Solid-state batteries
Solid-state batteries have the potential to offer increased safety, energy density and performance, over conventional lithium-ion batteries. However, technical challenges still exist. Learn about the science, potential and challenges here.
Why are we interested in
solid-state batteries?
Solid-state batteries have a number of potential advantages including increased safety, higher specific energy and faster charging, compared to traditional lithium-ion batteries. However, there are significant technical challenges still to overcome with lots of nuance in the technology.
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Battery thermal runaway in lithium-ion batteries
Before exploring how solid-state batteries work, it’s important to understand what happens during thermal runaway in traditional lithium-ion batteries since safety is key. Conventional lithium-ion batteries often use a graphite anode and a transition metal oxide cathode, which are separated by a polymer separator. All of these components are porous which allows a liquid electrolyte to soak all the regions; facilitating the movement of lithium-ions between the 2 electrodes.
The graphite anode is, however, thermodynamically unstable when in contact with the liquid electrolyte, which causes a solid electrolyte interphase, or SEI, to form. This is a protective layer which limits further reaction between the anode and the electrolyte.
If the battery heats up to over roughly 70°C, the SEI layer begins to decompose leading the electrolyte to react with new anode surfaces. This is an exothermic reaction which causes more heat to be generated. If the heat is not removed and goes over ~130°C, this can cause the polymer separator to melt which causes a short-circuit between the anode and cathode, leading to more heating due to the large current flow.
Above ~200°C the liquid electrolyte decomposes, and above ~300°C the cathode decomposes, leading to further heat release. The cathode in particular is important, as its decomposition also leads to oxygen being given off which is extremely flammable. Here, a key component of the thermal runaway reaction is the flammable liquid electrolyte, which is a major safety issue.
Adapted from Progress of enhancing the safety of lithium ion battery from the electrolyte aspect. Wang et al. Nano Energy 2018; 55 :93–114
What is a solid-state lithium-metal battery?
Motivated by the safety challenges posed by liquid electrolytes, as well as the desire for higher energy density batteries; research and development in solid-state batteries has been increasing in recently years, but what is a solid-state battery?
Traditional lithium-ion batteries have 2 porous electrodes which work via the insertion, or intercalation, of lithium-ions into their atomic structures. Here the organic liquid electrolyte allows the lithium-ions to move between the electrodes, with electrons flowing around an external circuit to provide useful electrical work.
In the case of a solid-state battery, the liquid electrolyte is replaced with a solid electrolyte which ideally is inflammable and safe.
In doing this, it also allows for the graphite anode to be replaced by lithium-metal which has about 10 times the specific capacity, giving the potential to increase cell level energy density. Other cell designs which use graphite, silicon and other anode materials are also possible.
However, focusing on lithium-metal, a major problem occurs during cycling when the lithium is stripped and deposited continually, which over time can lead to void formation, lithium dendrites or cracks in the solid electrolyte which can compromise the performance.
Adapted from: Performance and cost of materials for lithium-based rechargeable automotive batteries. Schmuch et al. Nature Energy. 2018
Solid-state battery architectures
Within the solid-state battery category, there are various different designs exists, each with their pros and cons.
Traditional lithium-ion batteries
Traditional lithium-ion batteries are now relatively mature with supply chains scaling up and a relatively easy manufacturing process; having benefitted from decades worth of research and development. However a major drawback of the technology is its relatively modest energy density and the safety concerns associated with the organic liquid electrolyte.
All-solid-state battery
Keeping the electrode materials the same and replacing all the liquid with a solid then leads to the all-solid-state battery (ASSB) design, which has the key advantage of being safer.
However, manufacturability is more difficult due to the potential need to sinter the solid material or challenges with handling the material. Furthermore, because a solid instead of a liquid is now used, the interface between the active material and electrolyte is more difficult to maintain since the solid does not flow as readily. This can lead to lower performance.
Furthermore, in this configuration with a traditional graphite electrode the specific energy is actually lower than a traditional lithium-ion battery. This is because simply replacing the density of the solid-electrolyte can be almost 5 times higher than liquid electrolyte. Lithium lanthanum zirconium oxide for instance has a density of 5.1 g/cm3 vs liquid electrolyte 1.3 g/cm3.
ASSB with a lithium-metal or silicon anode
The specific energy can be improved by changing the graphite anode with materials such as silicon or lithium-metal. However, this is still difficult to make because of the challenges with consolidating the electrolyte with the cathode active material which also leads to a higher resistance and thus a lower power battery. Also, the reversibility of the lithium-metal anode still remains a problem.
Hybrid solid-state batteries
To address the issue of interfaces, hybrid solid-state batteries have been proposed, whereby a solid electrolyte is still used to separate the anode and cathode, but a liquid or gel based catholyte/anolyte is used in the electrodes. This retains the high energy density from the lithium-metal/silicon, improves the cathode-electrolyte interface giving a lower resistance and also makes the battery easier to make than the ASSB design.
However, since a organic liquid, or gel, electrolyte is used, there are still some safety concerns as well as the fact that excess lithium-metal is used which is intrinsically reactive.
Hybrid anode-free batteries
Another investigated design is the hybrid anode-free battery. Here, no lithium-metal foil is used in the cell construction and instead forms on the first charge of the battery from lithium-ions in the cathode.
The advantage of this approach is again the good interface between the cathode and electrolyte but now that the lithium-metal foil has been removed this affords one of the highest energy density configurations and also removes one element of the manufacturing process, making them easier to make than other variants. Here, one might ask why lithium-metal foils are often used in the construction of lithium-metal batteries instead of just forming this on the first charge. The answer stems from the quality of the deposition, with it often being easier to deposit lithium on lithium, rather than lithium onto copper.
However, the drawback is that there are still some safety issues with the catholyte and the reversibility issues associated with the lithium plating and stripping is even higher, since there isn’t any excess lithium used.
How does a lithium solid-state battery work?
Focusing in on lithium-metal based systems, during discharge, the lithium is stripped from the anode to become a lithium-ion and electron. This lithium-ion then moves to the cathode to intercalate and the electron goes round the external circuit to extract useful electrical work.
When the battery is charged, much of the lithium has been stripped meaning that the thickness of the battery is smaller. This reaction is then reversed when charging to plate the lithium back onto the anode.
However, doing this repeatedly can lead to flaws in the lithium and solid-electrolyte. Therefore, there can be significant volume change in a lithium metal battery, which often requires additional compression to address for good lifetime and the volume of the cell is state-of-charge dependent, which can affect its stated energy density.
Solid-state electrolyte materials
Regardless of the configuration, common to all solid-state batteries is the fact that they replace some degree of the liquid electrolyte with a solid. Here, there are various types under development but sulfides, oxides and polymers are the most commercially mature
Sulfide electrolytes
Sulfide-based electrolytes which generally have excellent performance characteristics in terms of lithium-ion conductivity. Their mechanical properties are reasonable but because they are somewhat plastic, it makes them relatively easy to coat and process. However, the disadvantage is that they have relatively low oxidation stability, poor compatibility with traditional cathode materials and generate hydrogen sulfide when in contact which water, which is a poisonous, corrosive and flammable gas, giving them poor safety characteristics.
Oxide electrolytes
Oxides-based electrolytes generally have a lower conductivity but are generally more stable leading to excellent safety characteristics. However, because these oxide materials often require sintering to get good conductivity this makes them difficult to process, with the ceramic films generally being quite fragile and prone to fracture.
Polymer electrolytes
Polymer electrolytes generally have a poor conductivity and thus have to be operated at higher temperatures, however their manufacturability is excellent since they easily flow and are quite durable. However, these polymers have limited thermal stability and generally decompose at relatively low voltages, limiting the choice of high voltage cathodes. Here, polyethylene oxide (PEO) is one of the most researched types.
Adapted from: Solid-state batteries: The technology of the 2030s but the research challenge of the 2020s. The Faraday Institution. 2020
Adapted from: Lithium battery chemistries enabled by solid-state electrolytes. Manthiram et al. Nature Reviews Materials. 2016
Practical specific energy
The theoretical performance of a solid-state lithium-metal is worthy of investigation, however it is important to consider how practical considerations, limit the achievable performance.
If we consider a ASSB where the graphite has been replaced with a lithium-metal anode, and the liquid-electrolyte replaced with a solid-electrolyte, the theoretical performance and mass of the active materials gives an specific energy/energy density of over 1,000 Wh/kg and 5,000 Wh/L.
However if we consider that an excess lithium is needed for good lifetime and other practical constraints this decreases to just over 700 Wh/kg and 2,400 Wh/L.
Next if we add the mass of the electrolyte, this can significantly decrease the specific energy, with this being a strong function of the electrolyte thickness, where a thinner layer is ideal but harder to make.
Then if we include the mass of the current collectors and carbon black, as well as the mass of the casing material and tabs, we reach an energy density closer to 400 Wh/kg and just over 1,100 Wh/L. This of course is a welcome improvement on traditional lithium-ion batteries but far from their theoretical performance.
When looking at a mass breakdown, we can see the major factors are the cathode active material and the electrolyte, so careful selection of the cathode material will have a profound impact on the cell level energy density.
Adapted from: Theoretical versus Practical Energy: A Plea for More Transparency in the Energy Calculation of Different Rechargeable Battery Systems. Betz et al. Advanced Energy Materials. 2018
NCA – Nickel cobalt aluminum oxide , Al – Aluminum, Cu – Copper, Cell comp. – Cell components (e.g. case materials, tabs)
Comparing lithium-metal ASSBs with lithium-ion batteries
Comparing the analysis of the theoretical to practical performance for lithium-ion batteries to solid-state lithium-metal cells, we can see that whilst both battery types lose performance when considering the practical constraints, the solid-state lithium-metal cell does have improvements over traditional lithium-ion batteries.
In this specific example we looked at, this resulted in a 49% improvement in the specific energy and 80% improvement to the energy density. Acknowledging that these numbers will vary for different configurations and materials used.
Adapted from: Theoretical versus Practical Energy: A Plea for More Transparency in the Energy Calculation of Different Rechargeable Battery Systems. Betz et al. Advanced Energy Materials. 2018
Degradation mechanisms and challenges
Of course energy density isn’t the only consideration and one of the major barriers holding back solid-state batteries is their durability.
Here, one of the main considerations is that the solid-electrolyte does not perfectly block lithium filaments or dendrites from forming when charging. Here, a dense deposition of lithium on the anode is desirable, however this can instead grow through the electrolyte causing a short circuit if it reaches the cathode. When discharging the battery and dissolving the lithium, this can lead to interfacial delamination which causes spots on the anode which lose contact with the electrolyte. When cycled multiple times, this can cause surface flaws and voids in the lithium which is not ideal. Furthermore, dead lithium can also form in the electrolyte which leads to capacity loss since it is no longer electronically connected to the anode and not participating the electrochemical reactions. Another consideration is that lithium is a relatively soft metal and as such can experience mechanical creep of the metal which can cause issues such as short-circuits between layer and uneven anode loading.
Asides from the lithium, the electrolyte also has various issues such as stability when put in contact with the anode and cathode. At the cathode side, depending on the operating conditions, this can also lead to gas generation which can cause the cell to inflate and delaminate layers.
Certain solid-electrolyte materials are also very brittle and the stresses in the battery can cause fracture and cracking. This is also an issue in the cathode which often expands and contracts during cycling which can lead to cracking of the cathode and loss of active material. In the case of solid-state batteries where the electrolyte is not as flowable, this expansion and contraction can lead to void formation and loss of contact with the electronic and ionic conducting phases.
To address these durability issues various approaches have been considered. For delamination, additional compression (relative to lithium-ion batteries) is often added which increases system complexity and mass. For lithium reversibility issues, excess lithium is often used to improve lifetime but adds additional mass and cost. For poor interface formation at the cathode, a liquid or gel catholyte is often used which can potentially increase safety issues.
Adapted from Interactions are important: Linking multi-physics mechanisms to the performance and degradation of solid-state batteries. Pang et al. Materials Today. 2021
Innovation and scale-up take time
Finally, it is important to highlight that of innovation and scale-up in the battery field can take significant time. The Joint Centre for Energy Storage Research in the US have defined the battery technology readiness levels (BTRL) to highlight the various stages of a new technology.
This first starts with BTRL-1 where there is a scientific breakthrough or new innovation. This is followed by BTRL-2 in which the new class of material is synthesized which can take about 2 years. From here at BTRL-3 we want to prove the performance of this material in small coin cells which adds a few more years. Once, confident in the results, at BTRL-4 we then develop larger cells to prove the performance which are more representative of real cells, but might revisit the earlier synthesis and performance. Then, once we have this confidence at BTRL5-6 we then focus on how to scale-up the materials, manufacturing and performance at the pack and system level. All-in-all this can take upto 10 years to fully realise.
Adapted from: The joint centre for energy storage research: A new paradigm for battery research and development. Crabtree. AIP Conference Proceedings. 2015.