Supporting Information

Improved electrochemical performance of NaAlO2-coated LiCoO2 for lithium ion batteries

Bin Shen∙PengjianZuo*∙ Peng Fan∙ Jie Yang∙Geping Yin*∙Yulin Ma∙Xinqun Cheng∙Chunyu Du∙Yunzhi Gao

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, China

*Corresponding author. E-mail: ,

Prolonged cycling performance of pristine LCO and NA-coated LCO electrodes

Fig. S1 presents the prolonged cycling performance of pristine LCO and NA-coated LCO between 2.75 and 4.5 V at a current density of 100 mA g−1.As shown in Fig. S1, after 100 cycles the discharge capacity of the LiCoO2 sample coated by 2 wt.% NaAlO2 is 177.1 mAh g-1, corresponding to the capacity retention of 94.5%, which is higher than that of other samples. It can be seen that the samples deliver more and more capacity as the amount of coating NaAlO2 increasing except the LCO@NA-4% sample, resulting from the LCO covered by excessiveNaAlO2 particles. Additionally, the capacity decrease rates of NaAlO2 coted samples slow down after 50 cycles, whereas that of the pristine LCO sample increases. This indicates that the NaAlO2 coating is an efficient approach to improve the cycle performance of LiCoO2 at high cut-off voltage of 4.5V vs. Li+/Li.

Fig. S1 Cycling performance of pristine LCO and NA-coated LCO between 2.75 and 4.5 V at a current density of 100 mA g−1

Calculation of The lithium ion diffusion coefficient (DLi) from the EIS plots

The lithium ion diffusion coefficient (DLi) can be deduced from the EIS plots in thelow frequency region according to the following equation [S1]:

where DLi is the diffusion coefficient, R the gas constant, T theabsolute temperature, n the number of electrons per moleculeoxidized, A the surface area of electrode, F is the Faraday constant, CLi theconcentration of lithium ions in the solid, and σ is the Warburgfactor. The DLiof all samples before and after 50 cycles are calculated and the results are listed in Table S1. As shown, the DLiof the pristine sample before cycle is slightly bigger than that of the coated samples, which is resulting from that the activation process of the coated samples could continue for the first several charge/discharge cycles. After 50 cycles, the DLiof the pristine sample decreases by two orders of magnitude. The DLiof the NaAlO2 coated samples are five to ten times bigger than the pristine one. This result confirms that the NaAlO2 coating can kineticallyfacilitate the lithium-ion diffusion in the composite electrode.

Table S1The DLiof all samples before and after 50 cycles from the EIS spectra.

samples / DLi (cm2 s-1)
Before cycle / After 50 cycles
pristine LCO / 4.59*10-12 / 9.25*10-14
% / 3.14*10-12 / 4.40*10-13
LCO@NA-1% / 3.11*10-12 / 6.95*10-13
LCO@NA-2% / 1.72*10-12 / 9.60*10-13
LCO@NA-4% / 2.20*10-13 / 9.78*10-14

The surface morphology of electrode after 100 cycles

The SEM images of the pristine LCO and LCO@NA-2% electrode after 100 cycles are shown in Fig. S2. The formation of micro-cracks can been observed on the pristine LCO particles. Whereas the LCO@NA-2% sample shows the similar surface morphology as that before cycle. As mentioned bySujith Kalluri[S2], this kind of micro-cracks is mainlyattributed to the electrochemical/mechanical stresses and hugevolume expansions during Li-ion intercalation/de-intercalation at high cut-off voltage (>4.3V vs. Li+/Li), lead to the performance degradation. Which means coating NaAlO2 on the surface of LiCoO2 improved the mechanicalstability and chemical association with the host (bulk of LiCoO2)and the electrolyte, and resulting in excellent electrochemical performance.

Fig. S2Scanning electron microscopy images of(a), (b) pristine LCO, (c), (d) LCO@NA-2% after 100 cycles.

References:

[S1] Deng J, Xu Y, Li L, Feng T, Li L (2016) J Mater Chem A 4: 6561-6568

[S2]Kalluri S, Yoon M, Jo M, Park S, Myeong S, Kim J, S Dou S, Guo Z, Cho J (2016) Adv Energy Mater 1601507