Comparing the Effect of Oxygen Stoichiometry on the Behavior of Li2MnO3 in Pure Phase and Li-rich Forms

Lithium ion battery is a popular storage solution but, despite extensive research, only a handful of chemistries have been successfully commercialized. In this work we present a technique to rapidly investigate composite cathode schemes like the Li-rich chemistry.

       Various thin film deposition techniques have been used to prepare thin film Li ion batteries [1]. In addition to all solid state batteries, the flexibility of a pulsed laser deposition method allows for a variety of detailed investigations. We had previously evaluated single phase LiNiMnCoO₂ (NMC) cathode using a one step deposition process [2]. Recently, cathodes which combine two components are gaining popularity.  Manganese based chemistries are especially attractive due to its low cost and environmentally friendly nature. LiMO₂-Li₂MnO₃ is an attractive material with almost twice the capacity, of most commercial materials [3]. Various aspects of its operation like microstructure, electrochemical activity, phase as well as new combinations take time consuming experiments to decipher. Moreover, traditional solution based synthesis techniques, makes it difficult to isolate variables and design cathodes to conduct focused studies. We report several experimental models we synthesized to study aspects like micro-structure, phase and stiochiometry effects of Li-rich cathodes and its components.

 Pulsed laser deposition is a non-equilibrium technique that uses laser ablation to break down target materials into ionic, atomic and molecular species which is then deposited on the substrate. At high temperatures, we believe this should allow for a thermodynamically favorable phase to form. We employed this concept to design deposition process that should theoretically allow us to fabricate and test both the two phase and single phase models proposed by different researchers. This was achieved by using multiple targets to grow 2-phase cathodes and composite targets to simulate the single phase cathode [4]. To investigate the role of O and Li removal on electrochemical activity and activation of Li₂MnO₃, we varied O and Li stiochiometry of the cathode during deposition. Unlike acid etching this is done during the cathode synthesis step, allowing additional flexibility. The correlation between composition variation, microstructure and electrochemical behavior was investigated with cathodes deposited using two separate targets of Li₂MnO₃and NMC.

Electrochemical testing showed that an optimum performance was obtained for films with nano-domains of Li₂MnO₃ in a NMC matrix with a discharge capacity of 290 mAh/g. Li₂MnO₃  domain size clearly plays an important  role in the mechanism that  results in the high capacity of the Li-rich chemistry. Oxygen and Li stiochiometry variations in Li₂MnO₃ showed that an optimum capacity of  225 mAh/g was obtained for cathodes deposited at 50 mtor partial oxygen pressure. Samples with very low Li and O content showed formation of MnO₂  phase with lower capacity (115 mAh/g) and distinct absence of any redox peak at 3 V which were seen for samples with higher O and Li content. The effect of composition variation on domain size and electrochemical properties is critical for the performance of the Li-rich cathodes. 

 Thin film based model cathode synthesis approach allowed us to investigate different aspects of the Li-rich cathodes in a rapid and efficient manner. It can also be used to investigate new material schemes in a systematic manner.

References

[1] S. J. Dillon, et al., Curr. Opin. in Solid State and Mater. Sci., 2012, 16, 153.

[2] C. Jacob et al., Journal of Power Sources, 2013, 241, 410.

[3] D.Y. W. Yu et al., J. Electrochemical Soc., 2009, 156 , 6, A417.

 [4] C. Jacob et al., J. Mater. Chem. A, 2014, 2, 2283.