Lithium ions are inserted into manganese di, just as for Li/V2O5 cells, but only one discharge plateau is present. The system is low pressure and small format, low rate cells are in widespread use.
Spirally wound, high rate cells are presently gaining acceptance. However, the electrolyte used in these cells is highly inflammable (usually mixed solvents such as propylene carbonate and dimethoxyethane with lithium perchlorate) and a safer electrolyte is desirable. Cells have been known to burn fiercely for over two minutes following venting caused by electrical abuse. One method being used to avoid venting is the use of a porous separator that becomes impervious at a particular temperature, shutting down the cell reactions. Otherwise, Li/MnO2 may displace Li/SO2 in certain applications that do not require a high rate at low temperatures.
These batteries are ideal for electronic equipment requiring high rate pulses (e.g., photoflash), as well as other sophisticated devices requiring continuous operation or memory backup. Li/MnO2 cells are available in two configurations:
Cylindrical cells using spiral-wound electrodes for high current pulse or continuous drain applications
Coin-shaped cells for memory backup and small electronic device applications.
The Li/MnO2 coin cells range in capacity from 76 mAh to 500 mAh. The spiral-wound cylindrical cells are available in 160 mAh and 1.4 Ah sizes.
Voltage: The open circuit voltage of the Li/MnO2 cell is typically 3.2V, compared to 1 .5V for most zinc cells. Typical median operating voltage ranges from 3.0 to 2.5V.
Discharge Characteristics: Flat (stable voltage) discharge curves.
Energy Density: Depending on cell configuration, up to 230 Wh kg-1 and 550 Wh dm-3.
Effect of Discharge Load and Temperature: Capable of performance at low to high discharge rates on pulse or continuous loads; excellent low temperature capability. The typical operating temperature range for Li/MnO2 cells is -40°C to 60°C, with performance at wider temperature ranges possible.
Shelf Life: Capacity retention is over 95% after 5 years of room temperature storage.
Cathode: The cathode material used in Li/MnO2 cells is a mixture of heat-treated electrolytic manganese dioxide and conductive agents blended together for high conductivity. The conductivity of the MnO2 cathode results in higher initial cell voltage and operating voltage during discharge than achieved when using highly-resistive active cathode materials, such as poly-carbonmonofluoride. The thermodynamic stability of this specially processed MnO2 cathode ensures high reliability and performance, even after very long periods of storage.
The cell reaction involves the oxidation of lithium metal at the anode to produce positively charged lithium ions (Li+) and electrons (e-). The Li+ ions go into solution and diffuse through the electrolyte and separator to the cathode. Electrons travel through the external circuit and arrive at the cathode where MnO2, Li+ ions and electrons combine. The MnO2 is reduced from the tetravalent to the trivalent state. The solid discharge reaction product remains in the cathode. No gases are evolved during discharge to cause a pressurized condition. The LiMnO2 global cell reaction:
Li + MnIVO2 --> MnIIIO2(Li+)