Li4Ti5O12 Lithium Ion Cells at Elevated

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High Voltage LiNi0.5Mn1.5O4/Li4Ti5O12 Lithium Ion Cells at Elevated Temperatures: Carbonate- vs. Ionic Liquid-Based Electrolytes Xia Cao, Xin He, Jun Wang, Haidong Liu, Stephan Roeser, Babak Rezaei Rad, Marco Evertz, Benjamin Streipert, Jie Li, Ralf Wagner, Martin Winter, and Isidora Cekic-Laskovic ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07687 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 21, 2016

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ACS Applied Materials & Interfaces

High Voltage LiNi0.5Mn1.5O4/Li4Ti5O12 Lithium Ion Cells at Elevated Temperatures: Carbonate- vs. Ionic Liquid-Based Electrolytes Xia Caoa *, Xin Hea,b, Jun Wanga, Haidong Liua, Stephan Rösera, Babak Rezaei Rad Evertza, Benjamin Streiperta, Jie Lia, Ralf Wagnera, Martin Wintera,

b*

a,b

, Marco

and Isidora Cekic-

Laskovica [a] MEET Battery Research Center, Institute for Physical Chemistry, University of Münster, Corrensstraße 46, 48149 Münster [b] Helmholtz-Institute Münster, IEK-12, Forschungszentrum Jülich GmbH, Corrensstraße 46, 48149 Münster KEYWORDS: lithium ion battery; safety; high temperature; high voltage electrolyte; ionic liquids; LiNi0.5Mn1.5O4; Li4Ti5O12

ABSTRACT: Thanks to its high operating voltage, LiNi0.5Mn1.5O4 (LNMO) spinel represents a promising next-generation cathode material candidate for Lithium ion batteries. However, LNMO based full-cells with organic carbonate solvent electrolytes suffer from severe capacity fading issues, associated with electrolyte decomposition and concurrent degradative reactions at the electrode/electrolyte interface, especially at elevated temperatures. As promising alternatives,

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two selected LiTFSI/pyrrolidinium bis(trifluoromethane-sulfonyl)imide room temperature ionic liquid (RTIL) based electrolytes with inherent thermal stability were investigated in this work. Linear sweep voltammetry (LSV) profiles of the investigated LiTFSI/RTIL electrolytes display much higher oxidative stability compared to the state-of-the-art LiPF6/organic carbonate based electrolyte at elevated temperatures. Cycling performance of the LNMO/Li4Ti5O12 (LTO) fullcells with LiTFSI/RTIL electrolytes reveals remarkable improvements in respect to capacity retention and Coulombic efficiency. Scanning electron microscopy (SEM) images and X-ray diffraction (XRD) patterns indicate maintained pristine morphology and structure of LNMO particles after 50 cycles at 0.5C. Investigated LiTFSI/RTIL based electrolytes outperform the LiPF6/organic carbonate-based electrolyte in terms of cycling performance in LNMO/LTO fullcells at elevated temperatures.

INTRODUCTION Lithium ion batteries (LIBs) known for high energy density, are widely studied for electric (EV) and hybrid electric vehicle (HEV) applications. 1, 2 However, at the current stage of development, LIBs with higher energy, higher power and at the same time high safety still remain a big challenge.

3

One of the main research activities is associated with search for an advanced, so

called next generation materials. 4, 5 LiNi0.5Mn1.5O4 (LNMO) high-voltage spinel is considered as one of the most promising candidates as cathode material for LIBs due to its high operating voltage (4.7 V vs. Li/Li+), which is directly proportional to the energy density and power density of the battery.

6-9

Hand in hand with low material cost and excellent rate capability, LNMO has

received much attention over the last decade.

10-14

On the other hand, increasing the operating

temperature of the cell is an alternative approach to improve the achievable power density. 15


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State-of-the-art (SOTA) electrolytes based on mixtures of LiPF6 and organic carbonate solvents are not oxidatively stable at such a high voltage and represent one of the bottlenecks for the commercialization of high-voltage LIBs.

16-20

High temperature operation leads to an even

poorer cell performance scenario due to the accelerated electrolyte decomposition at elevated temperatures. 10, 21-23 In general, higher temperatures clearly increase the safety risk of the battery cell because of the thermal instability of LiPF6/carbonate-based electrolyte, evolution of toxic decomposition products, and electrodes. published.

23

6, 30-32

24-29

and accelerated decomposition reactions between the electrolyte

Over the last decade, scattered reports addressing this challenge have been

Nowadays, research mainly focuses on the modification of LNMO in view of

various morphologies, coating layers and doping elements, in order to stabilize the structure and suppress the well-known, inevitable side reactions with the electrolyte.

6, 30-32

To improve the

electrolyte performance, the application of high-voltage electrolyte additives has been extensively investigated and regarded as a feasible way to suppress the electrolyte decomposition.

33, 34

Solvents with higher oxidative stability have also been widely investigated

for high-voltage application. 17, 35 Nevertheless, articles reporting good LNMO cell performance at high temperature are scarce. Lu et al. reported severe capacity fading of the LNMO/graphite full cells after one week of storage at 55 °C in a fully discharged state. 22 In summary, a great challenge in respect to calendar and cycle life of LIBs at high voltage and high temperature still exists. With this in mind, the emphasis within this work is placed on understanding the related failure mechanisms and performance improvements of LNMO-based cells at elevated temperatures. Room temperature ionic liquid (RTIL)-based electrolytes with high oxidation stability,

36, 37

excellent thermal stability,

basically non-flammable nature

38

38

negligible vapor pressure and

are known as alternative candidates.

39

In recent years,


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different approaches have been pursued to investigate the performance of RTIL-based electrolytes in LIBs. Jin et al. reported on LiFePO4/Li cells with a LiTFSI/N-methyl-Nbutylpiperidinium bis(trifluoromethanesulfonyl)imide (PP14TFSI) based electrolyte, using vinylene carbonate (VC) as additive.

40

Nádherná et al. published results on the compatibility

between LiFSI/1-butyl-1-methylpyrrolidinium bis(trifluoromethane-sulfonyl)imide (Pyr14TFSI) electrolyte and graphite.

41

Kim et al. suggested binary electrolytes consisting of RTIL and

commercial carbonate electrolytes in the idea of improving both the safety and the lithium ion mobility of the resultant electrolyte formulation.

42

Similarly, Xiang presented the results with

this type of binary electrolytes by mixing RTILs and sulfone solvents.

43

Lately, Elia et al.

showed promising performance of LiTFSI/Pyr14TFSI with layered LiNi1/3Co1/3Mn1/3O2 cathode and a Sn/C anode at 40 oC.

44

Based on our previous work, ester modified methyl-

methylcarboxymethyl pyrrolidinium bis(trifluoromethane-sulfonyl)imide (MMMPyrTFSI) is considered as interesting and promising candidate for application in LIBs due to its wide temperature range and broad electrochemical stability window.

45

With this in mind, Figure 1

depicts chemical structures of Pyr14TFSI and MMMPyrTFSI, as the two RTILs investigated in the frame of this work.

Figure 1. Chemical structures of the investigated TFSI--based ionic liquids and the TFSI- anion


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Performance of the investigated electrolyte formulations was evaluated in LNMO-based fullcells. In a full-cell setup, the active Li+ loss issue is taken into consideration, which is known as an important factor for capacity fading during cycling.

8, 15, 46

To focus the investigation on the

high-voltage cathode side and to reduce side reactions at the anode, Li4Ti5O12 (LTO) was selected as the anode material. With ≈1.5 V vs. Li/Li+, the LTO anode operating potential than the graphite anode,

48

47

has a much higher

thus preventing electrolyte reduction and solid

electrolyte interphase (SEI) formation 49 at the anode/electrolyte interface. 32, 50, 51 RESULTS AND DISCUSSION Figure 2 displays the electrochemical stability window (ESW) of the investigated LiTFSI/RTIL-based electrolytes compared to the SOTA electrolyte 1M LiPF6 EC/DMC (1/1, by wt.) (from now on called benchmark electrolyte) on platinum electrode. At 20 oC, all investigated electrolytes display similar oxidative stability, as the bulk electrolyte oxidation process starts at ≈5.5 V vs. Li/Li+. Increasing the temperature to 40 oC, the benchmark electrolyte starts to oxidize at ≈4.4 V vs. Li/Li+, whereas the RTIL-based electrolytes LiTFSI/Pyr14TFSI and LiTFSI/MMMPyrTFSI show much higher oxidation stability, with an oxidative potential of 5.2 V vs Li/Li+ for LiTFSI/Pyr14TFSI and 5.5 V vs Li/Li+ for LiTFSI/MMMPyrTFSI. In case of the benchmark electrolyte, further increase of temperature to 60 oC, results in even lower oxidation onset potential of 3.9 V vs Li/Li+. On the other hand, LiTFSI/Pyr14TFSI and LiTFSI/MMMPyrTFSI electrolytes show stability up to 4.5 V vs Li/Li+. As for the reductive stability, all investigated electrolytes did not display an apparent reduction peak during the scan from open circuit potential to 0 V vs Li/Li+ at 20 oC and 40 oC. When increasing the temperature to 60 oC, reduction of the benchmark electrolyte occurs at ≈2.5 V vs Li/Li+. LiTFSI/RTIL


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electrolytes show lower reduction currents, with a reduction peak at ≈2.1 V vs Li/Li+ for LiTFSI/MMMPyrTFSI and ≈2.0 V vs Li/Li+ for LiTFSI/Pyr14TFSI. Overall, the electrochemical stability window of the electrolytes is temperature dependent and LiTFSI/RTIL electrolytes seem to be more suitable candidates for high voltage application at elevated operation temperatures.

Figure 2. Linear sweep voltammetry profiles of the investigated electrolytes 1M LiPF6 EC/DMC (1/1 by wt.), 0.7M LiTFSI Pyr14TFSI and 0.7M LiTFSI MMMPyrTFSI on platinum electrode at (a) 20 oC, (b) 40 oC and (c) 60 oC at the scan rate of 0.1 mV/s. However, the electrochemical stability window determined on inert electrodes is not directly applicable to real battery cell conditions and materials. 52 For this reason, the compatibility of the investigated electrolytes in LNMO/LTO full-cell at elevated temperatures (40 oC and 60 oC) was compared (Figure 3). The cell containing the benchmark electrolyte shows an initial specific discharge capacity of 60 mAh g-1 after the formation cycles and an obvious capacity fading during cycling at 0.5C, thus reaching 28% state of health (SOH) after 50 cycles at 40 oC. For the cell containing the LiTFSI/Pyr14TFSI electrolyte, the initial specific discharge capacity amounts to 90 mAh g-1 and the SOH to 47% after 50 cycles. For the cell containing LiTFSI/MMMPyrTFSI, the initial specific discharge capacity is 70 mAh g-1 and the SOH


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amounts to 69%. When increasing the operation temperature to 60 oC, the cell containing the benchmark electrolyte shows only low specific discharge capacities and reaches a SOH of 0% already after ≈35 cycles. However, for the cells with LiTFSI/RTIL-based electrolytes, the initial specific discharge capacity improves to approximately 105 mAh g-1, possibly due to the conductivity improvement at 60 oC compared to 40 oC.

45

Although capacity fading during

cycling is still observable, a significant improvement compared to the benchmark electrolyte is evident. Figures 3b and 3d display the corresponding Coulombic efficiencies of the aforementioned cells. Much higher Coulombic efficiency values upon cycling are achieved in cells with the LiTFSI/RTIL based electrolytes, especially for the cells cycled at 60 oC. Overall, the increased Coulombic efficiency values together with higher specific discharge capacities and higher capacity retention indicate the significant improvement of the LNMO/LTO full cell performance by employing the LiTFSI/RTIL-based electrolytes compared to the benchmark electrolyte. Furthermore, MMMPyrTFSI–based electrolyte shows better cycling performance than the Pyr14TFSI based electrolyte both at 40 °C and 60 °C, indicating that the ester group in MMMPyrTFSI cation can stabilize the cell cycling to a certain extent. Moreover, the obtained stable and reversible lithiation/de-lithiation of the LNMO/LTO full-cell at high operation temperature outperforms the results for similar systems published in literature so far. 10, 22, 53


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Figure 3. Comparison of (a, c) specific discharge capacity and (b, d) Coulombic efficiency vs. cycle number of the constant current charge/discharge cycling results for LNMO/LTO full-cells containing the investigated electrolytes at (a, b) 40 oC and (c, d) 60 oC at 0.5C after 2 formation cycles at 0.2C. Specific capacities refer to the active mass of the LNMO cathode material. Figure 4 displays the cell voltage (black curve) as well as the anode and cathode potential profiles vs. Li/Li+ (red curves) for the LNMO/LTO full cells (three-electrode Swagelok cell) during representative cycles (1st, 2nd, 3rd and 30th cycle). The cells were charged/discharged between 1.4 – 3.4 V (cathode vs. anode), and the potential (cathode vs. Li/Li+) was recorded at the same time. The potential (anode vs. Li/Li+) was calculated from the cell voltage and the cathode potential.


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In case of the benchmark electrolyte no reversible Li+ intercalation/de-intercalation is possible, due to severe electrolyte oxidation (Figure 4a). In contrast, the LiTFSI/RTIL containing cells show reversible Li+ intercalation/de-intercalation. However, LiTFSI/RTIL containing cells shown a continuous increase in over-potential during cycling, thus resulting in kinetically induced capacity fading.


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Figure 4. Cell voltage vs. time profile (black curve) and anode and cathode potential vs. time profiles (red curves) of the LNMO/LTO full cells containing the investigated electrolytes (a) 1M LiPF6 EC/DMC (1/1 by wt.), (b) 0.7M LiTFSI Pyr14TFSI and (c) 0.7M LiTFSI MMMPyrTFSI during representative cycles of the constant current charge/discharge cycling process at 0.5C after 2 formation cycles at 0.2C at 60 oC. To understand the capacity fading upon the charge/discharge process in the LNMO/LTO fullcells, electrochemical impedance spectroscopy (EIS) measurements were carried out to investigate the impendence evolution of the cells during cycling. The semicircle observed in the EIS spectra in Figure 5 represents the resistance of Li+ migration through the formed passivation layers, i.e. charge transfer resistance (Rct) 54, 55. For each cell, the Rct value of cell increases with the cycle number. However, the cell with the benchmark electrolyte shows a much faster increase of the Rct value compared to the cells with LiTFSI/RTIL-based electrolytes at 40 oC and 60 oC respectively. At 40 oC, the Rct value of the benchmark cell in the 50th cycle is almost two to three times higher compared to the LiTFSI/RTIL-based electrolytes containing cells. As for 60 oC, the difference is more pronounced between the cell with the benchmark electrolyte and the cells with LiTFSI/RTIL electrolytes. For the benchmark cell, the Rct value in the 50th cycle at 60 oC is about four times of that obtained at 40 oC, whereas it only shows a slight increase at 60 oC in the cells with LiTFSI/RTIL based electrolytes compared their corresponding Rct values at 40 oC. Consequently, the Rct value of the benchmark cell in the 50th cycle is about five to six times higher than the values obtained in the LiTFSI/RTIL-based electrolytes containing cells at 60 oC. Therefore, it can be concluded that LiTFSI/RTIL-based electrolytes outperform the benchmark electrolyte at elevated temperature with much lower impedance after cycling.


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Figure 5. EIS spectra of a LNMO/LTO full cell cycled with the investigated electrolytes (a, d) 1M LiPF6 EC/DMC (1/1 by wt.), (b, e) 0.7M LiTFSI Pyr14TFSI and (c, f) 0.7M LiTFSI MMMPyrTFSI at (a-c) 40 oC and (c-f) 60 oC. The insets show the magnification of the highfrequency semicircle. To get a deeper insight into the cathode electrolyte interphase (CEI) film formation at the electrode/electrolyte interface, cycled LNMO/LTO full-cells (after 50 charge/discharge cycles) were disassembled and the electrodes carefully harvested. After being rinsed with 2 mL DMC, the LNMO electrodes were analyzed by means of scanning electron microscopy (SEM). Figure 6 depicts the SEM micrographs of the pristine and cycled LNMO electrodes. The morphology of the pristine LNMO electrodes has a spherical structure (Figure 6a). For the LNMO electrodes recollected from the cell with the benchmark electrolyte (Figure 6b, 6e), the electrode surface is rough and blurry, covered with parasitic decomposition products. These products form a surface film, which increases the Rct value as displayed in Figure 5 and thus leads to capacity fading


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during cycling (Figure 3). On the contrary, as shown in Figures 6c-d and 6f-g, the LNMO electrodes cycled in the investigated LiTFSI/RTIL-based electrolytes reveal very clean surfaces, thus indicating less electrolyte oxidation and parasitic electrode/electrolyte reactions, which is in a good agreement with the electrochemical stability results and impendence measurements.

Figure 6. SEM images of (a) pristine LNMO electrode, and LNMO electrodes cycled in LNMO/LTO full-cells with investigated electrolytes (b, e) 1M LiPF6 EC/DMC (1/1 by wt.), (c, f) 0.7M LiTFSI Pyr14TFSI and (d, g) 0.7M LiTFSI MMMPyrTFSI at (b-d) 40 oC and (e-g) 60 oC. In addition to the SEM surface investigations, the bulk of the cycled LNMO electrodes was analyzed by means of X-ray diffraction (XRD). The representative patterns are shown in Figure 7, and pristine LNMO is illustrated for comparison, indexing to a spinel structure with a space group Fd3m 56. For the LNMO electrode cycled in the cell with the benchmark electrolyte at 40 o

C, the spinel structure of the electrodes is maintained with the space group Fd3m (Figure 7a).

Nevertheless, the diffraction peaks of the LNMO electrode shift to higher angle values compared to the pristine sample and each characteristic peak of the pristine LNMO divides into two peaks.


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This result indicates that a lattice spacing concentration takes place in the cycled LNMO in comparison to pristine LNMO. The peak split indicates that two states of delithiated Li1xNi0.5Mn1.5O4

are present in the case of the cell cycled at 40 oC. Rietveld refinement of the XRD

patterns gives, for one state, a cell volume of 512.36 Å3 with lattice parameter a of 8.00 Å, and for the other state, a cell volume of 529.43 Å3 with lattice parameter a of 8.09 Å. The cell volume of pristine LNMO is 545.46 Å3 with an a value of 8.17 Å. This difference is possibly caused by the consumption of active Li+ in side reactions, resulting in generation of a partly delithiated Li1-xNi0.5Mn1.5O4 instead of fully lithiated LNMO material. This irreversible Li+ consumption is considered as the main reason for the capacity fading of the cell with benchmark electrolyte. Figures 7b and 7c display the XRD patterns of the pristine and harvested LNMO samples re-collected from the cells with the LiTFSI/RTIL-based electrolytes cycled at different temperatures. Harvested LNMO electrodes have the same crystal structure as the pristine LNMO material. Rietveld refinement for the XRD patterns show that the cell volume of these recollected LNMO is in the range of 545.17-546.57 Å3 and a amounts to 8.17-8.18 Å, revealing a good structural maintainability of LNMO in case of the LNMO/LTO full-cells cycled with the LiTFSI/RTIL-based electrolytes.


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Figure 7. XRD patterns of LNMO electrodes cycled in the cell with electrolytes (a) 1M LiPF6 EC/DMC (1/1 by wt.), (b) 0.7M LiTFSI Pyr14TFSI and (c) 0.7M LiTFSI MMMPyrTFSI at different temperatures together with pristine LNMO as reference. The lithiation degree in recollected LNMO electrodes after 50 cycles was determined by means of inductively coupled plasma-optical emission spectrometry (ICP-OES). As shown in Table 1, the residual lithium in the LNMO electrode cycled in the cell with benchmark electrolyte amounts to only 49.2%, which means that more than 50% of Li+ was consumed during cycling. In case of the LiTFSI/RTIL electrolytes, much less Li+ loss occurs during cycling. Table 1. Lithiation degree of LNMO electrodes recollected from the cells with investigated electrolytes at 40 and 60 oC. The absolute deviation values are given by 3 replicates. Electrolyte

1M LiPF6 EC/DMC

0.7M LiTFSI

0.7M LiTFSI

(1/a by wt.)

Pyr14TFSI

MMMPyrTFSI

Temperature / oC

40

60

40

60

40

60

Lithiation degree/ %

49.2

-

80.1

79.3

80.4

85.4

Absolute deviation / %

1.3

-

1.4

0.7

1.1

0.8


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CONCLUSIONS In this work, two LiTFSI/RTIL-based electrolytes were investigated as promising alternatives to the SOTA electrolyte in high-voltage LNMO-based full-cells operated at elevated temperatures. Continuous formation of the cathode electrolyte interphase and concurrent Li+ loss are found to be the main reasons behind the capacity fading. In the cell with the benchmark electrolyte, severe electrolyte decomposition and electrode/electrolyte side reactions take place at 40 and 60 oC and are originally caused by the lower anodic stability and poor thermal stability of LiPF6/organic carbonate solvent-based electrolytes. As a solution, LiTFSI/RTIL-based electrolytes with improved anodic stability are investigated as potential electrolyte formulations that may replace the traditional carbonate-based electrolytes for elevated temperature application. A variety of advantages of LiTFSI/RTIL based electrolytes for LNMO/LTO full-cell manifested through less Li+ loss, enhanced electrode/electrolyte interface and lower charge transfer impedance were identified. In addition, the investigated LiTFSI/RTIL-based electrolytes show significant improvement of the electrochemical performance with higher specific capacity and better capacity retention at elevated temperatures compared to the benchmark electrolyte. Among them, LiTFSI/MMMPyrTFSI electrolyte shows the best electrochemical performance in the LNMO/LTO full-cell. Hand in hand with the inherent properties of RTILs, the investigated LiTFSI/RTIL-based electrolytes seem to be interesting and promising candidates for enabling LNMO/LTO full-cells at elevated operation temperatures. EXPERIMENTAL PROCEDURES 1.

Electrolytes and electrodes preparation


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MMMPyrTFSI was synthesized according to a method described in our previous work. purity of the synthesized MMMPyrTFSI was determined by means of 1H,

13

C,

19

45

The

F nuclear

magnetic resonance spectroscopy (NMR), as well as mass spectrometry (MS) measurements and Ion chromatography-mass spectrometry (IC-MS) analysis. No impurities were found.

45

Water

content in MMMPyrTFSI was ≤ 10 ppm as determined by coulometric Karl-Fisher analysis. 45 Pyr14TFSI (≥98.5%, Sigma-Aldrich) and the synthesized MMMPyrTFSI were dried under vacuum (5 × 10−8 mbar) at 120 °C for not less than 24 hours using a turbomolecular pump TPScompact (Varian Vacuum Technologies). LiTFSI (battery grade, 3M) was dried in a glass oven (drying oven 858, Büchi) at 120 °C for at least 24 hours. Investigated electrolyte formulations were prepared by dissolving 0.7 M LiTFSI in selected RTILs. As benchmark electrolyte, commercially available 1 M LiPF6 EC/DMC (1/1, by wt.) (BASF) electrolyte was used. All electrolytes were prepared and stored in a glove box under Ar atmosphere (O2 and H2O content below 0.5 ppm). LNMO electrodes were prepared with the composition of 85 wt.% of LNMO active material (Sigma-Aldrich), 8 wt.% conductive carbon Super C65 (Imerys) and 7 wt.% binder polyvinylidenfluoride (PVdF) (Kynar® FLEX 761A, Arkema Group). LTO electrodes were composed of 87 wt.% of LTO active material (SüdChemie, now Johnson Matthey), 5 wt.% of conductive carbon Super C65 (Imerys) and 8 wt.% of PVdF. The active mass loading of the LNMO and LTO electrodes was in the range of 2.5-3 mg cm-2. 2.

Electrochemical analysis

The oxidative stability window was determined by linear sweep voltammetry (LSV) performed on a VMP3 (BioLogic Science Instruments, GmbH) at a scan rate of 0.1 mV s-1. A threeelectrode Swagelok cell was used for this measurement, with platinum (Ø1 mm) as working


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electrode and lithium metal as counter (Ø12 mm) and reference (Ø5 mm) electrode (Rockwood Lithium). Glass microfiber filter (GF/D Whatman, Ø13 mm) were used as separator. Electrochemical charge/discharge measurements were carried out using CR2032 coin cells (Hohsen, Japan) and Swagelok cells, with LNMO as positive electrode and LTO as negative electrode (≈20% anode capacity excess), Li was used as the reference electrode in the three electrode Swagelok cells. Constant current charge/discharge cycling tests were performed on a multichannel Maccor series 4000 battery test system (MACCOR, INC). LNMO/LTO full-cell cycling experiments were carried out with a constant charge and discharge current of 0.2C for the first two cycles and 0.5C for subsequent cycling (1C = 147 mA g-1) in the cut-off voltage range from 1.4 to 3.4 V. AC impedance spectra were recorded in potentiostatic mode on a VMP3 (BioLogic Science Instruments, GmbH) in a frequency range of 100 KHz to 10 mHz with a 5 mV perturbation at 20 °C. The impedance was measured at open circuit voltage (OCV) at 0% state of charge (SOC). 3.

Morphology and structure analysis

Scanning electron microscopy (SEM, EVO® MA 10 microscopy, Zeiss) was used to investigate the morphology of the LNMO electrodes. The cycled LNMO electrodes (50 cycles) were disassembled from the investigated cells in the glove box, and rinsed with 2 mL DMC prior to SEM measurement. After SEM measurement, the structure of the cycled LNMO electrode was characterized by means of X-ray diffraction (XRD, Bruker) using a Bruker D8 Advance diffractometer with Ni filtered Cu Kα radiation, in the 2θ range of 15-55 degrees. TOPAS Ver. 4.1 program was used to refine the resulting diffraction pattern. The sample preparation and


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experimental setup for the inductively coupled plasma optical emission spectroscopy (ICP – OES) measurement is described in detail in our previous report. 57

AUTHOR INFORMATION Corresponding Author *Xia Cao e-mail: [email protected] *Martin Winter e-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This study was financially supported by the German Federal Ministry for Education and Research (BMBF) within the project Electrolyte Lab 4E (project reference 03X4632).

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Li4Ti5O12 Lithium Ion Cells at Elevated

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