
Indefatigable
Positive Connotation
Indefatigable is positive and suggests admirable persistence and energy in continuing activities. 
Intensity Emphasized
Use 'indefatigable' to express a very high level of energy or effort, more than 'hardworking'. 
Academic Tone
'Indefatigable' fits well in formal or academic writing, adding sophistication to the text. 