Charging a Lithium battery with a higher Lead-Acid charging voltage will cause the Lithium Battery's Battery Management System (BMS) to self-protect and disconnect the battery from the charging source. Additionally, determining state-of-charge and charge termination using voltage is more difficult with Lithium than with Lead-Acid.
A lithium-ion battery will still charge (slowly) at very low current. To avoid overcharge you must keep the voltage below 4.23V. Normally this is done by reducing charge current when it gets to 4.2V. I don't know what a 'shunt' battery charger is, but proper Li-ion charger IC's and modules are cheap and readily available.
The voltage output of the charger must meet the voltage requirements of the lithium battery pack to ensure safe and efficient charging. Using a charger with incorrect voltage output will result in overcharging or undercharging, which may damage the battery and shorten its life.
On the other hand, undercharging can cause irreversible capacity loss, negatively impacting battery performance and life. Discharging below the minimum voltage threshold of a lithium battery must be avoided to keep the battery healthy and ensure optimal functionality. Using a certified charger to charge lithium battery packs must be considered.
It’s a common belief that the voltage of a lithium-ion battery can accurately indicate its charge state. However, this is only partially true. The lithium-ion battery’s voltage increases as it charges, but the relationship is not linear. It can vary based on several factors, including the battery’s age and temperature.
Charge in an area with good ventilation Heat may be produced by lithium-ion batteries when they are charging. Charge it in a place with good ventilation to help dissipate this heat and keep the battery from overheating. Refrain from charging near combustible objects or in enclosed areas.