Investigating the Reversibility of Structural Modifications of Li_<i>x</i>Ni_<i>y</i>Mn_<i>z</i>Co_{1–<i>y</i>–<i>z</i>}O₂ Cathode Materials during Initial Charge/Discharge, at Multiple Length Scales

In this work, we investigate the structural modifications occurring at the bulk, subsurface, and surface scales of Li_<i>x</i>Ni_<i>y</i>Mn_<i>z</i>Co_{1–<i>y</i>–<i>z</i>}O₂ (NMC; <i>y</i>, <i>z</i> = 0.8, 0.1 and 0.4, 0.3, respectively) cathode materials during the initial charge/discharge. Various analytical tools, such as X-ray diffraction, selected-area electron diffraction, electron energy-loss spectroscopy, and high-resolution electron microscopy, are used to examine the structural properties of the NMC cathode materials at the three different scales. Cutoff voltages of 4.3 and 4.8 V are applied during the electrochemical tests as the normal and extreme conditions, respectively. The high-Ni content NMC cathode materials exhibit unusual behaviors, which deviate from the general redox reactions during the charge or discharge. The transition metal (TM) ions in the high-Ni content NMC cathode materials, which are mostly Ni ions, are reduced at 4.8 V, even though TMs are usually oxidized to maintain charge neutrality upon the removal of Li. It was found that any changes in the crystallographic and electronic structures are mostly reversible down to the subsurface scale, despite the unexpected reduction of Ni ions. However, after the discharge, traces of the phase transitions remain at the edges of the NMC cathode materials at the scale of a few nanometers (i.e., surface scale). This study demonstrates that the structural modifications in NMC cathode materials are induced by charge as well as discharge, at multiple length scales. These changes are nearly reversible after the first cycle, except at the edges of the samples, which should be avoided because these highly localized changes can initiate battery degradation

Identification and Catalysis of the Potential-Limiting Step in Lithium-Sulfur Batteries

The Li-S chemistry is thermodynamically promising for high-density energy storage but kinetically challenging. Over the past few years, many catalyst materials have been developed to improve the performance of Li-S batteries and their catalytic role has been increasingly accepted. However, the classic catalytic behavior, i.e., reduction of reaction barrier, has not been clearly observed. Crucial mechanistic questions, including what specific step is limiting the reaction rate, whether/how it can be catalyzed, and how the catalysis is sustained after the catalyst surface is covered by solid products, remain unanswered. Herein, we report the first identification of the potential-limiting step of Li-S batteries operating under lean electrolyte conditions and its catalysis that conforms to classic catalysis principles, where the catalyst lowers the kinetic barrier of the potential-limiting step and accelerates the reaction without affecting the product composition. After carefully examining the electrochemistry under lean electrolyte conditions, we update the pathway of the Li-S battery reaction: S8 solid is first reduced to Li2S8 and Li2S4 molecular species sequentially; the following reduction of Li2S4 to a Li2S2–Li2S solid with an almost constant ratio of 1:4 is the potential-limiting step; the previously believed Li2S2-to-Li2S solid–solid conversion does not occur; and the recharging reaction is relatively fast. We further demonstrate that supported cobalt phthalocyanine molecules can effectively catalyze the potential-limiting step. After Li2S2/Li2S buries the active sites, it can self-catalyze the reaction and continue driving the discharging process

Investigating Local Degradation and Thermal Stability of Charged Nickel-Based Cathode Materials through Real-Time Electron Microscopy

In this work, we take advantage of in situ transmission electron microscopy (TEM) to investigate thermally induced decomposition of the surface of Li_<i>x</i>Ni_0.8Co_0.15Al_0.05O₂ (NCA) cathode materials that have been subjected to different states of charge (SOC). While uncharged NCA is stable up to 400 °C, significant changes occur in charged NCA with increasing temperature. These include the development of surface porosity and changes in the oxygen K-edge electron energy loss spectra, with pre-edge peaks shifting to higher energy losses. These changes are closely related to O₂ gas released from the structure, as well as to phase changes of NCA from the layered structure to the disordered spinel structure, and finally to the rock-salt structure. Although the temperatures where these changes initiate depend strongly on the state of charge, there also exist significant variations among particles with the same state of charge. Notably, when NCA is charged to <i>x</i> = 0.33 (the charge state that is the practical upper limit voltage in most applications), the surfaces of some particles undergo morphological and oxygen K-edge changes even at temperatures below 100 °C, a temperature that electronic devices containing lithium ion batteries (LIB) can possibly see during normal operation. Those particles that experience these changes are likely to be extremely unstable and may trigger thermal runaway at much lower temperatures than would be usually expected. These results demonstrate that in situ heating experiments are a unique tool not only to study the general thermal behavior of cathode materials but also to explore particle-to-particle variations, which are sometimes of critical importance in understanding the performance of the overall system

Degradation of Lithium Iron Phosphate Sulfide Solid-State Batteries by Conductive Interfaces

The superionic solid-state argyrodite electrolyte Li6PS5Br can improve lithium and lithium-ion batteries’ safety and energy density. Despite many reports validating the conductivity of this electrolyte, it still suffers from passivating electrode degradation mechanisms. At first analysis, lithium iron phosphate (LFP) should be more thermodynamically stable in contact with sulfide electrolytes. However, without substantial improvements to interfacial engineering, we find that LFP is not inherently stable against Li6PS5Br. We hypothesize argyrodite oxidation favorably competes with LFP’s delithiation, insulating the electrolyte–electrode interface and causing large overpotential losses. We show that compared to LiNixMnyCozO2, LFP has no actual electrochemical stability advantage despite operating at a lower voltage. We utilize tender energy XAS and XPS to show that chemical reactions occur between LFP and the Li6PS5Br solid electrolyte and these reactions are exacerbated by cycling. We also show that electrochemical degradation occurs at the interface between the solid electrolyte ion conductor and any electron conductor, namely, the active material and carbon additives. We further demonstrate that LiNbO3 cathode coatings on LFP can delay electrochemical degradation by electronically insulating the LFP–sulfide electrolyte interface but not prevent its occurrence at the carbon–electrolyte interface

Surface Redox Pseudocapacitance of Partially Oxidized Titanium Carbide MXene in Water-in-Salt Electrolyte

Achieving pseudocapacitive intercalation in MXenes with neutral aqueous electrolytes and driving reversible redox reactions is scientifically appealing and practically useful. Here, we report that the partial oxidation of MXene intensifies pseudocapacitive Li+ intercalation into Ti3C2Tx MXene from neutral water-in-salt electrolytes. An in situ X-ray absorption near-edge structure analysis shows that the Ti oxidation state changes during the Li+ intercalation, indicating the presence of a surface redox reaction. The Ti oxidation/reduction is further confirmed by an in situ extended X-ray absorption fine structure analysis, which shows a reversible contraction/expansion of the Ti–C interatomic distance. The intensified Li+ pseudocapacitive intercalation can be explained by the higher oxidation state of Ti at the open circuit potential. This work demonstrates the possibility of tuning the pseudocapacitive intercalation by adjusting the initial oxidation state of the transition metal on the MXene and offers a facile way to enhance the pseudocapacitance of various MXenes

Oxygen-Release-Related Thermal Stability and Decomposition Pathways of Li_<i>x</i>Ni_0.5Mn_1.5O₄ Cathode Materials

The thermal stability of charged cathode materials is one of the critical properties affecting the safety characteristics of lithium-ion batteries. New findings on the thermal-stability and thermal-decomposition pathways related to the oxygen release are discovered for the high-voltage spinel Li_<i>x</i>Ni_0.5Mn_1.5O₄ (LNMO) with ordered (<i>o</i>-) and disordered (<i>d</i>-) structures at the fully delithiated (charged) state using a combination of in situ time-resolved X-ray diffraction (TR-XRD) coupled with mass spectroscopy (MS) and X-ray absorption spectroscopy (XAS) during heating. Both <i>o</i>- and <i>d</i>- Li_<i>x</i>Ni_0.5Mn_1.5O₄, at their fully charged states, start oxygen-releasing structural changes at temperatures below 300 °C, which is in sharp contrast to the good thermal stability of the 4V-spinel Li_<i>x</i>Mn₂O₄ with no oxygen being released up to 375 °C. This is mainly caused by the presence of Ni⁴⁺ in LNMO, which undergoes dramatic reduction during the thermal decomposition. In addition, charged <i>o</i>-LNMO shows better thermal stability than the <i>d</i>-LNMO counterpart, due to the Ni/Mn ordering and smaller amount of the rock-salt impurity phase in <i>o</i>-LNMO. Two newly identified thermal-decomposition pathways from the initial Li_<i>x</i>Ni_0.5Mn_1.5O₄ spinel to the final NiMn₂O₄-type spinel structure with and without the intermediate phases (NiMnO₃ and α-Mn₂O₃) are found to play key roles in thermal stability and oxygen release of LNMO during thermal decomposition

Quantification of Honeycomb Number-Type Stacking Faults: Application to Na₃Ni₂BiO₆ Cathodes for Na-Ion Batteries

Ordered and disordered samples of honeycomb-lattice Na3Ni2BiO6 were investigated as cathodes for Na-ion batteries, and it was determined that the ordered sample exhibits better electrochemical performance, with a specific capacity of 104 mA h/g delivered at plateaus of 3.5 and 3.2 V (vs Na+/Na) with minimal capacity fade during extended cycling. Advanced imaging and diffraction investigations showed that the primary difference between the ordered and disordered samples is the amount of number-type stacking faults associated with the three possible centering choices for each honeycomb layer. A labeling scheme for assigning the number position of honeycomb layers is described, and it is shown that the translational shift vectors between layers provide the simplest method for classifying different repeat patterns. It is demonstrated that the number position of honeycomb layers can be directly determined in high-angle annular dark-field scanning transmission electron microscopy (STEM-HAADF) imaging studies. By the use of fault models derived from STEM studies, it is shown that both the sharp, symmetric subcell peaks and the broad, asymmetric superstructure peaks in powder diffraction patterns can be quantitatively modeled. About 20% of the layers in the ordered monoclinic sample are faulted in a nonrandom manner, while the disordered sample stacking is not fully random but instead contains about 4% monoclinic order. Furthermore, it is shown that the ordered sample has a series of higher-order superstructure peaks associated with 6-, 9-, 12-, and 15-layer periods whose existence is transiently driven by the presence of long-range strain that is an inherent consequence of the synthesis mechanism revealed through the present diffraction and imaging studies. This strain is closely associated with a monoclinic shear that can be directly calculated from cell lattice parameters and is strongly correlated with the degree of ordering in the samples. The present results are broadly applicable to other honeycomb-lattice systems, including Li2MnO3 and related Li-excess cathode compositions

Quantification of Honeycomb Number-Type Stacking Faults: Application to Na₃Ni₂BiO₆ Cathodes for Na-Ion Batteries

Ordered and disordered samples of honeycomb-lattice Na₃Ni₂BiO₆ were investigated as cathodes for Na-ion batteries, and it was determined that the ordered sample exhibits better electrochemical performance, with a specific capacity of 104 mA h/g delivered at plateaus of 3.5 and 3.2 V (vs Na⁺/Na) with minimal capacity fade during extended cycling. Advanced imaging and diffraction investigations showed that the primary difference between the ordered and disordered samples is the amount of number-type stacking faults associated with the three possible centering choices for each honeycomb layer. A labeling scheme for assigning the number position of honeycomb layers is described, and it is shown that the translational shift vectors between layers provide the simplest method for classifying different repeat patterns. It is demonstrated that the number position of honeycomb layers can be directly determined in high-angle annular dark-field scanning transmission electron microscopy (STEM-HAADF) imaging studies. By the use of fault models derived from STEM studies, it is shown that both the sharp, symmetric subcell peaks and the broad, asymmetric superstructure peaks in powder diffraction patterns can be quantitatively modeled. About 20% of the layers in the ordered monoclinic sample are faulted in a nonrandom manner, while the disordered sample stacking is not fully random but instead contains about 4% monoclinic order. Furthermore, it is shown that the ordered sample has a series of higher-order superstructure peaks associated with 6-, 9-, 12-, and 15-layer periods whose existence is transiently driven by the presence of long-range strain that is an inherent consequence of the synthesis mechanism revealed through the present diffraction and imaging studies. This strain is closely associated with a monoclinic shear that can be directly calculated from cell lattice parameters and is strongly correlated with the degree of ordering in the samples. The present results are broadly applicable to other honeycomb-lattice systems, including Li₂MnO₃ and related Li-excess cathode compositions

Using Real-Time Electron Microscopy To Explore the Effects of Transition-Metal Composition on the Local Thermal Stability in Charged Li_<i>x</i>Ni_<i>y</i>Mn_<i>z</i>Co_{1–<i>y</i>–<i>z</i>}O₂ Cathode Materials

In this work, we use <i>in situ</i> transmission electron microscopy (TEM) to investigate the thermal decomposition that occurs at the surface of charged Li_<i>x</i>Ni_<i>y</i>Mn_<i>z</i>Co_{1–<i>y</i>–<i>z</i>}O₂ (NMC) cathode materials of different composition (with <i>y</i>, <i>z</i> = 0.8, 0.1, and 0.6, 0.2, and 0.4,and 0.3), after they have been charged to their practical upper limit voltage (4.3 V). By heating these materials inside the TEM, we are able to directly characterize near surface changes in both their electronic structure (using electron energy loss spectroscopy) and crystal structure and morphology (using electron diffraction and bright-field imaging). The most Ni-rich material (<i>y</i>, <i>z</i> = 0.8, 0.1) is found to be thermally unstable at significantly lower temperatures than the other compositionsthis is manifested by changes in both the electronic structure and the onset of phase transitions at temperatures as low as 100 °C. Electron energy loss spectroscopy indicates that (i) the thermally induced reduction of Ni ions drives these changes, and (ii) this is exacerbated by the presence of an additional redox reaction that occurs at 4.2 V in the <i>y</i>, <i>z</i> = 0.8, 0.1 material. Exploration of individual particles shows that there are substantial variations in the onset temperatures and overall extent of these changes. Of the compositions studied, the composition of <i>y</i>, <i>z</i> = 0.6, 0.2 has the optimal combination of high energy density and reasonable thermal stability. The observations herein demonstrate that real-time electron microscopy provide direct insight into the changes that occur in cathode materials with temperature, allowing optimization of different alloy concentrations to maximize overall performance

Quantification of Honeycomb Number-Type Stacking Faults: Application to Na₃Ni₂BiO₆ Cathodes for Na-Ion Batteries

Ordered and disordered samples of honeycomb-lattice Na₃Ni₂BiO₆ were investigated as cathodes for Na-ion batteries, and it was determined that the ordered sample exhibits better electrochemical performance, with a specific capacity of 104 mA h/g delivered at plateaus of 3.5 and 3.2 V (vs Na⁺/Na) with minimal capacity fade during extended cycling. Advanced imaging and diffraction investigations showed that the primary difference between the ordered and disordered samples is the amount of number-type stacking faults associated with the three possible centering choices for each honeycomb layer. A labeling scheme for assigning the number position of honeycomb layers is described, and it is shown that the translational shift vectors between layers provide the simplest method for classifying different repeat patterns. It is demonstrated that the number position of honeycomb layers can be directly determined in high-angle annular dark-field scanning transmission electron microscopy (STEM-HAADF) imaging studies. By the use of fault models derived from STEM studies, it is shown that both the sharp, symmetric subcell peaks and the broad, asymmetric superstructure peaks in powder diffraction patterns can be quantitatively modeled. About 20% of the layers in the ordered monoclinic sample are faulted in a nonrandom manner, while the disordered sample stacking is not fully random but instead contains about 4% monoclinic order. Furthermore, it is shown that the ordered sample has a series of higher-order superstructure peaks associated with 6-, 9-, 12-, and 15-layer periods whose existence is transiently driven by the presence of long-range strain that is an inherent consequence of the synthesis mechanism revealed through the present diffraction and imaging studies. This strain is closely associated with a monoclinic shear that can be directly calculated from cell lattice parameters and is strongly correlated with the degree of ordering in the samples. The present results are broadly applicable to other honeycomb-lattice systems, including Li₂MnO₃ and related Li-excess cathode compositions