Sample Preparation Techniques for Lithium Batteries

Due to their high energy and power densities compared to conventional batteries, lithium ion batteries (LIB) have found a wide application ranging from hybrid/full electric vehicles (H/EV), power tools, and portable electronics amongst others. 

The adoption of electric vehicle technology has been attributed to two main factors; first, their low impact on greenhouse gas emissions compared to traditional internal combustion engine primarily driven by low emission targets for ICU engines targeted at less that 95g/Km by 2021(Miller, 2015) (Tsiropoulos I., 2018). Secondly, is the increasing demand for a better range from a single charge that is at least more than half the current range achievable with ICU engines.

These factors have put more emphasis on cell chemistry and cell architecture, battery pack design, joining and manufacture. The selection of a particular type of lithium ion battery is thus dependent on the battery chemistry, which has a direct effect on its safety during use, its lifetime behavior, the charging/discharging time and the power or energy density. Battery packs consists of hundreds/thousands of individual cells assembled into packs, to provide the voltage and current density required for efficient running of H/EV cars.

The assembly of battery packs and/or individual cells can be metallographically evaluated to validate battery construction and chemistries. Various joining techniques used to make the packs can also be evaluated. This article highlights procedures that can be used to metallographically assess various battery components.

Sample Preparation Techniques

Sectioning

Prior to sectioning, the battery needs to be fully discharged, and no nominal voltage can be present. This is important as it minimizes the risks that are associated with short circuiting the cell. Lithium batteries should be sectioned in a dry state in order to avoid runaway exothermic reaction with the coolant and subsequent absorption into the laminate structure of the cell. The active components in both the cathode and anode laminates are made up of multi-materials which have a high affinity to water. This results in harmful corrosion products.

Sectioning was carried out using the IsoMet High speed, with the aid of double saddle chuck for cross-sections. A non-ferrous SiC abrasive blade was used. It is important to ensure that there is an extraction system fitted to the back of the machine to account for the dust generated.

Wet sectioning can be carried out for sectioning of the outer metallic casing without internal active cell elements. Wet sectioning was also used on welded terminal points. A diamond wafering wheel can be used instead of an abrasive blade, since the materials are mainly metallic (SumMet, 2018).

Showing sectioning on IsoMet high speed with (a) illustrating clamping configuration, (b) sectioned casing material without rolled cell element, (c) showing battery with cell element for cross-sectional view and (d) illustrating sectioned cell with characteristic laminate structure.

Figure 1 showing sectioning on IsoMet high speed with (a) illustrating clamping configuration, (b) sectioned casing material without rolled cell element, (c) showing battery with cell element for cross-sectional view and (d) illustrating sectioned cell with characteristic laminate structure.

Mounting

Lithium ion batteries can be prepared in both unmounted and mounted configurations. Unmounted samples should be hand prepared. For ease of handling, a jig fixture can be designed to hold the cross-sections in place when carrying out coarse to fine grinding steps. If the sample requires mounting, the castable mounting technique should be adopted. Epoxy-based resins are preferable, for example EpoxiCure 2 or Epothin 2, because of their low reactivity with battery constituent elements and low peak exotherm temperatures during curing.

Castable mounted samples are perfectly suited for dimensional checks, battery cap assembly weld evaluations and terminal weld evaluations. This is because the technique ensures weld integrity without the distortion and/or damage that occurs with a compression (hot) mounting process (SumMet, 2018).

Grinding and Polishing

Cell grinding should be carried out using SiC papers for several reasons. The lithium ion batteries consist of materials that have different abrasion rates and various polymeric materials, metallic components and compacted powders, among others. To ensure good planarity and uniform grinding, SiC papers offer the best alternative and do not cause too much subsurface damage to these materials.

A fine grit SiC paper is recommended for the initial grinding step, so long as it does not compromise the material removal process. Coarser grits can be used if high stock removal is desired. However, grits coarser than P600 (360 grit) should not be used. An additional advantage of using finer grits for initial grinding is that fewer subsequent steps are required.

In order to ensure better control of flatness without causing subsurface damage, dry grinding, at relatively low revolutions per minute of the platen, is recommended. During dry grinding, safety precautions must be considered because of the risk of dust inhalation. Using a dust mask and lower platen speeds significantly minimizes the risks associated with dry grinding. The mounted microsections were prepared as per the procedure in Table 1, using the Buehler EcoMet 30 (SumMet, 2018).

Table 1 shows the preparation routine used to prepare the LiCoO battery. This is applicable to other types of lithium ion battery chemistries.

Step No. Surface Abrasive Lubricant/
Extender
Force
(per specimen)
Time
(min:sec)
Platen Speed
(rpm)
Head
(rpm)
1 SiC Paper P600 Dry Moderate
Pressure
Until Plane 120 Manual
2 SiC Paper P1200 Dry Moderate
Pressure
Until Plane 120 Manual
3 SiC Paper P2500 Dry Moderate
Pressure
Until Plane 120 Manual
4 SiC Paper P4000 Dry Moderate
Pressure
Until Plane 120 Manual

 

Table 2 shows the preparation routine used to prepare battery casing components and welded interconnects for battery pack assembly.

Step No. Surface Abrasive Lubricant/
Extender
Force
(per specimen)
Time
(min:sec)
Platen Speed
(rpm)
Head
(rpm)
Rotation
1 SiC Paper P400 Water 25 N Until Plane 250 60 <<
2 SiC Paper P1200 Water 25 N 04:00 150 60 <<
3 PoliCloth 3 μm MetaDi MetaDi 30 N 04:00 150 60 <<
4 ChemoMet 0.05 μm MasterMet, 0.05 MasterPrep Water** 25 N 02:30 120 60 >>

 

Microscopy Analysis

Both optical and electron microscopical techniques were used to analyse the as-received cell, sectioned cells and micro-sections. For optical metallography, a Nikon LV150 upright microscope was used to give a detailed view of the laminate structure. A macro stand was used for photomicrography of the cell and its cross-sections. For Electron microscopy, energy dispersive spectroscopy (EDS) work was undertaken using a FEI electron microscope.

As the cell components are conductive, unmounted samples were ideal for electron microscopy work ensuring good elemental analysis of both the anode and cathode regions. Traditional techniques of metallographic preparation were carried out for external cell geometry without the active components.

Cell Structure and Description

The internal cell structure of a lithium ion battery is made up of an outer cylindrical casing which protects the internal active cell elements of a lithium metal oxide cathode, a graphite anode, an organic solvent and an electrolyte of lithium salt. The internal cell structure is common to all lithium batteries and the main variations are on the active cathode/anode chemistries. The rolled element is housed in the outer casing (Figure 2, a-e). Depending on the end application of the battery, either a polymeric outer sheath can be used (d and e), or the battery can be uncovered. The latter offers ideal surface for the battery to cool when assembled into a battery pack.

Illustrates the outer case structure of a cylindrical solid body lithium ion battery without the polymeric sheath with (a) showing the cathode, (b) anode side and (c) is a side view of the battery lid cap region, (d) and (e) illustrate the polymeric outer sheath.

Figure 2 illustrates the outer case structure of a cylindrical solid body lithium ion battery without the polymeric sheath with (a) showing the cathode, (b) anode side and (c) is a side view of the battery lid cap region, (d) and (e) illustrate the polymeric outer sheath.

The internal cell components consist of the cathode, a separator and the anode laminates, which are wound to make a cell element. The cathode terminal is connected to a PTC (positive temperature coefficient) switch and the battery lid, which has an in-built safety valve mechanism. This is illustrated in Figure 3. If the battery is used abnormally, for example excessively drawing current from the battery, the conductive polymer that makes up the PTC heats up and becomes resistive, stopping the current flow and acting as a short circuit protection. The PTC returns to a conductive state once it has cooled down, and this allows the battery to operate.

The safety valve mechanism consists of a membrane seal that ruptures under high pressure and relieves pressure build-up in the cell. However, this might cause the cell to leak and dry out. This issue has been mitigated by new designs which use a resealable vent. The safety value mechanism, the PTC (positive temperature coefficient) element and the battery lid are fixed to the open end of the battery. This is done by caulking them through the insulation sealing gasket, thereby sealing the inside of the battery can, Figure 3(a).

(a) schematic illustration of battery cross-section at the cap (open) end side showing the various components making up the outer casing, and (b) illustration of actual cell cross-section.

Figure 3 (a) schematic illustration of battery cross-section at the cap (open) end side showing the various components making up the outer casing, and (b) illustration of actual cell cross-section.

On the other hand, the laminate layer is wound around a metallic centrepin that consists of an anode and a cathode with their respective active elements, Figure 4. The cathode is made up of cathode active material layers on either side of a cathode current collector. The current collector is made of a metal foil, and this can be either stainless steel foil, aluminum foil or nickel foil.

Battery cross-section showing the laminate layers consisting of both the cathode and the anode, outer casing and centre pins. (B) shows a close-up of the active cell element layers around the centre pin.

Figure 4 Battery cross-section showing the laminate layers consisting of both the cathode and the anode, outer casing and centre pins. (B) shows a close-up of the active cell element layers around the centre pin.

The active material layer of the cathode is made of a formulation with a binder, conductive agent and ceramic. The ceramic is an inorganic oxide, for example SiO, Al2O, MgO, ZrO, TiO, NaO etc. Alumina is preferable because of superior diffusion of lithium. The active cathode materials include lithium sulphides, lithium oxides, or complex lithium-containing compounds like lithium phosphate compounds. Such compounds include lithium/nickel oxides (LiNiO), lithium/cobalt oxides (LiCoO), lithium/nickel/cobalt/manganese oxides (LiNiCoMnO) and lithium/cobalt/manganese oxides (LiNiCoO).

Additionally, they can be phosphoric acid compound based, for example lithium-iron-manganese-phosphoric acid compounds (LiFeMnPO) and lithium-iron-phosphoric acid compounds (LiFePO), Figure 6. Batteries are selected based on their power and energy densities, life time and safety. The most widely used batteries in the automotive industry are LiFePO, Titanate, NCM and LiCoO.

The conductive agent is made up of carbon-based materials for better conductivity, for example carbon black or graphite. Binders consist of fluorine-based polymers such as polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl fluoride and fluoride type rubbers.

The anode is a sandwich laminate that consists of a layer of anode active material between a current collector. The anode active material layer is made up of a conductive carbon doped with lithium, for example graphite, and bound with the same materials found on cathode laminate, Figure 7. The current collector is made of a metallic foil, for example nickel or copper coil.

The laminate structure can be assessed optically in order to evaluate the thickness, area or volume fraction of the anode/cathode layer material. Volume/area fraction analysis can give insights on energy density evaluation of active materials in Li-ion batteries, Figure 6. When electrode thickness was modelled as a function of energy density, the energy density was found to have a maximum point at a critical electrode thickness for a given discharge rate (Du, 2017). It was concluded that the limiting factor affected by Li-ion diffusion in active materials was cell polarization and underutilization of the active materials, as well as depletion in the electrolyte phase (Yu, 2013).

The area/volume fractions and thickness measurements were analyzed using OmniMet image analysis software. The software can be scripted to perform an overlay of multiple lines on the cathode and anode layers, irrespective of the geometry. For an in-depth analysis of the thickness, the software will also calculate feret diameter at a variety of angles (SumMet, 2018).

(a) Optical micrographs above illustrate the cell cross-sectional view. Figure A shows the outer casing and alternating laminate layers composed of anodic and cathodic components, (b) shows the structure towards the centrepin of the battery.

Figure 5 (a) Optical micrographs above illustrate the cell cross-sectional view. Figure A shows the outer casing and alternating laminate layers composed of anodic and cathodic components, (b) shows the structure towards the centrepin of the battery.

Illustrating active material area/volume area fraction analysis of (a) anode and (b) cathode layers, and (c) showing corresponding approximate thicknesses.

Illustrating active material area/volume area fraction analysis of (a) anode and (b) cathode layers, and (c) showing corresponding approximate thicknesses.

Illustrating active material area/volume area fraction analysis of (a) anode and (b) cathode layers, and (c) showing corresponding approximate thicknesses.

Figure 6 illustrating active material area/volume area fraction analysis of (a) anode and (b) cathode layers, and (c) showing corresponding approximate thicknesses.

Illustrates a higher magnification electron microscopy (EDS) analysis around the cathode laminate layer showing its compositional chemistry. It’s evident that remnant active and binder agent compositions have also been picked up.

Figure 7 illustrates a higher magnification electron microscopy (EDS) analysis around the cathode laminate layer showing its compositional chemistry. It’s evident that remnant active and binder agent compositions have also been picked up.

illustrates a lower magnification electron microscopy (EDS) analysis of the laminate structure of a Lithium ion battery with the main constituent components elemental map analysis shown. The micrographs clearly highlight the anode (carbon rich with Cu charge collector and cathode layers (cobalt rich) with Al charge collector.

Figure 8 illustrates a lower magnification electron microscopy (EDS) analysis of the laminate structure of a Lithium ion battery with the main constituent components elemental map analysis shown. The micrographs clearly highlight the anode (carbon rich with Cu charge collector and cathode layers (cobalt rich) with Al charge collector.

Cell Interconnects – Welding

Joining or welding technologies are areas of key development. There is increasing demand for low self-discharge, higher energy densities, and portability of battery packs as an alternative power source for H/EV’s. Typically, a battery pack consists of hundreds and/or thousands of individual lithium ion batteries, connected either in a series or in parallel configurations. This helps to achieve the required energy and power.

A battery pack can be a single module or several modules that are connected in series or parallel configurations with sensor and battery control systems encased in a housing structure. Robust and reliable interconnects are required for the module and/or packs. The joining methodology is dependent on the cell type and maintaining good mechanical, thermal and electrical behavior in use (Das, Li, Williams & Greenwood, 2018).

Resistance spot welding, laser beam welding and ultrasonic welding are the most common methods of connecting (welding) battery packs. The welding parameters and resultant welded area need to be of good quality, whilst not compromising the architecture of the battery casing. These techniques were investigated and it was shown that the laser welding technique had the lowest electrical contact resistance, with ultrasonic and spot techniques demonstrating considerably higher resistance which was attributed to imperfections (voids) that occur during the welding technique (Martin J. Brand, 2015).

It was concluded that different joining methodologies had distinct advantages and disadvantages. For example, joint partners with low conductivity such as nickel-plated steel conductors for 18650 cells, spot welding was ideal joining technique. For nearly all battery cell connections where welded components were clamped during the process of welding, ultrasonic welding was suitable.

However, ultrasonic welding of unclamped battery configurations led to crack formation in the conductors and damage to the battery cell. It was determined that laser welding was ideal for better quality welding and any battery cell could be connected, with fewer risk factors related to surface spatters and reflections (Steen, 2010).

During the weld tests, empty battery cans were joined using spot, laser and ultrasonic welding. This is shown in Figure 9. They were metallographically prepared in order to identify defects that affect electrical contact resistance and/or to investigate their microstructure. Interconnect samples made of copper were used because copper is highly conductive. As reported by Zhou (2000) and Martin J. Brand (2015), this poses a considerable challenge during welding, and it is likely that welding defects will be exhibited.

Illustrates the common welding techniques of lithium ion batteries with left - spot weld, middle - ultrasonic weld and right - laser beam welds interconnects on Li-Ion cylindrical cells, (Martin J. Brand, 2015).

Figure 9 illustrates the common welding techniques of lithium ion batteries with left - spot weld, middle - ultrasonic weld and right - laser beam welds interconnects on Li-Ion cylindrical cells, (Martin J. Brand, 2015).

Weld Preparation

The welded joins were prepared semi-automatically as per Table 3. This was to ensure that all component material types were prepared metallographically. Metallographic sections of a laser weld and a spot weld of a copper interconnect onto a battery can are shown in Figure 10.

Table 3 shows the preparation routine used to prepare the LiCoO battery. This will be  applicable to other types of lithium ion batteries.

Step No. Surface Abrasive Lubricant/
Extender
Force
(Per Specimen)
Time
(min:sec)
Platen Speed
(rpm)
Head Speed
(rpm)
Rotation
1 SiC/DGD equivalent P400 Water 5 lbs Until Plane 250 60 <<
2 SiC/DGD equivalent P1200 Water 5 lbs 01:30 250 60 <<
3 PoliCloth 3 μm MetaDi Fluid 5 lbs 04:00 150 60 <<
4 ChemoMet 0.06 μm MasterMet Water** 5 lbs 02:30 150 60 <<

 

A spot weld with its characteristic roots, attributed to the induced eddy current affecting the weld profile, is shown in Figure 11 (a). There are micro/macro cracks within the roots of the spot welded interconnect. These would compromise the integrity of the cell casing. The laser welded interconnect, shown in Figure 11 (b), had fewer microvoids. Furthermore, on certain areas it did not show weld defects that would affect the integrity of the cell casing. This confirms the observations made by Martin J. Brand (2015).

Examples of (a) ultrasonic and (B) laser beam welds before cross-sectioning and castable mounting to investigate the quality of the weld.

Figure 10 shows examples of (a) ultrasonic and (B) laser beam welds before cross-sectioning and castable mounting to investigate the quality of the weld.

Examples of polished cross-sections of (a) spot weld and (b) Laser weld. It’s evident that spot weld has more voids and appears to have penetrated deeper into the casing material and could compromise battery casing structure whereas the laser weld is moderately penetrating. This of course depends on welding parameters used and could be optimised to minimise observed defects.

Examples of polished cross-sections of (a) spot weld and (b) Laser weld. It’s evident that spot weld has more voids and appears to have penetrated deeper into the casing material and could compromise battery casing structure whereas the laser weld is moderately penetrating. This of course depends on welding parameters used and could be optimised to minimise observed defects.

Figure 11 shows examples of polished cross-sections of (a) spot weld and (b) Laser weld. It’s evident that spot weld has more voids and appears to have penetrated deeper into the casing material and could compromise battery casing structure whereas the laser weld is moderately penetrating. This of course depends on welding parameters used and could be optimised to minimise observed defects.

Summary

This article highlights the metallographic preparation of lithium ion battery cells found in battery packs/modules that are made up of the cylindrical 18650 cell type. The methodology discussed can also be applied to other battery types, for example those of pouch shaped and prismatic design. The safety precautions that need to be considered before sectioning cells are also discussed, as are the challenges in mounting and subsequent grinding/polishing procedures that are associated with this.

Furthermore, the metallographically prepared cell was also characterized optically, and elemental composition mapping of the cell structure was made possible with the use of an electron microscope. In addition, the cell interconnectivity through laser, spot and ultrasonic welds was prepared metallographically and characterized optically. Defects, including cracks and voids, relating to the welding methodology were also shown. This was to highlight their effect on performance behavior of the battery module/pack.

References

  • Das, A., Li, D., Williams, D., & Greenwood, D. (2018). Joining Technologies for Automotive Battery Systems Manufacturing. World Electric Vehicle, P22.
  • Du, Z. D. (2017). Understanding limiting factors in thick electrode performance as applied to high energy density Li-ion batteries. Journal of Applied Electrochemistry 47, 405-415.
  • Martin J. Brand, P. A. (2015). Welding techniques for battery cells and resulting electrical contact resistances. Journal of Energy Storage, 7-14.
  • Miller, P. (2015). State of the art and future developments in lithium-ion battery packs for passenger car applications. Johnson Matthey Technology Review.
  • Steen, W. M. (2010). Laser welding. In Laser Material Processing, 199-249.
  • SumMet, B. (2018). The Science Behind Materials preparation. Waukegan, Illinois, U.S.A. Retrieved from https://www. buehler.com/literature.php
  • Tsiropoulos I., T. N. (2018). Li-ion batteries for mobilityand stationary storage applications –Scenarios for costs and mar- ket growth. Publications Office of the European Union, 2018.
  • Yu, S. S. (2013). Model prediction and experiments for the electrode design optimization of lifepo 4/graphite electrodes in high capacity lithium-ion batteries. Bull. Korean Chem. Soc 34, 79.
  • Zhou, Y. P. (2000). Weldability of thin sheet metals during small-scale resistance spot welding using an alternating- current power supply. Journal of electronic materials 29, 1090-1099.

Acknowledgement

A special thank you to the University of Warwick – IARC
– energy innovation centre team for the provision of battery cans materials used in this article, and Advanced Manufacturing and Materials Centre for electron microscopy work.

This information has been sourced, reviewed and adapted from materials provided by Buehler.

For more information on this source, please visit Buehler.

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