Preparation And Modification Of Sulfate Cathode Materials For Li-Batteries

Since the 1970s,researchers had studied lithium-ion batteries.With the consumption of petrochemical energy and the vigorous development of electronic information technology,the demand for new energy storage technologies represented by lithium-ion batteries is increasing.After decades of efforts,a series of cathode materials LiCoO₂,LiMn₂O₄,LiFePO₄,and NMC have been commercialized.LiCoO₂ has high volumetric energy density but Co is toxic and expensive;Li Mn₂O₄ is low-cost but poor at cycle performance;The cycle performance of LiFePO₄ is excellent and environmentally friendly,but the operating voltage is relatively low;NMC cathode materials have the advantages of high voltage quality and high energy density,but there are side reactions.Therefore,researchers continue to explore new types of cathode materials while modifying mainstream cathode materials.Sulfate is a kind of polyanion with XO₄^2-group.Since S is more electronegative than P,the SO₄^2-group induction effect exhibits a higher working voltage.Sulfate that undergoes embedded reaction has a high operating voltage,but generally low capacity leads to low energy density.The sulfate that undergoes the conversion reaction can provide more capacity and higher energy density.This work mainly studies the sulfate that undergoes the conversion reaction and the corresponding reaction mechanism.The porous FeSO₄ is prepared by dehydration of FeSO₄·7H₂O.Within the voltage window of 0.01-3 V,the electrochemical reaction performance shows that the initial discharge specific capacity at 0.5C(1C=352 m A g^-1)is 888 m Ah g^-1.The discharge specific capacity in the 2^nd cycle is 470 m Ah g^-1 and the capacity retention rate after 100cycles is 68.5%relative to the 2^nd cycle.In order to improve conductivity of FeSO₄,Dopamine hydrochloride polymerized in situ on the surface of FeSO₄ and then was carbonized.By comparing the influence of different coating amount and heat treatment temperature on electrochemical performance,it’s determined that FeSO₄ treated at 15wt.%-300℃has the best performance.The conductivity of FeSO₄ is increased from 10^-9S m^-1 to 3.78×10^-5 S m^-1.The rate performance is significantly improved（≤5C）.Meanwhile,the capacity retention rate increases to 90.8%relative to the 2^nd cycle after100 cycles and stable cycle exceed 250 times.Finally,the kinetic performance and lithium-ion storage mechanism are studied.GITT shows that the overpotential decreases during discharge but increases during charging.The ion diffusion coefficient is on the order of 10^-11 cm²·S^-1.The EIS pattern shows that carbon coating effectively reduces the charge transfer resistance Rs.FeSO₄ is transformed into Fe+Li₂SO₄ during discharge,and the process is reversible during subsequent cycles.In the voltage window range of 0.01-3 V,the conversion reaction and the grain boundary storage of Fe|Li₂SO₄ jointly contribute a reversible capacity of 512 m Ah g^-1.The copper sulfate and its hydrate with higher working potential are further studied.The porous CuSO₄·n H₂O（n=0,1,3）is prepared by dehydration of CuSO₄·5H₂O.The initial voltage plateau of CuSO₄·nH₂O is 1.8 V,and the specific capacity is 430 m Ah g^-1,427 m Ah g^-1 and 360 m Ah g^-1,respectively.It is determined that only the CuSO₄·H₂O is stable by studying the influence of preparation process and electrolyte.GITT shows that the over-potential during the discharge process is 0.2 V less than the charge process.The ion diffusion coefficient is on the order of 10^-12 cm²·S^-1.EIS shows that the charge transfer impedance gradually increases as the charge and discharge proceed.During the initial discharge,CuSO₄·H₂O and Li undergo a two-electron transfer redox reaction.Amorphous Cu and Li₂SO₄·H₂O are formed,and part of Li₂SO₄·H₂O is further decomposed into Li₂SO₄ and H₂O.During the charging process,it is found that only Li₂SO₄·H₂O is reversible to obtain the CuSO₄·H₂O.During the subsequent cycles,Li₂SO₄·H₂O gradually decompose until it is converted into Li₂SO₄and H₂O,completely.