The lithium-ion battery of Samsung's mobile phone caught fire. Image source: SHAWN L. MINTER / AP
From laptops to lawn mowers, lithium-ion batteries are powering many appliances. However, due to the reliance on flammable components, lithium-ion batteries easily burn when damaged. Today, researchers report that they have redesigned these batteries so that they can use materials that are not flammable. In addition, the new battery may even store more power than existing models.
Gleb Yushin, a material scientist at the Georgia Institute of Technology in Atlanta, USA, who did not participate in the study, said the work was "absolutely significant progress." If commercialized, this new battery will help protect the safety of electric vehicle drivers, even if they are unfortunately in an accident.
Lithium-ion batteries contain three main components-two storage electrodes and a liquid organic electrolyte that separates them. During charging and discharging, the electrolyte transports lithium ions back and forth between the electrodes, but these electrolytes are flammable.
In recent years, researchers have tried to replace these organic electrolytes with solid electrolytes or non-flammable water-based electrolytes. However, if the operating voltage of these water-based batteries exceeds 1.23 volts (V)-which is even lower than the operating voltage of AA batteries of 1.5V, the electrode material will react with water molecules to decompose them into hydrogen and oxygen Usually causes an explosion.
However, when the researchers kept the operating voltage below the threshold of 1.23V, the energy stored in the final battery was much lower than that of the traditional lithium-ion battery, which had an operating voltage of about 4V.
In 2015, a research team led by Wang Chunsheng, a material scientist at the University of Maryland Parker, reported that they had developed a new type of salt-rich water-based electrolyte. This salty water electrolyte (WiSE) promotes the formation of a protective solid barrier around the electrode, which in turn prevents the electrode from tearing water molecules inside the electrolyte. However, the electrode materials in these batteries can only reach an operating voltage of 3V.
In 2017, when Wang Chunsheng and his colleagues reported that they had developed a cathode material that was compatible with 4V and could work with WiSE, the prospects were bright. Next, only the negatively charged electrodes are left.
Today, Wang Chunsheng and his colleagues have already done this. They reported in Nature magazine on May 8 that they have developed a graphite-based cathode that can work with WiSE at 4V or higher.
The new electrode materials include bromine and chlorine. By locking the reaction electrode material in solid salt particles around the electrode, it is protected from water-based electrolytes. The lithium in the battery is surrounded by solid lithium-bromine and lithium-chloride salt particles around a graphite electrode composed of a layer of carbon atoms. When the battery is charged, the bromine and chlorine atoms will abandon the lithium atoms, give the electrons to the cathode, and wedge between the graphite carbon layers to form another compact solid. Then, the voltage difference between the two electrodes drives the positively charged lithium ions through the water-based electrolyte to the anode, where they meet the electrons provided by the external circuit.
When the battery is discharged during use, lithium ions will give up these electrons and flow to the cathode. Electrons return to the cathode through an external circuit, where bromine and chlorine atoms will catch them. These charges will diffuse out of the graphite. Lithium ions will then grab them and reform the solid salt particles, which will stay in place until the next round of charging.
Wang Chunsheng and his colleagues noticed that their cathode materials can already store about 30% more charge than traditional cathode materials. But whether the full battery, including the new electrolyte, can eventually store more energy than commercial batteries remains to be seen.
The new WiSE battery will not require cobalt, a toxic metal found in traditional lithium-ion cathodes. In the Democratic Republic of the Congo, cobalt mining is associated with widespread deaths of miners, who are usually children. Cobalt mines in the Democratic Republic of Congo are relatively abundant. The new battery is not only safer for consumers, but also safer for miners and the environment. (Zhao Xixi)
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