Li Storage Properties And Mechanisms Of Li/Mg Silicides Prepared Through Hygrogen-driven Chemical Reaction Technique

Si has been considered as the most promising anode materials because of its extremely high theoretical capacity of 3579 mAh/g, low working potentials and natural abundance. However, the huge volume change (～300%) during lithiation and delithiation leads to severe electrode pulverization and subsequent electrical disconnection with cycling, consequently resulting in poor reversibility and rapid capacity fading, which impeded the practical implementation of Si anodes. Based on the review of the fading mechanisms of cyclic capacity and research development of strategies to improve cycle stability of Si-based anode materials, a hydrogen-driven chemical reaction (HDCR) approach for the preparation of lithium silicide and magnesium silicide was proposed to address the issues of low Coulombic efficiency and poor cycling stability. The preparation methods, sctructural characteristics and electrochemical properties of the object materials were studied systematically and the lithium storage and capacity fading mechanisms were elucidated.Li12Si7 with high purity was prepared through HDCR technique, and its lithium storage performances and lithiation/delithiation mechanisms were studied. A highly reversible and stable lithium storage behavior was observed for the as-prepared Li12Si7 in the voltage range of 0.02-0.6 V through lithium insertion/extraction between Li12Si7 and Li12+xSi7 (x≈4.8～5.3) upon discharging/charging. The capacity retention after 30 cycles was calculated to be 74.4% with an average Coulombic efficiency of～ 99%. Ex situ XRD analysis revealed that a new phase emerged when discharging to 0.11 V or charging to 0.45 V. Further charging to above 0.6 V, Li12Si7 gradually transformed into amorphous Si with delithiation. In particular, Li12Si7 exhibited a significantly improved cycling stability at 0.02-2.0 V with respect to the pristine Si due to the pre-expansion and reduced relative volume change during lithiation/delithiation. The maximum discharge capacity of the as-prepared Li12Si7 anode was 2648 mAh/g (based on the mass of Si) and the capacity retention of the anode was 41.8% after 20 cycles. However, the discharge capacity of the pristine Si anode was only 353 mAh/g and the corresponding capacity retention was only 12.7% after 20 cycles. Moreover, it was found that the ball milling treatment could reduce particle size and degree of crystallinity of Li12Si7. The 48-h-milled Li12Si7 exhibited a particle size of～1 μm and delivered a maximum charge capacity of 2908 mAh/g. The charge capacity of the 48-h-milled Li12S17 stayed at 853 mAh/g after 60 cycles and the corresponding capacity retention was 29.3%, which were about 7 times and 5 times those of the 48-h-milled Si, respectively. In addition, the degree of crystallinity was found to influence the discharge/charge curves. However, the cycling stability of Lii2Si7 remained nearly constants with increasing the degree of the crystallinity.The dependence of the charge transfer resistance on the depth of discharge/charge of amorphous Li12Si7 was analyzed. The result indicated that the charge transfer resistence was closely related to the lithium content. A dramatic increase in the charge transfer resistance due to the pulverization of the electrode material was observed while discharging to below 0.11 V or charging to above 0.7 V, which made the charge transfer resistance did not follow the U-type curve as a function of potential. Consequently, the optiamized working voltage window was determined to be 0.11-0.7 V, within which a significantly improved cycling performance was obtained. After 100 cycles, the capacity retention of the amorphous Li12Si7 was 57.6%at 0.11-0.7 V, which is more than a 2-fold enhancement compared to the capacity retention of the electrode cycled at 0.02-2.0 V.Structural evolutions of Mg2Si at diffrent charge/discharge states were studied and the underlying mechanism of the fast capacity fading of Mg2Si was elucidated. It was found that the charge process of the lithiated samples include two stages. First, Li-Mg alloy and Li2MgSi were delithiated to form Li^Mg2Si. And then, LixMg2Si was further delithiated to reform Mg2Si. Further analysis revealed that the capacity degradation of Mg2Si mainly originated from the degradation of the first stage because that the dissociated Mg during discharge was hard to be fully lithiated, which leads to the decrease of Mg2Si with cycling. Replacing Mg2Si with Li2MgSi could eliminate the dissociation of Mg, consequently improving the cycling stability of the electrode.On the basis of the above mechanism, a series of 7Mg2Si-xL12Si7 (0< x < 2) composite samples were designed and their electrochemical lithium storage properties as well as mechanisms were systematically investigated. It was found that the presence of Li12Si7 could effectively impede the dissociation of Mg and/or formation of Li-Mg alloy during discharge, consequently inducing significantly improved reversibility and cycling performance of the electrodes. The molar ratio of Mg and Si in 7Mg2Si-Li12Si7 is 1:1, which is identical to that of Li2MgSi. Therefore, the dissociation of Mg was completely impeded during discharge/charge and the 7Mg2Si-Lii2Si7 composite anode exhibited significantly improved electrochemical properties as it delivered a maximum charge capacity of 814 mAh/g and exhibited a capacity retention of 72.6%after 50 cycles. However, the cycling performance of Mg2Si-Si composite was almost identical to that of Mg2Si because the dissociation of Mg was not impeded. The lithiation of Mg2Si to convert into Li2MgSi and Mg occurred at-0.14 V, which is higher than the lithiation potential of the crystalline Si to convert into amorphous Li-Si alloy (-0.1 V). Consequently, the lithiation of the Mg2Si-Si composite anode was only a combination of the separated lithiation processes of Mg2Si and Si.Moreover, SiO was prelithiated by means of HDCR technique. After prelithiation, the initial irreversible capacity of SiO was significantly reduced and the initial Coulombic efficiency and cycling stability were improved. The initial Coulombic efficiencies of the as-prepared Li0.4-Si-O, Li0.75-Si-O and Li-Si-O were determined to 78.2%,87.8%and 90.7%, respectively, which were much higher than that of the pristine SiO (69.5%). Meanwhile, the volume expansion of Li-Si-O system during the first lithiation was lower than that of SiO. Therefore, the charge transfer resistance of the Li-Si-O sample was obviously smaller than that of the SiO sample, which leads to a higher capacity retention of Li-Si-O anode. After 50 cycles, the capacity retention of Li-Si-O anodes was 44%, which was twice that of the SiO sample. However, there was a Li2O layer at the surface of Li-Si-O composite, which is unfavorable for the electron transport. The electrochemical properties of Li-Si-O system was subsequently improved by adding Cu, which has a high electrical conductivity. The Li-Si-O/Cu composite which contains 10 wt%Cu exhibited an initial Coulombic efficiency of 92%and capacity retention of 62.9%after 50 cycles.