Overcharge protection systems for rechargeable batteries

#The present invention relates to electrochemical cells having a mechanism for protecting against damage from overcharge. More specifically, the invention relates to cells in which sulfur or a similarly acting species is oxidized during overcharge at a positive electrode and then shuttles to a negative electrode where it is reduced.

#Damage from overcharge presents a significant problem for many secondary batteries. Normal recharging is intended to be carried out until the cell on recharge reaches a defined voltage. If for any reason this voltage is exceeded, overcharge may result. Overcharge can cause various undesirable reactions such as destruction of the cell electrolyte, corrosion of current collectors, degradation of cell separators, and irreversible damage to the positive or negative electrode. Any one of these conditions can lead to the destruction of the cell. Further, overcharge can create unsafe conditions such as cell venting due to electrolyte gassing.

#The problem may be especially pronounced when a plurality of electrochemical cells are coupled together in series (e.g., to form a battery or battery pack). Typically, the individual cells will possess at least slightly varying capacities--reflected as the maximum number of coulombs (or ampere-hours) that the cell accept without overcharging. Those cells having lower capacities will become fully charged before other cells having higher capacities. If charging continues after the lower capacity cells reach full charge, the lower capacity cells will overcharge and possibly be damaged.

#FIG. 1 is an illustration of an arrangement of cell components for a lithium cell which may be employed in a preferred embodiment of the present invention.

#FIG. 2 is a mechanistic representation of an internal polysulfide overcharge protection mechanism of a lithium-sulfur cell.

#FIG. 3 is a graph showing how reaction rate increases with oxidation level of polysulfides (or other appropriate overcharge protectant) in accordance with the present invention.

#General Points

#The present invention provides an overcharge protection system and method for recharging electrochemical cells. In the following description, numerous specific details of the cells and mechanisms are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, that the present invention may be practiced without limitation to some of the specific details presented herein.

#Some suitable materials for use in fabricating certain sulfur-based cells of this invention, as well as fabrication methods employed therewith, are described in U.S. Pat. Nos. 5,523,179 (issued Jun. 4, 1996) and U.S. Pat. No. 5,582,623 (issued Dec. 10, 1996) as well as U.S. patent application Ser. No. 08/686,609 filed Jul. 26, 1996 (attorney docket no. PLUSP003). Each of these patent documents names May-Ying Chu as inventor, is assigned to the assignee of the present invention, and is incorporated herein by reference for all purposes.