Gardner

CHAPTER 20

AMBIENT-TEMPERATURE LITHIUM ANODE RESERVE BATTERIES
David L. Chua, William J. Eppley, and Robert J. Horning

20.1

GENERAL CHARACTERISTICS
The use of lithium metal as an anode in reserve batteries provides a significant energy advantage over the traditional reserve batteries because of the high potential and low equivalent weight (3.86 Ah / g) of lithium. A lithium reservebattery can operate at a voltage close to twice that of the conventional aqueous types. Due to the reactivity of lithium in aqueous electrolytes, with the exception of the special lithium-water and lithium-air batteries (see Sec. 38.6), lithium batteries must use a nonaqueous electrolyte with which lithium is nonreactive. The various ambient-temperature active (non-reserve) lithium batteries arecovered in Chap. 14. Of these systems, the ones demonstrating the higher energy densities and rate capabilities are Li / SO2, Li / V2O5, Li / SOCl2, and Li / LixCoO2. The discharge characteristics of these batteries are shown in Fig. 20.1. These are the electrochemical systems that are predominately employed in the reserve-type configurations.

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LIVE GRAPH

FIGURE 20.1Performance comparison of lithium anode primary systems at 20 C. Thionyl chloride (SOCl2)—3.6 V; vanadium pentoxide (V2O5)—3.4 V; sulfur dioxide (SO2)—2.9 V; precharged lithiated cobalt oxide (LixCoO2, x 0.4 0.5) 4.0 V.

20.1

20.2

CHAPTER TWENTY

In the reserve construction, the electrolyte is physically separated from the electrode active materials until the battery is used and it is stored ina reservoir prior to activation. This design feature provides a capability of essentially undiminished output even after storage periods, in the inactive state, of over 14 years. The reserve feature, however, results in an energy density penalty of as much as 50% compared with the active lithium primary batteries. Key contributors to this penalty are the activation device and the electrolytereservoir. In the selection of a lithium anode electrochemical system for packaging into a reserve battery, besides such important considerations as physical properties of the electrolyte solution and performance as a function of the discharge conditions, factors such as the stability of the electrolyte and the compatibility of the electrolyte with the materials of construction of the electrolytereservoir are of special importance.

20.2
20.2.1

CHEMISTRY
Lithium / Vanadium Pentoxide (Li / V2O5) Cell

The basic cell structure of this system consists of a lithium anode, a microporous polypropylene film separator, and a cathode that is usually composed of 90% V2O5 and 10% graphite, on a weight basis. When it is used in a reserve battery, the prevalent electrolyte is 2M LiAsF6 0.4M LiBF4in methyl formate (MF) because of its excellent stability during long-term storage. As shown in Fig. 20.1, the Li / V2O5 system has a two-plateau discharge characteristic. A net cell reaction, involving the incorporation of lithium in V2O5, has been postulated to account for the first plateau, Li V2O5 → LiV2O5 The initial voltage level ranges from 3.4 to 3.3 V, decreases to 3.3 to 3.2 V forapproximately 50% of the active life of the first discharge plateau, at which point the range again decreases to a level of 3.2 to 3.1 V, which is maintained for the balance of the first plateau of discharge. After completion of the first plateau, the Li / V2O5 system undergoes a rapid change in voltage to the second discharge plateau around a voltage range of 2.4 to 2.3 V. This step involves the formationof reduced forms of V2O5, although specific mechanisms remain unclear.1 This second plateau is relatively more sensitive to temperature and discharge rate, and it is for this reason that most Li / V2O5 cells (active and reserve) are designed to operate at only the first discharge plateau level.2 The long-term storage capability of Li / V2O5 reserve cells is heavily dependent on the stability of the...