High range, short charging times – no range anxiety. This is how simple the dream of electromobility could look. At least as far as the higher energy density is concerned, there is a broad consensus in the battery community that this will indeed be achieved. But what about short charging times? The answer can be found in this article.
Anyone who drives an electric car expects it to be possible to charge the car at the fast charging station in just a few minutes. Whereas a few years ago, fast-charge times were 40 minutes or more, today’s batteries can do it in half that time. To be competitive, a solid-state battery should be able to charge at speeds similar to today’s lithium-ion batteries. Whether this is the case is analyzed in this article. Both the state of the science and announcements from companies working on commercialization are examined.
Influence parameters of fast charging for Li-ion batteries
In order for a battery to be charged quickly, a number of requirements must be met. The following section describes the dominant influencing parameters that are specifically relevant for lithium-ion batteries in order to be able to charge a battery quickly. The following section then elaborates how the solid-state battery performs.
Macroscopic structure of the cell
It starts with the macroscopic structure of the cell. To ensure that a battery can be charged quickly, the electrodes (i.e. anode and cathode) should be as thin as possible so that the transport paths for the lithium ion are as short as possible. Small particles ensure that the transport paths within the particles are minimized. It is also important for the particles to have a very large surface area. As a result, the charge transfer reactions (chemical reaction of embedding in the particles) can take place particularly quickly and a higher current can flow. Thick current collectors to handle the high current density and a high amount of conductive agent (e.g. carbon black) for electrical conductivity support the high current levels. Figure 1 shows the difference in the structure of a high energy and high power cell for solid-state batteries [1].
Figure 1: Difference in the design of a solid-state battery optimized for high energy density and high power density (a) high energy density (b) high power density, own illustration according to [1].
Cell resistance
The internal resistance of the battery must not be too high. The internal resistance is composed of the resistance of the electrons, the resistance of the ions in the electrolyte, the charge transfer resistance (resistance resulting from the reaction of the lithium with the grain boundaries of the active material) and the resistance resulting from Li diffusion [2]. The movement of the anions also contribute a part to the resistance of the system. Figure 2 shows the resistance network for the negative electrode, separator and current collector.
Figure 2: Illustration of the resistor network of negative electrode, separator and current collector of a lithium-ion battery, own illustration according to [3].
High resistances cause the charging and discharging efficiency to drop. A larger part of the energy is not used for the actual chemical storage reaction, but is burned up in the battery as heat loss. This causes the battery to heat up considerably in some cases. In order not to overheat the battery, the charging rate must be throttled [4]. Active cooling worsens the charge/discharge efficiency, but can enable significantly higher charging currents.
Operating temperature
At cold temperatures, the resistance of the Li-ion cell increases sharply. Causes for the resistance increase are mainly the charge transfer resistance and the resistance due to the slowed down movement of the ions within the anode active material (In figure 2 RCT and ZW ). This results in increased polarization and a potential below 0 V at the anode, causing lithium to deposit on the surface of the anode active material) instead of penetrating into the active material. This process is called lithium plating. Figure 3 shows the this in detail. Some of the lithium still intercalates into the active material after charging and deposits in the anode as intended (Figure 3a). Another portion dissolves during discharge and migrates to the cathode, so the cell is not damaged (Figure 3b). However, lithium is very reactive, so it undergoes a parasitic chemical reaction with the electrolyte, the degradation products of which are deposited as an undesirable additional SEI layer (Figure 3c). The lithium at the top of the anode tends to pile up in layers, forming small spearheads that, in the worst case, can pierce the separator and cause a short circuit (Figure 3d). The dendrites in particular also tend to break off, forming so-called dead lithium. [5]. Since dendrite formation occurs especially at low temperatures, this temperature range should be completely avoided for fast charging.
Figure 3: Mechanisms of lithium plating. In lithium plating, lithium is deposited on the anode surface during charging. Subsequent processes depend on the individual case: (a) Some lithium still participates in the charge transfer reaction and intercalates into the cell, (b) During discharge, the lithium reversibly dissolves again and continues to participate in the intercalation reactions, (c) Lithium reacts with the electrolyte and the reaction products deposit as part of the SEI layer, (d) Li deposits can spear and potentially puncture the separator, (e) Parts of the deposited lithium can break off and become permanently inactive, Own illustration.
Conversely, high temperatures above 30°C lead to a higher proportion of undesirable reactions, the products of which settle as an additional SEI (Solid Electrolyte Interface) layer. The SEI layer is a protective layer that forms between the anode and the electrolyte and is important for minimizing parasitic reactions between the anode and the electrolyte. A thin layer is sufficient for its functionality. However, over the life cycle and especially during fast charging, the SEI layer becomes thicker due to the undesirable reactions.
There is therefore a conflict of interest: At low temperatures, there is increased lithium plating, and at high temperatures, generally higher cell kinetics with undesirable additional SEI formation. Since lithium plating is generally seen as more critical, SEI deposition is accepted as an undesirable side effect. It is therefore common practice to heat the cells during fast charging [4].
Chemical stability
The cell must be stable against the higher load due to fast charging. Lithium plating is not only favored by low temperatures, but also by fast charging in general. The reason for this is the different speed at which reactions take place in the battery, leading to large differences in concentration in the battery. The diffusion of the lithium within the anode active material prevents the differences from balancing out quickly, which increases the tendency of lithium plating.
The concentration differences caused by fast charging lead to a number of other stresses. One example is that the fast reactions near the current collectors can cause the conductive salt to become scarce, which means that the charge transfer reactions can no longer take place here and other areas of the cell are correspondingly overloaded, leading to increased current densities [4]. A good cell design optimized for the high currents can help deal with the high currents and concentration gradients.
Fast charging for solid-state batteries
But what about the fast charging capability of solid-state batteries? In fact, there are arguments for and against a higher charging speed.
Reasons for higher loading speeds
Unlike liquid electrolytes, solid-state batteries do not experience concentration differences during charge and discharge processes, which is one of the main reasons for low charging currents in Li batteries. In solid-state batteries, Li ions migrate along a fixed anion framework that does not move. Especially in sulfide-based systems, the ionic conductivity is just as good, or in some cases even slightly better, than in liquid electrolytes.
Since in solid-state batteries the flammable liquid electrolyte is replaced by a non-flammable, thermally much more stable solid electrolyte, these batteries can be used at significantly higher temperatures than batteries with liquid electrolyte. It is therefore no longer necessary to throttle the charging speed if the cells heat up too much during fast charging. The typical degradation effects that occur at an accelerated rate with liquid electrolytes at temperatures above 60°C are not to be expected with solid electrolytes. On the contrary, the higher temperature leads to a further improvement in ionic conductivity and thus to lower thermal losses. A high temperature also favors more uniform reactions at the interface between the anode and solid electrolyte, and vacancies in the Li metal are less likely to occur.
Solid electrolytes are also much better suited for Li-metal anodes, as they are less likely to cling to the anode due to the solid material. Due to the volume change during charging and discharging of the Li-metal anode, new electrolyte always reaches the anode active material in liquid systems, which leads to strong parasitic reactions and consumes lithium. With solid electrolytes, on the other hand, a passivation layer can build up so that undesirable processes are suppressed. Stable cell chemistry is the basic prerequisite for charging and discharging at high currents [6].
Reasons for lower fast loading speed
Despite the reasons described, however, there is considerable doubt as to whether solid-state batteries are fundamentally suitable for fast charging.
The first reason here is the very strong tendency of Li-metal solid-state batteries for dendrites to form, especially during rapid charging. In theory, the solid electrolyte should actually be less susceptible to their growth, since the solid electrolyte is mechanically very strong and elastic. In practice, however, while there is no active piercing through the solid separator, the dendrites can instead be deposited along the grain boundaries, leading to a short circuit along the way. Figure 4 shows the formation mechanism of the dendrites. This is different from the dendrite formation mechanism in Li-ion cells. Due to local potential differences, Li+ does not bond to the cathode during discharge, but instead deposits along the grain boundaries of the solid electrolyte. Over time, the Li intermixing in the solid electrolyte becomes larger. A short circuit occurs when the individual lithium deposits are large enough to combine and allow electron flow.
Figure 4: Course of dendrite formation in solid-state batteries, Own representation based on investigation of an LLZO electrolyte by [7].
The causes that promote lithium plating are not yet fully understood. However, it is assumed that too high electrical conductivity of the electrolyte favors the formation of the dendrites [7].
The tendency to form dendrites is one of the performance parameters of the cell. The maximum current density that a cell can deliver is specified in the form of CCD (critical current density). This is defined as the current above which dendrites form in the shape of the Li metal filaments. Below this current density, the growth of the dendrites does take place, but so slowly that it can be assumed that this does not critically limit the service life of the battery [8].
Another mechanism that takes place at the lithium metal anode makes high discharge currents difficult: rapid discharging causes uneven removal of the lithium from the anode. This can cause holes and voids to form. Although Li ions can migrate or creep within the anode (Li self diffusion) and reoccupy the holes, if the discharge current is too high, this does not happen quickly enough and permanent holes remain. During charging, this in turn leads to local current densities rising sharply and damaging the cell.
On the cathode side, the poor ionic conductivity of the cathode active material prevents high charging currents. To improve conductivity, the solid electrolyte is mixed into the cathode (see also Figure 1), but this reduces its energy density. Depending on the system, sometimes considerable amounts of electrolyte between 30-50 % are added. Lower admixtures (and thus a higher proportion of active material), on the other hand, quickly lead to a drop in conductivity, so that the cells must be weighed up to determine whether a high energy density or a high current density should be achieved. In addition, fast charging on the cathode side is made more difficult by the fact that contact between the cathode interfaces is increasingly lost at high currents. Although there are approaches to solve these problems (e.g. 3D templates along which the ionic and electrical paths can form), these are all still at an early stage of research [6].
Along with the lithium metal anode, silicon anodes are considered one of the most promising materials. While silicon anodes enable an exceptionally high theoretical energy density of 3572 mAh/g, the material suffers from several problems that hinder fast charging. Similar to Li-metal anodes, battery charging results in an extreme increase in the volume of the anode material. The volume change causes the anode material to break up and can create voids that cannot be filled due to the solid electrolyte. The volume change also causes the solid electrolyte and anode material to be in direct contact and there are permanent parasitic reactions that consume lithium and thicken the SEI layer. This leads to high resistances in the system, making high current densities difficult to realize so far [9].
While sulfur-based electrolytes achieve very good ionic conductivities that are of a similar order of magnitude to Li-ion batteries, the situation is much worse for polymer-based and oxide-based electrolytes, at least at room temperature. The conductivity of polymer-based electrolytes is based on the fact that polymer chain segments move. At room temperature, however, most polymers crystallize, which results in significantly poorer conductivity and makes fast charging at room temperature more difficult. This is also the reason why solid-state batteries with polymer electrolytes are generally used at high temperatures and the cells are heated during operation. Although oxide-based electrolytes perform somewhat better in terms of conductivity, they suffer from considerable interface problems [7].
Announced charging speeds of the solid-state startups
The data published so far on the charging speed of solid-state batteries also predominantly indicate that high charging speeds are rather not to be expected. Factorial, working on a 100 Ah polymer cell with Li anode, test their cells in charge and discharge at 1/3C. They also suggest allowing fast charging only occasionally to prevent accelerated aging of the cells [10].
QuantumScape has published a white paper on fast charging in which they claim to have achieved a residual capacity of well over 80% after 400 cycles using oxide-based electrolyte and a charge rate of 4C (discharge rate 1C). However, these are laboratory cells from only a single layer, which is potentially very easy to control, so it is not yet clear to what extent these values can also be achieved in a commercial cell [11] Solid Power, who are working on a solid-state battery with sulfur-based solid electrolyte, state that they have still achieved a residual capacity of 81 % after 650 cycles with a cell with silicon anode, in which every fifth cycle was fast-charged [12].
Most companies active in the solid-state battery space are working to improve the fast-charge capability of the cells. However, with the exception of QuantumScape, indications are that at least the first generation solid-state batteries will have significantly poorer fast-charge capabilities than today’s Li-ion batteries. While the measurement data presented by QuantumScape is impressive, it is unclear whether the measurement results are transferable to a series product or whether they are special laboratory cells optimized specifically for this demonstration.
Conclusion
The ability to fast-charge a battery is one of the key requirements placed on a battery for automotive use. However, for solid-state batteries, there are still some problems that need to be solved to enable high charging currents. The following problems have been identified:
- Solid-state batteries tend to form dendrites, which can lead to a short circuit of the cell. High charging currents massively increase this tendency.
- Poor contacting at the electrode interfaces and poor ionic conductivity (especially with polymer-based electrolytes) lead to strong self-heating and inhomogeneities in the material, so that the critical current density is not high
- In systems with Li anode, high discharge currents lead to the formation of cavities and voids in the anode, which locally increase the current densities
- Silicon-based anodes, on the other hand, suffer primarily from the strong volume change and the fact that interface surfaces are not stable.
- On the cathode side, high amounts of conductive material must be introduced because the conductivity of the pure cathode material is not good.
So far, it does not appear that all of these problems can be completely solved. The forecasts of most companies working on the commercialization of the solid-state battery tend to indicate that at least the first generation of solid-state batteries will not be optimized for fast charging. Most published measurement data from the companies shows that fast charging is actually associated with unacceptably high aging. The 800 full cycles usually required in the automotive sector have not yet been achieved. Further improvements are needed to make it suitable for use there.
Sources
[1] Vorlesung Institut für angewandte Materialien: Grundlagen poröser Elektroden, 2016, Karlsruhe Institut für Technologie
[2] Du, Z. et al. :Understanding limiting factors in thick electrode performance as applied to high energy density Li-ion batteries, 2017 Journal of Applied Electrochemistry
[3] Moskon, J., Zuntar, J. et al.: A Powerful Transmission Line Model for Analysis of Impedance of Insertion Battery Cells: A case Study on the NMC-Li System, 2020, Journal of The Electrochemical Society
[4] Tomaszewska, A., Chu, Z.: Lithium-ion battery fast charging: A review, 2019, eTransportation
[5] Liu, Q., Du, C. et al.: Understanding undesirable anode lithium plating issues in lithium-ion batteries, 2016, Royal society of chemistry
[6] Vishnugopi B., Kazyak E. et al. : Challenges and Opportunities for Fast Charging of Solid-State Lithium Metal Batteries. 2021, ACS Energy Letters
[7] Zhang, C., Hu, Q. et al.: Fast-Charging Solid-State Lithium-Metal Batteries: A review, 2022, Advanced Energy Sustainability Research
[8] Sarkar, S., Thangadurai V.:Critical Current Densities for High-Performance All-Solid-State Li-Metal Batteries: Fundamentals, Mechanisms, Interfaces, Materials, and Applications
[9] Lee, D., Lee, H. et al.: Toward High Rate Performance of Solid-State Batteries
[10] Yu, Alex: Challenges and Opportunities for Solid-State Manufacturing and Scale Up, 2023, Vortrag auf Battery Show 2023 Stuttgart
[11] Quantum Scape: White paper: A deep dive into QuantumScape’s fast-charging performance, 2022, https://www.quantumscape.com/resources/blog/white-paper-a-deep-dive-into-quantumscapes-fast-charging-performance/
[12] Jim Motovalli: How Solid-State EV Batteries May Lick The Fast-Charging Degradation Problem, https://www.autoweek.com/news/a39946624/solid-state-ev-batteries-fix-fast-charging-degradation-problem/
