Surface treatment of LiNi0.8Co0.2O2 cathode material for lithium secondary batteries
Introduction
The high voltage cathode materials that are being used in lithium secondary batteries are mainly the well-known LiCoO2, LiNiO2, LiMn2O4 and their derivatives [1]. In the batteries containing non-aqueous electrolyte, the cathode materials are contacting directly with the electrolyte. Some unwanted side reactions will occur at the interface, deteriorating the performance of the cathode materials and batteries [2].
Using the high voltage cathode materials, the cells must be charged up to voltages greater than 4 V so as to utilize their full capacity. At such high voltages, the electrolyte will be at the risk of being oxidized and decomposed. Fully charged, fairly delithiated positive electrode materials are strong oxidants, acting as catalytic agents toward electrolyte decomposition. The charged cathode materials are unstable because a large number of Li+-ions have deintercalated from their crystal lattice, resulting in the phase transformation, active materials decomposition and dissolution. The non-aqueous electrolyte will corrode the cathode materials and act as a catalytic agent toward charged cathode materials deterioration. The harmful interactions between the cathode materials and electrolyte will create deleterious by-products, such as insulating passive film, gases, etc. The side reactions will cause the self-discharge, irreversible capacity, capacity degradation, and unsafety within the lithium secondary batteries, especially at elevated temperature.
In order to minimize the side reactions between the cathode materials and electrolyte, the surface of the cathode contacting with the liquid electrolyte must be reduced. Amatucci and coworkers [3], [4] presented a surface treatment concept, in which a protective thin film was coated on the surface of the spinel LiMn2O4. The physical barrier between the oxidizing spinel and the electrolyte can reduce the area of electrolyte/cathode interface, protect the electrolyte from the catalytic effects of the LiMn2O4, and protect the LiMn2O4 from corrosion by the non-aqueous electrolyte. According to their reports, surface treatments are successful in improving the elevated temperature performance of spinel LiMn2O4. They also pointed out that the use of surface treatments might offer advantages to the other intercalation compounds such as LiCoO2 and LiNiO2 which were also suffering from catalytic process with the electrolyte in the charged state at elevated temperature similar to those observed with spinel LiMn2O4. However, the surface treatments of LiCoO2, LiNiO2 and their derivatives are rarely reported.
The layered LiNi0.8Co0.2O2 cathode material is considered as strong potential candidate to replace the actually commercialized LiCoO2 because of its attractive advantages, such as lower cost and higher reversible capacity than LiCoO2, easier preparation and better cycling stability than LiNiO2, etc. [5]. Before LiNi0.8Co0.2O2 being put into practice, the material’s comprehensive properties, especially at elevated temperature and charged state, should be further improved. It is expected that the surface treatment is an effective way.
According to the reports [3], [4] of Amatucci and coworkers, lithium boron oxide (LBO) glasses are particularly suitable for surface treatment. There are several reasons. The first of these is that molten LBO compositions exhibit good wetting properties with respect to the ceramics. The combination of good wetting properties and relatively low viscosity in the molten state allow easy processing and result in even coverage with the use of a minimal amount of material. Secondaly, LBO compositions have already been investigated as solid lithium ionic conductors which exhibit good ionic conductivity [6], [7]. Furthermore, electrochemical studies have shown that these materials are stable against the high oxidation potentials of the 4 V positive electrode materials used in Li-ion batteries today. Finally, such materials have processing temperatures consistent with those of LiNi0.8Co0.2O2.
In this paper, the composition of the coating is chosen as Li2O–2B2O3 glass. The preparation, structure and electrochemical performance of the surface-treated LiNi0.8Co0.2O2 cathode material have been discussed in comparison with the base one in detail.
Section snippets
Experimental
For the synthesis of base LiNi0.8Co0.2O2 powders used throughout this study, LiOH·H2O, Ni(OH)2, and Co(OH)2 were mixed in a stoichiometric ratio and heated to 750°C for 16 h in oxygen. The approach for applying coatings was to dissolve H3BO3 and LiOH·H2O (H3BO3/LiOH·H2O=2.0, molar ratio) in methanol. The base LiNi0.8Co0.2O2 powders were then added to the solution and the mixture was stirred and gently heated to 70–80°C until the solvent was completely evaporated. The powders were then heated at
Results and discussion
The surface composition of the base and surface-treated LiNi0.8Co0.2O2 powders was determined by XPS. The results in Table 1 show the surface composition of the base LiNi0.8Co0.2O2 is very close to the stoichiometric ratio (Li:Ni:Co:O=25:20:5:50, molar ratios). However, B and Li are enriched on the surface of the surface-treated LiNi0.8Co0.2O2, although the LiNi0.8Co0.2O2 powders were only coated with 1 wt.% of Li2O–2B2O3, as indicated in Table 1. It follows that the Li2O–2B2O3 does not solute
Conclusions
The use of surface treatment is an effective way to improve the comprehensive performance of LiNi0.8Co0.2O2 cathode material for lithium secondary batteries. The surface treatment is successful in minimizing the harmful side reactions within the batteries by placing a protective barrier layer between the oxidizing cathode material and the liquid electrolyte. The surface-treated LiNi0.8Co0.2O2 cathode material has the advantage of higher charge–discharge capacity, lower irreversible capacity,
Acknowledgements
This study is supported by the National Natural Foundation of China (Project no. 50002006) and the Basic Research Foundation of Tsinghua University (JC1999054).
References (7)
- et al.
On the LixNi0.8Co0.2O2 system
J. Solid State Chem.
(1998) - et al.
The structure and conductivity of binary and ternary glasses (B2O3)1−x−y(Li2O)x(Li2Cl2)y
J. Non-cryst. Solids
(1987) - R. Koksbang, J. Barker, H. Shi, et al., Cathode materials for lithium rocking chair batteries, Solid State Ionics 84...