Use of B.sub.2 O.sub.3 additive in non-aqueous rechargeable lithium batteries

#This invention pertains to non-aqueous rechargeable lithium batteries and to methods for improving the performance thereof. Specifically, it pertains to the use of B.sub.2 O.sub.3 in lithium batteries as a means for improving the capacity delivered from such ion batteries after extended cycling.

#Many varied types of non-aqueous rechargeable lithium batteries are used commercially for consumer electronics applications. Typically, these batteries employ a lithium insertion compound as the active cathode material, a lithium compound of some sort (eg. pure lithium metal, lithium alloy, or the like) as the active anode material, and a non-aqueous electrolyte. An insertion compound is a material that can act as a host solid for the reversible insertion of guest atoms (in this case, lithium atoms).

#Lithium ion batteries use two different insertion compounds for the active cathode and anode materials. Currently available lithium ion batteries are high voltage systems based on LiCoO.sub.2 cathode and coke or graphite anode electrochemistries. However, many other lithium transition metal oxide compounds are suitable for use as the cathode material, including LiNiO.sub.2 and LiMn.sub.2 O.sub.4. Also, a wide range of carbonaceous compounds is suitable for use as the anode material. These batteries employ non-aqueous electrolytes comprising LiBF.sub.4 or LiPF.sub.6 salts and solvent mixtures of ethylene carbonate, propylene carbonate, diethyl carbonate, and the like. Again, numerous options for the choice of salts and/or solvents in such batteries are known to exist in the art.

#In drawings which illustrate specific embodiments of the invention, but which should not be construed as restricting the spirit or scope of the invention in any way:

#FIG. 1 depicts a cross-sectional view of a preferred embodiment of a cylindrical spiral-wound lithium ion battery.

#FIG. 2 shows the capacity versus cycle number data for a representative battery of Comparative Example 1 comprising 0% wt. B.sub.2 O.sub.3 in the electrolyte.

#We have discovered unexpectedly that the capacity fade rate characteristic of non-aqueous lithium rechargeable batteries in general can be improved by dissolving a small amount of B.sub.2 O.sub.3 additive in the electrolyte. However, we have also discovered that some tradeoffs in other battery characteristics may occur with the use of such an additive. Thus, we have found that a balance must be struck between these characteristics when selecting the amount of B.sub.2 O.sub.3 additive to use.

#Typically, a non-aqueous lithium rechargeable battery employs a lithium insertion compound as the cathode and one of a variety of lithium compounds as the anode. Possible lithium compounds include lithium metal, lithium alloys, and lithium insertion compounds. Preferred embodiments are lithium ion batteries wherein the anode is also a lithium insertion compound. Currently, the majority of commercial lithium ion batteries employ LiCoO.sub.2 cathodes and carbonaceous anodes.

#Preferred electrolytes for lithium ion batteries comprise LiPF.sub.6 salt dissolved in a mixture of non-aqueous organic carbonate solvents (such as propylene carbonate, ethylene carbonate, ethyl methyl carbonate, and/or diethyl carbonate). This choice of salts can result in a safer, more stable, electrolyte than other salt choices.