Cells energy density

Irreversible Capacity. Because an SEI and surface film form on both the anode and cathode, a certain amount of electrolyte is permanently consumed. As has been shown in section 6, this irreversible process of SEI or surface layer formation is accompanied by the quantitative loss of lithium ions, which are immobilized in the form of insoluble salts such as Li20 or lithium alkyl carbonate. Since most lithium ion cells are built as cathode-limited in order to avoid the occurrence of lithium metal deposition on a carbonaceous anode at the end of charging, this consumption of the limited lithium ion source during the initial cycles results in permanent capacity loss of the cell. Eventually the cell energy density as well as the corresponding cost is compromised because of the irreversible capacities during the initial cycles. 

Since any compromise in cell energy density for the sake of safety would be undesired, most of the research efforts were concentrated on the reformulation of the electrolytes by using a flame-retarding additive or cosolvent, with the goal that its presence, kept at a minimum, could result in nonflammability or at least retarded flammability of the whole electrolyte system. 

The most obvious advantages of the oxygen cathode are that it has low weight and infinite capacity. Consequently, prototype D-size cells based on the zinc-air system have been shown to have twice the overall practical capacity of zinc-mercuric oxide cells (and 10 times that of a standard Leclanchd cell) when subjected to a continuous current drain of 250 mA. In the larger industrial cells, energy densities of up to 200 Wh/kg and specific capacities of 150 Ah/dm3 may be obtained. On the other hand, a catalytic surface must be provided for efficient charge transfer at the oxygen cathode, and by its nature the electrode is susceptible to concentration polarization. 

The theoretical energy of the 2Na + 3S —> Na S reaction is as high as 790 Wh/kg of reactants, but when allowance is made for the mass of the electrolyte, current collectors, container etc., practical cell energy densities of 250-350 Wh/kg are predicted... 

Energy Trends in Commercial Li Ion Cells Energy Density 18650 Cells... 

The major parameters used to measure battery performance are discharge voltage (or open circuit voltage e.g. 1.5 V for a common D" cell), energy density, usually expressed in mWh/g (milliWatt-hours/gram), specific capacity, usually expressed in mAli/g (A= amperes), number of deep discharge cycles obtainable (cyclability or... 

Table 1. Specific Energies and Energy Densities of Carbon-Zinc Cells ...

Performance. Alkaline manganese-dioxide batteries have relatively high energy density, as can be seen from Table 2. This results in part from the use of highly pure materials, formed into electrodes of near optimum density. Moreover, the cells are able to function well with a rather small amount of electrolyte. The result is a cell having relatively high capacity at a fairly reasonable cost. 

The energy density of the system depends on the type of cell as well as the current drain. Table 3 gives the specification for the various hthium systems. These coia cells have already been widely used ia electronic devices such as calculators and watches, whereas the cylindrical cells have found apphcations ia cameras. 

Lithium—Thionyl Chloride Cells. Lidiium—thionyi chloride cells have very high energy density. One of the main reasons is the nature of the ceU reaction. 

Table 4. Component Data and Energy Density for 300 A h Niekel Oxide—Iron Cells...

The positive plates are siatered silver on a silver grid and the negative plates are fabricated from a mixture of cadmium oxide powder, silver powder, and a binder pressed onto a silver grid. The main separator is four or five layers of cellophane with one or two layers of woven nylon on the positive plate. The electrolyte is aqeous KOH, 50 wt %. In the aerospace appHcations, the plastic cases were encapsulated in epoxy resins. Most usehil cell sizes have ranged from 3 to 15 A-h, but small (0.1 A-h) and large (300 A-h) sizes have been evaluated. Energy densities of sealed batteries are 26-31 W-h/kg. 

Tlie couple has a theoretical energy density of 172 W h/kg and complete cells are capable of deUvering 55-66 W-h/kg. Tlie cell reaction is... 

Zinc—Oxygen Cells. On the basis of reactants the zinc—oxygen or air system is the highest energy density system of all the alkaline rechargeable systems with the exception of the 2 Th reactants are cheap and abundant and therefore a number of attempts have been made to develop a practical rechargeable system. The reactions of this system are as follows ... 

Coin and Button Cell Commercial Systems. Initial commercialization of rechargeable lithium technology has been through the introduction of coin or button cells. The eadiest of these systems was the Li—C system commercialized by Matsushita Electric Industries (MEI) in 1985 (26,27). The negative electrode consists of a lithium alloy and the positive electrode consists of activated carbon [7440-44-0J, carbon black, and binder. The discharge curve is not flat, but rather slopes from about 3 V to 1.5 V in a manner similar to a capacitor. Use of lithium alloy circumvents problems with cycle life, dendrite formation, and safety. However, the system suffers from generally low energy density. 

---