Study On The Regulation And Mechanism Of Ion Transport At The Interface Of Li Metal Anode

Lithium metal battery（LMB）has been widely focused and studied in recent years due to its high theoretical energy density and capacity.The uneven Li⁺transfer and the following uneven Li deposition and Li dendrite growth lead to the instability of Li anode and battery failure,restricting the development of LMB.This dissertation focuses on solving the uneven transfer and distribution of Li⁺near anode surface and improving the stability of anode and the performance of LMB,via utilizing series of SEI/electrolyte modification strategies.Simultaneously,via analyzing the components and structures of modified SEI/electrolyte and the ion coordination/distribution states,effective regulation methods and strategies have been summarized,and valid understanding of ion transfer mechanism near anode surface has been established.The main researches are as follows:1.In order to solve the uneven Li⁺distribution and Li deposition caused by ion depletion and local space charge near anode surface,a metal-organic framework（MOF）with Lewis acidity,ZIF-67,is adopted as electrolyte additive to regulate ion coordination and study the influence of ion depletion on ion transfer and Li deposition.The unsaturated Co²⁺of ZIF-67 could adsorb TFSI^-,weaken the interaction between cations and anions,facilitate Li⁺solvation,improve mobility of Li⁺and restrict the mobility of anion,hence mitigating ion depletion and local space charge near anode surface,and realizing uniform Li⁺distribution and Li deposition.As a result,a long cycling performance for over 2000h at 3 m A cm^-2 under 3 m Ah cm^-2 for Li||Li,and an initial discharge capacity of 1.02 Ah g^-1 at 85 m A with a S loading of 1 g for Li-S pouch cell have been achieved.2.Based on the low stability of organic SEI phase,low flexibility of inorganic SEI phase,and the low homogeneity or integrity of multiphase SEI,an artificial coating synthesized via the reaction between lipoic acid（LA）and Li（LA@Li）is adopted to regulate the component and phase distribution of SEI and study the influence of SEI structure on ion transfer and Li deposition near anode surface.S-S of LA possesses self-healing effect which could coordinate with Li⁺to form Li_xS during cycling.O-C=O of LA could regulate Li⁺transfer and distribution.The carbon chain possesses good flexibility which could accommodate to volume expansion during Li deposition.LA@Li improves the ion transfer and the stability of SEI,and suppresses the decomposition of electrolyte,hence realizing the uniform ion transfer and improving the stability of Li anode.As a result,1500 h cycling performance of Li||Li cell at 5 m A cm^-2 under 5 m Ah cm^-2 and 70 stable cycles with the highest discharge capacity of 1.186 Ah g^-1 of Li-S pouch cell at 0.3 C with a S loading of 3 mg cm^-2 have been realized.3.Considering the dynamic exhaustion of reactive/essential components in both SEI and electrolyte,a MOF with strong adsorption ability and high reactivity is adopted as artificial anode coating layer to in-situ regulate ion coordination states and components of SEI and electrolyte during cycling,and to study the influence of the evolution of SEI/electrolyte on ion transfer and Li deposition.Highly reactive F functional group and high adsorption effect of University of Oslo-66-F₄（UIO-F）could increase the Li F content within SEI and suppress solvent decomposition,hence continuously regulating SEI/electrolyte and improving stability of SEI and Li anode.As a result,1000 h/2000 h cycling performances of Li||Li cell at 5 m A cm^-2 under 1 m Ah cm^-2/at 3 m A cm^-2 under6 m Ah cm^-2 respectively,and a stable discharge capacity of 1 Ah g^-1 of Li-S pouch cell at 0.1 C with a S loading of 6 mg cm^-2 and a cell area of 11.5 cm×15.5 cm have been realized.4.To solve the issue of low ion conductivity and low stability,and to overcome the difficulty in balancing them in polymer electrolyte,a zwitterion,3-（1-Pyridinio）-1-propanesulfonate（PPS）,is chosen to regulate the ion transfer and stability in poly（vinylidene fluoride）（PVDF）,and to study the interaction between ion transfer and stability of polymer electrolyte,improving stability of Li anode and PVDF.The adsorption role of PPS on Li TFSI could improve Li⁺transfer efficiency and prevent the attachment of Li⁺to-H/F in PVDF,hence suppressing the dehydro-fluorination reaction in PVDF and the etching of HF to Li anode,and improving stability of anode-electrolyte interphase.As a result,the stable electrochemical voltage window is improved to 5.3 V,the Li⁺transference number is improved to 0.59,and a stable cycling performance for over 1000 h of Li||Li cell at 0.2 m A cm^-2 under 0.1 m Ah cm^-2 have been achieved.