One, the heart! Vehicle energy storage system

1. Convenient! The car accepts the bottom line and core needs

According to the national standard "Terms and Meaning of Automobile and Trailer Types" (GB/T3730.12001), a car is a non-track-bearing vehicle driven by power and with 4 or more wheels. Based on the demand for convenient and comfortable transportation, users' (and society's) concerns about cars involve many aspects that are easy to quantify and not easy to quantify.

Convenience of use is the bottom line and core demand of users for cars in the vast majority of cases, which can be simply expressed as the driving time and charging time of a car under a certain working condition or a combination of working conditions, as well as the corresponding driving distance.

The longer the driving distance (corresponding to the endurance)/ the faster the driving speed (corresponding to the dynamic performance), the shorter the single charge time/the smaller the proportion of the charge time in the total time (corresponding to the charge capacity), the higher the convenience of the vehicle can be considered. Under the condition of inconvenient charging, the longer the driving distance of a single charging, the higher the convenience of using the whole vehicle.

2. Energy storage! The soil of a hundred year science and technology tree

The essential physical and chemical properties of the energy storage (and supporting power) system are the decisive factors of the vehicle's range, power performance and charging time. This has been confirmed by the evolution of the automobile for more than a century.

Fuel cars and electric cars take off at similar times. The former is marked by Daimler, Benz et al. 's invention of the internal combustion engine and its use in cars, while the latter is marked by Truff' s use in cars of power lithium-ion batteries (lead-acid batteries).

Compared with earlier petrol-powered cars, electric cars have many advantages: low environmental impact, smooth driving, almost no noise, simple operation, and the first power performance to achieve a top speed of more than 100km/h. At the beginning of the 20th century, electric cars in the United States rivaled petrol-powered cars in market share.

However, the energy density and charging time of lead-acid batteries are substantially different from those of fuel oil, and they are more dependent on infrastructure, which means that the potential for ease of use of electric vehicles at that time is far less than that of fuel oil vehicles. With the rapid rise of fuel output, the perfect layout of gas stations and highways, the invention of multi-cylinder high-speed internal combustion engines, and the application of air compressors, fuel passenger vehicles not only have a long range and a fast refueling speed, but also significantly reduce the unit energy cost, improve power performance, and significantly increase energy efficiency. In addition, the invention of the assembly line has greatly reduced the manufacturing cost and gradually improved the supporting infrastructure. For nearly a century, oil-powered vehicles have fully enjoyed the convenience dividend of the high energy density of fuel. The corresponding industries have formed a powerful path lock, which has greatly suppressed the development of electric vehicles.

It can be seen that more advanced power lithium batteries are the key for electric vehicles to attack fuel vehicles no matter they are trying to recover under strong off-grid conditions (energy density - range priority) or strong grid-connection conditions (charging time priority).

Two, look forward to! High performance pure electric vehicles

1. Battery! From lead acid to lithium ions

Typical components of power lithium battery include active materials such as positive electrode and negative electrode, electrolyte (liquid/solid), or auxiliary components such as diaphragm; Used at normal temperature or slightly deviate from normal temperature environment; Theoretically, there is no material exchange with the outside of the battery, and the corresponding chemical energy is released by the REDOX reaction of the electrode. Electrochemically rechargeable capability is required (for secondary batteries, as distinct from primary batteries).

Prior to the commercialization of lithium-ion batteries, lead-acid batteries and nickel-based batteries (such as nickel-cadmium and nickel-metal hydride) are important alternatives for secondary batteries. But from the end of the 20th century to the beginning of the 21st century, the lithium ion battery made of lithium cobalt acid, lithium manganese acid, lithium iron phosphate and lithium polymetallic acid as the positive electrode, graphite as the negative electrode, with electrolyte (electrolyte) and diaphragm (also known as liquid lithium ion battery because of the use of electrolyte) has greatly exceeded the performance of the original secondary battery. This, on the one hand, makes the NiMH battery out of lithium-ion battery become the standard configuration of 3C battery; on the other hand, it also means that lithium-ion battery can be used as the core energy storage device of power lithium-ion battery vehicles, providing an unrealized range of more than 200km and a shorter charge time to meet the basic use needs of passenger cars and commercial vehicles.

In lithium-ion batteries, the capacity of active substances with different positive and negative electrodes and the voltage to lithium are different, and the gram capacity of auxiliary components is different. Multiple factors jointly affect the energy density of the battery (measured in Wh/kg):

In supporting the charging infrastructure to power and popularity is not easy to happen overnight, the vehicle range anxiety long-standing driven by actual demand, high energy density batteries for electric cars must improve range, increase competitiveness, ternary nickel cobalt manganese cobalt/nickel anode aluminum material in combination with graphite/part of the silicon carbon negative, supplemented by supporting electrolyte, lithium ion battery become mainstream choice of the diaphragm.

Recovery! Pure electric passenger cars lead the way

Driven by the combination of power lithium battery technology progress and policies, new energy vehicles have made great progress both at home and abroad in terms of sales volume and quality.

New energy vehicle subsidies and double points policy are important factors to promote the development of the industry. Operating range, battery system energy density and vehicle quality - 100 km power consumption together determine the pure electric passenger vehicle subsidies. In the process of slope retreat, the threshold requirements for key parameters such as energy density of battery system and power consumption per 100 km of vehicle are gradually increased.

The double points policy is a long-term support policy in the form of quota after subsidy retreat. In terms of points setting, pure electric passenger car points are positively correlated with range, plug-in hybrid passenger car points are fixed, fuel-powered battery passenger car points are positively correlated with system rated power.

By the end of 2018, the sales volume of new energy vehicles in China had broken through the 1.2 million mark, and the ownership volume reached more than 2.6 million. Pure electric passenger car is an important part of it.

Inside the pure electric passenger car, the model structure has been adjusted to a considerable extent. After the subsidy transition period in June 2018, the monthly market share of A00 class vehicles decreased from more than 2/3 during the subsidy transition period in 2017 and 2018 to about 1/3 to 50%, and the annual market share decreased to less than 50%, while the proportion of A0 and A-class vehicles rose. In addition to the subsidy policy changes, the product mix is also affected by battery production, the supply and demand of specific models.

In terms of technical level, the range of the vehicle is increased, the energy density of the battery system is increased, and the power consumption of 100 kilometers is reduced. From the point of view of models in the recommended catalog of new energy vehicles of the Ministry of Industry and Information Technology, a total of 124 models in 2018 had a working condition range of more than 400km, far more than 17 models in 2017, and subsequent models with a working condition range of more than 500km also began to emerge. The energy density of the battery system of 53 models has exceeded 160Wh/kg, which was not achieved by any vehicle in 2017. In 2019, the highest energy density of the battery system has reached 182Wh/kg.

Among the models with A driving range of more than 500km, BYD Qin Pro (2019 model, or EV600) and GAC Aions are all A-class cars, and the selling price is controlled within 200,000 yuan after subsidies.

New energy vehicles are also supported to varying degrees abroad. In America, for example, the United States in 2008 passed the "energy independence and security act (EnergyImprovementandExtensionAct), Article 30 d of them specifically for new energy vehicles (Newqualifiedplug - inelectricdrivemotorvehicles) a special tax deduction. The provision by the American recovery and reinvestment act of 2009 (TheAmericanRecoveryandReinvestmentAct) and the American taxpayer bailouts act of 2013 (AmericanTaxpayerReliefAct, ATRA) revised perform today. According to the Act, American taxpayers can enjoy the corresponding tax rebate for newly purchased and qualified plug-in hybrid electric vehicles and pure electric vehicles after December 31, 2009. Specific calculation method of refund amount: starting from 5kWh of vehicle power lithium battery capacity, corresponding to $2,500; for the part greater than 5kWh, a progressive subsidy of $417 /kWh will be calculated, with a ceiling of $7,500. At the same time, the law, for a manufacturer, a quarterly statistics, when the cumulative sales of 200000 units in the United States, namely the trigger mechanism of subsidy TuiPo: starting from the standard after the second quarter, in the first and second quarter subsidies by half, the third and fourth quarter halved again, since then no longer enjoy subsidies.

In terms of products, the comprehensive performance and driving experience of Tesla Model3 have conquered many users. The range of the working condition is close to 600km, and the power consumption of 100km is about 12.5kWh, which basically represents the highest technical level of pure electric passenger cars at present. The Model3 was rated the most satisfactory car by ConsumerReports. Ranking as the top seller in California in the second half of 2018 also confirms the high performance pure electric passenger car's mass production and delivery capability. Tesla has already hit the subsidy adjustment conditions, starting a subsidy halving period in early 2019.

Based on the relevant progress at home and abroad, it can be concluded that pure electric passenger vehicles have initially had the ability to challenge fuel passenger vehicles from the aspects of technology, sales volume and vehicle structure completeness.

Bottleneck! Safety and energy density trade-offs

However, in the process of continuous progress in the use performance of power lithium batteries, continuous improvement in the vehicle operating range, effective control of power consumption in 100 kilometers, and gradual improvement in comprehensive performance, the safety problem of pure electric passenger vehicles always exists. Even Tesla, the star electric passenger car company, has had a number of safety incidents.

The occurrence of safety accidents is related to the behavior of the driver and the intrinsic safety of the vehicle. Power lithium battery itself is the core factor affecting the safety of pure electric vehicles.

As mentioned above, the power lithium battery consists of two components: active substance and auxiliary component. Active substances need to be directly used for energy storage, there are certain safety risks can not be prevented, and there is no possibility of substantial reduction; In theory, auxiliary components should only be used for auxiliary purposes, but liquid phase and solid phase composites such as electrolyte and diaphragm actually store more chemical energy inherently and are highly unstable, which have key negative uses for the triggering, expansion and eventual runaway of safety accidents.

The industry believes that the power lithium battery internal short circuit often means thermal runaway, making the battery safety accident.

Mechanical abuse, thermal abuse, and electrical abuse of the battery may lead to diaphragm failure, battery internal (positive, positive collector) and (negative, negative collector) short circuit, a large amount of heat release and ignites the electrode, electrolyte and diaphragm, causing irreparable battery thermal runaway. In this process, the lack of physical strength, thermal and chemical stability of the diaphragm is an important reason for the accident.

In the absence of an internal short circuit, the battery may also have an accident. The results show that the thermal runaway temperature (231 ℃) of the battery is lower than the failure temperature (257 ℃) of the membrane. The mechanism of the accident is that the continuous reaction between the cathode and the solvent in the electrolyte and the decomposition of lithium hexafluorophosphate in the electrolyte lead to the early temperature rise and performance degradation of the battery. The positive electrode and the electrolyte react to release oxygen at a higher temperature. After the metal ions corresponding to the positive electrode diffuse to the negative electrode, a large number of reactions generate heat, resulting in thermal runaway accidents. In other words, mild thermal abuse may cause the stability of the cathode - electrolyte - cathode system to be damaged; The lack of chemical stability of electrolytic liquefaction is an important driving force for safety accidents.

While the power lithium battery has made significant technological progress and the endurance of the corresponding model has been greatly improved, the chemical activity of the electrode material has been enhanced synchronously and the stability has gradually deteriorated. In order to meet the safety requirements, the modification complexity of all kinds of basic materials has been increasing continuously. The mass/volume ratio of auxiliary components decreases. We believe that under the traditional battery material system, it is increasingly difficult to balance the two goals of improving energy density and maintaining safety. The operating range of the corresponding models is also difficult to further improve significantly.

We also believe that under the same conditions, the safety of pure electric passenger cars using liquid lithium-ion power lithium batteries is difficult to reach the level of existing fuel passenger cars. At the same time, the energy density and safety of power lithium battery are greatly optimized, and the final way to make the whole vehicle products have a stronger competitiveness is to innovate the existing power lithium battery material system.

Three, solid! Lithium ion battery + steel steel bone

1. Hope! In the name of solid

Unlike liquid lithium-ion batteries, solid-state lithium-ion batteries replace the diaphragm and electrolyte with a solid electrolyte. Its biggest potential advantages are its high safety and high energy density.

In terms of safety, solid-state lithium ion batteries do not have the problem of gas generation in the reaction of electrolyte-electrode materials. Overcharging makes lithium metal deposit in the negative electrode and cause the possibility of puncture short circuit is low. The high temperature resistance of solid electrolytes is far superior to that of current electrolyte-diaphragm, and these properties make solid lithium-ion batteries far safer than liquid lithium-ion batteries.

In terms of energy density, the energy density of solid lithium-ion battery in the conventional positive and negative electrode system is similar to that of liquid lithium-ion battery. However, the compatibility potential of solid-state lithium-ion batteries for high-capacity and high-voltage electrode systems (lithium metal/alloy negative electrode, sulfur positive electrode, etc.) may be greater, which makes solid-state lithium-ion batteries expected to become the practical technical carrier of high-energy density batteries (350Wh/kg or above).

In addition, solid-state lithium-ion batteries do not contain electrolytes, and the battery post-treatment process can be greatly simplified. Based on the above advantages, the basic research of solid-state lithium ion batteries has been continuously promoted, and the ultimate industrial application, especially for vehicle use, has gradually become clear.

The road! Solid electrolyte

, in a conventional liquid lithium ion battery electrolyte - diaphragm system plays in the work is the basic purpose of WenYu lithium ion conductivity, electrical insulation, electrode infiltration/into, and prevent the electrode in direct contact, reflects the important effect in the energy storage system is to keep high energy efficiency in the process of charging and discharging, and does not constitute a power board. Although solid electrolytes have the potential to improve battery safety and energy density, they must first fulfill the basic purpose of conventional electrolytes, which is to have high ionic conductivity in the operating temperature range.

Based on the higher conductivity of lithium ion in the range from room temperature to slightly higher temperature, three types of organic polymers, oxides and sulfides are preliminarily formed in solid electrolyte material system. The lithium ion conductivity of solid electrolyte at room temperature should be at least 1/100 higher than that of conventional electrolyte.

In the case of suitable conventional lithium ion conductance, the solid electrolyte also has to solve the problem of high impedance between electrolyte and electrode interface. Corresponding methods include buffer layer coating, second phase doping modification, element substitution and so on.

In the end, the lithium ion conductivity, electronic insulation performance, is the cathode material compatibility, density, thickness, strength, interfacial impedance, raw material availability (the cost of raw materials), the manufacturing process (manufacturing costs), environmental impact (post-processing cost), and other technical parameters of the solid electrolyte material system can be a complete comprehensive evaluation.

Solid electrolyte: Organic polymer system

The electrolyte and separator used in conventional liquid lithium-ion battery are mainly organic, so the organic polymers also belonging to organic matter are the natural selection of solid electrolyte matrix. Organic polymer solid electrolyte system includes poly (ethylene oxide) (PEO) and polymers (poly (propylene oxide), poly (vinylidene chloride), poly (vinylidene fluoride), etc.

Poly (ethylene oxide) has become the mainstream choice of organic polymer solid electrolyte due to its good compatibility with lithium anode. In view of the fact that polyvinyl oxide does not inherently contain lithium, the lithium salt mentioned above should be doped first; The mechanism of lithium conduction is the induction of lithium ions by ether-oxygen bond/other atoms with high electronegativity, and the subsequent movement of lithium-rich chain segments in the amorphous region to realize the near-neighbor transfer of lithium ions. The final effect is that lithium ions enter from one side of the polymer layer and exit from the other side to realize the charge and discharge transport of lithium ions. The higher the crystallinity of polyvinyl oxide doped with lithium salt is, the higher its strength is, but the lower the conductivity of lithium ion is. Therefore, inorganic particle doping, polymer grafting, copolymerization, cross-linking modification and other methods to reduce the moderate crystallinity are also widely used by researchers. Up to now, the lithium ion conductance of polyvinyl oxide solid electrolyte at slightly higher temperatures has been accepted for practical use, and its low density, low interfacial impedance, and easy to thin lamination and mechanical processing.

However, the polyvinyl oxide solid electrolyte doped with lithium salt has poor resistance to high voltage and can be oxidized by ternary materials with conventional voltage, which restricts the choice of cathode materials and greatly limits the energy density of the final battery. In addition, the strength of polyvinyl oxide is relatively low, and its resistance to puncture short circuit is weaker than that of other solid electrolyte systems.

Solid electrolyte: Oxide system

The solid electrolyte of the oxide system mainly includes lithium lanthanum titanium oxide (LLTO) with perovskite structure, lithium lanthanum zirconium oxide (LLZO) with garnet structure, fast ion conductors (LISICON, NASICON) and so on. The lithium conduction mechanism is mainly that the material forms a stable lithium ion transport channel at the micro level. The greatest advantages of oxide solid electrolytes are derived from the intrinsic properties of inorganic oxides: high mechanical strength, high physical and chemical stability, strong pressure resistance, and low manufacturing complexity. At the same time, after partial element doping, the lithium ion conductance of the oxide solid electrolyte at slightly higher temperature (e.g., 80oC) can also be accepted in practice.

The deficiency of oxide solid electrolyte is also due to its intrinsic properties of inorganic oxides. For the electrode-electrolyte interface, poor interface contact ability and poor interface stability in the cycle process lead to the rapid increase of interface impedance in the cycle process, the insufficient effective capacity of positive and negative electrodes, and the rapid decay of battery life. Thin lamination is also difficult. Therefore, it is necessary to add some polymer components to the oxide solid electrolyte and combine with trace ionic liquid/high-performance lithium salt-electrolyte, or to manufacture the quasi solid battery by means of assisted in-situ polymerization, in order to retain some safety advantages and improve the interface contact between electrolyte and electrode.

Solid electrolyte: Sulphide system

The solid electrolyte of the sulfide system can be considered as a composite material composed of lithium sulfide, germanium, phosphorus, silicon, titanium, aluminum, tin and other sulfide elements, and the material phase covers both crystalline and amorphous states. The large ion radius of sulfur makes the lithium ion transport channel larger. The electronegativity is also suitable, so the lithium ion conductance of sulfide solid electrolyte is the best among all solid electrolytes, and the lithium ion conductance of Li-Ge-P-S system at room temperature can be directly compared with that of electrolyte. In addition, the mechanical strength of sulfide solid electrolyte is higher, and its compatibility with high capacity sulfur cathode is the best.

The important disadvantages of sulfide solid electrolyte include: the electronegativity of sulfur is lower than that of oxygen, which makes part of the electrolyte layer poor in lithium and increases the interfacial resistance when coupled with high voltage positive electrode; The impedance of SEI film with lithium electrode is also high. Sulfides are inorganic non-metallic particles, and there is a relatively serious deterioration of the electrolyte-electrode interface during the cycling process. In addition, the material system is very sensitive to water and oxygen, which is also flammable in the event of an accident. Thin lamination is also difficult. This makes its manufacturing process very demanding.

To sum up, different solid electrolyte materials have different properties and advantages and disadvantages, and no solid electrolyte with excellent comprehensive properties has been found. Precise control of composition and structure across basic types of materials may be the key to breakthroughs.

The thorns! Science, engineering and commercial reality

Based on the research and development status of solid electrolyte, it can be found that solid lithium-ion batteries have some key explicit/potential advantages, but there are still some important problems to be solved.

In terms of material system science, many technical indicators bring about complex and diverse requirements. The intrinsic properties of electrodes and electrolytes and interfacial interactions under different service conditions must be taken into account, which makes the study of solid-state lithium-ion batteries a real top composite system engineering. The energy storage, mass transfer cycle and ultimate failure mechanism of solid lithium ion battery materials system need a lot of scientific explanation. The acquisition of basic comprehensive performance and the choice of superior performance are very high for experiment and simulation calculation. The precise and ordered coupling of structure and function of electrolyte and electrode materials is challenging to achieve even at the laboratory level.

In terms of engineering practice and commercialization, consumers' high performance requirements for solid-state lithium-ion batteries and the urgent need for cost reduction make the boundary constraints for industry development very strong. The existing liquid lithium-ion battery material system research, battery material preparation, battery monomer production technology and supporting equipment production have gradually become mature, the vehicle level application has also been verified by a lot of different models of practice; However, the uncertainty of solid lithium-ion battery material system also brings the uncertainty of the process route, and the compatibility with the existing equipment system is still difficult to determine, and the final cost per kilowatt hour after large-scale is also difficult to effectively estimate. Therefore, the engineering practice and commercialization of solid-state lithium ion batteries are also full of uncertainties.

Four, must contend! Solid state lithium strategy

1. Planning! Long-term commanding height

Although still in its infancy, the tantalizing prospect of solid-state lithium has prompted the world's major economies to take long-term plans to promote technological progress and industrial development.

The U.S. support program for solid-state lithium battery is Battery500, which is supported by the U.S. Department of Energy, led by Northwest National Laboratory, and supported by a number of universities and companies as consultants or support. The specific path is to reduce auxiliary components such as electrolyte, increase the proportion and capacity of active substances and reduce the cost, and finally realize the economic and practical application of high-performance solid lithium battery: the energy density of the monomer will reach 275Wh/kg in 2023.

Japan's support plan for solid-state lithium battery is Rising-I, Rising-II, Solid-EV, etc., and many car companies, colleges and research institutions have joined the plan. The purpose of development in Japan is core technology oriented to mass production, standard setting and technology evaluation. The development idea is to start from the existing battery materials, optimize the battery structure and reduce auxiliary components, and simultaneously carry out solid-state replacement. While solving safety problems, the proportion and capacity of active substances are increased at the same time, so as to finally achieve the phased goal of solid-state lithium battery: the energy density of the single cell will exceed 300Wh/kg in 2025 and reach 400Wh/kg in 2030.

China's supporting policies for solid lithium battery are distributed in many top-level designs.

The Medium and Long Term Development Plan of the Automobile Industry requires the implementation of power lithium battery upgrade project. Give full play to the use of platforms such as Power Lithium Battery Innovation Center and Power Lithium Battery Industry Innovation Alliance, carry out joint research on key materials, single batteries, battery management system and other technologies of power Lithium Battery, and accelerate the revolutionary breakthrough of power Lithium Battery.

"Energy Saving and New Energy Vehicle Technology Roadmap" has a detailed description of solid lithium battery material system, interface problems, etc. The technical goal is to achieve 300Wh/kg energy density in 2020 and 400Wh/kg in 2025. The monomer capacity and group technology are simultaneously researched and developed, which will be promoted in the future

Exploit! A thousand miles travel one step

As an innovative field, the number of patents related to solid-state lithium battery is an important reference index of technical strength.

In terms of international patents, Japan, the United States, Germany, South Korea and other countries apply more; In terms of companies/organizations, Toyota filed the most, and carried out a multi-country patent layout.

In terms of domestic patents, research institutes and companies are involved, and the number of patents of research institutes is dominant. From the number of patent applications of international car companies such as Toyota and Hyundai and independent companies such as BYD, we can see that car companies attach great importance to solid-state lithium battery.

Globally, companies engaged in solid-state lithium battery R&D are mainly located in North America, Europe and East Asia. The technical route also covers polymers, oxides, sulfides and composite material systems.

Toyota hopes to achieve solid lithium-ion batteries by developing a sulphide/composite electrolyte system and corresponding batteries, with a planned industrialization date of 2020.

The company that took the lead in realizing the use of solid lithium tram is Bollore Group of France. However, its material system (lithium iron phosphate + polyvinyl oxide) limits the energy density of the battery system, making the vehicle's range less than 200km.

Solid state lithium technology is highly innovative, which means that startups with strong academic background and independent innovation ability are also likely to make a big difference. For example, the core team of SolidEnergy, which originated from MIT, hopes to eventually realize the commercial application of lithium cobaltate - polymer & ionic liquid - lithium metal solid lithium ion battery; The core team is from Tsinghua University School of Materials, Qingtao development and development of oxid-based solid state lithium ion batteries and related equipment; The core team is from the Institute of Physics, Chinese Academy of Sciences Veilan New Energy research and development of in-situ curing polymer based solid state lithium ion battery.