1. Continuous upgrading of energy and safety performance, outstanding advantages of solid lithium batteries
Solid state lithium battery is expected to be an important development direction of the next generation of lithium battery. Countries around the world have formulated research and development goals for lithium batteries with high energy density. The Japanese government takes the lead in proposing the goal of “the energy density of battery cells for pure electric vehicles to reach 250Wh/kg by 2020 and 500Wh/kg by 2030”. In November 2015, USABC raised the target of cell energy density from 220Wh/kg to 350Wh/kg in 2020. The technical targets set in made in China 2025 are 300Wh/kg for lithium batteries in 2020, 400Wh/kg in 2025 and 500Wh/kg in 2030
In order to achieve the goal of high energy density, all countries are actively carrying out the pioneering research of lithium sulfur battery, lithium air battery or lithium metal battery. From the current situation of continuous improvement of energy density and the progress of research and development, China’s proposed energy density requirement of 400Wh/kg in 2025 is relatively high, which is accelerating the development and application of new battery technology. At present, the energy density of all-solid lithium battery developed by some enterprises can reach 300-400wh /kg, which is expected to become an important development direction of the next generation of high-energy density power and energy storage battery technology.
Compared with traditional lithium battery, the difference of solid lithium battery lies in the solid electrolyte. All-solid lithium battery is the same as traditional lithium battery, including battery cells (positive electrode, negative electrode, electrolyte), and its working principle is the same as traditional lithium battery.
In electrolyte, solid state lithium battery with polymer, inorganic solid electrolyte replaced the traditional liquid electrolyte of lithium batteries (organic electrolyte), the current main thio – LISICON sulfide and oxide, polymer and lithium borohydride, etc as a solid electrolyte, which is the core of the difference, it is because of these differences, electrolyte salt, diaphragm and uf agent chemicals that are no longer being used, such as all solid state lithium battery structure is greatly simplified. At present, the research of electrolytes mainly focuses on the research and development of complex electrolytes with high conductivity.
In terms of the positive electrode, LiCoO2 is mainly used as the positive electrode material for all-solid lithium battery in previous studies. In addition, there are also traditional oxide positive electrode, such as LiFePO4, LiMn2O4, ternary material, etc., which can also be compatible with oxide positive electrode of higher voltage and high-capacity sulfide positive electrode. The research direction of the positive electrode focuses on reducing the interface impedance of the positive electrode and improving the performance of high rate discharge, such as in situ surface modification.
In terms of anode, In addition to graphite anode, a series of high-performance anode materials are also being developed and applied, including metal Li (li-in alloy), carbon group (such as carbon-based, silicon-based and tin-based), and oxide and other anode materials.
Solid – state lithium battery safety and high energy density performance advantages. On the basis of inheriting the advantages of traditional lithium batteries, solid lithium batteries have made great progress in terms of safety and energy density.
1) extremely high safety: compared with traditional lithium battery, the most prominent advantage of all-solid battery is safety. Liquid electrolytes are flammable and explosive, and the growth of lithium dendrites in the charging and discharging process is easy to Pierce the diaphragm, causing a short circuit in the battery, resulting in safety risks. Solid electrolyte is not flammable, no corrosion, no volatilization, there is no leakage problem, but also overcome the phenomenon of lithium dendrite, so all solid battery has high safety.
2) energy density improvement: first, the voltage platform is improved, and the battery energy density will increase. The electrochemical window of organic electrolyte is limited, which is difficult to be compatible with lithium metal anode and newly developed materials with high potential anode. However, solid electrolyte generally has a wider electrochemical window than organic electrolyte, which is conducive to further improving the energy density of battery. Second, solid electrolyte can block the growth of lithium dendrites, and the application range of materials has been greatly improved, laying a foundation for the new lithium battery technology with higher energy density space. Currently, all-solid lithium battery development can provide energy density up to 300-400wh /kg.
3) enhanced cycling performance: during charging and discharging, liquid electrolyte can have irreversible reaction with lithium ion, forming solid electrolyte interface film (SEI), which will lead to the loss of active substances and electrolytes, and reduce the coulomb efficiency. Solid electrolyte solves the problem of solid electrolyte interface film (SEI) and lithium dendrite phenomenon, and greatly improves the recycling and service life of lithium battery (for example, LIPON is an electrolyte material, and the cycling performance is excellent under ideal conditions, with about 45,000 cycles).
4) expansion of application scope: solid electrolyte Enders solid lithium batteries with features of compact structure, adjustable scale and large design flexibility. Solid batteries can be designed into thin film batteries with a thickness of only a few microns to drive micro electronic devices, and can also be made into large capacity batteries for power and energy storage fields. In addition, the inherent high and low temperature stability of solid materials provides basic guarantee for the operation of all-solid batteries in a wider temperature range (operating temperature range is about -25c to 60C).
2. Multiple technological paths coexist, accelerating the layout of global industries
Electrolyte material is the core of all solid state lithium battery technology. Electrolyte materials of all-solid lithium battery largely determine the performance parameters of solid lithium battery, such as power density, cycle stability, safety performance, high and low temperature performance and service life.
According to the solid electrolyte materials, they can be divided into polymer all-solid lithium batteries and inorganic all-solid lithium batteries. The performance of different types of electrolytes varies greatly (see table 2 below).
2.1. Polymer batteries have good high-temperature working performance and are the first to be commercialized
Polymer battery works well at high temperature and is the first to achieve small-scale industrialization. The electrolytes of all solid polymer batteries are mainly polyethylene oxide (PEO), polyacrylonitrile, polymethyl methacrylate, polyvinyl chloride, polyvinylidene fluoride, etc. Under the condition of high temperature, polymer (PEO) high ionic conductivity, can form a continuous and the anode composite ion conductive channel, and the metal lithium has high stability, at the same time, the polymer film easily, the flexible easy processing, both can be made into film type, type can also be made into large capacity, wide scope of application, so as the material performance and manufacturing process improvement, makes all solid state lithium polymer battery is also the first to realize the most easily small-scale commercial production. However, there are still limitations on the large-scale industrial development of polymer with low room temperature conductivity and low voltage. Bollore, CATL, Seeo and Qingdao institute of bioenergy and process, Chinese academy of sciences are the representatives of the development of polymer solid lithium battery.
Bollore produces all-solid secondary battery (LMP), with metal lithium as the anode material and polymer (PEO, etc.) thin film as the electrolyte. Currently, it has been applied in EV sharing service vehicle “Autolib” and small electric bus “Bluelus” in France, with a total application of more than 3,000 vehicles.
Seeo’s all-solid secondary batteries, which use dacang’s dry polymer film, provide a sample battery pack with an energy density of 130-150wh /kg, with an energy density of 300Wh/kg in 2017.
CATL has also developed rapidly in polymer field in China. At present, polymer cells with a capacity of 325 mah have been designed and manufactured, showing good high-temperature cycling performance.
In April 2017, the Qingdao institute of bioenergy and process, significant progress of the development of large capacity solid polymer lithium battery “green can I complete the deep-sea expedition,” it is understood that the energy density of more than 250 wh/kg, a 500 – cycle capacity to keep more than 80%, after several strict test conditions, such as acupuncture and extrusion keep very good safety performance. In addition, qingneng II has been successfully developed, with an energy density of up to 300Wh/kg.
2.2. Excellent performance parameters of sulfide, with great potential for development
Sulfide has good performance in performance parameters and is easy to process. The main electrolytes of all solid state batteries are thio-lisicon and LiGPS, LiSnPS, LiSiPS, etc.
Firstly, compared with polymers and oxides, the conductivity of sulfide compounds is relatively high. The conductivity at room temperature can reach 10-3~ 10-2s /cm, which is close to or even higher than that of organic electrolyte. Secondly, the electrochemical window is wide (more than 5V can be achieved) and the interface stability is relatively good after the formation of the film. Finally, the sulfide is similar to the polymer, and the sulfide is flexible and easy to process. The larger design elasticity broadens the application range of the sulfide all-solid lithium battery. The interface problem and the weak stability of sulfide ion environment are still faced. In general, sulfide has great potential for development. CATL, Toyota and other domestic and foreign enterprises have accelerated their layout.
2.3. Good cycling performance of oxide, suitable for thin film structure design
Oxide all-solid lithium battery: oxide cycle performance is good, high technical barriers, research is still in the early stages. Electrolytes of oxide all-solid battery mainly include LiPON and NASICON, among which LiPON is the most mature. When LiPON is used as electrolyte material, anode and cathode materials must be made into thin film electrode by magnetron sputtering, pulsed laser deposition, chemical vapor deposition and other methods, so as to make an all-solid lithium battery with thin-film structure.
The most outstanding oxide battery is its excellent battery multiplier performance and cycle performance, it can work at 50C, 45,000 cycles, capacity retention rate of more than 95%. Meanwhile, LiPON is stable to lithium metal, has an electrochemical window width (0~ 5.5v relative to Li+/Li), and is electrically insulated. In addition, the oxide electrolyte has high air and thermal stability, low raw material cost and easy to realize large-scale preparation in practical industrialization. However, oxide low room temperature conductivity and interface problems are the main obstacles to the development and application of oxide solid lithium batteries, which are in the early stage of research.
Oxide solid state lithium batteries are currently being developed by oak ridge national laboratory, QuantumScape, Sakti3 and the Chinese academy of sciences. Currently, solid-state batteries produced in small batches are mainly thin film batteries with amorphous LiPON as electrolyte. The interface problem of this technology is difficult to solve. Sakti3 claims that MWh thin film batteries can be assembled into kWh EV batteries by stacking and series of cells. Other companies have not yet found products that can be industrialized. At present, room temperature ionic conductivity and interface problems make it more difficult to develop pure oxide solid state batteries, which are still in the early stage of research.
In general, it has been a consensus in the industry that all-solid battery is an important development direction in the future. Global industrial giants have accelerated the layout pace, hoping to seize the first opportunity in the field of all-solid lithium battery.
1. Continuous upgrading of energy and safety performance, outstanding advantages of solid lithium batteries