Quantitative determination of solid electrolyte interphase and cathode electrolyte interphase homogeneity in multi-layer lithium ion cells

This X-ray photoelectron spectroscopy study of three multi-layer lithium ion cells is based on a total of 106 measurements. A formula is derived utilizing the mean relative standard deviation (mean rel. SD) of the surface composition to illustrate the solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) homogeneity within certain areas. Within an area of 1 cm², the mean rel. SD is 8% and 9% for negative and positive electrodes. Within an area of 5 cm², these values increase to 24% and 14%, indicating reduced homogeneity especially for negative electrodes. Negative electrode samples from the electrode sheet edge and outer sheet have no different homogeneity. Positive electrode samples, instead, have different composition at these positions. Sample washing increases homogeneity but also removed organic SEI components. Homogeneity between different cells is as similar as within one cell. Varying thicknesses of the organic SEI layer are identified as main factor for reduced SEI homogeneity (mean rel. SD 45%). The inorganic SEI has comparable thickness even between different cells (mean rel. SD 7%). This might indicate the importance of the inorganic layer for cell performance. The calculated CEI thicknesses are ∼0 nm, indicating only scattered surface reactions and no real passivation layer

Comparative X-ray Photoelectron Spectroscopy Study of the SEI and CEI in Three Different Lithium Ion Cell Formats

The solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) of three lithium ion cell formats, i.e., coin, lab-scale pouch and multi-layer pouch, are compared. Half the cells are additionally dried prior to electrolyte filling and cycling. The highest water content per cell, determined by Karl Fischer titration, is found for lab-scale pouch cells due to their disadvantageous ratio of cell housing area to electrode area. The water content influences the performance during electrochemical formation as well as the impedance. This is linked to increasing lithium fluoride concentration, as determined by X-ray photoelectron spectroscopy. For dried cells, this is not the case because there is less conducting salt hydrolysis. The CEI thickness decreases for dried pouch cells, while the organic SEI thickness increases in all cell formats for dried cells. It is concluded that the initial thickness of the porous organic SEI depends on the insulation of the dense inorganic SEI close to the electrode surface. Organic species are more likely to contribute to negative electrode passivation when the extent of conducting salt hydrolysis is low. For coin cells, the presence of atmospheric gases during formation results in thicker SEI and CEI, no matter whether cells are additionally dried

Unravelling charge/discharge and capacity fading mechanisms in dual-graphite battery cells using an electron inventory model

The dual-ion battery (DIB) and a subtype thereof, the dual-graphite battery (DGB), are considered as promising alternative options for stationary energy storage applications. Here, we show that not only the working principle of DGBs is fundamentally different from ion-transfer cells, such as the lithium ion battery (LIB), but the capacity fading mechanisms as well. In order to compare the charge/discharge mechanism and aging processes of LIBs and DIBs, which are associated with loss/change of the active species content, a unified “electron inventory model” applicable for both battery systems is introduced and comprehensively explained. This model regards all electron-consuming or -donating reactions in the cell at the respective electrodes. Using the model, two predominant aging variants of DGBs can be distinguished. Both of these originate from disparate (parasitic) electron consumption/donation at the negative or positive electrode, respectively. However, there is an “electron balance” or “electron charge neutrality” between the electrodes, which intrinsically does not allow this difference: This is a consequence of redox reactions that must take place simultaneously at both electrodes in order to guarantee electronic charge neutrality. For example, if electron-consuming reactions occur at the negative electrode that consume lithium cations, the positive electrode is forced to intercalate additional anions for charge compensation, i.e. to ensure charge neutrality in the electrolyte. These anions are irreversibly trapped in the positive electrode and can no longer contribute to reversible charge/discharge reactions. Suitable countermeasures including the pre-lithiation of the negative electrode are presented in this work

Al‐doped ZnO‐Coated LiNi1/3Mn1/3Co1/3O2 Powder Electrodes: The Effect of a Coating Layer on The Structural and Chemical Stability of The Electrode / Electrolyte Interface

Abstract LiNi1/3Mn1/3Co1/3O2 (NMC‐111) is one of the most popular cathode materials in Li‐ion batteries. However, chemical and structural instabilities of the cathode/electrolyte interface at high charge cut‐off voltages cause capacity fading. Surface modifications using metal oxides are promising candidates to suppress capacity fading. Here a systematic study on the degradation mechanism of an uncoated NMC‐111 powder electrode is presented. Moreover, the effect of an Al‐doped ZnO (Al:ZnO) coating layer on the structural and chemical stabilities of NMC‐111 electrode cycled at high charge cut‐off voltages is analyzed using X‐ray photoelectron spectroscopy, scanning electron microscopy and analytical transmission electron microscopy as well as electrochemical testing. The coating is applied to commercial NMC‐111 powder using a microwave‐assisted sol‐gel synthesis method. In the case of uncoated NMC‐111 electrodes, pitting corrosion due to hydrofluoric acid attacking the electrode surface, cation mixing, and an irreversible phase transformation from a trigonal layered to a rock‐salt phase occurs, causing capacity fading. While, in the case of Al:ZnO – coated NMC‐111 electrodes, pitting corrosion, cation mixing, and the irreversible phase transformation are mitigated. Therefore, the capacity retention and rate capability are improved as the coating layer protects the electrode surface from the direct electrolyte exposure

Al2O3 protective coating on silicon thin film electrodes and its effect on the aging mechanisms of lithium metal and lithium ion cells

In this work, an investigation of the effect of Al2O3-coating on the aging mechanisms of silicon anode thin films in lithium metal and lithium ion cells is presented. Aging mechanisms, namely: loss of lithium inventory, loss of silicon active material and loss of utilizable capacity due to an increase of cell resistance were determined for both, Li||Si and Si||LiFePO4 cells. Al2O3-coating was shown to be an effective strategy to reduce the loss of lithium inventory, while having a marginal effect on decreasing the loss of silicon active material. Indeed, in case of Si||LiFePO4 cells, where fading is governed by loss of lithium inventory, a 5 nm Al2O3-coating leads to a significant reduction (-64%) of the capacity fade per cycle. On the contrary, in case of Li||Si, where the aging mechanism is governed by the loss of active material, Al2O3-coated and uncoated silicon showed comparable tendencies regarding the capacity fade per cycle. It emerges, also, that loss of silicon active material and loss of lithium inventory are independent of each other. This indicates that the main contribution of loss of lithium inventory is not the lithium trapped in electrically insulated silicon, but rather lithium consumed in the ongoing SEI formation. Al2O3-coating could reduce the latter due the insulating nature of the coating. Ex situ investigations of the SEI by means of X-ray photoelectron spectroscopy confirmed a decrease in solvent decomposition in presence of the Al2O3-coating

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

The 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

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

On the Beneficial Impact of Li 2 CO 3 as Electrolyte Additive in NCM523 ∥ Graphite Lithium Ion Cells Under High‐Voltage Conditions

Lithium 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