Realizing High Voltage Lithium Cobalt Oxide in Lithium-Ion Batteries

Versatile Composite Binder with Fast Lithium-Ion Transport for LiCoO2 Cathodes

The low ionic conductivity of LiCoO2 limits the rate performance of the overall electrode. Here, a polymeric composite binder composed of poly(vinylidene fluoride) (PVDF) and poly(ethylene oxide) (PEO) is reported to efficiently improve the ion transport in the LiCoO2 electrode. This is where the lithium-ion transport channel constructed by oxygen atoms of PEO can afford the electrode a lithium-ion transport number (tLi+) as high as 0.70 with the optimized composite binder in a mass ratio of 1:1 (O5F5), significantly higher than that of traditional PVDF (0.44). As a result, the O5F5 binder endows the LiCoO2 electrode with an impressive capacity of 90 mAh g-1 even at 15 C, which is twice as high as the PVDF electrode. In addition, the initial Coulombic efficiency of the LiCoO2 electrode with the O5F5 binder is close to 100% and the capacity retention is 91% after 100 cycles at 1 C. This study overcomes the problem of slow ion conductivity of the LiCoO2 electrode, providing an easy method for developing high-rate cathode binders.

Structural Understanding for High-Voltage Stabilization of Lithium Cobalt Oxide

The rapid development of modern consumer electronics is placing higher demands on the lithium cobalt oxide (LiCoO2 ; LCO) cathode that powers them. Increasing operating voltage is exclusively effective in boosting LCO capacity and energy density but is inhibited by the innate high-voltage instability of the LCO structure that serves as the foundation and determinant of its electrochemical behavior in lithium-ion batteries. This has stimulated extensive research on LCO structural stabilization. Here, it is focused on the fundamental structural understanding of LCO cathode from long-term studies. Multi-scale structures concerning LCO bulk and surface and various structural issues along with their origins and corresponding stabilization strategies with specific mechanisms are uncovered and elucidated at length, which will certainly deepen and advance the knowledge of LCO structure and further its inherent relationship with electrochemical performance. Based on these understandings, remaining questions and opportunities for future stabilization of the LCO structure are also emphasized.

Aqueous Cold Sintering of Li-Based Compounds

Aqueous cold sintering of two lithium-based compounds, the electrolyte Li_6.25La₃Zr₂Al_0.25O₁₂ (LLZAO) and cathode material LiCoO₂ (LCO), is reported. For LLZAO, a relative density of ∼87% was achieved, whereas LCO was sintered to ∼95% with 20 wt % LLZAO as a flux/binder. As-cold sintered LLZAO exhibited a low total conductivity (10^-8 S/cm) attributed to an insulating grain boundary blocking layer of Li₂CO₃. The blocking layer was reduced with a post-annealing process or, more effectively, by replacing deionized water with 5 M LiCl during cold sintering to achieve a total conductivity of ∼3 × 10^-5 S/cm (similar to the bulk conductivity). For LCO-LLZAO composites, scanning electron microscopy and X-ray computer tomography indicated a continuous LCO matrix with the LLZAO phase evenly distributed but isolated throughout the ceramics. [001] texturing during cold sintering resulted in an order of magnitude difference in electronic conductivity between directions perpendicular and parallel to the c-axis at room temperature. The electronic conductivity (∼10^-2 S/cm) of cold sintered LCO-LLZAO ceramics at room temperature was comparable to that of single crystals and higher than those synthesized via either conventional sintering or hot pressing.

High-performance Li/LiNi0.8Co0.1Mn0.1O2 batteries enabled by optimizing carbonate-based electrolyte and electrode interphases via triallylamine additive

Lithium (Li) metal batteries (LMBs), paired with high-energy-density cathode materials, are promising to meet the ever-increasing demand for electric energy storage. Unfortunately, the inferior electrode-electrolyte interfaces and hydrogen fluoride (HF) corrosion in the state-of-art carbonate-based electrolytes lead to dendritic Li growth and unsatisfactory cyclability of LMBs. Herein, a multifunctional electrolyte additive triallylamine (TAA) is proposed to circumvent those issues. The TAA molecule exhibits strong nucleophilicity and contains three unsaturated carbon-carbon double bonds, the former for HF elimination, the later for in-situ passivation of aggressive electrodes. As evidenced theoretically and experimentally, the preferential oxidation and reduction of carbon-carbon double bonds enable the successful regulation of components and morphologies of electrode interfaces, as well as the binding affinity to HF effectively blocks HF corrosion. In particular, the TAA-derived electrode interfaces are packed with abundant lithium-containing inorganics and oligomers, which diminishes undesired parasitic reactions of electrolyte and detrimental degradation of electrode materials. When using the TAA-containing electrolyte, the cell configuration with Li anode and nickel-rich layered oxide cathode and symmetrical Li cell deliver remarkably enhanced electrochemical performance with regard to the additive-free cell. The TAA additive shows great potential in advancing the development of carbonate-based electrolytes in LMBs.

Phosphate-Rich Interface for a Highly Stable and Safe 4.6 V LiCoO2 Cathode

Increasing the upper cut-off voltage of LiCoO₂ (LCO) is one of the most efficient strategies to gain high-energy density for current lithium-ion batteries. However, surface instability is expected to be exaggerated with increasing voltage arising from the high reactivity between the delithiated LCO and electrolytes, leading to serious safety concerns. This work is aimed to construct a physically and chemically stable phosphate-rich cathode-electrolyte interface (CEI) on the LCO particles to mitigate this issue. This phosphate-rich CEI is generated during the electrochemical activation by using fluoroethylene carbonate and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyletherare as the solvents. Both solvents also demonstrate high thermal stability, reducing the intrinsic flammability of the commercial organic electrolyte, thereby eliminating the safety concern in the LCO-based systems upon high-voltage operation. This stable CEI layer on the particle surface can also enhance the surface structure by blocking direct contact between LCO and electrolyte, improving the cycling stability. Therefore, by using the proposed electrolyte, the LCO cathode exhibits a high-capacity retention of 76.1% after 200 cycles at a high cut-off voltage of 4.6 V. This work provides a novel insight into the rational design of high-voltage and safe battery systems by adopting the flame-retardant electrolyte.

Conformal Coating of a High-Voltage Spinel to Stabilize LiCoO2 at 4.6 V

The ever-growing demand for portable electronic devices has put forward higher requirements on the energy density of layered LiCoO₂ (LCO). The unstable surface structure and side reactions with electrolytes at high voltages (>4.5 V) however hinder its practical applications. Here, considering the high-voltage stability and three-dimensional lithium-ion transport channel of the high-voltage Li-containing spinel (M = Ni and Co) LiM_xMn_2-xO₄, we design a conformal and integral LiNi_xCo_yMn_2-x-yO₄ spinel coating on the surface of LCO via a sol-gel method. The accurate structure of the coating layer is identified to be a spinel solid solution with gradient element distribution, which compactly covers the LCO particle. The coated LCO exhibits significantly improved cycle performance (86% capacity remained after 100 cycles at 0.5C in 3-4.6 V) and rate performance (150 mAh/g at a high rate of 5C). The characterizations of the electrodes from the bulk to surface suggest that the conformal spinel coating acts as a physical barrier to inhibit the side reactions and stabilize the cathode-electrolyte interface (CEI). In addition, the artificially designed spinel coating layer is well preserved on the surface of LCO after prolonged cycling, preventing the formation of an electrochemically inert Co₃O₄ phase and ensuring fast lithium transport kinetics. This work provides a facile and effective method for solving the surface problems of LCO operated at high voltages.

A new sodium ion preintercalated and oxygen vacancy-enriched vanadyl phosphate cathode for aqueous zinc-ion batteries

At present, layered vanadium-oxygen structures have attracted wide attention for multivalent metal ion storage, especially in aqueous zinc-ion batteries (AZIBs), due to the attractive layered structure and large specific capacity based on V⁵⁺/V³⁺ double electron transfer. However, in addition to a large specific capacity, a high output voltage is necessary to achieve a high specific energy density. Vanadium oxide and vanadate usually feature low working voltages, serious structural degradation and limited practical. To alleviate these problems, some cathode modification strategies have been proposed that improve the operating voltage, structural stability and diffusion kinetics of multivalent metal ions. In this paper, vanadyl phosphate (Na_y(VO_1-x)₃(PO₄)₂) nanosheets preintercalated with sodium ions and modified with oxygen vacancies were prepared via a facile one-step liquid phase treatment. The Na_y(VO_1-x)₃(PO₄)₂ nanosheet cathode for AZIBs delivered a high specific capacity of 75.3 mAh g^-1 at 0.1 A g^-1 and retained 27.5 mAh g^-1 after 4000 cycles at 2 A g^-1. Subsequently, the as-prepared Na_y(VO_1-x)₃(PO₄)₂ nanosheets were physically and electrochemically characterized, and a possible mechanism of Zn²⁺ insertion/extraction and structural decomposition was proposed based on ex situ XRD and XPS characterizations. Our work provides a simple method for simultaneously introducing sodium ion preintercalation and oxygen vacancies into vanadyl phosphate structures, and provides some insights into the zinc storage mechanism.

Enhancing the Reversibility of Lithium Cobalt Oxide Phase Transition in Thick Electrode via Low Tortuosity Design

Lithium cobalt oxide (LCO) is a widely used cathode material for lithium-ion batteries. However, it suffers from irreversible phase transition during cycling because of high cutoff voltage or huge concentration polarization in thick electrode, resulting in deteriorated cyclability. Here, we design a low tortuous LiCoO₂ (LCO-LT) electrode by ice-templating method and investigate the reversibility of LCO phase transition. LCO-LT thick electrode shows accelerated lithium-ion transport and reduced concentration polarization, achieving excellent rate capability and homogeneous actual operating voltage. Moreover, LCO-LT thick electrode exhibits a durable phase transition between O2 and H1-3, mitigated volume expansion, and suppressed microcrack formation. LCO-LT electrode (25 mg cm^-2) delivers improved capacity retentions of 94.4% after 200 cycles and 93.3% after 150 cycles at cutoff voltages of 4.3 and 4.5 V, respectively. This strategy provides a new concept to improve the reversibility of LCO phase transition in thick electrode by low tortuosity design.

1,2,3,4-Tetrakis(2-cyanoethoxy)butane (TCEB)-Assisted Construction of Self-Repair Electrode Interface Films to Improve the Performance of 4.5 V Pouch LiCoO2/Artificial Graphite Full Cells Operating at 45 °C

1,2,3,4-Tetrakis(2-cyanoethoxy)butane (TCEB) is first evaluated as a functional electrolyte additive to increase the charge cutoff voltage and energy density of pouch LiCO₂ (LCO)/artificial graphite (AG) lithium-ion batteries (LIBs) at a high temperature of 45 °C. The charge (0.7 C) and discharge (1 C) tests show that TCEB effectively improves the cycle stability of cells under a high charge cutoff voltage of 4.5 V. At 25 °C, the capacity retention of the cells with TCEB increases from 0.0% to 72.1% after 1200 cycles. At 45 °C, the capacity retention of the cells without TCEB after 50 cycles is close to 0.0%, while the capacity retention of the cells with TCEB is still 81.6%, even after 350 cycles. The spectroscopic characterization results demonstrate that the TCEB electrolyte additive participates in the construction of a self-repair electrode/electrolyte interface film. Subsequently, low impedance and strong protective layers are formed on the two electrode surfaces. The quantitative analysis results and a theoretical calculation also show that TCEB effectively inhibits the dissolution of Co³⁺ and maintains the structural integrity of electrode materials. These results indicate that TCEB endows LIBs with excellent cycle stability and is a promising electrolyte additive for the high-voltage and high-temperature conditions of LCO-based LIBs.

Tailoring Co3d and O2p Band Centers to Inhibit Oxygen Escape for Stable 4.6 V LiCoO2 Cathodes

High-voltage LiCoO₂ delivers a high capacity but sharp fading is a critical issue, and the capacity decay mechanism is also poorly understood. Herein, we clarify that the escape of surface oxygen and Li-insulator Co₃ O₄ formation are the main causes for the capacity fading of 4.6 V LiCoO₂ . We propose the inhibition of the oxygen escape for achieving stable 4.6 V LiCoO₂ by tailoring the Co3d and O2p band center and enlarging their band gap with MgF₂ doping. This enhances the ionicity of the Co-O bond and the redox activity of Co and improves cation migration reversibility. The inhibition of oxygen escape suppresses the formation of Li-insulator Co₃ O₄ and maintains the surface structure integrity. Mg acts as a pillar, providing a stable and enlarged channel for fast Li⁺ intercalation/extraction. The modulated LiCoO₂ shows almost zero strain and achieves a record capacity retention at 4.6 V: 92 % after 100 cycles at 1C and 86.4 % after 1000 cycles at 5C.

Structural origin of the high-voltage instability of lithium cobalt oxide

Layered lithium cobalt oxide (LiCoO₂, LCO) is the most successful commercial cathode material in lithium-ion batteries. However, its notable structural instability at potentials higher than 4.35 V (versus Li/Li⁺) constitutes the major barrier to accessing its theoretical capacity of 274 mAh g^-1. Although a few high-voltage LCO (H-LCO) materials have been discovered and commercialized, the structural origin of their stability has remained difficult to identify. Here, using a three-dimensional continuous rotation electron diffraction method assisted by auxiliary high-resolution transmission electron microscopy, we investigate the structural differences at the atomistic level between two commercial LCO materials: a normal LCO (N-LCO) and a H-LCO. These powerful tools reveal that the curvature of the cobalt oxide layers occurring near the surface dictates the structural stability of the material at high potentials and, in turn, the electrochemical performances. Backed up by theoretical calculations, this atomistic understanding of the structure-performance relationship for layered LCO materials provides useful guidelines for future design of new cathode materials with superior structural stability at high voltages.

Advanced liquid electrolytes enable practical applications of high-voltage lithium-metal full batteries

High-voltage lithium metal batteries (HVLMBs) have received widespread attention as next generation high-energy-density batteries to meet the urgent demands of modern life. However, the unstable interphase between electrolytes and highly reactive electrodes is still an important threshold for practical applications. In this feature article, we review the formation mechanism of the electrode-electrolyte interphase in terms of cathodes and the Li metal anode, respectively, and summarize the surface modification methods to stabilize the interphase of HVLMBs. Electrolyte regulation strategies especially those using electrolyte additives are introduced, and the relationship between liquid electrolyte formulation, interphase engineering and the electrochemical performance of HVLMBs is analyzed. Finally, an industry-level evaluation is carried out and the remaining challenges are discussed for advanced electrolytes to guarantee the practical applications and commercialization of HVLMBs.

Frontier Orbital Energy-Customized Ionomer-Based Polymer Electrolyte for High-Voltage Lithium Metal Batteries

The development of gel polymer electrolytes (GPEs) is considered to be an effective strategy to drive practical applications of high-voltage lithium metal batteries (HLMBs). However, rare GPEs that can satisfy the demands of HLMBs have been developed because of the limited compatibility with lithium anodes and high-voltage cathodes simultaneously. Herein, a novel strategy for constructing polymer matrixes with a customized frontier orbital energy for GPEs is proposed. The as-investigated polymer matrix (P(CUMA-NPF₆))-based GPE (P(CUMA-NPF₆)-GPE) obtained via in situ random polymerization delivers a wide voltage window (0-5.6 V vs Li⁺/Li), large lithium-ion transference number (t_Li⁺, 0.61), and superior electrode/electrolyte interface compatibility. It is to be noted that such a t_Li⁺ of P(CUMA-NPF₆)-GPE, which is one of the largest t_Li⁺ among high-voltage GPEs in a fair comparison, results from the high dissociation of lithium salts and effective anion immobilization abilities of P(CUMA-NPF₆). Ultimately, the as-assembled HLMB delivers more enhanced cycle performance than its counterpart of commercial liquid electrolytes. It is also demonstrated that P(CUMA-NPF₆) can scavenge the active PF₅ intermediate generated in the electrolyte at the anode side, thus suppressing the PF₅-mediated decomposition reaction of carbonates. This work will enlighten the rational structure design of GPEs for HLMBs.

Stabilization of Li-Rich Disordered Rocksalt Oxyfluoride Cathodes by Particle Surface Modification

Promising theoretical capacities and high voltages are offered by Li-rich disordered rocksalt oxyfluoride materials as cathodes in lithium-ion batteries. However, as has been discovered for many other Li-rich materials, the oxyfluorides suffer from extensive surface degradation, leading to severe capacity fading. In the case of Li₂VO₂F, we have previously determined this to be a result of detrimental reactions between an unstable surface layer and the organic electrolyte. Herein, we present the protection of Li₂VO₂F particles with AlF₃ surface modification, resulting in a much-enhanced capacity retention over 50 cycles. While the specific capacity for the untreated material drops below 100 mA h g^-1 after only 50 cycles, the treated materials retain almost 200 mA h g^-1. Photoelectron spectroscopy depth profiling confirms the stabilization of the active material surface by the surface modification and reveals its suppression of electrolyte decomposition.