High energy-density lithium ion batteries (LIBs) are in demand for portable electronic devices and electrical vehicles. Since the energy density of these batteries relies heavily on the cathode material, a great deal of attention is being focused on developing alternative materials with a higher degree of Li utilization and specific energy density [1]. In particular, layered Ni-rich oxides can deliver higher capacities at lower cost than the conventional LiCoO₂(LCO). However, there are still several problems associated with their cycle life, thermal stability, and safety, which need to be solved [2]. In addition, these drawbacks increase with increasing Ni amount, and motivate major research efforts to understand, mitigate and overcome these effects. 

In stoichiometric layered lithium metal oxides, every vacant tetrahedral site is coordinated by either 3 lithium ions and 1 transition-metal (TM) ion, or 1 lithium ion and 3 TM ions. Effectively, only in the first case at least two Li sites are connected and can therefore sustain lithium migration. Hence, for each Li diffusion channel in the layered structure, exactly one gate site is a TM site, while the second one is a lithium site [3]. The barrier for lithium migration through such a 1-TM channel is correlated to the TM valence and the areal lithium-TM separation. The latter varies with the width of the Li layer (the distance between TM-O slabs), constraining the degree of local relaxation for the TM when lithium enters the activated state [4]. Nevertheless, the Li slab distance highly varies during discharge-charge processes and leads to cell volume fluctuation which is considered a potential source of electrode degradation, as it could favor the formation of microcracks [5]. Moreover, this fluctuation increases with increasing Ni amount.

One promising approach to control the Li slab distance in high Ni materials is anionic substitution [6]. In this work, the effect of N and F dopants is evaluated and a correlation between structural parameters and electrochemical performance was established. Moreover, the effect of N and F can be restricted to the materials surface. The stabilization of surfaces in high Ni materials is a fundamental challenge in order to control the aging processes of these materials.  

References

[1]. W. Liu et al., Angew. Chem. Int. Ed. 2015, 54, 4440.

[2]. L. Wu et al. Chem. Mater. 2011, 23, 3953.

[3]. A. Urban et al., Advanced Energy Materials 2014, 4, 1400478.

[4]. K. Kang et al. Phys. Rev. B 2006, 74, 094105.

[5]. R. Hausbrand et al., Materials Science and Engineering: B 2015, 192, 3.

[6]. F. Kong et al., J. Mater. Chem. A 2015, 3, 8489.