Lithium-ion batteries (LIBs) have been a reliable and scalable choice for electric vehicles, offering efficient energy storage. However, they face challenges that emerging solid-state batteries (SSBs) may solve. Although still in development, SSBs hold potential for the next generation of electric vehicles (EVs). To understand why, let’s compare the two batteries.
Figure 1. Solid-state batteries may be the future of electric vehicles. These batteries replace liquid electrolytes with solid materials, reducing the risk of overheating and boosting energy density. (All images: JOEL, Ltd.)
Lithium-ion batteries (LIB) perform charging and discharging by making lithium ions move between positive and negative electrodes. They have separators of polymer between positive and negative electrodes to prevent short circuits and an electrolyte of organic solvent is used to conduct ions.
Lithium-based rechargeable batteries are well-established as a reliable, cost-effective choice for electric vehicles (EVs), offering manufacturing scalability.
However, these batteries come with inherent risks. They’re prone to overheating, which can lead to thermal runaway. They also degrade over time, losing capacity and efficiency after repeated charge and discharge cycles, reducing the overall range of the EV.
Solid-state batteries (SSBs) are still in the R&D stage for use in EVs, but developments are advancing. SSBs perform charging and discharging like LIBs by using lithium ions. However, instead of a separator and electrolyte solution located between the positive and negative electrodes, a solid material electrolyte is used (Figure 1). This reduces the risks of smoking and ignition caused by the electrolyte solution (which could lead to thermal runaway), making them a safer choice than LIBs and an excellent potential option for EVs.
SSBs offer high energy density and safety and are expected to offer long service life. As an emerging technology, they’re still more costly and complex to manufacture at scale. Nevertheless, their expectations for next-generation EV batteries are high and are being developed for practical use by automakers and battery manufacturers.
The basic structure of LIBs
For the positive electrode, a compound of oxidized material containing lithium is used as the main active material. It’s created by kneading carbon materials as a conductive assistant with polymer binders that bind them. For the negative electrode, graphite carbons that can intercalate lithium are used and created by kneading the polymer binders (Figure 2).
Figure 2. The composition of a Lithium-ion battery.
Separator films for lithium-ion batteries are porous polymers with narrow pores. Separator films have a shutdown function as a safety switch in the case of thermal runaway, which works by closing the pores and preventing short circuits caused by any contact between the positive and negative electrodes. The electrolyte solution is created by dissolving lithium electrolytes using an organic solvent.
Since each material uses highly reactive lithium that alters in the air, it’s inevitable to manufacture them in an air-isolation environment. Additionally, for analysis of each material, specimen, preparation, observation, and analysis must be conducted in an air-isolation environment with air-isolation instruments.
The basic structure of SSBs
For solid-state batteries, instead of a polymer material separator and electrode solution between positive and negative electrodes, a solid electrolyte is used to conduct ions (Figure 3). As with LIBs, the positive electrode uses lithium transition metal (TM) oxide material and conductive assistants. But SSBs also require additional solid electrolytes.
Figure 3. The composition of a solid-state battery.
For the negative electrode, aside from the carbons used with LIBs, silicone materials have been attracting attention. This is because they’re expected to introduce nearly ten times more lithium than carbon materials. So, research into silicon negative electrodes is ongoing.
Understanding silicon negative electrodes (Si anode)
For rechargeable batteries that charge and discharge, anode materials that can store more lithium when charging are preferred. Carbon material (graphite) is primarily used in the anodes of lithium-ion batteries, with a theoretical capacity of 372 mAh/g. Si anodes have a theoretical capacity of 4,200 mAh/g, which is approximately ten times greater than graphite (Figure 4). Therefore, an increase in capacity is expected.
As a result, silicone is attracting attention because of its relatively low-action potential, and it offers abundant natural resources. On the other hand, silicone expands in volume to nearly three times during charging. It also has a shorter lifespan when used in LIBs. Its use as a complex with carbon material and silicone has been proposed, although its theoretical capacity would decline. Currently, nanoparticle silicone is being researched for use in SSBs to improve capacity and lifespan.
Figure 4. The SEM back-scattered electron image of an Si anode complex. The silicon particle (grey), solid electrolyte (white, and carbon (black) after charging.
Understanding solid electrolytes
A solid electrolyte has two functions as part of an electrolyte solution: to carry lithium ions between a cathode and an anode and as a separator to prevent short-circuiting. This material has high ion conductivity and electromechanical stability with a wide potential window in cyclic voltammetry (CV) despite its solid nature.
Additionally, since there are no anions other than lithium ions, side reactions do not occur, at least compared with an electrolyte solution. The result is a longer battery life.
Research is underway for two kinds of materials: oxide- and sulfide-based solid electrolytes. Oxide-based solid electrolytes have lower ion conductivity but are more stable in the air.
Concerns with sulfide-based solid electrolytes related to the generation of hazardous gases, such as hydrogen sulfide, exist. However, this type of electrolyte is still considered promising because it is soft and can suppress interface resistance.
Proper observation and analysis of SSBs
For batteries assembled with a positive electrode/negative electrode/solid electrode, it’s critical to evaluate the composite material before and after assembly. Disassembling the battery and using an electron microscope enables observation and analysis of the material property data. Batteries containing lithium and sulfur using sulfide-based solid electrolytes must be handled in an air-isolated environment to suppress material alteration. These batteries require air-isolated transfer for proper specimen preparation and observation.
Conclusion
In summary, while lithium-ion batteries remain the dominant choice for today’s EVs due to their scalability and efficiency, they face challenges that solid-state batteries (SSBs) might overcome. Additionally, ongoing research into silicon-based anodes, which have the potential to significantly increase energy capacity, is a key development in the push for more efficient battery technologies. It will be interesting to see how the next generation of EV batteries evolves.
Filed Under: Batteries, FAQs