Switch of the charge storage mechanism of LixNi0.80Co0.15Al0.05O2

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Switch of the charge storage mechanism of LixNi0.80Co0.15Al0.05O2 at overdischarge conditions Rosa Robert, and Petr Novák Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04784 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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Chemistry of Materials

Switch of the charge storage mechanism of LixNi0.80Co0.15Al0.05O2 at overdischarge conditions Rosa Robert and Petr Novák Paul Scherrer Institut, Electrochemistry Laboratory, CH-5232 Villigen PSI, Switzerland ABSTRACT: The specific charge “loss” during the first cycle of LiNi0.80Co0.15Al0.05O2 (NCA) is due to low Li+ ion diffusion kinetics on discharge down to 3.0 V. Discharge to 2.14 V pushes further reinsertion of Li+ into the layered NCA lattice, thus part of the specific charge “loss” can be regained. Additional discharge to even lower potentials switches the charge storage mechanism from a solid solution to a two-phase reaction process. Pushing the NCA to work at lower potentials than 2.14 V vs. Li metal irreversibly alters the material’s crystal structure and microstructure. The gradual overall change in the energy storage reaction mechanism, promoted by a repeated deep discharge, causes a decrease in the NCA’s performance and lifetime, thus challenging the validity of cycling over a wide potential window in practical cells.

Introduction The rechargeable lithium-ion battery (LIB) technology has enabled the technological revolution of cell phones, laptop computers, and tablets transforming thus global communication. Later, the LIB technology has demonstrated great potential for electro-mobility (EVs) and is continuously evolving (1). Furthermore, it is considered a key technology for electricity storage applications.(2) Certainly, lithium transition metal oxides with the layered LiTMO2 (TM = transition metal) structure are by far the most promising cathode materials to replace LiCoO2 in Li-ion batteries, as they can deliver a high reversible specific charge. Within the layered transition metals LiTMO2 family of materials, Li(Ni1–x–yCoxAly)O2 (NCA) and Li(Ni1–x–yMnxCoy)O2 (NMC)(3-6), have already reached commercialization. These cathodes operate at high potentials vs. Li+/Li and can therefore, enable high specific energy values of the cells. These cathodes typically exhibit irreversible specific charge “losses” after the first cycle. Several studies have been conducted to underpin the causes behind this specific charge “loss”, attributing it to reasons such as lack of full lithiation of the NCA material when discharged to 3.0 V versus lithium metal (7), formation of inactive domains within the electrode(8) which also lead to long-term specific charge fade(9), and side reactions such as electrolyte oxidation.(10, 11) Recent experiments by Mueller-Neuhaus et al.(12) and Kang et al.(13) had disclosed that layered oxides charged to 4.3 V would completely recover their specific charge “loss” on discharge if a following deep discharge step to 10 h at a constant potential (CP) and finally washed using DMC. The capillaries were then sealed with wax inside an argonfilled glove box. Operando XRD measurements were carried with an inhouse constructed cell in reflection geometry, cycled galvanostatically between 4.3 V and 1.2 V at a rate of 10 mA/g. The diffraction patterns were recorded continuously by step scanning (with a step size of 0.017°) over the 2Θ range of 16° ≤ 2Θ ≤ 23° and 34° ≤ 2Θ ≤ 47°. X-ray powder data was treated using the FullProf suite(15) and HighScore suite(16 ) softwares.

fications undergone during the first charge(17) and issues like loss of particle contact after long term cycling, the specific charge “loss” on discharge is also caused by a hampering of Li+ reinsertion on discharge. In other words, if complete lithium reinsertion should be possible, it would proceed with a very sluggish kinetics at the end of the process. Table 1. Structural data at room temperature for pristine LiNi0.80Co0.15Al0.05O2 and after charge to different upper cut+ off potentials and discharge to 3.0 V vs. Li /Li with a CP > 10 h. Pristine

4.3-3.0 V

4.3-3.0-4.33.0 V

a( )

2.86457(9)

2.8652(5)

2.8649(1)

c( )

14.17903(7)

14.1900(4)

14.1924(7)

100.76(3)

100.88(8)

100.88(2)

S.G. R-3m

V(

3

)

(104)

(018)

(110)

(101) (006) (012)

Ex situ XRD measurements were performed on LiNi0.80Co0.15Al0.05O2 electrodes charged to different upper cut-off potentials and discharged to 3.0 V vs. Li+/Li with a CP > 10 h. Figure 1 presents the XRD pattern of the cathode discharged to the 3.0 V cut-off potential after 1 cycle compared to the XRD pattern of the pristine electrode. The Bragg reflections are assigned based on the rhombohedral crystal system with
3 space group, which unit cell contains three MO2 sheets (M = Ni, Co, Al) with the Li+ ions occupying the octahedral sites with an oxygen stacking sequence of …ABCABC… along the c-axis. After the first cycle, the cycled NCA electrode presents the same crystal structure as the pristine electrode although the (003) Bragg reflection is broader compared to the (003) of the pristine, and is also shifted to lower 2θ angles. The (003) reflection is representative of the c lattice parameter, very sensitive to the amount of lithium between the MO2 layers, thus a shift towards lower 2θ angles tells that, despite the long constant potential step the complete relithiation of the NCA is not attained. An inspection of the lattice parameters of electrodes charged to 4.3 V and discharged to 3.0 V (CP > 10 h) vs. Li+/Li compared to the pristine sample (Table I) shows that, after the first electrochemical cycle, c increases significantly from the original value of 14.17903(7) Å to 14.1900(4) Å. A more significant increase of the c lattice parameter is observable after two full charge-discharge cycles. This observation is indicative of the presence of vacancies in the MO2 slabs after full discharge to 3.0 V. This trend, which increases with the cycle number, is a consequence of a non-complete Li re-insertion between the MO2 slabs after the full cycle. This lack of full lithiathion of the layered NCA at this stage of discharge (3.0 V vs. Li metal) is consistent with previous research.(7) Therefore, in addition to other factors such as irreversible structural modi-

(003)

Results & Discussion

Intensity [a.u.]

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cycle 1 CV @ 3V >10h Pristine

18

19

36

40

44 64

66

2Θ [°] (CuKα)

Figure 1. Ex situ diffraction patterns of NCA pristine material and NCA electrode charge to 4.3 V and discharged to 3.0 V with CP > 10 h.

Recent reports by Mueller-Neuhaus et al.(12) and Kang et al.(13) have disclosed a possible full recovery of the specific charge “loss” after the first cycle when NCA is put through a deep discharge cycling protocol. The particular reactions paths by which electrochemical reactions allow further charge transfer bellow 3.0 V are not clearly defined. Here, we identify the amount of the irreversible specific charge “loss” that is recovered from the layered LiNi0.8Co0.15Al0.05O2 (NCA) structure under overdischarge conditions and the impact that the low potential cycling has on the NCA material.

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Chemistry of Materials

Figure 2. a) Charge–discharge curves and b) dQ/dV vs. potential curves of LiNi0.85Co0.15Al0.05O2 cycled between 1.5 V and 4.3 V at a specific current of 10 mA/g.

Figure 2a shows the electrochemical cycling within a wider working potential window (4.3 V-1.5 V) over 10 cycles. The electrochemical reactions associated with the first discharge profile can be divided into two main processes defined by a sloped line from 4.3 V down to 3.0 V and a plateau at about 1.8 V vs. Li+/Li. The first process corresponds to the solid solution lithium intercalation process, with a contribution to the specific charge on discharge of 182 mAh/g (0.66 Li+ per formula unit) achieving a composition of Li0.92Ni0.80Co0.15Al0.05O2. The plateau on the potential–specific charge curve indicates a twophase coexistence region of a phase transition which starts from 1.91 V descending down to 1.5 V, and contributes to a specific charge on discharge of ~29 mAh g−1. Therefore, the total specific charge on the first discharge (lithiation) corresponds to 220 mAh/g, which is well over the charge delivered on the first charge (delithiation), about 202 mAh/g (Li0.28Ni0.80Co0.15Al0.05O2), with charge potential ascending from 3.0 V to 4.3 V. The following charge (2nd. charge after deep discharge) delivers a specific charge of 207 mAh/g and 218 mAh/g corresponds on reduction. After the ninth cycle the specific charge on charge corresponds to 190 mAh/g (x=0.68 Li in Li1xNi0.80Co0.15Al0.05O2) and to 192 mAh/g on discharge. Thus, during first charge/ discharge cycles we observe more reduction than oxidation which may be caused by irreversible parasitic processes such as electrolyte decomposition by reduction and to irreversible structural transformations of the NCA lattice. In the dQ/dV curve of the second charge, shown in Figure 2b, between 3.0 V and 4.3 V there are four oxidation peaks at 3.6 V, 3.74 V, 3.99 V, and 4.18 V, which can be assigned to the crystal structure rearrangements(17) that the NCA undergoes during (de)intercalation within this potential range and the oxidation of Ni3+ into Ni4+. These peaks are reversible on reduction. The second charge after deep discharge is characterized by a first oxidative process at 2.4 V (dQ/dV curve shown in Fig. 2b). The process at low potential is reversible and it is characterized by a relatively large hysteresis of 0.62 V with respect to the second discharge curve. In the subsequent cycles, the charge and discharge curves show similar profiles but it is clearly appreciable that the solid-solution process is diminished at expenses of the low potential “plateau” process, i.e. at discharge cycle 4, the delithiation on the “solid-solution” process starts at 0.4 V more negative potential compared

to the previous charge although the other transitions do not shift (Figure 2b). At cycle nine we see that, as the plateau processes increase and keep the same potential, the “solid solution “ process is clearly affected by the continuous overdischarge cycles so that the four oxidation/reduction couple peaks between ~3.5 V and 4.3 V are not visibly defined anymore implying that there is a serious crystal framework modification. Potential profiles and dQ/dV curves evidence a loss of contribution of the solid solution process and an overall switch of the energy storage reaction mechanism in favor of the “plateau” process.

Figure 3. a) Ex situ XRD plots of three different 2Θ regions of the pristine electrode and electrode samples cycled to 3.0 V after 1, 4, and 10 full charge/ deep discharge cycles, and b) Williamson-Hall plots of the patterns from a) according to the index of its reflection.

Figure 3a displays the ex situ XRD plots of three different 2Θ regions that comprise the (003), (101), (006), (012), (104), and the (110) and (018) diffraction peaks, of the pristine electrode and electrode samples cycled to 3.0 V after 1, 4, and 10 full charge/ deep discharge (down to 1.5 V) cycles and after an additional CP step of >10 h. A qualitative comparison of pristine NCA XRD pattern collected prior to the cell’s initial charge, with that of cycle 1, 4, and 10 full charge/ deep discharge cycles, after cycling down to 3.0 V, one can see that some of the crystallinity of the powder is lost, the diffraction lines are considerably broadened, and after nine full cycles the (003) and (104) diffraction lines shift to higher 2Θ angles. Nevertheless, no new phases or peaks can be observed. Furthermore, the integrated intensity ratio I(0 0 3)/I(1 0 4) of cycle nine is I(0 0 3)/I(1 0 4) = 0.96 for cycle 9 and I(0 0 3)/I(1 0 4) = 1.3 for the pristine electrode. This intensity ratio values are a good indication of a fairly ordered layered structure after nine deep discharge cycles and also indicate a quite significant amount of cation mixing. Figure 3b display the Williamson-Hall plots for the XRD patterns plotted in Figure 3a. Williamson-Hall method is used to determine the average volume weighted crystallite size and microstrain of the samples under study and it relies on the principle that size broadening βL and strain broadening βe vary quite differently with respect to Bragg angle Θ.(18) Using the simplified formula  cos Θ  ∙ sin Θ 

∙ 

(1)

by plotting βtotcosθ versus sinθ, the strain component is extracted from the slope (Ce) and the size component from the intercept (Kλ/L). The Williamson-Hall method

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has many assumptions but it is quite useful to identify trends when comparing different electrodes that have been cycled under different cycling conditions, giving thus a qualitative indication of sample microstructure. WH plot results in Figure 3b indicate a gradual increase of the crystallographic strain of the NCA electrode after each cycle. These results can be related to microstructural modifications in the samples; so that repetitive cycling down to 1.5 V would lead to particle fracturing and disconnection as a result of volume expansion/contraction which would lead to compositional inhomogeneities within the electrode. Particle fracturing and disconnection has been previously observed(19). In order to untangle the causes of the irreversible specific charge “loss” on the first discharge with respect to the first charge and to get insights into the nature and degree of reversibility of the processes occurring at deep discharge we used operando X-ray powder diffraction and operando electrochemical cell. Figure 4a shows the potential profile of the first charge to 4.3 V and first deep discharge down to 1.2 V vs. lithium metal. Figure 4b,c and d show selected 2Θ regions of the in situ XRD patterns for the first discharge to 1.2 V vs. Li+/Li. The XRD patterns, as the NCA is reduced, exhibit a growing new weak peak at about 17.7° 2Θ (see Figure 4b). Following the evolution of the (003) Bragg peak in Figure 4c that is related to the variation of the ‘‘chex.’’ axis and is intensely affected by the amount of lithium in the lattice, from the spectra taken at 3.3 V to the spectra taken at 1.2 V two main processes (Stage I and II) are mainly observed. Until a potential of 2.14 V (Stage I) is reached, the (003) line shifts to higher angles providing conclusive evidence that there is further lithiation of the NCA below 3 V. From this potential down to 1.2 V (Stage II) no clear shift towards higher 2-Theta angles is observed and the intensity of this reflection diminishes gradually and its peak width broadens. At the same time, the (101) reflection suffers a severe intensity reduction and peak’s width broadening consistent with the electrochemical onset of a structural change to an amorphous-like or more disordered *R-3m structure. At this stage II of two-phase transition reaction, clearly defined by the electrochemical potential plateau seen in Figure 2a, the visualization of a new Bragg reflection appearing at about 17.7° 2Theta (Figure 2b) is a strong evidence of an additional phase transformation. This new reflection can be assigned to the (001) reflection of the P3m1 space group for Li1+xNi0.80Co0.15Al0.05O2 phase, which has been already observed in other layered materials(20) (21). The observation of this phase indicates that, in some electrode regions, the NCA composition is such that further lithiation allows the accommodation of an additional Li into the host lattice and promotes the R-3m LiMO2 to P-3m1 Li2MO2 phase transformation (see Figure 5), the latter phase having hexagonal symmetry in which the oxygen ions are arranged in a hexagonally close-packed (hcp) array and the metal ions are located in all the octahedral sites of one layer and the Li ions are located in all the tetrahedral sites of adjacent layers.(20) At such very low potential a displacement reaction, in which the tran-

sition elements are reduced to their metallic state, can likely occur. For the NCA though, there is XRD evidence at 1.2 V of neither Ni nor Co reduction to the metal in the low-voltage two-phase region and at the end of discharge.

Figure 4. a) Galvanostatic curves for the first cycle of an NCA electrode cycled vs. Li at 10 mA/g at room temperature -the dots displaying specific potentials on the galvanostatic profile corresponding to the X-ray diffractograms that are shown in the following graphs; b, c, and d) in situ X-ray diffraction patterns at selected 2Θ regions for NCA with Bragg peaks according to space group R-3m and P-3m1.

Using X-ray powder diffraction within an operando cell provides conclusive evidence that, from the total irreversible charge “loss” on first discharge to 3.0 V (~10 mAh/g), about 5 mAh/g can be associated to the kinetically hindered but still possible relithiation process of NCA when cycled down to 2.14 V, while the additional 56 mAh/g of specific charge “loss” on the first cycle may be related to irreversible phase changes, parasitic reactions at the surface(22), and particle disconnection(9) that will lead to inactive material within the electrode.

Figure 5. Schematic representation of the LiMO2 with R-3m to Li2MO2 with P-3m1 phase transformation.

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Chemistry of Materials reversible as visible in Figure 7a and b. Repetitive cycling down to the two-phase region leads on one hand to the amorphisation of the layered NCA and on the other hand to particle fracturing due to the ~ 20% crystal volume expansion associated with the R-3m to P-3m1 two-phase reaction. These two effects hinder galvanostatic reversibility of the (de)lithiation of the NCA, as the initial layered structure is lost and eventually the particle-particle contacts, which in turn questions the usefulness of cycling over a wide potential window in practical cells.

Figure 6. a) Galvanostatic curves for the first cycle and second charge (in blue) of an NCA electrode cycled vs. Li at 10 mA/g at room temperature -the dots displaying specific potentials on the galvanostatic profile corresponding to the X-ray diffractograms that are shown in the following graphs; b, c, and d) in situ X-ray diffraction patterns at selected 2Θ regions for NCA with Bragg peaks according to space group R-3m and P-3m1.

Figure 6a shows the potential profile of the first cycle and second charge to 4 .3 V vs. lithium metal. Figure 6b,c, and d show selected 2Θ regions of the in situ XRD patterns for the second discharge to 1.2 V vs. Li+/Li that comprise the (001) diffraction peak of the P-3m1 phase and the (003) diffraction peak of the rombohedral phase. XRD results, while charging the cell, confirm the reversibility of the P-3m1 to R-3m two-phase transition reaction as the (001) P-3m1 line disappears gradually while the electrode is charged back to 3.0 V (Figure 4b). The (003) R-3m reflection (Figure 6c) initially shifts to higher 2Θ angles and gains intensity to get back to the NCA values at 3.0 V. Later on, as lithium is being de-inserted from the NCA lattice from 3.0 V to 3.68 V, the (003) line shifts to lower 2Θ angles, i.e. the c lattice parameter expands, as is typically observed for layered materials.(7) On this second charge after deep discharge we observe how the two rhombohedral phases, named R-3m and *R-3m, clearly seen in Figure 6d as well, de-lithiate via a solid solution mechanism up to a potential of 4.3 V. The two rhombohedral phases delithiate at different rates which it may also indicate slower lithiation kinetics for the new lowpotential *R-3m NCA phase. The distinctly different reaction kinetics observed during the second charge after deep discharge can be then attributed to crystal phase inhomogeneities within the NCA electrode attained at 1.2 V. Although the overlithiation reaction, accompanied by a structural change from R-3m to P-3m1 symmetry, initially occurred at some regions of the NCA electrode, as the electrode is further cycled within this operating potential window, the two phase reaction becomes more predominant. In turn, the (de)lithiation of the NCA becomes less

Figure 7. a) Charge–discharge curves in the potential window between 4.3 V and 1.2 V and b) dQ/dV vs. potential curves of LiNi0.80Co0.15Al0.05O2 at a specific current of 10 mA/g.

Conclusions The particular reactions paths by which electrochemical reactions allow further charge transfer of the layered NCA material bellow 3.0 V vs. Li+/Li were identified and divided in two stages. During stage I, between 3.0 V and 2.14 V vs. Li+/Li, the NCA’s specific charge “loss” of the first cycle is partially recovered by a solid solution mechanism. Bellow 2.14 V and throughout the plateau, a large specific charge on discharge is attained. The lithium storage reaction mechanism at this stage is identified as twophase reaction mechanism which involves the R-3m to P3m1 phase transition and the formation of a new *R-3m phase. Indeed, this clarification points to an additional lithiation process where the specific charge is not recovered by the original “standard” charge storage mechanism of LiNi0.80Co0.15Al0.05O2. Under such working conditions the material’s crystal structure and microstructure are irreversibly altered. Deep discharge empowers crystal structure heterogeneities of the NCA material that promote heterogeneous electrochemical reactions throughout the electrode on charge and discharge. The changes in the energy storage reaction mechanism, promoted by continuous deep discharge protocol, cause a decrease in the NCA’s performance and lifetime. Corresponding Author *[email protected]

Funding Sources The authors declare no competing financial interest.

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