The Thomas A. Edison Papers Digital Edition

Transcription

THE NEW EDISON STORAGE BATTERY
BY ARTHUR R. KENNELLY 
I take pleasure in bringing you the notice of the Institute, this evening, a novel type of storage battery, recently invented by Mr. Edison.
It is well known that the history of the storage cell is essentially that of the lead cell discovered by Plante in 1860,in which lead peroxide is the depolarizing substance. An enormous amount of labor has, in the aggregate, been expended upon the improvement of this cell in the hands of experimentalists. As a result of that labor, the storage battery has at last become a recognized adjunct to direct-current central stations, but it has limitations that seem to withstand further attempts toward improvement. Of recent years, hardly any success has been met with the direction of reducing its weight for a given energy-storage capacity, without detriment to endurance, and this weight is the great drawback of the storage battery in electric storage traction, and has been the principal obstacle to its advance in this direction for the past twenty years.
In practice, the storage energy per unit mass of the modern lead battery, is from four to six watt-hours per pound of battery (8.8 to 13.23 watt-hours per killogramme). Expressed in another way, a battery weighs from 124.5 to 186.5 pounds per horse-power-hour at its terminals (75.5 to 113.4 kilos per kilowatt-hour); of, if its stored energy available at terminals were all expended in gravitational work, a battery could raise its own weight through a vertical distance of from two to three miles (3.2 to 4.8 kilometres.) While it is possible to increase the energy per unit mass by making the electrodes very light, yet this is always found to be followed by a very heavy deterioration.
Many attempts have also been made to perfect storage cells of the alkaline-zincate type, but the great difficult of depositing zine in coherent form from the solution, as well as the lack of a depolarizer that shall be insoluble in the electrolyte, has stood in the way of this cell's success. 
Mr. Edison set himself the task of finding a cell which would possess the following advantages:
1. Absence of deterioration by work.
2. Large storage capacity per unit of mass.
3. Capability of being rapidly charged and discharged.
4. Capability of withstanding careless treatment.
5. Inexpensiveness. 
He believes that the cell here shown may claim these advantages
The negative pole, or positive element, corresponding to the zine of a primary cell, or the spongy lead of a secondary cell, is iron. The positive pole or negative element, corresponding to the carbon of a primary cell, or lead peroxide of a secondary cell, is a superoxide of nickel-iron cell, a name which suggests the structural material-nickel steel. The electrolyte is potash; viz., an aqueous solution containing from 10 to 40% by weight, but preferably 20% of potassium hydroxide, the freezing temperature of which is 20° below zero F or -30° C.
The initial discharge after recent charge is.....1.5 volts
The moon voltage of full discharge is approximately....1.1 volts
The normal discharging current rate per unit area of active element (positive or negative)......60 millamperes sq. inch
or...............8.64 amperes sq. foot
or..............0.93 amperes sq. decimetre
The storage capacity of the cell per unit of total mass of the cell
is.........14 watt-hours per pound
or..........30.84 watt-hours per kilo
Expressing the same statement in another way, the weight of battery per unit of electric energy at terminals is.
53.3 lbs. per E.H.P. hour
or.........39.4 kilos per kilowatt-hour
Or the battery gives energy at its terminal sufficient to lift its own weight through a vertical distance of approximately...........7 miles or 11.26 approximately
The mean normal discharging power-rate per unit mass of total cell is......4 watts per lb. or 8.82 watts per kilo:
Corresponding to a normal discharge period....3 1/2 hours The cell may, however, be discharged a relatively high rate, in approximately......... 1 hour: Corresponding to a discharging power rate per unit of total cell mass of.......12 watts per lb. or 26.46 watts per kilo. Charging and discharging rates are alike. That is to say, the cell may be charged at the normal rate in 3 1/2 hours; or it may be charged at a relatively high rate in one hour, with no apparent detriment beyond a somewhat lowered electrical charge efficiency. In other words, the cell does not appear to be injured by over-charging or discharging, and only suffers in electrical efficiency under such treatment. 
The positive and negative plates are mechanically alike, and can scarcely be distinguished by the eye. They differ only in the chemical contents of their pockets. The samples here exhibited, which are intended for automobile batteries, illustrate the construction. Each plate is formed of a comparatively thin sheet of steel, 0.024" (0.61 mm) in thickness, out of which rectangular holes are stamped, so as to leave a grid or frame somewhat resembling a window-frame. In the plate here shown, there are three tows of eight such rectangular holes or recesses, or 24 recesses in all.
Each opening or recess is filled with a pocket or shallow box containing the active material. These boxes correspond to the panes of glass in the window-frame analogy. The panes instead of being thinner than the frame as in an actual window, are thicker than the frame, or project slightly beyond the surface of the steel grid. They are perforated with numerous small holes to admit the electrolyte, by entirely conceal the contained active material from view. All that meets the eye, therefore, in any of the plates, is the steel frame, and its embedded "windows" of perforated steel.
The active material is made in the form of rectangular cakes or briquettes, and one such briquette is lodged in each pocket or "window pane" of the plate. Each ot the plate shows, therefore, supports, or contains 24 briquettes of active material, all in rigid contact with its own substance. Each briquette is placed in a shallow, closely fitting nickel-plated box of thin perforated crucible steel, cut from a long strip of that material 0.003" (0.075 mm.) thick. A cover or lid of the same material is then laid over it, so that the briquette is closely enveloped by the sides and walls of its perforated steel box. The boxes a are then placed in the openings or holes in the nickel-plated steel grid, and closely fit the same. The assembled plate is then placed in a hydraulic press, and subjected to a total pressure of about 100 tons. this pressure not only tightly closes the boxes, but it also forces their metal sides over adjacent sides of the recesses in the steel grid, thus clamping the whole mass into a single solid and rigid steel plate with the hollow "window panes" full of active material. The nickel-plating of both grids and boxes aids in securing good permanent electric connections between them. The finished plate has a grid thickness of 0.024" (0..56 mm.) and a "window" or pocket thickness of the plate at any point, but being of steel, the plate has ample rigidity.
The positive briquettes (zincs of a primary cell) are made by mixing a finely divided compound of iron obtained by a special chemical process with a nearly equal volume of thin flakes of graphite. The graphite does not enter into any of the chemical actions, but assists the conductivity of the briquettes. The graphite, is divided into very thin lamine by a chemical process, and these are passed through sieves or screens so as to leave a size or area of flake that is much larger than the area of the perforation in the steel windows. The mixture is then pressed into briquettes in a mould, under a hydraulic pressure of about two tons per square inch. The briquettes have a surface area of nearly 3"x1" on each face.
The negative briquette (carbon of a primary cell) are made by similarly mixing a finely divided compound of nickel, obtained by special chemical means, with a nearly equal bulk of fine flakes of graphite, and solidifying the mixture in a mould into briquettes of the same size as above. 
A suitable number of positive and negative plates are assembled together, being separated from one another only by a thin sheet of a perforated hard rubber.
The assembled plates are placed in a vessel or external containing cell of sheet steel containing the potash solution, which, of course, does not attack steel. There was however, much difficulty from the action of the potash on the soldered scams of the steel containing vessel. After many trials, however, Mr. Edison found a solder which seems to be entirely unaffected by alkali.
In charging, the current is, of course, sent into the positive pole and its attached negative nickel-plate, through the electrolyte, and into the positive plate of the iron compound which carries the negative pole. The current deoxidizes or reduces the compound to spongy metallic iron and carries the oxygen in the opposite direction against the forces of chemical affinity, from the iron to the nickel, and stores the energy in the reduced iron, which is, of course, unaffected and passive in the presence of the potash solution. On discharge, the current passes from the positive pole through the external circuit to the negative pole, and its attached iron or positive plate, and then through the solution to the negative or superoxide plate. In so doing the oxygen moves back against the current and partially reduces the nickel superoxide Ni O2 while oxidizing the spongy iron. The energy or burning the iron and oxygen would be developed as heat in the ordinary chemical process is now liberated in the circuit as electrical energy. 
The cell is an oxygen-lift. Charging pulls the oxygen away from the iron and delivers it temporarily to the nickel. The condition is then stable, until the circuit of the cell is completed. Discharge then allows the oxygen to fall back from the nickel to the iron with the natural affinity of iron and oxygen.
This action is very different from that which takes place in the lead storage cell. Here, neglecting complication, the action is usually regarded for practical purposes as being represented by the equation PhO2+2H2So4+Pb=PbSO4+2H2O+PbSO4+100 watt-hours, where the left hand side represents the condition of charge and the right hand side the condition of discharge. Here oxygen is not simply transferred in discharge from the peroxide to the spongy lead, but the solution is changed (theoretically) from an aqueous solution of sulphuric acid to plain water. Of course the discharge could not practically be carried to the point of denuding the solution of all sulphuric acid, and a surplusage of acid must be used. The equation gives a more theoretical outline of admittedly very complex reactions. In other words, the specific gravity of the sulphuric acid solution falls during the discharge, and the solution enters into the chemical combination. Theoretically, for every 445 grammes of active material on both plates, 196 grammes of sulphuric acid are required to effect the combination, or 44% by weight of the active elements, and in practice it is usual to allow a weight of sulphuric acid nearly equal nearly equal to half the weight of the elements, or about one-quarter of the total weight of the cell. 
In the new Edison cell, on the other hand, the theoretical action of the potash solution is merely to provide the proper channel through which the oxygen ions may travel in one direction or the other-positive plate to negative plate in charge, and negative plate to positive plate in discharge. Consequently, the amount of solution needs only to be sufficient to fulfill mechanical requirements. It is believe that the weight of solution will in practice be only about 20% of the plate weight or about 14% of the cell weight. In fact the cell may be worked in the same manner as the so-called primary "dry-cells." Moreover, if the solution should escape, or be carried away, by gasing in charging, the only detriment seems to be the loss of active surface thereby occasioned, and it will only be necessry to fill up the cells to the proper level with water from time to time, as evaporation or gasing may lower the level. For the same reasons the specific gravity of the electrolyte does not appreciably vary during charge and discharge. 
The briquettes of active material slightly expand on receiving oxygen, and slightly contract on delivering it, that is to say, the iron briquettes contract and the nickel briquettes expand during charge, while on discharge the iron briquettes expand and the nickel briquettes contract. The level of the solution is in this way scarcely affected. The expansions and contractions of the briquettes appear to be well within the elastic limits of the spring-steel containing boxes, and consequently the electric contact is always secure. The covers or sides of the window pockets merely approach to or recede form each other slightly during charge and discharge. Fortunately, steel is the metal which possesses the mechanical elasticity in a marked degree.
The action of the charging and discharging current upon the briquettes seems to be transferred from their external surface inwards in a manner similar to the transfer of carbon and oxygen in the process of making malleable cast-iron in the furnace on the principle of cementation. No active material has been found to be ejected from the briquettes through the window deforations, even under the deliberate overcharging and discharging. Such gas as is thereby produced makes its appearance on the external surface of the windows. 
If the nickel compound had no affinity for oxygen, so that energy was neither developed nor absorbed in the deoxidation of further oxidation of that substance, then the energy would be entirely that due to the energy of combination of oxygen and iron, stated to be 79.7 watt-hours and representing an E.M.F., theoretically obtainable, of 1.47 volts. If the combination of oxygen with the nickel compound be exothermic or energy releasing, then the watt-hours delivered (and the E.M.F.) will be loosened by the energy necessarily paid back to break up the combination.
If on the other hand the combination is endothermic or energy absorbing, then the watt-hours delivered (and the E.M.F.) will be increased by the energy restored on breaking up the combination. Since the superoxide seems not to have been known hitherto, no information concerning its energy of combination is obtainable. The electromotive force of the cell seems to be so near to that of the union of iron and oxygen as to suggest that the nickel superoxide is not far from being neutral, or that the nickel compound has but little affinity for oxygen, although the superoxide appears to be quite stable in the cell.
The new cell does not seem to be appreciably influenced by changes of temperature, and should stand a very low temperature without detriment. The electrolyte-potash-does not attack any of the ingredients of the cell, nor are any of the ingredients soluble therein. No local action occurs in the cell so far as yet been observed since the E.M.F. is below that necessary to decompose water. The cell may be fully discharged to the practical zero point of E.M.F. without detriment. In fact, a bell has not only been completely discharged, but recharged in the reverse or wrong direction, and after bringing it back to its originally charged state by proper restoration of the direction of charging current, the storage capacity remained unaffected. It would seem, therefore, that the cell should be capable of withstanding much abuse. Diagrams are appended giving the curves of discharge for experimental cells. 
Mr. Edison states that "the negative plate (nickel) either charged or discharged, can be removed from a working cell, and dried in the air for a week, without appreciably injuring it, and when the plate is finally replaced in the cell its charge is practically undiminished."
The positive (iron) plate, if similarly removed from the cell will be likewise uninjured, but it soon loses its charge by the oxidation of the spongy iron with accompanying liberation of heat and appreciable rise of temperature extending over a period of several hours. On replacing the electrode, however, in the cell the storage capacity is unaffected on recharge. 
As regards cost, Mr. Edison believes that after factory facilities now in course of preparation have been completed, he will be able to furnish the cells at a price per kilowatt hour not greater than the prevailing price of lead cells. 
Having now considered the action and properties of the cell, a brief description may be given of the difficulties encountered in developing it.
The phenomenon of passivity has probably kept inventors from finding this cell in the past. Mr. Edison believes that of all the very numerous compounds of iron, and of which he has tried many hundreds, the particular compound which he prepares, is perhaps the only one capable of being used.
If the dried hydrates, or oxides of iron native or artificial, are subjected to electrolyte reducing action in any alkaline solution, they remain inert and unaffected. 
On the other hand, if finely divided iron obtained by reducing a compound of iron under the action of a reducing agent, such as hydrogen, or carbonic monoxide is subjected to electrolytic oxidation in an alkaline solution it is inert and cannot be oxidized. It assumes the well known passive state. 
The same difficulty of passivity affects the use of nickel or the negative element. Finely divided nickel, reduced from a nickel compound, remains inactive when subjected to electrolytic oxygen in an alkaline solution. The monoxide and the black-oxide or peroxide are also inert. No oxide of nickel is active or can be made active by electrolytic action and the peroxide does not act as a depolarizer.