the composition of comets vary widely
the "atmosphere" around them is indeed a cloud of electrically charged ions
several processes involved
origin is 'outgassing' which varies widely due to composition of comet and temperature, of course as the comet nears the sun the temperature increases
ions are created by, solar radiation, cosmic radiation and triboelectric effects in proportion to the proximity to the sun. Far from the sun the ions are created primarily by triboelectric effects from outgassing and cosmic radiation.