Self-Passivation of a LiNiO2 Cathode for a Lithium-Ion Battery through Zr Doping Chong S. Yoon,† Un-Hyuck Kim,‡ Geon-Tae Park,‡ Suk Jun Kim,∥ Kwang-Ho Kim,§ Jaekook Kim,⊥ and Yang-Kook Sun*,‡ †
Department of Materials Science and Engineering, Hanyang University, Seoul 04763, South Korea Department of Energy Engineering, Hanyang University, Seoul 04763, South Korea ∥ School of Energy, Materials, and Chemical Engineering, Korea University of Technology and Education (KOREATECH), Cheonan 31253, South Korea § School of Materials Science and Engineering, Pusan National University, Busan 46241, South Korea ⊥ Department of Materials Science and Engineering, Chonnam National University, Gwangju 61186, South Korea
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‡
S Supporting Information *
ABSTRACT: A self-passivating Li2ZrO3 layer with a thickness of 5−10 nm, which uniformly encapsulates the surfaces of LiNiO2 cathode particles, is spontaneously formed by introducing excess Zr (1.4 atom %). A thin layer of Li2ZrO3 on the surface is converted into a stable impedance-lowering solid−electrolyte interphase layer during subsequent cycles. The Zr-doped LiNiO2 cathode with an initial discharge capacity of 233 mA·h·g−1 exhibited significantly improved capacity retention (86% after 100 cycles) and thermal stability, compared to the undoped LiNiO2. While the spontaneously formed Zr-rich coating layer provides surface protection, the Zr ions in the LiNiO2 lattice delay the detrimental phase transition occurring in the deeply charged state of LiNiO2 and partially suppress the anisotropic strain emerging from the phase transition. Further optimization of the proposed simultaneous coating and doping strategy can mitigate the inherent structural instability of the LiNiO2 cathode, making it a promising high-energydensity cathode for electric vehicles. Fe)15−23 and surface coating (Al2O3, AlPO4, Li3PO4, AlF3, Al(OH)3, ZrO2, and C)24−31 were proposed to extend the lifetime of the nickel-rich layered cathodes while utilizing the full DOD, the improvement in the cycling stability was either marginal, particularly in the cases of layered cathodes with Ni contents larger than 80%,32,33 or attained at the cost of a significant reduction of the discharge capacity of the cathodes.34−36 In this study, we demonstrate that LiNiO2 (LNO), which has a potential to deliver the largest discharge capacity among the nickel-rich layered oxides exceeding a value of 250 mA h g−14 at a low material cost (it is Co-free) but suffers from a rapid capacity fading, can be simultaneously doped and surface-coated by introducing excess Zr ions into the cathode particles to mitigate its inherent shortcomings: poor cycling and thermal stabilities. Excess Zr4+ ions beyond the solubility limit were added during the synthesis of the Ni(OH)2 hydroxide precursor prior to lithiation. During the lithiation,
N
ickel-rich layered oxides are promising cathode materials for lithium-ion batteries (LIBs) owing to their high energy density, which increases approximately proportionally to their Ni content; however, the inadequate cycling stability and poor thermal characteristics have been the main drawbacks for the commercialization of these cathodes. The low cycling stability of the Ni-rich layered oxides is generally attributed to surface degradation in the charged state emerging from the formation of chemically unstable Ni4+ ions, which spontaneously form an impedanceincreasing Ni−O impurity phase,1−6 and electrolyte decomposition catalyzed by the transition-metal ions on the cathode particles’ surfaces.7,8 In addition, it was recently shown that microcracks generated in the particle interior by internal strain caused by the anisotropic volume change in the deeply charged state (above 4.2 V) not only compromised the structural integrity but also facilitated the electrolyte penetration, which exposed the crack faces for further electrolyte attacks.9−12 For applications in electric vehicles, these challenges are typically avoided by limiting the depth of discharge (DOD), at the cost of a reduced capacity, which increases the battery weight and cost.2,13,14 Although various schemes involving both composition modification through doping (Co, Mn, Mg, Ti, Al, and © 2018 American Chemical Society
Received: May 16, 2018 Accepted: June 15, 2018 Published: June 15, 2018 1634
DOI: 10.1021/acsenergylett.8b00805 ACS Energy Lett. 2018, 3, 1634−1639
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Cite This: ACS Energy Lett. 2018, 3, 1634−1639
Letter
ACS Energy Letters
Figure 1. (a) Mosaic cross-sectional scanning transmission electron microscopy (STEM) dark-field image of Zr-LNO. (b) Bright-field image of Zr-LNO near the surface. (c) Surface-coating layer with a uniform thickness observed in the area outlined by the yellow box in (b). (d) High-resolution TEM image of the surface-coating layer. The upper inset shows the FT from the region in the yellow box; the simulated diffraction pattern in the [101] zone axis in the lower inset confirms the Li2ZrO3 structure.
Figure 2. (a) Cycling performance between 2.7 and 4.3 V at 0.5 C for the Zr-LNO and LNO cathodes; the inset shows the initial charge and discharge curves between 2.7 and 4.3 V at 0.1 C for a 2032-coin-type half-cell using Li metal anodes. (b) dQ·dV−1 vs V curves corresponding to the charge−discharge curves at the 1st, 50th, and 100th cycles of the LNO and Zr-LNO cathodes. (c) Comparison of the c-axis lattice parameters of the LNO and Zr-LNO cathodes as a function of the charge state. The relative weight fractions of the H2 phase were indicated when the H2 and H3 phases coexisted. (d) Charge-transfer resistances of the LNO and Zr-LNO cathodes as a function of the cycle number. The dependence of the charge-transfer resistance during the first six cycles is enlarged in the inset.
suggests that the Zr+ ions were not completely incorporated into the Ni(OH)2 precursor as a solid solution and that there may exist pockets of Zr hydroxide. In order to confirm the Zr distribution in the lithiated oxide particle after calcination, the same analysis was performed on a Zr-LNO particle after lithiation (Figure S2a). The EDS line scan across the Zr-LNO particle indicates that the Zr distribution became approximately uniform across the particle after calcination. However, the EDS line scan in Figure S2b along a primary particle at its surface shows that the edge of the primary particle (thickness: 100 nm) was slightly enriched with Zr (up to 2.0 atom %) as the excess Zr ions tended to diffuse to the particle surface during the heat treatment. Figure 1a shows a TEM image of a lithiated Zr-LNO particle, which contained a considerable fraction of pores at the particle center and crevices at the
Zr ions incorporated into the LNO structure, while the excess Zr ions diffused to the particles’ surfaces and reacted with LiOH to spontaneously form a stable self-passivating layer, which uniformly covered the entire LNO particles. The transmission electron microscopy (TEM) image of the Zr-doped Ni(OH)2 precursor in Figure S1 shows that the particle core is composed of primary nanoparticles, which acted as seed particles. From these core seed particles, increasingly large elongated primary particles grew. An energy-dispersive X-ray spectroscopy (EDS) line scan across the precursor particle indicates that the core was Zr-depleted and became Zr-enriched as the particle grew from the seed particles before the decrease below the nominal composition (1.4 atom %, measured by inductively coupled plasma atomic emission spectroscopy). The nonuniform concentration profile 1635
DOI: 10.1021/acsenergylett.8b00805 ACS Energy Lett. 2018, 3, 1634−1639
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Figure 3. (a) Mosaic cross-sectional STEM dark-field image of Zr-LNO after 100 cycles. (b) Bright-field TEM image of a cycled Zr-LNO primary particle aligned along the [100] direction with the EDS profile of the Zr concentration at the surface. (c) High-resolution TEM image showing the surface layer formed on the cycled Zr-LNO particle surface; the top-left inset shows the FT of the region in the yellow box on the particle surface, while the bottom-right inset shows the FT of the interior layered structure.
dQ·dV−1 profiles. During charging/discharging, the LNO cathode underwent a series of phase transitions, as indicated in Figure 2b. In particular, the intensity of the peak at 4.2 V associated with the H2 → H3 transition, which is mainly responsible for the irreversible structural degradation,4 rapidly decreased during the cycling, whereas the corresponding peak for the Zr-LNO cathode exhibited no significant change in intensity, demonstrating the intrinsic phase stability of ZrLNO, as the stability of the H2 → H3 transition peak correlated well with stable cycling observed in other Ni-rich NCM and NCA cathodes.4,43 In addition, the effect of Zr doping on the phase stabilization of LNO was quantitatively evaluated through a series of ex situ X-ray diffraction (XRD) profiles measured as a function of the state of charge from 4.1 to 4.5 V (Figure 2c). The c-axis lattice parameter of LNO abruptly decreased by 0.8 Å at 4.2 V owing to the H2 → H3 phase transition, consistent with previous reports.4,12,44 In the case of Zr-LNO, even at 4.5 V, the H2 phase still persisted as the main phase (weight fractions of Zr-LNO at 4.5 V: H2, 65.3% and H3, 34.7%), so that the abrupt contraction in the caxis was replaced by a smooth transition, allowing the particles to better accommodate the internal strain caused by the anisotropic lattice contraction (Figure S3, Table S1). Therefore, the ex situ XRD result demonstrated that the Zr doping delayed the deleterious phase transition and stabilized the structure in the deeply charged state. As the cycling proceeded, the beneficial effect of the Zr coating layer was apparent in the Nyquist plots of the electrochemical impedance (Figure S4). The estimated chargetransfer resistance (Rct) of the Zr-LNO cathode remained below 10 Ω throughout 100 cycles, whereas that of the LNO cathode continuously increased and reached a value of 85 Ω after 100 cycles (Figure 2d). In the initial cycle, Rct of the ZrLNO cathode was higher than that of LNO and progressively decreased, suggesting the formation of a stable solid− electrolyte interphase (SEI) on the Zr-LNO cathode surface during early cycles. The almost-constant resistance on the surface of Zr-LNO up to 100 cycles may be attributed to formation of the SEI layer by transformation of the Li2ZrO3 layer during the cycling. The TEM image of a cycled Zr-LNO cathode particle (after 100 cycles) in Figures 3a and S5 indicates that the particle remained largely intact without any visible major cracks, in contrast to the LNO cathode particles, which were almost pulverized after 100 cycles.4 In addition, the bright-field image of a [100] zone-aligned primary particle in Figure 3b verifies
particle periphery. This indicates that the pores and crevices are likely voids left behind by the Zr-rich regions from which Zr ions have diffused out, as justified by the absence of such pores and crevices in undoped LNO and increasingly low density of such defects observed in LNO cathodes doped with Zr at concentrations below 1%.37 The presence of Zr in the particle interior was confirmed by EDS. A thin layer with a thickness of 5−10 nm, approximately uniformly encapsulating the outer particle surface and crevices, was observed (Figure 1b−d). The Fourier transform (FT) pattern of the coating layer (Figure 1d, upper inset) well matched the calculated [101]-zone-axis electron diffraction pattern of Li2ZrO3 (Figure 1d, lower inset). Li2ZrO3 likely precipitated on the particle surface during the lithiation of Zr-LNO by heat treatment in O2 through a reaction with excess LiOH. The encapsulating Li2ZrO3 film forms a self-passivating layer, which can protect the cathode surface from electrolyte attacks. Li2ZrO3 was chosen by many research groups as a coating material to reduce the HF erosion of the cathodes in the electrolyte; in addition, Li2ZrO3 has a suitable structure for fast ion transportation.38−42 The thickness of the Li2ZrO3 layer previously reported was in the range of 5−30 nm, comparable to the one reported in this study. Though the percentage of coverage and homogeneity of the Li2ZrO3 layer were not measured in this study, the Li2ZrO3 layer formed in this study is expected to provide larger coverage and higher homogeneity because the Li2ZrO3 layer was spontaneously fabricated as a result of super-saturated Zr ions diffusing out from the matrix to the surface during calcination with Li source. Since the formation of the Li2ZrO3 layer was a spontaneous process, the coating layer was found nearly uniformly on the particle surface (even on the narrow micro-scale crevices as shown in Figure 1c) with homogeneity. Excess Zr doping of the LNO cathode substantially improved its cycling and thermal stabilities while maintaining an outstanding discharge capacity. The first-cycle discharge capacity of Zr-LNO was 232.6 mA·h·g−1, considerably higher than those of Li(Ni x Coy Mn 1−x−y )O 2 (NCM) and Li(NixCoyAl1−x−y)O2 (NCA) cathodes reported in the literature.11 The capacity retention of Zr-LNO at 0.5 C after 100 cycles, when cycled from 2.7 to 4.3 V, was 86%, significantly better than the value of 74% of the pristine LNO cathode cycled under the same conditions (Figure 2a). The improved cycling stability was partially attributed to structural stabilization of Zr-LNO through the introduction of Zr ions into the LNO lattice. The phase stability of Zr-LNO was confirmed by 1636
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capacity and its retention of the NCM and NCA cathodes investigated by our group11 as the Zr doping/coating substantially extended the cycle life while providing a high capacity equivalent to that of Li(Ni0.95Co0.025Mn0.025)O2. The thermal stability of Zr-LNO was also promising as the onset temperature of the thermal decomposition of Zr-LNO was higher than or equal to those of Li[Ni0.90Co0.05Mn0.05]O2 (195 °C)11,47 and Li[Ni0.85Co0.11Al0.04]O2 (200 °C).43 We believe that Zr-LNO can be an attractive cathode compared to the extremely Ni-rich NCM and NCA cathodes owing to the lower material cost (Co-free), simpler fabrication (single composition), and self-passivating surface coating. Further optimization of the coating layer such as modifying the layer thickness and/ or its phase is expected to provide larger thermal stability to LNO, which has hindered its use as a cathode. This study demonstrated that the Li2ZrO3 coating is a promising alternative approach to achieve a high energy density and extend the life of batteries with Ni-rich NCM and NCA cathodes.
the presence of the surface-protecting layer with a thickness of ∼20 nm, which was Zr-enriched (up to 6 atom %), compared to the bulk. The high-resolution TEM image and FT patterns (Figure 3c) demonstrate the presence of a surface layer, which was also suggested by the impedance data. One of the possible structures of the surface phase formed during the cycling is Zrand Li-doped Ni oxide. The FT pattern from the surface layer was consistent with the [110]-zone-axis diffraction pattern of NiO if the superlattice spots (NiO (1/2 1/2 1/2) plane) indicated by the yellow circles were ignored. NiO is also consistent with the often-observed rock-salt impurity phase in the cycled NCM and NCA cathodes.2−5 The superlattice spots likely emerged from the ordered occupation of Zr and Li atoms. It was previously reported that addition of Zr and Li into NiO influenced dielectric properties of NiO.45 In this study, a Zr and/or Li added NiO-like phase, i.e., a stable SEI layer, is expected to form by consuming Li2ZrO3 layer during the reaction with electrolyte. Thus, the Zr and/or Li added SEI layer stabilized the surface structure and decreased the Rct of Zr-LNO. Many research groups reported that coating layered cathodes with Li2ZrO3 reduced the Rct of the cathodes, but no detailed information on the SEI layer formation process was discussed.38−42 In our TEM analysis of the cycled cathode, we observed a new surface structure generated during cycling and it was surmised that the new structure is likely the SEI layer developed from the decomposition of the initial coating layer. In addition to the enhancement of the cycling stability, the thermal stability of LNO was improved by simultaneous doping and coating of LNO with Zr. The onset temperature and enthalpy of the exothermic reaction of the delithiated ZrLNO were 200 °C and 1182 J·g−1, respectively, while LNO began to exothermically decompose at 176 °C and gave off a significantly higher enthalpy of 1860 J·g−1 (differential scanning calorimetry (DSC) profiles in Figure S6.) It was demonstrated that the introduction of excess Zr into the LNO cathode led to simultaneous doping and coating and enabled the Zr-LNO cathode to partially resolve the trade-off between specific capacity and capacity retention typically observed in Ni-rich NCM and NCA cathodes.46 As illustrated in Figure 4 and Table S2, the Zr-LNO cathode does not obey the empirically determined linear relationship between the
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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/acsenergylett.8b00805.
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Experimental methods, precursor and lithiated oxide TEM image, XRD patterns at various cutoff voltages, Nyquist plots as a function of cycles, DSC profile, and lattice parameters (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Chong S. Yoon: 0000-0001-6164-3331 Jaekook Kim: 0000-0002-6638-249X Yang-Kook Sun: 0000-0002-0117-0170 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government Ministry of Education and Science Technology (MEST) (NRF-2018R1A2B3008794) and by the Global Frontier R&D Programme (2013M3A6B1078875) on the Center for Hybrid Interface Materials (HIM), by the Ministry of Science, ICT & Future Planning.
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REFERENCES
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DOI: 10.1021/acsenergylett.8b00805 ACS Energy Lett. 2018, 3, 1634−1639
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DOI: 10.1021/acsenergylett.8b00805 ACS Energy Lett. 2018, 3, 1634−1639
