In cases where excommunication is "reserved to the Apostolic See" (this includes most but not all cases of Latae Sententiae excommunication), a specific Cardinal, the Major Penitentiary, is able to lift excommunication even when there is no Pope.
From an EWTN article:
What happens to those in Office in the Roman Curia when the Pope dies?
Since supreme teaching, legislating and judicial authority rests with the Pope, all but the most ordinary business of the Holy See comes to a stop. The highest office holders, such as the Cardinals who are Prefects of Congregations and Presidents of Pontifical Councils and Commissions, all lose their offices with the death of the Pope.
There are two exceptions 1) the Cardinal who is Camerlengo or Chamberlain of the Holy Roman Church, an office which deals primarily with the period of the Papal Interregnum or Vacancy, and 2) the Cardinal who is the Major Penitentiary, and responsible for matters concerning the internal forum of conscience (e.g. absolution from excommunications reserved to the Holy See).