Elsevier

Journal of Energy Storage

Volume 17, June 2018, Pages 383-394
Journal of Energy Storage

Aging in 18650-type Li-ion cells examined with neutron diffraction, electrochemical analysis and physico-chemical modeling

https://doi.org/10.1016/j.est.2018.03.016Get rights and content

Highlights

  • Aging in NMC/C cells compared by neutron diffraction and electrochemical analysis.

  • A physico-chemical aging model is developed and validated for aging contributions.

  • Capacity loss is mainly due to loss of cyclable lithium into SEI layer growth.

  • Model and neutron data agree on cathode stoichiometry shift of 0.08.

  • Model and neutron data agree on anode stoichiometry shift of about 0.19.

Abstract

Aging in NMC/C cells (NMC-Liy(Ni0.33Mn0.33Co0.33)O2), cycled 1000 times at a 1C rate, has been characterized by in situ neutron diffraction and electrochemical analysis. These experimental results have been validated by a physico-chemical aging model, which attributes capacity fade to growth of a continuous SEI film on the anode. Neutron diffraction of the cells indicate a cyclable lithium loss corresponding to a capacity fade of about 23% in both electrodes of the cycled cell. The cycled cell suffers an anode stoichiometry shift from x = 0.84 to x = 0.65 in LixC6 (0 ⩽ x ⩽ 1) in its fully charged state and a cathode stoichiometry shift from y = 0.89 to y = 0.81 in Liy(Ni0.33Mn0.33Co0.33)O2 (0 < y ⩽ 1.05) in its fully discharged state. Anode (x = 0) as well as cathode stoichiometries (y = 0.54) remain practically unchanged in the cell's fully discharged and charged states, respectively. These stoichiometry shifts match well with those derived from the model, and both neutron diffraction and model are in good agreement to the electrically determined capacity fade of 21%. In fact, cyclable lithium losses slightly exceed this value. Thus, capacity fade in these cells is mainly due to loss of cyclable lithium into the continuous growth of a SEI film on the anode surface.

Introduction

Due to their high energy and power densities, Li-ion batteries are the most favored rechargeable systems in portable electronic devices [1]. Nowadays they are gaining popularity in electric transportation and stationary grid storage systems as well. These large-scale applications demand much longer battery lifespans and thus an understanding of aging mechanisms responsible for reducing lifetime or cycle life is essential [[2], [3]]. The most common cathode material used in commercial portable Li-ion batteries is LiCoO2 (LCO) due to its high energy density and good cycling performance. However, Co is expensive and considered toxic. For electric vehicles, Liy(Ni0.33Mn0.33Co0.33)O2 (NMC) seems to be the more preferred cathode materials as their layered structure is more stable, changes in lattice volume are smaller (for y > 0.5), and thereby safety and lifetime are enhanced. At the cost of a comparatively lower energy density, LiFePO4 (LFP) cathodes offer an even better cycling performance and safety, and are usually the optimal choice for stationary grid storage systems.

Several experimental methods are being used to understand Li-ion batteries during storage as well as during operation [4]. Analytical methods such as electrochemical impedance spectroscopy, microscopy, X-ray and neutron diffraction addressed aging and attributed loss of cyclable lithium and decay of electrode materials as the most important capacity fade mechanisms [[5], [6], [7], [8], [9], [10], [11], [12]]. These occurred due to solid-electrolyte interphase (SEI) layer growth, volume changes in the electrodes during Li-ion intercalation/deintercalation, blockage and structural degradation, Li plating, as well as undesirable phase transformations of active electrode materials. There are several studies which have investigated battery aging by comparing experimental results with simulation models [[13], [14], [15], [16], [17]]. For example, a SEI electron tunneling model, which attributed electron tunneling through the inner SEI layer as the rate determining step, was proposed and simulated to explain capacity fade during storage and cycling by Li et al. [[15], [16]]. This model was validated by experiments on commercial prismatic LFP/C cells. However, no study compared results from aging models to neutron diffraction data. In particular, there is no reported investigation where experimentally observed capacity fading results in 18650-type NMC/C cells using neutron diffraction are compared with a physico-chemical model. For non-destructive in situ studies of such large format Li-ion cells, neutron diffraction is a suitable and powerful method. Several types of Li-ion cells, such as LCO/C [[18], [19]], NMC/C [[20], [21]] and LFP/C [[22], [23], [24]] have been investigated using neutrons as a probe. These investigations addressed Li plating on the anode [[25], [26]], structural changes within the cathode [21] and spatially resolved inhomogeneities in current densities [[27], [28], [29]] in both prismatic as well as 18650-type cells. However, aging studies in 18650-type cells with neutron diffraction are rare [[11], [30], [12], [31], [32]]. In a recent investigation of aging in commercial 18650-type NMC/C cells with neutrons, capacity fade was attributed to loss of cyclable lithium and cathode material degradation [12]. Our studies investigate the cycling induced aging in similar cell chemistry in more details using additionally physico-chemical modeling and electrochemical analysis.

Section snippets

Cell description

Cylindrical 18650-type NMC/C cells, produced under commercial standards, were provided by the battery manufacturer (VW-VM Forschungsgesellschaft mbH and Co. KG, a joint venture between Volkswagen and VARTA Microbattery GmbH). Each cell consisted of a Li1.05(Ni0.33Mn0.33Co0.33)0.95O2 cathode, an organic carbonate based electrolyte (containing 1 M LiPF6 conducting salt as well as SEI forming and overcharge protecting additives), a monolayer polyolefin-based separator and a graphite anode. The

Electrical behavior

The aging procedure of cycling the NMC/C cell was initiated after the cell formation protocol was completed. A steady and an approximately linear decrease in the discharge capacity was observed as can be seen in Fig. 1. After completing 1000 cycles, the cycled cell had suffered a capacity loss of 21% relative to the capacity of the uncycled cell. A similar approximately linear trend was seen in the other set of cells which were simultaneously stored and cycled, which supports the

Development of a physico-chemical model

Physico-chemical modeling of Li-ion cells gains insights to investigate and describe the cell's behavior based on particle and electrode effects [41]. In contrast to first-principle or empirical models, physico-chemical models are suitable not only to describe surface and molecule processes in a phenomenological manner but also to describe particle and electrode domains in a mechanistical manner. We developed a physico-chemical aging model on the basis of a one-dimensional intercalation model

Discussion

For large format cells, neutron diffraction is one of the few experimental techniques that can give stoichiometric information of light elements like lithium without opening the cell. The anode stoichiometry shift, or reduction of x in LixC6 (0 ⩽ x ⩽ 1) on aging, was directly obtained from the phase fractions of LiC6 and LiC12 phases, which were the only lithiated graphite phases in the diffractogram. The cathode stoichiometry shift, or reduction in y in Liy(Ni0.33Mn0.33Co0.33)O2 (0 < y ⩽ 1.05)

Conclusion

In this study, the capacity loss in 18650-type NMC/C cells, produced under commercial standards, was experimentally investigated with in situ neutron diffraction and electrical tests, as well as physico-chemically modeling. The comparison of neutron diffraction experiments with electrochemical characterization reveals that the entire 21% capacity loss upon cycling is dominantly due to loss of cyclable lithium, which is evident from a reduction in available cyclable lithium in anode as well as

Acknowledgements

This work was financially supported by the German Federal Ministry of Education and Research (BMBF) under grant numbers 03X4633A, 03XP0081 and 03XP0034G. The authors thank the Heinz Maier-Leibnitz Zentrum (MLZ) for granting beam time at FRM II.

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