Rutgers University, North Brunswick, New Jersey 08902, United States. North Brunswick, New Jersey 08902, United States

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1 Surface Degradation of Li 1 x Ni.8 Co.15 Al.5 O 2 Cathodes: Correlating Charge Transfer Impedance with Surface Phase Transformations S. Sallis, 1 N. Pereira, 2 P. Mukherjee, 3 N. F. Quackenbush, 4 N. Faenza, 2 C. Schlueter, 5 T.-L. Lee, 6 Wanli Yang, 7 F. Cosandey, 3 G. G. Amatucci, 2 4, 1, a) and L. F. J. Piper 1) Materials Science & Engineering, Binghamton University, Binghamton, New York 1392, USA 2) Energy Storage Research Group, Department of Materials Science and Engineering, Rutgers University, North Brunswick, New Jersey 892, United States 3) Department of Materials Science and Engineering, Rutgers University, North Brunswick, New Jersey 892, United States 4) Department of Physics, Applied Physics and Astronomy, Binghamton University, Binghamton, New York 1392, USA 5) Diamond Light Source Ltd., Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 DE, UK 6) Diamond Light Source Ltd., Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 DE, UK 7) Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 9472, USA 1

2 The pronounced capacity fade in Ni-rich layered oxide lithium ion battery cathodes observed when cycling above 4.1V (versus Li/Li + ) is associated with a rise in impedance, which is thought to be due to either bulk structural fatigue or surface reactions with the electrolyte (or combination of both). Here, we examine the surface reactions at electrochemically stressed Li 1 x Ni.8 Co.15 Al.5 O 2 (NCA) binder-free powder electrodes with a combination of electrochemical impedance spectroscopy, spatially-resolving electron microscopy, and spatially-averaging x-ray spectroscopy techniques. We circumvent issues associated with cycling by holding our electrodes at high states of charge (4.1 V, 4.5 V and 4.75 V) for extended periods and correlate charge-transfer impedance rises observed at high voltages with surface modifications retained in the discharged state (2.7V). The surface modifications involve significant cation migration (and disorder) along with Ni and Co reduction, and can occur even in the absence of significant Li 2 CO 3 and LiF. These data provide evidence that surface oxygen loss at the highest levels of Li + extraction is driving the rise in impedance. a) Electronic mail: lpiper@binghamton.edu 2

3 Nickel-rich layered oxides as positive electrodes within lithium ion batteries offer high theoretical capacities but suffer from irreversible capacity fading with cycling at the highest levels of charge. The substitution of metals to stabilize the structure (Al 3+,Ti 4+, Mn 3+, Co 3+ ) along with synthesizing in lithium excess conditions have circumvented bulk issues, such as Ni/Li inter-site mixing and Ni 2+ formation, that hinder the capacity and cyclability of LiNiO 2. 1 Indeed, Li 1 x Ni.8 Co.15 Al.5 O 2 (NCA) and Li 1 x Ni x Mn y Co z O 2 (NMC) compounds are amongst best-in-class commercial cathodes capable of delivering high discharge capacities of up to 2 mahg 1, which are considered promising for electric vehicle applications. 2 However, this is still far below its theoretical capacity exceeding 265 mahg 1 because significant capacity fading occurs when charged beyond 4.1 V (vs. Li/Li + ). Increasing evidence has determined that the capacity retention issues in Ni-rich layered oxides is largely due to interactions at the electrode-electrolyte interface (EEI) that occur in the highly reactive delithiated phase. 3 9 This is in keeping with reports of dramatically improved capacity retention for layered oxide electrodes protected by surface coatings, especially insulating Al oxides Moreover, classical electrochemical impedance spectroscopy (EIS) has long identified the formation of a surface film on layered oxide cathodes as a source of the observed impedance rise upon cycling, 13 which has since been shown to be true for NCA. 14 Unfortunately, the EEI associated with the positive electrode is less understood than for the negative electrode. 15 At the negative electrode, a solid electrolyte interface (or SEI) layer is formed due to the electrochemical reduction of the electrolyte, the composition of the SEI layer is mostly carbonates and LiF. 15 However, Li 2 CO 3 and LiF species are still observed at cathode surfaces, including NCA. 6,9,15,16 This is surprising since electrolyte reduction should not occur at the cathode, and even electrolyte oxidation is not expected for most layered cathode materials based on the location of the highest occupied molecular orbital of most commercial electrolytes. 15 These surfaces species likely originate from undesired chemical reactions due to either air exposure (Li 2 CO 3 ) or reactions with HF from decomposed salt and residual water in the electrolyte (LiF). 15,16 While these species can harm the electrochemical performance and capacity retention, 17 pronounced phase transformations resulting in a rocksalt-like surface phase must also be accounted for. 3 9 Accelerated cycling tests at elevated temperatures have shown that Ni-rich layered oxides electrodes (including NCA) undergo significant power fading that coincide with the formation of electrochemically inactive NiO-like rocksalt phases at particle surface thought 3

4 to be due to loss of oxygen. 18,19 Recent transmission electron microscopy studies employing electron energy loss spectroscopy (STEM-EELS) have revealed that the formation of the NiO-like layer occurs sooner than previously thought i.e. at 5% depth of charge in the first cycle. 8 However, the presence of Li 2 CO 3 and LiF species makes it difficult to determine whether the surface species induce the phase transformations or if the surface phase transformations are inherently due to surface oxygen loss. Understanding this distinction between extrinsic chemical and intrinsic electrochemical origins is important for the rational design of next-generation high capacity-retention positive cathodes. Here, we present results of electrochemically stressed binder-free NCA powder electrodes from EIS, spatially-resolving transmission electron microscopy (TEM) and spatiallyaveraging x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS). We present evidence of a correlation between impedance rise and electrochemically-induced surface phase transformations involving Ni (and Co) reduction and Ni migration in the absence of organic build-up. The observation of surface phase transformations in the absence of traditional EEI components (such as Li 2 CO 3 and LiF), reveals that surface oxygen loss is the driving mechanism for the surface phase transformations that contribute to the impedance rise. Our studies employed powder electrodes of commercially available NCA active material with 2.5wt.% carbon black. The preparation and characterization of the NCA powder electrodes is provided in supplemental material (SM). For the LiCoO 2 parent material, PVDF binder in slurry electrodes has been shown to significantly accelerate surface Co reduction and dissolution into the electrolyte when employing a lithium hexafluorophosphate (LiPF 6 ) salt in ethylene carbonate and dimethyl carbonate (EC:DMC) electrolyte. 2 As a result, the PDVF-binder has been excluded from the electrode preparation of this study in order to circumvent chemically-induced surface reduction reactions and provide optimal samples for post electrochemistry characterization. We note that the absence of PVDF-binder did not harm the electrochemical performance of our powder electrodes in this study. The charge and discharge capacities for all the electrodes studied by TEM/XPS/XAS presented here are provided in SM (Tables SI-IV). Table I shows the results obtained in this study are consistent with results obtained with electrodes fabricated with PVDF-binder. Samples held at a constant voltage above 4.1V for 2 weeks exhibit charge capacities in excess of the 265 mahg 1 theoretical capacity (at.5 Li content) of NCA due to corrosion. Finally, we also 4

5 would like to note that the electrochemical performance was highly reproducible between duplicate batches for the TEM/XPS/XAS measurements. Figure 1 shows the EIS spectra evolution with time over 5 weeks at constant voltage after charge to 4.1, 4.5 and 4.75V. As expected, the impact of state of charge and potential is substantial. While the EIS spectra recorded remains fairly constant at low potential (i.e. up to 4.1V), significant development of charge transfer impedance is apparent by the evolution of the semicircle associated with the R3 intercept over time above 4.1V. The degree to which NCA is delithiated, as affected by the increase in voltage, clearly dictates the immediate and subsequent evolution of impedance with time. XPS studies of reference electrodes at various states of charge (SOC) immediately ruled out the build-up of organic EEI species, such as LiF and Li 2 CO 3, as the origin of the impedance growth. The Li 1s and C 1s core-level XPS analysis of our NCA electrodes is provided in SM (Fig. S2). Although LiF was observed in the held (and discharged) NCA electrodes, the LiF signal is almost below detectable limits for electrodes 4.5V (SOC) i.e. when the impedance growth is occurring in Fig. 1. Moreover, Li 2 CO 3 was undetectable in all of the electrodes (except for the 3.6V SOC electrode). No evidence of electrochemically-induced phase transformations were observed in our TEM studies of NCA powder electrodes held at 4.1V for 2 weeks (SM, refer to Fig S3), consistent with the corresponding impedance spectra showing very little impedance build up for this voltage (refer to Fig. 1a). Selected area electron diffraction (SAED) patterns from the interior (1) and surface (2) of the particle shown in Fig. S3 both display the layered R 3m diffraction pattern expected for NCA. In contrast, the NCA particle electrodes held at 4.5V exhibited two additional distinct phases to the layered R 3m phase at their surfaces. Figure 2 displays TEM images of three different surfaces of a representative NCA particle held at 4.5V for 2 weeks before discharging to 2.7V. Based on our SAED analysis, two of the surfaces displayed layered (R 3m) phases while the third shows a spinel (F d 3m) phase. We note that a subsurface spinel phase has previously been reported for Ni-rich layered oxides. 4,8,19 Further inspection of the third surface revealed a rocksalt NiO-like surface layer that was restricted to 2-3 nm of the surface, based on our HR-TEM and EELS analysis. The observation of surface phase transformations at this voltage agrees well with the observed onset in impedance rise with time at this voltage (Fig. 1). 5

6 Extended disordered phases are significantly more pronounced in the electrode held at 4.75V. They are found to extend up to 3 nm from the surface. Figure 3 displays two distinct high resolution HAADF-STEM images highlighting considerable cation migration into the Li layer (top figures) and with complete cation disordering (bottom figures). In the former, diffraction spots from Ni in the Li layer can be easily identified (note that these weaker interlayer spots due to Ni migration are absent for surfaces 1 and 2 of Fig. 2). The formation of insulating disordered phases would significantly hinder Li + transport and would contribute to the massive increase in impedance observed at this voltage (Fig. 1). High-resolution surface sensitive total electron yield (TEY) mode XAS was employed to obtain oxidation state information, to complement the TEM studies. Figure 4 displays the O K-edge, Ni and Co L 3,2 -edge XAS with an effective probing depth of 5 nm, determined from our epitaxial oxide studies. 21 Before discussing these data we note that the electrode held at 4.1V for 2 weeks (and discharged to 2.7V) is spectroscopically indistinguishable from a reference powder electrode at a SOC of 3.6V (refer to SM, Fig S4) in either TEY or bulksensitive total fluorescent yield (TFY) detection (refer to SM, Fig S4). This is expected for these electrodes since they should be layered R 3m phase throughout. This also confirms that the held electrodes recover to the nominally Ni 3+ line-shape in the bulk, consistent with their discharged Li contents (refer to SM, Fig. S4 and Tables SIII-SIV). As a result, any differences in the TEY mode XAS between the held electrodes held above 4.1V must reflect electrochemically-induced surface phase transformations. The Ni L 3,2 -edge in Fig. 4 shows increased spectra weight at 853 ev for electrodes held at 4.5V and 4.75V (compared to 4.1V), which reflects surface Ni 2+. This observation is consistent with the extended cation disordered phase with rocksalt-like contrast seen in the TEM studies for the NCA surfaces held at these voltages (Figs. 2 and 3). Further support is provided by closer inspection of the corresponding TEY mode O K-edge XAS spectra. The O K-edge can probe the hybridization between the unoccupied O 2p and metal 3d and 4sp states, and provides the same information as O K-edge EELS. Previous studies have employed O K-edge STEM-EELS to identify a rocksalt NiO -like surface phase in NCA. 4,8,19. Here, the TEY mode XAS provides better instrumental resolution whilst being highly surface-sensitive. In Fig. 4 the main peak at 529 ev is related to the O 2p and Ni e g (and Co e g ) hybridization of NCA. The spectra for the electrodes held at 4.5V and 4.75V display an additional feature at 532 ev (compared to the 4.1V), which is attributed 6

7 to the formation of surface NiO-like oxygen environment based on direct comparison with reference NiO thin films. In addition, our XAS in Fig. 4 present evidence of surface Co reduction occurring in tandem with surface Ni reduction. We note that the 4.1V held electrode reflects a Co 3+ lineshape, expected for the discharged state. The appearance of additional multiplet features is consistent with coexisiting Co 2+ containing surface species (most likely spinel Co 3 O 4 and related phases). Similar evidence of surface Co 2+ species have been reported in NMC electrodes. 5 Evidence of Co surface reduction at NCA electrodes has been suggested previously, 4,19 but Co reduction is rarely considered in recent studies of phase transformations of NCA. 8 As a result, the extended cation disordered surface phase may contain significant Co 2+ contributions and must be accounted for in computational studies. We note that Co reduction (and dissolution into the electrolyte) at LiCoO 2 surfaces is known to cause impedance growth. 22 To conclude, we present combined impedance, electron microscopy and x-ray spectroscopy results of electrochemically stressed NCA electrodes. We observe the formation of an extended surface cation disordered phase with reduced Ni and Co surface species above 4.1V, which correlates well with a pronounced impedance rise above this voltage. We note that our observed phase transformations occur at higher voltages than reported for slurry electrodes, 8 which we attribute to PVDF accelerating surface reduction in the latter. The use of binderfree powder electrodes also reveal that surface phase transformations can occur in the absence of significant chemically-induced electrode-electrolyte species, such as Li 2 CO 3 and LiF. This highlights surface oxygen loss at high levels of Li + extraction as the driving mechanism for the surface phase transformations and resultant charge transfer impedance growth. The formation of an extended cation disordered phase near the surface is likely to severely impede transport. In addition, the reduction and possible subsequent dissolution of Ni 2+ and Co 2+ into the electrolyte could also poison the counter electrode, as seen for Co-rich layered oxides. 7

8 I. SUPPLEMENTAL MATERIAL See supplementary information at url for a full description of the electrode preparation and electrochemical characterization along with details regarding the XPS, TEM and XAS. Additional XPS, TEM and XAS analysis is provided. II. ACKNOWLEDGEMENTS We thank Prof. Gerbrand Ceder for discussions. This work was supported as part of NECCES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC XAS experiments were performed at beamline 8..1 at the ALS and beamline I9 at Diamond Light Source. The work at ALS is supported by the Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC2-5CH We thank Diamond Light Source for access to beamline I9 (SI12764) that contributed to the results presented here. REFERENCES 1 A. Manthiram, J. C. Knight, S. T. Myung, S. M. Oh, and Y. K. Sun, Advanced Energy Materials 6, 511 (215). 2 M. S. Whittingham, Chem. Rev. 14, 4271 (24). 3 J. Li, L. E. Downie, L. Ma, W. Qiu, and J. R. Dahn, Journal of the Electrochemical Society 162, A141 (215). 4 S. Zheng, R. Huang, Y. Makimura, Y. Ukyo, C. a. J. Fisher, T. Hirayama, and Y. Ikuhara, Journal of The Electrochemical Society 158, A357 (211). 5 F. Lin, I. M. Markus, D. Nordlund, T.-C. Weng, M. D. Asta, H. L. Xin, and M. M. Doeff, Nature communications 5, 3529 (214). 6 M. Shikano, H. Kobayashi, S. Koike, H. Sakaebe, E. Ikenaga, K. Kobayashi, and K. Tatsumi, Journal of Power Sources 174, 795 (27). 7 L. Baggetto, D. Mohanty, R. a. Meisner, C. a. Bridges, C. Daniel, D. L. Wood III, N. J. Dudney, and G. M. Veith, RSC Advances 4, (214). 8

9 8 S. Hwang, W. Chang, S. M. Kim, D. Su, D. H. Kim, J. Y. Lee, K. Y. Chung, and E. A. Stach, Chemistry of Materials 26, 184 (214). 9 S. Watanabe, M. Kinoshita, T. Hosokawa, K. Morigaki, and K. Nakura, Journal of Power Sources 258, 21 (214). 1 S. T. Myung, K. Izumi, S. Komaba, Y. K. Sun, H. Yashiro, and N. Kumagai, Chem. Mater 17, 3695 (25). 11 A. T. Appapillai, A. N. Mansour, J. Cho, and Y. Shao-Horn, Chemistry of Materials 19, 5748 (27). 12 L. Daheron, R. Dedryvere, H. Martinez, D. Flahaut, M. Menetrier, C. Delmas, and D. Gonbeau, Chemistry of Materials 21, 567 (29). 13 M. G. S. R. Thomas, P. Bruce, and J. B. Goodenough, Journal of The Electrochemical Society 132, 1521 (1985). 14 T. Hayashi, J. Okada, E. Toda, R. Kuzuo, N. Oshimura, N. Kuwata, and J. Kawamura, Journal of the Electrochemical Society 161, A17 (214). 15 M. Gauthier, T. J. Carney, A. Grimaud, L. Giordano, N. Pour, H.-H. Chang, D. P. Fenning, S. F. Lux, O. Paschos, C. Bauer, et al., The Journal of Physical Chemistry Letters 6, 1463 (215). 16 K. Edström, T. Gustafsson, and J. O. Thomas, Electrochimica Acta 5, 397 (24). 17 D.-H. Cho, C.-H. Jo, W. Cho, Y.-J. Kim, H. Yashiro, Y.-K. Sun, and S.-T. Myung, Journal of the Electrochemical Society 161, A92 (214). 18 A. M. Andersson, D. P. Abraham, R. Haasch, S. MacLaren, J. Liu, and K. Amine, Journal of The Electrochemical Society 149, A1358 (22). 19 D. P. Abraham, R. D. Twesten, M. Balasubramanian, J. Kropf, D. Fischer, J. McBreen, I. Petrov, and K. Amine, Journal of The Electrochemical Society 15, A145 (23). 2 E. Markevich, G. Salitra, and D. Aurbach, Electrochemistry Communications 7, 1298 (25). 21 N. Quackenbush, H. Paik, J. Woicik, D. Arena, D. Schlom, and L. Piper, Materials 8, 5452 (215). 22 G. Amatucci, Solid State Ionics 83, 167 (1996). 9

10 2 - Im(Z) / ohm V 5 weeks 4.1V R Re(Z) / ohm 1 - Im(Z) / ohm V time Re(Z) / ohm FIG. 1. Impedance spectra recorded every 12 hours over 5 weeks at constant voltage after charge to 4.1 V, 4.5 V and 4.75 V. Only selected representative spectra were plotted for clarification. 1

11 FIG. 2. TEM and SAED images of a NCA electrode particle held at 4.5V. Three distinct phases can be identified between the interior, subsurface and surface. Three surfaces are shown (top left), with surfaces 1 and 2 displaying a layered (R3 m) phase up to the surface. Surface 3 displays a spinel (F d3 m) phase, with a disordered rocksalt phase in the first 2-3 nm (top right). 11

12 FIG. 3. High-resolution HAADF-STEM images of two subsurface structures of a representative NCA electrode particle held at 4.75 V. (Top left) Layered R3 m structure with extensive cation migration into the Li layer and (bottom left) cation disordered phase with rocksalt like contrast observed up to 3 nm from the surface. Enlarged respective figures with highlighted structure R3 m and F m3 m models are shown on the right. 12

13 Ni L 3,2 -edge XAS Held at 4.1 V Held at 4.5 V Held at 4.75 V NiO NiO O K-edge XAS Co 2+ Co L 3,2 -edge XAS Photon Energy (ev) Photon Energy (ev) Photon Energy (ev) FIG. 4. High resolution TEY mode XAS of the (left) Ni L 3,2 (middle) O K- and (right) Co L 3,2 - edges of three electrodes held at 4.1V, 4.5 V, 4.75 V for 2 weeks and measured in the discharged state of 2.7 V vs. Li/Li +. A reference NiO film is included as a reference Ni 2+ lineshape. The arrows indicate evidence of surface NiO and Co 2+ contributions for electrodes held at 4.5 V and 4.75 V. 13

14 Voltage Duration Charge Discharge Li held (weeks) Capacity Capacity content (V) (mahg 1 ) (mahg 1 ) TABLE I. Representative electrochemical data of NCA powder electrodes. Figure 1: Impedance spectra recorded every 12 hours over 5 weeks at constant voltage after charge to 4.1 V, 4.5 V and 4.75 V. Only selected representative spectra were plotted for clarification. Figure 2: TEM and SAED images of a NCA electrode particle held at 4.5V. Three distinct phases can be identified between the interior, subsurface and surface. Three surfaces are shown (top left), with surfaces 1 and 2 displaying a layered (R 3m) phase up to the surface. Surface 3 displays a spinel (F d 3m) phase, with a disordered rocksalt phase in the first 2-3 nm (top right). Figure 3: High-resolution HAADF-STEM images of two subsurface structures of a representative NCA electrode particle held at 4.75 V. (Top left) Layered R 3m structure with extensive cation migration into the Li layer and (bottom left) cation disordered phase with rocksalt like contrast observed up to 3 nm from the surface. Enlarged respective figures with highlighted structure R 3m and F m 3m models are shown on the right. Figure 4: High resolution TEY mode XAS of the (left) Ni L 3,2 (middle) O K- and (right) Co L 3,2 -edges of three electrodes held at 4.1V, 4.5 V, 4.75 V for 2 weeks and measured in the discharged state of 2.7 V vs. Li/Li +. A reference NiO film is included as a reference Ni 2+ lineshape. The arrows indicate evidence of surface NiO and Co 2+ contributions for electrodes held at 4.5 V and 4.75 V. Table 1: Representative electrochemical data of NCA powder electrodes. 14

15 2 - Im(Z) / ohm V 5 weeks 4.1V R Re(Z) / ohm 1 - Im(Z) / ohm V time Re(Z) / ohm

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18 Ni L 3,2 -edge XAS Held at 4.1 V Held at 4.5 V Held at 4.75 V NiO NiO O K-edge XAS Co 2+ Co L 3,2 -edge XAS Photon Energy (ev) Photon Energy (ev) Photon Energy (ev)