In commercial lithium batteries, polyolefin separators with poor dimensional stability and flammable and leaky organic electrolytes are important causes of battery thermal runaway. Modifying them is the most direct way to improve battery safety. The Solid State Energy System Technology Center of the Qingdao Institute of Energy Research has developed a series of new flame-retardant and heat-resistant shrinkable separators based on its previous experience in electrolyte/additive research, including aramid separators, cellulose-based composite separators, and polyarylsulfonamide separators. and polyimide separators (NanoEnergy, 2014, 10, 277-287; J. Electrochem. Soc. 2015, 162, A834-A838; Prog. Polym. Sci., 2015, 43, 136-164); at the same time, developed Cyclotriphosphazene flame retardant additives (ethoxy pentafluoro cyclotriphosphazene, phenoxy pentafluoro cyclotriphosphazene, hexaallylamine-based Cyclophosphazene and phosphorus-based oligomers, etc.) (Adv. Energy Mater. 2018, 8, 1701398; J. Electrochem. Soc. 2021, 168050511), effectively improve the safety performance of lithium batteries. On the other hand, in trying to solve the safety hazard of electrolyte leakage, the team innovatively used the important component of 502 glue (ethyl cyanoacrylate (PECA)) as a starting point to utilize the strong nucleophilicity in the lithium-sulfur battery system. The sulfide fast ion conductor (Li6PS5Cl) was used to attack PECA to prepare in-situ polymerization large anion-regulated ether electrolyte. The electrolyte can be anchored to the polymer skeleton through hydrogen bonds, which can effectively prevent electrolyte leakage while achieving high conductivity, improve the safety of the battery, and open up new ideas in the field of electrolyte leakage prevention for lithium-sulfur batteries. (Angew. Chem. Int. Ed., 2021, 202103209).

　　Although the preparation of highly thermally stable separators and flame-retardant electrolytes can effectively delay or slow down the violent heat release behavior of batteries, it still cannot fundamentally prevent battery thermal runaway accidents. Tracing back to the source, understanding the exothermic characteristics of lithium batteries from a microscopic level and analyzing the triggers and evolution paths of thermal runaway chain exothermic reactions are important prerequisites for building a high-safety battery system. Based on a full summary of the thermal stability and thermal characteristics of battery materials, researchers from this team proposed that the thermal compatibility between battery materials (electrode materials/electrolytes/additives, etc.) is crucial to battery safety. Simply improving a certain group Poor thermal stability cannot ensure the improvement of the overall safety performance of the battery (Energy Storage Mater., 2020, 31, 72–86). In view of this, the team explored the failure mechanism of ternary high-nickel batteries at the material-battery level through in-situ/ex-situ coupling methods. They used an isotope titration-mass spectrometry online gas detection device to pioneer the use of NCM ternary battery anodes. The existence of H- ions was discovered on the side, and it was confirmed that this component and the electrolyte have poor thermal compatibility, becoming an important trigger for inducing chain exothermic reactions during the battery heating process. Moreover, through the self-designed in-situ gas shuttle test device and method for detecting thermal runaway battery materials, it was proved that H2 appearing on the negative electrode side can shuttle to the positive electrode side, thereby accelerating violent exothermic behavior and causing thermal runaway of the battery (Adv.Sci., 2021, 2100676). At the same time, in the lithium metal battery system that has attracted much attention, the team used synchrotron radiation X-ray three-dimensional imaging technology and the quenching reaction principle of fluorescent molecules and lithium metal to directly reveal the lithium metal morphology, active derivatization and battery performance during the cycle. Decay and failure are closely related (Angew.Chem.Int.Ed., 2019, 58, 5936; Mater. Today, 2020, 38, 7–9). In order to further analyze the microscopic failure mechanism of lithium metal anodes, after three years of hard work, the research team found that the failed and pulverized lithium metal anodes contained a large amount of lithium hydride (LiH) with poor conductivity induced by R-H+ in the electrolyte. Moreover, the cycle performance of practical lithium metal batteries is negatively correlated with the accumulation of LiH in the lithium metal anode. It also reveals that the generation and decomposition of LiH is a temperature-sensitive chemical balance, which affects the electrochemical performance and safety performance of the battery (Angew.Chem.Int .Ed., 2021, 60, 7770–7776).

　　Based on the above findings, the Solid State Energy System Technology Center proposed that the battery negative electrode interface layer and its derivatives have an important impact on causing battery failure and thermal runaway. How to effectively suppress the emergence of H2 and the accumulation of LiH is the key to solving battery safety issues from the intrinsic nature of materials. Based on this, researchers from the team optimized the design of the battery material system from the molecular level and developed a hydrogen-free solid electrolyte lithium battery system. This strategy is expected to fundamentally solve the problem of hydrogen production in lithium batteries, cut off the source of thermal runaway, and prepare a lithium battery system with the essential characteristics of high safety and high reliability.

　　Low temperature lithium iron phosphate battery 3.2V 20A -20℃ charging, -40℃ 3C discharge capacity ≥70%

　　In solid-state systems, how to effectively solve the problems of low conductivity and high interface impedance of solid-state electrolytes is a prerequisite for preparing high-performance solid-state batteries. Based on many years of rich experience in the development of polymer electrolytes, the Solid State Energy System Technology Center has proposed a new "rigid and flexible" in-situ polymerization strategy to build an integrated electrode/electrolyte structure and combine the polymer precursor solution ( Polyethylene glycol methyl ether acrylate) is cast into a self-supporting three-dimensional porous fast-ion ceramic (Li1.3Al0.3Ti1.7(PO4)3) framework. The composite solid electrolyte obtained through in-situ polymerization effectively improves the conductivity. Reduce the contact resistance of the solid-solid interface (Adv. Sci., 2021, 8(9), 2003887). At the same time, SeS2 was used as a pore-forming agent to prepare a self-supporting three-dimensional porous sulfide Li6PS5Cl (p-LPSCl) percolation skeleton with high electrical conductivity for the first time. The polymer precursor was cast into the self-supporting three-dimensional porous sulfide skeleton and in-situ polymerization was initiated. A composite solid electrolyte was obtained, which effectively optimized the electrode/electrolyte interface compatibility, and the assembled all-solid-state battery showed high discharge specific capacity and excellent cycle performance (Adv. Funct. Mater., 2021, 2101523).

　　Figure 1. Solid State Energy System Technology Center’s research progress on high specific energy and high safety battery systems.

　　On the other hand, in solid-state electrolyte batteries, there are interfacial electrochemical reactions and high ion migration barriers between the electrode materials and the solid-state electrolyte, which seriously restrict the improvement of the energy density, lifespan and power density of solid-state lithium batteries. To address the above problems, the Team researchers proposed innovative solutions from the aspects of interface microstructure design and multi-field coupling. For the first time, a design method for constructing a bidirectionally compatible buffer layer is proposed. Through first-principles calculations and various in-situ/ex-situ testing methods, it is proven that the NASICON structure LixZr2(PO4)3 has good electrochemical compatibility at the cathode/solid electrolyte material interface and can significantly reduce the lithium ion migration barrier. The solid-state battery prepared by this bidirectionally compatible buffer layer has high specific capacity and excellent long-term cycle stability (Adv. Energy Mater., 2021, accepted). In addition, a design strategy is proposed to couple the built-in electric field and chemical potential to regulate the interface charge distribution. Using finite element simulation and in-situ scanning transmission electron microscopy differential phase contrast imaging technology testing, it was confirmed that by constructing an epitaxially grown ferroelectric single crystal at the interface between the cathode material and the solid electrolyte, the interface charge can be induced to redistribute, thereby effectively suppressing the formation of the space charge layer and Its use to hinder lithium ion transport significantly improves the energy density and rate performance of solid-state batteries (Nat. Commun., 2020, 11, 5889). The above research has deepened the understanding of key basic scientific issues of solid-state lithium batteries, and has important guiding significance for promoting the commercialization of high-specific-energy all-solid-state batteries.

　　Based on the above research foundation and technology accumulation, the Solid State Energy System Technology Center has formed unique material system preparation methods and large-scale manufacturing technology in building high-specific energy, high-safety battery systems. Related results and technologies have applied for 6 PCT patents. , applied for more than 190 national patents and authorized more than 90, forming a high-security solid-state battery system core technology with completely independent intellectual property rights, and was selected into the 2020 "Global New Energy Vehicle Frontier Technology" to promote the core technology of automobiles He has made important contributions to key research and large-scale applications.

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