High-voltage capabilities of ultra-thin Nb₂O₅ nanosheet coated LiNi_1/3Co_1/3Mn_1/3O₂ cathodes

Abstract

In this study, we propose coating the surface of LiNi_1/3Co_1/3Mn_1/3O₂ (NCM) electrodes with 1.1 nm Nb₂O₅ nanosheets as a new way for enhancing their high-voltage capabilities in galvanostatic charge–discharge performances and long-term storage of the charged state at 60 °C. The coatings suppress the oxidative decomposition of the electrolyte and the disordering of the layered structures on the NCM surface, which result in impedance growth. After 100 cycles at voltage ranges 4.6–2.8 V at 1 C rate, more than 70% of the initial discharge capacity was retained in the coated electrodes. In comparison, the discharge capacity completely faded in the bare NCM cathode. Furthermore, the nanosheet coatings stabilize the delithiated NCM lattice charged at 4.6 V and inhibit current generation based on the electrode reactions at 60 °C for 300 h.

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Introduction

Layered LiNi_1/3Co_1/3Mn_1/3O₂ (NCM) has been extensively studied as a high-energy cathode active material for lithium ion rechargeable batteries (LIBs) because of its high specific capacity, low volume change during charging and discharging, and the minimal use of high-cost cobalt.^1–6 The practical use of NCM, specifically in plug-in hybrid electric vehicles and electric vehicles has been delayed because of its limited rate capability and capacity fading at high temperatures and operation voltages. The reduction of Ni⁴⁺ to Ni²⁺ at highly charged states releases oxygen atoms from the NCM lattice, which causes a thermal runaway because of reaction with the flammable electrolytes, leading to LIB failure.^7–9 Furthermore, the oxidative reaction between the NCM electrode surface and the electrolyte at high voltages >4.3 V promotes the formation of LiF and transition metal–F bonds at the surface, as well as the disordering of the layered atomic arrangement to form spinel structures.^10–12 These changes at the NCM surface result in impedance growth.

Surface modification with inorganic oxide layers can potentially endow NCM-based cathodes with high-voltage capabilities by stabilizing the interface via reducing the contact area between the NCM cathode and the electrolytes and by enhancing the chemical stability.^13–16 The demand for durable cathode materials has stimulated the use of many coating methods, such as chemical vapor deposition and plasma layer deposition for depositing Al₂O₃, ZrO₂, AlPO₄, and Li₄Ti₅O₁₂ to protect the electrodes.^17–22 These deposition processes, which use vacuum conditions, have been widely used to coat electrodes with a variety of materials with a high thickness controllability (ranging from a few nanometers to micrometers). Due to the high surface roughness and porosity of the cathode surfaces, these dry processes, however, lead to inhomogeneous coatings. Lithium ions are transported across the coating during charging and discharging and the inhomogeneous thickness distribution in the coating layer presents a risk of accelerating resistance increase and capacity degradation. Furthermore, these dry processes require enormous equipment and energy inputs in comparison to wet processes.

Inorganic nanosheets are promising candidate materials for thin coating and stabilizing functional materials without any suppression of their original physicochemical properties due to their ultimate thinness. In general, exfoliation of layer-structured materials with the assistance of organic templates results in the formation of nanosheet structures having sub-millimeter scale dimensions with atomic layer thickness. The inorganic nanosheet has a large surface area and many hydroxyl groups and/or other functional groups on the surface, therefore they show strong affinity to electrostatically charged surfaces of metal and metal oxides. Very recently, Sugimoto et al. demonstrated that RuO₂ nanosheet coating on a Pt₁Ru₁/C and Pt/C catalyst surface resulted in the suppression of CO adsorption on the catalyst surface, leading to an improvement in CO tolerance.^23–25

In this paper, we demonstrate the coating of NCM electrode surfaces with Nb₂O₅ nanosheets by using a wet process and examine the effect of the protecting layer in enhancing the high-voltage capabilities of the electrodes. Systematic analyses, including charge–discharge tests, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy were used to understand the effects of the Nb₂O₅ nanosheets on the surface chemistry of the NCM electrodes. To the best of our knowledge, this is the first work to report the effects of nanosheet coatings on NCM electrodes, providing a fresh perspective on suppressing the impedance growth of these electrodes at high-voltage (4.6 V) operation.

Experimental

Idiomorphic KNb₃O₈ crystals were grown from molten KCl at 800 °C with average dimensions of 1 μm × 20 μm, as characterized by SEM. The KNb₃O₈ crystals were exfoliated using tetrabutylammonium hydroxide (TBA⁺OH⁻) and HCl solution (the details of the process have been described in previously published articles^26,27). Finally, the concentration of Nb₂O₅ nanosheets dispersed in pure water was adjusted to 0.1, 0.5 and 1 wt% by diluting with pure water. The pH was fixed at ca. 7. In addition, the thickness of the exfoliated nanosheets was measured by atomic force microscopy (AFM, Nanocute, SII). The aqueous solution was spin-coated on the NCM electrodes deposited on Al foil with a diameter of 12 mm (Hosen Corp.). The mixing percentage of NCM/acetylene black/PVDF was controlled to be 92/5/3 by weight. The loading amount of NCM particles was 10.2 mg cm⁻², and electrode density (tap density) was 2.75 g cm⁻³. The NCM electrodes were annealed at 150 °C for 12 h to remove the residual TBA⁺OH⁻ molecules and water completely. As control samples, the uncoated NCM (bare NCM) electrodes were also prepared using an identical procedure without the nanosheet coating. An aqueous solution of HCl and TBA⁺OH⁻ without the Nb₂O₅ nanosheets was used for spin-coating.

The surface morphologies and elemental mapping of Nb were obtained by field emission SEM (FE-SEM, JSM-7000F, JEOL Ltd.) and EDX (MiniFlex2, RIGAKU) at an operating accelerating voltage of 2 kV. The surface chemical states of the NCM electrodes were characterized by XPS (JPS-9010, JEOL) with a monochromic Al source. The beam-diameter was ca. 5 mm. All the binding energies measured in the XPS studies were referenced to the C_1s hydrocarbon peak at 284.5 eV. Laser Raman spectroscopy (LabRAM HR Evolution) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FT-IR, JASC FT/IR-6000) were performed at room temperature. The galvanostatic charge–discharge properties of all the NCM electrodes were studied by using a coin-type cell (R2032). The NCM electrodes contain acetylene black (AB) and polyvinyldene fluoride (PVDF) added as electron conductivity and adhesion enhancement agents, respectively. The electrodes were dried under vacuum at 120 °C for 12 h prior to cell assembly. No significant changes were observed in the AB and polyvinyldene fluoride after the annealing. Lithium metal foil and polypropylene film were used as the counter electrode and separator, respectively. A solution of 1 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC) solution (EC : DMC = 3 : 7 in volume) was used as the electrolyte. The coin-type cells were assembled in an argon-filled glove box with <1 ppm of H₂O and O₂. The charge–discharge tests, including cycles, C rates, and capacity fade holding at 60 °C were evaluated using a potentio/galvanostat at 2.8–4.6 V (vs. Li⁺/Li) at 25 °C. Moreover, the charge–discharge tests of the nanosheets uncoated/coated electrodes were performed at 0.25 C rate (0.58 mA g⁻¹) and 1 C rate (2.31 mA g⁻¹). To analyze the surface chemical states of the NCM electrodes after the charge–discharge tests, the coin-type cells were disassembled in the glove box. The NCM electrodes were washed with DMC to remove any residual electrolyte, dried under vacuum for 10 h, and subjected to characterization.

Results and discussion

We used a Nb₂O₅ nanosheet prepared from KNb₃O₈ crystals as a model compound due to its electrochemical inertness at 4.6 V (vs. Li⁺/Li), electronic insulating property, Li⁺ conducting property, and easy handling. The Nb₂O₅ nanosheets were prepared by the exfoliation of KNb₃O₈ crystals using tetrabutylammonium hydroxide (TBA⁺OH⁻) and HCl solution. The average thicknesses of the individual Nb₂O₅ nanosheets were evaluated by atomic force microscopy (AFM), as 1 nm and 0.5 × 3 μm, respectively (Fig. S1†). The height difference between the nanosheet surface and a Si substrate is ca. 1.1 nm, indicating that the exfoliated nanosheets formed a single atomic layer. Powder XRD pattern of proton-exchanged KNb₃O₈ was consistent with that of the reference data of H₃ONbO₈ (ICDD PDF 44-0672). Therefore, the major component of the Nb₂O₅ nanosheet is considered to be Nb₂O₅ (+5). Furthermore, Wu et al. reported that the experimental atomic ratio of Nb/O in the Nb₂O₅ nanosheet prepared from a KNb₃O₈ precursor was 0.39 (calculated from EDX results), which is close to the theoretical Nb/O ratio in Nb₂O₅.²⁶ The XPS-Nb_3d spectrum taken from the Nb₂O₅ nanosheets coated on NCM electrodes was primary assigned to Nb₂O₅ (Fig. S2†).^27,28

Fig. 1 shows the secondary electron and backscattered electron (BSE) SEM images acquired from the NCM electrodes coated with Nb₂O₅ nanosheets under different conditions. The surface of the bare NCM electrodes is highly porous. The number of nanosheets on the NCM electrode surface seemed to be increased with increase in concentration, leading to higher coverage. The surface distribution of Nb₂O₅ nanosheets was not clearly detected by energy dispersive X-ray spectroscopy coupled with SEM (SEM-EDS) because the deposited concentration of the Nb₂O₅ nanosheet was small, wet processing resulted in relatively homogeneous coating of inorganic thin layers to modify substrate surfaces independent of substrate surface roughness and porosity. The weight of the nanosheet layers deposited on the NCM electrodes from 0.1, 0.5, and 1.0 wt% solutions was 0.002 mg cm⁻², 0.007 mg cm⁻², and 0.03 mg cm⁻², respectively, assuming that the Nb₂O₅ nanosheets were densely deposited on the NCM electrodes on increasing the concentration of the colloidal Nb₂O₅ nanosheet solution. In order to evaluate the thickness of the nanosheet layers deposited on the top layer of the NCM cathodes, the changes in the Nb_3d5/2 spectra as a function of Ar sputtering number were further studied (see in Fig. S3†). Etched thickness per Ar sputtering was controlled to be 2 nm, which was calculated in terms of silicon wafer. It is found that the thickness was increased with increasing concentration. From the changes in the Nb_3d5/2 spectra, assuming that the Nb₂O₅ nanosheets are homogeneously deposited on the NCM electrodes, the thicknesses of the nanosheet layers deposited from 0.1, 0.5, and 1.0 wt% solutions can be roughly estimated as 4 nm, 10 nm, and 12 nm, respectively. We believe that the data from XPS depth profiles are possibly explicit microscopic evidence to confirm the formation of a nanometer-scale Nb₂O₅ nanosheet layer on the electrode because our X-ray with beam-diameter of 5 mm was exposed to almost the whole surface of electrode-samples with dimensions of 3 × 3 mm².

Fig. 1 (a–d) SEM, (e–h) BSE images of the bare (a and e) and Nb₂O₅ nanosheet coated NCM electrode surfaces. The coatings were prepared from aqueous dispersions with Nb₂O₅ nanosheet concentrations of (b and f) 0.1 wt%, (c and g) 0.5 wt%, and (d and h) 1 wt%.

To investigate the effect of the Nb₂O₅ nanosheet coating on the initial resistance, the initial potential profiles of the coated and bare NCM electrodes were examined. The typical initial potential profiles at the third cycles are shown in Fig. 2. The absence of clear differences among the profiles of the various NCM electrodes indicates that the Nb₂O₅ nanosheets were electrochemically inactive. Interestingly, the discharge capacities tended to improve regardless of the nanosheet thickness. We have also summarized the changes in the discharge capacity of the NCM electrodes as a function of the C rates on the nanosheet coated NCM electrodes prepared from 1 wt% nanosheets solutions (Fig. 3). The C rate capability was significantly improved by the Nb₂O₅ nanosheet coating. We are currently considering plausible mechanisms to explain this behavior; however, we conjecture that the nanosheets may contribute to the rapid diffusion of Li⁺ ions at the electrolyte interface, leading to a reduction in the impedance of the composite electrodes when operated at high current densities.

Fig. 2 Galvanostatic charge–discharge curves of bare and coated NCM electrodes obtained from 0.1 wt% (red), 0.5 wt% (green), and 1 wt% (blue) Nb₂O₅ nanosheet solutions.

Fig. 3 C rate capability of bare and coated NCM electrodes obtained from 1 wt% (blue) NbO_x nanosheet solutions.

The half-cell cycling performances at 1 C rate (20 mA h g⁻¹) of the coated NCM electrodes obtained from nanosheet solutions of various concentrations are compared. Fig. 4 shows the changes in the discharge capacity retention as a function of cycle numbers. The Nb₂O₅ nanosheets coated NCM half-cells exhibited better cycling capability at 4.6 V regardless of the nanosheet solution concentration in comparison to bare NCM. The coated NCM half-cell retained 65% of the initial capacity (98 mA h g⁻¹) at the maximum. In comparison, the bare NCM electrode-based half-cell showed severe fading. The capacity retention dropped abruptly, beginning at 70 cycles. Finally, the discharge capacity had almost completely faded after 100 cycles. The coated NCM electrodes prepared from 0.1 wt% nanosheet solutions did not show any changes in terms of the high voltage capability, indicating that the surface coverage of the NCM electrodes with the Nb₂O₅ nanosheets critically influenced the high voltage capabilities. It is interesting to note that retention of dark pigments was observed on the polypropylene separator after 100 cycles. Furthermore, the batteries are reusable after the replacement of an old separator with a new one. These results strongly suggest that high voltage operation poses a risk for higher capacity fade due to impedance growth and short circuit formation caused by oxidative decomposition of electrolytes following LiF and metal–F₂ formation and Mn²⁺ dissolution during the cycles.^29–32

Fig. 4 Changes in discharge capacity of bare (black) and coated NCM electrodes obtained from 0.1 wt% (red), 0.5 wt% (green), and 1 wt% (blue) Nb₂O₅ nanosheet solutions as a function of the cycle number.

The nanosheets covered only the electrode surface in this study, uncoated internal porous electrode materials freely access the organic electrolyte. Therefore, the morphologies of the nanosheets, including shape and size will be expected to greatly affect the improvement of the high voltage capability of the NCM electrodes. For instance, we predict that the nanosheets prepared form K₄Nb₆O₁₇ crystals might exhibit better high voltage capability due to their platelet shape than the stripe shaped nanosheets prepared from KNb₃O₈ crystals.

In order to realize the nanosheet effects on the cycle capabilities, the changes in the surface chemical state of the NCM electrodes after 100 cycles at 1 C rate were studied by XPS, in addition to ATR-FT-IR and Raman spectroscopies. Fig. 5 shows the XPS-F_1s core level profiles acquired from the all NCM electrodes. The sharp peak centered at 687.5 eV is assigned to the C–F bond of PVDF in the as-prepared bare NCM electrode. After 100 cycles, an intense new peak appeared at 685.8 eV, which could be assigned to LiF and MeF₂ (Me: Ni, Co, Mn).^33,34 With an increase in the weight of the Nb₂O₅ nanosheets deposited, the contribution of LiF and MeF₂ to the peak area decreased. This suggests that the nanosheets suppressed the formation of both LiF and MeF₂ at the surface, which does not conduct lithium ions well and leads to an impedance growth. LiF and MeF₂ are formed via the oxidation decomposition of the electrolyte by the NCM electrode when it is exposed to the organic electrolyte containing LiPF₆.^35,36 ATR-FT-IR spectroscopy also supports the LiF and MeF₂ formation on the NCM electrodes (Fig. S4†). The corresponding XPS O_1s core level profiles are also shown in Fig. S5,† which indicate that the Nb₂O₅ nanosheet coatings inhibit the formation of polyether derivatives on the NCM electrode surface after the charge–discharge tests.

Fig. 5 Changes in F_1s core level XPS profiles of the (a) bare and the Nb₂O₅ nanosheet coated NCM electrode surfaces obtained from (b) 0.1 wt%, (c) 0.5 wt%, and (d) 1 wt% Nb₂O₅ nanosheet solutions after 100 cycles at 1 C rate (20 mA h g⁻¹).

To further understand the structural changes in the NCM electrode during cycling at high voltage, Raman spectroscopy was carried out. Layered NCM, which crystallizes in the hexagonal R3̄m space group, offers only two Raman-active modes i.e., A_1g and E_g, corresponding to the out-of-plane M–O stretching and in-plane O–M–O bending, respectively. The Raman spectra of the pristine NCM electrode in Fig. 6(a) shows two peaks centered at 609 cm⁻¹ and 503 cm⁻¹ assigned to the A_1g and E_g modes derived from the R3̄m space group, respectively. Some additional poorly resolved bands were observed from the monoclinic LiMnO₂, which crystallizes in the C2/m space group with low symmetry because of Jahn–Teller distortions. Since NCM can be thought of as a solid solution of LiNiO₂, LiCoO₂, and LiMnO₂, the corresponding spectral features shown in Fig. 6(a) can be considered to be reasonable and in close agreement with Ruther’s results.³⁷ A sharp high frequency mode centered at 654 cm⁻¹ appeared in the bare NCM electrode after the cycling tests. This high frequency mode cannot be characterized as an ideal layered phase with R3̄m or C2/m symmetry. According to the literature, this high frequency mode can be expected from related oxides, such as cubic spinel Li_1.33Mn_1.67O₄ and low temperature modified spinel LiNi_0.5Co_0.5O₂.^38–41 Thus, the appearance of this high frequency mode is evidence for the spinel phase growth in a layered phase during the charge–discharge cycling at high voltages. As shown in Fig. 6(b), the growth of a spinel phase is absent within the cycling conditions used in the case of the Nb₂O₅ nanosheet coated NCM electrode. These results also highlight the effects of the Nb₂O₅ nanosheet coating on the high voltage durability of the NCM electrodes operated at 4.6 V.

Fig. 6 Changes in the Raman spectra of the (a) bare and (b) the Nb₂O₅ nanosheet coated NCM electrodes surface before (black) and after the 100 cycles at 1 C rate (blue). The nanosheet coated NCM electrodes were prepared from 1 wt% solutions.

We further found that the nanosheet coating could possibly be useful at high temperature. Fig. 7 shows the time course of the voltage change in half-cells maintained at 60 °C. The cell voltage significantly reduced with retention time in the bare NCM electrodes. The bare NCM electrode delivers voltage of 3.7 V after 300 h, stored at 60 °C. This voltage drop might be attributed to the current generation via the irreversible reaction of delithiated NCM electrode with electrolyte. The nanosheet coated NCM electrode shows a higher voltage retention than that of the bare NCM electrode. The voltage was maintained at 4.35 V after 300 h, stored at 60 °C. The nanosheet helps to form an insulating layer to direct contact with electrolyte for electron conducting, promoting oxidative decompositions of organic electrolytes.

Fig. 7 Time course of the voltage in the NCM electrodes charged at designated voltage. The coated NCM electrodes were prepared from 1 wt% nanosheet solutions.

Conclusion

The effects of coating NCM electrodes with Nb₂O₅ nanosheets on the high voltage durability, rate capability, and high temperature stability when operated at 4.6 V have been studied systematically by XPS, ATR-FT-IR spectroscopy, Raman spectroscopy, and half-cell based charge–discharge tests under various conditions. We found the Nb₂O₅ nanosheets prevent the direct contact of the NCM electrode with the commonly used electrolyte containing LiPF₆, resulting in the suppression of the formation of LiF, MeF₂, and polyether derivatives on the electrode surface and the structural evolution of the spinel phase in the layered phase of NCM during the charge–discharge tests. Furthermore, for the first time, we also demonstrate that the nanosheet coating results in the enhancement of the calendar life of the half-cells at 60 °C when discharged at 4.6 V. Note that the C rate enhanced significantly because of the Nb₂O₅ nanosheet coating. All the results demonstrated in this study strongly depend on the Nb₂O₅ nanosheet coating conditions. These results prove that Nb₂O₅ nanosheet could be used as a potential ultimately thin water-soluble protecting layer for the enhancement of high voltage capabilities of NCM electrode in LIBs. To understand the mechanism of how the nanosheet coating affects the electrode performance, further studies, specifically on Li ion transfer at the NCM electrode/electrolyte interface across the nanosheet layer are required. By acquiring further insights into the chemical phenomena occurring at the surface, we believe that optimum conditions of nanosheet coating, such as thickness, cover ratio, and material, and the upper limit of the high voltage capabilities can be explored.

Acknowledgements

This work was partially supported by Core Research for Evolution Science and Technology and Super Cluster Program from the Japan Science and Technology Agency.

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Footnote

† Electronic supplementary information (ESI) available: The changes in the Nb_3d5/2, C_1s, and O_1s, core level XPS profiles of the Nb₂O₅ nanosheet coated NCM electrodes after the charge–discharge test, the AFM image of the Nb₂O₅ nanosheet spread on a Si wafer, and spectral changes in the ATR-FT-IR of the NCM electrodes after the charge–discharge tests. See DOI: 10.1039/c6ra10155k

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