Improved Cycling Stability Research Articles

Observation and suppression of metallic and metallic-like plating on hard carbon for high-performance sodium-ion batteries

Increasing the lifespan of Na-ion batteries (NIBs) is one of the primary requirements for stationary energy storage. Metallic and metallic-like Na plating on hard carbon (HC) could be one of the main causes for degraded performance during cycling, but the mechanism remains unclear. Here, we systematically investigate the plating phenomenon on HC under various cycling conditions in NIBs. We reveal that underpotential deposition (UPD) could potentially trigger the metallic-like plating on the surface of HC electrodes. In-situ distribution of relaxation times (DRT) analysis provides an effective indictor to identify the occurring of metallic and metallic-like Na plating on HC electrode by probing the time constant for charge transfer process. Based on the understanding, we apply a uniform nano porous coating layer on HC electrode to restrict potential UPD on the HC electrode surface but allow uniform Na+ transport across interface, which greatly suppress the plating phenomenon, leading to significantly improved cycling stability of NIBs.

Regulation of fiber surface nucleation kinetics via ordered nitrogen-rich carbon dots

Three dimensional (3D) carbon-based current collectors have attracted much attentions as host for Li metal anodes (LMAs) owing to their lightweight, flexible, and wide sources etc. However, the lithiophobic nature of carbon makes it difficult to achieve low nucleation overpotential and dendrite-free Li deposition. Herein, carbon dots (CDs) with nitrogen (N)-rich were introduced to modify carbonized cotton fiber (CCF) coated by reduced graphene oxide (CD@RGCCF). Coating graphene oxide (GO) increases the active site of cotton fiber (CF), which facilitates the growth of CDs on the surface of CF fiber. The introduction of ordered structure CDs drives fast migration of Li+ in the “highway-like” channels and induces the uniform nucleation of Li in the initial stage. As a result, the obtained CD@RGCCF decreases the nucleation overpotential of Li, and achieves dendrite-free Li deposition. the CD@RGCCF|Li half cells exhibit extending cycling life over 5000 h with a voltage hysteresis of 10.5 mV at 2 mA cm−2. As coupled with the LiFePO4 (LFP), the Li@CD@RGCCF||LFP composite anode enables a high reversible specific capacity and improved cycle stability. This work provides a low-cost modification method for carbon substrate to realize dendritic-free LMAs.

Air-Stable and Low-Cost High-Voltage Hydrated Eutectic Electrolyte for High-Performance Aqueous Aluminum-Ion Rechargeable Battery with Wide-Temperature Range.

Aqueous aluminum-ion batteries (AAIBs) are considered as a promising alternative to lithium-ion batteries due to their large theoretical capacity, high safety, and low cost. However, the uneven deposition, hydrogen evolution reaction (HER), and corrosion during cycling impede the development of AAIBs, especially under a harsh environment. Here, a hydrated eutectic electrolyte (AATH40) composed of Al(OTf)3, acetonitrile (AN), triethyl phosphate (TEP), and H2O was designed to improve the electrochemical performance of AAIBs in a wide temperature range. The combination of molecular dynamics simulations and spectroscopy analysis reveals that AATH40 has a less-water-solvated structure [Al(AN)2(TEP)(OTf)2(H2O)]3+, which effectively inhibits side reactions, decreases the freezing point, and extends the electrochemical window of the electrolyte. Furthermore, the formation of a solid electrolyte interface, which effectively inhibits HER and corrosion, has been demonstrated by X-ray photoelectron spectroscopy, X-ray diffraction tests, and in situ differential electrochemical mass spectrometry. Additionally, operando synchrotron Fourier transform infrared spectroscopy and electrochemical quartz crystal microbalance with dissipation monitoring reveal a three-electron storage mechanism for the Al//polyaniline full cells. Consequently, AAIBs with this electrolyte exhibit improved cycling stability within the temperature range of -10-50 °C. This present study introduces a promising methodology for designing electrolytes suitable for low-cost, safe, and stable AAIBs over a wide temperature range.

Single-Crystal P2-Na0.67Mn0.67Ni0.33O2 Cathode Material with Improved Cycling Stability for Sodium-Ion Batteries.

Layered oxides constitute one of the most promising cathode materials classes for large-scale sodium-ion batteries because of their high specific capacity, scalable synthesis, and low cost. However, their practical use is limited by their low energy density, physicochemical instability, and poor cycling stability. Aiming to mitigate these shortcomings, in this work, we synthesized polycrystalline (PC) and single-crystal (SC) P2-type Na0.67-δMn0.67Ni0.33O2 (NMNO) cathode materials through a solid-state route and evaluated their physicochemical and electrochemical performance. The SC-NMNO cathode with a large mean primary particle size (D50) of 12.7 μm was found to exhibit high cycling stability leading to 47% higher capacity retention than PC-NMNO after 175 cycles at 1C rate in the potential window 4.2-1.5 V. This could be attributed to the effective mitigation of parasitic side reactions at the electrode-electrolyte interface and suppressed intergranular cracking induced by anisotropic volume changes. This is confirmed by the lower volume variation of SC-NMNO (ΔV ∼ 1.0%) compared to PC-NMNO (ΔV ∼ 1.4%) upon charging to 4.2 V. Additionally, the SC-NMNO cathode displayed slightly higher thermal stability compared to PC-NMNO. Both cathodes exhibited good chemical stability against air and water exposure, thus enabling material storage/handling in the ambient atmosphere as well as making them suitable for aqueous processing. In this regard, PC-NMNO was investigated with two low-cost aqueous binders, carboxymethyl cellulose, and sodium trimetaphosphate, which exhibited higher binding strength and displayed excellent electrochemical performance compared to PVDF, which could potentially lead to significant cost reduction in electrode manufacturing.

Reconstruction of Electric Double Layer on the Anode Interface by Localized Electronic Structure Engineering for Aqueous Zn Ion Batteries

AbstractThe electric double layer (EDL) at the electrode/electrolyte interface plays a crucial role to the electrochemical reactions of zinc ion batteries. For Zn anode, the EDL consists of H2O dipoles, which can cause Zn corrosion and passivation. Herein, the localized electronic‐rich (LER) structure performing as soild electrolyte interphase (SEI) changes the electron distribution, leading to the rapid capture of Zn2+, thus promoting the desolvation of the cH2O shell. Moreover, the LER generates an electrostatic repulsion effect to SO42−. Consequently, a unique H2O‐poor EDL is reconstructed with the distribution of Zn2+‐H2O‐SO42−, which inhibits side reactions and improves the deposition kinetics of Zn2+. In situ Raman intuitively confirms that the zinc‐ion‐flux is uniform during the whole electroplating process. LER as regulator for EDL structure, leads to smooth and fast Zn2+ deposition. The performance enhancement is demonstrated by LER@Zn//LER@Zn cells, which exhibit exceptional lifespan for 4800 h. Furthermore, the LER@Zn///MnO2 cell shows improved cycling stability over 1500 cycles, with a high capacity of 124 mAh g−1 at 5 C.

Nonflammable Succinonitrile-Based Deep Eutectic Electrolyte for Intrinsically Safe High-Voltage Sodium-Ion Batteries.

Intrinsically safe sodium-ion batteries are considered as a promising candidate for large-scale energy storage systems. However, the high flammability of conventional electrolytes may pose serious safety threats and even explosions. Herein, a strategy of constructing a deep eutectic electrolyte is proposed to boost the safety and electrochemical performance of succinonitrile (SN)-based electrolyte. The strong hydrogen bond between S═O of 1,3,2-dioxathiolane-2,2-dioxide (DTD) and the α-H of SN endows the enhanced safety and compatibility of SN with Lewis bases. Meanwhile, the DTD participates in the inner Na+ sheath and weakens the coordination number of SN. The unique solvation configuration promotes the formation of robust gradient inorganic-rich electrode-electrolyte interphase, and merits stable cycling of half-cells in a wide temperature range, with a capacity retention of 82.8% after 800 cycles (25°C) and 86.3% after 100 cycles (60°C). Correspondingly, the full cells deliver tremendous improvement in cycling stability and rate performance.

A novel carbon-induced-porosity mechanism for improved cycling stability of magnesium hydride

MgH2 has been extensively studied as one of the most ideal solid hydrogen storage materials. Nevertheless, rapid capacity decay and sluggish hydrogen storage kinetics hamper its practical application. Herein, a Ni/C nano-catalyst doped MgH2 (MgH2Ni/C) shows an improved hydrogen absorption kinetics with largely reduced activation energy. Particularly, the MgH2Ni/C displays remarkable cycling stability, which maintains a high capacity of 6.01 wt% (98.8 % of initial capacity) even after 50 full hydrogen ab/desorption cycles, while the undoped MgH2 counterpart retains only 85.2 % of its initial capacity. Detailed microstructure characterizations clearly reveal that particle sintering/growth accounts primarily for the deterioration of cycling performance of undoped MgH2. By comparison, MgH2Ni/C can maintain a stable particle size with a growing porous structure during long-term cycling, which effectively increases the specific surface of the particles. A novel carbon-induced-porosity stabilization mechanism is proposed, which can stabilize the proportion of rapid hydrogen absorption process, thus dominating the excellent cycling performance of MgH2Ni/C. This study provides new insights into the cycling stability mechanism of carbon-containing Mg-based hydrogen storage materials, thus promoting their practical applications.

Improved Cycling Stability of LFP by W-Ti Co-Doping Strategy for Li-Ion Batteries

Olivine-type lithium iron phosphate (LFP) is used widely as a cathode material with excellent cycling stability, thermal stability, and low cost for lithium-ion batteries in power vehicles and energy storage. However, it has inherent drawbacks that limit its further development, such as low electronic conductivity and lithium ion diffusion rate. Here, secondary spherical particles LFP-W/Ti@C were synthesized using a simple sanding and spray drying method. The pyrolysis of glucose forms a uniform and dense amorphous carbon on the surface of LFP, and the constructed three-dimensional conductive network accelerates the transfer of electrons and ions while mitigating the volume expansion during the lithiation-delithiation process. In addition, the synergistic co-doping of W/Ti achieves nanosizing of primary particles with more active sites providing locations for lithium ions storage, while reducing polarization and internal resistance and enhancing the diffusion kinetics of lithium ion. The LFP-W/Ti@C obtained after optimization still has a discharge capacity of 107.7 mAh g−1 (with a cycle retention rate of up to 100%) after 500 cycles at 5 C. This simple W/Ti co-doping strategy provides excellent rate performance and shows great potential for fast charging in electric vehicles.

Remarkably improved cycling stability of commercial micron-sized silicon enabled by a novel organic/inorganic hybrid binder

Silicon (Si) anode with ultra-high capacity and low cost are highly desirable for next-generation high-energy lithium-ion batteries, whereas suffers from severe volume change that causes rapid capacity decay upon lithiation/delithiation. The development of high-performance binders to alleviate volumetric expansion and maintain integrity of Si particles remains a significant challenge. Here, reported is a novel organic/inorganic hybrid binder composed of partial neutralized polyacrylic acid (PAAS) and sodium metaborate tetrahydrate (SMTH), which achieves in-situ crosslinking upon drying wafer electrodes. The robust crosslinked network structure with borate ester bonds is successfully attained, and can be modulated by changing the content of sodium metaborate. The strong covalent bond network endows the hybrid binder system with superior mechanical properties and powerful adhesion, which improve the contact and integrity of wafer silicon electrode by suppressing volume expansion and exfoliation, therefore facilitating Li-ion migration and enhancing electrochemical performance. The resulting commercial micro-silicon wafer electrode with PAAS/0.08SMTH binder remains a reversible specific capacity of 1670 mAh g−1 after 150 cycles at 300 mA g−1, largely superior to that prepared with the original PAAS binder because its reversible specific capacity was determined to be only 1046 mAh g−1 under identical conditions. The in-situ crosslinked 3D network structure provides a facile feasible way to improve the high-capacity anode materials for lithium-ion batteries.

Multiwalled-Carbon-Nanotube-Modified Li2ZnTi3O8 as Anode with Improved Cycling Stability and Rate Capability for Lithium-Ion Batteries

Due to its better electronic conductivity and larger lithium intercalation capacity, a multiwalled-carbon-nanotube (MWCNT)-modified Li2ZnTi3O8 (LZTO) has significant potential for use as an anode material for Li-ion batteries. The rapid and affordable ball-mill-aided solid state approach is used to synthesize LZTO with MWCNT-modified Li2ZnTi3O8 (LZTO@MWCNTs). Electrochemical performance tests of the anode material were carried out using cyclic voltammetry (CV), galvanostatic charge-discharge measurement, and electrochemical impedance spectroscopy (EIS). The CV experiments reveal that the LZTO@MWCNT electrode has a lower anodic and cathodic peak potential difference (0.184 V) than the LZTO electrode (0.211 V) and LZTO@MWCNT electrode has better electrochemical reversibility than that of pristine electrode after five cycles utilizing the same procedure. LZTO@ MWCNTs anode material has a higher charge-discharge capacity than LZTO anode material at 0.1–5 C rates. After 100 cycles, the initial discharge capacities of LZTO@MWCNT electrode are 227 and 142 mAh g−1 at the 1C–5C rate, respectively, whereas the initial discharge capacities of LZTO electrode are only144 and 28 mAh g−1 in the same condition. The charge transfer and SEI resistance of the LZTO anode are 19.31 and 62.74 ohms, respectively, after 30 cycles at 1C, according to the EIS data. Additionally, the values for the LZTO@MWCNTs anode are 7.93 and 12.06 ohms for charge transfer and SEI resistance, respectively.

Bulk doping of tungsten improves the structure stability and electrochemical performance of layered Ni-rich material

Tungsten doping has attracted great attention to improve the electrochemical performance of layered Ni-rich cathodes. However, the role of tungsten dopant remains unclear since tungsten is usually rich on surface rather than incorporated into bulk lattices in layered Ni-rich particles, therefore the improved performance is mostly ascribed to the W coating layers or preferred orientation microstructures. In this work, as low as 0.1 mol% of tungsten bulk doping is introduced into a layered Ni-rich material during the coprecipitation of precursor. As a result, the tungsten bulk-doped LiNi0.8Co0.1Mn0.1O2 (NCM811-W) cathode shows significantly improved cycling stability at 1 C, with a capacity retention of 95.06 % after 200 cycles (compared to 70.83 % for the undoped NCM811 cathode). The improved performance of NCM811-W cathode is mainly attributed to strong W–O bonding within the lattices of NCM811, which can increase the reversibility of the two-phase reaction (H2–H3) of layered Ni-rich material and reduce the migration energy barrier of lithium ions. This work represents a step forward in understanding the high-valence metal ions-doping for high-performance Ni-rich cathodes.

Suppressed phase separation in high-voltage LiNi0.5Mn1.5O4 cathode via Co-doping

Spinel LiNi0.5Mn1.5O4 (LNMO) is a promising cathode material with high operating voltage and energy density. However, the poor rate performance caused by the two-phase reaction during charging and discharging limits its large-scale application. In this work, Co3+ doped LNMO cathode materials are prepared by co-precipitation method, and the effect of Co3+ doping replacing Mn4+, as well as the amount of Co3+ doping (LiNi0.5CoxMn1.5-xO4) are explored. Co3+ doping increases the content of Mn3+, thus improving the electronic conductivity of LNMO. Co3+ doping suppresses the phase transition of LNMO during discharging, which results in a smaller volume change and improves cycling stability. The presence of a solid solution intermediate allows for more uniform changes within the particles during the discharge process, thus increasing the diffusion rate of Li+ ion. LiNi0.5Co0.05Mn1.45O4 exhibits the best electrochemical performance compared to other samples, with a discharge specific capacity of 111.1 mAh g−1 after 500 cycles at 1 C and a capacity retention of 92.4%. At 10 C high rate, the specific discharge capacity is 106.8 mAh g−1. Our research reveals that Co3+ doping can improve the electrochemical performance of LNMO and provide a deeper insight into Co3+ doping of other cathode materials.

The hierarchical dense structure induced high stability to NiCoB-based electrode for electrochemical energy storage

The construction of stable hierarchical surfaces through structural engineering is the key to improve reactive active sites and cycle stability to achieve high cycle performance of supercapacitors (SCs). In this work, the NiCo-LDH nanoflower as a structure guide agent was used to support NiCoB nanosheets to form a dense and stable hierarchical structure, thereby exposing more active sites and improving cycle stability. Due to the hierarchical stable surface structure, the NiCoB-0.3@NiCo-LDH-30 electrode has an excellent specific capacitance of 2710F g−1 at 1 A/g due to the excellent electrochemical active surface area (1259 mF cm−2), improving the OH– diffusion coefficient (2.4 × 10−9 cm2 s−1) and decreasing ionic diffusion barrier. After 5000 cycles, NiCoB-0.3@NiCo-LDH-30 electrode still has 92.6 % initial specific capacitance. In order to balance the energy density decrease caused by the capacitance imbalance between positive and negative electrodes, the cubed carbon (Co-C) derived from cobalt metal organic frameworks (Co-MOFs) as cathode with a good specific capacitance of 220F g−1 at 1 A/g is prepared. The assembled NiCoB-0.3@NiCo-LDH-30//Co-C hybrid SCs (HSCs), which are assembled with NiCoB-0.3@NiCo-LDH-30 electrode as anode and Co-C electrode as cathode, displays an energy density of 75 Wh kg−1 at a power density of 741 W kg−1.

Largely improved cycling stability of CoV2O6 supercapacitor electrode synthesized by deep eutectic solvent

Transition metal oxides composed of cobalt-based and vanadate configurations are widely used in supercapacitor electrodes due to the abundant nature of the various elements and their outstanding electrochemical performance. However, redox and ion intercalation mechanisms are involved between the electrodes and electrolyte, which inevitably lead to large differences in capacity retention and long-term cycling ability of synthetic electrodes by different preparation methods. In this work, the CoV2O6 electrode with a capacity of 194.0 C g−1 and a capacitance retention of 122.2 % after performing 100,000 cycles was synthesized by using deep eutectic solvent. In addition, the underlying mechanism of its excellent cycling stability was systematically studied. It was demonstrated that the continuous growth of particles on the electrode surface could lead to the formation of a stable polyhedron structure. More specifically, the generation of the CoOOH phase with strong reversibility on the electrode surface is considered the key to attaining the long cycling life of the CoV2O6 electrode. A hybrid supercapacitor (CoV2O6//AC HSC) was assembled for the evaluation of practical applications. It delivers a specific capacity of 119.3 C g−1 as well as a rate capability with 92.3 % capacity retention and good cycling durability, and the energy density of the HSC can reach up to a high energy density of 20.6 W h kg−1 at 373.4 W kg−1. A new cobalt vanadate electrode synthesis method was proposed here to examine the changes in the specific capacity and capacitance retention after performing ultra-long cycles, which provide a solid experimental basis for the development of supercapacitors for practical applications.

Averting H+-Mediated Charge Storage Chemistry Stabilizes the High Output Voltage of LiMn2O4-Based Aqueous Battery.

H+ co-intercalation chemistry of the cathode is perceived to have damaging consequences on the low-rate and long-term cycling of aqueous zinc batteries, which is a critical hindrance to their promise for stationary storage applications. Herein, the thermodynamically competitive H+ storage chemistry of an attractive high-voltage cathode LiMn2O4 is revealed by employing operando and ex-situ analytical techniques together with density functional theory-based calculations. The H+ electrochemistry leads to the previously unforeseen voltage decay with cycling, impacting the available energy density, particularly at lower currents. Based on an in-depth investigation of the effect of the Li+ to Zn2+ ratio in the electrolyte on the charge storage mechanism, a purely aqueous and low-salt concentration electrolyte with a tuned Li+/Zn2+ ratio is introduced to subdue the H+-mediated charge storage kinetically, resulting in a stable voltage output and improved cycling stability at both low and high cathode loadings. Synchrotron X-ray diffraction analysis reveals that repeated H+ intercalation triggers an irreversible phase transformation leading to voltage decay, which is averted by shutting down H+ storage. These findings unveiling the origin and impact of the deleterious H+-storage, coupled with the practical strategy for its inhibition, will inspire further work toward this under-explored realm of aqueous battery chemistry.

Trigger a multi-electron reaction by tailoring electronic structure of VO2 toward more efficient aqueous zinc metal batteries

VO2 (B) is recognized as a promising cathode material for aqueous zinc metal batteries (AZMBs) owing to its remarkable specific capacity and its unique, expansive tunnel structure, which facilitates the reversible insertion and extraction of Zn2+. Nonetheless, challenges such as the inherent instability of the VO2 structure, poor ion/electron transport and a limited capacity due to the low redox potential of the V3+/V4+ couple have hindered its wider application. In this study, we present a strategy to replace vanadium ions by doping Al3+ in VO2. This approach activates the multi-electron reaction (V4+/V5+), to increase the specific capacity and improve the structural stability by forming robust V5+O and Al3+O bonds. It also induces a local electric field by altering the local electron arrangement, which significantly accelerates the ion/electron transport process. As a result, Al-doped VO2 exhibits superior specific capacity, improved cycling stability, and accelerated electronic transport kinetics compared to undoped VO2. The beneficial effects of heterogeneous atomic doping observed here may provide valuable insights into the improvement electrode materials in metal-ion battery systems other than those based on Zn.

Sulfur loads under high temperature on the surface of C/SnO2 to enhance first platform capacity of lithium-sulfur batteries

Polysulfides are easily dissolved during the redox reaction process of Li-S batteries, which may be severe when current density and cycle number increase. Increasing capacity of the first platform effectively is beneficial for improving cycling stability. Physical sulfur loads are benefit for achieving high-capacity storage. However, the influence of composite electrodes with sulfur loads under high temperature on cycling stability of the first platform capacity has not been studied. Sulfur particles are loaded on the surface of C/SnO2 under high temperature in this work. C/SnO2/S and C/SnS/S composite electrodes are obtained by controlling sulfur contents. Results show that C/SnS/S composite electrodes display better cycling stability (1161 mAh⋅g−1) than C/SnO2/S composite electrodes at 1C after 100 cycles, especially in P1 platforms (724 mAh⋅g−1). When cycling number increases to 1000, C/SnS/S composite electrodes still possess good cycling stability (274.8 mAh⋅g−1) at 1C. Good cycling stability is ascribed to strong surface activities of sulfur in SnS and rapid electron transportation abilities during the cycling process. Studying sulfur-loading methods under high temperature is beneficial for obtaining high cycling performance composite electrodes for lithium-sulfur batteries.

Reconstruction of solvation structure at the cathode-electrolyte interface via partial de-solvation in a metal-organic framework

The formation of cathode-electrolyte interphase (CEI) is closely related to the solvation structure in direct contact with the cathode. The de-solvated solvent molecules are subject to the attack of highly catalytic transition metal ions in the cathode materials (such as Ni-rich LiNi0.8Co0.1Mn0.1O2, Ni-rich NCM), especially at high voltage or high temperature, leading to the poor CEI structure that accelerates the deterioration of rechargeable batteries. Considerable strategies have been proposed to solve this issue. Herein, we have constructed a porous Mg-MOF buffer layer on the surface of Ni-rich NCM, which not only greatly reduces the contact between solvent molecules and cathodes to alleviate the extensive interface reactions, but also promote the de-solvation of Li+ solvation structure through strong affinity between solvents and Mg2+. The partially de-solvated structure induces the anion to participate in the solvation structure and contributes to the formation of a uniform LiF-rich CEI with superior mechanical integrity and chemical stability. Therefore, the NCM with Mg-MOF buffer layer exhibits improved cycling stability at both elevated temperature of 45 °C and high voltage of 4.5 V with lithium as anode. Moreover, the modified NCM/graphite full cells also deliver ultra-stable rechargeable capability with a remarkable capacity retention of 87.3% over 1000 cycles as compared to the only 21.7% of pristine NCM based cells.

Additives strategies to improving cycling stability of Li metal batteries by modifying electrode electrolyte interphases

Li metal batteries (LMBs) have attracted much attention due to the ultra-high theoretical capacity of the Li metal anode (LMA). However, the uncontrollable dendrites growth and low coulombic efficiency hinder their practical application. Here, we explore 3,5-difluoropyridine (2F-BD) as a novel electrolyte additive to enable high-performance LMBs. The 2F-BD additive participates in and modifies the solvation structure of Li ions by reducing the coordinated number of PF6− at the electrode surface, leading to the formation of a LiF and Li3N-rich solid electrolyte interphase (SEI). The LiF component increases the robustness of SEI and suppresses the formation of Li dendrites, while the Li3N component facilitates the transportation and reaction kinetics of Li ions. As a result, the Li||Li symmetric cell presents a stable cycling performance of up to 500 ​h at a current density of 1 ​mA ​cm−2. After coupling with LiFePO4 cathode, the obtained full cell achieves high specific capacities of 106.06 ​mA ​h ​g−1 and 84.98 ​mA ​h ​g−1 at 2.55 ​mA ​cm−2 and 5.10 ​mA ​cm−2, respectively, and maintains a high capacity retention of 84.0% after 1000 cycles at 0.13 ​mA ​cm−2, with an average cycling CE of 99.58%.

Exploring the Voltage Stability of Birnessitic Bi-MnO2 Cathodes Formed In Situ in NaOH and KOH-Based Electrolytes

MnO2, in its many phases, is abundant, non-flammable, non-toxic, reliable, made with abundant materials using simple manufacturing methods, and can have a high theoretical capacity for some phases (up to 617 mAh g−1). Here we have investigated the sensitivity of the performance ofbirnessite—produced in situ—to the presence of Bi2O3, depth-of-discharge, electrolyte salt type, and relative electrolyte volume. We prepared cathodes composed of 45 wt% MnO2, 22.5 wt% Bi2O3, and 22.5 wt% carbon additives, and compared cycling stability in two electrolytes—6.6 M KOH (27 wt%) and 6.6 M NaOH (21 wt%), and two types of 3-electrode test fixtures (flooded beaker or electrolyte-lean T-cell). Our results showed that birnessite can be synthesized electrochemically in NaOH, and cycling the cathode in NaOH improves its stability when compared to cycling in KOH. We tested the cathode in electrolyte-lean environments and found a drastic improvement in cycling stability in NaOH. The cathode exhibited higher initial capacity in lean amounts of KOH, but capacity retention plummeted after the first 20 cycles. In contrast, the cathode in NaOH delivered 65% of the theoretical capacity for over 400 cycles.