Article pubs.acs.org/cm
Cite This: Chem. Mater. 2018, 30, 4694−4703
Elucidation of LixNi0.8Co0.15Al0.05O2 Redox Chemistry by Operando Raman Spectroscopy Eibar Flores,† Nathalie Vonrüti,‡ Petr Novaḱ ,† Ulrich Aschauer,‡ and Erik J. Berg*,†,§ †
Electrochemistry Laboratory, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Department of Chemistry and Biochemistry, University of Bern, CH-3012 Bern, Switzerland
‡
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S Supporting Information *
ABSTRACT: The local structure evolution of LixNi0.8Co0.15Al0.05O2 (NCA) is linked to its electrochemical response during cycling (and overcharge) by operando Raman spectroscopy with findings supported by complementary techniques, such as online electrochemical mass spectrometry (OEMS) and density functional theory (DFT) phonon calculations. The vibrational motion of lattice oxygens is observed to be highly dependent on the local LixMO2 lattice environment, e.g. MO bonding strength/length and state of lithiation x. All vibrational modes generally harden upon delithiation due to MO bond character (ionic → covalent) evolution (disregarding an early bond softening due to Li+ vacancy formation) and evidence the important influence of the local structural lattice configuration on the electrochemical response of NCA. Although the intensities of all Raman active bands generally increase upon delithiation, a major inflection point at x = 0.2 marks the onset of a partly irreversible fundamental transition within NCA that is most likely related to electron removal from MO bonding states and partial oxidation of oxygen sublattice, which is also indicated by the observed concomitant O2 release from the particle surface. Operando Raman spectroscopy with higher time resolution provides unique possibilities for detailed studies of how chemical parameters (Li+ vacancy formation, transition metal cation concentration, and lattice doping, etc.) may govern the onset and nature of processes (such as bond character evolution and stability) that define the performance of the LixMO2 class of oxides. The further insights thus gained can be exploited to guide the development of next-generation layered cathodes for Li-ion batteries operating stably at higher voltages and capacities.
1. INTRODUCTION
NCA has a rombohedral layered crystal structure belonging to the R3̅ m space group and consists of octahedrally coordinated cations forming alternating LiO6 and (Ni/Co/ Al)O6 slabs. Nickel, cobalt, and aluminum are initially in a trivalent state and homogeneously occupy the 3b Wyckoff sites, while Li+ occupy the 3a sites. The lattice parameters of NCA evolve on charge, in which the delithiation follows a solid solution type process. Ni(III)/Ni(IV) was found to be the predominantly active redox couple providing a practical specific charge up to ∼215 mAh/g (C/10 rate) when the electrode potential is cut at ∼4.3 V vs Li+/Li.7,8 Operation at higher cell voltages could unlock NCA’s full theoretical specific charge (279 mAh/g) and significantly improve the energy density of the Li-ion battery.9 However, the propensity for structural transformations in the layered oxides increases dramatically at higher potentials leading to lattice instability and poor Li+ de/intercalation reversibility, thus compromising both the safety and cycle life of the Li-ion cell.10 These structural transformations typically nucleate at the surface or in the near-surface region of the oxide and are
The energy content, power output capability, and cycle life of Li-ion batteries (LIBs) are strongly influenced by the choice of the positive electrode.1 The family of LiMO2 (M = Co, Ni, Mn) layered oxides represent today the highest performing positive electrode materials for LIBs.2 LiCoO2 is the flagship member of the family and its implementation led to extensive commercialization of LIBs.2 LiNiO2 could provide both cheaper and higher energy density Li-ion cells compared to LiCoO2, but has been discarded largely due to challenging synthesis conditions and rapid performance loss. Gliding of the oxygen slabs and the presence of divalent nickel in lithium sites leads to structural phase transformations and chemical instabilities. Cobalt substitution in LiNiO2 is well-known to improve the structural stability and electrochemical reversibility, but at the expense of reduced energy density.3,4 Further substitution of aluminum as a nonredox active dopant has been shown to increase the power performance of LiMO2 based electrodes by reducing the cell impedance growth.5 An optimized composition, LiNi0.8Co0.15Al0.05O2 (NCA) was commercialized by Panasonic and SAFT and is currently one of the most frequently used cathode materials in Li-ion cells for electric vehicles (e.g., Tesla).6 © 2018 American Chemical Society
Received: April 4, 2018 Revised: June 25, 2018 Published: June 25, 2018 4694
DOI: 10.1021/acs.chemmater.8b01384 Chem. Mater. 2018, 30, 4694−4703
Article
Chemistry of Materials
dynamic vacuum at 80 °C overnight, and finally introduced into an argon filled glovebox without air exposure. Raman Spectro-Electrochemical Cell Assembly. The operando Raman measurements were performed using a custom-made Raman spectro-electrochemical cell (Figure S1). The cell was assembled in a coin-cell configuration inside an argon-filled glovebox (O2, H2O < 1 ppm). Before assembly, the NCA composite electrode and the Celgard 2400 separator (Ø17 mm) were wetted for several minutes in LC30 electrolyte (1.0 M LiClO4 in 1:1 (w/w) ethylene carbonate (EC)/dimethyl carbonate (DMC)). The NCA working electrode was pressed against an aluminum mesh (Ø17 mm, 5 Al 7-125, Dexmet corporation, CT, US) for electric contact. Lithium metal disks (0.2 mm thick, Ø12 mm, Sigma-Aldrich, Germany) were used as counter electrode. The cell was cycled galvanostatically at room temperature with a computer-controlled galvanostat (CCCC Hardware, Astrol Electronic, Switzerland) at an applied current of ∼10 mA/gNCA normalized to the weight of the NCA active material in the electrode. Raman Spectroscopy. The Raman spectra were acquired using a Labram HR800 Raman microscope (Horiba-Jobin Yvon) with a He− Ne excitation laser (632.8 nm). A grating was used as dispersion element with a groove density of 600 g/mm that attains a 2 cm−1 spectral resolution. The hole and slit of the confocal system were fixed at 1000 and 100 μm, respectively. The laser was focused on the sample using a 50× (numerical aperture 0.55) objective, which produced a laser spot of ca. 4 μm diameter with an estimated sampling depth of around 2 μm.17 However, opaque samples such as LixMO2 oxides strongly damp the laser intensity profile in accordance with the dielectric properties of the sample, thus the sampling depth is theoretically estimated to be around 160 nm in our case.18 The nominal laser power was filtered down to 2 mW to avoid sample overheating. The probed sample spot was continuously focused during the experiment using an autofocus function. Every spectrum recorded resulted from the average of 6 acquisitions of 110 s each (a spectrum every 11 min). Data Analysis. The data treatment was a two-step process as follows: first a custom-made Matlab graphical user interface (GUI) was used to visually evaluate several sample spectra in search of optimum fitting parameters such as the baseline polynomial degree, number of peaks, and the initial guesses and boundaries of peak position and width of the Lorentz-type deconvolution profiles. Second, the optimum fitting parameters were input in a script to treat and fit each spectrum consistently in order to retrieve band position, intensity, and width of all Lorentz-type peaks. The script also retrieved the corresponding electrochemical parameters from the cycling data file. These parameters were compared to the state of lithiation x, which is defined as the fraction of the specific charge at a given time relative to the theoretical specific charge of 279 mAh/g:
believed to be enhanced by Li/Ni cation intermixing and anionic oxygen redox reactions, which leads to the formation of reactive oxygen species and O2 release.11−13 This surface reconstruction process triggers, in turn, electrolyte decomposition, whereby the side-products deposit as thick surface layers that obstruct lithium diffusion and irreversibly trap Li.8,14 Structural transformations of these layered oxides and the associated electrolyte decomposition are two major causes of performance fade of high-energy Li-ion batteries today.15 Further progress in the field is, however, hampered by the lack of fundamental understanding of the redox chemistry of LiMO2 oxides and how it relates to their structural stability. Ultimately, a complete description of the spatial and electronic structure is desired in order to identify degradation mechanisms and establish directions of improvement. Traditional ex situ characterization techniques in the field are already approaching their limitations and efforts are increasingly focused on advanced operando techniques investigating battery materials under operation. However, characterizing surface structures in their working environment is a challenge considering the complex nature of electrochemical solid/ electrolyte interfaces and the dynamic character of the cycling process. Raman spectroscopy represents a versatile, nondestructive tool to characterize the local cation environment in the nearsurface region of electrode materials during cycling. Measurements can be done operando due to the optical nature of the analytical probe and the fast recording times allow for timeresolved experiments during cell operation. Several studies of layered positive electrodes have already revealed a clear influence of dis-/charge of the LiMO2 oxides on the Raman spectra, such as changes in the positions (in wavenumbers), widths, and intensities of the vibrational bands depending on lithium content, but the interpretations are circumstantial at best, and often contradictory.16 Ideally, the interpretation of the operando Raman features of layered LiMO2 relies on (i) high spectral quality to identify and assign the bands, (ii) high time resolution for assessing the rate of spectral changes, (iii) complementary supporting experimental techniques, and (iv) a theoretical framework linking the lattice dynamics of LiMO2 with its reaction mechanism. The aim of the present study is to elucidate the links between the electrochemical response of NCA and its nearsurface structural changes. For this purpose a dedicated Raman spectro-electrochemical cell and a robust data analysis automated procedure are developed and foreseen to offer improved spectral quality, higher time resolution, and facilitated spectral deconvolution. Interpretation of the data is supported by complementary operando techniques, such as online electrochemical mass spectrometry (OEMS), and DFT calculations to rationalize the observed relationship between the electrochemistry and the spectral response of NCA.
x(in LixNi 0.8Co0.15Al 0.05O2 ) =
It /279 mactive
where I (mA) is the galvanostatic current, t (h) is the elapsed cycling time, and mactive (g) is the mass of active material in the composite electrode. Online Electrochemical Mass Spectrometry. The OEMS setup operated with a quadrupole mass spectrometer (QMS 200, Pfeiffer) for partial pressure measurements, a pressure transducer (PAA-33X, Keller Druck AG) for total cell pressure, temperature, and internal volume determination, stainless steel gas pipes and Swagelok fittings (3-mm compression tube fittings, Swagelok) to connect the OEMS cell, a set of solenoid valves (2-way magnetic valve, Series 99, silver-plated nickel seal, Parker), and a scroll pump (nXDS15i, EDWARDS GmbH) for efficient flushing. The magnetic valves were electronically controlled with a solid state relay module (NI 9485 measurement System, National Instruments) connected to a computer with a LabView Software (NI Labview 2013, National Instruments). For partial pressure and gas evolution rate analysis 1.3 mL of gas was extracted from the headspace (∼4 mL) of the cell and replaced by pure Ar (quality 5.0). Calibration gas bottles were utilized to relate the MS ion-current signals at m/z = 32 and 44 to known
2. EXPERIMENTAL METHODS Electrode Preparation. The NCA composite electrodes were prepared from mixed slurries of 89 wt % LiNi0.8Co0.15Al0.05O2 (NCA, stored in an Ar-filled glovebox, received from Leclanché SA), 5 wt % polyvinylidene difluoride (PVdF Kynar HSV 9000, Arkema), 4.6 wt % amorphous carbon Super C65, and 1.4 wt % graphite SFG6 (Imerys Graphite and Carbons) dispersed in n-methyl pyrrolidone solvent (NMP, Sigma-Aldrich). The slurries were coated onto Celgard 2400 (Celgard LLC) sheets by doctor blading at a 100-μm wet thickness. The coated sheets were dried for 10 h under dynamic vacuum at 80 °C, punched to 14-mm diameter electrodes, further dried under 4695
DOI: 10.1021/acs.chemmater.8b01384 Chem. Mater. 2018, 30, 4694−4703
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Figure 1. (a) Galvanostatic dis-/charge potential profile and the corresponding −dx/|dE| of NCA during two cycles (3.0−4.3 V vs Li+/Li) and a third overcharge cycle (3.0−4.8 V vs Li+/Li) along with the operando CO2 and O2 evolution analysis (bottom, enlarged plot in Figure S2). (b) Raman spectra of a single NCA particle at OCP, 3.87 V, and 4.30 V during cycling, together with the Raman active modes (orange bars and asterisks) and atomic displacements (insets) expected from DFT phonon calculations. All spectra have the same intensity scale (in a.u.). concentrations of O2 and CO2 (1000 ppm in Ar), respectively, before and after the measurement. Computational Methods. Our DFT calculations were performed with the VASP code19−22 using the Perdew−Burke−Ernzerhof (PBE) exchange-correlation functional23 in conjunction with a DFT+U24 correction of 4 eV applied to the Ni and Co 3d states. Wave functions were expanded in planewaves up to a kinetic energy of 500 eV while using PAW potentials25,26 with Li(1s, 2s), Ni(3p, 3d, 4s), Co(3p, 3d, 4s), and O(2s, 2p) valence states. The reciprocal space was sampled using Γ-centered meshes with dimensions 6 × 6 × 6 for the rhombohedral 8-atom R3̅m cell and 10 × 10 × 2 for the Jahn−Teller distorted monoclinic 8-atom C/2m cell. Phonons were computed using the finite displacement approach as implemented in phonopy27 and Raman intensities determined in a procedure similar to ref 28 from mode-dependent changes of the dielectric constant evaluated within density-functional perturbation theory, while applying a peak broadening of 10 cm−1.
any noticeable increase of overpotentials. In the third overcharge cycle (4.8 V cutoff potential), the overpotential in the subsequent discharge (∼100 mV) and irreversible charge loss (∼4%), however, increase markedly. Overcharging of layered oxides (>4.3 V vs Li+/Li) is well-known to be detrimental for their performance and has been attributed to several degradation processes: (i) enhanced electrolyte oxidation and surface-deposition of resistive electrolyte byproducts,35 (ii) structural defects such as cation mixing that impedes Li+ diffusion,36 (iii) microstructural changes such as cracking and electrical isolation of active particles,37 and (iv) promoted surface reconstruction leading to nonion-conducting and electronically insulating phases.38 Important to note is that each degradation process could produce specific Raman spectroscopic signatures during cycling. The differential capacity (E vs −dx/|dE|) plots show five distinct pairs of peaks centered at 3.54, 3.72, 3.98, 4.18, and 4.60 V vs Li+/Li that agree with the redox peaks observed by cyclic voltammetry (CV).31,39 Redox processes up to 4.3 V vs Li+/Li derive mainly from Ni(III) ↔ Ni(IV) oxidation/reduction and the individual current peaks have been associated with intermediate transitions in the electronic and/or lattice structure (such as local Li+ ordering) and their effect on the rate of Li+ diffusion in NCA during cycling.3,40,41 All Ni cations are expected to be in a tetravalent oxidation state when about 0.8 Li are extracted per NCA formula (i.e., x = 0.2, Figure 1a), such that the redox activity >4.3 V vs Li+/Li (x < 0.2) would have to predominantly involve a different charge compensation mechanism, possibly Co(III) ↔ Co(IV) and/or anionic lattice oxygen redox. Indeed, oxygen oxidation is evidenced by the release of O2 positive to 4.2 V (Figure 1a bottom). In fact, NCA follows the same trend as previously reported for Ni-rich NCMs: after all Ni cations presumably reach their tetravalent oxidation state, further Li+ removal (x < 0.2 for NCA) at least partly involves oxygen oxidation and release.42 This overcharge process is believed to promote surface reconstruction and the formation of oxygen-deficient rock-salt and spinel-type phases (MO2 → MO + O or 1/3M3O4 + 2/3O). Most of the released oxygen appears to immediately react with the electrolyte
3. RESULTS AND DISCUSSION 3.1. Electrochemistry of NCA. Figure 1a shows the cell voltage profiles and the corresponding −dx/|dE|, as well as the O2/CO2 gas evolution of a NCA half-cell during the 3 first cycles. A specific charge of 237 mAh/g is obtained in the first charge (10 mA/g, 4.3 V vs Li+/Li cutoff potential) of which about 93% is recovered during discharge. The initial overpotential on the first charge (area in gray) is commonly observed for Ni-rich Li-ion battery cathodes and proposedly caused by a nm-thin surface layer on NCA active material containing Li2-/NiCO3 and/or other inorganic species (such as LiOH) that are residual from the synthesis and/or formed due to air exposure of the powder during storage/handling.29,30 The Li2-/NiCO3 layer degrades in the oxidizing environment as the electrode potential increases; thus, the kinetic barrier for Li-deinsertion is reduced and the overpotential disappears as the charge proceeds.31 Apart from the irreversible charge associated with the Li2-/NiCO3 layer dissolution, processes such as conductive carbon retreat,32 particle cracking,33 electrolyte decomposition,34 and surface reconstruction14 would all add to the initial capacity loss (16 mAh/g). The Coulombic efficiency is significantly higher in the second cycle; a specific discharge capacity of 221 mAh/g is obtained without 4696
DOI: 10.1021/acs.chemmater.8b01384 Chem. Mater. 2018, 30, 4694−4703
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Figure 2. −dx/d|E| profiles (top) along with the fitted band positions P, derivative curves −dP/dx, and band intensities of the Eg and A1g Raman bands of NCA as a function of x during the 1st and 3rd (overcharge) cycle, cf. Figure 1. The intensities are normalized to the highest measured during a cycle. The inset shows the step increase of peak intensity during the initial stages of the first charge.
carbonate solvents to form CO2 (120 μmol/gNCA), while only a minor fraction forms molecular O2 (2.4 μmol/gNCA), judging from the clear correlation between the evolution rates of the two gases. Direct electrochemical oxidation of the electrolyte has previously been shown to be comparatively negligible in the same potential range.42 In summary, the electrochemical de-/lithiation of NCA displays several electrochemical features in the form of dx/dE peaks (centered at 3.54, 3.72, 3.98, 4.18, and 4.60 V vs Li+/Li) of which the last two at least partly involve lattice oxygen release, and irreversible structural changes with increased dis-/charge overpotentials as a result. 3.2. Raman Spectra of NCA. Operando Raman spectroscopy was performed in order to further understand the basis of the electrochemical response and its relation to the structural evolution of NCA during cycling. A group factor analysis predicts that the layered LiMO2 family of oxides with the R3̅m space group allows two Raman active modes involving oxygen motions only: the doubly degenerate Eg mode where oxygens in adjacent parallel layers move in opposite directions in a O
MO shear-like fashion and the A1g mode where oxygens in MO stretch parallel to the c axis (inset in Figure 1b).43 For LiCoO2, polarized Raman spectra have assigned the Eg mode to a band at 486 cm−1 and the A1g mode to a band at 595 cm−1.44,45 Therefore, the lower and higher wavenumber bands generally found for the mixed transition metal Li(Ni/Co/Mn/ Al)O2 series of compounds have been similarly assigned.43,44 Figure 1b shows the Raman spectra acquired from a single NCA particle at selected stages of cycling (x = 1.0, 0.50, and 0.15). The spectra were consistent and reproducible when comparing several particles in the composite electrode probed at the same x. The two main bands in the spectra are most suitably deconvoluted into two Lorentzian peaks, which again are tentatively assigned to the Eg mode at 488 cm−1 and the A1g mode at 556 cm−1. Compared to the spectra of the Li(Ni/ Co)O2 series, the inclusion of Al3+ does not appear to add any noticeable spectral features.43 Previous attempts to deconvolute the Raman spectra of the layered mixed transition metal oxides have employed multiple peaks for the fitting, e.g. 4697
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observed by X-ray diffraction.31 However, a local Li+ ordering could still be thermodynamically favorable and observable by Raman spectroscopy even though the average lattice properties would not be sufficiently influenced to be observed by XRD. The appearance of a local Li+ ordering was previously proposed to explain the unexpected minimum of Li+ diffusivity observed at x = 0.5.50 Likewise, a suppressed Li+ ordering was proposed to explain the broadening of NCA’s CV features on Mg2+ doping.57 In conclusion, four bands are observed in the course of the cycling: a pair below 600 cm−1 assigned to the Eg/A1g modes of the R3̅m NCA structure, and additional pair of Raman-active modes above 600 cm−1 (x < 0.6) arising from vibrations of oxygen toward vacant Li sites and likely related to the redox current peak at 3.71 V vs Li+/Li. Raman spectroscopy thus provides unique possibilities for operando studies of local processes, such as Li+ vacancy formation, and how they may influence the performance of the LixMO2 class of oxides. 3.3. Raman Band Positions on Charge/Discharge. Figure 2 shows the differential capacity (−dx/|dE|) profile and the band positions P(Eg) and P(A1g) of the Eg and A1g bands during the first and the third charge/discharge cycle to 4.3 and 4.8 V, respectively. P responds immediately to changes in x as follows: P(Eg) follows a U-shape trend with a minimum close to x = 0.6, whereas P(A1g) increases monotonically to higher wavenumbers before reaching a maximum at x = 0.2, hereafter P(A1g) again decreases. From the general perspective of molecular mechanics, the band position (or vibration frequency) of a given vibrational mode i is proportional to the square root of the spring constant ki of the corresponding stretching or bending MO bond configuration according to 58 P ∼ k1/2 i . Therefore, P gauges the propensity of the vibrating atoms (oxygens in this case) to remain in their equilibrium position. Although frequency shifts are typically associated with varying lattice parameters,43 the average crystallographic length scales and symmetries are not necessarily representative of bond lengths and coordinations on the local scale relevant for MO6 vibrations. Indeed, the OMO angles and MO bond distances of the MO6 octahedra (as derived from the a and c lattice parameters obtained by operando XRD,39 Figure S5) display important inflection points close to x = 0.5, whereas neither P(Eg) nor P(A1g) show any dramatic changes in the same range (Figure 2). Conversely, P(Eg) rather reaches a minimum at x = 0.6 and P(A1g) goes through a maximum at x = 0.2, at which point no dramatic changes in the bond distances nor angles are observed by XRD (Figure S5). Because P is evidently uncorrelated to the average crystallographic parameters, we emphasize that the observed frequency shifts should rather reflect local changes in bonding strength and coordination within the MO6 unit upon cycling. Generally, our DFT calculations predict a hardening (blue shift) of both the A1g and Eg modes upon oxidation (when disregarding delithiation), which is expected from the increased MO bond hybridization (i.e., more covalency)59 when the electrons are removed from antibonding states of the MO6 unit (Figure S3, left).52 However, when a concomitant delithiation (x < 1) of LixMO2 is considered, a red-shift of P(Eg) by 18 cm−1 between 1.0 > x > 0.5 is predicted, which also roughly agrees with experimental observations (∼16 cm−1, Figure 2). P(A1g) also red-shifts, though to a much lesser extent (∼4 cm−1, Figure 2) compared to the Eg mode, thus agreeing with the very subtle shift predicted by our DFT calculations (∼2 cm−1, Figure S3, right). The Eg eigenvector
assuming each transition metal (Ni/Co/Mn) contributes a pair of unique Eg and A1g modes.46−48 However, neither visual inspection nor implementation of a multiple peak fitting procedure of our spectra suggest the existence of further bands, hence a 2-peak Lorentzian model is considered sufficient to capture the contributions of the two main Raman modes. The expected band positions were determined from DFTbased phonon calculations of the layered LiMO2 structures with M being either Ni or Co for computational feasibility. For R3̅m LiNiO2 the calculations result in Eg and A1g frequencies of 345 and 514 cm−1 respectively. However, the calculated electronic structure of LiNiO2 in the R3̅m space group is metallic49 and the band positions are, due to the inherently different bonding, expected to be less representative for NCA, which is a semiconductor.50 Semiconducting LiNiO2 can be obtained by permitting the Jahn−Teller distortion of the NiO6 octahedra,51,52 which however also lifts the degeneracy of the Eg modes resulting in three nondegenerate modes at 312, 372, and 565 cm−1 respectively, in the monoclinic C2/m structure, hence deviating more from the experimental findings. We therefore propose that R3̅m LiCoO2, which is a semiconductor without a Jahn−Teller distortion, provides a more realistic estimate for phonon frequencies in NCA. Indeed, for this structure we compute the Eg modes at 468 cm−1 and the A1g mode at 575 cm−1, which are illustrated in Figure 1b (orange bars, highlighted by asterisks) and in reasonably good agreement with the experimental band positions. Upon delithiation of NCA (x < 1.0, Figure 1b), both the position and the relative intensity of the Raman bands evolve with a general agreement between the experimental and calculated results. In the phonon calculations, an additional minor doubly degenerate mode appears at 567 cm−1 (Figure S3, right), but is hidden in the A1g peak due to about an order of magnitude smaller intensity, thus not observed in our experiment. Interestingly, when x = 0.5 (Figure 1b) both experiment and calculation show new bands above 600 cm−1. Two bands at 609 and 630 cm−1 are clearly observed experimentally; they grow on delithiation and reversibly disappear upon lithiation with no additional spectral features being found. To exclude the origin of these new bands as a result of phase transformations of NCA at low x, a separate investigation by heat-treating the delithiated NCA powder at 500 °C was pursued in order to accelerate the degradation process (Figure S4). Thermally induced disorder results, however, in broad Raman spectral bands dissimilar to any feature in the spectra of electrochemically delithiated NCA. Important to note is that the formation of oxygen-deficient surface layers,7,8 as anticipated from the O2 release described above, corresponds only to a couple of monolayers on the oxide surface42,53,54 and no additional bands are expected due to the micrometer-scale signal probing depth of the Raman microscope. In the DFT calculations, the eigenvector of the mode at 623 cm−1 (Figure S3) is similar to the A1g mode (Figure 1b, inset), but show stronger displacements toward Li+ vacant sites (VLi). The appearance of the Raman band above 600 cm−1 would thus be associated with formation of a Lideficient MO6 environment in NCA, which in turn could be tentatively correlated to the dx/dE peak at 3.71 V (x = 0.6, Figure 1a). In LixNiO2 (the parent compound to NCA) a strong redox activity at x = 0.6 arises from a R3̅m to C2/m lattice symmetry transformation driven by a collective Liordering.55,56 In NCA, Co and Al incorporation frustrates the long-range ordering56 such that no symmetry transformation is 4698
DOI: 10.1021/acs.chemmater.8b01384 Chem. Mater. 2018, 30, 4694−4703
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Chemistry of Materials (450 cm−1) in the x = 0.5 structure (Figure S3, right) displays much larger O displacements neighboring an empty Li layer compared to those atoms next to Li-filled layers, whereas the A1g eigenvector is hardly affected by the empty lithium layer. An anisotropic redistribution of electron density around M and O atoms several coordination shells away from VLi was previously proposed based on ab initio calculations,49 hence the Eg and A1g modes are expected to respond differently to delithiation. In conclusion, the experimentally observed Ushaped trend of P(Eg) (Figure 2) results from a crossover of two phenomena: an initial bond softening due to VLi formation (x > 0.7) and a later continuous bond hardening due to the oxidation of MO6. These findings demonstrate the unique and complementary utility of Raman spectroscopy to probe highly local changes in bonding strength and coordination rather than global average lattice changes normally determined by diffraction techniques. Beyond the general trends described above, the highly resolved P profiles also display minor plateaus that correlate clearly with the current peaks in the differential capacity (−dx/ |dE|). The link is more easily discerned when examining the first derivative of the band positions (−dP/dx, Figure 2, middle) with respect to x: maxima in the differential capacity −dx/|dE| are directly associated with minima in −dP/dx. Our interpretation of this detailed correlation between the electrochemical and Raman signal response is that the delithiation and consecutive emptying of the electronic bands in the electron partial density of states (PDOS) of NCA60 follow a stepwise oxidation process and an equally stepwise evolution of the bond character (ionic → covalent) within the MO6 unit. The electrochemical potential E is, after all, expected to be strongly related to the bond character and its bonding strength ki considering their mutual dependence on the free energy G of the LixMO2 lattice (E ∼ ∂G/∂x and ki ∼ ∂2G/∂δi2 for a given displacement δi in length or angle of the vibrating MO bond). More importantly, Raman spectroscopy demonstrates experimentally that oxidation (i.e., bond character evolution) and delithiation of NCA indeed leads to structural transitions analogous to the structural evolution of the lattice (hexagonal ↔ monoclinic) observed by XRD for LiNiO2.55 Although the coexistence of multiple MO lengths within the mixed Ni, Co, and Al transition metal oxides suppresses such global structural transitions (as detectable by XRD),3 local changes in MO bond length and coordination within the MO6 unit evidently affect the electrochemical response of NCA. Interestingly, P(A1g) decreases again at x < 0.2, thus possibly indicating a loss in MO bonding strength, which coincides with the evolution of O2 from the MO2 lattice at the surface of NCA (Figure 1a). Electron depletion from highly hybridized (covalent) MO states would both weaken the MO bond and in the absence of kinetic hindrance leave O prone to escape the lattice. X-ray absorption spectroscopy studies have in fact demonstrated a significant participation of anionic redox within these type of Ni-rich layered oxides via a charge compensation mechanism involving both bulk and surface oxygens, despite qualitative differences in MO coordination between two.40,61,62 Although the Raman spectroscopy approach presented herein mainly probes the near-surface but bulk-like region of NCA (∼160 nm deep) and O2 releases from the immediate surface (∼1 nm),18,42 this correlation is intriguing and motivates further studies. Unlike the first and second cycle, where both P(Eg) and P(A1g) generally overlap
on charge−discharge (Figure S6), charging the NCA up to 4.8 V vs Li+/Li in the third cycle induces a hysteresis in P, which further supports the presence of irreversible structural rearrangement processes upon overcharge. The position of the bands above 600 cm−1 (Figure S7), tentatively assigned to the formation of VLi, have trends difficult to identify and fit because of their low intensity. Both bands do however appear to reach an inflection close to x = 0.3: the band initially at 607 cm−1 reaches a minimum while the band at 623 cm−1 reaches a maximum, and both bands display highly reversible trends upon cycling, given the noise level. In summary, operando Raman spectroscopy provides evidence that local structural evolution within NCA is strongly correlated to its redox activity. Unlike most alternative operando structural characterization techniques,63 such local structural transitions are immediately observable and the onset of irreversible processes on overcharge can be more readily discerned. 3.4. Raman Band Intensity on Charge/Discharge. Figure 2 also displays the normalized band intensities I(Eg) and I(A1g) of the Eg and A1g bands, respectively, during the first (bottom left panel) and the third cycle (bottom right panel). On charge, both bands display a progressive increase in intensity until the maxima at x ∼ 0.2, hereafter the intensities again decrease. These band intensity maxima further confirm the presence of a fundamental structural transition of NCA at x ∼ 0.2, which, as discussed above, is related to a transition in the electrochemical charging process from predominantly Ni3+ → Ni4+ to most likely O2− → O−/O. Apart from the increase in I upon oxidation and the maxima at x ∼ 0.2, no further correlation to the electrochemical response was found, which shows that the band positions and band intensities have most probably different origins. The intensity of the bands above 600 cm−1 (Figure S9) mirror the trends of the Eg and A1g bands despite being more scattered. They show negligible intensity (within noise) for x > 0.6, but a progressive increase as further delithiation proceeds (0.6 < x < 0.2) and reach maxima close to x = 0.2. Changes in Raman band intensity of this type of layered oxides have been foremost claimed to originate from changes in the electronic conductivity.43,44,64−67 This argument is based on the so-called skin effect, that is, the penetration depth of the laser light shallows as the material becomes more conductive. For NCA, however, both the electronic conductivity50 and the Raman band intensity increase on delithitation. We therefore rather propose and discuss four distinct, but somewhat complementary, mechanisms by which the intensity of the Raman bands could be influenced by lithium content: Mechanism 1: MO Bond Character (Ionic → Covalent) Evolution. Increases in the MO interatomic electron population during the vibration has been related to a larger Raman scattering cross section of the corresponding mode and thus stronger Raman bands.18 Increasing I on oxidation (Figure 2) would simply be another manifestation of higher Ni−O bond covalency. Weaker Raman intensity x < 0.2 would accordingly reflect electron depletion from bonding MO states (with concomitant oxygen loss). However, increases in bond covalency are unlikely the sole origin since no correlation to dx/|dE| peaks was found (c.f. discussion on band positions above). Mechanism 2: Electronic Resonance Raman Effect. When the excitation (laser) frequency approaches the energy of an 4699
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could be triggered by an electrochemical process that occurs between 3.74 and 3.78 V vs Li+/Li. Second, although both I(A1g) and I(Eg) display the same trend and go through similar inflection points at x ∼ 0.2, their maxima are slightly offset by Δx ∼ 0.03 (Figure 2, bottom), hence indicating that the origin of the intensity changes of the Eg and A1g modes is somewhat different. Similar trends are observed during the third cycle (right panel), even though the Eg band displays an apparently erratic, but reproducible, behavior on overcharge (x < 0.2). All the trends show a negligible charge−discharge hysteresis in I except again the A1g band on the third cycle when NCA is charged beyond 4.3 V vs Li+/Li. Clearly, in addition to the O2 release observed above, Raman spectroscopy shows that the structural integrity of NCA is strongly affected by overcharge. In summary, the intensities of all Raman active bands of NCA generally increase upon oxidation, which largely can be explained by increasing covalency and electron polarizability along the MO bonds. Band intensities are likely modulated by further effects, such as electronic resonance, lattice disorder, and/or strain, as no immediate correlation to the electrochemical dx/dE peaks was found. However, the band intensities drop for x < 0.2 and mark the onset of a fundamental transition within NCA that is most likely related to electron removal from MO bonding states and overoxidation of oxygen sublattice (thus explaining the observed O2 release as discussed above). The overcharge process (x < 0.2) causes irreversible changes on the local lattice environment as evidenced by the charge−discharge hysteresis in the intensity trends in the third cycle.
electronic transition, the intensity of some Raman bands can be greatly enhanced. The degree of enhancement depends on how well the laser wavelength matches the band gap of the material.68 Electronic resonance effects have previously been measured on LiNiO2,69 LiCoO2,70 LiNi0.5Mn1.5O4,71 and in particular on spinel λ-MnO2 where the A1g band reaches maximum intensity close to 620 nm laser excitation wavelength (2.0 eV)72 which is close to the experimentally measured band gap (2.6 eV).73 I(Eg) and I(A1g) would thus probe the xdependent changes of the materials band gap relative to the laser excitation energy (here 632 nm ∼1.96 eV). An xdependent resonance-enhancement is, however, not evident as the band gap of Ni-rich layered oxides (∼1.85 eV) is reported to narrow (i.e., deviate more from the laser’s 1.96 eV) upon oxidation;74 hence, a general band-intensity weakening, not the observed growth, would be expected. Mechanism 3: Site Disorder Induced by Jahn−Teller (JT) Distortion. The JT effect breaks the symmetry of the degenerate ground state of low-spin NiIII (t2g)6(eg)1 by distorting the NiIIIO6 environment and modulating the Ni− O distances.75 The distortion itself and its propagation to neighboring sites should result in broader and weaker Raman bands. Once delithiation proceeds, Ni oxidizes to low-spin NiIV with (t2g)6(eg)0 configuration, which is JT-inactive, and the resulting symmetric NiIVO6 environment76 is anticipated to yield sharper and stronger bands. The observed growth of I on delithiation would thus reflect the rise of the NiIV/NiIII proportion. Note that the JT effect could be present in LiMO2 without any symmetry lowering from R3̅m to C2/m because the distortions could be highly localized.75,77 Mechanism 4: Lattice Strain. Raman band intensities of LCO have been observed to increase with applied external pressure, thus inducing higher polarizability of the MO bonds due to compression.78 Increases in I may be caused by the buildup of internal, anisotropic lattice strains upon oxidation.37,79 These mechanisms are not mutually exclusive. For instance, a particular electronic structure could at the same time modify a bond’s polarizability, favor JT distortion, permit electronic excitations, and change the electronic conductivity of NCA. Despite the possible influence from electronic excitations (mechanism 2), JT distortion (mechanism 3), and lattice strain (mechanism 4), changes in bond covalency (mechanism 1) are, according to our phonon calculations (Figure 1b), a significant (but not the sole) contributor to band-intensity growth upon delithiation. Although Raman band-intensity changes of NCA were previously reported in situ,80 our fit procedure of the spectra with a higher time resolution now reveals further details and two observations are noteworthy, as follows. First, during the first cycle a step increase in I at 3.74 V vs Li+/Li (x = 0.8) on charge and a step decrease in I at 3.58 V vs Li+/Li on discharge (inset on Figure 2, bottom) are observed, which could possibly be related to the dissolution/formation of a −CO3 surface layer (inducing the initial overpotential, as discussed above, Figure 1a).31,39,61 Such layers are typically nm-thin and Raman signatures of these carbonates are thus neither expected nor observed. Reformation of −CO3 layers has, however, previously been observed by XRD.39 We also examined I(Eg) and I(A1g) for NCA particles stored in air (Figure S10), where we expect thicker −CO3 layers,31 and a step increase at fairly close electrode potential (3.78 V) was equally observed, so we conclude that surface-layer breakdown
4. CONCLUSIONS Local structural features of LixNi0.8Co0.15Al0.05O2 (NCA) governing its electrochemical response are explored by operando Raman spectroscopy during practically relevant cycling conditions (600 cm−1 associated with Li+ vacancy (VLi) formation appear. All modes generally harden upon delithiation (disregarding the initial Eg mode softening due to VLi formation), that is, the respective band positions P blue-shift due to MO bond character (ionic → covalent) evolution within the MO6 octahedra. Both P(Eg) and P(A1g) vs x profiles display small plateaus that correlate to the electrochemical response (dx/dE redox peaks) of NCA, hence linking the sequential e− depletion from MO states in the PDOS of NCA to local structural transitions defined by bond character transformations and equilibrium bond length rearrangements within the MO6 units. Unlike conventional diffraction techniques, Raman spectroscopy reveals that localized structural transitions, analogous to the (monoclinic ↔ hexagonal) phase transformations found for the LiNiO2 parent compound, are likely intimately related to electrochemical response of NCA. The intensities of all Raman active bands of NCA generally increase upon oxidation, which can be largely explained by increasing covalency and electron polarizability along the MO bonds. The band position and intensity profiles display a major 4700
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inflection at x = 0.2, thus marking the onset of a fundamental transition within NCA that is most likely related to electron removal from MO bonding states and oxidation of oxygen sublattice (possibly explaining the observed O2 release from the surface). The overcharge process (x < 0.2) leads to irreversible changes on the local lattice environment as evidenced by the charge−discharge hysteresis in the intensity trends in the third cycle. Operando Raman spectroscopy of the local lattice structure provides unique possibilities for detailed studies of how processes, such as bond character evolution and Li+ vacancy formation, may influence the stability and performance of the LixMO2 class of oxides. Insights into the underlying nature and onset potential of such processes can thus be exploited to guide the development of LiMO2 based cathode materials in next-generation Li-ion batteries operating at higher voltages.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01384.
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Article
Operando Raman cell design and validation; Raman and X-ray diffraction patterns of thermally treated samples; DFT-predicted peak shifts, intensities and eigenvectors of LixCoO2 (x = 1.0, x = 0.5); peak positions and intensities of all observed bands for three full cycles; crystallographic lattice parameters, MO6bond lengths and angles during cycling; intensity step-increase during first cycle of composite electrodes comparing NCA powder stored on air and inert atmosphere (PDF)
AUTHOR INFORMATION
Corresponding Author
*
[email protected]. ORCID
Ulrich Aschauer: 0000-0002-1165-6377 Erik J. Berg: 0000-0001-5653-0383 Present Address §
Department of Chemistry, Ångström Laboratory, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS E.F. and E.J.B. acknowledge Swiss National Science Foundation (SNSF) under the “Ambizione Energy” funding scheme (Grant 160540). We thank Dr. Rosa Robert for fruitful discussions, Dr. Bing Sung for the XRD measurements on the thermally treated charged samples, and Mr. Hermann Kaiser for technical assistance. N.V. and U.A. were supported by the SNSF Professorship Grant PP00P2_157615. Calculations were performed on UBELIX (http://www.id.unibe.ch/hpc), the HPC cluster at the University of Bern. 4701
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Article
Chemistry of Materials
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DOI: 10.1021/acs.chemmater.8b01384 Chem. Mater. 2018, 30, 4694−4703
