Quantitative Characterization of the Surface Evolution for LiNi0. 5Co0

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Quantitative characterization of the surface evolution for LiNi0.5Co0.2Mn0.3O2/graphite cell during long-term cycling Huiyuan Zheng, Qunting Qu, Guobin Zhu, Gao Liu, Vincent S. Battaglia, and Honghe Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00427 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on March 26, 2017

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ACS Applied Materials & Interfaces

Quantitative characterization of the surface evolution for LiNi0.5Co0.2Mn0.3O2/graphite cell during long-term cycling Huiyuan Zheng†, Qunting Qu†,*, Guobin Zhu†, Gao Liu‡, Vincent S. Battaglia‡, Honghe Zheng†,* †

College of Physics, Optoelectronics and Energy, Soochow University, Suzhou, Jiangsu 215006, P R China ‡

Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Berkeley, CA 94720, USA

Abstract: Many factors have been brought forward to explain the capacity degradation mechanisms of LiNixCoyMnzO2 (NCM)/graphite cells at extreme conditions such as under high temperature or with high cut-off voltage. However, the main factors dominating the long-term cycling performance under normal operations remain elusive. Quantitative analyses of the electrode surface evolution for a commercial 18650 LiNi0.5Co0.2Mn0.3O2 (NCM523)/graphite cell during ca. 3000 cycles under normal operation are presented. Electrochemical analyses and inductively coupled plasma-optical emission spectroscopy (ICP-OES) confirm lithium inventory loss makes up for ca. 60% of the cell’s capacity loss. Electrochemical deterioration of the NCM523 cathode is identified to be another important factor, which accounts for more than 30% of the capacity decay. Irregular primary particle

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cracking due to the mechanical stress and the phase change aroused from Li-Ni mixing during repetitive cycles are identified to be the main contributors for the NCM cathode deterioration. The amount of transition metal dissolved into electrolyte is determined to be quite low and the resulted impedance rise after about 3000 cycles is obtained to be twice that of the reference cell, which are not very significant affecting the long-term cycling performance under normal operations.

Keywords: Lithium ion cells, LiNi0.5Co0.2Mn0.3O2 cathode, capacity loss, lithium inventory loss, solid electrolyte interphase.


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1. Introduction The demands of lithium ion batteries (LIBs) for large scale energy storage and for electric vehicle (EV) applications have motivated the development of extremely long cyclability.1-5 The state-of-the-art LIBs consist of carbonaceous anode, layered transition metal oxide cathode and liquid organic electrolyte. Ternary transition metal oxides such as LiNixCoyMnzO2 (x + y + z = 1, NCM) exhibit high energy density, low cost and wide operating potential range.6-8 The presence of Ni provides high reversible capacity and Co improves the rate capability. Mn helps to improve the structural and thermal stability of the oxide. With lithium insertion and extraction, this material shows about 3% volume change during charge-discharge process.6 Upon long-term cycling, many factors including impedance rise, active material loss, structure transformation, and reversible lithium loss have been brought forward to explain the capacity degradation of the LIBs based on NCM cathodes.9-16 Amine9 et al. investigated the electrochemical degradation of a Li1.1(Ni1/3Mn1/3Co1/3)0.9O2 /graphite cell at elevated temperature and they found that the major reason is the dissolution of transition metal ions which migrate from the NCM cathode to the graphite anode. With LiNi1/3Mn1/3Co1/3O2+LiMnO4 composite cathode, Dubarry13 et al. discovered that lithium inventory loss was the main reason for capacity decay in early stage and the cell suffered an active material loss in the subsequent cycles. Zeng14 reported that the structure of LiNi1/3Mn1/3Co1/3O2 gradually transformed from hexagonal phase to layer rock salt phase during long-term cycling. In our previous study, we saw a dramatic impedance rise at the graphite anode aroused from the deposit of dissolved transition metal ions at cut-off voltage higher than 4.5V and it is specified to be the main contributor for the cell degradation.17



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Schappacher18 et al. investigated the degradation of NCM523 cathode and found several factors including particle cracking, phase change and transition metal dissolution coexist at the electrode. Here it can be concluded that, under extreme operating conditions such as at high temperature or high cut-off voltage, dissolution of transition metal ions is specified to be an important reason for the cell failure. In hence, ultrathin inert material coating on NCM has been proposed to suppress electrolyte oxidation and metal ion dissolution, particularly under extreme operation conditions.19 Under normal operations when the dissolution of transition metal is effectively suppressed, the cell is able to last thousands of deep charge-discharge cycles. Then, what is the most important reason for the capacity degradation? As the matter of fact, to maximize the battery life, it is of utmost importance to specify the contribution of each factor under normal operation condition. Within a LIB, lithium ion de-inserts from the NCM cathode, transports through the electrolyte and eventually inserts into graphite interlayer during charge. In the subsequent discharge, the backwards reaction occurs. Based on this concept, the amount of reversible lithium ions determines the cell capacity. On the electrode surface, a passivation film is formed during initial charge-discharge cycles through decomposition of the electrolyte components.20 If the passivation film, which is known as solid electrolyte interphase (SEI) formed on graphite anode or on the cathode, is not very stable, a steady growth and repair of it in repetitive cycles will continuously consume the transferable lithium ions in the cell. It means

a

fraction

of

mobile

lithium

maybe

irreversibly

immobilized

at

the

electrode/electrolyte interface. As the reversible lithium is the capacity determining species in LIBs, evolution of the electrode/electrolyte interface strongly affects the capacity retention of


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the cell.21 It can be calculated that, for a LIB of 2500 cycle-life, lithium inventory loss must be kept within 0.006% per cycle. This clearly shows the importance of suppressing lithium inventory loss for LIBs in cycling and aging. However, so far, the role of lithium inventory loss due to the surface evolution for NCM/graphite cell chemistry remains elusive. Although many factors such as active material loss, structural change, transition metal dissolution and impedance rise have been found to be important affecting long-term cycling behavior of the cell under different operation conditions, quantitative characterization of these factors during normal cycling or aging has rarely been investigated. There is no doubt, the results are of enormous importance for understanding the cell performances and will be very helpful for taking effective measures to maximize the cell’s service life. In this work, commercial 18650-type NCM523/graphite cells were subject to deep charge-discharge cycling test under normal operation condition. At different cycling stages (with capacity loss of 10%, 20% and 30%, respectively), quantitative characterization of the most important factors relating to the capacity decay was conducted through electrochemical and inductively coupled plasma (ICP) analyses. We herein elucidated the effects of different factors on the capacity decay of the cell during deep charge-discharge cycling. During the 3000 electrochemical cycles under normal operation, the immobilization of active lithium due to SEI growth on the graphite anode accounts for ca. 60% of its capacity loss. Meanwhile, deterioration of the NCM523 cathode makes up for more than 30%. Metal dissolution into the electrolyte and the resulted impedance rise of the cell are not very significant affecting its long-term cycling performance.



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2. Experimental section 2.1 Cycling performance of theNCM523/graphite cell The NCM523/graphite 18650-type cells (about 45 g per cell) with a nominal capacity of 1.6 Ah were provided by a manufacturer. The cell chemistry is based on NCM523 cathode and graphite anode. Formation of the cells was done at 0.05C for three charge-discharge cycles between 3.0 and 4.2V. Some of the formed cells were used as the reference and they were subject to thorough discharge at 3.0V for 5 h before any further analyses. Long-term cycling test of the formed cells was carried out between 3.0 and 4.2 V at 1C charge-discharge rate on a battery cycler (Maccor S4000, USA) at 30ºC. A slow electrochemical cycle at 0.1C rate was applied once after 100 cycles at 1C rate. The test was terminated till the capacity loss attained 10%, 20% and 30% of its initial capacity, respectively. 2.2 Electrochemical investigations At different cycling stages, hybrid pulse power characterization (HPPC) tests were conducted for the NCM523/graphite cells from 10% to 80% depth of discharge (DOD). At each DOD state, the cell was discharged at a pulse current of 5C for 10 s and then rested for 60 s. Afterwards, a 3.75C regen pulse was applied for 10 s. After another 60 s rest, the cell was subject to constant current discharge at 1C to remove 10% of its capacity. From the voltage difference before and at the end of the discharge pulse, area specific impedance (ASI) of the cell can be calculated by taking the area of the electrode (874 cm2) into consideration. Electrochemical properties of the NCM cathode and the graphite anode at different cycling stages were investigated separately. To conduct the electrochemical measurements, both the cathode and the anode were harvested from the dismantled reference and cycled cells


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in a glove box. Before disassembly, all the full cells were thoroughly discharged at 3.0V for 5h. Electrode discs with diameter of 14 mm (area of 1.54 cm2) were punched out randomly from the harvested NCM cathode and graphite anode, respectively. The mass loading of the NCM523 cathode is determined to be about 12.5 mg/cm2 while the loading of the graphite anode is ca. 6.7 mg/cm2. The electrode material on one side of the electrode disc was carefully wiped off. With the retrieved electrode discs, new half-cells (here half-cell means using lithium foil as the counter electrode) were assembled. A polypropylene film (Celgard 2500) was adopted as the separator and 1 mol L-1 LiPF6/ethylene carbonate (EC) + ethyl methyl carbonate (EMC) (1:1 by weight ratio) solution was used as the electrolyte. Real capacity of the NCM cathode and the graphite anode is determined at 0.1C rate (equivalent to a current density of 0.185 mA/cm2). 2.3 Surface and structural characterizations At different cycling stages, the amounts of Li, Ni, Co and Mn deposit on the graphite anode were determined by Inductively Coupled Plasma-Optical Emission Spectrometer (ICPOES, Optima 8000, PerkinElmer, USA). Specifically, 10 pieces of the graphite anode discs were rinsed with high purity dimethyl carbonate (DMC) solvent for 3 times to remove the residual electrolyte within the electrode. After drying overnight under vacuum, the electrode discs were put into a tube containing 40 ml HCl aqueous solution (2.4 mol L-1). The tube was sealed and placed into a thermostatic chamber at 60 º C for 5 h. To ensure thorough dissolution of the surface deposits into the HCl aqueous solution, the tube was shaken once in a while in the chamber. After centrifuge separation at 3000 rpm for 5 min, 10 ml of the supernatant was transferred into a 100 ml volumetric flask and the solution was diluted to 100



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ml by adding appropriate amount of deionized water. Calibrations were done with a blank and three Merck standard solutions containing 0.1, 1 and 10 ppm of the determined elements. The spectrum relating to Li, Ni, Co and Mn element was obtained by detecting the peak at the wavelength of 610.362, 231.604, 228.616 and 257.610 nm, respectively. According to the ICP protocol, determination of the Li, Ni, Co and Mn concentration in the solution was conducted. Morphologies of the electrodes at different cycling stages were observed by Fieldemission scanning electron microscope (FESEM, SU8010, Hitachi, Japan) at 10 kV accelerating voltage. The elemental analyses were performed by affiliated energy dispersive X-ray spectroscopy (EDX) of the Hitachi S-8010 operated at 15 kV accelerating voltage. Crystal structures for the NCM cathodes and the graphite electrodes at different cycling stages were characterized by X-Ray Diffraction (XRD, D/MAX-2000PC, Rigaku, Japan) using Cu K(α) radiation. The diffraction angle (2θ) was set between 10ºand 80ºwith an augment of 6ºmin-1.

3. Results and discussion Figure 1 presents the long-term cycling behavior of the commercial 18650 NCM523/graphite cells. As seen in Figure 1a, the initial reversible capacity at 0.1C was measured to be ca.1.62 Ah. With increasing electrochemical cycles, the cell capacity decreases steadily. A linear relationship between the cell capacity and the cycle number is observed and the capacity-fading trend for these cells appears to be very identical. This shows good consistency between the different test cells. The cycling test was terminated when the discharge capacity at 0.1C is decreased to 1.45, 1.30 and 1.13 Ah, corresponding to 90%,


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80% and 70% of their initial capacity, respectively. It takes almost 3000 cycles for the cell to attain 30% capacity loss, illustrating excellent cyclability of the NCM523/graphite cell. It should be noted that, at the early cycles, the capacity difference for the cell cycled between 0.1C and 1C is relatively small. It increases gradually as the electrochemical cycle proceeds, reflecting an increase of the cell polarization.



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Figure 1. (a) Cycling performances of the NCM523/graphite cells, (b) comparison of discharge curves, and (c) differential capacity versus cell voltage at different cycles.

Figure 1b shows the discharge curves of the NCM523/graphite cell at different electrochemical cycles. As seen in this figure, a significant capacity loss occurs at the 3.6V plateau with increasing electrochemical cycles. This plateau is known to be related to the Ni2+/Ni4+ couple in the electrochemical process.7 Moreover, a slight drop of the discharge profile is observed with increasing cycle number, illustrating larger polarization of the cell with prolonged cycles. As differential capacity versus cell voltage offers great sensitivity to probe the cell degradation over long-term cycling test22-23, Figure 1c depicts the differential capacity dQ/dV versus the cell voltage for the NCM523/graphite cell with increasing cycle number. As the cycling proceeds, the dQ/dV peaks for the typical lithium de-intercalation shift to lower voltages. More importantly, the magnitude of the typical dQ/dV peak at ca. 3.6V remarkably decreases with prolonged cycles. This implies that the most important reason for the capacity decay of the cell is associated with the decrease of the Ni2+/Ni4+ couple. By contrast, shift of the dQ/dV peak toward lower voltage due to polarization of the cell seems not enough to induce a significant capacity loss. To quantitatively characterize the internal resistance change of the cell over various electrochemical cycles, HPPC test was conducted and the results are depicted in Figure 2. For the formed NCM/graphite cell before cycling, the internal resistance is obtained to be a little more than 60 Ω·cm2. At the early cycling stage, relatively quick increase of the internal resistance is observed. ASI of the cell with 10% capacity loss (ca. 1000 cycles) increased to almost 90 Ω·cm2. However, the resistance rise is mitigated at the end of the


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cycling. ASI of the cell with 30% capacity loss (ca. 3000 cycles) increased to about 120 Ω·cm2, which is about twice of the reference cell. Taking the current density of the cell at 1C (about 1.85 mA/cm2) into consideration, voltage drop of the discharge curve resulted from the resistance rise is only a little more than 0.1V. Clearly, impedance rise is not the most important factor for the cell’s capacity degradation during ca. 3000 cycles under normal operations.

Figure 2. Area specific impedance (ASI) of the NCM/graphite cell at different cycling stages.



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Figure 3. The first charge-discharge curves of the retrieved (a) NCM523cathodes and (b) graphite anodes from the cells at different cycling stages.

To specify the origin of the capacity loss, the initial charge/discharge curves of the retrieved NCM523 and graphite electrodes at different cycling stages are shown in Figure 3. As the full cells were thoroughly discharged before disassembly, all the transferrable lithium in the cell is assumed to be inserted in the cathode. As shown in Figure 3a, for the NCM523 electrode retrieved from the reference cell, the first charge capacity and discharge capacity are very identical, manifesting that the “mass balance” of the active lithium between the anode and the cathode is well kept. For the cycled cells with different capacity losses, the first


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charge capacity of the NCM523 cathode (refers to the de-inserted transferable lithium from the cathode) decreases with increasing cycle number. 2.56, 2.28, and 2.01 mAh of the first charge capacity were obtained for the cells with 10%, 20% and 30% capacity losses, respectively. The capacity obtained in the first charge reflects the total reversible lithium and the real capacity of the corresponding full cell. Compared to the reversible capacity of 2.82 mAh obtained from the reference cell, the capacity loss at the cathode accounts for 9.1%, 19.2% and 28.2% of the cell’s capacity loss, manifesting most of the capacity loss is originated from the NCM cathode side. In the subsequent discharge, as lithium foil is used as the counter electrode, the missed lithium at the cathode can be replenished with the lithium reservior. The first discharge capacity of the NCM cathode (refers to the available lithium sites on the cathode) is obtained to be 2.73, 2.64 and 2.56 mAh for the cell with 10%, 20% and 30% capacity loss, respectively. The discrepancy is attributed to the lithium inventory loss, which is defined as the capacity difference between the first charge (extracted lithium) and discharge capacity (lithium vacancies) for the retrieved cathode in this figure. This means there are more available lithium sites than the transferable lithium ions at the NCM cathode. With increasing charge-discharge cycles, more and more lithium vacancies cannot be effectively lithiated when the NCM523/graphite cell is subject to deep discharge. This explains the shrinkage of the 3.6V discharge plateau for the full cell with electrochemical cycles. Due to lithium deficiency, some of the Ni4+ cannot be reduced to Ni2+ in the NCM523/graphite cell. It can be calculated that lithium inventory loss makes up for 6.0%, 12.8% and 19.5% for the cell with 10%, 20% and 30% capacity losses, respectively. Clearly,



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lithium inventory loss makes up ca. 60% of the capacity loss almost at each cycling stage of the cell. Deterioration of the NCM523 cathode is also observed in Figure 3a. Decrease of the discharge capacity with electrochemical cycles indicates the loss of the available lithium vacancies in the NCM structure. Three important reasons have been brought forward to explain the deterioration of NCM cathode. One is the loss of active material due to dissolution of the transition metal cations.24-25 Another one is associated with the particle cracking aroused from an anisotropic stress within the NCM particle.18 The last one is related to the structural change of the NCM material, especially the Li-Ni mixing due to the mobility of the nickel ions in the NCM structure.14 From the first discharge capacity of 2.73, 2.64 and 2.56 mAh obtained in this figure, it can be calculated that the deterioration of the NCM cathode accounts for 3.1%, 6.4% and 8.7% for the full cells with 10%, 20% and 30% capacity losses, respectively. It illustrates that deterioration of the NCM523 cathode makes up for ca. 30% of the capacity decay. Table 1 lists capacity loss of the full cell, capacity loss at the NCM cathode, lithium inventory loss and deterioration of the NCM cathode, respectively. Clearly, lithium inventory loss and deterioration of the NCM cathode make up the majority of the cell’s capacity loss. Figure 3b shows the first charge-discharge profiles of the retrieved graphite electrodes from the reference and cycled cells. Even after ca. 3000 cycles with 30% capacity loss, the capacity loss and the polarization at the graphite anode is not very significant. The capacity obtained for the graphite anode is significantly higher than that of the corresponding cathode obtained in Figure 3a. Thus it can be concluded that the graphite anode has little impact on the capacity decay of the NCM/graphite cell.


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Table 1. List of the capacity loss of the full cell, capacity loss at the NCM523 cathode, lithium inventory loss and deterioration of the NCM523 cathode.

Capacity loss in

Capacity loss at

Lithium inventory

Degradation of

18650 cells

the cathode

loss

the cathode

10%

9.1%

6.0%

3.1%

20%

19.2%

12.8%

6.4%

30%

28.2%

19.5%

8.7%

Figure 4. XRD patterns of the retrieved (a) NCM523 cathodes and (b) graphite anodes harvested from the cells at different cycling stages.



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Figure 4 displays the XRD patterns of the NCM cathode and graphite anode at different cycling stages. Figure 4a shows similar patterns in accordance with that of LiCoO2 (αNaFeO2 type). As the cycling proceeds, position of the diffraction spots is not considerably changed. A slight decrease of the (003) peak intensity indicates random distribution of the Li and Ni cations. Due to the mobility of the nickel ions in the NCM structure, Li-Ni mixing occurs even at the first charge, leading to partial phase changes and a loss of available lithium sites.26 Li-Ni mixing may also lead to the formation of a thin passivation layer on the particle surface contributing to an impedance rise at the cathode.27 As seen in the inserted figure, several typical peaks ((113), (110) etc.) shift toward high angle direction. This is associated with the decrease of the lattice parameter due to lithium deficiency in the structure. As seen in Figure 4b, no considerable peak shift is observed in the diffraction patterns, showing no significant structural change for the graphite anode occurs during the long-term cycling process. However, the intensity decline of (002) peak appears to be significant, illustrating a growth and thickening of surface film (SEI layer) on the graphite surface.28-29 No sign of the existence of transition metal is observed as reported in literature. This is because the content of transition metal deposited on graphite electrode is too low to be detected with this technique.


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Figure 5. Morphologies of the retrieved NCM523 cathodes from the (a) reference cell and cycled cells with the capacity losses of (b) 10%, (c) 20% and (d) 30%, respectively.

Post-mortem analyses of the cycled electrodes via SEM reveal different degradation effects on the cathode and anode surface. As shown in Figure 5a, the NCM active material consists of spherical secondary particles with a diameter of about 8 micron. The secondary particles are consisted of agglomerated nano-scale primary particles. For the NCM cathodes retrieved from the cycled 18650 cells with different capacity losses (Figure 5b-d), irregularly distributed particle cracking is observed on the secondary particle surface. Some of the primary particles are cracked and missing. This can be explained by the expansion and contraction of NCM particle due to lithium insertion and extraction.6 The volume change induces mechanical stress within the secondary particle. If the anisotropic stress between neighboring primary particles in the secondary particle cannot be compensated, it will eventually cause the particle cracking.18 With increasing cycle number, the active material cracking and the resulted active material loss become more serious. From the structural and



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morphological studies, active material loss and the phase change due to Li-Ni mixing are the main contributors for the electrochemical degradation of the NCM523 cathode. Figure 6 shows the morphologies of the retrieved graphite electrodes harvested from the full cells at different cycling stages. Before long-term cycling (Figure 6a), graphite particles and even the conductive additive are clearly observed, indicating the surface film (SEI) developed during the cell formation is thin and uniform. Not too much Li salt is precipitated on the graphite surface at this stage. For the graphite anode retrieved from the cycled 18650 cell with 10% capacity loss (as seen in Figure 6b), thickening of the SEI film is observed. The conductive carbon particle is buried under the surface film. However, the shape of graphite particles can still be identified. With increasing electrochemical cycles, more thick and dense passivation film is developed on the graphite surface. The conspicuous morphological change reveals a significant surface evolution on the graphite anode during electrochemical cycling. As lithium insertion into graphite induces a basically uniaxial volume expansion of about 10%,30-31 the large volume change is able to induce mechanical cracking of the SEI film. As the result, a repair and growth of the SEI film occurs through the continuous decomposition reactions of the electrolyte, which definitely contributes to the thickening of the passivation film. Of course, the side reactions consume the limited transferable lithium in the cell.21, 32 In this sense, lithium inventory loss at the NCM cathode is directly related to the SEI growth on the graphite.


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Figure 6. Morphologies of the retrieved graphite anodes from the (a) reference cell and cycled 18650 cells with (b) 10%, (c) 20% and (d) 30% capacity losses, and (e) EDX results of the graphite harvested from the cell with 30% capacity loss.

Figure 6e shows the elemental analysis conducted by EDX technique on a special area on the graphite retrieved from the cell with 30% capacity loss. C, O, F, and P can be easily detected on the graphite surface. O is coming from the SEI components of various Li salts, and F is related to the PVDF binder in the electrode and the decomposition of PF6- anions. The appearance of P may also be attributed to the decomposition of PF6- anions. All these elements are closely related to SEI components on the graphite surface. Besides the C, O, F, and P elements, Mn, Ni, Co, and even Al were also detected. Obviously, these elements come from the NCM cathode. During long-term electrochemical cycles, Ni, Co, and Mn in the



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cathode dissolve into the electrolyte and eventually deposited on the graphite anode through reduction reactions.24, 33-34 The appearance of Al on the graphite surface implies that corrosion of Al current collector also takes place when the cell is subject to long-term cycling.35

Figure 7. Amounts of Li, Ni, Mn and Co elements on the graphite surface obtained at different cycling stages.

The amounts of Li, Ni, Mn and Co elements deposited on graphite surface at different cycling stages were determined by ICP technique and the results are presented in Figure 7. For the reference NCM523/graphite cell, Li concentration of the solution is determined to be 3.98 mg L-1. For the cycled cells with 10%, 20% and 30% capacity losses, Li concentrations are determined to be 7.19, 9.63 and 12.26 mg L-1, respectively. There is no doubt some residual electrolyte within the voids and pores of the graphite laminate cannot be thoroughly wiped off simply by washing in DMC solvent and trace lithium trapped in the graphite cannot be completely extracted through constant voltage discharge due to kinetic inhibitions. However, our purpose in this work is focused on the growth of the SEI film (Li salt increase) during the long-term cycling process. The system error can be simply eliminated by assuming


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that the residue lithium is not very different at each cycling stage. After subtracting the baseline (3.98 mg L-1 obtained in reference solution), the amount of Li deposits on the graphite can be obtained. The immobilized lithium on the graphite anode can be easily converted to capacity according to Faraday's law: CLi= 26.8 n Where CLi represents the capacity loss resulted from immobilization of lithium on the graphite surface and n is the mole of the immobilized lithium. As the result, it is calculated that the immobilized Li on the graphite makes up for 8.7%, 15.3% and 22.4% of the capacity loss for the cells with 10%, 20% and 30% capacity loss, respectively. The obtained lithium deposit on the graphite is considerably higher than the obtained lithium inventory loss at the cathode from Figure 3. This is because we only consider the effective lithium vacancies and active lithium in the calculation of lithium inventory loss at the cathode. The released lithium from the NCM cathode due to Li-Ni mixing and transition metal dissolution is neglected. This amount of lithium may also participate the surface evolution on the graphite anode. Basically, it can be concluded that the missed active lithium at the cathode is most probably immobilized at the graphite surface.36 The content of Mn on the graphite surface also increases with more cycle number. The significantly high concentration of dissolved manganese species compared to that of nickel and cobalt is in good accordance with the results in literature which shows that Ni and Co is barely detected in electrolyte solutions.37-38 It is known that, for NCM cathode at de-lithiated state, most of Ni2+turns to Ni4+ and Co3+ turns to Co4+. This brings about Ni-O and Co-O bond length decrease. However, the Mn-O bond length doesn’t considerably change and



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becomes to be the weakest one.39-40 Therefore, manganese ions preferentially dissolve into the electrolyte from the NCM structure. If the dissolution of a Mn induces the electrochemical inactivity of 3.33 active lithium according to the stoichiometric coefficient of the LiNi0.5Co0.2Mn0.3O2 compound, the capacity loss of the NCM cathode resulted from Mn dissolution can be calculated to be ca. 1.6% for the cell with 30% capacity loss. This is quite low compared to the 8.7% of the electrochemical degradation of the NCM cathode obtained in Figure 3. The result is consistent with the conclusion that, under normal cycling conditions, the amount of dissolved transition metal ions is very low reported by Schappacher et al.18The very low content of Mn on the graphite surface illustrates that the main reason for the electrochemical degradation of the NCM cathode is the active material loss and the phase change during long-term cycling. Most of the relevant literature has shown the dissolution of transition metal cations is of special significance affecting the cell’s cycle life.36, 41 While this conclusion is still under debate, our result illustrates that this is not an important factor for the NCM523/graphite cell operated under normal operations.

Figure 8. The capacity loss, lithium inventory loss and electrochemical degradation at the NCM523


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cathode, and the immobilized lithium on the graphite anode for the NCM523/graphite cell at different cycling stages.

In order to visualize the various factors relating to the capacity decay of the NCM523/graphite cell, Figure 8 demonstrates a comparison between the capacity loss, lithium inventory loss and electrochemical degradation at the cathode, and the immobilized lithium on the graphite anode. Clearly, the majority of capacity loss for the full cell is ascribed to the NCM523 cathode. Lithium inventory loss at NCM523 electrode takes a major proportion (ca. 60%) of the capacity decay while the electrochemical degradation of the cathode accounts for ca. 30% of the capacity loss. The missed lithium at the NCM cathode is most probably immobilized on the graphite surface. The Li salt accumulation on the graphite anode is determined more than that of the lithium inventory loss found at the cathode. This is because lithium ions coming from the transition metal dissolution and the phase change due to Li-Ni mixing may also find their way on the graphite surface. Other factors such as transition metal dissolution and the resulted impedance rise may be related to the rest 10% of the capacity loss. There is no doubt, they are not important factors for the cell’s capacity decay under normal operations.

4. Conclusion To quantitatively characterize the surface evolution and their effects on the capacity decay forNCM523/graphite cell, long-term cycling test of the cell was carried out under normal operation conditions. The cell is able to last ca. 3000 deep charge-discharge cycles and a linear capacity-fading with electrochemical cycles was obtained. Surface evolution and the related capacity degradation mechanisms of the cell were investigated through



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electrochemical, morphological and ICP studies. The results manifest that lithium inventory loss at the cathode is the most important factor for the capacity decay of the cell, which accounts for ca. 60% of the cell’s capacity loss at different cycling stages. The missing transferable lithium is immobilized on the graphite surface due to the SEI growth. Electrochemical degradation of the NCM cathode, which is mainly aroused from the irregular primary particle cracking and Li-Ni mixing, makes up for ca. 30% of the capacity decay. The transition metal dissolution into the electrolyte and the resulted impedance rise are not very significant affecting the long-term cycling performance under normal operations. The results enable us to get deep insight into the degradation mechanisms of the cell under normal operations and the quantitative analyses are helpful for us to take effective measures to maximize the cell’s service life.


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Author information *Corresponding Authors

E-mail: [email protected] (H. Zheng), [email protected] (Q. Qu)

Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (NSFC) under Contract no.’s 21473120, 51272168 and 21403148.

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Table of Contents


Quantitative Characterization of the Surface Evolution for LiNi0. 5Co0

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