I assume the lithium is added to lessen audionervosa. 
Or due to material science:
"Role of Lithium in a Lithium-Magnesium Tweeter:
1. Driver mass: Lithium is one of the lightest metals (atomic number 3), which reduces the mass of the tweeter diaphragm. A lighter diaphragm can respond more quickly to electrical signals, improving transient response and enabling the tweeter to handle high frequencies (typically 2 kHz to 20 kHz or higher) with precision.
2. Stiffness and Rigidity: When alloyed with magnesium, lithium contributes to a material that balances stiffness and low density. Magnesium is already lightweight and strong, but adding lithium enhances the alloy’s rigidity-to-weight ratio. This minimizes unwanted flexing or resonance in the diaphragm, ensuring cleaner sound reproduction and reducing distortion.
3. Damping: The lithium-magnesium alloy can provide good internal damping, which helps control vibrations and prevent unwanted resonances that could color the sound. This results in clearer, more accurate high-frequency output.
4. Propagation Velocity: lithium-magnesium alloy has a high speed of sound propagation due to its low density and high stiffness. This property is critical for tweeters, as it allows the diaphragm to efficiently transmit vibrational energy, producing crisp and detailed high frequencies.
5. Thermal and Electrical Stability: Lithium’s inclusion in the alloy can improve the material’s stability under the thermal and electrical stresses of high-frequency operation, ensuring durability and consistent performance over time."
