Surface Characterization of Li-Substituted Compositionally

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C: Energy Conversion and Storage; Energy and Charge Transport

Surface Characterization of Li-Substituted Compositionally Heterogeneous NaLi Cu Fe Mn O Sodium-Ion Cathode Material 0.045

0.185

0.265

0.505

2

Muhammad Mominur Rahman, Yan Zhang, Sihao Xia, Wang Hay Kan, Maxim Avdeev, Linqin Mu, Dimosthenis Sokaras, Thomas Kroll, Xi-Wen Du, Dennis Nordlund, Yijin Liu, and Feng Lin J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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The Journal of Physical Chemistry

Surface

Characterization

of

Li-Substituted

Compositionally

Heterogeneous

NaLi0.045Cu0.185Fe0.265Mn0.505O2 Sodium-Ion Cathode Material Muhammad Mominur Rahman,1 Yan Zhang,2,3 Sihao Xia,3 Wang Hay Kan,4,5 Maxim Avdeev,5 Linqin Mu,1 Dimosthenis Sokaras,3 Thomas Kroll,3 Xi-Wen Du,2 Dennis Nordlund,3 Yijin Liu,3 Feng Lin1* 1. Department of Chemistry, Virginia Tech, Blacksburg, VA 24060, USA E-mail: [email protected] 2. School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China 3. Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA 94035, USA 4. China Spallation Neutron Source, Institute of High Energy Physics, Chinese Academy of Sciences, Dongguan Institute of Neutron Science, Dongguan 523803, People's Republic of China 5. Australian Nuclear Science and Technology Organisation (ANSTO), New Illawarra Road, Lucas Heights, New South Wales 2234, Australia

Abstract The understanding of surface chemical and structural processes can provide some insights into designing stable sodium cathode materials. Herein, Li-substituted and compositionally heterogeneous NaLi0.045Cu0.185Fe0.265Mn0.505O2 is used as a platform to investigate the interplay between Li substitution, surface chemistry, and battery performance. Li substitution improves the initial discharge capacity and energy density. However, there is no noticeable benefit in the long-term cycling stability of this material. The Li substitution in the transition metal layer also seems to influence the transition metal (TM)3d–oxygen (O)2p 1 ACS Paragon Plus Environment

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hybridization. Upon desodiation, the surface of active particles undergoes significant transition metal reduction, especially Mn. Furthermore, the presence of electrolyte drastically accelerates such surface degradation. In general, the Li-substituted material experiences severe surface degradation, which is partially responsible for the performance degradation upon long-term cycling. While some studies have reported the benefits of Li substitution, the present study suggests that the effectiveness of the Li substitution strategy depends on the TM compositional distribution. More efforts are needed to improve the surface chemistry of Lisubstituted sodium cathode materials.

1. Introduction Energy storage devices based on intercalating electrode materials have revolutionized the modern electronics industry. In particular, lithium-ion batteries have dominated the market due to their high energy density that enabled their widespread applications in electronic devices1 and electric vehicles.2 However, Li minerals are rather resource-limited, scarcely distributed and largely expensive.3 Thus, to maintain the long-term sustenance of battery technologies, developing alternatives to lithium-ion batteries has become a dire need. Moreover, applications such as large-scale stationary energy storage call for significantly reduced cost of batteries.3,4 Thus, sodium-ion batteries offer one of the most feasible alternatives to meet these particular demands. Layered oxide materials are regarded as a technologically important class of cathode materials due to their excellent reversibility in cycling the alkali metal ions5 and relatively smaller degree of lattice volume change.6 However, the energy density delivered from sodium layered oxides is typically inferior to that from their lithium-ion counterparts.7 Therefore, novel design strategies must be undertaken to reduce the performance gap between the two classes of energy storage devices. Conventionally, homogeneous distribution and gradient distribution8–10 of the transition metals have been considered as the most potent ways of designing layered transition metal 2 ACS Paragon Plus Environment

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oxide cathodes. However, recently, our group have demonstrated that a sodium layered oxide material with a high degree of compositional heterogeneity could also deliver stable electrochemical performance.11 This opened up plenty of room for further development of cathode materials for sodium-ion batteries following similar design methods. Meanwhile, Li substitution has become a popular method to lessen the degree of bulk phase transformations in sodium layered oxide cathodes.12,13 The cycling stability and rate capability of sodium layered oxide materials are also reported to be improved upon Li-substitution.14 Importantly, multiscale processes collectively determine the performance of these substituted materials, not only limiting to the bulk phase transformations. Most of the reported studies in Li-substitution on sodium layered oxide cathodes focus on the influece of Li-substitution in the bulk properties of the materials. However, recent studies have demonstrated that the surface chemistry of layered oxides also governs the electrochemical characteristics.15–17 Largely observed phenomena such as transition metal dissolution18, segregation and migration of transition metals11, crack formation19,20, and electrolyte decomposition21 contribute to the capacity fading of the cathode materials in alkali metal ion batteries upon long-term cycling. Many of these phenomena originate from the surface degradation of the cathode particles, related to the the orbital hybridization between transition metal cations (TM) and oxygen anions (O).11,17 Meanwhile, several studies have suggested that lithium ions can occupy the transition metal sites in the lattice.12,13,22,23 Therefore, we hypothesize that lithium can potentially influence the TM–O hybridization, thus altering the cathode–electrolyte interfacial chemistry. Moreover, in order to identify ways to stabilize layered oxide materials, it is imperative to understand the onset of processes that causes the surface stability issues. Therefore, it is crucial to investigate what changes take place at the surface of the cathode particles during the electrochemical cycling. Moreover, various transition metal ions can be incorporated in the transition metal layer, giving considerable flexibility to the choice of the transition metals.24 The different choices of transition metals make it challenging to obtain the 3 ACS Paragon Plus Environment


perfectly homogeneous distribution in multielemental sodium layered oxide materials because of their differences in the ionic size and electronic properties.11 Several studies have shown that chemically heterogeneous materials could still deliver the expected performance.8,11 Therefore, studying the influence of Li-substitution in a compositionally heterogeneous material will represent a plausible case in sodium layered oxide cathodes. In this study, we focus on understanding the role of Li substitution in altering the TM3d–O2p hybridization and the surface chemistry of compositionally heterogeneous NaLi0.045Cu0.185Fe0.265Mn0.505O2 cathode particles. We expect that enlightening the influence of Li-substitution from atomic and molecular orbital points of view and unraveling the onset of capacity fading through surface chemistry of cathode particles will inform desigining more stable sodium layered oxide cathode materials.

2. Experimental Section 2.1.

Material Synthesis The precursor of NaLi0.045Cu0.185Fe0.265Mn0.505O2 with Cu, Fe, and Mn as the transition

metals (the overall ratio between Cu, Fe and Mn are 0.2, 0.28, and 0.52) was synthesized following a co-precipitation method as described in a recent publication.11 Salt solutions of CuSO4.5H2O (Sigma Aldrich, 98%), MnSO4.H2O (Sigma Aldrich, 99%) and FeSO4.7H2O (Sigma Aldrich, 99%), and NaOH as a base solution were utilized for the synthesis. Fe and Mn hydroxides were first precipitated together followed by the precipitation of Cu(OH)2, thus coating the Cu(OH)2 on the mixed hydroxide of Fe and Mn. The precursor after the precipitation was collected through filtration and dried in a vacuum oven at 100 °C overnight. The dried precursor was then mixed with nanosized Na2CO3 (half of the as many moles of precursor hydroxide) and LiOH (4.5 mol% of the total Na+), first by grinding in a mortar and pestle and then in a Speedmixer at 1500 rpm for sufficient amount of time. The mixed

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precursor with Na2CO3 and LiOH was then calcined in a box furnace at 850 °C for 10 hours to get the final electroactive layered oxide material. Chemical Na+ removal from the lattice of the material was performed by employing NO2BF4 as the oxidant using acetonitrile as the solvent. Solutions of NO2BF4 in acetonitrile were prepared with the oxidant percentage being equal to 25 mol%, 50 mol% and 100 mol% of NaLi0.045Cu0.185Fe0.265Mn0.505O2. The powder of the material was mixed with the designated amount of the oxidant solution and stirred in an argon-filled Kimble bottle for 24 hours. Upon completion of the reaction, the powder was separated from the mixture through centrifugation and dried in a vacuum oven at 100 °C overnight. The dried powder was then pressed into a pellet for synchrotron X-ray Raman spectroscopic measurements.

2.2.

Materials Characterization

The morphological features of the material were investigated with scanning electron microscopy (LEO FESEM) at an acceleration voltage of 5.0 kV. The compositional measurements of the material were performed through energy dispersive X-ray spectroscopy (EDS) on an FEI Quanta 600 FEG SEM operated at an acceleration voltage of 20 kV. The overall elemental composition of the material was evaluated using an SPECTRO ARCOS ICP-AES analyzer. For sequential multiple elemental measurement, the relative standard deviation and the coefficient of variation are less than 2% for concentration higher than the background equivalent concentrations. The background equivalent concentrations are Na = 0.028 mg/L, Li = 0.022 mg/L, Cu = 0.004 mg/L, Fe = 0.009 mg/L, and Mn = 0.001 mg/L. 2D x-ray spectroscopic imaging analysis of NaLi0.045Cu0.185Fe0.265Mn0.505O2 was performed using the full-field transmission X-ray microscope (TXM) at the beamline 6-2c at SSRL.25 The energy range of the instrument is ∼4.5 to 13 keV. 2D elemental mapping was achieved through acquiring the signal above and below the X-ray absorption edges of the transition metals. An in-house developed software package known as TXM-Wizard was used for the analysis 5 ACS Paragon Plus Environment


of the TXM data.26 XRD pattern on the pristine powder material was acquired through a PANalytical Empyrean X-ray diffractometer with a Cu Kα radiation (λ=1.54 Å) X-ray source and Bragg-Brentano HD divergent beam optic with an energy resolution of about 450 eV. A 0.02 rad Soller slit, 10 mm beam mask, fixed anti-scatter slit (1/2°) and fixed divergence slit (1/8°) were included in the incident beam path. 32 mm low-background Si wafer samples holders with 0.2 mm wells (PANalytical) were utilized for sample loading and rotated in proper position during the analysis. Data acquisition was performed from an initial 2θ of 10° to final 2θ of 80° with approximate step size of 0.0018° using a GaliPIX3D area detector system operating in line mode (501 active channels). The crystal structure refinement was carried out through Rietveld method as implemented in the Fullprof software package. XPS characterization was performed on a PHI VersaProbe III scanning XPS microscope using monochromatic Al K-alpha X-ray source (1486.6 eV). For the sputter depth profiling, 3 keV Ar+ ion bombardment at a raster size of 3 mm × 3 mm and a 5 min sputter interval was used, which gives a sputter rate of 9.3 nm/min for SiO2. XPS Spectra were acquired at 200 µm/50 W/15 kV using 26 eV pass energy, which gives a Ag3d5/2 full width at half maximum of 0.59 eV. All binding energies were referenced to C-C peak at 284.8 eV. Soft X-ray absorption spectroscopy (XAS) measurements on chemically desodiated and electrochemically cycled materials were acquired on the 31–pole wiggler beamline 10–1 at Stanford Synchrotron Radiation Lightsource (SSRL). A ring current of 500 mA in addition to a 1000 l·mm spherical grating monochromator with 20 μm entrance and exit slits were utilized which yielded ∼1011 ph·s–1 at 0.28 eV resolution in a 1 mm2 beam spot. Data acquisition was performed under ultrahigh vacuum (10–9 Torr) in a single load at room temperature in both total electron yield (TEY) and fluorescence yield (FY) mode. The sample drain current was collected for the TEY mode. The probing depths of the TEY and FY are approximately

Surface Characterization of Li-Substituted Compositionally

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