A solid-state sodium battery comprising: a) a positive compartment comprising: a.1) a positive electrode, said positive electrode comprising a compound of formula NaFe x Mn y Ni z O 2, wherein x is more than 0 and equal or less than 1, y is more than 0 and equal or less than 1, z is more than 0 and less or equal than 1, and wherein said compound is a neutral oxide; and a.2) a catholyte comprising: - a salt of formula NaX, wherein Na is a sodium cation and X is an anion selected from the group consisting of bis(trifluoromethanesulfonyl)imide; (fluorosulfonyl)(trifluoromethanesulfonyl)imide; and - a polymer, wherein the polymer is selected from a polyether, polyester, polyethylene, polycarbonates, phosphazene, siloxanes and mixtures thereof; b) a negative compartment comprising a negative electrode, said negative electrode comprising solid metallic sodium or an alloy comprising solid metallic sodium; and c) an electrolyte composition; wherein said electrolyte composition comprises a solid sodium-ion conductive composition that conducts sodium ions during use between the negative and the positive electrodes; wherein the mass loading of said compound of formula NaFe x Mn y Ni z O 2 is equal or higher than 1.0 mAh·cm -2; wherein the mass loading being equal or higher than 1.0 mAh·cm -2 was calculated by multiplying the amount of said compound of formula NaFe x Mn y Ni z O 2 per area of the positive electrode (g/cm 2) by the expected capacity for said compound (mAh/g); and wherein the expected capacity is 130 mAh/g at a C-rate of C/10. a) a positive compartment comprising: a.1) a positive electrode, said positive electrode comprising a compound of formula NaFe x Mn y Ni z O 2, wherein x is more than 0 and equal or less than 1, y is more than 0 and equal or less than 1, z is more than 0 and less or equal than 1, and wherein said compound is a neutral oxide; and a.2) a catholyte comprising: - a salt of formula NaX, wherein Na is a sodium cation and X is an anion selected from the group consisting of bis(trifluoromethanesulfonyl)imide; (fluorosulfonyl)(trifluoromethanesulfonyl)imide; and - a polymer, wherein the polymer is selected from a polyether, polyester, polyethylene, polycarbonates, phosphazene, siloxanes and mixtures thereof; a.1) a positive electrode, said positive electrode comprising a compound of formula NaFe x Mn y Ni z O 2, wherein x is more than 0 and equal or less than 1, y is more than 0 and equal or less than 1, z is more than 0 and less or equal than 1, and wherein said compound is a neutral oxide; and x is more than 0 and equal or less than 1, y is more than 0 and equal or less than 1, z is more than 0 and less or equal than 1, and wherein said compound is a neutral oxide; and a.2) a catholyte comprising: - a salt of formula NaX, wherein Na is a sodium cation and X is an anion selected from the group consisting of bis(trifluoromethanesulfonyl)imide; (fluorosulfonyl)(trifluoromethanesulfonyl)imide; and - a polymer, wherein the polymer is selected from a polyether, polyester, polyethylene, polycarbonates, phosphazene, siloxanes and mixtures thereof; - a salt of formula NaX, wherein Na is a sodium cation and X is an anion selected from the group consisting of bis(trifluoromethanesulfonyl)imide; (fluorosulfonyl)(trifluoromethanesulfonyl)imide; and - a polymer, wherein the polymer is selected from a polyether, polyester, polyethylene, polycarbonates, phosphazene, siloxanes and mixtures thereof; b) a negative compartment comprising a negative electrode, said negative electrode comprising solid metallic sodium or an alloy comprising solid metallic sodium; and c) an electrolyte composition; wherein said electrolyte composition comprises a solid sodium-ion conductive composition that conducts sodium ions during use between the negative and the positive electrodes; wherein the mass loading of said compound of formula NaFe x Mn y Ni z O 2 is equal or higher than 1.0 mAh·cm -2; wherein the mass loading being equal or higher than 1.0 mAh·cm -2 was calculated by multiplying the amount of said compound of formula NaFe x Mn y Ni z O 2 per area of the positive electrode (g/cm 2) by the expected capacity for said compound (mAh/g); and wherein the expected capacity is 130 mAh/g at a C-rate of C/10. wherein the mass loading of said compound of formula NaFe x Mn y Ni z O 2 is equal or higher than 1.0 mAh·cm -2; wherein the mass loading being equal or higher than 1.0 mAh·cm -2 was calculated by multiplying the amount of said compound of formula NaFe x Mn y Ni z O 2 per area of the positive electrode (g/cm 2) by the expected capacity for said compound (mAh/g); and wherein the expected capacity is 130 mAh/g at a C-rate of C/10.