Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-04-06
2001-04-17
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S304000, C029S623100, C029S623500
Reexamination Certificate
active
06218048
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to secondary lithium or lithium-ion batteries and, in particular, to secondary lithium and lithium-ion batteries with improved cycling, storage and high temperature performance, and methods of preparing same.
BACKGROUND OF THE INVENTION
Lithium intercalation compounds such as Li
1+X
Mn
2−X
O
4+Y
, LiCoO
2
, and LiNiO
2
have been used in positive electrodes for 4 V and higher than 4 V secondary lithium and lithium-ion batteries. These batteries typically include (1) a positive electrode including a lithium intercalation compound, (2) a negative electrode formed of lithium metal, a lithium alloy or a carbon compound, (3) an electrolyte based on an inorganic lithium salt dissolved in organic solvents, and (4) an appropriate separator.
One of the main drawbacks of 4V or higher secondary lithium and lithium-ion batteries is electrolyte decomposition during the charging process or during the shelf life of the battery in its charged state. The negative effects of this decomposition are considerably accelerated at elevated temperatures. Accordingly, to decrease electrolyte decomposition in conventional cells, low voltage limits are strictly used during the cell charge process.
Another drawback of electrolyte decomposition is that the decomposition products either tend to polymerize or tend to initiate electrolyte polymerization, particularly in solvents containing cyclic esters. This polymerization can block the compartment between the electrodes and cause failure of the cell.
Furthermore, when manganese-rich and cobalt-rich lithiated metal oxides are used as positive electrode materials, manganese and cobalt dissolution can occur in the cell. This dissolution is observed in the electrolyte and results in a reduction in the capacity and cycleability of the cell. In particular, the negative effect of manganese dissolution is more pronounced because it is believed that the dissolved manganese catalyzes electrolyte polymerization and/or decomposition.
SUMMARY OF THE INVENTION
The present invention provides a method of preparing secondary lithium and lithium-ion batteries with improved coulombic efficiency, improved cycling and storage performance at elevated temperatures, and lower rates of transition metal dissolution in the electrolyte, by forming a thermally activated thin passivating film on the positive electrode-electrolyte interface. The passivating film can comprise a polymer electrolyte interface (PEI), a solid electrolyte interface (SEI) or a combination thereof, between the positive electrode material and electrolyte that is permeable to lithium ions but not permeable to the electrolyte solution. In place of or in addition to the PEI or SEI, the passivating film can comprise a polymer interface (PI) covering predominantly the surface of carbon additives distributed throughout the positive electrode composite.
The present invention includes a method of pretreating a positive electrode for a secondary lithium or lithium-ion cell comprising thermally treating a positive electrode in its discharge state and in a liquid nonaqueous electrolyte at a temperature of between 50° C. and 120° C., and preferably between 65° C. and 75° C., to thereby create a thin, passivating film with lithium ion conductivity on the positive electrode. Preferably, the positive electrode is thermally treated for a period of from about one hour to about two months. The positive electrode can either be heat treated prior to producing the secondary lithium or lithium-ion cell or the entire cell can be thermally treated. Alternatively, the positive electrode material can be thermally treated prior to forming the positive electrode composite.
The secondary lithium or lithium-ion cells that can be treated in accordance with the invention are preferably 4 V or higher than 4 V cells and typically include (1) a positive electrode including a lithium intercalation compound, (2) a negative electrode formed of lithium metal, a lithium alloy or a carbon compound, (3) an electrolyte based on an inorganic lithium salt dissolved in organic solvents, and (4) an appropriate separator. The positive electrode material for the secondary lithium and lithium-ion cells can be a lithium intercalation compound and is preferably a lithium manganese spinel. The electrolyte is preferably a liquid, non-aqueous electrolyte that includes a lithium salt such as LiPF
6
, LiClO
4
or LiBF
4
dissolved in organic solvents. The organic solvents can include at least one cyclic ester solvent selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC) and &ggr;-butyrolactone (GBL). For example, the electrolyte can include LiPF
6
dissolved in an ethylene carbonate/dimethyl carbonate solvent.
The present invention further provides a secondary lithium or lithium-ion cell comprising a liquid nonaqueous electrolyte, a negative electrode and a positive electrode, at least a portion of a surface of the positive electrode covered with a thermally-activated thin passivating film having lithium ion conductivity such as those described above. The passivating film is formed by thermally treating the positive electrode in a liquid electrolyte preferably containing a lithium salt at a temperature between 50° C. and 120° C., and preferably between 65° C. and 75° C.
In accordance with the invention, the secondary lithium or lithium-ion cell preferably has a coulombic efficiency (C
eff
) of greater than 99% after 250 charge/discharge cycles as defined by the formula:
C
eff
=
Q
reversible
Q
reversible
+
Q
side
reactions
100
%
wherein Q
reversible
is the reversible capacity of the intercalation process and Q
side reactions
is the capacity of side reactions such as electrolyte decomposition and includes the irreversible capacity loss. When a lithium manganese spinel is used as the positive electrode material, the secondary lithium or lithium-ion cell preferably has a manganese dissolution of less than 1.5% after 400 cycles at 55° C. In addition, the cell preferably has a manganese dissolution of less than 0.2% after 200 cycles at 23° C.
The present invention also includes a positive electrode material comprising particles of a lithium intercalation compound such as lithium manganese spinel particles and a thermally-activated passivating film on the surface of the particles. The passivating film can be an SEI or PEI and is formed by thermally treating the positive electrode material in a liquid electrolyte solution containing a lithium salt at a temperature between 50° C. and 120° C.
These and other features and advantages of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description and accompanying drawings, which describe both the preferred and alternative embodiments of the present invention.
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European Search Report, Aug. 26, 1999.
Barnette D. Wayne
Engel John Francis
Faulkner Titus
Manev Vesselin
Alston & Bird LLP
FMC Corporation
Weiner Laura
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