Regulation On Electrolytes And Interphases For Lithium/Sodium Batteries

The extensive application of lithium batteries has put forward new requirements on battery performance,such as high energy density,long cycle life,high rate capability and low cost.Using high-capacity electrode materials and elevating the upper cut-off voltage of the cathodes are effective ways to improve the energy density of batteries.However,the bulk and interfacial instabilities will induce rapid performance degradation.In addition,the uneven distribution and limited terrestrial reserves of lithium have raised concerns on the affordability for next-generation energy storage systems.As low-cost alternative to lithium batteries,sodium-ion batteries suffer from more serious structural damage and faster performance degradation due to the larger radius of sodium ions.Therefore,it’s urgently demanded to develop new strategies for material design,stabilizing electrode surface,and improving reaction kinetics.Herein,the thesis focuses on electrolyte regulation to provide better kinetics,construct a high-quality interface between electrodes and electrolytes,and improve cycling stability,thus understanding the fundamental interactions between electrolyte compositions and the structure-function relationship.The detailed contents for investigation are listed as follows:（1）A robust Li F-rich cathode-electrolyte interphase（CEI）was constructed on the surface of lithium cobalt oxide（Li Co O2）cathode by potentiostatic reduction of fluorinated electrolytes,which significantly suppressed the structural degradation and Co ion dissolution at high voltage of 4.6 V,thus decreasing polarization and interfacial impedance.The Li Co O2 with Li F-rich CEI delivered a capacity of～200 m Ah g^-1 at0.5C(1C rate corresponds to a specific current of 200 m A g^-1),and achieved an excellent cyclability with a high capacity retention of 63.5%over 400 cycles as compared to the Li F-free Li Co O2 with only 17%of capacity retention.The method provides a universal strategy for the realization of high-energy-density lithium-ion batteries with long cycle life.（2）An electrolyte that can be reduced at a high voltage of 2.4 V was designed through regulating the interaction between ion and solvent.When applied to lithium-fluorinated carbon（CFx）batteries,the electrolyte acts as both an ionic conductor and an active component to contribute capacity.The designed electrolyte provided 2 times of CFx capacity（based on CFx weight）,higher than that of traditional electrolyte.Furthermore,the energy density of a single-layer pouch cell based on the total mass of lithium anodes,CFx cathodes and electrolytes increased by 44%more than traditional electrolytes,showing great potential for application.In addition,at an extremely low temperature of-70°C,Li-CFx cell delivered an unprecedented capacity of 1356 m Ah g^-1.The design strategy provides a new avenue to develop high energy lithium primary batteries.（3）Based on the difference in reductive behaviors of ester/ether solvents and salts,we pre-engineered a“foreign solid-electrolyte interphase（SEI）”from an ester-based electrolyte or introduced a small amount of ester solvents into an ether-based electrolyte,successfully constructing a stable SEI on hard carbons（HCs）and achieving superior electrochemical performance.The HC anodes delivered an extraordinary cycling performance with a remarkable capacity of 211 m Ah g^-1 after 2000 cycles at a high rate of 1 A g^-1,corresponding to a quite high capacity retention of 95.6%.The work shed light on an effective strategy to achieve more efficient sodium storage.（4）By investigating the effect of ether-based and ester-based electrolytes on the sodium insertion behaviors in hard carbons,along with the structural design,the“adsorption-filling”storage mechanism of sodium in HCs was elucidated.The similar sodium insertion behaviors of HCs with ether-based and ester-based electrolytes indicated that no Na⁺ions intercalate between the graphene layers.In addition,filling sulfur into the micropores of HCs can remove the low-voltage plateau,providing solid evidence for its association with the pore-filling mechanism.Lastly,the linear relationship between the decreased concentration of defects/heteroatoms in HCs and the sloping capacity confirmed the adsorption mechanism in the sloping region.The determined“adsorption-filling”mechanism encourages the design of high-performance HC anodes for sodium-ion batteries.