Article pubs.acs.org/cm
Electron Spectroscopy Study of Li[Ni,Co,Mn]O2/Electrolyte Interface: Electronic Structure, Interface Composition, and Device Implications Gennady Cherkashinin,* Markus Motzko, Natalia Schulz, Thomas Spaẗ h, and Wolfram Jaegermann Institute of Materials Science, Surface Science Division, TU Darmstadt, Jovanka-Bontschits Str. 2, D-64287 Darmstadt, Germany S Supporting Information *
ABSTRACT: In recent years, there have been significant efforts to understand the role of the electronic structure of redox active materials according to their performance and thermodynamic stability in electrochemical storage devices and to develop novel materials with higher energy density and higher power. It is generally recognized that transition metal compounds used as a positive electrode determine the specific capacity and the energy density of rechargeable batteries, while the charge transfer resistance at the electrolyte−electrode interface plays a key role in delivering the power of the electrochemical cell. In the present work, we study the stability of LixNi0.2Co0.7Mn0.1O2 thin films through the evolution of the occupied and unoccupied density of states as a function of the charging state of the electrode as well as the physicochemical conditions influencing the ionic transport across the electrode−electrolyte interface. A comprehensive experimental quasi in situ approach has been applied by using synchrotron X-ray photoelectron spectroscopy (SXPS) and O K-edge and Co, Ni, Mn L-edges XANES. Our experimental data demonstrate the change of the Fermi level position with Li+ removal and Ni2+ → Ni4+ and Co3+ → Co4+ changes of oxidation state for the charge compensation in the bulk of the material. As is evidenced by the experimentally determined energy band diagram of Lix≤1.0Ni0.2Co0.7Mn0.1O2 vs the evolution of the Fermi level, no hole transfer to the O2p bands is observed up to a charging state of 4.8 V, which evidences the thermodynamic stability of Lix≤1.0Ni0.2Co0.7Mn0.1O2 under high charging voltage in contrast to LiCoO2. A very thin solid electrolyte interface layer (less than 30 Å thickness) on the Lix≤1.0Ni0.2Co0.7Mn0.1O2 film is formed in a decomposition reaction of the electrolyte also involving the transition metal oxide. The enhanced concentration of lithium in the interface layer correlates evidently with the electron transfer to the transition metal sites changing their electronic configuration. It is concluded that Lix≤1.0Ni0.2Co0.7Mn0.1O2 can serve as a high energy density cathode material, but the delivery of high power, which is a critical parameter for an electric vehicle, is strongly influenced by the physicochemical conditions at the solid electrolyte interface, which can suppress Li+ diffusion or even block the Li+ paths across the interface. involved in TMN (dn) ↔ TMN+1 (dn−1) or TMN (dn) ↔ TMN+2 (dn−2) redox processes (here, N is a formal oxidation state of the TM ion, dn is the occupation of the related d electron states) is limited by the energy of the TM (dn; dn−1; dn−2) states with respect to the top of the O2p bands. The Li + deintercalation and related hole transfer during the charging process leads to a changed TM (3dn) occupation of electron states, which is dependent on the TM-O(3dn-2p6) hybridization of the original state and possible changes in the electronic band structure with Li+ deintercalation (rigid band vs nonrigid band behavior).6 For the oxides, where the 3d states contain a large O2p fraction, the increase of oxidation potential or the absolute value of the electron chemical potential μ of the TM oxide (equivalent to a lowering of the Fermi level position in the standard electron band energy diagrams) caused by the hole transfer accompanying a Li+ removal can lead to the situation
1. INTRODUCTION The state of the art in the field of renewable energy storage devices demands the development of high power and high energy density batteries, which are often based on Li-ion intercalation compounds in a “rocking-chair” arrangement.1 High power is achieved by a high diffusion transport rate of electrons and positively charged Li ions through the bulk material and across its interfaces, whereas the energy density is defined by the capacity of the active material, as well as by the voltage difference between the cathode and anode of the battery. Transition metal (TM) oxides are widely used as cathode materials in rechargeable Li-ion batteries (LIB)1 and capacitors,2,3 where the key factors are internal redox reactions of the oxide defining the high voltage range and the ability to store a high energy density defined by the reversible exchange of Li.4 The energy density of any cathode material is governed by its intrinsic voltage limit.5 This quantity is inherent for a relevant material, since it depends on the electronic structure given by the coupling of the metal 3d states and p states of the anion. The voltage window of any transition metal oxide © XXXX American Chemical Society
Received: December 25, 2014 Revised: March 24, 2015
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DOI: 10.1021/cm5047534 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
the Lix≤1Ni0.2Co0.7Mn0.1O2 (LNCMO) thin film upon Li+ deintercalation to shed light on the thermodynamic stability of LNCMO at a high voltage and to find out which of the conditions at the cathode−electrolyte interface may lead to solid electrolyte interface formation and may lead to limitations of Li+ diffusion rate. The local electronic structure at the TM and oxygen sites is in situ studied by using O K-edge and Co, Ni, Mn L-edges XANES and soft X-ray photoelectron spectroscopy (SXPS). Starting from a pristine Li1.0Ni0.2Co0.7Mn0.1O2 film formed in situ by plasma sputtering, we study the evolution of the occupied and unoccupied states of the LNCMO film and build up the energy band diagram versus Li+ deintercalation. To our best knowledge, this study is the first comprehensive experimental in situ analysis of the evolution of the electronic configuration of LNCMO with different depth resolution as a function of the charging state/Li content. An advantage of thin film electrode materials prepared under high vacuum conditions compared to powder electrodes, where active particles are composed with polymeric binders and conductive carbon, is the possibility to study the elementary physicochemical processes occurring at the interface without detrimental effects of the supporting material. In the absence of impurities and of unwanted chemical reactions under ambient conditions, the information experimentally obtained on the electronic properties of the TM oxide is unambiguous, whereas the chemical compatibility of powder electrodes with the liquid electrolytes is not unambiguous.14 The results obtained on the LNCMO film can perfectly be compared to electronic structure calculations, e.g., by using DFT approaches.15,16
where the electron removal from the occupied 3d state crosses the top of the O2p band, involving the oxygen states in the oxidation, which leads to the decomposition of the cathode material. Thus, the electronic structure of the transition metal oxides plays a key role in the thermodynamic stability of the whole battery cell under reversible electrochemical Li-ion extraction/insertion from/to the host material.5 Recently, we have experimentally determined the intrinsic voltage limit of LiCoO2 (4.2 V), which evidences that this cathode material cannot be used as a high energy density storage device, even for all solid state batteries.6 The concept of tailoring the voltage window of the cathode material by the replacement/ substitution of the TM ions in the host material was widely discussed [see, for example the work of Goodenough et al., refs 5 and 7, as well as refs 6 and 8−10]. Accordingly, the partial substitution of the original TM by a well chosen substitutional TM′ tunes the fraction of covalency from the 3d−O2p bond as well as the hole transfer to nonbonding or antibonding TM′ dn states. This may lead to the preferential situation, where the 3d states involved in the TMN ↔ TMN+1 redox process barely overlap with oxygen or lies above the O2−(2p) band, i.e., which leads to stronger electron localization on the TM ion, thereby increasing the stable voltage window of the cathode material by suppressing oxygen loss at deep Li+ extraction. To understand these effects in their mechanistic origin, DFT calculations combined with detailed experimental investigations of the electronic structure are needed. The high power of a LIB cell is achieved by a high Li+ ion rate diffusion through the bulk material and across the electrode−electrolyte interface. Nowadays, most of the cathode materials are able to deliver high energy power which is realized by the synthesis of nanosized active particles, where the Li+ path in the bulk material is significantly reduced.11 However, the passivating solid electrolyte interface layer which is usually formed after the contact of the cathode material with the electrolyte and during electrochemical cycling is a crucial issue influencing the diffusion rate and permeability of Li+ ions. The chemical reactions of the electrolyte with the surface of the TM oxide can strongly modify the electronic structure at the interface resulting in the suppression or blocking of the Li+ transport across the interface. For estimating the chance of such redox reactions, the electrochemical potentials of the holes in the electrode versus the highest occupied molecular orbitals (HOMO) of the electrolyte must be known. 5 Only, simultaneous realization of both factors, namely, delivering of high energy density and high power, would make a cathode material suitable for a chemical energy storage device of a plugin hybrid electric vehicle. Thus, the knowledge of the surface composition of the transition metal oxides and its evolution under charging−discharging conditions, as well as the understanding of the physicochemical processes occurring at the cathode−electrolyte interface, allow us to predict the functioning of electrochemical storage devices under working conditions, as well as to develop novel materials suitable for energy conversion. LiNi1/3Mn1/3Co1/3O2 (NMC)a layered material of R3m ̅ space group has gained attention as the positive electrode for LIBs due to its high specific capacity (∼200 mAh g−1).12,13 It is generally accepted that NMC is thermodynamically stable at a high charging state due to the favorable electronic configuration of the Mn4+ (3d) band in relation to the energy of the O2p band to allow complete oxidation of the Ni2+/Ni4+ couple. In this paper, we study the variation of the electronic structure of
2. EXPERIMENTAL SECTION The deposition of the LNCMO thin film, in situ electron spectroscopy, and electrochemical experiments were carried out at the SoliAS experimental station for solid liquid interface analysis at the synchrotron source BESSY-II (Berlin) [for a detailed description of the setup, see ref 17]. The UHV thin film deposition chamber was transported to BESSY-II from Darmstadt and connected directly to SoLiAS. The base pressure, pbase, in the ultra-high vacuum (UHV) deposition chamber was below 5 × 10−8 mbar. The LNCMO thin film was deposited on Ti foil (99.95%, Alfa Aesar) at room temperature by using radio frequency (RF) co-sputtering.6 Two-inch commercial LiNiCoMnO2 (99.99% Kurt J. Lesker Company Ltd.) and Li2O (99.9%, FHR Anlagenbau GmbH) targets were used for this aim. The thickness of the LNCMO thin films was around d ∼ 100−200 nm. The LMNCO films were postdeposition annealed at Tann = 600 °C to obtain the crystallized R3m ̅ layered structure. Due to this treatment, the stoichiometric composition of the TM ions is changed from their original low temperature values. More details of the LNCMO thin film parameter deposition will be reported in a future publication. High purity LNCMO films were transferred under UHV conditions to the analysis chamber (p < 10−9 mbar). SXPS, O K-edge and TM L-edges XANES quasi-in-situ experiments have been carried out at the undulator beamline U49/2 PGM-2 with a plane grating monochromator. The beamline is equipped with the integrated UHV system.17 The spectrometer is equipped with a SPECS PHOIBOS 150 MCD9 electron analyzer with an accessible energy range of hν = 86− 1890 eV. The binding energies of the spectra have been referenced to the Fermi level of a clean Ag-polycrystalline foil with the energy resolution better than 300 meV for photon energies below 400 eV and around 600 meV for photon energy B
DOI: 10.1021/cm5047534 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
with acetonitrile in order to remove remains of the electrolyte followed by a drying of the sample under Ar, the sample has been in situ retransferred into the UHV system using the transfer chamber for the SXPS and XANES analysis. The cathode materials were charged from 3.7 to 4.8 V at room temperature.
of 1000 eV. The inelastic mean free path (IMFP) of electrons, λ, vs their kinetic energy, Ekin, was estimated from the equation describing the universal curve for inorganic compounds taking into account the average atomic number.18 The estimation of λ by using the Tanuma, Powell, and Penn (TPP − 2M) formula [ref 19] leads to similar values as given by the universal curve. The photoelectron and XANES spectra were collected perpendicularly to the surface. The photon energies for the SXPS experiments were selected in such a manner to collect the photoelectrons from the same depth. In this connection, two types of the experiments, namely, surface sensitive and bulk sensitive ones, were separately carried out. To achieve a maximal surface sensitivity, the photoelectrons of all analyzed photoemissions were collected with the same kinetic energy providing λ ∼ 6 Å, whereas a maximal yield sensitivity was achieved by measuring the photoelectrons with the kinetic energy corresponding to λ ∼ 20 Å. Taking into account the sampling (information) depth dXPS = 3λ, the maximal analyzed thickness was
