Electrolyte Design And Electrochemical Performance Research For Wide Temperature And High-power Sodium/Lithium Batteries

Sodium/lithium-ion batteries have become important electrochemical energy storage technologies due to their high energy conversion efficiency,energy density,cycle life,and other advantages.Sodium-ion batteries（SIBs）have advantages over lithium-ion batteries（LIBs）in terms of resource abundance and safety.However,slow kinetics at low temperatures,severe interface side reactions at high temperatures,and severe polarization at high power severely limit their application in wide temperature ranges and high-power conditions.Compared with SIBs,LIBs have higher energy density and cycle life,and have been widely used in consumer electronics,electric vehicles,and other fields.However,LIBs technology suitable for special application scenarios such as wide temperature range and high power still needs further development.Electrolyte optimization design is one of the most important strategies to improve the performance of SIBs and LIBs.Collaborative regulating bulk electrolyte and electrode-electrolyte interface,and clarificating their structure-activity relationship with battery electrochemical performance,are the focus and difficulty of electrolyte research.The thesis focuses on the design of functional electrolytes for a wide temperature range and high-power sodium/lithium-ion batteries.From the perspective of electrolyte solvation structure,bulk ion transport,interfacial film composition control,new strategies such as weak solvated electrolyte,supramolecular elastomer solid electrolyte,high power electrode/electrolyte interface,etc.were constructed,and a series of new high-performance SIBs and LIBs electrolytes were obtained.The micromechanisms of electrolytes in enhancing ion conductivity,migration number,kinetic transport,thermodynamic stability,and electrochemical performance of batteries were revealed from the bulk to the interface.On this basis,Ah-level commercial pouch LIBs are constructed to realize the practical application of electrolytes at wide temperatures and high power filed.The main research content and achievements of this thesis are as follows:（1）Using ethylene glycol diethyl ether（DEE）as the main solvent of SIBs electrolyte and non-polar n-Hexane as diluent,the weakly solvated electrolytic liquid system was prepared to improve the desolvation kinetics and interfacial film components of Na⁺on tin（Sn）anode.The Sn/Na cell cycled stably for more than 500 weeks at-30℃with a capacity of about 700 m Ah g^-1.Moreover,moderate polarity tetrahydrofuran（THF）was added to DEE to improve the low-temperature conductivity of the electrolyte and the solvation structure of Na⁺,which is favorable for enhancing the insertion/extraction kinetics of solvated Na⁺on graphite（Gr）anode,and enabling the Gr/Na cell to show stable cycling performance at-40℃.（2）Based on the equal molar ratio Na TFSI,Li TFSI salts,and hexafluorobutyl acrylate（HFBA）monomers,a solid electrolyte of SIBs with high fracture strength,good thermal stability,wide electrochemical window,high ionic conductivity,and high diffusion coefficient were prepared.The fluorinated groups in HFBA can fix TFSI^-,which promotes the dissociation of salts and freedom degree of cations.Theoretical calculation showed that the carboxyl group in HFBA can combine with Li⁺to form an ion-dipole effect,which is conducive to improving the thermal stability of the electrolyte.The relatively weak binding ability with Na⁺is conducive to improving the freedom of Na⁺in the electrolyte.As a result,the electrolyte can achieve a dissociation temperature of 287.8°C,a high migration number of up to 0.64 for Na⁺at room temperature,and a conductivity of 0.21 m S cm^-1.The electrolyte can promote the Na₃（VOPO₄）₂F（NVPF）∥Na to have a stable cycle performance at 25℃and 94.4%capacity retention after 500 cycles at 60℃,enabling the symmetrical sodium metal cell to cycle stably for 879 h at 90℃.（3）A solid electrolyte interface layer（SEI）rich in uniformly dispersed inorganic substances was constructed by introducing self-sacrificing Li TFSI into the carbonate electrolyte for SIBs.Among them,Li⁺and TFSI^-can preferentially adsorb in the inner Helmholtz layer（IHP）of sodium metal and form initial SEI,producing a large amount of Li₃N.As an active catalytic center,lone pair electrons are transferred from the N atom in Li₃N to FEC,promoting the reduction of FEC.Since the decomposition of Li TFSI is inhibited by both concentration and reduction kinetics,the reaction is relatively mild,and the reductive decomposition of FEC can produce large amounts of amorphous organics.Therefore,the inorganic substances such as Li₃N,Li F,and Na F generated by the decomposition of the two are cross-generated and exist in a multi-point dispersed state.The unique distribution forms a Na⁺rapid transport microregion.It also provides abundant interfacial transmission channels and high-flux nucleation sites,which can inhibit the formation of sodium dendrites,and promote the unprecedented ultra-high rate and long cycle performance of sodium metal batteries.Thanks to this high ion conductivity interface film,the Na‖Na₃V₂（PO₄）₃（NVP）coin cell had a high capacity retention rate of 89.15%after 10,000 cycles at an ultra-high rate of 60 C;The capacity retention rate of Na‖NVP pouch battery is 92.05%after 2000 cycles at 10 C rate.（4）A new additive,5-dicyano-1,3-dimercaptan 2-one（DDO）was introduced into the carbonate electrolyte,riching in C=O,C-S,-CN,and other groups.After electrochemical impedance spectroscopy（EIS）,cyclic voltammetry（CV）,scanning electron microscopy（SEM）,and X-ray photoelectron spectroscopy（XPS）tests,it was confirmed that the additive has a high HOMO（Highest Occupied Molecular Orbital）level and can be preferentially oxidized and decomposed at the positive electrode interface.The internal multi-electron groups can be oxidized and decomposed at the positive electrode to form a stable and uniform CEI film.Thus,the integrity of the positive particles is protected,and the cyclic stability and dynamic characteristics of the Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）‖Li cell were improved.Adding 0.1 wt.%DDO to the electrolyte enabled the NCM811‖Li cell to have75.59%capacity retention after 200 cycles.（5）Based on the design idea of electrolyte phase to interface,an ester electrolyte with three components（DDO,1,3,2-Dioxathiolane 2,2-dioxide,and lithium difluorophosphate）was used in Li Ni_0.9Co_0.05Mn_0.05O₂（NCM9055）‖Li and Gr‖Li coin cells to verify its cycle and rate performance.Then,by combining the fast-charging ternary cathode and the mixed high-power hard carbon anode,Ah-level pouch LIBs were prepared.The cell showed a capacity of 3.20 Ah,a median voltage of 3.65 V,and an energy density of 154 Wh kg^-1.Additionally,the cell had the characteristics of a wide temperature range and high power.During the test,the cell kept a stable capacity retention rate of 99.58%after 250 cycles at 25℃and 98.98%after 150 cycles at 25℃.At 30 C,the discharge capacity is 2.79 Ah and the discharge medium voltage is 3.2 V.Moreover,it can support low SOC discharge at-28℃,providing a theoretical foundation for the engineering design and development of wide-temperature range/high-power batteries.