Doping and Defect Chemistry in LiCoO2 By First-Principles Calculation

Introduction

In order to improve the performance of lithium-ion batteries, doping to electrode active materials is often carried out. The doped ions are extrinsic point defects in the active materials, and thus the doping exerts an influence on native point defects in the materials. Since the point defects are associated with many properties of the active materials, information on the doped ions as well as the native defects is of importance to control the materials. In this study, we made comprehensive first-principles calculations to estimate solubility of various species doped into LiCoO₂and to discuss effects of the doping on the defect chemistry.

Computation method

As the doped species to LiCoO₂, several metals such as Na, Mg, and Al, were selected. Both the Li and Co sites were considered as the substitution positions of the doped ions. Vacancies, interstitial cations and antisite cations were examined as the native defects. Defect formation energy was evaluated by first-principles calculations using a supercell consisting of 144 atoms with a single defect. The first-principles calculations were carried out using the plain-wave basis PAW method and the GGA+U exchange correlation functional. Equilibrium defect concentrations were estimated by the Boltzmann distribution under given chemical conditions and charge neutrality.

Results and discussion

Figure 1(a) illustrates equilibrium defect concentrations in Al-doped LiCoO₂ as a function of temperature when LiCoO₂ coexists with LiAlO₂ and Li₅AlO₄ under an oxygen pressure of 0.2 atm. Al preferentially substitutes for Co (Al_Co) as shown in the figure, and thus this condition can be considered as a nominal composition of Li(Al_xCo_1-x)O₂ with a small amount of excess Li. At this chemical condition, one can expect high Al solubility of ~50% with low concentrations (<1%) of the native defects such as Co_Li under usual synthetic conditions (800-900 °C in air). Since defect formation energy and equilibrium defect concentration depend on the chemical condition, more Li-rich and Al-rich conditions were also examined. Figure 1(b) illustrates equilibrium defect concentrations at the more Li-rich condition, where LiCoO₂ coexists with Li₂O and Li₅AlO₄. At this condition, the concentration of Al_Co is less than 10%. This is because excess Li reacts with Al, resulting in suppression of the Al-doping to LiCoO₂. On the other hand, when LiCoO₂ coexists with LiAlO₂ and CoAl₂O₄ as the Al-rich condition, the concentration of Co_Li becomes ~6% at 1100 K as shown in Fig. 1(c). The Al-rich condition means Li-poor condition, and thus Li deficiency in LiCoO₂ is increased. These calculations suggest that the chemical conditions are of great importance to achieve both the increase in the Al solubility and the suppression of the Li deficiency.   The solubility and effects on the defect chemistry of other species are discussed at the meeting.

Acknowledgement

This work was supported by RISING project of NEDO.