How safe are solid-state batteries actually? Do they keep their promise of absolute safety? How does a solid-state battery behave when abused? This article provides the answers.
One of the limitations of today’s lithium-ion batteries with liquid electrolytes, apart from the restrictions in energy density, is their safety. When used, and especially when cells are abused, but also due to defects in the manufacture of the cell, there is a risk of uncontrolled release of the energy contained in the battery, resulting in an explosion of the cell, which can then also affect neighboring cells.
The solid-state battery is said to be significantly safer than conventional Li-ion batteries. Whether this is true is examined in more detail in this article:
Thermal runaway in Li-ion batteries with liquid
electrolytes:
To find out whether thermal runaway can occur in solid-state batteries, it is useful to first understand why thermal runaway occurs in today’s cells:
The basic sequence of a thermal runaway in Li batteries with liquid electrolyte is well known in the literature and is described, e.g., by Ruiz and Pfrang [1]. Precise temperatures for the single steps are listet in [2]. The exact sequence always depends on the exact cell chemistry and temperatures only give an approximate order of magnitude, but the basic sequence is as follows:
If the battery is heated to above 85 °C due to a trigger situation, the SEI layer first starts to decompose and at 120 °C the anode begins to react with the electrolyte. This is an exothermic reaction that continues to heat the battery at 150 J/g. At 130 °C, the separator begins to melt in an endothermic reaction. The electrolyte begins to dissolve exothermically at 250 J/g at 200°C. Decomposition of the cathode then begins. The decomposition temperature depends on the type of cathode: for LCO, decomposition starts at 150°C, for LMO at 265°C, and for LFP not until 310°C. Bates, Preger et.al. [3][Bates,A., Preger,Y., u.a: Are solid-state batteries safer than lithium-ion batteries, Louisville, 2022] have described the decomposition of the cathode for NMC cathode. The nickel, cobalt and manganese of the NMC cathode decompose (essentially) according to the following chemical equations:
2 NiO2→2 NiO+O2 (Eq. 1)
2 CoO2→2 CoO+O2 (Eq. 2)
2 MnO2→2 MnO+O2 (Eq. 3)
The decisive factor here is the oxygen molecule that is formed in each case. This reacts with the electrolyte in the cell and releases a lot of heat in a short time (450-600 J/g, depending on the electrolyte), which causes the cell temperature to rise sharply. For EMC (ethyl methyl carbonate) – a typical electrolyte material – the equation looks like this:
2 C4H8O3 + 9 O2 →8 CO2 + 8 H2O (Eq. 4)
Further reactions occur, such as the decomposition of the electrolyte and the melting of the current collectors. The decomposition reactions in the cell lead to the formation of flammable gases, which on the one hand increase the pressure in the cell and on the other hand can also result in an explosion if the venting channels in the cell are not opened beforehand. The thermal runaway cascade for lithium-ion cells with liquid electrolyte is visualized in figure 1 (a).
Figure 1: Thermal runaway cascade comparing (a) conventional Li-ion batteries with liquid electrolytes and (b) all-solid-state batteries. Exothermic (energy releasing) reactions that further accelerate the thermal runaway are marked in red, endothermic (energy consuming) reactions that slow down further heating are marked in blue, own illustration.
Causes of potential thermal runaways in solid-state batteries:
Solid-state batteries also have mechanisms and decomposition reactions when a battery is placed in harmful operating conditions. X. Yu, R. Chen et.al. [4] expect significantly improved safety due to the mechanical, electrical and chemical strength of the system with solid electrolytes. Bates, Preger et al [3], on the other hand, come to the conclusion that damage can also occur in solid-state batteries and distinguish between different damage mechanisms:
- For strong external heating, they assume that, as with Li-ion batteries with liquid electrolyte, decomposition of the cathode can occur (if an NMC cathode is used, equations 1,2,3 apply). However, due to the solid electrolyte, the oxygen does not react further and the reaction cascade is stopped.
- However, if the solid electrolyte becomes permeable to the oxygen from the cathode decomposition, e.g. due to mechanical damage, the oxygen reaches the lithium metal on the anode side. The oxygen reacts with the lithium according to the following equation: 4Li + O2 → 2 Li2O(Eq. 5). The reaction is so strongly exothermic that it leads to melting of the Li metal anode.
- If, due to dendrite formation on the anode side, the solid electrolyte is broken, resulting in a short circuit between the anode and cathode, all the chemically stored energy is converted into heat within the cell. The higher the state of charge, the more lithium can migrate from the anode to the cathode side.
Dendrite formation is considered to be a particularly critical damage, especially because significantly higher energy densities are associated with Li-metal anodes, which leads to extreme heating in the cell. There are already approaches to prevent the occurrence of dendrites: Zhang, Wang et.al. [5] suggest, for example, pressing solid-state batteries, as this should prevent dendrites from forming. Wu, Wang et.al. [6] show that the correct choice of separator can prevent the occurrence of dendrites.
What about other typical damage mechanisms such as overcharging, external short circuit and mechanical damage? Solid Power, one of the companies researching solid-state batteries, have carried out tests on this issue. [7]: During the overcharge test, 15 V was applied to the cell. The temperature of the cell then climbed to an average of 69 °C and the cell voltage increased slightly. However, no further damage was detected. In the short-circuit test with external loads between 0.1 – 0.5 Ohm, a temperature increase to 112 °C and a voltage drop of max. 1.27 V were detected. To test the effect of mechanical damage, a nail penetration test (A conductive needle is inserted into the battery) was performed, which resulted in a slight drop in voltage. No other damage occurred. Solid-Power’s results should be taken with a grain of salt due to its prototype status and only a small batch size of three cells. Figure 2 shows the possible causes comparing solid-state battery and liquid electrolyte cell.
Figure 2: Overview of the causes of thermal runways for Li-ion batteries and solid-state batteries, Own illustration.
Sequence of Thermal Runaway in Solid State Batteries:
What reactions are now possible in a solid-state battery to cause a thermal runaway? Basically, it should be noted that it can be assumed that the operating range of a solid-state cell is significantly larger than for liquid electrolyte cells. X. Yu, R. Chen et al [4] expect that solid-state batteries will also survive hot-box tests up to 200 °C and thus prove to be much more tolerant of higher temperature than conventional cells. However, with pure Li metal anodes, there is the problem that lithium becomes liquid already at 180°C. The extent to which liquid lithium represents a safety risk remains to be investigated. However, there are approaches in the literature to use lithium compounds such as Li5Ba4 instead of pure lithium, which would increase the melting temperature to 325°C (see [8]).
The decomposition of the cathode (at least for an NMC cathode) is endothermic to slightly exothermic for nickel manganese and cobalt (vgl. Shurtz und Hewson [9]) so that it should be rather unlikely that these reactions already initiate the thermal runaway cascade. Since there is no liquid electrolyte, X. Yu, R. Chen et al [4] conclude that there is no formation of flammable gases, which would significantly increase safety. Chen, Nolan, et.al. [10] note, however, that it is possible that solid separators can also decompose at higher temperatures and release oxygen, which then reacts with the Li of the lithium metal anode and heats up the cell. However, they acknowledge that the lithium-oxygen reaction is expected to proceed much more slowly than when flammable gases have formed in liquid cells and they burn explosively. With solid electrolytes, the lithium is in solid form and thus reactions can only occur at the Li anode surface. Figure 1 (b) shows the thermal runaway cascade with the reactions known today. Since the chemical composition is still being actively researched, it can be assumed that not all possible parasitic reactions are known today.
Conclusion: Safer yes, but...
In a comparison of the solid-state battery with Li-ion batteries with liquid electrolytes, the solid-state battery represents the safer system overall. There are indications that the solid-state battery can also be used at higher temperatures without the risk of thermal runaway. Liquid electrolyte and its reaction with oxygen in the cathode poses the greatest safety risk in conventional cells today. This reaction is not possible for all-solid-state batteries.
However, the use of lithium metal anodes introduces new risks: The potential reaction with oxygen must be prevented, as this reaction potentially causes even more damage than burning gases in liquid electrolyte cells due to the generally higher energy density in solid-state batteries.
An internal short circuit caused by dendrites is just as dangerous in solid-state batteries as for conventional cells. The Li-metal anode in particular tends to form dendrites very quickly, so that this poses an increased risk. Overcharging and external short circuits do not lead to thermal runaway, at least in test series to date.
It remains to be seen which safety risks can be eliminated before series production. However, at least for the first generation of solid-state batteries in vehicles, it is unlikely that safety systems such as module heating and cooling or mechanical protection devices can be dispensed with.
Sources
[1] Ruiz, V., Pfrang. A.: Safer Li-ion batteries by preventing thermal propagation, JRC technical reports, European Commision, Luxembourg, 2018
[2] Loveridge, M., Remy, G. u.a.: Looking Deeper into the Galaxy (Note 7), Warwick University, 2018
[3] Bates,A., Preger,Y., u.a: Are solid-state batteries safer than lithium-ion batteries, Louisville, 2022
[4] X. Yu, R. Chen, L. Gan, H. Li, L. Chen, Battery Safety: From Lithium-Ion to Solid-State Batteries, Engineering (2022), doi: https://doi.org/10.1016/j.eng.2022.06.022
[5] Zhang, X., Wang, Q.J., Harrison, K.L., Jungjohann, K., Boyce, B.L., Roberts, S.A., Attia, P.M., and Harris, S.J. (2019). Rethinking how external pressure can suppress dendrites in lithium metal batteries. J. Electrochem. Soc.166, A3639–A3652. https://doi.org/10.1149/2.0701914jes.
[6] Wu, B., Wang, S., Lochala, J., Desrochers, D., Liu, B., Zhang, W., Yang, J., and Xiao, J. (2018). The role of the solid electrolyte interphase layer in preventing Li dendrite growth in solidstate batteries. Energy Environ. Sci. 11, 1803–1810. https://doi.org/10.1039/C8EE00540K.
[7] SolidPower: Are Solid-State Cells Safer?, 19.10.22, Are Solid-State Cells Safer? – Solid Power (solidpowerbattery.com)
[8] Fu, L., Wan, M. et al.: A Lithium Metal Anode Surviving Battery Cycling Above 200 °C,2020, https://doi.org/10.1002/adma.202000952
[9] Randy C. Shurtz and John C. Hewson 2020 J. Electrochem. Soc. Review.materials Science Predictions of Thermal Runaway in Layered Metal-Oxide Cathodes: A Review in Thermodynamics
[10] Chen, R., Nolan, A.M., Lu, J., Wang, J., Yu, X., Mo, Y., Chen, L., Huang, X., and Li, H. (2020). The thermal stability of lithium solid electrolytes with metallic lithium. Joule 4, 812–821. https://doi.org/10.1016/j.joule.2020.03.012.
