Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
2000-12-28
2003-01-14
Langel, Wayne A. (Department: 1754)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C423S277000
Reexamination Certificate
active
06506516
ABSTRACT:
The subject matter of the invention is lithium-bisoxalatoborate, Li[(C
2
O
4
)
2
B], two methods for the production thereof, and the use of lithium-bisoxalatoborate as a conducting salt in lithium ion batteries.
At present, lithium hexafluorophosphate (LiPF
6
) is used as a conducting salt in all commercial lithium ion batteries. This salt has the necessary prerequisites for use in high-energy cells, i.e. it is easily soluble in aprotic solvents, it leads to electrolytes having high conductivities, and it has a high level of electrochemical stability. Oxidative decomposition first occurs at potentials of >approximately 4.5V. LiPF
6
, however, also has serious disadvantages, which are mainly to be attributed to its lack of thermal stability. In solution, a dissociation into LiF and PF
5
takes place, even if only slight, which can lead to a cationic polymerisation of the solvent, caused by the Lewis acid PF
5
. Upon contact with moisture, caustic hydrofluoric acid is released, which, on the one hand makes handling more difficult, because of its toxicity and corrosiveness, and, on the other hand, can lead to the (partial) dissolution of the transition-metal oxides (for example LiMn
2
O
4
) used as cathode material. In this way, the cycle stability of the respective electrochemical energy store is affected.
With this background in mind, intensive efforts are being made with the aim of developing alternative conducting salts. As such, lithium salts with perfluorated organic radicals are being tested above all. In particular, lithium trifluoromethane sulphonate, lithium bis(trifluoromethane sulphonyl)imide and the lithium methides, the most fundamental of which is lithium bis(trifluoromethane sulphonyl)methide, are to be mentioned. These salts also have disadvantages, which hitherto prevented their use in commercial lithium batteries. The first-mentioned salt does not give the electrolytes produced with it a sufficiently high conductivity. The last-mentioned salts admittedly have a conductivity which is equal to that of LiPF
6
, but because of the costly production methods are not of interest commercially. Additionally, the imide has a corrosive effect on aluminium sheets, which are used as current diverters in many battery systems. Apart from this, because of the high fluorine content of the compounds, under unfavourable conditions exothermal reactions with lithium are to be feared.
Lithium organoborates were tested as a further class of compound for use as a conducting salt. However, their use in lithium ion batteries was not seriously taken into consideration because of the low oxidation stability, the safety problems linked with the formation of triorganoboranes as well as their high price.
The lithium borate complex salts [(R
1
O)
2
B(OR
2
)
2
]Li described in DE 19633027 A1 represent a substantial step forward. In this connection, R
1
and R
2
are the same or different, R
1
and R
2
are, if appropriate, connected to each other by a single bond or a double bond, R
1
and R
2
may be, individually or jointly, an aromatic ring from the group phenyl, naphthyl, anthracenyl or phenanthrenyl, which can be unsubstituted or substituted one to four times by A or Hal, Hal standing for fluorine or chlorine and A meaning alkyl with 1 to 8 C-atoms, which in turn can be halogenised one to four times.
A disadvantage of these compounds is, on the one hand, the stabilities of the non-fluorinated derivatives which, although improved, are in no way sufficient for the 3V systems required. Thus, for example, the unsubstituted lithium-bis[1,2-benzenediolato(2-)-O,O{grave over ( )}] borate(1-) decomposes when an anodic potential of 3.6 V is exceeded. This value lies clearly below that of the standard conducting salt LiPF
6
(approximately 4.5V). As a result of increasing fluorine substitution of the organic radical, the oxidation stability rises to a value of approximately 4V for the perfluorated compound. However, these values are still lower than in the case of the standard salt LiPF
6
. The stability of the borates which are described, however, increases further because of a top layer formation during cyclisation, so that for some compounds almost sufficient stabilities are achieved. The stable compounds, however, have high molar masses (for example 378 g/mol for the perfluorated catecholate compound). Also, the preliminary stages required for the synthesis are not commercially available, but instead have to be produced in a costly way. Finally, compounds with CF bonds represent a potential safety risk, because they are not thermodynamically stable with respect to metallic lithium.
The underlying object of the invention is therefore to eliminate the disadvantages of the prior art and to develop an electrochemically stable lithium compound which has a good solubility in the aprotic solvents used by the battery industry, and also a method for the production thereof.
The object is achieved by the lithium compound lithium-bisoxalatoborate, Li[(C
2
O
4
)
2
B], indicated in claim 1. The independent claims 2 and 11 indicate two different methods for the production of lithium-bisoxalatoborate, claims 3 to 10 and 12 to 13 develop the method further and claim 14 indicates a use of the compound lithium-bisoxalatoborate.
Surprisingly, although it does not have any fluorine substituents, lithium-bis(oxalatoborate) has an excellent oxidation resistance. Thus, solutions of this salt in a mixture of ethylene carbonate (EC) and 1,2-dimethoxyethane (DME) are stable up to a voltage of 4.6V.
Furthermore, the conductivities which can be achieved with the salt in accordance with the invention are note worthy. Thus, a 0.56 m solution in a 1:1 mixture of EC and DME has a conductivity of 10.3 mS/cm at room temperature. In the usual solvent mixture propylene carbonate (PC)/DME (1:1), the conductivity of lithium-bisoxalatoborate in the case of different concentrations was measured (FIG.
1
). It can be inferred from the measurement results that with concentrations of up to 15% by weight, conductivities of up to 14 mS/cm are achieved (see FIG.
1
). These values are at the same level as, or even above, the conductivities which can be achieved with LiPF
6
. Thus, for 1 m solutions of LiPF
6
in dimethyl carbonate (DMC)/EC, 11.0 mS/cm is achieved.
The molar mass of 193.8 g/mol is admittedly approximately 27% above that of the LiPF
6
, but clearly below that of the borates described in DE 19633027 A1. This is not problematic, however, because electrolytes with lithium-bis(oxalatoborate) are also sufficiently conductive at lower concentrations (for example approximately 0.5 mol/l).
The lithium-bis(oxalatoborate) is easily soluble in water and in many polar aprotic solvents. In tetrahydrofuran (THF), approximately 42% by weight dissolves at 50° C. and approximately 30% by weight dissolves at 23° C. It has a solubility of at least 15% by weight in diethylene glycol dimethyl ether (diglyme) and mixtures of diglyme and carbonates.
According to thermogravimetry experiments, lithium-bis(oxalatoborate) is fully stable at up to approximately 300° C.
The lithium-bis(oxalatoborate) in accordance with the invention can be produced by reacting a lithium compound, such as lithium hydroxide (anhydrous or the hydrate) or lithium carbonate or a lithium alkoxide, with oxalic acid or an oxalate and a boron compound, such as boron oxide or boric acid or a boric acid ester.
The reaction can be carried out in a solvent, but does not necessarily have to be.
Preferably, lithium hydroxide or lithium carbonate is reacted with a stoichiometric amount of oxalic acid and a stoichiometric amount of boric acid or boron oxide in water, for example:
The reaction of lithium oxalate with oxalic acid and boric acid or boron oxide in water is also preferred, for example:
The sequence in which the components are added does not play a significant role. Preferably, oxalic acid is placed in an aqueous solution and the calculated amount of lithium base is added, or lithium oxalate is mixed with the 3-fold molar amount of
Lischka Uwe
Wegner Marion
Wietelmann Ulrich
Langel Wayne A.
Metallgesellschaft Aktiengesellschaft
Strickland Jonas N.
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