Lifetime of solid state batteries: Better than Li-Ion?

You are currently viewing Lifetime of solid state batteries: Better than Li-Ion?

A long service life of batteries in automotive use is important in order to generate environmental advantages compared to combustion technology. Li-ion batteries have not always been completely convincing in this. But what about solid-state batteries?

For the solid-state battery to be mass-produced, it is necessary to ensure that the battery remains efficient over its entire life cycle and only loses a small amount of its capacity. The minimum requirements are based on today’s cell chemistries. As a rule, the end of the life cycle is reached when either 80% of the original storage capacity is still available, or when the internal resistance (this affects the maximum performance of the cell) has doubled compared to a new cell. The current state of research on the lifetime of solid-state batteries is summarized in this article and compared with the conventional Li-ion battery.

Comparison of aging mechanisms in solid-state batteries to lithium-ion cells

An overview of the main aging mechanisms in solid-state batteries is shown in Figure 1. A comparison is made between the aging of Li-ion batteries with liquid electrolyte and solid-state batteries. For each damage mechanism, the colors indicate the severity of aging. The individual causes are discussed below.

Figure 1: Comparison of the damage mechanisms of solid-state batteries. The strength of the aging effect can be seen from the color coding. Red: massive aging effect, Orange: relevant aging effect, Yellow: slight-moderate aging effect, Green: aging negligible, Own illustration.

Irreversible deposits:

In Li-ion batteries, the battery charging and discharging process is not fully reversible. Instead, there are small side reactions that cause active lithium to be permanently bound and deposited on the surface of the anode, increasing the size of the solid electrolyte interface (SEI) layer. The SEI layer is actually a protective layer that is built up during initial charging and protects the anode from decomposition. However, due to the degradation products of the side reactions, this layer becomes larger and larger, which on the one hand increases the cell resistance, but also reduces the capacity. (cf. Korthauer [1]). For solid-state batteries, there are no extensive studies yet on whether degradation reactions will occur.

 

Volume changes:

Volume changes are a challenge for both solid-state batteries and classic cells. The migration of the lithium ions during charging causes the volume of the cathode to decrease, while the volume of the cathode increases. The volume change of the cathode for Li-Ion-cells is small at 1 % (cf. Käbitz [2]), the anode growth is significantly larger at 10 % (cf. Jossen [3]).

Solid-state electrolytes allow the use of Li-metal anodes. However, these suffer from volume changes to a much greater extent than is the case with liquid electrolytes. This is due in particular to the tendency for vacancies to form and for inactive “dead” lithium to be deposited. For lithium metal anodes, it is stated that the volume more than quadruples. (cf. Oh, Yun et. al. [4]). Therefore, there are approaches to use a kind of 3D framework for the anode, into which the lithium can then intercalate, limiting volume changes. (See Ye, Zhang, et. al [5]). It remains to be seen what number of cycles can be achieved with this approach.

Unstable cathode/electrolyte interface:

A current problem of solid-state batteries is the interface stability between the cathode and the solid-state electrolyte. The cathode and electrolyte are both in a solid aggregate state, which makes it difficult to achieve a good mechanical connection between the two. Cheng, Kushida et. al [6] demonstrated in a study of a coated LLZO solid-state electrolyte/LCO cathode combination that cracking is the cause of poor cycling stability. Shi, Zhag et. al. [7] were able to demonstrate that applying an external pressure to the anode and cathode can partially prevent the loss of contact.  

For conventional cells, the liquid electrolyte ensures that there is always a good mechanical and electrical connection. Interface problems therefore usually do not exist with conventional cells.

Unstable interface (Li-)anode/electrolyte:

Analogous to the cathode/electrolyte interface, the Li-anode/electrolyte interface is also a significant factor in cell aging. Again, the problem is limited to solid-state batteries because of the solid-solid connection. The lithium does not mix well with the electrolyte and it is the subject of research to increase the compatibility of the materials. The cause is especially also in the mechanical stress due to volume change during charging and discharging.  Approaches to improve the interface quality include using lithium alloys instead of pure lithium, inserting intermediate layers, or using lithiated graphite [8].

 

Dendrite growth:

Dendrite growth is an aging effect that occurs in both Li-ion and solid state batteries. Dendrites are spear-like deposits that form on the top of the anode when the cell is charged, and there is a risk that they will eventually grow large enough to puncture the separator (in Li-ion batteries) or solid-state electrolyte (in solid-state batteries), causing a short circuit. In the case of Li-ion batteries, it is known that low temperatures and high charging currents favor the formation of the dendrites(cf. Korthauer [1]). 

Solid-state batteries are particularly susceptible to dendrite formation because the Li metal/electrolyte interface is uneven and leads to an inhomogeneous distribution of lithium ions. On the one hand, this increases resistance; on the other hand, it reinforces tendencies for dendrites to grow preferentially or for parts of the lithium to break off and become inactive. Differences in volume during cycling are thus amplified, causing cells to age more rapidly (see Ye, Zhang, et. al [9]). In a paper by Ye and Li[10] it is concluded that dendrite formation cannot be avoided with previous design. With the help of an extra electrolyte layer, the researchers managed to run through 10,000 cycles with a residual capacity of over 80%. However, the number of cycles achieved is a major exception in a literature comparison and it remains to be seen whether the study results can be repeated in other work.

 

Calendar aging:

Calendar aging describes the aging when a battery is at rest and not cycling. This progresses continuously over the life cycle and ensures that batteries degrade over time even when not in use. This is caused by parasitic side reactions which also occur on a small scale in the resting state. (cf. Korthauer [1]).

For solid-state batteries, there is an expectation that, due to the solid electrolytes, the parasitic reactions will take place to a significantly reduced extent and that calendar aging will therefore not represent a dominant aging effect.  (cf. Fraunhofer Institute [11]).

Expected cycle stability in solid-state startups:

In addition to the information on cycle stability from the literature, there is initial information from solid-state startups on the predicted durability of solid-state cells. In particular, an overview of the market is provided by QuantumScape in an investor presentation from August 2020[12]. Figure 2 shows the cycle numbers achieved. Quantumscape claims to have achieved over 1000 cycles with their cells at 30 °C test temperature and a ceramic separator. Similar cycle numbers are achieved by Samsung with a sulfide-based electrolyte – but at higher test temperatures of 60 °C. ProLogium is said to have achieved a cycle life of 1300. However, no more detailed information about the test parameters is known. SES achieved 750 cycles with a semi-solid-state battery, and for Ionic Materials it is known that they were able to achieve 20 cycles at 30°C with a polymer electrolyte.

Figure 2: Data from various solid-state companies on the number of cycles achieved with their prototypes. Source: QuantumScope [12], own illustration.

QuantumScape states on their website that they expect a longer overall lifetime than current cell variants because there should be no loss of capacity on the anode side [13]. Solid Power justifies higher cycling stability with generally higher resistance and better performance at high temperatures [14].

Conclusion:

According to current knowledge, it is not yet foreseeable that the cycle stability of solid-state batteries will be significantly higher than that of Li-ion batteries. Apart from a notable success at Cambridge University, in which a cycle number of 10,000 was achieved, most studies tend to show cycle stability in the double to lower triple-digit range.

The dominant aging mechanisms of solid-state batteries differ from Li-ion batteries. While for Li-ion batteries the decomposition of the cathode and the electrolyte and the resulting growth of the SEI layer are considered the main drivers of aging, for solid-state batteries it is mainly the strong volume change of the anode due to charging and discharging and the resulting mechanical stress on the cell. The change in volume and poor interface compatibility between Li anode and solid electrolyte also result in a much higher tendency to form dendrites and short circuits.

Published cycle numbers working on the commercialization of the solid-state battery indicate that significantly improved lifetime is not expected, at least in the first generation of solid-state batteries.

So, at least in the medium term, it can be expected that the cycle numbers of today’s systems will be achieved, but not surpassed.

Sources

[1] [Rainer Korthauer: Handbuch Lithium-Ionen-Batterien, Springer Vieweg, 2012, Heidelberg

[2] Stefan Käbitz: Untersuchung der Alterung von Lithium-Ionen-Batterien mittels Elektroanalytik und elektrochemischer Impedanzspektroskopie, 2016, RWTH Aachen

[3] Jossen,Weydanz: Moderne Akkumulatoren richtig einsetzen, 2006, Leipheim/München

[4] Oh, P., Yun, J. et. al.: Development of High-Energy Anodes for All-Solid-State Lithium Batteries Based on Sulfide Electrolytes, 2022, Angewandte Chemie

[5] Xe, L., Li, X.: A dynamic stability design strategy for lithium metal solid state batteries, 2021, Nature

[6] Cheng, E., Kushida, Y. et. al. : Degradation Mechanism of All-Solid-State Li-Metal Batteries Studied by Electrochemical Impedance Spectroscopy, 2022, Applied Materials & Interfaces

[7] Shi, T., Zhang,Y. et al.: Characterization of mechanical degradation in an all-solid-state battery cathode, 2020, Journal of Materials Chemistry

[8] Dai, Q., Zhao, J. et. al.: Ultrastable Anode/Electrolyte Interface in Solid-State Lithium-Metal Batteries Using LiCux Nanowire Network Host, 2021, Applied Materials & Interfaces

[9] Ye, H., Zhang, Y., et. al. : An Outlook on Low-Volume.Change Lithium Metal Anodes for Long-Life-Batteries, 2020, ACS Central Science

[10] Xe, L., Li, X.: A dynamic stability design strategy for lithium metal solid state batteries, 2021, Nature

[11] Fraunhofer Institute for Systems and Innovation Research ISI: Solid-State Battery Roadmap 2035+, Karlsruhe, 2022

[12]QuantumScape: Investor Presentation 08/12/2021, https://s29.q4cdn.com/884415011/files/doc_presentation/2021/08/Investor-Presentation_Sep_2021.pdf, 2023

[13] QuantumScape: Delivering on the promise of solid-state technology, https://www.quantumscape.com/technology/, 2023

[14] SolidPower: All-Solid-State Battery Cell Technology, https://www.solidpowerbattery.com/batteries/, 2023