PL205377B1 - Electrolyte for lithium ion batteries, and the batteries with graphite anode in particular as well as lithium ion cell - Google Patents

Description

Description of the invention

The present invention relates to an electrolyte for lithium-ion batteries, in particular batteries with a graphite anode, and a lithium-ion cell consisting of a graphite anode, a lithium oxide cathode and a transition metal, and an electrolyte containing a lithium salt and a polar solvent.

The technology of producing reversible lithium cells (Li-ion batteries), i.e. cells containing an anode made of graphite or another type of carbon and an appropriate electrolyte, and capable of multiple charging and discharging, is undergoing dynamic development. This is due to the ever-growing importance of wireless electrical devices such as laptops, mobile phones and cameras. Li-ion batteries are also widely regarded as the most future-proof power source for electric vehicles, medical equipment and space technology. One of the most important technological problems standing in the way of obtaining the optimal Li-ion cell is the selection of the appropriate electrolyte.

Currently, in industrial practice, electrolytes containing ethylene carbonate (EC) are most often used as a film-forming component (i.e. ensuring adequate passivation of the carbon electrode) and lowering the viscosity, e.g. diethyl carbonate or dimethyl carbonate. This solution has some limitations, because due to the relatively high melting point of ethylene carbonate, electrolytes of this type cannot be used for operation at very low temperatures.

In lithium cells, propylene carbonate is also used, which is characterized by a high dielectric constant, low viscosity, low melting point and high boiling point. It has very favorable properties as a solvent for lithium salt (e.g. lithium hexafluorophosphate - LiPF6). While in primary lithium cells (these are cells containing a metallic lithium anode and can only be discharged once), propylene carbonate has found wide application as the main component of electrolytes, its use in lithium-ion batteries has encountered serious difficulties. During the first charging of the Li-ion battery, propylene carbonate particles cointercalate with lithium cations between the graphene layers of the graphite material, combined with its decomposition. The gaseous propylene released in this process causes the exfoliation (delamination) of the graphite scales, which is tantamount to the destruction of the electrode structure. In the graphite electrode constant current charging curves it manifests itself as an extensive potential plateau around 800 mV (relative to the Li ⁺ / Li redox system), followed by the characteristic plateau of lithium ion intercalation into the graphite lattice, which is the basic graphite electrode reaction in a Li-ion cell.

The described disadvantageous processes have become the reason to search for additives to propylene carbonate preventing the exfoliation of graphite scales, and thus causing the proper formation of the passive layer on the electrode. Among the proposed chemicals to fulfill this role were, inter alia, vinyl carbonate and crown ethers. However, the need to find effective passivating (film-forming) additives is still large. These substances should be low cost, non-toxic, non-flammable and should not adversely affect the electrochemical characteristics of graphite.

The subject of the invention is an electrolyte for lithium-ion batteries, in particular batteries with a graphite anode, containing a lithium salt and a polar solvent, preferably propylene carbonate, and according to the invention from 5 to 80% by weight of at least one difunctional Si-podand having the general formula, in which R denotes a substituent containing π electrons in an alkenyl or aryl group, amines take values from 1 to 3, the sum n being equal to 4. The essence of the invention is also a lithium-ion cell, consisting of a graphite anode, a lithium oxide cathode and a transition metal and an electrolyte containing a lithium salt and a polar solvent, preferably propylene carbonate, wherein the electrolyte also contains from 5 to 80% by weight of at least one difunctional Sipodand of the general formula, wherein R is a substituent containing% electrons in an alkenyl or aryl group, the amines assume the values of from 1 to 3, where the sum of n and m equals 4.

The compounds used in the electrolyte, and thus in the cell according to the invention, belong to the group of difunctional silicon subanders containing one or more functional groups having π electrons (e.g. multiple bonds or aromatic groups) and one, two or more polyoxaethylene chains capable of complexing lithium cations. . They dissolve lithium salts and can be used alone or mixed with other solvents as electrolytes in Li-ion batteries.

PL 205 377 B1

The lithium-ion cells constructed according to the invention, containing the Si-podands according to the invention in the electrolyte composition, are characterized by high stability and reliability. It has been found experimentally that they retain the required operating parameters when subjected to even thousands of charges and discharges.

The invention is characterized by an example.

Two electrolytes were prepared: 1M lithium hexafluorophosphate (LiPF6) in a mixture of propylene carbonate and vinyltrimethoxyethoxysilane (PC / H2C = CHSi (OCH2CH2OCH3) 3 <10%>) and 1M lithium hexafluorophosphate (LiPF6) in a mixture of propylene propylene and propylene carbonate (OCH2CH2OCH3) 3 <10%>).

Then, two-electrode electrochemical cells were built, in which the tested electrode was typical synthetic graphite and the opposite electrode was metallic lithium. Prepared electrolytes were used in these cells. The cells were charged and discharged in the potential range of 0 V - 2 V at a constant current density of 10 mAh / g.

The electrochemical characteristics of the tested electrolytes are shown in the table:

Electrolyte Charging capacity in the first cycle [mAh / g] Discharge capacity in the first cycle [mAh / g] Electric charging efficiency in the first cycle [%] 1M LiPF6 in PC frame of reference <x> 0 0 1MLiPF6 in PC / (PC / H2C = CHSi (OCH2CH2OCH3) 3 <10%>) proposed composition 1 502 352 70 1M LiPF6 in PC / PhSi (OCH2CH2OCH3) 3 <10%>) proposed composition 2 807 374 46

Patent claims

Claims (2)

1. Electrolyte for lithium-ion batteries, in particular graphite anode batteries, containing a lithium salt and a polar solvent, preferably propylene carbonate, characterized in that the electrolyte also contains from 5 to 80% by weight of at least one difunctional Si-podand of the general formula, where R is a pi-electron substituent on an alkenyl or aryl group, the amines range from 1 to 3 with the sum of n and 4. 2.Lithium-ion cell, consisting of a graphite anode, a lithium oxide cathode and a transition metal and an electrolyte containing a lithium salt and a polar solvent, preferably propylene carbonate, characterized in that the electrolyte also contains from 5 to 80% by weight of at least one difunctional Si-suband of the general formula in which R is a substituent containing π electrons in an alkenyl or aryl group, amines range from 1 to 3, with the sum of n equal to 4.