This discovery may make your phone more durable!

Produced by: Science Popularization China

Author: Xie Congxin and Li Xueyang (Dalian Institute of Chemical Physics, Chinese Academy of Sciences)

Producer: China Science Expo

If you ask what remote controls and electric cars have in common, it's probably that they both use batteries. Batteries, a device that we cannot do without in our daily lives, have taken on a variety of colors in this era of calling for new energy, and the most famous of them is the lithium-ion battery.

Others generate electricity with love, we generate electricity with metal

Before talking about lithium batteries, let's review what batteries are.

A battery is a device that uses chemical reactions to generate electrical energy. People usually call batteries that can both be charged and discharged secondary batteries, such as lithium-ion batteries; and batteries that can only be discharged are called primary batteries, such as dry batteries.

At the end of the 18th century, Italian scientist Volta soaked zinc and silver sheets in salt water and found that the two metals could generate electricity. So he stacked the silver and zinc sheets together with flannel soaked in salt water to create the earliest primary battery - the voltaic cell.

A typical battery structure mainly includes key components such as positive electrode, negative electrode, electrolyte and separator (Figure 1). For example, the silver sheet and zinc sheet in the voltaic cell are the positive and negative electrodes of the battery, the salt water is the electrolyte, and the flannel is the separator of the battery, which plays a role in preventing the positive and negative electrodes of the battery from short-circuiting. During the discharge process of the voltaic cell, the positive electrode consumes the electrolyte to produce hydrogen, and the negative electrode metal zinc dissolves to produce zinc ions.

Figure 1. Typical battery structure

(Image source: Reference [1])

In addition to silver and zinc, scientists have discovered that other different metals can also generate currents, and the magnitude of the current is related to the "energy difference" between the metals. This "energy difference" between different metals is usually called the potential difference, and the potential difference between the positive and negative electrodes constitutes the battery voltage. The greater the potential difference between the positive and negative electrodes, the higher the battery voltage.

Figure 2. (a) Voltaic cell (left) and its schematic diagram (right); (b) Various common batteries in life; (c) Overview of the development history of common chemical batteries.

(Image sources: (a) from Wikipedia; (b) from Veer Gallery; (c) from Zheshang Securities Research Institute)

Why lithium?

Lithium-ion batteries can be found everywhere in our lives. There are so many metals, so why choose lithium?

Here we need to mention a concept called "energy density", which is the energy that can be released per unit mass/volume of the battery. Among the many metal elements, lithium has the lightest atomic mass. Lithium is located in the third position of the periodic table, and its relative atomic mass is only 7, which means that for metals of the same mass, metallic lithium can release the highest energy. Therefore, the research on lithium batteries came into being.

Figure 3. (a) and (b): Common 18650 type LiCoO2 lithium-ion battery; (c) The development history of lithium-ion batteries.

(Image sources: (a) and (b) are from reference [2]; (c) is from veer gallery)

The first thing scientists developed was a lithium primary battery, which uses metallic lithium as the negative electrode. Similar to the zinc negative electrode of the voltaic battery mentioned earlier, the metallic lithium negative electrode will generate lithium ions and enter the electrolyte during the discharge process; the charging process is the opposite, and the lithium ions in the electrolyte will deposit on the electrode to form metallic lithium. Because the "lithium" deposited during the charging process is uneven, its shape is similar to that of a tree branch, and is academically called a "dendrite", which can easily pierce the diaphragm and cause the battery to short-circuit and catch fire. Therefore, although lithium metal batteries have a high energy density, they are difficult to charge and discharge.

Figure 4. Dendrite morphology of lithium metal anode

(Image source: from reference [3])

Nowadays, common lithium batteries generally use graphite materials as negative electrodes. During the charging process, lithium-containing positive electrode materials (lithium iron phosphate, lithium cobalt oxide, and lithium manganese oxide positive electrodes, etc.) release lithium ions and penetrate the diaphragm to embed into the lattice of negative electrode graphite; the discharge process is the opposite, and lithium ions are released from the graphite negative electrode and return to the positive electrode material. Therefore, the common lithium batteries in our lives are also called lithium-ion batteries. The use of graphite negative electrodes greatly improves the safety of batteries. Today, lithium-ion batteries are widely used in mobile phones, computers, electric vehicles and other fields.

The 2019 Nobel Prize in Chemistry was awarded to three scientists, American scientist John B. Goodenough, British scientist Stanley Whittingham and Japanese scientist Akira Yoshino, in recognition of their outstanding contributions to lithium-ion batteries.

Figure 5. Three scientists who won the Nobel Prize in Chemistry in 2019 for their great contributions to the field of lithium-ion batteries.

(Image source: Graphic news)

However, compared with metallic lithium, the energy that graphite can release as a negative electrode of the battery is limited, which restricts the energy density of the battery. Therefore, researchers have to re-study the lithium metal negative electrode, trying to further improve the energy density of the battery through the use of lithium metal negative electrodes.

How are “lithium dendrites” produced?

For lithium metal batteries, the lithium negative electrode is accompanied by deposition/dissolution reactions during the charging and discharging process. During the deposition process, since the metal electrode itself cannot be absolutely flat, more negative charges will accumulate at the protruding parts, causing metal lithium to deposit preferentially. The more lithium is deposited, the higher the protrusion is (Figure 6a), the more charges accumulate, and the more lithium ions are exposed. All of these cause the originally small protrusions to become higher and higher, similar to the positive feedback process in biology. When it reaches a certain level, it will pierce the diaphragm and cause the battery to short-circuit.

Figure 6. (a) Li metal anode dendrites and (b) SEI layer formation process

(Image source: (a) from reference [4]; (b) from reference [5])

At the same time, although lithium metal is not as active as sodium and potassium metals in the same main group, it is also worthy of being called an active metal. When it comes into contact with the electrolyte, lithium metal will react with the electrolyte to form an interface layer on the surface, which is called the SEI layer (Figure 6b).

Similar to the common metal corrosion in our lives, if this interface layer is as dense and stable as alumina, it can isolate the lithium metal and the electrolyte, and the lithium metal battery can be well protected; but if SEI is as loose as rust, the electrolyte will always be in contact with lithium, constantly reacting until it is completely corroded.

Therefore, SEI is also an important research direction for metal lithium batteries. In addition, during the process of lithium deposition and dissolution, the volume of the battery is constantly changing. Compared with the graphite intercalation reaction, the volume change of lithium deposition/dissolution is more significant, which further puts forward requirements for the uniform deposition of lithium and the performance of the SEI film.

A new method is here to solve the lithium metal anode problem with one click

The three major problems of dendrites, corrosion, and volume change are ultimately problems at the interface between electrodes and electrolytes, so the researchers' solution is to start from the interface.

They thought of two methods: they could use electrode materials and structural design to make the charge distribution more uniform, thereby inhibiting the growth of lithium dendrites; they could also optimize the electrolyte composition and use the reaction between the electrolyte and metallic lithium to form a stable SEI film.

Recently, a paper published in the academic journal Nature reported a lithium deposition technology to prevent lithium metal corrosion. The researchers avoided the formation of the SEI layer by rapidly depositing lithium at a high current density. The results showed that without the formation of SEI, lithium atoms were orderly arranged to form rhombic dodecahedrons, and no obvious dendrites were generated, which overturned the common perception that high current density exacerbates the formation of lithium dendrites.

Figure 7. Lithium atoms are arranged in an orderly manner to form a rhombic dodecahedron

(Image source: Reference [6])

The research results mean that the lithium dendrite problem can be completely overcome. For example, by regulating the electrolyte composition to avoid corrosion of the lithium negative electrode by the electrolyte, thereby avoiding the formation of SEI, lithium ions will be evenly deposited, greatly promoting the application of lithium metal negative electrodes and improving the energy density of batteries.

Conclusion

With the iterative development of technology, the energy density of lithium batteries will continue to increase, which will make lithium batteries more widely used in electric vehicles, drones and other fields. In addition to lithium batteries, many other battery technologies are also making rapid progress, such as fuel cells and zinc-based batteries. The development of battery technology will also greatly promote the large-scale use of new energy and provide a technical basis for achieving the strategic goals of "carbon neutrality" and "carbon peak".

References

1.Ghiji, M., et al., A review of lithium-ion battery fire suppression. Energies, 2020. 13(19): p. 5117.

2.Chen, M., et al., Study of the fire hazards of lithium-ion batteries at different pressures. Applied Thermal Engineering, 2017. 125: p. 1061-1074.

3. Whittingham, MS, Inorganic nanomaterials for batteries. Dalton Transactions, 2008(40): p. 5424-5431.

4.Yang, C., et al., Protected Lithium‐Metal Anodes in Batteries: From Liquid to Solid. Advanced Materials, 2017. 29(36): p. 1701169.

5.Spotte-Smith, EWC, et al., Toward a Mechanistic Model of Solid–Electrolyte Interphase Formation and Evolution in Lithium-Ion Batteries. ACS Energy Letters, 2022. 7(4): p. 1446-1453.

6. Yuan, X., et al., Ultrafast deposition of faceted lithium polyhedra by outpacing SEI formation. Nature, 2023. 620(7972): p. 86-91.