Studying the Effects of Carbon Coatings on the Electrochemical Performance of LiNi_1/3Co_1/3Mn_1/3O₂

Abstract

Over the past few years LiNi1/3Co1/3Mn1/3O2 (NCM) has become the material of interest in lithium ion batteries because of its low cost, low toxicity and higher energy density than the commercial LiCoO2. NCM can achieve practical capacitiy of ~170 mAh/g at room temperature up-to 4.3 V vs Li/Li+ cut-off potential. If a cut-off potential of more than 4.3 V vs Li/Li+ is applied to achieve a higher discharge capacity, structural instability of the layered NCM structure suffers mechanical stress and phase changes. This deteriorates the crystal structure and causes oxygen release. Furthermore, high cut-off potentials facilitate the dissolution of metal ions into the electrolyte. The dissolved ions can migrate through the separator to the anode and deposit at the surface which worsens the anode performance and cause accelerated capacity fading.(1, 2) If the cell is exposed to higher temperatures, the electrolyte starts to decompose and the products lead to further reactions with the NCM. This results in deterioration of the cycling performance and rate capability. To overcome these issues, different methods and techniques have been applied by different research groups such as doping, coating or addition of additives in electrolytes. Herein, coating of the active material has an advantage because it can stop the direct contact between cathode and electrolyte. This is known to stop dissolution of ions into the electrolyte as well as other side reactions.(3) Different materials have been coated on NCM such as oxides (TiO2, ZrO2, Al2O3), lithium containing compounds (LiFePO4, Li3PO4), carbon and solid state electrolytes (LiTaO3).(4) Among those, a carbon coating can increase the electronic conductivity and improve the C-rate performance while allowing sufficient lithium-ion diffusion through the macro-porous structure.(5) In addition, the performance at higher temperatures as well as improved Coulombic efficiencies during formation might be realized with such coatings. To realize a carbon coating, pyrolysis of e.g. sugar or poly-vinyl alcohol have been used in air atmosphere. Pyrolysis in inert atmosphere leads to the decomposition of the active material due to oxygen release from the NCM structure and the subsequent reaction with the carbon precursor, which happens at temperatures below 400 °C. There are other studies which used physical methods such as microwave plasma chemical vapor deposition (MPCVD) which coated crystalline carbon on NCM. To the best of our knowledge, there is no electrochemical study on carbon coated active material at temperatures higher than room temperature. In this study a carbon coating on NCM was realized using a hydrothermal method. Stirring was applied to achieve a uniform precursor coating from a formalin-phenol polymer reaction. Post-annealing of the organic precursor coating was used for carbonization. Different temperatures for post-annealing were used to combust the organic precursors in air in order to create different characteristics of the coating with regard to coating thickness and surface chemistry. The synthesized powders were analyzed with regard to their thermal properties during annealing, surface morphology and chemistry, as well as coating thickness, and the possible influence on the active material by means of thermogravimetric analysis (TGA), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and powder X-ray diffraction (XRD). Electrochemical performance was investigated in Li-metal cells using pristine NCM material as reference. The thickness and properties of the coating vary with the annealing temperature which consequently affected the electrochemical performance. Therein, higher annealing temperatures improved the combustion of the organic precursor resulting in a decreased coating thickness and lower internal cell resistance. Here, annealing temperatures higher than 1000 °C could not be applied due to irreversible degradation of the active material. To study the effect on the formation Li-ion cells were build and the Coulombic efficiencies were compared by cycling the cells to an upper cut-off voltage of 4.3 V at different temperatures. Figure 1