There are two main types of lithium storage battery: lithium-metal batteries, where the negative electrode is made of lithium metal (which material causes problems with safety when in the presence of a liquid electrolyte), and lithium-ion batteries, where the lithium remains in the ionic state.
Lithium-ion batteries consist of at least two conductive Coulombic electrodes of different polarities, the negative electrode or anode and the positive electrode or cathode, between which electrodes a separator is located, which separator consists of an electrical insulator imbibed with an aprotic electrolyte based on Li+ cations ensuring the ionic conductivity. The electrolytes used in these lithium-ion batteries usually consist of a lithium salt, for example of formula LiPF6, LiAsF6, LiCF3SO3 or LiClO4, which is dissolved in a mixture of non-aqueous solvents such as acetonitrile, tetrahydrofuran, or more often a carbonate, for example of ethylene or propylene.
The active material of the anode of a lithium-ion battery is typically graphite (capacity of 370 mAh/g and redox potential of 0.05 V relative to the Li+/Li couple) or, as a variant, mixed metal oxides, among which are included the lithiated titanium oxides of formula Li4Ti5O12, LixTiO2, or even compounds, for example, of formula LixCuVO4, LixSnO2, LixMoO2, LixMoS2, where 0<x<5. The reversible insertion/removal of lithium into/from these active anode materials takes place over electrochemical potentials that are generally higher than that of graphite.
The active material of the cathode of a lithium-ion battery is conventionally an oxide of a transition metal, such as an oxide of vanadium, nickel, manganese or cobalt, or, as a variant, it may be a lithiated iron phosphate.
The respective active materials of the cathode and anode of a lithium-ion battery allow reversible insertion/removal of lithium into/from these electrodes, and the higher the mass fraction of the active materials in the electrodes, the higher their capacity. These electrodes must also contain an electrically conductive compound, such as carbon black and, in order to provide them with sufficient mechanical cohesion, a polymer binder. A lithium-ion battery is thus based on the reversible exchange of lithium ions between the anode and the cathode during the charging and discharging of the battery, and, for a very low weight, by virtue of the physical properties of lithium, such a battery has a high energy density.
The anodes of lithium-ion batteries are most often manufactured using a process comprising, in succession, a step of dissolving or dispersing the various ingredients of the anode in a solvent, a step of spreading the obtained solution or dispersion on a metallic current collector, and then lastly a step of evaporating this solvent.
Processes for manufacturing the anodes of lithium-ion batteries that use an organic solvent have many drawbacks with respect to the environment and safety. In particular, it is, in this case, necessary to evaporate large amounts of such solvents, which are toxic or inflammable.
As for processes that use an aqueous solvent to manufacture these anodes, their major drawback is that the anode must be very thoroughly dried before they can be used, traces of water being known to limit the useful lifetime of lithium storage batteries. Mention may, for example, be made of document EP-B1-1 489 673, which describes a process for manufacturing an anode based on graphite and an elastomer binder, and using an aqueous solvent.
It is therefore highly desirable, for lithium-ion batteries, to prepare anodes that are manufactured without using solvents. It is in this context that processes for manufacturing anodes for lithium-ion batteries using melt processing techniques (for example extrusion) have been described in the literature.
Unfortunately, these melt processes cause major difficulties in the case of lithium-ion batteries, which, as is known, require a mass fraction of active material in the polymer mixture of the anode of at least 85% for the latter to have sufficient capacity in the lithium-ion battery. However, at such contents of active material the viscosity of the anode polymer mixture becomes very high, and leads to a risk of the mixture over-heating and losing its mechanical cohesion once it is in use.
Document US-B2-6 939 383 describes the extrusion of a polymer composition comprising a poly(ethylene oxide)-polypropylene oxide)-poly(glycidyl ether) copolymer by way of ionically conducting polymer, for solventless processing of an anode or cathode for a lithium-polymer battery. However, the mass fraction of active material in the single polymer composition prepared in this document is only 64.5% and, furthermore, it relates to a cathode.
Document US-A-5 749 927 discloses a process for the continuous preparation of lithium-polymer batteries by extrusion, which process comprises compounding the anode or cathode active material with an electrical conductor and a solid electrolyte composition comprising a polymer, a lithium salt and a propylene carbonate/ethylene carbonate mixture greatly in excess over this polymer. In this document, the mass fraction of active material present in the only anode polymer composition obtained is also below 70%.
Thus, a major drawback of these known melt processes for manufacturing anodes for lithium storage batteries is that the mass fractions of active material in the anode polymer composition are insufficient to obtain high-performance anodes specifically for lithium-ion batteries.