ESP8266 Power Management Circuit

I need help with voltage regulation to ensure my ESP-12F module runs continuously while being powered by a solar-charged lithium-ion battery.

Current Setup:
- ESP-12F module powered by a 3.7V lithium-ion cell.
- 6V, 5W solar panel with a digital MPPT circuit to charge the battery up to 4.2V.
- Circuit stops when battery voltage drops to 3.5V.

Requirements:
- Improve voltage regulation to prevent circuit stoppage at 3.5V.
- Integrate power management features to optimize performance and ensure continuous operation.

Ideal Skills and Experience:
- Expertise in voltage regulation and power management circuits.
- Experience with solar charging systems and MPPT technology.
- Familiarity with ESP8266 and related components.

Please provide a solution that ensures uninterrupted operation.