Overwhelmed by his fervor, I couldn't argue back.
The players' spirits were high before the match.
The demonstrators marched, shouting rallying cries with great spirit.
The team's victory boosted everyone's fighting spirit.
士気 specifically means morale within a group, often military or organizational, whereas 気勢 can describe the spirited energy of an individual or a crowd more broadly.
Standard kanji spelling; commonly used in news and writing.
Compound of 気 (spirit, energy) and 勢 (force, momentum). The combination conveys a sense of vigorous spirit or energetic momentum.