Stability of Carbon Coating Surrounding LiMPO4 (M=Fe, Co) Particles in Water Free and Water Containing Electrolytes

LiMPO₄ (M=Fe, Co) materials are known as possible high voltage materials leading to high energy density batteries. LiFePO4 is electrochemically active below 4V vs. Li/Li⁺ with a theoretical capacity of 170 mAh/g, and it is well known for its good stability upon cycling [1]. Conversely, LiCoPO₄, which works at higher potential (4.85 V), exhibits poor performance on cycling in common alkyl carbonate electrolytes [2]. In the literature, two hypotheses can be found which try to explain the low stability of LiCoPO₄: the Li salt is corrosive for the material [3] and/or the electrolyte oxidation at high voltage plays a role. Tsiouvaras et. al. [4] have shown that CO₂ is the main gas evolved by the anodic oxidation of water free alkyl carbonate based electrolytes (e.g. PC) at high electrode potentials (≥ 4.5 V vs. Li/Li⁺). Moreover, a recent study by our group [5], has demonstrated that CO₂ is already evolved at lower potentials if the electrolyte contains traces of water.
In this work we will investigate the evolution of CO₂ at high electrode potentials (≥ 4.5 V vs. Li/Li⁺) for LiFePO₄/C and LiCoPO₄/C by on-line electrochemical mass spectrometry (OEMS). It can be due to the electrochemical oxidation of the electrolyte and/or the carbon, according to the well-known reaction:
C + 2H₂O → CO₂ + 4H⁺ + 4e^- (1)
Both materials are synthetized in house by a solid state route for LiCoPO₄ and a solvothermal route for LiFePO₄.
Firstly, a preliminary experiment on LiFePO₄/C is done in order to study the stability of the carbon coating of LiFePO₄ at high potentials. Glucose ¹³C is used as carbon precursor in the LiFePO₄ synthesis to obtain an isotopically labelled carbon coating. In this case, the signal from the electrolyte decomposition and from the carbon of the electrode can be deconvoluted. By adding a defined amount of water (4000 ppm) to the electrolyte one can mimic the effect of trace water that could unintentionally be introduced to the cell through the active materials.
Secondly, in order to compare the stability of carbon coating from LiFePO₄ and LiCoPO₄, isotopically labeled ¹³C electrolyte (1M LiPF₆ in labelled DMC) is used to deconvolute the formation of CO₂ from the carbon and the electrolyte. To distinguish carbon from the electrode and from the coating of particles, the electrode is designed with ¹³C carbon instead of the common Super P (Timcal) as conductive additive. The potential limit of the charge is fixed to 5.5 V to allow comparison on carbon coating stability between both, LiCoPO₄ and LiFePO₄.

References:
[1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough – J. Electrochem. Soc. 144, 1188 (1997)
[2] N. Bramnik, K. Bramnik, T. Buhrmester, C. Baehtz, H. Ehrenberg, H. Fuess - J. Solid State Electrochem. 8, 558 (2004).
[3] E. Markevich, R. Sharabi, H. Gottlieb, V. Borgel, K. Fridman, G. Salitra, D. Aurbach, G. Semrau, M. A. Schmidt, N. Schall, C. Bruenig – Electrochem. Comm. 15 (2012) 22.
[4] N. Tsiouvaras, S. Meini, I. Buchberger, and H. A. Gasteiger, J. Electrochem. Soc., 160, A471 (2013).
[5] R. Bernhard, S. Meini, and H. A. Gasteiger, J. Electrochem. Soc., 161, A497 (2014).

Acknowledgements:
This work is financially supported by BMW AG.