Research On Transition Metal Oxide Cathode Materials For Lithium-ion Batteries

Lithium-ion batteries have been sufficiently developed since their birth and have a profound impact on people’s daily lives.Lithium-ion batteries are often named after the category of cathode materials,which reflects the importance of the cathode materials.Transition metal oxide cathode materials are a very important class of cathode materials,as they dominate the majority of the market in consumer electronics,transportation and electrochemical energy storage due to their broad compositions and structures.Li Co O₂（LCO）and Li Mn₂O₄（LMO）were the first batch of cathode materials to be commercialized and today they continue to play an irreplaceable role due to their unique advantages.With increasing market demand,both of two are facing significant challenges caused by material failures,which is severely limiting their further progress.Therefore,it is of great practical value to investigate the failure mechanisms of these two cathode materials and to build material modification schemes thereafter.It is generally accepted that the dissolution of Mn²⁺ions from the cathode and their deposition on the anode via the electrolyte is the main reason for the failure of the LMO batteries,but there is still no unified conclusion on the exact process of Mn dissolution from the LMO cathode.On the one hand,this is due to the complexity of the interface reactions within lithium-ion batteries,and on the other hand,this is due to lack of in-situ instruments.We have investigated the Mn dissolution process in the LMO cathode with the aid of the powerful ab initio molecular dynamics（AIMD）method.After identifying that Mn dissolution tends to occur at the high charge state,we found that Mn dissolution is the result of interface evolution involving electrolyte decomposition and surface reconstitution,where the central factors are surface oxygen loss and EC decomposition.By decoupling these two factors,we found that:Surface oxygen loss is an intrinsic behavior that provides the initial driving force for Mn dissolution;EC decomposition requires the transfer of electrons to Mn atoms with high valence on the surface,and its decomposition products promote further Mn dissolution.After clarifying the key role of surface oxygen loss and EC decomposition in Mn dissolution of the LMO cathode,we continued to investigate how surface doping suppresses Mn dissolution.Using first-principles calculations,we systematically evaluated the effects of Mg,Ca,Sr,Al,Ga,In and all 3,4d transition metal elements and then screened out six elements,namely Nb,Ru,Mo,V,Tc and Ti,that could improve the stability of surface oxygen atoms.In addition,for Mg,Cu and Zn,we also examined their Li-site doping and found that their Li-site doping was more capable of stabilizing the surface oxygen atoms than their Mn-site doping.These dopants,which stabilize the surface oxygen atoms,are expected to inhibit Mn dissolution of the LMO cathode.We then confirmed the reliability of these computational results using uniformly prepared Mg,Mo and Nb surface doping LMO samples.During experiments,we found that surface dopants can modulate the morphology of the LMO particles,which also affects Mn dissolution.For rapid,accurate and cost-effective detection of Mn²⁺concentration in electrolytes,we have also explored novel efficient binary chromogenic reagents,namely PAN-PS,for ultraviolet-visible spectroscopy analysis.The failure mechanism of the high-voltage LCO cathode is more complex.There is no universally accepted dominant factor,but most likely a full range of structural deterioration from the bulk phase to the surface and then to the particle and the electrode.To develop the long-life high-voltage LCO cathode,it is necessary to understand the regulation mechanism of structural stability in the high-voltage LCO.We have systematically studied the overall influence of 61 elements（including Co）on the bulk LCO by using the first-principles calculations.The decomposition energy reflecting the octahedral adaptability of dopants,the lithium layer spacing reflecting the ease of Li⁺migration,Δθreflecting the lattice distortion,the anti-site energy reflecting the migration tendency of dopants and Co atoms,the charge compensation reflecting the redox activity of dopants and lattice oxygen atoms,the average voltage and Coulomb-like forces reflecting the stability of lattice oxygen atoms have been calculated.These computational results were summarized in the periodic tablet,which are important references for the construction of doping modification schemes for the high-voltage LCO.The results on LMO and LCO fully demonstrate the importance of lattice oxygen stability in transition metal oxide cathode materials.If we can break through the constraints of the existing cathode material system and investigate the factors affecting the lattice oxygen stability in a wider compositional and structural space,it will be of great importance for the modification of existing cathode materials and the development of new cathode materials.For this purpose,we firstly defined a simple descriptor for evaluating lattice oxygen stability in Li-TM-O compounds.We then brought this descriptor into the data mining and machine learning processes.The following are the primary results we obtained:（1）The overall oxygen stability decreases as the lithium content of the material increases.（2）When the stoichiometric ratio is fixed,the overall oxygen stability of the material is correlated with the transition metal species.The 3d transition metal elements increase and then decrease the overall oxygen stability from left to right and the 4,5d transition metal elements are generally better than the 3d transition metal elements.（3）We have screened out a number of stable cathode and cathode coating materials,balanced cathode materials and ultra-high-capacity cathode or lithium supplement materials.（4）For individual lattice oxygen atoms,the higher the coordination number and the higher the percentage of lithium atoms in the ligand,the less stable they are in general.（5）A model has been trained to predict the lattice oxygen stability with input parameters containing only simple information on material compositions and structures.