Preparation Of Germanium And Silicon Based Materials By Electrolysis In Molten Chlorides

Electrochemical reduction of solid compounds to metals and their alloys in molten salts is suitable not only for conventional metals, but also for semiconductor materials such as germanium and silicon. Reduction of SiO2 to Si has been widely studied, while less attention was paid to the correlative research on the reduction of GeO2. The concrete mechanism and preparation of the reduction of GeO2 should be further discussed. Meanwhile, germanium and silicon based materials show typically high capacity as anodes in lithium-ion battery. Various methods to prepare these materials have been established extensively, but preparation by electrolysis in molten salt has not been reported. Herein, the reduction mechanism of solid GeO2 in molten salt has been studied by cycle voltammetry, constant potential and cell voltage electrolysis. Combined with previous studies, a series of germanium and silicon based materials were obtained by electrolysis in molten salt. Our research also provided a novel method for preparing composites. The main results of the research are summarized as follows:1. Cyclic voltammetry, potentiostatic and constant cell voltage electrolysis together with metal cavity electrode, XRD, EDX analyses and SEM observation were used for discussing the mechanism for the reduction of solid GeO2 in CaCl2-NaCl melt at 750?. It suggested that the mechanism included (1) electrochemical reduction of GeO2 to Ge; (2) chemical formation of calcium germanates ((CaO)x(GeO2)y) and (3) electrochemical reduction of (CaO)x(GeO2)y to Ge. Although the first step can start at a potential (-0.5 V vs. Ag/Ag+) about 1.8 V more positive than that of cathodic decomposition of the electrolyte, the reduction released O2 together with the Ca2+ from the electrolyte react immediately with remaining GeO2, generating the germanates whose reduction occurs at potentials more negative than -1.0 V with a relatively slow speed. An increased polarization is expected to speed up the reduction of germanates. Therefore pure Ge can be prepared at a potential range between -1.10 V and -1.40 V in 4 hours. However, potentials exceeding -1.60 V will lead to the formation of Ca-Ge or Na-Ge intermetallic compounds. Based on these understandings, rapid electrolysis of GeO2 to pure Ge has been carried out at a cell voltage of 2.5 V. A current efficiency as high as 92% and an elemental recovery of about 96.8% were achieved. The electrolytic Ge exhibits a mixture of micrometer particles and sub-micrometer nodular particles with the primary particle sizes of around 100 nm. These findings promise a new approach for the metallurgy of GeO2,as well as preparation of nanometer Ge powders.2. Electrolytic preparation of Cu-Ge compounds and carbon coated Ge-based materials in CaCl2-NaCl melt at 750? and 600? was achieved, and the electrochemical performance of these materials was also evaluated. It was found that the electrolytic Ge at 750? and Cu3Ge/Ge prepared from CuGeO3 show poor cycle performance. When carbon was introduced to Ge-based precursors, carbon coated Ge or Ge/GeO2 composites were obtained by electrolysis, which did better at electrochemical tests than pure Ge. High temperature of melts is beneficial to the rapid deoxidization process, but will cause the sintering of product. Lowering the temperature and the extension of electrolytic time could improve the electrochemical performance of materials. The existence of coated carbon would prevent the spread of the "3PI", which slow the deoxidation process. Hybrid GeO2/Ge/G composite could be prepared by electro reduction in molten salt when using germanate/graphene oxide as the precursor, which shows good cycle stability. The addition of AgCl in precursors might produce internal doped Ag in Ge-based materials, partly solving the problem of low electrical conductivity in electrolysis and cell tests. Ag doping GeO2/Ge/G material exhibited a good cycling stability and rate capability. A charge capacity of 660 mAh/g was found at the 60th cycle.3. Pure Ge was prepared by electrolysis at -1.45 V in 5 hours in MgCl2-NaCl-KCl melt at 700?. Cyclic voltammetry, potentiostatic electrolysis together with metal cavity electrode, XRD, EDX analyses and SEM observation were used for discussing the mechanism for the reduction of solid GeO2. It suggested that the mechanism included (1) chemical formation of calcium germanates ((MgO)x(GeO2)y) and (2) electrochemical reduction of (MgO)x(GeO2)y to Ge. Because of the high hygroscopic ity of MgCl2, the existing MgO in molten salt will react with the immersed GeO2 pellet, generating germanates whose reduction occurs at potentials more negative than-1.40 V (vs. Ag/Ag+). Insoluble MgO would stay inside the pellets, causing a slow reduction. An increased polarization is expected to speed up the reduction of germanates. Potentials exceeding -1.60 V will lead to the formation of Mg-Ge intermetallic compounds, showing porous and crystalline structure after washed by HC1 solution. These findings promise a new approach for the metallurgy of GeO2.Ge prepared in MgCl2-NaCl-KCl melt shows better performance than that in CaCl2-NaCl melt because of the internal existing MgO, which could be further improved by coating graphene. Carbon coated Ge or GeO2/Ge composites can been prepared using the same precursors in CaCl2-NaCl, and deoxidation process was also impeded. Hybrid GeO2/Ge/G composite using germanate/graphene oxide as the precursor shows better cycle stability than other materials. And the obtained Cu/Ge/CuGe composite also show better performance than that in in CaCl2-NaCl system.4. Electrolytic preparation of Si-based composites using Si or SiO2 together with carbon sources and sulfides was achieved in CaCl2 melt at 850?. It was found that Si mixed with graphene oxide or glucose can form carbon-coated SiC/Si composites. Amorphous carbon from glucose is conductive to form SiC to composite Si, but the increase of inactive SiC would lead to poor performance of material. If using GO, highly active SiC/Si/G composites could be prepared. It should be noted that the coated carbon and generated SiC would fower the electroreduction of SiO2/C precursors and the electrochemical performance of product. A better conductivity in precursor pellets and product would be realized if using acetylene black as carbon source, but it is not promising as coating materials. Si/XSi2(X=Mo, W) composite was also prepared from Si and sulfide XS2, whose performance can be enhanced with graphene coating.