An electrochemical device includes a positive electrode. The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes a lithium manganese oxide. The lithium manganese oxide includes aluminum and sodium. Based on a weight of the positive active material, a weight percentage of the aluminum is A %, a weight percentage of the sodium is B %, and the electrochemical device satisfies 0.01≤A≤2 and 0.001≤B≤1.