New energy vehicles have been put on the agenda, solid lithium batteries compared to traditional lithium batteries

At present, vigorously developing new energy vehicles has become a consensus for countries to achieve energy conservation and emission reduction and cope with climate change. Many countries have raised the development of new energy vehicles to the national strategic level. United States, Europe, Japan and other countries have launched major automotive group their development plans, such as Volkswagen presented "2025 strategy", is expected to launch in 2025 more than 30 models of electric vehicles, sales and strive to reach 300 million. Especially since 2016, major auto powers have increased their support for the new energy auto industry:

The German government and industry have provided a total of 1.2 billion euros in subsidies and implemented a special purchase subsidy strategy;

The US government provided a $4.5 billion loan guarantee to promote the construction of electric vehicle infrastructure and invest in the development of high-energy-density batteries.

In this context, as of 2016, the global sales of new energy vehicles exceeded 2 million, of which China accounted for more than 50%, making substantial contributions to energy conservation and global climate change.

However, the current large-scale application of electric vehicles is still subject to many constraints such as driving range, safety, cost, etc. For example, for the driving range of vehicles, simply increasing the number of batteries will cause the whole vehicle to gain weight, which in turn will cause 100 kilometers. The obvious increase in power consumption, followed by the increase in carbon emissions throughout the life cycle, the vehicle price will also rise, so the fundamental solution still needs to significantly improve the performance of all aspects of the battery. The United States launched Modle S Tesla electric car, for example, in order to solve the problem "range anxiety", using nearly 7000 18650 3.1 Ah lithium-ion battery, so mileage of more than 400 km, but its battery weighs 500 kg, the car's price of up to 79,000 US dollars, to some extent inhibited its promotion in the market.

A significant increase in battery performance every time is essentially a major change in the battery material system. From the first generation of nickel-metal hydride batteries and lithium manganate batteries, the second generation of Lithium Iron Phosphate Batteries, to the current third-generation ternary batteries, which are widely used and expected to last until around 2020, their energy density and cost are respectively presented. A clear trend of rising and falling steps. Therefore, the battery system used in the next generation of automotive batteries is critical to achieving the battery target of 2020-2025.

At present, the frequency of “all-solid-state Lithium Batteries” in various public places in the field of new chemical power sources is getting higher and higher, and the industry has basically formed a consensus: all solid-state lithium batteries are expected to enter the market as the next-generation power source, but what is it? All solid state Lithium Battery?

1. Overview of all solid state lithium batteries

Conventional lithium-ion batteries use organic liquid electrolytes. In the case of abnormalities such as overcharging and internal short-circuiting, the battery is prone to heat, causing the electrolyte to swell, spontaneously ignite or even explode, posing serious safety hazards. The all-solid-state lithium battery based on solid electrolyte developed in the 1950s, due to the use of solid electrolytes, does not contain flammable and volatile components, completely eliminates the safety hazards such as smoke and fire caused by leakage of batteries. For the safest battery system.

For the energy density, the United States and Japan government wants to develop in 2020 400 ~ 500 Wh / kg of prototype devices, 2025 and 2030, mass production. To achieve this goal, the most recognized one is the use of lithium metal anodes. Metal lithium has dendrites, powders, SEI (solid electrolyte interface film) instability, surface side reactions in traditional liquid lithium ion batteries. Many technical challenges, and the compatibility of solid electrolytes with lithium metal makes it possible to use lithium as a negative electrode, thereby significantly achieving an increase in energy density.

Table 1 Comparison of characteristics of different types of lithium-based batteries

Table 1 compares the traditional lithium battery and the all-solid lithium battery, from which the basic characteristics of the solid lithium battery can be understood. Further, as shown in Table 2, for the desired requirements of automotive battery applications, based on their own characteristics, solid-state battery systems give possible solutions one by one.

Table 2 Battery application requirements and solid-state battery system solutions

From the point of view of the time node, the all-solid metal lithium battery is earlier than the liquid lithium-ion battery, but in the early stage, the electrochemical performance, safety and engineering manufacturing of the all-solid metal lithium battery have not been able to meet the application requirements.

Liquid lithium-ion batteries have been continuously improved, and comprehensive technical indicators have gradually met the needs of consumer electronics market applications, and have been accepted by more markets.

From the perspective of technological development trends, compared with liquid lithium ion batteries, all solid metal lithium batteries may have the advantages of good safety performance, high energy density and long cycle life.

In recent years, solid electrolyte materials, especially sulfide electrolyte materials, have made major breakthroughs in ionic conductivity. Therefore, all-solid-state lithium battery technology has gradually begun to attract the attention of R&D institutions and large enterprises worldwide.

2. Classification of all solid lithium batteries

Along with the rise of all-solid-state lithium battery heat, various "all solid" or "solid" lithium batteries have emerged, and there is a current state of confusion. The seven types of concepts related to solid-state lithium batteries have been sorted out and a preliminary summary has been made.

Liquid lithium battery: A lithium battery that does not contain a solid electrolyte during the manufacturing process and contains only a liquid electrolyte, including a liquid lithium ion battery and a liquid metal lithium battery.

Gel electrolyte lithium battery: The liquid electrolyte in the cell exists in the form of a gel electrolyte, which does not contain a solid electrolyte, which is actually in the category of liquid lithium ion batteries.

Semi-solid lithium battery: In the electrolyte phase, half of the mass or volume is a solid electrolyte, and the other half is a liquid electrolyte; or one end of the cell is all solid and the other end contains a liquid.

Quasi-solid lithium battery: The electrolyte of the battery contains a certain solid electrolyte and a liquid electrolyte, and the mass or volume of the liquid electrolyte is smaller than that of the solid electrolyte.

Solid-state lithium batteries: batteries with a high mass or volume ratio of solid electrolytes and batteries containing a small amount of liquid electrolyte are called "solid-state lithium batteries" by some researchers, but this is not actually an all-solid lithium battery.

Mixed solid-liquid lithium battery: There are both a solid electrolyte and a liquid electrolyte in the battery. The above-mentioned semi-solid, quasi-solid, solid lithium battery and the like are all one of mixed solid-liquid lithium batteries. Since it is not necessary to artificially classify according to the ratio of solid to liquid, and it does not cause ambiguity, it is recommended to use this term, which can also be called "mixed solid-liquid electrolyte lithium battery".

All solid-state lithium battery: The battery core is composed of solid electrode and solid electrolyte material. The battery core does not contain any mass and volume fraction of liquid electrolyte in the working temperature range. It can also be called “all solid electrolyte lithium battery”. The charge and discharge cycle can be further referred to as an "all solid lithium secondary battery" or an "all solid electrolyte lithium secondary battery".

In summary, lithium batteries can be divided into liquid lithium batteries, mixed solid-liquid lithium batteries and all-solid lithium batteries according to different electrolytes. According to the difference of the negative electrode, it can be divided into a lithium metal battery in which the negative electrode is metallic lithium, and a lithium ion battery in which the negative electrode does not contain metallic lithium.

Table 1: Types and characteristics of mixed solid-liquid lithium batteries and all-solid lithium secondary batteries with different electrolyte types [1]

3. Possible advantages of all-solid lithium secondary batteries

The reason why all-solid-state lithium secondary batteries will make international giants look because it is expected to solve the two "challenges" that currently plague the power battery industry - safety hazards and low energy density. The advantages of an all-solid-state lithium battery over a liquid lithium-ion battery include:

(1) High security performance

Since the liquid electrolyte contains a flammable organic solvent, the sudden rise in temperature during internal short circuit is likely to cause combustion or even explosion. It is necessary to install a safety device structure that is resistant to temperature rise and short circuit, which increases the cost, but still cannot completely solve the safety problem. . Known as BMS to achieve the best Tesla in the world, there have been two serious fires in the Model S in China this year.

Many inorganic solid electrolyte materials are non-flammable, non-corrosive, non-volatile, and have no leakage problems, and are also expected to overcome lithium dendrite. Therefore, all-solid lithium secondary batteries based on inorganic solid electrolytes are expected to have high safety characteristics.

Polymer solid electrolytes still have a certain risk of burning, but the safety is also greatly improved compared to liquid electrolyte batteries containing flammable solvents.

(2) High energy density

At present, the energy density of lithium-ion battery cells used in the market is up to 260Wh/kg, and the energy density of lithium-ion batteries being developed can reach 300-320Wh/kg. For all-solid-state lithium batteries, if the negative electrode is made of metallic lithium, the battery energy density is expected to reach 300-400 Wh/kg, or even higher.

It should be noted that since the solid electrolyte density is higher than that of the liquid electrolyte, the lithium battery energy density of the liquid electrolyte is significantly higher than that of the all solid lithium battery for the same system of the positive and negative materials. The reason why the all-solid lithium secondary battery has a high energy density is because the negative electrode may be made of a metallic lithium material.

(3) Long cycle life

The solid electrolyte is expected to avoid the problem of continuous formation and growth of the solid electrolyte interface film during the charging and discharging process of the liquid electrolyte and the problem of the lithium dendrite piercing the membrane, which may greatly improve the cycle and service life of the metal lithium battery.

The film-type all-solid metal lithium battery has been reported to be able to circulate 45,000 times, but currently there is no report on long-cycle life of large-capacity lithium metal batteries, mainly due to the current cycle performance of high-surface-capacity metal lithium electrodes (> 3 mAh/cm2). difference.

(4) Wide operating temperature range

If all solid-state lithium batteries use inorganic solid electrolytes, the maximum operating temperature is expected to increase to 300 ° C or even higher. At present, the low-temperature performance of large-capacity all-solid lithium batteries needs to be improved. The operating temperature range of a specific battery is mainly related to the high and low temperature characteristics of the electrolyte and the interface resistance.

(5) Electrochemical window width

The all-solid-state lithium battery has an electrochemically stable window width, which is likely to reach 5V, and is suitable for a high-voltage electrode material, which is advantageous for further increasing the energy density. At present, a thin film lithium battery based on lithium nitride phosphate can work at 4.8V.

(6) Flexibility advantage

All-solid-state lithium batteries can be fabricated into thin-film batteries and flexible batteries, which can be applied to smart wearable and implantable medical devices in the future. The package is easier and safer than the flexible liquid electrolyte lithium battery.

(7) Easy recycling

Battery recycling is generally two methods, one is wet and the other is dry. The wet method is to take out the toxic and harmful liquid core inside, and the dry method is, for example, crushing to extract the effective ingredients. The advantage of an all-solid-state lithium battery is that it has no liquid in itself, so theoretically there should be no waste liquid, which is relatively simple to handle.

4. Existing defects and partial solutions for all-solid lithium secondary batteries

Although all-solid-state lithium secondary batteries show obvious advantages in many aspects, there are also some urgent problems to be solved: the ionic conductivity of solid electrolyte materials is low; the interface impedance between solid electrolytes/electrodes is large, and the interface compatibility is poor. At the same time, the volume expansion and contraction of each material during charging and discharging, resulting in easy separation of the interface; the electrode material to be designed and constructed to match the solid electrolyte; the current cost of battery preparation is higher. In response to these problems, the researchers made various attempts and gave some possible solutions.

Table 2: Current defects and solutions for all-solid lithium secondary batteries

5. Introduction of core materials

(1). Solid electrolyte

The solid electrolyte is the core component of the all-solid lithium secondary battery, and its progress directly affects the industrialization process of the all-solid lithium secondary battery. At present, the research of solid electrolytes mainly focuses on three types of materials: polymers, oxides and sulfides.

Table 3: Main systems and properties of three types of solid electrolytes

Polymer solid electrolyte (SPE) consisting of polymer matrix (such as polyester, polyether and polyamine) and lithium salt (such as LiClO4, LiAsF6, LiPF6, etc.) since 1973 PV Wright in alkali metal salt complex After the discovery of ionic conductivity, the polymer material has received extensive attention due to its solid electrochemical properties such as light weight, good elasticity, and excellent machinability. SPE was also the first solid electrolyte to achieve practical application. As early as 2011, the French company Bolure began to deliver Autolib electric cars to Paris, which is based on SPE-based all-solid lithium battery system.

Oxide solid electrolytes can be classified into crystalline and amorphous states according to their structure. Among them, crystalline electrolytes include perovskite, anti-perovskite, garnet, NASICON, LISICON, etc., amorphous oxidation. The research hotspots are LiPON type electrolytes and partially crystallized amorphous materials used in thin film batteries.

The sulfide solid electrolyte is derived from an oxide solid electrolyte in which an oxygen element in an oxide body is replaced by a sulfur element. Since the electronegativity of sulfur is smaller than that of oxygen, the binding of lithium ions is small, which is beneficial to obtain more free-moving lithium ions. At the same time, the radius of the sulfur element is larger than that of the oxygen element. When the sulfur element replaces the oxygen element, the lattice structure is expanded to form a larger lithium ion channel and the conductivity is improved, and the room temperature can reach 10-4-10-2 S/cm. .

(2). Cathode material

The positive electrode of the all-solid lithium secondary battery generally adopts a composite electrode, and includes a solid electrolyte and a conductive agent in addition to the electrode active material, and functions to simultaneously transport ions and electrons in the electrode. LiCoO2, LiFePO4 and LiMn2O4 are more common. Later, it is possible to develop high-nickel layered oxides, lithium-rich manganese-based and high-voltage nickel-manganese spinel-type positive electrodes. At the same time, attention should be paid to the research and development of new cathode materials without lithium.

(3). Anode material

The anode materials of all-solid lithium secondary batteries are mainly concentrated in three categories: metal lithium anode materials, carbon group anode materials and oxide anode materials. The three materials have their own advantages and disadvantages, among which the metal lithium anode materials are high in capacity and low. The advantage of potential is one of the most important anode materials for all-solid lithium batteries.

Table 4: Main systems and properties of three types of anode materials

6. Capacity division of all-solid lithium secondary battery and corresponding application fields and preparation processes

Figure 2: Flexible film all solid lithium secondary battery

From the form of the all-solid lithium secondary battery, it can be divided into two types, a film type and a large capacity type. The cell packaging technology of all types of all-solid lithium batteries is similar, and the main difference lies in the preparation of pole pieces and electrolyte membranes.

The thin film type all-solid lithium secondary battery sequentially prepares various elements of the battery in the order of the positive electrode, the electrolyte, and the negative electrode on the substrate, and finally encapsulates into a battery. In the preparation process, it is necessary to separately prepare the film layers of the battery by using corresponding techniques. Generally, the negative electrode is selected from lithium metal and is prepared by vacuum thermal vapor deposition (VD) technology; the negative electrode of the electrolyte and the positive electrode including oxide can be splashed. Injection techniques, such as RF sputtering (RFS), RF magnetron sputtering (RFMS), etc., have also been studied to produce films using 3D printing technology.

Figure 3: Large-capacity all-solid secondary lithium battery

Large-capacity all-solid-state lithium secondary batteries, due to the wide application range and large market, need to be prepared at a rapid and low-cost scale, and high-speed extrusion coating or spraying technology widely used in liquid lithium ion batteries can be used for reference.

The preparation of a large-capacity all-solid lithium secondary battery based on a polymer solid electrolyte is close to the winding process of the existing lithium ion battery.

However, considering the current poor flexibility of the inorganic solid electrolyte membrane, the lamination process is more often used in the preparation of the all-solid lithium secondary battery, and it is specifically used to separately prepare the electrolyte and the positive and negative membranes. The double-layer or multi-layer coating is used to prepare the composite layer of the electrolyte and the positive electrode, and the technical route suitable for large-scale production needs further research.

Table 5: Capacity, application and possible preparation process of all-solid lithium secondary battery

Although the production equipment of all-solid lithium secondary batteries is quite different from the traditional lithium-ion battery cell production equipment, there is no revolutionary innovation from the objective point of view. It is possible that 80% of the equipment can continue the production equipment of lithium-ion batteries. Only in the production environment has higher requirements, it needs to be produced in a higher-level drying room, which is sensitive to air with super capacitors, lithium ion capacitors, nickel-cobalt aluminum, pre-lithiation, lithium titanate, etc. For companies with devices or materials, the manufacturing environment is compatible, but the corresponding production environment costs are significantly higher.

7. All-solid lithium secondary battery development event

Figure 4: Overview of the development of all-solid lithium battery

(There are only some of the major events in the all-solid-state battery industry. If there are any omissions, please add)

8. Prospects for all-solid lithium secondary batteries

At present, the development of new energy vehicles has clearly risen to the national strategic level, in which power batteries are the most critical core components of new energy vehicles, and the key level can be seen.

Figure 5: Comparison of China-US-Japan Power Battery National Project Indicators

According to China's "Technology Roadmap for Energy Saving and New Energy Vehicles", the energy density target for pure electric vehicle power batteries in 2020 is 300Wh/kg, the target for 2025 is 400Wh/kg, and the target for 2030 is 500Wh/kg.

According to public data, the current energy density limit of liquid electrolyte-powered lithium-ion battery using ternary cathode material and graphite anode material is about 250Wh/kg, while silicon-based composite material is introduced instead of pure graphite as anode material, liquid electrolyte-powered lithium-ion battery. The energy density of the cell can reach 300Wh/kg, and the upper limit is about 350Wh/kg (the Panasonic 21700 battery that has been used on the Tesla Model 3, the nickel-cobalt-aluminum ternary material for the positive electrode, and the silicon-based composite material for the negative electrode. The density has exceeded 300Wh/kg).

"If the energy density is further increased, we must consider all-solid-state lithium batteries from now on." Academician Chen Liquan of the Chinese Academy of Engineering said in a recent public speech that "the long-term development of the electric vehicle industry requires technical reserves, and all-solid lithium batteries are expected." It has become the leading technology route for the next generation of vehicle power batteries in China. It is imperative to develop all-solid-state lithium batteries!"

From a global perspective, almost all of the old powerhouses have already established new energy vehicle development plans. On September 7, the Scottish National Party (SNP) leader Nikola Stutkin said in the parliament that it will fight for 2032. The sale of gasoline and diesel vehicles was stopped in the year to reduce air pollution.

In fact, not only Scotland, Norway, the Netherlands, Germany, the United Kingdom, and Belgium have all introduced or are preparing to introduce a policy on abolishing fuel vehicles.

Therefore, we can imagine that by 2050, traveling to Europe, traveling, looking around, running new energy vehicles on the road. On the other hand, our country has made relevant development plans based on actual conditions. In the already published “Long-term Plan for the Automotive Industry”, China’s automobile industry aims to achieve 30 million vehicle sales and sales by 2020, including 2 million new energy vehicles. By 2025, the production and sales volume of automobiles will reach 35 million, including 7 million new energy vehicles, accounting for 20%.

In response to the increasingly urgent high-performance demands of new energy vehicles, countries have begun to deploy high-energy-density lithium batteries. As proposed by the Japanese government, the energy density of power battery cells will reach 250Wh/kg in 2020 and 500Wh/kg in 2030. The United States Advanced Battery Association (USABC) proposed to increase the energy density of batteries in 2020 from 220Wh/kg to 350Wh/kg; China's State Council issued "China Made 2025" clearly stated that China's power battery ratio in 2020 The energy reaches 300Wh/kg, reaches 400Wh/kg in 2025, and reaches 500Wh/kg in 2030.

The Battery500 project in the United States proposes to develop a power battery sample with an energy density of 500 Wh/kg in 2020. To improve the energy density of the battery core, it is necessary to take into consideration the safety. Therefore, the development of all-solid lithium secondary battery technology is of great significance.

Figure 6: Distribution diagram of some all-solid lithium secondary battery research institutions and enterprises worldwide

(There are only some global solid-state secondary lithium battery research institutions and enterprises listed in the figure. If there are any omissions, welcome to add)

At present, there are more than 20 manufacturing companies, start-up companies and university research institutes around the world dedicated to solid-state battery technology.

Most of the institutions of higher learning focus on material-level research. The foreign countries are represented by the Argonne National Laboratory in the United States. In China, the Chinese Academy of Sciences set up a solid-state pilot program in 2013, hoping to realize solid-state battery industrialization within 5 years; some research institutions, Start-up And the new energy company has unique technology in material research and development and preparation, battery samples are mainly manual / semi-manual, only a small part of the demonstration vehicle. In terms of large-scale enterprises, Japan is represented by Toyota, Hitachi Shipbuilding and Idemitsu Kosan, and is a world leader in the field of solid-state batteries.

The industrialization of solid-state batteries is closely related to the high consistency of the cells and the difficulty of large-scale preparation:

As far as the preparation process is concerned, in view of the poor flexibility of the current solid electrolyte membrane, the solid state battery assembly is more biased toward the lamination than the winding process, but the subdivision process is still unknown;

As far as manufacturing equipment is concerned, although there is a big difference between solid-state batteries and conventional lithium-ion batteries, there is no fundamental difference, but only customized equipment is required in the processes of coating, packaging, etc., and the manufacturing environment needs to be higher. It is carried out in a drying room.

Therefore, the realization of solid-state battery industrialization fundamentally depends on the breakthrough of material technology, including key materials, pole pieces, positive and negative electrodes and electrolyte matching material technology. At present, the interface resistance is reduced, metal lithium is high in capacity and high. Solutions for rate and low volume changes, as well as mature preparation techniques for solid electrolyte membranes with both ion conductance and mechanical properties, lack effective solutions.

Therefore, the industrial application of high-energy-density all-solid-state batteries is expected to be gradually realized in three stages of semi-solid batteries, solid-state batteries and all-solid-state batteries, which is expected to take 5 to 10 years.