Inhomogeneous temperature distribution in a large-format lithium-ion cell or between cells in a module/pack may cause a non-uniform current distribution, causing a local difference in aging, and potentially faster global aging (capacity fade and impedance rise) of the module. To study this effect, LiNi1/3Mn1/3Co1/3O2/graphite lithium-ion pouch cells were cycled at 32, 36, and 40 degrees C as single cells and in parallel connection, representing uniform and non-uniform temperature distributions. The results show that the current distribution becomes less uniform after cycling at a higher rate and in a narrower state-of-charge range. Cycling with non-uniform temperature at 3C rate results in aging similar to that at the maximum uniform temperature, while at 1C rate the non-uniform aging follows the trend at the average temperature. The performance decay of the cells cycled at 3C is mainly driven by the cell at 40 degrees C which shows 30 % more capacity loss than the corresponding cell cycled singularly. This leads to additional considerations when designing for cycle life and reliability in fast charging applications and high-power applications such as in electric vehicles or frequency regulation in stationary storage.

In this study, Synchrotron X-ray diffraction (XRD) radiography was utilized to investigate the ageing heterogeneity in 48 Ah prismatic lithium-ion cells with Ni-rich LiNi0.8Mn0.1Co0.1O2 (NMC811) as the positive electrode active material and graphite as the negative electrode active material after similar to 2800 cycles. The study revealed that the area closest to the positive electrode tab is most vulnerable to degradation, particularly impacting the NMC material. Application of principal component analysis allowed to differentiate and visualize part of positive electrode material that has a different degradation due to the lithium plating. A comparison of non-destructive X-ray diffraction-based methods and electrochemical characterization method which was performed on the opened cell has shown an importance of a complementary approach. Our results highlight the feasibility of employing non-destructive techniques to study large prismatic cells, thereby presenting extensive opportunities for advancements in battery research and industry.

The Tesla Model 3 is currently one of the most popular electric vehicle (EV) and was the best selling EV in 2020. In this article, performance and degradation of 21700 cylindrical cells, taken from a new vehicle, were studied by cycling within 10% State of charge (SOC) windows. Cells tested in either very high and very low SOC windows show faster degradation than at moderate SOC. In particular, the shortest service life was for cells cycled below 25% SOC. The ageing mechanisms of the cells cycled in these most extreme windows have been monitored by non-destructive electrochemical methods including analyses of differential voltage, incremental capacity, and voltage hysteresis. The combination of loss of lithium inventory (LLI) accelerated in early cycling by SiOx utilization, paired with loss of active material (LAM) of SiOx are responsible for the most rapid ageing, which is observed in the cells cycled in the 5%–15% SOC window. Calendar ageing, however, is not accelerated by storage at low SOC. The results from this study offer an understanding of the distinct, SOC-dependent ageing patterns observed in the cells. This understanding of the ageing mechanisms in different cycling and storage conditions can be used to recommend improved customer usage patterns and substantially extend the lifetime of lithium-ion batteries in operation.

Lithium ion batteries (LIB) have become a cornerstone of the shift to electric transportation. In an attempt to decrease the production load and prolong battery life, understanding different degradation mechanisms in state-of-the-art LIBs is essential. Here, we analyze how operational temperature and state-of-charge (SoC) range in cycling influence the ageing of automotive grade 21700 batteries, extracted from a Tesla 3 Long Range 2018 battery pack with positive electrode containing LiNixCoyAlzO2 (NCA) and negative electrode containing SiOx-C. In the given study we use a combination of electrochemical and material analysis to understand degradation sources in the cell. Herein we show that loss of lithium inventory is the main degradation mode in the cells, with loss of material on the negative electrode as there is a significant contributor when cycled in the low SoC range. Degradation of NCA dominates at elevated temperatures with combination of cycling to high SoC (beyond 50%).

Conditions such as the temperature and pressure experienced by lithium-ion battery components are dependent on cell geometry and can vary widely within a large cell. The resulting uneven degradation is challenging to study at the full cell level but can be revealed upon disassembly and post mortem analysis. In this work, we report localized lithium plating in automotive-grade, prismatic lithium-ion cells, also under cycling conditions generally considered to be mild (e.g., 5-65 %SOC, 23 degrees C, 0.5C cycle rate). Dead lithium content is quantified using 7Li nuclear magnetic resonance spectroscopy in both electrode and separator samples, corresponding to substantial capacity fade (26-46%) of the full cells. Severe lithium plating is typically initiated in regions near the positive tab, in which both the separators and negative electrodes are ultimately deactivated. High pressure arises during cycling, and we propose a deactivation mechanism based on high local stress due to electrode expansion and external constraint. Further, we develop a model to demonstrate that component deactivation can result in lithium plating even under mild cycling conditions. Notably, components harvested from regions with no detected lithium plating maintained adequate electrochemical performance.

The energy density and lifetime of lithium-ion cells for automotive applications have both substantially improved in recent years. This enables electric vehicles in both the consumer and commercial sectors that have greater range, better reliability, and longer service lives [1]. These factors all contribute to the widespread adoption of electric vehicles. However, inhomogeneities exist within cells that contribute to uneven internal degradation and, when severe, can trigger rapid failure. These inhomogeneities can be related to manufacturing tolerances or distributions of component parameters [2], as well as gradients in internal or external conditions. As such, the reality of heterogeneous aging is a complex, multivariate problem to solve. In this latest work, we cycle automotive-grade prismatic cells for several thousand cycles (until 10-30 % capacity fade) and harvest components for post mortem aging characterization and experimental parameter identification. The constituent electrodes are a Ni-rich layered oxide (approximately LiNi0.76Mn0.15Co0.09O2) and graphite. In addition to microscopy and electrochemical testing on harvested electrodes, we quantify local lithium plating on graphite with our recently developed method utilizing ex-situ, 7Li nuclear magnetic resonance spectroscopy [3]. We believe this to be the first report of spatially-resolved lithium plating quantification in commercial lithium-ion cells. Clear patterns emerge (see Figure 1), reflecting the geometry of the cell and showing reproducible heterogeneity that cannot be attributed solely to defects in manufacture. Several tests even confirm local lithium plating and unusually rapid degradation at comparatively mild cycle conditions (slow charge/discharge, low state-of-charge, and room temperature). The wound jellyroll design in contemporary prismatic cells enables very high packing efficiency and energy density, but includes inherently weak points from where lithium plating and internal stresses [4] can propagate. In some cases, cells achieve satisfactory lifetimes without accelerated aging. In others, heterogeneities cause extreme local degradation that triggers end-of-life even though other regions may remain relatively intact. We explore the causes of these patterns and question how such damage can occur, even under cycling conditions usually considered safe. References [1] P. Svens et al., IEEE Trans. Transp. Electrif., (2022), doi:10.1109/TTE.2022.3158838. [2] D. Beck et al., Energies, 14 (2022), 3276, doi:10.3390/en14113276. [3] Y. Fang et al., manuscript submitted, (2022). [4] P. Gupta and P. Gudmundson, J. Power Sources, 511 (2021), 230465, doi:10.1016/j.jpowsour.2021.230465. Figure 1

A key degradation mechanism in lithium-ion batteries (LIBs) is the irreversible loss of cyclable lithium during cycling. At the graphite negative electrode, this loss occurs through the deposition of lithium-containing compounds in the solid-electrolyte interphase (SEI) and through plating of metallic lithium, resulting in so-called dead lithium. The separate quantification of SEI and dead lithium has so far been a challenge in post mortem analysis of commercial LIBs. Here we report a simple and fast Li-7 nuclear magnetic resonance spectroscopy (NMR) protocol applied to solid-state samples derived from lab-built batteries to independently quantify these and other lithium species in graphite electrodes without the need for specialized cell design nor knowledge of prior charging history. The metallic lithium content is corroborated by electrochemical calculations; the total amount of lithium is also determined from Li-7 liquid-state NMR and inductively coupled plasma optical emission spectroscopy (ICP-OES) in suitably digested samples. Factors influencing accuracy like the sample handling process, the radiofrequency skin effect, and re-intercalation losses are investigated. Measurements on samples from commercial cells aged under realistic conditions demonstrate quantification of dead lithium and remaining ionic species (SEI), and further reveal lithium dendrites entrained in the separator following cell disassembly. The method uses conventional and widely available NMR instrumentation and is applicable to samples from lab-scale test cells or commercial batteries, thereby presenting a vast improvement over prior post mortem methods.

Ni-rich oxide cathode active materials are today key components in Li-ion batteries used for electric vehicles (EV). The high capacity of LiNi x Co y Mn z O 2 and LiNi x Co y Al z O 2 (NCA) with x≥0.6 has enabled a continued increase in energy density and decrease in cost at the pack level, enabling the rapid growth of the EV market in recent years. Despite a wide use, rather fast material ageing results in the batteries being a source of considerable environmental impact [1]. Various degradation sources are highlighted in the literature, these include transition metal dissolution, surface layer reconstruction, particle cracking, etc [2]. However, when extrapolating the knowledge from a model cell to a commercial battery the relevance and the degree of degradation impact can drastically change [3]. The importance of approaching real conditions is a key to a full understanding of the ageing processes occurring in commercial cells. In the given work, we analyse commercial state-of-art 2170 cylindrical cells. The cells were aged at various temperatures and state of charge windows, targeting divergent usage scenarios. The degradation processes were accessed with a wide range of analytical techniques applied to extracted electrodes (x-ray diffraction, inductively coupled plasma atomic emission spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, cycling voltammetry, incremental capacity analysis, electrochemical impedance spectroscopy and intermittent current interruption). In the current work we show that depending on the operating conditions, influence of ageing sources varies. Additionally, ageing mapping within cell geometry is presented highlighting “hot spots” of the cell [4]. References [1] W. Liu et al., “Nickel-Rich Layered Lithium Transition-Metal Oxide for High-Energy Lithium-Ion Batteries,” Angew. Chemie Int. Ed., vol. 54, no. 15, pp. 4440–4457, Apr. 2015. [2] T. Li, X.-Z. Yuan, L. Zhang, D. Song, K. Shi, and C. Bock, Degradation Mechanisms and Mitigation Strategies of Nickel-Rich NMC-Based Lithium-Ion Batteries, vol. 2018, no. January 2017. Springer Singapore, 2019. [3] M. Lucu et al., “Data-driven nonparametric Li-ion battery ageing model aiming at learning from real operation data – Part A: Storage operation,” J. Energy Storage, vol. 30, p. 101409, Aug. 2020. [4] Mikheenkova et al., in manuscript .

During the last decade, the market interest for electrified vehicles has increased considerably alongside global climate initiatives. This has coincided with vast improvements in automotive-grade, lithium-ion battery performance. This has increased the range of battery electric vehicles and plug-in hybrids, but lifetime remains a challenge. Aging during fast charging is especially difficult to understand due to its nonlinear dependence on charge rate, state-of-charge, and temperature. We present results from fast charging of several energy-optimized, prismatic lithium-ion battery cell generations with a nickel manganese cobalt (NMC)/graphite chemistry through comparison of capacity retention, resistance, and dQ/dV analysis. Changes in cell design have increased energy density by almost 50% over six years of cell development and acceptable cycle life can be expected, even under fast charging, when restricting the usage of the available capacity. Even though this approach reduces the useable energy density of a battery system, this tradeoff could still be acceptable for vehicle applications where conventional overnight charging is not possible. The tested cell format has been used for a decade in electrified vehicles. The ongoing development and improvement of this cell format by several cell manufacturers suggests that it will continue to be a good choice for future vehicles.

Ni-rich Li[NixCoyAlz]O2 (NCA) positive electrode materials are today found in the state-of-the-art high-energy Li-ion batteries [1]. Considering the high production cost of these batteries combined with the scarcity and environmental impact of the included transition metals, large benefits would be gained by prolonging the lifetime of the Li-ion cells. Although NCAs are the key to a high energy density, they are generally believed to be the main culprit behind the failure of the cell [2]. The NCAs are subjected to adverse structural phase transformation and display increased surface reactivity towards the electrolyte upon electrochemical charging [3]. Particle cracking, surface reconstruction and associated electrolyte decomposition are well-known processes leading to both Li+ inventory loss and increased cell impedance. Yet, several aspects governing these underlying ageing mechanisms remain to be explained, particularly for Ni-rich NCAs contained in state-of-the-art commercial Li-ion cells today. Herein we present a combined structural and electrochemical study based on detailed X-ray diffraction analysis, electron microscopy, and intermittent current interruption cycling. The extent and impact of the various ageing mechanisms at each stage of battery life will be assessed and discussed [4]. Reference s : [1] Yuliya Preger et al 2020 J. Electrochem. Soc. 167 120532 [2] A. Manthiram, B. Song, and W. Li, Energy Storage Mater. 6 (2017) pp. 125–139. [3] E. Flores, P. Novák, U. Aschauer, and E. J. Berg, Chem. Mater. 2020, 32, 1, 186–194 [4] Mikheenkova et al. In preparation.

An important step toward safer and more reliable lithium-ion battery systems is the development of better methods for detection and characterization of battery degradation. For a method to be suitable for online application (e.g., onboard an electric vehicle), it must be simple, explanatory, and non-invasive. In this work, we develop and track aging indicators over the life of 18650-format lithium-ion batteries with a blended NMC532-LMO positive electrode and graphite negative electrode. Cells are cycled until reaching 80 % of their original capacity under combinations of four different cycling conditions: ambient and sub-ambient temperatures (29 and 10 °C) and fast and mild rates (2.7 and 1.0C). Loss of lithium inventory dominates aging for all cases, with additional loss of NMC capacity under the combination of sub-ambient temperature and mild rate. A novel, easily acquired polarization factor (supported by electrochemical impedance spectroscopy) complements capacity fade analysis; it correlates well with ultimate cell lifetime and indicates changes in active aging processes. These processes are further revealed by differential voltage analysis (DVA) and incremental capacity analysis (ICA). New indicators and aging scenarios are evaluated for these techniques and supported by post mortem analysis. From in operando cycling data and a single, slow discharge curve, these four methods (capacity fade, polarization factor, DVA, and ICA) comprise a simple, explanatory, and non-invasive toolbox for evaluating aging online in lithium-ion battery systems.