Li/SO2 cells were the first lithium cells to be commercialized. Their main features are:
These cells are produced in spirally wound configurations, to exploit their power capabilities.
The anode is Li foil, while the cathode is Teflon-bonded acetylene black. The solvent is CH3CN in which LiBr and SO2 are dissolved.
Li2S2O4 (Li dithionite) precipitates into the pores of the carbon cathode. The stability of this cell (and of other cells with a soluble cathode) is connected with the formation of a passivating film on the Li surface as soon as Li is exposed to SO2. The film growth rate increases during storage of partially discharged cells.
The presence of SO2 requires a special construction: hermetic seals are used to prevent SO2 loss. The cell is pressurized (2 atm) to keep the electrolyte in the liquid state and a safety vent is incorporated in the cell to cope with pressure values exceeding certain limits (e.g. 24 atm). The acetylene black-Teflon mix, supported on Al screen, is characterized by high values of conductivity, surface area and porosity. The last feature ensures that Li2S2O4 precipitating on the cathode does not cause its clogging in early reaction stages.
The electrolyte contains 70% SO2 and has a high conductivity even at 50C. This affords the use of Li/SO2 batteries in applications prohibited to other chemistries. They can maintain a high proportion of their capacity even at the 1-h discharge rate, whereas the capacity of aqueous batteries with a Zn anode starts declining at the 20 to 50 h rates.
Early design cells had a Li/SO2 ratio as high as 1.5:1. However, it was ascertained that this ratio greatly impaired the cell safety. Indeed, in deeply discharged cells, when the SO2 concentration is below 5% and the passivating film removed, the reaction of Li with CH3CN occurs. Therefore, cells with a Li/SO2 ratio close to 1 are now preferred: in these balanced cells, Li remains passivated as there is a sufficient amount of SO2.
Li/SO2 cell are fabricated in cylindrical sizes with capacities ranging, for standard cells, from 0.86 Ah (1/3C size) to 11.0 Ah (F size). This last cell can stand continuous currents of 8.0A and pulse currents of 60 A.
A drawback, common to all soluble-cathode cells, is the so-called voltage delay. Extended storage, particularly at high temperatures, favours formation of a thick film on the anode; therefore, discharges at high rates and low temperatures start with a lower voltage. The time needed to resume the standard voltage depends on the length and temperature of storage. This effect is not appreciated in room-temperature operations and can be eliminated by a pulse discharge at high rate to depassivate the anode.
Safety concerns arise on overdischarge and thermal abuse, thus imposing the presence of controlling components in the cells/batteries. If the cell is to deliver high currents, a fuse is necessary; if there is any possibility of cell (or battery) charging, a diode is also needed. The use of a microporous polypropylene separator allows electrolyte flow in its 200-nm channels, while blocking carbon particles that might reach the anode and short the cell. For a more reliable control, an electronic circuit is needed, especially for larger cells on heavy duty.
Li/SO2 cells have a number of military and civilian applications, including radio communications, lighting/night vision, automotive electronics, professional electronics, meteorology/space, toll-gate systems.
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