The Theoretical Studies On Rare Earth Doped Cathode Materials Of Li-Ion Batteries

Experimental studies have made great improvements on the performance of cathode materials for Li-ion batteries by rare earth doping.However,the effects of rare earth elements on some basic physical properties are unrevealed,which needs intrinsic analysis from microscopic aspect.In order to shed light on them,we study the rare earth doping issues of cathode materials by first-principles method,analyzing the effects of rare earth doping on the physical properties of cathode materials for Li-ion batteries.We focus on two common cathode materials for Li-ion batteries: LiCoO₂ and LiMn₂O₄.The structural,electronic,Li ion migration properties,and some other properties of rare earth doped Li CoO₂ and LiMn₂O₄ are systematically studied respectively.1)As for structural properties,the lattice constants increase after rare earth doping,regardless of LiCoO₂ or LiMn₂O₄.These results theoretically solve the qualitative contradiction of experimental studies?lattice constants increase or decrease?.2)As for electronic properties,the electronic structures obtained by our calculations indicate that the cathode materials remain semiconducting characteristics except for the smaller band gaps and some edge states near the Fermi level after doping.To some extent,these explain the experimental results that rare earth doping improves the electronic conductivity of cathode materials.3)As for Li mobility,the Li ion migration barrier is determined by two factors.On the one hand,Li ion migration is affected by the size of the migration channel resulting from the rare earth doping;On the other hand,Li ion migration is affected by the variation of the local potential energy surface brings by rare earth elements.Herein,we perfect the oversimple influence mechanism proposed by experimental studies,which was simply based on the one factor?the size of the migration channel?.4)As for some other properties,we study the mechanical properties,the delithiated state and the delithiation/lithiation voltage of rare earth doped LiMn₂O₄.We conclude that LiMn₂O₄ remains brittle and the rigidity is weakened after doping.During the delithiation progress,the volume change is weakened and the average delithiation voltage is increased after doping,which means that the cycling stability and energy density of LiMn₂O₄ can be improved by rare earth doping.Overall,we theoretically study several basic issues of rare earth doped cathode materials,revealing the microscopic influence mechanism of rare earth doping on cathode materials,and giving the explanation to some experimental results.