Mechanism of Performance Improvement in LiNi0.5Mn1.5-XTixO4 High Voltage Spinel Full-cells paired with Graphite and Li4Ti5O12 Negative Electrodes

The LiNi_0.5Mn_1.5O₄ (LNMO) high voltage spinel has been recognized as a tantalizing option for the cath­ode in next generation of Li-ion batteries. The combina­tion of high operating voltage (4.75 V vs Li) and ex­cellent rate capability, which arises from three-dimensional Li⁺ ion diffusion, makes this material particularly attractive for automotive applications. Recent studies, however, suggest that the most critical barrier for the com­mercializa­tion of LNMO in Li-ion batteries is electrolyte decomposition and the concurrent degrada­tive reactions at electrode/electrolyte interfaces, which consume active Li⁺ ions and reduce cycle life.[1,2] Elemental substitution for Ni and/or Mn in LNMO has been the most widely accept­ed strategy to control the crystallographic properties and stabilize the performance of high voltage spinel/Li half-cells. Despite demonstrating excellent half-cell perfor­mance, LiNi_0.5-xM_xMn_1.5O₄/graphite full-cells (x = 0.05 and 0.1, M = Fe, Co, Cu, Al, Ga, and Mg) deliver poor cycle live, similar to that of the pristine LNMO; i.e., ca­pac­ity retention after 100 cycles falls to the range of 45 – 53 %, and shows similar rates of  capacity fading ( Fig. 1).

In contrast, Noguchi et al.[3] reported that Ti-sub­stitution for Mn in LNMO improves its cycle life in full-cells paired with amorphous carbon anodes. They showed that the capacity retention of the LiNi_0.5Mn_1.5-xTi_xO₄ full-cells improved with increasing Ti content in the range from x = 0 to 0.19. In our recent work[4], this improve­ment was also observed for higher Ti contents, up to x = 0.4, as shown in Fig. 2. The LiNi_0.5Mn_1.5-xTi_xO₄/graphite full-cells also showed less oxidation of the electrolyte during cycling compared with that of Ti-free LNMO, as evi­denced by higher Coulombic efficiency and lower self-discharge rates.[4] However, the improve­ment mecha­nism of the LiNi_0.5Mn_1.5-xTi_xO₄full-cells has not been determined.

In an attempt to identify the improvement mech­a­nism operant in the LiNi_0.5Mn_1.5-xTi_xO₄ full-cells, various surface and bulk analyses of cycle-aged and/or HF-etched cathodes were undertaken, including TEM, TOF-SIMS, FT-IR, and ICP in combination with AC-impedance anal­ysis. Figure 3 shows that LiNi_0.5Mn_1.5-xTi_xO₄ particle sur­faces are Ti and O rich after etching in 1 wt% HF solu­tion, and Mn and Ni deficient. In contrast, Ti-free LNMO does not show any elemental gradient after etching under the same conditions. This result suggests that the Ti-rich surface, which is formed as a result of the Mn dissolution, may passivate the LiNi_0.5Mn_1.5-xTi_xO₄ during prolonged cy­cling. The detailed post-mortem analyses results for LiNi_0.5Mn_1.5-xTi_xO₄will be discussed in comparison with those for Ti-free LNMO to support the above hypothesis.

Since the LiNi_0.5Mn_1.5-xTi_xO₄spinel is a rela­tively promising new material, its electrochemical perfor­mance needs to be optimized by tuning chemical compo­si­tions and synthesis parameters. Therefore, we will also discuss what influence these factors have on crystal struc­ture, phase purity, and electrochemical perfor­mance.

References

[1] D. Aurbach et al., J. Power Sources, 162, 780–789 (2006).

[2] N. P. W. Pieczonka, Z. Liu, P. Lu, K. L. Olson, J. Moote, B. R. Powell, J.-H. Kim, J. Phys. Chem. C, 117, 15947–15957 (2013).

[3] T. Noguchi, I. Yamazaki, T. Numata, and K. Utsugi,

Electrochem. Soc. Meet. Abstr., 1378 (2011).

[4] J.-H. Kim, N. P. W. Pieczonka, Y. K. Sun, and B. R. Powell, J. Power Sources, in press