$Li_{3-x}Co_xN$ has been reported to be the most promising candidate which can be substituted for graphite. However, it has an unsatisfactory cycle life and the research on the degradation mechanism has not been done. Therefore, this work was focused on the manifestation for degradation mechanism of cycle life of $Li_{3-x}Co_xN$ and its improvement. In order to optimize the composition and synthetic temperature of $Li_{3-x}Co_xN$, the charge/discharge cycling test for $Li_{3-x}Co_xN(0.2 \le x \le 0.6)$ synthesized at various temperature(600∼800℃) was performed. As a result, $Li_{2.6}Co_{0.4}N$ synthesized at 650℃ showed the best anode performance. It showed very high capacity of 1024mAh/g, good rate capability(1C/0.2C = 94.94%) and extraordinary initial Coulometric efficiency of 96%. In addition to the excellent capacity of this material, its rate capability was much superior to that of graphite(82∼88%). However, it can`t be commercialized because its capacity loss after 30 cycles is around 40%. Furthermore, the degradation mechanism of its cycle life has not been manifested yet. Thus, in order to demonstrate the degradation mechanism of $Li_{2.6}Co_{0.4}N$, diverse analyses for surface and structure of $Li_{2.6}Co_{0.4}N$ were carried out before and after cycling. XRD patterns after 1 and 100 cycles showed the amorphization of structure in $Li_{2.6}Co_{0.4}N$ system and it was confirmed by TEM diffraction patterns. While the degradation of cycle life in $Li_{2.6}Co_{0.4}N$ advanced gradually, the change of structure after 1st cycle wasn`t observed. This phenomenon implies that the change of structure in $Li_{2.6}Co_{0.4}N$ is not the cause of its cyclic degradation. From EDS and ICP analyses, the dissolution of cobalt and nitrogen from $Li_{2.6}Co_{0.4}N$ into the electrolyte wasn`t detected. It indicates that the dissolution of $Li_{2.6}Co_{0.4}N$ into the electrolyte doesn`t cause the cyclic degradation of $Li_{2.6}Co_{0.4}N$, either. When EIS analyses of $Li_{2.6}Co_{0.4}N$ were performed after 20, 50 and 100 cycles, two semicircles related to the surface film and Li absorption/desorption respectively increased very much.Especsially, the first semicircle that represents the formation of surface film increased to approximately 243%. Through SEM analysis, the formation of thick surface film after cycling was confirmed. AES depth profile notified that the surface film on $Li_{2.6}Co_{0.4}N$ consisted of fluorine and phosphorus which are evolved from the electrolyte decomposition. The film was characterized as $CoF_2$ by XRD. Becuase $CoF_2$ has low electron conducticity and inactivity, these properties are thought to induce the cycle degradation of $Li_{2.6}Co_{0.4}N$. Based on this study, iron doping in $Li_{2.6}Co_{0.4}N$ was tried to restrain the formation of $CoF_2$ film on the surface of $Li_{2.6}Co_{0.4}N$. $Li_{2.6}Co_{0.36}Fe_{0.05}N$ had a little lower capacity(about 900mAh/g) than $Li_{2.6}Co_{0.4}N(1024mAh/g)$, but showed much better cycle life than $Li_{2.6}Co{0.4}N$(35\%\to\60% after 50 cycles). SEM analyses showed that the formation of surface film on $Li_{2.6}Co_{0.35}Fe_{0.05}N$ decreaced. It could be confirmed by AES depth profile which showed that after cycling, the quantity of fluorine and phosphorus on $Li_{2.6}Co{0.36}Fe_{0.05}N$ was much lower than that on $Li_{2.6}Co_{0.4}N$. From the Nyquist plot of EIS, it was found that iron doping decreased the resistance of the first semicircle related to the surface film. This abrupt decrease of the first semicircle in $Li_{2.6}Co{0.36}Fe_{0.05}N$ system electrochemically proved that iron doping plays an important rule in restraining the formation of $CoF_2$ on the surface. In short,the degradation of cycle life of $Li_{2.6}Co_{0.4}N$ is due to inactive surface. film($CoF_2$) formed by the reaction between electrolyte and $Li_{2.6}Co_{0.4}N$. By doping iron in $Li_{2.6}Co_{0.4}N$, its cycle life could be improved.
