Nan Cewen: Lithium battery still has a lot of room for development

On March 1, 2017, the four ministries and commissions issued the Action Plan for Promoting the Development of Automotive power battery Industry, which set clear requirements for battery performance, capacity, safety, materials and equipment. In the face of the leaps and bounds of 100 billion watts, how can the industry face the new technology that emerges in an endless stream? This article recently interviewed Nan Cewen, academician of the Chinese Academy of Sciences and dean of the Institute of Materials Science and Engineering at Tsinghua University. All-solid-state lithium batteries will greatly improve safety and performance, but before industrialization, there is still a need to continuously upgrade existing technologies. In the future, with the continuous discovery of new materials, lithium battery technology and industry development space is infinite.

The "golden key" to solve security problems

Energy Review: In 2017, with the rapid expansion of the scale of electric vehicles, safety issues have received unprecedented attention. What do you think is the advantage of an all-solid lithium battery compared to a conventional lithium battery?

Nan Cewen: In simple terms, the traditional lithium-ion battery is a structure in which the positive and negative electrodes are separated by a diaphragm and poured into an organic electrolyte. The electrolyte is easy to seep out, especially when it encounters short circuit or overcharge of positive and negative electrodes, which will cause the temperature to rise rapidly, the electrolyte will evaporate and decompose, and a large amount of gas will be generated, which will cause safety problems of the battery and even cause the battery to burn and explode.

The all-solid-state lithium battery uses the all-solid electrolyte to achieve a two-in-one function, replacing the diaphragm and electrolyte in the traditional battery to solve the safety problem. At the same time, after using an all-solid electrolyte, metal lithium can be used as the negative electrode to increase the energy density.

The security issue is the key and foundation of the industry development, and also the fundamentals of the survival of the battery industry; energy density is a core of the industry's research and development, and the prospects for the development of the industry. From the perspective of solving safety problems and using existing materials to improve energy density, all-solid-state lithium batteries can be expected to meet the needs of industrial development and are worthy of vigorous development.

Energy Review: Based on existing technology, can we solve security problems better?

Nan Cewen: There are a variety of techniques available to improve the safety of lithium-ion batteries, such as battery management systems (BMS). However, BMS is a means of "treating the symptoms", and the "rule of governance" needs to start from the battery material itself. Among them, the use of ceramic diaphragm is a good direction to improve the safety of lithium-ion batteries. It is coated with a layer of nano-ceramic (Al2O3) particles on the substrate of the separator, which increases the mechanical strength and heat shrinkage of the separator, and reduces the possibility of direct short-circuit between the positive and negative electrodes, thereby improving safety. The new generation ceramic diaphragm product is a nano ceramic fiber coated diaphragm (produced by Jiangsu Qingtao Energy), which has better heat resistance and is more effective for improving the safety of lithium ion batteries. The second generation of products is an active ceramic fiber coated diaphragm. The use of ceramic electrolyte fibers, in addition to improving safety, also increases the lithium ion conduction rate, thereby improving battery rate performance. The general idea is to improve the safety of existing lithium-ion batteries through ceramic diaphragms, and gradually develop to replace the diaphragm and electrolyte with all-solid electrolytes in order to completely solve the safety problem.

"Energy Review": In this way, all-solid electrolytes can be called the "golden key" to solve battery safety. Based on the current industrial layout and research and development, what kind of development strategy do you think the industry should choose?

Nan Cewen: At present, Bolloré of France, Sakti3 of the United States and Toyota of Japan represent the typical research and development direction of three solid electrolytes of polymers, oxides and sulfides. In fact, the combination of several methods is also an idea. For example, combining inorganic materials with organic materials, the general principle is to try in the middle of multiple schemes. The more likely development strategy in the future is to slowly transition and gradually reduce the amount of electrolyte, for example, from 20% to 30% to 5% to 10%, or even 0, from semi-solid to all solid.

Although the current solid-state battery type is “not far from being thirsty”, it is still not industrialized. However, before this, the industry has been continuously improving the existing technology to gradually improve the safety and energy density of existing batteries, such as improvement. Existing material ratios, improved electrolyte performance, battery management systems (BMS), and more.

Scientific research and industrialization: from 1% to 100%

Energy Review: What are your expectations for the industrialization agenda of all-solid-state lithium-ion batteries?

Nan Cewen: For industrialization, the domestic formulation is generally achieved by 2020~2025, and some experts have proposed to strive for industrialization within five years. This goal requires everyone to work together to make it possible. Of course, it also depends on the industrialization standards, and to what extent and on a large scale. For example, it is reported that the goal of the German BMW company is that by 2028, Toyota of Japan has not announced a commercialization schedule, but it has invested heavily in the field of all-solid-state batteries earlier, and has been vigorously working hard.

Energy Review: In the future, what areas will all-solid-state lithium-ion batteries used in?

Nan Cewen: All solid-state batteries are currently used in some special industries, such asspecial and medical, which have absolute requirements for safety. The future has good prospects in the fields of power and energy storage.

"Energy Review": As a new technology, all-solid-state lithium batteries will inevitably have problems such as insufficient technology and high cost. How do you evaluate the view that the high cost is the biggest bottleneck of its industrialization?

Nan Cewen: The overall low-rate performance of all-solid-state lithium batteries is a scientific and technical problem that needs to be solved slowly. The cost issue is not the biggest bottleneck. In fact, any new technology and new products are costly at the beginning. Once the production technology is mature and the output goes up, the cost can naturally come down. Therefore, the cost is solved by the industry, not the problem that the academic world can solve. .

At the same time, the goals pursued by laboratory research and industrialization are different. Doing research to pursue the possibility and feasibility of 1%, you can find new materials through continuous trial and error innovation, as long as there is a possibility, even 1% can be; the industry is pursuing 99% or even 100% reliability and consistency. Sexuality, not bad at all, and all aspects must be considered thoughtfully, so to change 1% into 99% or even 100%, there is still a need for a bridge and process of transformation, which needs to be gradually improved from the laboratory and the pilot. Then zoom in and mature to achieve full controllability.

Development without "ceiling"

Energy Review: A breakthrough in chemical batteries, relying on innovation in materials technology. From this perspective, how do you evaluate the direction of R&D in all-solid-state lithium batteries?

Nan Cewen: Unlike ordinary people, lithium-ion batteries are not the same as ordinary electronic components. They are actually a very complicated system. For example, the positive electrode and the negative electrode are composite materials of various materials, and the electrolyte and the separator may also be a mixture of various kinds.

Solid-state batteries seem simple, but they are also very complicated. For example, the positive electrode layer of a liquid lithium ion battery contains a plurality of components such as a positive electrode active material, a conductive agent, an electrolyte solution, a binder, and the like, and if it is replaced with an all-solid electrolyte, since there is no electrolyte penetration in the positive electrode layer, various components The problem of matching combinations can be complicated. Doing liquid lithium-ion batteries is like we are mixing the sand with cement. Adding water can reconcile stones, sand and cement, but there is no liquid substance involved in the all-solid battery, how to solve the interface problem between solid and solid materials and ensure effective substances Activity, the challenge is great.

"Energy Review": What kind of evolutionary pattern do you think will be the technical route from lithium iron phosphate, ternary, high-nickel, and all-solid-state batteries?

Nan Cewen: The energy density of a single battery should reach 300 watt-hours/kg and it is not difficult to develop new products on the existing technology system once it exceeds 400-500 watt-hours/kg, new breakthroughs are needed, technically, the evolutionary route is carried out according to time, but batteries of different skill levels are not a relationship between you and the living, and may be a pattern of coexistence and symbiosis. This means that after a new generation of batteries, other batteries will be completely eliminated, which may be a gradual process, and may also coexist for a long time. Taking lead-acid batteries as an example, although the energy density is low and the pollution is large, so far, lead-acid batteries have not been completely replaced by lithium-ion batteries, and they have developed very well. The reason is that the cost is low, the safety is ok, and the problems of recycling and recycling are better solved, so that it has been coexisting with lithium ion batteries. Different batteries have different characteristics and exist in different application fields suitable for them. Energy Review: as far as energy density is concerned as an element ranked third in the periodic table lithium metal batteries can theoretically reach 700 watt-hours/kg is it the limit of battery energy storage?

Nan Cewen: This is certainly not the limit. The energy density of the battery requires a comprehensive consideration of the positive and negative materials. If a new positive electrode material is found, the specific capacity and voltage are much higher than the ternary or existing materials, and the energy density of the battery is also increased. The limits of lithium batteries, or ceilings, are at least not technically visible at present. If you have to determine the relative limits, as a lithium-ion battery that is more than one order of magnitude higher than the current lithium-ion battery energy density, it may be imaginable as the limit (theoretical energy density is about 3500 watt-hours/kg), but 700 watt-hours/kg is not limit.

The page contains the contents of the machine translation.