Is lying to one's husband about performing supererogatory acts of worship (such as praying Qiyam al-Layl or voluntary fasting) a sin, especially if the lie is to avoid his negative reactions or his disapproval without a valid reason?

Lying is forbidden except in specific situations, among them, a husband and wife lying to each other. However, this is restricted to what brings affection, such as showing love. You are not permitted to lie to your husband in situations other than these. You can use tawriya (ambiguity) and ta'rid (allusion) instead of outright lying. As for voluntary fasting, if your husband is ill and does not need conjugal relations, and the fast does not harm your body or prevent you from fulfilling his rights, then you are not required to seek his permission.