Research in rechargeable lithium ion battery (LIBs) systems has recently focussed on improving the operating voltage, specific capacity as well as safety. In this regard, nickel substituted spinel (LiNi_0.5Mn_1.5O₄, LNM) is attractive as a cathode with an operating voltage of ~4.7V vs. Li/Li⁺. Nickel substitution in spinel LiMn₂O₄ was initially performed with the aim of stabilizing the material against phase transformation that arose due to co-operative Jahn-Teller distortion of the lattice.  But it was discovered that the Ni²⁺/Ni⁴⁺ redox couple was active at a potential of ~4.7 V in a reversible manner[1]. This generated intense interest in this material. Although the material has proven to be robust and maintains structural stability during cycling, the existing electrolytes are unable support such high operating voltages and continue to remain a hurdle in the development of batteries based on this spinel. In this work, we are interested in understanding another redox couple based on Mn³⁺/Mn⁴⁺ which operates at ~2.8V vs Li/Li⁺ [1,2]. This redox couple has not been explored in detail and understandably so due to the attractive high voltage characteristics offered by the nickel redox couple.  But the operating potential (~2.8V vs Li/Li+) makes it an ideal candidate as anode for aqueous rechargeable lithium battery systems (which would operate at ~ -0.24V vs SHE). Furthermore, the effect of co-operative Jahn-Teller distortion that occurs due to the average manganese valence reducing below 3.5 is minimized to a significant extent compared to the counterpart spinel LiMn₂O₄. It would be possible to insert approximately 0.75 mols of lithium before the average manganese valence reaches 3.5. This would give it a capacity of roughly 110 mAh.g^-1. However there are only a few studies that investigate systematically the electrochemical performance of the Mn³⁺/Mn⁴⁺ redox couple in LNM.

In this work, we have investigated the effect of synthesis method and conditions on the electrochemical performance of LNM while storing and extracting lithium using the manganese redox couple in non-aqueous electrolytes. It was found that synthesis conditions had a significant effect on the specific capacity that could be obtained for the manganese redox couple. Lower temperatures of synthesis resulted in higher capacities. This is in contrast to the performance of the nickel redox couple where higher temperatures offered better capacities. Cyclic voltammetry, galvanostatic cycling and impedance measurements were performed to understand the origin of the differences in the behaviour. More results will be presented at the meeting.

References

[1]         K. Amine, H. Tukamoto, H. Yasuda, Y. Fujita, J. Power Sources. 504 (1997) 604–608.

[2]         K. Amine, H. Tukamoto, H. Yasuda, et al., J. Electrochem. Soc. 143 (1996) 1607–1613.