1. Field of the Invention
The present invention relates to a block copolymer of the BA or BAB type, where A is a block of the ethylene oxide or derivative type and B is an anionic polymer block based on lithium bis(trifluoromethylsulfonyl)imide, a method of preparation thereof, as well as its uses, notably for preparing an electrolyte composition for lithium-metal-polymer (LMP) batteries.
It applies to the field of the manufacture of lithium-metal-polymer batteries. This type of battery is in the form of an assembly of coiled thin films (coiling of the following unit {electrolyte/cathode/collector/cathode/electrolyte/lithium} on n turns or of n stacked thin films (cut and superposed, or n stacks of the aforementioned unit). This stacked/complexed unit has a thickness of the order of about a hundred micrometers. Four functional sheets are included in its composition: i) a negative electrode (anode) generally consisting of a sheet of metallic lithium or of a lithium alloy, ii) an electrolyte composed of a polymer (generally based on poly(ethylene oxide) (PEO)) and of lithium salts, iii) a positive electrode (cathode) composed of an active electrode material whose working potential is under 4V vs Li+/Li, for example based on metal oxide or based on phosphate of the type LiMPO4 where M represents a metal cation selected from the group Fe, Mn, Co, Ni and Ti, or combinations of these cations, for example LiFePO4, of carbon and of polymer, and finally iv) a current collector generally consisting of a sheet of metal and providing electrical connection.
2. Description of Related Art
The polymers included in the composition of the electrolytes must combine good properties of ionic conductivity and good mechanical properties of elasticity and plasticity to be able to be used satisfactorily in LMP batteries.
Polymer solid electrolytes offer many advantages, namely high thermal stability, improved safety, design of batteries that are thin, flexible and of various shapes, low cost of the material and of its application. Moreover, polymer solid electrolytes allow lithium metal to be used as the anode, offering higher energy densities than lithium ion anodes. Polymer electrolytes are also very interesting owing to their low reactivity with respect to lithium metal and their potential for blocking the growth of dendrites. However, despite these many advantages, the advance of polymer electrolytes has been held back by the inability to develop an electrolyte that has both high ionic conductivity and good mechanical durability. These difficulties arise because high conductivity requires great mobility of the polymer chains, which has the converse effect of producing polymers with low mechanical strength.
Various types of polymers have already been proposed in the literature. In particular, the use of polymers consisting of units of ethylene oxide (EO) has been widely known since the end of the 1970s, but it has been found that they do not have sufficient conductivity at room temperature. For example, poly(ethylene oxide) (PEO) of high molecular weight doped with lithium salt has very good mechanical properties at room temperature but is also a semicrystalline polymer. The crystalline structure restricts the mobility of the chains and reduces the ionic conductivity of the polymer. Above the inciting point of PEO (Tm˜60-65° C.), ionic conductivity increases considerably, but at these temperatures PEO becomes a viscous liquid and loses its dimensional stability.
Since then, research and development of polymer solid electrolytes possessing improved ionic conductivity as well as good mechanical properties, in particular good flexibility, has continued. Attempts to reinforce the mechanical properties of PEO by adding hard colloidal particles, by increasing the number-average molecular weight of the PEO or by crosslinking, have often caused a decrease in ionic conductivity. Similarly, tests for improving the conductivity of PEO by adding plasticizers have led to deterioration of the mechanical properties.
In order to increase the ionic conductivity of the PEO matrix of polymer solid electrolytes, it has also been envisaged in the literature to copolymerize PEO macromers or produce sequence or block copolymers based on PEO. This latter strategy has been the basis of the main research efforts with considerable development of techniques of controlled radical polymerization such as ATRP (Atom Transfer Radical Polymerization).
The PEO block copolymers used in polymer solid electrolytes can be A-B diblock copolymers or A-B-A triblock copolymers.
For example, diblock copolymers in which the first block is a poly(alkyl methacrylate), notably poly(lauryl methacrylate) (PLMA), poly(n-butyl methacrylate) (PnMBA), or poly(methyl methacrylate) (PMMA), and the second block is poly(polyethylene glycol methacrylate), comprising 9 units of ethylene oxide (PMAPEG), have been proposed, notably by Sadoway D. R. (J. Power Sources, 2004, 129, 1-3). As an example, the copolymer PLMA-b-PMAPEG doped with LiCF3SO3 has a conductivity of the order of 8.10−6 S/cm at room temperature, which is insufficient.
More recently, Niitani et al. (Electrochemical Solid-State Letters, 2005, 8(8), 1385-A388; J. Power Resources, 2005, 146, 386-390 and EP 1 553 117), described a triblock polymer composed of PMAPEG (23 units of EO) as central block and polystyrene (PS) as outer blocks. According to this document, the polymer having the best ionic conductivity at 30° C. (2.10−4 S/cm) is a copolymer doped with LiClO4 with a ratio EO:Li=20. This ionic conductivity is correct but this corresponds to a viscoelastic liquid and not to a solid. Moreover, the lithium ion transfer number is low, which leads to poor power-handling capability and a large drop in capacity beyond C/10.
The international application WO 00/05774 describes a polymer solid electrolyte including a block copolymer with micro-phase separation comprising an ionic conducting block, a second block not miscible with the ionic conducting block, an anion immobilized on the electrolyte polymer, and a cationic species (Li+) providing neutrality of the polymer and ionic mobility. The use of such a copolymer obviates the need to use an additional lithium salt. In this copolymer, the anion is preferably immobilized on the second block, which induces micro-phase separation of the cations and anions of the electrolyte polymer with the aim of improving the lithium ion transfer number (t+) to a value above 0.5. The ionic conducting block can notably consist of polyethylene oxide (PEO), polyethylene glycol (PEG), polypropylene oxide (PPO) or polypropylene glycol (PPG). The number-average molecular weight of the ionic conducting block is above 50 kg/mol, and especially preferably above 200 000 kg/mol. The second block is not miscible with the first block and can consist of a non-ionic conducting block such as a polyalkyl acrylate of the methacrylate type, a polydimethylsitoxane, a polybutadiene, a polyisoprene, modified polystyrenes with flexible alkylfluorocarbon or siloxane side chains attached to the phenyl groups, etc. The anion is preferably joined to the polymer by a covalent bond and can be selected from carboxylates, sulfonates and phosphates. These polymers can be used in any type of batteries and have operating temperatures varying between 20 and 100° C.
Moreover, polymers comprising a delocalized polyanion attached to the polymer backbone, replacing polymer electrolytes consisting of a simple mixture of a polymer of the PEO type and a salt based on lithium bis(trifluoromethylsulfonyl)imide, in particular with the aim of increasing the transfer number of the Li+ions, which is only of the order of about 0.2 for this conventional type of electrolyte, have also been proposed, notably by R. Meziane et al. (Electrochimica Acta, 2011, in press, available on-line doi:10.1016/j.electacta.2011.03.074). The polymer is a polystyrene bearing sulfonyl(trifluoromethylsulfonyl)imide groups (PSTFSI (a)), which is obtained by radical polymerization from monomers of the sodium 4-styrene-sulfonyl(trifluoromethylsulfonyl)imide type. This polymer is then used simply mixed with PEO to make an electrolyte membrane that does not contain additional lithium ions. For comparison, membranes prepared with PSTFSI obtained by chemical modification of polystyrene sodium sulfonate (PSTFSI (b)) mixed with PEO, as well as with lithium poly(styrene sulfonate) (PSSO3Li) mixed with PEO have also been tested for their ionic conductivity. The results obtained show equivalent conductivity at 70° C. between the membranes consisting of the mixture PSTFSI (b)/PEO and of the mixture PSSO3Li/PEO whereas that obtained with the membrane PSTFSI (a)/PEO is 10 times higher (of the order of 9.5×10−6 S cm−1). However, the lithium ion transfer number is not stated, Nevertheless, the authors point out that the conductivity of the PSTFSI (a)/PEO membrane is still insufficient relative to the desirable ionic conductivity, which is of the order of 10−5 S cm−1 at room temperature. Moreover, as the two polymers are not joined together covalently, a macro-phase separation leading to a drop in conductivity over time is probable.