2.10 Features and Attributes of the Basic Series

Series SL-300
Keywords: standard use and stand-by

• Excellent shelf life (10 years)
• Extremely low self-discharge (less than 0.5 % per year on shelf)
• Suited for long-term use with low current
• For operation at low current levels with long stands
• Intermittent discharge with medium current level provided the average is not below the active current level
• Temperature range from -55 °C to +85 °C (flat batteries up to +75 °C)

Series SL-500
Keyword: extended temperature range

• Extension of temperature range up to +130 °C
• Somewhat smaller capacity
• Otherwise like series SL-300

Series SL-700
Keyword: enhanced start

• Major improvement of voltage delay at the start of discharge at medium current levels (TMV)
• Best results if used after no more than 3 years of storage
• Intermittent discharge at medium current levels
• Otherwise like series SL-300

Computer batteries

• Supplied with plastic outer case, cable and connector
• Classified by UL as "user-replaceable"
• Some versions with current and/or voltage limitation
• Some versions contain two or more cells.
• Otherwise like series SL-300
• Please request selection list for PC's.

3.1 Cell Components and Materials

Anode
The anode is made of a battery grade lithium foil, which is rolled against the inner surface of the cell can to provide a mechanically sound and reliable electrical connection.

Cathode
The cathode is made of highly porous Teflon-bonded carbon black whose electronic conductivity is needed for the charge transfer to take place. Thionyl chloride cathodic reduction is catalyzed by the cathode surface when a load is connected. The pores of the carbon cathode retain both the reactants and the products of this process.

Separator
The separator, between the anode and the cathode, prevents internal short-circuits and hence immediate discharge while enabling ions to move freely between the electrodes. It is made of non-woven glass, carefully selected for compatibility with the chemical system during prolonged storage and operation.

Electrolyte
The electrolyte is basically a solution of lithium aluminum tetrachloride in thionyl chloride, which retains its ionic conductivity over the entire temperature range. Thionyl chloride freezes only at -105°C. The electrolyte thus contributes essentially to the outstanding low temperature performance of the batteries. From the standpoint of the electrochemical reaction, thionyl chloride also forms the active depolarizer. The electrolyte is therefore often referred to as catholyte.

Current Collector
A metal current collector provides the electrical connection between the porous carbon cathode and the positive terminal of the battery. Different forms of current collector are used for small cells (1/2AA, 2/3AA, and AA), big cells (C, D, and DD), and flat cells (BEL, 1/10D, 1/6D).

Can and Cover
The cell can and cover are made of nickel-plated cold-rolled steel. The can is designed to withstand the mechanical stresses that would be encountered over the anticipated wide range of environmental service conditions.

3.2 Mechanical Design

Sonnenschein Lithium thionyl chloride batteries are manufactured in two distinct mechanical designs, the cylindrical bobbin type, and flat cells. These two designs differ in the ratio of height and diameter as well as in the way anode and cathode are arranged with respect to each other.

Bobbin Design
In the bobbin design (Fig. 3-1), the cathode is cylindrical in shape. The anode is rolled against the inner wall of the battery case. This offers several advantages from the standpoint of safety. In the event of an unintentional short-circuit the discharge currents cannot exceed a limit that prevents hazardous situations. The heat generated, primarily at the contact surface between the anode and cathode, can easily be dissipated to the outside. The design leads to a safe battery that needs no additional rupture vent.

Flat Cells
In the flat cells (Fig. 3-2), the anode is pressed onto the bottom of the case, and the cathode, having the shape of a disk, is situated on top of the anode. The design has the same advantages with respect to intrinsic safety as that of the bobbin version.

Jelly-Roll Design
When the moderate rate capability of the bobbin type design is not sufficient, the so-called jelly-roll design may present a solution. In this type of cell, the anode and cathode material have the shape of long thin strips that are wound like a jelly-roll. This yields a larger electrode surface area and a greater current capability. A comparable standard of safety is achieved in this version by means of additional design measures, like a rupture vent and a fuse.

Hermetic Seal
Sonnenschein Lithium engineers have carefully designed the sealing between the positive (+) cell terminal and the cell cover, which has the same electric potential as the negative (-) terminal. Hermiticity is ensured by a glass-to-metal seal using the compression seal technology. In addition, the cell cover is welded to the cell can by a LASER beam welding process. In contrast to most systems using crimp seal techniques or polymer materials, the sealing and insulating system of Sonnenschein Lithium Batteries is not sensitive to temperature and humidity changes of any kind within the range of operating conditions. It is thus a major contributor to the excellent shelf and operating lives obtained.

Safety Vent
A safety vent is sometimes incorporated in hermetically sealed batteries in order to reduce the burst pressure of the cell case. This has not been found to be an advantage with Sonnenschein Lithium Batteries. The majority are therefore not vented. Under all conditions of use, internal pressure stays far below the burst pressure. However, under extreme conditions of abuse, like e.g. heating in fire or by forcing a large current through the cell, internal pressure may reach a critical value. Experience has proven that it is possible to avoid these conditions successfully. No incidence was reported from the field within more than 20 years of experience with this cell design. Obviously, a close and straightforward customer consultation is necessary to this end. It should be noted that safety vents are compulsory for user-replaceable batteries and for spirally wound cells. In these cases, the draw-backs of a vent with respect to long-term reliability and cost effectiveness are acceptable. Sonnenschein Lithium is prepared to supply most battery types with a vent if required.

	Figure 3-1 Cross sectional view of a ½AA size cell (bobbin version)
	Figure 3-2 Cross sectional view of a WD size cell (flat cell)

3.3 Chemical Reaction and Protective Layer The generally accepted overall discharge reaction during current flow is as follows: Anodic oxidation: 4 Li l 4 Li+ + 4e- Cathodic reduction: 2 SOCl2 l SO2 + S + 4 Cl- - 4e- Overall reaction: 4 Li + 2 SOCl2 + 4 LiCl + S + SO2 Most of the sulphur dioxide formed during discharge is dissolved in the electrolyte by complex formation. This results in a low internal pressure before, during and after normal discharge. A protective layer on the lithium surface is responsible for the excellent shelf life of lithium thionyl chloride batteries since it effectively prevents self-discharge. The layer basically consists of lithium chloride crystals that are formed as soon as the electrolyte comes into contact with the lithium anode during cell manufacture. As the layer grows, it prevents further reaction. If an external load is connected to the battery, lithium ions formed on the anode surface can migrate through the layer which contains a sufficient number of vacancies needed for this process. If the current drain is increased the motion of the lithium ions will disturb the ionic lattice of the layer and eventually disrupt it or even break it up completely. At each level of this process, the conductivity of the layer is increased. The internal resistance thus decreases allowing for the voltage to reach a stable value. The process of adaptation usually takes some time and is responsible for the voltage delay. The protective layer can be considered as consisting of two distinct parts. The one which is on the lithium surface is compact and thin. It is referred to as the solid electrolyte interface (SEI). On top of this layer there is a more porous layer of corrosion products which, to some extent, blocks the surface of the anode but does not take part in the electrochemical processes. It is often referred to as secondary porous layer (SPL). The morphology, thickness, mechanical strength, and porosity of the layer influence the voltage behaviour when the battery is first loaded. The most severe voltage delay will be encountered for batteries stored for long periods at elevated temperature, discharged at low temperature (or during the cooling down period), and at high current density. Figure 3-4 is a schematic overview of the reactions taking place in a lithium thionyl chloride cell.

Figure 3-4 Reaction mechanism of lithium thionyl chloride batteries. The circles are enlarged views of the anode surface (lower left) and cathode surface (upper right). The anode surface is covered with the SEI (solid electrolyte interface) and the SPL (secondary porous layer) on top of it. On the cathode and on the separator, both lithium chloride and sulfur crystals have formed as reaction products.