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The doping of Al into layered Li transition metal (TM) oxide cathode materials, LiTMO 2 , is known to improve the structural and thermal stability, although the origin of the enhanced properties is not well understood. The effect of aluminum doping on layer stabilization has been investigated using a combination of techniques to measure the aluminum distribution in layered LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) over multiple length scales with 27 Al and 7 Li MAS NMR, local electrode atom probe (APT) tomography, X-ray and neutron diffraction, DFT, and SQUID magnetic susceptibility measurements. APT ion maps show a homogeneous distribution of Ni, Co, Al, and O 2 throughout the structure at the single particle level in agreement with the high-temperature phase diagram. 7 Li and 27 Al NMR indicates that the Ni 3+ ions undergo a dynamic Jahn−Teller (JT) distortion. 27 Al NMR spectra indicate that the Al reduces the strain associated with the JT distortion, by preferential electronic ordering of the JT lengthened bonds directed toward the Al 3+ ion. The ability to understand the complex atomic and orbital ordering around Al 3+ demonstrated in the current method will be useful for studying the local environment of Al 3+ in a range of transition metal oxide battery materials.

A combination of soft and hard synchrotron-based spectroscopy with firstprinciples density functional theory within the GGA + U framework is used to investigate the distortion of the Mn local environment of Li x MnPO 4 as a function of electrochemical delithiation (x = 1.0, 0.75, 0.5, 0.25) and its effect on the electron and hole polaron formation. Analysis of the soft X-ray absorption spectroscopy (XAS) of the Mn L 3,2 -edges confirmed the evolution from the Mn 2+ to the Mn 3+ charge state as a two-phase reaction upon delithiation; the corresponding Mn K-edge extended X-ray fine structure measurements clearly revealed a splitting of the Mn−O nearest-neighbor distances with increasing Mn 3+ character. In addition, the O K-edge absorption and emission spectra confirmed the corresponding orbital lifting of degeneracy accompanying the distortion of the MnO 6 octahedra in the Mn 3+ state. Our GGA + U calculations show that the distortion is not a strict Jahn−Teller distortion but is instead a preferential elongation of two of the equatorial Mn−O bonds (edge-sharing with the PO 4 ), which results in a Mn−O−P induction driven hybridization of the unoccupied states (i.e., a pseudo Jahn−Teller distortion). Excellent agreement between the calculated electronic structure and our soft X-ray measurements of the electrochemically delithiated Li x MnPO 4 nanoparticles verifies the link between the preferential structural distortion and the resultant hybridization of the unoccupied 3d d xz and d x 2 −y 2 orbitals. Our analysis of the corresponding calculated electron and hole polaron supports claims that the elongation of the equatorial bonds (edge-sharing with the PO 4 ) in the Mn 3+ charge state (i.e., the pseudo Jahn−Teller distortion) is responsible for increasing the activation energy for polaron migration and the formation energy of the electron (hole) lithium ion (vacancy) complex of the Mn olivine compared to the Fe olivine.

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