Environmental protection, energy saving, unlimited traffic, and cheap electricity! Although new energy vehicles have so many advantages, we often see the news that "new energy vehicles suddenly run out of power on the highway and can't hold on to the next service area" and "multiple cars waiting for a charging pile, queuing for 4 hours to charge for 1 hour". The three most complained points by new energy car owners are "low number of charging piles", "long charging time" and "short range", which are also the long-term constraints on the development of new energy vehicles. The "three big mountains". How to overcome these "three mountains", to achieve a wider, faster and better popularity of new energy vehicles? You must have thought of the battery!

The ideal lithium battery runs far, charges fast and is safer

Ideally, the battery of a new energy vehicle should at least look like this: first, its capacity should be high to ensure that the car can run far; second, it should be charged quickly to ensure a short waiting time; third, its stability should be strong to ensure that it is safer on the road. In this way, the development goal is clear, that is, to develop a new generation of "large capacity" "high rate" "long cycle" battery.

In order to achieve such a goal, since the commercialization of lithium-ion batteries in 1991, as the core components of lithium-ion batteries, scientists in the lithium-ion battery electrode materials to carry out a lot of research work.

All batteries have positive and negative electrodes, and lithium batteries are no exception. Regardless of positive or negative electrode materials, the ideal electrode materials should have: good reversibility of de-lithium embedded lithium, high mass ratio capacity, smooth redox potential platform, high electronic conductivity, ionic conductivity and lithium ion diffusion coefficient and good stability. The difference between positive and negative materials lies in the high potential of lithium ion embedding, with the higher embedding potential being the positive material and the lower embedding potential being the negative material.

The history of the development of cathode and anode materials for lithium-ion batteries is also quite a story. Since the first commercialization of lithium-ion batteries by Sony in the early 1990s, a variety of cathode material systems have been developed after more than two decades of development. The earliest commercialized cathode material is lithium cobaltate, which is also the longest and most mature lithium-ion battery cathode material and has been widely used so far. However, lithium cobaltate is not a panacea. Although the lithium cobaltate system has high energy density, high specific capacity, considerable cycle life and safety, it is slightly less stable, and the battery capacity decays more seriously under high voltage conditions.

Subsequently, researchers have developed lithium manganate system, which can solve the problem of the lack of stability of lithium cobaltate, but there is a huge defect of trivalent manganese dissolution, has gradually faded out of the stage of lithium-ion battery cathode materials. Lithium iron phosphate system due to the stability of the structure before and after lithium ion de-embedding, good cyclability, slow capacity decay after lithium ion cycling, low toxicity, from the beginning of the birth of the battery is considered the most ideal cathode material for electric vehicles, however, the system has a low electronic conductivity, greatly affecting the overall performance of the battery.

Positive electrode materials composed of two metals cannot meet the demand well, scientists have again turned their attention to ternary materials. The ternary material lithium nickel cobalt manganate is prepared by doping lithium cobaltate, which has higher safety than lithium cobaltate. Ternary materials are prone to oxidation in air to form unstable surfaces, structural defects and nickel-lithium mixed row, making the material internal resistance increases, electrochemical activity decreases, generating intergranular cracks and micro-strain, forming additional insulating film, increasing the material impedance, so that the ternary material performance decreases. At present, the mature commercialization of ternary materials still has a long way to go.

Overall, lithium battery cathode materials are moving toward the development of high specific capacity, high safety, high cycle efficiency, traditional materials, despite the maturity of the technology, but has been unable to meet the constant demand in the field of power batteries, the future in the field of cathode materials will appear more breakthrough technology.

Lithium metal is good but "thorny rose"

The negative electrode material of lithium batteries is also the key. It has a direct impact on the first cycle efficiency, cycle life, multiplier performance and safety performance of the battery. The first generation of lithium-ion battery cathode material directly using lithium metal, but in the charge and discharge process is prone to dendrites. Lithium metal will grow dendrites on the surface after long time charging and discharging. This is like a smooth plane suddenly grows millions of thorns, you can imagine that this "thorny rose" may eventually poke through the battery, causing a short circuit, or even cause an explosion.

The second generation of anode materials uses lithium-aluminum alloy to solve the problem of lithium metal dendrites, but the volume of the material changes greatly during the cycle, and the main body of the material is easily pulverized and falls off, making the cycle poor. The third generation of anode materials is the use of layered graphitic carbon materials, the material in the lithium de-embedding process potential close to the potential of lithium itself, the layered structure is conducive to the embedding of lithium off, greatly improving the cycle and safety performance of lithium-ion batteries. To date, large-scale commercialization of anode materials are still mainly graphite-based carbon materials and lithium titanate two categories.

Although graphite-based carbon materials and lithium titanate are more mature in commercialization, both materials have an inherent defect of low theoretical specific capacity, which makes the current energy density of lithium-ion batteries can not meet the higher requirements of power batteries.

Therefore, the future development of lithium-ion battery cathode materials may show a "two-legged" trend, one is to return to the original heart, reuse lithium metal as the cathode material, the focus of research on how to overcome the problem of lithium metal dendrites in the long time charging and discharging process; the other road is for the current urgent demand for high energy density, improve the existing battery system, there is a high demand for high energy density. The other way is to improve the existing battery system for the current urgent demand for high energy density, targeted replacement of electrode materials, and to find the anode materials that can be truly industrialized and have application prospects.

Copper house to isolate the "thorns" of lithium metal

After a lot of comparisons, our team finally locked in lithium metal as the focus of research on anode materials, because we found that the theoretical capacity of lithium metal is more than 10 times the current commercial lithium battery anode materials, and its good electrical conductivity, is one of the most ideal anode materials. If the dendritic problem can be solved properly, it will be one step closer to producing lithium batteries with high capacity and fast charging.

How to solve the dendrite problem? One of the common solutions is to build up a three-dimensional copper collector. The negative electrode of lithium metal requires copper as a collector, and lithium metal will grow dendrites after a long time charging and discharging, which may penetrate the diaphragm and cause a short circuit or even cause an explosion. Research shows that if the flat copper into three-dimensional copper, can reduce the absolute current density, thereby inhibiting the growth of lithium dendrites; at the same time, the three-dimensional structure of the copper collector can effectively induce lithium deposition in the interior of the substrate, thereby avoiding dendrites penetrate the diaphragm. This is like building a copper house, so that the "thorns" grow inside the house, so that they can not penetrate the room.

But the problem is that building this house directly is not only time-consuming and labor-intensive, but also very expensive and cannot be produced on a large scale. Therefore, this research remains in the laboratory, greatly limiting the commercialization of lithium metal. Therefore it is quite a challenging research topic to find out how to make three-dimensional copper in a cost-effective and reproducible way.

We have tried various methods, such as hydrothermal and vapor deposition, but the results are not satisfactory. When we were puzzled, a common and interesting phenomenon caught our attention.

When I was studying at Harvard, I often bought Boston lobster because the local area was very famous. Steamed lobster has a red color, but its red color is not natural, but due to the high temperature that turns the greenish-black lobster into red. It was this all-too-common phenomenon that made me think: If the red color of lobster is not natural but transformed later, then why should we insist on preparing red 3-D copper directly? What if we could get a cheap 3-D structure to transform into copper? We immediately redirected our research: transformation! Instead of building a copper house directly from scratch, we could build a cheap cloth house and then paint it with a layer of copper to turn it into a red copper house. The point of stone into gold can not be achieved, but the point of cloth into copper has the feasibility.

Light and thin paper can also be used to make lithium batteries

In the process of searching, another interesting little creature - the mussel came into our sight. The shell secretes a sticky protein that acts as a binder and allows the mussel to be firmly attached to the bottom of the ship. For the ship, the mussel is not popular, if the original smooth bottom full of mussels will make the resistance greatly increased, increasing fuel consumption and even the bottom of the steel plate also has a corrosive effect.

But the fishermen's headache of the little guy, but we are very inspired, can not imitate the mussel, to the lithium metal to build a surface firmly adsorbed copper "house" it?

In nature, mussels secrete a mucin that is firmly attached to the surface of almost any material. The core component of mussel mucin is similar to dopamine, so a dopamine solution can be used instead. Can dopamine be firmly adsorbed on the surface of materials by soaking inexpensive and readily available glass fiber cloth, for example, in a solution of dopamine?

Based on such an idea, we propose a novel transformation idea: to cover the surface of ordinary porous materials with a copper layer, thus turning the substrate material into a three-dimensional copper skeleton. The whole process is divided into two steps: loading of the polydopamine coating and deposition of copper monomers. Firstly, the substrate material is immersed in dopamine solution, and the polydopamine coating is loaded on the surface of the material by using the in situ polymerization of dopamine; in the second step, the chelation of polydopamine and copper ions is used, and dimethylamine borane is added to enhance the reduction effect, so that the copper monomers are successfully coated on the fiber surface uniformly by electroless deposition.

After the test, the white cloth house really turned into a black house with adsorbed dopamine. Then the reducing agent and copper ion solution were added, and after 24 hours of reaction, the black dopamine house really turned into a red copper house.

The surface of the material eventually turned reddish brown, which could visually confirm the successful deposition of copper monomers. The whole process is simple, efficient and non-polluting to the environment.

Not only that, replacing the glass fiber cloth with other more common materials, the conventional inorganic and organic porous materials such as glass fiber, nickel foam, polycarbonate filter membrane, and rice paper have successfully completed the construction of three-dimensional lithium storage copper skeleton by simple soaking, and satisfactory results were also obtained. This confirms the high efficiency of the method and also greatly broadens the selectivity of the materials available. This means that this conversion method does not require special chemicals and instrumentation to convert a wide range of materials (inorganic, organic polymers, etc.) into three-dimensional lithium storage skeletons.

The prepared new cell was tested electrochemically, and the Coulomb efficiency remained at 94% after 600 hours of cycling, with significantly improved long-cycle performance. This simple but universal method can transform common porous materials into efficient lithium storage skeletons, providing a new solution for building three-dimensional fluid collectors and significantly reducing battery costs. Meanwhile, the three-dimensional lithium storage structure can effectively regulate the lithium ion deposition behavior, fundamentally regulate the lithium metal nucleation and growth process, effectively inhibit dendrite formation, and promote the commercialization of lithium metal anode secondary batteries.

Based on this, we have achieved a major breakthrough in the energy density, safety and charge/discharge rate of lithium-ion batteries, and significantly reduced the battery production cost, providing a reasonable and feasible new idea for the design and development of new generation power batteries. The relevant research work has been successfully applied for international patents and published in several high-level papers in international well-known journals.

It is conceivable that in the near future, we can hope that the light and thin paper in our hands, suitably modified, can also be used to make large-capacity and low-cost batteries.

Expect lithium batteries in the future to help better life

In today's rapid development of science and technology, lithium batteries have long been into thousands of households, the current application areas of lithium batteries are mainly concentrated in electric vehicles, electronic products and aerospace and other major categories.

In the field of electric vehicles, lithium battery is still the preferred energy source with the highest market share, and its clean and zero-emission advantages will be further amplified under the "double carbon" policy. With the progress of technology, the current lithium-ion battery electric vehicles in the mileage, range and safety have been greatly improved; electronic products is the traditional advantage of lithium-ion battery field, cell phones, digital cameras, laptop batteries are all inseparable from the lithium-ion battery.

With the continuous improvement of charge and discharge performance, the future application of lithium-ion batteries in the field of power tools will be more extensive. Although the application in the space field is not often heard, but in fact, as early as 2004 lithium-ion batteries were used in the Mars lander and Mars rover, the current application of lithium-ion batteries in the space field is mainly in the launch, flight correction, night operations, etc. to provide support.

Half a century of lithium battery development history has been dramatic. Today, researchers are still exploring for the development of better batteries, expecting that lithium batteries will better contribute to the better life of mankind in the future.

Translated with www.DeepL.com/Translator (free version)