| 1. |
- Aktekin, Burak, et al.
(författare)
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Concentrated LiFSI-€“Ethylene Carbonate Electrolytes and Their Compatibility with High-Capacity and High-Voltage Electrodes
- 2022
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Ingår i: ACS Applied Energy Materials. - : American Chemical Society (ACS). - 2574-0962. ; 5:1, s. 585-595
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Tidskriftsartikel (refereegranskat)abstract
- The unusual physical and chemical properties of electrolytes with excessive salt contents have resulted in rising interest in highly concentrated electrolytes, especially for their application in batteries. Here, we report strikingly good electrochemical performance in terms of conductivity and stability for a binary electrolyte system, consisting of lithium bis(fluorosulfonyl)imide (LiFSI) salt and ethylene carbonate (EC) solvent. The electrolyte is explored for different cell configurations spanning both high-capacity and high-voltage electrodes, which are well known for incompatibilities with conventional electrolyte systems: Li metal, Si/graphite composites, LiNi0.33Mn0.33Co0.33O2 (NMC111), and LiNi0.5Mn1.5O4 (LNMO). As compared to a LiTFSI counterpart as well as a common LP40 electrolyte, it is seen that the LiFSI:EC electrolyte system is superior in Li-metal–Si/graphite cells. Moreover, in the absence of Li metal, it is possible to use highly concentrated electrolytes (e.g., 1:2 salt:solvent molar ratio), and a considerable improvement on the electrochemical performance of NMC111-Si/graphite cells was achieved with the LiFSI:EC 1:2 electrolyte both at the room temperature and elevated temperature (55 °C). Surface characterization with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) showed the presence of thicker surface film formation with the LiFSI-based electrolyte as compared to the reference electrolyte (LP40) for both positive and negative electrodes, indicating better passivation ability of such surface films during extended cycling. Despite displaying good stability with the NMC111 positive electrode, the LiFSI-based electrolyte showed less compatibility with the high-voltage spinel LNMO electrode (4.7 V vs Li+/Li).
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| 2. |
- Aktekin, Burak, et al.
(författare)
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How Mn/Ni Ordering Controls Electrochemical Performance in High-Voltage Spinel LiNi0.44Mn1.56O4 with Fixed Oxygen Content
- 2020
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Ingår i: ACS Applied Energy Materials. - : AMER CHEMICAL SOC. - 2574-0962. ; 3:6, s. 6001-6013
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Tidskriftsartikel (refereegranskat)abstract
- The crystal structure of LiNi0.5O4 (LNMO) can adopt either low-symmetry ordered (Fd (3) over barm) or high-symmetry disordered (P4(3)32) space group depending on the synthesis conditions. A majority of published studies agree on superior electrochemical performance of disordered LNMO, but the underlying reasons for improvement remain unclear due to the fact that different thermal history of the samples affects other material properties such as oxygen content and particle morphology. In this study, ordered and disordered samples were prepared with a new strategy that renders samples with identical properties apart from their cation ordering degree. This was achieved by heat treatment of powders under pure oxygen atmosphere at high temperature with a final annealing step at 710 degrees C for both samples, followed by slow or fast cooling. Electrochemical testing showed that cation disordering improves the stability of material in charged (delithiated) state and mitigates the impedance rise in LNMO parallel to LTO (Li4Ti5O12) and LNMO parallel to Li cells. Through X-ray photoelectron spectroscopy (XPS), thicker surface films were observed on the ordered material, indicating more electrolyte side reactions. The ordered samples also showed significant changes in the Ni 2p XPS spectra, while the generation of ligand (oxygen) holes was observed in the Ni-O environment for both samples using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). Moreover, high-resolution transmission electron microscopy (HRTEM) images indicated that the ordered samples show a decrease in ordering near the particle surface after cycling and a tendency toward rock-salt-like phase transformations. These results show that the structural arrangement of Mn/Ni (alone) has an effect on the surface and "nearsurface" properties of LNMO, particularly in delithiated state, which is likely connected to the bulk electronic properties of this electrode material.
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| 3. |
- Aktekin, Burak, et al.
(författare)
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Understanding the Capacity Loss in LiNi0.5Mn1.5O4-Li4Ti5O12 Lithium-Ion Cells at Ambient and Elevated Temperatures
- 2018
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 122:21, s. 11234-11248
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Tidskriftsartikel (refereegranskat)abstract
- The high-voltage spinel LiNi0.5Mn1.5O4, (LNMO) is an attractive positive electrode because of its operating voltage around 4.7 V (vs Li/Li+) and high power capability. However, problems including electrolyte decomposition at high voltage and transition metal dissolution, especially at elevated temperatures, have limited its potential use in practical full cells. In this paper, a fundamental study for LNMO parallel to Li4Ti5O12 (LTO) full cells has been performed to understand the effect of different capacity fading mechanisms contributing to overall cell failure. Electrochemical characterization of cells in different configurations (regular full cells, back-to-back pseudo-full cells, and 3-electrode full cells) combined with an intermittent current interruption technique have been performed. Capacity fade in the full cell configuration was mainly due to progressively limited lithiation of electrodes caused by a more severe degree of parasitic reactions at the LTO electrode, while the contributions from active mass loss from LNMO or increases in internal cell resistance were minor. A comparison of cell formats constructed with and without the possibility of cross-talk indicates that the parasitic reactions on LTO occur because of the transfer of reaction products from the LNMO side. The efficiency of LTO is more sensitive to temperature, causing a dramatic increase in the fading rate at 55 degrees C. These observations show how important the electrode interactions (cross-talk) can be for the overall cell behavior. Additionally, internal resistance measurements showed that the positive electrode was mainly responsible for the increase of resistance over cycling, especially at 55 degrees C. Surface characterization showed that LNMO surface layers were relatively thin when compared with the solid electrolyte interphase (SEI) on LTO. The SEI on LTO does not contribute significantly to overall internal resistance even though these films are relatively thick. X-ray absorption near-edge spectroscopy measurements showed that the Mn and Ni observed on the anode were not in the metallic state; the presence of elemental metals in the SEI is therefore not implicated in the observed fading mechanism through a simple reduction process of migrated metal cations.
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| 4. |
- Aktekin, Burak, et al.
(författare)
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Understanding the Capacity Loss in LiNi0.5Mn1.5O4 - Li4Ti5O12 Lithium-Ion Cells at Ambient and Elevated Temperatures
- 2017
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Konferensbidrag (refereegranskat)abstract
- The high voltage spinel LiNi0.5Mn1.5O4 (LNMO) is an attractive positive electrode due to its operating voltage around 4.7 V (vs. Li/Li+) arising from the Ni2+/Ni4+ redox couple. In addition to high voltage operation, a second advantage of this material is its capability for fast lithium diffusion kinetics through 3-D transport paths in the spinel structure. However, the electrode material is prone to side reactions with conventional electrolytes, including electrolyte decomposition and transition metal dissolution, especially at elevated temperatures1. It is important to understand how undesired reactions originating from the high voltage spinel affect the aging of different cell components and overall cycle life. Half-cells are usually considered as an ideal cell configuration in order to get information only from the electrode of interest. However, this cell configuration may not be ideal to understand capacity fading for long-term cycling and the assumption of ‘stable’ lithium negative electrode may not be valid, especially at high current rates2. Also, among the variety of capacity fading mechanisms, the loss of “cyclable” lithium from the positive electrode (or gain of lithium from electrolyte into the negative electrode) due to side reactions in a full-cell can cause significant capacity loss. This capacity loss is not observable in a typical half-cell as a result of an excessive reserve of lithium in the negative electrode.In a full-cell, it is desired that the negative electrode does not contribute to side reactions in a significant way if the interest is more on the positive side. Among candidates on the negative side, Li4Ti5O12 (LTO) is known for its stability since its voltage plateau (around 1.5 V vs. Li/Li+) is in the electrochemical stability window of standard electrolytes and it shows a very small volume change during lithiation. These characteristics make the LNMO-LTO system attractive for a variety of applications (e.g. electric vehicles) but also make it a good model system for studying aging in high voltage spinel-based full cells.In this study, we aim to understand the fundamental mechanisms resulting in capacity fading for LNMO-LTO full cells both at room temperature and elevated temperature (55°C). It is known that electrode interactions occur in this system due to migration of reaction products from LNMO to the LTO side3, 4. For this purpose, three electrode cells have been cycled galvanostatically with short-duration intermittent current interruptions5 in order to observe internal resistance for both LNMO and LTO electrodes in a full cell, separately. Change of voltage curves over cycling has also been observed to get an insight into capacity loss. For comparison purposes, back-to-back cells (a combination of LNMO and LTO cells connected electrically by lithium sides) were also tested similarly. Post-cycling of harvested electrodes in half cells was conducted to determine the degree of capacity loss due to charge slippage compared to other aging factors. Surface characterization of LNMO as well as LTO electrodes after cycling at room temperature and elevated temperature has been done via SEM, XPS, HAXPES and XANES.ReferencesA. Kraytsberg, Y. Ein-Eli, Adv. Energy Mater., vol. 2, pp. 922–939, 2012.Aurbach, D., Zinigrad, E., Cohen, Y., & Teller, H. Solid State Ionics, 148(3), 405-416, 2002.Li et al., Journal of The Electrochemical Society, 160 (9) A1524-A1528, 2013.Aktekin et al., Journal of The Electrochemical Society 164.4: A942-A948. 2017.Lacey, M. J., ChemElectroChem. Accepted Author Manuscript. doi:10.1002/celc.201700129, 2017.
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| 5. |
- Björklund, Erik, et al.
(författare)
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How the Negative Electrode Influences Interfacial and Electrochemical Properties of LiNi1/3Co1/3Mn1/3O2 Cathodes in Li-Ion Batteries
- 2017
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Ingår i: Journal of the Electrochemical Society. - : The Electrochemical Society. - 0013-4651 .- 1945-7111. ; 164:13, s. A3054-A3059
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Tidskriftsartikel (refereegranskat)abstract
- The cycle life of LiNi1/3Co1/3Mn1/3O2 (NMC) based cells are significantly influenced by the choice of the negative electrode. Electrochemical testing and post mortem surface analysis are here used to investigate NMC electrodes cycled vs. either Li-metal, graphite or Li4Ti5O12 (LTO) as negative electrodes. While NMC-LTO and NMC-graphite cells show small capacity fading over 200 cycles, NMC-Li-metal cell suffers from rapid capacity fading accompanied with an increased voltage hysteresis despite the almost unlimited access of lithium. X-ray absorption near edge structure (XANES) results show that no structural degradation occurs on the positive electrode even after >200 cycles, however, X-ray photoelectron spectroscopy (XPS) results shows that the composition of the surface layer formed on the NMC cathode in the NMC-Li-metal cell is largely different from that of the other NMC cathodes (cycled in the NMC-graphite or NMC-LTO cells). Furthermore, it is shown that the surface layer thickness on NMC increases with the number of cycles, caused by continuous electrolyte degradation products formed at the Li-metal negative electrode and then transferred to NMC positive electrode.
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| 6. |
- Björklund, Erik, et al.
(författare)
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Influence of state-of-charge in commercial LiNi0.33Mn0.33Co0.33O2/LiMn2O4-graphite cells analyzed by synchrotron-based photoelectron spectroscopy
- 2018
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Ingår i: Journal of Energy Storage. - : Elsevier BV. - 2352-152X .- 2352-1538. ; 15, s. 172-180
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Tidskriftsartikel (refereegranskat)abstract
- Degradation mechanisms in 26 Ah commercial Li-ion battery cells comprising graphite as the negative electrode and mixed metal oxide of LiMn 2 O 4 (LMO) and LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC) as the positive electrode are here investigated utilising extensive cycling at two different state-of-charge (SOC) ranges, 10–20% and 60–70%, as well as post-mortem analysis. To better analyze these mechanisms electrochemically, the cells were after long-term cycling reassembled into laboratory scale “half-cells” using lithium metal as the negative electrode, and thereafter cycled at different rates corresponding to 0.025 mA/cm 2 and 0.754 mA/cm 2 . The electrodes were also analyzed by synchrotron-based hard x-ray photoelectron spectroscopy (HAXPES) using two different excitation energies to determine the chemical composition of the interfacial layers formed at different depth on the respective electrodes. It was found from the extensive cycling that the cycle life was shorter for the cell cycled in the higher SOC range, 60–70%, which is correlated to findings of an increased cell resistance and thickness of the SEI layer in the graphite electrode as well as manganese dissolution from the positive electrode.
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| 7. |
- Björklund, Erik, et al.
(författare)
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Investigation of dimethyl carbonate and propylene carbonate mixtures for LiNi0.6Mn0.2Co0.2O2-Li4Ti5O12 cells
- 2019
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Ingår i: ChemElectroChem. - : Wiley. - 2196-0216. ; 6:13, s. 3429-3436
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Tidskriftsartikel (refereegranskat)abstract
- It has recently been shown that ethylene carbonate (EC) experience poor stability at high potentials in lithium-ion batteries, and development of electrolytes without EC, not least using ethyl methyl carbonate (EMC), has therefore been suggested in order to improve the capacity retention. In this context, we here explore another alternative electrolyte system consisting of propylene carbonate (PC) and dimethyl carbonate (DMC) mixtures in NMC-LTO (LiNi0.6Mn0.2Co0.2O2, Li4Ti5O12) cells cycled up to 2.95 V. While PC experience wettability problems and DMC has difficulties dissolving LiPF6 salt, blends between these could possess complementary properties. The electrolyte blend showed superior cycling performance at sub-zero temperatures compared to EC-containing counterparts. At 30 degrees C, however, the PC-DMC electrolyte did not show any major improvement in electrochemical properties for the NMC-LTO cell chemistry. Photoelectron spectroscopy measurements showed that thin surface layers were detected on both NMC (622) and LTO electrodes in all investigated electrolytes. The results suggest that both PC and EC will react on the electrodes, but with EC forming thinner layers comprising more carbonates. Moreover, the electrochemical stability at high electrochemical potentials is similar for the studied electrolytes, which is surprising considering that most are free from the reactive EC component.
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| 8. |
- Björklund, Erik, et al.
(författare)
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Sulfolane-Based Ethylene Carbonate-Free Electrolytes for LiNi0.6Mn0.2Co0.2O2-Li4Ti5O12 Batteries
- 2020
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Ingår i: Batteries & Supercaps. - : Wiley. - 2566-6223. ; 3:2, s. 201-207
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Tidskriftsartikel (refereegranskat)abstract
- Most electrolytes in today's lithium-ion batteries contain a large proportion of ethylene carbonate (EC) mixed with other alkyl carbonate-based solvents. EC has, however, been shown to be unstable at the high potentials at which several novel cathode materials are electrochemically active. Here, different mixtures of sulfolane and DMC are investigated in this context. The electrochemical stability is explored in addition to galvanostatic cycling of LiNi0.6Mn0.2Co0.2O2-Li4Ti5O12 (NMC-LTO) cells. The measurement of the ionic conductivity showed that mixing 25 % sulfolane into DMC improved the electrolyte properties as compared to pure DMC, making the conductivity similar to EC:DEC electrolytes and therefore fully functional. Moreover, the addition of sulfolane slightly enhanced the capacity retention, likely caused by formation of thinner and more stable surface layers on the LTO electrodes as determined by X-ray photoelectron spectroscopy (XPS). The cycling performance is especially improved for sulfolane-based electrolytes during cycling at sub-zero temperatures.
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| 9. |
- Björklund, Erik, et al.
(författare)
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Temperature dependence of electrochemical degradation in LiNi1/3Mn1/3Co1/3O2/Li4Ti5O12 cells
- 2019
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Ingår i: Energy Technology. - : Wiley. - 2194-4288 .- 2194-4296. ; 7:9
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Tidskriftsartikel (refereegranskat)abstract
- Aging mechanisms in lithium‐ion batteries are dependent on the operational temperature, but the detailed mechanisms on what processes take place at what temperatures are still lacking. The electrochemical performance and capacity fading of the common cell chemistry LiNi1/3Mn1/3Co1/3O2 (NMC)/Li4Ti5O12 (LTO) pouch cells are studied at temperatures 10, 30, and 55 °C. The full cells are cycled with a moderate upper cutoff potential of 4.3 V versus Li+/Li. The electrode interfaces are characterized postmortem using photoelectron spectroscopy techniques (soft X‐ray photoelectron spectroscopy [SOXPES], hard X‐ray photoelectron spectroscopy [HAXPES], and X‐ray absorption near edge structure [XANES]). Stable cycling at 30 °C is explained by electrolyte reduction forming a stabilizing interphase, thereby preventing further degradation. This initial reaction, between LTO and the electrolyte, seems to be beneficial for the NMC–LTO full cell. At 55 °C, continuous electrolyte reduction and capacity fading are observed. It leads to the formation of a thicker surface layer of organic species on the LTO surface than at 30 °C, contributing to an increased voltage hysteresis. At 10 °C, large cell‐resistances are observed, caused by poor electrolyte conductivity in combination with a relatively thicker and LixPFy‐rich surface layer on LTO, which limit the capacity.
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| 10. |
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