Small / Medium Collaborative Project GC-SST Grant Agreement no.:

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1 Small / Medium Collaborative Project 7 th Framework Programme GC-SST Grant Agreement no.: Project OSTLER Optimised Storage Integration for the Electric Car Work package WP 3 Crashworthy battery pack and vehicle crash inv. Task number WT 3.2 Physical testing of battery packs to evaluate potential damage Deliverable number D3.15 Report on validation of FE simulation from WT3.1 Author(s) M. Funcke, S. Schäfer, R. Wohlecker (IKA/FKA) D. Dufaut, D. Sturk, K. Vavalidis (ALV) Filename Status Distribution OSTLER R D3.15 Report on validation of FE simulations from WT3_1 Final Issue date 11 th March 2014 Project start and duration 1 st of June, months Project co-funded by the European Commission DG-Information Society and Media in the 7 th Framework Programme D3.15 Report on validation of FE simulations of WT3.1 Page 1 of 42

2 DOCUMENT REVISION AND HISTORY Version Date Author Comment DRAFT 03/04/2014 M. Funcke Draft for review DRAFT 09/04/2014 M. Funcke Reviewed by David Sturk (ALV) DRAFT 28/04/2014 M.Funcke Draft for peer review DRAFT 05/06/2014 M Watkinson Peer Review FINAL 23/06/2014 M. Funcke Final D3.15 Report on validation of FE simulations of WT3.1 Page 2 of 42

3 EXECUTIVE SUMMARY The storage elements in an electric vehicle (EV) remain a key challenge to wide-scale, successful deployment of EVs that are appealing to customers and are adequately functional (e.g. in terms of range and driveability). It is estimated that state-of-the-art Lithium-Ion battery chemistries, which are the technology of choice for EVs, offer approximately 0.5 km range of driving per 1 kg of battery pack mass. It is technically feasible to develop EVs with a driving experience comparable to hydrocarbon-fuelled vehicles, in order to achieve significant range in zero-emissions mode (> 100 km). In that case battery packs with a mass exceeding 200 kg have to be integrated into the vehicle. For the vehicle manufacturers, current EV designs are frequently based around adapting an existing vehicle platform. Although some mass is removed if there is no internal combustion engine, there are frequently significant constraints on the location of the battery packs in order to anticipate issues such as handling and crashworthiness. On the other hand, many new market entrants are proposing technologies for EVs with little cognisance of the issues to be overcome in integrating the battery packs into other on-board systems such as thermal and electrical systems. The OSTLER project will take a modular approach for integrating energy storages into EVs. Among the benefits is the fact that purpose-built EVs can readily be designed around their energy storage and, crucially from a market attractiveness point of view, will give manufacturers the flexibility to offer model variants of an EV (or a plug-in hybrid vehicle) based on green attributes, specifically locally cleaner cities, rather than speed or acceleration as might be the case with conventional vehicles. The concept responds to the challenge of reconciling utility with pushing down the base level of vehicle emissions in densely populated city regions. The objective of this work package is to investigate new smart physical integration and protection concepts for batteries in vehicles aiming at, by means of battery pack external protective structures, allowing reduced total battery system weight without compromising crashworthiness. With regard to the modular approach of the OSTLER project it is important to prevent damage of the battery even if it is placed in a region that is not crash optimized e.g. utilizing a larger width of the floor area i.e. closer to the vehicle sill. The aim is to develop a protective concept that will allow nonrestrictive placement and to bring the battery pack further out into more frequent deformation zones. In WP3 two different concepts, energy absorbing elements as well as inflatable elements (i.e. active structures), will be developed and evaluated in terms of their ability to shield the pack from intruding objects. For the preliminary design of these systems using the finite element (FE) method corresponding models of the cells are required. For this purpose, the cell structure and chemistry is analyzed in a first step within this report. The second step is the determination of the expected loadcases and loads on cell level using the electrified full vehicle FE model of WT3.1 which includes the traction battery and the determined critical full vehicle loadcase which was identified within the deliverable D3.14. The identified loadcases are tested in WT3.2 on real cells and afterwards these results are used to develop a suitable FE model of the cells. D3.15 Report on validation of FE simulations of WT3.1 Page 3 of 42

4 TABLE OF CONTENTS DOCUMENT REVISION AND HISTORY... 2 EXECUTIVE SUMMARY... 3 TABLE OF CONTENTS... 4 LIST OF FIGURES... 6 NOMENCLATURE INTRODUCTION Scope of Deliverable Work Package Work Task Work Task Cell structure Electrolytes in Li-Ion Batteries Cylindrical Cells Prismatic Cells Pouch Cells Resulting Loadcases of WT Cylindrical Cells Prismatic Cells Pouch Cells Validation of Pouch cell model Test Bench FE Model Pouch Cell Flat Impactor (Loadcase 1) Three-Point Bending (Loadcase 2) D3.15 Report on validation of FE simulations of WT3.1 Page 4 of 42

5 3.5 Clamped Battery (Loadcase 3) Conclusions References D3.15 Report on validation of FE simulations of WT3.1 Page 5 of 42

6 LIST OF FIGURES Figure 1: Cross-section of the battery core material composition exemplifying dimensions of components (2) Figure 2: Three types of vehicle battery cell geometries Figure 3: Lithium-ion cell with mechanical safeguard systems Figure 4: Heat production from combustion (15) Figure 5: Cross section of a lithium-ion cylindrical cell (17) Figure 6: cylindrical cell (17) Figure 7: Cross-section of a Prismatic Cell (17) Figure 8: Example of dimensions of one commercially available pouch-cell for HEV Figure 9: Cross-section of a three-layered foil protecting the core of the pouch-cell (18) Figure 10: Melting points of the plastic components of the cell-container (18) Figure 11: Swelling pouch cell after being exposed to abusive testing Figure 12: Battery packs Figure 13: Deformed battery pack cylindrical cells Figure 14: Resulting loadcases cylindrical cells Figure 15: Deformed battery pack prismatic cells Figure 16: Resulting loadcases prismatic cells Figure 17: Interior pouch cell battery system Figure 18: Deformation Battery Pack Pouch Cells Figure 19: Resulting loadcases pouch cells Figure 20: Current density microscopic simulation (22) Figure 21: Analysed loadcases (23) Figure 22: Servo hydraulic test bench for validation process at ika Figure 23: Test bench properties Figure 24: Controlling VI Figure 25: Pouch cell testing with round (left) and flat (right) impactor Figure 26: FE model pouch cell Figure 27: FE model pouch cell with boundary area Figure 28: Loadcase 1 (flat impactor) Figure 29: Testing vs. simulation flat impactor Figure 30: Loadcase three-point bending Figure 31: Testing vs. simulation three-point bending Figure 32: Deformation characteristic three-point bending Figure 33: Testing vs. simulation three-point bending 90 turned Figure 34: Deformation characteristic three-point bending 90 turned Figure 35: Battery cell with two holders Figure 36: Test bench for clamped battery Figure 37: Boundary conditions testing and FEM Figure 38: Testing vs. simulation clamped battery D3.15 Report on validation of FE simulations of WT3.1 Page 6 of 42

7 NOMENCLATURE BASt BEV BMS BPS BS CID DEC DMC E EC EMC EV ESS ESP FAT FE FEV FHWA GIDAS GW HEV HV IC l krit LTMO LTO Bundesanstalt für Straßenwesen Battery Electric Vehicle Battery Management System Battery Pack System Battery System Current Interrupt Device Diethyl Carbonate Dimethyl Carbonate Young s Modulus Ethylene Carbonate Ethyl Methyl Carbonate Electric vehicles Energy Storage Systems Electronic Stability Control Forschungsvereinigung Automobiltechnik Finite Element(s) Full Electric Vehicle Federal Highway Administration German In-Depth Accident Study George Washington University Hybrid Electric Vehicle High Voltage Interface Cradle Critical element length Lithium Transition Metal Oxide Lithium Titanium Oxide D3.15 Report on validation of FE simulations of WT3.1 Page 7 of 42

8 LV MBP NCAC NE NHTSA PC PE PEL PHEV PP PVDF RM RP SEI SOC TR WP WT ρ Δt krit Low Voltage Main Battery Pack National Crash Analysis Center Negative Electrode National Highway Traffic Safety Administration Propylene Carbonate Polyethylene Positive Electrode Plug-in Electric vehicles Polypropylene Polyvinylidene Fluoride Removable Module Removable Pack Solid Electrolyte Interface State of Charge Technical Report Work Package Work Task Density Critical time step D3.15 Report on validation of FE simulations of WT3.1 Page 8 of 42

9 1 INTRODUCTION 1.1 Scope of Deliverable The traction or main battery is a system that must be adapted to each specific car model. Depending on the car size and specific body construction, its dimensions, implementation and positioning will change. Also, each car model will have different requirements in terms of voltage, power delivery and energy stored. Moreover, each OEM has his specific electrical and mechanical standards. The basic idea of OSTLER is to use modular battery packs which easily can be replaced within an authorised repair shop. Therefore the battery must be placed in an area with good accessibility. The customer has the opportunity to choose between different battery manufacturers as well as different energy contents of the battery system and changing the battery system. As highlighted within deliverable D3.14, three different cell types can be used for the battery systems. There are cylindrical, prismatic and pouch cells available which can withstand a certain amount of load and require specific battery housing design each. The work tasks WT3.1 and WT3.2 are running in parallel. Within WT3.2 the different cells are tested and in WT3.1 a simulation model for the cells is created. The second deliverable of work package 3 (D3.15) summarizes the validation of the FE simulations. 1.2 Work Package 3 The objective of OSTLER WP3 is to develop smart concepts of the crashworthy physical integration of battery packs in structures of electric vehicles. Apart from intelligent passive solutions, inflatable elements will be investigated to provide protection to the battery itself and to the occupants of the vehicle. This includes the provision of protection in the vehicle for the battery pack installation, taking into account various potential pack-to-vehicle designs. Moreover the focus is also on the external crashworthiness of the battery pack in order to prevent passenger cell intrusions and to examine the effect of battery pack mass and stiffness on overall vehicle crashworthiness including occupant loading. 1.3 Work Task 3.1 The main WT3.1 objective is to study the effect of integrated battery packs on vehicle crashworthiness. The contributions from the partners are: ALV: An accurate design of the battery pack will be developed and its structure will be analysed from the structural and crashworthiness stand point. ALV will define a battery position according to a comprehensive study of the crash statistics from the GIDAS report. D3.15 Report on validation of FE simulations of WT3.1 Page 9 of 42

10 CRF: The contribution of CRF to WP3.1 and 3.2 will focus primarily on the evaluation of the crashworthiness of the battery packs when installed in the vehicle, utilizing their broad expertise in crashworthiness, and specifically in the electrification of combustion engine vehicles. Since the safety of battery is one of the key aspects of this WP, CRF will coordinate this and cooperate actively in the development of innovative and secure solutions that could lead to safer and more reliable energy storage systems (ESS). An accurate design of the battery pack will be developed and its structure will be analysed from the structural and crashworthiness standpoint, which will be optimised in order to be as intrinsically safe as possible in accordance to weight specifications and the specified mission profiles. CRF is responsible for one of the main battery packs and provides the CAD of the modular battery. ZF-fka and ika: Both will build up a FE model of one of the battery packs to be investigated, integrate it into a full vehicle model at different suitable locations, run the corresponding simulations and evaluate the results. 1.4 Work Task 3.2 The main WT3.2 objective is the physical testing of battery packs to evaluate potential damage. The contributions from the partner are: ALV: Assisting in physical testing using their expertise and equipment available. ALV will assist in the validation of the FE model by testing a larger module compound. ZF-fka and ika: Both will derive critical load cases as well as parameters for physical tests from the results of WT3.1, develop one of the test configurations to be applied on the battery packs investigated. They will build up a corresponding test set up and carry out the tests. The results will be evaluated deriving limit values for crush distances, battery accelerations, etc. The FE model of the battery pack will be validated based on the test results. ika: Will provide the partners with battery packs for the tests and provide information about potential hazards during crash tests. D3.15 Report on validation of FE simulations of WT3.1 Page 10 of 42

11 2 CELL STRUCTURE In order to understand the behaviour, an overview of the general structure of the different cell types is given. A typical Li-ion battery consists of a graphite based (LiC 6 ) porous negative electrode (NE), a polymer membrane separator, a porous positive electrode (PEL) made of lithium transition metal oxide (LTMO) (1), and an electrolyte mixture composed of salt and organic solvents providing a medium for Li-Ions to shuttle between the NE and the PEL. 1 Common dimensions of the core material elements of a Li-ion cell are at the level of 5 to 40 micro meters. Exemplifying dimensions are given in Figure 1. As material for the current collectors copper and aluminium are commonly used. Figure 1: Cross-section of the battery core material composition exemplifying dimensions of components (2) Anodes (NE) made of LTO (e.g. lithium titanium oxide LTO) or pure lithium also exist but lithiated graphite is used widespread as it provides higher potential than LTO anodes and is more stable than pure lithium. As for cathodes (PEL), numerous variations of materials are under development and research (3), but conventional cells regularly use variations originated from the LTMO groups of LiCoO 2 (e.g. LiNi 0.85 Co 0.15 O 2, LiNi 0.80 Co 0.15 Al 0.05 O 2, and LiNi 1/3 Co 1/3 Mn 1/3 O 2 ) and LiMnO 2 or LMO (e.g. LiNi 0.5 Mn 0.5 O 2 ), spinel (e.g. LiMn 2 O 4 ), or metal-phosphate based such as LiFePO 4. Electrodes thus consist of those active materials bonded into a porous matrix using a polymer binder. Although non-fluorinated binders may offer higher thermal stability (4), the most commonly used electrolyte binder is polyvinylidene fluoride (PVDF). It is considered as the overall best alternative with good capability of binding together the electrically conducting graphite and the active material while also generating good porosity and providing chemical compatibility between these materials and the electrolyte of the cell. (5) 1 The poles of the NE and the PEL are always constant while the location of the anode and the cathode depends on where oxidation and reduction of the charge carrying ions occur (Li + /Li for Li-ion cells). Hence, when a battery cell is being discharged the NE is defined as the anode and the PEL as the cathode. The opposite is valid when charging a battery, i.e. NE is defined cathode and the PEL will be the anode. As this report will focus on mechanical abuse (which is not related to charging) the NE and PEL will be defined as the anode and the cathode respectively, throughout this report. D3.15 Report on validation of FE simulations of WT3.1 Page 11 of 42

12 The separators of Li-ion cells often consist of polymer membrane material, such as semi-crystalline polyolefin materials including polyethylene (PE), polypropylene (PP) and their blends PE/PP or the tri-layered PP/PE/PP. (6) Li-ion battery separators provide some protection against overcharge and short circuit in the cells. This is due to the inherent capability of the membranes to exhibit a large increase in impedance at temperatures about 130 C that effectively stops ionic transport between the electrodes. (7) As for electrolyte, Li-ion batteries generally use mixtures of alkyl carbonates (as solvents) and lithium hexafluorophoshate LiPF 6 (as salt). Examples of solvents frequently referred to in scientific literature are ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). Additional salts exist, e.g. LiClO 4, LiCF 3 SO 3, LiTFSI, TEABF 4, LiBF 4, and LiBOB (8) (9), but all of these have lower stability than LiPF 6 which is currently the predominant salt among conventional Li-ion cell chemistries. (8) There are three major types of battery cells used from vehicle manufacturers: prismatic, cylindrical and pouch cells (Figure 2). A representative prismatic cell is protected by a case made of metal or hard plastic. Cylindrical cell cases often consist of aluminium (nickel plated steel cans exist but are more common for small A-series battery cells). And pouch cells use lightweight cases made of a layered foil consisting of sheets of polymer and aluminium welded together around the edges of the flat cell. Figure 2: Three types of vehicle battery cell geometries Conventional Li-ion cells incorporate various safety systems to prevent high power discharge, internal heat generation and internal pressure build-up etc. The poly-thermal switch (PTC) responds to a temperature rise in the cell by increasing the resistance in the switch, and thus inhibits high current surges. Shut-down polymer membranes respond to high temperatures and will inhibit high currents. The membrane s morphology will change at elevated temperatures increasing the resistance to ionic transport across the separator. A current interrupt device (CID) can be used to open the electric path if the internal cell pressure is increased to 10 bars for example. Such pressure rise can be the consequence of an excessively high charge voltage. A variety of safety systems, including the CID, are shown in Figure 3 on the example of a cylindrical cell. D3.15 Report on validation of FE simulations of WT3.1 Page 12 of 42

13 The first safety measure is a disc, marked with A, which consists of a temperature-sensitive polymer that resists electron flow as the temperature increases. If this is not sufficient to balance the battery and the pressure increases as a result of the high temperature, the CID opens breaking the cell circuit (see upper right image). A further increase in the pressure causes the CID to vent to the cap, which is shown in the lower right image. Simultaneously the polymer sheet between the anodic and cathodic foils (left image) melts at a given temperature, stopping the electron flow (10). Figure 3: Lithium-ion cell with mechanical safeguard systems Besides the mechanical safeguards inside the cells, an electronic protection circuit connected externally to the cells enhance the safety. (3) To further enhance the safety and stability of a Li-ion battery various additives can be mixed into the cell chemistry. Amongst other things additives may stabilise the anode solid electrolyte interface (SEI) mitigating its exothermal decomposition reactions during thermal runaway, impart beneficial effects to the electrolyte such as flame resistance (by flame-retardant agents) or overcharge protection and protecting the cathode materials e.g. by creating a barrier to solubility or oxidation of the solvent. (11) (12) 2.1 Electrolytes in Li-Ion Batteries As mentioned, Li-Ion batteries often use electrolytes composed of mixtures of alkyl carbonates solutions and a salt (13). In a study performed by Huang et al. (2008), three common electrolytes and their heat of combustion were examined. As can be seen in Figure 4 below, the magnitude of their average heat generated was estimated to be close to one third of the heat produced when combusting diesel approximately 42 kj/g. (14) D3.15 Report on validation of FE simulations of WT3.1 Page 13 of 42

14 Heat [kj/g] Contract N Solvents only Solvents + LiPF6 Figure 4: Heat production from combustion (15) The electrolytes studied consisted of ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) together with the salt LiPF 6. Many other salts have been studied and presented in research literature, but LiPF 6 has long been the most widely used alternative. This is due to the fact that it meets more of the requirements (high conductivity, low cost, thermal stability, etc.) for Li-Ion battery electrolytes than most of its competitors. (16) Considering the presented levels of heat generation from combustion of electrolyte, it is reasonable to pay attention to the design and durability of Li-Ion battery cell-containers. 2.2 Cylindrical Cells The cylindrical cell continues to be one the most widely used packaging styles for primary and secondary batteries. The advantages are ease of manufacture and good mechanical stability. The tubular cylinder has the ability to withstand internal pressures without deforming. (17) Figure 5 shows a cross section of a cylindrical cell. The cell consists of an outer casing often made of stainless steels which in addition can be used as negative cap. The layers of positive and negative electrodes as well as separator layers are in this type of battery rolled up inside the casing. The PTC as well as the CID are visible in the upper area of the cell. The cylindrical cell design has good cycling ability, offers a long calendar life, is economical but is heavy and has low packaging density due to space cavities. D3.15 Report on validation of FE simulations of WT3.1 Page 14 of 42

15 Typical applications for the cylindrical cell are power tools, medical instruments and laptops. Nickelcadmium offers the largest variety of cell choices, and some popular formats have spilled over to nickel-metal-hydride. To allow variations within a given size, manufacturers use fractural cell length, such as half and three-quarter formats. (17) The established standards for nickel-based batteries did not carry over with lithium-ion and the chemistry has established its own formats. One of the most popular cell packages is the 18650, as illustrated in Figure 6. Eighteen denotes the diameter and 65 is the length of the cell in millimetres. Figure 5: Cross section of a lithium-ion cylindrical cell (17) A Li-manganese version has a capacity of 1,200 1,500 mah, and a Li-cobalt version 2,400 3,000 mah. The larger cells have a diameter of 26 mm with a length of 65 mm and deliver about 3,200 mah in the manganese version. This cell format is used in power tools and some hybrid vehicles. (17) Figure 6: cylindrical cell (17) D3.15 Report on validation of FE simulations of WT3.1 Page 15 of 42

16 2.3 Prismatic Cells Introduced in the early 1990s, the prismatic cell satisfies the demand for thinner sizes and lower manufacturing costs. Wrapped in packages resembling a box of chewing gum or a small chocolate bar, prismatic cells make optimal use of space by using the layered approach. These cells are predominantly found in mobile phones with lithium-ion. No universal format exists and each manufacturer designs its own. If the housing design allows a few millimetres extra in a cell phone or laptop, the manufacturer designs a new pack for the sake of higher capacity. High volume justifies this move. Prismatic cells are also encountering into larger formats. Packaged in welded aluminium housings, the common cells deliver capacities of 20 to 40 Ah or even more power and are primarily used for electric powertrains in hybrid and electric vehicles. Figure 7 shows cross section of a prismatic cell. Figure 7: Cross-section of a Prismatic Cell (17) The prismatic cell improves space utilization and allows flexible design but may be more expensive to manufacture, less efficient in thermal management and have a shorter cycle life than the cylindrical design. 2.4 Pouch Cells One of the most common cell designs used in high capacity battery stacks for electric vehicles, due to its light weight and low cost, are flat pouch-cells (see Figure 8) with cell-containers made of two three-layered foils welded together around the cell core. (18) D3.15 Report on validation of FE simulations of WT3.1 Page 16 of 42

17 Figure 8: Example of dimensions of one commercially available pouch-cell for HEV A cross-section of such a foil is illustrated in Figure 9. It is showing the composition of polyamide (PA), aluminium and polypropylene (PP). The welded seam around the cell core has a melting point of 166 C, as the inward surfaces of the two foils welded together consist of polypropylene. Figure 9: Cross-section of a three-layered foil protecting the core of the pouch-cell (18) Hence, the cell seam risks to be critically damaged if temperature exceeds that point. However, the tab-seals, which consist of a mixture of three unspecified polymers, are critically damaged for the same reason already at 105 C (see Figure 10). However, this melting point by the tabs may be used as the safety feature allowing a pouch cells to vent any build-up of internally pressurized for any failure reason. Hence, a pouch cell can avoid explosion at higher pressures. Figure 10: Melting points of the plastic components of the cell-container (18) The pouch cell makes the most efficient use of space and achieves a 90 to 95 percent packaging efficiency, the highest among battery packs. Eliminating the metal enclosure reduces weight but the cell needs some alternative support in the battery compartment. The pouch cell finds applications in consumer, military, as well as automotive applications (17). Still, there are no applicable standards D3.15 Report on validation of FE simulations of WT3.1 Page 17 of 42

18 concerning the dimensions of pouch cells. However, recent attempts have been made to standardize pouch cells within business organizations e.g. Verband deutscher Automobilhersteller (VDA, Association of German Automobile Manufacturers) (19) Today, many vehicles with electric drive train and traction battery are equipped with pouch cells (e.g. GM Volt and Volvo Plug-in Hybrid). One downside with battery packs using pouch cells is that such cells need to be firmly clamped together in order to not loose cycle life due to swelling. During charging respectively discharging the anode respectively cathode may swell a little due to transport phenomenon during the process of lithiating each electrode. This may affect the cell s life length and consequently it is important to keep pouch cells steadily clamped together to reduce such swelling. Nevertheless, some swelling may occur due to the choice of active material as some compositions have proven more prone to being gradually irreversibly damaged during the cycling and thus reducing the cells lifetime. In some worst case examples among early pouch cell manufacturers some dealers experienced failures due to swelling of as much as three percent on certain batches which naturally could not be acceptable in use for vehicle applications. Figure 11: Swelling pouch cell after being exposed to abusive testing 2.5 Resulting Loadcases of WT3.1 The loadcases on cell level are the major spin-off of the deliverable D3.14 Conclusions concerning the effect of large battery packs on vehicle crashworthiness. Within this previous deliverable a conventional car was electrified virtually. This included a replacement of the combustion engine through an electric motor with its controller and the integration of a traction battery pack, which was designed by MIRA. The initial installation locations of those components and the loadcases to be simulated on full vehicle level were determined using the results from the investigation and analysis of the GIDAS crash statistics. However, in the end, with regard to the aim of the OSTLER project to investigate active and passive safety systems for battery casings, the main battery pack was intentionally not placed in a traditional safe zone but rather in an area that presents higher frequency of intrusions i.e. battery pack under front seats and stretching out into areas close to the vehicle sill. To represent all occurring crash loads four loadcases for the simulation were identified: D3.15 Report on validation of FE simulations of WT3.1 Page 18 of 42

19 - FMVSS 301 rear impact at 80 km/h - EuroNCAP pole side with 50 km/h and varying pole position (NB. 29 km/h in original version) - EuroNCAP ODB frontal impact at 64 km/h - FMVSS 208 (USNCAP) at 56 km/h Due to the placement of the battery case under the front seats no damage of the battery cells and only minor deformations of the battery housing occur within the front and rear impacts. During the side pole impact with 50 km/h, the vehicle structure stays more intact than in the reference vehicle, due to the battery restricted deformation space,. Despite, the limited available space, the battery is deformed. A significant deformation of the battery casing and plastic deformation of the cells themselves was evident. The lateral pole impact was rated as the most critical loadcase for the battery pack. In order to investigate the effects of the pole impact on the different cell types, the MIRA battery with prismatic cells is adapted as casing used for cylindrical and pouch cells as well. Each cell type requires an adjustment of the inner structure of the battery case. Figure 12 shows the battery housing and the three different interiors. While the prismatic cells can be simply placed on the bottom plate of the system, the cylindrical and the pouch cells require special fixtures. Battery housing Battery Models: Interior: cells & cell holders Prismatic Prismatic Cylindrical Cylindrical Pouch Pouch Figure 12: Battery packs Because significant deformations on the battery packs occur only for the side pole impact, all loadcases on cell level are the result from the EuroNCAP pole impact with the increased velocity of 50 km/h. D3.15 Report on validation of FE simulations of WT3.1 Page 19 of 42

20 2.5.1 Cylindrical Cells Figure 13 shows the deformations occurring on the battery pack running the full vehicle simulation using cylindrical cells. In the upper left figure the deformations of the battery housing are shown. Clearly visible is the transfer of the pole shape through the sill onto the housing. Deformed battery pack Plastic strain battery cells Battery housing Deformed battery interior (side view/bottom view) Cell fixture Moving cells Pole impact Cells Inner walls Figure 13: Deformed battery pack cylindrical cells More relevant than the outer deformations are the deformations of the cells inside. The lower pictures show the movement of the cells and inner walls. It can be seen, while the first two rows of batteries are strongly shifted, the shift decreases to the crash opposite side. The analysis of plastic deformation in the upper right of the screen shows where the cells are loaded and deformed. Here, the scale ranges from white to black. While white represents a slight plastic deformation, black indicates high deformation. It can be seen that loads occur mainly at the battery terminals and the edges of the terminal ends. Two loadcases for this battery configuration was selected for single cell mechanical abuse testing, based on these plastic deformations (see Figure 14). In both loadcases, the battery is clamped at one battery terminal end while the load is applied on the opposite terminal. Within the loadcase on the left side a flat impactor with a defined angle hits the battery. The displacement of the impactor is parallel to the battery axis. A flat impactor is also used within the second loadcase. In this case the angle is defined as 90 to the battery longitudinal axis which means the connecting axis of the poles forms the normal vector of the impactor. D3.15 Report on validation of FE simulations of WT3.1 Page 20 of 42

21 Loadcase 1: Loadcase 2: s s Impactor s + - s s Impactor + - Load application (displacement s) Clamping Load application (displacement s) Clamping Figure 14: Resulting loadcases cylindrical cells Prismatic Cells Figure 15 shows the deformations occurring on the battery pack equipped with prismatic cells. In the upper left figure the deformations of the housing can be seen. Again the transfer of the pole shape through the sill onto the housing is clearly visible. Deformed battery Pack Plastic strain battery cells Battery housing Deformed battery interior (side view/bottom view) Inner walls Pole impact Cells Inner walls Figure 15: Deformed battery pack prismatic cells D3.15 Report on validation of FE simulations of WT3.1 Page 21 of 42

22 The lower pictures show the deformation and movement of the cells and inner walls. The inner behaviour is similar to the behaviour of the pack with the cylindrical cells. The first two rows of batteries are moving and the shift decreases to the crash opposite side. The analysis of plastic deformation in the upper right of the screen shows that only the cells in line with the load receive plastic deformation. The load is localised to a few cells, which is due to the close-fitting packaging of the cells in the battery pack. The loadcases of the cylindrical cell can be transferred without changes to the prismatic battery, see Figure 16. Loadcase 1: Loadcase 2: Impactor s Impactor s + - s s + - s Load application (displacement s) Clamping s Load application (displacement s) Clamping Figure 16: Resulting loadcases prismatic cells In both loadcases, the battery is placed terminal upwards and clamped at one of the small areas while the load is applied on the opposite side. Within the loadcase on the left side a flat impactor hits the battery with an angle 45. The displacement of the impactor is parallel to the battery axis again. A flat impactor is also used within the second loadcase. In this case the angle is defined as Pouch Cells To conduct the loadcases for the pouch cells, the construction of the associated battery pack will be explained first as it differs from the previous battery packs. The battery pack consists of 144 pouch cells, twelve cells are each combined into a module. It follows that a total number of twelve modules are used within the pack. Each module consists of 13 plastic brackets which are used to clamp the cells between and two closure plates. The closure plates prevent the intrusion of sharp objects in the cells and support the clamping of the plastic parts. Within the pack the modules are placed on D3.15 Report on validation of FE simulations of WT3.1 Page 22 of 42

23 the ground plate and connected to the bottom via a frictional connection. The overall construction is shown in Figure 17. Module Plastic brackets Bottom plate Rail for frictional connection Figure 17: Closure plates Interior pouch cell battery system Figure 18 shows the deformations occurring on the battery pack equipped with the pouch cells. In the upper left figure the deformations of the housing can be seen. Again the transfer of the pole shape through the sill onto the housing is clearly visible. Deformed battery pack Plastic strain battery cells Battery housing Deformed battery interior (side view/ bottom view) Pole impact Figure 18: Plastic brackets Deformation Battery Pack Pouch Cells D3.15 Report on validation of FE simulations of WT3.1 Page 23 of 42

24 The lower both pictures show the deformation and movement of the cells and inner walls. Contrary to the small movement of the cells within the other two battery systems, the individual modules of the pouch cells show larger movements during the crash. The deformation of the modules and cells inside the battery pack decreases toward the side facing away from the impact. This hypothesis is supported by the observation of the plastic strains, seen in the upper right figure. High deformations that result in high plastic strains occur only in the first module closest to the impact point of the pole. Figure 19 shows the loadcases for the pouch cells which are generated out of the described loading. Loadcase 1 (flat impactor): Load application (displacement s) s s s Impactor s Loadcase 2 (three-point bending): Impactor s Load Application (displacement s) Abutment Abutment Loadcase 3 (clamped battery): Load application (displacement s) Impactor Clamping Figure 19: Resulting loadcases pouch cells Because of the absence of an own housing, the cells must first be examined how they respond to squeezing. Therefore the cells are placed on top of a support and loaded over the full area. This loadcase can be seen in the upper right corner in the figure above. With this loading condition it is also possible to determine the stiffness for the simulation model. Since the plastic deformation at the centre of the battery is the largest, it is recommended to apply the overall load in this area. This burden, which is comparable to the common three point bending test, arises because of the roundish deformation by the penetrating pile. In this loadcase, shown D3.15 Report on validation of FE simulations of WT3.1 Page 24 of 42

25 above on the left side, the battery rests on two round abutments and is centrally loaded by a round impactor. Since the battery cells are trapped in the real installation condition between the plastic brackets, a similar load case is used which takes into account these clamping. Therefore the cells are clamped at two to three sides and loaded in the centre by an impactor (see bottom of Figure 19). D3.15 Report on validation of FE simulations of WT3.1 Page 25 of 42

26 3 VALIDATION OF POUCH CELL MODEL Since the pouch cells are the most energy dense and lightest cells, as well as lack both the mechanical safeguard systems (Figure 1) and its own structurally hard housing, and moreover due to the larger deformations, as described in Chapter 2.5.3, it is regarded by the WP3 group as the one cell type in largest need of additional protection. Thus, the development for the FE models focuses on this cell type. The generated FE model will be used to build up future systems for testing and demonstration activities within the OSTLER project. The FE simulation of lithium ion battery and also the simulation of pouch cells have already been treated in publications. Some of these publications address the thermal or chemical behaviour e.g. (20), (21) or (22). For thermal simulations, the cells may be analysed in two different ways. First, the cells are defined as solids with orthotropic material properties in order to deal with the different thermal conductivity of the electrodes and electrolyte. Coupled with a one-dimensional fluid flow analysis, this approach allows a quick examination of the heat flux on cell level. In more detailed analyses, finely meshed volumes are used (see Figure 20). Figure 20: Current density microscopic simulation (22) With these models detailed temperature distribution as well as chemical changes in the cell, such as the local density distribution, are calculated. However, both approaches are not suitable for the mechanical analysis performed in WP 3 since the mapping of the three-dimensional behaviour is essential as well as the resolution of the elements in chemical analysis is too high. This leads to a large number of elements and the associated high computational time. The same limitation arises in the analysis of the found publications dealing with the modeling of the mechanical behavior of lithium-ion batteries. Thus, the structure of a FE model for lithium batteries in pouch design is described in the article (23). According to the authors, it is possible to map the mechanical behaviour for selected loadcases. The used element edge length is 1 mm which is, as previously described, too small for the simulations of WP 3. The element edge length target for the metallic layers of the battery, see Chapter 3.2, is according to the Courant criterion 6 mm (24). The D3.15 Report on validation of FE simulations of WT3.1 Page 26 of 42

27 Courant criterion describes the stability of explicit FE calculations based on the relationship between critical time step, critical element edge length and the material parameters density and Young s modulus. Eq. 3-1 with: Critical element edge length Critical simulation time step (1.112x10-6 s) Young s Modulus Aluminium (70000 MPa) Density Aluminium (2.7x10-9 ton/mm 3 ) In addition, the considered loadcases unconfined compression and punch indentation do not reflect the mounting position used in the OSTLER project (see Figure 21). Figure 21: Analysed loadcases (23) D3.15 Report on validation of FE simulations of WT3.1 Page 27 of 42

28 However, the resulting model within this paper provides initial starting points for the simulation model used within OSTLER. Another EU project which addresses the safety of electric cars and includes the modelling of pouch cells is EVERSAFE (25). The evaluation of the battery system is done via exclusion of intrusions into the system and the analysis of the acceleration curves of the modules (26). In contrast, an intrusion into the battery system is accepted in the OSTLER project and limited to a safe level, which offers additional lightweight potential. Another possibility to investigate battery systems is the separation of the simulations in a macroscopic and a microscopic approach (27). The following chapter describes the development and validation process of an adequate FE model for the used cells. Therefore, a short overview of the adapted test bench is given. 3.1 Test Bench For the validation process ika developed a new test bench set-up with a servo hydraulic cylinder. A hydraulic power unit with an operating pressure of 200 bars enables the movement of the cylinder. The combination of this hydraulic power unit and the servo hydraulic cylinder enables a maximum force of 30 kn. The test bench set-up is freestanding and consists of a vertically arranged hydraulic test cylinder that has a carrier plate mounted on the cylinder s upper side. The plate faces an area of 200 mm by 200 mm. Two linear guides on the sides enforce a movement only in vertical direction. With the help of these linear guides any shear forces on the servo hydraulic cylinder are avoided. On the top side of the test set-up a counter bearing including a load cell with an interchangeable impactor is mounted. Up to now two different impactor types are in use, a cylindrical and flat impactor. The load cell is replaceable and can be adapted according to the expected load for the test scenario. Figure 22 shows the test bench set-up with a 20 kn load cell. Load cell Impactor Carrier plate Linear guides Test cylinder Figure 22: Servo hydraulic test bench for validation process at ika D3.15 Report on validation of FE simulations of WT3.1 Page 28 of 42

29 The test bench is integrated in a closed housing, so in case of fire or explosion during the test no persons get harmed. If necessary tests can be recorded by a high speed camera system allowing a maximum of 3,800 fps at the maximum resolution of 1280 x 1024 pixels. Therefore a disposable windowplane is mounted on the front of the housing. Any escaping gas will get vacuumed by the built-in ventilation. Figure 23 shows a table giving the limitations for tests on the servo hydraulic test bench. Physical Quantity Velocity v Force F Travel s Area A Range/Size [mm/s] 0 30 [kn] [mm] 200 x 200 [mm²] Figure 23: Test bench properties The controlling of the servohydraulic test bench is done with a CompactRIO and LabVIEW. This system of National Instruments enables a realtime controlling of the test bench and minimizes any kind of phase deviation of the control loop. Right now the controlling is done by the displacement but in general a controlling by force is possible but not implemented yet. Necessary parameters such as displacement of the cylinder which equals the intrusion of the impactor in the specimen are prompted in a graphical user interface implemented in a software architecture called VI (Figure 24). This makes the input comfortable and failsafe. Pressure On/Off Displacement target PID Controller Oscilloscope Figure 24: Controlling VI Apart from the controlling a second computer system is installed. This second systems is used for the data acquisition during a test. Possible measurement parameters during a test are force, D3.15 Report on validation of FE simulations of WT3.1 Page 29 of 42

30 displacement, cell voltages or temperature. The measurement parameters are recorded via DIAdem. The software is also used for visualizing and creating custom reports. For the process of validation different cell type geometries in different load cases can be investigated. Figure 25 shows tests with pouch cells. The picture on the left side shows the test bench set-up for a three point bending test with a cylindrical impactor, the picture on the right side illustrates the test set-up of a squeezing test with a flat impactor. Figure 25: Pouch cell testing with round (left) and flat (right) impactor 3.2 FE Model Pouch Cell Due to the fact that the full vehicle model of the Toyota Yaris, used for the analyse in Chapter 2.5, is built up in the non-linear finite element solver LS Dyna version 971, the same solver is used for the FE model of the pouch lithium ion battery. The basic model consists of 11 shell layers which represent the copper and aluminium foil. These layers are connected by ten solid layers using a node to node connection. This means, the nodes of the hexagonal solid elements are the same as used for the shell element definition. Figure 26 shows the basic FE model in two different ways. The above picture shows the model while the lower picture visualise an exploded view. Clearly visible are the various layers of shells and the solid layers. In the contact definition of all following loadcases only the different layers of the foils are involved, the solid layers are not considered. This is valid because of the above mentioned node to node connection between solids and shells. Figure 26 also shows the dimensions of the active area which are modelled in the above described way. The model is supplemented by the boundary area, which is not filled with active layers. In this area corresponding shell elements are added (see Figure 27). The connection of these shell elements with the active area of the cell is realised using a Contact_Tied_Nodes_to_Surface. Here, the selected nodes in Figure 27 are connected through a virtual contact force to the active region. This connection transmits node forces in all three spatial directions but is not considering rotations. The following loadcases, determined in Chapter 2.5.3, are used to define material parameters for solids and shells in the active region and for the shells in the boundary area as well as the thickness of the different shell layers. Due to the design of pouch cells, the start is made by the loadcase uniaxal compression with a flat impactor. D3.15 Report on validation of FE simulations of WT3.1 Page 30 of 42

31 t = 10.3 mm Exploded view Shell Solid Solid layer Shell layer Figure 26: FE model pouch cell Topview Nodes used for Contact_tied_nodes_ to_surface 8.5 mm 8.5 mm Sideview 6.8 mm 5 mm 193 mm 239 mm Figure 27: FE model pouch cell with boundary area D3.15 Report on validation of FE simulations of WT3.1 Page 31 of 42

32 3.3 Flat Impactor (Loadcase 1) When considering the construction of the pouch cell with its different layers (see Chapter 1.1) it is assumed that the cell behaviour in the normal direction depends more on the active material and less on the aluminium and copper foils. This means, transferring the force vs. displacement curves from the uniaxial compression tests to stress-strain curves and using them as input curves for the solid material a good agreement between simulation and experiment is assumed. Figure 28 shows the loadcase uniaxial compression realized within FE. Impactor Displacement Support plate Pouch cell Figure 28: Loadcase 1 (flat impactor) The impactor and the support plate are modelled by rigid plates with limited degrees of freedom. In addition, a prescribed motion of 2.25 mm is applied to the impactor which is the maximum deflexion within the cell tests. Figure 29 shows the comparison of testing and the final simulation results. Figure 29: Testing vs. simulation flat impactor D3.15 Report on validation of FE simulations of WT3.1 Page 32 of 42

33 All tests are numbered consecutively. In this case the tests 020 till 022 include the flat impactor. The simulation results are plotted in red and named Simulation. A good agreement between test and simulation can be seen. The force-displacement curve of the simulation is in the corridor formed by the test results. Up to a distance of one millimeter, the force increases only slightly. From this point it rises depending on the impactor intrusion non-linearly to a maximum value of 30 kn. As best suitable material model for the solid layers Mat_57_Low_Density_Foam was identified. Its Young s modulus does not affect the simulation outcome because the stress is a result of the specified stress-strain curve which is used as input parameter. The same is true for the density since at this point a quasi-static simulation is used where mass effects such as inertia do not occur. The density and the Young s modulus can be adjusted according to the time step of the overall vehicle model taking into account the total mass of the cell. The total mass of the cell model has to be lower or equal to the real mass. If necessary, additional mass can be applied using an Element_Mass_Part or an Assign_Mass. 3.4 Three-Point Bending (Loadcase 2) The simulation model fulfilling the load curves from the uniaxal tests and simulations is used as the basis for the three-point bending simulations. The boundary conditions of the 3-point-bending test are shown in Figure 30. Impactor and abutments have a diameter of 40 mm. The distance between both abutments is 150 mm, the load is carried out centrally between both supports. As loading a prescribed motion is applied to the impactor which is also used to measure the occurring forces by a force transducer contact. Figure 30: 75 mm Loadcase three-point bending 150 mm Figure 31 shows the force displacement curves of this loadcase. The black line represents the final simulation results while the curves BQS012 to 014 stand for the testing results. The experimental data lies in a narrow corridor; after an initially steep slope in impactor force follows a linear force increase with lower pitch to the maximum intrusion which is reached by about 35 mm. In comparison to the test results, the total force is growing faster within the simulation. The reason D3.15 Report on validation of FE simulations of WT3.1 Page 33 of 42

34 for this is the compliance. While within the simulations, all contact surfaces are in direct succession and both supports and impactor are adamant, settling phenomena occur in the tests as well as there is flexibility left in the tests. In addition to this initial difference there is a good correspondence between tests and simulations concerning the impactor force as well as the deformation characteristic shown in Figure 32. Figure 31: Simulation Testing vs. simulation three-point bending Testing Figure 32: Deformation characteristic three-point bending As material for the shell layers aluminium with a yield stress of 160 MPa is used. The sheet thickness of the shell layers was determined to 0.2 mm. Additionally, the shape parameter and HU have been added to the solid material, which influence the unloading behaviour of the material. This is necessary because in this experimental set-up individual areas go through pressure and tension. D3.15 Report on validation of FE simulations of WT3.1 Page 34 of 42

35 Although the cells in the current battery system are installed upright, which means the terminals face upside, the 90 rotated mounting position is checked in addition to rule out a direction depending on the load for this loadcase and to validate the found parameters. The results of these tests, BQS015 to BQS019, the simulation result and one result from the previous positioning (BQS013) are shown in Figure 33. Figure 33: Testing vs. simulation three-point bending 90 turned Comparing the curves BQS15 to BQS019 with the curve BQS013 it is found that the force level and the curve characteristics are independent of the positioning of the battery. It also shows again a good agreement between the force-displacement curves of testing and simulation. The deformation characteristic is shown in Figure 34. Simulation Testing Figure 34: Deformation characteristic three-point bending 90 turned D3.15 Report on validation of FE simulations of WT3.1 Page 35 of 42

36 3.5 Clamped Battery (Loadcase 3) In the battery system, each battery cell is clamped as described in chapter between two plastic brackets (see Figure 35). Regarding installation recommendations of the manufacturer, the battery is clamped over the boundary areas. Battery Battery holder Clamping direction Figure 35: Battery cell with two holders Based on this clamping, the boundary conditions for testing and simulation, shown in Figure 36 and Figure 37, are derived. Upper and lower constraints are identical except in the bright coloured area at the bottom due to the asymmetry of the cell holder in this area. The bright area represents the lower constraint, the bright and the adjacent dark area represent the upper constraint. The impactor diameter is 40 mm which is already used in the three-point bending test. Due to the impactor has to fit between the clamping the impactor length is limited to 194. An impactor length of 180 mm is chosen. This is a clearance between impactor and clamping of 7 mm to each side which ensures the free movement of the impactor at high penetration depths. Figure 36: Test bench for clamped battery D3.15 Report on validation of FE simulations of WT3.1 Page 36 of 42