Most of the rechargeable batteries found in our laptops, phones and tablets today are lithium-ion batteries.

These types of batteries use a liquid as an electrolyte to work, but these liquid electrolytes can be flammable and lead to fires, so using solid electrolytes could be safer.

While attempts to make such a battery have faced challenges as they tend to behave erratically, researchers at MIT have discovered that it's actually the smoothness of the electrolyte - instead of its firmness - that matters the most in preventing short circuits, and the discovery could potentially double a lithium-ion battery's energy capacity.

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Researchers at MIT discovered that it's the smoothness of a solid electrolyte, instead of its stiffness, that prevents dendrites from forming: Protrusions of metal that build up from one electrode and can lead to short-circuit. Bottom right is an image of a smoothed solid electrolyte surface, and bottom left is a picture of a rough solid electrolyte surface

WHAT THEY FOUND Commonly used liquid electrolytes can be flammable, and they're also prone to forming dendrites - thin, finger-like protrusions of metal that build up from one electrode and, if they reach all the way across to the other electrode, can create a short-circuit that can damage the battery. To solve the problem of flammability and dendrites, researchers have focused on using solid electrolytes instead, and they focused on the electrolytes stifness, thinking that these would be more resistant to dendrites. However, new MIT research has revealed that researchers were focusing on the wrong properties of a solid electrolyte - it's actually the smoothness of the electrolyte's surface that is the most important factor in preventing dendrites. With solid electrolyte batteries, lithium from one of the electrodes begins to be deposited, through an electrochemical reaction, onto any tiny defect that exists on the electrolyte’s surface, including pits, cracks, and scratches, so focusing on smooth electrolyte surfaces can prevent dendrites. Advertisement

The electrolyte in a battery is the material in between the positive and negative electrodes.

When a battery is charge or drained, ions cross through the electrolyte from one electrode to the other.

Commonly used liquid electrolytes can be flammable, and they're also prone to forming dendrites - thin, finger-like protrusions of metal that build up from one electrode and, if they reach all the way across to the other electrode, can create a short-circuit that can damage the battery.

To solve these problems, researchers have tried to use solid electrolytes instead - for example some forms of ceramic.

While this could eliminate flammability risk and provide offer other benefits, tests have shown that these solid electrolytes can perform erratically and are more prone to short-circuits than expected.

But according to a new MIT study, researchers were focusing on the wrong properties of a solid electrolyte.

They thought that the material's firmness determined whether dendrites could penetrate into the electrolyte, buy the study revealed that it's the smoothness of the electrolyte's surface that is the most important factor in preventing dendrites.

Microscopic scratches on the electrolyte's surface can provide a foundation for dendrites to start to force their way in.

Using smooth, solid electrolytes could eliminate the flammability problem and make it possible to use a solid lithium metal electrode, which could potentially double a lithium-ion battery's energy capacity - which is important for vehicles and portable devices.

'The formation of dendrites, leading to eventual short-circuit failures, has been the main reason that lithium-metal rechargeable batteries have not been possible,' said Dr Yet-Ming Chiang, the corresponding author of the research.

Lithium-metal electrodes are commonly used in nonrechargeable batteries, but that’s because dendrites only form during the charging process.

According to Dr Chiang, the problem of dendrite formation in lithium rechargeable batteries was first identified in the early 1970's, and '45 years later that problem has still not been solved.

'But the goal is still tantalizing,' said Dr Chiang, because of the potential to double a battery’s capacity by using lithium metal electrodes.

Researchers were focusing on the wrong properties of a solid electrolyte. They thought that the material's firmness determined whether dendrites could penetrate, but an MIT study found that the smoothness of the electrolyte is the most important factor in preventing dendrites

In recent years, different research groups have been trying to develop solid electrolytes as a way of enabling the use of lithium metal electrodes.

These research efforts focused on the though that the electrolyte material needed to be stiff, not elastic.

Dr Chiang said that this idea made sense - a stiffer material could more easily resist something trying to press into its surface.

But the new MIT research, which tested samples of four different solid electrolytes and how they performed during charging and discharging cycles, showed that the way dendrites form in stiff, solid electrolytes follows a different process than how they form in liquid electrolytes.

Liquid electrolytes in batteries can be flammable, and they're also prone to forming dendrites - thin, finger-like protrusions of metal that build up from one electrode and, if they reach all the way across to the other electrode, can lead to a short-circuit that can damage the battery

On the solid surfaces, lithium from one of the electrodes begins to be deposited, through an electrochemical reaction, onto any tiny defect that exists on the electrolyte’s surface, including pits, cracks, and scratches.

Once the initial deposit forms on such a defect, it continues to build — and the buildup extends from the dendrite’s tip, not from its base, as it forces its way into the solid, acting like a wedge as it goes and opening a wider crack.

These materials are 'very sensitive to the number and size of surface defects, not to the bulk properties' of the material, Dr Chiang says.

'It’s the crack propagation that leads to failure. … It tells us that what we should be focusing on more is the quality of the surfaces, on how smooth and defect-free we can make these solid electrolyte films.'