In the last article, we introduced the origin of lithium batteries, and also introduced the development trend of positive and negative electrode materials. This issue will talk more about revolutionary all-solid-state battery technology, and will also focus on the evolution of battery structures beyond the material system.



1, looking forward to the all-solid era



In 1993, Guyomard and Tarascon proposed the EC/DMC/LiPF6 electrolyte system, which is still the standard formula for electrolytes. In practice, multiple solvents are usually mixed in proportion to meet multi-dimensional performance requirements, for example, formulations such as PC+DEC or EC+DMC are usually used to balance viscosity and ionic conductance. Subsequently, a rich additive system has been developed to improve battery performance, including improving SEI film, improving ion conductance, internal overcharge protection, flame retardant, and so on.



Electrolyte material system



Improving the safety of electrolytes is essential for batteries. Traditional liquid organic electrolytes have inherent safety risks. When abnormal conditions such as overcharge or short circuit occur, the electrolyte is prone to thermal runaway, resulting in spontaneous combustion or explosion. Solid electrolytes have better intrinsic safety and can also be better compatible with metal lithium anode materials, which has been the ultimate battery material pursued by academia and industry.



Electrolyte is the key to the thermal runaway of lithium batteries



As a result, solid-state batteries have become a high ground for governments and companies to chase. Governments attach great importance to it, such as Japan's national efforts to develop solid-state batteries, intending to achieve cornering overtaking on solid-state batteries. According to incomplete statistics, more than 50 startups and academic institutions worldwide are committed to the research and development of solid-state batteries, and startups in the field of solid-state batteries have also become investment hotspots, and large amounts of financing have continued.



Solid-state battery industry policies and start-ups by country



Solid electrolyte materials have been related to the development of systems since 1950, in recent years, solid electrolyte academic research is very active, papers emerge in an endless stream, the current formation of four mainstream systems: polymer, film, sulfide and oxide systems.



Development status of solid electrolyte



In 2012, Bollore commercialized polymer all-solid-state batteries in France. This polymer all-solid-state battery dissolves LiFSI lithium salts in polyethylene oxide PEO for high safety and long cycle life. However, because the oxidation potential of PEO is only 3.8V, it can only match low-voltage cathode materials such as lithium iron phosphate, so the energy density of the cell is only 220Wh/kg. PEO needs to have a high ionic conductivity at 60-85 ° C, so it needs to be equipped with a heating device to work normally, the energy density at the system level is only 110-130Wh/kg, the safety of the polymer itself is not as good as the thermal stability of sulfide and oxide, and the phenomenon of fire and combustion will occur at high temperatures. Because the performance is not outstanding, the current research on polymer solid-state batteries is relatively silent.



In the 1990s, American researchers prepared lithium phosphorus oxygen nitrogen (LiPON) electrolyte film by magnetron sputtering at Oak Ridge National Laboratory. LiPON film solid-state battery has good safety and long cycle life. However, LiPON film is essentially a glassy metal oxide, the material is easy to crack, can not be made into a multi-layer cell, the monomer cell capacity is small, the preparation process is complex, the cost is high, there is no good mass production prospects.



Sulfide materials are the systems with the highest ionic conductivity at room temperature, so they have received extensive attention and research, mainly including glass-phase and glass-ceramic phase materials (Li2S-P2S5, etc.), thio-LISICON, silver germanium sulfide (LPSCl, etc.), LGPS series and layered series. Among them, LGPS (lithium Germanium phosphorus sulfur), which was first reported in 2011, has an ultra-high ionic conductivity of up to 12mS/cm at room temperature, even exceeding some organic electrolytes. Sulfide has high thermal stability, good safety, and has a wide electrochemical stability window, which can be well matched with positive and negative electrode materials. However, the fatal disadvantage of sulfide is that it is easy to react with air and water to produce highly toxic hydrogen sulfide gas, which makes the preparation conditions of sulfide all-solid-state batteries extremely harsh and costly. In 2018, Japan NEDO united with dozens of enterprises and scientific research institutions, the nation's effort to overcome the technical problems of sulfur, including Toyota, Nissan, Honda and other automakers, Panasonic, Hitachi and other battery companies, Mitsui metal and other chemical companies and dozens of scientific research institutions, has now built a ten-ton production line, It is expected that the mass production of sulphide all-solid-state batteries can be achieved in 2025. China's research in this area started late, but there are a number of domestic scientific research teams and startup companies committed to tackling the sulfide material system. If breakthroughs in interface compatibility and preparation processes can be achieved in the next five to ten years, sulfides are the most likely all-solid-state battery technology to be commercialized.



Japan has completed the prototype preparation and testing of sulfide all-solid-state batteries



Oxide materials are very rich in basic scientific research, mainly including garnet structure (LLZO), NASICON structure (LAGP and LATP), perovskite structure (LLTO) and LISICON structure (LZGO). The oxide is essentially a ceramic material with excellent electrochemical and mechanical stability, but the ceramic particles are very hard and the material is easy to be brittle, so it is difficult to prepare a large area or multi-layer cell. In addition, there is a serious solid-state contact problem between ceramic particles and positive and negative electrodes, and the interface conductor-lithium performance is very poor, so the oxide solid-state battery is theoretically difficult to achieve, and the industry is more inclined to solid-liquid mixed semi-solid technology. The advantage of the semi-solid battery is to use the electrolyte to infiltrate the gap of ceramic particles to form a complete guide lithium channel, and the electrolyte content is greatly reduced, and the safety of the battery will be greatly improved.



The liquid content of the semi-solid battery will be gradually reduced



From a realistic point of view, the progressive development path may be the most feasible way to promote the industrialization of solid electrolytes. Semi-solid batteries can use mature liquid lithium battery production lines to achieve large-scale mass production in the short to medium term. For example, the ET7 long-life version listed by NIO this year will use the semi-solid state battery of Wei Blue, and the Dongfeng E70 demonstration operation vehicle also uses the semi-solid state battery of Ganfeng lithium battery. As solid-state electrolyte technology matures, the electrolyte content will gradually decrease until it eventually changes to an all-solid-state system.



There are currently production models equipped with semi-solid state batteries



Solid electrolyte to achieve real industrial resistance and resin. In the future, scientific research breakthroughs in solid electrolytes and metal lithium are expected, and the realization of solid state batteries will lay a solid foundation for higher specific energy lithium-sulfur and lithium-air batteries, setting off a new round of battery revolution.