The Impact of Oxygen Vacancies on Lithium Vacancy Formation and Diffusion in Li2MnO3

Introduction

The high capacity cathode material xLi₂MnO₃∙(1-x)LiMO₂ (where M=Ni, Co and/or Mn) is a promising candidate for lithium-ion batteries (LIBs). However, the kinetics are limited by the Li₂MnO₃ component,¹ there is capacity loss during cycling, and the material undergoes voltage fade. Furthermore, it is generally agreed that oxygen vacancies are being introduced into the Li₂MnO₃ component during the initial charging cycle above 4.4 V.² However, the amount and location of these vacancies are not yet understood and difficult to quantify using experimental techniques. These oxygen vacancies alter the structure on an atomic scale and therefore will significantly impact the capacity, structural evolution and lithium diffusion within the material. Atomistic computational techniques provided a unique tool to introduce oxygen vacancies at various concentrations. Therefore, this study focuses on using atomistic computation to analyze the impact of oxygen vacancies on the properties of the Li₂MnO_3-δ component such as diffusion of lithium and capacity.

Method

Density functional theory (DFT) calculations with GGA+U approximations were performed on a 2x1x2 cell of Li_2-xMnO_3-δ with x incrementing by 0.125 from 0 to 0.500 from starting systems with different oxygen vacancy concentrations (δ=0, 0.125, 0.250). Preliminary calculations on a smaller cell showed that the lithium vacancies tend to form near the oxygen vacancies. Therefore, the configurations with lithium vacancies (V_Li­) in all positions within an average of about 4 Å from the oxygen vacancies (V_O), and a couple of V_Li a larger distance from V_O for comparison, were sampled.

Ab-initio molecular dynamics simulations (AIMD) were on a 2x1x2 cell with the Nosé thermostat at a temperature of 1500 K so that diffusion could be observed on the computed time scale of 50 ps without the crystal melting. The AIMD simulations were performed starting from the structures with the minimal energy determined by the above DFT calculations. Three concentrations were investigated: (x=0.125, δ=0), (x=0.125, δ=0.125), and (x=0.250, δ=0.125).

Results

The DFT calculations showed that the V_Li prefer to form in the neighboring sites next to the V_O with the energy of formation of the V_Li increasing with increasing distance from the V_O. The open circuit voltage was also calculated from the minimal energy structures, Figure 1, which showed that the oxygen vacancies caused an increase in capacity at voltages around and below 4.0V (the approximate maximum voltage used to avoid electrolyte oxidation reactions³).

The AIMD simulation of the system with only V_Li  but no V_O (x=0.125, δ=0) showed significant movement of the lithium vacancy hopping within the lithium layer. The system with x=0.125 and δ=0.125 showed little diffusion as the lithium vacancy stayed in that optimal configuration next to the V_O. The only movement observed within 50ps is  the V_Li hopping to a neighboring site and then hopping back after 5 ps.  The last simulation contained (x=0.250, δ=0.125). Both V_Li were in sites neighboring the V_O, one in the transition metal layer and the other in the lithium layer at the same optimal site as the first V_Li . The V_Li in the lithium layer stayed in place during the 50ps, only hopping to a neighboring position and back briefly twice. In contrast, the V_Li in the transition metal layer first moved to another lithium layer via a trans-layer hopping and then diffused tremendously within that layer.

Conclusion

These DFT and AIMD results showed that there is a strong correlation between V_Li ­and V­­_O. Introducing V_O into the Li₂MnO₃ structure increased the capacity by making the Li sites near the V_O less energetically favorable but hindered Li diffusion because these sites were higher in energy, therefore less accessible for Li.

References

1.            Yu, H. J.; Wang, Y. R.; Asakura, D.; Hosono, E.; Zhang, T.; Zhou, H. S., Electrochemical kinetics of the 0.5Li(2)MnO(3)center dot 0.5LiMn(0.42)Ni(0.42)Co(0.16)O(2) 'composite' layered cathode material for lithium-ion batteries. Rsc Advances 2012, 2(23), 8797-8807.

2.            Thackeray, M. M.; Kang, S. H.; Johnson, C. S.; Vaughey, J. T.; Benedek, R.; Hackney, S. A., Li2MnO3-stabilized LiMO2 (M = Mn, Ni, Co) electrodes for lithium-ion batteries. Journal of Materials Chemistry 2007, 17(30), 3112-3125.

3.            Lithium-Ion Batteries: Advanced Materials and Technologies. CRC Press: Boca Raton, Florida, 2012; p 406.