High-voltage LiCoO2 Research Articles

A synergetic promotion of surface stability for high-voltage LiCoO2 by multi-element surface doping: a first-principles study.

The utilization of high-voltage LiCoO2 is an effective approach to break through the bottleneck of practical energy density in lithium ion batteries. However, the structural and interfacial degradations at the deeply delithiated state as well as the associated safety concerns impede the application of high-voltage LiCoO2. Herein, we present a synergetic strategy for promoting the surface stability of LiCoO2 at high voltage by Ti-Mg-Al co-doping and systematically study the effects of the dopants on the surface stability, electronic structure and Li+ diffusion properties of the LiCoO2 (104) surface using first-principles calculations. It is found that Ti, Mg and Al dopants can be facilely introduced into the Co sites of the LiCoO2 (104) surface. Furthermore, the co-doping could significantly stabilize the surface oxygen of LiCoO2 at a high delithiation state. Particularly, by aggregating Ti-Mg-Al co-dopant distribution in the surface layer, surface oxygen loss is dramatically suppressed. In addition, analysis of the electronic structure indicates that Ti-Mg-Al co-doping can enhance the electronic conductivity of the LiCoO2 (104) surface and greatly inhibit the charge deficiency of the superficial lattice O atoms at a highly delithiated state. In spite of a negligible improvement in the surface Li+ diffusion kinetics, the Ti-Mg-Al surface-modified LiCoO2 is expected to exhibit improved electrochemical performance at high voltage due to its superior surface stability. Our results suggest that aggregating Ti, Mg and Al co-dopant distribution in the surface layer is a promising modulation strategy to synergistically promote the surface oxygen stability of LiCoO2 at high voltages.

High-voltage stability and electrochemical performance of polyacrylic acid–xanthan gum copolymer-reinforced LiCoO2 cathode material

High-voltage LiCoO2 (LCO) cathode materials are in increasing demand in industry, but their stability is greatly affected by serious irreversible phase transitions and interfacial reactions at high voltages.

Enhancing the reaction kinetics and structural stability of high-voltage LiCoO2via polyanionic species anchoring

Anchored polyanionic species acting as micro funnels boost the Li+ kinetics and enhance the structural stability of high-voltage LiCoO2.

Suppressing Surface Ligand-to-Metal Charge Transfer toward Stable High-Voltage LiCoO2.

Increasing the charging cutoff voltage is a viable approach to push the energy density limits of LiCoO2 and meet the requirements of the rapid development of 3C electronics. However, an irreversible oxygen redox is readily triggered by the high charging voltage, which severely restricts practical applications of high-voltage LiCoO2. In this study, we propose a modification strategy via suppressing surface ligand-to-metal charge transfer to inhibit the oxygen redox-induced structure instability. A d0 electronic structure Zr4+ is selected as the charge transfer insulator and successfully doped into the surface lattice of LiCoO2. Using a combination of theoretical calculations, ex situ X-ray absorption spectra, and in situ differential electrochemical mass spectrometry analysis, our results show that the modified LiCoO2 exhibits suppressed oxygen redox activity and stable redox electrochemistry. As a result, it demonstrates a robust long-cycle lattice structure with a practically eliminated voltage decay (0.17 mV/cycle) and an excellent capacity retention of 89.4% after 100 cycles at 4.6 V. More broadly, this work provides a new perspective on suppressing the oxygen redox activity through modulating surface ligand-to-metal charge transfer for achieving a stable high-voltage ion storage structure.

Towards Extreme Fast Charging of 4.6 V LiCoO2 Viamitigating High-Voltage Kinetic Hindrance

High-voltage LiCoO2 (LCO) is an attractive cathode for ultra-high energy density lithium-ion batteries (LIBs) in the 3 C markets. However, the sluggish lithium-ion diffusion at high voltage significantly hampers its rate capability. Herein, combining experiments with density functional theory (DFT) calculations, we demonstrate that the kinetic limitations can be mitigated by a facial Mg2++Gd3+ co-doping method. The as-prepared LCO shows significantly enhanced Li-ion diffusion mobility at high voltage, making more homogenous Li-ion de/intercalation at a high-rate charge/discharge process. The homogeneity enables the structural stability of LCO at a high-rate current density, inhibiting stress accumulation and irreversible phase transition. When used in combination with a Li metal anode, the doped LCO shows an extreme fast charging (XFC) capability, with a superior high capacity of 193.1 mAh g−1 even at the current density of 20 C and high-rate capacity retention of 91.3% after 100 cycles at 5 C. This work provides a new insight to prepare XFC high-voltage LCO cathode materials. Figure 1

Nonflammable Fluorinated Ester-Based Electrolytes for High Voltage Li Batteries with LiCoO2 Positive Electrode

LiCoO2 is widely used as a positive electrode material for LIBs. The theoretical capacity of LiCoO2 is calculated to be 274 mA h g–1 upon full delithiation, but the practical reversible capacity is generally restricted to approximately 140 mA h g–1 to avoid capacity deterioration when cycled at higher voltages (>4.2 V vs Li/Li+). To solve this limitation, functional electrolytes that possess a high oxidative stability and form a stable and effective passivation layer on the LiCoO2, are desired.In this study, we report that the cyclability of LiCoO2 under high-voltage operation is drastically improved by using a fluorinated ester, methyl 3,3,3-trifluoropropionate (MTFP), as an electrolyte solvent.1 Notably, the combined use of MTFP-based electrolyte and mixed sodium carboxymethyl cellulose/styrene-butadiene rubber binder synergistically stabilize the LiCoO2 electrode surface, resulting in the almost no capacity fading for 50 cycles even with the upper cutoff voltage of 4.5 V. Note that, in general, LiCoO2 shows significant capacity deterioration when cycled at higher voltages over 4.2 V vs Li/Li+. Moreover, our study reveals that superior low-temperature performance and higher thermal stability is evidenced for the LiCoO2 with MTFP-based electrolyte when compared with that with conventional electrolyte. XPS analysis reveals that thinner and uniform passivation layer derived from PF6 – anion and MTFP is formed on the LiCoO2 electrode surface, which is the origin for improved cyclability and thermal stability of high-voltage LiCoO2 with MTFP-based electrolyte.Reference[1] Y. Ugata et al., and N. Yabuuchi, Chemistry of Materials, in press.

Mitigating Lattice Distortion of High-Voltage LiCoO2 via Core-Shell Structure Induced by Cationic Heterogeneous Co-Doping for Lithium-Ion Batteries

HighlightsA simple two-step multi-element co-doping strategy is proposed to fabricate core-shell structured LiCoO2 based on the different diffusivities of dopant ions.The high diffusivity Al3+/Mg2+ ions occupy the core of single-crystal grain while the low diffusivity Ti4+ ions enrich the shell layer. In-situ XRD demonstrates the mitigated structural distortion under a high cut-off voltage of 4.6 V, resulting in a significantly improved cycling stability.

Improved electrochemical performance of high voltage LiCoO2 with Al doping and ZrO2 coating

Surface coating and element doping are the key strategies in LIBs development to achieve a high and stable battery performance. In this paper, ZrO2 is directly sputtered on the Al-doped LiCoO2 (Al-LCO) composite electrodes, enabling a controllable oxide coating on the surface of the electrode. With a sputtered coating layer of ∼6 nm, the discharging capacity of the Al doped LCO coated with ZrO2 (Al-LCO-ZrO2) is 141.04 mAh g−1 at the 100th cycle within 3.0–4.7 V at 1 C, while the Al-LCO (without coating layer) is 126.66 mAh g−1. After 100 cycles of charging-discharging at high voltage region, the capacity retention of the Al-LCO-ZrO2-3 (90.7%) is also higher than the Al-LCO (75.57%). It is worth noting that, in this paper, due to the Al doping and the ZrO2 coating, we increase the operation voltage up to 4.7 V, in which the batteries can obtain a higher energy density.

Electrodes for Li-ion batteries: From high-voltage LiCoO2 to Co-reduced/Co-free layered oxides with potential anodes

Electrodes for Li-ion batteries: From high-voltage LiCoO2 to Co-reduced/Co-free layered oxides with potential anodes

Insight into the Contribution of the Electrolyte Additive LiBF4 in High-Voltage LiCoO2||SiO/C Pouch Cells.

High-voltage pouch cells using an LiCoO2 cathode and SiO/C anode are regarded as promising energy storage devices due to their high energy densities. However, their failure is associated with the unstable, high-impedance cathode electrolyte interphase (CEI) film on the cathode and the solid electrolyte interphase (SEI) film on the anode surface, which hinder their practical use. Here, we report a novel approach to ameliorate the above challenges through the rational construction of a stable, low-impedance cathode and anode interface film. Such films are simultaneously formed on both electrodes via the participation of the traditional salt, lithium tetrafluoroborate (LiBF4), as electrolyte additive. The application of 1.0% LiBF4 enhances the capacity retention of the cell from 26.1 to 82.2% after 150 cycles between 3.0 and 4.4 V at 1 C. Besides, the low-temperature discharge performance is also improved by LiBF4 application: the discharge capacity of the cell with LiBF4 is 794 mAh compared with 637 mAh without LiBF4 at 1 C and -20 °C. The excellent electrochemical performance of pouch cells is ascribed to the contribution of LiBF4. Especially, the low binding energy of LiBF4 with the oxygen on the LiCoO2 surface leads to the enrichment of LiBF4 that forms the protective cathode interface, which fills the blanks of previous research.

Surface-interspersed nanoparticles induced cathode-electrolyte interphase enabling stable cycling of high-voltage LiCoO2

The development of high-voltage LiCoO2 (LCO) is crucial for achieving lithium-ion batteries with a high volumetric energy density. However, LCO experiences accelerated degradation at high voltages due to severe interface and structure instability. A uniform and robust cathode-electrolyte interphase (CEI) serves as a vital barrier for protecting the interface. However, the smooth surface of single-crystalline LCO, lacking grain boundaries, poses challenges in generating an effective CEI. Herein, ZrO2 nano-rivets are constructed on the surface of LCO to provide phase boundaries for preferential film-forming sites, inducing the formation of a uniform and robust CEI. The high-quality CEI derived from ZrO2 nano rivets stabilizes the fragile surface of high-voltage LCO and facilitates lithium-ion diffusion. In addition, a Zr diffusion layer is simultaneously built in the LCO bulk. Zr diffusion into the LCO lattice not only effectively suppresses unfavorable phase transitions to stabilize the bulk structure, but also mitigates oxygen charge deficiencies at the highly delithiated state, therefore stabilizing lattice oxygen. Consequently, the modified LCO exhibits excellent capacity retention of 80% after 700 cycles at 4.6 V. This work emphasizes the significance of material surface properties in CEI formation and provides new insights for the design of CEI.

Facile Solid-State Synthesis to In Situ Generate a Composite Coating Layer Composed of Spinel-Structural Compounds and Li3PO4 for Stable Cycling of LiCoO2 at 4.6 V.

Due to its high energy density, high-voltage LiCoO2 is the preferred cathode material for consumer electronic products. However, its commercial viability is hindered by rapid capacity decay resulting from structural degradation and surface passivation during cycling at 4.6 V. The key to achieving stable cycling of LiCoO2 at high voltages lies in constructing a highly stable interface to mitigate surface side reactions. In this study, we present a facile in situ coating strategy that is amenable to mass production through a simple wet-mixing process, followed by high-temperature calcination. By capitalizing on the facile dispersion characteristics of nano-TiO2 in ethanol and the ethanol dissolubility of LiPO2F2, we construct a uniform precoating layer on LiCoO2 with nano-TiO2 and LiPO2F2. The subsequent thermal treatment triggers an in situ reaction between the coating reagents and LiCoO2, yielding a uniform composite coating layer. This composite layer comprises spinel-structured compounds (e.g., LiCoTiO4) and Li3PO4, which exhibit excellent chemical and structural stability under high-voltage conditions. The uniform and stable coating layer effectively prevents direct contact between LiCoO2 and the electrolyte, thereby reducing side reactions and suppressing the surface passivation of LiCoO2 particles. As a result, coated LiCoO2 maintains favorable electronic and ionic conductivity even after prolonged cycling. The synergistic effects of spinel-structured compounds and Li3PO4 contribute to the superior performance of LiCoO2, demonstrating a high capacity of 202.1 mA h g-1 (3.0-4.6 V, 0.5 C, 1 C = 274 mA g-1), with a capacity retention rate of 96.7% after 100 cycles.

Revealing the Grain-Boundary-Cracking Induced Capacity Decay of a High-Voltage LiCoO2 at 4.6 V.

During a practical battery manufacture process, the LiCoO2 (LCO) electrodes are usually rolled with high pressure to achieve better performance, including reducing electrode polarization, increasing compact density, enhancing mechanical toughness, etc. In this work, a high-voltage LCO (HV-LCO) is achieved via modulating a commercialized LCO with an Al/F enriched and spinel reinforced surface structure. We reveal that the rolling can more or less introduce risk of grain-boundary-cracking (GBC) inside the HV-LCO and accelerate the capacity decay when cycled at 3-4.6 V vs Li/Li+. In particular, the concept of interface structure is proposed to explain the reason for the deteriorated cycle stability. As the GBC is generated, the interface structure of HV-LCO alters from a surface spinel phase to a hybrid of surface spinel plus boundary layer phases, leading to the exposure of some the nonprotective layer phase against the electrolyte. This alternation causes serious bulk structure damage upon cycles, including expanding GBC among the primary crystals, forming intragranular cracks and inactive spinel phases inside the bulk regions, etc., eventually leading to the deteriorated cycle stability. Above all, we realize that it is far from enough to achieve a eligible high-voltage LCO via only applying surface modification. This work provides a new insight for developing more advanced LCO cathodes.

Improving high-voltage electrochemical performance of LiCoO2 by Nd2O3 modification

High-voltage LiCoO2 (LCO) is currently the most promising cathode material for portable electronic devices, however, its further development is hindered by the accelerated structural decline that results from increasing the cut-off voltage. This article presents a facile solid-phase sintering approach for synthesizing Nd2O3-modified LCO in a one-step process. The crystal parameters of LCO after Nd2O3 modification increase, and a coating layer of Nd2O3 was formed on the surface. Meanwhile, a fraction of Nd3+ ions diffused into the surface layer of LCO to replacing Li + sites, which leads to an increase in lattice parameters. The electrochemically inert Nd3+ can serve as an anchoring pillar within the Li layer, playing a role in enhancing lattice stability. Moreover, the proportion of lattice oxygen in the Nd4000 structure was determined to be 56.82%, which is significantly higher than that of bare LCO (51.81%), leading to an improved structural stability. The Nd2O3 modification also significantly suppressed the side reactions at the electrode-electrolyte interface, thereby reducing the thickness of the CEI film and increasing the Li+ diffusion rate, ultimately resulting in superior electrochemical performance. Notably, the optimized Nd4000 exhibited a capacity retention of 86.19% after 300 cycles at 1.0C and 3.0∼4.5 V, while bare LCO only retained 38.63% of its initial capacity.

Direct current internal resistance decomposition model for accurate parameters acquisition and application in commercial high voltage LiCoO2 battery

Accurate modeling and simulation of lithium-ion batteries are beneficial to predict the battery performance. However, the acquisition and verification of parameters are still the main factors that determine the accuracy of the model simulation results. In this paper, direct current internal resistance (DCR) is systematically decomposed differ from the timescale response characteristics and thus DCR decomposition model is constructed. The numerical expressions and the key parameters of different resistances components are reasonably evaluated. Based on the estimated original results, a series of parameters are successfully collected and the strong correlation to DCR is further defined. When applied the proposed DCR model to a commercial high voltage 4.45 V LiCoO2 pouch battery, the simulated results can well match with the experiments, implying the workable of this strategy. This paper shed light on guiding electrochemical parameters design, especially for high energy-density commercial batteries.

Synthesis of co-doped high voltage lithium cobalt oxide with high rate electrochemical performance

LiCoO2 can further develops its capacity potential by increasing cut-off voltage, but the irreversible phase transition and structural damage at high voltage will lead to severe capacity fading. Here, we propose a method of multi-doping of four elements Ti, Mg, Al, and Y, which realizes the stable cycling of LiCoO2 at a cut-off voltage of 4.5 V through the synergistic effect among the four elements. The cell delivers the increased initial discharge capacity of 173.5 mAh/g, with the capacity retention rate as high as 91.1% after 100 charging-discharging cycles at the rate of 1 C in the voltage ranging from 3.0 to 4.5 V. Moreover, the capacity retention is still as high as 74.2% after 300 cycles. At the same time, the capacity can still be maintained close to 70% after 100 cycles at the temperature of 50 ℃. The specific discharge capacity of the cathode is 130.0 mAh/g, even if at high rate of 5 C. More than 5 times that of unmodified LiCoO2. We verified the effect of four-element co-doping on phase and surface stability by characterizing the materials before and after the cycle. At the same time, the positive effects of four doping elements on structural stability and electronic conductivity were clarified by DFT method. This work provides a promising idea of multi-element synergistic doping and contributes to solving problem of stable high voltage LiCoO2.

Structural stabilizing action of terbium cation and phosphate anion to layered transition-metal oxide cathodes

The capacity decay and safety issues of layered cathodes caused by structural deterioration upon deeply delithiated state, have impeded the further large-scale commercial-used of layered transition-metal cathodes in advanced lithium-ion batteries. In this work, the terbium cation and phosphate anion are doped homogeneously into LiCoO2 and LiNi0.83Co0.11Mn0.06O2 cathodes via the facile solid phase reaction method. We found that the lattice oxygen can be stabilized by the introduction of the Tb element, leading to the mitigation of structural degradation. More impressively, the PO43− polyanion possesses a benign binding affinity for Li-ions, further accelerating Li-ion transportation. Theoretically, the incorporation of the Tb element into the crystal lattice can alleviate oxygen redox activity in the high delithiation state. Thus, the introduction of an appropriate content of anion and cation dopants synergistically modulates the robust layered structure to suppress structural collapse during cycling. Consequently, the Tb element and PO43− polyanion co-doped LiNi0.83Co0.11Mn0.06O2 (a capacity of 179.3 mAh g−1 after 100 cycles with capacity retention of 96%) and LiCoO2 (169.4 mAh g−1 after 400 cycles and with capacity retention of 95%) illustrate prominent cycling stability. This study proposes a feasible and powerful strategy for enhancing the structural stability and cycling durability of high-voltage LiCoO2 and nickel-rich ternary layered cathode materials.

Reducing Co/O Band Overlap through Spin State Modulation for Stabilized High Capability of 4.6 V LiCoO2

High-voltage LiCoO2 (LCO) attracts great interest because of its large specific capacity, but it suffers from oxygen release, structural degradation, and quick capacity drop. These daunting issues root from the inferior thermodynamics and kinetics of the triggered oxygen anion redox (OAR) at high voltages. Herein, a tuned redox mechanism with almost only Co redox is demonstrated by atomically engineered high-spin LCO. The high-spin Co network reduces the Co/O band overlap, eliminates the adverse phase transition of O3 → H1-3, delays the exceeding of the O 2p band over the Fermi level, and suppresses excessive O → Co charge transfer at high voltages. This function intrinsically promotes Co redox and restrains O redox, fundamentally addressing the issues of O2 release and coupled detrimental Co reduction. Moreover, the chemomechanical heterogeneity caused by different kinetics of Co/O redox centers and the inferior rate performance limited by slow O redox kinetics is simultaneously improved owing to the suppression of slow OAR and the excitation of fast Co redox. The modulated LCO delivers ultrahigh rate capacities of 216 mAh g-1 (1C) and 195 mAh g-1(5C), as well as high capacity retentions of 90.4% (@100 cycles) and 86.9% (@500 cycles). This work sheds new light on the design for a wide range of O redox cathodes.

A scalable one-pot strategy for the development of polymer electrolytes adaptable to room-temperature high-voltage lithium batteries

Poly(ethylene oxide) (PEO) polymer electrolytes (PEs) have been commercially applied in LiFePO4||Li solid-state lithium batteries (SSLBs). However, it remains challenging to develop PEO-based PEs applicable to the high-voltage SSLBs with higher energy density, owing to the poor electrochemical stability of PEO. Herein, we report a scalable strategy for fabricating PEO-based PEs with high-voltage compatibility, by exploiting a new mechanism to stabilize the cathode-electrolyte interface in the high-voltage SSLBs. The protocol only involves a one-pot synthesis procedure to covalently crosslink the PEO chains, in the presence of high-content lithium bis(trifluoromethylsulphonyl)imide (LiTFSI) salts and N,N-dimethylformamide (DMF). LiTFSI-DMF supramolecular aggregates are formed and firmly embedded in the polymer network, endowing the PE with high room-temperature ionic conductivity. The dissociated and highly concentrated TFSI− anions can enter the Helmholtz layer close to the high-voltage cathode, leading to the formation of a thin and homogeneous cathode electrolyte interface (CEI), mainly composed of LiF, on the cathode. The CEI with high electrochemical stability can effectively stabilize the cathode-electrolyte interface, enabling long-term stable cycling of the high-voltage LiCoO2||Li and nickel-rich NCM622||Li batteries at room temperature. The simplicity and scalability of the strategy makes the reported PEO-based PE potentially applicable in high-voltage SSLBs in practice.

B-/Si-containing electrolyte additive efficiently establish a stable interface for high-voltage LiCoO2 cathode and its synergistic effect on LiCoO2/graphite pouch cells

An effective electrolyte additive, 3-(tert-Butyldimethylsilyoxy) phenylboronic acid (TBPB), is proposed to significantly improve the cycle stability of high voltage LiCoO2 (LCO) cathode. Experimental and computational results show that TBPB has a relatively higher oxidation activity than base electrolyte, and preferentially constructs a stable cathode electrolyte interphase (CEI) containing B-/Si- components on LCO surface. Theoretical calculation, XPS and NMR data show that TBPB-derived CEI layer contains B-F species and has the function of eliminating HF. The as-formed CEI effectively inhibits the detrimental side reactions from electrolyte decomposition and LCO surface structure reconstruction. The capacity retention of LCO/Li half-cell increases from 38.92% (base electrolyte) to 83.70% after 150 cycles at 1 C between 3.0 V and 4.5 V by adding 1% TBPB. Moreover, TBPB is also reduced prior to base electrolyte, forming an ionic conducting solid electrolyte interphase (SEI) on graphite surface. Benefiting from the synergistic effect between CEI layer on LCO cathode and SEI layer on graphite anode to effectively decrease the electrolyte decomposition, the capacity retention of commercial LCO/graphite pouch cell with 1% TBPB increases from 10.44% to 76.13% after 400 cycles at 1 C between 3.0 V and 4.5 V. This work demonstrates that TBPB can act as an effective film-forming additive for high energy density LCO cathode at high voltage, and provides novel insights for its commercial application from the aspect of synergistically interfacial stability.