In an aspect, the present disclosure provides a heat-treated transition metal carbonitride MXene film annealed at high temperatures and a polymer composite comprising the same. In another aspect, the present disclosure provides a method for producing a heat-treated transition metal carbonitride MXene film comprising: obtaining a MXene aqueous solution containing dispersed 2-dimensional (2D) MXenes through an acid etching process; filtering the obtained MXene aqueous solution through a vacuum filtration process to produce a free-standing film; and annealing the produced free-standing film at high temperatures to obtain a heat-treated transition metal carbonitride MXene film. In still another aspect, the present disclosure provides an electromagnetic interference (EMI) shielding method comprising: superposing a coating comprising a heat-treated transition metal carbonitride MXene film on at least one surface of an object in a contact or non-contact manner.