Lithium ion conducting lithium sulphur oxynitride thin film, and a process for the preparation thereof

Abstract: The disclosure herein relates to a lithium ion conducting electrolyte. This electrolytic material has improved ionic conductivity. The material disclosed herein is an amorphous compound of the formula LixSMwOyNz wherein x is between approximately 0.5 and 3, y is between 1 and 6, z is between 0.1 and 1, w is less than 0.3 and M is an element selected from B, Ge, Si, P, As, Cl, Br, I, and combinations thereof. The material can be prepared in the form of a thin film. The electrolyte material can be used in microbatteries and elctronic systems. (end of abstract)

Agent: Buchanan, Ingersoll & Rooney PC - Alexandria, VA, US
Inventors: Philippe Vinatier, Alain Levasseur, Brigitte Pecquenard, Kyong-Hee Joo
USPTO Applicaton #: #20070172738 - Class: 429321 (USPTO)

Lithium ion conducting lithium sulphur oxynitride thin film, and a process for the preparation thereof description/claims

The Patent Description & Claims data below is from USPTO Patent Application 20070172738, Lithium ion conducting lithium sulphur oxynitride thin film, and a process for the preparation thereof.

Brief Patent Description - Full Patent Description - Patent Application Claims

[0001]The present invention relates to a lithium ion conducting electrolyte in a thin film, as well as a method for producing it. This electrolyte can be used in particular in microbatteries and electrochromic systems.

[0002]Lithium microbatteries can be used in various technological fields. A microbattery is produced by stacking a positive electrode material, an electrolytic material and a negative electrode material in thin layers. A microbattery is charged and discharged respectively with a transfer of lithium ions from one electrode to the other through the electrolyte, while electrons circulate via the external circuit. An improvement in the ionic conductivity of the electrolyte brings about a reduction in the internal resistance of the battery which can then be charged or discharged at a high current without an unacceptable ohmic drop.

[0003]Electrochromic systems can be formed by depositing an electrochemical cell, similar to that which forms a microbattery, onto a substrate made of glass or of transparent polymer covered with a transparent conducting layer. An electrochromic system is transparent and colorless in the charged state and colored in the discharged state or vice versa. Its operation relies on a transfer of ions from one electrode to another, as rapidly as possible, so as to minimize the response time of the system, said rapid transfer being promoted by an electrolyte having a high conductivity.

[0004]It is known to prepare electrolytes in thin film form by cathode sputtering. In this type of method, a target is used to obtain an electrolyte film made of amorphous material, it being understood that the amorphous structure generally enables the ionic conductivity of the electrolyte to be improved.

[0005]U.S. Pat. No. 5,512,147 and U.S. Pat. No. 5,597,660 describe the preparation and use of a thin film of Li.sub.xPO.sub.yN.sub.z (with x around 2.8, 2y+3z.apprxeq.7.8 and 0.16.ltoreq.z.ltoreq.0.46) as a solid electrolyte in a microbattery. The film is prepared by cathode sputtering of an Li.sub.3PO.sub.4 target in an atmosphere of nitrogen. The ionic conductivity of the film is around 2.times.10.sup.-6 S/cm.

[0006]K. H. Joo, P. Vinatier et al., [Solid State Ionics, 2003, Vol. 160, No. 1-2, pp. 51-59] describe electrolytes formed from a thin film (1-2 .mu.m) of (1-x)LiBO.sub.2-xLi.sub.2SO.sub.4, x being 0.4 to 0.8. The maximum conductivity is obtained for x=0.7 and is 2.5.times.10.sup.-6 S/cm.

[0007]EP-1 365 470 describes an electrolyte formed from LIPS containing, in at %, 20% to 60% Li, 3% to 20% P, 30% to 60% S and optionally up to 5% O and/or N. The film forming the electrolyte can be deposited on a substrate that will serve as an electrode. Deposition can be carried out in particular by cathode sputtering. In an electrochemical cell that comprises an anode formed of a 10 .mu.m layer of lithium, a thin layer (0.5 .mu.m) of electrolyte obtained from a "78% Li.sub.2S-21.5 P.sub.2S.sub.5-0.5 Li.sub.3PO.sub.4" target, the ionic conductivity is 1.3.times.10.sup.-3 S/cm at 25.degree. C. (Cf. p. 6, .sctn.42 and 44). The target used for cathode sputtering is a target consisting of sulfides sensitive to atmospheric moisture. Such a target is therefore brittle and unsuited to use on an industrial scale.

[0008]S. J. Lee et al., [Electrochem. Commun., 2003, Vol. 5, No. 1, pp. 32-35] describe a battery in which the electro-lyte is Li.sub.1.9Si.sub.0.28P.sub.1.0O.sub.1.1N.sub.1.0, the active material of the positive electrode is LiCoO.sub.2 and the negative electrode is Si.sub.0.7V.sub.0.3. The conductivities obtained for the electrolyte are around 8.8.times.10.sup.-6 S/cm, a value that is quite close to those obtained in the present invention. The thin film of LiSiPON is obtained by cathode sputtering using a target consisting of Li.sub.3PO.sub.4 and Li.sub.2SiO.sub.3.

[0009]The object of the present invention is to provide an electrolytic material that has an improved ionic conductivity compared with materials of the prior art, and/or that can be prepared from stable targets suitable for industrial scale manufacture.

[0010]The material according to the present invention consists of an amorphous compound having the atomic composition Li.sub.xSM.sub.wO.sub.yN.sub.z in which x is between approximately 0.5 and approximately 3, y is between approximately 1 and approximately 6, z is between 0.1 and 1, w is less than 0.3 and M represents one or more elements chosen from B, Ge, Si, P, As, Cl, Br and I.

[0011]In one particular embodiment, x is between approximately 0.5 and approximately 2, y is between approximately 1 and approximately 4, z is between 0.1 and 1 and w=0. The material then consists of 100% Li.sub.xSO.sub.yN.sub.z.

[0012]In another embodiment, w is not zero and the compound contains a small proportion of one or more M elements.

[0013]The material defined in this way has a high ionic conductivity, of the order of 10.sup.-5 S/cm, and good electrochemical stability between 0 and 5.5 volts. By way of examples, mention may be made of the compounds LiSO.sub.1.4N.sub.0.2 and LiSO.sub.1.3N.sub.0.3. Their ionic conductivity at 25.degree. C. is 1.times.10.sup.-5 S/cm and 2.times.10.sup.-5 S/cm respectively. The electronic conductivity of the material is of the order of 10.sup.-13 S/cm.

[0014]The amorphous character of the material obtained can be confirmed by transmission electron microscopy or by X-ray diffraction.

[0015]The composition of the material obtained can be determined by various techniques: atomic absorption spectroscopy, Rutherford backscattering spectroscopy and energy dispersive X-ray spectroscopy.

[0016]The ionic conductivity can be determined by impedance measurement.

[0017]A material according to the invention can be prepared in the form of a thin film deposited on a substrate, by radiofrequency magnetron sputtering, deposition being carried out under the following conditions:

[0018]the plasma used for cathode sputtering is a gas consisting of nitrogen and optionally oxygen and/or argon, the minimum nitrogen content being 30 at %; and

[0019]the target used for cathode sputtering is a target consisting of at least 80 at % Li.sub.2SO.sub.4 in the crystallized form.

[0020]The substrate used for deposition may consist of aluminum, silicon, carbon, stainless steel, a positive electrode material (in particular TiS.sub.2 or LiCoO.sub.2), a negative electrode material (in particular metallic lithium or a lithium alloy).

[0021]The minimum content of 30% nitrogen in the plasma is necessary for obtaining the amorphous structure of the material. In a preferred embodiment, the gas mixture of which the plasma consists contains 0 to 20 at % argon or oxygen.

[0022]The target may consist of the stable compound Li.sub.2SO.sub.4 in the commercially available crystallized form. It may also consist of a mixture containing at least 80 at % of crystallized Li.sub.2SO.sub.4, and at most 20 at % of one or more additives chosen from compounds acting as glass network formers, compounds acting as vitrification modifiers, and compounds that are the source of lithium ions. Among glass network formers, mention may be made of oxides, sulfides and nitrides of boron, germanium, silicon, arsenic or phosphorus. Among the compounds that are sources of lithium, mention may be made of lithium halides. Among the vitrification modifiers, mention may be made of the oxides, sulfides and nitrides of lithium, which are also sources of lithium. The respective proportions of Li.sub.2SO.sub.4 and an additive or additives are chosen so that the atomic composition of the mixture formed of Li.sub.2SO.sub.4 and the additive or additives is Li.sub.xSM.sub.wO.sub.yN.sub.z, x being between approximately 0.5 and approximately 3, y between approximately 1 and approximately 6, z between 0.1 and 1, w less than 0.3 and M representing one or more elements chosen from B, Ge, Si, P, As, Cl, Br and I.

[0023]In a particularly preferred embodiment, the substrate used for deposition of the layer of material according to the invention is one of the electrodes of the electrochemical device in which the layer of material is used as an electrolyte.

[0024]The present invention is described in greater detail with the aid of the following examples, to which it is however not limited.

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