House is not built from Roof.
If you make Battery, then you need to start from current supply Electrodes, that is, from Current Collector, which should provide:
• Low contact resistance of the metal-active electrode;
• High adhesion of active electrode;
• High chemical resistance;
• Flexural and tear strength;
• Low weight.
Current-carrying Electrodes of Battery – aluminum and copper foil. Thickness of foil is determined by its mechanical strength and amount of current that it must pass. Thin foil reduces weight of Battery. The thinner the foil, the lower its mechanical strength and the magnitude of the permissible electric current. These two factors affect choice of foil thickness.
Adhesion of Active Electrode to metal foil influences the choice of metal foil and the preparation of its surface.
Different metals have different adhesion. There is this for a number of reasons:
Firstly, strength of “coating-to-metal surface” bond depends on the ratio of the atomic mass of the metal to its density: the higher it is, the worse the adhesion. This figure for aluminum is higher than for iron and copper.
Secondly, the reason for low adhesion of coatings to a number of metals (lead, aluminum) lies in weak bond strength with oxides of these metals, which are always contained on their surface. Therefore, peeling off of active material takes place mainly along oxide layer.
To increase adhesion, the processes of “flowing” of suspension of active material into micropores and cracks on the foil surface are important. For this reason, etched foil can be used to increase the adhesion of Battery or Supercapacitor. But this method of increasing adhesion has already approached the physical limit of further improvement, since deeper etching leads to a decrease in mechanical characteristics of etched foil, such as bending strength and tensile strength.
In chemical sources of current: Batteries and Supercapacitors, a very important parameter is magnitude of transient contact resistance between metal current tab (foil) and material of Active Electrode. Magnitude of contact resistance depends on area of ohmic contact, is determined by microrelief of foil surface, and also by total area and thickness of insulating oxide film on foil surface. When Active Electrode is applied, oxide film on foil is not destroyed, and very quickly forms on surface of purified metal. Oxide film thickness of 7 nm is formed on surface of pure aluminum in 20 seconds. At the same time, on copper surface, thickness of oxide film reaches 2 nm.
An important factor in selection of Current Collector foil is chemical resistance. Usually, chemical resistance of metal in working environment is determined by rate of corrosion. For pure aluminum in acidic solutions, it is 250 μm / year, copper – more than 500 μm / year, and for silver, gold and graphite – less than 15 μm per year.
From numerous attempts to create Current Collector that corresponds to its ideal characteristics, it is possible to isolate Current Collector with applying fine-grained graphite-carbon black on surface of aluminum or copper foil. Soot, as an intermediate layer, is applied between metal of foil and Active Electrode. Electrical conductivity of acetylene black is an order of magnitude higher than the conductivity of graphite and 1000 times worse than the conductivity of aluminum. But this is already enough to reduce contact resistance and improve the adhesion of Active Electrode.
Adhesion is higher if the roughness created on the foil surface is commensurate with the particle size of the acetylene black. Size of soot particles does not exceed a few hundredths of a micron. Due to small size of particles, soot has greatest hiding power.
Magnitude of contact resistance of Сurrent Сollector with soot decreases, transient processes associated with degradation of Battery are slowed down. But the presence of oxide film on metal surface does not allow contact across entire surface between foil and Active Electrode. High porosity of soot does not protect Current Collector metal from contact with electrolyte and subsequent chemical corrosion.
Current Collector made using vacuum magnetron sputtering (or deposition) technology is available from listed disadvantages – Vacuum Carbon Technologies owns and promotes High-Density Current Collector (HD4CT) to overcome them. Contact resistance of such Current Collector is much lower than the best analogs. High density of carbon coating provides protection against electrochemical corrosion. Modified surface of aluminum or copper foil provides high adhesion with Active Electrode of Battery or Supercapacitor. Absence of chemical etching increases the mechanical strength of foil, as a Current Collector.