Why Soft Pure Nickel Strip Is the Only Right Answer?

To understand the material selection logic, we must first recognise the actual operating conditions of battery interconnects. Whether for cylindrical cells, prismatic aluminium housings, or pouch cells, the interconnect (tab) faces three rigid constraints:

 Welding constraint

It must form a solid‑state weld with dissimilar metals (aluminium for the positive terminal, copper for the negative), with weld areas often only a few square millimetres. Any microscopic non‑conformity leads to a sharp increase in contact resistance and localised heating.

 Forming constraint

The interconnect must be bent through 90°, S‑curves, or U‑bends to accommodate cell layout, with bend radii often less than twice the material thickness-severe sharp‑angle bending.

 Lifetime constraint

During vehicle operation or charge/discharge cycling, the interconnect is subjected to ±0.5 mm to 1 mm cyclic vibration displacement, along with thermal shock from ‑40 °C to 85 °C, demanding long‑term dimensional stability and resistance to stress relaxation.

Under these three constraints, the performance gap between soft and half‑hard tempers is fundamental and irreconcilable.

2.Four Core Mechanisms Where Soft State Outperforms Half‑Hard State

  Welding Reliability – From "Point Contact" to "Area Fusion"

Ultrasonic welding is essentially a solid‑state joining process where high‑frequency mechanical vibration enables atomic diffusion across the interface. Soft nickel strip, with its low hardness (HV ≤ 110), undergoes sufficient local plastic deformation under sonotrode pressure, filling the microscopic surface irregularities of the cell tab and transforming "point contact" into "area fusion" – achieving an effective weld area of over 90%. In contrast, half‑hard strip (HV ≥ 160) resists deformation, forming only limited "line contact" at the weld spot, with actual fusion area often below 70%. This translates directly to: soft‑state welds deliver stable peel strengths ≥ 15 N/mm, while half‑hard batches show wide fluctuations between 4 and 10 N/mm – precisely the physical root cause of the "false weld" plague on production lines.

Moreover, soft nickel strip has a thinner and more uniform surface oxide layer, requiring no additional pickling or grinding before welding – it can go straight to the line. This not only saves pre‑treatment steps but also avoids weld‑consistency variations caused by uneven surface preparation. For automated lines, this consistency means fewer machine adjustments and higher overall equipment effectiveness (OEE).

 Bending Formability – The Toughness Baseline for 180° Folds Without Cracking

Battery interconnects routinely require 180° fold‑over or U‑bends. Soft nickel strip offers elongation ≥ 40%, allowing bend radii as tight as 0.5t (where t is thickness) without crack initiation. Its equiaxed grain structure and low dislocation density enable smooth slip deformation during bending, with minimal springback – facilitating subsequent assembly dimensional control. Half‑hard strip, however, has elongation plummeting below 15%. Its fibrous grains, when subjected to tensile stress on the outer bend surface, lack sufficient mobile dislocations and readily nucleate microcracks at grain boundaries – cracks that may be invisible to the naked eye but rapidly propagate into fracture sites during service. Third‑party test data show: under identical bending cycles, soft state withstands 12–15 repeated folds, while half‑hard typically fractures within 3–5 cycles.

Equally important, soft‑state material releases internal stress fully after bending, ensuring excellent dimensional stability – no "creeping springback" over time. This is critical for tab alignment when assembling multiple cells in parallel. Half‑hard, with its higher residual stress, exhibits unpredictable springback that often demands repeated tooling adjustments to maintain acceptable yield.

 Vibration Fatigue Resistance – Flexible Absorption vs. Rigid Stress Amplification

Continuous vibration during vehicle operation subjects the interconnect to cyclic bending fatigue. Soft nickel strip, with its low yield strength (≈150 MPa), undergoes initial micro‑elastic deformation under vibratory loads, dissipating vibration energy and effectively attenuating stress peaks – achieving a fatigue limit of approximately ±80 MPa at 10⁷ cycles. This is due to its low dislocation density and intact grain boundaries, which allow coordinated deformation through slip and rotation within grains, preventing stress concentration at any single location.

Half‑hard strip, boasting a yield strength of 380 MPa, might seem "stronger," but its excessive rigidity leaves vibration energy with no cushion. The vibratory stress concentrates directly at geometric discontinuities – the bend root or weld edge – creating a significant stress amplification effect.Furthermore, the cold‑worked microstructure of half‑hard contains numerous dislocation tangles and sub‑grain boundaries, which serve as preferential sites for crack initiation under cyclic loading. Fatigue tests demonstrate that under identical vibration conditions, half‑hard strip delivers less than one‑third the fatigue life of soft strip. This counter‑intuitive fact, validated by extensive engineering practice, holds true: in battery interconnects, flexibility outlasts rigidity. Soft state "absorbs" impact energy through its own compliant deformation, while half‑hard, by "fighting" the stress head‑on, amplifies it and accelerates fatigue failure.

 Contact Resistance Stability – Long‑Term Interface Integrity

The contact resistance after welding depends not only on the initial fusion area but also on thermal cycling and mechanical creep. Soft nickel strip, with its thermal expansion coefficient differing from aluminium/copper tabs, accommodates mismatch through its own plastic compliance, keeping the weld interface tightly mated over the long term – contact resistance variation under ‑40 °C to 85 °C thermal cycling is less than 5%. Meanwhile, under continuous current‑induced heating, soft material undergoes slight stress relaxation, which paradoxically helps maintain constant interfacial pressure and prevents contact loosening.

Half‑hard strip, due to its strong elastic recovery, tends to develop micro‑gaps at the weld interface during thermal cycling. Moreover, its higher yield strength, when subjected to thermal stresses, can promote fretting wear at the interface, accelerating irreversible contact resistance drift. For the Battery Management System (BMS), contact resistance drift directly translates into voltage‑sensing errors, compromising State‑of‑Charge (SOC) estimation accuracy – an outcome absolutely unacceptable for high‑end power batteries.

3. Selection Decision Matrix – One Table Settles the Debate

From a total‑cost‑of‑ownership perspective, soft strip carries a modest price premium over half‑hard of the same specification (roughly 3%–5% higher per kilogram). However, when factoring in improved weld yield, reduced bend scrap, saved rework labour, and long‑term reliability assurance, soft state actually delivers lower lifecycle costs.For safety‑critical products like power batteries, any compromise in material selection is a disservice to the end user.

4. Conclusions and Actionable Recommendations

A battery interconnect is not a structural load‑bearing component – it is a functional element that integrates electrical conduction, thermal dissipation, vibration damping, and fatigue resistance. In this application, soft pure nickel strip is irreplaceable, thanks to its outstanding weld compatibility, superior bend formability, excellent vibration fatigue resistance, and long‑term stable electrical contact performance.

We recommend that procurement and engineering teams explicitly specify in their technical datasheets: "Material temper: Soft (M), hardness range HV 80–110, elongation ≥ 40%, and accompanied by a mechanical property test report for each lot."In addition, perform a simple bend verification on every incoming lot – fold a sample 180°; soft state should show a smooth surface with no cracks. Any visible cracks or fracture indicate non‑conforming material temper and should be rejected.