The demonstrators marched, shouting rallying cries with great spirit.
Overwhelmed by his fervor, I couldn't argue back.
試合前の選手たちは気勢が上がっていた。
The players' spirits were high before the match.
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.