It is not recommended to directly supply power to a 6V DC motor with a 9V battery

1. What problems may arise
The voltage is 50% higher, the motor speed exceeds the standard significantly, and it rotates rapidly
The working current increases, causing the motor to overheat severely and significantly shorten its lifespan, making it easy to burn the coils
The torque increases, the noise increases, and the load is slightly higher, making it more prone to stalling and burning out
Long term overvoltage operation of brushed DC motors results in rapid wear of carbon brushes

2. Can it be used reluctantly?
case by case
Can be used temporarily for a short period of time (not recommended for long-term use)
Low power toy motor, short time jog, without heavy load, can rotate occasionally, but not suitable for long-term continuous work.
Absolutely cannot be used directly
Model motors, reduction motors, and 6V motors that operate under load for a long time are easily scrapped if they are directly 9V.

3. The correct and prudent approach
Series step-down diode
Using silicon diodes like 1N4007, each with a voltage drop of approximately 0.7V:
Pressure drop required:
9−6=3V
String 4 diodes, with a voltage drop of about 2.8V, dropping to nearly 6V, simple enough.
Use voltage reduction module
Buy a DC-DC 9V to 6V voltage regulator module, which can output stable 6V without damaging the motor and maintaining a constant speed. It is highly recommended.
Using current limiting resistors (not recommended)
High heat generation, voltage changes with load, only suitable for toys with extremely low current.
riepilogo
Temporary play can be used directly, but for long-term and serious use, it is necessary to reduce the voltage to 6V. It is most reliable to use a series diode or a voltage regulator module.