Surface preparation of natural graphite and the effect of impurities on grinding and the particle distribution

Abstract

The present invention relates to the physical or chemical specific purification of natural mineral graphite. This purification is preferably applied to the surface of natural graphite in order to allow the formation of a passivation film during the first electrical discharge or the insertion of lithium in the graphite when the latter is used in a lithium-ion cell. The grinding to a small size before purification allows the optimization of the distribution of the particles, resulting in a more uniform electrode. This grinding is carried out in the presence of the natural impurities of the graphite that play the role of a micro-abrasive and result in a hardness of the graphite that increases its mechanical properties.

Description

FIELD OF INVENTION [0001] The present invention relates to the physical and chemical purification specific to the mineral of natural graphite with the goal of generating a purified graphite which is particularly advantageous for use in carbon-lithium anodes. This purification is applied preferentially to the surface of natural graphite to permit the formation of a passivating layer with the first electrical discharge or insertion of lithium in the graphite when the graphite is used in a lithium-ion battery. Grinding to a very small size before the purification permits the optimisation of the size distribution of the particles, which gives rise to an electrode which is much more homogenous. This grinding is done in the presence of the natural impurities of the graphite which play a role as a micro abrasive and give a hardness to the graphite which increases its mechanical properties. It goes without saying that the purified graphite particles can have many other usages, notably in th...

AI Technical Summary

Benefits of technology

[0021] The present invention relates to a purification method, either chemical or physical, of impurities found on the surface of the natural graphite, i.e. where the passivating film is formed. The present method permits the removal of the impurities which can harm the formation of the passivating film and the cycling of the carbon-lithium anode. The grinding process is advantageously conducted before purification, this permits a better control of the size and the size distribution of the particles, where a more uniform powder does not require filtration to remove the oversized and undersized particles.

[0024] To eliminate compounds containing silicium, an acid treatment is used, for example with H2SO4, HNO3, HCl, HF or their mixtures thereof A treatment with HF or a fluoridated derivative allowing the generation of HF in the medium represents a particularly preferred embodiment. This treatment equally causes an interaction between the fluoride and the calcium already present in the mineral, leading to the formation of calcium fluoride, a compound which is strongly ionic, an electric insulator as well as a good anionic conductor at high temperatures. Moreover, the presence of calcium fluoride will not alter the formation of the passivating layer.

[0032] only one step using two different compounds, where one is HF or a fluoridated derivative as described above, and the other may consists of a non fluoridated acid such as HCl, HNO3, H2SO4 etc. The use of two different compounds has the advantage of extracting more impurities that with one single acid or fluoridated derivative;

[0035] It will be noted that the addition of HNO3 to the purification solvents (H2SO4—NH4F) or (HCl—NH4F) allows us to obtain in one step a purified and exfoliated graphite.

[0037] During the purification by chemical means, it is very important to choose concentrations of compounds added to achieve this end, as well as the operating conditions, in order to avoid all physical changes or exfoliation of the natural graphite. In this way, the concentration of HF or of fluoridated derivatives added for the purification has to be preferably between 10% and 50% (by weight) and at a process temperature which preferably does not exceed 250° C. in order to maximise the output. In the case where another acid is used, the concentration of this acid will vary preferably between 10 and 30%.

Problems solved by technology

This major problem has slowed the arrival of lithium batteries in the classical formats (AA, C, D, etc.) to the public at large.

The lithium thus inserted has, moreover, a more substantial mass and apparent volume, which reduces its mobility as well as its maximum concentration inside the layers.

In addition, in propylene carbonate medium, the ternary compound is very unstable, the solvent reacts to become gaseous propene that can cause a violent degradation of the battery.

Electricity consumed during this step cannot be recovered when the current is inverted.

The faraday output of this first cycle is therefore poor.

The graphite used conventionally as electrode material in a lithium-ion battery is generally obtained from two distinct sources: synthetic graphite or natural thermally highly purified graphite, preferably treated at temperatures higher than 2,500° C. This type of graphite although of excellent quality, is however very costly, and this has a direct impact on the cost of the final product eventually sold in the market.

Moreover, the graphite is only reduced to the powder state after having been purified or synthesised, this causes certain problems during the grinding process.

In effect, the uniformity of the size distribution of the particles in the powder is markedly altered, since pure graphite is very fragile.

In fact, it can be said that the distribution is non-uniform.

The alternative is then to filter the graphite and to collect only those particles having the wanted size, which involves additional process steps, and an increase in the cost of the resulting material.

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