Application of orthosilicates Li₂MSiO₄ (M=Fe, Mn, Co, Ni) compounds as insertion materials for LIBs was firstly proposed by Prof. Goodenough [1]. The materials mainly crystallize in orthorhombic system of Pmn2₁ space group in olivine structure [2-4]. The Li₂MSiO₄ olivine structure may be described as wavy layers of [SiMO₄]_∞ on ac axis plane and connected along b axis by LiO₄ tetrahedra [2]. Within the layers every SiO₄ tetrahedron shares its corner with four next MO₄ tetrahedra. Lithium ions occupy the tetrahedral sites (LiO₄) between two layers and share 3 and 1 oxygen atoms with the layers. In fact, diffusion of lithium ions in this structure is possible only through the canals formed by LiO₄ tetrahedra. Li₂MSiO₄ silicates reveal a possibility of reversible insertion of two lithium ions per molecule, which leads to exchange of two electrons by transition metal. As a results the silicates reveal high theoretical capacity, up to 330 mA/g for Li₂MnSiO₄ (LMS). LMS, which works at potentials 4.2 and ~4.5 V for first and second redox reactions respectively, seems to be the most useful from all of dilithiumorthosilicates family compounds. The main disadvantage of LMS is its extremely low electrical conductivity [2,5] and structural instability [7-11]. While the conductivity issues can be fixed by the preparation of nanograined material and its modification by carbon coating, similarly to the procedures applied in case of LiFePO₄, the reasons of structural instability of LMS are not entirely known. There is a number of reports showing that Li₂MnSiO₄ cathode material undergoes amorphization in the first few charging/discharging cycles but there is no comprehensive explanation of this phenomenon. The degradation of crystalline structure can be caused by Jahn–Taller distortion associated with changes in lattice parameters during Mn³⁺→Mn⁴⁺ transition [8]. Another explanation may be the occurrence of secondary reactions between electrolyte and delithated forms of lithium manganese silicate (LiMnSiO₄ and MnSiO₄). The aforementioned structural changes of the material entail variations in electrochemical properties of Li₂MnSiO₄[9].

The goal of this studies is to examine structural instability of Li₂MnSiO₄which occurs in first electrochemical charging/discharging cycle via microscopic analysis of individual grains at different stages of electrochemical reaction.

The Li₂MnSiO₄material was prepared using Pechini's sol–gel method which was previously reported by our group [9-11]. The structural changes in the material was studied using in-situ X-Ray diffraction as well as ex-situ transmission electron microscopy.

To determine degradation mechanism of Li₂MnSiO₄ and C/Li₂MnSiO₄ in working Li-ion cell two different aspects of LMS performance were analyzed. Firstly corrosive properties of liquid electrolytes were taken under consideration. Solubility of manganese from LMS and C/LMS samples (occurring during electrochemical reaction) in LiPF₆solution in EC:DMC (1:1) was studied. On the other hand using in-situ XRD and ex-situ TEM analysis in different SOC (in initial cycle in 1.5-4.8V potential range) the process of amorphization of crystalline LMS was examined.

ACKNOWLEDGMENT

This work is supported by National Science Centre, Poland under research grant no. 2014/13/B/ST5/04531.

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