Wendell Johnson Applications Engineer, Resistance and Solid-State Welding

1. Reversible Battery Tab Attachments Development of Resistance Heating-Based Process September 14, 2011 Wendell Johnson Applications Engineer, Resistance and Solid-State Welding Email: wjohnson@ewi.org Phone: 614.688.5172

2. Rising Importance of Battery Systems • Energy sources transition from fossil fuels • According to US DOE 1.2 million plug-in EVs on road by 2015 • Li-ion cells with voltage of about 3.7 V will require many cells connected into battery pack (system) • Li-ion cells use aluminum electrode tab and copper is typical of buss material • Aluminum-to-copper interconnects • Must be cost effective • Repeatable made • Best when repairable

3. Battery System Reliability • A battery system • Serial string for higher voltages • Parallel circuit groups for extending charge storage capacity • Protection circuits and control • Interconnects between cells • Auxiliary systems including cooling • Reliability of a serial string = 90% • For 50 cells and 50 connections • Each with 1/10th of 1% failure probability • Modularity adds replacement capability • Single cell or connection point replacement is ultimate

4. Battery System Assembly • Collection of cells and connections • View shows 24 battery cells • Two parallel groups • Connected serial 12 units • Single or multiple cells attached to a single point • Combinations ultimately lead to dissimilar metal joints – aluminum to copper

5. Experiences Related to Aluminum Tab to Copper Buss Attachment • Range of technologies have been demonstrated • Percussion welding • Ultrasonic welding • Laser welding • Deformation welding • Permanent connection • Can only be applied once to a joint • Development of reversible attachments • Ultrasonic tinned aluminum with lead-free solder • Then reflow to copper conductor

6. Value of Reversible Attachments • Large systems with serial connections can have a low system reliability • A 100 serial component system will have a combined reliability of 90% when each component has a failure probability of 1/10th of 1% • Low voltage cells reduce battery performance • Replaceable cells can maintain battery peak performance • Reduce pollution impact by scrapping a single cell versus a system of cells • Lower scrap reduces system cost and improves buyer acceptance

7. Multiple Tabs in a Joined Stack • Multiple cells attached to single connection point • Cell tabs can be joined by CHRSW • Four tabs placed between SS caps • Placed in spot welder the current heats SS • The process heats from outside in and cools is reverse • Differing from spot welding the reverse solidification can move contaminants to surface Four Loose Tabs SS Heaters on Stack Consolidated Tab Stack Peel Test Tab Separation

8. Process Conditions and Outcome • Aluminum tab stack • Four tabs of Alloy 1100 • 0.12-mm thick • 12-mm wide by 30-mm long • 304 SS tab heaters • 3-mm thick • 12-mm square • 100-kVA AC spot welder • 6-kA weld current • 1.1-kN welding force • Weld time 6 cycles or 100 ms • A 3-cycle downslope after weld

9. Pre-Tinning Aluminum Tab • Setup Conditions • SS wire brush 13 × 13 mm • Heat tab to about 250°C • Monitor with contact probe • Set iron temp to flow solder • Use SAC 305 • 60-kHz sonic horn set at about 3 W • Process Steps • Tip the iron with solder • Contact iron to brushed area • As tab wets apply sonic agitation • Move tip over surface watching for active surface

10. Tinning Continued • Single Tabs • Tab 0.13-mm thick × 13-mm wide • Tinned using smaller iron • Copper buss clips tinned using rosin core SAC 305 • Multi Tabs • Four joined tabs like above • Tinned using larger iron and hot plate • Copper buss clips tinned using mechanical brushing

11. Heating Tool Interface • Tool design • Close back access 12 mm above and below • Pinching action • High resistance on copper • Low resistance on aluminum • Cooling circuit near electrodes • Consideration for de-soldering

12. Setup to Reflow Solder • Parameters • Squeeze Force 378 N (85 lbf) • On time – 33 cycles (0.5 s) • Current – 6 kA • Hold time – 5 s • Setup • Anvilloy contact with Cu • RWMA-2 contact with Al • Electrode contacts are 10-mm diameter by 3-mm thick

13. Reflow Trials • First four trials with 4-tab tab stacks • Trials to produce assemblies for desolder testing • More assemblies made with hot plate reflow • Trials after No. 4 were with single tab • Additional assemblies for setup made using pinch tool

14. Temperature Trace for Reflow • Top trace probes were both in solder joint • Blue trace front of center • Purple trace near tab edge • Temp below 100°C in 1.5 s • Lower trace probe on Al tab 2 mm away from joint • Temperature remains below 50°C • Max reached at about 3 s

15. Results • Pulling tab axially or at right angle to bond plane • Single tab – failure at edge • Axial load 77 N (17.3 lb) • 90-degree load 52 N (11.7 lb) • Multi tab – failure tear of tabs at CHRSW bond edge

16. Removal of a Single Cell or Connection • Cut away cell from tab with special shear or knife tool • Engage the junction with resistance heating tool • Clamp device to separate tab from buss

17. De-Solder Tool – Cell Interface

18. Results of a De-Solder Trial Before Heating After Heating Close-up After Heating Removed From Tool Solder Bridge

19. De-Soldering Trials • De-soldering or de-attaching tabs with a mechanical clip • Clip was attached to the tab with tool clamped • Assembly heated without cooling of heads joint heated to flow point • Pinch jaws opened immediately on heating cycle completion • Separation of lower jaw pulls tab away from copper buss

20. Conclusions • Multiple aluminum tabs could be effectively joined into a stack using CHRW • Ultrasonic solder deposition could be used to pre-tin both aluminum and copper components • Reflow soldering facilitated the dissimilar Al-Cu attachment • Resistance de-attachment was also demonstrated using a wedging device • Tool must be specifically designed to fit in a confined space • A single new battery cell can be field replaceable and attached to the buss by using solder joints • Next steps are • Demonstrate with real battery system • Advance the design of tab removal mechanics • Demonstrate function with other tab configurations

21. Questions? Wendell Johnson Applications Engineer, Resistance and Solid-State Welding Email: wjohnson@ewi.org Phone: 614.688.5172