Method for detecting and dosing hydrofluoric acid in an electrolyte containing lithium hexafluorophosphate LIPF6 for lithium batteries

Abstract

A method for detecting and dosing hydrofluoric acid content of an electrolyte containing lithium hexafluorophosphate LiPF6 in lithium batteries, including measuring a variation in weight of a material that can undergo a surface reaction with the hydrofluoric acid in the electrolyte, the variation being determined by a quartz microbalance.

Claims

1. A process for detecting and assaying hydrofluoric acid in an electrolyte based on lithium hexafluorophosphate LiPF.sub.6, comprising: (i) bringing the electrolyte into contact with a first layer comprising a material M that reacts at a surface with hydrofluoric acid HF, the hydrofluoric acid originating from the reaction between lithium hexafluorophosphate and water; (ii) measuring variation in weight of the first layer by bringing the first layer into contact with an electrode of a quartz crystal microbalance, the variation in weight of the first layer being due to the reaction between the material M of the first layer and the hydrofluoric acid HF to form, at a surface, a second layer comprising a fluorinated compound having the structure MF.sub.n, with n corresponding to an integer strictly greater than 0; (iii) calculating the content by weight of hydrofluoric acid from the variation in weight of the first layer determined in (ii).

2. The process as claimed in claim 1, further comprising (iv) determining an amount of water in the electrolyte.

3. The process as claimed in claim 1, wherein the material M of the first layer is chosen from elements of Groups IIIa, IIIb and Ib of the Periodic Table of the Elements.

4. The process as claimed in claim 1, wherein the material M of the first layer is chosen from aluminum, boron, and silicon.

5. The process as claimed in claim 1, wherein the material M of the first layer is chosen from metals of Group IIIa of the Periodic Table of the Elements.

6. The process as claimed in claim 1, wherein the first layer is a layer of aluminum.

7. The process as claimed in claim 1, wherein the material M of the first layer is chosen from aluminum, boron and silicon, and the second layer comprises a fluorinated compound having the structure MF.sub.n, with M corresponding to an element chosen from aluminum, gold, boron and silicon and n corresponding to an integer ranging from 1 to 4.

8. The process as claimed in claim 6, wherein the second layer is a layer of aluminum fluoride AlF.sub.3 or a layer of silicon tetrafluoride SiF.sub.4.

9. The process as claimed in claim 1, wherein the first layer is deposited on an electrode of a quartz crystal microbalance which is subsequently brought directly into contact with the electrolyte by immersion of the electrode in the electrolyte to measure the variation in weight of the first layer.

10. The process as claimed in claim 1, wherein the first layer is weighed on an electrode of a quartz crystal microbalance, then immersed in the electrolyte and, after a time necessary for the reaction between the hydrofluoric acid and the material of the first layer, the first layer is deposited on the electrode of the quartz crystal microbalance to measure the variation in weight of the first layer.

Description

(1) Other characteristics and advantages of the invention will become apparent on the detailed examination of an embodiment taken by way of example of a process for detecting and assaying hydrofluoric acid within an electrolyte based on LiPF.sub.6 and illustrated by the appended drawings, in which:

(2) FIG. 1 represents a graph in which the variation in weight of the layer (a) has been represented as a function of the amount of water for contents ranging from 0 to 25 μl;

(3) FIG. 2 represents a graph in which the variation in weight of the layer (a) has been represented as a function of the amount of water for contents ranging from 35 to 130 μl.

(4) Four aluminum disks having a surface area of 1.53 cm.sup.2 were prepared from aluminum sheets intended to act as current collector. These disks were washed, dried and then weighed with a quartz crystal microbalance. Each sample was subsequently immersed in an electrolyte as described below.

(5) The electrolyte studied corresponds to a 1 mol.Math.l.sup.−1 solution of LiPF.sub.6 dissolved in a carbonate mixture of EC (1), PC (1) and DMC (3) type. In a glove box, 10 ml of the solution are introduced into four plastic flasks; each amount of electrolyte is carefully and accurately weighed. A sample as mentioned above is introduced into each flask.

(6) An increasing amount of water is introduced into each flask using a 5 μl micropipette, in particular 4, 9, 15 and 22 mg of water, and also a control solution without addition of water.

(7) The lithium hexafluorophosphate thus reacts according to the following reaction:
LiPF.sub.6+H.sub.2O.fwdarw.LiF+OPF.sub.3+2HF

(8) The solutions are subsequently placed in a desiccator filled with dehydrating agent (silica gel) which has been dried beforehand, for 7 days; the desiccator is placed under reduced pressure the time of the experiment for 24 hours.

(9) At the end of this period, the aluminum disks are removed from the electrolyte and are rinsed with dimethyl carbonate and then with acetone and are dried at 60° C. for 24 hours. Once dry, the disks are again weighed using the quartz crystal microbalance.

(10) As shown in FIG. 1, the variation in weight of the aluminum disk is proportional to the amount of water introduced.

(11) The results of the variation in weight of the aluminum disks are shown in the following table:

(12) TABLE-US-00001 Weight of Weight of Variation in aluminum aluminum the weight of Number of (mg) before (mg) after the aluminum the sample reaction reaction disks in mg 1 8.413 8.415 0.002 2 8.369 8.681 0.312 3 8.389 9.279 0.89 4 8.395 9.473 1.078

(13) Other tests were carried out with greater amounts of water ranging from 35 to 130 μl and a linear behavior was also observed here, as shown in FIG. 2.

(14) The visual observation of the aluminum disks indicates that the surface has indeed been modified, given that the presence of a white surface on the disks is found.

(15) Furthermore, the photographs taken using scanning electron microscopy (SEM) prove that a surface reaction has taken place.

(16) The EDX analysis indicates that the chemical nature of the compounds formed is of the AlF.sub.3 type, which clearly confirms the surface reaction at the surface of the electrode of the quartz crystal microbalance.

(17) The experiment shows that a linear law exists between the amount of water present in the electrolyte and the increase in weight of the layer a. Consequently, for a person skilled in the art, it is sufficient to draw up nomograms which make it possible, by simple confirmation of the weight of the layer b, to go back to the amount of water and of hydrofluoric acid in the electrolyte.