High Voltage Spinel Lmno/Lto Li-Ion Batteries: Ageing Processes and Role of Electrolyte Additives on Interfaces

The combination of high voltage spinel oxides LiMn_2-xNi_xO₄ (LMNO, that can be charged up to 5V vs. Li⁺/Li) with Li₄Ti₅O₁₂ (LTO, 1.5V vs. Li⁺/Li) is a very attractive solution to obtain Li-ion systems with a high power density and an increased safety with respect to systems using carbon negative electrodes [1,2]. LTO authorizes higher charge rates without any problem of lithium dendrites formation, which is suitable for electric vehicles (EVs) or hybrid electric vehicles (HEVs) for instance.

However, conventional electrolytes made up of LiPF₆ salt dissolved in a mixture of carbonates are not stable above 4.2 - 4.5V vs. Li⁺/Li and lead to a reactive cathode/electrolyte interface at high voltage [3]. This interfacial reactivity results in capacity fading and fast self-discharge of LMNO/LTO batteries, preventing up to now the commercial use of these systems [4,5]. To reduce this electrode/electrolyte reactivity at the cathode surface and to improve the electrochemical performances, it is possible to introduce additives in the electrolyte in order to stabilize interfaces. In this work we have focused on an organic additive: glutaric anhydride. A small amount (2 %) of this additive was found to show a beneficial effect by reducing the self-discharge and improving the cycle life and rate capability of these systems.

To better understand the ageing processes in these systems and explain the beneficial role of this additive, we have investigated the cathode and anode/electrolyte interfaces by two complementary techniques: X-ray Photoelectron Spectroscopy (XPS) and Electrochemical Impedance Spectrocopy (EIS), which allow a chemical and an electrochemical analysis of the interfaces, respectively.

Our study has shown that, whereas LTO is usually considered as a passivation-free electrode material, large amounts of organic and inorganic species are deposited at the surface of this electrode upon cycling in a conventional electrolyte, due to a cross-talk process with the LMNO cathode.

The additive was found to significantly change the chemical composition of the passivation layer at both electrodes surfaces. In this case a large amount of oxygenated organic species accumulates at the surface of the cathode (and also at the anode side) upon cycling or storage in charged state. After long cycling, the characteristic XPS signals of LMNO strongly decrease, showing that the electrode's surface has been covered by a thick layer coming from degradation species of the additive (see figure 1). EIS electrical measurements support a more covering/homogenous and ionically conducting surface film on the cathode when the additive is used, suggesting that the passivation film behaves like a Polymer Electrolyte Interface (PEI).

The deposition of this surface film is accompanied by an improvement of the electrochemical performances, and does not hinder Li⁺ ions diffusion but decreases the interfacial reactivity of the electrode towards the electrolyte.

Moreover, the deposition of a thick layer was also observed at the surface of LTO negative electrode, but the nature of the oxygenated species is different, as shown by XPS spectra. This shows that two different mechanisms of degradation of the additive are involved at the cathode and the anode.

References

[1] M. Imazaki, K. Ariyoshi, T. Ohzuku, J. Electrochem Soc., 2009, 156, A780

[2] A. Du Pasquier, C. C. Huang, T. Spitler, J. Power Sources, 2009, 186, 508

[3] K. Kanamura, T. Umegaki, M. Ohashi, S. Toriyama, S. Shiraishi, Z. Takehara, Electrochim. Acta, 2001, 47, 433

[4] S. Patoux, L. Daniel, C. Bourbon, H. Lignier, C. Pagano, F. Le Cras, S. Jouanneau, S. Martinet, J. Power Sources, 2009, 189, 344

[5] R. Dedryvère, D. Foix, S. Franger, S. Patoux, L. Daniel, D. Gonbeau, J. Phys.Chem. C, 2010, 114, 10999