Why Is Li2MnSiO4 Unstable in Li-Ion Battery Cell? Structural Studies at Different Stages of Electrochemical Reaction
Authors: Swietoslawski Michal , Gajewska Marta , Molenda Marcin 
Published: February 2020

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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