| Silicon composite, making method, and non-aqueous electrolyte secondary cell negative electrode material -> Monitor Keywords |
|
|
 
Silicon composite, making method, and non-aqueous electrolyte secondary cell negative electrode materialRelated Patent Categories: Chemistry: Electrical Current Producing Apparatus, Product, And Process, Current Producing Cell, Elements, Subcombinations And Compositions For Use Therewith And Adjuncts, Electrode, Chemically Specified Inorganic Electrochemically Active Material ContainingSilicon composite, making method, and non-aqueous electrolyte secondary cell negative electrode material description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060003227, Silicon composite, making method, and non-aqueous electrolyte secondary cell negative electrode material. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATION [0001] This non-provisional application claims priority under 35 U.S.C. .sctn.119(a) on patent application No. 2004-195586 filed in Japan on Jul. 1, 2004, the entire contents of which are hereby incorporated by reference. TECHNICAL FIELD [0002] This invention relates to a silicon composite powder having a capacity controlled to compensate for the drawback of silicon which is believed useful as lithium ion secondary cell negative electrode active material; a method for preparing the same; and a non-aqueous electrolyte secondary cell negative electrode material comprising the powder. BACKGROUND ART [0003] With the recent remarkable development of potable electronic equipment, communications equipment and the like, a strong demand for high energy density secondary batteries exists from the standpoints of economy and size and weight reductions. One prior art method for increasing the capacity of secondary batteries is to use oxides as the negative electrode material, for example, oxides of V, Si, B, Zr, Sn or the like or complex oxides thereof (see JP-A 5-174818 and JP-A 6-060867 corresponding to U.S. Pat. No. 5,478,671), metal oxides quenched from the melt (JP-A 10-294112), silicon oxide (Japanese Patent No. 2,997,741 corresponding to U.S. Pat. No. 5,395,711), and Si.sub.2N.sub.2O and Ge.sub.2N.sub.2O (JP-A 11-102705 corresponding to U.S. Pat. No. 6,066,414). Conventional methods of imparting conductivity to the negative electrode material include mechanical alloying of SiO with graphite, followed by carbonization (see JP-A 2000-243396 corresponding to U.S. Pat. No. 6,638,662), coating of silicon particles with a carbon layer by chemical vapor deposition (JP-A 2000-215887 corresponding to U.S. Pat. No. 6,383,686), and coating of silicon oxide particles with a carbon layer by chemical vapor deposition (JP-A 2002-42806). None of these patents relates to the method of alleviating a substantial volume change of a silicon negative electrode during charge/discharge cycles which is an outstanding problem characteristic of the silicon negative electrode nor the method of reducing the current collection associated with the volume change. It remains an important task to establish such techniques. [0004] The prior art methods using silicon as such or making its surface conductive for improving the cycle performance of negative electrode material are successful in increasing the charge/discharge capacity and energy density, but are not necessarily satisfactory because of failure to fully meet the characteristics required in the market, particularly the cycle performance of importance in mobile phone and other applications. There is a desire for further improvement in cycle performance. [0005] In particular, Japanese Patent No. 2,997,741 uses silicon oxide as the negative electrode material in a lithium ion secondary cell to provide an electrode with a high capacity. As long as the present inventors have confirmed, there is left a room for further improvement as demonstrated by a still high irreversible capacity on the first charge/discharge cycle and cycle performance below the practical level. With respect to the technique of imparting conductivity to the negative electrode material, JP-A 2000-243396 suffers from the problem that solid-to-solid fusion fails to form a uniform carbon coating, resulting in insufficient conductivity. In the method of JP-A 2000-215887 which can form a uniform carbon coating, the negative electrode material based on silicon undergoes excessive expansion and contraction upon adsorption and desorption of lithium ions, meaning impractical operation, and loses cycle performance. Thus, the charge/discharge quantity must be limited. In JP-A 2002-42806, despite a discernible improvement of cycle performance, due to precipitation of silicon crystallites, insufficient structure of the carbon coating and insufficient fusion of the carbon coating to the substrate, the capacity gradually lowers as charge/discharge cycles are repeated, and suddenly drops after a certain number of charge/discharge cycles. This approach is thus insufficient for use in secondary cells. SUMMARY OF THE INVENTION [0006] An object of the present invention is to provide a silicon composite which maintains the high initial efficiency inherent to silicon, has excellent cycle performance, and has alleviated a substantial volume change during charge/discharge cycles so that it is effective as active material for lithium ion secondary cell negative electrodes; a method for preparing the same; and a non-aqueous electrolyte secondary cell negative electrode material comprising the silicon composite. [0007] The inventor has discovered a silicon composite which maintains the high initial efficiency inherent to silicon, has excellent cycle performance, and has alleviated a substantial volume change during charge/discharge cycles and which is thus effective as the active material for lithium ion secondary cell negative electrodes. [0008] The development of an electrode material having an increased charge/discharge capacity is very important and many engineers have been engaged in the research and development thereof. Under the circumstances, silicon and amorphous silicon oxides represented by the general formula SiO.sub.x wherein 1.0.ltoreq.x<1.6 are of great interest as the negative electrode active material for lithium ion secondary cells because their capacity is large. Only few of them have been used in practice because of their shortcomings including substantial degradation upon repeated charge/discharge cycles, that is, poor cycle performance and in particular, low initial efficiency. [0009] Making investigations from such a standpoint with the target of improving cycle performance and initial efficiency, the inventor found that CVD treatment of silicon oxide powder led to a substantial improvement in performance as compared with the prior art. However, this approach starting with silicon oxide left the problem of low initial efficiency due to the presence of oxygen atoms. The problem could, of course, be solved by some means, for example, by the addition of phenyl lithium which is known as a method for compensating for the low initial efficiency. These solutions, however, invited side issues that the cell manufacture process becomes complex and unnecessary materials are left within the cell. [0010] In contrast, a silicon powder characterized by the absence of oxygen is expected to have a far greater charge/discharge capacity than the silicon oxide. On the other hand, the silicon powder undergoes a substantial volume change while occluding and releasing a large amount of lithium, which can cause separation between silicon and binder, breakage of silicon particles, and even separation between the electrode film and the current collector. This leads to such problems as a failure of current collection and cycle degradation. Among approaches contemplated to solve these problems, electrically controlling the capacity is effective as a method of alleviating the volume change by expansion and contraction of silicon, but impractical. Under the circumstances where a charge/discharge capacity as large as that of silicon is not necessary, there is a need for a silicon base material having the advantages of a less volume change and good adhesion to binders or the like, notwithstanding a lower energy density than silicon. [0011] Making extensive investigations from this standpoint on a material which undergoes a less volume change upon occlusion and release of lithium even during full charge/discharge operation and has highly adhesive surfaces, the inventor has found that the above problems of lithium ion secondary cell negative electrode active material are overcome by coating silicon particles or micro-particles with an inert robust material, that is, silicon carbide. The resulting material has an initial efficiency comparable to or surpassing the existing carbonaceous materials and an extremely greater charge/discharge capacity than the carbonaceous materials and achieves drastic improvements in cyclic charge/discharge operation and efficiency thereof. [0012] In one aspect, the present invention provides a silicon composite comprising silicon particles whose surface is at least partially coated with a silicon carbide layer. [0013] In preferred embodiments, the silicon particles have an average particle size of 50 nm to 50 .mu.m; the silicon particle surface is at least partially fused to silicon carbide; a diffraction line attributable to silicon is observed when the silicon composite is analyzed by x-ray diffractometry; the silicon composite contains free carbon in an amount of up to 5% by weight; the silicon composite contains 5 to 90% by weight of zero-valent silicon capable of generating hydrogen gas when reacted with an alkali hydroxide solution; and after the silicon composite is treated with a mixture of hydrofluoric acid and an oxidizing agent and heat dried, silicon carbide is left as the evaporation residue. [0014] In another aspect, the present invention provides a method for preparing the silicon composite defined above, comprising the steps of subjecting a silicon powder to thermochemical vapor deposition treatment with an organic hydrocarbon gas and/or vapor at 900.degree. C. to 1,400.degree. C., and heating the powder for removing a surface excess free carbon layer through oxidation. [0015] Also contemplated herein is a negative electrode material for a non-aqueous electrolyte secondary cell, comprising the silicon composite defined above. [0016] The silicon composite of the present invention maintains the high initial efficiency inherent to silicon, has excellent cycle performance, and alleviates a substantial volume change during charge/discharge cycles so that it is effective as the active material for lithium ion secondary cell negative electrodes. When the silicon composite is used as the active material for a lithium ion secondary cell negative electrode, the resulting lithium ion secondary cell negative electrode material is adherent to a binder, has a high initial efficiency, alleviates a volume change during charge/discharge cycles, and is improved in repeated cyclic operation and efficiency thereof. BRIEF DESCRIPTION OF THE DRAWINGS [0017] FIG. 1 diagrammatically illustrates in cross section the structure of a silicon composite particle of the invention. [0018] FIG. 2 is a chart of x-ray diffraction (Cu--K.alpha.) on a silicon composite obtained by starting with a silicon powder, conducting thermal CVD using methane gas, and conducting oxidative decomposition for removing carbon. DESCRIPTION OF THE PREFERRED EMBODIMENTS Continue reading about Silicon composite, making method, and non-aqueous electrolyte secondary cell negative electrode material... Full patent description for Silicon composite, making method, and non-aqueous electrolyte secondary cell negative electrode material Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Silicon composite, making method, and non-aqueous electrolyte secondary cell negative electrode material patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. Start now! - Receive info on patent apps like Silicon composite, making method, and non-aqueous electrolyte secondary cell negative electrode material or other areas of interest. ### Previous Patent Application: Lithium secondary battery and method for producing same Next Patent Application: Sheet anodes based on zinc-aluminum alloys and zinc-air batteries containing the same Industry Class: Chemistry: electrical current producing apparatus, product, and process ### FreshPatents.com Support Thank you for viewing the Silicon composite, making method, and non-aqueous electrolyte secondary cell negative electrode material patent info. IP-related news and info Results in 0.13462 seconds Other interesting Feshpatents.com categories: Novartis , Pfizer , Philips , Polaroid , Procter & Gamble , 174 |
 * Protect your Inventions * US Patent Office filing 
PATENT INFO |
|
