Constructing Elastic SEI Film By In-situ Polymerization Of Small Organic Salts To Suppress Lithium Inventory Loss In Lithium Ion Batteries

During the long-term cycling process of lithium batteries,the limited active lithium-ions are continuously consumed by the side effects at the electrode/electrolyte interface,which is considered to be one of the most important capacity responsible for the capacity fading of lithium ion batteries.This dissertation focuses on the construction of elastic solid electrolyte interphase（SEI）on graphite materials.We chose a variety of small organic salts for use as a starting material for SEI formation and investigated the suppressing effects of the film on the active lithium ion consumption in lithium ion batteries.To realize the goal of constructing elastic SEI film on graphite anode,we selected many small organic salts including sodium maleate（SM）,sodium acrylyte（SA）,sodium vinylsulfonate（SVS）et al.and organic macromolecules such as sodium alginate（NaAlg）,sodium polyacrylate（PAA）and polyvinylidene fluoride（PVDF）et al to coat a natural graphite.After coating,systematic tests and characterizations were used to analyze the performances of the electrodes modified with these material.As shown in the TEM and SEM images,we can gain a controllable,stable and uniform coating layer through a sample liquid coating technique.The organic salts are relatively insoluble in organic solvents and electrolyte,thus the coating layer is stable and can be well preserved in the processes of the electrodes preparation and long-life electrochemical tests.Carboxylic and sulfonic groups of the organic small molecules are beneficial for the formation of a uniform and stable SEI film,which improves the first coulombic efficiency and long-term cyclic performance of the batteries.Moreover,in-situ polymerization of these small molecules by unsaturated bonds can form an organic skeleton during electrochemical processes,contributing to a flexible organic-inorganic composite SEI film.The film is able to take up the volume change of graphite due to lithium intercalation and de-intercalation.Coating layers of small organic salts exhibits better toughness compared to those of inorganic materials and eliminate the negative influence on the electrode conductivity caused by organic macromolecular coating layers.As the result,the first coulombic efficiency is increased by 2.0%to 4.0%with the help of the coating layers of sodium maleate（SM）and sodium acrylyte（SA）.Coating with 3.0 w.t.%sodium acrylate,the capacity retention of the graphite at the rate of 50C is 50%,obviously higher than the capacity retention of only 20%for the graphite anode without any coating.The dQ/dV plots and the test results of CV indicate that the electrochemical polarization of the electrodes is reduced by the small organic salt coating layer,which is distinct from the film forming additives such as VC and FEC.Electrochemical impedance spectroscopy（EIS）test manifests the impedance of the electrode becomes smaller and smaller after repeating charging-discharging tests.After 200cycles of electrochemical test at 0.1C,the capacity retention of the anodes coated with SM and SA are all more than 95%,and the capacity retention of the samples with optimal thickness are almost up to 100%.What’s more,the capacity retention of the full cell assembled with LiFePO₄（LFP）cathode and graphite anode is also significantly improved.After 500 cycles,the capacity retention shows an improvement of more than 10%as a result of the small organic salt coating layers on the surface of the graphite particles.The novel surface decoration technology is facial and controllable.It contributes to a simultaneous improvement of the electrochemical properties in terms of the first coulombic efficiency,rate capability and cycling stability.The new method is very promising for use with next generation of lithium ion batteries of high electrochemical performances.