| 1. |
- 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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| 2. |
- 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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| 3. |
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| 4. |
- Aktekin, Burak, et al.
(författare)
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Understanding the Rapid Capacity Fading of LNMO-LTO Lithium-ion Cells at Elevated Temperature
- 2017
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The high voltage spinel LiNi0.5Mn1.5O4 (LNMO) has an average operating potential around 4.7 V vs. Li/Li+ and a gravimetric charge capacity of 146 mAh/g making it a promising high energy density positive electrode for Li-ion batteries. Additionally, the 3-D lithium transport paths available in the spinel structure enables fast diffusion kinetics, making it suitable for power applications [1]. However, the material displays large instability during cycling, especially at elevated temperatures. Therefore, significant research efforts have been undertaken to better understand and improve this electrode material.Electrolyte (LiPF6 in organic solvents) oxidation and transition metal dissolution are often considered as the main problems [2] for the systems based on this cathode material. These can cause a variety of problems (in different parts of the cell) eventually increasing internal cell resistance, causing active mass loss and decreasing the amount of cyclable lithium.Among these issues, cyclable lithium loss cannot be observed in half cells since lithium metal will provide almost unlimited capacity. Being a promising full cell chemistry for high power applications, there has also been a considerable interest on LNMO full cells with Li4Ti5O12 (LTO) used as the negative electrode. For this chemistry, for an optimized cell, quite stable cycling for >1000 cycles has been reported at room temperature while fast fading is still present at 55 °C [3]. This difference in performance (RT vs. 55 °C) is beyond most expectations and likely does not follow any Arrhenius-type of trend.In this study, a comprehensive analysis of LNMO-LTO cells has been performed at different temperatures (RT, 40 °C and 55 °C) to understand the underlying reasons behind stable cycling at room temperature and rapid fading at 55 °C. For this purpose, testing was made on regular cells (Figure 1a), 3-electrode cells (Figure 1b) and back-to-back cells [4] (Figure 1c). Electrode interactions (cross-talk) have been shown to exist in the LTO-LNMO system [5] and back-to-back cells have therefore been used to observe fading under conditions where cross-talk is impossible [4]. Galvanostatic cycling combined with short-duration intermittent current interruptions [6] was performed in order to separately observe changes in internal resistance for LNMO and LTO electrodes in a full cell. Ex-situ characterization of electrodes have also been performed using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES).Our findings show how important the electrode interactions can be in full cells, as a decrease in lithium inventory was shown to be the major factor for the observed capacity fading at elevated temperature. In this presentation, the effect of other factors – active mass loss and internal cell resistance – will be discussed together with the consequences of cross-talk.References[1] A. Kraytsberg et al. Adv. Energy Mater., vol. 2, pp. 922–939,2012.[2] J. H. Kim et al., ChemPhysChem, vol. 15, pp. 1940–1954, 2014.[3] H. M. Wu et al. J. E. Soc., vol. 156, pp. A1047–A1050, 2009.[4] S. R. Li et al., J. E. Soc., vol. 160, no. 9, pp. A1524–A1528, 2013.[5] Dedryvère et al. J. Phys. C., vol. 114 (24), pp. 10999–11008, 2010.[6] M. J. Lacey, ChemElectroChem, pp. 1–9, 2017.
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| 5. |
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| 6. |
- Nordh, Tim, 1988-
(författare)
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A Quest for the Unseen : Surface Layer Formation on Li4Ti5O12 Li-Ion Battery Anodes
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The electric vehicle itself today outlives its battery, necessitating battery replacement. Lithium titanium oxide (LTO) has, in this context, been suggested as a new anode material in heavy electric vehicle applications due to intrinsic properties regarding safety, lifetime and availability.The work presented here is focused on the LTO electrode/electrolyte interface. Photoelectron spectroscopy (PES) has been applied to determine how and if the usage of LTO could prevent extensive anode-side electrolyte decomposition and build-up of a surface layer. The presence of a solid electrolyte interphase (SEI) comprising LiF, carbonates and ether compounds was found in half-cells utilizing a standard ethylene:diethylcarbonate electrolyte with 1 M LiPF6. Via testing of symmetrical LTO-LTO cells, the stability of the formed SEI was put in to question. Moreover, the traditional polyvinylidene difluoride (PVdF) binder was replaced by more environmentally benign carboxylmethyl cellulose (CMC) and polyacrilic acid (PAA) binders in LTO electrodes, and it was found that CMC helped to form a more stable surface-layer that proved beneficial for long term cycling.Following the half-cell studies, full-cells were investigated to observe how different cathodes influence the SEI of LTO. The SEI in full-cells displayed characteristics similar to the half-cells, however, when utilizing a high voltage LiNi0.5Mn1.5O4 cathode, more electrolyte decomposition could be observed. Increasing the operational temperature of this battery cell generated even more degradation products on the LTO electrodes. Mn was also found on the anode when using Mn-based cathodes, however, it was found in its ionic state and did not significantly affect the composition or behavior of the observed SEI layer. Furthermore, by exchanging the electrolyte solvent for propylene carbonate, the thickness of the SEI increased, and by replacing the LiPF6 salt for LiBF4 the stability of the SEI improved. Thus is it demonstrated that such a passivation can be beneficial for the long-term surface stability of the electrode. These findings can therefore help prolong the lifetime of LTO-based battery chemistries.
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| 7. |
- Nordh, Tim, 1988-, et al.
(författare)
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Depth profiling the solid electrolyte interphase on lithium titanate (Li4Ti5O12) using synchrotron-based photoelectron spectroscopy
- 2015
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Ingår i: Journal of Power Sources. - : Elsevier BV. - 0378-7753 .- 1873-2755. ; 294, s. 173-179
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Tidskriftsartikel (refereegranskat)abstract
- The presence of a surface layer on lithium titanate (Li4Ti5O12, LTO) anodes, which has been a topic of debate in scientific literature, is here investigated with tunable high surface sensitive synchrotron-basedphotoelectron spectroscopy (PES) to obtain a reliable depth profile of the interphase. LijjLTO cells with electrolytes consisting of 1 M lithium hexafluorophosphate dissolved in ethylene carbonate:diethyl carbonate (LiPF6 in EC:DEC) were cycled in two different voltage windows of 1.0e2.0 V and 1.4e2.0 V. LTO electrodes were characterized after 5 and 100 cycles. Also the pristine electrode as such, and an electrode soaked in the electrolyte were analyzed by varying the photon energies enabling depth profiling of the outermost surface layer. The main components of the surface layer were found to be ethers, PeO containing compounds, and lithium fluoride.
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| 8. |
- Nordh, Tim, 1988-, et al.
(författare)
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Different Shades of Li4Ti5O12 Composites : The Impact of the Binder on Interface Layer Formation
- 2017
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Ingår i: ChemElectroChem. - : Wiley. - 2196-0216. ; 4:10, s. 2683-2692
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Tidskriftsartikel (refereegranskat)abstract
- Replacing the traditional PVdF(-HFP) electrode binder by water-soluble alternatives can potentially render electrode fabrication more environmentally benign. Herein, the surface layer formation of stored and cycled samples of two water-based Li4Ti5O12 composites employing either poly(sodium acrylate) (PAA-Na) or sodium carboxymethyl cellulose (CMC-Na) as binders are studied by X-ray photoelectron spectroscopy. In all three formulations, the surface layer composition formed upon storage differed notably from the solid-electrolyte interphase (SEI) layer formed on cycled samples. The surface layer under open-circuit conditions seems to originate mostly from the electrolyte salt (LiPF6) degradation. The comparison with cycled samples after 10 and 100 cycles shows a continuous build-up of an SEI layer on PAA-Na and PVdF-HFP electrodes. In contrast, on CMC-Na containing electrodes the SEI composition remains nearly unchanged. The results correlate well with the electrochemical behavior.
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| 9. |
- Nordh, Tim, 1988-
(författare)
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Lithium titanate as anode material in lithium-ion batteries : -A surface study
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The ever increasing awareness of the environment and sustainability drives research to find new solutions in every part of society. In the transport sector, this has led to a goal of replacing the internal combustion engine (ICE) with an electrical engine that can be powered by renewable electricity. As a battery for vehicles, the Li-ion chemistries have become dominant due to their superior volumetric and gravimetric energy densities. While promising, electric vehicles require further improvements in terms of capacity and power output before they can truly replace their ICE counterparts. Another aspect is the CO2 emissions over lifetime, since the electric vehicle itself presently outlives its battery, making battery replacement necessary. If the lifetime of the battery could be increased, the life-cycle emissions would be significantly lowered, making the electric vehicle an even more suitable candidate for a sustainable society. In this context, lithium titanium oxide (LTO) has been suggested as a new anode material in heavy electric vehicles applications due to intrinsic properties regarding safety, lifetime and availability. The LTO battery chemistry is, however, not fully understood and fundamental research is necessary for future improvements. The scope of this project is to investigate degradation mechanisms in LTO-based batteries to be able to mitigate these and prolong the device lifetime so that, in the end, a suitable chemistry for large scale applications can be suggested. The work presented in this licentiate thesis is focused on the LTO electrode/electrolyte interface. Photoelectron spectroscopy (PES) was applied to determine whether the usage of LTO would prevent anode-side electrolyte decomposition, as suggested from the intercalation potential being inside the electrochemical stability window of common electrolytes. It has been found that electrolyte decomposition indeed occurs, with mostly hydrocarbons of ethers, carboxylates, and some inorganic lithium fluoride as decomposition products, and that this decomposition to some extent ensued irrespective of electrochemical battery operation activity. Second, an investigation into how crossover of manganese ions from Mn-based cathodes influences this interfacial layer has been conducted. It was found, using a combination of high-energy x-ray photoelectron spectroscopy (HAXPES) and near-edge x-ray absorption fine structure (NEXAFS) that although manganese is present on the LTO anode surface when paired with a common manganese oxide spinel cathode, the manganese does little to alter the surface chemistry of the LTO electrode.
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| 10. |
- Nordh, Tim, 1988-, et al.
(författare)
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Manganese dissolution and electrochemistry of alternative electrolytes
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 1M lithium tetrafluoroborate (LiBF4) in propylene carbonate (PC) is here presented as an electrolyte alternative to 1M lithium hexafluorophosphate (LiPF6) in ethylene:diethylcarbonate (EC:DEC) for use in lithium titanate (Li4Ti5O12, LTO) based cell chemistry. Manganese dissolution was found to depend on the presence of LiPF6 through dissolution studies with inductive coupled plasma – atomic emission spectroscopy (ICP-AES). 1M LiBF4 in PC was used as an electrolyte for LTO|litium manganese oxide (LiMn2O4) cells and initial cell cycling looks promising and preliminary measurements with x-ray photoelectron spectroscopy show a more homogeneous and stable solid electrolyte interphase (SEI) than observed for the traditional EC:DEC 1M LiPF6 electrolyte.
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