High Voltage Cycling Research Articles

An All‐Fluorinated Electrolyte Toward High Voltage and Long Cycle Performance Dual‐Ion Batteries (Adv. Energy Mater. 19/2022)

Dual-Ion Batteries In article number 2103360, Denis Y. W. Yu and co-workers report the development of an all-fluorinated electrolyte which enables highly reversible PF6− and Li+ storage towards high performance dual-ion batteries (DIBs). Because of its high-power capability, the DIB is suitable for modulating power supply and the demands of renewables, and for fast-charging of electric buses within minutes.

Investigating CO2 streamer inception in repetitive pulsed discharges

In this study, we investigate the responsible species and processes involved in repetitive pulsed streamer inception in CO2. We applied a 10 kV high-voltage (HV) pulse with a repetition frequency of 10 Hz and pulse width of 1 ms to a pin electrode which is placed 160 mm apart from the grounded plane electrode. We measured the inception times by a photo-multiplier tube (delay between the rising edge of the HV pulse and the rising edge of the photo-multiplier waveform) for 600 high voltage cycles. We observed one peak in the histogram of inception times with a median of 1.2 μs. To identify the source of this peak, we applied a negative or positive low-voltage (LV) pulse before the main HV pulse to manipulate the leftover space charges. Three different phenomena are observed: (1) drift, (2) neutralization, and (3) ionization in the LV pulse. At low LV amplitude and pulse width, the peak starts to drift toward the faster and slower inception times under a positive and negative LV pulse, respectively. However, under the same LV pulse configuration for positive and negative LV pulse, the observed shift in inception times is not the same. We present a hypothesis to explain this asymmetry based on the difference of the detachment processes between air and CO2.

One-step calcination reaction to synthesize Li2MnO3 coating layers for LiNi0.8Co0.1Mn0.1O2 to improve cycling performances under high-voltage for Li-ion batteries

Poor cycling performances restrict the large-scale application of Ni-rich LiNixCoyMnzO2 (x + y + z = 1, x ≥ 0.6) cathode materials, especially during high cut-off voltage cycling. Surface modification is proposed as a feasible method to improve the electrochemical performances for Ni-rich cathode materials. However, traditional coating method is defective for final products. Here, we propose an innovative strategy to synthesize coating layers on the surface of Ni-rich cathode materials by one-step calcination, for example, Li2MnO3 layer is coating on the surface of LiNi0.8Co0.1Mn0.1O2, and invest the optimum ratio of the coating layers to the materials. A diversity of characterization methods demonstrated the existence of Li2MnO3 on the surface of LiNi0.8Co0.1Mn0.1O2. The LiNi0.8Co0.1Mn0.1O2 with 5 % (mole ratio) Li2MnO3 coating layers exhibits much enhanced cycling performance when compared with the bare material. When charge-discharged at 1C (200 mA g−1) in the voltage range of 2.8–4.6 V, it delivers 169.6 mAh g−1 at the 100th cycling with capacity retention of 90.6%, whereas the bare material shows capacity retention of 81.1%. The enhanced electrochemical performance of the materials should be attributed to the Li2MnO3 coating layers, which not only protect the active cathode material from the electrolyte, but also suppress the phase transition of the materials from H2 to H3.

High-Voltage Induced Surface and Intragranular Structural Evolution of Ni-Rich Layered Cathode.

Layered Ni-rich lithium transition metal oxides are promising cathode materials for high-energy-density lithium-ion batteries. These cathodes, however, suffer from rapid performance decay under high-voltage operation. In this work, the electrochemical properties and structural evolution of the LiNi0.8 Mn0.1 Co0.1 O2 (NMC811) cathode upon high-voltage cycling are investigated. The results show that the NMC811 cathode not only experiences surface evolution with the formation of Li-deficient rock-salt layers, but also suffers from drastic intragranular structural changes inside bulk grains after high-voltage cycling. Direct evidence for the formation of transition-metal/Li disordering domains with uneven Li content and lattice plane distortion at the internal grains of 4.6V-cycled NMC811 are provided with their atomic ordering and spatial distribution clearly resolved. The complex intragranular structural changes impede Li+ diffusion inside bulk material, resulting in kinetic limitation and capacity loss. The results demonstrate that the high-voltage cycling would induce severe structural degradation at the grain interior of the cathode material beyond surface evolution, which contributes significantly to the rapid performance decay of the NMC811 cathode. The findings provide new insights for developing effective countermeasures to mitigate this degradation pathway.

Boosting the ionic transport and structural stability of Zn-doped O3-type NaNi1/3Mn1/3Fe1/3O2 cathode material for half/full sodium-ion batteries

The O3-type NaNi1/3-xMn1/3Fe1/3ZnxO2 (x = 0, 0.005, 0.01) materials were prepared by ball milling-spray drying process. XRD, EDS and XPS results proved that the Zn element was successfully embedded in the lattice and replaced the position of Ni. The sample with Zn doping amount x = 0.005 (NMF-Zn 0.005) exhibited significant improvements in long-term stability and rate capability, the initial capacity was 123.3 mAh g−1 with the capacity retention of 86.53% after 200 cycles at 1 C. Even at higher 5 C, the discharge specific capacity was still up to 102.5 mAh g−1 with the capacity attenuation of 13.96%. Moreover, the full-cell with NMF-Zn 0.005 as cathodes and hard carbon as anodes had been studied, which could deliver excellent cycling of 82.08% over 100 cycles at 1 C. Finally, combining the results of ex-situ XRD and EDS spectrum, the modification mechanism of Zn under high voltage cycling was explained in detail.

Mg,Ti-base surface integrated layer and bulk doping to suppress lattice oxygen evolution of Ni-rich cathode material at a high cut-off voltage

The Nickel-rich layered cathode materials charged to 4.5 V can obtain a specific capacity of more than 200 mAh g−1. However, the nickel-rich layered cathode materials suffer from the severe capacity fade during high-voltage cycling, which is related to the phase transformation and the surface sides reactions caused by the lattice oxygen evolution. Here, the simultaneous construction of a Mg, Ti-based surface integrated layer and bulk doping through Mg, Ti surface treatment could suppress the lattice oxygen evolution of Ni-rich material at deep charging. More importantly, Mg and Ti are co-doped into the particles surface to form an Mg2TiO4 and Mg0.5–xTi2–y(PO4)3 outer layer with Mg and Ti vacancies. In the constructed surface integrated layer, the reverse electric field in the Mg2TiO4 effectively suppressed the outward migration of the lattice oxygen anions, while Mg0.5–xTi2–y(PO4)3 outer layer with high electronic conductivity and good lithium ion conductor could effectively maintained the stability of the reaction interface during high-voltage cycling. Meanwhile, bulk Mg and Ti co-doping can mitigate the migration of Ni ions in the bulk to keep the stability of transition metal–oxygen (M−O) bond at deep charging. As a result, the NCM@MTP cathode shows excellent long cycle stability at high-voltage charging, which keep high capacity retention of 89.3% and 84.3% at 1C after 200 and 100 cycles under room and elevated temperature of 25 and 55 °C, respectively. This work provides new insights for manipulating the surface chemistry of electrode materials to suppress the lattice oxygen evolution at high charging voltage.

Regulation of Surface Defect Chemistry toward Stable Ni-Rich Cathodes.

Surface reconstruction of Ni-rich layered oxides (NLO) degrades the cycling stability and safety of high-energy-density lithium-ion batteries (LIBs), which challenges typical surface-modification approaches to build a robust interface with electrochemical activity. Here, a strategy of leveraging the low-strain analogues of Li- and Mn-rich layered oxides (LMR) to reconstruct a stable surface on the Ni-rich layered cathodes is proposed. The new surface structure not only consists of a gradient chemical composition but also contains a defect-rich structure regarding the formation of oxygen vacancies and cationic ordering, which can simultaneously facilitate lithium diffusion and stabilize the crystal structure during the (de)lithiation. These features in the NLO lead to a dramatic improvement in electrochemical properties, especially the cyclability under high voltage cycling, exhibiting the 30% increase in capacity retention after 200 cycles at the current density of 1C (3.0-4.6V). The findings offer a facile and effective way to regulate defect chemistry and surface structure in parallel on Ni-rich layered structure cathodes to achieve high-energy density LIBs.

Artificial cathode electrolyte interphase for improving high voltage cycling stability of thick electrode with Co-free 5 V spinel oxides

Spinel-type cathode LiNi0.5Mn1.5O4 (LNMO) has intrigued the transportation industry due to its high operating voltage and total elimination of the expensive cobalt element. However, LNMO cathode with high mass loading (> 3 mAh/cm2 in areal capacity) has suffered from excessive capacity degradation upon long cycling. Here, a robust Al2O3 surface layer is introduced to the thick LNMO electrode via atomic layer deposition (ALD). The capacity retention in full cells with the graphite anode is improved from 46.3% to 75.3% after 300 cycles with cutoff voltage up to 4.85 V, while enabling average Coulombic efficiency of 99.9% during the cycling. The post-mortem analyses reveal that the Al2O3 surface layer would convert to Al-O-F /Al-F species upon cycling, offering stable interphase to protect the cathode material. These results demonstrate the significance of surface modification enabling high voltage cathode for next-generation LIBs.

P2‐Na2/3Ni2/3Te1/3O2 Cathode for Na‐ion Batteries with High Voltage and Excellent Stability

Air‐stable layered structured cathodes with high voltage and good cycling stability are highly desired for the practical application of Na‐ion batteries. Herein, we report a P2‐Na2/3Ni2/3Te1/3O2 cathode that is stable in ambient air with an average operating voltage of ~3.8 V, demonstrating excellent cycling stability with a capacity retention of more than 92.7% after 500 cycles at 20 mA g−1 and good rate capability with 91.9% capacity utilization at 500 mA g−1 with respect to capacity at 5 mA g−1 between 2.0 and 4.0 V. When the upper cutoff voltage is increased to 4.4 V, P2‐Na2/3Ni2/3Te1/3O2 delivers a reversible capacity of 71.9 mAh g−1 and retains 91.8% of the capacity after 100 cycles at 20 mA g−1. The charge compensation during charge/discharge is mainly due to the redox couple of Ni2+/Ni3+ in the host with a small amount of contribution from oxygen. The stable structure of the material without phase transformation and with small volume change during charge‐discharge allows it to give excellent cycle performance especially when the upper cutoff voltage is not higher than 4.2 V.

Boosting high voltage cycling of LiCoO2 cathode via triisopropanolamine cyclic borate electrolyte additive

Energy density of lithium-ion batteries (LIBs) can be promoted by raising the charge cutoff voltage, which faces the challenges of accelerated structural and interfacial degradation. Introducing a functional electrolyte additive is considered as an effective approach by forming a robust and protective cathode electrolyte interphase (CEI). Herein, triisopropanolamine cyclic borate (TPCB) additive is probed for the first time as an electrolyte additive to significantly enhance the interfacial stability of LiCoO2 (LCO) cathode at high charge cutoff voltages (≥4.6 V). With optimized 0.5% TPCB in electrolyte, a thin (about 8 nm) and dense CEI layer, containing C–N, B–O/B–F species and less LiF, is generated to suppress the decomposition of electrolyte at high voltages and alleviate structure damage of LCO during long-term cycling. As a result, the LCO cathode in the TPCB-modified electrolyte displays remarkably enhanced capacity retention of 82.2% after 200 cycles at 1C at high charge cut-off 4.6 V, exceeding that of 63.2% in the Base electrolyte.

High-Potential Cathodes with Nitrogen Active Centres for Quasi-Solid Proton-Ion Batteries.

Although proton-ion batteries have received considerable attention owing to their reliability, safety, toxin-free nature, and low cost, their development remains in the early stages because of lacking proper electrolytes and cathodes for facilitating a high output voltage and stable cycle performance. We present a novel cathode based on active nitrogen centre, which provides a flat discharge plateau at 1 V with a capacity of 115 mAh g-1 and excellent stability. Moreover, a quasi-solid electrolyte was developed to overcome the issue of corrosion, broaden the potential window of the electrolyte, and prevent the active material from dissolving. While using the unique as-developed electrolyte, the newly designed cathode retained 89.67 % of its original capacity after 2000 cycles. Finally, we demonstrated the excellent cycle performance of the as-developed metal-free, flexible, soft-packed battery. Notably, even when a portion of the battery was cut off, it continued to function normally.

High‐Potential Cathodes with Nitrogen Active Centres for Quasi‐Solid Proton‐Ion Batteries

AbstractAlthough proton‐ion batteries have received considerable attention owing to their reliability, safety, toxin‐free nature, and low cost, their development remains in the early stages because of lacking proper electrolytes and cathodes for facilitating a high output voltage and stable cycle performance. We present a novel cathode based on active nitrogen centre, which provides a flat discharge plateau at 1 V with a capacity of 115 mAh g−1 and excellent stability. Moreover, a quasi‐solid electrolyte was developed to overcome the issue of corrosion, broaden the potential window of the electrolyte, and prevent the active material from dissolving. While using the unique as‐developed electrolyte, the newly designed cathode retained 89.67 % of its original capacity after 2000 cycles. Finally, we demonstrated the excellent cycle performance of the as‐developed metal‐free, flexible, soft‐packed battery. Notably, even when a portion of the battery was cut off, it continued to function normally.

Enhanced rate capability and high-voltage cycling stability of single-crystal nickel-rich cathode by surface anchoring dielectric BaTiO3

The single-crystal Ni-rich Li(NixCoyMn1−x−y)O2 cathode (NCM) demonstrates better cycle performance, enhanced tap density and improved mechanical structure stability, compared with polycrystalline NCM.However, limited Li+ transports, (003) plane slips and microcracks in large single particles hinder rate capability and cycle performance. To overcome these shortcomings,single-crystal NCM cathodes have been modified by nanosized tetragonal BaTiO3. Due to the dielectric properties, BaTiO3 particles induce electric field concentration at the BaTiO3-NCM-electrolyte interface. Thus, a large amount of lithium vacancies can be formed, providing sufficient sites for the hopping diffusion of lithium ions, thereby significantly enhancing the diffusion coefficient of Li+. Moreover, the redistribution of charges can inhibit the formation and accumulation of cathode-electrolyte-interface. Owing to the synergetic effect of BaTiO3, the BT-modified single-crystal NCM with the optimized loading shows a remarkable initial discharge capacity of 138.5 mAh g−1 and maintains 53.8% of its initial discharge capacity after 100 cycles under 5C at 4.5 V cut-off voltage. Overall, the proposed dielectric cathode-electrolyte-interface strategy can enhance Li+ ion transport and stabilize the interface structure, leading to improved rate performance. Meanwhile, the diffusion-induced state of charge gradient can also be inhibited, resulting in high structure stability of single-crystal NCMs under high rate and cut-off voltage cycling.

Enhanced interfacial stability of Ni-rich cathode for Li-ion batteries towards excellent high-voltage performance

Ni-rich layered oxide LiNi0.8Co0.1Mn0.1O2 (NCM811) is one of the most promising cathode materials in advanced lithium-ion batteries. However, it suffers from poor cycling and rate capabilities due to the interfacial instability and structural degradation especially at high-voltage operation (＞4.3 V which are the key barriers for developing high-energy density cathode materials. Herein, we present an effective modification strategy toward the LiNi0.8Co0.1Mn0.1O2 through coating a rather uniform AZO protective layer on its surface, which can effectively passivate the deleterious side reactions at the cathode/electrolyte interface and improve structural stability of the layered oxides at 4.5 V high-voltage cycles. As a result, the AZO-modified Ni-rich cathode (AZO-NCM811) exhibits a great capacity retention of 86.3% after 100 cycles over 3.0–4.5 V at room temperature, which is superior than those for the pristine electrode (only 69.9%). Moreover, the AZO coating with a high ionic/electronic conductivity also endows LiNi0.8Co0.1Mn0.1O2 with a high-rate performance. This work provides a practical approach for developing the commercial application of Ni-rich cathode materials, more importantly, it may also be applied to resolve the interface instability issues occurring for other high-voltage cathode materials.

A melatonin-inspired coating as an electrolyte preservative for layered oxide cathode-based lithium batteries

Layered transition metal oxides are key cathode materials for obtaining high energy density lithium batteries by virtue of their high specific capacities and operating voltages. However, harmful active species (e.g. reactive oxygen and free radicals) give rise to severe electrolyte decomposition during high-voltage cycling, and subsequently result in rapid battery capacity decay. Inspired by the reactive species-cleanup activity of melatonin, we first build up an endogenous chemically protective cathodic polymer coating as a preservative to relieve electrolyte decomposition. This coating is prepared by casting the homopolymer of 2,2,6,6-tetramethyl-4-piperidyl methacrylate on the cathode via a spin-coating method. Detailed experimental and simulation studies reveal that the as-casted coating can efficiently capture reactive species, sequentially favorable construct compatible cathode/electrolyte interface. Moreover, it is demonstrated that electrolyte solvent decomposition routes can be tuned by this chemically protective coating, compared with traditional Al2O3 coating. Resultantly, the polymer coating-based LiNi0.8Co0.1Mn0.1O2 cathode shows 85% capacity retention after 700 cycles in half cells under 2.7–4.5 V at 1C, evidently outperforming that (35%) of the pristine one. Notably, this polymer coating is also effective for other layered oxide cathode-based lithium batteries. This chemical protection design strategy marks an important milestone to address electrolyte decomposition involving reactive species in rechargeable batteries.

Reversible Redox Chemistry in Pyrrolidinium‐Based TEMPO Radical and Extended Viologen for High‐Voltage and Long‐Life Aqueous Redox Flow Batteries

AbstractAqueous organic redox flow batteries (AORFBs) are regarded as a promising candidate for grid‐scale, low‐cost and sustainable energy storage. However, their performance is restricted by low aqueous solubility and the narrow potential gap of the organic redox‐active species. Herein, a highly‐soluble organic redox pair based on pyrrolidinium cation functionalized TEMPO and extended viologen, namely Pyr‐TEMPO and [PyrPV]Cl4, which exhibits high cell voltage (1.57 V) and long cycling life (over 1000 cycles) in AORFBs is reported. The intrinsic hydrophilic nature of the pyrrolidinium group enables high aqueous solubilities (over 3.35 m for Pyr‐TEMPO and 1.13 m for [PyrPV]Cl4). Furthermore, the interaction of nitroxyl radicals with water is observed, which may be helpful to prevent collision‐induced side reactions or structure decomposition. Notably, the assembled AORFBs realize a high energy density of 16.8 Wh L‐1 and a peak power density of 317 mW cm‐2. The evidence is provided to clarify the capacity degradation mechanism of TEMPO/viologen AORFB systems by a series of comprehensive characterizations. Furthermore, the reversible consumption and re‐generation of the nitroxyl radicals upon charging and discharging are well understood. This work presents effective electrochemical and spectroscopic approaches to clarify the redox chemistry and capacity degradation mechanism of radical incorporating AORFB systems.

An All‐Fluorinated Electrolyte Toward High Voltage and Long Cycle Performance Dual‐Ion Batteries

AbstractThe dual‐ion battery (DIB) is a promising energy storage system that demonstrates high‐power characteristics and fast‐charging capability. However, conventional electrolytes are not compatible with the high‐voltage graphite cathode and the reactive Li metal anode, thus leading to the poor cycle stability and low Coulombic efficiency of the DIB. Here, an all‐fluorinated electrolyte is reported that can enable a highly stable operation of the graphite||Li DIB up to 5.2 V by forming robust and less‐resistive passivation films on both electrodes to reduce side reactions. The electrolyte allows reversible PF6– anion insertion/extraction and Li+ cation plating/stripping in the graphite||Li battery, achieving stable cycling with 94.5% capacity retention over 5000 cycles at 500 mA g–1, high capacity utilization of 91.8% of the available charge capacity at 50 C (5000 mA g–1), and also minimal self‐discharge. At a low temperature of 0 °C, this all‐fluorinated electrolyte exhibits 97.8% of the room temperature reversible capacity, along with ≈100% capacity retention after more than 3000 cycles, at 5 C. This work sheds a new light on the development of fluorinated electrolytes for high voltage and long‐lasting DIBs.

A three-in-one engineering strategy to achieve LiNi0.8Co0.1Mn0.1O2 cathodes with enhanced high-voltage cycle stability and high-rate capacities towards lithium storage

Recently, the Ni-rich layered LiNi0.8Co0.1Mn0.1O2 (NCM811) is established as a promising cathode material for next-generation lithium-ion batteries (LIBs). However, it always suffers from severe bulk structural and interfacial degradation during electrochemical operation. To well address the issues, herein, a three-in-one engineering strategy is devised to construct a stable Nb-doped full concentration-gradient NCM811 coated with rhombohedral LiNbO3 (denoted as N-FCG-NCM811@LNO). In the strategy, each secondary particle possesses a Ni-rich core and a Co/Mn-rich outer surface, along with the Li+-conductive LNO coating layer and surface gradient Nb doping. It is the synergistic contributions from three-aspect design (i.e., FCG construction, surface coating and heteroatom doping) that renders NCM811 cathodes with stable crystalline structure and surface, mitigated microcracks, accelerated Li+ diffusion kinetic, and suppressed interfacial phase transition over cycling. The optimum N-FCG-NCM811@LNO achieves the improved capacity retention at high cut-off voltages (∼85.1% after 300 cycles between 3.0 and 4.4 V, and ∼80.5% after 200 cycles between 3.0 and 4.5 V both at 1C rate) and better high-rate capability (∼162.4 mAh g−1 at 10C). More meaningfully, the instructive electrode design concept here can be extended to other Ni-rich cathodes for high-energy/power LIBs via the simple yet scalable fabrication process.

Revisiting the capacity-fading mechanism of P2-type sodium layered oxide cathode materials during high-voltage cycling

P2-type sodium layered oxide cathode (Na2/3Ni1/3Mn2/3O2, P2-NNMO) has attracted great attention as a promising cathode material for sodium ion batteries because of its high specific capacity. However, this material suffers from a rapid capacity fade during high-voltage cycling. Several mechanisms have been proposed to explain the capacity fade, including intragranular fracture caused by the P2-O2 phase transion, surface structural change, and irreversible lattice oxygen release. Here we systematically investigated the morphological, structural, and chemical changes of P2-NNMO during high-voltage cycling using a variety of characterization techniques. It was found that the lattice distortion and crystal-plane buckling induced by the P2-O2 phase transition slowed down the Na-ion transport in the bulk and hindered the extraction of the Na ions. The sluggish kinetics was the main reason in reducing the accessible capacity while other interfacial degradation mechanisms played minor roles. Our results not only enabled a more complete understanding of the capacity-fading mechanism of P2-NNMO but also revealed the underlying correlations between lattice doping and the moderately improved cycle performance.

Elucidating and Mitigating High‐Voltage Degradation Cascades in Cobalt‐Free LiNiO2 Lithium‐Ion Battery Cathodes (Adv. Mater. 3/2022)

Lithium-Ion Batteries In article number 2106402, Mark C. Hersam and co-workers show that lattice oxygen loss plays a critical role in the O3–O1 stacking transition in cobalt-free LiNiO2 lithium-ion battery cathodes, which subsequently induces Ni-ion migration and irreversible stacking faults, microscale electrochemical creep, cracking, and even bending of layers after high-voltage cycling. By suppressing oxygen evolution, hermetic graphene coatings arrest this degradation cascade, resulting in substantially improved high-voltage capacity retention.