The Future of the Lithium-ion Battery
How can energy densities of traction batteries be further increased? Some misunderstandings and misinterpretations can be seen in the answers and various arguments. But exactly this field of tension is ideally suited to sound out the directions of future cell research and to better assess today’s battery systems and cell technologies. However the three important performance criteria such as power density, service life and safety do not even have to be taken into account, as an article by the University of Stuttgart explains. Sensitization for the appropriate assessment variables The measure of energy density (voltage times specific capacity) is usually the main development driver for the next and future generations of batteries. At first glance, this makes a lot of sense and is understandable. However, not only gravimetric energy density (Wh/kg) should be used as a unit of measurement, but also volumetric energy density (Wh/l). This applies in any case to high-voltage batteries that have to be integrated in the limited space of an automobile. Another lately not ignored aspect is the correlation between energy density and costs. The latter are usually given in euro/kWh or cent/Wh. For example, if energy density can be increased without switching to a completely new material system, costs will also fall. Costs and benefits must be weighed very sensitively against each other. High energy densities are claimed to be expensive to buy if one underestimates the complex structure of effects. Rather, expensive means: One is dealing with massive security problems or other conflicts, a consequence of an exclusive evaluation of theoretical energy densities. There is straight forward clear directive in the selection of systems and materials as well as cell chemistry, but the suitable development paths are becoming clearer. Value creation at cell level increases Although there is an immense know-how and further research requirement in the entire battery system, the cell represents about 70 % of the added value. This value will rise in the future. Apart from the dependence of the quality of supplied cells, this may be one of the main arguments for striving for cell production developed in Germany and by Germans. After unsuccessful attempts to achieve this, the decision-making pressure increases for another reason: Cell materials account for 70 % of the added value. Only 30 % is attributable to production. The material value will also continue to rise. A considerable proportion of the materials fall on the cathode (positive electrode), the anode (negative electrode) and the electrolyte. In the a sum, you could say that cell production means the refinement of raw materials. Therefore, access to cobalt and nickel deposits is of particular importance for the production of modern lithium(Li)-ion cells. China went ahead on this topic by investments in mining globally. Theoretical energy densities Against this background, the efforts to develop Li-sulfur and Li-air cells appear in a special light. Sulphur is an industrial waste product and extremely cheap compared to current cathode materials in Li-ion cells. Oxygen from the air is free, so far. However, this is precisely where the misunderstanding of a purely material- and energy-density driven approach begins, because often only so-called theoretical energy densities are considered. While commercial Li-ion cells already reach more than 250 Wh/kg, for Li-air (correct Li-oxygen) cells, one often finds values of 11.140 Wh/kg in the literature and for Li-sulphur still considerable approximately 2600 Wh/kg. Here, however, values from reaction equations on paper are compared with practical values, meaing apples are compared with pears, so to speak. In fact, however, a technology must also be assessed in terms of what values can be achieved at cell level (commercial product) and ultimately at battery level. An example of the “energy density decline” is a lead accumulator. It should be emphasized that there is actually only about 3 to 4 % by weight of metallic lithium in a Li-ion accumulator, the rest is somewhat loosely speaking “fitting” this metal. The theoretical energy density of pure Li-metal would be around 16.000 Wh/kg, which is even higher than the energy density of gasoline or diesel; only hydrogen is the leader with approximately 33.000 Wh/kg, and this is the reason why fuel cells are attractive for mobile use. 3 % lithium of 16.000 Wh/kg are about 500 to 640 Wh/kg, which again corresponds very well with the theoretical value of the reaction equation of a Li-ion cell. This is where the name Li-ion comes from — but it should be distinguished from Li-metal cells — because the Li-ions are stored and removed in the electrodes like water in a sponge. A special feature of Li-ion cells is that the volumetric energy density is at least twice as high as the gravimetric one, which is a decisive finding. Energy densities of different cell types. The energy densities shown are not adequately transferable to battery system levels. (© University of Stuttgart) Exclusion of Li-sulfur cells for use in passenger cars Although Li-sulfur cells (approximately 300 to 400 Wh/kg) still have a (small) gravimetric advantage over Li-ions (approximately 250 to 300 Wh/kg for cylindrical cells with high energy density), volumetrically Li-ions have a clear advantage (600 Wh/l and more for commercial cells). Sulphur is virtually “dead” electrochemically and therefore not insignificant amounts of conductivity-improving carbon and electrolyte are required to activate the material sufficiently. The volumetric disadvantages alone prohibit the use of Li-sulfur cells in the vehicle. With this rigorous exclusion, other aspects such as the poorer performance density and cycle ability of Li-sulfur cells compared to Li-ion cells can be neglected. Metallic lithium counts more than solid state electrolyte Li-ion cells currently reveal dominance by their practical energy density versus many so-called future systems. Combination of different systems with regard to practically achievable energy densities at cell level (© University of Stuttgart) Future systems are often referred to as post-lithium generation, solid batteries or simply “next generation of battery cells.” It should first be noted that solid systems make use of the possibility of using metallic lithium as anode. The high specific capacity of metallic lithium of 3380 mAh/g and the most electronegative potential of all elements make up the energy density — and not, the solid ion conductors. Solid systems could now suppress the known side reactions during cyclization (charging and discharging) of metallic lithium in systems with liquid organic electrolytes and thus effectively use the energy density boost through the use of Li-metal. This is a decisive advantage over Li-sulfur and Li-air, for which a liquid organic electrolyte is usually the agent of choice. Not the solid ion conductor but the possible successful use of metallic lithium in a rechargeable system shows the perspectives of more energy-rich cells in the future. If this were also possible with a liquid or gel-like electrolyte, solid ion conductors could become obsolete at this point. Challenges with solid ion conductors Experience with solid ion conductors goes back to decades. ABB developed a sodium (Na) sulfur battery in the 1970s, also known as a Zebra battery (Na-NiCl2). However, the initially successful but safety-critical systems required working temperatures of around 300 °C, which ultimately led to a halt in development after a serious fire. A few years ago, Bolloré commercialized a solid system based on metallic lithium and lithium iron phosphate (LFP) and a PEO (polyethylene oxide) based polymer solid electrolyte. LFP operates at voltages around 3.2 to 3.3 V against lithium, so the PEO is stable. The Californian start-up Seeo took up the idea and worked on ways to make standard cathodes (NCM, lithium nickel-manganese-cobalt oxides) of Li-ion cells with voltages up to 4.2 V accessible to such a system. This can be achieved in two ways: by coating the NCM and by modifying and/or replacing the PEO at least on the cathode side. Successes along this path may have been the reason for the acquisition by Bosch at the time. The great challenge of making such systems sufficiently efficient for temperatures below 60 °C has not yet been met. Even the path via battery heating via the internal resistance did not lead to success, since the current systems did not allow sufficient performance (current acceptance) for the heating at temperatures around 20 °C, not even the limit of 40 °C could be reached. This means that a complex external heating and thermostatting system is still necessary. What else makes the development of a solid-state cell so challenging? Cell production involves complex processing of metallic lithium from roll to roll. The undesired dendrite suppression (growth of fine “Li-needles” by the separator during cell operation) in polymeric solid electrolytes is also undissolved. What are the next steps? The challenges which have been shortly summarized lead to the assessment that the degree of maturity of “lithium metal batteries with solid state electrolyte” is lower than often assumed. A longer-term and more cost-intensive development is to be expected. The technical aspects alone should justify Bosch’s withdrawal from Seeo and the withdrawal from high-voltage battery development, which shows the complexity. The Volkswagen Group dares a new attempt, as was recently reported. The Germany-based company secured further shares in QuantumScape Corporation for the equivalent of 86 million euros. The establishment of a joint venture is said to have been decided. Founded in 2010 and headquartered in San José, California (USA), QuantumScape is expected to hold over 200 patents and patent applications for solid state battery cell technology. The planned joint venture is intended to enable large-scale production of solid batteries and to build corresponding production facilities by 2025. In general, the breakthrough of solid ion conductors can be measured essentially by the challenge of “operating temperature” and the suppression of metallic Li-dendrites during operation. In addition, ceramic solid ion conductors present challenges in terms of weight and material costs (for example, lanthanum and zirconium are components). These solid electrolytes are by no means always easy to handle. Since the development goal was stability against metallic lithium, these materials can in principle easily release lithium again (“supersaturation with lithium”), which in contact with water leads to the formation of highly corrosive lithium hydroxide, which has an insulating effect in the cell and can trigger many side reactions. Consistent research into material alternatives to Li-ion cells in the coming years is promising. The following development advances compared to, BUT also for Li-ion-based systems are: — more efficient cooling systems at battery level — a comparatively simple and cheaper heating system — optimized insulations — improvements in high temperature stability (> 60° C) — improved safety and cycle ability. However, solid ion conductors still not perfect. Some contain toxic substances, they are not available in unlimited quantities and are not generally inert. This means there are reactions with the electrodes that have to be controlled. And due to the smaller contact surfaces compared to liquid electrolytes and the high sensitivity of mechanical decontacting during reaction (while it cycles) and the lowest volume thrusts. This means, the development of such cells presents considerable challenges that must not be underestimated. Plea for Li-Ion cell production in Germany As mentioned, a cell is actually the value-adding “refinement” of raw materials. This added value can take place in Germany in contract manufacturing of foreign cells (for example CATL factories) or contract manufacturing in so-called foundries (known from the semiconductor industry). Li-ion cells must generally be manufactured close to the customers worldwide, also in Germany and Europe, because the strict transport regulations add costs, long transport routes and even sometimes damage to cells (during transport by sea and sometimes high temperatures in uncooled containers). Purchasing cells always means a cost disadvantage which speaks in favour of in-house developments by German industry. Another sustainable argument counts: If the battery means diversification in electric cars, as (previously) the combustion engine does, then this can only be achieved through an individual (electrochemical) cell design. This know-how should not be given to the outside world carelessly, nor should it even create a dependence on high-performance cells from others. Germany has only a deficit in the mass production of cells, but not in the know-how of cell research and cell design, of mechanical engineering, nor in other components of the cell value-added chain. In contrary I think Germany has even an advantage in the value chain due to the machinery supply to build cells. A further quality offensive for Li-Ion cells is necessary With demands for Li-ion cells not only increasing but exploding in the future, increasing the quality of individual cells in production is of considerable strategic importance. Modern batteries consist of a large number of individual cells. Tesla batteries contain 7000 to 10000 cells, stationary batteries in the range of 1 MWh and more need even much more cells. Added to this is the statistical failure “x cells per number of units.” This value remains almost constant, so the losses increase proportionally with the increasing quantities. Drastic failures and anomalies in batteries are to be expected. Such an increase in quality can be achieved by means of digitization and industrial 4.0-compatible approaches. In this way, data from cells can be traced and thus introduced into production, quality defects can be detected much earlier and production can even be designed in a self-learning and self-optimizing manner. This could be a unique opportunity for Germany and Europe to provide value. It is not the development of (questionable) future cell technologies but production technologies of the next generation that should be at the forefront again and the control of the cell during operation within a battery pack. I have recently (at AABC in Strasbourg) seen some promising ideas by German Universities. Perspectives of Li-Ion cells In the big contrast to the semiconductor industry, Moore’s law cannot be applied to mathematically calculated steadily increasing energy densities for batteries. Solid-state based systems will be more a complement than a complete replacement; especially in performance oriented applications and/or lower temperatures, there will be limits. In principle, it is not a matter of the solid body but of increasing energy density by using Li-metal with the help of solid electrolytes, which could also be achieved one day by a consistent evolution of Li-ion cells with liquid organic electrolytes. Therefore, important future development goals for lithium-ion cells can also address the electrolyte. If liquid electrolytes are found which make cathodes with voltages higher than 4.2 V accessible against lithium and/or electrolytes with higher temperature stability up to 80 °C, this enables: — higher energy density at cell level — using a passive cooling instead of a complex active cooling also higher energy densitiy on battery level; here in particular, clear jumps are still possible to lose significantly less energy density from the cell to the battery. In roll-to-roll processing, for example, Li-metal will pose an extreme challenge to drying room conditions and the production of very thin Li electrodes in the range of 20 μm. Li-ions with graphite electrodes have a very clear advantage here, since the lithium remains in the cathode until the first charge. These electrodes can be produced and processed easily without drying rooms. With the expected growth in demand for cells, safe, high-quality and reproducible cell production will come to the fore: however, the safe, i.e. high-quality and reproducible production of cells will come absolutely to the fore. I think in the mass production of top-quality Li-ion cells, German industry could establish a competitive and strong position internationally.