Use of additives in electrolyte for electrochemical cells

Details Use of additives in electrolyte for electrochemical cells Use of additives in electrolyte for electrochemical cells

2000-03-13

2003-04-15

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S322000, C252S062200

Reexamination Certificate

active

06548212

ABSTRACT:

BACKGROUND
The invention relates to the use of salt-based compounds as additives in electrolytes for improving the properties of electrochemical cells.
Lithium ion batteries are amongst the most promising systems for mobile applications. The areas of application extend from high-quality electronic equipment (for example mobile telephones, camcorders) to batteries for electrically driven vehicles.
These batteries consist of a cathode, an anode, a separator and a non-aqueous electrolyte. The cathode is typically Li(MnMe
z
)
2
O
4
, Li(CoMe
z
)O
2
, Li(CoNi
x
Me
z
)O
2
or other lithium intercalation and insertion compounds. Anodes can consist of lithium metal, carbon, graphite, graphitic carbon or other lithium intercalation and insertion compounds or alloy compounds. The electrolyte can be a solution containing lithium salts, such as LiPF
6
, LiBF
4
, LiClO
4
, LiAsF
6
, LiCF
3
SO
3
, LiN(CF
3
SO
2
)
2
or LiC(CF
3
SO
2
)
3
and mixtures thereof, in aprotic solvents.
A multiplicity of additives for use in lithium ion batteries is mentioned in the literature. For example, in EP 0759641 and U.S. Pat. No. 5,776,627, organic aromatic compounds, such as biphenyl, substituted thiophenes and furans, and in EP 0746050 and EP 0851524, substituted anisole, mesitylene and xylene derivatives are added to the electrolyte in order to increase the safety of the battery in the case of overcharging. For the same purpose, U.S. Pat. No. 5,753,389 uses organic carbonates as additives. In order to improve the cycle stability, organic boroxines are added in EP 0856901. However, all these additives have some crucial disadvantages. Organic substances, as used in the specifications mentioned here, generally have low flash points and low explosion limits.
Additive
Explosion limit [%]
Flash point [° C.]
Thiophene
1.5-12 
−9
Anisole
0.34-6.3 
43
Mesitylene
1-6
54
Furan
 2.3-14.3
−35
Since the use of electrochemical cells and in particular the occurrence of faults (for example short-circuiting, mechanical damage) is always accompanied by warming, escape of the electrolyte represents an additional source of danger.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to provide additives whose volatility is low and whose flash points are relatively high. This object according to the invention is achieved by the use of organic alkali metal or tetraalkylammonium salts as additive.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
The organic alkali metal salts are dissolved in electrolytes which are usually employed in non-aqueous secondary lithium batteries.
It has been found that the additives participate in formation of the coating layer on the anode and cathode. The coating layer results in passivation of the electrodes and thus in an increase in the cyclability of the electrodes. Film formation on the cathode can in addition serve increase safety in the event of overcharging, since after release of a mechanical safety means, for example by a disconnector, as described in U.S. Pat. No. 5,741,606, the voltage is dissipated by “internal self-discharge”.
The additives are distinguished by very high thermal decomposition points. A crucial advantage over the additives used hitherto is the formation of a glass-like, polymeric layer on thermal decomposition, which can be caused, for example, by a short-circuit.
The invention therefore relates to an electrolyte for non-aqueous secondary lithium batteries which improves the performance, such as, for example, the coating layer formation, cyclability, safety, conductivity and low-temperature behaviour, through the addition of specific additives.
Surprisingly, it has been found that lithium salts which actively participate in the formation of a passivating coating layer on the graphite electrode are suitable for improving the passivation of the anode. It has been found that the quality of the coating layer is crucially improved. Reduction of the additive gives a film which is permeable to lithium ions on the anode. This film leads to improved cyclability of the anode from only the second cycle.
It has furthermore been found that these additives decompose oxidatively at potentials above the charge potential of the selected cathode and thus form a passivating film on the cathode. These films are permeable to lithium ions and protect the selected solvent and conductive salt against oxidative decomposition.
Use in battery systems based on LiCoO
2
and LiNiO
2
appears particularly interesting. It is known that these electrode materials are unstable in the overcharged state. This can result in a vigorous reaction with the electrolyte, with corresponding safety risks occurring. The state of the art consists of internal safety mechanisms, such as, for example, so-called disconnectors. On overcharging of a battery, gaseous components are generally liberated, with evolution of heat. The resultant pressure increase results in the disconnector breaking the contact between electrode and current conductor and thus preventing further overcharging of the battery. A problem here is that the battery remains in the charged, unstable state.
External discharge is no longer possible owing to the irreversible breaking or contact.
The aim is, through addition of selected additives, to apply a film to the cathode in the event of overcharging, i.e. at potentials above the charge voltage, which film reacts with the cathode in a controlled manner after addressing of the disconnector and thus dissipates the “excess potential” through internal self-discharge.
A general example of the invention is explained in greater detail below.


REFERENCES:
patent: 5660947 (1997-08-01), Wuhr
patent: 5691083 (1997-11-01), Bolster
patent: 5741606 (1998-04-01), Mayer et al.
patent: 5753389 (1998-05-01), Gan et al.
patent: 5776627 (1998-07-01), Mao et al.
patent: 6022643 (2000-02-01), Lee et al.
patent: 631340 (1994-12-01), None
patent: 0746050 (1996-12-01), None
patent: 0759641 (1997-02-01), None
patent: 0851524 (1998-07-01), None
patent: 0856901 (1998-08-01), None
patent: 2704099 (1994-10-01), None
patent: 09-283176 (1997-10-01), None
patent: WO-98/07729 (1998-02-01), None
patent: 9828807 (1998-07-01), None
Barthel J.: “a new class of electrochemically and thermally stable lithium salts for lithium battery electrolytes” J. Electrochem. Soc., vol. 144, No. 11, Nov. 1997, XP002140970.
Barthel J.: “New Class of Electrochemically and Thermally Stable Lithium Salts OFR Lithium Battery Electrolytes” Journal of the Electrochemical Society, vol. 147, No. 1, Jan. 1, 2000 p. 21-24 XP002140971.

Affiliated with

Also associated with

Rating

Use of additives in electrolyte for electrochemical cells does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Use of additives in electrolyte for electrochemical cells, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Use of additives in electrolyte for electrochemical cells will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3035725