Research Of Multifunctional Electrolytes For High-voltage Electrochemical Energy Storage Devices

Along with the prompt development of a range of application areas such as industrial automation systems,electrical vehicles,and electrical utility storage systems in the 21st century,there has been a strong need for energy storage devices with a high energy density,power density,long cycle life,and safety concurrently.However,the overall performance of the currently available electrochemical energy storage devices（including activated carbon-based super capacitors and lithium-ion batteries）cannot satisfy this rapidly increasing demand,which,thereby,has triggered the research and development of new-generation high-performance electrochemical energy storage devices to simultaneously possess a high energy density,power density,long cycle life,and safety.Referring to the essential components of an electrochemical energy storage device（i.e.,electrolyte,electrode,and device configuration）,in this thesis,we proposed and investigated a variety of approaches in designing and optimizing multifunctional electrolytes,compositing electrodes with carbon nanomaterials,and investigating devices with various cathode/anode designs.With these new strategies,we have studied and developed novel high-voltage asymmetric lithium-ion capacitors（LICs）,high-voltage lithium-ion batteries（LIBs）,and high-voltage lithium metal batteries（LMBs）having the enhanced overall performance in energy density,power density,long cycle life,and safety.The comprehensive research performed and obtained results in this thesis is summarized as follows:⑴Study on a multifunctional phosphite electrolyte additive for high-voltage LICsIn order to overcome the shortcomings of the low energy density of supercapacitors and the poor power characterisitcs of lithium-ion batteries,we developed high-voltage asymmetric LICs by coupling the high-potential lithium manganese nickel oxide（LNMO）as cathode and activated carbon（AC）as anode,i.e.,AC//LNMO.By introducing an appropriate amount of triphenyl phosphite（TPPi）as additive into a conventional carbonate electrolyte,the resultant electrolyte scavenged the electrolyte impurities and formed robust,uniform,and thin cathode/electrolyte interface（CEI）at cathode and solid-electrolyte interphase（SEI）at anode.These can efficiently suppress the continueous oxidative/reductive decomposition of the electrolyte,the metal dissolution from LNMO cathode,and the solvent decomposition inside AC anode to achieve a high operating voltage for LICs.Moreover,the electrodes were composited with carbon nanotubes（CNTs）and the cathode/anode capacity ratio was modulated.As a result,the optimized high-performace LIC showed a high operating voltage of 3.45 V,high energy density of 61.6 Wh kg^-1,high power density of 52.5 k W kg^-1,and long cycle life with a capacity retention of 91.8%after 6000 cycles.⑵Study on a multifunctional dual-additive electrolyte for high-voltage LIBsIn order to boost the operating voltage to enhance the energy density and power density over currently available LIB techonlogy,we incorporated the LNMO cathode with a graphite（Gr）anode to develop high-voltage LIBs,i.e.,Gr//LNMO.The simultaneous addition of tris（trimethylsilyl）phosphite（TMSP）and fluoroethylene carbonate（FEC）into a conventional carbonate electrolyte resulted in a new multifunctional dual-additive electrolyte,which can form a robust,uniform,and thin CEI at the LNMO cathode and a SEI at the Gr anode as well as suppress the decomposition,hydrolysis,and reduction of Li PF₆,the continueous oxidative/reductive decomposition of the electrolyte,and the metal dissolution from the LNMO cathode,leading to a high operating voltage and cycling stability for LIBs.Furthermore,compositing the electrodes with multidimensional carbon nanomaterials（including one-dimensional CNT and two-dimensional graphene）established a well-defined three-dimensional electrical conduction network and thus improved the rate capability for both the LNMO cathode and Gr anode,boosting the power charatceristics for LIBs.Synergistically,these realized a high-performance LIB with a high operating voltage of 4.85V,high energy density of 359.6 Wh kg^-1,high power density of 2.8 k W kg^-1,and long cycle life with a capacity retention over 80%after 300 cycles.⑶Study on a multifunctional in situ-formed Lewis acid-base complex as electrolyte additive for high-voltage LMBsThe uncontrollable lithium dendrite growth,low Coulombic efficiency,short cycle life,and poor safety of the Li anode are the issues seriously hindering the practical applications of LMBs.In this regard,we comprehensively studied the function mechanism of the aforementioned dual-additive electrolyte and incorporated it with the LNMO cathode to develop high-voltage LMBs,i.e.,Li//LNMO.The theoretical calculation,NMR characterization,and electrochemical investigation confirmed the formation of a Lewis acid-base complex between TMSP and FEC（i.e.,TMSP:FEC）in situ in the electrolyte.This complex can be preferentially（and electrochemically）decomposed to produce an inorganic Li F and organic F-/P-/Si-rich SEI at the Li anode and an organic F-/P-/Si-rich CEI at the LNMO cathode,both being ionic conductive,mechanically strong,and uniform.In conjunction with the electrolyte-impurity-scavenging function of TMSP and the salt-stabilizing capability of FEC,the SEI and CEI can effectively restrain the dendrite growth at the Li anode and protect the LNMO cathode,thereby,significantly improving the Coulombic efficiency and cycling stability for LMBs.Incorporating with this complex-containing electrolyte,the LMBs fabricated with three representative cathode materials(all at a moderately high mass loading of 11.0 mg cm^-2),including LNMO,Li Ni_0.8Mn_0.1Co_0.1O₂（NMC811）,and Li Fe PO₄（LFP）,have successfully demonstrated superior cycle lives（capacity retention,Li//LNMO:87.2%after 500 cycles;Li//NCM811:87.3%after200 cycles;Li//LFP:87.0%after 390 cycles）.Among them,the Li//LNMO battery exhibited a high operating voltage of 4.95 V,energy desnity of 511.3 Wh kg^-1,and power density of 8.3k W kg^-1.In summary,through the innovative conception and systematic study,we have developed a range of multifunctional high-voltage electrolytes.Along with the electrode compositing with carbon nanomaterials and various cathode/anode designs,we have incorporated these new electrolytes to develop high-performance electrochemical energy storage devices ranging from high-voltage LICs（3.45 V）to high-voltage LIBs（4.85 V）and high-voltage LMBs（4.95V）with significantly enhanced energy densities,power densities,cycle lives,and safeties over their previously reported counterparts.The achievments made in the present work have shown a substantial potential for practical commercialization and would provide a remarkable theoretical foundation and instructive significance to the research and development of new-generation high-performance electrochemical energy storage devices.