Hydrothermal Stripping Synthesis Of α-LiFeO₂ And β-LiFe₅O₈ As Potential Cathodes For Lithium-ion Batteries

Lithium-ion batteries（LIBs）possessing many virtues indeed fit development of current society,while cathode materials play a key role in performance breakthrough and cost reduction to a great extent.However,commercial cathode materials still have a great demand for LiCoO₂ or other cobalt compounds though Co shows expensive and poisonous shortcomings,limiting its development.Iron-based compounds,both LiFeO₂ and LiFe₅O₈,deserve wide attention in that sense as promising cathode materials for abundance,low cost,and green of Fe element.LiFeO₂ has caught great interests for its high theoretical capacity of 282 mA h g^-1.Here,according to process coupling principle,a facile,controllable,green and economic two-phase reaction method,hydrothermal stripping,was successfully developed to obtainα-Li FeO₂ andβ-LiFe₅O₈.Using cheap Qinghai salt lake brine as raw materials,Fe³⁺and Li⁺were simultaneously extracted to organic phase of tributyl phosphate first,andα-Li FeO₂was synthesized by adding LiOH in water to have hydrothermal stripping reaction with the former.Impurity introduced into sample from the salt lake brine was removed by adjusting washing method.XRD analysises showed thatα-Li Fe O₂ can be obtained within 3 hours at 200²10℃with Li/Fe molar ratio≥30 when LiOH was added to water phase singly.Substituting LiOH with KOH partially,Li/Fe molar ratio could reduce to 20.SEM and TEM images exhibited that small and uniform squares-like particles with diameter of³00 nm were obtained with 1³ h.Changes of molar ratio,temperature and raw materials controlled the particle size significantly.Particles after calcination revealed the best crystallinity at 700℃and nearly spherical with more uniform size.FTIR spectra suggested that organic phase was stable with no decomposition under experimental conditions.A low cost and green concept were achieved in this method as aqueous phase and organic phase used in this method can be recycled.It also greatly reduced the process flow from lithium source of salt lake brine toα-LiFeO₂ product,and provides a new way for application area expansion of saline lake brine,in line with requirements of innovation-driven development strategy.In addition,Fe³⁺singly entered into organic phase of naphthenic acid by extracting Fe（NO₃）₃ solution,and then hydrothermally stripped by LiOH solution to synthesizeβ-LiFe₅O₈.XRD patterns showed thatβ-LiFe₅O₈ was obtained with Li/Fe molar ratio of 15 at 150,180 and 200℃for 4 h.SEM,TEM and HR-TEM images revealed that samples all were similar quasi-spherical shape particles with diameter of5 nm,15 nm and 20 nm,respectively.Initial discharge capacity of samples prepared at both 150 and 180℃reached 200.0 mA h g^-11 at 0.2 C(56.4 mA g^-1),much higher than that of 200℃(127.5 mA h g^-1).Sample obtained at 150℃gave 91.9 mA h g^-11 after50 cycles and capacity retention was around 77.3%with reference to the 10th cycle,higher than that of 180℃（69.5%）,exhibiting a better cycle stability.The electrochemical performance of the sample is also better than that ofα-Fe₂O₃,α-Li FeO₂ andγ-Fe₂O₃ synthesized in this method andα-Li Fe O₂ prepared by some methods in literatures.