Research On Inhibition Strategies Of Lithium Plating On The Anode Of Secondary Lithium Batteries

With the rapid development of lithium ion batteries(LIBs)in recent years,the high demand for energy storage continues to grow,and people have put forward much higher expectations for the future of LIBs.On the one hand,while ensuring the safety performance and cycle life,LIBs should be able to work under extremely harsh environments such as low temperature.On the other hand,to meet the requirements of the electric vehicles field,we need to improve the battery charging rate and the energy density simultaneously.It is worth noting that these battery performances that people are diligently seeking are closely related to the side reaction of lithium plating.The aging process of the battery and the kinetic changes of the anode reaction caused by lithium plating of the anode greatly affect the performance of the battery.During a typical charging process of LIBs,Li-ions are firstly extracted from the cathode,and then intercalated into the anode materials after been transported by the electrolyte and the separator.However,the kinetic upper limit of the anode to accept Li-ions will be exceeded when the distribution of Li-ions on the anode surface is uneven or the intercalation resistance is too large.And the Li-ions that cannot be inserted into the anode can only obtain electrons on the surface of the anode,and the lithium metal is precipitated.Especially in the case of high current fast charging,when the anode potential drops closely to or below 0 V vs.Li0/Li+,lithium plating is further aggravated,and may cause serious safety problems such as battery explosion.Lithium plating on the anode not only reduces the battery capacity and Coulombic efficiency,but also greatly shortens the cycle life.Based on the essential reasons of lithium plating,this thesis proposes two research ideas,that is,to adjust the distribution of Li-ions on the surface of the anode;or keep the process of Li-ion intercalation away from the potential of lithium plating,such as organic anode materials with a designable structure.Herein,we discussed the typical cases of lithium deposition process at lithium metal anode and a high-potential organic anode material.Based on this,the main results of the research are summarized as follows:(1)Adjusting the distribution of Li-ions based on the Galton board mechanism.The concentration distribution of Li-ions on the surface of the lithium metal anode is finely tuned by introducing a COF-LZU1 functional modification layer between the separator and the anode to achieve dendrite-free lithium deposition.COF-LZU1 crystallites consist of ordered nanochannels that can regulate Li-ion transport while hindering the movement of anions,and the obtained Li-ion transference number is much high as 0.77.The transport of Li-ions within the COF-LZU1 layer can be considered as similar to that of the pellets passing through the Galton board model and conforming to the normal distribution statistics,thus obtaining a uniform distribution of Li-ions.In this way,the growth of lithium dendrites is suppressed,thereby avoiding short circuits and obtaining excellent battery performance.(2)A Janus-type separator is designed to regulate the Li-ion flow reaching the surface of the lithium metal anode,thereby achieving dendrite-free lithium deposition.By introducing a molecular sieve(MCM-41 or SAPO-34)modification layer on the surface of the polypropylene separator as the redistribution layer of Li-ions,a uniformly distributed Li-ion flow can be obtained on the surface of the lithium metal anode due to the nano-or sub-nano pores of the molecular sieve,and the Li-ion transference number is significantly enhanced.We determined the relative concentration of Li-ions after passing through the Janus separator by an intuitive simulation.The presence of micropores in the molecular sieves can provide a desolvation effect of Li-ions,thereby improving the reaction kinetics.In addition,the functional modification layer based on molecular sieve also improves the mechanical strength of the composite separator and its wettability to the electrolyte,which is more conducive to the cycling stablity and safety.It also provides a facile but effective strategy to realize high-performance lithium metal anodes.(3)A novel porous organic framework was designed as a high-potential anode material for LIBs.The reaction mechanism and electrochemical performance are highly controllable for organic electrode materials,and a higher lithiation potential than lithium plating side reaction can be obtained through structural design.We theoretically and experimentally investigate the electrochemical performance and reaction mechanism of this type of organic anode materials with the redox centers containing azo groups and adjacent lithiophilic adsorption sites.POF-AN is a porous amorphous polymer connected by an O-N=N structure,which has a lithium intercalation potential far away from the occurrence of lithium plating,as well as good Li-ion conductivity and high specific capacity.The azo group can be used as an electrochemical active site for reversible bonding with Li-ions,and the adjacent oxygen atoms can undergo charge adsorption with Li-ions to promote the charge transfer between Li-ions and redox centers.This structural design is beneficial for synergistically enhancing the adsorption and intercalation of Li-ions,and further improves the lithium storage capacity and cycling stability.The reaction system can also be extended to more reaction substrates,and we obtained POF-BN with similar structure by replacing phloroglucinol with 1,3,5triaminobenzene.It has been verified that both POF-AN and POF-BN can be used as stable anode materials for LIBs.