Lithium-ion Batteries Material Strategy and Positioning. Energy Storage HARDWARE

Transcription

1 HARDWARE Energy Storage Lithium-ion Batteries Material Strategy and Positioning Lithium-ion batteries are to replace the nickel-metal hydride batteries that are currently being used in hybrid motor vehicles in order to give electric vehicles, among other things, a much longer travelling range. The automobile industry is conducting extensive studies to examine different cell and system designs, as well as various material pairings, and these have advanced to varying levels of development. The independent battery and fuel cell research institute ZSW (Zentrum für Sonnen energie und Wasserstoffforschung Baden-Württemberg) is supporting these orientation endeavours. 22 ATZelektronik 05I2008 Volume 3

2 1 Introduction The best suited battery technologies are a much discussed topic within the current trend towards hybrid and battery vehicles. Attention is being focused on Lithium-ion technologies (Li-ion) for hybrid drives since such batteries have already become part and parcel of consumer electronics. However, the material systems used for batteries for consumer electronics cannot be simply transferred at a one-to-one ratio to the automobile world Cost and safety reasons invoke many limitations. The energy content is larger by a factor of at least 10 to 100 so that motor vehicle developers and battery producers are confronted with enormous problems in their efforts to fulfil the requirements of the automobile industry. These include such factors as stability and safety under extreme ambient temperatures from -30 C to +80 C, shock, vibration and crash resistance, the highest possible number of recharging cycles at extremely low depths of discharge, minimal degradation and a service life that is, ideally, more than ten years (US norm). Current work for the development of new Li-ion batteries is primarily concerned with a synthesis of cost-effective and safe active materials, adapted cell geometries and an optimised battery management system. 2 Comparison of Rechargeable Batteries Until this day the lead accumulator has remained the most used rechargeable battery. Turnover solely for motor vehicle applications is more than 10 billion Euros. However, the lead accumulator does not provide the required energy content (at an acceptable weight). Moreover, the lead accumulator does not cope with the necessary cyclic stress. The nickel-metal hydride (NiMH) battery has double the energy density of the lead accumulator, Figure 1. Consequently, this material combination has become firmly established throughout the branch in the order of a billion cells per year. Marketable hybrid vehicles are therefore currently exclusively based on NiMH batteries. This technology has been hitherto favoured on account of its relatively high specific performance (up to 1,500 W/kg) and a longer service life at a smaller cyclic depth (> 100,000 cycles, cycle depth <5%). The Li-ion battery currently has the highest energy density of all available rechargeable batteries and, in the medium term, this type is expected to replace the NiMH battery. The nominal voltage of the Li-ion battery (3.6V) is three times higher than that of the nickel-metal-hydride accumulator, and its energy density is approximately three times higher than that of the lead accumulator. Other points favouring the Li-ion battery include its excellent charging/discharging efficiency (low losses due to internal bat- tery resistance), excellent cyclic performance (regular deep discharging does not cause irreversible damage to the battery). However, the development of this favoured storage technology has not yet been sufficiently advanced to the maturity level required for application in automobiles: New active components still have to be tested, work on the optimisation of the cell and module technology has to continue, and vital production know-how has to be acquired. 3 Lithium-ion Batteries 3.1 Functional Principle The functional principle of the Li-ion bat- tery is based on two electrodes (anode Figure 1: Comparison of rechargeable batteries The Author Prof. Dr. Werner Tillmetz leads as board director the business unit electro chemical energy at head at ZSW (Zentrum für Sonnenenergie und Wasserstoffforschung Baden-Württemberg) in Ulm (Germany). ATZelektronik 05I2008 Volume 3 23

3 HARDWARE Energy Storage Figure 2: Li-ion cell forms: Round cells and Li-ion foil cells and cathode) which can reversibly store lithium in a so-called host grid. Most of the customary cells used nowadays have an anode consisting of a 100 to 200 μm thick, porous layer of carbon (as the host grid). The cathode consists of a lithium transition metal oxide usually cobalt oxide (LiCoO 2 ). The charging process transfers lithium ions from the lithium transition metal oxide and transports them to the anode where they are stored in the carbon host grid. The process is reversed during the discharging process. The two electrodes are separated by an electrolyte for instance LiPF6 (lithium hexafluorophosphate) in a mixture of organic carbonate. A 20 μm thin separator mechanically separates the anode and the cathode. 3.2 Categories of Lithium Cells The following categories currently apply to rechargeable lithium batteries, Figure 2: Round lithium-ion cells (solid case) Prismatic lithium-ion cells (solid case) Prismatic lithium-polymer cells (flexible case) Rechargeable button cells (niche application). The annual output of prismatic cells is in the order of one billion cells primarily for mobile telephones with usually one cell per battery pack. They are character-r ised by a flat design, better heat dissipation, more uniform temperature distribution, flexible dimensioning, simpler battery design, and tightness of the case and pole lead-through points. The cells become inflated at elevated internal pressure levels, but they remain tight. Cylindrical lithium-ion cells usually in the size are used in laptops, digital cameras and entertainment electronics. They are characterized by their simple, reliable production technology (electrode winding), pressure-resistant case (40 bar), a defined opening pressure of the bursting disk, dependable tightness and a high temperature gradient within the cell. The disadvantage of cylindrical battery cells: Their poor packing density, particularly when many cells are circuited into a single battery system. Except for button cells the afore-listed forms are used in motor vehicles. Par- allel or series circuiting of small cells makes it possible to realize virtually any voltage and capacity. With regard to costs, reliability and installation space (and many other factors), current cell geometries are being modified to the specific application. Depending on the given vehicle, voltage levels of more than 400 V are envisaged for cars and more than 700 V for commercial vehicles. The capacity of an individual cell must be significantly higher than that of consumer cells namely between 10 and 200 Ah per cell, depending on the cell configuration of the hybrid drive. This is associated with an increase in diameter of round cells, but also in higher gradients with internal temperature distribution. With prismatic cells the base area is enlarged. Cooling is simpler and more homogenously realized. However, higher demands are expected of the mechanical stability of the cell and of current derivation from the cell. Both shapes round and prismatic are being further developed for use in vehicles, and each battery manufacturer is pursing its own philosophy. 3.3 Materials The most important materials that are being tested today or that are still being researched are shown with their respective potential positions in Figure 3: It is clearly apparent that materials suitable for the negative electrode lie in the vicinity of lithium metal. The potential 0 V is used as reference. Materials suitable for the positive electrode are substances with a high potential, i.e. 3.5 V against the lithium metal reference. Nowadays the rechargeable lithium cell market is Figure 3: Potential ranges of different active materials for lithium-ion cells (the red framed section represents the state of the art) 24 ATZelektronik 05I2008 Volume 3

4 determined by the lithium-cobalt oxide material combination (LiCoO 2 ) as the positive electrode and graphite as the negative electrode (see Figure 3, highlighted red). This active material combination is frequently used for mobile phone and laptop cells. Next to the potential position, other important criteria are the capacity determined by the number of charges the material can endure as well as the deep discharging ability and cyclic strength. Further assessment criteria are the discharge curve, Figure 4, the thermal stability of the active materials as well as electric and ion conductivity. To this must be added such positive aspects as price and environmental compatibility: For instance the price of cobalt oxide has experienced a sharp rise in recent years. Under certain conditions the material can decompose, thereby giving rise to safety problems. 3.4 Material Structures With regard to the active materials for the cathodes of lithium-ion cells a distinction is made between three material structures, namely the layer structure, the spinel structure and the olivine structure, Figure 5. The currently dominating material class is the layer structure (classic LiCoO 2 ). To reduce costs and increase reliability, the cobalt in the layer structure is partly or entirely replaced by nickel or manganese cations. The LiCo 0,33 Ni 0,33 Mn 0,33 O 2 often referred to as 1:1:1 material is now commercially available. The group of materials with a spinel structure include LiMn 2 O 4 and LiMn 1.5 Me 0.5 O 4. LiMn 2 O 4 is the so-called manganese spinel and is a lithic manganese oxide a material that is not associated with any decomposition reactions. Furthermore, it is noted for its very reliable behaviour at high temperatures. Another advantage is the price and availability. Manganese spinels are used commercially, in some cases also mixed with layer oxides. At a charge of up to 4.3V it is possible to derive up to 110 mah/g. The manganese spinel is suitable for cells in larger systems or for high-current applications where not only the energy density is decisive. The disadvantage of this material is its limited service life (corrosion processes) at higher application Figure 4: Cathode materials currently being developed temperatures, as encountered in the automobile industry. Many specialist circles consider materials with an olivine structure to be the most attractive class of material for motor vehicle applications. Lithium-iron phosphate (LiFePO 4 ) and lithium-manganese phosphate (LiMnPO 4 ) are favoured. These materials are characterised by excellent thermal and chemical stability and high reliability. Furthermore, they fulfil the important criteria of economic efficiency and availability of the raw material. The principle problem of lithium-iron phosphate is its limited electrical and ionic conductivity due to the material s structure which only provides a one-dimensional tunnel for lithium diffusion. This can be compensated by nano-structuring and coating techniques. The first products are already available on the market. Developers are now working intensively on creating new active materials within all levels of the value-added chain from the cell right up to the complete vehicle. 4 Ready-Made Sizes and Battery Design The term hybrid denotes a technology that covers a wide scope of energy management within a vehicle, Figure 6: The so-called micro-hybrids feature a start/ stop function in conjunction with an increasing number of electrically powered secondary units which, in their totality, cannot be realized with conventional lead accumulators. Moreover, the mildhybrids also cover the functions of torque support and energy recovery (energy contents of the batteries 0.5 to 1 kwh). The full-hybrid drive additionally enables the function of electric driving (currently for a maximum distance of two kilometres with a car). For this purpose the bat- tery must provide up to 3 kwh. The plugin hybrid supplements the double-drive variant with batteries that have an ener- gy content of 6 to 8 kwh. Next to the combustion engine that charges the batteries by a generator, the batteries can also be charged from the home power socket. Electric vehicles derive their energy from at least 20, ideally more than 50 kwh, depending on the vehicle size. Cell design varies according to the given application profile. Full-hybrid vehicles require high-power cells (highpower design). For instance, these cells require high conductivity rates for the electrolytes (ion lines) and active masses (little resistance within and between the particles). The thickness of the separators and active-mass layers should be minimal, and the routes to the power leads should be as short as possible. The compact dimensions also result in strin- ATZelektronik 05I2008 Volume 3 25

5 HARDWARE Energy Storage Figure 5: Active material structures (from the left): layer structure, spinel structure, olivine structure and they are indicating the correct path forwards. Furthermore, it is imperative that basic research regarding every aspect of electrochemical energy storage be systematically re-established in Germany. The industrial basis for the implementation of Li-ion technology in vehicles is still relatively slender, especially in the western world. Often there is a lack of know-how concerning the assessment of the complex interaction between materials and the electrical properties. Appropriate specifications and correspondingly adapted testing regulations still have to be developed. The Centre for Solar Energy and Hydrogen Research is support- ing this process. Moreover, the Research Institute is testing all conceivable battery concepts as a partner of the automobile industry. References [1] ZEV-CARB Report 2007 [2] W. Weydanz, A. Jossen, Ulm: Moderne Akkumultoren richtig einsetzen,2006 Figure 6: Potential of different battery technology in hybrid and electric vehicles [1] gent demands regarding manufacturing tolerances and the thermal design of the battery pack. Electric vehicles, on the other hand, require high-energy batteries (high-energy design) to ensure electric travel over long distances. As much energy as possible must be stored in each cell and, at the same time, it is essential to keep costs and weight within acceptable limits. These so-called high-energy cells require thicker electrodes with a high mass coverage. Electrode reactions should be used at high utilisation rates. Passive components should be dispensed with (current leads, conductance additives). 5 Summary and Prospects The Lithium-ion battery is still a very young technology in terms of use in the automobile industry. The prospects are excellent and the commitment of the participating industry is enormous. An enduring success will be decided by the correct strategy for product development and the long-term, consistent improvement of battery technology. The current battery research programs of the German Federal Ministry for Research and Education (BMBF) and the German Federal Ministry of Economics and Technology (BMWi) are making an important contribution in this respect 26 ATZelektronik 05I2008 Volume 3

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