CA2143767C

CA2143767C - Process for producing manganese dioxide

Info

Publication number: CA2143767C
Application number: CA002143767A
Authority: CA
Inventors: Enoch I. Wang; Lifun Lin; William L. Bowden
Original assignee: Duracell Inc USA
Current assignee: Duracell Inc USA
Priority date: 1992-09-28
Filing date: 1993-08-04
Publication date: 1999-02-16
Anticipated expiration: 2013-08-04
Also published as: RU2118292C1; JPH08503918A; NO950803D0; CZ75195A3; CN1036515C; NO950803L; FI105180B; DE69322788D1; AU4997193A; CA2143767A1; KR100268066B1; CN1085189A; JP2941059B2; DK0664768T3; WO1994008056A3; EP0664768B1; RU95110667A; EP0664768A4; US5348726A; SG49861A1

Abstract

The invention relates to the manufacture of manganese dioxide by a chemical process. The resulting manganese dioxide product takes the form of particles characterized by filament-like protrusions jutting out from its surface. The manganese dioxide particles having such surface features can be manufactured by reacting manganese sulfate with sodium peroxodisulfate in an aqueous solution. The process can be controlled to yield high density manganese dioxide. The manganese dioxide formed in the process can be deposited directly onto the surface of electrolytic manganese dioxide (EMD). The manganese dioxide product is particularly suitable for use as a cathode active material in electrochemical cells.

Description

~1 43767 PRO~S FOR PROD~CTNG MANG~ ~ DTOXTD~

The invention relates to a ~Lo~Lss for production of manganese dioxide, particularly for use as a cathode active material in ele_~ochemical cells.

Manganese dioxide is commonly employed as a cathode active material in commercial batterie~. Such manganese dioxide has been derived from naturally o~..ing manganese dioxide (NND) and synthetically pro~ce~ manganese dioxide which includes electrolytic manq~n~Fe dioxide (EM~v) and chemical manganese dioxide tCMD). NMD has a high impurity content and cannot be employed in alkaline or lithium cells.

EMD is typically manufactured by direct electrolysis of manganese sulfate and sulfuric acid. It's high purity and high density make gamma manganese dioxide desirable for use as a cathode active material in alkaline and lithium cells.

The features of the product of the invention will be better appreciated with reference to the following figures:

Fig. lA is an electron photomic~oy.aph showing the MnO2 particles from the process of the invention carried out at slow rate of heating of reactants.

Fig. lB is an electron photomi~ -aph of the particles in Fig. lA enlarged to show the filament-like surface protrusions.

Fig. 2A is an electron photomi~.vgr~ph showing smaller sized~
- MnO2 particles from the process of the invention carried out at fast rate of heating of the reactants.

~- W094/080~6 2 1 ~ 3 7 6 7 PCT/VS93/07333 Fig. 2B is an electron photomi~.Gy.~ph of the particles in Fig. 2A enlarged to show the filament-like surface protrusions.

Fig. 3A is an electron photomi~ -dph of EKD particles (prior art).

Fig. 3B is an electron photomi~G~.aph of the EMD particles in Fig. 3A enlarged to show the characteristically irregular particle shape and smooth surface structure.

Fig. 4A is an electron photomi~G~L~ph of EMD particles coated with MnO2 produced by the process of the invention ( P-CMD) .

Fig. 4B is an electron photomicLG~.dph of coated EMD
particles in Fig. 4A enlarged to show the filament-like surface protrusions.

Fig. 5A is an electron photomi~.u~,aph of prior art chemical manganese dioxide (CMD) particles.

Fig. 5B is an electron photomi~-G~,aph of the particles in Fig. 5A enlarged to show surface features.

Fig. 6A is a graphical plot of the voltage profile (voltage versus service hours) in an alkaline AA cell at 3.9 ohm constant load, comparing performance of the P-CMD with conventional EMD.

Fig. 6B is a graphical plot of the voltage profile (voltage versus milli amp-hour per gram MnO2) in a flooded alkaline cell at o. 3 milli-amp/cm2 current drain rate, comparing performance of the P-CMD with conventional EMD.

' W094/08056 2 1 4 3 7 6 7 PCT/US93/07333 Fig. 7A is a graphical plot of the voltage profile (voltage versuC milli a~p-hour per gram MnO2) in a lithium cell at 0.17 milli-amp/cm2 ~ el~L drain rate comparing performance of P-CMD
with conventional EMD.

Fig. 7B ic a graphical plot of the voltage profile (voltage versus milli amp-hour per gram MnO2) in a lithium cell at 1.0 milli-amp/cm2 current drain rate, comparing performance of P-CMD
with conventional EMD.

Fig. 8A is a graphical plot of the voltage profile (voltage versus milli amp-hour per gram MnO2) in a lithium cell at 0.17 milli-amp/cm2 current drain rate, comparing performance of P-CMD
with conventional CMD (wsLi).

Fig. 8B is a graphical plot of the voltage profile (voltage versus milli amp-hour per gram MnO2) in a lithium cell at 1.0 milli-amp/cm2 current drain rate, comparing performance of the MnO2 product of the invention (P-CMD) with conventional CMD
(WSLi).

The present invention involves a ~LoceDs for production of battery grade chemical manganese dioxide (CMD). P-CMD when used in electrochemical cells, particularly alkaline and lithium cells, provides these cells with higher capacity and energy density per gram than are obtainable from the same cells employing electrolytic manganese dioxide (EMD). The process of the invention allows for great-r control of properties such as density, surface area and particle ize than is possible with present processes for the manufacture of conventional forms of EMD or CMD. The process of the invention therefore allows for production of high purity CMD which can be made to have properties more nearly optimal for a given electrochemical cell type. The high density of our MnO2 product is comparable to -'' W094/08056 2 1 4 3 7 6 7 PCT/US93/07333 . .
that obtained from electrolytic man~n~-? dioxide (EMD), yet the surface area of each MnO2 particle ~ greater than that obtained from cG.I~el.Lional EMD and CMD pL-_e~-~ . The high useful surface area of each particle allows for better performance, particularly in lithium cells cont~ining MnO2. By "useful"
surface area we refer to the surface area which is accessible to the electrolyte.

The process of the invention for production of battery grade manganese dioxide is- carried out principally by reacting an aqueous solution of manganese sulfate with sodium pero~o~isulfate.

The reaction may be le~_ented as follows:
MnS0~ + NA2C2Q~ + 2H20 Z Mn~2 + Na2S~~ + 2H2S~~ (I) When an aqueous solution of manganese sulfate (MnS0~) is reacted with sodium pero~o~isulfate (~ ), a gamma crystal structure of MnO2 is directly obtainable as a reaction product in the form of a precipitate. The MnO2 precipitate tends to form spherical particles having filament-like ~o~sions emanating outwardly from each particle surface and are substantially uniformly distributed over the particle surface. The term "filament-like"
as used herein shall be cG~.aL~ed as including thin, elongated, protruding structures such as but not limited to filaments, hairs, needles and fibers. The ~filament-like" protrusions are characterized by a length to width ratio between about 2:1 and 20:1, typically between about 3:1 and 10:1, wherein the width and length refer to those portions of the ~o~-sions which are visible from the particle surface. The average length of the "fila~ent-like~ protrusions is typically between 0.3 to 1 micron and the average width is typically between 0.1 to 0.3 micron.
These dimensions are measurable at a magnification of about 40,000 times actual size. The "filament-like" protrusions result in high surface area of the MnO2 particle. The MnO2 ~ W094/080S6 2 1 4 3 7 6 7 PCT/US93/07333 particleO of the invention as above described may be referred to in the Opecification drawings and claimO as P-C~D.

Unlike the well known Sedema ~.oceOO as discloOed in U.S
patent 2,956,860, the present invention permits the average particle size and density of the HnO2 product to be altered by regulating the rate of the above reaction (I). This can be accompliOhed by simply ~o..~.olling the amount or rate of heat supplied to the reaction- Unlike the Sedema ~-oce_O the present reaction does not reguire a catalytic MnO2 substrate for receiving the HnO2 product. In fact no catalyOt is required and the MnO2 product forms into dense, discrete particles without the need of a substrate material. Ps~.ver, it has been discovered that the reaction mixture can be Feed~ with almost any nonreactive oOlid material including metals and such material will act as a substrate for the P-CMD. That is, the MnO2 reaction product will precipitate directly on the solid material.

It has been discovered that the above reaction mixture can be seeded with particles of electrolytic manganese dioxide (EMD) and the MnO2 reaction product will depoOit directly on the EMD.
This results in a very high density hybrid gamma MnO2 whose outer surface comprises a P-CMD coating having filament-like protrusions and high surface area, while the overall particle shape and interior structure is that characteristic of EMD.
This hybrid form of MnO2 can advantageously be used as cathode active material in elec~rochemical cells, particularly alkaline or lithium cells. It is especially attractive for use in lithium cells, since the expoOure of the EMD particles to H2SO~
during the reaction of the invention, leaches out small amounts of sodium that iô trapped within the EMD particleO. Thiô
reduce~ the amount of sodium impurity in the P-CMD product, which is particularly advantageous if it is to be used as W094/080~6 2 1 4 3 7 6 7 PCT/US93/07333 -cathode active material in lithium cells. It has also been di~covered that the reaction mixture can advantageously be ~ with graphite or carbon black particlec- In such case the MnO2 reaction product will deposit directly onto the surface of the~e particle~ to form a hybrid particulate material which may also be used a~ cathode active material in ele~L~o~hemical cells.

The above reaction (I) may typically be carried out in a temperature range between about 30 and 100~ C, preferably between 70 and 90~ C. The reaction (I) is prefer~bly carried out in a temperature range between about 70~ C and 80~ C when the intenA~A
use of the P-CMD is as a cathode active material in an alkaline cell, and between about 80 and 90~ C when the int~nA~A uce iF as a cathode active material in a lithium cell. (For end application of the P-CMD in alkaline cells it is preferable to keep the final temperature below 85~ C in order to obtain a gamma MnO2 product with higher rl~n~in~ voltage and capacity than EMD.) After the reaction is complete, the MnO2 precipitate is collected and rinsed with distilled water until it has a pH of 7. It may tben be dried at room temperature if its inte~A~A use i~ as a cathode active material in an alkaline cell. Alternatively, it may be dried at elevated temperature for more thorough drying, if its intended use is as cathode active material in a lithium cell. The resulting dry gamma MnO2 has a high purity and low sodium content of less than about 500 ppm. The dry P-CMD
contains at least 95% gamma MnO2 in particulate form. (No other crystalline forms of MnO2 have been detected in the dry P-CMD
of the invention, but 95% is the limit of resolution of the x-ray diffraction analysis employed for MnOz.) Every MnO2 particle made by the process of the invention, when observed between 200 and 2000 times actual ~ize, Appe~rs to have filament-like protrusions radiating outwardly from the particle surface and tbese protrusions appear to be uniformly distributed around the W094~08056 2 1 4 3 7 6 7 PCT/US93/07333 particle surface. The P-CMD so proA~c~ may 6~h-~quently be heat treated in conventional manner to convert it to a gamma-beta variety, if desired. This treatment i6 preferred if the end use of the MnO2 i6 as cathode active material in lithium cells. The heat treatment i8 well known, a suitable heat treatment process being disclosed in U.S. patent 4,921,689.

It hAs been determined that various properties of the MnO2 product can be altered and controlled by c~,.LLolling the rate at which the reaction mixture ic heated. In general a ~n-~r P-CHD is obtAin~ if the reaction is carried out at a slower rate, e.g., if the reaction temperature is increased at a slower rate. In a slower reaction individual particles of MnO2 have time to grow to form larger, more compact particles. In a f_ster reaction, e.g. produced by faster increase of the reaction temperature, the individual particles of the P-CMD do not have sufficient time to grow to for~ larger particles.
Therefore the individual particles are smaller and less compact.
They have a fluffier appearance and lower average density than particles obtAine~ from a slower rate of heating.

A sufficiently low reaction rate to provide a P-CMD product bulk density of about 15 to 32 g/in3 (0.9 and 2 g/cm3) SAD (Scott Apparent Density) is obtained if the aqueous reaction mixture of MnS0~ and Na2S20~ is maintained at an initial temperature of about 50~ C for about 18 hours and then slowly increased at nearly constant rate for between about 5 and 10 hours until a final temperature of between about 70 to 90~ C is ob~in~. The reaction mix may then be left to stand for about 1 hour at this final reaction temperature, to obtain a maximum yield, typically about 70% of the ~toichiometric amount of MnS0~ converted to P-CMD. In thi~ manner battery grade P-CMD can be obtained having densities comparable to or even higher than the density of electrolytic manganese dioxide (EHD) which typically i~ at a -level of about 25 to 28 g/in3 (1.5 to 1.7 g/cm3) SAD (Scott Apparent Den6ity). In general A bulk den~ity of the P-CMD
between _bout 15 and 32 g/in3 (0.9 and 2 g/cm3) can be achieved by heating the aqueous solution of MnSO~ and 2~C~Q~ from an initial temperature between about 40~ C and 70~ C for a period during reaction at an average rate of less than about 7~ C per hour for at least 5 hours, typically between about 1~ C per hour and 7~ C per hour for at least 5 hourc.

A sufficiently fast reaction rate to achieve a P-CMD
product bulk density of between about 8 to 15 g/cm3 (0.5 to 0.9 g/cm3) (Scott Apparent Density) i8 obtained if the agueous reaction mixture of MnSO~ and 2~aCc~ is heated at about constant rate from room temperature so that a final temperature of between 70 and 90~ C is achieved in about one to two hours. The reaction mixture may be left to stand for about one hour at this final temperature, to obtain a maximum yield, typically about 70% of the stoichiometric amount of manganese in MnSO~ converted to MnO2. In general a bulk density of the HnO2 product between about 8 g/in3 and 15 g/in3 (0.5 and 0.9 g/cm3) can be achieved by heating the aqueous solution of MnSO~ and N~C~Q~ from an initial temperature between about 30~ C and 100~ C for a period during reaction at an average rate greater than 7~ C per hour for less than about 5 hours, typically between about 7~ C and 20~ C per hour for less than about 5 hours.

It has been determined that the stoichiometric yield of MnO2 can be dramatically increased to about 95S by slowly adding a suitable alkaline base slowly to the reaction mixture. As the reaction procee~ the base reacts with the H2SO~ as it forms, thereby improving the reaction kinetics and ultimate yield of MnO2. A preferred base is Li2CO3. Alternative bases can be employed to react with the H2SO~ to produce the s_me increase in yield of HnO2. Such compounds include Na2CO3, LiOH, NaOH And HgO.

W094/08~6 2 1 4 3 7 6 7 PCT/US93/07333 For ultimate use of the MnO2 product in lithium cells it would be preferred to add comro~ such as T-i 2co~ and LiOH to the reaction mixture to increase yield. For ultimate use of the HnO2 product in alkaline cells it would be preferred to add Na2CO3 or NaOH to the reaction mixture. If such comro-~nAr ~re added, they should be added 810wly to the reaction mixture to ~.~ve..L the pH
of the mixture from abruptly increasing to a pH greater than about 3.

The MnO2 reaction product of the invention takes the form of discrete particles having a ~pherical shape and gamma crystalline stru~Lu.e. The particle size of the P-CMD can also be controlled by varying the rate at which the reaction mixture is heated. If the reactron mixture is heated to produce a constant rate of increase in temperature then the MnO2 pArticle size distribution will be uniform, that is, there will not be muck variance in the diameter of individual HnO2 particles. If the reaction mixture is slowly heated at constant rate, e.g., of betwee~ about 1~ C and 7~ C for at least 5 hours, the MnO2 product will take the form of relatively large uniform spherical particles as above ~entioned. If the reaction mixture is rapidly heated at a fast constant rate, e.g., between About ~7~
C per hour and 20~ C per hour for less than about 5 hours, the P-CMD product will tend to take the form of relatively small spherical particles. If -the reaction mixture is initially heated at a slow constant rate of temperature increase and later at a fast constant rate of temperature increase, the reaction product will contain a distribution_of both large and small MnO2 particles.

The following examples illustrate the method of preparation of battery grade MnO2 by the the ~& _ e c of the invention. All parts are parts by weight unless specified otherwise.

ExamDle 1:
High density gamm MnO2 is prepared by the ~-_eF- of the invention as follows:
120 g of MnS0~, HzO is di6solved in 1800 ml of diE;tilled water.
Then, stoichiometric amount of 1'~ 0~ (169 g) i~; added to the clear pi n~ solution to foml a reactant ~olution. While stirring, the temperature of the ~olution i~ raised over a period of about 2 hours from room temperature (20~ C) to 50~ C
and i8 mainta i n~l at a temperature of 50~ C overnight (about 18 hrs) while contin~ y ctirring. This enh-n~ec the nucleation The reaction ~ vce~ C according to reaction (I) above referenced. The clear pi nlri ch solution ~lowly turns brown and then eventually turns a black color ac more MnO2 is precipitated.
After the 18 hour period the solution is heated from about 50~ C to produce a constant rate of temperature increase of about 25~ C per hour for about 1 hour to a temperature of about 75~ C
and i6 maintained at 75~ C for about 3 hours. The solution is then heated at constant rate of about 10~ C per hour for about 1 hour to a temperature of 85~ C and maintained at 85~ C for 1 hour. The solution is again heated at a constant rate 30~ C per hour for about 1/2 hour to a temperature of about 100~ C and maintained at 100~ C for about 1 1/2 hours at which time the r,un is ended. The pH of the solution at the end of the run is less than 0.5. The solution is then cooled to room temperature (20~
C) in about one hour. The solution is filtered and the solid MnO2 is contin-~-lly rin~ied with distilled water until the filtrate ~;tream has a neutral pH of about 7. The resulting black powder i~; dried at 100~ C to drive off surface water. The overall yield of P-CHD is 41 g or 67% of theoretical yield.
The re~ulting product is battery grade MnO2 at least 959c of which is verified by x-ray diffraction to be of the gamma cry~talline structure. (No other type HnO2 cry~;talline structure was detected, the 95% threshold being the limit of resolution of the x-ray diffraction analysis.) The P-CHD product has a high W094/08056 ~ PCT/US93/07333 bulk density of about 23 g/in3 (1.4 g/cm3) SAD (Scott Apparent Density). An el~_L.~.. photomi~Gy,aph ~ 6-ntative of thi~
MnO2 product is shown in Figs. lA and lB. The uniform cpherical structure of the P-CMD particles (e.g. particle 10) i~ shown in Fig. la t_ken at l99X magnification. The fil~ent-like (e.g.
hair-like) ~.uL.~sionc 15 em_n_ting from the ~urface of each spherical particle are clearly visible in Fig. lB, which show6 an individual particle at 2,030X magnification. By comparison the electron photomi~ .aphs of the commercial battery grade CMD (WSLi) particle6 are ~hown in Figs. 5A and 5B, which are taken _t 202X and 2060X magnification, re~pect~vely. (The WSLi brand of CMD i~ available from Sedem_, a divicion of Sadacem, S.A., Terte, Belgium.) It i~ cle~r from Figs. 5A and 5B that representative particles 70 do not exhibit fil_ment-like protrusions characteristic of the MnO2 product of the invention (Figs lA _nd lB).

le 2:
Lower dencity gamm_ MnO2 is prepared by the process of the invention as follows:
The P-CMD is made in a similar manner _s described in example l, except that rate of heating is f_ster 1~A~;ng to smaller ~ize and less dense particles. Specifically, the ~ame method of preparation and conditions _s in example l are employed ex~e~L
the reactant solution is heated from 50~ C to 100~ C at rate of about about 17~ C per hour for a period of less than 5 hours, namely about 3 hours. Figures 2A and 2B are electron photomi~ ph~ of the resulting MnO2 product. The product sample represented in Figs. 2A and 2B had a bulk density of about 8.7 g/in3 tO.53 g/cm~) (Scott Apparent Density) and i~ at least 95% gamma MnO2.
The filament-like (e.g. h_ir-like) surfAce ~Ll~sion~ 20 and 25 of the individual particles may be seen in Figc. 2A ~d 2B, respectively. The P-CMD particles as described in this cxample WO 94/080~6 2 1 4 3 7 6 7 PCr/US93/07333 may be used as cathode active material in ele_Llo~;hemical cells, particularly alkaline and lithium cells. If intonA~~ for use in lithium cell the gamma MnO2 may be heated at a temperature between about 300-400~ C, typically for about 6 hours at 350~ C
or 32 hours at 300~ C to convert the gamma MnO2 to gamma-beta cry~;talline ~;tructure and to _~a~GL~te any residual moi~;ture entrapped within the MnO2 particles.

Example 3:
P-CMD is proAt~ in a manner similar to that de~cribed in Example 1 ex~,e~L that Li2C03 is added to the reaction mixture in order to increase the yield of MnOz. 583 g of MnSO~ H20 is first dissolved in 8 liter of distilled water in a 12 liter round bottom flask. Then stoichiometric amount of r~oO (822 g) is added to the ~ilightly pinl~ir~l solution. The solution is heated at a con~tant slow rate for 6 hour~ from room temperature (20~
C) to 55 ~C. Then 23 g of Li2Co3 is then slowly added and the solution is maint~ at a temperature of about 55~ C for 18 hour~ while contin~ y mixing. An additional 69 g of Li2Co3 is added after the 18 hour period and the solution i~ heated at a constant rate of about 6~ C per hour for about 2.5 hours up to a temperature of 70~ C. Another 36 g of Li2Co3 i6 then added ~nd the solution is heated at a constant rate of about 5~ C per hour for 2 hours up to a temperature of about 80~ C. The solution is then heated at a reduced constant rate of about 3.3~ C per hour for 3 more hourc up to a temperature of 90~ C. The solution temperature i~; held for about 18 hours and then cooled in about 1 hour to room temperature (20~ C). P-CHD is L~ ered and dried in the manner described in example 1. The yield of MnO2 is 270 g or 90% of the theoretical yield. At least 95% of the HnO2 product i8 verified by x-ray diffraction to be gamma HnO2. The bulk denl;ity of the P-CMD is measured as 20 g/in3 (1.2 g/cm3) (Scott Apparent Density). This P-CHD product can be heat treated as in Example 1 whe,eliE,o.- it become~ particularly W094/080~6 2 1 4 3 7 6 7 PCT/US93/07333 suitable for use ag a cathode active material in lithium cells.

E~ le 4:
Thi~ example demo~-D~Lates the use of EMD particles as a substrate for the precipitation of HnO2 made in accordance with the invention.
120 g of MnS0~ H20 i8 dissolved in 1.6 liter of distilled water in a 2 liter beaker by stirring. 120 g of ~fc~~a and 20 g of D (from Rerr-McGee) are then added to the slightly pi n~ clear solution.
The heating regimen is as follows. The whole mixture is first heated from room temperature t20~ C) to 55~ C in about 2 hours and held at this temperature for 18 hours while con~in~ ly mixing. The whole mixture is then heated ~lowly at constant rate for about 5.S hourc to a temperature of 75~ C. Then the whole mixture is heated for another hour at constant rate to a temperature of 100~ C. The.eu~.. the mixture is cooled to room temperature (20~ C) in about 1 hour.
The hybrid MnO2 product is rinsed with distilled water until neutral. Then it is filtered and dried at 100~ C to remove surface water. The total yield of hybrid MnO2 prqduct is 60 g and its bulk density is 24 g/in3 (1.5 g/cm3) (Scott Appare,nt Density). The hybrid MnO2 product contains about 67 wt% of the deposited gamma MnO2 and 33 wt~ D .
The MnO2 product consists of gamma MnO2 deposited uniformly over the surface of the individual EMD particles to form a hybrid MnO2 product. Each particle of the hybrid MnO2 product retain6 the overall irregular shape of the EMD particle, but exhibits a surface for,med of uniformly distributed filament-like protrusions characteristic of the gamma MnO2 made in accordance with the proces6 of the invention. Representative electron photomi~u~phs of the hybrid MnO2 particles are shown in Figs.
4A and 4B. By way of comparison Figs. 3A and 3B are electron photomic~oy~aphs of the EMD particles. These figures clearly ., show the irregular shape and smooth surface of each EMD
particle. Fig. 4A shows the overAll shape of each hybrid particle, e.g., particle 60 (at a magnification of 450 times actual), as resembling the shape of the EMD particles, e.g.
particle 50 (Fig 3A). u~w-~er~ a~ may be seen from Fig. 4B, the surface features of each hybrid MnO2 particle exhibit filament-like protrusions, e.g. ~oL~sions 65, emanating from and uniformly covering the surface of each hybrid particle. This is the result of the deposition of the gamma MnO2 of the present ~ oce6s onto the EMD particles. An advantage of this hybrid is that it has higher surface area than EMD, but yet also has high bulk density. It is also cheaper to manufacture than an equivalent weight of gamma MnO2 pr~ A by the ~l~ess of the invention. The hybrid MnO2 o pro~ce~ can be used as cathode active material in electrochemical cells. If heat treated before application, e.g. as in Ex~mple 1, it can be employed as cathode active material in lithium cells.

amDle 5:
This example demonstrates the preparation of high density P-CMD specifically for use as cathode active material in alkaline cell~.
583 g of MnSO~ H2O are dissolved in 8000 ml of distilled water contained in a 12 liter round bottom flask. Then, stoichiometric amount of Na2S20~ (822 g) is added to the clear pinkish solution. The solution is heated from room temperature (20~ C) to 50~ C in about 2 hours. The solution is then slowly heated from 50~ C to 65~ C over a period of eight hours and maintained at a temperature of 65~ C for 18 hours while continually stirring. The reaction p~o.~ according to reaction (I) above referenced. The clear pinkich solution slowly turn~ to a brown and then eve..Lually black color as more MnO2 is deposited. Following the 18 hour period the solution is then finally heated slowly at about a constant rate from 65~ C

W094/08056 21 ~ 376 7 PCT/US93/07333 to 80~ C over a period of eight hours. The solution is cooled to roo~ temperature (20~ C) in about 1 hour. The gamma MnO2 product is ,~ red by filtering the final solution and contin~ ly rinsing with di~tilled water until the filtra_e has a neutral pH of ahout 7. The resulting black powder is dried a~
in the ~ ~r~ling ex~ples to drive off surface water. The resulting product is battery grade MnO2 which ic verified by x-ray diffraction to be of the gamma crystalline structure. The MnO2 ~Lu~-L has a high bulk density of about 28 g/in3 (1.7 g/cm3) SAD (Scott Apparent Den~ity). (For us~ge in an alkaline cell, the MnO2 product of the invention preferably should ex~ibit a high SAD, preferably of at least 25 g/in3 (1.5 g/cm3) which in turn has been found to result in ~ high load voltage and capacity.) Perfor~Ance Tests:
FyAmDle 6:
The P-CMD product of the invention ( P-CMD) is evaluated for its electrochemical performance in an AA cell. The performace of the P-CMD a~ cathode active material in an alkaline AA cell is shown in figure 6A and compared to ~ e"~ional EMD cathode active material (from Kerr-McGee Corp.) for the same type cell.
It is clear that P-CMD exhibits a slightly higher r~n~jng voltage and a greater capacity (amp-hrc) than obt~ hle for the same cell using EHD as cathode material. The P-CMD product is believed to be the first ~MD that exhibits better performance in alkaline cells than EMD.
Example 7:
P-CMD is evaluated for its performance in a flooded alkaline cell. Thi~ cell utilizes cG-Iv~,,Lional zinc anode and XOH
electrolyte and paper separator as employed in commercial alkaline cells. The flooded cell is in the shape of a disk of same diameter as that of a Duracell AA cell. The flooded cell is cathode limited, thus ~rYces~ electrolyte (1.5 g) and eY~e~r zinc (5.6 g) are used in order to evaluate the intrinsic W O 94/08056 2 1 4 3 7 6 7 PC~r/US93/07333 ._ performance of the ~nO2 product (0.17 g) ag cathode active m_terial. The flooded cell is f_bric_ted by fir6t pouring a mixture of MnO2 powder, graphite _nd ROH (60 wt% MnO2, 34.5 wt%
graphite and 5.5 wt% ROH) into the bottom of an empty AA gize nickel coated ctainle~s steel can which i~ open at one end and closed at the other. The HnO2 powder i~ then compacted into a disk-like shape. A paper separator i~ then pl~ce~ on top of the MnO2 disk. The separator ic then filled with the KOH
electrolyte and the rem~in;ng volume of the can then filled with a zinc slurry. The open end of the can is covered with a stainles6 steel cap. The cap is in electrical contact with the zinc slurry through a nail penetrating from the cap into the slurry.
Two flooded cells are made as above described, but with one containing P-CMD a~ cathode material and the other cont~ini ng convention_l battery gr_de D (from Xe~L Il_Gee Co.) _s cathode material. The performance of the two cell~ are compared at a current drain rate of 0.3 milli-amp/cm2 _nd the results shown in Figure 6B. It may be ~een from the voltage profiles ~_~olLed in Figure 6B that the perform_nce of the flooded al~Aline cell utilizing P-CMD is 6uperior to that employing the D .
F~YAmDle 8:
The P-CMD product obtained by the ylOCe~ described in example 3 is heated at about 350~ C for _bout six hours to convert the gamma MnO2 to a-gamma-beta phase.
A coin sh_ped cell is fabric_ted utilizing a cathode active materi_l prepared by mixing MnO2 ~ gr_phite _nd polytetrafluoroethylene binder in a weight r_tio of 6:3:1. The cathode mixture is compacted by press molding it onto a stainless steel mesh and spot welding it onto a steel case which forms the positive electrode. The positive ele_L.o~e contAining the cathode material is immersed in a conventionAl lithium salt electrolyte composed of lithium hexafluo~ol~hG-l~h~te (~iPF6) dissolved in propylene carbonate and dimethoxyethane organic WO 94/080~6 2 1 4 3 7 6 7 PCT/US93/07333 solvents. Other ~G..~el,Lional lithium salt electrolytes cuch as lithium perchlorate and organic solvents such as propylene carbonate, ethylene cArhon~te~ dinetho~Lhane and mixLu~
thereof can al o be used. F~ce-F ~oount of lithium i~ employed for the negative el~_L.c'e. The negative el~_L,6de i~ formed by pres~ molding a lithium foil onto a stainless steel mesh which in turn is cpot welded to a steel case. A ~eparator composed of a n~.. ww en cloth is applied over the lithium foil. The positive ele_L~ode is assembled over the negative ele~L~e with the ~eparator therebetween. The as~embly i8 performed in an argon filled dry chamber. The entire a~embly i~ filled with the liguid electrolyte and then ~ealed by crimping the edge of the cell.
Two lithium coin-~ap~A cell~ made in the above manner are ~ h~ged down to 1.2 volts with current drain rates of 0.17 and 1 milliamp/cm2, respectively. The resulting voltage profiles for these cell~ using P-CHD are shown in figures 7A and 7B for drain rates at 0.17 and 1 milliamp/cm2, respectively. Each figure ~lso show~ comparative voltage profiles obtained for a like cell at same c~,,c~,L drain rate~, but instead using conventional D cathode active material (from Xerr-McGee Corp.) which is heat treated and pres~ molded for u e in the lithium cell. As may be seen from the figures, P-CM~
exhibits a greater capacity (milliamp-hr/g) than the D . The capacity improvement of the P-CMD over that of EMD at ~ e,-~drain rates of 0.17 and 1 milliamp/cm2 are about 20% and 28%, respectively. The P-CMD, thus, shows performance improvement over D in lithium cells, particularly at the higher ~,el-~rates.

A~nl e 9:
The ~ame tests are performed as in example 8 using the coin-shaped lithium cells assembled, as above described, exc~ that the performance of the P-CHD is compared to that of CMD. The CMD chosen is a commercially available CMD from Sedema int~n~
for specific use in lithium cells.

Two coi~. -h~r~ lithium cell~ are prepared ag in example 8 but with one cell containing Sedema CND and the other containing the P-CMD a~ cathode active material. The voltage profiles for these two cell~ are given at ~LLe~L drain rates of 0.17 and 1.0 milliamp/cm2 as illustrated in figure~ 8A and 8B, l~--rertively.
As can be ~een from these figures, the P-CMD has significantly greater rapa~ity (milliamp-hr/g) than the Sedema CMD at the ~LLe~ drain rates tested.

Al~ho~lgh the ~L ~ ~nt invention has been described with reference to specific embodiments, it should be ~o-~l.i7ed that variations are posgible within the scope of the invention.
Therefore, the invention is not inten~~~ to be limited to specific embodiments, but rather is defined by the claims and eguivalents thereof.

Claims

WHAT IS CLAIMED IS:

1. A process for manufacture of gamma manganese dioxide comprising the steps of:
a) reacting MnSO4 and Na2S2O8 in a solution to produce a reaction product mixture comprising a precipitate of gamma MnO2;
b) removing the gamma MnO2 precipitate from said reaction product mixture; and c) drying said precipitate.

2. The process of claim 1 wherein the gamma MnO2 precipitate is comprised of particles characterized by filament-like protrusions radiating outwardly from the surface of said particles.

3. The process of claim 2 wherein the filament-like protrusions are visible at a magnification between about 200 and 2,000 times actual size.

4. The process of claim 1 wherein the solution is characterized by an aqueous solution and said aqueous solution during step a) is heated to a temperature between about 30° C and 100° C.

5. The process of claim 4 and further characterized by bringing the temperature of said aqueous solution to between 40 and 70° C and then raising the temperature thereof during a period of at least 5 hours at an average rate of less than 7° C
per hour, whereby the bulk density of the resulting gamma MnO2 will be between 15 and 32 g/in3 (0.9 and 2 g/cm3).

6. The process of claim 4 and further characterized by bringing the temperature of said aqueous solution to between 30 and 100° C and then raising the temperature thereof during a period of less than 5 hours at an average rate of greater than 7° C per hour, whereby the bulk density of the resulting gamma MnO2 will be between 8 and 15 g/in3 (0.5 and 0.9 g/cm3).

7. The process of claim 1 further comprising adding a compound reactive with H2SO4 to said solution during or prior to step a) to increase the yield of MnO2, wherein said compound is selected from the group consisting of Li2CO3, Na2CO3, LiOH, NaOH
and MgO.

8. The process of claim 1 further comprising adding carbon particles to said solution during or prior to step a), wherein the carbon particles are selected from the group consisting of graphite and carbon black and wherein the MnO2 precipitate deposits on the surface of the carbon particles.

9. The process of claim 1 further comprising adding electrolytic MnO2 (EMD) particles to said solution during or prior to step a), wherein the MnO2 precipitate deposits on the surface of the EMD particles.

10. A hybrid MnO2 material comprising gamma MnO2 deposited on the surface of electrolytic manganese dioxide (EMD), wherein said gamma MnO2 material has filament-like protrusions radiating outwardly from its surface, said protrusions being visible at a magnification between about 200 and 2,000 times actual size.

11. A hybrid MnO2 material comprising gamma MnO2 deposited on the surface of carbon material selected from the group consisting of graphite and carbon black, wherein said gamma HnO2 material has filament-like protrusions radiating outwardly from its surface, said protrusions being visible at a magnification between about 200 and 2,000 times actual size.

12. A hybrid MnO2 material comprising gamma MnO2 deposited on the surface of another material wherein said gamma MnO2 material has filament-like protrusions radiating outwardly from its surface, said protrusions being visible at a magnification between about 200 and 2,000 times actual size.

13. An electrochemical cell having MnO2 cathode active material in said cell, characterized in that at least 95% of said MnO2 material comprises gamma MnO2 particles wherein said gamma MnO2 particles have filament-like protrusions radiating outwardly from the surface of said particles, said filament-like protrusions being visible at a magnification of between 200 and 2,000 times actual size.

14. An electrochemical cell having MnO2 cathode active material in said cell, characterized by said electrochemical cell having an anode comprising lithium, and further characterized by said HnO2 material comprising gamma MnO2 particles having filament-like protrusions radiating outwardly from the surface of said particles, said filament-like protrusions being visible at a magnification of between about 200 and 2,000 times actual size.

15. The electrochemical cell of claim 14 wherein the gamma MnO2 particles comprise at least 95% of said MnO2 material.

16. The electrochemical cell of claim 14 characterized by the filament-like protrusions being substantially uniformly distributed over the surface of said particles and the filament-like protrusions having a length to width ratio between about 2:1 and 20:1.