Practical energy densities

System Specific energy (Wh kg 1) Theoretical Practical Energy density (practical) (Wh L 1)... 

Figure 19. Depiction of the components of a lead acid battery showing the differences between theoretical and practical energy density of a lead acid battery and source of the differences.

Table 3.2 Practical energy density of miniature alkali button cells (diameter, 11.6 mm height, 4.2 mm) with zinc anodes at a discharge rate of 5-10 pA...

Cells are also formed into batteries with nominal OCV of 7.2 V for electric fence activators. Alternatively, banks of 11 individual cells are used to operate 10 V railway signal motors such applications require a current drain of 3 A for 5-10 s, perhaps 100 times per day. Multicell batteries are used in the US Space Shuttle programme for crew communication. Practical energy densities of up to 310 Wh/kg may be obtained. 

At room temperature, discharge curves are exceptionally flat (Fig. 4.35), even at rates of up to 3 A for D-sized cells. As noted above, SOCl2 utilization is much higher at low current drains. Low rate cells manufactured by Mallory with a spiral configuration (4.45 cm X 25-38 cm electrodes) produced practical energy densities of 661 Wh/kg (1240 Wh/dm3)... 

It is probable that a range of soluble species such as Fe(OH)2 and Fe02 are involved, and it is known that Fe(OH)3 or Fe304 may be formed on deep discharge. The practical energy density of conventional tubular plate cells is 20-30 Wh/kg with the more recent cells which use press-sintered iron electrodes, values of 40-60 Wh/kg have been reported. 

As discussed below, there are problems with morphological changes and passivation reactions at lithium metal negative electrodes in secondary cells, which reduce cycle life and the practical energy density of the system, and may in some circumstances introduce safety hazards. A more recent development involves the replacement of the lithium metal anode by another insertion compound, say C Dm. In this cell, the electrochemical process at the negative side, rather than lithium plating and... 

Over 300 cycles at current densities of 5 mA/cm2 were obtained, with efficiencies of 80-90% for a 400 mAh rated cell. The practical energy density was 280 Wh/kg. 

The lithium/carbon monofluoride cell possesses very high theoretical (2260 Wh/ kg) and practical energy densities [26], The cathode material is made by the reaction of carbon with fluorine. The overall reaction is... 

The theoretical energy density of lead-acid batteries is only 171 W h/kg, as a result of the high atomic weight of lead. The practical energy density depends on the rate of discharge, as seen in Fig. IM, but even at low rates it does not exceed about 40 W h/kg. This... 

Figure 4 shows a comparison of practical energy density and specific energy of primary batteries. Two groups can be distinguished the zinc-based and lithium-based systems. 

Figure 4. Practical energy density and specific energy of primary systems.

Switching to lithium-alloy negative electrodes, some voltage loss must be noted. LiAl has Uu = -1-385 mV, Li4.5Pb has Uu = 388 mV. Entries 18-20 in Table 10(b) represent three examples of rechargeable cells, which have been, at least temporarily, commercialized. The first (No. 18) is due to a lithium alloy/carbon black battery conunercialized by the Matsushita Co. [248]. The lithium alloy components are Pb -I- Cd -I- Bi -h Sn (Wood s alloy). Button cells in the range 0.3 to 2.5 mAh were offered. The electrolyte was LiC104 in an unknown solvent. The practical energy densities, 2Wh/kg, were rather low. The c.b. positive electrode acts as a double... 

It should be remembered that the practical energy density Eg in metal-free batteries is often closer to the theoretical value than in the case of conventional systems. This can be rationalized in terms of higher active mass utilizations through thin-layer technology (via thermoplastic binders, for instance), lighter current collectors (at least in bipolar systems), and so on. The lead-acid accumulator has a ratio a = Eg sh)/Es,th 15% thus Eg — 25 Wh/kg. But a metal-free system with s,th = 80 Wh/kg may allow Eg = 40 Wh/kg, if a is 50% in this way. 

Table 3.1 shows the thermodynamic energy densities and practical energy densities for several battery systems and fuel cells. Fuel cells potentially offer 5-10 times greater energy densities than rechargeable batteries [4]. 

Rechargeable batteries Electrochemical reaction Energy density (Wh/L) Practical energy density (W h/L)... 

The lead-acid battery Pb/H2S04/Pb02 [1] provides a fast cell discharge at 2.0 V, it is relatively low-cost, and 98% of the batteries used in the USA are recycled. This battery has dominated the market for rechargeable batteries, but Pb is heavy and the practical energy density of the battery is only about 25% of its limited theoretical energy density (Wh/kg). Therefore, other secondary batteries are used for handheld devices and contend for the electric-vehicle market. 

This type of calculation can be useful as a means of comparing the theoretical specific energies of different batteries. Practical energy densities are design dependent and typically deliver no more than 20-40% of the theoretical specific energy. 

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