Reconstruction Of Alloy Electrode/Electrolyte Interfacial Behavior For Sodium Ion Batteries

Due to the shortage of lithium resources,sodium-ion batteries have gradually become the partial substitute and supplement for lithium-ion batteries.However,the theoretical energy density of sodium-ion batteries is lower than lithium-ion batteries.If the alloy which has high capacity is used as the electrode,the energy density of the sodium-ion batteries can be effectively improved.As of late,the research on the alloy materials used in sodium-ion batteries mainly focuses on the control of the morphology and structure of the materials,to solve the volume expansion problem and improve the electrochemical performance.However,reducing the size of the alloy material and constructing the hole structure will cause the reduction of the volume energy density of batteries,and the preparation process is relatively complex.The main content of this work is to analyze the electrochemical behavior of the interface by adjusting and controlling the interface between the alloy electrode and the electrolyte,which can reveal the cause of the degradation of the alloy material properties.The sodium-ion batteries with micrometer alloy electrode which have long cycling stability can be obtained by adjusting the solvation structure of the electrolyte and inhibiting the side reactions at the interface of the alloy electrode.The main research results are as follows:1.It is proposed that the interfacial side reaction causes the failure of the micron alloy electrode(the non-traditional volume effect),where the anions play a key role.The interfacial side reactions which are induced by anions(electrolyte decomposition,corrosion of metal by anions)lead to the continuous decomposition of electrolyte and the failure of alloy materials.Raman and NMR characterization of the electrolyte revealed the probability of different anions around Na+ is different.The DFT simulation also verified that the probability of the occurrence of anions at the electrode interface is different.We reduced the probability of anions approaching the electrode by adjusting the electrolyte and got the micron alloy(Sn,Bi)electrodes which have a specific capacity higher than 650 mAh/g and 500 cycles stability.Based on this,we propose that the side reactions which were caused by anions lead to the failure of the alloy electrode,and establish an interface model to predict and analyze the stability of the alloy electrode.2.Anions induce side reactions at the alloy electrode interface and reduce the stability of the alloy electrode.We stabilize the alloy electrode by reducing the concentration of salt.The reduction of the salt concentration brings three benefits:(1)reduce the cost of the electrolyte;(2)reduce the viscosity of electrolyte and improve the interface wettability;(3)reduce the interface side reaction. Compared with PF6-,CF3SO3-and ClO4-,BPh4-has the lowest probability of weak interaction with Na+near the electrode because of its large space resistance.Micron Sn and Bi electrodes can run 900 cycles at 1 C in 0.1 M BPh4-electrolyte and the specific capacity is higher than 600mAh/g,where it can also operate at-20? with good performance.This work further confirms that the interfacial side reaction caused by anions leads to the failure of the alloy electrode,and also confirms that the interfacial impedance is dominant,and the alloy electrode with good rate performance can be obtained in the electrolyte with low conductivity.3.To solve the problem of the low energy density of sodium-ion batteries,we used a new electrolyte to optimize the batteries' structure and improve the energy density of sodium-ion batteries.We used ultra-thin Sn foil as a working electrode and current collector to avoid the use of traditional binder and conductive agent,where it makes the preparation process simplified.The full cell was constructed with Sn foil,1.0 MNaPF6 DME,and Na3VPO4 as the cathode,electrolyte,and anode respectively,and the battery can remain more than 94%and 65%of the capacity after 200 cycles and 1000 cycles.It shows excellent performance in different working conditions.The characterization reveals that the regulation of electrolytes inhibits the interface side reaction,and the released sodium from the positive electrode will be stored in the Sn sheet temporarily.During the whole reaction process,the sodium source will be released layer by layer by infiltration.The new modification of electrolytes for sodium-ion batteries improves the energy density of the batteries and simplifies the manufacturing process of the traditional batteries.4.During the electron donation process of electrode,metal ions(Li+/Na+/K+)will have two behaviors after gaining an electron.(1)The alloy materials get electrons and react with metal ions,the electrochemical performance is stable;(2)at this time,the electrolyte decomposes,and the continuous decomposition of the electrolyte leads to the failure of the electrode.The interaction of anions,solvents,and metal ions was revealed by spectral characterization of the electrolyte.The composition of a solvation unit near the electrode is also deduced.DFT calculation was used to simulate the stability of an electron at the electrode interface of a solvated structural unit of electrolyte to evaluate the tendency of electron selection.Based on this,a standard for evaluating the stability of electrolyte to alloy materials was established.