Solid-State Battery: The Magic of Diffusion
In recent years, rapid advancements in material science and molecular physics has led to extraordinary developments in battery technology.
Playing mix-and-match with different compounds to create new materials or uniquely arrange materials to improve the overall battery capacity may have manifested in yet another crazy but effective outcome — solid state battery technology.
Actively or passively, scientists tend to nomenclate their inventions in a way that is self-explanatory. The term “solid state” bolsters the claim as it points to the solid state of the electrolyte in the battery.
I’m sure most of us are familiar with the three popular states of matter — solids, liquids and gases and the primary difference between them. The molecules in solids are tightly packed and practically immobile unlike its liquid and gaseous counterparts.
Conventional batteries use liquid electrolytes to transfer charge between the electrodes, well, due to its liquid nature and mobility. However, the new solid state batteries use solid electrolytes which are known to transport greater amount of charge than liquid electrolytes, particularly in the case of the current leader, lithium-ion batteries.
Ironical, isn’t it?? Keep reading to find out how!
History of Solid State Batteries
Difficulties Faced in Liquid Electrolyte Batteries
On disassembling a cell (rather an electrochemical cell), it basically consists of:
- Two electrodes namely, anode and cathode
- A liquid called electrolyte that “corrodes” one of the two electrodes to generate charge and transports it to other electrode for current to flow
Note — Though the word “battery” and “cell” are used synonymously, they are not the same. Read my article on Introduction to Batteries to know the difference.
While there are other essential components (which I’ve described in my article on Types of Batteries) for the cell to deliver best results, it is impossible to build a cell without the two electrodes and an electrolyte.
Almost every cell on the planet uses an electrolyte formed by dissolving chemicals in water proportionately. Even the so called dry cell contains a paste made of ammonium chloride (or any other relevant compounds) embedded with some amount of moisture. This minuscule amount of moisture is enough to not make the paste a solid!
Hence,the interest towards building a solid state battery stemmed from the challenges faced when working with the current batteries using liquid electrolyte. Some of the primary issues are:
- Loss of battery capacity due to continuous charging and discharging cycles
- Leakage of liquid electrolyte
- Flammability of the electrolyte
- Disposal of electrolyte and plausible harmful effects to the environment
And a few pesky issues with our modern hero, lithium-ion battery are:
- They require excessive secondary materials for cooling and sophisticated battery management systems
- Formation of dendrites which make them prone to explosion — Dendrites are spiky structures that typically grow on the electrodes and affect the physical structure.
For example, they protrude through the separator (or barrier) and reach the other electrode causing an unwanted short circuit (see Fig 2) thereby leading to a battery failure.
In fact, such drawbacks of lithium-ion batteries are held responsible for inventing the solid state battery.
The Solid State
The word “solid” indicates that the electrolyte is in solid state. In other words, now, all the basic components of the battery are solids.
Fig 3 shows a lithium based solid state battery. During discharge positive lithium ions flow from anode to cathode via the electrolyte and the reverse occurs during charging process.
The major advantage is that they are considerably less-hazardous as compared to their liquid counterparts. In fact, they are strong competitors to coin cells for powering cardiac pacemakers as they are innocuous, long lasting and have a high energy density.
Charge transfer in this type of battery takes place through diffusion as the molecules in solids do not move. To understand what diffusion looks like, dip a paint brush with tiny amounts of green color at its tip in a glass of water. Subsequently, the green will move to regions that are less green (or not green) thus making them greener.
As a result of diffusion, the electrolyte needs to have a high ionic conductivity. In simple words, the electrolyte should be receptive to and conduct positive charge (like lithium-ions).
Some of the commonly used materials in lithium based batteries are listed in Table 1.
Engineering Design Requirements for the Electrolyte
When compared to the other batteries, the solid state batteries have higher energy and power densities thus delivering sufficiently large power for long periods of time.
Hence, selection of materials for the electrolyte plays a massive role. A few traits that satisfy the designer’s greedy yet stringent requirements are:
- High ionic conductivity
- Compatibility with the electrodes as they are solids
- Thermal, chemical and electrochemical stability to prevent unwanted reactions at different ambient temperatures
- High mechanical stability
Advantages and Challenges Faced in Solid State Battery Technology
While the primary factor driving research and commercial interest in solid state batteries is safety, other notable advantages include:
- Low self-discharge when compared to the current liquid electrolyte based batteries
- High energy and power density
- Stability
- Low electrolyte volatility as it is in solid state
As you may have already guessed, the advantages are interlinked to and are directly derived from the choice of material. Make the right choice and reap the benefits!
As good as it may sound, there are a few concerns that need to be addressed for the battery technology to become commercial:
- Since the entire setup is composed of solids, lattice defects affect the structure of the material. By definition, lattice is a 3 D arrangement of atoms (or ions) and a defect essentially indicates a misplaced atom (or ion).
A good analogy is the bricks used to build a house. The arrangement of bricks is a framework (lattice) that supports the entire house and any misplaced or eccentric brick causing a deviation in the structure is a defect that threatens the stability of the house. - Manufacturing process of the material — Most of the materials used in the solid electrolyte are not naturally available and are required to be processed with utmost care, and at scale.
- High manufacturing cost of the materials
- Meeting stringent size requirements for a captivating engineering finish
- May require a new category of battery management systems that work on different parameters
- Sustainability across different applications as the energy and power requirements vary with the application
Commercial Market
Any technology during its infancy is equipped with problems that are seemingly impossible to solve. However, history has shown that humans overcame multiple such problems and will continue to do so.
Fortunately or unfortunately, technological advancements requires money and in huge chunks and the need is visible in modern day scientific advancements more than ever before.
A few notable investments from industry giants are:
- In 2018, Volkswagen made an investment of $100 million in QuantumScape, a Stanford University spinout developing solid state battery for commercial use. Impressed with the results, Volkswagen invested an additional $200 million in 2020 to accelerate the growth of the technology.
- Solid Power is another startup working on solid-state battery technologies. Nine months after a deal with BMW to produce solid-state batteries for electric vehicles, the startup raised capital from Hyundai and Samsung.
- In 2018, Hyundai CRADLE, Hyundai Motor Company’s venturing and open innovation business invested in a US-based startup Ionic Materials, another player in this space.
Such financial investments and faith are clear signs of cash flowing into developing the technology and painting a promising future.
Conclusion
In conclusion, solid state battery technology is still in the research phase and requires considerable resources in terms of financial capital and human labour.
As the technology becomes popular and catches the eye of investors and venture capitalist funds, the rate of development will increase and may very well drive the technology into the market in the next decade.
We just have to sit tight and watch the show!
Thank you everyone for your time.
