Doping LiMnPO4 with Cobalt and Nickel: A First Principle Study

Abstract

Abstract:A density functional theory (DFT) study has been carried out on transition metal phosphates with olivine structure and formula LiMPO 4 (M = Fe, Mn, Co, Ni) to assess their potential as cathode materials in rechargeable Li-ion batteries based on their chemical and structural stability and high theoretical capacity. The investigation focuses on LiMnPO 4 , which could offer an improved cell potential (4.1 V) with respect to the reference LiFePO 4 compound, but it is characterized by poor lithium intercalation/de-intercalation kinetics. Substitution of cations like Co and Ni in the olivine structure of LiMnPO 4 was recently reported in an attempt to improve the electrochemical performances. Here the electronic structure and lithium intercalation potential of Ni-and Co-doped LiMnPO 4 were calculated in the framework of the Hubbard U density functional theory (DFT+U) method for highly correlated materials. Moreover, the diffusion process of lithium in the host structures was simulated, and the activation barriers in the doped and pristine structures were compared. Our calculation predicted that doping increases Li insertion potential while activation barriers for Li diffusion remain similar to the pristine material. Moreover, Ni and Co doping induces the formation of impurity states near the Fermi level and significantly reduces the band gap of LiMnPO 4 .

Density Functional Theory Investigation of Mixed Transition Metals in Olivine and Tavorite Cathode Materials for Li-Ion Batteries

“…The calculated lattice parameters of LMP were a = 10.51283 Å, b = 6.12553 Å, and c = 4.74066 Å, with a unit-cell volume of 305.28298 Å 3 . These values are in good agreement with previous DFT and experimental results. ,, …”

“…To the best of our knowledge, Ni-substitution studies in LMP and LMPF cathodes have been seldom reported. Previously, Sgroi and Wang et al have reported the investigation of metal-doped LMP using different elements or concentrations. , Therefore, the present study was aimed at bridging this gap by performing DFT calculations to understand the geometry and electronic structure of LiMn 0.5 Ni 0.5 PO 4 and LiMn 0.5 Ni 0.5 POF cathode materials. Li diffusions within the structures were also investigated.…”

Energetics and cathode voltages of LiMPO4 olivines ( M=Fe , Mn) from extended Hubbard functionals

“…The relevance of the electronic and ionic degrees of freedom within the single particles or grains of the electrodes, and their role in determining the performance of Li-ion batteries (e.g., rate capability, energy density), has made the use of first-principles calculations fundamental in the understanding of their functionality and increasingly more common for the characterization and design of battery materials [28,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54]. Since the open-circuit voltage corresponds to the redox potential of the electro-chemically active species changing their oxidation state during the charge/discharge of the battery (these are often transition-metal ions), it is crucial for the energetics of various phases and compositions involved in the charge/discharge processes to be predicted accurately and reliably.…”

“…[75][76][77]); this work showed that DFT+U with effective interactions computed from first-principles [78], albeit averaged over different Li contents, can promote a more pronounced localization of d electrons and predict average voltages (with respect to Li/Li + ) in closer agreement with available experimental data, while also recovering the correct thermodynamics between phases. This effort led to Hubbard-corrected DFT becoming a standard computational tool to perform predictive first-principles calculations on Li-ion cathode materials, and over the last decade its use on these systems has been broad and successful [40,42,[79][80][81][82][83][84][85][86][87]. Although the accuracy of the original application of DFT+U to these materials was largely due to the possibility to compute the Hubbard parameter U from first principles, its semiempirical evaluation was often preferred, probably due to the complexity to evaluate U reliably and efficiently on every system of interest.…”

“…This failure results from the incomplete localization of valence electrons on the d states due to the defective cancellation of the electronic self-interaction, that is a well known problem of most approximate DFT functionals. DFT+U is effective in fixing this problem[38][39][40][41][42][75][76][77] and, favoring integer occupations of atomic states, stabilizes a charge-disproportionated ground states thus leading to a clear distinction between 2+ and 3+ Fe ions. In fact, as evident from the DFT+U ave results in TableIII, consistently with previous work, half of the Fe ions in Li 0.5 FePO 4 have an occupation that closely resembles that of the (2+) Fe ions in LiFePO 4 , the other half that of the (3+) Fe ions in FePO 4 .…”