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The Structural and Chemical Origin of the Oxygen Redox Activity in Layered and Cation-Disordered Li-Excess Cathode Materials

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The Structural and Chemical Origin of the Oxygen Redox Activity in Layered and Cation-Disordered Li-Excess Cathode Materials ARTICLES PUBLISHED ONLINE: 30 MAY 2016 | DOI: 10.1038/NCHEM.2524 The structural and chemical origin of the oxygen redox activity in layered and cation-disordered Li-excess cathode materials Dong-Hwa Seo1,2†, Jinhyuk Lee1,2†, Alexander Urban2, Rahul Malik1,ShinYoungKang1 and Gerbrand Ceder1,2,3* Lithium-ion batteries are now reaching the energy density limits set by their electrode materials, requiring new paradigms for Li+ and electron hosting in solid-state electrodes. Reversible oxygen redox in the solid state in particular has the potential to enable high energy density as it can deliver excess capacity beyond the theoretical transition-metal redox- capacity at a high voltage. Nevertheless, the structural and chemical origin of the process is not understood, preventing the rational design of better cathode materials. Here, we demonstrate how very specific local Li-excess environments around oxygen atoms necessarily lead to labile oxygen electrons that can be more easily extracted and participate in the practical capacity of cathodes. The identification of the local structural components that create oxygen redox sets a new direction for the design of high-energy-density cathode materials. ver the past two decades, lithium-ion battery technology has why it is observed with some metals and not with others. The contributed greatly to human progress and enabled many of specific nature of our findings reveals a clear and exciting path Othe conveniences of modern life by powering increasingly towards creating the next generation of cathode materials with capable portable electronics1. However, with the increasing com- substantially higher energy density than current cathode materials. plexity of technology comes a demand for batteries with higher energy density, which thus leads to a requirement for cathode Results and discussion materials with higher energy densities2–4. Appearance of labile oxygen states in Li metal oxides. It is The traditional design paradigm for Li-ion battery cathodes has generally understood that the relative energy of the TM versus been to create compounds in which the amount of extractable Li+ oxygen states determines which species (TM versus O) are oxidized is well balanced with an oxidizable transition metal (TM) species upon delithiation1,19. As the energy level of those states depends on (such as Mn, Fe, Co or Ni) to provide the charge-compensating elec- the species in a compound and their chemical bonding, local trons, all contained in an oxide or sulfide host. Transition metals have environments in a crystal structure can play a critical role in the been considered the sole sources of electrochemical activity in an redox processes and the participation of oxygen in them10. Unlike intercalation cathode, and as a consequence the theoretical specific in conventional stoichiometric layered cathode materials, which are capacity is limited by the number of electrons that the transition- well ordered and in which only a single local environment exists metal ions can exchange per unit mass5–9. Recent observations have for oxygen ions, a variety of local oxygen environments exist in brought this simple picture into question and argued that oxygen Li-excess materials or materials with cation disorder. By ions in oxide cathodes may also participate in the redox reaction. systematically calculating and analysing the density of states (DOS) This oxygen redox is often ascribed to covalency, following theoreti- and charge/spin density around oxygen ions in various local cal10,11 and experimental12–16 work in the last two decades that has environments using density functional theory (DFT) (ref. 20), we demonstrated large electron density changes on oxygen when the demonstrate that the Li-excess content and the local configuration transition metal is oxidized. However, covalency cannot lead to a sensitively affect oxygen redox activity in the oxide cathodes. To higher capacity than would be expected from the transition accurately study the oxygen redox activity, all calculations were metal alone, as the number of transition-metal orbitals remains performed with DFT using the Heyd–Scuseria–Ernzerhof (HSE06) unchanged when they hybridize with the oxygen ligands. The hybrid functional21, which can correct the self-interaction errors more important argument for the future of the Li-ion battery field (SIEs) for both metal (M) and O atoms. Note that the generalized is whether oxygen oxidation can create extra capacity beyond what gradient approximation (GGA) and GGA+U, which are frequently is predicted from the transition-metal content alone, as has been used in DFT calculations, cannot properly predict the oxygen redox 22 argued for several Li-excess materials, such as Li1.2Ni0.2Mn0.6O2, activity, because they cannot correct the SIE for O atoms . Li2Ru0.5Sn0.5O2 and Li1.3Mn0.4Nb0.3O2 (refs 3,17,18). We start with LiNiO2 (containing site disorder) and Li2MnO3 as In this Article, we use ab initio calculations to demonstrate which two simple model systems before moving on to more complex fi speci c chemical and structural features lead to electrochemically materials. LiNiO2 is one of the most studied materials and Ni is the active oxygen states in cathode materials. Our results uncover a dominant redox active species in many technologically important fi speci c atomistic origin of oxygen redox and explain why this cathodes such as LiNi1/3Mn1/3Co1/3O2 and LiNi0.8Co0.15Al0.05O2 oxygen capacity is so often observed in Li-excess materials and (refs 23,24). In perfectly layered LiNiO2, oxygen ions are exclusively 1Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. 2Department of Materials Science and Engineering, UC Berkeley, Berkeley, California 94720, USA. 3Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. †These authors contributed equally to this work. *e-mail: [email protected] NATURE CHEMISTRY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturechemistry 1 © 2016 Macmillan Publishers Limited. All rights reserved ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.2524 a 0.5 b 0.5 c 0.5 d O 2p O 2p O 2p EF EF EF 0.0 0.0 0.0 Ni –0.5 –0.5 –0.5 O –1.0 –1.0 –1.0 Li Energy (eV) –1.5 –1.5 –1.5 –2.0 –2.0 –2.0 Li Ni O –2.5 –2.5 –2.5 Density of states (a.u.) Density of states (a.u.) Density of states (a.u.) Figure 1 | Effect of local atomic environments on the electronic states of O ions in cation-mixed layered LiNiO2. Cation mixing introduces various local environments around oxygen. a–c, Projected density of states (pDOS) of the O 2p orbitals of O atoms in cation-mixed layered LiNiO2 coordinated by two Li and four Ni (a), three Li and three Ni (b) and four Li and two Ni (c). Insets: coordination of the O ion. d, Isosurface of the charge density (yellow) around the oxygen coordinated by four Li and two Ni (c), in the energy range of 0 to −1.64 eV. Increased pDOS can be found near the Fermi level for the O ion coordinated by four Li and two Ni, which originates from the particular Li–O–Li configuration. coordinated by three Li and three Ni (Fig. 1b). The other two environ- (M = Ni, Co, and so on) and Li2Ru0.5Sn0.5O3 are layered Li-excess 17,28 ≈ ments in Fig. 1 are created by Li/Ni exchange (anti-sites): two Li and materials , and Li1.25Mn0.5Nb0.25O2 ( Li1.3Mn0.4Nb0.3O2) and four Ni (Fig. 1a) and four Li and two Ni (Fig. 1c). Li1.2Ni1/3Ti1/3Mo2/15O2 are cation-disordered Li-excess The projected DOS (pDOS) of the oxygen 2p states of the three materials18,29,30. For each of the compounds we constructed unit oxygen environments are shown in Fig. 1a–c. Although there is not cells that take into account as much as is known about the much change in the oxygen pDOS between the 4Ni/2Li and 3Ni/3Li structures. Fragments of these unit cells are shown in the top row configurations, the oxygen pDOS changes substantially when four of Fig. 3 (for more details see Supplementary Section ‘Preparation Li ions are near the oxygen (Fig. 1c). In particular, a much greater of the structure models’). All compounds were delithiated beyond pDOS between 0 and −2.5 eV of the Fermi level is found for the the conventional limit from TM redox. oxygen ion coordinated with four Li and two Ni ions (Fig. 1c). Figure 3a–d plots the isosurface of the spin density around oxygen fi The origin of this increased DOS can be identi ed by visualizing in partially delithiated Li1.17–xNi0.25Mn0.58O2 (x =0.5, 0.83), the charge density around the oxygen ion for the energy range Li2–xRu0.5Sn0.5O3 (x = 0.5, 1.5), Li1.17–xNi0.33Ti0.42Mo0.08O2 (x =0.5, − between 0 and 1.64 eV (Fig. 1d). This energy range corresponds 0.83) and Li1.25–xMn0.5Nb0.25O2 (x = 0.75, 1.0), respectively. To sim- to the extraction of one electron per LiNiO2. As seen in the isosur- plify the presentation, the spin densities around metal ions are not face plot, a large charge density resembling the shape of an isolated drawn in the figures. In all cases, we observe a large spin density O2p orbital is present along the direction where oxygen is linearly from the oxygen ions along the Li–O–Li configuration with bonded to two Li (Li–O–Li configuration). This result indicates that the shape of an isolated O 2p orbital, indicating a hole along the the labile electrons from the O ion in the local Li-excess environ- Li–O–Li configuration. These holes along the Li–O–Li configurations ment originate from this particular Li–O–Li configuration.
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