Adding bumpy anode to Lithium-ion batteries boosts performance in the cold

A Chinese research team has found that adding a bumpy carbon-based electrode as an anode in lithium-ion cells can make the batteries last longer in frigid temperatures, well below the freezing point of water. 

Thanks to their high energy storage capacities, lithium-ion batteries are now widely used, ranging from consumer electronics to the automotive industry. Although these batteries work well in most conditions, their performance falls severely short when temperatures dip below 32 degrees Fahrenheit (0 degrees Celsius). 

As temperatures decline, smartphones need more frequent charging while electric cars see significant drops in their range as the anode or the positively charged terminal of the battery has trouble holding charge. This is why electric batteries are not preferred for space explorations and are also feared by those residing in extremely cold regions and wanting to transition to electric vehicles.  

Why do Lithium-ion batteries work poorly in extreme cold? 

Some types of Lithium-ion batteries use a liquid electrolyte that can become viscous or freeze in extremely cold temperatures, affecting battery performance. However, batteries that do not use a liquid electrolyte have also been found to suffer in low temperatures.

Previous research has also shown that ions that usually enter the anode of the batteries easily have warmer temperatures and find it harder to do so at freezing temperatures. Researcher Xi Wang and his team sought to learn if changing the anode surface material and structure would make a difference. 

To create a new material, the researchers first heated a cobalt-containing zeolite imidazolate framework (ZIF-67) at very high temperatures to obtain 12-sided carbon nanospheres with bumpy surfaces and had good capabilities of transferring electric charge.

Using it as a new anode

Then the researchers trialed the material as an anode in a coin-shaped battery with lithium metal as cathode. When tested for charging and discharging abilities between temperatures of  77 F to -4 F (25^oC to -20^oC), the anode demonstrated stability and, even in below-freezing temperatures, held 85.9 percent of the charge it could hold at the normal temperature. This is a major achievement since regular lithium-ion batteries can hold almost no charge at these temperatures. 

During one of their experiments, the researcher dropped the air temperature to -31 F (-35^oC) and found that the anode made from this new material still held the capacity to recharge the battery. The researchers then discharged the battery and found that almost 100 percent of the charge that was put in was released at those temperatures. 

By using a slightly different carbon material for the anode in a lithium-ion battery, the researchers were able to resolve the greatest limitation of this battery which is widely used around the world. 

The research findings and their application in the future could have profound effects on how batteries are deployed in the planet's coldest regions and maybe even beyond. 

The research was published in the journal ACS Central Science.

Abstract

Since sluggish Li⁺ desolvation leads to severe capacity degradation of carbon anodes at subzero temperatures, it is urgently desired to modulate electron configurations of surface carbon atoms toward high capacity for Li-ion batteries. Herein, a carbon-based anode material (O-DF) was strategically synthesized to construct the Riemannian surface with a positive curvature, which exhibits a high reversible capacity of 624 mAh g^–1 with an 85.9% capacity retention at 0.1 A g^–1 as the temperature drops to −20 °C. Even if the temperature drops to −35 °C, the reversible capacity is still effectively retained at 160 mAh g^–1 after 200 cycles. Various characterizations and theoretical calculations reveal that the Riemannian surface effectively tunes the low-temperature sluggish Li⁺ desolvation of the interfacial chemistry via locally accumulated charges of non-coplanar sp^x (2 < x < 3) hybridized orbitals to reduce the rate-determining step of the energy barrier for the charge-transfer process. Ex-situ measurements further confirm that the sp^x-hybridized orbitals of the pentagonal defect sites should denote more negative charges to solvated Li⁺ adsorbed on the Riemannian surface to form stronger Li–C coordinate bonds for Li⁺ desolvation, which not only enhances Li-adsorption on the curved surface but also results in more Li⁺ insertion in an extremely cold environment.