| 1. |
- Björklund, Erik
(författare)
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Avoiding ageing : Surface degradation of commercial electrode materials in lithium-ion batteries
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The battery market today expands rapidly, not least for electric vehicles. But to compete against the combustion engine, the cost of batteries must be reduced. After years of usage, the batteries degrade and need to be exchanged, increasing the cost over the vehicle lifecycle. This can be mitigated by tailoring the usage conditions and the battery materials. Understanding and avoiding ageing can be key to a more sustainable transport system. This thesis contains studies on degradation processes in Li-ion batteries utilizing the LiNixMnyCozO2 (NMC) cathode material, and suggests strategies for the improvement of battery life time.When cycling different negative electrodes – including graphite, lithium foil and lithium titanium oxide (LTO) – against NMC electrodes, only minor capacity fading was observed in the NMC-LTO and NMC-graphite cells, in contrast to the NMC-Li-metal cells. The capacity fading for Li-metal cells was determined to be caused by degradation products formed at the lithium foil which thereafter diffused to the NMC electrode, leading to a higher resistance. Commercial NMC/LiMn2O4-graphite cells were also investigated after cycling in limited state of charge (SOC)-intervals. The cycle life was far longer in the low-SOC cell than in the high-SOC cell. Photoelectron spectroscopy revealed increased manganese dissolution in the high-SOC cell, likely causing a less stable solid electrolyte interphase layer on the negative electrode. This, in turn, limits the capacity. How temperature influence ageing in NMC-LTO was analysed in cells cycled at -10 °C, 30 °C and 55 °C. It was found that the initial side reactions at the LTO electrode limited the cell capacity, but that these also stabilized the NMC electrode. At 55 °C, excessive side reactions at LTO caused capacity fading due to loss of active lithium. At -10 °C, high cell resistance limited the capacity. Switching to a PC based electrolyte allowed stable low temperature cycling, although it was found that PC degraded and formed thick electrode surface layers. Also sulfolane-based electrolytes were investigated, showing thinner surface layers than the EC containing reference electrolyte at high potentials, thus indicating a more stable electrolyte system.
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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- 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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| 7. |
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| 8. |
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| 9. |
- Mogensen, Ronnie, et al.
(författare)
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Sodium bis(oxalato)borate (NaBOB) in trimethyl phosphate : a fire-extinguishing, fluorine-free, and low-cost electrolyte for fullcell sodium-ion batteries
- 2020
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Ingår i: ACS Applied Energy Materials. - : American Chemical Society (ACS). - 2574-0962. ; 3:5, s. 4974-4982
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Tidskriftsartikel (refereegranskat)abstract
- Sodium-ion batteries based on all-naturally abundant elements, in which no cobalt, nickel, copper, and fluorine is used, can lead to a major breakthrough in making batteries more sustainable. Safety aspects-in particular, flammability of electrolytes-in the state-of-the-art battery technology is another important concern, especially for applications in which large numbers of cells are employed. Nonflammable battery electrolytes studied so far are based on highly fluorinated compounds or high salt concentrations, which suffer from high cost and toxicity. We here propose an electrolyte based on a single solvent and low-cost and fluorine-free salt at a lower range of "standard" concentrations. Our results show-for the first time-that sodium bis(oxalato)borate (NaBOB) is soluble in the nonflammable solvent trimethyl phosphate (TMP). This finding enables a nonflammable electrolyte with high ionic conductivity and promising electrochemical performance in full-cell sodium-ion batteries. An electrolyte of 0.5 M NaBOB in TMP provides an ionic conductivity of 5 mS cm(-1) at room temperature, which is comparable to the commonly used electrolytes based on sodium hexafluorophosphate (NaPF6) and organic carbonate solvents. The proposed electrolyte shows a Coulombic efficiency of above 80% in the first cycle, which increased to about 97% from the second cycle in sodium-ion battery full-cells consisting of a hard carbon anode and a Prussian white cathode. This work opens up opportunities to design safe electrolytes which can further be optimized with electrolyte additives such as vinylene carbonate for industrial applications.
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| 10. |
- Naylor, Andrew J., et al.
(författare)
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Depth-dependent oxygen redox activity in lithium-rich layered oxide cathodes
- 2019
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry. - 2050-7488. ; 7:44, s. 25355-25368
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Tidskriftsartikel (refereegranskat)abstract
- Lithium-rich materials, such as Li1.2Ni0.2Mn0.6O2, exhibit capacities not limited by transition metal redox, through the reversible oxidation of oxide anions. Here we offer detailed insight into the degree of oxygen redox as a function of depth within the material as it is charged and cycled. Energy-tuned photoelectron spectroscopy is used as a powerful, yet highly sensitive technique to probe electronic states of oxygen and transition metals from the top few nanometers at the near-surface through to the bulk of the particles. Two discrete oxygen species are identified, On− and O2−, where n < 2, confirming our previous model that oxidation generates localised hole states on O upon charging. This is in contrast to the oxygen redox inactive high voltage spinel LiNi0.5Mn1.5O4, for which no On− species is detected. The depth profile results demonstrate a concentration gradient exists for On− from the surface through to the bulk, indicating a preferential surface oxidation of the layered oxide particles. This is highly consistent with the already well-established core–shell model for such materials. Ab initio calculations reaffirm the electronic structure differences observed experimentally between the surface and bulk, while modelling of delithiated structures shows good agreement between experimental and calculated binding energies for On−.
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