Self-Discharge Phenomena in LiCoPO4 Electrodes

Polyanion-type materials, like lithium iron phosphate, are one of the recent great success in the research field of applied electrochemistry. Within this family, LiFePO₄ (LFP) is a now mature material and its properties have been largely optimized thus opening the door to its commercial exploitation as cathode material in lithium-ion cells for power tools. In recent years the attention of the scientific community is focusing the great advantage of the substitution in the LFP lattice of Fe with Mn, Co or Ni. In fact the Mn³⁺/Mn²⁺, Co³⁺/Co²⁺ and Ni³⁺/Ni²⁺ couples show increasing redox potentials, all higher than the Fe³⁺/Fe²⁺, thus opening the door to large improvements in the energy performances. Among them LiCoPO₄(LCP) is the best compromise: it shows in lithium cells an higher working potential compared to LFP, still within the working limits of the carbonate-based liquid electrolytes. However many fundamental aspects of the LCP properties and reaction mechanisms in lithium cells are not completely understood. Owing to this, large room for improvements in the performances in Li-cells of LCP is expected by assessing the synthesis route, the coatings, the doping/substitution and the electrolyte additives.

The aim of this communication is to present our recent results concerning the study of one of the most detrimental phenomenon observed in the use of LCP in lithium-ion cells: self-discharge.

Upon charge in lithium cells LCP undergoes to the following reversible two step oxidation:

LiCoPO₄ → Li_0.7CoPO₄+0.3Li⁺+0.3e^- → CoPO₄+ Li⁺ + e^-

At the end of charge, in absence of flowing current, the self-relaxation of the charged LCP electrode is accompanied by a change in the OCV very similar to the discharge curve at constant current. Apparently prolonged OCV relaxation leads to the re-incorporation of lithium ions into the electrode that can be newly electrochemically charged. Self-discharge is a spontaneous phenomenon and has big technological consequences as it limits the use of LCP in commercial electrodes.

Here we discuss the results of our study of LCP self-discharged electrodes by ex-situ synchrotron powder diffraction, synchrotron infrared spectroscopy and transmission electron microscopy. Self-discharge apparently occurs by the following spontaneous reactions in OCV conditions:

CoPO₄ + 0.7 Li⁺+electrolyte (reduced form) → Li_0.7CoPO₄ + electrolyte (oxidized form)

Li_0.7CoPO₄ + 0.3 Li⁺+electrolyte (reduced form) → LiCoPO₄ + electrolyte (oxidized form)

The two self-discharge steps are accompained by accumulation of electrolyte decomposition by-products on the electrode surfaces.