Implementation of Lithium-Rich Layered-Layered Oxides As Cathodes: Thermal Stability Insight
Authors: Geder Jan , Yu Denis Yau Wai , Garche Juergen 
Published: February 2020

Lithium-rich layered-layered oxide materials aLi[Li1/3Mn2/3]O2·(1-a)Li[NixCoyMn1-x-y]O2 (LMO-NCM) are seen as promising candidates for cathode material in next generation Li-ion batteries [1]. This is due to their high specific capacity (> 250 mAh g-1) [2] and low price due to lower cobalt content. On the other hand, the major obstacles for implementation of these materials are high irreversible capacity in first cycle, voltage fade upon cycling due to structural changes, and poor rate capability [1].

For cathode material, thermal stability is particularly relevant as it influences safety. In addition to well-known tendency of layered oxides to decompose exothermally in charged state and emit O2 [3], the oxygen in above mentioned group of materials also participates in charge-discharge reaction [4]. This raises additional safety concerns. We have shown for 0.5Li[Li1/3Mn2/3]O2·0.5Li[Ni1/3Co1/3Mn1/3]O2 that presence of oxidized oxygen in lattice after first cycle increases decomposition enthalpy of the cathode [5]. On the other hand, reversible oxygen and manganese participation in reactions of first cycle also stabilize the structure, which results in a higher onset of decomposition reaction [5].

It has been found that during the activation of Li[Li1/3Mn2/3]O2 (LMO) component in the first cycle, oxygen is emitted from the surface of the material, and oxygen in the bulk is oxidized to compensate for the removal of lithium [6]. Furthermore, the crystal structure changes while the material is held at higher voltage for longer time [7]. These phenomena affect not only performance, but also thermal stability. Knowing that the LMO component decisively impacts the thermal stability of the cathode, the effects of cycling regime, cycling history, ratio between LMO/NCM component, and active material particle size/surface area on thermal stability are investigated.

In this research, cathodes are made using LMO-NCM as active material. They are built into half-cells vs. lithium metal with 1 M LiPF6 in flouroethylenecarbonate/diethylcarbonate (1:1) as electrolyte. Cells are conditioned electrochemically at certain current. At certain state of charge, the cathodes are taken out, washed and dried. Thermal stability is investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Evolved gasses are characterised by mass spectrometry. Stability of cathode-electrolyte system is addressed by accelerating rate calorimetry (ARC).

Results of the investigations will be presented at the meeting and are expected to elucidate the safety behaviour of LMO-NCM cathodes and help define the required synthesis conditions and microstructure of the cathode active material with regards to safety. Furthermore, appropriate charging regime and cut-off voltage can be derived from the findings.

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