The Metallurgical Maze: Challenges in Battery Pack Manufacturing

Dr. Evans Mogire

Lithium-ion battery packs, considered as one of the pre-EV motor components are at the core of electric vehicle (EV) revolution, but behind their silent hum lies a battleground of metallurgical challenges. As engineers strive for higher energy density, faster charging rates, and longer life cycles, they must navigate a minefield of materials science hurdles.

The anatomy of a battery pack

At its core, the battery pack is an intricate assembly of individual cells organized into modules, housed within a robust pack structure. It all begins with the battery cell, comprising:

• Cathode (the energy workhorse): Common materials include nickel-manganese-cobalt oxide (NMC), Lithium Iron phosphate (LFP).
• Anode (the ion host): Typically made of graphite, hosting lithium ions during charging.
• Electrolyte (the lithium highway): Can be a solution, gel, or solid-state material in next-gen batteries.
• Separators (the silent guardians): Prevent direct contact between the cathode and anode while allowing lithium ions to pass.

Each component presents unique metallurgical challenges. Scaling up to the battery pack level, considerations extend to current collectors, busbars, and cooling plates, where welding and material selection become increasingly complex. This article will highlight the key areas where metallographic analysis is fundamental to the understanding of these components.

1.???? The Great Dissimilar Metal Dilemma

EV battery packs are a cocktail of metals - aluminium (Al), copper (Cu), nickel (Ni), and steel (Fe) - each with its own personality.

• Aluminium is light but stubbornly oxidizes.
• Copper is an electrical maestro but loves to reflect laser beams.
• Nickel is corrosion-resistant but introduces residual stresses.
• Steel? Strong but refuses to bond well with aluminium.

Trying to weld these together is like persuading cats and dogs to live in harmony. The culprit? Intermetallic compounds (IMCs) - hard, brittle phases that form at the interface of dissimilar metals, reducing mechanical strength and increasing electrical resistance. A joint filled with IMCs is the metallurgical equivalent of a ticking time bomb. Additionally, presence of voids and/or microcracks create a deleterious zone affecting the performance of the weld and need to be minimised/eliminated as much as possible.

Recent studies highlight the role of process optimizations, such as laser beam offset towards steel in Fe-Cu welding, and the use of interlayers like Ni, Zn, and Sn to mitigate Fe-Al reactions. In Al-Cu welding, pulsed lasers and sinusoidal beam oscillation have been shown to suppress excessive IMC growth, enhancing mechanical and electrical properties.

2.???? Welding Woes: Too Hot to Handle?

In the battery world, the wrong welding technique can mean higher resistance, weaker joints, or a catastrophic failure. The three main contenders in EV battery welding are:

• Laser Beam Welding (LBW): Precise and fast but struggles with reflectivity (looking at you, copper!) and deep penetration. New green and blue lasers are improving energy absorption on Cu and Al surfaces.
• Ultrasonic Welding (UW): Ideal for delicate aluminium-copper joints, but excessive vibration can damage battery cells. This is used extensively in Tesla battery connections.
• Resistance Spot Welding (RSW): Reliable but less effective on high-conductivity materials like aluminium and copper.

Recent findings emphasize the importance of beam offset and wobbling techniques to control heat distribution and IMC formation. Additionally, hybrid welding techniques combining laser and ultrasonic welding are proving to be effective in reducing defects.

3.???? Thermal Expansion Tension: The Hot & Cold War

Thermal expansion mismatch is another villain lurking in battery assembly. Materials like aluminium expand twice as much as steel when heated. If not accounted for, the stress induced at weld joints can lead to cracking, delamination, or fatigue failures over time. Imagine your battery pack expanding and contracting with every charge cycle - those joints take a beating!

Using Ni-coated interlayers has been shown to minimize stress in Al-Cu and Fe-Al welding, improving overall joint longevity. Cooling techniques, such as Cu backing plates, have also been effective in controlling thermal expansion effects.

4.???? Corrosion Catastrophes: When Metal Goes Rogue

Battery modules live in a harsh environment - exposed to temperature swings, humidity, and even road salts. Galvanic corrosion is a major issue, especially where aluminium meets copper or steel.

• Steel-to-Aluminium joints? A classic recipe for corrosion.
• Copper busbars in contact with aluminium? Expect electrochemical chaos.

Recent studies highlight the use of Zn coatings and interlayers to prevent Al-Fe corrosion and silver coatings for Al-Cu connections. Additionally, Pb and Sn interlayers have shown improvements in corrosion resistance and mechanical stability.

5.???? The Future: Smarter Metallurgy, Smarter Batteries

To tame these challenges, engineers are experimenting with:

• Hybrid welding techniques (combining laser and ultrasonic welding).
• Advanced coatings and interlayers to mitigate IMC formation.
• Real-time weld monitoring with AI for defect detection.
• New alloy compositions that improve metallurgical compatibility.
• Laser beam wobbling and sinusoidal oscillation techniques to control IMC growth and heat distribution.

Final Thoughts: Metallurgy Matters!

EV battery packs might look sleek on the outside, but inside, they are a masterclass in materials science, thermodynamics, and precision engineering. The road to better battery manufacturing is paved with smarter metallurgy, optimized welding, and a deep understanding of how different metals interact.

The next series of articles will focus on the best practices on sample preparation and metallurgical evaluation of battery pack components.

Have you worked on battery module or pack metallographic analysis? Share your experiences and insights. For further queries please contact us on [email protected] [email protected] or our website at www.buehler.com and for solutions applicable to automotive industry.

Let’s collaborate - one microstructure at a time.

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