Zhiyang Luoa, Hong Zhanga,⁎, Le Yub, Donghai Huangb, Jiaqi Shena
a School of Mechanical and Electrical Engineering, Shannxi University of Science & Technology, Xi'an 710021, PR China
b Guangzhou Tinci Materials Technology Co. Ltd., Guangzhou Key Laboratory of New Functional Materials for Power Lithium-ion Battery, Guangzhou 510760, PR China
A B S T R A C T
Layered structure Ni-rich LiNi0.8Co0.15Al0.05O2 cathode has been considered as a higher energy density candidate, but the problems of poor interface stability and cyclic performance restrain its further application. Herein, a functional vinyl (eC]Ce) group allylboronic acid pinacol ester (ABAPE) is introduced as a film forming additive. Linear sweep voltammetry test shows ABAPE could be oxidized prior to blank electrolyte on the cathode and prevent the electrolyte decomposition. The LiNi0.8Co0.15Al0.05O2/graphite pouch cells which contained 1.5% ABAPE shows remarkable cyclic performance of 87.7% capacity retention compared with 71.8% of blank electrolyte after 500 cycles. This is attributed to the protective surface layer which prevents the further decomposition of electrolyte and transition metal ion dissolution, thus stabilizing the electrode-electrolyte interface. Scanning electron microscopy and X-ray photoelectron spectroscopy results also demonstrate that there are less unwanted lithium compounds on the additive-contained cathode.
1. Introduction
Presently, lithium ion batteries (LIBs) are deemed as an optimal approach of generating clean and commercial energy system. Due to its high energy density and considerable cycle performance, lithium ion battery has been wildly applied to small electronic devices, electric vehicles (EVs), hybrid electric vehicles (HEVs) and other fields. However, the fast updating of these products is exigent for the superior battery system with higher specific capacity and longer cycle life. Notably, cathode material is the key to determine the electrochemical performances of Li-ion batteries [1], such as LNMO, LCO and LNCM cathode [2–10]. Among all kinds of cathode materials, Ni-rich layered LiNi0.8Co0.15Al0.05O2 (NCA) cathode is generally recognized as a higher specific capacity alternative. Since Ni is the main redox reaction element, the specific capacities of LIBs will increase with the growth of Ni content [11]. Compared with the specific capacity of traditional LiCoO2 material (150 mAh g−1), Ni-rich layered NCA cathode with 80% Ni can provide more than 180 mAh g−1 of specific capacity when fully charged. Unfortunately, several disadvantages of Ni-rich layered structure NCA cathode tend to limit the applications in the highly demanding field. First, the poor electrode-electrolyte interface stability results in continuous capacity fading and cycle life shortening. It is well acknowledged that the valence of Ni elements increases with the Ni content increasing, so active Ni4+ in Ni-rich NCA cathode tends to revert to more stable Ni3+, thus the electrolyte would be decomposed and exert a lot of non-conducting lithium compounds on the particle surface such as LiF, Li2CO3, ROCO2Li [12,13]. The solid electrolyte interphase (SEI) will be thicker to restrain Li+ transportation and deteriorate the cycle life of battery. Furthermore, opulent oxygen, carbon dioxide and other gases will be generated by these adducts, consequently cause the battery swelling and leave the safety issue in the repeated lithiation condition [14]. During long-term cycling process, the electrode-electrolyte interface will be constantly deteriorated with the continuous dissolution of transition metal ions and decomposition of electrolyte. More importantly, Ni2+ and Li+ have similar ionic radius, so Ni2+ tends to occupy the particle spot of Li+, which will intensify in longterm cycle process. This material characteristic results in an irreversible transformation of the crystal structure from the R3m phase to the rock salt phase. In which case the active lithium layer in the main lattice is destroyed, and leads to the permanent capacity fading [15]. It has been documented through numerous research studies that introducing film forming additives in electrolyte is an economical and efficient approach to solve these issues in electrode-electrolyte interface [16–19]. Yim T et al. have done numbers of researches on the NCM721 cathode, where they used divinyl sulfone (DVS) as film forming additive, and significantly improved the cycle performance from 71.7% to 91.9% after 100 cycles at elevated temperature. In the linear sweep
voltammetry (LSV) test, it can be observed that there is an obvious current peak around 3.8 V vs. Li/Li+, indicating that DVS decomposed ahead of the electrolyte, thus protecting cathode surface from further oxidative decomposition. Their further researches focused on a boron containing additive triphenyl borate (TPB), cells with 2% TPB exhibit a remarkable capacity retention of 88.6% (compared to 80.4%) after 100 cycles at 60 °C because of reduced side reaction on the cathode surface. Notably, there is very little research on improving interface characteristics of Ni-rich LNCA (Ni content > 80%) cathode by using electrolyte additives. In this experiment, allylboronic acid pinacol ester (ABAPE) was firstly reported in the NCA cathode and explored the longterm cycle performance at room temperature. The molecular structure was showed in Fig. 1. As measured, the long cyclic performance is obviously improved in pouch cells with 1.5% ABAPE; meanwhile, the charge-discharge plot demonstrated the characteristics of lower polarization and better cyclic reversibility when introducing ABAPE.
2. Experimental section
All theoretical calculations were conducted on the Gaussian 09W package [27]. The ground-state molecular structure of ABAPE was optimized by the DFT method and using B3LYP/6-311G as basis set.The frontier molecular orbital energy of each solvent in the electrolyte was calculated at B3LYP/6-311G (d, p) level. Then the Hartree constant of 27.21 was used for unit conversions to acquire the final Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) values. NCA cathodes were prepared as follows: mixing LiNi0.8Co0.15Al0.05O2 (Shenzhen BTR Co. Ltd.) and the binder poly(vinylidene fluoride) (PVDF) and CNTs (as conductive agent) in the mass ratio of 97.8%:1.2%:1%, then the slurry was coated on Al foil after stirring for 8 h with constant revolving speed (RPM). After that, put the semi-finished cathode plate in a vacuum oven at 120 °C for 24 h. As for the anode, the graphite (Shenzhen BTR Co. Ltd.) was used for the active substance. Firstly, mixing the active substance with carboxymethyl cellulose (CMC) binder, conductive carbon black (as conductive agent) and SBR binder in the mass ratio of 95%:1.5%:1.5%:2%. Then the slurry was coated on Cu foil, the stirring process was consistent with cathode. At last, the anode plate was dried off at 100 °C for 24 h. In addition, polyethylene membrane was used as a separator (Shenzhen Xingyuancaizhi Co. Ltd., SD216202, China). The blank electrolyte consisting of 1.2 M lithium hexafluorophosphate (LiPF6), ethylene carbonate (EC), ethylmethyl carbonate (EMC) and diethyl carbonate (DEC) in mass ratio of 3:5:2 were purchased in Guangzhou Tinci Materials Technology Co. Ltd., China, compared with other different samples added 1%, 1.5%, 2% ABAPE, respectively. Pouch cells were assembled in the Argon-filled glovebox after dried in the 85 °C vacuum oven for 48 h, then 6.5 g electrolyte was injected into the cells and sealed for a day before test. The expected specific capacity was 185 mAh g−1. Coin cells were accomplished in the same process as pouch cells. In order to demonstrate the improvement of the NCA cathode interface by adding ABAPE, many electrochemical tests were conducted as following. To observe the decomposition of ABAPE in the initial charge state of battery, linear sweep voltammetry (LSV) was measured by electrochemical workstation (CHI660, Chenhua, China) in scan rate of 0.1 mV s−1 and a voltage range of 3–6 V. A platinum electrode was used as the working electrode, Li plate served as the reference electrode. Chronoamperometry tests were to demonstrate the protective effect on the cathode. The NCA/Li coin cells after 3 cycles were charged to 4.5 V, then maintained a constant voltage of 4.5 V for 10 h at room temperature and the corresponding current values were recorded. All coin cells were assembled in an Argon-filled glove box and placed for one day in order to ensure the electrolyte fully infiltrated the entire battery. The electrochemical behavior in different rates of 0.5C, 1C, 2C, 3C, 5C for every 5 cycles was investigated by pouch cells. The long-term cyclic performance for 500 cycles was also investigated at room temperature over the range of 2.75–4.2 V by battery test instrument (Neware, CT-3008W-5V/6A, China). The charge-discharge profiles from 1st to 500th cycles were observed for the cell polarization in different lithiation periods. Electrochemical impedance spectroscopy (EIS) tests were conducted with frequency response analyzer (FRA, Solartron 1455A, Solartron, England), CHI660 after 5 cycles and 500 cycles. Each cell was measured at a frequency range of 10 mHz to 100 kHz with an amplitude of 10 mV in fully charged state. To further explore the interface characteristic and element distribution on the NCA cathode, the cycled cells were disassembled in the Argon-filled glove box. The electrode plates were rinsed with DMC for three times in order to remove residual lithium salts and electrolyte on the cathode surface, then left in the vacuum state for 12 h. Morphology features of the fresh cathode, cycled cathode with and without additive were investigated by scanning electron microscopy (SEM, ZEISS Sigma500, Carl Zeiss, Germany). X-ray photoelectron spectroscopy (XPS, Thermo SCIENTIFIC ESCALAB 250Xi, America) were conducted with Al-K α source to observe the degree of unwanted reaction and the content of each side reaction product. Inductively Coupled Plasma (ICP) was conducted on an Agilent720-OES analyzer for investigating the content of transition metal ion on the cycled electrode surface. The 100 times cycled NCA/graphite cells were disassembled and washed with DMC, then 0.1 g anode was dissolved in 10 mL aqua regia at high temperatures. Finally, the obtained solution was diluted with DI water to conduct the ICP analysis.
3. Results and discussion
The oxidation behavior of ABAPE conducted by LSV was illustrated in Fig. 2a. A distinct anodic peak around 4.1 V was related to the decomposition of ABAPE. Furthermore, the electrolyte with ABAPE began to be oxidized at about 5.5 V, which was later than the blank electrolyte of 5 V. This indicated ABAPE could be preferentially oxidized and form a protect surface layer on NCA cathode, thus suppressed the decomposition of electrolyte. Oxidation stability of electrolyte is the key to electrochemical performance of batteries, so the chronoamperometry tests of electrolytes with and without ABAPE were conducted where the plot was showed in Fig. 2b. In blank electrolyte, a sharp current peak was illustrated in the initial stage and then followed by continuous current signals, which represented drastic electrolyte decomposition during the whole experiment. In contrast, the lower current signals of ABAPE-contained cells illustrated that ABAPE could generate a stable protect layer on cathode surface and greatly suppressed the decomposition of electrolyte. The HOMO and LUMO values showed the same
conclusion. As suggested in Table 1, ABAPE had the Highest HOMO and LUMO, which meant ABAPE also had a strong tendency to be preferentially oxidized among other solvents in electrolyte, meanwhile it was very hard to be reduced on graphite prior to other components. To investigate the effect of additives on electrochemical properties of pouch cell, a long-term cycling test at room temperature was conducted as depicted in Fig. 3. All cells were cycled in 1C rate and voltage range of 2.75–4.2 V. As presented, the initial capacity of cells with ABAPE was little lower than the blank group in the first few cycles, this might be due to the formation of SEI in the initial process. With continuous cycling, all cells with different concentration of ABAPE displayed excellent cycling behavior compared with the blank group. The capacity retention after 500 cycles for the cells with 1%, 1.5% and 2% ABAPE was 79.6%, 87.7% and 82.3%, compared to blank group of 71.8%. Notably, the capacity loss of blank group began to intensify after about 300 cycles, this might be attributed to more irreversibly Li+ dissolution in the electrolyte during continuous charge-discharge process, hence the cells without stable SEI finally led to the structure collapse in the main lattice of secondary particle. Besides, some new discovery could be found notably in the coulombic efficiency plot asshowed in Fig. 3b, which the coulombic efficiency of the blank cells was unsteady during continuous charge-discharge process, and the same trend occurred in the 1% and 2% ABAPE-contained group. This might be attributed to continuous side reaction in electrolyte-electrode interface which greatly effected Li+ intercalation and de-intercalation between electrodes. However, the cells with 1.5% ABAPE provided a more stabilized coulombic efficiency in the whole cyclic process, corresponding to good charge-discharge reversibility and a stable electrode interface. This indicated ABAPE participated in the formation of more stable SEI which protected the cathode surface and significantly reduced the capacity loss of the pouch cell. According to above results, we chose cells with 1.5% ABAPE as main research object to demonstrate in the subsequent experiment. The EIS results of NCA cathode in different concentrations of electrolyte were presented in Fig. 4. All cycled cells were fully charged to 4.2 V and tested at room temperature. For the initial 3 cycles, there wasn't significant change in high frequency semicircle [20] of SEI film impedance (Rsei) and middle-low frequency semicircle of charge
transfer reaction impedance (Rct). It illustrated that the presence of ABAPE didn't influence the electrode interface state in the initial cycling process. Then it could be observed that the Rct evidently decreased in the cells with 1.5% ABAPE after 500 cycles compared to the blank group. This indicated moderate amounts of ABAPE were involved in the formation of protective SEI film which effectively relieved the side reaction during continuous charging and discharging, so the cyclic performance could be significantly improved by the stabilized electrodeelectrolyte interface. Charge-discharge ability in different period of cycling process was usually considered as a side reaction degree in electrolyte-electrode interface [21]. In this case, charge-discharge curves in 1st cycle and 500th cycle were studied over range of 2.75–4.2 V and 1C rate with blank electrolyte, 1%, 1.5% and 2% ABAPE, respectively. After first cycle (see in Fig. 5a), the cells with ABAPE all provided more discharge capacity than blank group, and 1.5% ABAPE-contained group showed greatest charge-discharge behavior. This phenomenon might be ascribed to the forming of stable SEI under the action of additive, so the hinder of Li+ transmission was reduced. After 500 cycles, obvious changes could be observed as shown in Fig. 5b. The blank group displayed a more dramatic polarization and capacity fading, which indicatedthe cells without additive suffered severe deterioration during such long-term cycling process. Notably, 1.5% ABAPE-contained cells kept a considerable discharge capacity (163.5 mAh g−1) compared to blank group of 136.8 mAh g−1. Moreover, the charge-discharge platform of cells with ABAPE was wilder. This demonstrated that the existence of ABAPE could effectively prevent the electrolyte decomposition and active metal ion dissolution, for which the permanent capacity loss could be relieved. As for rate performance (Fig. 5c), each group was conducted in 0.5C, 1C, 2C, 3C for 5 cycles. The cells with 1% and 1.5% ABAPE showed a relatively similar tendency with blank group in different scan rate, it indicated that the addition of ABAPE has no negative effect on the rate performance. Note that the cells with 2% ABAPE got a worse behavior than blank group, the main reason was that ABAPE was not electrically conductive, which led to the increase of interfacial resistance. This result was matched up with the previous EIS results (see in Fig. 4). SEM test was used to investigate the morphology on the surface of electrodes with the introduction of ABAPE. As displayed in Fig. 6a, the surface of the uncycled cathode was rather clean and complete, and the contour of the secondary particle can be vividly observed as well. After 500 cycles, the blank group (see in Fig. 6b) exhibited poor interface characteristics. There were plenty of lithium compounds derived from continuous side reactions on electrolyte-electrode interface, which were poor conductors thus effecting the cycling stability and capacity retention. In contrast, the cells with ABAPE (Fig. 6c) showed the positive effect on the NCA cathode, which suggested smooth and clean surfaces, together with much less electrolyte decomposition adducts coated on the second particle. It was contributed to the protective function of ABAPE on the electrolyte-electrode interface. To further understand the effect of ABAPE on NCA cathode, X-ray photoelectron spectroscopy (XPS) of blank and 1.5% ABAPE-controlled group was conducted to analyze the chemical element distribution as illustrated in Fig. 7. All cycled cells were disassembled and washed with DMC to remove residual lithium salts. In the C 1s spectra of cycled cells with blank and 1.5% ABAPE-contained electrolyte, four characteristic peaks were detected in both two groups. The peak at 284.9 eV belonged to CeC bond corresponding to the CNTs. Another two peaks at 286.1and 288.8 eV were attributed to CeO and C]O, respectively. CeF bond located in 290.5 eV was assigned with PVDF binder [22]. Obvious differences could be observed in O 1s spectra. The characteristic peak of MeO (529.5 eV) and Li2CO3 (531.9 eV) which represented the degree of transition metal (Ni, Co, Al) dissolution and solvent decomposition [23] all decreased in the 1.5% ABAPE cells. This illustrated the presence of ABAPE effectively protected both electrolyte and electrode from further degradation. F 1s spectra showed three main peaks. First one peak at 685.2 eV was assigned with LiF. Next peak PeF bond at 687.4 eV was generated from the decomposition of lithium salt. Last
PVDF binder was located at 688.1 eV. Notably, the peak of LiF of cells with 1.5% ABAPE was lower than blank electrolyte. Knowing that the LiF represented the extent of side reaction between electrolyte and NCA cathode [24,25], which came from two possible reaction steps as following
Eqs. (1) and (2).
Moreover, HF was Lewis acid which had very strong corrosivity, it could easily oxidize transition metal ions in the cathode particle and produce H2. As a result, cathode structure was totally collapsed with the huge swelling of cells. All these side reactions would cause capacity fading and safety issue during continuous cycling process. Therefore, the introduction of ABAPE leads to positive effects on the stabilization of NCA cathode interface. To further confirm ABAPE was involved in the formation of SEI on the NCA cathode, B 1s spectra of 500 cycled cells with 1.5% ABAPE was recorded. There were two main peaks detected at 191.8 and 194 eV corresponding to the BeO bond [26]. This was the most direct evidence that ABAPE participated in the formation of SEI and protected the NCA cathode. The dissolution of transition metal ions was an important factor to evaluate the electrode stability. It also was a symbol that suggesting how far the side reactions went on the electrode. The contents of Ni, Co and Al on the cycled graphite anode with blank electrolyte and 1.5% ABAPE were demonstrated in Fig. 8. Obviously, the cycled cathode with ABAPE had lower contents in all transition metal ions than the blank electrolyte. This was mainly because of ABAPE which promoted forming a more stable SEI film and thus protected the stability of the electrode. Judging from the above experiment and research, it would be reasonable to affirm that the existence of ABAPE significantly improved prolonged cycling performance and interface characteristics of NCA/ graphite full cells.
4. Conclusions
In this report, allylboronic acid pinacol ester (ABAPE) was introduced as the film-forming electrolyte additive in Ni-rich LiNi0.8Co0.15Al0.05O2/graphite cells. ABAPE could be oxidized ahead of other solvents and provide a stable SEI layer on the cathode surface due to its unsaturated (eC]Ce) group, the prolonged cyclic performance was greatly improved by 87.7% capacity retention after 500 cycles. Some interface characterization such as SEM and XPS analysis demonstrated that the introduction of ABAPE would lead to less dissolution of transition metal ions and side reaction products on the cathode surface by limiting the direct contact between electrolyte and electrode. The BeO bond in XPS profile confirmed that ABAPE was involved in the formation of SEI in the initial charging process. All above experimental methods and results could provide a reasonable approach in modifying cathode material of lithium battery by introducing the functional additive.
Acknowledgement
This work is financially supported by the industry-university-research collaborative creation projects of Guangzhou (program number: 201704030011) and the science and technology projects of Guangzhou (201704030020).
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