Identifying surface structural changes in layered Li-excess nickel manganese oxides in high voltage lithium ion batteries: A joint experimental and theoretical study† Bo Xu,‡ a Christopher R. Fell,‡ b Miaofang Chi c and Ying Shirley Meng * ab Received 5th February 2011, Accepted 30th March 2011 DOI: 10.1039/c1ee01131f High voltage cathode materials Li-excess layered oxide compounds Li[Ni x Li 1/32x/3 Mn 2/3x/3 ]O 2 (0 < x < 1/2) are investigated in a joint study combining both computational and experimental methods. The bulk and surface structures of pristine and cycled samples of Li[Ni 1/5 Li 1/5 Mn 3/5 ]O 2 are characterized by synchrotron X-Ray diffraction together with aberration corrected Scanning Transmission Electron Microscopy (a-S/TEM). Electron Energy Loss Spectroscopy (EELS) is carried out to investigate the surface changes of the samples before/after electrochemical cycling. Combining first principles computational investigation with our experimental observations, a detailed lithium de-intercalation mechanism is proposed for this family of Li-excess layered oxides. The most striking characteristics in these high voltage high energy density cathode materials are 1) formation of tetrahedral lithium ions at voltage less than 4.45 V and 2) the transition metal (TM) ions migration leading to phase transformation on the surface of the materials. We show clear evidence of a new spinel-like solid phase formed on the surface of the electrode materials after high-voltage cycling. It is proposed that such surface phase transformation is one of the factors contributing to the first cycle irreversible capacity and the main reason for the intrinsic poor rate capability of these materials. Introduction The rechargeable Lithium Ion Battery (LIB) is one of the most important energy storage technologies today. Layered transition metal oxides, based on either LiCoO 2 or LiNiO 2 are currently used in portable electronic devices due to their high operating voltage and high specific capacity 140–160 mAh/g. To enable LIB as the main on-board storage technology in plug-in hybrid electric vehicles (PHEVs) or electric vehicles (EVs), higher energy density materials such as the ‘‘Li-excess’’ layered oxides, formed as the composites between Li[Li 1/3 Mn 2/3 ]O 2 and LiMO 2 (M ¼ Ni, Mn, Co), are promising candidates as they offer much higher capacity (>250 mAh/g) and better safety with much reduced cost. 1–3 It is now well known that in the bulk of the pristine a Department of NanoEngineering, University of California San Diego, La Jolla, CA, 92037, USA. E-mail: shirleymeng@ucsd.edu b Department of Materials Science and Engineering, University of Florida, Gainesville, FL, 32611, USA c Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA † Electronic supplementary information (ESI) available: Rietveld refinement of the high resolution synchrotron X-ray diffraction of pristine Li[Ni 1/5 Li 1/5 Mn 3/5 ]O 2 ; Cation arrangements and energies of all the calculated models of Li[Ni 1/4 Li 1/6 Mn 7/12 ]O 2 ; The projected DOS of Mn ions in Li n/12 Ni 1/4 Mn 7/12 O 2 (n ¼ 14, 8, 0). See DOI: 10.1039/c1ee01131f ‡ These two authors contributed equally. Broader context High voltage high energy density Li-excess layered oxide compounds are one of the most promising candidate cathode materials used in lithium ion batteries (LIB) for on-board storage technology in plug-in hybrid electric vehicles (PHEVs) or electric vehicles (EVs). Low rate capability and large first-cycle irreversible capacity have been preventing their commercial application. In this work, the mechanisms of cation migration and subsequent surface phase transformation are described in details at the atomistic level by combining first principles computation with synchrotron X-ray diffraction (XRD), aberration corrected scanning transmission electron microscopy (a-STEM) imaging and electron energy loss spectroscopy (EELS). Stable electrode/electrolyte interface is one of the key components in designing new high energy density electrode materials for energy storage. Our research findings provide significantly new insights on understanding the complex surface chemistry in oxide materials for energy storage in LIB. This journal is ª The Royal Society of Chemistry 2011 Energy Environ. Sci. Dynamic Article Links C < Energy & Environmental Science Cite this: DOI: 10.1039/c1ee01131f www.rsc.org/ees PAPER Downloaded by University of California - San Diego on 03 May 2011 Published on 03 May 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01131F View Online