Overcharge Protection of Secondary, Non-Aqueous Batteries

Accordingly, an object of this invention is to provide a means of chemical overcharge protection to secondary non-aqueous cells by the use of redox reagents.

The invention features a rechargeable electrochemical cell with includes an anode, a cathode, and an electrolyte. The electrolyte is a non-aqueous solvent or a mixture of non-aqueous solvents in which one or more salts and the redox reagent are dissolved. The redox reagent is present in an amount sufficient to maintain proper mass transport for the desired steady overcharge current for the cell.

A particular class of redox reagents for overcharge protection are metallocenes, in which cyclic .pi. electron donors such as the pentahaptocyclopentadienyl (h.sup.5 -C.sub.5 H.sub.5) and hexahaptobenzene (h.sup.6 -C.sub.6 H.sub.6) and related molecles combined with metal atoms to form complexes of the general formula: ##STR1## where, M represents metals such as iron, cobalt, nickel, chromium or tungsten and R.sup.1 through R.sup.6 stand for H or alkyl groups such as methyl, ethyl or butyl.

In preferred embodiments the redox shuttle is ferrocene, or n-butylferrocene, having the structural formulas shown below, and the rechargeable cell is a Li/TiS.sub.2 cell containing a mixed ether electrolyte of the composition described in the teachings of U.S. Pat. No. 4,489,145. ##STR2##

It has been discovered that ferrocene and its derivatives, such as n-butyl-ferrocene, 1,1-dimethylferrocene, N,N-dimethylaminomethylferrocene and decamethylferrocene, are particularly useful as overcharge protection additives for the Li/TiS.sub.2 cell.

The choice of a particular redox reagent will vary with the cathode material used in a rechargeable Li cell. Thus, nickelocene would be most suited for use with cells in which the cathode is fully charged by 2.65V and cobaltocene for cells in which the cathode is fully charged by 1.70V.

Other features, objects and advantages will become apparent from the following specification when read in connection with the accompanying drawing, the single figure of which shows a cycling curve showing overcharge for an exemplary embodiment of the invention.

Cyclic voltammetry was used to screen candidates for use as redox shuttle reagents for overcharge protection in rechargeable Li cells. Results of such experiments are given in Table 1.

The reactions which are believed to be responsible for the suitability of these materials as redox shuttle reagents for overcharge protection are given in the following equations, illustrated with n-butylferrocene:

At positive electrode (cathode): n-butylferrocene .fwdarw.(n-butylferrocene).sup.+.sub.+e.sup.-

At negative electrode (anode):

(n-butylferrocene).sup.+.sub.+e.sup.- .fwdarw.n-butylferrocene

In Li/TiS.sub.2 cells containing ether electrolytes, oxidation of the redox reagent should take place between .about.2.8 and 3.5V vs. Li.sup.+ /Li. Of the compounds listed in Table 1, ferrocene and its derivatives are the most likely candidates for overcharge protection of such cells.

In a high energy density ambient temperature cell, the positive electrode material preferably consists of titanium disulfide (TiS.sub.2). However, the positive electrode material may comprise other transition metal compounds and notably other insoluble chalcogenides of titanium, zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, tungsten, rhenium, platinum, germanium, iron, tin and lead, e.g., TiS.sub.3, TiSe.sub.2, TiSe.sub.3, ZrS.sub.2, ZrS.sub.3, HfS.sub.2, HfS.sub.3, VS.sub.2, VS.sub.3, NbS.sub.2, TaS.sub.2, TaS.sub.3, ZrSe.sub.2, ZrSe.sub.3, HfSe.sub.2, HfSe.sub.3, VSe.sub.2, Vse.sub.3, NbSe.sub.2, NbSe.sub.3, TaSe.sub.2, TaSe.sub.3 and MoSe.sub.3 S and alloys of the foregoing cations such as ZrHfSe.sub.2. Other positive electrode materials may comprise mixed sulfides such as Fe.sub.x V.sub.1-x S.sub.2 and Cr.sub.x V.sub.1-x S.sub.2 (x<1) and sulfide compounds such as NiPS.sub.3 and metal oxides such as MoO.sub.3 V.sub.6 O.sub.13, V.sub.2 O.sub.5 and Cr.sub.x O.sub.y where y/x is between 1.5 and 3. Soluble and partially soluble positive electrode materials also may be used, notably I.sub.2, Br.sub.2, Cl.sub.2, SO.sub.2, S, CuC1, CuC1.sub.2, AgCl, FeCl.sub.2, FeCl.sub.3, and other transition metal halides. Other soluble positive electrode materials that may be used are lithium polysulfide (Li.sub.2 Sn) and organic compounds such as chloranil and fluoranil. The requirement for the selection of a metallocene for use as redox shuttle reagent for a given positive electrode is that the oxidation potential of the metallocene be slightly higher than the full charge limit of the cell containing the cathode.

The concentration of the solute (ionic conductor) in the solvent is not critical. Usually, an amount sufficient to yield the desired level of conductivity is used. By way of example, the salt concentration should be such as to give a specific conductivity of about 5.times.10.sup.-3 10.sup.-3 ohm.sup.- 1at about 22.degree. C. The preferred salt is LiAsF.sub.6. However, other lithium salts such as LiClO.sub.4, LiBR.sub.4 (where R=alkyl or aryl groups), LiPF.sub.6, LiAlBr.sub.4, LiSCN, LiAlCl.sub.4, LiBF.sub.4, and lithium salts of organic acids such as trichloroacetic, trifluoromethane sulfonic and formic acids can be used.

The preferred electrolyte solvents are THF and 2-Me-THF. However, embodiments of the invention are equally applicable with other organic solvents such as dioxolane, 1,2-dimethoxyethane (DME), diethylether, dimethoxymethane, trimethoxymethane, tetrahydropyran, 2-methyltetrahydropyran, 3-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, and 2,5-dimethyltetrahydropyran, propylene carbonate, ethylene carbonate, butyrolactone and mixtures of these solvents.

The cells used to demonstrate the advantages of this invention contained 7.1 Ah of Li, 2.3 Ah of TiS.sub.2, and at lest one of the shuttle reagents described above dissolved in a mixed ether electroylte solution comprised of LiAs.sub.6 in the THF: 2-Me-THF: Me-F mixed ethers of the teachings of U.S. Pat. NO. 4,489,145.

The Li/TiS.sub.2 cells were cycled galvanostatically with the aid of standard cycling equipment. The potential limits for normal cycling were 1.6V for discharge, and 2.8V for charge. The cells were discharged to a 100% depth at 0.75 or 1.5 mA/cm.sup.2, and charged at 0.38 or 0.75 mA/cm.sup.2.

Overcharge experiments were performed either by changing the upper voltage limit to a value between 3 and 3.5V versus Li.sup.+ /Li or by potentiostatically setting the positive electrode (TiS.sub.2) at a potential in the range between 3 and 3.5V versus Li.sup.+ /Li.

Finally, the occurrence of oxidation reactions of the type in equation [1]above was demonstrated by electroylsis experiments followed by spectroscopic analysis of the resulting solutions.

The following illustrative examples further clarify the invention to those skilled in the art.

A Li/TiS.sub.2 cell of the description given above was filled with about 8 grams of electrolyte solution containing LiAsF.sub.6 in a mixed ether (THF/2MeTHF/2MeF) and 1.0 molar n-butylferrocene. The cell was cycled at a discharge current density of 0.75 mA/cm.sup.2 and a charge current density of 0.38 mA/cm.sup.2 at room temperature. After 34 cycles the upper voltage limit was raised to 3.5V, and the cell was allowed to overcharge. Defining overcharge as charge obtained at a potential greater than 2.8V vs. Li, 866 mAh of overcharge was obtained. As FIG. 1 makes apparent, the overcharge was obtained at a constant potential of 3.25V vs. Li. A shuttle mechanism is evident since the overcharge is four times the charge expected for oxidation of the 8 mmol of n-butylferrocene contained in the cell.

A Li/TiS.sub.2 cell of the description given above was filled with an electrolyte solution containing 0.25 molar ferrocene. The first cycle with this cell was performed between 1.6 and 2.8V at current densities of 1.5 mA/cm.sup.2 for discharge and 0.75 mA/cm.sup.2 for charge. The discharge capacity was 2.32 Ah, and 2.24 Ah were regained in the first charge. On the second cycle the potential limit was increased to 3.5V. An overcharge of 120 mAh was observed at a potential of 3.35V. After two more cycles under the regular cycling conditions, the cell was again overcharged. At a charge rate of 0.25 mA/cm.sup.2, 114 mAh of overcharge (twice the charge expected for oxidation of the ferrocene) was observed, at a potential of 3.3V. The original cycling conditions were resumed, and a total of 124 cycles were obtained. The cycle life of the cell is the same as that obtained in similar cells without ferrocene. No adverse effect of either the overcharge or ferrocene on the cycle life of the Li/TiS.sub.2 cell was observed.

A series of cells of the above description containing electrolyte solutions with n-butylferrocene were maintained at a constant potential, and the resulting current was observed.

A cell which contained the baseline ether mixture and LiAsF.sub.6 with 0.15 molar n-butylferrocene, was maintained at 3.35V for 2.5 h. The initial current, 700 mA, decayed to a limiting value of 63 mA during the course of the experiment. The total overcharge was 324 mAh, or 10 times the number of coulombic equivalents expected for one-electron oxidation of n-butylferrocene.

A cell containing the same electrolyte solution, except that the n-butylferrocene concentration was increased to 0.5 M, was charged at 3.3 and 3.5V. At 3.3V, the current was 100 mA. The cell was held at 3.3V long enough to produce 8 mAh of overcharge. The voltage was then increased to 3.50V and held there for two hours. The limiting current was 124 mA and the charge was 319 mAh. The total overcharge was 327 mAh or 3 times the theoretical coulombic charge for this one-electron oxidation of the 4 mmol of n-butylferrocene present.

Electrolysis experiments confirmed that oxidation of the n-butylferrocene occurs at the cathode. A three compartment cell with the compartments separated by glass frits was filled with a 0.2 molar solution of n-butylferrocene in the mixed ether electrolyte with LiAsF.sub.6 as the electrolyte.

The working electrode was a Teflon-bonded carbon electrode having 1.4cm.sup.2 /side, and the counter and reference electrodes were Li. The glass frit separator prevented significant mixing of the anolyte and the catholyte. The open circuit potential was 3.09V. When a constant charging current of 0.5 mA (.about.0.3 mA/cm.sup.2) was applied, the oxidation potential was constant at 3.3V vs. Li.sup.+ /Li. During charging for 18 h (9 mAh), the solution in the cathode compartment turned blue-green. Whereas UV-VIS specta of the starting solution showed two absorbance maxima at 440 and 320 nm, the blue-green solution showed a strong absorption maximum at 630 nm. The latter absorption is due to the formation of the oxidized form of n-butylferrocene.

Also, electrolytes containing redox shuttle reagents prepared in accordance with this invention may be used where the anode essentially includes or is a material other than lithium, e.g., sodium, potassium, magnesium, calcium or zinc, or mixtures thereof with or without lithium. Other modifications and advantages will be obvious to persons skilled in the art, and are within the following claims.