What happens to the excess voltage?
There is no excess.
Forget about volts for a moment.
The solar panel can deliver, with good solar energy, 110 watts of electrical power. The MPPT controller circuits collect the power from the panel and convert to suitable voltages and current for the battery, its a power converter.
There are many power converters that convert power at one voltage level to power at a different voltage level, for example your AC powered phone charger, the MPPT is just a specialised type of power converter.
All battery chargers apply a charge voltage suitable for the battery type and the battery state of charge.
Regard voltage as a measure of 'push' and current as a measure of 'flow'. Usefully the product of voltage and current is the amount of energy flowing in a circuit at that moment in time, measured in watts.
Neglecting any losses in the system, 18 volts applied to the MPPT with the panel capable of delivering 6.11 amps, gets 110 watts of energy into the energy 'store' inside the controller.
If we assume the battery needs charging, the charger electronics in the MPPT 'push' amps into the battery. The degree of 'push' will be the difference in volts between charger output and the battery. If, for example the battery needs 14 volts to charge, then the current available from the 'store' will be watts divided by volts, 110/14 = 7.86 amps.
In real life the solar energy falling on the panel wont be at the maximum and will vary due to weather conditions.
The MPPT controller will respond to changing conditions by 'loading' the panel optimally, thus being able to transfer via the conversion process, the maximum amount of power available to the battery. Once the battery reaches full charge the controller no longer 'loads' the panel for maximum power but just enough for the battery needs.
Mike