Recovery Of High-quality Silicon From Wafer-slicing Waste As High-performance Lithium-ion Battery Anodes

Due to the advantages of high theoretical specific capacity(4200 mAhg^-1), low charge/ discharge voltage plateaus, massive reserves and environmental benignity, silicon is regarded as the most promising candidate for commercial graphite. However, Si undergoes significant volume effect associated with lithiation/delithiation processes, which results in irreversible pulverization, rearrangement and excessive growth of solid electrolyte interphase（SEI） and the loss of conductive environment. To address these bottlenecks, many efforts have been made to improve the electrochemical performance of silicon based anodes, for instance, reducing particle size to nanoscale, including Si nanoparticles, Si nanowires, Si nanotubes, Si hollow nanospheres, Si porous nanostructure, and constructing “yolk-shell” Si@void@C composites. Nevertheless, the above mentioned strategies need complicated procedures and specialized equipments（such as CVD） with low yield and high cost, making it difficult to commercialize Si based anode materials.Hence, for promoting the practical application of Si-based anodes, it is necessary to combine two different solutions together: 1) explore novel strategy to prepare Si micro/nano materials in a high-efficient way at industry level; 2) based on the above manufacture techniques, develop reasonable purification and surface modification approaches to optimize systematically the electrochemical performance of Si. Taking the above into consideration, starting with interdisciplinary research, this paper chooses crystalline high-purity Si sawdust waste of photovoltaic industry as cheap Si source. In the last few years, the photovoltaic industry has entered the fast development period and the output of global single-crystal Si is more than 60 thousand tons per annum. To fabricate solar cells, the solar grade high-purity Si ingots need to be sliced into Si wafers. During the wafer sawing, 44 wt% of the solar grade Si feedstock ends up as high-purity micro-sized sawdust and goes into the slurry waste according to theoretical calculation, however, in practical production as high as 50-55 wt% crystalline Si has been lost as waste, which means over 30 thousand tons of Si sawdust waste will be generated in PV industry. Obviously, if the Si sawdust waste of PV industry can be successfully applied to lithium ion battery（LIB）, it will bring about enormous environmental and economic benefits without doubt.The main work of this paper includes the following two aspects:1) By adopting high-purity Si sawdust waste of PV industry as LIB Si-based anodes,in-situ construct an ultrathin and compact silicon oxide layer on the Si surface through cleaning, purification and effective surface modification; employing alginate sodium as functional binder, the strong hydrogen bonding between the hydroxylated Si and alginate carboxylic groups results in a firmly-anchored alginate coating layer onto Si surface, which could significantly enhance the interactions between binder and Si and help to increase the attachment of conductive additives to Si. Thus, Si-based electrodes can accommodate the repeatedly expansion/contraction associated with charge/discharge processes without destroying the conductive network during long-time cycling, leading to excellent cycling and rate performance. After 200 cycles, reversible capacity of the Si-based electrode maintains at 2815.5 mAhg^-1, which corresponds to ⁹3.7% capacity retention. Reversible capacities of the Si electrode at 1C, 5C, 10 C and 20 C are found to be 2432, 2276, 1998 and 1561 mAhg^-1, respectively, corresponding to capacity retentions of ⁷7.5,72.6,63.7 and 49.8% of that obtained at 0.05C(0.21 A g^-1).2) Si/graphite hybrid materials are prepared as LIB anodes by mixing purified Sisawdust waste with graphite homogenously with different weight ratios. This method can combine the advantages of stable electrochemical performance of graphite and high capacity of Si together: on the one hand, Si can compensate for the low energy density of graphite; on the other hand, as supporting materials, graphite can accommodate the mechanical strength of Si particles and improve the electrical conductivity of electrodes at the same time, moreover, the void space between graphite can provide extra room for the volume expansion of Si, thereby stable cycling performance can be maintained. After 150 cycles, reversible capacities of electrodes with 10, 20, 30 and 40 wt% silicon content are found to be 430.6, 751.1, 882.2 and 440.4 mAhg^-1, respectively, corresponding to capacity retentions of 98.22, 90.95, 89.38 and 39.33% of that obtained at 30 th charge/discharge cycles(438.4, 825.8, 987 and 1119.6 mAhg^-1, respectively). According to the electrochemical performance analysis, when Si content in Si/graphite hybrid electrodes is below 30wt%, stable long-term cycling performance can be achieved.From the point of interdisciplinary research, the combining of PV industry and LIBs can solve the post-processing problems of Si sawdust waste and produce battery grade Si materials in a high-efficient way at the same time, providing a new route for the large scale production and commercial application of Si-based anodes in LIBs.