referring to fig1 and 2 , a battery sheath 10 includes a ferrite sus layer 11 , a first insulation layer 12 formed on a surface of the ferrite sus layer 11 and a second insulation layer 13 formed on the other surface of the ferrite sus layer 11 . the ferrite sus layer 11 has an approximately planar or a completely planar first surface 11 a and an approximately planar or a completely planar second surface 11 b opposite the first surface 11 a . in addition , the thickness of the ferrite sus layer 11 between the first and second surfaces 11 a , 11 b ranges from about 10 μm to about 60 μm , which is less than the thickness of prior art sheaths by several microns to tens of microns . namely , since the ferrite sus layer 11 has increased mechanical strength because of the material characteristics , it may have more reduced thickness than that of the prior art sheaths . furthermore , the ferrite sus layer 11 doesn &# 39 ; t need to increase the thickness in order to enhance the elongation ratio related to workability because of the material characteristics . the ferrite sus layer 11 , therefore , may not only reduce the thickness thereof but also keep up the high mechanical strength . for a comparative reference , in the case of using an austenite sus layer , it needs to increase its thickness in order to enhance the elongation ratio , thus it is difficult for its thickness to stay less than 60 μm . therefore , in accordance with the present invention more electrode assemblies ( not shown ) can be contained within the same volume . that is , the capacity of the battery increases . the ferrite sus layer 11 may include an alloy having from about 84 % to about 88 . 2 % iron , about 0 . 5 % or less carbon , from about 11 % to about 18 % chromium , and from about 0 . 3 % to about 0 . 5 % manganese . furthermore , the ferrite sus layer 11 may include a material selected from the group consisting of korean industrial standard ( ks ) sts430 and japanese industrial standard ( jis ) sus430 . however , it is understood that any suitable material may be used for the ferrite sus layer 11 . since the ferrite sus layer has high mechanical strength and high resistance to chemical corrosion , it increases the mechanical strength of the battery sheath 10 and increases the resistance to the electrolyte . the ferrite sus layer 11 , of course , prevents moisture from penetrating the battery . the ferrite sus layer 11 has an elongation ratio of about 10 % to about 60 %, enabling easy formation of a cavity ( not shown ). this elongation ratio prevents the ferrite sus layer 11 from being damaged during formation of the cavity . the cavity is formed to a predetermined depth by a die , and contains the electrode assembly . for example , the ferrite sus layer 11 may be annealed in an inactive gas atmosphere at a temperature of hundreds of degrees celsius to maintain the elongation ratio at about 10 % to about 60 %. of course , since the ferrite sus layer 11 has excellent workability , it has high elongation ratio by itself and there may be no need to be annealing . furthermore , the characteristics of the ferrite sus layer 11 enable suppression of swelling which may occur at higher temperatures after battery assembly . therefore , deformation of the thickness and shape of the battery is sufficiently prevented . more particularly , massive gas may be generated by decomposition of the electrolyte at high temperature after assembling the battery . and then , the swelling phenomenon , wherein the battery sheath swells outwardly , may occur because of the massive gas . however , since the battery sheath in accordance with the present invention uses the ferrite sus layer 11 having high mechanical strength , the swelling phenomenon is sufficiently prevented from deforming the battery . the first insulation layer 12 which is applied to the first surface 11 a of the ferrite sus layer 11 may be a cpp layer . a cpp layer with a thickness of about 30 μm to about 40 μm may be applied to the first surface 11 a of the ferrite sus layer 11 . the cpp layer may have a thickness slightly greater than that of the ferrite sus layer 11 because the cpp layer directly contacts to the electrode assembly and is thermally bonded to each other . the second insulation layer 13 which is applied to the second surface 11 b of the ferrite sus layer 11 may be one selected from a nylon layer and a pet layer . for example , the nylon layer or the pet layer is applied to the second surface 11 b of the ferrite sus layer 11 by lamination at high temperature . the nylon or the pet layer with a thickness of about 5 μm to about 10 μm is applied to the second surface 11 b . the pet layer as the second insulation layer 13 may include an alloy film . more particularly , the pet layer may further include rubber particles for enhancing resistance to impact , a solubilizer surrounding the rubber particles for enhancing adherence , and an adhesive . the rubber particles increase the elongation ratio and the resistance to impact . the solubilizer improves adherence to the ferrite sus layer 11 , and particularly to the second surface 11 b of the ferrite sus layer 11 . the adhesive , previously applied to the pet layer enables direct lamination of the pet layer at high temperature without applying any special adhesive to the ferrite sus layer 11 . this further simplifies the manufacturing process of the battery sheath 10 . the pet layer may not include an adhesive . in that case , an adhesive is previously formed on the second surface 11 b of the ferrite sus layer 11 . the pet layer is then applied to the ferrite sus layer 11 . fig3 a is a perspective view of a battery sheath 110 according to one embodiment of the present invention . the sheath 110 includes a cavity 116 for containing an electrode assembly . fig3 b is a magnified view of region 3 b in fig3 a . referring to fig3 a and 3 b , the battery sheath 110 includes a first region 117 a and a second region 117 b which are folded together such that their edges are thermally bonded . the first region 117 a may include a cavity 116 having a predetermined width and depth for containing an electrode assembly ( not shown ). the electrode assembly includes at least one positive electrode collector , at least one negative electrode collector and at least one separator between the positive and negative electrode collectors . the second region 117 b may also include a cavity ( not shown ). a ferrite sus layer 111 , which is the main material of the sheath 110 , has an elongation ratio of about 10 % to about 60 % for preventing the sheath 110 from being damaged during formation of the cavity 116 . the thickness of the first layer 112 is greater than the thickness of the ferrite sus layer 111 , and the thickness of the ferrite sus layer 111 is greater than the thickness of a second insulation layer 113 , such as a pet layer . the first insulation layer 112 is the thickest because the portion of the first insulation layer 112 on the outer peripheral edges of the first and second regions 117 a , 117 b , respectively , are thermally bonded to each other . fig4 is a perspective view of a rechargeable battery 200 according to another embodiment of the present invention . fig5 is a cross - sectional view of the rechargeable battery taken along line 4 - 4 in fig5 . as shown , the rechargeable battery 200 includes an electrode assembly 221 , a sheath 210 , and a protective circuit module 223 . the electrode assembly 221 is formed by laminating at least one positive electrode collector 221 a , at least one negative electrode collector 221 b , and at least one separator 221 c between the positive and negative electrode collectors 221 a , 221 b , respectively . the positive electrode collector 221 a includes lithium cobalt oxide ( licolo 2 ) on aluminum ( al ) foil . the negative electrode collector 221 b includes graphite on copper ( cu ) foil . the separator 221 c includes a gel - type polymer electrolyte . at least one positive electrode tab 222 a of aluminum is bonded to the aluminum foil of the positive electrode collector 221 a , and at least one negative electrode tab 222 b of nickel is bonded to the copper foil of the negative electrode collector 221 b . the positive and negative electrode tabs 222 a , 222 b extend a predetermined length from the exterior of the sheath 210 . the sheath 210 includes a first region 217 a having a cavity 216 of a predetermined depth for containing the electrode assembly 221 , and a second region 217 b for covering the cavity 216 of the first region 217 a . the sheath 210 includes a ferrite sus layer 211 . a first insulation layer 212 , such as a cpp layer , is applied to a surface of the ferrite sus layer 211 and a second insulation layer 213 , such as a pet layer , is laminated at high temperature on the other surface of the ferrite sus layer 211 . an adhesive ( not shown ) may optionally be applied between the ferrite sus layer 211 and the first insulation layer 212 . the other adhesive ( not shown ) may also be optionally applied between the ferrite sus layer 211 and the second insulation layer 213 . the first insulation layer 212 surrounds the electrode assembly 221 , and the second insulation layer 213 is positioned on the outermost surface of the sheath 210 . the first insulation layers 211 on the outer peripheral edges 217 c of the first and second regions 217 a , 217 b , respectively , of the sheath 210 , are thermally bonded to each other and can be folded such that the volume of the sheath 210 is minimized . the remaining features of the sheath 210 are similar to those described above with reference to fig1 through 3 b . the protective circuit module 223 is attached to a front side of the sheath 210 to protect the battery 200 from voltage or current generated during overcharging or over - discharging . the protective circuit module 223 is electrically connected to the positive and negative electrode tabs 222 a , 222 b , respectively . as shown in fig5 , the positive electrode 221 a is positioned on the outer surface of the electrode assembly 221 . therefore , although the first insulation layer 212 is formed so that the positive electrode 221 a contacts the ferrite sus layer 211 , the ferrite sus layer isn &# 39 ; t corroded . namely , since the ionization tendency of the positive electrode 221 a is greater than that of the ferrite sus layer 211 , the positive electrode may be corroded but the ferrite sus layer 211 is not corroded . therefore , the electrolyte doesn &# 39 ; t leak through the ferrite sus layer 211 . as described above , the battery sheath includes a ferrite sus layer having high mechanical strength such that the sheath stably protects the battery from external impact . the high mechanical strength of the sheath enables to have a reduced battery thickness and an increased volume of the electrode assembly . this increases battery capacity . the high mechanical strength of the sheath also suppresses a swelling phenomenon and prevents a deformation of the thickness and shape of the battery . the excellent workability of the battery sheath makes it possible to easily form the cavity for containing the electrode assembly . the high resistance to chemical corrosion of the battery sheath enables the battery to stably prevent from the resistance to the electrolyte and an external acid solution . exemplary embodiments of the present invention have been described for illustrative purposes . however , those skilled in the art will appreciate that various modifications , additions and substitutions may be made to the described embodiments without departing from the scope and spirit of the invention as disclosed in the accompanying claims .