In Situ Neutron Powder Diffraction Using Custom-made Lithium-ion Batteries

Summary

We describe the design and construction of an electrochemical cell for the examination of electrode materials using in situ neutron powder diffraction (NPD). We briefly comment on alternate in situ NPD cell designs and discuss methods for the analysis of the corresponding in situ NPD data produced using this cell.

Abstract

Li-ion batteries are widely used in portable electronic devices and are considered as promising candidates for higher-energy applications such as electric vehicles.^1,2 However, many challenges, such as energy density and battery lifetimes, need to be overcome before this particular battery technology can be widely implemented in such applications.³ This research is challenging, and we outline a method to address these challenges using in situ NPD to probe the crystal structure of electrodes undergoing electrochemical cycling (charge/discharge) in a battery. NPD data help determine the underlying structural mechanism responsible for a range of electrode properties, and this information can direct the development of better electrodes and batteries.

We briefly review six types of battery designs custom-made for NPD experiments and detail the method to construct the ‘roll-over’ cell that we have successfully used on the high-intensity NPD instrument, WOMBAT, at the Australian Nuclear Science and Technology Organisation (ANSTO). The design considerations and materials used for cell construction are discussed in conjunction with aspects of the actual in situ NPD experiment and initial directions are presented on how to analyze such complex in situ data.

Introduction

Rechargeable lithium-ion batteries provide portable energy for modern electronics and are important in high-energy applications such as electric vehicles and as energy storage devices for large-scale renewable energy generation.^3-7 A number of challenges remain to achieve widespread use of rechargeable batteries in vehicular and large-scale storage, including energy densities and safety. The use of in situ methods to probe atomic and molecular-scale battery function during operation are becoming increasingly common as the information gained in such experiments can direct methods to improve existing battery materials, e.g. by identifying possible failure mechanisms,^8-10 and by revealing crystal structures that could be considered for the next generation of materials.¹¹

A primary goal of in situ NPD is to probe the crystal structure evolution of the components inside a battery as a function of charge/discharge. In order to measure the crystal structure evolution the components must be crystalline, which focuses such studies on crystallographically-ordered electrodes. It is at the electrodes that the charge carrier (lithium) is inserted/extracted and such changes are followed by NPD. In situ NPD offers the possibility to “track” not only the reaction mechanism and lattice parameter evolution of the electrodes, but also the insertion/extraction of lithium from the electrodes. Essentially the charge carrier in lithium-ion batteries can be followed. This gives a lithium-centered view of the battery function and has been recently demonstrated in only a few studies.^11-13

NPD is an ideal technique to examine lithium-containing materials and lithium-ion batteries. This is because NPD relies on the interaction between a neutron beam and the sample. Unlike X-ray powder diffraction (XRD), where the interaction of the X-ray radiation is predominantly with the electrons of the sample and thus varies linearly with atomic number, in NPD the interaction is mediated by neutron-nuclei interactions that result in a more complex and apparently random variation with atomic number. Thus, in situ NPD is particularly promising for the study of lithium-ion battery materials due to factors such as the sensitivity of NPD towards lithium atoms in the presence of heavier elements, the non-destructive interaction of neutrons with the battery, and the high penetration depth of neutrons enabling the examination of the bulk crystal-structure of the battery components within whole batteries of the size used in commercial devices. Therefore, in situ NPD is particularly useful for the study of lithium-ion batteries as a result of these advantages. Despite this, the uptake of in situ NPD experiments by the battery-research community has been limited, accounting for only 25 publications since the first report of using in situ NPD for battery research in 1998.¹⁴ The limited uptake is due to some major experimental hurdles, such as the need to account for the large incoherent neutron-scattering cross section of hydrogen in the electrolyte solutions and separator in the battery, which is detrimental to the NPD signal. This is often overcome by substituting with deuterated (²H) electrolyte solutions and replacing the separator with alternate hydrogen-free or poor materials.¹⁵ Another hurdle is the need to have sufficient sample in the neutron beam, a requirement that often necessitates the use of thicker electrodes which in turn limits the maximum charging/discharging rate that can be applied to the battery. A more practical concern is the relatively small number of world-wide neutron diffractometers relative to X-ray diffractometers, and their capabilities — e.g. time and angular resolution. As new neutron diffractometers have come online and the abovementioned hurdles overcome, in situ NPD experiments have grown in number.

There are two options to conduct in situ NPD experiments, using either commercial or custom-built cells. Commercial cells have been demonstrated to reveal structural information, including the evolution of lithium content and distribution in electrodes.^8-11,16-20 However, using commercial cells limits the number of electrodes that can be studied to those already commercially available, and where manufacturers or select research facilities are engaged to produce commercial-type cells with as yet un-commercialized materials. The production of the commercial-type cells is dependent on the availability of sufficient quantities of electrode material for cell manufacture, typically of the order of kilograms and significantly higher than that used in battery research, which can be a barrier to cell production. Commercial cells typically feature two electrodes that evolve during charge/discharge and the evolution of both electrodes will be captured in the resulting diffraction patterns. This is because the neutron beam is highly penetrating and can penetrate the single lithium-ion cells (e.g., the entire volume of 18,650 cells). The evolution of the two electrodes can make the data analysis complicated, but if sufficient Bragg reflections of both electrodes are observed these can be modeled using whole powder-pattern methods. Nonetheless, custom-made half cells can be constructed in which one electrode is lithium and should not structurally change during charge/discharge and therefore act as an (or another) internal standard. This leaves only one electrode that should exhibit structural change, simplifying data analysis. Care must also be taken to ensure that all electrode reflections of interest are not overlapping with reflections from other components undergoing structural change in the cell. The advantage of a custom-made cell is that components can be swapped to alter reflection positions in diffraction patterns. Furthermore, custom-made cells allow researchers the option to, in principle, improve signal-to-noise ratios and to investigate materials that are made in smaller-scale research batches and thereby permitting the in situ NPD study of a larger variety of materials.

To date there have been six electrochemical cell designs for in situ NPD studies reported, including three cylindrical designs,^14,15,21,22 two coin-type cell designs^23-26 and a pouch cell design.^12,27 The first cylindrical cell design was limited in use to very low charging/discharging rates due to the large quantities of electrode materials used.^14,21 The roll-over design,¹⁵ detailed below, and modified version of the original cylindrical cell,²² have overcome many of the problems associated with the first cylindrical design, and can be used for reliably correlating the structure of electrode materials with their electrochemistry. Coin-cell designs for in situ NPD also allow similar quantities of electrode materials to be probed relative to the roll-over cell, while featuring subtle differences in terms of construction, applicable charging rates, and cost.¹⁵ In particular, the coin-cell type was recently reported to have been constructed using a Ti-Zn alloy as the casing material (null-matrix) which produces no signal in the NPD patterns.²⁶ This is similar to the use of vanadium cans in the roll-over design described below. A key factor that can influence applicable charge/discharge rates (and polarization) is electrode thickness, where typically thicker electrodes require the application of lower current. The cell designs that are now becoming more popular are the pouch cells with sheets of multiple individual cells connected in parallel, or sheets that are rolled in a similar manner to the construction of lithium-ion batteries found in mobile electronics.^12,27 This cell is rectangular (a pouch) that can function at higher charge/discharge rates than the roll-over or coin-type cells. In this work, we focus on the ‘roll-over’ cell design, illustrating the cell construction, use, and some results using the cell.

The electrode preparation for the roll-over design batteries is practically similar to the electrode preparation for use in conventional coin-cell batteries. The electrode can be cast onto the current collector by doctor blading, with the biggest difference being that the electrode needs to span dimensions larger than 35 x 120-150 mm. This can be hard to uniformly coat with every electrode material. Layers of the electrode on current collector, separator, and lithium metal-foil on current collector are arranged, rolled, and inserted into vanadium cans. The electrolyte used is LiPF₆, one of the most commonly used salts in lithium-ion batteries with deuterated ethylene carbonate and deuterated dimethyl carbonate. This cell has been used successfully in four reported studies and will be described in greater detail below.^15,28-30

Protocol

1. Cell Components Required Prior to Construction

NOTE: A vanadium can is conventionally used for NPD experiments and it is a wholly-vanadium tube that is sealed at one end and open at the other. There is virtually no signal in NPD data from vanadium.

1. Cut a piece of lithium metal-foil to dimensions matching the volume of the vanadium can. For example, cut a piece approximately 120 x 35 mm for a 9 mm diameter vanadium can. In addition, use thinner lithium foil to minimize neutron absorption, noting that thicknesses below 125 µm may be difficult to handle without tearing.
2. Pre-select the type of separator to be used. Cut a sheet of separator such that the dimensions are slightly larger than the electrodes, e.g. 140 x 40 mm.
   NOTE: While porous polyvinyl-difluoride (PVDF) membrane readily soaks up electrolyte, it is expensive and can be easily damaged and torn if not handled carefully during construction. Alternatively, commercially available polyethylene-based sheets are more robust, however they do not soak up electrolyte as readily and generally reduce the signal-to-noise due to the larger hydrogen content.
3. Make the positive electrode by following the guidelines set out by Marks et al.³¹ Namely, combine PVDF, carbon black, and the active material at a selected ratio. Typically, use a ratio of 10:10:80 of PVDF:carbon:active material, but adjust this depending on the material under investigation. Grind the mixture and add n-methyl pyrrolidone (NMP) dropwise until a slurry forms, then stir overnight.
4. Spread the mixture onto aluminum foil (20 µm thickness) using the doctor blade technique.
   1. Adhere the current collector sheet of dimensions 200 x 70 mm to a smooth surface (e.g. glass) by applying a few drops of ethanol on to the surface and placing the current collector on the surface. Alternatively, use an instrument which can pull a slight vacuum on the current collector from the smooth surface. Smooth out the current collector to ensure that there are no crinkles or creases prior to applying the slurry.
   2. Place a tooth or wide semi-circle shaped puddle of the slurry on one end of the current collector. Using a notch bar, roller or specially designed coater (a notch bar with a pre-defined height above the current collector, e.g. 100 or 200 µm is typically used) spread the slurry over the current collector by sliding the chosen device across the current collector and slurry, resulting in the spreading of the slurry onto the current collector surface.
   3. Gently remove the current collector from the smooth surface and place the current collector and spread slurry into a vacuum oven for drying.
      NOTE: The spreading technique is described in greater detail in Marks et al.³¹
5. Cut the positive electrode prepared in step 1.3 such that the dimensions match the lithium foil. Ensure that there is a “tab” of uncoated metal current collector approximately 0.5 cm in length at one end. To improve battery performance, press the dried positive electrode film into the current collector using a flat plate press.
   NOTE: Figure 1 shows the relative sizes of the separator and positive electrode components. Minimum active material quantity in the electrode is 300 mg, however, the larger the quantity (relative to other battery components), the better the NPD signal. A larger signal may allow more detailed information to be extracted from the NPD data and better temporal resolution.
6. Pre-prepare 1 M lithium hexafluorophosphate in a 1/1 vol% mixture of deuterated ethylene carbonate and deuterated dimethyl carbonate. Ensure that all the LiPF₆ is dissolved and the electrolyte is thoroughly mixed prior to use.
7. Cut a piece of current collector of the same dimensions as the positive electrode in step 1.5 and weigh the current collector and positive electrode. Subtract these masses to obtain the mass of the electrode mixture. Multiply the mass of the electrode mixture by 0.8 to give the mass of the active material.

2. Cell Construction

1. Prior to assembling the cell inside an argon filled glovebox, lay down either a plastic tray or some other non-metallic covering on the base of the gloxebox.
2. Stack the individual components in the following order: A long strip of separator, positive electrode with the slurry facing up and aluminum rod (or copper wire) wound in the “tab” at one end, the second strip of separator, and finally the lithium metal with copper wire wound on the end of the lithium metal (the same end as the aluminum rod).
3. Start rolling the layers from the end with the aluminum rod and copper wire, ensuring that the two electrodes do not come into contact.
4. If a polyethylene-based sheet was selected as the separator, occasionally add several drops of electrolyte to the separator between the lithium metal and positive electrode along the entire length of the stack. Alternatively, add the drops gradually during the rolling process. If PVDF membrane was used as the separator this step is not necessary.
5. Take care to ensure that the electrode is rolled tightly and that the layers remain aligned.
   NOTE: If the layers become misaligned the rolling process may need to be restarted, however, caution must be taken as the electrolyte solution is highly volatile and more may need to be added.
6. Ensure that the longer piece of separator completely wraps around the stack or roll such that the electrodes are not exposed (i.e. the electrodes don’t touch the vanadium housing).
7. Insert the rolled stack into the vanadium can such that the copper wire and aluminum rod protrude 2-3 cm beyond the top of the vanadium can. Add the remaining electrolyte dropwise into the top of the vanadium can, use 1.5 ml in total.
8. Add a rubber stopper with notches cut in the sides for the aluminum rod and copper wire into the top of the vanadium can. Seal the can by melting dental wax over the top of the can and around the end of the plastic sheath of the copper wire. Check that the final cell appears as shown in Figure 2.
9. Allow the cell to “age” or “wet” horizontally for 12-24 hr. Prior to use, test the open-circuit potential by connecting the aluminum rod and the copper wire to the terminals of a multi-meter and measuring the potential of the constructed cell. Also ensure that there are no leaks by visual inspection.

Results

We have demonstrated the versatility in using this roll-over cell in the literature^15,28-30 and here we present an example with the Li_0.18Sr_0.66Ti_0.5Nb_0.5O₃ electrode.³²

Prior to attempting a sequential Rietveld refinement (Rietveld refinements as a function of state-of-charge), a single refinement of a multiphase model to the first data set was performed, with this data collected for the pristine cell prior to curren...

Discussion

When designing and performing an in situ experiment, either with the “roll-over” neutron diffraction cell or another design, there are a number of aspects that must be carefully controlled to ensure a successful experiment. These include careful choice of the type and quantity of cell components, ensuring that the prepared electrode and final constructed cell are of high quality, choosing appropriate diffraction conditions, planning the electrochemical cycling steps to be performed in advance, and fi...

Materials

Name	Company	Catalog Number	Comments
Slurry Preparation
PVDF	MTI Corporation	EQ-Lib-PVDF	http://www.mtixtl.com/PVDFbinderforLi-ionbatteryelectrodes80g/bag-EQ-Lib-PVDF.aspx
Active Electrode Material	Researcher makes*		This is dependent on the electrode under investigation, typically made in-house by the researcher and varies every time
Carbon black	MTI Corporation	EQ-Lib-SuperC65	http://www.mtixtl.com/TimicalSUPERC65forLithium-IonBatteries80g/bag-EQ-Lib-SuperC65.aspx
NMP	MTI Corporation	EQ-Lib-NMP	http://www.mtixtl.com/N-Methyl-2-pyrrolidoneNMPsolventforPVDF 250g/bottleLib-NMP.aspx
Magnetic stirrer	IKA	C-MAG HS 7 IKAMAG	http://www.ika.in/owa/ika/catalog.product_detail?iProduct=3581200
Electrode Fabrication
Doctor blade (notch bar)	DPM Solutions Inc.	100, 200, 300 & 400 micron  4-Sided Notch Bar
Al or Cu current collectors	MTI Corporation	EQ-bcaf-15u-280	http://www.mtixtl.com/AluminumFoilforBatteryCathodeSub strate-EQ-bcaf-15u-280.aspx
Vacuum Oven	Binder	e.g. VD 53	http://www.binder-world.com/en/vacuum-drying-oven/vd-series/vd-53/
Flat-plate press	MTI Corporation	EQ-HP-88V-LD	http://www.mtixtl.com/25THydraulicFlat HotPress-EQ-HP-88V.aspx
Roll-over cell construction
V can
electrode on Al/Cu	MTI Corporation	EQ-bcaf-15u-280	http://www.mtixtl.com/AluminumFoilforBatteryCathodeSub strate-EQ-bcaf-15u-280.aspx
polyethylene-based or PVDF membrane	MTI Corporation	EQ-bsf-0025-400C	http://www.mtixtl.com/separatorfilm-EQ-bsf-0025-400C.aspx
LiPF6	Sigma-Aldrich	450227	http://www.sigmaaldrich.com/catalog/product/aldrich/450227?lang=en&region=AU
deuterated dimethyl carbonate	Cambridge Isotopes	DLM-3903-PK	http://shop.isotope.com/productdetails.aspx?id=10032379&itemno=DLM-3903-PK
deuterated ethylene carboante	CDN Isotopes	D-5489	https://www.cdnisotopes.com/as/products/specifications/D-5489.php?ei=YWVraWmjoJ1i0lZ7nkr0RpwHr Hxc9ornu14O4WUtZKbZWZrcq6j55 G0lOab3Wi0dMZ7xc+0Yse1leWVtZ LnrGKvta7v591o4JrnkbRowHt/r
Li metal foil	MTI Corporation	Lib-LiF-30M	http://www.mtixtl.com/Li-Foil-30,000 ml-35 mmW-0.17 mm Th.aspx
Rubber stopper cut to size	generic eraser		cut a generic eraser to size
dental wax	Ainsworth Dental	AIW042	http://www.ainsworthdental.com.au/catalogue/Ainsworth-Modelling-Wax-500g.html
Copper wire (insulated)	generic		sheathed Cu wire that can be cut to size
Aluminum rod (<2 mm diameter)	generic		cut to size as required
Glovebox	Mbraun	UNILab	http://www.mbraun.com/products/glovebox-workstations/unilab-glovebox/
Scissors	generic
Soldering iron	generic
In situ NPD
Appropriate neutron diffractometer	ANSTO	Wombat	http://www.ansto.gov.au/ResearchHub/Bragg/Facilities/Instruments/Wombat/
Potentiostat/galvanostat	Autolab	PGSTAT302N	http://www.ecochemie.nl/Products/Echem/NSeriesFolder/PGSTAT302N
Connections to battery from potentiostat/galvanostat	generic
Training of NPD instrument and use
Data analysis
Data visualisation and peak fitting, .e.g. LAMP suite	ILL	LAMP	http://www.ill.eu/instruments-support/computing-for-science/cs-software/all-software/lamp/
Rietveld analysis software, e.g. GSAS	APS	GSAS	https://subversion.xray.aps.anl.gov/trac/EXPGUI