Recent developments in solid-state battery technology has drawn a lot of attention from electric vehicle manufacturers around the world. In this blog post, we will cover the latest insights in this exciting technology and answer the following questions:

• How do solid-state batteries work and what are the differences to conventional lithium-ion batteries?
• What are the advantages of solid-state batteries?
• Is solid-state battery technology ready for the market?
• Why are solid-state batteries interesting for electric vehicles?

How does a battery work?

Batteries convert electrical energy into chemical energy via reversible electrochemical reactions called redox reactions (reduction + oxidation). These controlled chemical reactions move positively charged ions through an electrolyte and negatively charged electrons through an external circuit. This process is reversible, providing electrical energy during discharging and storing energy during charging.

In general, many different ions could be considered for batteries, however, lithium ions (Li⁺) currently dominate due to their superior energy density and performance. Other battery technologies use sodium ions (Na⁺), potassium ions (K⁺), magnesium ions (Mg²⁺), calcium ions (Ca²⁺), lead ions (Pb²⁺) or aluminum ions (Al³⁺).

Solid-State Battery Technology

Solid-state vs. conventional lithium-ion batteries

A solid-state battery basically consists of the same components as a regular lithium-ion battery, the two electrodes (cathode & anode), electrolyte, current collectors and packaging. However, solid-state batteries utilize a solid electrolyte as a separator simultaneously instead of a liquid electrolyte with a porous separator, which has several advantages.

Solid Electrolyte Materials:

There are 3 classes of solid electrolyte materials currently considered as the most promising for solid-state battery technology:

1. Polymer electrolytes: Polymers are inexpensive and easy to process. Their moderate ionic conductivity can be enhanced by adding liquid electrolyte, which is then considered as semi-solid, quasi-solid or hybrid electrolytes. Due to the high flexibilty of polymers, a good electrode/electrolyte interface can be maintained, however the oxidation stability with high-voltage cathode materials can be challenging.
   
2. Sulfide electrolytes: Sulfides can possess comparably high ionic conductivities as liquid electrolytes and good electrochemical stability. The cost of sulfide electrolytes are reasonable but their high reactivity with environmental humidity is problematic due to the formation of the toxic gas hydrogen sulfide (H₂S). This has to be considered during handling and processing of sulfide electrolytes.
   
3. Oxide electrolytes: The main advantages of oxide electrolytes are good electrochemical stability towards the electrode materials and their high mechanical properties. However, their complex processing, moderate ionic conductivities at room temperature and interfacial stability are still challenges in their application.
   
   Both, oxide and sulfide electrolytes, usually need external pressure to maintain good electrode/electrolyte contact over several hundreds of charging/discharging cycles.

Anode Materials for Solid-State Batteries:

Many solid-state battery technologies replace the conventional graphite anode with lithium metal or silicon, both providing about 10x higher specific capacity compared to graphite. This means a reduction in battery weight and volume, resulting in higher energy densities.

However, both anode materials are facing challenges in their long-term stability. Lithium dendrite formation can cause internal shorting of batteries, posing a risk of thermal runaway. The volumetric expansion of silicon of about 300% during lithium insertation results in capacity fading and a shorter lifetime of the battery.

Solid-state batteries with oxide electrolytes can be operated at elevated temperatures, use residual stresses or interphases between anode and electrolyte to mitigate the dendride growth of lithium metal.

Silicon nanostructres and silicon/graphite mixtures are promising approaches to compensate for the volumetric expansion of silicon during charging/discharging cycles.

Advantages of Solid-State Batteries

1. Enhanced Safety:

• Reduced Risk of Leakage: Solid electrolytes are non-flammable and less likely to leak compared to liquid electrolytes.
  
• Lower Fire Risk: Solid-state batteries are less prone to thermal runaway, a condition that can cause fires and explosions in liquid electrolyte batteries.

2. Higher Energy Density:

• Increased Storage Capacity: Solid-state batteries can potentially store more energy per unit volume, allowing for longer battery life and greater energy storage capacity.
  
• Compact Size: Higher energy density enables the design of smaller and lighter batteries, which is beneficial for portable electronic devices and electric vehicles.

3. Longer Lifespan:

• Reduced Degradation: Solid electrolytes are more stable and less prone to forming dendrites (metal filaments that can cause short circuits) which extends the battery life.
  
• Better Cycling Stability: These batteries can withstand more charge and discharge cycles without significant performance degradation.

4. Wide Operating Temperature Range:

• Better Performance in Extreme Conditions: Solid-state batteries can operate efficiently over a wider temperature range, making them suitable for applications in extreme environments.

5. Improved Performance:

• Faster Charging: Solid electrolytes can support faster ion transport, potentially leading to quicker charging times.
  
• Higher Voltage: Solid-state batteries can operate at higher voltages, improving their overall efficiency and energy output.

6. Greater Design Flexibility:

• Form Factor Versatility: The solid nature of the electrolyte allows for more flexible and innovative battery designs, which can be tailored to specific applications and space constraints.

7. Environmental Benefits:

• Reduced Use of Hazardous Materials: The elimination of liquid electrolytes can reduce the reliance on toxic and flammable materials, leading to a safer and potentially more environmentally friendly battery technology.

Market Portential of Solid-State Battery Technology

Solid-state batteries can improve safety and performance compared to conventional lithium-ion batteries with liquid electrolytes. This could be an advantage in several industries, inclduing mobile transportation, aerospace and satellites. On the long term, medical devices, consumer electronics and energy storage systems could also benefit from advanced solid-state battery technologies.

Are Solid-State Batteries Ready for the Market?

Currently, solid-state battery technology is in a very early stage and still faces many challenges. Several manufacturers have announced the production of solid-state batteries in the near future, including Samsung, Solid Power, QuantumScape, Toyota, CATL, etc., but there are still some issues that have to be addressed.

Challenges of Solid-State Batteries:

1. Particularly, the composite cathode requires a minimum stack pressure for stable long-term operation. This pressure should ideally not exceed 0.1 MPa (~1 atm) but most solid-state batteries need several MPa, which is technically challenging to realize.
   
2. Modern batteries should deliver both, high-rate performance for fast charging/discharging and high energy density to reduce weight. This can only be achieved with high capacity electrodes, such as lithium metal or silicon anodes. However, both still face stability issues that have to be solved for large-scale industrial manufacturing.
   
3. The effective ionic conductivity of solid electrolytes should be high at room temperature but oxide-based electrolytes often require increased temperatures to operate. Additionally, solid electrolytes are currenlty much more difficult to manufacture on a large scale compared to conventional liquid electrolytes and polymer separators.
   
4. A low interface resistance between solid electrolyte and electrode materials has to be maintained on the long-term to ensure the required cycle stability for their applications. Many researchers have addressed this issue, however, more sustainable and scaleable approaches are necessary for the market adoption of solid-state batteries.
   
5. Low-cost and large-scale production of solid-state batteries has to be realized to compete with other battery technologies on the market. This could still take a long time since todays lithium-ion battery manufacturing has been optimized for several decades.

Electric Vehicles with Solid-State Batteries

In recent years, all major EV manufacturers around the world have expressed their interest in solid-state battery technology.

Toyota, Panasonic (Japan):

Toyota has been working on solid-state batteries since 2010 and with over 1300 patents is considered as one of the leading companies in this field. This new technology could push the range of an EV to 700-1000 miles on a single charge.

However, Toyota’s announcements keep on postphoning the realization of solid-state technology in their cars year after year. Toyota partnered up with Panasonic to accelerate the development and plan to equip some of their vehicles with solid-state batteries in 2027-2028.

CATL, BYD, NIO (China):

China’s battery giant CATL has been developing solid-state batteries for 10 years now but still expresses concerns about the long-term stability and safety of this technology. Especially, when combined with a lithium metal anode, the exposure of lithium to air during a car accident can form toxic lithium hydroxide (LiOH).

However, CATL expects to overcome these issues and commercialize solid-state batteries by 2030. In the meantime, CATL is heavily investing in sodium-ion batteries (SIB) and semi-solid battery technology. CATL globally supplies batteries for automakers, such as Tesla, Volkswagen, BMW, Volvo and Ford.

BYD, the largest EV manufacturer in China, has partnered with CATL and NIO in the development of solid-state batteries. The alliance should ensure China’s leading position in electric vehicles and solid-state technology.

Hyundai, Samsung, LG (South Korea):

South Korean market leaders Hyundai, Samsung and LG partnered up to boost EV sales and to compete in the race to commercialize solid-state battery powered EVs. The US-based company Factorial Energy also works with the Hyundai Motor Group (Hyundai, Kia, Genesis) to integrate their novel solid-state battery technology in EVs.

Conclusion

• Solid-state battery technology has a great potential to revolutionize electric transportation and other industries.
  
• Current electric vehicles (EV) issues, such as limited mileage and safety concerns could be solved with solid-state batteries.
  
• However, some key challenges still have to be solved before a wide market adoption of solid-state batteries can be realized.
  
• Major automakers are collaborating with large battery manufactureres to solve these issues and accelerate the development of solid-state battery technology.
  
• Competing market leaders include companies in Japan, China and South Korea, predicting limited manufacturing capabilities of solid-state batteries for EVs between 2027-2030.

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