Effect Of Lithium Plating Behavior On Deterioration Of Energy Storage Characteristics Of Graphite Negative Electrode

As a widely commercialized secondary energy storage device,lithium-ion battery has many advantages,such as safety,stability,high energy density,long life and so on.However,lithium evolution reaction is easy to occur when charging at high rate or high or low temperature.The deposit of metal lithium and its non-uniformity and instability can lead to the loss of active lithium capacity and internal micro short circuit,resulting in rapid capacity attenuation and serious safety problems.In order to bridge the gap between current technology and future goals,we must understand the scientific problem of limiting the rapid charging of lithium-ion batteries.Under extreme conditions,such as high temperature and low temperature,rapid charging will greatly reduce its service life.At present,it is generally believed that graphite negative electrode is the primary reason to hinder the high rate charging of lithium-ion batteries under extreme conditions.This paper explores the problems of graphite negative electrode.Firstly,overcharge the capacity of graphite negative electrode to 500 m Ah g^-1,actively guide the precipitation of metal lithium,explore the influence of lithium evolution behavior on the decay of energy storage characteristics of graphite negative electrode,and study the influence of charging conditions and electrolyte components on the lithium evolution behavior of graphite negative electrode.In addition,explore the influence of external physical field on the decay of different order capacity of graphite negative electrode.The main research results are as follows:（1）New insight on graphite anode degradation induced by Li-platingIn this work,we describe the evolution mechanism of lithium precipitation from intercalation in graphite to formation of metal lithium on graphite particles through excessive lithium cycle test,in-situ XRD and titration gas chromatography.We overcharged the capacity of graphite negative electrode to 500 m Ah g^-1,actively guided the precipitation of metal lithium,and studied the disappearance mechanism of lithium intercalation/stripping process in graphite particles.The results show that with the continuous precipitation of metal lithium,graphite intercalation compounds(GIC:Li C₁₂,Li C₆,etc.)gradually become inactive and are wrapped by dead lithium or side reaction deposits.Lithium ions cannot be embedded/detached into the layered structure of graphite,which directly leads to the capacity attenuation of graphite.The relationship between lithium ion embedding and lithium metal deposition is further clarified by excessive lithium.In addition,the precipitation behavior of lithium metal is closely related to the number of cycles.Two direct effects of metal lithium precipitation on graphite lithium half cell are clarified:on the one hand,in the process of lithiation,dead lithium or side reaction deposits block the intercalation/detachment channel of ions,resulting in the inability of lithium ions to be separated from intercalation compounds;On the other hand,once the lithium precipitation behavior occurs,the process is more likely to occur in the subsequent cycle and enter a vicious cycle,resulting in the rapid deactivation of graphite in the subsequent lithium/de lithium process.This work provides a new insight into the degradation of graphite anode caused by lithium-plating.（2）The effects of charging conditions and electrolyte components on the performance degradation caused by lithium precipitation of graphite negative electrode were investigated.We investigated the effect of lithium precipitation on the capacity degradation of graphite anode.This paper focuses on the factors affecting the lithium precipitation behavior of graphite negative electrode,and makes a preliminary exploration from four aspects:excessive lithium amount,electrolyte,temperature and magnification.Firstly,the capacity of excessive lithium will rapidly deteriorate the lithium intercalation graphite stage.A large number of dead lithium and side reaction deposits fill the space inside the graphite and accelerate the blockage of the ion transmission channel of lithium ions,resulting in the disappearance of the lithium ion insertion/detachment mechanism.Secondly,electrolyte additives can guide the uniform deposition of metal lithium precipitated by excessive lithium,reasonably fill the space inside graphite particles,and delay the blockage of lithium ion insertion/removal channel,so as to alleviate the disappearance of lithium ion insertion/removal mechanism.In addition,the charge transfer and solid-phase diffusion become slower at low temperature.The phase of lithium intercalated graphite decays faster than that at room temperature.The polarization is increasing.After lithium ions get electrons,they are transformed into lithium metal and precipitate on the graphite surface.The rapid increase of polarization leads to the aggravation of side reactions,which makes the embedding/stripping mechanism of graphite negative electrode decline rapidly.It is also found that the charging current ratio determines the lithium ion flux on the negative electrode material per unit area.When the solid-phase diffusion process of Li⁺in the negative electrode is slow,lithium evolution reaction will occur on the surface of the negative electrode.When the current density increases to a certain threshold,the influence on the interior of graphite also changes.The capacity contribution of metal lithium precipitated from the surface of graphite negative electrode is positively correlated with the excessive lithium current,while the lithium potential of graphite negative electrode is negatively correlated with the lithium current.（3）Influence of external physical field on different order capacity degradation of graphite negative electrodeThe influence of magnification and temperature on the capacity attenuation mechanism of graphite negative electrode is completely different.Therefore,we will compare the influence of magnification and temperature on the capacity attenuation of graphite negative electrode.The intercalation compound of graphite is divided into four stages.Accordingly,the intercalation capacity is also divided into four stages and reflected in the lithium curve.For this,we have made a detailed statistics of the intercalation capacity of graphite negative electrode in the four stages of lithium process.Through in-depth analysis and exploration of the capacity of the four intercalation stages inside graphite,we have found some important laws.The rapid capacity decay stage of graphite negative electrode generally starts from the first-order compound,the capacity of the fourth-order compound can be maintained to the end,and the influence on the capacity polarization of the four embedding stages of graphite is different,and the capacity decay of the four embedding stages is also different.