cathode coatings. Journal of Materials Chemistry A 3, 17248-17272 (2015).

2          Hao, S. Q. & Wolverton, C. Lithium Transport in Amorphous Al2O3 and AlF3 for Discovery of Battery Coatings. J. Phys. Chem. C 117, 8009-8013, (2013).