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Commercial battery cells with Li4Ti5O12 negative electrode and composite metal oxide positive electrode have been analyzed with respect to aging mechanisms. Electrochemical impedance spectroscopy (EIS), differential capacity analysis (dQ/dV), differential voltage analysis (dV/dQ) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX) were used to identify different ageing mechanisms such as lithium inventory loss, loss of active electrode material and surface film growth. The active material of the positive electrode was also examined by X-ray diffraction (XRD). Aging mechanisms were studied for both calendar-aged and cycle-aged cells. Data from half cells prepared from post mortem harvested electrode material, using lithium foil as negative electrode and pouch material as encapsulation, were used as reference to full cell data. Electrochemical analysis of full and half cells combined with material analysis showed to be a powerful method to identify aging mechanisms in this type of commercial cells. The calendar-aged cell showed insignificant aging while the cycle-aged cell showed noticeable loss of positive electrode active material and loss of cyclable lithium, but only minor loss of negative electrode active material. The results imply that Li4Ti5O12 negative electrode material is a good alternative to other materials if high energy density is not the primary goal.

Aging of power-optimized commercial 2.3 Ah cylindrical LiFePO4//graphite cells to be used in hybrid electric vehicle is investigated and compared for three different aging procedures; (i) using a simulated hybrid electric vehicle cycle within a narrow SOC-range, (ii) using a constant-current cycle over a 100% SOC-range, and (iii) stored during three years at 22 degrees C. Postmortem analysis of the cells is performed after full-cell electrochemical characterization and discharge. EIS and capacity measurements are made on different parts of the disassembled cells. Material characterization includes SEM, EDX, HAXPES/XPS and XRD. The most remarkable result is that both cycled cells displayed highly uneven aging primarily of the graphite electrodes, showing large differences between the central parts of the jellyroll compared to the outer parts. The aging variations are identified as differences in capacity and impedance of the graphite electrode, associated with different SEI characteristics. Loss of cyclable lithium is mirrored by a varying degree of lithiation in the positive electrode and electrode slippage. The spatial variation in negative electrode degradation and utilization observed is most likely connected to gradients in temperature and pressure, that can give rise to current density and potential distributions within the jellyroll during cycling.

The complex structural transformations of Li 0.5Ni 0.25TiOPO 4 during electrochemical lithiation have been examined by in situ X-ray diffraction. During the first lithiation two structural changes take place: first a transition to a second monoclinic phase (a = 9.085(4), b = 8.414(5), c = 6.886(5), β = 99.85(4)) and secondly a transition to a third phase with limited long-range order. The third phase is held together by a network of corner sharing Ti-O octahedra and phosphate ions with disordered Ni-Li channels. During delithiation the third phase is partially transformed back to a slightly disordered original phase, Li 0.5Ni 0.25TiOPO 4 without formation of the second intermediate phase. These phase transitions correspond well to the different voltage plateaus that this material shows during electrochemical cycling.

The tavorite-type polymorph of LiFeSO₄F has recently attracted substantial attention as a positive elec- trode material for lithium ion batteries. The synthesis of this material is generally considered to rely on a topotactic exchange of water (H₂O) for lithium (Li) and fluorine (F) within the structurally similar hy- drated iron sulfate precursor (FeSO₄·H₂O) when reacted with lithium fluoride (LiF). However, there have also been discussions in the literature regarding the possibility of a non-topotactic reaction mechanism between lithium sulfate (Li₂SO₄) and iron fluoride (FeF₂) in tetraethylene glycol (TEG) as reaction medium. In this work, we use in situ X-ray diffraction to continuously follow the formation of LiFeSO₄F from the two suggested precursor mixtures in a setup aimed to mimic the conditions of a solvothermal autoclave synthesis. It is demonstrated that LiFeSO₄F is formed directly from FeSO₄·H₂O and LiF, in agreement with the proposed topotactic mechanism. The Li₂SO₄ and FeF₂ precursors, on the other hand, are shown to rapidly transform into FeSO₄·H₂O and LiF with the water originating from the highly hygroscopic TEG before a subsequent formation of LiFeSO₄F is initiated. The results highlight the importance of the FeSO₄·H₂O precursor in obtaining the tavorite-type LiFeSO₄F, as it is observed in both reaction routes.