Graphite Lithium-ion Cells Research Articles

Understanding the Role of Commercial Separators and Their Reactivity toward LiPF6 on the Failure Mechanism of High‐Voltage NCM523 || Graphite Lithium Ion Cells

AbstractNCM523 || graphite lithium ion cells operated at 4.5 V are prone to an early “rollover” failure, due to electrode cross‐talk, that is, transition metal (TM = Mn, Ni, and Co) dissolution from NCM523 and deposition at graphite, subsequent formation of Li metal dendrites, and, in the worst case, generation of (micro‐)short‐circuits by dendrites growing to the cathode. Here, the impact of different separators on the high‐voltage performance of NCM523 || graphite cells is elucidated focusing on the separators’ structural properties (e.g., membrane vs fiber) and their reactivity toward LiPF6 (e.g., ceramic‐coated separators). First, the separator architecture has a major impact on cycle life. Fiber‐structured separators can prevent the “rollover” failure by a more homogeneous deposition of TMs and formation of Li metal dendrites, thus, hindering penetration of dendrites to the cathode. In contrast, porous membrane‐structured separators cannot prevent the cell failure due to inhomogeneous TM deposits/Li metal dendrites. Second, it is demonstrated that different types of ceramic‐coated separators (Boehmite (γ‐AlO(OH)) vs α‐Al2O3) exhibit different reactivities toward LiPF6. While α‐Al2O3 shows a minor reactivity toward LiPF6, the γ‐AlO(OH) coating leads to in situ formation of the beneficial difluorophosphate anion in high amounts due the high reactivity toward LiPF6 decomposition, which significantly improves cycle life.

Li‐Ion Batteries: Understanding the Outstanding High‐Voltage Performance of NCM523||Graphite Lithium Ion Cells after Elimination of Ethylene Carbonate Solvent from Conventional Electrolyte (Adv. Energy Mater. 14/2021)

In article number 2003738, Martin Winter, Tobias Placke, Johannes Kasnatscheew and co-workers report that a simple elimination of ethylene carbonate (EC) from conventional electrolytes counterintuitively boosts the high voltage performance in Li-ion batteries. It is attributed to the beneficially formed LixPOyFz-species, which can scavenge the hazardous transition metals (Ni, Co, Mn) dissolved from NCM cathodes.

Graphite Lithium‐Ion Cells: On the Beneficial Impact of Li2CO3 as Electrolyte Additive in NCM523 ∥ Graphite Lithium Ion Cells Under High‐Voltage Conditions (Adv. Energy Mater. 10/2021)

Graphite Lithium‐Ion Cells: On the Beneficial Impact of Li₂CO₃ as Electrolyte Additive in NCM523 ∥ Graphite Lithium Ion Cells Under High‐Voltage Conditions (Adv. Energy Mater. 10/2021)

STEM-EELS analysis of improved cycle life of lithium-ion cells with Al2O3-coated LiNi0.8Co0.1Mn0.1O2 cathode active material

Abstract The effect of Al2O3 on the cycle performance of 1 Ah-class LiNi0.8Co0.1Mn0.1O2 (NCM811)/graphite lithium-ion cells was studied by exploiting STEM-EELS analysis. Al2O3 was coated on NCM811 particles by a mechanochemical process with the coating from 0.5 to 3 wt%. All cells with the Al2O3-coated NCM811 cathode demonstrated better cycle performance than that with bare NCM811 independent of the coating amount. Analysis by STEM-EELS for the cells after 200 cycles revealed that the 3 wt%- Al2O3-coated NCM811 cathode had a solid solution layer of Al and NCM and also AlF3 on the surface of NCM particles. Moreover, the Al2O3-coated NCM811 cathode showed suppression of the growth of NiO-like layer on the Ni-based cathode material surface to degrade the cell performance. It is, therefore, concluded that the cycle performance was improved mainly by the solid solution and AlF3 layers to suppress the growth of the surface deposit and NiO-like layer formed by the reaction between NCM and electrolyte solution.

Case study of N-carboxyanhydrides in silicon-based lithium ion cells as a guideline for systematic electrolyte additive research

Incorporation of silicon into negative electrodes is pursued widely to increase the energy density of lithium ion batteries (LIBs). However, severe volume changes of silicon during (de)lithiation cause consumption of active lithium and electrolyte by continuous solid electrolyte interphase (SEI) formation, resulting in deterioration of cell performance. Electrolyte additives offer an unprecedented way to improve LIB cell performance by effective interphase formation. Here, we report a class of electrolyte additives based on substituted N -carboxyanhydrides ( N -CAs) designed to effectively tailor the SEI formed in LiNi 0.5 Co 0.2 Mn 0.3 O 2 ( NCM523) ∥ SiO x /C full cells, which are evaluated in pouch cells with application-relevant electrolyte per-cell capacity ratios. The working mechanism is elucidated systematically by use of complementary postmortem techniques, correlating cycling and storage performance data, gas formation, SEI composition, and electrolyte degradation. With successful additive functionalization, several N -CAs even outperform the state-of-the-art additive fluoroethylene carbonate in terms of capacity retention. Tailored design of N -carboxyanhydrides with synergistic functional moieties Significant performance improvements of NCM523 ∥ SiO x /graphite lithium ion cells Relevant performance data by a reduced ratio of electrolyte per cell capacity Analyses of electrolyte degradation and SEI and gas formation are a versatile approach Use of electrolyte additives is a straightforward approach to improve the performance of silicon-based lithium ion batteries (LIBs). Schmiegel et al. introduce a new class of electrolyte additives based on substituted N -carboxyanhydrides with synergistic functional moieties that significantly improve LIB cell performance by effective solid electrolyte interphase formation.

Understanding the Outstanding High‐Voltage Performance of NCM523||Graphite Lithium Ion Cells after Elimination of Ethylene Carbonate Solvent from Conventional Electrolyte

AbstractThe increase of specific energy of current Li ion batteries via further increase of the cell voltage, for example, to 4.5 V is typically accompanied by a sudden and rapid capacity fade, known as “rollover” failure. This failure is the result of Li dendrite formation triggered in the course of electrode cross‐talk, that is, dissolution of transition metals (TMs) from the cathode and deposition on the anode. It is shown herein, that the elimination of ethylene carbonate (EC) from a state‐of‐the‐art electrolyte, that is, from 1.0 m LiPF6 in a 3:7 mixture of EC and ethyl methyl carbonate prevents this failure in high‐voltage LiNi0.5Co0.2Mn0.3O2||graphite cells, even without any electrolyte additives. While the oxidative stability on the cathode side is similar in both electrolytes, visible by a decomposition plateau at 5.5 V versus Li|Li+ during charge, the anode side in the EC‐free electrolyte reveals significantly less TM deposits and Li metal dendrites compared to the EC‐based electrolyte. The beneficial effect of EC‐free electrolytes is related to a significantly increased amount of degraded LiPF6 species, which effectively trap dissolved TMs and suppress the effect of detrimental cross‐talk, finally realizing rollover‐free performance under high voltage conditions.

On the Beneficial Impact of Li2CO3 as Electrolyte Additive in NCM523 ∥ Graphite Lithium Ion Cells Under High‐Voltage Conditions

AbstractLithium ion battery cells operating at high‐voltage typically suffer from severe capacity fading, known as ‘rollover’ failure. Here, the beneficial impact of Li2CO3 as an electrolyte additive for state‐of‐the‐art carbonate‐based electrolytes, which significantly improves the cycling performance of NCM523 ∥ graphite full‐cells operated at 4.5 V is elucidated. LIB cells using the electrolyte stored at 20 °C (with or without Li2CO3 additive) suffer from severe capacity decay due to parasitic transition metal (TM) dissolution/deposition and subsequent Li metal dendrite growth on graphite. In contrast, NCM523 ∥ graphite cells using the Li2CO3‐containing electrolyte stored at 40 °C display significantly improved capacity retention. The underlying mechanism is successfully elucidated: The rollover failure is inhibited, as Li2CO3 reacts with LiPF6 at 40 °C to in situ form lithium difluorophosphate, and its decomposition products in turn act as ‘scavenging’ agents for TMs (Ni and Co), thus preventing TM deposition and Li metal formation on graphite.

Cell-to-cell variation of calendar aging and reversible self-discharge in 18650 nickel-rich, silicon–graphite lithium-ion cells

Abstract The origin of the voltage imbalance in lithium-ion battery packs is often linked to different self-discharge rates. However, there is no information regarding the variance of self-discharge currents nor possible implications for cells connected in series. In this study, initial cell parameter distributions were analyzed via differential voltage analysis (DVA) and variations of calendar aging and reversible self-discharge rates of 24 commercial 18650 nickel-rich/SiC cells were statistically evaluated. After initial characterization, all cells were stored for 10 month at 70% state of charge (SOC) at 25 °C, whereby last two weeks of storage were used for reversible self-discharge determination via voltage decay. The results obtained in this work reveal that initial capacity and impedance variations do not depend on electrode balancing, nor storage capabilities of NMC, silicon or graphite. Furthermore, the evaluation of calendar aging showed, that all cells exhibited constant but different aging rates. However, whereas the relative variance of the impedance remained almost the same, the relative variance of the capacity almost doubled during the storage. The average self-discharge current amounted to 4.2 µA (3.5 Ah cell) with the relative variance of 10%. Model based assessment of different self-discharge currents revealed almost no influence on the voltage imbalance in battery packs.

Improving the Cycling Performance of High-Voltage NMC111 || Graphite Lithium Ion Cells By an Effective Urea-Based Electrolyte Additive

In order to further increase the energy density of lithium ion batteries (LIBs), it is of utmost importance to develop advanced electrode materials in combination with suitable electrolytes, which deliver either higher capacities and/or can be operated at high cell voltage with sufficient cycling stability. Here, we introduce (1H-imidazol-1-yl)(morpholino)methanone (MUI) as a cathode electrolyte interphase (CEI) forming electrolyte additive for LiNi1/3Co1/3Mn1/3O2 (NMC111) || graphite cells operated at high cell voltage of up to 4.6 V. The addition of MUI to the carbonate-based reference electrolyte leads to a superior cycling stability in comparison to those with the pure reference electrolyte. The working mechanism of MUI is comprehensively elucidated with various ex situ analytical techniques. A reduction of MUI at the graphite anode is observed starting at a potential of ≈0.9 V vs. Li/Li+, resulting in a slightly higher kinetic impairment of lithium ion intercalation/deintercalation into/from graphite. To prevent the unfavorable reduction of MUI at the graphite anode, Li4Ti5O12 (LTO)-based composite anodes are also used to analyze the function of the additive at the cathode surface. We can clearly confirm that a protective film at the cathode surface is formed, helping to suppress parasitic side reactions at the cathode/electrolyte interface during long-term cycling.

Investigation of the Storage Behavior of Shredded Lithium-Ion Batteries from Electric Vehicles for Recycling Purposes.

Shredding of the cells is often the first step in lithium-ion battery (LIB) recycling. Thus, LiNi1/3 Mn1/3 Co1/3 O2 (NMC)/graphite lithium-ion cells from a field-tested electric vehicle were shredded and transferred to tinplate or plastic storage containers. The formation of hazardous compounds within, and being released from, these containers was monitored over 20 months. The tinplate cans underwent fast corrosion as a result of either residual charge in the active battery material, which could not fully be discharged because of contact loss to the current collector, or redox reactions between the tinplate surface and metal parts of the shredded material. The headspace compositions of the containers were investigated at room temperature and 150 °C using headspace-gas chromatography-mass spectrometry (HS-GC-MS). Samples of the waste material were also collected using microwave-assisted extraction and the extracts were analyzed over a period of 20 months using ion chromatography-electrospray ionization-mass spectrometry (IC-ESI-MS). LiPF6 was identified as a conducting salt, whereas dimethyl carbonate, ethyl methyl carbonate, and ethylene carbonate were the main solvent components. Cyclohexylbenzene was also detected, which is an additive for overcharge protection. Diethyl carbonate, fluoride, difluorophosphate and several ionic and non-ionic alkyl (fluoro)phosphates were also identified. Importantly, dimethyl fluorophosphate (DMFP) and diethyl fluorophosphate (DEFP) were quantified using HS-GC-MS through the use of an internal standard. DMFP, DEFP, and related compounds are known as chemical warfare agents, and the presence of these materials is of great interest. In the case of this study, these hazardous materials are present but in manageable low concentrations. Nonetheless, the presence of such compounds and their potential release during an accident that may occur during shredding or recycling of large amounts of LIB waste should be considered.

Effect of Rapid Charging or Discharging on Performance of High Energy Li- and Mn-Rich NMC Full Cells

It is commonly accepted that rapid charging shortens the cycle life of full cells much more than rapid discharging when equal magnitudes of cell current are applied. The reason for this behavior is the low operating voltage of the graphite anode once it is partially lithiated during charging. Polarization below the redox voltage of Li metal triggers Li plating on the anode surface; the Li deposits then react with the electrolyte irreversibly leading to capacity loss and impedance rise. The goal of this presentation is to show how high charge and discharge currents affect the cycle life of Li-ion full cells that contain a Li- and Mn-rich transition metal oxide (LMR-NMC) [1]. Asymmetric cycle life ageing using different charge and discharge currents is the key experimental procedure: all cells were subjected asymmetrically to a low charge and a high discharge current - or vice versa. The low current corresponds to approximately a C/14 rate for a symmetrically cycled cell. Three different current levels were chosen for the corresponding rapid charge or discharge, corresponding to a rate of C/3, 1C, and 3C. All tests were conducted in 2032-sized coin cells (1.6 cm2 area electrodes) at 30°C. The positive electrodes contained Li1.2Mn0.55Ni0.15Co0.1O2, and the negative electrodes contained graphite as the active component. Data on full cells containing Li4Ti5O12 (LTO)-based negative electrodes were also obtained during our study.Figures 1A and 1B show the asymmetric cycle-life performance of six different tests: rapid charging and rapid discharging, each with three different current levels during the high current half-cycles. After every eight rapid cycles, two low current cycles are implemented to track the mobile Li inventory of the cells. Significant differences between the three current levels are seen for the fast discharge tests (bottom), while relatively small differences are seen for the fast charge tests (top). Regardless of fast charging or discharging, the low current capacity decreases continuously, almost linearly, as all cells continue to age – indicating a continuous loss of mobile Li in the cells, which is believed to result from Li trapping in the negative electrode SEI. The decrease in discharge capacity with increasing current is related to the slow Li-intercalation kinetics and the hysteretic behavior intrinsic to the LMR-NMC. The abrupt drop in the high-current capacity that is encountered during fast discharging after ~30 cycles is due to a significant increase in cell impedance, and not due to excessive Li trapping, as the low current capacity is nearly identical for the two cases. In summary, the findings show that, contrary to common belief, the effect of rapid discharging on the cycle life can be more detrimental than that of rapid charging for some cycling conditions and cell chemistries, such as the ones described in this study. Reference: [1] Understanding Long-Term Cycling Performance of Li1.2Ni0.15Mn0.55Co0.1O2–Graphite Lithium-Ion Cells, Y. Li, M. Bettge, B. Polzin, Y. Zhu, M. Balasubramanian, D. P. Abraham, J. Electrochem. Soc. 160 (5) A3006-A3019 (2013) Figure 1

Influence of temperature and upper cut-off voltage on the formation of lithium-ion cells

The influences of temperature on the formation losses and subsequent electrical performance of Lix(Ni1/3Co1/3Mn1/3)yO2 (NCM)/graphite lithium-ion cells were investigated. It was shown that the total capacity loss during formation of a full cell at 25 °C consists of losses on the positive and negative electrode to about one half each. The losses of the negative electrode (ca. 10%) are due to solid electrolyte interphase (SEI) formation on graphite but are masked by the losses of the positive side (ca. 20%) which are mainly caused by a kinetic inhibition of NCM and are theoretically reversible when the cell is discharged to very low potentials. The total loss of a full cell fits with the loss of the positive electrode. With increased temperature the ratio of losses on positive and negative electrode decreases as the diffusion coefficient of lithium in NCM increases. In total, an elevated formation temperature leads to increased irreversible losses on both electrodes and significantly lower cell performance of graphite. The upper cut-off voltage has an influence on the positive electrode formation losses in a reversible manner. The constituents of the SEI identified via the combination of XPS and FTIR are mainly RCH2OCO2Li, RCOOLi and LiF for the outer SEI and mainly Li2CO3, Li2O and LiF for the inner SEI.

Surface study of electrodes after long-term cycling in Li1.2Ni0.15Mn0.55Co0.1O2–graphite lithium-ion cells

Lithium- and manganese-rich layered-oxides (LMR-NMC) have potential application for electric vehicles due to the high energy density. Electrochemical measurements and ex-situ surface analysis of cells containing Li1.2Ni0.15Mn0.55Co0.1O2-based positive electrodes and graphite-based negative electrodes has been conducted. Electrochemical cycling reveals significant capacity fade and voltage decrease after 1500 cycles. Ex-situ surface analysis including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) were conducted for cells with long term cycling at 30 °C. The surface analysis supports the formation of surface film on both cathodes and anodes after formation cycling. Upon long term cycling, both the cathode and anode surface films become thicker although more significant changes appear to be occurring to the anode. Fresh LMR-NMC powder has been stored in the presence of electrolyte, 1.2 M LiPF6 in EC/EMC 3:7, at 85 °C for 7 days. The surface film on the LMR-NMC powder after storage is similar to the surface film after cycling. The presence of the LMR-NMC powder inhibits the thermal decomposition of the electrolyte.

Understanding Long-Term Cycling Performance of Li1.2Ni0.15Mn0.55Co0.1O2–Graphite Lithium-Ion Cells

Lithium-ion cells containing lithium– and manganese– rich layered-oxides (LMR-NMC) have gained significant attention in recent years because of their ability to deliver high energy densities. In this article we report on a comprehensive performance and degradation study of cells, containing Li1.2Ni0.15Mn0.55Co0.1O2–based positive electrodes and graphite–based negative electrodes, on extended cycling. In addition to electrochemical measurements on full cells, characterization data on harvested electrodes by techniques that include scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), Raman spectroscopy, and secondary ion mass spectrometry (SIMS) are discussed. Our data show that cell capacity fade mainly results from lithium trapping in the solid electrolyte interphase (SEI) of the negative electrode. In addition, cell impedance rise and voltage fade mainly arise at the positive electrode and result from degradation processes in its oxide and carbon constituents. Processes that include the accumulation of transition metal elements at the negative electrode, and increasing misalignment of electrode capacity windows on extended cycling, also have a deleterious effect on cell performance. Identifying sources of performance degradation has enabled strategies to extend cell life, which include improved cell fabrication protocols, positive electrode coatings, and bifunctional electrolyte additives.

LiPF 6 and lithium oxalyldifluoroborate blend salts electrolyte for LiFePO 4/artificial graphite lithium-ion cells

The electrochemical behaviors of LiPF 6 and lithium oxalyldifluoroborate (LiODFB) blend salts in ethylene carbonate + propylene carbonate + dimethyl carbonate (EC + PC + DMC, 1:1:3, v/v/v) for LiFePO 4/artificial graphite (AG) lithium-ion cells have been investigated in this work. It is demonstrated by conductivity test that LiPF 6 and LiODFB blend salts electrolytes have superior conductivity to pure LiODFB-based electrolyte. The results show that the performances of LiFePO 4/Li half cells with LiPF 6 and LiODFB blend salts electrolytes are inferior to pure LiPF 6-based electrolyte, the capacity and cycling efficiency of Li/AG half cells are distinctly improved by blend salts electrolytes, and the optimum LiODFB/LiPF 6 molar ratio is around 4:1. A reduction peak is observed around 1.5 V in LiODFB containing electrolyte systems by means of CV tests for Li/AG cells. Excellent capacity and cycling performance are obtained on LiFePO 4/AG 063048-type cells tests with blend salts electrolytes. A plateau near 1.7–2.0 V is shown in electrolytes containing LiODFB salt, and extends with increasing LiODFB concentration in charge curve of LiFePO 4/AG cells. At 1 C discharge current rate, the initial discharge capacity of 063048-type cell with the optimum electrolyte is 376.0 mAh, and the capacity retention is 90.8% after 100 cycles at 25 °C. When at 65 °C, the capacity and capacity retention after 100 cycles are 351.3 mAh and 88.7%, respectively. The performances of LiFePO 4/AG cells are remarkably improved by blending LiODFB and LiPF 6 salts compared to those of pure LiPF 6-based electrolyte system, especially at elevated temperature to 65 °C.

Lithium oxalyldifluoroborate/carbonate electrolytes for LiFePO 4/artificial graphite lithium-ion cells

The electrolytes based on lithium oxalyldifluoroborate (LiODFB) and carbonates have been systematically investigated for LiFePO 4/artificial graphite (AG) cells, by ionic conductivity test and various electrochemical tests, such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge–discharge test. The conductivity of nine electrolytes as a function of solvent composition and LiODFB salt concentration has been studied. The coulombic efficiency of LiFePO 4/Li and AG/Li half cells with these electrolytes have also been compared. The results show that 1 M LiODFB EC/PC/DMC (1:1:3, v/v) electrolyte has a relatively higher conductivity (8.25 mS cm −1) at 25 °C, with high coulombic efficiency, good kinetics characteristics and low interface resistance. With 1 M LiODFB EC/PC/DMC (1:1:3, v/v) electrolyte, LiFePO 4/AG cells exhibit excellent capacity retention ∼92% and ∼88% after 100 cycles at 25 °C and at elevated temperatures up to 65 °C, respectively; The LiFePO 4/AG cells also have good rate capability, the discharge capacity is 324.8 mAh at 4 C, which is about 89% of the discharge capacity at 0.5 C. However, at −10 °C, the capacity is relatively lower. Compared with 1 M LiPF 6 EC/PC/DMC (1:1:3, v/v), LiFePO 4/AG cells with 1 M LiODFB EC/PC/DMC (1:1:3, v/v) exhibited better capacity utilization at both room temperature and 65 °C. The capacity retention of the cells with LiODFB-based electrolyte was much higher than that of LiPF 6-based electrolyte at 65 °C, while the capacity retention and the rate capacity of the cells is closed to that of LiPF 6-based electrolyte at 25 °C. In summary, 1 M LiODFB EC/PC/DMC (1:1:3, v/v) is a promising electrolyte for LiFePO 4/AG cells.

The development of low cost LiFePO 4-based high power lithium-ion batteries

Pouch type LiFePO 4-natural graphite lithium-ion cells were cycled at constant current with periodic pulse-power testing in several different configurations. Components were analyzed after cycling with electrochemical, Raman and TEM techniques to determine capacity fade mechanisms. The cells with carbon-coated current collectors in the cathode and LiBOB-salt electrolyte showed the best performance stability. In many cases, iron species were detected on the anodes removed from cells with both TEM and Raman spectroscopy. The LiFePO 4 electrodes showed unchanged capacity suggesting that the iron is migrating in small quantities and is acting as a catalyst to destabilize the anode SEI in these cells.