Article pubs.acs.org/JPCC
Correlating Local Structure with Electrochemical Activity in Li2MnO3 Rose E. Ruther,*,† Hemant Dixit,† Alan M. Pezeshki,†,‡ Robert L. Sacci,† Valentino R. Cooper,† Jagjit Nanda,*,†,‡ and Gabriel M. Veith† †
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States
‡
S Supporting Information *
ABSTRACT: Li2MnO3 is believed to be a critical component of the high capacity Li-rich−manganese-rich oxide materials; however, the mechanism of its electrochemical activity remains controversial. Here, Raman spectroscopy and mapping are used to follow the chemical and structural changes that occur in Li2MnO3 during electrochemical cycling. Conventional composite electrodes cast from a slurry and thin films are studied as a function of the state of charge (voltage) and cycle number. Thin films have similar electrochemical properties as electrodes prepared from slurries but enable spectroscopy of uniform samples without carbon additives and binder. First-principles density functional theory is used to calculate the phonon spectra and identify the Raman-active modes. On the basis of the calculations of phonon spectra for pristine Li2MnO3 and structures with Li vacancies, we discuss the origin of Raman-active peaks observed during the electrochemical cycling. The spectral changes correlate well with the electrochemical behavior and support a mechanism whereby capacity is lost upon extended cycling due to the formation of new manganese oxide phases.
■
INTRODUCTION Li2MnO3-stabilized LiMO2 (M = Mn, Ni, Co) is being developed for high voltage, high energy density cathodes for next-generation lithium ion batteries.1 These lithium- and manganese-rich oxides have a chemical formula xLi2MnO3·(1 − x)LiMO2 and are considered either a solid solution2,3 or a composite structure with nanoscale domains of Li2MnO3 and LiMO2 character.4,5 While lithium-rich cathodes can deliver capacities >200 mAh/g, significant capacity and voltage fade occur during electrochemical cycling.6,7 Since both the initial high energy and long-term energy fade are correlated with the Li2MnO3 component,8 understanding the electrochemical behavior of the pure phase Li2MnO3 may provide insights into the promising class of lithium-rich oxides. Interestingly, Li2MnO3 was long considered to be electrochemically inert because all the manganese ions are tetravalent and cannot be oxidized further.9 Subsequent studies showed that Li2MnO3 can cycle significant amounts of lithium (>200 mAh/g), especially in nanocrystalline samples or at elevated temperature.10−14 The mechanism of electrochemical activity, though, remains highly controversial. Oxidation of Mn4+ to Mn5+ was initially proposed10 but was subsequently ruled out from X-ray photoelectron and X-ray absorption spectroscopies.15−17 Oxygen may also participate in the redox chemistry, resulting in evolution of O2 gas and oxygen vacancy formation.14,17−21 A third mechanism involves oxidation of the electrolyte during charge at high voltages (>4.5 V vs Li/Li+). Electrolyte decomposition is thought to be accompanied by © XXXX American Chemical Society
proton/lithium exchange and a change in the stacking sequence of the oxygen layers from O3 to P3 due to hydrogen bonding.15,17,19,22 The transformations that occur in Li2MnO3 during discharge and after long-term electrochemical cycling are also debated in the literature. It is unclear if manganese is reduced during discharge12,14,23 or remains electrochemically inactive.17 Several groups have proposed that the structure changes from layered to spinel with cycling,12,14,22,24,25 while others conclude that the structure remains very similar to the pristine material.11,17 Vibrational spectroscopies such as infrared and Raman are very sensitive to changes in structure and bonding and are therefore powerful tools to follow electrochemical reactions in battery materials.26 Raman spectroscopy in particular has been applied to Li2MnO3 and related lithium-rich oxides previously.12,13,25,27−31 This study builds on previous work by exploring in detail the changes that occur both in the initial electrochemical cycles (activation process) and long-term cycling of Li2MnO3. Conventional electrodes cast from a slurry of micron-sized particles and thin film electrodes are compared. The thin films provide a platform to understand the fundamental properties of Li2MnO3 without the complications that may arise from inactive electrode materials such as polymer binders and conductive carbon additives. This is especially Received: April 23, 2015 Revised: July 13, 2015
A
DOI: 10.1021/acs.jpcc.5b03900 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
a solid-state 532 nm excitation laser, a 20× objective, and a 600 grooves/mm grating. The laser spot size is approximately 1 μm. The 50 μm × 50 μm maps were acquired in 1 μm × 1 μm steps. Acquisition times for each spectrum were typically 15 s. Spectral deconvolution of the Raman bands was performed using Lorentzian line profiles after a linear baseline subtraction. Calculation of Raman-Active Modes. All calculations are based on density functional theory (DFT) with the Perdew− Burke−Ernzerhof (PBE)32 generalized gradient approximation (GGA), employing the projector-augmented-plane-wave (PAW)33,34 method, as implemented in the Vienna Ab Initio Simulation Package (VASP 5.2).35,36 The PAW potentials used explicitly treated 3 valence electrons for Li (2s22p1), 13 for Mn (3p63d54s2), and 6 for oxygen (2s22p4). A cutoff energy of 520 eV was used to terminate the plane-wave expansion. We considered a 24-atom unit cell to calculate the phonon frequencies at the Γ point. Structural optimizations were achieved by allowing the atoms in the unit cell to relax until all the forces on each atomic site were below 2 meV/Å while simultaneously achieving a total energy convergence of 10−6 eV. This convergence was obtained with a 4 × 2 × 4 Monkhorst− Pack special k-point grid. The vibrational frequencies and modes of the considered crystals were calculated using the “frozen phonon” method as implemented in the VASP code. In the calculation of phonon dispersions, a 4 × 2 × 4 grid was used to integrate over the vibrational Brillouin zone. The results presented in this paper were tested (and found robust) against finer k-point grids.
important to understand Li2MnO3, the performance of which can change significantly depending on electrode processing and the formation of good electrical contacts.23 The experimental results are complemented by computational efforts to assign the Raman-active modes in Li2MnO3 (with and without lithium vacancies) for the first time using density functional theory (DFT).
■
EXPERIMENTAL SECTION Sample Preparation. Li2MnO3 powder was used to prepare the slurry electrodes and as a target material for sputtering thin films. The Li 2 MnO 3 powder was synthesized from a stoichiometric mixture of Li2CO3 (Mallinckrodt, analytical grade) and MnCO3 (Sigma-Aldrich, 99.9%) ball milled in isopropyl alcohol for 24 h. The powder was dried and annealed in air at 550 °C for 12 h. The material used for the composite electrodes was reground and annealed in a crucible at 850 °C in air for 6 h and then slowly cooled to room temperature, resulting in a red powder that was then reground before electrode fabrication. Slurry electrodes were prepared from 80% Li2MnO3 powder with 3% carbon black (C65 - Timcal), 11% graphite (KS-6 - Timcal), and 6.0% PVdF dispersed in Nmethyl-2-pyrrolidinone (NMP, Sigma-Aldrich). Electrodes were cast on Al foil with a 2 mil doctor blade and dried. Cathode disks (1/2 in. diameter) with 0.6−2.3 mg of active material were stored in an argon-filled glovebox prior to assembly in coin cells consisting of a Li foil (0.75 mm, Alfa Aesar, 99.9%) counter electrode, 1.2 M LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC) electrolyte, and Dreamweaver Gold separator (Dreamweaver International Au40). Slurry electrodes were cycled at room temperature on a Maccor battery tester at a constant current of 10 μA (8 μA/ cm2) between 2.5 and 4.8 V. Thin film electrodes were prepared using a homemade sputter target. After annealing at 550 °C, the Li2MnO3 powder was fabricated into a 2 in. diameter 1/8 in. thick pellet and annealed at 850 °C for 12 h to form a sputter target. The target was sputtered using a radiofrequency source at an applied power of 80 W using an Ar plasma (Air Liquide, 99.9995%) at a pressure of 30 mTorr. The films were deposited on Pt-coated Al2O3 substrates (10 mm diameter, Valley Design) and annealed at 850 °C for 2 h in air to obtain the final films. The composition of the films was determined using a Thermo Jarrell Ash IRIS inductively coupled plasma (ICP) optical emission spectrometer (OES). The electrode was dissolved in freshly prepared aqua regia (3:1 mixture of hydrochloric acid and nitric acid) for analysis and diluted with a 5% HNO3 solution prepared in deionized water (18.2 MΩ). The resulting solution was diluted to obtain a solution in the linear sensitivity range of the ICP. ICP standards were prepared by the serial dilution of Li and Mn standards purchased from Alfa-Aesar. Swagelok type cells were constructed using the thin film as the working electrode, a Li foil (0.75 mm, Alfa Aesar, 99.9%) counter electrode, 1.2 M LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC) electrolyte, and Dreamweaver Gold separator (Dreamweaver International - Au40). Thin film electrodes were cycled at room temperature on a Maccor battery tester at a constant current of 4 μA (5 μA/cm2) between 2.5 and 4.8 V. X-ray Diffraction (XRD). Powder X-ray diffraction studies were performed using a Scintag Pad IV diffractometer utilizing a Cu Kα source. Raman Spectroscopy. Raman spectra were acquired with an Alpha 300 confocal Raman microscope (WITec, GmbH) using
■
RESULTS AND DISCUSSION Figure 1 shows the XRD patterns and Raman spectra for pristine (uncycled) Li2MnO3 powder and thin films. Both the XRD and Raman results are in excellent agreement with the literature and confirm the formation of the desired monoclinic phase.37,38 The XRD pattern of the Li2MnO3 thin film (Figures 1a) has several additional strong reflections from the platinum
Figure 1. (a) XRD patterns of Li2MnO3 powder and thin film. Additional reflections due to the thin film platinum current collector and alumina substrate are indicated. (b) Enlarged view of the low angle reflections shown in panel a. (c) Experimental Raman spectra of Li2MnO3 powder and thin film with symmetries assigned using DFT calculations. B
DOI: 10.1021/acs.jpcc.5b03900 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C current collector and alumina substrate. Interestingly, the thin film material exhibits a higher degree of Li−Mn ordering in the transition metal layer than the powder material as evidenced by the better defined superstructure reflections between 20° and 30° 2θ (Figure 1b).39 ICP results further confirm the stoichiometry of Li2MnO3 films (Li2.02±0.01Mn1.00±0.01O3). Li2MnO3 has monoclinic symmetry space group C2/m and its point group symmetry is C2h. The irreducible representations of the phonons at the Brillouin zone center (Γ point) are
Table 1. Frequencies of Raman-Active Modes
Γ = 7A g + 8A u + 8Bg + 13Bu
symmetry
frequency (cm−1) theory
frequency (cm−1) experiment
Ag
621.41 593.12 577.03 533.73 448.43 414.44 260.01 498.39 496.74 425.98 376.98 338.17 315.24 309.61 221.75
621
Bg
where 7Ag and 8Bg optical modes are Raman-active and 7Au and 11Bu optical modes are infrared-active as predicted from group theory.40 The Ag modes result from symmetric vibrations of cations along the b-axis, whereas Bg modes result from symmetric vibrations of cations simultaneously along the a- and c-axis. Furthermore, out of these 15 Raman-active modes, 3 modes (1 Ag and 2 Bg) are due to vibrations of Mn atoms at the 4g site, 3 modes (1 Ag and 2 Bg) are due to vibrations of Li atoms at the 4h site, 3 modes (2 Ag and 1 Bg) are due to vibrations of oxygen atoms at the 4i site, and the remaining 6 modes (3 Ag and 3 Bg) can be assigned to oxygen atoms at the 8j site in terms of Wyckoff positions. It should be noted that the Li atoms in the Li layer (2b and 2c sites) do not contribute to any Raman-active modes. The cation movements are typically out of phase with the anion motions as illustrated in Figure 2. Thus, the Raman-active modes are linear combinations of parallel and perpendicular movements of cations and anions along the crystal axes.
574 442 252 502 422 377 330
Figure 3. (a) Charge and discharge capacity as a function of cycle number for thin film and slurry electrodes. (b) Charge and discharge profiles for slurry electrodes. (c) Charge and discharge profiles for thin film electrodes.
capacity, followed by significant fade (Figure 3a). The maximum in capacity occurs around 10−20 cycles. Such an initial capacity rise has been observed before for some Li2MnO3 samples12,21 and suggests that the activation process and related changes in the electrode structure can occur over several cycles. The charge and discharge capacities for the thin film samples are significantly different, primarily due to the oxidation of electrolyte on the platinum current collector at high voltage. Both thin films and slurries exhibit similar voltage profiles (Figures 3b and 3c), confirming that the thin film studies are relevant to understanding more practical slurry electrodes. More capacity is extracted from the thin films, which have a uniform thickness of less than 1 μm. The thin film geometry enables short diffusion distances for lithium transport and good electrical contact with the current collector. The primary particle size in the slurry electrodes ranges from
