The standard format for cylindrical Li-ion cells is about to change from 18650-type cells (18mm diameter, 65mm height) to 21700type cells (21mm diameter, 70mm height). We investigated the properties of five 18650 cells, three of the first commercially available 21700, and three types of the similar 20700 cells in detail. In particular, the (i) specific energy/energy density and electrode thickness, (ii) electrode area and cell resistance, (iii) specific energy as a function of discharge C-rate, as well as (iv) heating behavior due to current flow are analyzed. Finally, the production effort for cells and packs are roughly estimated for 21700 cells compared to 18650 cells.

Li-ion cells are used in a variety of mobile and stationary applications. Their use must be safe under all conditions, even for aged cells in second-life applications. In the present study, different aging mechanisms are taken into account for accelerating rate calorimetry (ARC) tests. 18650-type cells are cycled at 0 • C (Li plating expected) and at 45 • C (SEI growth expected). After extensive evaluation of the electrochemical results (voltage curve analysis, capacity fade, energy fade, Coulombic efficiency), the cells are tested by PostMortem analysis (CT, GD-OES, SEM) to reveal the main aging mechanisms and by ARC to test the safety behavior. Besides typical ARC results such as onset-of-self-heating, onset-of-thermal runaway and maximum temperatures, as well as acoustic responses of thermal runaway are evaluated and a method is developed to compare fresh cells and cells aged until different SOHs. It turns out that the safety of aged cells is not simply a function of the SOH. However, safety is strongly affected by the main aging mechanism and to the history of operating parameters during the life-time of the cell. Driven by the demand for higher capacity, lower volume, and lower weight, Li-ion technology was developed in the 1980s.1,2 Nowadays, Li-ion technology is used for energy storage in a large variety of applications, e.g. smartphones, tablets, or drones. In particular, electric vehicles in combination with renewable energy sources are promising for reducing climate change and the corresponding glacier melting and sea level rise which is influenced by burning fossil fuels.3-6 For all applications, the safety behavior under all specified operating conditions is most important.7-9 The same is true for aged Li-ion cells used in second-life applications, for example cells which are utilized in stationary energy storage applications after their usage in electric cars.In the past, failure of Li-ion cells led to product recalls which are very expensive for the manufacturers and unsettle customers, even if such incidents happen only in very few cases (ppm range).10 At the moment, safety tests have to be performed with Li-ion prototypes cells before market release. In these tests, fresh cells are always used, however, aged cells are usually neglected. Even if fresh cells show an acceptable safety behavior, this can change for aged cells. 7,[11][12][13][14][15] While some authors observed decreases for certain safety properties, 7,11,12 others found improvements. 12,13,15 It is well known that aging mechanisms change the properties of the materials inside Li-ion cells. [16][17][18][19] Operating conditions and materials have a strong influence on these aging mechanisms. 12,[20][21][22][23] We recently reported on a change of the aging mechanism with temperature for cycling aging of commercial 18650-type cells with graphite anodes and NMC/LMO cathodes. 20 This mechanism change is expressed by a V-shape in Arrhenius plots of the capacity fade rates obtained from cycling at different ambient temperatures. 20 This c...

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Due to their extreme volume expansion, Si/C-composites suffer from fracture or delamination and consequent capacity fading during the Li-ion cell operation. One approach to reduce the electrical contact loss and improve the performance is the application of mechanical pressure on the cell. Therefore, a comprehensive aging study of Si/C|NMC811 pouch cells is conducted with cells in different compression configurations as uncompressed and under flexible and fixed compression at pressure levels in the range of 0.08 MPa, 0.42 MPa, and 0.84 MPa. In-situ swelling measurements by dilation as well as in-operando mapping of the pressure distribution on the cell surfaces reveal the positive influence of the low pressure fixed and the middle pressure flexible compression on the cycle life. For the heavily fixed compressed cells, an inhomogeneous pressure distribution and occurring pressure hot -spots close to the cell current collectors of up to 5.2 MPa are found. An extensive post-mortem analysis including SEM cross-sectioning and EIS measurements of the aged anodes and the separator confirms cell failure by different aging mechanisms depending on the type of compression. Aging experiments of Si/C|NMC811 cylindrical 18650-cells show local differences along the jelly roll which are explained by the help of the pouch cell results.

The addition of Si compounds to graphite anodes has become an attractive way of increasing the practical specific energies in Li‐ion cells. Previous studies involving Si/C anodes lacked direct insight into the processes occurring in full cells during low‐temperature operation. In this study, a powerful combination of operando neutron diffraction, electrochemical tests, and post‐mortem analysis is used for the investigation of Li‐ion cells. 18650‐type cylindrical cells in two different aging states are investigated by operando neutron diffraction. The experiments reveal deep insights and important trends in low‐temperature charging mechanisms involving intercalation, alloying, Li metal deposition, and relaxation processes as a function of charging C‐rates and temperatures. Additionally, the main aging mechanism caused by long‐term cycling and interesting synergistic effects of Si and graphite are elucidated.