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.

An important step toward safer and more reliable lithium-ion battery systems is the improvement of methods for detection and characterization of battery degradation. 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 cycling conditions: ambient and sub-ambient temperatures (29 degrees C and 10 degrees 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 complements capacity fade analysis; it correlates well with impedance and galvanostatic cycle life 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 in lithium-ion battery systems.

The Tesla Model 3 is currently one of the most popular electric vehicles (EV)and was the best selling EV in 2020. In this article, performance and degradation are studied for 21700 cylindrical cells from a Tesla Model 3 cycledwithin nine, sequential state of charge (SOC) windows, each using 10% ofthe cell capacity is studied. Cells tested in either very high and very low SOCwindows show faster degradation than at moderate SOC. In particular, theshortest service life was for cells cycled below 25% SOC. The ageing mechanisms of the cells cycled in these most extreme windows have been monitoredby non-destructive electrochemical methods including analyses of differentialvoltage, incremental capacity, and voltage hysteresis. The combination ofloss of active material (LAM) and loss of lithium inventory (LLI) are responsible for the most rapid ageing observed in the cells cycled in the 5-15%SOC window. Calendar ageing, however, is not accelerated by storage atlow SOC. The results from this study offer an understanding of the distinct,SOC-dependent ageing patterns observed in the cells. This understanding ofthe ageing mechanisms in different cycling and storage conditions can be usedto recommend improved customer usage patterns and substantially extendthe 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%).

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 7Li 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 7Li 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. 

Conditions such as the temperature and pressure experienced by lithium-ion battery components are dependent oncell geometry and can vary widely within a large cell. The resulting uneven degradation is challenging to study at thefull cell level but can be revealed upon disassembly and post mortem analysis. In this work, we report localizedlithium plating in automotive-grade, prismatic lithium-ion cells, also under cycling conditions generally consideredto be mild (e.g., 5–65 %SOC, 23 ◦C, 0.5C cycle rate). Dead lithium content is quantified using 7Li nuclear magneticresonance 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 boththe separators and negative electrodes are ultimately deactivated. High pressure arises during cycling, and wepropose 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 undermild cycling conditions. Notably, components harvested from regions with no detected lithium plating maintainedadequate electrochemical performance.