The invention relates to a mixed 
ion-
electron conductor with a garnet structure and application of the mixed 
ion-
electron conductor in an 
energy storage device. The general formula of the conductor is(LimM)xLa3(Zr<1-a>Xa)2O12, wherein m is greater than 0 and less than or equal to 1, n is greater than or equal to 0 and less than or equal to 0.5, the sum of m and n is less than or equal to 1, and xis greater than or equal to 3 and less than or equal to 7.5; M comprises at least one element selected from H, Na, K, Rb, Mg, Ca, Sr, Ba, Y, La, Ti, Zr, Zn, B, Al, Ga, In, C, Si, Ge, P, S and Se; a is greater than or equal to 0 and less than 1; and X comprises at least one element selected from Ta, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Si, Ge, Sn, Pb, As, Sb and Se. The conductor provided bythe invention has high 
ionic conductivity and 
electronic conductivity, and can be applied to an 
electrode / 
electrolyte interface layer in a battery or a 
capacitor, a positive 
electrode layer and a negative 
electrode layer. The mixed 
ion-
electron conductor can be applied to a 
lithium ion battery, a rechargeable 
metal lithium battery, a 
lithium flow battery and a 
lithium ion capacitor, and the energystorage devices have wide application.