Study of Lithium Silicide Nanoparticles as Anode Materials for Advanced Lithium Ion Batteries

The development of high-performance silicon anodes for the next generation of lithium ion batteries (LIBs) evokes increasing interest in studying its lithiated counterpart-lithium silicide (Li_xSi). In this paper we report a systematic study of three thermodynamically stable phases of Li_xSi (x = 4.4, 3.75, and 2.33) plus nitride-protected Li_4.4Si, which are synthesized via the high-energy ball-milling technique. All three Li_xSi phases show improved performance over that of unmodified Si, where Li_4.4Si demonstrates optimum performance with a discharging capacity of 3306 (mA h)/g initially and maintains above 2100 (mA h)/g for over 30 cycles and above 1200 (mA h)/g for over 60 cycles at the current density of 358 mA/g of Si. A fundamental question studied is whether different electrochemical paradigms, that is, delithiation first or lithiation first, influence the electrode performance. No significant difference in electrode performance is observed. When a nitride layer (Li_xN_ySi_z) is created on the surface of Li_4.4Si, the cyclability is improved to retain the capacity above 1200 (mA h)/g for more than 80 cycles. By increasing the nitridation extent, the capacity retention is improved significantly from the average decrease of 1.06% per cycle to 0.15% per cycle, while the initial discharge capacity decreases due to the inactivity of Si in the Li_xN_ySi_z layer. Moreover, the Coulombic efficiencies of all Li_xSi-based electrodes in the first cycle are significantly higher than that of a Si electrode (∼90% vs 40-70%).