Crystal structure Control and Application to electrode materials of Low-expanding Composite Oxides

以锂钴氧化物、LiCoO_2为正极，碳材料为负极的二次锂电池（即所谓的“锂离子电池”）已经实现了商业化，并迅速扩大生产规模。然而，钴的自然资源非常有限（不到900万吨），因此人们强烈期望新的阴极材料取代钴。锰是锂-锰尖晶石氧化物的丰富元素之一，LiMn_2O_4也可以成功地作为锂二次电池的可逆阴极，但与LiCoO_2相比，该化合物在长期充放电循环中的稳定性不理想。在本项目中，我们的工作重点是通过用其他金属元素取代LiMn_2O_4中的部分锰离子，即LiM_yMn_<2-y>O_4，来稳定LiMn_2O_4的晶格。作为M元素，选择了与Mn-O相比对氧化离子有更强亲和力的金属，如Cr、Ni、co等。LiM_yMn_<2-y>O_4的充放电循环性能得到了显著改善。特别是当y=1/6， M=Co时，循环200次后仍能保持115 mAh/g初始放电容量的90%以上，而对于LiMn_2O_4，相同循环次数后的放电容量几乎为初始容量的60%。当M=Ni时，虽然初始容量不大（95mAh/g），但循环容量的下降幅度也很小。测定了LiM_yMn_<2-y>O_4中锂的扩散系数（D_<Li>）随锂含量x的变化。用CPR方法还发现，x=0.5处的D_<Li>值比其他x区域的D_<Li>值高两个数量级（10^<-7.5>cm^2s^<-1>），而其他x区域的锂离子被认为是有序排列的。

Lithium secondary batteries using lithium-cobalt oxide, LiCoO_2 as the cathode, and carbon materials as the anode (so called "lithium ion batteries") have been commercialized, and the production is repidly expanding. However, the natural resources for cobalt is so limited (less than 9 million tons) that new cathode materials which replace cobalt are strongly expected. Manganese is one of the abundant elements, and lithium-manganese spinel oxide, LiMn_2O_4 can also successfully work as the reversible cathode of lithium secondary batteries, however, the stability of this compound in a long term charge-discarge cycling is not satisfactory when it is compared with LiCoO_2. In the present project, we have focused our efforts on the stabilization of the LiMn_2O_4 crystal lattice by substituting other metal elements for a part of manganese ion in LiMn_2O_4, i.e., LiM_yMn_<2-y>O_4. As the M elements, such metals that have stronger affinities to oxide ions compared to that of Mn-O have been chosen, e.g., Cr, Ni, Co.Notable improvement in the charge-discharge cycling properties have been observed for LiM_yMn_<2-y>O_4. Especially, when y=1/6 and M=Co, more than 90% of the initial discharge capcity of 115 mAh/g was maintained even after 200 cycle, while in the case of LiMn_2O_4, the discharge capacity after the same cycle number was almost 60% of initial capacity. When M=Ni, although the initial capacity was not large (95mAh/g), reduction in the cycling capacity was also small. Variation of the diffusion coefficient of lithium (D_<Li>) with the lithium content x in LiM_yMn_<2-y>O_4 were also measured. It was also found by using CPR method that the D_<Li> value at x=0.5 was higher by two orders of magnitude (10^<-7.5>cm^2s^<-1>) compared with those in other x region, where lithium ions were expected to be arranged in a ordered form.