Can we connect battery negative terminal to oscilloscope ground?
Most likely you'll be fine. A battery is isolated from the mains earth ground. Shorting them will simply set the battery earth and main earth as the same reference.

The only issue will be if the battery ground is somehow otherwise referenced to a potential that's not mains earth isolated.

For example if you have 2 batteries in series and one battery's ground is connected to mains earth, you need to be careful about which battery ground you connect your probe ground to.

You can also use a differential probe. These probes measure the differential voltage between the two test leads, there's not low impedance path from either lead to mains earth.