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Surface Coating of 5V Spinel LiNi0.5Mn1.5O4 Cathodes By Carbon Materials for Li-Ion Batteries Applications
As applications of Lithium ion batteries (LIBs) are focused, high energy density active materials are needed. Therefore, LiNi0.5Mn1.5O4(LNMO) is one of the promising active materials for LIBs because of it has high working voltage about 4.7 V and good cyclability. Carbon coating is one of the best methods to prevent surface side reactions of electrolyte. However, oxide cathode materials are usually synthesized at high temperature, so that carbon coating is limited because carbon can be oxidized easily. To overcome this problem, we performed re-oxidized process to make RO-MWCNTs. We expected this can assign resistivity against oxidation to MWCNTs. Continually, RO-MWCNTs are coated on LNMO by solid-state coating method. Several electrochemical tests have performed to identify effects of RO-MWCNTs coating on LNMO powder.
Experimental
To prepare RO-MWCNTs, pristine MWCNTs were reduced at 900℃ for 6 hours under Ar gas which contains 5wt% of H2. Reduced MWCNTs are re-oxidized at 300℃ for 2 hours. As prepared MWCNTs and RO-MWCNTs have coated 1wt% on LNMO by the solid-state coating at Hosokawa. Proportion of coated materials were specified by TGA and EA. Electrode slurry has been made of 96% of active materials, 2% of Denka black, 2% of PVdF in NMP solvent. Total amounts of conductive materials and coated MWCNTs are fixed for 2%. The slurry was casted on Al foil for current collector and assembled with 2032 coin cell.
Results and discussion
Figure 1. shows rate capability test of pristine, MWCNTs coating, RO-MWCNTs coating LNMO samples. C-rates were changed at various rates. According to rate capability test result, RO-MWCNTs coating has improved electrical conductivity of electrode.
Figure 2. shows cyclability test of samples which had been stored at 60℃ oven for 7 days. We could observe RO-MWCNTs coated sample has much better capacity retention than MWCNT coated and pristine samples. We could identify that re-oxidized process have affected MWCNTs to resist against oxidation at high voltage.