Shuaifeng Lou‡
,
Bin Shen‡,
Pengjian Zuo*,
Geping Yin*,
Lijie Yang,
Yulin Ma,
Xinqun Cheng,
Chunyu Du and
Yunzhi Gao
Institute of Advanced Chemical Power Sources, School of Chemical Engineering and Technology, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, China. E-mail: zuopj@hit.edu.cn; yingeping@hit.edu.cn; Fax: +86-451-86403807; Tel: +86-451-86413721
First published on 9th September 2015
Electrochemical performance degeneration of LiCoO2 electrodes under high state of charge (SOC) during long-term cycling was studied using LiCoO2/MCMB batteries. The batteries were charged/discharged at 0.6C with 30% depth of discharge (DOD) for 100, 400, 800, 1600, 2000 and 2400 cycles, respectively, and then disassembled to analyze the evolution of morphology, element content, microstructure and electrochemical performance. Through energy dispersive spectrometer (EDS), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM) characterization, it was confirmed that the formation of discontinuous solid electrolyte interface (SEI) layer consisting of Li2CO3, RCOOLi and LiF led to the increase of electrochemical charge transfer resistance (Rct). Although the X-ray diffraction (XRD) refined results showed that there was no new phases were formed during the long-term cycling, the actually increased Li/Co exchange ratio of LiCoO2 from 1.6% at 800th to 2.1% at 2400th resulted in the decrease of lithium ion diffusion coefficient and deterioration of the rate performance.
Solid electrolyte interface (SEI) layer has been proved to be the most critical factor for the capacity degradation of the carbon anode by many literatures.3–7 For the most widely used cathode material of LiCoO2, clarifying the evolution mechanism of the cathode materials is also very important to understand the degradation of the electrochemical performance for lithium ion batteries. Actually, the structure & interfacial characterization of LiCoO2 cathode under overcharged/overdischarged conditions8,9 and high temperature environment10–12 have been investigated. However, these reported research are mostly based of full charge/discharge (100% DOD) conditions within hundreds of cycles, and few of them focused on the degradation process of LiCoO2 at high SOC, especially in the long-term cycling more than 2000 cycles. In practical applications, the capacity of Li1−xCoO2 is 140 mA h g−1 for x = 0.5 when charged up to 4.2 V, corresponding to ∼50% of the theoretical capacity of 274 mA h g−1. The limitation of discharge capacity is owing to the structural instability of Li1−xCoO2 (x > 0.5), where the crystal structure of Li1−xCoO2 undergoes phase transformation from monoclinic to hexagonal phase.13,14 As well known, the x value in Li1−xCoO2 is proportional with the state of charge (SOC) of batteries, so there will be more lithium vacancies existing in Li1−xCoO2 materials at high SOC, which has an important effect on the evolution of structure and electrochemical performance.15,16 With the progress of power source technology, LIBs will be applied in various specific working environments where the batteries will be kept at high SOC constantly, such as energy storage equipment for smart-grid electricity systems, vehicle-carried lithium ion accumulator and so on, where the batteries will be kept at high SOC constantly leading to the long-term operating condition of the lithium-poor state for Li1−xCoO2. Hence, in order to understand the degradation mechanism of LiCoO2/MCMB batteries more comprehensively, it is meaningful to study the electrochemical performance and structure evolution of LiCoO2 electrodes with high SOC in long-term cycles.
In this work, the LiCoO2/MCMB batteries cycled with 30% DOD at room temperature were used to investigate the capacity fading mechanism of the LiCoO2 cathode at high SOC. The structure of LiCoO2 materials and the electrochemical performance of the full batteries was studied to confirm the main factors leading to the degeneration of the LiCoO2 with high SOC in long-term cycling.
The cycled LiCoO2/MCMB cells after different cycles were disassembled at completely discharge state (2.75 V) in an argon glove box. The obtained LiCoO2 electrodes were soaked in fresh dimethyl carbonate (DMC) solvent repeatedly to remove the lithium salts absorbed on the electrodes surface and then dried in the glovebox spontaneously. The surface morphologies and element distribution of the electrodes after different cycles were observed by SEM and XPS, and the interface layer of LiCoO2 particles was observed by HRTEM. Meanwhile, species on electrode surface were analyzed by attenuated total reflectance-Fourier transform infrared spectrometry (ATR-FT-IR). XRD measurements were carried out on a D/max-γB diffractometer with CuKα radiation, and the XRD data was refined by Rietveld method using GSAS/EXPGUI software17 to obtain the detailed structural information.
To investigate the electrochemical performance evolution of LiCoO2 electrodes as the cycle number increases, the LiCoO2/Li half cells was assembled as follows: the LiCoO2 electrodes obtained from the dismantled MCMB/LiCoO2 cells with different cycles were punched to obtain disc electrodes with diameter of 14 mm, subsequently, the 2025 coin-type cells were fabricated with disc LiCoO2 electrodes as working electrodes and pure lithium metal was used as the counter and reference electrode. Celgard 2400 membrane was used as the separator, and 1 M LiPF6 in ethylene carbonate, diethyl carbonate, and ethylmethyl carbonate (1:1:1, in volume) was used as the electrolyte.
The calibration capacity of LiCoO2 electrode was obtained by charging and discharging the coin-type LiCoO2/Li cells within the voltage range of 3.0–4.2 V at 0.1C, and rate performance was obtained by charging and discharging the half cells within 3.0–4.2 V at 0.1C, 0.5C and 1C, respectively. The electrochemical impedance spectroscopy (EIS) was performed at the state of fully discharge state (3.0 V vs. Li+/Li) with an AC amplitude of 5 mV over a frequency range from 10 kHz to 10 mHz. The Li+ diffusion coefficients (D) were measured by the potentiostatic intermittent titration technique (PITT) method, and then calculated by the following equation:18
Fig. S1 (ESI†) shows the morphology and size of LiCoO2 electrodes after different cycles. We can find that the size distribution of LiCoO2 particles is from 2 μm to 10 μm, and there is flocculent acetylene black on the surface of LiCoO2 particles. Meanwhile, no obvious change can be found for the surface morphology of LiCoO2 particles after different cycles. In order to analyze the surface elements and functional group of the LiCoO2 particles, LiCoO2 electrode after activation was investigated by EDS and FT-IR-ATR. Fig. 2(a) shows the selected area image and EDS results of the LiCoO2 surface. The detailed EDS data are listed in Table 1, and it can be found that the amount ratio of oxygen and cobalt atoms is 2.34 (57.64/24.64), which is higher than the theoretical O/Co ratio of 2.00 for LiCoO2. That the higher O/Co ratio indicates that there are excessive oxygenic components in the interface layer for the LiCoO2 materials after the activated process. From the FT-IR-ATR result as shown in Fig. 2(b), the peaks at 860 cm−1 and 1425 cm−1 are attributed to the stretching vibration of C–O bond and bending vibration of CO32−, respectively. The peaks at 1597 cm−1 derives from the symmetrical stretching vibration of CO in RCOOLi.22 The above-mentioned results indicate that there are inorganic and organic lithium salts on the positive electrodes, which means that the SEI layer has formed during the activation process for the LiCoO2/MCMB cells.
Fig. 2 (a) The selected area for EDS test and (b) FT-IR-ATR spectra of LiCoO2 electrode activated only. |
Element | Weight (%) | Atomic (%) |
---|---|---|
C k | 8.23 | 17.72 |
O k | 35.65 | 57.64 |
Co k | 56.12 | 24.64 |
Totals | 100.00 | 100.00 |
In order to further investigate the evolution of various elements on the surface of LiCoO2, the XPS of the LiCoO2 electrode after different cycles were tested, and the element contents of C, O, F and Co are listed in Table 2. It can be seen that the oxygen content related to the SEI films increases gradually with the increasing cycle number. Fig. 3 shows the XPS spectra of C 1s, O 1s, and F 1s with different cycles respectively. In the C 1s spectra, the strong peak centered at 284.6 eV can be assigned to C–C bond from acetylene black and adsorbed carbon, and peaks at 285.8 and 289.4 eV are related to CO bond and CO32−, respectively, corresponding to RCOOLi and Li2CO3 from the SEI films on the surface of LiCoO2 particles. The peak at 290.4 eV is related to C–F bond in PVDF binder. The two peaks in the O 1s spectra at 532.2 and 533.5 eV correspond to CO and C–O bond in RCOOLi and Li2CO3 respectively, and the peak at 529.7 eV comes from the LiCoO2 materials.23,24 F 1s spectra show two peaks at 685.3 and 688.1 eV, corresponding to the PVDF binder in the electrode and the LiF in SEI layer on the surface of LiCoO2 respectively. The fluctuation of the peak intensities of Li–F bond is mainly related to the different intensities of C–F bond deriving from different content of PVDF in the XPS test zone.
Cycle | C 1s (%) | O 2p (%) | F 1s (%) | P 2p (%) | Li 1s (%) | Co 2p (%) |
---|---|---|---|---|---|---|
Activated | 59.05 | 17.34 | 19.39 | 2.68 | 0.00 | 1.54 |
800 | 55.4 | 22.12 | 18.79 | 1.57 | 0.00 | 2.1 |
1600 | 55.07 | 24.89 | 14.38 | 1.41 | 1.58 | 2.67 |
2400 | 55.53 | 24.51 | 15.32 | 1.18 | 0.00 | 3.46 |
Fig. 3 C 1s, O 1s and F 1s spectra of LiCoO2 electrodes after different cycles: (a) activated, (b) 800, (c) 1600, (d) 2400. |
The formation of discontinuous solid electrolyte interface (SEI) layer on the surface of LiCoO2 particles was observed after 2400 cycles with TEM examination (Fig. 4). Fig. 4(a) shows the LiCoO2 particle with marked locations on the surface, and it can be found the boundary of particle can be clearly distinguished. Fig. 4(b) shows a enlarged drawing of the corresponding area, and a composite structure of crystal and amorphous components is observed on the edge of the area. Interestingly, we can find that there is an obvious amorphous layer coating on the surface of LiCoO2 particles with a thickness of about 15 nm in Fig. 4(c), confirming the existence of SEI layer on the surface of the cycled LiCoO2 particles. However, the clear boundary on the edge of particle can be seen in Fig. 4(d), indicating that no SEI layer is generated on the selected LiCoO2 surface area. Therefore, it is believed that SEI layer exists and distributes discontinuously on the surface of LiCoO2 particle after long-term cycling.
Fig. 4 (a) TEM image of LiCoO2 particle after 2400 cycles, and (b)–(d) HRTEM images in different marked locations. |
To investigate the evolution of SEI films on the surface of LiCoO2 during the long-term cycling, the EDS and EIS tests of the electrodes after different cycles were carried out. It can be seen that O/Co ratio rises from 2.34 to 2.96 gradually as the cycle proceeds in Fig. 5(a), indicating that the oxygen content related to the SEI growth increases slowly during cycling. Fig. 5(b) shows the EIS of LiCoO2 electrodes after different cycles in LiCoO2/Li cells. The typical EIS profile consists of the sloping straight line in the low frequency and two semicircles in the high and middle frequency regions, which represent the Warburg diffusion impedance, passivation film formation impedance and charge-transfer impedance, respectively. Different semicircle diameter of the Nyquist plots indicates the change of electrochemical characteristics of LiCoO2 electrodes. In order to calculate the corresponding resistance value of LiCoO2 electrodes after different cycles, the equivalent circuit was introduced as shown in Fig. 5(b) inset. Rs represents the contact resistance depending on electrolyte solution, active material, current collector and so on. Rp and CPE1 at high frequency are the resistance and capacitance of lithium ions transportation through the SEI films. Rct and CPE2 at middle frequency are the charge transfer resistance and double layer capacitance. In the equivalent circuit, constant phase element (CPE) was introduced to offset the depression of semicircle. From the fitting results shown in Table 3, we can find that the fitting value of Rp exhibits an increasing tendency with cycling, and the Rp increases from 71 Ω to 124 Ω with the cycle number from 0 to 2400, revealing that the increasing amount of SEI leads to the increase of ohmic resistance and electrochemical transfer charge impedance, and thus results in increasing polarization in long-term cycling.
Fig. 5 (a) Oxygen/cobalt atom amount ratio of LiCoO2 electrodes after different cycles and (b) EIS and equivalent circuit (inset) of LiCoO2 electrodes after different cycles. |
Cycle number | Rp (Ω) | Rct (Ω) |
---|---|---|
Activated | 71.47 | 32.89 |
100 | 73.67 | 36.60 |
400 | 90.25 | 43.27 |
800 | 80.28 | 50.25 |
1600 | 114.80 | 47.67 |
2000 | 142.30 | 35.12 |
2400 | 124.10 | 74.02 |
Fig. 6 (a) XRD patterns of LiCoO2 electrodes after different cycles and Rietveld patterns of LiCoO2 sample after (b) 800 and (c) 2400 cycles. |
Cycle number | a (Å) | c (Å) | c/a | I(003)/I(104) |
---|---|---|---|---|
a JCPDS no. 50-0653. | ||||
Activated | 2.81720 | 14.08582 | 4.99994 | 1.27671 |
100 | 2.81509 | 14.07444 | 4.99964 | 1.30673 |
800 | 2.81820 | 14.09873 | 5.00274 | 1.42884 |
1600 | 2.81802 | 14.09431 | 5.00149 | 1.23228 |
2000 | 2.81691 | 14.10560 | 5.00747 | 1.13242 |
2400 | 2.81679 | 14.09736 | 5.00476 | 1.1363 |
Standarda | 2.81498 | 14.04930 | 4.99091 | — |
Fig. 6(b) and (c) show the X-ray diffraction patterns and Rietveld refinement results of LiCoO2 sample with 800th and 2400th cycles, and the detailed structural parameters are shown in Table 5. The values of Rp, Rwp, χ of 800th and 2400th sample are 5.48%, 2.17%, 1.49 and 5.73%, 2.21%, 1.47, respectively, which means that the results calculated using the structure model of space group Rm are in good agreement with the experimental results. The parameters c and the Li/Co exchange ratio increase from 14.1110(6) Å, 1.6% to 14.1444(7) Å, 2.1% for the samples after 800 and 2400 cycles, respectively, indicating that the cation disorder increases during cycling.
Cycle number | a (Å) | c (Å) | Li/Co exchange (%) | Rp | Rwp | χ |
---|---|---|---|---|---|---|
Activated | 2.8220(4) | 14.0774(4) | 1.4 | 11.18% | 5.03% | 2.58 |
800 | 2.8190(9) | 14.1110(6) | 1.6 | 5.48% | 2.17% | 1.49 |
2400 | 2.8175(8) | 14.1444(7) | 2.1 | 5.73% | 2.21% | 1.47 |
The Li+ diffusion coefficients (D) obtained by PITT at different potentials of the LiCoO2 electrodes with different cycles are shown in Fig. 7(a). It can be seen that D values range from 4.1 × 10−12 to 3.8 × 10−11 cm2 s−1 at various electrode potentials. In general, the values of D in LiCoO2 varied from 10−13 to 10−7 cm2 s−1, and the difference is mostly attributed to different assumptions of the geometrical factors used in the calculation of D, such as diffusion length and cross-sectional surface area.30,31 The chemical diffusion coefficients increase gradually during the charging process, which is due to the fact that the improving Li concentration in LiCoO2 crystal lattice affects Li+ migration in the solid phase. Li+ diffusion in the Li layer occurs via tetrahedral sites by a divacancy mechanism,32 and the increasing electrode potential leads to the Li concentration reduction and thus more lithium vacancies, which results in the increase of D as a function of electrode potential.
In order to analyze the lithium diffusion in the LiCoO2 cathode during cycling, three curves of D vs. cycle number are extracted. Fig. 7(b)–(d) show the variation of D at 3.95 V, 4.05 V and 4.2 V during cycling, respectively. The three curves of the D vs. cycle number of the LiCoO2 electrode at 3.95 V, 4.05 V and 4.2 V reveal that the diffusion coefficients of Li+ in LiCoO2 electrode decline linearly with increasing cycles, correspondingly, the values of D decrease from 7.9 × 10−12, 2.2 × 10−11, 3.8 × 10−11 cm2 s−1 to 4.1 × 10−12, 1.3 × 10−11, 1.8 × 10−11 cm2 s−1 at potential of 3.95 V, 4.05 V, 4.2 V, respectively. From Table 4, it can be found that the variation of D and ratio of I(003)/I(104) have the similar trend as a function of cycle number, which implies a certain internal relation between these two parameters.
The rate performance of LiCoO2 with different cycle numbers at the charge/discharge rates of 0.1, 0.5 and 1C are compared in Fig. 7(e). When charged and discharged at 0.1C and 0.5C, the LiCoO2 electrodes with various cycles show the approximate discharge capacity of around 130 mA h g−1 and 120 mA h g−1 respectively, and the specific capacity declines slowly during cycling. When charged and discharged at 1C, the LiCoO2 electrodes after the activation, 400, 800 and 1600 cycles show good electrochemical performance with the discharge capacities of 114.1, 113.7, 114.2 and 109.6 mA h g−1, respectively, while the discharge capacities of the LiCoO2 after 2000, 2400 cycles decrease to 82.9, 73.0 mA h g−1, respectively. The results are in good agreement with the evolution of Li+ diffusion coefficients with different cycles for the LiCoO2 cathode materials.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra13841h |
‡ S. L. and B. S. contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2015 |