A team of researchers led by Harvard John A. Paulson School of Engineering and Applied Science’s (SEAS) Associate Professor of Materials Science, Xin Li, has developed a stable lithium-metal solid-state battery that has the potential to improve the lifetime of electric vehicles (EVs) by 10 to 15 years longer than cars running on gasoline. With this new battery technology, it may be possible for EV owners to charge their vehicles in 10 to 20 minutes. The team’s research was originally published in Nature.

 

The design of the battery developed by Li and colleagues allows for dendrite formation within the graphite layer and first electrolyte (green), but not the second electrolyte (brown). Image used courtesy of SEAS 
The design of the battery developed by Li and colleagues allows for dendrite formation within the graphite layer and first electrolyte (green), but not the second electrolyte (brown). Image used courtesy of SEAS 

 

EV Market Projections

Lithium-ion batteries (LiBs) are considered to be the material of choice for powering EVs. Together, with the knowledge that EVs are generally more environmentally sustainable than internal combustion engine vehicles, advances in technology, and government subsidies, EVs have increased in global demand. 

 

A graph depicting the projected growth of EV stock. Image used courtesy of Communications Materials
A graph depicting the projected growth of EV stock. Image used courtesy of Communications Materials

 

In an article published in Nature’s Communications Materials, it was reported that EVs may make up 8–14% of the total light-duty vehicle fleet by 2030. This includes numbers for battery electric vehicles (BEVs) at 89–166 million and plug-in hybrid electric vehicles (PHEVs) at  46–71 million. These projections are based on two scenarios of the International Energy Agency (IEA) until 2030: the Stated Policies (STEP) scenario, which incorporates existing government policies, and the Sustainable Development (SD) scenario, which aligns with the climate goals of the Paris agreement. The SD scenario also includes the target of reaching a 30% global sales share for EVs by 2030.

 

The Research

Scientists and automotive manufacturers continue to invest time and investment into research and development (R&D) for lithium-ion battery technology. Typically, LiBs use a liquid electrolyte (LE) to regulate ion flow, whereas solid-state batteries use a solid electrolyte (SE). All-solid-state LiBs can provide higher energy densities than conventional batteries, and mitigate safety hazards associated with conventional LiBs. Conventional LiBs with a LE are known to cause fires and even explode. This is due to the instability and flammable nature of the LE.

 

The researchers describe their battery technology as having multiple layers like a BLT sandwich. Image used courtesy of SEAS
The researchers describe their battery technology as having multiple layers like a BLT sandwich. Image used courtesy of SEAS
 

Additionally, when a LiB has an anode made of lithium metal, tree-like structures known as dendrites can form on the metal’s surface. They can grow into the electrolyte and pierce the barrier that separates the cathode and the anode. This can also cause a fire to start and/or the battery to short. 

To prevent the issue caused by the dendrites, Li and his research team developed a battery with multiple layers, which can be described using the analogy of a BLT sandwich. The lithium-metal anode represents the bread, the graphite represents the lettuce, and a layer of bacon represents the second electrolyte which is surrounded by tomatoes (or the first electrolyte). 

The last layer of bread represents the cathode. The formation of the battery allows dendrites to penetrate the graphite and the first electrolyte but they stop at the second electrolyte, which appears to be immune to their formation. So, the dendrites pass through the lettuce and tomato layer and stop at the layer of bacon. This layer stops the dendrites from growing further and shorting the battery. The battery’s chemistry also allows for self-maintenance through the backfilling of holes created by the dendrites. 

In a recent news release, Li added: “This proof-of-concept design shows that lithium-metal solid-state batteries could be competitive with commercial lithium-ion batteries,” said Li. “And the flexibility and versatility of our multilayer design make it potentially compatible with mass production procedures in the battery industry. Scaling it up to the commercial battery won’t be easy and there are still some practical challenges, but we believe they will be overcome.”


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