Lithium-rich Oxide Cathode Research Articles

Phase segregation and nanoconfined fluid O2 in a lithium-rich oxide cathode

Lithium-rich oxide cathodes lose energy density during cycling due to atomic disordering and nanoscale structural rearrangements, which are both challenging to characterize. Here we resolve the kinetics and thermodynamics of these processes in an exemplar layered Li-rich (Li1.2–xMn0.8O2) cathode using a combined approach of ab initio molecular dynamics and cluster expansion-based Monte Carlo simulations. We identify a kinetically accessible and thermodynamically favourable mechanism to form O2 molecules in the bulk, involving Mn migration and driven by interlayer oxygen dimerization. At the top of charge, the bulk structure locally phase segregates into MnO2-rich regions and Mn-deficient nanovoids, which contain O2 molecules as a nanoconfined fluid. These nanovoids are connected in a percolating network, potentially allowing long-range oxygen transport and linking bulk O2 formation to surface O2 loss. These insights highlight the importance of developing strategies to kinetically stabilize the bulk structure of Li-rich O-redox cathodes to maintain their high energy densities.

The role of PrPO4 in improving the electrochemical performance of cobalt-free Lithium-rich oxide cathode

The utilization of lithium-rich layered oxides (LLOs) materials shows promising possibilities for the innovation of high-capacity Li-ion batteries with enhanced energy density. However, their practical applications are limited by continuous voltage decay, inferior cycling stability, and poor rate capability. In this work, a novel doping strategy is proposed, where PrPO4 is introduced during the high-temperature solid-state reaction to prepare Co-free Li-rich layered oxides. It is found that other than as a dopant, PrPO4 can activate Li2MnO3 to form well-distributed Pr0.96Mn0.982O3 and Li3PO4 in LLOs particles, which availably suppresses the phase transition. The obtained material increases the initial coulombic efficiency (ICE) from 75.8% to 84.1%, and improves capacity retention from 74.6% to 90.9% compared with the pristine material. Meanwhile, the average voltage decay decreases from 2.33 mV/cycle to 1.25 mV/cycle over 200 cycles. This work provides valuable insights into the development of LLOs using rare-earth phosphate, offering valuable knowledge for further advancements in this field.

Ring-shaped all manganese-based lithium-rich oxide cathode with high performance and stability via biomineralization method

As free from cobalt and nickel, all manganese-based lithium-rich layered oxides (AMLROs) are promising cathode materials with low cost and high theoretical energy density. This study introduced yeast as a biomineralization template to regulate the crystallization and growth of precursors at the molecular level. The mechanism of structural evolution was investigated in detail, offering a new approach to control the morphology of lithium-rich materials. The resulting AMLRO-54 exhibited a micro-nano composite ring-shaped structure with particle size around 2 μm. The unique hierarchical particles provided large flexible volume, leading to superior structural stability and high rate capability. The reversible capacity of ring-shaped AMLRO-54, without any other modifications, was 274.1 mAh·g−1 at 0.1 C. Once extra carbon material was added to improve the inferior electronic conductivity, the initial capacity of AMLRO-54 at 1 C reached 242.9 mAh·g−1 with a retention rate of 90.9 % after 300 cycles (221.0 mAh·g−1). Furthermore, this cobalt and nickel-free cathode material significantly mitigates material costs and environmental hazards, thereby promoting the development of lithium-ion batteries in the future.

Effect of compaction pressure on electrochemical performance of layered lithium-rich oxide

Lithium-rich oxide cathode material focuses the attention of many researchers due to its excellent electrochemical performance for lithium–ion batteries. Further improvement on the performance with a strong operability approach can be beneficial to its industrialization and commercialization. In this work, the effect of powder compaction pressure in sintering process on the electrochemical performance of Li[Formula: see text]Mn[Formula: see text]Ni[Formula: see text]Co[Formula: see text]O2 cathode material is investigated. Four cathode materials under different powder compaction pressure are synthesized by sol–gel method. The XRD analysis results reveal their consistency on the crystal structure. But there is a slight difference on particle size and microstructure found by SEM analysis. Electrochemical tests show that a cathode material under medium compaction pressure exhibits the best performance with a discharge capacity of 170.3 mAh⋅g[Formula: see text] at the rate of 200 mA⋅g[Formula: see text] with 89.35% retention after 100 cycles. This work reveals that the powder compaction pressure does have influence on the cycle and rate performance and this relation is not proportional and linear.

Enabling Superior Electrochemical Performance of Lithium-Rich Li1.2Ni0.2Mn0.6O2 Cathode Materials by Surface Integration

Lithium-rich cathode oxides exhibit extraordinary specific capacities that are mainly ascribed to the accumulated redox reactions of anions and cations at high operating potentials. However, rapid ...

Unravelling high-temperature stability of lithium-ion battery with lithium-rich oxide cathode in localized high-concentration electrolyte

Abstract Lithium (Li)-rich manganese (Mn)-rich oxide (LMR) cathode materials, despite of the high specific capacity up to 250 mAh g−1 suffer from instability of cathode/electrolyte interfacial layer at high working voltages, causing continuous voltage decay and capacity fading, especially at elevated temperatures. In various battery systems, localized high-concentration electrolytes (LHCEs) have been widely reported as a promising candidate to form effective electrode/electrolyte interphases. Here, an optimized LHCE is studied in graphite (Gr)-based full cells containing LMR cathode, being cycled at 25, 45 and 60 °C with the reference of a conventional LiPF6-based electrolyte. It is revealed that the LHCE can effectively suppress continuous electrolyte decompositions and mitigate the dissolution of Mn ions due to the formation of more protective electrode/electrolyte interphases on both anode and cathode, which, in turn, lead to significantly improved cycling stability and enhanced rate capability under the selected temperatures. The mechanistic understanding on the failure of the conventional LiPF6-containing electrolyte and the function of the LHCE in Gr||LMR cells under high temperatures provides valuable perspectives of electrolyte development for practical applications of LMR cathodes in high energy density batteries over a wide temperature range.

Suppressing the voltage decay of lithium-rich cathode for Li-Ion batteries via Pt nanoparticles surface modification

Lattice oxygen undergoes redox reaction to achieve high specific capacity of the material in lithium-rich cathode oxides. However, irreversible oxygen loss causes a change in the crystal structure, and the cations migrate in the transition metal layer, resulting in a rearrangement of the electronic structure and ultimately a severe voltage decay. Herein, we introduce Pt nanoparticles with good catalytic activity and electrical conductivity into lithium-rich cathode materials to improve the loss of lattice oxygen for the first time. We have revealed that the evolution of the lattice structure after the lattice oxygen redox reaction is relatively stable in the lithium-rich oxide with Pt nanoparticles, which is in stark contrast to the apparently deformed crystal structure in the lithium-rich oxide without Pt. Pt-containing electrodes exhibit excellent high-capacity retention rate (more than 80% after 200 cycles), and voltage decay is significantly reduced (less than 0.4 V after 200 cycles). Our results highlight the role of Pt nanoparticles in alleviating the loss of lattice oxygen and stabilizing the crystal structure, which opens up the field of vision for the design of high-energy-density lithium rich cathode oxides with stable structure.

Harbinger of hysteresis in lithium-rich oxides: Anionic activity or defect chemistry of cation migration

This work presents findings on the mechanisms of hysteresis in lithium-rich cathode oxides. It is generally reported that the energy inefficiency, observed during electrochemical cycling of Li-rich electrodes, is a direct result of anion redox processes. Similarly, the coupled phenomenon of cation migration has been cited as being a mere consequence of that process with no essential ramification to hysteresis. Such works, however, have only shown correlations and not causation. Specifically, studies that clearly preclude the complete absence of either anion redox or cation migration (down to defect-level concentrations) are not reported yet. Herein, we present first principles calculations and experimental data on a bespoke Li-rich system that is indubitably immune to oxygen activity but yet exhibits significant energy inefficiency. Additionally, the results directly implicate transition metal migration as the fundamental, sole process responsible for hysteresis in this material. This study is of particular importance to the design and development of new Li-rich materials aimed at eliminating the role of oxygen and enhancing the effective cationic redox contribution to the capacity and points out the essential need to develop materials devoid of transition metal migration to interstitial sites in both layered and disordered oxides alike.

Surface substitution of polyanion to improve structure stability and electrochemical properties of lithium-rich layered cathode oxides

Lithium-rich layered cathode oxides attract enormous attention due to their superior capacity, high voltage and environmentally friendliness. However, it is considerable that they suffer from serious capacity fade, voltage attenuation and phase transition deriving from inevitable oxygen evolution. In this work, polyanions are induced on the surface of the oxides via the method of surface substitution, considering synergy effects of bulk substitution and surface modification to stabilize crystal structure and promote electrochemical properties. The material doped with 5 wt% PO43− exhibits excellent specific capacity of 217.1 mAh g−1 with capacity retention of 88.43% and average discharge voltage drop of 0.4846 V after 200 cycles at 0.5 C significantly superior to the pristine material. And the discharge capacity of the material can reach 105.6 mAh g−1 at 10 C owing to enlarged interlayer spacing and improved ionic conductivity. Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS) of the material before and after cycle verify that polyanions are beneficial to supply more active oxygen, restrain transition metal migration, reduce surface microstrain, and enhance electronic and ionic conductivities. All above indicate that it is effective for polyanions to promote lithium-rich cathode oxides to give full play to their electrochemical advantages.

Triethyl borate and tripropyl borate as electrolyte additives for 4.8 V high voltage layered lithium-rich oxide cathode with enhanced self-discharge suppression performance: A comparative study

Trimethyl borate (TMB) has been identified as electrolyte additive to suppress self-discharge for high voltage layered lithium-rich oxide cathode (LRO) for lithium ion batteries in our previous work. To expand pervious findings, its derivatives, triethyl borate (TEB) and tripropyl borate (TPB) with increased alkyl chains, have been considered as electrolyte additives to mitigate self-discharge behavior of LRO cathode for lithium ion batteries and we make a contrast to explain which one is better and why is better. It is found that fully charged (4.8 V, vs. Li/Li+) LRO cathode suffers severe self-discharged issue upon storage in standard (STD) electrolyte (1 mol L−1 LiPF6-EC/EMC/DEC (3:5:2, in weight)). In contrast, this detrimental self-discharge behavior of LRO cathode at high voltage can be effectively depressed by introducing TEB or TPB into the electrolyte system. Combinations of electrochemical methods and multi spectroscopy techniques have been employed to evaluate effects of TEB and TPB on the LRO cathode surface. They reveal that the B–O and B–F bonds species in the film formed by TEB and TPB prevents electrolyte decomposition and protects LRO from structure transformation. These species in the film formed by TEB is much more than TPB, which explains the better effect for TEB.

Stabilizing a High-Voltage Lithium-Rich Layered Oxide Cathode with a Novel Electrolyte Additive.

We report a novel electrolyte additive, bis(trimethylsilyl)carbodiimide, that effectively stabilizes high-voltage lithium-rich oxide cathode. Charge/discharge tests demonstrate that even trace amounts of bis(trimethylsilyl)carbodiimide in a baseline electrolyte improve the cycling stability of this cathode significantly, either in Li-based half cells or graphite-based full cells, where the capacity retention after 200 cycles between 2 and 4.8 V at 0.5C is enhanced from 40 to 72% and 49 to 77%, respectively. Analyses using physical characterization and theoretical calculations reveal that this additive not only builds a protective film on the cathode but also eliminates detrimental hydrogen fluoride via its strong coordination with hydrogen fluoride or protons.

Understanding the Discrepancy of Defect Kinetics on Anionic Redox in Lithium-Rich Cathode Oxides.

Reversible anionic (oxygen) redox in lithium-rich cathode oxides has been becoming a blooming research topic to further boost the energy density in lithium-ion batteries. There are numerous experimental observations and theoretical calculations to illustrate the importance of defects on anionic redox activity, but how the defects on the surface and bulk control the kinetics of anionic redox is not well understood. Here, we uncover this intriguing ambiguity on the correlation among defects states, Li-ion diffusion, and oxygen redox reaction. It is found that the surface-defective microstructure has fast Li-ion diffusion to achieve superior cationic redox activities/kinetics, whereas the bulk-defective microstructure corresponds to a slow Li-ion diffusion to result in poor cationic redox activities/kinetics. By contrast, both surface and bulk defects can be of benefit to the enhancement of oxygen redox activities/kinetics. Moreover, a positive correlation is also established among charge-transfer resistance, interface reaction charge-transfer activation energy, and oxygen redox activity in these electrode materials. This study on defect-anionic activity provides a new insight for controlling anionic redox reaction in lithium-rich cathode materials for real-world application.

Cover Feature: Unravelling the Enhanced High‐Temperature Performance of Lithium‐Rich Oxide Cathode with Methyl Diphenylphosphinite as Electrolyte Additive (ChemElectroChem 12/2018)

The Cover Feature illustrates the protection effects of the cathodes interfacial layer after adding the methyl diphenylphosphinite (MDP) additive into the electrolyte. MDP participates in the interfacial reaction of cathodes through the peroxidation behavior, and forms a robust protective film with interaction between the P−O species and transition-metal elements. The film prevents the cathodes being attacked by HF in the electrolyte, leading to an enhanced high-temperature performance of lithium-rich oxide cathodes. More information can be found in the Article by Y. Ma and co-workers on page 1569 in Issue 12, 2018 (DOI: 10.1002/celc.201800061).

High-capacity lithium-rich cathode oxides with multivalent cationic and anionic redox reactions for lithium ion batteries

Lithium-rich cathode oxides with capability to realize multivalent cationic and anionic redox reactions have attracted much attention as promising candidate electrode materials for high energy density lithium ion batteries because of their ultrahigh specific capacity. However, redox reaction mechanisms, especially for the anionic redox reaction of these materials, are still not very clear. Meanwhile, several pivotal challenges associated with the redox reactions mechanisms, such as structural instability and limited cycle life, hinder the practical applications of these high-capacity lithium-rich cathode oxides. Herein, we review the lithium-rich oxides with various crystal structures. The multivalent cationic/anionic redox reaction mechanisms of several representative high capacity lithium-rich cathode oxides are discussed, attempting to understand the origins of the high lithium storage capacities of these materials. In addition, we provide perspectives for the further development of these lithium-rich cathode oxides based on multivalent cationic and anionic redox reactions, focusing on addressing the fundamental problems and promoting their practical applications.

Anionic Redox in Rechargeable Lithium Batteries.

The extraordinarily high capacities delivered by lithium-rich oxide cathodes, compared with conventional layered oxide electrodes, are a result of contributions from both cationic and anionic redox processes. This phenomenon has invoked a lot of research exploring new kinds of lithium-rich oxides with multiple-electron redox processes. Though proposed many years ago, anionic redox is now regarded to be crucial in further developing high-capacity electrodes. A basic overview of the previous work on anionic redox is given, and issues related to electronic and geometric structures are discussed, including the principles of activation, reversibility, and the energy barrier of anionic redox. Anionic redox also leads to capacity loss and structural degradation, as well as voltage hysteresis, which shows the importance of controlling anionic redox reactions. Finally, the techniques used for characterizing anionic redox processes are reviewed to aid the rational choice of techniques in future studies. Important perspectives are highlighted, which should instruct future work concerning anionic redox processes.

Effect of Lithium Content on Spinel Phase Evolution in Lithium-Rich Layered Oxides LixNi0.25Co0.10Mn0.65O(3.4+x)/2 (0.8 ≤ x ≤ 1.6) for Li-Ion Batteries

Lithium-rich oxides LixNi0.25Co0.10Mn0.65O(3.4+x)/2 (x=1.6, 1.4, 1.2, 1.0, 0.8) were synthesized and characterized for their structural, morphological, and performance as cathode materials in Li-ion batteries. The baseline composition x=1.4 had been previously determined as having the best electrochemical performance due to its small primary particle size and homogeneous distribution of nano-domains consisting of (C2/m) and (R3¯m) phases. In this study, the Rietveld refinement results indicate the presence of two phases at high lithium levels (x=1.6 and 1.4): Li2MnO3 (C2/m) and Li M O2 ( M = Ni, Co, Mn) (R3¯m); the latter contains Ni2+ and Ni3+. At low lithium levels (x=1.2, 1.0, and 0.8) an additional spinel phase Li M 2O4 (Fd3¯m) emerges, which is known to affect the electrochemical performance of the oxide. Structural analysis reveals that the spinel phase contains mixed transition metals Ni, Co, and Mn as [Li+,Co2+][Ni2+,Co3+,Mn4+]2O4. A low lithium level is found to induce primary particle growth, as well as Co and Ni segregation within the secondary particles. These results are expected to contribute to material optimization and commercialization of lithium-rich oxide cathodes.

Effect of lithium content on spinel phase evolution in the composite material LixNi0.25Co0.10Mn0.65O(3.4 + x) / 2 (0.8 ≤ x ≤ 1.6) for Li-ion batteries

The composite material LixNi0.25Co0.10Mn0.65O(3.4+x)/2 (x=1.6, 1.4, 1.2, 1.0, 0.8) were synthesized and characterized for their structural, morphological, and performance as cathode materials in Li-ion batteries. The Rietveld refinement results indicate the presence of two phases at high lithium levels (x=1.6 and 1.4): Li2MnO3 (C2/m) and LiMO2 (MNi, Co, Mn) (R3¯m); the latter contains Ni2+ and Ni3+. At low lithium levels (x=1.2, 1.0, and 0.8) an additional spinel phase LiM2O4 (Fd3¯m) emerges, which is known to affect the electrochemical performance of the oxide. Structural analysis reveals that the spinel phase contains mixed transition metals Ni, Co, and Mn as [Li+,Co2+][Ni2+,Co3+,Mn4+]2O4. A low lithium level is found to induce primary particle growth, as well as Co and Ni segregation within the secondary particles. These results are expected to contribute to material optimization and commercialization of lithium-rich oxide cathodes.

Insight into self-discharge of layered lithium-rich oxide cathode in carbonate-based electrolytes with and without additive

Self-discharge behavior of layered lithium-rich oxide as cathode of lithium ion battery in a carbonated-based electrolyte is understood, and a simple boron-containing compound, trimethyl borate (TMB), is used as an electrolyte additive to suppress this self-discharge. It is found that layered lithium-rich oxide charged under 4.8 V in additive-free electrolyte suffers severe self-discharge and TMB is an effective electrolyte additive for self-discharge suppression. Physical characterizations from XRD, SEM, TEM, XPS and ICP-MS demonstrate that the crystal structure of the layered lithium-rich oxide collapses due to the chemical interaction between the charged oxide and electrolyte. When TMB is applied, the structural integrity of the oxide is maintained due to the protective cathode film generated from the preferential oxidation of TMB.

Improving High Voltage Interfacial and Structural Stability of Layered Lithium-Rich Oxide Cathode by Using a Boracic Electrolyte Additive

In this work, a boracic electrolyte additive trimethyl borate (TMB) was investigated to improve the interfacial and structural stability of layered lithium-rich oxide (LLO) at high operating potential. The LLO in electrolyte with 2% TMB additive shows high capacity retention of 84.5% after 300 cycles at 0.5 C and good rate capability of 120 mAh g−1 at 4 C rate, comparing to 17.9% after 250 cycles and 65 mAh g−1 at 4 C rate of baseline electrolyte. The results from DFT calculation, dQ/dV, EIS, chronoamperometry and physical characterizations indicate that TMB has higher oxidation activity than baseline electrolyte. The preferential oxidation of TMB generates an effective protective film on the LLO surface, which greatly restrains the continuous oxidation of baseline electrolyte and enhances the structural stability of electrode, leading to excellent cyclic and rate performances of LLO at high operating potential.

Tris(trimethylsilyl)borate as an electrolyte additive for improving interfacial stability of high voltage layered lithium-rich oxide cathode/carbonate-based electrolyte

Tris(trimethylsilyl)borate (TMSB) is used as an electrolyte additive for high voltage lithium-rich oxide cathode of lithium ion battery. The interfacial natures of Li[Li0.2Mn0.54Ni0.13Co0.13]O2/carbonate-based electrolyte are investigated with a combination of electrochemical measurements and physical characterizations. Charge/discharge tests show that the cyclic performance of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 in a mixed carbonate electrolyte is significantly improved by using TMSB. After 200 cycles between 2 V and 4.8 V (vs. Li/Li+) at 0.5 C rate, the capacity retention of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 is only 19% in the blank electrolyte, while it is improved to 74% when 0.5% TMSB is applied. The results from physical characterizations demonstrate that this excellent cyclic performance is attributed to the improved interfacial stability of Li[Li0.2Mn0.54Ni0.13Co0.13]O2/electrolyte due to the thin and protective film generated by TMSB.