Operando Monitoring of Early Ni-mediated Surface Reconstruction in

Jun 6, 2017 - Nickel-rich layered lithiated Ni–Co–Mn oxides (NCMs) are emerging as .... Hollow symbols and dashed lines represent data up to 4.3 V...
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Operando Monitoring of Early Ni-mediated Surface Reconstruction in Layered Lithiated Ni−Co−Mn Oxides Daniel Streich,† Christoph Erk,§ Aurelie Guéguen,† Philipp Müller,§ Frederick-Francois Chesneau,§ and Erik J. Berg*,† †

Electrochemistry Laboratory, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland BASF SE, 67056 Ludwigshafen, Germany

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S Supporting Information *

ABSTRACT: Nickel-rich layered lithiated Ni−Co−Mn oxides (NCMs) are emerging as the most promising candidates for next-generation Li-ion battery cathodes. Progress, however, is hindered by an incomplete understanding of processes that lead to performance-limiting impedance growth and reduced cycling stability. These processes typically involve surface reconstruction and O2 release at the cathode surface, both of which are difficult to monitor in the working cell. We demonstrate that online electrochemical mass spectrometry can be used to measure the gas release from NCMs of varying Ni content at practically relevant potentials and under operando electrochemical conditions. We find that for cathode potentials up to 4.3 V (vs Li+/Li) there is virtually no trade-off between Ni-mediated specific-charge enhancement and parasitic surface reactions. However, at potentials greater than 4.3 V, surface-reconstruction processes giving rise to substantial CO2 and O2 release occur, implying that surfacereconstructed layers a few nanometers thick may form already after the first charge. Ni content and the Ni/Co ratio are found to govern the onset, rate, and extent of these surface-reconstruction processes. These results provide novel insights into the role of Ni in governing the surface stability and performance of Li-ion layered oxides.

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INTRODUCTION Layered lithiated transition metal oxides (LiMO2, M = transition metal) have been the leading cathode active materials (CAMs) in Li-ion batteries ever since their conception 25 years ago.1 Among these, the mixed Ni−Co−Mn oxides (NCMs) stand out as the highest-performing CAMs today, and their nickel-rich variants (NR-NCMs) are the most promising nextgeneration CAMs for high-energy demanding applications, such as electric vehicles (e.g., NCA in Tesla Model X).2 NR-NCMs provide substantial gains in energy density compared to traditional CAMs (e.g., LiCoO2) by displaying a higher reversible specific charge (>160 mAh/g) and enabling denser electrode packing.3 However, nickel enrichment has also been extensively reported to accelerate adverse structural phase transitions and enhance reactivity of the cathode toward the carbonate electrolytes commonly employed in Li-ion batteries, which results in cell-impedance growth as well as in reduced cycling and thermal stability.4,5 Resolving these performancerelated issues is of uttermost importance for electrochemical energy storage in general and for the widespread realization of electric mobility in particular. Current academic and industrial research activities, therefore, focus intensively on improving the stability of NR-NCMs without compromising energy density by designing electrolyte additives, surface coatings, and optimized electrode structures.6,7 Progress is however hampered by the lack of fundamental understanding regarding the underlying mechanisms and interrelations that bring about the Ni-related impairment effects.4,8 The study of Ni-promoted phase transitions, in © 2017 American Chemical Society

particular Li/M mixing and formation of defect structural phases, has benefited from recent developments in advanced structural analytics (e.g., TEM).9,10 Most importantly, several independent studies have evidenced the formation of rock-salt and spinel-type phases at grain boundaries and particle surfaces of layered oxide materials during cycling,11,12 particularly at high potentials, temperatures, and/or current densities.13,14 The low ionic conductivity of these surface-reconstructed phases is generally believed to be a major contributor to the increased impedance (and thus also capacity loss) of the cell,11 and thereby constitutes a major hurdle for the application of highenergy NCM-based Li-ion batteries.15 The knowledge about Liion cathode surface reactivity remains incomplete due to the hitherto limited availability of suitable characterization techniques. Clearly, in situ and operando techniques are desirable as most processes decisive for surface reactivity are initiated electrochemically and often solely observable immediately during cycling. Online electrochemical mass spectrometry (OEMS) is such an operando analytical tool and provides unrivalled quantitative information about electrochemical interface reactions during operation of the battery cell based on the highly sensitive analysis of gaseous decomposition products, such as CO2 and O2.16 CO2 occurs as a side-product to virtually any reaction taking place under oxidizing conditions in a system made up of C- and O-containing components, Received: March 10, 2017 Revised: May 18, 2017 Published: June 6, 2017 13481

DOI: 10.1021/acs.jpcc.7b02303 J. Phys. Chem. C 2017, 121, 13481−13486

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The Journal of Physical Chemistry C

Figure 1. OEMS analysis. All materials were galvanostatically cycled at a specific current of 15 mA per g of CAM between 2.0 and 4.7 V vs Li+/Li in 3:7 (w/w) EC/DEC electrolyte containing 1 M LiPF6. (a) Specific charge and CO2 and O2 evolution rate profiles vs cathode potential during first and second charge (see Figure S6 in Supporting Information for discharge data). (b) Ni content dependence of gas evolution rates during first (red) and second (blue) charge. Average CO2 (top) and O2 (bottom) evolution rates were normalized with respect to both average potential and electrode specific BET surface area. Hollow symbols and dashed lines represent data up to 4.3 V; filled symbols and solid lines correspond to data between 4.3 and 4.7 V. The lines are included as guides to the eye.

hexafluorophosphate (LiPF6) in 3:7 (w/w) ethylene carbonate (EC)/diethyl carbonate (DEC) were employed (BASF SE, water contents ≤20 ppm as verified by coulometric Karl Fischer titration). The OEMS setup comprised a quadrupole mass spectrometer (MS, QMS 200, Pfeiffer PrismaTM, Germany) for partial pressure measurements, a pressure transducer (PS, PAA-33X, Keller Druck AG, Switzerland) for cell pressure, temperature, and internal volume determination, stainless steel gas pipes, and Swagelok fittings (3 mm compression tube fittings, Swagelok, OH, US) for OEMS cell attachment, a set of solenoid valves (two-way magnetic valve, Series 99, silver-plated nickel seal, Parker, US) and a scroll pump (nXDS15i, EDWARDS GmbH, Germany) for efficient flushing. The magnetic valves were automatically opened/closed with a Solid State Relay Module (NI 9485 measurement System, national Instruments, TX, US) connected to a computer with a homemade LabVIEW software (NI LabVIEW 2013, National Instruments, TX, US). For partial pressure and gas evolution rate analysis 1.3 mL of gas was extracted from the headspace (∼4.0 mL) of the cell and replaced by pure argon (quality 6.0). To enable quantitative comparability the partial pressures derived from the ioncurrents at the MS detector were all normalized by the homemade LabVIEW software with respect to the partial pressure of argon as determined from the ion-current at 36 m/ z. Gas evolution rates were calculated in the same software. Before and after each measurement, the MS ion-current signals at m/z = 32 and 44 were related to known concentrations of O2 (1000 ppm) and CO2 (1000 ppm) using an argon-based calibration gas, enabling precise gas quantification for O2 and CO2. Further m/z channels were monitored to ensure consistent temporal evolution of different fragments derived from the same molecular species. Full details of the OEMS setup, including drawings and calibration protocols, are described elsewhere.17

rendering it an extraordinarily serviceable indicator for organic electrolyte instability. Likewise, O2 is a valuable marker for decomposition reactions involving the CAM alone rather than the electrolyte or other auxiliary constituents of an electrochemical cell. This is, because O−O bond formation requires not only sufficiently reactive but also properly distanced oxygen species, a combination of prerequisites that will predominantly be met within the CAM itself. A systematic analysis of CO2 and O2 release is thus foreseen to further clarify the impact of Ni on the electrochemical stability and surface reactivity of NCM based CAMs.

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EXPERIMENTAL METHODS Electrode Preparation. All electrodes were prepared from slurries containing 93 wt % cathode active material (NCM materials: BASF SE), 3 wt % polyvinylidene fluoride (PVDF, Kynar HSV 900, Arkema), 2.64 wt % Super C65 (Imerys), and 1.36 wt % graphite SFG6 (Imerys Graphite and Carbon, Switzerland) dispersed in N-methylpyrrolidone (NMP, SigmaAldrich). Celgard 2400 (Celgard) sheets were coated ∼200 μm thick with slurry by doctor blading and dried for 8 h under dynamic vacuum at 80 °C. Eighteen millimeter diameter electrodes (approximate cathode active material loading: 18 mg/cm2) were punched and further dried under dynamic vacuum at 80 °C for ≥12 h prior to introduction into an argon filled glovebox. Online Electrochemical Mass Spectrometry (OEMS). For all OEMS measurements a custom-made two-electrode cell was employed, and cell assembly was performed inside an argon filled glovebox. Twenty millimeter diameter disks of 0.2 mm thick lithium metal foil (Sigma-Aldrich) were used as counter electrodes. Twenty-two millimeter diameter Celgard 2400 disks, dried under dynamic vacuum at 80 °C for ≥12 h prior to introduction into the glovebox, were used as separators. As electrolyte, 120 μL of ready-made mixtures of 1 M lithium 13482

DOI: 10.1021/acs.jpcc.7b02303 J. Phys. Chem. C 2017, 121, 13481−13486

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Figure 2. Interface reactivity, surface reconstruction, and gas evolution. (a) Schematic overview of selected reactions resulting in surface reconstruction as well as CO2 and O2 formation. (b) Dependence of CO2 (top) and O2 (bottom) evolution on the state of Ni oxidation (SNOX) during 1st charge. The dashed gray lines indicate the SNOX range into which the gas evolution onsets fall. Sigmoidal fit curves of the O2 data are represented as dashed black lines. (c) Linear correlation between centers, SNOXi, of O2 data and CAM nickel content. (d) Linear dependencies of rise coefficients, χ, and upper limits, rO2,lim, of O2 data on Ni/Co ratio.

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RESULTS AND DISCUSSION We performed operando OEMS gas evolution measurements on a series of composite NCM electrodes in half-cell configuration with CAM Ni contents of 33% (NCM 111), 50% (NCM 523), 60% (NCM 622), and 80% (NCM 811) during the two first galvanostatic cycles. Figure 1a shows the specific charge as well as the CO2 and O2 evolution rate profiles during the first and second charge in dependence of the cathode potential vs Li+/ Li. These profiles verify the alleged3 Ni mediated specific charge enhancement mentioned above and reveal two distinct regimes of gas evolution with respect to the cathode potential, E: E < 4.3 V: Hardly any CO2 and O2 evolution regardless of CAM Ni content. E > 4.3 V: Both CO2 and O2 gas evolution are substantial and correlate with each other as well as with CAM Ni content. The small quantities of CO2 that do evolve below 4.3 V are likely due to oxidation of Li2CO3, a well-known residual impurity typically amounting to a few wt % in lithium containing layered oxides.4 A control experiment subjecting a carbon black electrode to a comparable potential profile reveals that CO2 evolution from a carbon-only electrode is insignificant in the absence of the CAM oxide (see Figure S1 in Supporting Information). Figure 1b shows the average gas evolution rates, also taking into account differences in electrochemical potential and surface area between the CAMs. This data further underlines the strong positive correlation between the CAM Ni content and surface reactivity and corroborates the coupling of CO2 to O2 evolution. Exposure of the NCMs to potentials >4.3 V likely leads to the formation of reactive surface oxygens, which via side reactions with the organic electrolyte and structural oxygen loss evolve both CO2 and O2, respectively.

For instance, partly oxidized surface oxygens, such as super- or peroxides, are well-known to be strongly reactive toward organic carbonate electrolyte solvents and may in addition to CO2 generate a range of oligomeric R−O intermediates (Figure 2a).16 Experiments with an oxygen-free model electrolyte provide clear evidence for structural oxygen loss being the sole origin of O2 and for the evolving oxygen being reactive toward the electrolyte (compare the CO2/O2 evolution in Figure S2 with 1 M LiPF6 in acetonitrile, Supporting Information). These findings are further supported by previous reports providing mostly ex situ experimental evidence of the formation of Odeficient reconstructed surface layers on layered oxides (LiMO2) during cycling.14 Most of the product phases are either of rock-salt (→ MO) or spinel (→ M3O4) type structures,14,18 both of which are believed to significantly enhance electrode impedance and limit cell cycle-life.11,15 By simply assuming that oxygen release and surface reconstruction give rise to a homogeneous MO shell around a spherical NCM particle, the lower limit surface layer thickness, d, estimated from the total amount of O released in the form of CO2 and O2 during both electrochemical cycles is on the order of about 3 to 5 nm (see Supporting Information for calculation details and tabulated values in Table S1). Although the estimated thicknesses roughly agree with previous ex situ TEM observations (cf. Figures S6−S8 in Supporting Information),12,19 we notice that the reconstructed phases are not homogeneously distributed over the NCM surfaces (some crystal facets may be more vulnerable). Interestingly, the drastically reduced CO2 and O2 evolution rates during the second charge (Figure 1) imply that surface reconstruction, at the same time, might have a considerable and even possibly desirable surface passivation effect (e.g., less electrolyte 13483

DOI: 10.1021/acs.jpcc.7b02303 J. Phys. Chem. C 2017, 121, 13481−13486

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The Journal of Physical Chemistry C decomposition), which in fact has been recently proposed.19 Once initiated, the reconstructed phases, however, appear to expand inward during cycling (Figure S8) and most likely reduce Li+ mobility and affect the cycling performance of the NCMs. The growth rate is, however, considerably less, compared to the first cycle, which would explain the absence of an O2 gas signal in the second (and subsequent) cycle, as the O2 release rate would be below the sensitivity of our OEMS under the current experimental conditions (∼10−11 mol/min, at which the gas signal is 10% above the background). Our results clearly demonstrate that, unlike the techniques applied in most previous studies, OEMS enables measuring the onset, rate, and extent of these early phase transitions operando and with extraordinarily high sensitivity. The capability to estimate structural stability and propensity for oxygen release of CAMs renders OEMS a unique tool for the development of stable high-energy Li-ion cathodes. Figure 2b reveals, after further analysis of the OEMS data, that the state of Ni oxidation (SNOX = specific charge/ theoretical Ni capacity, assuming the Ni and Co redox potentials are well separated;20,21 see section 3 in Supporting Information for details) rather than the potential or the state of charge (SOC) provides the most consistent predictor of the observed CO2 and O2 evolution characteristics across all investigated materials (see also Figure S3). All CO2 and O2 evolution onsets fall into a very narrow SNOX range (between 85−100%), and evolution rates clearly correlate with CAM Ni content. The fact that O2 release coincides with the end of Ni and the onset of Co oxidation suggests that Ni and/or Co species are intimately associated with the formation of reactive oxygen. Furthermore, the shapes of the O2 evolution traces in Figure 2b appear to be sigmoidal upon visual inspection. Numerical values for their centers, SNOXi, their rise coefficients, χ, and their upper limits, rO2,lim, can be obtained by means of curve-fitting (dashed lines in O2 evolution data, Figure 2b) using ⎛ ⎞ 1 rO2(SNOX) = rO2,lim⎜1 − ⎟ (SNOX SNOX ) χ − i ⎝ ⎠ 1+e

Figure 3. Qualitative DOS diagrams for NCM111 (left of ordinates) and NCM811 (right of ordinates), based on a set of independent computational studies20−23 and extended by Mn−O2*, Ni−O2*, and Co−O2* surface states (zoom-in) at (a) full lithiation, (b) maximum nickel oxidation, and (c) complete delithiation. The dashed horizontal line indicates the Fermi level, EF. Occupied and unoccupied states are represented by filled and hollow ovals, respectively.

lithiation, (b) maximum nickel oxidation, and (c) complete delithiation. The relative band sizes reflect the TM composition of the CAM. Traditionally, the highest energy bands of NCMs in such diagrams comprise antibonding Ni eg as well as bonding Co t2g, Ni t2g, Mn t2g, and O 2p states.18,20,21 During charging, these bands are sequentially becoming depleted of electrons, progressively moving the Fermi level, EF, toward lower energies. Since gas evolution originates from the immediate surface of the CAM, a consistent explanation of the experimentally observed trends should necessarily also take the energy states of surface species into account, referred to as M−O2* in the following discussion. Incomplete oxygen coordination and defects associated with these surface species have been shown to position their states at ∼1.5 eV higher energies, likely overlapping with some of the bulk M t2g states (M = Co, Ni, Mn).20,26−28 In line with the observed accelerated CO2 and O2 evolution at the onset of Co oxidation (Figure 2b), competing oxidation processes may arise between such overlapping bulk and surface states once Ni eg oxidation approaches completion. Considering that layered Mn-oxide materials are generally known to preferentially transform into spinel-type phases29 and realizing that the Mn states are energetically more stable than the corresponding Co and Ni states (Figure 3), it appears reasonable to assume that reactive oxygen formation triggered by Mn t2g or Mn−O2* oxidation is of minor importance. Furthermore, reactive oxygen formation is likely to be considerably slower based on Co t2g and Co−O2* than based on Ni−O2* oxidation. For instance, the formation of the most stable oxide phase of cobalt, Co3O4, requires electronic interaction of several Co centers, which will be substantially impaired by the dilution of Co within an excess of Ni centers at Ni/Co > 1. Furthermore, several reports claim that the activation barriers for TM migration between adjacent octahedral and tetrahedral sites are substantially higher for Co compared to Ni centers in layered oxides,18 rendering Co kinetically less prone to

(1)

as an empirical fit equation. The rise coefficient, χ, quantifies how rapidly the O2 evolution rates increase with respect to SNOX (note that, unlike the inflection point slope, χ is independent of variations in the upper limits rO2,lim of the respective curves). A comprehensive cross-correlation analysis encompassing all fit parameters and the CAM composition reveals that SNOXi decreases linearly with the Ni content (Figure 2c), whereas both χ and rO2,lim increase linearly with the Ni/Co ratio of the CAM (Figure 2d). Important to note is that the onset, rate, and extent of O2 release from the surface are thus all clearly related to the bulk NCM composition. Since the bulk composition defines the electronic structure of the CAM, these fit-parameter dependencies strongly suggest that electronic property variations between the different investigated CAMs are responsible for the observed differences in their gas evolution and thus also in their reactive oxygen formation and surface reconstruction behavior. Based on diagrams of Goodenough et al.24,25 and recent density functional theory calculations of lithiated layered oxides,20−22,26 we propose a phenomenological model describing the evolution of the valence band structure of NCMs during charging. Figure 3 qualitatively illustrates electronic density of state (DOS) diagrams for NCM111 and NCM811 at (a) full 13484

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structural rearrangements than Ni. Therefore, we conclude that the rate of reactive oxygen formation, and thus r(CO2) and r(O2), is most likely controlled by the rate of electron depletion from the Ni−O2* surface states, which, in turn, is attenuated by competitive oxidation of the Co t2g bulk and, to a smaller extent, the Co−O2* surface states. Consequently, the observed positive correlations between Ni/Co ratio and both rO2,lim and χ can be rationalized by lower numbers of available occupied Co t2g states limiting competition and thus favoring the oxidation of Ni−O2* surface states at higher Ni/Co ratios. Similarly, the negative correlation between CAM Ni content and SNOXi could be attributed to stronger overlap between the Ni eg and the M−O2* DOS bands (M = Co, Ni) shifting the onset of M− O2* oxidation toward lower SNOX at high Ni content.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Erik J. Berg: 0000-0001-5653-0383 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS E.J.B. acknowledges Swiss National Science Foundation (SNSF) under the “Ambizione Energy” funding scheme (Grant No. 160540). The authors would like to thank Petr Novák and Minglong He for fruitful discussions as well as Hermann Kaiser and Christoph Junker for technical assistance.

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CONCLUSIONS We find that operando detection of early gaseous side products provides unique opportunities for studying performancelimiting surface-reconstruction processes (e.g., LiMO2 → MO + 1/2O2) in state-of-the-art and next-generation Li-ion battery CAMs. Onset, rate, and extent of surface reconstruction are found to strongly correlate with CAM Ni content and Ni/Co ratio. The correlation between CO2 and O2 release shows that surface reconstruction is a main contributor to electrolyte decomposition. Combining our results with a phenomenological model linking the observed gas-evolution behavior with the electronic properties of NCM, we conclude that Ni−O2* (and to a minor extent Co−/Mn−O2*) surface states and their interplay with high energy electronic states of the other transition metals, specifically Co, are likely to influence the structural stability of NCMs, but further theoretical/experimental studies are required. Considering the revealed potential-dependent nature of surface reconstruction it is clear that the regular Li-ion cell charge cutoff voltage of 4.2 V already pushes the cathode close to the potential limit at which O2 and associated CO2 evolution set in and that the gradual shift of cathode potential due to voltage slippage will eventually take the cathode outside its stability range during long-term cycling. Apart from O2 release, surface reconstruction leads to the formation of low Li+-conductive surface phases (e.g., MO), which likely explains the gradual growth of cell impedance (and capacity loss) during long-term cycling, particularly under high-temperature and/or high-current charging conditions. Our findings provide further understanding of the intimate relationship between the composition and structural/surface stability of CAMs that is necessary for a rational development (e.g., by compositional tuning, surface coatings, and/or electrolyte additives) of next-generation highperformance Li-ion battery materials, and OEMS is a superior tool to guide this development.

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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.jpcc.7b02303. OEMS Experiments with CAM-free electrodes and oxygen-free electrolyte, derivation of state of Ni oxidation (SNOX), estimation of reconstructed surface layer thickness, X-ray powder diffraction, transmission electron microscopy, and complementary OEMS data evaluations (PDF) 13485

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