Characterization of Surface-Modified LiMn2O4 Electrodes Using Surface-Enhanced Raman Spectroscopy

To date, lithium-ion batteries still represent the most promising electrical energy storage option for many emerging technologies; for instance, the electrification of transportation hinges on the availability of batteries with significantly improved energy and power density, extended lifetime, and reduced cost. In particular, the positive electrode materials used in lithium-ion batteries contribute more than any other component to the overall cell cost and limit the total cell energy density for a given mass because of their substantially lower specific capacities compared to negative electrode material. Thus, many research efforts have been devoted to developing positive electrode materials with low cost, high performance, and improved safety for future lithium-ion systems.

One material of interest is LiMn₂O_4, however capacity fading in the spinel-structured oxide has been reported for many years, limiting its commercial application [1]. The primary mechanism for the observed capacity fading is commonly attributed to a chemical reaction originally proposed by J.C. Hunter in 1981. The Hunter reaction is well accepted in the literature as one of the key features of LiMn₂O₄ capacity loss in LiPF₆ containing carbonate electrolytes, wherein LiPF₆ can react with adventitious water in the electrolyte to form highly corrosive HF. Direct observations of the LiMn₂O₄ electrode after electrochemical cycling have been made which support the Hunter reaction. For example, spontaneous λ-MnO₂ formation after storing LiMn₂O₄ films in an LiPF₆ containing carbonate electrolyte at elevated temperatures was observed using ex situ surface-enhanced Raman spectroscopy (SERS) [2]. Applying passivating coatings on the surface of the spinel active material is one approach to limit electrode capacity fading. Specifically, layers of amphoteric oxides, particularly ZrO₂, TiO₂ and Al₂O₃, have been shown empirically to be very effective [3].

Very little has been reported on the role of ampohteric oxides toward reducing capacity fading of LiMn₂O₄ positive electrodes, and while it has been suggested that they act as local getters of HF in the electrolyte, their influence on the surface chemistry of modified electrodes has not been specifically investigated. Recently, our group has developed electrochemically stable, SERS-active Ag@SiO₂ core-shell nanoparticles via a solution route [4] which dramatically enhanced the sensitivity to surface species and incipient phases on the electrode surface at temperatures up to 450 °C. This presentation will highlight the application of in operando SERS to probing the electrode/electrolyte interfaces of thin-film LiMn₂O₄ model electrodes with and without a thin layer of amphoteric oxide. Development of advanced surface characterization techniques such as in operandosurface-enhanced Raman spectroscopy will enable us to directly correlate the surface chemistry and structure with electrochemical performance, offering information vital to the rational design of these devices for future high power, long lifetime lithium-ion batteries.

[1] G. Amatucci, J.M. Tarascon, J. Electrochem. Soc., 149 (2002) K31-K46.

[2] Y. Matsuo, R. Kostecki, F. McLarnon, J. Electrochem. Soc., 148 (2001) A687-A692.

[3] J.S. Kim, C.S. Johnson, J.T. Vaughey, S.A. Hackney, K.A. Walz, W.A. Zeltner, M.A. Anderson, M.M. Thackeray, J. Electrochem. Soc., 151 (2004) A1755-A1761.

[4] X. Li, J.-P. Lee, K.S. Blinn, D. Chen, S. Yoo, B. Kang, L.A. Bottomley, M.A. El-Sayed, S. Park, M. Liu, Energy Environ. Sci., (2013).