Changing Established Belief on Capacity Fade Mechanisms

Article pubs.acs.org/JPCC

Changing Established Belief on Capacity Fade Mechanisms: Thorough Investigation of LiNi1/3Co1/3Mn1/3O2 (NCM111) under High Voltage Conditions Johannes Kasnatscheew,*,† Marco Evertz,† Benjamin Streipert,† Ralf Wagner,† Sascha Nowak,† Isidora Cekic Laskovic,†,‡ and Martin Winter*,†,‡ †

MEET Battery Research Center/Institute of Physical Chemistry, University of Münster, Corrensstrasse 46, 48149 Münster, Germany ‡ Helmholtz-Institute Münster, IEK-12, Forschungszentrum Jülich GmbH, 48149 Münster, Germany ABSTRACT: The further development of lithium ion batteries operating at high voltages requires basic understanding of the occurring capacity fade mechanisms. In this work, the overall specific capacity loss with regard to reversible and irreversible processes for LiNi 1/3 Co 1/3 Mn 1/3 O 2 (NCM111)/Li half cells, cycled at a charge cutoff potential of 4.6 V vs Li/Li+, has been investigated in detail. By means of total X-ray fluorescence (TXRF) technique it was shown that specific capacity losses associated with the amount of dissolved transition metals are negligible, implying a still intact NCM111 active material after 53 cycles. It was demonstrated that the specific capacity fade during cycling at constant specific currents can be mainly attributed to the increase of the delithiation (charge) hindrance, whereas lithiation (discharge) hindrance is only present after a specific current increase, leading to apparent specific capacity losses and to decreased Coulombic efficiencies. This could be proven by the determination of the NCM lithiation degree in the discharged state with inductively coupled plasma optical emission spectroscopy (ICP−OES). Moreover, by decreasing the kinetic hindrance in the NCM material, it was shown that most of the observed specific capacity losses after 53 cycles are reversible. The influence of the active material and the cathode electrolyte interphase (CEI) on the specific capacity fade has been discussed. The results of the X-ray photoelectron spectroscopy (XPS) studies revealed that the CEI thickness is predominately dependent on the applied temperature (thermal-chemical origin) rather than the applied electrode potential (electrochemical origin). Finally, the absence of a fade in specific capacity for LiNi0.5Mn1.5O4 (LNMO) at an even higher charge cutoff potential of 4.95 V vs Li/Li+ points to a strong active material dependence than solely to the impact of electrolyte decomposition and CEI formation.

1. INTRODUCTION Future generations of rechargeable lithium ion batteries (LIBs) with increased energy and power density are required to power myriad portable electronic devices, store electricity from renewable sources, and as traction batteries in electric vehicles.1 The thermodynamic value of the energy content of a LIB cell is determined by the difference in the electrode potentials of the positive and negative electrode materials and the delivered capacity.2 Graphite is commonly used as a negative electrode in LIBs since it has a high specific capacity (372 mAh g−1) and a desirably low working potential, which is close to that of lithium metal.3,4 Consequently, the type of employed positive electrode material dictates the output energy of the LIB cell. Among different positive electrode material classes, the layered transition metal oxides (LiMO2, with M = Co, Ni, Mn, etc.) are predominately used since the commercial launch of the LIB in 1991.5−7 Due to their high theoretical specific capacity of more than 270 mAh g−1, layered transition metal oxides are superior to olivine LiMPO4 (M = Fe, Mn, Ni, Co, © 2017 American Chemical Society

etc.) and spinel LiM2O4 (M = Mn, etc.) material classes with regard to increase the output energy density of the LIB cell.8−10 The nature of layered transition metal oxides as solid solution materials theoretically implies that an increase of the specific capacity (higher amount of extracted lithium) also leads to an increase in the operation potential.11 Bearing in mind that both potential and capacity determine the output energy density of a LIB cell, a higher lithium extraction amount has a double beneficial effect on the energy density12,13 However, exceeding the material-specific lithium extraction limit leads to the instability of the electrode and thus to structurally induced irreversible specific capacity losses.13,14 Hence, to keep cycle life and safety of the LIB cell, the amount of delithiated lithium, the related working potential, the resulting specific capacity, and thus eventually the achievable specific energy are limited. Received: November 22, 2016 Revised: December 19, 2016 Published: January 2, 2017 1521

DOI: 10.1021/acs.jpcc.6b11746 J. Phys. Chem. C 2017, 121, 1521−1529

Article

The Journal of Physical Chemistry C

cutoff potential was held at 3.0 V vs Li/Li+ for 24 h. Further variations of the test procedure are indicated in the text. The sample preparation and experimental setup for the inductively coupled plasma optical emission spectroscopy (ICP−OES),13 total X-ray fluorescence (TXRF),28 and the X-ray photoelectron spectroscopy (XPS)29 investigations are described in detail in our previous reports.

Among the different layered transition metal oxides, the commercially used LiCoO2 (LCO) electrode represents a satisfying compromise concerning working potential, rate capability, and specific capacity. Unfortunately, LCO is expensive and toxic and suffers from safety hazards and structural instability when utilizing more than half of the theoretical specific capacity.15,16 For large battery cells, development and application activities have therefore moved from LCO to its derivatives in which Co ions are partially substituted by more abundant and less toxic transition metal ions. A well-established material is LiNi1/3Co1/3Mn1/3O2 (NCM111),17 as it is superior in terms of electrochemical performance, thermal stability, safety, and costs compared to LCO.7,10,18,19 Besides, the NCM111 charge cutoff potential can be further increased with less safety concerns, which enables higher specific capacities and thus the desired increase of the LIB cell output specific energy.20 However, the increase in the charge cutoff potential adversely affects the cycle life.21 For the further improvement of the energy density owing to the increase of the charge cutoff potential, it is therefore indispensable to understand the failure mechanisms taking place during LIB cell operation. In our recent publication, we listed a variety of literatureknown capacity fade mechanisms of NCM111 charged to elevated cutoff potentials.22 Among them, the mostly discussed mechanisms are oxidative electrolyte decomposition,23−25 transition metal dissolution,21,26 and active material related phase changes.8,27 However, the significance of each process with regard to the specific capacity fade during constant charge/discharge cycling is not yet fully understood. In our previous work the focus was set on the deconvolution of reversible and irreversible parts of the capacity loss of NCM111/Li half cells observed in the first cycle.13 In the frame of this work, the significance of parasitic reactions (e.g., the electrolyte oxidation) for NCM111 charged up to 4.6 vs Li/ Li+ was neglected.13 It was shown that the observed specific charge and discharge capacities can be completely attributed to the delithiation and the lithiation amount, respectively.13 Thus, the observed specific capacity losses could be proven to be the result of an incomplete relithiation process and disconfirmed to be the result of irreversible parasitic reactions, such as electrolyte oxidation.13 The present work focuses on the evaluation and elucidation of different capacity fade mechanisms during 53 constant current charge/discharge cycles. We believe that our findings can contribute to a better understanding and thus contribute to an improvement of the LIB cycle life at high voltage application.

3. RESULTS AND DISCUSSION The NCM111 specific capacity evolution of NCM111/Li half cell during constant current charge/discharge cycling in the potential range of 4.6−3.0 V vs Li/Li+ is depicted in Figure 1.

Figure 1. Constant current charge/discharge cycling of NCM111/Li half cell in the potential range 4.6−3.0 V vs Li/Li+. The respective estimated specific capacity losses associated with charge (red numbers) and discharge (blue numbers) are discussed in the text. The specific capacity decay of 4 and 24 mAh g−1 (black numbers) during constant current cycling will be investigated in detail.

In the used test procedure, the specific current is lower for the initial three charge/discharge cycles (30 mA g−1) and higher for the subsequent 50 charge/discharge cycles (150 mA g−1). Based on our previous work, the specific charge and discharge capacities can be solely attributed to structural delithiation and lithiation amounts, respectively.13 The complete delithiation of the theoretical specific capacity of 278 mAh g−1 would cause severe irreversible structural changes in the NCM111 host material.30 This is the reason behind the required limitation of the practical specific capacity, which is determined by the applied charge cutoff potential and specific current. Despite the limitation of the specific capacity, further specific capacity losses are present during ongoing cycling. Based on our previous work, the specific capacity losses in the first and fourth cycle are associated with the specific current increase, formally from 0 to 30 mA g−1 and from 30 to 150 mA g−1, respectively.13 The corresponding delithiation and lithiation overpotential increase is thus responsible for the respective specific charge and discharge capacity losses in these cycles. However, this effect cannot explain the specific capacity fade origin of the residual cycles as the specific current is held constant (30 mA g−1 for cycles 1−3; 150 mA g−1 for cycles 4−53). This specific capacity decay of ∼28 mAh g−1 (4 + 24 mAh g−1) after 53 cycles requires a detailed investigation. Several reports focused on the investigation of transition metal (TM) dissolution of the positive electrode active materials.31−33 Some reports correlated the amount of dissolved TMs to the observed specific capacity loss of

2. EXPERIMENTAL SECTION BMW Group provided NCM111 electrodes, which had an active mass loading of ∼15 mg cm−2. Battery grade electrolyte 1 M LiPF6 in EC/EMC (1:1, by wt) (LP50 Selectilyte) from BASF was used as received. Constant current charge/discharge cycling experiments were performed in a three-electrode cell setup (Swagelok), enabling the monitoring of respective electrode potentials. Lithium metal (Rockwood Lithium) was chosen as counter- and reference electrode. Maccor Series 4000 was used as battery cell test system. The constant current charge/discharge cycling was performed in the potential range between 4.6−3.0 V vs Li/Li+. For the initial three cycles and subsequent 50 cycles a specific current of 30 and 150 mA g−1 was used, respectively. In selected experiments, the discharge 1522


Article

The Journal of Physical Chemistry C NCM111.21,26 The amount of dissolved TMs in NCM/Li half cells after 53 cycles was determined by means of total X-ray fluorescence (TXRF), and the results are summarized in Table 1. The total amount of dissolved Ni, Co, and Mn found in the

Subsequently, the charged host material exhibits vacant Li+ sites. The overall number of Li sites, which is expressed by the theoretical capacity of NCM111 (278 mAh g−1) is equal to the sum of vacant Li+ sites (= the specific charge capacity) and the residual Li+ in the host material structure. In the ideal reversible case, the subsequent relithiation during discharge would completely refill the vacant Li+ sites, thus resulting in a specific discharge capacity value equal to the previous specific charge capacity. However, in practical applications an incomplete relithiation during discharge leads to a lower specific discharge capacity, which results in remaining Li+ vacancies in the discharged state of the NCM111 host material, known as the specific capacity loss responsible for decreased Coulombic efficiencies (CE).13 It is important to note that the maximum delithiation amount during the subsequent charge can only be as high as the lithiation amount during previous discharge. The continuous growth of the lithiation hindrance during ongoing charge/discharge cycling thus leads to a decrease in the lithiation amount during the subsequent discharge and simultaneously to an increase of Li+ vacancies in the discharged state. Following this simple deduction, the observed specific capacity fade can be explained by a steady decrease of the lithiation amount and a concomitant increase of vacant Li+ sites in the discharged state. The quantity of the charge and discharge specific current influences the lithiation and delithiation amount during cycling, respectively. Therefore, a careful adaption of the specific currents can be used to simulate the capacity fade mechanism depicted in Figure 2. By increasing the specific discharge current, while keeping the specific charge current constant, a specific discharge capacity fade of the NCM111/Li half cell is observed (Figure 3). The increased lithiation hindrance during cycling leads to an increased specific capacity difference between charge and discharge in each cycle after the raise of the specific discharge current in cycles +2, + 3, + 4, and +5 (among the galvanostatic charge/discharge cycling only the experiment relevant cycles are selected and highlighted with “+”). The value of the specific charge capacity (delithiation amount) of a given cycle is limited to the value of the in the previous cycle obtained specific discharge capacity (lithiation amount), as proposed by Figure 2. Furthermore, a decrease in specific discharge current to the initial value (i.e., 150 mA g−1 in

Table 1. Amount of Dissolved TMs (Ni, Co, and Mn) in NCM111/Li Half Cells after 53 Constant Current Charge/ Discharge Cycles in the Potential Range of 4.6−3.0 V vs. Li/ Li+ as Determined by TXRF and the Calculated Amount of TMs in Pristine NCM Electrodes with an Active Mass Loading of ∼15 mg dissolved TM (μg) pristine electrode TM (mg) ratio (%)

Ni

Co

Mn

total

5.1 (±0.2) 3.04

3.4 (±0.1) 3.06

4.9 (±0.3) 2.85

13.4 8.95

0.17

0.11

0.17

0.15

cell amounts to 13.36 μg. Considering that the electrode’s total active mass loading is ∼15 mg, the total TM amount in the electrode can be calculated for the LiNi1/3Co1/3Mn1/3O2 stoichiometry to be ∼8.95 mg. The ratio of the dissolved TMs after 53 cycles to the total amount of TMs in the electrode is only 0.15%, which corresponds to a specific capacity loss of 0.42 mAh g−1. Therefore, the influence of TM dissolution, per se, on the observed capacity fade in NCM111/ Li half cells seems not to be significant, which is also in accordance with the results reported for full cells.34 In theory, the specific capacity fade can be induced either reversibly by kinetic limitations or irreversibly by possible phase changes taking place in the host material, or both.8 The low amount of dissolved TMs implies an in first approximation overall intact active material after 53 cycles. Therefore, it is reasonable to assume that the specific capacity fade is more likely induced by a continuously increasing hindrance of lithiation, delithiation, or both. A specific capacity fade mechanism induced by a hindered lithiation process is schematically depicted in Figure 2. This kind of specific capacity loss is already known to take place after specific current increase13,35 as also denoted in Figure 1. During charge a certain Li+ amount is removed (specific charge capacity), which is controlled by the applied cycling conditions, such as operation potential window, specific current, and temperature.

Figure 2. Scheme of the assumed specific capacity fade mechanism due to impeded relithiation upon constant current charge/discharge cycling. 1523


Article


Figure 3. Specific capacity development after a specific discharge current increase for NCM111/Li half cells after formation. From cycle +1 to +5 the specific discharge current was stepwise increased from 150 to 750 mA g−1, whereas the specific charge current was set constant to 150 mA g−1. Afterward (cycles +6 and +7) the specific charge and discharge current was both set to 150 mA g−1 again. Figure 4. Schematic overview of specific capacity losses in the 4th and 53rd cycle, as well as calculated (cycles 4 + 53) and by ICP−OES detected (53rd cycle) lithiation degrees after constant current charge/ discharge cycling in NCM111/Li half cells. If the lithiation hindrance mechanism is responsible for the observed specific capacity fade (28 mAh g−1) during constant current cycling then the lithiation degree would be low (72.7%). If the specific capacity fade is not influenced by this mechanism the lithiation degree would remain the same as after the 4th cycle (82.7%). The experimentally measured value is 82.3%, thus neglecting the proposed mechanism in Figure 2.

cycle +1) for cycles +6 and +7 also leads to the specific discharge capacity achieved in the first cycle. Any associations to a kind of a “memory effect” in the NCM material can thus be excluded. As seen in the first and fourth cycle of Figure 1, the increase of the specific discharge current leads to an increased kinetic lithiation hindrance and thus to a specific capacity loss, which in total amounts to ∼48 mAh g−1 (sum of specific capacity losses in the first and fourth cycle). Considering a theoretical specific capacity of 278 mAh g−1 for NCM111, this corresponds to a lithiation degree of ∼82.7% in discharged state of the NCM111 host material after the fourth cycle (Figure 4). In comparison, the total specific capacity loss of the subsequent cycles at a constant specific current (cycles 1−3 and cycles 4−53) amounts to ∼28 mAh g−1 as illustrated in Figure 1. When this specific capacity loss would be completely attributable to the proposed lithiation hindrance mechanism, the lithiation degree of the NCM111 host material should be further decreased to 72.7% after 53 cycles in the discharged state as schematically shown in Figure 4. If this is not the main mechanism, the lithiation degree would be higher. In complete absence of this mechanism, the lithiation degree would remain at the value of ∼82.7% (= lithiation degree after the fourth cycle). Our previous work showed that the lithiation degree of NCM111 can be easily determined by means of ICP−OES technique by measuring the Li+ amount in relation to the normalized TM amount of the discharged NCM111.13 Here, the lithiation degree was also determined after 53 cycles.28 The detected lithiation degree by means of ICP−OES amounts to 82.3% (±1.2%), which is almost equal to the calculated lithiation degree of 82.7% in the fourth cycle (Figure 4). For this reason, in Figure 2 the proposed mechanism could be excluded as a predominant reason for the observed specific capacity fade. The vacant Li+ sites in the discharged state can be electrochemically refilled with Li+ by implementing a constant potential (CP) step at the discharge cutoff potential.13 However, our previous work showed that still ∼14 mAh g−1 of the first cycle specific capacity loss of NCM111/Li half cells could not be recovered at 20 °C.13 This specific capacity loss was attributed to a change in structural properties in the NCM111 host material during the initial cycle, which hinders

the reinsertion of Li+.13,36 Nevertheless, these results also indicate that it should be possible to partially refill the Li+ site vacancies, thus reducing the specific capacity loss from ∼48 mAh g−1 (Figure 4) to ∼14 mAh g−1, as finally determined in ref 13, which corresponds to an increase in the lithiation degree from ∼82.7% to 95.0%, respectively. As expected, the detected lithiation degree after 53 cycles including a CP step at the discharge cutoff potential in the 53rd cycle by means of ICP− OES amounts to 95.3% (±1.4%). It can be concluded that in the 53rd cycle nearly the same number of Li+ site vacancies is present as compared to the first cycle. The related NCM111 intrinsic structural changes that hinder the residual lithiation, thus resulting in ∼14 mAh g−1 specific capacity loss, occur predominately in the first cycle. These results clearly reveal that specific capacity losses can only be attributed to the presence of lithiation hindrances when the specific discharge current is increased (e.g., in the first and fourth cycle). This leads to the conclusion that the specific capacity fade during charge/discharge cycling (cycles 1−3 and 4−53) is not primarily attributable to lithiation hindrances since the specific currents are constant. The opposite, namely, the delithiation hindrance mechanism, is possibly more responsible and is discussed in the following. The mechanism of specific capacity fade caused by delithiation hindrances during charge is schematically depicted in Figure 5. During charge of the NCM111/Li half cell, the presence of delithiation hindrances lowers the delithiation amount, thus leading to a lower specific charge capacity, which means that more Li+ remains within the NCM111 host structure. Assuming the subsequent discharge as ideally reversible, the specific charge and discharge capacities in each cycle are equal, which means a complete relithiation of the 1524


Article


Figure 5. Scheme of the assumed delithiation hindered specific capacity fade mechanism upon constant current charge/discharge cycling.

Due to the fact that the structure of the NCM111 host material is mostly maintained after 53 cycles (Table 1) and the lithiation degree in the discharged state is not significantly changed after the 53rd cycle compared to the fourth cycle (Figure 4), it is reasonable to assume that an increase in delithiation hindrance is predominately responsible for the specific capacity fade observed in the cycles 1−3 and 4−53 (Figure 1). In agreement with our previous report, a higher delithiation hindrance could also be observed by detailed investigation of the overpotentials.35 It was also pointed out that the lithiation hindrance is only higher for the cycles after the specific current increase, in accordance to the here presented results.35 Based on this, also the specific charge capacity loss of ∼10 mAh g−1 (= specific charge capacity difference between third and fourth cycle) in the fourth cycle (Figure 1) can also be assigned to the delithiation hindrance. In analogy to the specific current increase experiments in Figure 6, this finding is a result of the increase of the specific charge current from 30 to 150 mA g−1. Several additional evidence confirm the coherence of the specific capacity fade and the delithiation hindrance. In Figure 7 (left), the constant current charge/discharge cycling with and without the addition of a CP step at 3.0 V vs Li/Li+ for 24 h in each cycle is shown. The addition of CP steps leads to higher specific discharge capacities because of a facilitated lithiation, thus counteracting a possible increase in lithiation hindrance.13 Nevertheless, there is no significant difference in terms of the observed specific capacity fade, which points to the delithiation hindrance origin. Furthermore, even though the overpotentials of the delithiation and lithiation profiles are intertwined,35 the charging potential comparison between the fifth and 53rd cycle, shown in Figure 7 (right), points to an apparent delithiation hindrance increase. Following these considerations, the specific capacity fade during constant current charge/discharge cycling (Figure 1) can now be deconvoluted, and the specific capacity losses can be assigned to the respective capacity fade mechanisms as schematically depicted in Figure 8. In the fully lithiated state, NCM111 has a theoretical specific capacity of 278 mAh g−1. After 53 cycles, the amount of vacant Li+ sites corresponds to a total specific capacity of ∼48 mAh g−1, which originates from the lithiation hindrance mechanism taking place in the first and fourth cycle when the specific discharge current is increased.

NCM111 host material takes place. The further increase of the delithiation hindrances in each cycle leads also to a decrease of the specific discharge capacity and in turn to a further decrease of the specific charge capacity. In conclusion, the specific capacity fade can be solely attributed to the presence and the increase of delithiation hindrances. The specific capacity fade mechanism due to delithiation hindrance can be simulated in a constant current charge/ discharge cycling experiment by increasing the specific charge current in each cycle while keeping the specific discharge current constant (Figure 6). The specific charge/discharge

Figure 6. Specific capacity development after specific charge current increase for NCM111/Li half cells after formation steps in the potential range 4.6−3.0 V vs Li/Li+. From cycle +1 to +5 the specific charge current was increased from 150 to 750 mA g−1, whereas the specific discharge current was set to 150 mA g−1. Afterward (cycles +6 and +7) the specific charge and discharge current was both set to 150 mA g−1 again.

capacities are decreasing with each cycle as expected. Furthermore, the specific discharge capacities are limited by the specific charge capacities in each cycle, indicating high CE values in contrast to the lithiation hindrance mechanism depicted in Figure 2. In analogy to the discussion above on Figure 3, the decrease in specific charge current to the initial value (150 mA g−1 in cycle +1) for cycles +6 and +7 also leads to the initially obtained specific charge and discharge capacities. This observation additionally excludes any influence of a memory effect. 1525


Article


Figure 7. (Left) Constant current charge/discharge cycling of NCM111/Li half cells in the potential range 4.6−3.0 V vs Li/Li+ with and without a CP step at the discharge cutoff potential. The CP step facilitates the relithiation process during discharge and excludes the effect of a kinetic lithiation hindrance on the specific capacity fade, that is observed in the case of no CP step applied. (Right) The respective charging profiles of the 5th and 53rd cycle.

Figure 8. Summary of the specific capacity losses occurring during a constant current charge/discharge cycling for 53 cycles of NCM111/Li half cells in the potential range 4.6−3.0 V vs Li/Li+.

The residual Li+ in the NCM111 host structure, which could not be reversibly delithiated after 53 cycles at the given operation conditions, corresponds to a total specific capacity of ∼96 mAh g−1. This value originates from three incomplete delithiation steps: (a) the incomplete delithiation of 58 mAh g−1 in the first cycle (278 mAh g−1 theoretical specific capacity; 220 mAh g−1 experimentally achieved specific charge capacity in the first cycle), (b) an additional specific charge capacity loss of 10 mAh g−1 due to the specific charge current increase in the fourth cycle, and (c) the total specific capacity fade of 28 mAh g−1 during charge/discharge cycling at constant specific current (cycle 1−3 and 4−53). Consequently, the remaining Li+ amount corresponding to a specific capacity of ∼134 mAh g−1 is the reversibly cycled Li+ amount (specific discharge capacity), which is finally remaining in the 53rd cycle (Figure 1). The specific capacity losses induced by the lithiation hindrance mechanism have predominately a reversible nature. However, a specific capacity of 14 mAh g−1 arising from the first cycle specific capacity loss could not be recovered at 20 °C during a CP step at the discharge cutoff potential. The degree of reversibility of the specific capacity losses caused by the delithiation hindrance can be analogously investigated by facilitating the delithiation kinetics. In this regard, an additional charge/discharge cycle is performed after 53 cycles in which the

charge conditions are varied, while the cell is discharged at a specific current of 30 mA g−1 including a CP step at 3.0 V vs Li/Li+ for 24 h. The addition of a CP step at the discharge cutoff potential while keeping the specific charge current at 150 mA g−1 leads to a specific discharge capacity of 168 mAh g−1 in the 54th cycle. The specific discharge capacity gain of ∼34 mAh g−1 (specific discharge capacity difference between 53rd and 54th cycles) could be expected as they agree with our above considerations about vacant Li+ sites (48 mAh g−1; vacant Li+ sites, 14 mAh g−1 truly irreversible first cycle specific capacity loss at RT). By decreasing the specific charge current from 150 down to 30 mA g−1, the specific discharge capacity is further increased to 188 mAh g−1. However, when applying the same cycling conditions to a fresh NCM111/Li half cell, the first cycle specific discharge capacity amounts to even 205 mAh g−1. The specific discharge capacity difference of 17 mAh g−1 after 54 cycles has both, an irreversible (not recoverable) and a reversible but kinetically inhibited (thus recoverable) origin. In order to facilitate the delithiation kinetics, a CP step at the charge cutoff potential was introduced in the 54th cycle. In Figure 9, the specific discharge capacities as a function of the charge CP step time are depicted. The specific discharge capacities asymptotically increase with increasing CP step charge time. A specific discharge capacity of 208 mAh g−1 is reached in the 54th cycle after applying the charge CP step for 1526


Article


Figure 9. Specific discharge capacities as a function of the charge CP step time for fresh (in the 1st cycle) and cycled (in the 54th cycle) NCM111/Li half cells.

Figure 10. Normalized specific capacity of NCM111/Li and LNMO/ Li half cells charged to the respective cutoff potential of 4.6, 4.8, and 4.95 V vs Li/Li+ as indicated.

15 h. Compared to the fresh NCM111/Li half cell under the same charge/discharge constant current conditions, a specific discharge capacity difference of only ∼4 mAh g−1 is observed. Therefore, it can be concluded that at the most ∼4 mAh g−1 are irreversibly lost after 54 cycles since they cannot be recovered under the applied cycling conditions. Hence, the overall observed specific capacity losses during ongoing constant current charge/discharge cycling are predominately of reversible nature. The fact that facilitating the delithiation kinetics after 53 cycles results in an almost complete specific discharge capacity recovery additionally confirms the delithiation hindrance mechanism proposed in Figure 6. Both, the lithiation and delithiation hindered specific capacity fade mechanisms originate from a steady increase in kinetic limitations within the NCM111/Li half cell, which is in literature often assigned to the presence and growth of more resistive surface layers at the electrode/electrolyte interfaces, often called cathode electrolyte interphase (CEI),13,22 thus inhibiting the Li+ transport kinetics. It is worth noting that already in the pristine state of the cathode active material its surface is partially covered with native surface species, which arise from the reaction with atmospheric CO2 and H2O during processing.23,37 The correlation between the NCM111 specific capacity fade and the applied charge cutoff potential, which has been reported in literature for NCM111/Li half cells would imply an electrochemically induced change in the resistivity of the CEI.26 The growth and change in composition as a function of the charge cutoff potential is known from the solid electrolyte interphase (SEI) formation on graphite.23,38 In Figure 10, the specific capacity retention during constant current cycling in NCM111/Li half cells is compared for different charge cutoff potentials. The increase of the charge cutoff potential from 4.6 to 4.8 V vs Li/Li+ leads to a larger specific capacity fade. For reasons of comparison, the specific capacity fade of high voltage LiNi0.5Mn1.5O4 (LNMO)/Li half cells cycled to an elevated charge cutoff potential of 4.95 V vs Li/Li+ is added to Figure 10. LNMO/Li half cells show only a small specific capacity fade, despite they were charged to a higher charge cutoff potential. Neglecting the presence of catalytic effects, when comparing both active materials, it has to be mentioned that electrolyte oxidation and the formation of a more resistive CEI cannot be the primary reason for the observed difference in specific capacity fade. Therefore, it is assumed that the continuous increase in kinetic limitations during constant current cycling of NCM111/Li half cells at a

charge cutoff potential of 4.6 V vs Li/Li+ is not arising from a electrochemically formed resistive CEI. This conclusion is supported by our previous work where severe electrolyte oxidation could be basically excluded for a NCM111/Li half cell up to the charge cutoff potential of 4.6 V vs Li/Li+.13 Additionally, some reports claim oxidative electrolyte stabilities even up to 4.8 V vs Li/Li+ and higher.11,39 It is therefore assumed that most likely changes associated with the active material are responsible for the rise in kinetic hindrance and thus specific capacity fade, which will be discussed in detail in an upcoming full publication. It seems that changes of the CEI layer are rather independent from electrode potential changes. This assumption can be supported by detailed surface layer investigation after cycling using X-ray photoelectron spectroscopy (XPS).29 Focusing on the LNMO electrode having the highest charge cutoff potential, the surface layer and composition of the pristine and cycled electrode are depicted in Figure 11. Even though the chemical composition of the surface layer changes, the layer thickness remains almost constant. The precipitation of the oxidative decomposed electrolyte products seems not to be significant after 53 cycles even for the high charge cutoff potential of 4.95 V vs Li/Li+. The observed compositional changes of the surface layer are rather supposed to be of chemical intrinsic nature than

Figure 11. Surface layer thickness for the pristine and cycled LNMO electrodes investigated by XPS. Absolute contents of decomposition products are given in each column. The surface layer thickness is rather dependent on the temperature than on the electrode potential. 1527


Article

The Journal of Physical Chemistry C electrochemical reactions.23,40 It is literature known that the native surface layer based on Li2CO3 diminishes after electrolyte exposure yielding LiF products.23,24,41 Furthermore, it was already pointed out that the precipitation related thickness increase is rather dependent from the storage time and temperature than from the applied electrochemical potential.23 Indeed, by increasing the cycling temperature (T) from 20 to 60 °C, the thickness increases significantly as demonstrated in Figure 11. Additionally, the results in this work have shown that predominately the delithiation kinetics are responsible for the impedance rise, while the lithiation kinetics are almost unchanged during constant current cycling. This would imply that the Li+ transport kinetics within the surface layer would have a directional preference. However, such diode-like behavior of a surface film is rather unlikely as already stated for the analogous surface layer on graphite.23,42 Overall, it can be concluded that the apparent impedance rise, which is responsible for the specific capacity fade, most likely originates from changes within the investigated active NCM111 material.

(XPS). Thus, it is assumed that the observed CEI is rather formed by reactions actively involving both the cathode material (in particular its surface) and the electrolyte, being dependent on the type of cathode material and the type of electrolyte. In other words, simple oxidative electrolyte decomposition and deposition of the decomposition products on an inert cathode surface, being dependent on the applied electrode potential and the electrolyte composition, reveal to be unlikely the main origin for the steadily resistance increase. Considering all these results, the specific capacity fade at high electrode potentials is predominately attributable to changes in/at the active material and less by simple electrolyte decomposition and CEI formation.

■

AUTHOR INFORMATION

Corresponding Authors

*Tel: +49 251 83-36737. Fax: +49 251 83-36032. E-mail: [email protected]. *Tel: +49 251 83-36031. Fax: +49 251 83-36032. E-mail: [email protected] or [email protected]. ORCID

4. CONCLUSION During constant current charge/discharge cycling of LiNi1/3Co1/3Mn1/3O2 (NCM111)/Li half cells, apparent specific capacity losses can be observed at an elevated charge cutoff potential of 4.6 V vs Li/Li+. Transition metal dissolution as a possible failure mechanism was determined by means of the total X-ray fluorescence (TXRF) technique. In relation to the active material mass, merely 0.15% dissolved transition metals could be detected, which corresponds to an overall specific capacity loss of 0.42 mAh g−1 and thus basically excludes its significance on the observed much higher specific capacity losses after 53 cycles. The intact structure of the host material implies that the specific capacity fade is caused by steady hindrance increase of either the lithiation (discharge) or the delithiation (charge) processes, or both. The obtained results showed that the specific capacity losses induced by the lithiation hindrance are only present for cycles in which the specific charge current is increased compared to the previous cycle. The corresponding Li+ site vacancies in the discharged state of the host material could be confirmed by inductive coupled plasma optical emission spectroscopy (ICP−OES) analysis. Any impact of the presence and growth of a lithiation hindrance on the specific capacity fade for charge/discharge cycling at a constant current could hence be disproved. In this regard, a capacity fade mechanism caused by delithiation hindrance was proposed. The kinetic facilitation via a constant potential (CP) step for charge and discharge after the investigated 53 cycles revealed a nearly complete specific discharge capacity recovery by comparing the NCM111 after one cycle under the same electrochemical conditions. This result indicates that the specific capacity fade origin has predominately a reversible nature. Furthermore, a specific capacity fade originating only from the steady resistivity growth of a CEI (cathode electrolyte interphase) as a result of continuous electrolyte oxidation during cycling was not found. The negligible specific capacity fade for LiNi 0.5Mn1.5O4 (LNMO) at a charge cutoff potential of 4.95 V vs Li/Li+ disconfirms any electrode potential-dependent surface layer changes to be responsible for the resistivity rise. Moreover, it could be shown that the surface layer thickness is more influenced by the temperature than the electrode potential, as measured by means of X-ray photoelectron spectroscopy

Johannes Kasnatscheew: 0000-0002-8885-8591 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors J.K., B.S., and R.W. gratefully acknowledge the financial support for this work by BMW Group and the material support from BASF. M.E., S.N., and I.C.L. acknowledge funding from German Federal Ministry for Education and Research (BMBF) within the project Electrolyte Lab 4E (project reference 03X4632). M.W. received funding from the Federal Government.

■

REFERENCES

(1) Wagner, R.; Preschitschek, N.; Passerini, S.; Leker, J.; Winter, M. Current Research Trends and Prospects among the Various Materials and Designs Used in Lithium-Based Batteries. J. Appl. Electrochem. 2013, 43, 1−16. (2) Goodenough, J. B.; Park, K. S. The Li-Ion Rechargeable Battery: A Perspective. J. Am. Chem. Soc. 2013, 135, 1167−1176. (3) Olivier, J. P.; Winter, M. Determination of the Absolute and Relative Extents of Basal Plane Surface Area and ″Non-Basal Plane Surface″ Area of Graphites and Their Impact on Anode Performance in Lithium Ion Batteries. J. Power Sources 2001, 97−8, 151−155. (4) Kasnatscheew, J.; Schmitz, R. W.; Wagner, R.; Winter, M.; Schmitz, R. Fluoroethylene Carbonate as an Additive for γButyrolactone Based Electrolytes. J. Electrochem. Soc. 2013, 160, A1369−A1374. (5) Nagaura, T.; Tozawa, K. Prog. Batteries Sol. Cells 1990, 9, 209. (6) Ozawa, K. Lithium-Ion Rechargeable Batteries with LiCoO2 and Carbon Electrodes - the LiCoO2 C System. Solid State Ionics 1994, 69, 212−221. (7) Andre, D.; Kim, S. J.; Lamp, P.; Lux, S. F.; Maglia, F.; Paschos, O.; Stiaszny, B. Future Generations of Cathode Materials: An Automotive Industry Perspective. J. Mater. Chem. A 2015, 3, 6709− 6732. (8) Liu, W.; Oh, P.; Liu, X.; Lee, M. J.; Cho, W.; Chae, S.; Kim, Y.; Cho, J. Nickel-Rich Layered Lithium Transition-Metal Oxide for HighEnergy Lithium-Ion Batteries. Angew. Chem., Int. Ed. 2015, 54, 4440− 4457. (9) Marom, R.; Amalraj, S. F.; Leifer, N.; Jacob, D.; Aurbach, D. A Review of Advanced and Practical Lithium Battery Materials. J. Mater. Chem. 2011, 21, 9938−9954. 1528


Article

The Journal of Physical Chemistry C (10) Robert, R.; Villevieille, C.; Novak, P. Enhancement of the High Potential Specific Charge in Layered Electrode Materials for LithiumIon Batteries. J. Mater. Chem. A 2014, 2, 8589−8598. (11) Yin, S. C.; Rho, Y. H.; Swainson, I.; Nazar, L. F. X-Ray/Neutron Diffraction and Electrochemical Studies of Lithium De/Re-Intercalation in Li1‑XCo1/3Ni1/3Mn1/3O2 (X = 0 -> 1). Chem. Mater. 2006, 18, 1901−1910. (12) Huggins, R. A. Advanced Batteries; Springer: 2009. (13) Kasnatscheew, J.; Evertz, M.; Streipert, B.; Wagner, R.; Klopsch, R.; Vortmann, B.; Hahn, H.; Nowak, S.; Amereller, M.; Gentschev, A.; et al. The Truth About the 1st Cycle Coulombic Efficiency of LiNi1/3Co1/3Mn1/3O2 (NCM) Cathodes. Phys. Chem. Chem. Phys. 2016, 18, 3956−3965. (14) Manthiram, A.; Choi, J.; Choi, W. Factors Limiting the Electrochemical Performance of Oxide Cathodes. Solid State Ionics 2006, 177, 2629−2634. (15) Amatucci, G. G.; Tarascon, J. M.; Klein, L. C. CoO2, the End Member of the LixCoO2 Solid Solution. J. Electrochem. Soc. 1996, 143, 1114−1123. (16) Chebiam, R. V.; Kannan, A. M.; Prado, F.; Manthiram, A. Comparison of the Chemical Stability of the High Energy Density Cathodes of Lithium-Ion Batteries. Electrochem. Commun. 2001, 3, 624−627. (17) Ohzuku, T.; Makimura, Y. Layered Lithium Insertion Material of LiCo1/3Ni1/3Mn1/3O2 for Lithium-Ion Batteries. Chem. Lett. 2001, 30, 642−643. (18) He, P.; Yu, H. J.; Li, D.; Zhou, H. S. Layered Lithium Transition Metal Oxide Cathodes Towards High Energy Lithium-Ion Batteries. J. Mater. Chem. 2012, 22, 3680−3695. (19) Ellis, B. L.; Lee, K. T.; Nazar, L. F. Positive Electrode Materials for Li-Ion and Li-Batteries. Chem. Mater. 2010, 22, 691−714. (20) Kraytsberg, A.; Ein-Eli, Y. Higher, Stronger, Better··· A Review of 5 V Cathode Materials for Advanced Lithium-Ion Batteries. Adv. Energy Mater. 2012, 2, 922−939. (21) Zheng, H.; Sun, Q.; Liu, G.; Song, X.; Battaglia, V. S. Correlation between Dissolution Behavior and Electrochemical Cycling Performance for LiNi1/3Co1/3Mn1/3O2-Based Cells. J. Power Sources 2012, 207, 134−140. (22) Wagner, R.; Streipert, B.; Kraft, V.; Reyes Jiménez, A.; Röser, S.; Kasnatscheew, J.; Gallus, D. R.; Börner, M.; Mayer, C.; Arlinghaus, H. F.; et al. Counterintuitive Role of Magnesium Salts as Effective Electrolyte Additives for High Voltage Lithium-Ion Batteries. Adv. Mater. Interfaces 2016, 3, 1600096. (23) Xu, K. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104, 4303−4417. (24) Eriksson, T.; Andersson, A. M.; Bishop, A. G.; Gejke, C.; Gustafsson, T.; Thomas, J. O. Surface Analysis of LiMn2O4 Electrodes in Carbonate-Based Electrolytes. J. Electrochem. Soc. 2002, 149, A69− A78. (25) Yang, L.; Ravdel, B.; Lucht, B. L. Electrolyte Reactions with the Surface of High Voltage LiNi0.5Mn1.5O4 Cathodes for Lithium-Ion Batteries. Electrochem. Solid-State Lett. 2010, 13, A95−A97. (26) Gallus, D. R.; Schmitz, R.; Wagner, R.; Hoffmann, B.; Nowak, S.; Cekic-Laskovic, I.; Schmitz, R. W.; Winter, M. The Influence of Different Conducting Salts on the Metal Dissolution and Capacity Fading of NCM Cathode Material. Electrochim. Acta 2014, 134, 393− 398. (27) Liu, S.; Xiong, L.; He, C. Long Cycle Life Lithium Ion Battery with Lithium Nickel Cobalt Manganese Oxide (NCM) Cathode. J. Power Sources 2014, 261, 285−291. (28) Evertz, M.; Lurenbaum, C.; Vortmann, B.; Winter, M.; Nowak, S. Development of a Method for Direct Elemental Analysis of Lithium Ion Battery Degradation Products by Means of Total Reflection X-Ray Fluorescence. Spectrochim. Acta, Part B 2015, 112, 34−39. (29) Niehoff, P.; Winter, M. Composition and Growth Behavior of the Surface and Electrolyte Decomposition Layer of/on a Commercial Lithium Ion Battery LiNi1/3Co1/3Mn1/3O2 Cathode Determined by Sputter Depth Profile X-Ray Photoelectron Spectroscopy. Langmuir 2013, 29, 15813−15821.

(30) Whittingham, M. S. Lithium Batteries and Cathode Materials. Chem. Rev. 2004, 104, 4271−4301. (31) Choi, W.; Manthiram, A. Comparison of Metal Ion Dissolutions from Lithium Ion Battery Cathodes. J. Electrochem. Soc. 2006, 153, A1760−A1764. (32) Amatucci, G. G.; Tarascon, J. M.; Klein, L. C. Cobalt Dissolution in LiCoO2-Based Non-Aqueous Rechargeable Batteries. Solid State Ionics 1996, 83, 167−173. (33) Wang, L. F.; Ou, C. C.; Striebel, K. A.; Chen, J. J. S. Study of Mn Dissolution from LiMn2O4 Spinel Electrodes Using Rotating RingDisk Collection Experiments. J. Electrochem. Soc. 2003, 150, A905− A911. (34) Evertz, M.; Horsthemke, F.; Kasnatscheew, J.; Börner, M.; Winter, M.; Nowak, S. Unraveling Transition Metal Dissolution of Li1.04Ni1/3Co1/3Mn1/3O2 (NCM 111) in Lithium Ion Full Cells by Using the Total Reflection X-Ray Fluorescence Technique. J. Power Sources 2016, 329, 364−371. (35) Kasnatscheew, J.; Rodehorst, U.; Streipert, B.; Wiemers-Meyer, S.; Jakelski, R.; Wagner, R.; Laskovic, I. C.; Winter, M. Learning from Overpotentials in Lithium Ion Batteries: A Case Study on the LiNi1/3Co1/3Mn1/3O2 (NCM) Cathode. J. Electrochem. Soc. 2016, 163, A2943−A2950. (36) Seidlmayer, S.; Buchberger, I.; Reiner, M.; Gigl, T.; Gilles, R.; Gasteiger, H. A.; Hugenschmidt, C. First-Cycle Defect Evolution of Li1−XNi1/3Mn1/3Co1/3O2 Lithium Ion Battery Electrodes Investigated by Positron Annihilation Spectroscopy. J. Power Sources 2016, 336, 224−230. (37) Aurbach, D.; Levi, M. D.; Levi, E.; Teller, H.; Markovsky, B.; Salitra, G.; Heider, U.; Heider, L. Common Electroanalytical Behavior of Li Intercalation Processes into Graphite and Transition Metal Oxides. J. Electrochem. Soc. 1998, 145, 3024−3034. (38) Winter, M. The Solid Electrolyte Interphase - the Most Important and the Least Understood Solid Electrolyte in Rechargeable Li Batteries. Z. Phys. Chem. 2009, 223, 1395−1406. (39) Tarascon, J. M.; Guyomard, D. New Electrolyte Compositions Stable over the O-V to 5-V Voltage Range and Compatible with the Li1+XMn2O4 Carbon Li-Ion Cells. Solid State Ionics 1994, 69, 293−305. (40) Balasubramanian, M.; Lee, H. S.; Sun, X.; Yang, X. Q.; Moodenbaugh, A. R.; McBreen, J.; Fischer, D. A.; Fu, Z. Formation of Sei on Cycled Lithium-Ion Battery Cathodes - Soft X-Ray Absorption Study. Electrochem. Solid-State Lett. 2002, 5, A22−A25. (41) Andersson, A. M.; Abraham, D. P.; Haasch, R.; MacLaren, S.; Liu, J.; Amine, K. Surface Characterization of Electrodes from High Power Lithium-Ion Batteries. J. Electrochem. Soc. 2002, 149, A1358− A1369. (42) Huang, C. K.; Sakamoto, J. S.; Wolfenstine, J.; Surampudi, S. The Limits of Low-Temperature Performance of Li-Ion Cells. J. Electrochem. Soc. 2000, 147, 2893−2896.

1529


Changing Established Belief on Capacity Fade Mechanisms

Recommend Documents