First-principles Study On The Redox Reactions In ACrX₂(A= Li,Na;X=O,S) As Cathode Materials For Lithium/Sodium-Ions Batteries

The rapid development of science and technology has brought not only the prosperity for the society,but also the problems of energy shortage and environmental pollution.Therefore,the development of clean energy and large-scale energy storage has become the main task at the present stage.In this context,lithium/sodium ion batteries,as one of the electrochemical energy storage methods,have received attentions extensively from all circles.However,a thorny issue in the development of lithium/sodium ion batteries is that the capacity of the cathode material has not been able to be greatly improved.In order to meet the increasing global demand for the lithium/sodium ion batteries,it is necessary to develop new cathode materials with high-capacity and good cycle stability.No matter what battery is,capacity is one of the most important parameters.Over the past decades,a great deal of efforts has been made to improve the capacity of cathode materials.The anionic redox may increase the capacity of cathode materials,therefore,extensive studies have been made on the anionic redox reactions from both the experimental and theoretical sides.It is of great significance for scientific researchers to deeply understand the mechanism of the charge and discharge processes and explore the mechanism behind the redox reactions.In this work,comparative studies on the electronic structures and redox reactions in oxides(LiCrO2,NaCrO2)and sulfides(LiCrS2,NaCrS2)are performed.Firstprinciples method has been used to calculate the Bader charge transfer,the electronic structures and the magnetic moments during the entire delithiation/desodiation processes.The analysis on the Bader charge indicates that both the anions and cations participate in the redox reactions when the lithium/sodium ions are extracted from LiCrX2 and NaCrX2(X=O,S).Compared with cations,the anions play prominent roles in the charge compensation processes.Moreover,the loss of electrons of S ions are visibly larger than those of O ions,which is consistent with their electronegativities.It is noted that Cr ions present different characteristics in sulfides and oxides.Cr ions lose electrons in the oxides while gain electrons(less than 0.1e)in the sulfides during the extraction of Li/Na ions.For sulfides,the electrons gained by chromium ions are originated from the S-3p band.Calculations show that all the charge transfer happens between the S-3p/O-2p and the Cr-3d bands.The redox reactions of O or S ions originate from the energy levels of O/S being pushed towards/cross the Fermi surface due to the strong p-d hybridization.