Structural Degradation of Layered Cathode Materials in Lithium-Ion Batteries Induced by Ball Milling

: Layered LiNi 0.4 Mn 0.4 Co 0.18 Ti 0.02 O 2 cathode powders were ball-milled for various lengths of time. The structural properties of the pristine and milled powders, which have different particle sizes were examined with X-ray diffraction, soft X-ray absorption spectroscopy, and transmission electron microscopy to determine the effect of milling on structure. Electrochemical testing in half-cells was also carried out and shows that milling plays an important role in the performance of these cathode materials; as milling time increases, there is a decrease in initial discharge capacity. The first cycle irreversible capacity also increases for milled samples, as does capacity loss upon cycling under some regimes.The electrochemical degradation is strongly correlated with damage to the lamellar structure of cathode particles induced by milling, and lithium carbonate formation.

currently the most promising cathode materials for lithium ion batteries intended for vehicular applications, due to the reduced cobalt content, lower cost, and increased safety compared to LiCoO 2 (LCO). Other advantages include the significantly higher capacities and better rate capability compared to LCO. 3,4 However, improvement of the performance of NMC at high cutoff voltages and at high current densities is still needed. For the latter, the rate capability is limited by the movement of lithium ions in the active material and specifically to the slow lithium diffusion kinetics within the grains and the low intrinsic electronic conductivity. 5,6 In theory, rate capability can be improved by utilizing a cathode material with smaller particle size. 7 The reduced particle size shortens the diffusion pathways for lithium ions and also creates a large contact area between the active material and conductive additives. 8,9 Zheng et al. reported that Li-ion diffusion within the electrode is the rate-determining step for the discharge process at high rates. 10 The beneficial effect of smaller particle sizes has been shown using various cathode materials such as LiNi 0.50 Mn 0.50 O 2 , LiNi 0.33 Mn 0.33 Co 0.33 O 2 , and LiNi 0.40 Mn 0.40 Co 0.20 O 2 . 11,12 However, smaller particle size is usually associated with high surface area, which may cause higher capacity loss due to faster ageing and side reactions within the cell, 13 as has been found with nano-sized LiCoO 2 used as the cathode material for high power applications. 14, 15 An optimal particle size in terms of performance is believed to exist in the submicron particle size range for cathode materials.
Therefore, methods that can reduce the particle size of NMC to improve the performance are of significant interest.
In general, the electrochemical performance of NMCs depends on details of the preparation and synthesis conditions, which can result in a wide range of powder properties (e.g., particle size, density, and surface area) and electrochemical properties (e.g., rate capability and irreversibility). Ball milling has been shown to be an effective mechanical process to decrease particle size to the sub-micron size domain for a wide range of energy related materials 16,17 and is commonly used in preparation of electrode materials. In principle, ball milling is a simple and scalable process, which results in variation of the stoichiometry, degree of intermixing, and crystallite size by controlling milling time. Previous work has shown that ball milling effectively reduced the particle size of NMC cathode materials and related compounds 18,19 and improved the performance of LiMnPO 4 20 and LiMn 2 O 4 21 cathodes. However, the effect of ball milling conditions on the structure and properties of NMC cathodes has not been thoroughly investigated. The effective mechanism of particle size reduction induced by ball milling on the performance of the cathode is still unclear. The conditions used for ball milling processes are particularly important and can significantly affect material structure and surface area, which can in turn affect the electrochemical properties. It has been previously shown that lithium-ion battery electrodes evolve with cycling (for example, in situ surface reconstruction and formation of a surface reaction layer can cause deterioration in electrochemical performance). [13][14][15][22][23][24]

Materials Characterization
A Phillips X'Pert diffractometer with an X'celerator detector using copper K α radiation was used to obtain powder x-ray diffraction (XRD) patterns. The XRD patterns were typically scanned over an angular range of 10-90• (2θ)) with a step size of 0.0167•. Rietveld refinement was carried out using MDI Riqas software. Scanning electron microscopy images (SEM) were collected by a field-emission microscope (JEOL JSM-7500F) equipped with a Thermo Scientific Inc. energy dispersive X-ray spectroscopy (EDS) detector to examine particle size and morphology and to determine sample composition.

Electrochemical Properties of NMC
The ball milling treatment has a critical influence on the charge-discharge performance of NMC cathodes in the first cycle. The galvanostatic profiles in the first cycle of lithium half-cells containing the NMC cathodes with different particle sizes are presented in Figure 1 and Figure 2.
All the cells were charged and discharged between 2.0 and 4.7, 4.5, or 4.3 V at a current density of 0.055 mA/cm 2 . The differences between the pristine and ball-milled samples are immediately apparent. The discharge profiles after charging to 4.7 V are typical of stoichiometric NMCs, but the pristine sample delivers higher specific capacities than those treated by the ball milling process. The initial charge curve for the cells containing pristine NMC cathodes show a gradually sloping profile even at high voltage ranges. In contrast, the cell with NMC cathode material ball milled for three hours shows an obvious voltage plateau at 4.5 V, when charged to 4.7 V (Figure 1).
Since the NMC was ball milled in air with acetone, this suggests that there was lithium carbonate formation, which is known to oxidize at potentials Ball milling NMC results in the reduction of both discharge capacities and Coulombic efficiencies (CE) during the first cycle for cells containing these materials. As indicated in Figure 1a, the first charge capacities of all three NMC cathodes all exceed 210 mAh/g. However, the first discharge capacities vary considerably between the cells containing the pristine material and those containing ball-milled NMC. More than 220 mAh/g (4.7 V limit) was obtained for the former, while the discharge capacities of the NMC cathodes after one and three hours of ball milling were approximately 170 and 140 mAh/g, respectively. First cycle irreversible capacities are also higher for cells containing milled materials (38% and 32% compared to 19% for the one containing the pristine material, when the charge limit was 4.7 V). The  Figure 1b shows the discharge capacities obtained over 20 cycles at a current density of 0.055 mA/cm 2 between 4.7-2.0 V for all three types of NMC cathodes. At these high potentials, electrolyte oxidation can occur, resulting in increased cell impedance, and apparent capacity fading. 22 The smaller particle size and higher surface area of the milled NMC samples may exacerbate these processes, although the rate of fading in cells containing these materials is not markedly different than that of cells containing pristine NMC (instead, the milling results in an overall reduction of capacity on every cycle). Exacerbated capacity fading for cells containing milled materials was, however, observed when voltage limits of 4.5-2.0 V or 4.3-2.0 V for the ball-milled NMC cathodes, as seen in Figure 2. These results suggest that the effects of milling on electrochemical performance are not solely due to increased surface area and reactivity with electrolyte, as electrolyte oxidation should be minimal using these voltage cutoffs, particularly for 4.3 V. Figure 4 shows the XRD patterns of the pristine and ball-milled samples.

NMC materials characterization
The XRD results confirm that the NMC pristine material had the expected R3 ̅ m layered structure (red line in XRD pattern) with strong crystalline peaks, particularly the 2=19 o (003) peak. However, the ball-milled NMC materials show very obvious differences in the crystallinity as evidenced by peak broadening. There is also an impurity peak at around 2=30 o marked with asterisks, attributable to Li 2 CO 3 , 33 observable in the pattern of NMC ballmilled 3 hours. The zoomed-in sections in Figure 4b show the peak broadening induced by ball milling, and the merging of the doublet near 2=65, which suggests a loss of lamellarity. In well-layered structures, the (018) and (110) reflections are clearly separated, but in rock salt or spinel phases, only one peak near 2=65is observed. Another qualitative indication of how layered a structure is, is to determine the ratios between the integrated intensities of (003) and (104) peaks, which approximate the amount of cation mixing. 3,32,[34][35][36][37] Here, the ratio decreases with extended ball milling time (Table 1). Table 1

SEM and TEM analysis
The chemical compositions and morphologies of the NMC materials were studied by SEM-EDS. The EDS analysis provided an estimate of the chemical composition for the pristine materials of NMC: Ni ~43.0%, Mn ~37.8%, Cõ 6.9%, and Ti~2.4%, somewhat more Ni-rich than the targeted chemical composition, and similar to that reported by Kam et al. using a similar synthetic protocol. 32 The micrographs (SEM) of these NMC samples are shown in Figure 5. The pristine NMC sample is made up of large secondary particles, containing primary particles of irregular shapes. These ranged in size from 110-130 nm in diameter, and the boundaries between different particles are clearly distinguishable (Figure 5a). The primary particle size for the sample ball-milled for one hour is only slightly reduced to about 80-100 nm (Figure 5b), in agreement with the XRD data. The NMC particles are still agglomerated into larger secondary particles. After 3 hours of ball milling, there is a further reduction in primary particle size to about 50-60 nm ( Figure   5c). The small particles agglomerate to form considerably larger secondary particles compared to the pristine materials after three hours ball milling.
Brunauer-Emmett-Teller (BET) characterization was conducted to obtain the specific surface areas of the pristine and ball-milled samples through N 2 physisorption. The surface areas of NMC particles are roughly related to the average particle size and increased with milling time (Table 1), with the largest differences seen between the pristine material and the one milled for one hour. The higher surface area of the ball-milled samples may contribute to the increased first cycle Coulombic inefficiency observed for cells containing these materials, charged to 4.7 V and discharged to 2.0 V, due to amplified irreversible electrolyte oxidation.
The XRD and SEM results indicate that structural changes occurred during the ball milling of NMC. To better understand these effects, the sample ballmilled for 3 hours was further studied by high resolution TEM. Previous studies on the pristine material 22 show that it has a well-ordered lamellar structure. Figure 6a shows STEM images taken on pristine NMC particles. The pristine NMC materials exhibit the expected R3 ̅ m layered structure, with well-organized transition metal layers observed in the image. Figure 6b shows a particle of the sample milled for three hours. After 3 hours milling, some of the particles converted almost completely into the rock salt structure. One of these NMC particles is shown in Figure 6b. The fast Fourier transform of Figure 6b shows that TEM imaging was performed along the Fm3 ̅ m [110] zone axis. The FFT patterns from the region marked by a yellow dashed square in Fig. 6b shows evidence of the conversion of the R3 ̅ m structure to the Fm3 ̅ m structure. The selected area Fast Fourier transform pattern of the bulk (Figure 6b) also shows considerable streaking consistent with disorder. This adds to the evidence that ball milling induced structural changes in the NMC phase, not only at the surface but also in the bulk.

Materials characterization-soft (XAS)
A limited number of particles can be studied using STEM, making it a challenge to obtain information on the average bulk structure. In contrast, soft X-ray absorption spectroscopy may be used to obtain ensembleaveraged information about the electronic structure of materials; 38  based on previous XAS studies. 40,41 The XAS spectra show L 3 doublet peaks for Ni 2+ and Mn 4+ and a single peak for Co 3+ , as expected. A lower energy shoulder is observed for the Co L 3 edge for both ball-milled samples ( Figure   7a), consistent with reduction of some trivalent Co 3+ to the divalent state. This is observed in both the TEY and FY modes, indicating that reduction happened in the bulk as well as at the surface. A very slight reduction at both sampling depths was also observed for Mn, as evidenced by the marginally increased intensity on the low energy side of the L 3 peaks. For Ni, the ratio between the L 3high and L 3low peaks is a sensitive indicator of the relative oxidation state. Table 2 lists these values. The lower ratios for the milled samples indicate that reduction of Ni occurred, particularly near the surface. According to the EDS elemental analysis, this sample was slightly Nirich, so that the initial average oxidation state of Ni in the pristine state was probably slightly greater than +2. Reduction of transition metals is consistent with the formation of rock salt observed in the microscopy discussed above.
The O K-edge data for the NMC materials after different ball milling durations is shown in Figure 8. The O K-edge includes a higher energy region (>535 eV) basically associated with O 1s transitions to hybridized TM4sp-O2p states and a lower energy region (<535 eV) mainly originating from O1s transitions to TM3d-O2p hybridized states. 42 The lower energy region of O Kedge is of most interest here. Three peaks or shoulders (labeled 1, 2, and 3 in Fig. 8a and 8b) can be assigned by comparison to standard samples. Peak 1 around 530 eV can be attributed to Mn 4+ 3d-O2p, Ni 3+ 3d-O2p, or Co 3+ 3d-O2p hybridization. Peak 2 (532 eV) mainly arises from the contribution of Mn 4+ 3d-O2p or Ni 2+ 3d-O2p bonds. Peak 3 (534 eV) corresponds to the presence of Li 2 CO 3 . In all regions, there is some overlap of TM-O signals, making definitive assignment difficult, as discussed in reference 43. The peak at 534 eV in the TEY spectra for the ball-milled NMC materials is much more prominent than for the pristine material, and also appears in the FY spectra, while it is absent for the pristine material. Thus, it is reasonable to assign this to the presence of Li 2 CO 3 in the ball-milled materials, as also evidenced by the XRD data. The Li 2 CO 3 compound is frequently detected as a stable by-product on the particle surfaces of nickel-rich lithium-host materials. [44][45][46] The XAS data suggests that a small amount of Li 2 CO 3 may have already been present on surfaces of the pristine material, to a depth of about 5 nm, but the amount greatly increased after ball-milling, judging from the signal seen in FY mode, probing the bulk.
The integrated intensities of the TM3d-O2p hybridization peaks in FY and TEY mode appear to be somewhat lower for the milled NMC than for the pristine material, indicating that the bulk in the NMC-442 particles has fewer TM3d-O2p unoccupied states, consistent with metal reduction. This is particularly true for peak 1, which has some contributions from Ni 3+ . milling also results in a reduction in average primary particle size and an increase in surface area, but this, somewhat surprisingly, has little effect on capacity retention when a 4.7V charging limit is used, suggesting that the structural changes and formation of lithium carbonate have the largest negative effects on the electrochemistry.

Conclusions
In summary, we have shown that the effects of structural changes caused by ball milling have deleterious effects on the performance of NMC-442 cathode materials during electrochemical cycling. Along with a reduction in particle size, ball milling causes disordering of the layered structure, reduction of transition metals, and the formation of rock salt phases and lithium carbonate, as evidenced by XRD, soft XAS, and TEM results. These changes are correlated with higher Coulombic inefficiencies, lower initial capacities, and capacity fading during cycling to high voltage, but harmful effects were also observed even when upper voltage limits of 4.5 or 4.3 V were used.

Notes
The authors declare no competing financial interest.