Internal short circuit

Spontaneous low resistance internal short circuits can develop in silver—zinc and nickel—cadmium batteries. In high capacity cells heat generated by such short circuits can result in electrolyte boiling, cell case melting, and cell fires. Therefore cells that exhibit high resistance internal short circuits should not continue to be used. Excessive overcharge that can lead to dry out and short circuits should be avoided. 

An imperative condition for materials being used as an electrolyte is the complete (or practically complete) absence of electronic conduction. Such conduction would amount to internal short-circuiting, leading to unproductive power consumption and/ or waste of electrode materials. 

The advantage of this type of cell is its cojmpactness, due to the small distance between the electrodes, and its lightness, due to the fact that it is made throughout (with the exception of the insulating strips) of mild steel sheet However, the small distance between, tlie electrodes necessitates care being taken to prevent an internal short circuit in the individual cells. 

As for the other electrochemical storage/conversion devices, the fuel cell electrolyte must be a pure ionic conductor to prevent an internal short circuit of the cell. It may have an inert matrix that serves to physically separate the two electrodes. Fuel cells may contain all kinds of electrolytes including liquid, polymer, molten salt, or ceramic. 

Asahi Chemical Industry carried out an exploratory investigation to determine the requirements for cellulose based separators for lithium-ion batteries. In an attempt to obtain an acceptable balance of lithium-ion conductivity, mechanical strength, and resistance to pinhole formation, they fabricated a composite separator (39—85 /cellulosic fibers (diameter 0.5—5.0 /pore diameter 10—200 nm) film. The fibers can reduce the possibility of separator meltdown under exposure to heat generated by overcharging or internal short-circuiting. The resistance of these films was equal to or lower than the conventional polyolefin-based microporous separators. The long-term cycling performance was also very comparable. 

Figure 12. Typical nail penetration behavior of a 18650 lithium-ion cell with shutdown separator. This test simulates internal short circuit of a cell. Key (a) cell passed nail penetration test (b) cell failed nail penetration test.

Such displacements result in the formation of local couples where hydrogen evolution and zinc dissolution can occur at a greatly accelerated rate (Fig. 3.8). (Where the concentration of such impurities is very high, displacement reactions have been known to produce such extensive dendritic growths that the cells have become internally short-circuited.)... 

A two- or multilayer separator is generally used in which a strong fibrous element is incorporated to prevent internal short circuits by zinc dendrites... 

Alkaline batteries are capable of high current discharges and accidental short circuits should be avoided. Spontaneous low resistance internal short circuits can develop in silver-zinc and nickel-cadmium batteries. 

As described in Section II.2 variants of the permeation techniques use systems in which electronic and ionic pathways are locally separated, e.g., an ion conductor which is internally short-circuited by percolating metallic inclusions (or materials in which grain boundary and bulk possess different conductivity types (e.g., nano- Ce02)). In all these cases the permeation flux is determined by the lowest partial conductivity. 

Apart from this important feature of a corroding system, there is another characteristic that arises from the short-circuit condition of the corrosion cell and of the equivalent cell. (It will be recalled that the electron sources and sinks in the corroding metal are internally short-circuited the two electrodes in the equivalent cell are externally short-circuited.) The total potential difference V across the equivalent cell is zero. But this cell potential is composed of the absolute potential differences across the interfaces at the two electrodes and the potential drop IR in the electrolyte ... 

At the same time, it was found that Li metal is an unsafe negative electrode material [11, 13]. Upon charging a battery, lithium is electrodeposited on the Li metal electrode. During this process, formation of dendrites was observed, and after multiple charge/discharge cycles, those dendrites penetrated the separator and led to an internal short circuit of the cell with intense heat formation and sometimes even combustion of the cell. 

The equilibrium situation in an electrochemical cell is obtained, if the electrical current is interrupted, if all local actions (e.g. transport in the electrode) have come to an end and no internal short circuits occur. Then, as mentioned (Figure 3.5.10), the cell voltage is determined by the difference in the lithium potential (chemical potential of lithium) between the left-hand side (Ihs) and right-hand side (rhs) of the electrochemical cell (E - open cell voltage, F - Faraday constant) ... 

The electrolyte in an SOFC must consist of a good ion conductor, which has essentially no electronic conductivity. Otherwise the cell will be internally short-circuited. An often-used electrolyte material is yttria-stabilised zirconia (YSZ). The electrodes must pos.scss good electron conductivity in order to facilitate the electrochemical reaction and to collect the current from the cell. The fuel electrode usually contains metallic nickel for this purpose. The anodic oxidation of the fuel (H or CO) can only take place in the vicinity of the so-called three-phase boundary (TPB), where all reactants (oxide ions, gas molecules and electrons) are present. Thus, it is advantageous to extend the length and width of the TPB zone as much as possible. One way to do this is by making a composite of Ni and YSZ called a Ni-YSZ-cermet. Another way is to use a mixed ionic and electronic conductor, which in principle can support the electrochemical reaction all over the surface as illustrated in Fig. 15.1. Partially reduced ceria is a mixed ionic and electronic... 

Figure 36 Self-driven electrochemical promotion of CH3OH oxidation on Pt using conductor (YSZ) with external catalyst-counterelectrode short-circuiting (a) and a mixed ionic (0 )-electronic conductor (Ti02) with internal short-circuiting.

Isothermal emf of cell (VI) in a sodium borate melt as a function of temperature (dynamic measurements) showing validity of Equation (26-33) and thermodynamically correct functioning of the cell with respect to the inherent oxygen/oxide system of the melt, (b) Effect of internal short circuit of platinum measuring electrode during a 10 h halt of the dynamic measurement after (a). 

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