Aging in 18650-type Li-ion cells examined with neutron diffraction, electrochemical analysis and physico-chemical modeling
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.
References (66)
- et al.
In situ methods for Li-ion battery research: a review of recent developments
J. Power Sources
(2015) - et al.
Identifying battery aging mechanisms in large format Li ion cells
J. Power Sources
(2011) - et al.
Calendar aging of a graphite/LiFePO4 cell
J. Power Sources
(2012) - et al.
Surface structural disordering in graphite upon lithium intercalation/deintercalation
J. Power Sources
(2010) - et al.
Cycle aging of commercial NMC/graphite pouch cells at different temperatures
Appl. Energy
(2015) - et al.
Calendar and cycle life study of Li(NiMnCo)O2-based 18650 lithium-ion batteries
J. Power Sources
(2014) - et al.
Degradation analysis of 18650-type lithium-ion cells by operando neutron diffraction
J. Power Sources
(2016) - et al.
Degradation mechanisms of C6/LiFePO4 batteries: experimental analyses of cycling-induced aging
Electrochim. Acta
(2016) - et al.
(Invited) Electron tunneling based SEI formation model
ECS Trans.
(2014) - et al.
Structural changes in a commercial lithium-ion battery during electrochemical cycling: an in situ neutron diffraction study
J. Power Sources
(2010)
Understanding structural changes in NMC Li-ion cells by in situ neutron diffraction
J. Power Sources
Real-time investigation of the structural evolution of electrodes in a commercial lithium-ion battery containing a V-added LiFePO4 cathode using in-situ neutron powder diffraction
J. Power Sources
Structural evolution in LiFePO4-based battery materials: in-situ and ex-situ time-of-flight neutron diffraction study
J. Power Sources
Lithium plating in lithium-ion batteries at sub-ambient temperatures investigated by in situ neutron diffraction
J. Power Sources
Lithium plating in lithium-ion batteries investigated by voltage relaxation and in situ neutron diffraction
J. Power Sources
Spatially resolved in operando neutron scattering studies on Li-ion batteries
J. Power Sources
In-situ observation of inhomogeneous degradation in large format Li-ion cells by neutron diffraction
J. Power Sources
“In-operando” neutron scattering studies on Li-ion batteries
J. Power Sources
Aging behavior of lithium iron phosphate based 18650-type cells studied by in situ neutron diffraction
J. Power Sources
Effect of fatigue/ageing on the lithium distribution in cylinder-type Li-ion batteries
J. Power Sources
High-resolution neutron powder diffractometer SPODI at research reactor FRM II
Nucl. Instrum. Methods Phys. Res. Sect. A: Accel. Spectrom. Detect. Assoc. Equip.
Current-dependent electrode lattice fluctuations and anode phase evolution in a lithium-ion battery investigated by in situ neutron diffraction
Electrochim. Acta
Low-temperature charging of lithium-ion cells Part I: Electrochemical modeling and experimental investigation of degradation behavior
J. Power Sources
A generalized cycle life model of rechargeable Li-ion batteries
Electrochim. Acta
Simulation of capacity loss in carbon electrode for lithium-ion cells during storage
J. Power Sources
Development of a physics-based degradation model for lithium ion polymer batteries considering side reactions
J. Power Sources
Controls oriented reduced order modeling of solid-electrolyte interphase layer growth
J. Power Sources
Ageing mechanisms in lithium-ion batteries
J. Power Sources
Modeling mechanical degradation in lithium ion batteries during cycling: solid electrolyte interphase fracture
J. Power Sources
Improvement of electrochemical properties of layered LiNi1/3Co1/3Mn1/3O2 positive electrode material by zirconium doping
Solid State Ion.
Issues and challenges facing rechargeable lithium batteries
Nature
Electrochemistry and the future of the automobile
J. Phys. Chem. Lett.
Future generations of cathode materials: an automotive industry perspective
J. Mater. Chem. A
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