Is it Haram or Halal for a husband to lie to his wife about taking another wife to prevent her from becoming upset or leaving the marriage as a result of his new marriage?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
No doubt that speaking is a means to achieve some goals. So, any good and praiseworthy goal that could be achieved through truth should not be achieved otherwise. But if there is no other way to achieve this goal but through lying, then lying becomes lawful. This is the opinion of Imam Al Nawawi. Lying is lawful between spouses if it is done for benefit that is accepted by the Sharia. It is reported by Muslim from Oumekelthoum Bint Uqba that the Prophet said: "The liar is not the one who reconciles between people by saying good or reporting good". She added: "I have not heard him (the Prophet) permitting saying lies in anything except in three situations: people's reconciliation, war and the talking of the husband to his wife and of the wife to her husband".
So, your lying to your wife by denying having a second wife is lawful and there is no harm in it as long as your goal is to preserve your companionship with her as this goal is conform to Sharia.
Allah knows best.