Local Environment of Oxygen in Paramagnetic Battery Materials: A First-principles investigation of ¹⁷O NMR in Li₄Ti₅O₁₂ Spinal Structure

One of the great current global challenges is to develop alternative renewable energy (RE) sources due to the limited fossil fuel reservoirs. Rechargeable batteries comprise one of the areas of RE research, and Li-ion batteries (LIBs), which have good capacity and high power density, have shown great promise [1]. Indeed, the LIBs have already shown its feasibility in the area of practical RE applications. A critical component of any LIB is the anode materials, and Li₄Ti₅O₁₂ is a very appealing alternative due to its negligible volume change and stability upon repeated charge/discharge cycles [2].

In this work, the local environment of oxygen in the spinel-structured Li₄Ti₅O₁₂ has been characterized via solid state ¹⁷O NMR. The experimental ¹⁷O NMR spectrum showed two distinct oxygen peaks (408 and 545 ppm), raising the question regarding the nature of the local oxygen coordination sphere. To elucidate the source of the oxygen peaks, we performed first-principles density functional theory (DFT) calculations of the chemical shifts of O in Li₄Ti₅O₁₂. The theoretical calculations adopted both a pseudo-potential and all-electron plane-wave strategy. Using these strategies we faithfully reproduce the main features of the NMR spectrum (448 and 536 ppm), and also propose an explanation for the two observed oxygen peaks. Specifically, we ascribe the two oxygen peaks to two distinct oxygen coordination sites. At the one site, the tetrahedral oxygen is surrounded by two Li-ions and two Ti-ions (O@Li₂Ti₂), whereas at the alternative tetrahedral site the oxygen is surrounded by one Li-ion and three Ti-ions (O@LiTi₃). Analysis of the electron density, using the Bader charge analysis approach, suggests that the upfield peak corresponds to the more shielded O@LiTi₃ environment (Bader charge -1.21 a.u.), whereas the downfield peak corresponds to the less shielded O@Li₂Ti₂ site (Bader charge -1.18 a.u.).

Keywords: Li-ion rechargeable batteries, Nuclear Magnetic Resonance, Chemical Shifts, Density Functional Theory

References:

1. Scrosati, B., et al., Journal of Power sources 195, 2419-2430 (2010).
2. Ping-Chun Tsai et al., Journal of the Electrochemical Society, 146 (3) 857-861 (1999).