Elliott Investment Management, an activist investment group, said on Wednesday it had amassed a $1-billion stake in one of the largest U.S. refiners, Phillips 66, and expressed dissatisfaction with the company’s underperformance in recent years. Apart from disclosing its $1 billion investment in Phillips 66, the activist investment firm sent a letter to the Board of Directors of Phillips 66, in which it outlined its ideas of how the refiner could regain trust with investors and unlock significant and sustainable value for its…