<jats:p>Structural, computational and electrochemical investigations are combined to study the intercalation properties of tunnel-type Na<jats:sub>0.44</jats:sub>MnO<jats:sub>2</jats:sub> and Cu-substituted Na<jats:sub>0.44</jats:sub>MnO<jats:sub>2</jats:sub>.</jats:p>