Page 30 - European Energy Innovation - autumn 2017 publication
P. 30

30  Autumn 2017 European Energy Innovation


Modern battery engineering

By Prof. Dr.-Ing. Kai Peter Birke and Christoph Bolsinger, Electrical Energy Storage Systems, University of Stuttgart

B  attery development for                   is also developing and there are           the cell as a self-supporting part. One
              electromobility is driven by  standards such as 18650 and 21700          cylindrical cell may fail under static
              continuous energy density     for cylindrical cells, and for prismatic   load tests. However, a multiple array
              enhancement (Wh/kg, Wh/l).    ones that standardization committees       easily sustains the automotive standard
 This applies for weight and volume as      are at last accepting.                     deformation test conditions from
 well as for the costs of batteries in €/                                              different directions (x, y, z) for traction
 kWh. If more energy can be stored by       However, the situation on battery          batteries. This is an important result
 using the same raw materials, batteries    level (assembled cells) is surprisingly    which has already been achieved
 become more cost effective.                lacking in focus. As a rule of thumb,      in this project. Such a cell array is
                                            only half of the energy density remains    depicted in Figure 1. On module level
 Lately the focus has been more and         on battery level for most of the           180 Wh/kg can still be preserved and
 more on volumetric energy density.         batteries which are actually installed in  this shows the power of the concept
 Here Li-Ion cells show a unique            electric vehicles. This means that, from   of self-supporting cell arrays. The final
 property since volumetric energy           200 Wh/kg on cell level only, 100 Wh/      battery housing has thus no tasks to
 density is double that of gravimetric      kg are available on battery level. For     prevent against deformation and can
 energy density. This is why Li-sulfur      volumetric energy density it is often      be constructed with very lightweight
 cannot be a competitive technology         less than one third. Additionally, those   materials. The cooling board will be
 any more for applications in electric      batteries are not easily recyclable.       made out of modern plastics. The final
 vehicles. Even Li-air (on realistic cell                                              target of the project is to reduce the
 level, not by theoretical calculation)     The reasons may be seen in battery         loss of energy density from a factor 2
 may not be able to compete any more        constructions which satisfy firstly the     (if gravimetric energy density from cell
 in this field.                              safety and lifetime. First generation      is divided through energy density of
                                            Li-Ion batteries for hybrid vehicle        the battery) to at least 1.5, preferably
 Generally, the focus of energy density     application show a very rigid housing      1.3. The level from cell to module (as
 improvement is still extremely targeted    which withstands harsh deformation         shown in Figure 1) is actually in the
 on cell level. The latest potential        from outside, and thus satisfies safety     regime of 1.2.
 advancement can be seen in the             demands. For the lifetime of the
 renaissance of Li-metal solid state        battery, usually welded connections of     Additionally, a recycling concept on
 cells. The level of standardization        the cells are employed. All this makes     cell level is successfully employed.
                                            recycling very difficult.                   The modules can be completely
 Figure 1                                                                              disassembled since the cells are
                                            However, looking at the tremendous         screwed on and clamped. Here,
                                            potential of energy density                experimental results show that this way
                                            enhancement by avoiding senseless          of cell connection is fully competitive
                                            losses from cell to battery level, new     with the conventional cell welding
                                            construction methods are necessary.        methods. However, these results still
                                            It is important to consider that energy    have to be verified over lifetime. The
                                            density enhancement of Li-based cells      recycling concept is supported by a pure
                                            will be more and more exhausted            optical based communication concept
                                            within the next decade.                    to the Battery Management System.

                                            Here the project “LIBELLE”, founded        Generally, battery production
                                            by the “VECTOR-Stiftung” shows             concepts still show a huge lack of
                                            new directions. The project has been       industrialisation which, surprisingly,
                                            undertaken by the Electrochemical          lacks the focus applied to
                                            Energy Storage Systems team at the         standardisation efforts on cell level.
                                            University of Stuttgart in Germany,        A Giga-Factory usually means a cell
                                            under the supervision of Professor Kai     factory. Looking at the cost structure,
                                            Peter Birke. The basic idea is to use      the ratio between cell and battery

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