Stability of Carbon Coating Surrounding LiMPO4 (M=Fe, Co) Particles in Water Free and Water Containing Electrolytes
Authors: Metzger Michael , Marino Cyril , Haering Dominik , Kavakli Cüneyt , Alarcon Carlos , Puravankara Sreeraj , Nilges Tom , Stinner Christoph , Gasteiger Hubert A. 
Published: February 2020

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.