All-solid-state batteries have made major breakthroughs in stages


Wang Renmei


In the field of scientific research, the development of solid-state batteries focuses on the three major technical routes of sulfides, oxides and polymers, and the industrialization of each route is full of challenges. So scientific institutions around the world are concentrating their efforts on breakthroughs. Among them, the sulfide solid electrolyte is favored by many researchers because of its excellent conductivity characteristics and suitable hardness and softness. However, although sulfide solid-state batteries show significant advantages in laboratory-level research, there are still many difficulties to overcome on the engineering road.


Recently, Cui Guanglei's team at the Qingdao Institute of Bioenergy and Process of the Chinese Academy of Sciences has made important scientific research achievements in the field of sulfide solid-state batteries, which marks a major breakthrough in the development of solid-state battery technology in China.


Cui Guanglei said in an interview with a reporter from China Automotive News that the research and development team has made a significant breakthrough in scientific research and successfully designed a homogenized zero-strain positive electrode material, which effectively solves the problems faced by sulfide solid-state batteries in materials. At the same time, the team also used melt bonding technology to prepare an ultra-thin sulfide solid electrolyte film with excellent flexibility through the dry preparation process, breaking through a major technical bottleneck in the engineering application of sulfide solid batteries.


01 Interface stability defects greatly limit the practical application of sulfide solid state batteries



Industry experts generally hold a consensus on solid-state batteries, which have significant safety performance, high energy density, excellent cycle performance, and a wide range of applications. At the current stage, although sulfide, oxide and polymer solid-state batteries have reached the expected standards in some performance indicators, there are still key technical problems to be broken through.


Zhang Yongzhu, a professor at Central South University, pointed out in an interview with reporters that the interface impedance problem has always been a major challenge in the field of solid-state batteries. Because of its excellent contact, the sulfide makes the overall ionic conductivity performance excellent. In the current human development of solid-state battery materials, sulfide is the only material that can exceed the level of ionic conductivity of liquid electrolyte. However, despite the excellent performance of sulfide materials in terms of electrical conductivity, its deadly shortcomings have become an obstacle to its engineering application.


Because of its high chemical activity, sulfide is easy to produce violent chemical reactions with air, organic solvents and positive and negative active materials. Especially in contact with water, it will quickly produce toxic and pungent odor H₂S, which poses a serious threat to human health. The high chemical activity of sulfide leads to its interface stability is extremely low, although it has good electrical conductivity, but the lack of interface stability will make its performance rapidly decay, which brings great challenges to production, transportation and processing. The serious defects in the interfacial stability of sulfide greatly limit its widespread application in practical applications.


"Sulfides are extremely environmentally demanding, and the specific conditions of the laboratory environment are not directly applicable to everyday life." Cui Guanglei clearly pointed out.


02 Homogenized zero strain positive electrode strategy provides a new idea


It is not feasible to rely solely on sulfide materials to make solid-state batteries. Faced with the challenge, many researchers have tried to limit its activity through other methods or materials, but it is very difficult to find a suitable technical path or material. Cui Guanglei's team not only successfully found suitable materials, but also explored effective technical methods.


₇₅Ti₂(Ge₀.₂₅P₀.₇₅S₃.₈Se₀.₂)₃ A combination of high ionic conductivity (0.2 mS cm⁻¹), high electronic conductivity (225 mS cm⁻¹), and high specific capacity (250 mA h g⁻¹). Its ionic and electronic conductivity is 3 to 5 orders of magnitude higher than that of traditional layered oxide cathode materials, and the specific capacity is higher than that of current high nickel cathode materials. The positive construction of this material does not require additional introduction of heterogeneous conductive additives, and fundamentally solves the problem of electric-chemical force mismatch between heterogeneous components.


In the process of charging and discharging, the volume of solid state batteries will change, and expansion and contraction will easily lead to crystal fragmentation. The new materials and new methods created by Cui Guanglei's team also have a better solution for this. According to reports, the material only occurs 1.2% volume deformation during the charge and discharge process, which is 50% lower than the traditional layered oxide cathode material, and matches the energy level of the sulfide electrolyte, thus giving the all-solid-state battery excellent charge and discharge reversibility.


"Our proposed positive electrode homogenization strategy can overcome the bottleneck problem of further improvement in energy storage devices in terms of energy, power and life, and pave the way for practical and commercial devices." Cui Guanglei said. The homogenized zero-strain positive electrode material designed by the team has subvert the current paradigm that the positive electrode of all-solid-state lithium batteries requires composite ions and electronic conductors, and fundamentally solves the bottleneck problem of electric-chemo-force coupling failure caused by the positive electrode composite, and further produces an all-solid-state lithium battery with high energy density, high power density, long cycle life and wide operating temperature. The homogenized zero-strain positive electrode strategy provides a new solution for the further breakthrough of all-solid-state battery performance.


03 Great progress has been made in engineering applications


New technologies and new materials must be engineered before they can be commercialized, otherwise they can only stay in the laboratory stage. In fact, as early as more than 10 years ago, Japan's Toyota Motor research and development personnel found the characteristics of sulfide materials, but has not been able to engineering, resulting in Toyota Motor announced the launch of electric vehicles repeatedly delayed.


Cui Guanglei's team has also made significant progress in the engineering of sulfide all-solid-state batteries. The team has used melt bonding technology to prepare an ultra-thin sulfide solid electrolyte film with excellent flexibility by dry method. Its excellent mechanical properties, ionic conductivity and stress dissipation characteristics can effectively inhibit the mechanical force failure caused by uneven internal stress of the battery. "The preparation of polymer/sulfide composite thin-layered electrolytes is one of the most critical technologies to significantly increase the energy density and mass production of these batteries." Cui Guanglei said.


At present, the dry preparation process had the problem of uneven dispersion of components. Cui's team proposed a melt bonding strategy for low pressure preparation. The researchers premitized low viscosity thermoplastic polyamide (TPA) and sulfide Li₆PS₅Cl in viscous flow, then formed them hot. The ultra-thin sulfide solid electrolyte film with excellent flexibility, thermoplasticity, bend-ability, stretchability and high ionic conductivity was prepared.


"We used synchrotron radiation X-ray tomography (SX-CT) to observe the symmetric battery after the cycle, and found that the ultra-thin film can effectively inhibit the interface separation and electrolyte fragmentation caused by the expansion of the electrode volume during the cycle, and maintain the interface stability. This proves that building a complete polymer percolation network inside the solid electrolyte not only facilitates thin layering, but also dissipates the non-uniform internal stress during battery operation and reduces the risk of mechanical failure." Cui Guanglei said.


In the engineering process, Cui Guanglei's research team used pure silicon negative electrode to prepare an integrated all-solid-state battery based on the interface fusion strategy of the developed new homogeneous positive electrode material and thin layer electrolyte. According to reports, the energy density of the battery exceeds 390 Wh/kg (660 Wh/L), the capacity retention rate is greater than 80% after 4000 charge and discharge cycles, and the service life can exceed 10,000 hours. After testing, the high voltage Bipolar and soft pack batteries prepared by this strategy have high practical value and industrialization potential, which is of great significance for the commercialization of sulfide all-solid-state batteries, and can provide a strong reference for the future scientific research and process technology development of all-solid-state batteries.


China Automotive News, July 8, 2024