Inhomogeneous degradation of graphite anodes in automotive lithium ion batteries under low-temperature pulse cycling conditions

Understanding the mechanical activity of lithium-ion cells during cycling and its connection with aging phenomena is essential to improve cell design and operation strategies. Previous studies of lithium-ion pouch cells [B. Rieger et al., Journal of Energy Storage, 8, 1 (2016)] have shown non-uniform swelling with local displacement overshoots during charging. In this experimental work, a novel three-dimensional laser scanning method is used to investigate local reversible and irreversible thickness changes of six commercial LiCoO 2 /graphite cells during a cyclic aging experiment. Three cycle scenarios were included and two cells each were exposed to a specific temperature and charging rate. The cells showing local displacement overshoots also exhibit non-uniform distributions of irreversible thickness change. Post-mortem analysis showed largely inhomogenously degraded surfaces of the single anode layers. It is shown that the cells' irreversible thickness change correlates with capacity fade and internal resistance increase monitored via electrochemical impedance spectroscopy. Lithium-ion batteries have become the most promising energy storage technology for small electronic devices such as smartphones or laptops as well as for battery packs in electric vehicles. Although their relatively high density in power and energy make Li-ion batteries the technology of choice for many applications, its aging and degradation behavior likewise limits their use in applications that require extreme safety standards and cycle life.2 In order to quantify the decay of batteries, the state of health (SOH) 3 is used which refers to the capacity fade by relating the current capacity of the cell to its initial capacity. This relation can be seen as an overall concept introducing a measurable quantity of aging effects occurring during battery lifetime.Detecting the cell's SOH by measuring external stress and strain on the surface of the cell's housing 2,4 is a recent method based on the mechanical behavior of Li-ion batteries in form of volume change effects during charge and discharge processes. These are largely caused by electrode swelling, 5-8 polymer deformation 5,9 and film growth. 10,11Estimating the cell's SOH by a single point measurement implies a homogeneous stress and strain distribution over the housing of the cell. Hence, the utilization of the cell would need to be homogeneous. Local variations in current density and electrode potential occurring during cell operation along the current collector foils 12-15 object this assumption. Considering an inhomogeneous utilization of the electrodes described by local variations in state of charge (SOC), 15 an overall estimation of SOH seems inappropriate to gain a deeper understanding of aging mechanisms occurring in commercial Li-ion cells. Consequently, there is a need for local detection of aging mechanisms to account for design specific inhomogeneous load across the cell.As already presented in previous work, 1 extending the measurement of strain or stress to a local re...

Section: Resultssupporting

confidence: 83%

mentioning

confidence: 99%

Non-Destructive Detection of Local Aging in Lithium-Ion Pouch Cells by Multi-Directional Laser Scanning

Sturm

Spingler

Rieger

et al. 2017

“…In some cases, a resistance increase was observed with 18650 cells by EIS measurement because of the unevenness of the coating quality of electrode. 16 By using small electrodes, we may avoid such uncertainty and achieve steady results in any experiment. The whole resistance for these cells persisted within approximately 30 throughout the cycles.…”

Section: Resultsmentioning

confidence: 99%

Study of Li Metal Deposition in Lithium Ion Battery during Low-Temperature Cycle Using In Situ Solid-State⁷Li Nuclear Magnetic Resonance

Arai

Nakahigashi

2017

The influences of temperature and cell operation conditions on Li metal deposition in lithium ion batteries were studied by in-situ solid-state 7 Li nuclear magnetic resonance (NMR) spectroscopy. The rates of Li metal deposition during the low-temperature cycle with two charge-discharge operation modes, i.e., continuous current and pulse current, were estimated for temperatures of 5, 0, and −5 • C. Close values of activation energies were obtained for the capacity fading rate and the Li metal deposition rate, suggesting that Li metal deposition is the main cause of capacity fading in low-temperature cycles. The amounts of Li stored in the negative electrode and Li metal deposited during the low-temperature cycle were estimated and are discussed to understand the capacity fading mechanism. The pulse current mode cycle did not lead to any Li metal deposition at low temperatures. Lithium ion batteries (LIBs) are superior energy storage devices from the viewpoint of energy density, wide range of cell voltages derived from a variety of electrode materials, and further possibility of energy density improvement by developing innovative electrode materials.1,2 LIBs have been widely used in portable applications, and their use is extending to electric vehicles (EVs) and hybrid electric vehicles (HEVs). However, these EV applications require high energy densities to realize long driving distances, a long battery life to preserve the performance of vehicles during their lifecycle, and cost reduction. The temperature performance of LIBs also influences the vehicle performance, especially the limitation of low-temperature regeneration.3,4 This prevents full usage of LIB performance. LIBs are degraded by damage to the active material at high temperatures and by Li metal deposition at low temperatures.5-8 Moreover, the safety of LIBs is worsened by Li metal deposition.9,10 Thus, understanding the behavior of Li metal deposition at low temperatures is crucial for optimizing charging protocols and assuring safe and durable use of LIBs.Li metal deposition has been studied using optical microscopy and scanning electron microscopy with specially designed electrochemical cells at ambient temperature, which has allowed observation of the growth of dendritic Li deposition and estimation of the volume during operation.11-13 However, those cells were open structures that did not consider actual cell circumstances, such as the limited volume of electrolyte and the cell pressure, which restrains the movement of Li ions. Li metal deposition has also been evaluated by weighing the electrode and measuring the nuclear magnetic resonance (NMR) of a commercial 18650 battery after cycling.14-16 However, these studies are post molten analysis with disassembling the cell and affected the results by sample preparation.17 Furthermore, deposited Li metal sometimes intercalates into graphite, thus missing the chance to acquire a true sample. 16 We developed an in-situ solid-state 7 Li NMR method using a small laminated full cell composed of actual ele...

“…Lower temperatures favor lithium plating at the end of the charging process. Many researchers [9][10][11][12][13] have identified lithium plating using either in-situ or ex-situ methods in cycled lithium-ion cells at low temperature, but none of them has reported cathode degradation under this condition.…”

mentioning

confidence: 99%

Impact of Temperature and Discharge Rate on the Aging of a LiCoO₂/LiNi_0.8Co_0.15Al_0.05O₂Lithium-Ion Pouch Cell

Keil

Schuster

et al. 2017

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.