Method of preparation of lithium battery

A method for preparing lithium-ion and lithium-ion polymer batteries to reduce water content and cell impedance. A battery having improved calendar life is prepared by electrochemically treating the activated cell by applying a voltage to the cell to react oxygen provided or trapped in the cell with moisture present as an unavoidable impurity. The electrochemical treatment of the present invention decreases the water content in the cell, thereby lowering cell impedance and extending battery life.

DETAILED DESCRIPTION The present invention provides a method for preparing lithium batteries, in particular lithium-ion and lithium-ion polymer batteries having improved power, cycling and storage performance. To this end, and in accordance with the present invention, the lithium-ion cell is electrochemically treated prior to formation of a passivating film on a carbon-containing electrode, typically the negative electrode (anode). First, an anode (negative electrode) is positioned opposite a lithium-retention cathode (positive electrode) with a lithium salt-based electrolyte/separator therebetween. One or both of the electrodes must contain carbon. Moisture is present in the cell as an unavoidable impurity. In one embodiment of the present invention, oxygen is provided in the cell, such as by dissolving oxygen in the electrolyte, or by adsorbing oxygen onto at least one of the electrodes. Alternatively, oxygen may become trapped in the cell from the environment in which the cell is assembled, which is typically a dry room and/or dry box. At a minimum, oxygen is present in the cell as a component of H 2 O, present as an unavoidable impurity. Prior to the first discharge-charge cycle of the cell, the cell is electrochemically treated by applying a voltage to the cell. The voltage is selected to keep the potential of the carbon-containing electrode(s) more positive than the potential necessary for SEI formation by reduction of electrolyte on the carbon-containing electrode(s), and is further selected to keep the potential of the carbon-containing electrode(s) more negative than the equilibrium potential of the oxygen to insure that oxygen reduction occurs on the carbon-containing electrode(s). For example, the voltage applied for the electrochemical pretreatment is in the range of 0.1-2.8V, selected to keep the carbon-containing electrode potential in the range of 1.0-3.0V versus a Li/Li &plus; reference electrode. The voltage may be applied by an external source. Further, the electrochemical pretreatment may be a float charge or float discharge, or a combination thereof at the predetermined voltage. Alternatively, the electrochemical pretreatment may be a voltage sweep or voltage pulse, or a combination thereof at the predetermined voltage. The applied voltage may be for a period of about 1 hour to 5 days, for example. More particularly, the voltage is applied for a period of time sufficient to cause a reaction between the oxygen in the cell and the moisture impurity in the cell. In an embodiment of the present invention, the partial pressure of oxygen of the cell prior to the electrochemical treatment is maintained in the range of about 0.0001-1.0 atm, for example in the range of about 0.001-0.1 atm. In yet another embodiment of the present invention, the electrochemical pretreatment is performed at a temperature in the range of about −40 to about 120° C. During operation or cycling of a battery cell, additional oxygen and moisture are generated. Thus, the electrochemical treatment may be repeated periodically throughout the calendar life of the battery to reduce the moisture, thereby further prolonging the battery calendar life. Alternatively, the initial pre-formatting treatment may be skipped, but with periodic treatments throughout the calendar life of the battery. In one example of the present invention, a carbon anode is positioned opposite a transition metal chalcogenide cathode with a lithium salt-based electrolyte/separator therebetween. Again, moisture is present in the cell as an unavoidable impurity. Optionally, additional oxygen may be provided in the cell. Prior to the first discharge-charge cycle of the cell, the cell is electrochemically treated by a applying a voltage to the cell. The voltage is selected to keep the potential of the negative carbon electrode more positive than the potential necessary for SEI formation by reduction of electrolyte on the negative electrode, and is further selected to keep the potential of the negative carbon electrode more negative than the equilibrium potential of the oxygen to insure that oxygen reduction occurs on the negative carbon electrode. For example, the voltage applied for the electrochemical pretreatment is in the range of 0.1-2.8V, selected to keep the negative electrode potential in the range of 1.0-2.9V versus a Li/Li &plus; reference electrode. In another example of the present invention, an anode is positioned opposite a transition metal chalcogenide composite cathode containing a carbon additive with a lithium salt-based electrolyte/separator therebetween. Again, moisture is present in the cell as an unavoidable impurity. Optionally, additional oxygen may be provided in the cell. Prior to the first discharge-charge cycle of the cell, the cell is electrochemically treated by a applying a voltage to the cell. The voltage is selected to keep the potential of the positive carbon-containing electrode more positive than the potential necessary for SEI formation by reduction of electrolyte on the carbon additive in the positive electrode, and is further selected to keep the potential of the positive carbon-containing electrode more negative than the equilibrium potential of the oxygen to insure that oxygen reduction occurs at least on the carbon additive in the positive electrode. For example, the voltage applied for the electrochemical pretreatment is in the range of 1.0-2.8V, selected to keep the positive electrode potential of 1.0-3.0V versus a Li/Li &plus; reference electrode. A battery prepared by the method of the present invention exhibits a decrease in cell impedance and a respective increase in cell power capability. This improvement in electrochemical performance is believed to be caused by a decrease in the water content within the cell. This decrease in water content may be caused by the electrochemical reduction of oxygen on the carbon-containing electrode, for example the negative carbon electrode. A negative carbon electrode is known to be one of the best catalysts for oxygen reduction. In the presence of water in the voltage range described above, the oxygen reduction may occur on the negative carbon electrode according to the following reaction: 2H 2 O&plus;O 2 &plus;4e − →4OH  (1) The simultaneous respective reaction on the positive electrode will be as follows: LiMO 2 →Li 1-x MO 2 &plus;xLi &plus; &plus;xe − (2) where M represents the transition metal or solid solution of transition metals typically used as positive electrode materials in lithium-ion batteries, and x represents the number of electrons or respectively the number of lithium ions exchanged. In the presence of lithium ions, which are present in lithium salt-based electrolytes, the generation of OH— ions on the carbon surface is expected to produce precipitation of solid state compounds, such as LiOH. The lithium ions used for the LiOH precipitation will be compensated in the cell electrolyte by reaction (2). It is anticipated that the LiOH precipitated on the carbon surface will be covered during cell formation cycles by the solid electrolyte interface (SEI) passivation layer formed during this process on the carbon electrode active material. Thus, it is likely that the SEI layer will encapsulate the LiOH precipitated on the carbon surface. The believed reaction mechanism in a system having a lithiated nickel oxide cathode, the respective LiOH precipitation and the expected encapsulation by the SEI layer formation is illustrated in FIG. 1 . A similar mechanism is believed to occur where the positive electrode contains a carbon additive. However, the present invention is not limited to said proposed mechanism. 
 EXAMPLE 1 The decrease in cell impedance is further illustrated in FIG. 2 where the cell impedance measured by electrochemical impedance spectroscopy (EIS) of four cells treated according to the present invention versus the impedance of untreated reference cells is depicted. The cell impedance was measured using 2 mV amplitude in the frequency range 10 5 -10 −2 Hz. As illustrated, the impedance of the four treated cells by a float charge in the voltage range of 1.2-1.5V for 48 hours is substantially lower than the impedance of the four untreated reference cells. FIG. 3 shows the charge and discharge power of the cell treated by a float charge at 1.4V for 48 hours compared with the power of the untreated reference cell as measured using a hybrid pulse power characterization (HPPC) test. As shown in FIG. 3 , the treated cell according to the present invention has about 70% higher power than the power of the untreated reference cell. In each of FIGS. 2 and 3 , the polarization amplitude was 2 mV. 
 EXAMPLE 2 A lithium-ion PVdF polymer 250 cm 2 Bellcore type test cell was used for comparative measurement. An Al doped LiCo0.2Ni0.8O 2 positive electrode and a natural graphite negative electrode were used. The two electrodes were separated with a PVdF based polymer separator. The cell was activated with 1 M LiPF 6 electrolyte dissolved in EC:EMC (ethylene carbonate:ethyl-methyl carbonate). The cell was then hermitically closed and a 1V voltage was applied from an external power source to the cell for a period of about 48 hours. A standard negative electrode formation cycle was then applied at a charge-discharge rate of C/5 (i.e., current&equals;cell capacity/5 hours). The cell was then subjected to BIS impedance measurement in the frequency range of 10 4 -5×10 −2 Hz and 5 mV amplitude. The cell impedance measured is shown by the solid line in FIG. 4 . The cell electrolyte was then extracted and the water content in the cell electrolyte measured using a Karl Fischer titration method. The water content measured in the cell electrolyte was 24±10 ppm. A reference cell with the same chemistry described above was used for comparative measurement. The cell was activated with the same 1 M LiPF 6 dissolved in EC:EMC electrolyte. The cell was then closed and a standard negative electrode formation cycle was applied at a charge-discharge rate of C/5. The cell was then subjected to EIS impedance measurement in the same frequency range at 10 4 -5×10 −2 Hz and 5 mV amplitude. The cell impedance measured is shown by the dashed line in FIG. 4 . The cell electrolyte was then extracted and the water content in the cell electrolyte was measured using the same Karl Fischer titration method. The water content was 158±10 ppm. The treated cell according to the present invention exhibits substantially lower cell impedance than the untreated reference cell and about six times lower water content than the untreated reference cell. 
 EXAMPLE 3 A 20 cm experimental lithium-ion cell was prepared according to the present invention as described in Example 2, but with a 1.5V pretreatment in accordance with the present invention. A comparative reference cell was also prepared as described in Example 2. The impedance response of the treated cell of the present invention was measured after 0, 30, 60 and 90 days calendar life test at 55° C. Impedance response of the reference cell was measured after 0, 30 and 60 days calendar life test at 55° C. The results are provided in FIG. 5 . After a 90 day calendar life test at 55° C., the lithium-ion cell of the present invention exhibited an even lower impedance than the untreated reference cells after only a 30 day calendar life test. FIG. 5 demonstrates that the cell prepared according to the present invention has a significantly longer calendar life than an untreated reference cell. Further, lithium-ion cells prepared according to the present invention exhibit significantly better elevated temperature calendar life than untreated reference cells. While the present invention has been illustrated by the description of an embodiment thereof, and while the embodiment has been described in considerable detail, it is not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope or spirit of applicant's general inventive concept.