Characterization of Electrode Materials for Lithium Ion and Sodium Ion Batteries Using Synchrotron Radiation Techniques

Summary

We describe the use of synchrotron X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) techniques to probe details of intercalation/deintercalation processes in electrode materials for Li-ion and Na-ion batteries. Both in situ and ex situ experiments are used to understand structural behavior relevant to the operation of devices

Abstract

Intercalation compounds such as transition metal oxides or phosphates are the most commonly used electrode materials in Li-ion and Na-ion batteries. During insertion or removal of alkali metal ions, the redox states of transition metals in the compounds change and structural transformations such as phase transitions and/or lattice parameter increases or decreases occur. These behaviors in turn determine important characteristics of the batteries such as the potential profiles, rate capabilities, and cycle lives. The extremely bright and tunable x-rays produced by synchrotron radiation allow rapid acquisition of high-resolution data that provide information about these processes. Transformations in the bulk materials, such as phase transitions, can be directly observed using X-ray diffraction (XRD), while X-ray absorption spectroscopy (XAS) gives information about the local electronic and geometric structures (e.g. changes in redox states and bond lengths). In situ experiments carried out on operating cells are particularly useful because they allow direct correlation between the electrochemical and structural properties of the materials. These experiments are time-consuming and can be challenging to design due to the reactivity and air-sensitivity of the alkali metal anodes used in the half-cell configurations, and/or the possibility of signal interference from other cell components and hardware. For these reasons, it is appropriate to carry out ex situ experiments (e.g. on electrodes harvested from partially charged or cycled cells) in some cases. Here, we present detailed protocols for the preparation of both ex situ and in situ samples for experiments involving synchrotron radiation and demonstrate how these experiments are done.

Introduction

Lithium ion batteries for consumer electronics presently command an $11 billion market worldwide (http://www.marketresearch.com/David-Company-v3832/Lithium-Ion-Batteries-Outlook-Alternative-6842261/) and are the premier choice for emerging vehicular applications such as plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs). Analogs to these devices utilizing sodium ions rather than lithium are in earlier stages of development, but are considered attractive for large scale energy storage (i.e. grid applications) based on cost and supply security arguments^{1, 2}. Both dual intercalation systems work on the same principle; alkali metal ions shuttle between two electrodes acting as host structures, which undergo insertion processes at different potentials. The electrochemical cells themselves are relatively simple, consisting of composite positive and negative electrodes on current collectors, separated by a porous membrane saturated with an electrolytic solution usually consisting of a salt dissolved in a mixture of organic solvents (Figure 1). Graphite and LiCoO₂ are the most commonly employed negative and positive electrodes, respectively, for lithium ion batteries. Several alternative electrode materials have also been developed for specific applications, including variants of LiMn₂O₄ spinel, LiFePO₄ with the olivine structure, and NMCs (LiNi_xMn_xCo_1-2xO₂ compounds) for positives, and hard carbons, Li₄Ti₅O₁₂, and alloys of lithium with tin for negatives³. High voltage materials like LiNi_0.5Mn_1.5O₄, new high capacity materials such as layered-layered composites (e.g. xLi₂MnO₃·(1-x)LiMn_0.5Ni_0.5O₂), compounds with transition metals that can undergo multiple changes in redox states, and Li-Si alloy anodes are currently subjects of intense research, and, if successfully deployed, should raise practical energy densities of lithium ion cells further. Another class of materials, known as conversion electrodes, in which transition metal oxides, sulfides, or fluorides are reversibly reduced to the metallic element and a lithium salt, are also under consideration for use as battery electrodes (primarily as replacements for anodes)⁴. For devices based on sodium, hard carbons, alloys, NASICON structures, and titanates are being investigated for use as anodes and various transition metal oxides and polyanionic compounds as cathodes.

Because lithium ion and sodium ion batteries are not based on fixed chemistries, their performance characteristics vary considerably depending on the electrodes that are employed. The redox behavior of the electrodes determines the potential profiles, rate capabilities, and cycle lives of the devices. Conventional powder X-ray diffraction (XRD) techniques can be used for initial structural characterization of pristine materials and ex situ measurements on cycled electrodes, but practical considerations such as low signal strength and the relatively long times needed to collect data limit the amount of information that can be obtained on the discharge and charge processes. In contrast, the high brilliance and short wavelengths of synchrotron radiation (e.g. λ=0.97 Å at the Stanford Synchrotron Radiation Lightsource's beamline 11-3), combined with the use of high throughput image detectors, permit acquisition of high-resolution data on samples in as little as 10 sec. In situ work is performed in transmission mode on cell components undergoing charge and discharge in hermetically sealed pouches transparent to X-rays, without having to stop operation to acquire data. As a result, electrode structural changes can be observed as "snapshots in time" as the cell cycles, and much more information can be obtained than with conventional techniques.

X-ray absorption spectroscopy (XAS), also sometimes referred to as X-ray Absorption Fine Structure (XAFS) gives information about the local electronic and geometric structure of materials. In XAS experiments, the photon energy is tuned to the characteristic absorption edges of the specific elements under investigation. Most commonly for battery materials, these energies correspond to the K-edges (1s orbitals) of the transition metals of interest, but soft XAS experiments tuned to O, F, C, B, N and the L_2,3 edges of first row transition metals are also sometimes carried out on ex situ samples⁵. The spectra generated by XAS experiments can be divided into several distinct regions, containing different information (see Newville, M., Fundamentals of XAFS, http://xafs.org/Tutorials?action=AttachFile&do=get&target=Newville_xas_fundamentals.pdf). The main feature, consisting of the absorption edge and extending about 30-50 eV beyond is the X-ray Absorption Near Edge Structure (XANES) region and indicates the ionization threshold to continuum states. This contains information about the oxidation state and coordination chemistry of the absorber. The higher energy portion of the spectrum is known as the Extended X-ray Absorption Fine Structure (EXAFS) region and corresponds to the scattering of the ejected photoelectron off neighboring atoms. Fourier analysis of this region gives short-range structural information such as bond lengths and the numbers and types of neighboring ions. Preedge features below the characteristic absorption energies of some compounds also sometimes appear. These arise from dipole forbidden electronic transitions to empty bound states for octahedral geometries, or dipole allowed orbital hybridization effects in tetrahedral ones and can often be correlated to the local symmetry of the absorbing ion (e.g. whether it is tetrahedrally or octahedrally coordinated)⁶.

XAS is a particularly useful technique for studying mixed metal systems such as NMCs to determine initial redox states and which transition metal ions undergo redox during delithiation and lithiation processes. Data on several different metals can be obtained rapidly in a single experiment and interpretation is reasonably straightforward. In contrast, Mossbauer spectroscopy is limited to only a few metals used in battery materials (primarily, Fe and Sn). While magnetic measurements can also be used to determine oxidation states, magnetic coupling effects can complicate interpretation particularly for complex oxides such as the NMCs.

Well-planned and -executed in situ and ex situ synchrotron XRD and XAS experiments give complementary information and allow a more complete picture to be formed of the structural changes occurring in electrode materials during normal battery operation than what can be obtained via conventional techniques. This, in turn, gives a greater understanding of what governs the electrochemical behavior of the devices.

Protocol

1. Planning of Experiments

1. Identify beam line experiments of interest. Refer to beam line webpages as guides. For SSRL XAS and XRD, these are: http://www-ssrl.slac.stanford.edu/beamlines/bl4-1/ and http://www-ssrl.slac.stanford.edu/beamlines/bl4-3/ and http://www-ssrl.slac.stanford.edu/beamlines/bl11-3/
   1. Contact beam line scientist and discuss details of experiment.
2. Check deadlines and requirements for proposals by going to the relevant website.
3. Write beam time proposal and submit.
4. After the proposal has been scored, schedule beam time.
5. Follow instructions provided by the facility to prepare for beam time. Consider the details of the experiment, transport of materials (especially of devices containing alkali metals) and equipment, and any safety concerns. Safety training is generally required for new users.

2. Preparation of Materials, Electrodes, and Cells

1. Synthesize or obtain active material of interest.
2. Characterize material by conventional X-ray powder diffraction, using steps 2.2.1-2.2.9.
   1. Grind powder and sieve to ensure uniform particle size distribution.
   2. Load sample into sample holder. Remove backplate from holder and place it against a glass slide. Fill cavity with powder, then attach backplate, flip holder and remove slide. This ensures that the powder is even with the surface of the holder and that the surface is flat.
   3. Log into logbook for the diffractometer.
   4. Insert sample holder into diffractometer and align.
   5. Close doors of diffractometer.
   6. Using Data Collector program on computer attached to Panalytical diffractometer, increase voltage and current to values appropriate for measurement. Select slits and beam masks for the experiment. Select or modify program for scan.
   7. Start program and name datafile. Lock diffractometer doors by swiping badge when prompted by the program. Collect data.
   8. Analyze pattern using High Score program. In particular, look for the presence of impurities (extra reflections) and whether pattern matches that of reference materials or calculated patterns.
   9. Remove sample from diffractometer. Turn down current and voltage, and close doors. Log out, noting any unusual conditions.
3. Obtain scanning electron micrographs to assess particle morphologies, using steps 2.3.1-2.3.10.
   1. Prepare sample by attaching carbon tape to an aluminum stub, and sprinkling sample powder onto sticky side. Test for magnetism by holding a kitchen magnet over the sample.
   2. Insert sample into SEM chamber via airlock.
   3. Once vacuum is established, turn accelerating voltage on.
   4. In low magnification mode, adjust contrast and brightness. This is most conveniently done using the ACB button.
   5. Find area of interest by manually scanning in the x and y directions.
   6. Switch to SEM or gentle beam modes if higher magnification is desired. Select desired detector, and set working distance to values appropriate for the experiment.
   7. Adjust contrast and brightness using ACB knob.
   8. Focus image with stage z control.
   9. Align beam, correct astigmatism and focus using x and y knobs.
   10. Take pictures as desired, using photo button, and save to appropriate folder on the computer.
   11. When finished, turn off accelerating voltage. Move sample to exchange position and remove from chamber via airlock.
4. Conduct elemental analysis by ICP if needed, and characterize materials with any other desired techniques such as IR or Raman spectroscopy.
5. Fabricate electrodes, using steps 2.5.1-2.5.8.
   1. Make a solution of 5-6% (wt.) polyvinylidene fluoride (PVDF) in N-methylpyrolidinone (NMP).
   2. Mill together active material and conductive additive (acetylene black, graphite, etc.).
   3. Add NMP solution from step 2.3.1 to dry powder from step 2.3.2 and mix. Proportions vary depending on the nature of the active material, but a final dry composition of 80:10:10 (active material:PVDF:conductive additive) is common.
   4. Using a doctor blade and (optionally) a vacuum table, cast electrode slurry onto an Al or Cu current collector. Carbon coated Al foil may be used for Li-ion battery cathode materials and all Na-ion electrode materials, and Cu foil is used for Li-ion anode materials.
   5. Allow electrodes to air-dry.
   6. Dry electrodes further using an IR lamp, hot plate, or vacuum oven.
   7. Cut or punch electrodes to the size needed. Weigh electrodes.
   8. Transfer electrodes to an inert atmosphere glovebox. An additional drying step using a vacuum heated antechamber attached to the glovebox is recommended to remove all residual moisture.
6. Assemble electrochemical devices (usually coin cells, but other configurations can be used for electrochemical characterization) for initial characterization, ex situ samples, and/or beam line experiment, using steps 2.6.1-2.6.7.
   1. Gather all needed components in the inert atmosphere glovebox.
   2. Cut lithium or sodium foil to the desired size.
   3. Cut microporous separator to the desired size.
   4. Layer components in this order in the device: electrode, separator, electrolytic solution, and Li or Na foil.
   5. Add spacers and wave washers as needed.
   6. Seal cell using a coin cell press.
   7. For in situ XRD experiments, attach tabs to either side of coin cell and seal device in polyester pouch.
7. Perform electrochemical experiment for initial characterization or ex situ work, using steps 2.7.1-2.7.6.
   1. Connect leads from the potentiostat/galvanostat or cycler to device and measure open circuit potential.
   2. Write program for the electrochemical experiment desired or select an archived program.
   3. Run experiment and collect data.
   4. For ex situ experiments, disassemble the device in glovebox, taking care not to short-circuit it. For coin cells, use either a coin cell disassembler tool or pliers wrapped with Teflon tape.
   5. Rinse electrodes with dimethylcarbonate to remove residual electrolyte salt. Allow them to dry.
   6. Cover electrodes for ex situ study with Kapton foil for XRD experiments or scotch tape for XAS and store in the glovebox until the experiment is carried out.
8. Powders intended for study by XAS should be sieved to ensure particle size homogeneity. They may then be sprinkled onto several pieces of scotch tape. A series of samples can then be prepared by stacking progressively more numerous pieces of the powdered tape together. This is particularly useful if the user is uncertain about the amount of powder needed for the optimal signal.
   1. Alternatively, powders for XAS measurements may be diluted with BN if the user is confident about what will result in the optimum signal.

3. Performance of Experiments at the Synchrotron Facility

1. Several days before the experiment is to begin, plan transport of materials and equipment to the facility.
   1. For devices containing alkali metal anodes, shipping is required to avoid hazards associated with transportation in personal or public vehicles.
   2. Equipment such as portable galvanostat/potentiostats and laptop computers and nonhazardous samples such as electrodes for ex situ work may be brought to the facility by the individual carrying out the experiments in any convenient fashion.
2. Check in and register at the facility.
3. For both in situ and ex situ XRD experiments, take a reference pattern of LaB₆ for purposes of calibration.
   1. Contact beamline scientist and personnel for instructions.
   2. Calibrate beam to find right beam conditions.
   3. Measure reference pattern of LaB₆.
4. For in situ XRD experiments, set up device and start experiment following steps 3.4.1-3.4.6.
   1. Insert pouch into Al pressure plates and ensure that holes are properly aligned to allow the X-ray beam to transmit.
   2. Find optimum beam position and exposure time. Prolonged exposure can lead to oversaturation. Decide whether sample will be rocked or stationary.
   3. Take initial pattern before electrochemistry is started.
   4. Attach leads from galvanostat/potentiostat to device.
   5. Start electrochemistry experiment.
   6. Obtain data. Once experiment is under way, data collection is automatic, and user need only to oversee to make sure experiment is going as planned.
5. Set up XAS experiments.
   1. Check in and contact beamline scientist and personnel for instructions.
   2. Insert sample and foil reference material (depending on metal that is being measured; e.g. Ni for Ni K edge).
   3. Align sample.
   4. Determine energy of specific metal edge using IFEFFIT's Hephaestus. Tune monochromator, then de-tune by about 30 % to eliminate higher order harmonics. Change gains to adjust I₁ and I₂ measure offsets.
   5. Take measurement. Two or more scans should be taken and merged for the element of interest.
   6. Repeat steps 3.5.3 to 3.5.5 for additional elements, as needed.

4. Data Analysis

1. For XRD work, calibrate the LaB₆ image.
   1. Download Area Diffraction Machine, which is available through the Google code (http://code.google.com/p/areadiffractionmachine/).
   2. Open the image for LaB₆ diffraction and use initial calibration values from the file header.
   3. Open the reference Q (=2π/d) values of LaB₆.
   4. Calibrate the LaB₆ diffraction image with the Q values and the initial guess of the calibration values.
   5. Obtain correct calibration values by image fitting.
   6. Save the calibration values into the calibration file.
2. Calibrate the data images from the experiment.
   1. Open the diffraction images from the experiment.
   2. Open the calibration file from the LaB₆ reference (saved in step 4.1.6).
   3. Open the reference Q (=2π/d) values of Al or Cu (current collectors for the electrodes) and use them as internal references.
   4. Calibrate the pattern images by image fitting.
   5. Integrate the image to Q vs. Intensity data (line scans).
   6. Fit patterns using the desired fitting program (CelRef, Powdercell, RIQAS, GSAS, etc.).
3. Process electrochemical data using any convenient plotting program (Excel, Origin, KaleidaGraph, Igor, etc.).
4. For XAS data, use ARTEMIS/ATHENA in the IFEFFIT software package for analysis.
   1. Calibrate data using the first peak in the derivative of the absorption spectra of the reference metals.
   2. Merge like scans.
   3. Subtract background and normalize data.
   4. Use the AUTOBK function to isolate the EXAFS data.
   5. Fourier transform the EXAFS data.
   6. Use a least squares fit to the Fourier transformed spectra in R or k space to extract structural information.

Results

Figure 2 shows a typical sequence used for an in situ experiment. After synthesis and characterization of active material powders, composite electrodes are prepared from slurries containing the active material, a binder such as polyvinylidene fluoride (PVDF) and conductive additives such as carbon black or graphite suspended in N-methylpyrrolidinone (NMP), cast onto either aluminum or copper foil current collectors. Aluminum is used for lithium ion battery cathodes and all sodium ion battery ele...

Discussion

Analysis of XANES data indicates that as-made LiNi_xCo_1-2xMn_xO₂ (0.01≤x≤1) compounds contains Ni²⁺, Co³⁺, and Mn⁴⁺.¹⁰ A recent in situ XAS study on LiNi_0.4Co_0.15Al_0.05Mn_0.4O₂ showed that Ni²⁺ was oxidized to Ni³⁺ and, ultimately, Ni⁴⁺ during delithiation, but that redox processes involving Co³⁺ contributed some capaci...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Equipment
Inert atmosphere glovebox	Vacuum Atmospheres	Custom order, contact vendors	Used during cell assembly and to store alkali metals and moisture sensitive components. (http://vac-atm.com)
Inert atmosphere glovebox	Mbraun		Various sizes (single, double) available, many options such as mini or heated antechambers oxygen/water removal systems, shelving, electrical feedthroughs, etc. (http://www.mbraunusa.com)
X-ray powder diffractometer (XRD)	Panalytical	X'Pert Powder	X'Pert is a modular system. Many accessories available for specialized experiments. (www.panalytical.com)
X-ray powder diffractometer (XRD)	Bruker	Bruker D2 Phaser	Bruker D2 Phaser is compact and good for routine powder analyses. (www.bruker.com)
Scanning Electron Microscope (SEM)		JSM7500F	High resolution field emission scanning electron microscope with numerous customizable options. JEOL (http://www.jeolusa.com) Low cost tabletop versions also available. Contact vendor for options.
Pouch Sealer	VWR	11214-107	Used to seal pouches for in situ work. (https://us.vwr.com)
Manual crimping tool	Pred Materials	HSHCC-2016, 2025, 2032, 2320	Used to seal coin cells. Match size to coin cell hardware. (www.predmaterials.com)
Coin cell disassembling tool	Pred Materials	Contact vendor	Used to take apart coin cells to recover electrodes for ex situ work. Needlenose pliers can also be used. Cover ends with Teflon tape to avoid shorting cells. (www.predmaterials.com)
Film casting knives	BYK Gardner	4301, 4302, 4303, 4304,4305,2325, 2326,2327,2328, 2329	Used to cast electrodes films from slurries. Different sizes available, with either metric or English gradations. Bar film or Baker-type applicators and doctor blades are less versatile but lower cost options. Can be used by hand or with automatic film applicators. (https://www.byk.com)
Doctor blades, Baker applicators	Pred Materials	Baker type applicator and doctor blade. Film casting knives also available.	Used to cast electrodes films from slurries. Different sizes available, with either metric or English gradations. Bar film or Baker-type applicators and doctor blades are less versatile but lower cost options. Can be used by hand or with automatic film applicators. (www.predmaterials.com)
Automatic film applicator	BYK Gardner	2101, 2105, 2121, 2122	Optional. Used with bar applicators, doctor blades, or film casting knives for automatic electrode film production. Films can also be made by hand but are less uniform. (https://www.byk.com)
Automatic film applicator	Pred Materials	Contact vendor	Optional. Used with bar applicators, doctor blades, or film casting knives for automatic electrode film production. Films can also be made by hand but are less uniform. (www.predmaterials.com)
Potentiostat/Galvanostat	Bio-Logic Science Instruments	VSP	Portable 5 channel computer-controlled potentiostat/galvanostat used to cycle cells for in situ experiments. (http://www.bio-logic.info)
Potentiostat/Galvanostat	Gamry Instruments	Reference 3000	Portable single channel computer-controlled potentiostat/galvanostat used to cycle cells for in situ experiments. (www.gamry.com)
The Area Diffraction Machine		Free download	Used for analysis of 2D diffraction data. Mac and Windows versions available. http://code.google.com/p/areadiffractionmachine/
IFEFFIT		Free download	Suite of interactive programs for XAS analysis, including Hephaestus, Athena, and Artemis. Available for Mac, Windows, and UNIX. http://cars9.uchicago.edu/ifeffit/
SIXPACK		Free download	XAS analysis program that builds on IFEFFIT. Windows and Mac versions. http://home.comcast.net/~sam_webb/sixpack.html
CelRef		Free download	Graphical unit cell refinement. Windows only. http://www.ccp14.ac.uk/tutorial/lmgp/celref.htm and http://www.ccp14.ac.uk/ccp/web-mirrors/lmgp-laugier-bochu/
Reagent/Material
Electrode active materials	various		Synthesized in-house or obtained from various suppliers.
Synthetic flake graphite	Timcal	SFG-6	Conductive additive for electrodes. (www.timcal.com)
Acetylene black	Denka	Denka Black	Conductive additive for electrodes. (http://www.denka.co.jp/eng/index.html)
1-methyl-2-pyrrolidinone (NMP)	Sigma-Aldrich	328634	Used to make electrode slurries. (www.sigmaaldrich.com)
Al current collectors	Exopack	z-flo 2650	Carbon-coated foils. Coated on one side. (http://www.exopackadvancedcoatings.com)
Al current collectors	Alfa-Aesar	10558	0.025 mm (0.001 in) thick, 30 cm x 30 cm (12 in x 12 in), 99.45% (metals basis), uncoated (http://www.alfa.com)
Cu current collectors	Pred Materials	Electrodeposited Cu foil	For use with anode materials for Li-ion batteries. (www.predmaterials.com)
Lithium foil	Rockwood Lithium	Contact vendor	Anode for half cells. Available in different thicknesses and widths. Reactive and air sensitive. Store and handle in an inert atmosphere glovebox under He or Ar (reacts with N2). (www.rockwoodlithium.com)
Lithium foil	Sigma-Aldrich	320080	Anode for half cells. Available in different thicknesses and widths. Reactive and air sensitive. Store and handle in an inert atmosphere glovebox under He or Ar (reacts with N2). (www.sigmaaldrich.com)
Sodium ingot	Sigma-Aldrich	282065	Anodes for half cells. Can be extruded into foils. Reactive and air sensitive. Store and handle in an inert atmosphere glovebox under He only. (www.sigmaaldrich.com)
Electrolyte solutions	BASF	Selectilyte P-Series contact vendor	Contact vendor for desired formulations. (http://www.catalysts.basf.com/p02/USWeb-Internet/catalysts/en/content/microsites/catalysts/prods-inds/batt-mats/electrolytes)
Dimethyl carbonate (DMC)	Sigma-Aldrich	517127	Used to wash electrodes for ex situ experiments. (www.sigmaaldrich.com)
Microporous separators	Celgard	2400	Polypropylene membranes (http://www.celgard.com)
Coin cell hardware (case, cap, gasket)	Pred Materials	CR2016, CR2025, CR2320, CR2032	Match size to available crimping tool, Al-clad components also available. (www.predmaterials.com)
Wave washers	Pred Materials	SUS316L	(www.predmaterials.com)
Spacers	Pred Materials	SUS316L	(www.predmaterials.com)
Ni and Al pretaped tabs	Pred Materials	Contact vendor	Sizes subject to change. Inquire about custom orders. (www.predmaterials.com)
Polyester pouches	VWR	11214-301	Used to seal electrochemical cells for in situ work. Avoid heavy duty pouches because of strong signal interference. (https://us.vwr.com)
Kapton film	McMaster-Carr	7648A735	Used to cover electrodes for ex situ experiments, 0.0025 in thick (www.mcmaster.com)
Helium, Argon and 4-10% hydrogen in helium or argon	Air Products	contact vendor for desired compositions and purity levels	Helium or argon used to fill glovebox where cell assembly is carried out and alkali metal is stored. (http://www.airproducts.com/products/gases.aspx)
Do not use nitrogen because it reacts with lithium. Use only helium if sodium is being stored. Purity level needed depends on whether the glovebox is equipped with a water and oxygen removal system. Hydrogen mixtures needed to regenerate water/oxygen removal system, if present or any other suitable gas supplier