Spinel-type LiNi_0.5Mn_1.5O₄ (LNMO) have been attracted many attentions as a cathode material for 5V-level lithium-ion secondary batteries. LNMO is possibly crystalized into two different atomic arrangements (space groups). These two differences showed different characteristics. Ni/Mn ordered P4₃32 offers high specific energy density and disordered Fd-3m provide high electron conductivity. The origin of their electron conductivity in the Fd-3m type LNMO is strongly connected with the oxygen deficiencies in the spinel lattice, which is accompanying the reduction of Mn⁴⁺ to Mn³⁺. In order to realize electron conductivity enhancement of the P4₃32 type LNMO, we performed both theoretical and experimental studies on phase stabilities of metal-doped oxygen-deficient LiNi_0.5Mn_1.5O_4-δ crystalized in spinel lattices and corresponding electrochemical properties.

We point out that LiNi_0.5Mn_1.5-yCu_yO_4-δ is the most plausible candidate which provides stable P4₃32 phase with oxygen deficiency. Computational studies of difference in the defect formation energies between P4₃32 and Fd-3m revealed that the Fd-3m became lower than that in P4₃32. Especially, the partial substitution of Cu²⁺ with Mn⁴⁺ effectively contributed to reduce oxygen vacancy formation energy in P4₃32, comparing to that of Fd-3m. In addition, Mn³⁺ is absent when oxygen vacancy concentration is equivalent to Cu²⁺ concentration, even though oxygen deficiency was formed in the spinel lattice. P4₃32 type LiNi_0.5Mn_1.5-yCu_yO_4-δ potentially responsible to offer no appearance of plateau at 4 V in the charge-discharge profiles, no dissolution of Mn³⁺ species during the charge-discharge cycling and improve electronic conductivity due to the d-electron doping by Cu²⁺ substitution.

Based on our theoretical calculation, we performed the flux growth of LiNi_0.5Mn_1.5-yCu_yO_4-δ crystals. X-ray diffraction pattern and Raman spectroscopy revealed that the substation of Cu²⁺ highly stabilized the P4₃32 type LiNi_0.5Mn_1.5-yCu_yO_4-δ. The galvanostatic charge-discharge tests at 25 ̊C, ranging from 3.5 and 4.8 V of cut-off voltage showed the appearance of flat plateau at 4.7 V attributed to Ni^2+/4+ redox couple. The discharge capacities were 137 mAh g^-1 at 0.2C rate and 100 mAh g^-1 at 10C rate, respectively. Furthermore, no current peaks from the Mn^3+/4+ redox couples were observed at 4 V by cyclic voltammetry measurements. These findings strongly suggest that the substitution of Cu²⁺ with Mn⁴⁺ LNMO stabilize P4₃32 type structure even though the oxygen deficient may contain in the spinel lattice.

Acknowledgement

This work was partially supported by CREST, JST.