In-situ Optical Microscopy Study Of The Deposition Behaviors Of Liquid Na-K Alloy Batteries

Today’s demand for highperformance energy storage systems is increasingly urgent,and alkali metal batteries have become a research hotspot in the field of high-energy-density batteries.Despite the good prospects of alkali metal batteries,the safety problems caused by dendritic growth hinders their large-scale commercial applications.The Na-K alloy is liquid within a certain concentration range,and the liquid alloy electrode can theoretically avoid the formation of solid dendrites.The use of liquid Na-K alloy electrodes is one of the feasible strategies to tackle the dendriteissues.However,Na-K alloy electrodes also have their own problems.For example,only one of the alkali elements in Na-K alloy batteries is able to work,which causes great energy loss;At a high current density,dendrite growth will also occur on the Na-K alloy electrode.This thesis,aims to screen out the effective electrolyte to achieve sodium-potassium co-deposition,as well as to study the related dendrite growth issues.In-situ optical microscopy was employed to analyze the influence of different electrolytes such as 0.8 M KPF6 in EC/DEC,1 M KFSI in DME,1 M NaClO4 in EC/DEC in Na-K alloy batteries,complemented by the transmission electron microscopy(TEM)to analyze the composition of the deposited product,so as to find the best electrolyte that enables the sodium-potassium co-deposition.In addition,the factors that influence the nucleation and growth of dendrites in Na-K alloy batteries were also explored.The main research results are summarized as follows:For the first time,we successfully accomplished the partial Na-K co-deposition in Na-K alloy batteries by using the electrolyte of 1M KFSI in DME.It is confirmed by experiments that potassium-ion electrolyte is more effective to achieve sodium-potassium co-deposition in comparison to sodium-ion electrolyte.In-situ optical microscopy studies show that the Na-K co-deposition in the potassium electrolytic solution proceeds in following steps:in the beginning,K dendrites are first generated at the copper substrate;then Na is deposited at the tip of the K dendrites due to a tip effect;finally they fuse with each other to form a liquid Na-K alloy,leading to the dissolution of K dendrites.Besides,the type of potassium electrolytic solutions also affect the realization of Na-K co-deposition.The SEI formed in the KFSI electrolyte is relatively strong and can withstand the stress caused by the solid-liquid transition of the deposits during Na-K co-deposition.Whereas the SEI in the KPF6 electrolyte will rupture,resulting in the loss of a certain amount of Na-K alloy.Therefore,KFSI electrolyte is more effective to maximize the co-deposition of sodium and potassium.The in situ optical microscopy was also used to observe the galvanostatic deposition behaviors of the Na-K alloy symmetric battery.The current density plays a determining role in the growth of dendrites.As the current density increases,dendrites are produced,and increase in numberand growth rate.When the current density is in the range of 1 mA cm-2-2 mA cm-2,the dendrite clusters generated on the Na-K alloy electrode will gradually shrink andfuse into the liquid alloy,and some of them can completely disappear.The shrinkage of dendrite clusters is mainly influenced by the three factors:(1)The oxide shells of the Na-K alloy prevent the dendrite clusters from contacting the liquid alloy;(2)The rate of dendrite integration into the Na-K alloy is slow;(3)In the KFSI electrolyte,a liquid alloy can be generated at the tips of the dendrite to accelerate the shrinkage rate of the dendrite cluster.Carbon paper(asubstrate material used to support Na-K alloy)is also helpful tosuppress dendrite growth.That is because carbon paper can react with K to form a guide layer on the surface with strong wettability to Na-K alloy.This layer can efficiently guide the liquid alloy generated at the tips of the dendrite into the carbon paper(need to cooperate with the KFSI electrolyte),so that the dendrites completely shrink and are retained.The above results are the first step toward Na-K co-deposition,and offer helpful insight for realizing the complete Na-K co-deposition in the future.The mechanistic study on dendrite growth and its suppression is of great significance to promote the practical applications of Na-K alloy electrodes.