Structural Evolution and Electrochemical Performance of Li2MnSiO4/C Nanocomposite as Cathode Material for Li-Ion Batteries

High capacity Li2MnSiO4/C nanocomposite with good rate performance was prepared via a facile sol-gel method using ascorbic acid as carbon source. It had a uniform distribution on particle size of approximately 20？nm and a thin outlayer of carbon. The galvanostatic charge-discharge measurement showed that the Li2MnSiO4/C electrode could deliver an initial discharge capacity of 257.1？mA？h？g？1 (corresponding to 1.56 Li+) at a current density of 10？mA？g？1 at 30°C, while the Li2MnSiO4 electrode possessed a low capacity of 25.6？mA？h？g？1. Structural amorphization resulting from excessive extraction of Li+ during the first charge was the main reason for the drastic capacity fading. Controlling extraction of Li+ could inhibit the amorphization of Li2MnSiO4/C during the delithiation, contributing to a reversible structural change and good cycling performance. 1. Introduction Environmental pollution and energy crisis promote people to search for renewable resources and energy storage device, such as batteries. Rechargeable Li-ion secondary batteries are attracting more and more attention due to their high energy density and environmental friendly features [1]. Recently, orthosilicate Li2MSiO4 (M = Fe, Co, Mn) materials were regarded as the candidate for cathodes [2–4] due to their high theoretical capacity (e.g., Li2MnSiO4: 330？mA？h？g？1). Among this family, Li2MnSiO4 is much easier to achieve the transformation of Mn2+/Mn3+ and Mn3+/Mn4+ to carry out extraction of two Li+ in comparison to other orthosilicates [5, 6]. Li et al. [7] prepared Li2MnSiO4/C through a solution route. The composite electrode exhibited a discharge capacity of 209？mA？h？g？1 at the initial cycle and 140？mA？h？g？1 after ten cycles. Bhaskar et al. [8] fabricated Li2MnSiO4/C by a facile nanocomposite gel precursor route. This composite electrode showed an initial discharge capacity of 330？mA？h？g？1 and a stable discharge capacity of 115？mA？h？g？1 for 30 cycles at room temperature. Devaraju et al. [9] adopted supercritical fluid process to synthesize Li2MnSiO4/C with a mean particle diameter of 4-5？nm. It showed a high capacity of about 320？mA？h？g？1 at the first cycle and 190–220？mA？h？g？1 after 50 cycles. Carbon coating plays a critical role in the improvement of the electrical conductivity of the Li2MnSiO4 and consequently enhancement of discharge capacity. However, this material still has a large irreversible capacity loss and low capacity retention rate upon cycling. Structure change resulting from J-T effects and the dissolution of Mn caused by the disproportionate reaction (Mn3+ → Mn4+ + Mn2+)