To answer your question, one must first define both Impressionism and Expressionism.
Impressionism was a period which emerged in the Nineteenth century and was prominent during the 1870s and 1880s. Impressionistic literature characteristically detailed the author's impression (idea, opinion, or feeling about something) regarding a scene. Impressionism is sometimes related to symbolism given that impressionists characteristically offer a description of life using emotions and sensations.
Expressionism was a period which emerged during the late Nineteenth century and moved into the early Twentieth. This movement was most prominent from 1910 to 1925. Expressionistic literature characteristically detailed
distorted objective features of the sensory world using symbolism and dream-like elements in their works illustrating the alienating and often emotionally overwhelmed sensibilities.
While both movements relied upon the senses, Expressionists focused upon the overwhelming aspects of emotions while Impressionists used emotions to describe life in general.
Basically, Expressionists are using their emotions to define their words and Impressionists are using their words to define their emotions.
Impressionism in literature grew out of the French literary movements of Realism and Naturalism. Both of these valued objective, scientific observation and accurate representation making the literary work as true to the observed as possible. Exact imitation of detail was important to both literary movements. Impressionism shares these traits with Realism and Naturalism, with the distinction that for the Impressionist, the observation is a "flash" in time, a mere "sensation." Thus the accurate true-to-detail imitation is an impression of what was observed. This introduces an "element that is entirely subjective" (Gibbs, 176). Impressionism was early identified as having an "emphasis on the subjectivity of experience" and an "intensified sensitivity" (van Gunsteren). Flaubert and Balzac are cited as representative of Impressionism, and it dominated in France.
Expressionism in literature is different from Impressionism because it developed later, with its height in the early 20th century. Expressionism began in Europe before World War I, then continued throughout the war up to the 1920s. Expressionism is different also because it began as a movement of rebellion against the two literary movements leading up to Impressionism: Realism and Naturalism. Hostile toward materialism, a burgeoning wealthy middle class and the rural drain of urbanization, expressionists decrying the sordid representation of accurate details of sordid life, as in Zola's Realism novel Nana, sought to represent the inner mental state of characters.
Characters were situated in vague physical surroundings, a significantly different approach from Impressionism, because surroundings were less important than a character's inner spiritual state, as illustrated in Kafka's "Metamorphosis." Expressionism, producing a style different from that of Impressionism, was identified with conveying "general truths" and "representative symbolic types" (Encyclopedia Britannica). James Joyce and Eugene O'Neill are also representative of Expressionism, though it dominated in Germany.
Beverly Jean Gibbs, "Impression as a Literary Movement"
Julia van Gunsteren, Katherine Mansfield and Literary Impressionism
The expressionism and impressionism literary movements occurred alongside the art movements of the same names and shared some of the same characteristics.
Impressionism as a literary movement, much as the art movement of the same name, referred to works that were not true to realism. In a sense, they were the opposite of it. The impressionist style was not meant to represent reality but instead to give the author's impression of reality. With attention to the descriptions of color and movement, the impressionist author painted his view of the world with descriptive words and figurative language that drew the reader in and allowed the reader to have the same impression of the text.
Expressionism similarly has the intent of depicting the author's view of reality but rather than using color and movement, the expressionist author attempts to elicit strong emotion from the reader that give the text a particular meaning. Events and characters are described in such as way as to cause the reader to experience shock or other strong emotions.