In this study, the calendar aging of lithium-ion batteries is investigated at different temperatures for 16 states of charge (SoCs) from 0 to 100%. Three types of 18650 lithium-ion cells, containing different cathode materials, have been examined. Our study demonstrates that calendar aging does not increase steadily with the SoC. Instead, plateau regions, covering SoC intervals of more than 20%-30% of the cell capacity, are observed wherein the capacity fade is similar. Differential voltage analyses confirm that the capacity fade is mainly caused by a shift in the electrode balancing. Furthermore, our study reveals the high impact of the graphite electrode on calendar aging. Lower anode potentials, which aggravate electrolyte reduction and thus promote solid electrolyte interphase growth, have been identified as the main driver of capacity fade during storage. In the high SoC regime where the graphite anode is lithiated more than 50%, the low anode potential accelerates the loss of cyclable lithium, which in turn distorts the electrode balancing. Aging mechanisms induced by high cell potential, such as electrolyte oxidation or transition-metal dissolution, seem to play only a minor role. To maximize battery life, high storage SoCs corresponding to low anode potential should be avoided.

Calendar aging of lithium-ion cells is investigated by storing commercial 18650 cells with NCA cathode and graphite anode at different states of charge and temperatures. The resulting capacity fades are analyzed by differential voltage analysis (DVA) and coulometry. DVA reveals that the capacity fade results mainly from a shift in the electrode balancing due to a reduced inventory of cyclable lithium. Moreover, DVA confirms that the capacity fade strongly correlates with the anode potential. The observed loss of cyclable lithium is further analyzed by coulomb tracking, which stands for creating a continuous ampere-hour balance from all individual measurements performed with an examined cell and tracking the slippage of charging and discharging endpoints. It reveals the extent of anodic and cathodic side reactions during the storage periods and their effect on the inventory of cyclable lithium. Anodic side reactions, which are related to electrolyte reduction and passivation layer growth, can be identified as the main driver of capacity fade. Coulomb tracking also discloses that increasing cathodic side reactions can reduce the irreversible capacity fade, particularly for storage at very high SoC, which is likely to be misinterpreted as decelerated aging reactions. Evaluating also the reversible capacity fade prevents such a misconception. Calendar aging comprises all aging processes that lead to a degradation of a battery cell independent of charge-discharge cycling. It is an important factor in many applications of lithium-ion batteries where the operation periods are substantially shorter than the idle intervals, such as in electric vehicles. 1Parasitic side reactions at the electrode-electrolyte interfaces are considered to be the predominant degradation processes of calendar aging.2,3 They cause electrolyte reduction at the negative electrode and electrolyte oxidation at the positive electrode. 4,5 The electrolyte reduction at the anode is generally associated with a growth of the solid electrolyte interphase (SEI), the passivation layer which separates the anode active material from the electrolyte. 6 In addition to electrolyte decomposition, transition metals are dissolved from the cathode at higher voltage and get deposited at the anode, which in turn increases anodic side reactions. 7,8 There are many studies on calendar aging of lithium-ion batteries which present the capacity fade of the cells over time but do not provide explicit investigations on anodic or cathodic side reactions causing the capacity fade.9-17 Furthermore, calendar aging is mostly examined only for a few SoCs: Refs. 11-17 examine three SoCs or fewer. By contrast, this paper presents investigations on calendar aging with a large number of SoCs examined to obtain a comprehensive understanding of the dependency of the capacity fade on SoC. Moreover, side reactions are analyzed.In this paper, two experimental studies on calendar aging of nickel cobalt aluminum oxide (NCA) lithium-ion batteries are presented and evaluated. Differential vo...

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In this paper, we present an aging study of commercial 18650-type C/LiNi 0.33 Mn 0.33 Co 0.33 O 2 lithium-ion cells. The test procedure comprises varying charging currents, discharging currents and resting times between cycles. The cells show a nonlinear capacity fade after a few hundred equivalent full cycles, if cycled with a standard charging and discharging rate of almost 1C, and different resting times. By increasing the discharging current or decreasing the charging current, the lifetime improves and results in a linear capacity fade. The neutron diffraction experiment reveals a loss of lithium inventory as the dominant aging mechanism for both linearlyand nonlinearly-aged cells. Other aging mechanisms such as the structural degradation of anode or cathode active materials, or the deactivation of active materials, cannot be confirmed. With ongoing aging, we observe an increasing capacity loss in the edge area of the electrodes. Whereas the growth of the solid electrolyte interphase defines the early stage, linear aging, marginal lithium deposition is supposed to cause the later stage, nonlinear aging. Capacity recovery caused by lithium stripping and chemical intercalation is shown to be dependent on the cell's state of health.

A single-layered NMC/graphite pouch cell is investigated by means of differential local potential measurements during various operation scenarios. 44 tabs in total allow for a highly resolved potential measurement along the electrodes whilst the single layer configuration guarantees the absence of superimposed thermal gradients. By applying a multi-dimensional model framework to this cell, the current density and SOC distribution are analyzed quantitatively. The study is performed for four C-rates (0.1C, 0.5C, 1C, 2C) at three temperatures (5 • C, 25 • C, 40 • C). The maximum potential drop as well the corresponding SOC deviation are characterized.The results indicate that cell inhomogeneity is positively coupled to temperature, i.e. the lower the temperature, the more uniform the electrodes will be utilized. Within the past decades, demand for lithium-ion batteries in mobile applications has significantly increased. Due to their well proven performance as well as their stability in long-term usage, lithium-ion batteries became the technology of choice for electrochemical energy storage devices.1,2 Still, the specific energy density as well as cycle life are constantly being optimized by either commercializing new active materials, electrolytes and additives or by reducing the fraction of non-active parts within a battery. Often, this corresponds to thicker electrodes or larger form factors leading to capacities of up to 100 Ah per cell. In these large format cells, severe gradients in current density and temperature distribution can occur along the electrode stack, 3-9 which might provoke a performance loss during operation due to inhomogeneous utilization. Also non-properly adapted thermal conditioning can have a crucial impact on the performance of larger cells. [10][11][12] Modeling of internal distributions of potential and temperature along the electrodes is quite challenging, since even to calculate only a few cycles, a lot of computational resources are required for fully resolved models. In literature, there are many examples for spatially resolved multi-dimensional modeling approaches, 8,9,[13][14][15][16] which aim at representing the cell's internal behavior in terms of potential, current density, state of charge (SOC) and temperature distribution. Unfortunately, all of these examples lack a detailed, i.e. spatially resolved experimental validation, which is capable of tracking internal variables instead of measuring the surface temperature at a few spots and considering the overall battery's terminal voltage. Also, only a few examples of direct measurements of the internal current density distribution were published so far. Zhang et al.6,7 built a specific LFP/graphite prototype cell for this purpose. A segmented cathode was used for analyzing the current distribution during discharge at varying C-rates and temperatures. This setup allows for a precise monitoring of the current of each electrode element individually. Large deviations in SOC of up to several percent were identified during the process...

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This paper presents a lithium-ion battery aging study in which pouch cells comprising a LiCoO 2 /LiNi 0.8 Co 0.15 Al 0.05 O 2 blended cathode and a graphite anode are examined. The study focuses on the impact of temperature and discharge rate on the cycle life of the tested cells. Compared to the aging behavior of other lithium-ion cells in the literature, the cells tested here are less sensitive to the discharge rate but more vulnerable to low temperature cycling. The vulnerability to low temperature mainly comes from cathode degradation, especially of the LiCoO 2 component. This is identified by electrochemical impedance spectroscopy, differential voltage analysis and incremental capacity analysis. The cells are able to achieve 3000-5000 cycles before reaching a capacity fade of 20%, also at higher discharge rates up to 5C. All in all, the high discharge rate capability could be a general advantage of pouch cells due to less mechanical and thermal stress in their geometry. Furthermore, more attention should be paid to the cathode health in low temperature applications of lithium-ion cells containing layered oxides. This paper focuses mainly on non-invasive aging detection methods for lithium-ion cells. Post-mortem results will be published in a following paper.