First-principles Calculations On Li₂MSiO₄(M=Fe,Mn) Cathode Materials For Lithium-ion Batteries

Lithium-based silicate compounds, Li2MSiO4 (M = Mn, Fe, Co, Ni, etc.), have been intensivly studied as potential cathode materials for lithium ion batteries because of their low cost and high safty properties. Among these candidates, Li2FeSiO4 has shown the best electrochemical performance, which exhibited a high reversible specific capacity of 125 mAhg-1 at 2C rate. In the early stage of study, Li2FeSiO4 has been considered to have aβ-Li3PO4 related structure, with the space group of Pmn21. In 2008, Nishimura et al. prepared phase pure Li2FeSiO4 crystals and studied the crystal structure of the material detailed by X-ray diffraction. Now it is widely accepted that Li2FeSiO4 has a monoclinic structure with the space group of P21. In this work, we calculated the electronic structure of Li2FeSiO4 using first principle calculations based on this P21 structure to study the physical and electochemical properties of the material. We also study the related properties of similar Li2MnSiO4 and Li2Mn0.5Fe0.5SiO4 compounds using the first principle calculations.Firstly, we made a geometry optimization for the crystal model of Li2FeSiO4. Based on this the electronic structure of the material was calculated by GA approximation without considering the electron spin polarization. The result showed that the material was a conductor, which is obviously inconsistent with the actual situation. Therefore, when calculating the electronic structure of Li2FeSiO4 , we must consider the Fe-3d electronic spin polarization. After considering the spin polarization, we obtained an energy band gap of 0.359 eV, which shows that the material was a semiconductor. Based on the analysis to the total density of states (TDOS) and part density of states (PDOS), it is showed that the Fe2+ ion in Li2FeSiO4 were in high spin state. In addition, the DOS near Fermi level was very small, most of which were contributed from the Fe-3d electron. This indicates the weak electronic conductivity of the material.Only one Li+ can be extracted from Li2FeSiO4 because the oxidation state of Fe in the material was +2. As we know, there are four different Li atoms in the structure of Li2FeSiO4. It is difficult to determine which one could be extracted during electrochemical process based on experimental studies. For discussing this issue in theory, we respectively calculated the total energy of LixFeSiO4 when different lithium-ion is extracted. The results show that, when lithium-ion located in Li(1) or Li(2) is extruded, total energy of system is lower. After calculating the electronic structure of LixFeSiO4, we found that only when Li(1) or Li(2) is extruded, the lost charge are mainly providesd by the Fe (1)-ion located near the Li (1) or Li (2). At this point, the band gap between Fe (1) and Fe (2) is almost 0 eV, which makes the conductivity increase. As continue extracting lithium ions, the material's band gap get wider, so the conductivity of the system got lower again. The analysis to the TDOS and PDOS indicates, with the lithium ions extracting from the Li2FeSiO4, the O-2p orbital obviously moved to the high energy level. This makes the band gap between Fe-3d and O-2p decrease. This may increase the total energy of the system, so that the architecture becomes unstable, which may affect the electrochemical cycling performance of the mateiral.Through comparing the lattice constant of off-lithium LixFeSiO4 system, we found that with the Li-ion extrusion, the material's lattice constant showed different trends along all directions. As the first lithium ion is extracting, it is not obvious about the a-parameter increasing and the c-parameter reducing. However, with the second lithium-ion extracting, the a-parameteri increased sharply, while the c-parameter decreased sharply. Li2FeSiO4 had a typical two-dimensional layered structure in a axis direction, with layers combine with each others depending on the electrostatic interactions of Li. Therefore, the lithium-ion extraction will certainly have an enormous impact on the crystal structure of the material. It is particular important that, with the second lithium-ion extraction, the lattice constant changed obviously as the cell volume even increased after the extraction of the second lithium-ion. This dramatic structural change is one of main reasons that lead to the reduce of cycling stability. In addition, we used first principles pseudopotential method to calculate the prolapse voltage when LixFeSiO4 system is in different charging state.When 1 < x < 1.5,the theoretical average voltage of Li/LixFeSiO4 battery is 2.45V when Li-ion extracted either from Li (1) position or from Li (2), which fit well to the experimental results (2.78 V). When 1.5 < x < 2, it is 3.05V and fit well to the experimental results (3.10V). If the second extracted lithium is Li (3) or Li (4), the theoretical average voltage was 6.07 V and 5.97V, respectively. It is very different with the experimental results, which verified that only Li (1) and Li (2) ions can be extracted in the electrochemical process.Both Li2MnSiO4 and Li2FeSiO4 belong to theβ-Li3PO4 orthogonal structure, thus they are easy to form a Li2MnxFe1-xSiO4 solid solution. Subject to restrictions on the number of transferable electron, Li2FeSiO4 can only achieve reversible insertion and extraction of one Li ion (Fe2 +→Fe3 +). However, Li2MnSiO4 allow insertion and extraction of two lithium ions (Mn2 +→Mn4 +), so the material had a higher theoretical capacity. In this paper, we also studied the electronic structure of Li2MnSiO4 and Li2Mn0.5Fe0.5SiO4. It is similar with Li2FeSiO4, when we calculated the electronic structure of Li2Mn0.5Fe0.5SiO4 and Li2MnSiO4, we must consider the spin polarization of Fe-3d or Mn-3d electrons. Calculation shows that the band gap of Li2MnSiO4 and Li2Mn0.5Fe0.5SiO4 were 1.865 eV and 0.367 eV, respectively. Obviously, the band gap of Li2MnSiO4 significantly larger than Li2FeSiO4, this indicates that the former had a lower electronic conductivity. When Mn of Li2MnSiO4 is replaced by Fe, the band gap of Li2Mn0.5Fe0.5SiO4 was smaller than Li2MnSiO4. This shows that the electronic conductivity has been greatly improved compared with Li2MnSiO4. After analyzing the TDOS and PDOS of Li2Mn0.5Fe0.5SiO4, we can see that, Mn2 + and Fe2 + in the materials are in high spin states. The states density near the Fermi level comes mainly from Fe-3d electrons. This shows that the conductivity of Li2Mn0.5Fe0.5SiO4 primarily comes from the addition of Fe2 + ions.