Lithium-manganese-tin oxide cathode active material and lithium secondary cell using the same

Abstract

A cathode thin film for a lithium secondary cell, which uses a cathode active material substituting Sn for Mn in lithium manganese oxide, has a high discharge capacity and an improved cycle property.

1 . A cathode active material comprising a lithium-manganese-tin oxide of formula (I):
   LiSn x/2 Mn 2−x O 4  (wherein, 0< x≦ 0.05)   (I)   
   
   
       2 . The cathode active material of  claim 1 , whose density is 3.4 to 4.4 g/cm 3 . 
   
   
       3 . The cathode active material of  claim 1  prepared by a method comprising the steps of:
 a) mixing powders of Li, Mn and Sn compounds to obtain a mixed powder;   b) first-milling the mixed powder;   c) calcining the first-milled powder;   d) second-milling the calcined powder;   e) pressing the second-milled powder; and   d) sintering the pressed mixture.   
   
   
       4 . The cathode active material of  claim 3 , wherein the Li compound is selected from the group consisting of Li 2 CO 3 , LiOH, CH 3 COCH═C(OLi)CH 3 , LiOOCCH 3 , LiOOCCH 3 .2H 2 O, LiCl, LiCl.xH 2 O (0.005≦x≦1), LiO 2 CCH 2 C(OH)(CO 2 Li)CH 2 CO 2 Li.xH 2 O (1≦x≦6), Li 2 S, Li 2 SO 4 .H 2 O, and a mixture thereof. 
   
   
       5 . The cathode active material of  claim 3 , wherein the Mn compound is selected from the group consisting of MnO 2 , (CH 3 CO 2 ) 2 Mn, (CH 3 COO) 2 Mn.4H 2 O, (CH 3 COO) 2 Mn.2H 2 O, [CH 3 COCH═C(O)CH 3 ] 2 Mn, Mn(C 5 H 7 O 2 ) 3 , MnCO 3 , MnCl 2 , MnCl 2 .xH 2 O (1≦x≦5), [C 6 H 11 (CH 2 ) 3 CO 2 ] 2 Mn, MnF 2 , Mn(NO 3 ) 2 , Mn 3 O 4 , MnO, Mn 2 O 3 , Mn(ClO 4 ) 2 .6H 2 O, MnSO 4 .xH 2 O (1≦x≦6), MnS, and a mixture thereof. 
   
   
       6 . The cathode active material of  claim 3 , wherein the Sn compound is selected from the group consisting of SnO, Sn(CH 3 CO 2 ) 2 , SnCl 2 , SnCl 2 .2H 2 O, [CH 3 (CH 2 ) 3 CH(C 2 H 5 )CO 2 ] 2 Sn, SnF 2 , SnC 2 O 4 , SnSO 4 , SnS, and a mixture thereof. 
   
   
       7 . The cathode active material of  claim 3 , wherein the amounts of the Li, Mn and Sn compounds used in step a) are 18.0 to 26.0 wt %, 72.4 to 81.9 wt %, and 0.1 to 1.6 wt %, respectively, based on the total weight of the mixed powder. 
   
   
       8 . The cathode active material of  claim 3 , wherein the calcination process in step c) is conducted at 400 to 800° C. for 1 to 5 hours under an ambient condition. 
   
   
       9 . The cathode active material of  claim 3 , wherein the second-milled powder is pressed in step e) to form a pellet. 
   
   
       10 . The cathode active material of  claim 3 , wherein the sintering process in step f) is conducted at 800 to 1,300° C. for 1 to 24 hours under an ambient condition. 
   
   
       11 . A lithium secondary cell which comprises a lithium-manganese-tin oxide of formula (I) as a cathode active material:
   LiSn x/2 Mn 2−x O 4  (wherein, 0< x≦ 0.05)   (I)   
   
   
       12 . The lithium secondary cell of  claim 11 , comprising a cathode thin film prepared by depositing the lithium-manganese-tin oxide of formula (I) on a substrate using an LCVD (laser-assisted chemical vapor deposition) process using the compound of formula (I) as the target. 
   
   
       13 . The lithium secondary cell of  claim 12 , wherein the LCVD process is conducted at a substrate temperature of 350 to 550° C., while maintaining the distance between the substrate and the target at 3 to 5 cm and the oxygen partial pressure at 0.05 to 0.25 Torr. 
   
   
       14 . The lithium secondary cell of  claim 12 , wherein the LCVD process is conducted with a laser power density of 1 to 4 J/cm 2 , a beam area of 2 to 5 mm 2  and a spot repetition of 3 to 10 Hz, for 30 to 120 minutes.