Investigation Of High Performance Layered Materials For Sodium Ion Batteries

Recently,the attention to sodium-ion batteries has been refocused on large-scale energy storage applications,due to sodium’s infinite abundance and similar physical and chemical properties to lithium.However,the radius and mass of sodium ions are larger than that of lithium,resulting in a decrease in battery energy density,difficulty in sodium ion transmission,and poor material stability.Therefore,research and development of high-performance electrode materials are the key factors to promote the development of sodium ion batteries.Compared with alloying and conversion anode materials,Na_0.7Ni_0.35Sn_0.65O₂ based on the intercalation reaction has better cycle performance,and it has the advantages of abundant resources,stable structure,safety and non-toxicity.Therefore,Na_0.7Ni_0.35Sn_0.65O₂ is a very promising material in the anode materials.However,the initial coulombic efficiency of Na_0.7Ni_0.35Sn_0.65O₂ is usually lower than 60%,which will cause a significant decrease in battery capacity,and electrolyte decomposition is an important factor that leads to its low initial coulombic efficiency.Among the cathode materials for sodium ion batteries,NaNiO₂ has been widely studied because of its high reversible capacity,high potential,and simple synthesis process.However,during the charging and discharging process of this material,it is easy to cause the slip of the transition metal layer and the irreversible phase change of the structure,resulting in its poor cycle performance.Therefore,it is particularly important to improve the cycle performance of layered transition metal oxides and suppress irreversible phase transitions.Based on the above background,the research contents of this paper are as follows:（1）In order to explore the influence of electrolyte on the electrochemical performance of Na_0.66Li_0.22Ti_0.78O₂,a series of different electrolytes are obtained by changing the solvent,additives,sodium salt and concentration of electrolyte.Then the performance of Na_0.66Li_0.22Ti_0.78O₂ in different electrolytes is studied,and it is found that the initial coulombic efficiency of Na_0.66Li_0.22Ti_0.78O₂ is obviously affected by the electrolyte.（2）Considering the irreversible phase transition of NaNiO₂ during charge and discharge,Na_0.7Ni_0.35Sn_0.65O₂ is synthesized by high temperature solid phase method.Due to the layered transition metal oxide is very sensitive to the synthesis conditions,we discuss the effects of different ball milling conditions,sintering temperature and time,and cooling rate.It is found that high performance Na_0.7Ni_0.35Sn_0.65O₂ is synthesized at a higher speed,an appropriate amount of sodium source and rapid cooling.Electrochemical performance tests show that Sn substitution can suppress multiple phase transitions,and the material has an average voltage of 3.7 V.