2Outline Developing EV Market Joining Issues for Vehicle Batteries Project with OSU Center for Automotive ResearchUltrasonic Metal WeldingLaser WeldingResistance Spot WeldingNondestructive EvaluationSummary and Acknowledgements
32011 Commercial EV and PHEV GM Plans 50,000+ Volts Nissan LeafChevrolet VoltGM Plans 50,000+ VoltsNissan plans 200,000+ EVsTesla working on Model SVolt is PHEV with 40 mile nominal electric range. Market price is about $40,000 with numerous state and Federal incentives for early adopters. GM has low lease rates to encourage potential buyers to lease instead of buy.Leaf is electric vehicle with 100 mile nominal range. Market price is about $32,500.Tesla has more than 6800 cells because it was designed when the only lithium cells commercially available were cylindrical cells designed for laptop computers. The range of the Tesla EV is more than 250 miles. Cost is over $100,000.GM and Nissan are committed to the EV market. Both have announced plans to manufacture more than 50,000 vehicles in 2012 even though sales in 2011 have been less than 1,000 vehicles per month due mostly to limited battery pack manufacturing capacity.Tesla Roadster
4EVs 2011-2014 Manufacturer Vehicle Audi eTron EV (2012); PHEV (2014) BMWMiniE EV (2012); City Car (2013)BYDE6 EV (2012); F3DM PHEV (2012)CodaSedan EV (2011)Chrysler/FiatFiat 500 EV (2012)FiskerKarma EV (2011)FordFusion HEV (commercial); Transit Connect EV (commercial); Escape HEV (commercial); Focus EV (2011); CMax PHEV (2013)GMVolt PHEV (commercial); Ampera PHEV (2011); Cadillac SRX HEV (2012)HondaInsight HEV (commercial); Civic HEV (commercial); Fit EV (2012)MazdaMazda 2 EV (2012)MitsubishiiMEV EV (2011)NissanLeaf EV (commercial); other platformsTeslaModel S EV (2012);ToyotaPrius HEV (commercial); Prius PHEV (2012); RAV4 HEV (2012)VolkswagenEup EV (2013); Gold EV (2013); Jetta EV (2013)Almost every major automotive manufacturer has plans to introduce a hybrid, plug-in hybrid or electric vehicle between now and Several start ups are also planning to bring vehicles to market.HEV have about cells/pack. Operational demand on the battery is not high for most systems. Nickel-metal hydride batteries can compete for these vehicles, but lithium batteries will gain market share.PHEV have cells per pack. EV have up to 300 cells per pack (range dependent).Cell manufacturers have capacity coming on line in 2011 and 2012 supported by the Federal government grants as part of the ARRA program in 2010.To get a return on investment, manufacturers will need to develop lower cost manufacturing and higher reliability processes. (Note that some estimates are that 30-40% of cells do not pass the first quality check.)Pike research anticipates cumulative PHEV + EV sales 5.2M by 2017 (up from 114,000 in 2011) with an additional 8.7M HEV.
5Vehicle Electrification Challenge Scale factor (size, capacity)Cell phone 4 WLaptop 80 WHEV 1,500 WPHEV 10,000 WEV 45,000 WDesign Life/Life Cycle CostCell phone monthsLaptop monthsHEV, PHEV, EV >120 monthsNew demands require new manufacturing industryWorking environmentState-of-charge windowRapid charge and dischargeLithium cells have been manufactured for cell phones, music players and laptops for several years. These cells are much smaller than those required for automotive applications and the performance requirements are not as stringent as those for automotive.For example, a cell phone or laptop battery is expected to last about months (max). Replacement may cost about $200 for a laptop. Automotive packs cost about $10,000 in Consumers will not be willing to replace packs every months.Cost is a major issue. To achieve a reasonable payback period for vehicle electrification, battery cost will need to be less than half what it is today. The service life of the pack must also be extended to at least 10 years in the vehicle for PHEV and EV to be successful in the broader market.HEV all NiMH batteries today. Economically these batteries have performance window that is a good fit for HEV requirements. The market will shift to Li as costs decline. Market predictions anticipate about 35% of HEV will be lithium by 2020.Battery companies have done a good job inventing new chemistry to improve cell performance. They have been less successful at designing cells and modules for manufacturing.
6Cells to Modules to Packs Can be 100s to 1000s of electrical joints per packBus barsInterconnectsCollectorsPouch/cell sealVoltage sensor leadsBalance of plantMotor connectionsThermal managementBattery managementThere are 100s or 1000s of electrical joints per pack. This include joints in the cell between the foil (+/-0.001” thick) current collectors and the tabs (0.005” thick), tab to tab connections and tab to bus (>0.025” thick) connections. Since many of these are in series, all must be good for the cell, module, and pack to function properly.This project focused on foil to tab, tab to tab and tab to bus connections. Most of the results to be presented will be on tab-to-tab joints.
7Joining Issues No single process dominates UltrasonicLaserResistanceSolderingAdhesivesComplex material combinationsCopper (native, plated)AluminumNickelSteelDissimilar combinationsNeedSpeedHigh reliabilityDurabilityLow heat inputNDE approachEWI has been working with consumer battery and automotive companies for many years. As the automotive industry began to develop electrified vehicles, we noted that large scale manufacturing of lithium batteries will require the development of fast, inexpensive and reliable joining processes.As part of a Symposium on battery manufacturing EWI did a survey of attendees about their concerns related to battery manufacturing. The results of the survey are captured here. Lithium batteries are complex assemblies. There are several joining processes involved and these processes must be capable of joining a wide range of materials. This includes making joints between dissimilar materials like nickel, aluminum, and copper.Reliability (at 6σ % yield) 1/2000 packs will fail.Modules and packs have a wide range of designs so joints have varying geometries.Packs must have 10+ year service life to be economically viable.Joints must not impede current flow either in operation or during fast charging.Most companies do not admit to any NDE beyond electrical continuity testing. Is this sufficient?7
8Substrate Comparison Property Cu Al Ni Thermal conductivity (W/m-°K)39022970Melting point (°C)10806521430Thermal expansion coefficient (ppm/°C)17.324.112Heat capacity (J/kg-°C)386900456Absorption (at 1064 nm%)2-5832Conductivity (106 S/m)573418Resistivity (10-6 -cm)2.112.879.5Specific heat (J/kg/°K)238455Latent heat of fusion (J/g)205388298Electrochemical potential (V)0.34-1.66-0.257Thermal Diffusivity (cm2/s)1.140.910.11These substrates also present challenges because their physical properties are very different (particularly the low melting point of aluminum). This makes dissimilar combinations difficult to assemble.Copper and Aluminum form a range of intermetallic compoundsProblem for electronics industryShows up in wire bonding/encapsulationMajor problem for fusion weldsHas been observed in solid state weldsComposition variable CuAl2; CuAl; Cu4Al3; Cu3Al2; Cu9Al4These compounds are brittle and low electrical conductivityEvidence shows phases can grow over hours to days at 150°CReports of growth at temperatures as low as 120°CReports that growth is influenced by electric currentNickel plating may prevent intermetallic formation and agingAdds process step and costProperty mismatch makes direct welding difficult
9OSU CAR EWI Welding Study Process screening study for module/pack assemblyLaser, resistance and ultrasonic metal weldingCopper, aluminum, nickel, nickel-plated copper (electro- and electroless-)Foil (0.001 in.); tab (0.005 in.); bus (0.032 in.)Mechanical and electrical propertiesShear strengthPeel strength (T peel)Resistance/conductivity/thermal profileMetallographyNon-destructive evaluation/process monitoringElectrical cycling (OSU CAR)Mechanical fatigue (Phase 2)The study here was designed in conjunction with The Center for Automotive Research at The Ohio State University. Working with faculty members Giorgio Rizzoni and Yann Guezenec, we devised a program to evaluate welding of copper, aluminum, nickel, and nickel-plated copper combinations appropriate for the assembly of battery cells, modules and packs. The mechanical and electrical properties of each weld were determined and the results were analyzed to help manufacturers select the best welding process and substrate for their product. EWI also explored a non destructive technique for monitoring weld quality.OSUCAR designed and is testing the cycling performance of small battery packs to determine how much impact the welded connection has on pack performance.Ultimately we would like to characterize the fatigue performance of these joints, but that was beyond the scope of the present study.Let’s look at each process in turn starting with ultrasonic metal welding and proceeding to laser and resistance welding. I will finish with a brief description of our efforts to develop NDE and a summary.
10Ultrasonic Metal Welding (UMW) Static ForceSonotrodeVibrationWorkpiecesAnvilWeld ZoneAdvantagesSolid-state, low heat inputWelds through contaminantsLow powerNo filler or cover gasFastExcellent for Al, Ni, CuDisadvantagesUnfamiliar processLap joints, thin sheet onlyDeforms partsLarge weld sizeRequires open accessNoiseSubstrate-horn adhesionOxides, ContaminantsAsperitiesUltrasonic metal welding is becoming very popular in several assembly industries. It has several advantages. Since it is a solid state process, it can be adapted to dissimilar materials combinations and avoids most concerns about formation of intermetallic compounds. It is ideally suited to welding the highly conductive materials used in batteries including plated copper.On the other hand it has limitations. Since high forces are involved, it creates large deformations in the weld zone. This looks bad and can create residual stresses. It is also a relatively new process that manufacturers do not have confidence in.The process is relatively simple. US energy (10,000 Hz+) is used to rub two parts together. This scrubbing breaks off oxide and contamination on the surface and rubs off surface asperities creating two ‘smooth’ clean metal surfaces. Once these contact, a weld is formed. This understanding is common in the adhesion community. Adhesive scientists always mention that the best bond would be between two clean flat metal surfaces. US welding creates something approaching that situation.
11USMW Previous Results No Cu-Cu bonding observed Ni-plated Cu 110Aluminum 1100AlNi-plated Cu 110Ni-plated Cu 110Ni-plated Cu 110Ni-plated Cu 110Ni-plated Cu 110No Cu-Cu bonding observedNi-plating broken or thinned in some areas, but never removedProfile of the horn and anvil are importantCross sections show ultrasonic metal welds done as part of a senior research Capstone project at Ohio State completed about 2 years ago. Welds are between plated copper substrates and it is interesting to note that even when the base substrate is highly deformed, the Ni plating is not disrupted.Ni-plated Cu 110Ni-plated Cu 110
12USMW OSU Preliminary Results Tab to BusAluminum tabs to all bus materials (Al, Cu, and Ni-plated Cu) result in weld joints with similar mechanical strengthNi-plated copper tabs to all bus materials-lower than expected peel strengthCopper tab to aluminum bus shows low peel but high tensile strengthTab to TabAluminum and copper join wellAluminum to other substrates less successfulFoil to tabUSW can easily join multiple thin layers in a single stepPreliminary resultsFor tab to bus welds aluminum tab welds have similar strength regardless of what the bus material is (Al, Cu, Ni, Ni plated Cu).Nickel plated copper tab peel strength was lower than expectedCopper tab to aluminum bus has acceptable tensile shear strength but low peel. This may require careful design if aluminum bus is required for weight or cost control.For tab to tab joints, US metal welding works well for aluminum to copper welds. It is less successful for welds between aluminum and nickel surfaces. One reason for this is that aluminum tends to stick to the horn and anvil face during welding. This would be a detriment during production.For foil to tab joints, US metal welding is a good technique for joining thin (0.001) foils to tabs in multiple layers. Welds with 10 foils were made in a single step for both copper and aluminum.
13USMW Tab to Tab Shear Peel Aluminum controls the weld behavior in dissimilar welds. Aluminum has lowest properties so this makes sense.Nickel plated copper shows low strength. This may be due to non optimized welding parameters for nickel containing samples. May also reflect fact that harder materials are more difficult to weld. Better welds are being evaluated.Copper to dissimilar substrates higher strength reflecting better properties than aluminum. Not so hard that USMW struggles to make the weld.PeelShear
14Generic Set-Up for Direct Beam Laser Welding Conduction Mode Welding Lasers use a focused beam of light to create weldsKeyhole Mode WeldingGeneric Set-Up for Direct Beam Laser WeldingConduction Mode Welding
15Nickel Plated Copper on Copper-Shaped Weld LW Advantages/DisadvantagesDisadvantagesLaser cost $$Need line-of-sight accessRequires good fit-up, toolingHeating starts on the surfaceLimited weld penetration especially on copperMakes fusion weldsWelds very narrowEye safety hazardAdvantagesPrecise location ofsmall weldsLow heat inputMinimal distortionHigh speedNon-contactCan weld “shapes”Nickel Plated Copper on Copper-Shaped Weld
17LW Sample Cross Sections Aluminum on Nickel-Electroplated Copper-VoidsNickel-Electroplated Copper on AluminumAluminum welded to other metals produced the weakest weldsIncomplete mixing of metalsCopper on Nickel
18Resistance Spot Welding Resistive heating of workpieces or electrodesCommonAdaptableLow cycle time and heat inputSelf-fixturingSelf-monitoring equipmentBlock diagram of AC welding system.Resistance welding is a classic technique for joining metal sheets.
19RSW Variants Solid state is preferred for battery assembly Advantages Rapid cycle timeLow heat inputMultiple welds easyProcess monitoring possibleDisadvantages for batteriesDissimilar metalsLow resistanceHigh conductivityCurrent path can limit geometryAccess can be limitedElectrodes orWelding TipsSpot Weld
20RSW Process Development Produce a weld matrix to determine process limitsCurrentTimeForceAcceptance requirementsApplication definedWeld strengthWeld sizeExpulsionAcceptableNuggetsSmall NuggetsMinimum Nugget DiameterWeld CurrentLevelTimeASmaller“Brittle”LobeCurveNugget DiameterWeld TimeTime AProcess development usually done to generate weld lobes that define parameters that product acceptable weld strength without burn through or metal expulsion. Strength evaluation is commonly done by a hand peel test to determine if the weld persists after the substrate fails. This is pulling a nugget.Need OSU project specific results to insert showing what works and what does not work. Strength results and cross sections would be good.
21RSW Tensile Shear Results Weld force and current important for Al and CuForce and current become less important for Ni and Ni-plateWeld time less important for al and cu becomes important for Ni plateWelding aluminum to copper works best at average settings between aluminum-aluminum and copper to copper.Welding to nickel plated copper requires use of parameters that work best for nickel plated copper.
22RSW Peel Test ResultsForce, current, and time equally important for Al and CuWeld time becomes more important for Ni and Ni plate
23Non Destructive Evaluation Can excite welds with external source.
24NDE X-Ray vs Thermal Signature Bad WeldGood WeldX-ray image showing weld nuggets(controlled specimen)
25Summary Batteries for motive power have numerous joints Material combinations increase complexityElectrical testing is not sufficient to determine if a weld is goodConductivity/resistance good even if weld is weakSeveral processes are usedUltrasonic metal weldingExcellent for Al, Cu, NiGood for multiple layersNeed to complete metallurgy and data analysisLaser weldingFlexibleMay be limited to like-to-like weldsNeed to look for intermetallic compound formationResistance WeldingMost combinations can be weldedParameter selection can be based on like-to-like resultsNeed to finish metallurgical analysisNondestructive evaluation approaches can be used for process development and perhaps production
26Buckeye Bullet “Hood Up” Assembled Battery Packs Buckeye Bullet 2.5 August 2010Buckeye Bullet “Hood Up”New international land-speed record for battery-powered vehicles of mphOver 1500 BatteriesEWI Laser Work CellAssembled Battery PacksEWI is about to start efforts with OSU CAR to assemble the Buckeye Bullet 3.0. this vehicle will have a new frame, a new drive train and new batteries. Goal is 350 mph+. Look for records in 2012 or 2013!Battery
27AcknowledgementsSupport of the Department of Energy through the Ohio State University Center for Automotive ResearchDOE Award DE-EENational Center of Excellence for Energy StorageTeam effortTim FrechMitch MathenyJay EastmanSam LewisWarren PetersonBarb ChristelNancy PorterMike Ryan
28Questions? Dr. David Speth Senior Engineer-Materials Phone: