(a) Specific discharge capacity and coulombic efficiency of NMC111-graphite cells and (b) charge-averaged mean discharge voltage (s. Eq. 1) of the NMC111 cathode (≡ ¯ V cathode discharge ; solid lines) and the graphite anode (≡ ¯ V anode discharge ; dashed lines) vs. cycle number in LP57 electrolyte (1 M LiPF 6 in EC:EMC 3:7) operated with different upper cutoff voltages (4.2 V, 4.4 V, 4.6 V) and a constant lower cutoff voltage of 3.0 V. Formation was done at a rate of 0.1 C. Cycling was performed at 1 C and 25 • C. For each condition, two independent cells were run and the data in the figure always represent the average of two cells (the error bars in (a) represent the standard deviation between the two cells). 

Contexts in source publication

Context 1

... the cells were cycled with a lithium reference electrode, ¯ V discharge can be determined independently for the NMC111 cathode (≡ ¯ V cathode discharge ) and the graphite anode (≡ ¯ V anode discharge ) for each end-of- charge voltage as a function of the cycle number, which is depicted in Figure 1b by the solid and dashed lines, respectively. While the energy fading of the cells is further detailed in the Discussion section, it may be noted here that the discharge energy for each cycle corresponds to the product of capacity and ¯ V discharge = ¯ V cathode discharge − ¯ V anode discharge . Un- der conditions where the loss of cyclable lithium is the only aging mechanism, i.e., in the absence of an impedance buildup, ¯ V cathode discharge for cathode active materials with a strongly sloping charge/voltage curve like NMC would be expected to gradually increase with the number of cycles. This can indeed be seen when cycling with an upper cutoff potential of 4.2 V (solid black line in Figure 1b). On the other hand, when impedance buildup becomes dominant, ¯ V cathode discharge decreases with the number of cycles, as can be seen when the upper cutoff poten- tial reaches 4.6 V (solid light gray line in Figure 1b). Interestingly, the charge-averaged mean discharge voltages of the graphite anodes ( ¯ V anode discharge ) remain fairly constant over the complete number of cycles, even at high end-of-charge voltages. This suggests that a crucial con- tributing factor for the fast capacity fading of the NMC111-graphite cells at an upper cutoff of 4.6 V is a strong impedance buildup on the NMC111 cathode rather than on the graphite anode. In fact, pre- vious reports in the literature showed a drastic rise of the low fre- quency semicircle in the impedance spectra of NMC111-graphite 11 and NMC442-graphite cells, 43,44 which was attributed to the positive electrode. Later, Petibon et al. showed that the increase of impedance in NMC442-graphite cells operated at high cutoff potentials, indeed stems from the positive electrode, proven by using symmetric cells. 45 Even though these results are consistent with our observations on the charge-averaged mean discharge voltage (Figure 1b) one has to be careful since an additive-containing electrolyte was used in References 43-45, which likely causes a different surface film forma- tion and impedance. A detailed discussion about the reason for the rise in the polarization of NMC111 with upper cutoff potential is given in the Discussion section. Figure 2a shows the cycling stability of NMC622-graphite cells. Similar to the case of the NMC111-graphite cells also NMC622- graphite cells can be cycled stably to upper cutoff voltages of 4.2 V and 4.4 V with excellent coulombic efficiencies of >99.9%, whereas at an upper cutoff potential of 4.6 V, the capacity fades rapidly and the coulombic efficiency decreases to ∼99.6% (before the rollover- failure), as was observed for NMC111. In analogy to the cells with NMC111, the occurrence of a rollover-failure 41,42 at 4.6 V cutoff indi- cates growing polarization and causes the large error bars at high cycle numbers as described above. Also with respect to the mean discharge voltages, NMC622 (s. Figure 2b) stability is observed. In order to aid the comparison between the dif- ferent NMCs, the capacity retentions measured between the 5 th and the 300 th cycles at a 1C-rate for all cells presented in Figures 1-3 are summarized in Table I. Stable cycling with capacity retentions ≥90 % is possible for NMC111 and NMC622 up to 4.4 V and for NMC811 only up to 4.0 V, whereby its capacity retention is still clearly lower than that for the cells with NMC111 and NMC622 cycled to 4.4 V. It is interesting to note that the measured specific capacity of NMC811 at a 4.2 V cutoff is similar to the one of NMC622 at 4.4 V (see values in parentheses in Table I), however, with the latter one still having a stable cycling performance. The impact of the different cutoff volt- ages on the specific energy of the cells will be picked-up again in the Discussion section. The coulombic efficiencies for the NMC811- graphite cells are >99.9% at 4.0 V cutoff potential, and even at 4.1 V and 4.2 V, where pronounced capacity fading is observed, their coulombic efficiency remains at ∼99.9%, i.e., similar to that of the NMC111 and NMC622 cells at 4.4 V. The fact that the latter display substantially lower capacity fading suggests that its origin must be an enhanced cathode and/or anode impedance ...

Context 2

... the cells were cycled with a lithium reference electrode, ¯ V discharge can be determined independently for the NMC111 cathode (≡ ¯ V cathode discharge ) and the graphite anode (≡ ¯ V anode discharge ) for each end-of- charge voltage as a function of the cycle number, which is depicted in Figure 1b by the solid and dashed lines, respectively. While the energy fading of the cells is further detailed in the Discussion section, it may be noted here that the discharge energy for each cycle corresponds to the product of capacity and ¯ V discharge = ¯ V cathode discharge − ¯ V anode discharge . Un- der conditions where the loss of cyclable lithium is the only aging mechanism, i.e., in the absence of an impedance buildup, ¯ V cathode discharge for cathode active materials with a strongly sloping charge/voltage curve like NMC would be expected to gradually increase with the number of cycles. This can indeed be seen when cycling with an upper cutoff potential of 4.2 V (solid black line in Figure 1b). On the other hand, when impedance buildup becomes dominant, ¯ V cathode discharge decreases with the number of cycles, as can be seen when the upper cutoff poten- tial reaches 4.6 V (solid light gray line in Figure 1b). Interestingly, the charge-averaged mean discharge voltages of the graphite anodes ( ¯ V anode discharge ) remain fairly constant over the complete number of cycles, even at high end-of-charge voltages. This suggests that a crucial con- tributing factor for the fast capacity fading of the NMC111-graphite cells at an upper cutoff of 4.6 V is a strong impedance buildup on the NMC111 cathode rather than on the graphite anode. In fact, pre- vious reports in the literature showed a drastic rise of the low fre- quency semicircle in the impedance spectra of NMC111-graphite 11 and NMC442-graphite cells, 43,44 which was attributed to the positive electrode. Later, Petibon et al. showed that the increase of impedance in NMC442-graphite cells operated at high cutoff potentials, indeed stems from the positive electrode, proven by using symmetric cells. 45 Even though these results are consistent with our observations on the charge-averaged mean discharge voltage (Figure 1b) one has to be careful since an additive-containing electrolyte was used in References 43-45, which likely causes a different surface film forma- tion and impedance. A detailed discussion about the reason for the rise in the polarization of NMC111 with upper cutoff potential is given in the Discussion section. Figure 2a shows the cycling stability of NMC622-graphite cells. Similar to the case of the NMC111-graphite cells also NMC622- graphite cells can be cycled stably to upper cutoff voltages of 4.2 V and 4.4 V with excellent coulombic efficiencies of >99.9%, whereas at an upper cutoff potential of 4.6 V, the capacity fades rapidly and the coulombic efficiency decreases to ∼99.6% (before the rollover- failure), as was observed for NMC111. In analogy to the cells with NMC111, the occurrence of a rollover-failure 41,42 at 4.6 V cutoff indi- cates growing polarization and causes the large error bars at high cycle numbers as described above. Also with respect to the mean discharge voltages, NMC622 (s. Figure 2b) stability is observed. In order to aid the comparison between the dif- ferent NMCs, the capacity retentions measured between the 5 th and the 300 th cycles at a 1C-rate for all cells presented in Figures 1-3 are summarized in Table I. Stable cycling with capacity retentions ≥90 % is possible for NMC111 and NMC622 up to 4.4 V and for NMC811 only up to 4.0 V, whereby its capacity retention is still clearly lower than that for the cells with NMC111 and NMC622 cycled to 4.4 V. It is interesting to note that the measured specific capacity of NMC811 at a 4.2 V cutoff is similar to the one of NMC622 at 4.4 V (see values in parentheses in Table I), however, with the latter one still having a stable cycling performance. The impact of the different cutoff volt- ages on the specific energy of the cells will be picked-up again in the Discussion section. The coulombic efficiencies for the NMC811- graphite cells are >99.9% at 4.0 V cutoff potential, and even at 4.1 V and 4.2 V, where pronounced capacity fading is observed, their coulombic efficiency remains at ∼99.9%, i.e., similar to that of the NMC111 and NMC622 cells at 4.4 V. The fact that the latter display substantially lower capacity fading suggests that its origin must be an enhanced cathode and/or anode impedance ...

Context 3

... the cells were cycled with a lithium reference electrode, ¯ V discharge can be determined independently for the NMC111 cathode (≡ ¯ V cathode discharge ) and the graphite anode (≡ ¯ V anode discharge ) for each end-of- charge voltage as a function of the cycle number, which is depicted in Figure 1b by the solid and dashed lines, respectively. While the energy fading of the cells is further detailed in the Discussion section, it may be noted here that the discharge energy for each cycle corresponds to the product of capacity and ¯ V discharge = ¯ V cathode discharge − ¯ V anode discharge . Un- der conditions where the loss of cyclable lithium is the only aging mechanism, i.e., in the absence of an impedance buildup, ¯ V cathode discharge for cathode active materials with a strongly sloping charge/voltage curve like NMC would be expected to gradually increase with the number of cycles. This can indeed be seen when cycling with an upper cutoff potential of 4.2 V (solid black line in Figure 1b). On the other hand, when impedance buildup becomes dominant, ¯ V cathode discharge decreases with the number of cycles, as can be seen when the upper cutoff poten- tial reaches 4.6 V (solid light gray line in Figure 1b). Interestingly, the charge-averaged mean discharge voltages of the graphite anodes ( ¯ V anode discharge ) remain fairly constant over the complete number of cycles, even at high end-of-charge voltages. This suggests that a crucial con- tributing factor for the fast capacity fading of the NMC111-graphite cells at an upper cutoff of 4.6 V is a strong impedance buildup on the NMC111 cathode rather than on the graphite anode. In fact, pre- vious reports in the literature showed a drastic rise of the low fre- quency semicircle in the impedance spectra of NMC111-graphite 11 and NMC442-graphite cells, 43,44 which was attributed to the positive electrode. Later, Petibon et al. showed that the increase of impedance in NMC442-graphite cells operated at high cutoff potentials, indeed stems from the positive electrode, proven by using symmetric cells. 45 Even though these results are consistent with our observations on the charge-averaged mean discharge voltage (Figure 1b) one has to be careful since an additive-containing electrolyte was used in References 43-45, which likely causes a different surface film forma- tion and impedance. A detailed discussion about the reason for the rise in the polarization of NMC111 with upper cutoff potential is given in the Discussion section. Figure 2a shows the cycling stability of NMC622-graphite cells. Similar to the case of the NMC111-graphite cells also NMC622- graphite cells can be cycled stably to upper cutoff voltages of 4.2 V and 4.4 V with excellent coulombic efficiencies of >99.9%, whereas at an upper cutoff potential of 4.6 V, the capacity fades rapidly and the coulombic efficiency decreases to ∼99.6% (before the rollover- failure), as was observed for NMC111. In analogy to the cells with NMC111, the occurrence of a rollover-failure 41,42 at 4.6 V cutoff indi- cates growing polarization and causes the large error bars at high cycle numbers as described above. Also with respect to the mean discharge voltages, NMC622 (s. Figure 2b) stability is observed. In order to aid the comparison between the dif- ferent NMCs, the capacity retentions measured between the 5 th and the 300 th cycles at a 1C-rate for all cells presented in Figures 1-3 are summarized in Table I. Stable cycling with capacity retentions ≥90 % is possible for NMC111 and NMC622 up to 4.4 V and for NMC811 only up to 4.0 V, whereby its capacity retention is still clearly lower than that for the cells with NMC111 and NMC622 cycled to 4.4 V. It is interesting to note that the measured specific capacity of NMC811 at a 4.2 V cutoff is similar to the one of NMC622 at 4.4 V (see values in parentheses in Table I), however, with the latter one still having a stable cycling performance. The impact of the different cutoff volt- ages on the specific energy of the cells will be picked-up again in the Discussion section. The coulombic efficiencies for the NMC811- graphite cells are >99.9% at 4.0 V cutoff potential, and even at 4.1 V and 4.2 V, where pronounced capacity fading is observed, their coulombic efficiency remains at ∼99.9%, i.e., similar to that of the NMC111 and NMC622 cells at 4.4 V. The fact that the latter display substantially lower capacity fading suggests that its origin must be an enhanced cathode and/or anode impedance ...

Context 4

... the cells were cycled with a lithium reference electrode, ¯ V discharge can be determined independently for the NMC111 cathode (≡ ¯ V cathode discharge ) and the graphite anode (≡ ¯ V anode discharge ) for each end-of- charge voltage as a function of the cycle number, which is depicted in Figure 1b by the solid and dashed lines, respectively. While the energy fading of the cells is further detailed in the Discussion section, it may be noted here that the discharge energy for each cycle corresponds to the product of capacity and ¯ V discharge = ¯ V cathode discharge − ¯ V anode discharge . Un- der conditions where the loss of cyclable lithium is the only aging mechanism, i.e., in the absence of an impedance buildup, ¯ V cathode discharge for cathode active materials with a strongly sloping charge/voltage curve like NMC would be expected to gradually increase with the number of cycles. This can indeed be seen when cycling with an upper cutoff potential of 4.2 V (solid black line in Figure 1b). On the other hand, when impedance buildup becomes dominant, ¯ V cathode discharge decreases with the number of cycles, as can be seen when the upper cutoff poten- tial reaches 4.6 V (solid light gray line in Figure 1b). Interestingly, the charge-averaged mean discharge voltages of the graphite anodes ( ¯ V anode discharge ) remain fairly constant over the complete number of cycles, even at high end-of-charge voltages. This suggests that a crucial con- tributing factor for the fast capacity fading of the NMC111-graphite cells at an upper cutoff of 4.6 V is a strong impedance buildup on the NMC111 cathode rather than on the graphite anode. In fact, pre- vious reports in the literature showed a drastic rise of the low fre- quency semicircle in the impedance spectra of NMC111-graphite 11 and NMC442-graphite cells, 43,44 which was attributed to the positive electrode. Later, Petibon et al. showed that the increase of impedance in NMC442-graphite cells operated at high cutoff potentials, indeed stems from the positive electrode, proven by using symmetric cells. 45 Even though these results are consistent with our observations on the charge-averaged mean discharge voltage (Figure 1b) one has to be careful since an additive-containing electrolyte was used in References 43-45, which likely causes a different surface film forma- tion and impedance. A detailed discussion about the reason for the rise in the polarization of NMC111 with upper cutoff potential is given in the Discussion section. Figure 2a shows the cycling stability of NMC622-graphite cells. Similar to the case of the NMC111-graphite cells also NMC622- graphite cells can be cycled stably to upper cutoff voltages of 4.2 V and 4.4 V with excellent coulombic efficiencies of >99.9%, whereas at an upper cutoff potential of 4.6 V, the capacity fades rapidly and the coulombic efficiency decreases to ∼99.6% (before the rollover- failure), as was observed for NMC111. In analogy to the cells with NMC111, the occurrence of a rollover-failure 41,42 at 4.6 V cutoff indi- cates growing polarization and causes the large error bars at high cycle numbers as described above. Also with respect to the mean discharge voltages, NMC622 (s. Figure 2b) stability is observed. In order to aid the comparison between the dif- ferent NMCs, the capacity retentions measured between the 5 th and the 300 th cycles at a 1C-rate for all cells presented in Figures 1-3 are summarized in Table I. Stable cycling with capacity retentions ≥90 % is possible for NMC111 and NMC622 up to 4.4 V and for NMC811 only up to 4.0 V, whereby its capacity retention is still clearly lower than that for the cells with NMC111 and NMC622 cycled to 4.4 V. It is interesting to note that the measured specific capacity of NMC811 at a 4.2 V cutoff is similar to the one of NMC622 at 4.4 V (see values in parentheses in Table I), however, with the latter one still having a stable cycling performance. The impact of the different cutoff volt- ages on the specific energy of the cells will be picked-up again in the Discussion section. The coulombic efficiencies for the NMC811- graphite cells are >99.9% at 4.0 V cutoff potential, and even at 4.1 V and 4.2 V, where pronounced capacity fading is observed, their coulombic efficiency remains at ∼99.9%, i.e., similar to that of the NMC111 and NMC622 cells at 4.4 V. The fact that the latter display substantially lower capacity fading suggests that its origin must be an enhanced cathode and/or anode impedance ...

Context 5

... the cells were cycled with a lithium reference electrode, ¯ V discharge can be determined independently for the NMC111 cathode (≡ ¯ V cathode discharge ) and the graphite anode (≡ ¯ V anode discharge ) for each end-of- charge voltage as a function of the cycle number, which is depicted in Figure 1b by the solid and dashed lines, respectively. While the energy fading of the cells is further detailed in the Discussion section, it may be noted here that the discharge energy for each cycle corresponds to the product of capacity and ¯ V discharge = ¯ V cathode discharge − ¯ V anode discharge . Un- der conditions where the loss of cyclable lithium is the only aging mechanism, i.e., in the absence of an impedance buildup, ¯ V cathode discharge for cathode active materials with a strongly sloping charge/voltage curve like NMC would be expected to gradually increase with the number of cycles. This can indeed be seen when cycling with an upper cutoff potential of 4.2 V (solid black line in Figure 1b). On the other hand, when impedance buildup becomes dominant, ¯ V cathode discharge decreases with the number of cycles, as can be seen when the upper cutoff poten- tial reaches 4.6 V (solid light gray line in Figure 1b). Interestingly, the charge-averaged mean discharge voltages of the graphite anodes ( ¯ V anode discharge ) remain fairly constant over the complete number of cycles, even at high end-of-charge voltages. This suggests that a crucial con- tributing factor for the fast capacity fading of the NMC111-graphite cells at an upper cutoff of 4.6 V is a strong impedance buildup on the NMC111 cathode rather than on the graphite anode. In fact, pre- vious reports in the literature showed a drastic rise of the low fre- quency semicircle in the impedance spectra of NMC111-graphite 11 and NMC442-graphite cells, 43,44 which was attributed to the positive electrode. Later, Petibon et al. showed that the increase of impedance in NMC442-graphite cells operated at high cutoff potentials, indeed stems from the positive electrode, proven by using symmetric cells. 45 Even though these results are consistent with our observations on the charge-averaged mean discharge voltage (Figure 1b) one has to be careful since an additive-containing electrolyte was used in References 43-45, which likely causes a different surface film forma- tion and impedance. A detailed discussion about the reason for the rise in the polarization of NMC111 with upper cutoff potential is given in the Discussion section. Figure 2a shows the cycling stability of NMC622-graphite cells. Similar to the case of the NMC111-graphite cells also NMC622- graphite cells can be cycled stably to upper cutoff voltages of 4.2 V and 4.4 V with excellent coulombic efficiencies of >99.9%, whereas at an upper cutoff potential of 4.6 V, the capacity fades rapidly and the coulombic efficiency decreases to ∼99.6% (before the rollover- failure), as was observed for NMC111. In analogy to the cells with NMC111, the occurrence of a rollover-failure 41,42 at 4.6 V cutoff indi- cates growing polarization and causes the large error bars at high cycle numbers as described above. Also with respect to the mean discharge voltages, NMC622 (s. Figure 2b) stability is observed. In order to aid the comparison between the dif- ferent NMCs, the capacity retentions measured between the 5 th and the 300 th cycles at a 1C-rate for all cells presented in Figures 1-3 are summarized in Table I. Stable cycling with capacity retentions ≥90 % is possible for NMC111 and NMC622 up to 4.4 V and for NMC811 only up to 4.0 V, whereby its capacity retention is still clearly lower than that for the cells with NMC111 and NMC622 cycled to 4.4 V. It is interesting to note that the measured specific capacity of NMC811 at a 4.2 V cutoff is similar to the one of NMC622 at 4.4 V (see values in parentheses in Table I), however, with the latter one still having a stable cycling performance. The impact of the different cutoff volt- ages on the specific energy of the cells will be picked-up again in the Discussion section. The coulombic efficiencies for the NMC811- graphite cells are >99.9% at 4.0 V cutoff potential, and even at 4.1 V and 4.2 V, where pronounced capacity fading is observed, their coulombic efficiency remains at ∼99.9%, i.e., similar to that of the NMC111 and NMC622 cells at 4.4 V. The fact that the latter display substantially lower capacity fading suggests that its origin must be an enhanced cathode and/or anode impedance ...

Context 6

... Eq. 4 it becomes clear that once the release of oxygen becomes slower (due to the growing thickness of the oxygen depleted surface layer), the local concentration of 1 O 2 decreases, so that the ratio of O 2 /CO 2 released to the gas phase (and detected by the mass spectrom- eter) is predicted to decrease over cycling, as indeed is observed. The gradually decreasing release of lattice oxygen over cycling, which we ascribe to a growing thickness of an oxygen depleted surface layer is also consistent with the observed decrease of the mean discharge potential of the cathode shown in the Figures 1-3 In a recent publication by Li et al. on NMC811 it was suggested that the c-axis contraction of the unit cell at potentials of ∼4.2 V may not be the reason for the poor cycling stability. 78 Instead, a rapid increase of the parasitic heat flow above 4.2 V vs. Li/Li + was detected and it was hypothesized that the highly delithiated cathode surface be very reactive toward the electrolyte causing an increased cathode impedance. 78 Our observation of a growing polarization is consistent with the study by Li et al., however, we believe that it might be the chemical reaction of the released oxygen with the electrolyte that drives the parasitic heat flow, rather than the direct electrochemical oxidation of the electrolyte on the surface. Additionally, Imhof et al. reported CO 2 evolution for LNO already at 4.2 V and ascribed it to the reactivity of the surface toward electrolyte. 79 However, since this onset potential coincides with the H2 → H3 phase transition, 47 we believe that it is more likely related to a release of oxygen from the layered LNO structure, followed by its chemical reaction with the electrolyte to CO 2 rather than to an electrochemical oxidation of the electrolyte at potentials as low as 4.2 ...

Context 7

... energy densities of NMC111, NMC622 and NMC811.- In the previous sections it was demonstrated that the release of oxygen from the NMC surface has a very detrimental impact on the material stability, as it causes significant gas evolution (Figures 7-9) as well as a significant increase of the polarization of the cathode material most probably due to the oxygen depleted surface layer (Figures 1-3). For NMC811, the oxygen release occurs already at potentials as low as 4.3 V vs. Li/Li + , whereas for NMC111 and NMC622 it occurs roughly at 4.7 V vs. Li/Li + . These values limit the end-of-charge voltage that can be applied to achieve a stable cycling and they have therefore a severe impact on the achievable specific energy of these materials. We want to highlight that additional aging mechanisms will be occurring in parallel, like lithium loss due to SEI growth on the anode, elec- trochemical electrolyte oxidation at high potentials on the cathode, metal dissolution from the cathode, etc.; however, when cycling up to potentials where oxygen release occurs, the formation of a resis- tive surface layer is the most severe aging mechanism under these conditions, causing significant capacity and discharge voltage fading during extended charge/discharge cycling (Figures 1-3). The specific energies of the cells shown in the Figures 1-3 are depicted in Figure 11 with the full bars representing the specific energy of the 5 th cycle at a 1C-rate. The dashed bars indicate the remaining specific energy after 300 cycles. As discussed before, stable cycling was possible for NMC111 and NMC622 up to 4.4 V and up to 4.0 V for NMC811. This is also clearly visible in Figure 11, as the differences between the spe- cific energies in the 5 th and 300 th cycle are fairly low for these voltage limits, but increase significantly for the others. The highest specific energy with stable cycling was achieved with NMC622 cycled up to 4.4 V. Comparing only the cells with stable cycling performance, it becomes clear that NMC811 reaches the lowest specific energy, which is due to the very low applicable end-of-charge voltage of only 4.0 V. This rather sobering outlook for NMC811 emphasizes the need to prohibit the oxygen release from the surface. Our results suggest that one way of making use of the high capacities of NMC811 and achieving stable cycling at the same time might be possible by either a core-shell structure in which the core consists of NMC811 with i) a shell that has a Ni-content of up to 60 % (surface like NMC622) and does not release oxygen until >4.4 V or ii) a shell consisting of an ordered spinel like high-voltage spinel (LNMO) that does hardly evolve any gases ( Figure 5) due to the absence of oxygen release. In both cases the shell would need to be thick enough to prevent oxygen loss from the core structure via the limited diffusion of the oxygen anions. Indeed, these approaches have been used by several research groups and we believe that the prevention of oxygen release explains the successful use of core-shell 85-88 materials possessing Ni- contents in the core and shell of 80% and ≤55%, respectively, and full concentration gradient 89-91 materials with Ni-contents of ≥75% and ≤56% in the particle center and the surface, respectively. Additionally, also a superior performance of LiMn 2 O 4 coated NMC over uncoated samples was reported by Cho et al. 92 Conclusions . NMC-graphite cells were cycled to different end- of-charge potentials and it was demonstrated that stable cycling is possible up to 4.4 V for NMC111 and NMC622 and only up to 4.0 V for NMC811. The capacity fading rates observed at 4.6 V for NMC111 and NMC622 and 4.1 V and 4.2 V for NMC811 are due to a significant increase in the polarization of the NMC electrode as evidenced by charge/discharge cycling in a 3-electrode setup with a lithium reference electrode. In contrast, the polarization of the graphite electrode remained rather constant. By a dq/dV analysis we demonstrated that the significant rise in the impedance occurs when the NMC materials are cycled up to a high-voltage feature at ∼4.7 V vs. Li/Li + for NMC111 and NMC622 and up to the H2 → H3 phase transition at ∼4.3 V vs. Li/Li + for NMC811; we hypothesize that this is caused by oxygen release from the NMC ...

Context 8

... energy densities of NMC111, NMC622 and NMC811.- In the previous sections it was demonstrated that the release of oxygen from the NMC surface has a very detrimental impact on the material stability, as it causes significant gas evolution (Figures 7-9) as well as a significant increase of the polarization of the cathode material most probably due to the oxygen depleted surface layer (Figures 1-3). For NMC811, the oxygen release occurs already at potentials as low as 4.3 V vs. Li/Li + , whereas for NMC111 and NMC622 it occurs roughly at 4.7 V vs. Li/Li + . These values limit the end-of-charge voltage that can be applied to achieve a stable cycling and they have therefore a severe impact on the achievable specific energy of these materials. We want to highlight that additional aging mechanisms will be occurring in parallel, like lithium loss due to SEI growth on the anode, elec- trochemical electrolyte oxidation at high potentials on the cathode, metal dissolution from the cathode, etc.; however, when cycling up to potentials where oxygen release occurs, the formation of a resis- tive surface layer is the most severe aging mechanism under these conditions, causing significant capacity and discharge voltage fading during extended charge/discharge cycling (Figures 1-3). The specific energies of the cells shown in the Figures 1-3 are depicted in Figure 11 with the full bars representing the specific energy of the 5 th cycle at a 1C-rate. The dashed bars indicate the remaining specific energy after 300 cycles. As discussed before, stable cycling was possible for NMC111 and NMC622 up to 4.4 V and up to 4.0 V for NMC811. This is also clearly visible in Figure 11, as the differences between the spe- cific energies in the 5 th and 300 th cycle are fairly low for these voltage limits, but increase significantly for the others. The highest specific energy with stable cycling was achieved with NMC622 cycled up to 4.4 V. Comparing only the cells with stable cycling performance, it becomes clear that NMC811 reaches the lowest specific energy, which is due to the very low applicable end-of-charge voltage of only 4.0 V. This rather sobering outlook for NMC811 emphasizes the need to prohibit the oxygen release from the surface. Our results suggest that one way of making use of the high capacities of NMC811 and achieving stable cycling at the same time might be possible by either a core-shell structure in which the core consists of NMC811 with i) a shell that has a Ni-content of up to 60 % (surface like NMC622) and does not release oxygen until >4.4 V or ii) a shell consisting of an ordered spinel like high-voltage spinel (LNMO) that does hardly evolve any gases ( Figure 5) due to the absence of oxygen release. In both cases the shell would need to be thick enough to prevent oxygen loss from the core structure via the limited diffusion of the oxygen anions. Indeed, these approaches have been used by several research groups and we believe that the prevention of oxygen release explains the successful use of core-shell 85-88 materials possessing Ni- contents in the core and shell of 80% and ≤55%, respectively, and full concentration gradient 89-91 materials with Ni-contents of ≥75% and ≤56% in the particle center and the surface, respectively. Additionally, also a superior performance of LiMn 2 O 4 coated NMC over uncoated samples was reported by Cho et al. 92 Conclusions . NMC-graphite cells were cycled to different end- of-charge potentials and it was demonstrated that stable cycling is possible up to 4.4 V for NMC111 and NMC622 and only up to 4.0 V for NMC811. The capacity fading rates observed at 4.6 V for NMC111 and NMC622 and 4.1 V and 4.2 V for NMC811 are due to a significant increase in the polarization of the NMC electrode as evidenced by charge/discharge cycling in a 3-electrode setup with a lithium reference electrode. In contrast, the polarization of the graphite electrode remained rather constant. By a dq/dV analysis we demonstrated that the significant rise in the impedance occurs when the NMC materials are cycled up to a high-voltage feature at ∼4.7 V vs. Li/Li + for NMC111 and NMC622 and up to the H2 → H3 phase transition at ∼4.3 V vs. Li/Li + for NMC811; we hypothesize that this is caused by oxygen release from the NMC ...

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... energy densities of NMC111, NMC622 and NMC811.- In the previous sections it was demonstrated that the release of oxygen from the NMC surface has a very detrimental impact on the material stability, as it causes significant gas evolution (Figures 7-9) as well as a significant increase of the polarization of the cathode material most probably due to the oxygen depleted surface layer (Figures 1-3). For NMC811, the oxygen release occurs already at potentials as low as 4.3 V vs. Li/Li + , whereas for NMC111 and NMC622 it occurs roughly at 4.7 V vs. Li/Li + . These values limit the end-of-charge voltage that can be applied to achieve a stable cycling and they have therefore a severe impact on the achievable specific energy of these materials. We want to highlight that additional aging mechanisms will be occurring in parallel, like lithium loss due to SEI growth on the anode, elec- trochemical electrolyte oxidation at high potentials on the cathode, metal dissolution from the cathode, etc.; however, when cycling up to potentials where oxygen release occurs, the formation of a resis- tive surface layer is the most severe aging mechanism under these conditions, causing significant capacity and discharge voltage fading during extended charge/discharge cycling (Figures 1-3). The specific energies of the cells shown in the Figures 1-3 are depicted in Figure 11 with the full bars representing the specific energy of the 5 th cycle at a 1C-rate. The dashed bars indicate the remaining specific energy after 300 cycles. As discussed before, stable cycling was possible for NMC111 and NMC622 up to 4.4 V and up to 4.0 V for NMC811. This is also clearly visible in Figure 11, as the differences between the spe- cific energies in the 5 th and 300 th cycle are fairly low for these voltage limits, but increase significantly for the others. The highest specific energy with stable cycling was achieved with NMC622 cycled up to 4.4 V. Comparing only the cells with stable cycling performance, it becomes clear that NMC811 reaches the lowest specific energy, which is due to the very low applicable end-of-charge voltage of only 4.0 V. This rather sobering outlook for NMC811 emphasizes the need to prohibit the oxygen release from the surface. Our results suggest that one way of making use of the high capacities of NMC811 and achieving stable cycling at the same time might be possible by either a core-shell structure in which the core consists of NMC811 with i) a shell that has a Ni-content of up to 60 % (surface like NMC622) and does not release oxygen until >4.4 V or ii) a shell consisting of an ordered spinel like high-voltage spinel (LNMO) that does hardly evolve any gases ( Figure 5) due to the absence of oxygen release. In both cases the shell would need to be thick enough to prevent oxygen loss from the core structure via the limited diffusion of the oxygen anions. Indeed, these approaches have been used by several research groups and we believe that the prevention of oxygen release explains the successful use of core-shell 85-88 materials possessing Ni- contents in the core and shell of 80% and ≤55%, respectively, and full concentration gradient 89-91 materials with Ni-contents of ≥75% and ≤56% in the particle center and the surface, respectively. Additionally, also a superior performance of LiMn 2 O 4 coated NMC over uncoated samples was reported by Cho et al. 92 Conclusions . NMC-graphite cells were cycled to different end- of-charge potentials and it was demonstrated that stable cycling is possible up to 4.4 V for NMC111 and NMC622 and only up to 4.0 V for NMC811. The capacity fading rates observed at 4.6 V for NMC111 and NMC622 and 4.1 V and 4.2 V for NMC811 are due to a significant increase in the polarization of the NMC electrode as evidenced by charge/discharge cycling in a 3-electrode setup with a lithium reference electrode. In contrast, the polarization of the graphite electrode remained rather constant. By a dq/dV analysis we demonstrated that the significant rise in the impedance occurs when the NMC materials are cycled up to a high-voltage feature at ∼4.7 V vs. Li/Li + for NMC111 and NMC622 and up to the H2 → H3 phase transition at ∼4.3 V vs. Li/Li + for NMC811; we hypothesize that this is caused by oxygen release from the NMC ...

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... energy densities of NMC111, NMC622 and NMC811.- In the previous sections it was demonstrated that the release of oxygen from the NMC surface has a very detrimental impact on the material stability, as it causes significant gas evolution (Figures 7-9) as well as a significant increase of the polarization of the cathode material most probably due to the oxygen depleted surface layer (Figures 1-3). For NMC811, the oxygen release occurs already at potentials as low as 4.3 V vs. Li/Li + , whereas for NMC111 and NMC622 it occurs roughly at 4.7 V vs. Li/Li + . These values limit the end-of-charge voltage that can be applied to achieve a stable cycling and they have therefore a severe impact on the achievable specific energy of these materials. We want to highlight that additional aging mechanisms will be occurring in parallel, like lithium loss due to SEI growth on the anode, elec- trochemical electrolyte oxidation at high potentials on the cathode, metal dissolution from the cathode, etc.; however, when cycling up to potentials where oxygen release occurs, the formation of a resis- tive surface layer is the most severe aging mechanism under these conditions, causing significant capacity and discharge voltage fading during extended charge/discharge cycling (Figures 1-3). The specific energies of the cells shown in the Figures 1-3 are depicted in Figure 11 with the full bars representing the specific energy of the 5 th cycle at a 1C-rate. The dashed bars indicate the remaining specific energy after 300 cycles. As discussed before, stable cycling was possible for NMC111 and NMC622 up to 4.4 V and up to 4.0 V for NMC811. This is also clearly visible in Figure 11, as the differences between the spe- cific energies in the 5 th and 300 th cycle are fairly low for these voltage limits, but increase significantly for the others. The highest specific energy with stable cycling was achieved with NMC622 cycled up to 4.4 V. Comparing only the cells with stable cycling performance, it becomes clear that NMC811 reaches the lowest specific energy, which is due to the very low applicable end-of-charge voltage of only 4.0 V. This rather sobering outlook for NMC811 emphasizes the need to prohibit the oxygen release from the surface. Our results suggest that one way of making use of the high capacities of NMC811 and achieving stable cycling at the same time might be possible by either a core-shell structure in which the core consists of NMC811 with i) a shell that has a Ni-content of up to 60 % (surface like NMC622) and does not release oxygen until >4.4 V or ii) a shell consisting of an ordered spinel like high-voltage spinel (LNMO) that does hardly evolve any gases ( Figure 5) due to the absence of oxygen release. In both cases the shell would need to be thick enough to prevent oxygen loss from the core structure via the limited diffusion of the oxygen anions. Indeed, these approaches have been used by several research groups and we believe that the prevention of oxygen release explains the successful use of core-shell 85-88 materials possessing Ni- contents in the core and shell of 80% and ≤55%, respectively, and full concentration gradient 89-91 materials with Ni-contents of ≥75% and ≤56% in the particle center and the surface, respectively. Additionally, also a superior performance of LiMn 2 O 4 coated NMC over uncoated samples was reported by Cho et al. 92 Conclusions . NMC-graphite cells were cycled to different end- of-charge potentials and it was demonstrated that stable cycling is possible up to 4.4 V for NMC111 and NMC622 and only up to 4.0 V for NMC811. The capacity fading rates observed at 4.6 V for NMC111 and NMC622 and 4.1 V and 4.2 V for NMC811 are due to a significant increase in the polarization of the NMC electrode as evidenced by charge/discharge cycling in a 3-electrode setup with a lithium reference electrode. In contrast, the polarization of the graphite electrode remained rather constant. By a dq/dV analysis we demonstrated that the significant rise in the impedance occurs when the NMC materials are cycled up to a high-voltage feature at ∼4.7 V vs. Li/Li + for NMC111 and NMC622 and up to the H2 → H3 phase transition at ∼4.3 V vs. Li/Li + for NMC811; we hypothesize that this is caused by oxygen release from the NMC ...

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... energy densities of NMC111, NMC622 and NMC811.- In the previous sections it was demonstrated that the release of oxygen from the NMC surface has a very detrimental impact on the material stability, as it causes significant gas evolution (Figures 7-9) as well as a significant increase of the polarization of the cathode material most probably due to the oxygen depleted surface layer (Figures 1-3). For NMC811, the oxygen release occurs already at potentials as low as 4.3 V vs. Li/Li + , whereas for NMC111 and NMC622 it occurs roughly at 4.7 V vs. Li/Li + . These values limit the end-of-charge voltage that can be applied to achieve a stable cycling and they have therefore a severe impact on the achievable specific energy of these materials. We want to highlight that additional aging mechanisms will be occurring in parallel, like lithium loss due to SEI growth on the anode, elec- trochemical electrolyte oxidation at high potentials on the cathode, metal dissolution from the cathode, etc.; however, when cycling up to potentials where oxygen release occurs, the formation of a resis- tive surface layer is the most severe aging mechanism under these conditions, causing significant capacity and discharge voltage fading during extended charge/discharge cycling (Figures 1-3). The specific energies of the cells shown in the Figures 1-3 are depicted in Figure 11 with the full bars representing the specific energy of the 5 th cycle at a 1C-rate. The dashed bars indicate the remaining specific energy after 300 cycles. As discussed before, stable cycling was possible for NMC111 and NMC622 up to 4.4 V and up to 4.0 V for NMC811. This is also clearly visible in Figure 11, as the differences between the spe- cific energies in the 5 th and 300 th cycle are fairly low for these voltage limits, but increase significantly for the others. The highest specific energy with stable cycling was achieved with NMC622 cycled up to 4.4 V. Comparing only the cells with stable cycling performance, it becomes clear that NMC811 reaches the lowest specific energy, which is due to the very low applicable end-of-charge voltage of only 4.0 V. This rather sobering outlook for NMC811 emphasizes the need to prohibit the oxygen release from the surface. Our results suggest that one way of making use of the high capacities of NMC811 and achieving stable cycling at the same time might be possible by either a core-shell structure in which the core consists of NMC811 with i) a shell that has a Ni-content of up to 60 % (surface like NMC622) and does not release oxygen until >4.4 V or ii) a shell consisting of an ordered spinel like high-voltage spinel (LNMO) that does hardly evolve any gases ( Figure 5) due to the absence of oxygen release. In both cases the shell would need to be thick enough to prevent oxygen loss from the core structure via the limited diffusion of the oxygen anions. Indeed, these approaches have been used by several research groups and we believe that the prevention of oxygen release explains the successful use of core-shell 85-88 materials possessing Ni- contents in the core and shell of 80% and ≤55%, respectively, and full concentration gradient 89-91 materials with Ni-contents of ≥75% and ≤56% in the particle center and the surface, respectively. Additionally, also a superior performance of LiMn 2 O 4 coated NMC over uncoated samples was reported by Cho et al. 92 Conclusions . NMC-graphite cells were cycled to different end- of-charge potentials and it was demonstrated that stable cycling is possible up to 4.4 V for NMC111 and NMC622 and only up to 4.0 V for NMC811. The capacity fading rates observed at 4.6 V for NMC111 and NMC622 and 4.1 V and 4.2 V for NMC811 are due to a significant increase in the polarization of the NMC electrode as evidenced by charge/discharge cycling in a 3-electrode setup with a lithium reference electrode. In contrast, the polarization of the graphite electrode remained rather constant. By a dq/dV analysis we demonstrated that the significant rise in the impedance occurs when the NMC materials are cycled up to a high-voltage feature at ∼4.7 V vs. Li/Li + for NMC111 and NMC622 and up to the H2 → H3 phase transition at ∼4.3 V vs. Li/Li + for NMC811; we hypothesize that this is caused by oxygen release from the NMC ...

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... mean discharge voltages versus cycle number of the NMC811 cathodes and the graphite anodes are shown in Figure 3b. Table I. Measured capacity retentions between the 5 th and 300 th cycle of the NMC-graphite cells shown in Figures 1-3. The values in brackets are the specific capacities in units of mAh/g NMC of the 5 th and the 300 th cycles. 4.6 V for NMC111 and NMC622, indicating that the observed strong cathode impedance growth sets in at ∼0.4 V lower cutoff potentials for NMC811. On the other hand, the mean discharge potentials for the graphite anode in the NMC811-graphite cells ( ¯ V anode discharge ) behave similarly as for the NMC111 and the NMC622 cells, showing negligi- ble increase with cycle number for all cutoff potentials. In summary, the observed capacity decay at >4.0 V cutoff potential for NMC811 full-cells and at >4.4 V for NMC111 and NMC622 full-cells seems to be largely related to the onset of a strong cathode impedance growth (i.e., a strong fading of ¯ V cathode discharge ) above these cutoff potentials. The above results clearly demonstrate a similarity between NMC111 and NMC622, but a big difference to NMC811 with re- spect to the onset of the cathode impedance growth. To investigate the origin of this difference and to find the reason for the instability occurring for NMC111 and NMC622 at 4.6 V and for NMC811 at 4.1-4.2 V, a dq/dV plot of the delithiation and lithiation of the three NMC materials in NMC-graphite cells of the 3 rd cycle is depicted in Figure 4. The voltage region up to 3.8 V is very similar for all three NMC compositions, with two anodic peaks between 3.4 V and 3.8 V. While the first one originates from the lithium intercalation into the graphite anode, the second one stems from the phase transition from a hexagonal to a monoclinic (H1 → M) lattice of the NMC. 18,[46][47][48][49] In the region >3.8 V, it becomes very obvious that the dq/dV curve for the NMC811 cell deviates substantially from that of the NMC111 and NMC622 cells. In particular, NMC811 has a small anodic fea- ture at ∼3.95 V and a large anodic peak at ∼4.15 V, both of which are absent for the other NMCs. The first one belongs to the M → H2 phase transition and the latter one corresponds to the H2 → H3 phase transition as was reported before for LiNiO 2 46-48 and Ni-rich NMC 18,49 materials. In contrast, for NMCs with Ni-contents <80% the M → H2 and H2 → H3 phase transitions have not been reported. Accordingly, for NMC111 and NMC622 such distinct features are not observed. However, for NMC622 a broad peak around 4.1 V is visible, which might indicate an M → H2 phase transition. For both NMC111 as well as NMC622, a clear redox peak is observed at 4.6 V, which could correspond, in analogy to NMC811, to a H2 → H3 phase tran- sition or could also indicate an oxygen redox feature, a process which has been suggested for Li 2 Ru 1-y Sn y O 3 50 and Li 2 IrO 3 51 by Tarascon's group and was investigated theoretically using DFT. 52, 53 The vertical dotted lines mark the upper cutoff voltages which were chosen for the cells presented in the Figures 1-3. Note that up to the onset of the H2 → H3 phase transition of NMC811 at >4.0 V and up to the onset of the redox feature at >4.4 V of NMC111 and NMC622, the capacity retention of the materials is very stable. In other words, stable cycling was only possible if the cutoff voltage was below the onset of the last peak in the dq/dV plot. The early onset of the H2 → H3 transition at >4.0 V (NMC811) explains why NMC811 cannot be cycled stably at >4.0 V cutoff voltages, whereas NMC111 and NMC622 cells show an excellent performance at potentials as high as 4.4 V. The detri- mental effect of the H2 → H3 phase transition was already described before for LiNiO 2 and NMC811 and was explained by a significant reduction of the unit-cell volume upon this phase transition, which we will critically review in the Discussion section. 18,47 Figure 5 shows the results of On-line Electrochemical Mass Spectrom- etry (OEMS) measurements with NMC-Li and LNMO-Li half-cells. For these experiments, metallic lithium was chosen as a counter- electrode in order to achieve a stable reference potential. Figure 5a displays the voltage profiles of NMC111 (black), NMC622 (red), NMC811 (green) as well as LNMO (blue) upon the first charging from OCV to 5 V at a 0.05 C-rate and 25 • C as a function of the state- of-charge (SOC) (note that 100% SOC is defined as the removal of all lithium from the cathode materials; s. Experimental section). The three lower panels show the total moles of evolved gas, normalized to the BET surface area of the cathode active material (CAM) in units of μmol/m 2 CAM for O 2 (Figure 5b), CO 2 ( Figure 5c), and CO ( Figure 5d). Note that normalization of the gassing data to the BET surface area is meant to account for the differences in the available surface area for electrochemical oxidation reactions. Figure 5b demonstrates that for all three NMC compositions a release of oxygen can be detected near a state-of-charge of ∼80-90%, corresponding to onset potentials for O 2 evolution of ∼4.3 V vs. Li/Li + (or ∼4.2 V cell voltage in a full-cell vs. graphite) for NMC811 and of ∼4.7 V vs. Li/Li + (or ∼4.6 V cell voltage in a full-cell vs. graphite) for NMC111 and NMC622 (this will be seen more clearly later, when discussing Figure 6). The observed onset for O 2 evolution on NMC111 at ∼80% SOC during electrochemical delithiation (s. Figure 5) is in surprisingly good agreement with the observed onset for oxygen loss upon the chemical delithiation of NMC111 (with NO 2 BF 4 ), which was found to initiate at a lithium content corresponding to ∼75% SOC. 6 The scatter in the reported O 2 concentration of ca. ±0.5 μmol O 2 /m 2 CAM for NMC111 and NMC622 and of ca. ±1 μmol O 2 /m 2 CAM for NMC811 corresponds to our experimental error in quantifying the O 2 concentration of ca. ±10 ppm. As was already reported previously, 54 no O 2 evolution is observed for the LNMO half-cell up to 5.0 ...

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... mean discharge voltages versus cycle number of the NMC811 cathodes and the graphite anodes are shown in Figure 3b. Table I. Measured capacity retentions between the 5 th and 300 th cycle of the NMC-graphite cells shown in Figures 1-3. The values in brackets are the specific capacities in units of mAh/g NMC of the 5 th and the 300 th cycles. 4.6 V for NMC111 and NMC622, indicating that the observed strong cathode impedance growth sets in at ∼0.4 V lower cutoff potentials for NMC811. On the other hand, the mean discharge potentials for the graphite anode in the NMC811-graphite cells ( ¯ V anode discharge ) behave similarly as for the NMC111 and the NMC622 cells, showing negligi- ble increase with cycle number for all cutoff potentials. In summary, the observed capacity decay at >4.0 V cutoff potential for NMC811 full-cells and at >4.4 V for NMC111 and NMC622 full-cells seems to be largely related to the onset of a strong cathode impedance growth (i.e., a strong fading of ¯ V cathode discharge ) above these cutoff potentials. The above results clearly demonstrate a similarity between NMC111 and NMC622, but a big difference to NMC811 with re- spect to the onset of the cathode impedance growth. To investigate the origin of this difference and to find the reason for the instability occurring for NMC111 and NMC622 at 4.6 V and for NMC811 at 4.1-4.2 V, a dq/dV plot of the delithiation and lithiation of the three NMC materials in NMC-graphite cells of the 3 rd cycle is depicted in Figure 4. The voltage region up to 3.8 V is very similar for all three NMC compositions, with two anodic peaks between 3.4 V and 3.8 V. While the first one originates from the lithium intercalation into the graphite anode, the second one stems from the phase transition from a hexagonal to a monoclinic (H1 → M) lattice of the NMC. 18,[46][47][48][49] In the region >3.8 V, it becomes very obvious that the dq/dV curve for the NMC811 cell deviates substantially from that of the NMC111 and NMC622 cells. In particular, NMC811 has a small anodic fea- ture at ∼3.95 V and a large anodic peak at ∼4.15 V, both of which are absent for the other NMCs. The first one belongs to the M → H2 phase transition and the latter one corresponds to the H2 → H3 phase transition as was reported before for LiNiO 2 46-48 and Ni-rich NMC 18,49 materials. In contrast, for NMCs with Ni-contents <80% the M → H2 and H2 → H3 phase transitions have not been reported. Accordingly, for NMC111 and NMC622 such distinct features are not observed. However, for NMC622 a broad peak around 4.1 V is visible, which might indicate an M → H2 phase transition. For both NMC111 as well as NMC622, a clear redox peak is observed at 4.6 V, which could correspond, in analogy to NMC811, to a H2 → H3 phase tran- sition or could also indicate an oxygen redox feature, a process which has been suggested for Li 2 Ru 1-y Sn y O 3 50 and Li 2 IrO 3 51 by Tarascon's group and was investigated theoretically using DFT. 52, 53 The vertical dotted lines mark the upper cutoff voltages which were chosen for the cells presented in the Figures 1-3. Note that up to the onset of the H2 → H3 phase transition of NMC811 at >4.0 V and up to the onset of the redox feature at >4.4 V of NMC111 and NMC622, the capacity retention of the materials is very stable. In other words, stable cycling was only possible if the cutoff voltage was below the onset of the last peak in the dq/dV plot. The early onset of the H2 → H3 transition at >4.0 V (NMC811) explains why NMC811 cannot be cycled stably at >4.0 V cutoff voltages, whereas NMC111 and NMC622 cells show an excellent performance at potentials as high as 4.4 V. The detri- mental effect of the H2 → H3 phase transition was already described before for LiNiO 2 and NMC811 and was explained by a significant reduction of the unit-cell volume upon this phase transition, which we will critically review in the Discussion section. 18,47 Figure 5 shows the results of On-line Electrochemical Mass Spectrom- etry (OEMS) measurements with NMC-Li and LNMO-Li half-cells. For these experiments, metallic lithium was chosen as a counter- electrode in order to achieve a stable reference potential. Figure 5a displays the voltage profiles of NMC111 (black), NMC622 (red), NMC811 (green) as well as LNMO (blue) upon the first charging from OCV to 5 V at a 0.05 C-rate and 25 • C as a function of the state- of-charge (SOC) (note that 100% SOC is defined as the removal of all lithium from the cathode materials; s. Experimental section). The three lower panels show the total moles of evolved gas, normalized to the BET surface area of the cathode active material (CAM) in units of μmol/m 2 CAM for O 2 (Figure 5b), CO 2 ( Figure 5c), and CO ( Figure 5d). Note that normalization of the gassing data to the BET surface area is meant to account for the differences in the available surface area for electrochemical oxidation reactions. Figure 5b demonstrates that for all three NMC compositions a release of oxygen can be detected near a state-of-charge of ∼80-90%, corresponding to onset potentials for O 2 evolution of ∼4.3 V vs. Li/Li + (or ∼4.2 V cell voltage in a full-cell vs. graphite) for NMC811 and of ∼4.7 V vs. Li/Li + (or ∼4.6 V cell voltage in a full-cell vs. graphite) for NMC111 and NMC622 (this will be seen more clearly later, when discussing Figure 6). The observed onset for O 2 evolution on NMC111 at ∼80% SOC during electrochemical delithiation (s. Figure 5) is in surprisingly good agreement with the observed onset for oxygen loss upon the chemical delithiation of NMC111 (with NO 2 BF 4 ), which was found to initiate at a lithium content corresponding to ∼75% SOC. 6 The scatter in the reported O 2 concentration of ca. ±0.5 μmol O 2 /m 2 CAM for NMC111 and NMC622 and of ca. ±1 μmol O 2 /m 2 CAM for NMC811 corresponds to our experimental error in quantifying the O 2 concentration of ca. ±10 ppm. As was already reported previously, 54 no O 2 evolution is observed for the LNMO half-cell up to 5.0 ...

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... Figures 7-9, the amount of released oxygen is largest in the first cycle and decreases in the subsequent cycles. This fits to the hypothesis that the oxygen is released only from surface-near regions and is therefore fastest in the first cycle, and lower in subsequent cycles, since then it has to diffuse through the already formed disordered spinel or rock-salt layer. In summary, a clear correlation can be made between the structural rearrangement of the NMC particle surface and the release of oxygen. Additionally, the spinel or rock-salt surface layer is very likely the cause of the increase in the polarization (represented by a decrease in the charge-averaged mean discharge voltage of the cathode, ¯ V cathode discharge ) observed during cycling in the Figures ...

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... mechanism of EC oxidation by reactive oxygen.-As discussed above, our data indicate that the majority of the evolved CO and CO 2 are actually a consequence of the reaction of the released oxygen from the NMC, which is likely very reactive in the moment it is released from the material (see Figures 7-9). In Figure 10, we demonstrated that the carbon source for the CO and CO 2 formation is not the conductive carbon in the electrode. The only remaining carbon source in the cell is therefore ethylene carbonate (EC) and possibly the binder. In Scheme 1, we propose a mechanism for how oxygen might react with EC, whereby it is clear that oxygen in its triplet ground state does not react with EC. As the reaction requires the oxygen to be reduced, there are only the two carbon atoms bound to the hydrogen which can be potentially oxidized (the carbonyl- carbon is already in its maximum oxidation state). Our proposed mechanism starts with an electrophilic attack on the carbon by the O 2 molecule, yielding a peroxo group carrying the proton which was initially bound to the carbon. The rather unstable peroxo group would immediately decompose, forming a carbonyl group and releasing a water molecule. This molecule could potentially decompose forming CO, CO 2 and formaldehyde, in which case, however, the predicted CO 2 /CO ratio would be 1/1, which does not match the observed ratios in Figures 7-9 nor did we observe any formaldehyde in the mass spectrometer. Instead, a second 1 O 2 molecule could attack the other carbon atom if the EC molecule attacked in the first step is assumed to be adsorbed at the NMC surface forming another carbonyl group and releasing another molecule of water. The formed molecule would readily decompose, yielding two molecules of CO 2 , one molecule CO, aside with the previously formed two H 2 O molecules. The formation of water upon the reaction of electrolyte with oxygen was already hypothesized before. 81,82 The overall proposed reaction would thus be EC + 2 O 2 → 2 CO 2 + CO + 2 H 2 O, predicting a CO 2 to CO ratio of 2:1. Ex- amining the evolved amounts summarized in Table II, a somewhat higher CO 2 :CO ratio ranging from 2.2:1 to 2.4:1 was measured. Con- sidering that water is a reaction product, several follow-up reactions are likely to occur: i) H 2 O can be reduced at the graphite anode, yielding H 2 and OH − , as was reported previously by our group 64 and which would be consistent with the observed continuous evolution of H 2 in Figures 7-9; ii) OH -produced by the reduction of H 2 O at the anode was shown to lead to rather high rates of EC hydrolysis, pro- ducing CO 2 gas; 80 iii) chemical reaction of LiPF 6 with H 2 O can yield Li x PO y F z species, which are frequently reported as surface species at the interface between electrolyte and the NMC cathode. 15,16 A com- bination of i) and ii) would lead to additional CO 2 evolution (as well as to the observed ongoing H 2 evolution) and therefore to a higher CO 2 :CO ratio than the ratio of 2:1 predicted by Scheme 1, consistent with our observations (s . Table II). In order to check if the reduction of water forming H 2 and OH − can be a reasonable, we will calculate the total amount of water which can be formed according to Scheme 1 and compare it to the H 2 formed at potentials ≥ 4.6 V in Figures 7-9. For the following calculation, we will use the values obtained for the NMC111-graphite cell as an example. As stated in Table II, ∼80 μmol/m 2 NMC CO are formed. Assuming the stoichiometry in Scheme 1, this would imply that ∼160 μmol/m 2 NMC H 2 O be formed at the same time. Multiplying this value with the active material mass of the NMC electrode (16.69 mg) and the BET-surface area of the NMC111 (0.26 m 2 /g), one obtains a total of 0.7 μmol H 2 O (≡ 12.5 μg H 2 O ). Analo- gous to the calculation in the previous section, this would correspond to an increase of the H 2 O content in the electrolyte by 21 ppm. The re- duction of this in-situ formed water at the negative graphite electrode via H 2 O + e − → 0.5 H 2 + OH − could yield 0.35 μmol H 2 which, when normalized to the NMC or carbon surface area would amount to 80 μmol/m 2 NMC and 3.8 μmol/m 2 C , respectively. Since water can be formed as soon as oxygen is released for the first time, we examine the hydrogen signal in Figure 7 from this point until the end of the measurement: the amount of H 2 increases from 6.6 μmol/m 2 C to 11.7 μmol/m 2 C , i.e., by 5.1 μmol/m 2 C , which may be compared to the above predicted value of 3.8 μmol/m 2 C . Analogous estimates can be made for the NMC622 and the NMC811 cells, for which the agreement is also within a factor of ∼2. Considering that besides the reduction of the formed water several additional reactions occur simultaneously like hydrogen formation from initially present trace water reduction (see Results section), hydrolysis of EC, as well as decomposition of LiPF 6 , the calculated maximum of hydrogen from the reduction of in-situ generated water actually fits astonishingly well to the experi- mentally observed amount. Table III, the oxygen depleted surface layer thickness was estimated as a compact, homogeneous layer around the NMC particles in a similar way as reported by Strehle et al. 36 We considered both scenarios, a layered to disordered spinel (Eq. 5) and a layered to rock-salt (Eq. 6) transformation using, in analogy to the literature, 21,22,36,83 the follow- ing general equations with M = (Ni, Mn, ...

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... presented substantial evidence that the CO/CO 2 evolution at high potentials is mostly caused by a chemical reaction of the released lattice oxygen, the question remains whether the evolved CO/CO 2 derive from its reaction with the electrolyte or with the con- ductive carbon in the NMC electrode. Therefore, an NMC622 elec- trode with 4.4 % wt 13 C-labeled carbon as conductive additive was pre- pared, replacing the Super C65 conductive carbon, such that a reaction of released lattice oxygen with carbon would result in 13 CO/ 13 CO 2 , while its reaction with electrolyte would result in 12 CO/ 12 CO 2 . The NMC622-graphite cell with 13 C conductive carbon was charged to 4.8 V and subsequently discharged to 2.6 V (see Figure 10a). The capacity reached during the CC-phase was only 198 mAh/g NMC , i.e., ∼17% lower than for the NMC622 electrode with Super C65 (see Figure 8); this inferior electrode performance is likely caused by the strongly agglomerated structure of the 13 C-carbon, resulting in a poor electronic accessibility of the active material particles in the cathode. Nevertheless, also for this electrode, the release of oxygen can be clearly seen. It is shifted to a higher potential of 4.75 V, compared to the 4.54 V for NMC622 with Super C65 (s. Figure 8), which can be rationalized by the fact that the material contains more lithium at ∼4.6 V due to the worse cathode performance, which in turn renders it more stable at this voltage. Additionally, the cutoff potential is only 50 mV above the O 2 onset, which is the reason for the overall lower oxygen ...

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... carbonate impurity oxidation (CO 2 ). This raises the question, whether CO and CO 2 derive from the chemical reaction of the released lattice oxygen with the electrolyte. A significant reaction of the evolved oxygen with conductive carbon can be excluded, since it was shown in Figure 10 that no 13 CO and 13 CO 2 was evolved when 13 C labeled carbon was used as conductive additive in the NMC electrode instead of conventional carbon (Super C65). Another interesting observation is that in the case of NMC811-graphite cells, O 2 , CO, and CO 2 evolve already at ∼4.2 V. At this potential, no gas evolution is observed for the analogous cells with NMC111 (onset of O 2 evolution at ∼4.57 V) or NMC622 (onset of O 2 evolution at ∼4.54 V), so that it is too low to ascribe the evolved gases to the electrochemical oxidation of the electrolyte, which strongly supports our hypothesis that the evolution of O 2 , CO, and CO 2 are of the same ...

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... to the differences in the BET surface area of the three NMCs, the obtained layer thicknesses turn out to be very similar. For model I, the calculated layer thicknesses for the spinel and rock-salt trans- formations are 11.6 nm-12.1 nm and 6.9 nm-7.3 nm, respectively. Also for the calculation based on model II, the obtained values are very close to the ones obtained using model I (12.7 nm-14.6 nm and 7.8 nm-8.2 nm, respectively). These calculated layer thicknesses are in good agreement with previous reports in the literature on NMC, 9 LiNi 0.8 Co 0.2 O 2 28 , NCA 30 and Li-rich NMC. 36 As expected, a trans- formation to the spinel phase leads to a thicker surface layer than the transformation to the rock-salt phase, since the former contains more oxygen in its structure than the latter. Moreover, it is very likely that for the different NMC materials different ratios of spinel and rock- salt phases occur with higher rock-salt ratios for Ni-richer NMC as they tend to form rock-salt rather than spinel phases. 9,70 However, this would not significantly affect the estimated surface layer thickness, as shown in Table III. Additionally, as it was shown in Figures 7-9, an increase in the oxygen signal cannot be observed anymore after a few cycles; however, we believe that the oxygen release is ongoing also in subsequent cycles, but is only detected as CO and CO 2 . This would also explain the steady decrease of the charge-averaged mean cathode discharge potential shown in Figures 1-3 (solid lines in panels b). All in all, we demonstrated in this section that the film thicknesses deduced from the gas evolution data from OEMS yield values which are consistent with microscopy data from the ...

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... only the amount of oxygen is cut in half, but also the amounts of CO 2 and CO are cut in half, which shows once more that these gases are linked to the oxygen evolution. Finally, Figure 10 clearly shows that neither the evolution of 13 CO nor 13 CO 2 was observed, prov- ing that the carbon additive in the cathode is stable at potentials of 4.8 V and also stable against the released oxygen from the NMC lat- tice. Therefore, the observed CO/CO 2 formation at high potentials can be ascribed to the oxidation of EC (possibly also the binder) rather than of the conductive carbon by released lattice ...

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... cycling of NMC-graphite cells.- Figure 1a shows the cycling stability of NMC111-graphite full-cells with differ- ent upper cutoff voltages of 4.2, 4.4, and 4.6 V. The cells have a very stable cycling performance for upper cutoff voltages of 4.2 V and 4.4 V (black and gray lines), however, cycling to 4.6 V leads to a fast ca- pacity fading (light gray line), which is in agreement with previous reports in the literature. 9,11,13 While the error bars are hardly visibly for cutoff voltages ≤4.4 V and at low cycle numbers at a cutoff of 4.6 V, the error bars at higher cycle numbers significantly increase, which is due to the delayed onset of the so-called rollover-failure for the two cells. This failure mechanism was described previously by Dubarry et al. and Burns et al. and was shown to be due to growing kinetic resistances or more generally an impedance buildup. 41,42 In our data the increasing polarization stems almost exclusively from the NMC cathodes, which will be discussed below. The coulombic efficiencies (right axis in Figure 1a) for cells cycled to 4.2 V and 4.4 V are in average >99.9%, indicating the absence of major side reactions. When the end-of-charge voltage is increased to 4.6 V, the coulombic efficiency decreases to ∼99.6% (before the onset of the rollover-failure), reflecting an increasing loss of cyclable lithium. A further decrease of the coulombic efficiency is observed at the on- set of the rollover-failure. On the other hand, any increase in the polarization during cell discharge can be monitored by plotting the charge-averaged mean discharge voltage, defined ...

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... cycling of NMC-graphite cells.- Figure 1a shows the cycling stability of NMC111-graphite full-cells with differ- ent upper cutoff voltages of 4.2, 4.4, and 4.6 V. The cells have a very stable cycling performance for upper cutoff voltages of 4.2 V and 4.4 V (black and gray lines), however, cycling to 4.6 V leads to a fast ca- pacity fading (light gray line), which is in agreement with previous reports in the literature. 9,11,13 While the error bars are hardly visibly for cutoff voltages ≤4.4 V and at low cycle numbers at a cutoff of 4.6 V, the error bars at higher cycle numbers significantly increase, which is due to the delayed onset of the so-called rollover-failure for the two cells. This failure mechanism was described previously by Dubarry et al. and Burns et al. and was shown to be due to growing kinetic resistances or more generally an impedance buildup. 41,42 In our data the increasing polarization stems almost exclusively from the NMC cathodes, which will be discussed below. The coulombic efficiencies (right axis in Figure 1a) for cells cycled to 4.2 V and 4.4 V are in average >99.9%, indicating the absence of major side reactions. When the end-of-charge voltage is increased to 4.6 V, the coulombic efficiency decreases to ∼99.6% (before the onset of the rollover-failure), reflecting an increasing loss of cyclable lithium. A further decrease of the coulombic efficiency is observed at the on- set of the rollover-failure. On the other hand, any increase in the polarization during cell discharge can be monitored by plotting the charge-averaged mean discharge voltage, defined ...