Layered Oxide Cathode Materials For Sodium-Ion Batteries And Their Industrial Exploration

Due to the depletion of fossil energy,the development of clean energy and the related large-scale energy storage system is getting urgent.Sodium ion batteries,owing to sodium abundance and low cost,are promising candidates for energy storage.Nowadays,many enterprises both at home and abroad,have started to lay out sodium ion batteries and its commercialization is in the near futureHowever,several problems should be cared during the industrialization:the development of better cathode material with higher capacity,lower cost and better stability in air and electrolyte;the research and development of electrolyte with wider temperature range and better stability.We thus mainly focus on the industrialization of sodium ion batteries,and explore new cathode material,the air-stability and the optimization of electrolyte.Based on the reported results,we find that O3 layered oxide usually have higher capacity but limited cycling performance,such as NaNi_xMn_1-xO₂.We thus put forward the combination of two optimization methods:reduce the sodium content so as to reduce the stress during the intercalation and de-intercalation of sodium ion;break the ordering structure by Ti doping.The optimized material,Na_0.9Ni_0.4Mn_0.4Ti_0.2O₂ and Na_0.9Ni_0.4Mn_0.3Ti_0.3O₂,can deliver an initial discharge capacity of 120 m Ah/g in the voltage range of 2.5-4.2 V with good capacity retention after 100 cycles.In-situ XRD is adopted to investigate the structure change during charge and discharge,and the volume change is found to be as low as 2%.Better performance confirmed the validity of our design.Hybrid phase with O3/P2 structure is obtained when the sodium content is further reduced,and it is getting an increasing interest by researchers due to better cycling performance.We systematically study the relationships among sodium content,sintering temperature,transition metal species,structure formed and electrochemical performance.The optimized material,Na_0.78Ni_0.2Fe_0.38Mn_0.42O₂,can deliver a capacity of 86 mAh/g and 66%of its capacity maintain when the current rate is increased to 10C.Moreover,the capacity retention is 90%after 1500 cycles,proving the best comprehensive performance.We first proposed and verified that the radius of transition metal has influence on the structure finally formed:larger weighted radius of transition metal makes it easier to form O3 layered structure.The effect of temperature is also studied by ex situ quench method.This research results provide new insight in designing novel cathode materials with improved performance for sodium-ion batteries.More issues should be taken into account during the industrialization of sodium-ion batteries,such as the stability of the cathode material,as it is highly related with the final cost.We thus compared air-stability of two promising materials,Na_0.9Cu_0.22Fe_0.30Mn_0.48O₂?CFM?and NaNi_1/3Fe_1/3Mn_1/3O₂?NFM?.The structure,morphology and electrochemical performance were studied weekly for 3 months.CFM can maintain its structure and electrochemical property for about two weeks.However,structure damage and large capacity loss happened within one week for NFM.CFM thus is thought to have better potential for application for its Ni and Co free composition and better air-stability.Moreover,re-heating is an effective mean to partial recover the structure and performance.One of the most important components of sodium-ion battery is electrolyte,and mini-change will lead to great performance change.Through the control of solvent species and addition of additives,we optimized the electrolyte so as to obtain better interface between electrolyte and cathode material.The full battery with optimized electrolyte has smaller interfacial impedance and thus better capacity delivery and better rate performance.XPS and SIMS were adopted to study the interface of cathode,and we found that better CEI was formed than the control group.The designed electrolyte also has better high temperature stability,and the battery has larger capacity retention after 60 ^oC for 7 days.