The Influence of the Grain Size of Li2MnSiO4 Cathode Material on Its Stability in Li-Ion Battery Cell

Abstract

Application of orthosilicate Li2MSiO4 (M = Fe, Mn, Co, Ni) compounds as cathode materials for LIBs was firstly proposed by Goodenough’s group [1,2]. The materials crystallizes mainly in orthorhombic system (Pmn21 space group) in olivine structure [3-5]. The Li2MSiO4 structure may be described as distorted layers of [SiMO4]∞ on ac axis plane and connected along b axis by LiO4 tetrahedra. Within the layers every SiO4 tetrahedron shares its corners with four 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. Such a structure gives the Li2MSiO4silicates many advantages, e.g.: - possibility of reversible insertion of two lithium ions per molecule and exchange of two electrons by transition metal which leads to very high theoretical capacity, up to 333 mAh/g (for Li2MnSiO4, LMS); - high working voltage, for Li2MnSiO4 (LMS) the potential of the first electrochemical reaction: Li2M2+SiO4 → □LiM3+SiO4 + Li+ is around 4.2 V, while the potential of second reaction: □LiM3+SiO4 → □2M4+SiO4 + Li+(2) is above 4.5 V; - high thermal stability provided by very strong covalent bonding of Si-O; - inexpensive elements (Mn, Si); - environmental friendliness. Unfortunately, the structure of LMS material does also entail some disadvantages: - very low electrical conductivity (~10-13 S cm-1), as in all olivine structure type cathode materials; - limited structural stability in electrochemical reaction which causes amorphisation of the LMS material upon first few charging/discharging cycles [6]; The low conductivity of LMS can be overcome by preparation of the cathode material in the form of nanocomposite where nanosized LMS is uniformly covered with conductive layer. Thus, the finest material should provide the best electrochemical properties. But, with the lower grain size the reactivity of the material surface starts to play an important role. In this study we are focusing on investigation of influence of the grain size of LMS material on its capacity and stability in Li-ion battery cell. Studied materials were obtained using sol-gel synthesis and the grain size control was achieved by controlled heat treatment. The TEM and SEM were used to analyze electrodes before and after galvanostatic charge/discharge cycles. ACKNOWLEDGMENT This work is supported by National Science Centre, Poland under research grant no. 2014/13/B/ST5/04531.