Lithium Nickel Manganese Oxide Research Articles

Temperature-controlled synthesis of spinel lithium nickel manganese oxide cathode materials for lithium-ion batteries

In this work, we successfully synthesized series of LiNi0.5Mn1.5O4 (LNMO) cathode materials with spinel structure by using a facile sol-gel method and then calcined at various temperature ranging from 600 to 1000 °C. The application of different calcination temperatures significantly influenced the surface morphology, stoichiometry and crystalline nature of the as-synthesized LNMO material. According to the results of physical characterizations, the LNMO materials calcined at various temperatures mainly revealed the stoichiometric disordered Fd-3m structure with a small amount of well-ordered P4332 phase. The structural analysis also exhibited that the control of the calcination temperature contributed to the higher crystalline nature. Moreover, the morphological investigations indicated that the increasing calcination temperatures caused the formation of large micron-sized LNMO material. In turn, the electrochemical evaluations revealed the impact of the calcination temperatures on enhancing the electrochemical performances of the LNMO electrode materials up to 900 °C. The LNMO electrode calcined at 900 °C exhibited an impressive initial discharge specific capacity of ca. 142 mAh g−1 between 3.5 and 4.9 V vs. Li/Li+, and remarkably improved capacity retention of 97% over 50 cycles. Those excellent electrochemical properties were associated with the presence of the dominant Fd-3m phase over the P4332 phase. Additionally, the results of the corrosion and dissolution tests which were performed for all calcined LNMO materials in order to estimate the amount of manganese and nickel ions leached from them, proved that the micro-sized LNMO calcined at 900 °C was the most stable.

Microwave Fusion of the Composite LiMn1.6Ni0.4O4-LiFePO4 /C to Improve the Stability of Spinel Phase

Lithium-ion batteries are a promissory technology for energy storage devices in electric vehicles and renewable energy generation systems. Lithium nickel manganese oxides (LMNO, spinel structure) are promising cathode materials for lithium-ion batteries because of their high potential (4.7V vs. Li/Li+, 147 mAhg-1), moderate thermal stability and environmental friendliness. However, their chemical stability and electronic conductivity needs to be improved for practical applications. The microwave fusion of composite material LiMn1.6Ni0.4 (LMNO, spinel) coated with LiFePO4 /C (LFP, olivine structure) is studied in order improve the stability of active material without reducing the active sites for lithium intercalation. Synchrotron diffractions shows superior crystallographic order and no further impurities precipitation for the composite material heated at 10W (see Figure 1). Electrochemical characterizations shows high cycled stability for LMNO-2LFP/C heated at 10W (97% after 100th cycles) (see Figure 2). The composite materials heated at 100W presents lower discharge capacity due to the changes in the lattice parameters and impurities precipitation during microwave heating. Sincrotron diffraction after 60th cycle in composite material heated at 10W shows the same lattice parameters for spinel phase and the reductions of lattice parameters for olivine structure in “b” direction. Figure 1

Enhanced Stability at the Electrode-Electrolyte Interface in Ionic Liquid Electrolyte for 5 V Lithium-Ion Rechargeable Batteries

Lithium nickel manganese oxide (LiNi0.5Mn1.5O4) is a promising cathode material for next-generation lithium-ion rechargeable batteries due to its high operating voltage of 4.7 V vs. Li/Li+. However, there are lots of drawbacks in using LNMO as a cathode material because of the liquid electrolyte instability. When carbonate-based electrolytes are decomposed at high cut-off voltage, they generate a hydrofluoric acid which attacks the surface of LNMO. This unwanted parasitic reaction at the electrode-electrolyte interface accelerates the decay in cycle life. In this work, Ionic liquid (1M LiFSI in Pyr13FSI) is adopted as an electrolyte for LNMO cathode material. Highly stable ionic liquid mitigates dissolution of Ni and Mn even in the high cut-off voltage of 5.1 V vs. Li/Li+. X-ray fluorescence mapping analysis confirmed the mitigation of dissolution by building up a 2D image of the varying composition of a sample.

Investigations on sputter deposited lithium nickel manganese oxide thin film cathodes for micro battery applications

Abstract Lithium manganese oxide thin films have been deposited on nickel (Ni) and platinum (Pt) coated stainless steel substrates at room temperature using powder target by Radio Frequency (rf) reactive magnetron sputtering. The samples are exposed to heat treatment at 500 °C to form crystalline phase. Nickel and platinum thin film coatings have been carried out using direct current (DC) sputtering. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and electrochemical characterizations have been carried out. XPS spectra indicate the presence of all elements present in the powder target. A discharge capacity of 54 µAh µm−1 cm−2 and 48 µAh µm−1 cm−2 has been obtained from charge discharge studies in the potential range 2.0 to 4.4 V for the thin film samples deposited on Ni and Pt coated SS substrates correspondingly Charge discharge cycles are conducted up to 40 cycles.

Self-discharge tests to measure side-reaction currents of a Li[Li1/3Ti5/3]O4 electrode

The lifetime performance of lithium-ion batteries is a critical issue for automobile and stationary applications. The difference in the side-reaction current (ISR) of electrodes causes deviations of the state of charge (SOC) of the electrodes leading to the capacity fading of lithium-ion batteries. Establishment of a method to measure the ISR is important for understanding the capacity fading mechanism. We report herein that a novel and simple method to determine the ISR in lithium-ion batteries, the self-discharge test, was developed and applied to lithium-ion cells with lithium titanium oxide (Li[Li1/3Ti5/3]O4, LTO), lithium aluminum manganese oxide (Li[Li0.1Al0.1Mn1.8]O4, LAMO), and lithium nickel manganese oxide (Li[Ni1/2Mn3/2]O4, LiNiMO) as electrodes. According to the self-discharge test results, the ISR of LTO is affected by another electrode of LAMO or LiNiMO. The ISR of LTO in LTO/LiNiMO cells larger than that in LTO/LAMO cells is explained by the additional-ISR of LTO, which results from side reactions such as the reduction of oxidized products generated at the positive electrode. The side reactions at the positive electrode are accelerated with increasing electrode potential, meaning that the higher potential of the positive electrode resulted in the larger additional-ISR of LTO. The real side-reaction current of the LTO electrode in lithium-ion cells is the sum of the intrinsic and the additional current (real-ISR = intrinsic-ISR + additional-ISR).

Mn2+ or Mn3+ ? Investigating transition metal dissolution of manganese species in lithium ion battery electrolytes by capillary electrophoresis.

A new CE method with ultraviolet-visible detection was developed in this study to investigate manganese dissolution in lithium ion battery electrolytes. The aqueous running buffer based on diphosphate showed excellent stabilization of labile Mn3+ , even under electrophoretic conditions. The method was optimized regarding the concentration of diphosphate and modifier to obtain suitable signals for quantification. Additionally, the finally obtained method was applied on carbonate-based electrolytes samples. Dissolution experiments of the cathode material LiNi0.5 Mn1.5 O4 (lithium nickel manganese oxide [LNMO]) in aqueous diphosphate buffer at defined pH were performed to investigate the effect of a transition metal-ion-scavenger on the oxidation state of dissolved manganese. Quantification of both Mn species revealed the formation of mainly Mn3+ , which can be attributed to a comproportionation reaction of dissolved and complexed Mn2+ with Mn4+ at the surface of the LNMO structure. It was also shown that the formation of Mn3+ increased with lower pH. In contrast, dissolution experiments of LNMO in carbonate-based electrolytes containing LIPF6 showed only dissolution of Mn2+ .

A new strategy to improve the cyclic stability of high voltage lithium nickel manganese oxide cathode by poly(butyl methacrylate-acrylonitrile-styrene) terpolymer as co-binder in lithium ion batteries

High voltage LiNi0.5Mn1.5O4 material is a promising cathode for lithium ion battery, but the unsatisfied cycle stability hinders its further development in higher energy density battery. Amorphous terpolymer poly(butyl methacrylate-acrylonitrile-styrene) (P(BMA-AN-St)), which possesses good affinity with carbonated liquid electrolyte and high adhesion ability, is used together with traditional poly(vinylidene fluoride) (PVDF) binder to improve the cyclic performance of LiNi0.5Mn1.5O4 cathode. The retention ability of discharge capacity is 92% for Li/LiNi0.5Mn1.5O4 coin cell after 300 cycles at 1C rate when the cathode uses the co-binders of PVDF and P(BMA-AN-St) (2:1 in weight), compared with 55% capacity retention for the cathode fabricated with PVDF binder alone. From the physical and electrochemical characterizations of the cycled electrode, it is found that the introduced terpolymer enhances the liquid electrolyte uptake ability of LiNi0.5Mn1.5O4 cathode and the adhesion strength inside the particles by building a better interconnected conductive structure. Furthermore, the improved cycle stability of co-binder based cathode is contributed to the passivating layer formed by the terpolymer on the surface of LiNi0.5Mn1.5O4 particles, which effectively suppresses the decomposition of liquid electrolyte at high voltage and maintains the structural integrity of active material in the repeated intercalation/de-intercalation process.

A Study of Particle Size Effect on the Voltage Behavior of LixNi0.5Mn1.5O4 Electrode

Lithium nickel manganese oxide (LNMO, LixNi0.5Mn1.5O4) is an attractive high potential cathode material for rechargeable lithium ion batteries to meet toady’s pressing need of high energy density. One of the main issues faced by LNMO electrode is that its voltage could be invariant with state of charge in charge and discharge operations of battery. This is believed to be originated from its two consecutive phase transition processes during cycling. In this study, a mathematical model is developed to understand the voltage behavior of LNMO electrode in the charge and discharge process by coupling phase transition process with lithium transport process. Effect of electrode particle size on the phase distribution and stability is simulated by the developed model. Results indicate that the voltage behavior of electrode in the charge and discharge can be significantly influenced by different LNMO particle size. Further, electrode with mixed size particles is also investigated by the developed model. Through comparison, limitations of current existing models on predicting LNMO electrode voltage behavior as well as their underlying reasons are discussed based on the modeling results.

Microstructured Electrodes Supported on Serpentine Interconnects for Stretchable Electronics

In this work, we report the fabrication of lithium nickel manganese oxide (LNMO) micropillar electrodes on Al serpentine interconnects that can be stretched up to 70% without structural damaging. Unlike compact and continuous electrode thin-films, we show that under mechanical strains, arrays of vertical micropillar supported on serpentines are carrying empty spaces that can prevent the formation of cracks and the electrode delamination. This innovative approach opens interesting perspectives for the fabrication of truly stretchable devices including Li-ion microbatteries, solar cells, transistors, sensors, etc.

Interfacial reaction during co‐sintering of lithium manganese nickel oxide and lithium aluminum germanium phosphate

AbstractUnderstanding interface evolution during co‐sintering of solid Li‐ion conducting electrolytes and Li‐metal oxide cathodes is crucial to development of solid state batteries. In this work, X‐ray photoelectron spectroscopy, X‐ray diffraction (XRD), and thermal gravimetric analysis/differential scanning calorimetry (TGA/DSC) document changes at the lithium manganese nickel oxide (LMNO)/lithium aluminum germanium phosphate interface during sintering. Measurements are performed as a function of sintering gas environment (air vs N2). Upon sintering, manganese is reduced from +4 to a mixture of +4, +3, and +2, while nickel is oxidized from +2 to a mixture of +2 and +3. The Mn2+ species does not arise when LMNO is sintered alone. XRD identifies formation of new chemical phases, LiGe0.5M0.5PO4 (where M = Mn3+ or Ni3+), AlPO4, and NiMn2O4, also not observed when LMNO is sintered alone. The emergence of resistive interfacial phases may offset the increase in conductivity expected when Mn3+ is formed.

Microstructured electrodes supported on serpentine interconnects for stretchable electronics

In this work, it is reported the fabrication of highly stretchable electrodes on a polydimethylsiloxane substrate. A laser ablation technique is used to design lithium nickel manganese oxide micropillars supported on serpentine Al interconnects. Morphological, mechanical, and chemical analyses have been investigated by scanning electron microscopy, optical microscopy, and energy dispersive X-Ray spectroscopy. We show that unlike compact and continuous electrode thin-films, vertical micropillar structures supported on Al serpentines can be stretched up to 70% without structural damage, which opens a new alternative for the fabrication and development of truly stretchable devices such as stretchable micro-batteries.

Electrodeposition of Polymer Electrolyte Into Porous LiNi0.5Mn1.5O4 for High Performance All-Solid-State Microbatteries.

We report the electrodeposition of polymer electrolyte (PMMA-PEG) in porous lithium nickel manganese oxide (LiNi0.5Mn1.5O4) cathode layer by cyclic voltammetry. The cathode-electrolyte interface of the polymer-coated LNMO electrode has been characterized by scanning electron microscopy and electrochemical techniques. Electrochemical measurements consisting of galvanostatic cycling tests and electrochemical impedance spectroscopy revealed a significant improvement of the capacity values and the increase of the operating voltage. These effects are attributed to the total filling of pores by the electrodeposited polymer that contributes to improve the reversible insertion of Li+. A complete all-solid-state microbattery consisting of electropolymerized LNMO as the cathode, a thin polymer layer as the electrolyte, and TiO2 nanotubes as the anode has been successfully fabricated and tested.

Investigations on the Electrochemical Characteristics of Rechargeable MCMB-LiNi0.5Mn1.5O4 Pouch Cells

In the present work, we have investigated the electrochemical characteristics of MCMB-LiNi0.5Mn1.5O4 pouch cells. It is demonstrated that microstructure engineered, auto-combustion synthesized, lithium manganese nickel oxide (LMNO) spinel cathode yield decent capacity (∼70 mAh/g at 10C rate) and cycleability. The superior rate performance is explained to be due to the truncated octahedral surface morphology of LMNO powder with affluence in (100) and (111) facets which expedite the Li+ ion diffusion during charge-discharge. MCMB-LMNO full cells yield 300 mAh capacity and ∼80% capacity retention after 50 charge-discharge cycles at C/4 rate. It was claimed that faceted growth of particles might have retarded the dissolution of transition metal cations from the positive electrode and improved the discharge capacity and capacity retention characteristics of the fabricated cells. We have suggested that, decomposition of electrolyte during charge discharge needs to be impeded to further improve the electrochemical characteristics of these pouch cells.

Improvement of Interfacial Stability for LiNi0.5Mn1.5O4 Cathode: Insight into the Effect and Mechanism of Additive with Special Structure

AbstractThe development of a suitable high voltage electrolyte system is of great importance for high energy density system. However, solving the interfacial stability of high voltage cathode materials remains a challenge. Herein, methyl benzenesulfonate (MBS) with a special structure as a “victim” additive has been introduced to construct a protective interface film on the surface of lithium nickel manganese oxide (LiNi0.5Mn1.5O4, LNMO) cathode. Charge‐discharge cycling and rate capability performances demonstrate that introducing MBS additive has positive effect on interfacial stability, especially for power property. In 3.0 C rate, the cell with MBS electrolyte has about 63.5 % of capacity retention compared with the discharge capacity using 0.2 C rate, nevertheless, the baseline electrolyte is only 24.2 %. Besides, electrolyte with MBS additive shows some advantages in suppressing overcharge. The excellent performance can be attributed to the special molecular structure of MBS, which can further optimize the properties of interface film. The preferential oxidizability of MBS is able to construct a protective film on the electrode to hinder the further breakdown of electrolyte. The aromatic ring ensures the chemical stability of interface film and the sulfur provides the interface film with high ionic conductivity. These contributions are confirmed by further electrochemical measurements, detailed physical characterizations and theoretical calculations.

Thermal Conductivity Changes Due to Degradation of Cathode Film Subjected to Charge–Discharge Cycles in a Li Ion Battery

A battery device with graphene platelets as anode, lithium nickel manganese oxide as cathode, and solid-state electrolyte consisting of layers of lithium phosphorous oxynitride and lithium lanthanum titanate is assembled on the stainless steel substrate. The battery in a polymer enclosure is subjected to several electrical tests consisting of charge and discharge cycles at different current and voltage levels. Thermal conductivity of the cathode layer is determined at the end of charge-discharge cycles using transient thermoreflectance. The microstructure and composition of the cathode layer and the interface between the cathode, the anode, and the electrolyte are characterized using scanning electron microscopy and elemental mapping. The decrease in the thermal conductivity of the same cathode observed after each set of electrical test cycles is correlated with the volume changes and formation of low ionic and thermal conductivity lithium oxide and lithium oxychloride at the interface and along porous regions. The interface between the metal current collector and the cathode is also found to be responsible for the increase in thermal resistance. The results indicate that changes in the thermal conductivity of the electrodes provide a measure of the resistance to heat transfer and degradation of ionic transport in the cathode accompanying the charge–discharge cycles in the batteries.

Fluorinated phosphazene derivative – A promising electrolyte additive for high voltage lithium ion batteries: From electrochemical performance to corrosion mechanism

The development of novel electrolytes for next-generation high voltage lithium ion battery is of primary importance. In this work, a fluorinated phosphazene derivative, ethoxy-(pentafluoro)-cyclotriphosphazene (PFN), is proposed as a novel electrolyte additive for improving the electrochemical performance and safety of lithium nickel manganese oxide (LiNi0.5Mn1.5O4) cathode. With the addition of PFN, the electrolyte can be preferentially oxidized and decomposed, thus producing some linear polymers, multi-ring polymers, LiNO3, RONO2Li (RONO2: nitrate ester functional group, with R standing for any organic residue), Li3PO4, and ROPO3Li (ROPO3: monoester phosphate) simultaneously. These as-generated materials form a dense, uniform, and thin protective layer on the surface of the cathode material, which suppresses the decomposition of electrolyte and electrode corrosion, correspondingly protecting the LiNi0.5Mn1.5O4 from structural destruction. Due to the coverage by the protective film and corrosion suppression, charge and discharge tests demonstrate that PFN is effective for improving the cycling stability of LiNi0.5Mn1.5O4. The discharge capacity of a battery with 5 wt% PFN is 124.4 mAh g−1 and 99.8 mAh g−1 after 100 cycles at the current rates of 0.2 C and 1 C, respectively, which is much better than the performance without PFN. Meanwhile, because of the combined structure of the nonflammable cyclophosphazene and fluorine, the PFN creates a highly synergistic flame retardant effect, and a low content of PFN can almost completely extinguish burning electrolyte, leading to excellent safety performance for the lithium ion battery.

Mixed Mn3+/Mn4+ in high-voltage nano-structured lithium nickel manganese oxide cathode with a P4332 structure

In this paper, we have synthesized LiNi[Formula: see text]Mn[Formula: see text]O4 with P4332 space group by carbonate co-precipitation. TG/DSC, TG*, ICP-OES, a gas displacement pycnometer using He gas, XPS, and XRD refinement results show that oxygen vacancies exist in the LiNi[Formula: see text]Mn[Formula: see text]O[Formula: see text] with a P4332 space group. There is a mixture of Mn[Formula: see text] and Mn[Formula: see text] in the P4332 structured LiNi[Formula: see text]Mn[Formula: see text]O[Formula: see text]. Despite the large amount of Mn[Formula: see text] in the spinel structure, no 4.0[Formula: see text]V plateau appears in charge–discharge curves. This result indicates that the Mn[Formula: see text] in LiNi[Formula: see text]Mn[Formula: see text]O[Formula: see text] does not participate in electrochemical reactions during the charge–discharge process.

Nanoscale Manipulation of Spinel Lithium Nickel Manganese Oxide Surface by Multisite Ti Occupation as High-Performance Cathode.

A novel two-step surface modification method that includes atomic layer deposition (ALD) of TiO2 followed by post-annealing treatment on spinel LiNi0.5 Mn1.5 O4 (LNMO) cathode material is developed to optimize the performance. The performance improvement can be attributed to the formation of a TiMn2 O4 (TMO)-like spinel phase resulting from the reaction of TiO2 with the surface LNMO. The Ti incorporation into the tetrahedral sites helps to combat the impedance growth that stems from continuous irreversible structural transition. The TMO-like spinel phase also alleviates the electrolyte decomposition during electrochemical cycling. 25 ALD cycles of TiO2 growth are found to be the optimized parameter toward capacity, Coulombic efficiency, stability, and rate capability enhancement. A detailed understanding of this surface modification mechanism has been demonstrated. This work provides a new insight into the atomic-scale surface structural modification using ALD and post-treatment, which is of great importance for the future design of cathode materials.

Improved Electrochemical Performance of an All Solid-State Microbattery By Electrodeposition of Polymer Electrolyte into the Nanostructured Electrodes

The miniaturization of Lithium ion batteries (LIBs) as a power source to drive small devices such as smartcards, medical implants, sensors, radio-frequency identification (RFID) tags etc. has been continuously developed to meet the market requirements of portable applications1. In this direction 3D microbatteries have been considered to satisfy the requirements of these portable devices. All solid-state microbatteries involving self-supported titania nanotubes (TiO2 nts) have been considered, due to their high surface area and short diffusion lengths for Li+ ion transport2. It has been reported that the electropolymerization of the polymer electrolyte through cyclic voltammetry (CV) leads to the conformal deposition of the electrolyte into the 3D TiO2 nts 3,4. So far, the electropolymerization and its influence on the capacity was reported with regard to TiO2 nts only5. This work, along with electropolymerized TiO2 nts, includes the electropolymerization of the polymer electrolyte onto the Lithium nickel manganese oxide (LNMO) composite cathode and its influence on the capacity of the whole microbattery. A drop cast layer of the monomer methyl methacrylate-polyethylene glycol (MA-PEG) serves as the separator and ion conducting medium. Fig.1 a) and b) shows the CV of the electropolymerization of the polymer electrolyte into the TiO2 nts and LNMO respectively. The cathodic current density decreases with cycle number which indicates that the thin polymer layers is deposited onto the TiO2 nts wall which helps in using the effective surface area of the nanotubes. For the LNMO composite electrode, the higher current density observed is due to the high rugosity of the cathode layer. This suggests that the polymer layer formed at the initial cycles allows to follow the rugosity of the LNMO electrode, which is important to establish a good electrode-electrolyte interface. A comparison of the capacities by electropolymerized electrodes (TiO2nts(EP)/Polymer/LNMO(EP)) and non-electropolymerized electrodes (TiO2nts/Polymer/LNMO) shows the positive influence of the electrodeposition of the polymer electrolyte on the capacity of the microbattery which is shown in Fig. 1 c) and d). TiO2nts(EP)/Polymer/LNMO(EP) microbattery delivers a capacity of 169 mAh.g-1 (82 μAh cm−2 μm−1) at the first cycle and 150 mAh.g-1 (70 μAh cm−2 μm−1 ) for the tenth cycle at C/10 rate. Comparison of the capacities at C/10 and C/2 with TiO2nts/Polymer/LNMO showed an increment of around 100% even after 100 cycles.6 Fig. 1 Cyclic voltammetry of the electropolymerization on the (a) TiO2nts and (b) LNMO in aqueous solution of 0.5 M MMA-PEG + 0.5 M LiTFSI vs. Ag/AgCl (saturated) in the potential window -0.35 V to -1 V at a scan rate of 10 mV.s-1. Galvanostatic charge/discharge profiles of different microbatteries at C/10 for (c) TiO2nts(EP)/Polymer/LNMO(EP); (d) TiO2nts/Polymer/LNMO. References 1) B.L. Ellis, P. Knauth, T. Djenizian, Three-Dimensional self-supported metal oxides for advanced energy storage, Adv. Mater. 26 (2014) 3368-3397. 2) T. Djenizian, I. Hanzu, P. Knauth, Nanostructured negative electrodes based on titania for Li-ion microbatteries, J. Mater. Chem. 21 (2011) 9925-9937. 3) G. F. Ortiz, I. Hanzu,, T. Djenizian, P. Lavela,, J. L. Tirado, P. Knauth, Alternative Li-Ion Battery Electrode Based on Self-Organized Titania Nanotubes, Chem. Mater. 21 (2009), 63–67. 4) N. Plylahan, N.A. Kyeremateng, M. Eyraud, F. Dumur, H. Martinez, L. Santinacci, P. Knauth, T. Djenizian, Highly conformal electrodeposition of copolymer electrolytes into titania nanotubes for 3D Li-ion batteries, Nanoscale Res. Lett. 7 (2012) 349-353. 5) N. Plylahan, M. Letiche, M.K.S. Barr, B. Ellis, S. Maria, T.N.T. Phan, E. Bloch, P. Knauth, T. Djenizian, High energy and power density TiO2 nanotube electrodes for single and complete lithium-ion batteries, J. Power Sources 273 (2015) 1182-1188. 6) G. D. Salian , C. Lebouin , A. Demoulin , M.S. Lepihin , S. Maria ,A.K. Galeyeva , A.P. Kurbatov,T. Djenizian, Electrodeposition of polymer electrolyte in nanostructured electrodes for enhanced electrochemical performance of thin-film Li-ion microbatteries, J. Power Sources, 340 (2017), 242-246. Figure 1

Fundamental Electrochemical Study of Manganese Redox States in Nonaqueous Battery Solvents

Next-generation transition metal oxide cathodes such as lithium nickel manganese oxide (LNMO) are needed to meet the increasing demand for high-voltage, high-power lithium-ion batteries. However, these manganese- and nickel-containing cathode materials accelerate capacity fade due to transition metal dissolution from the electrode. The role of these metals, particularly manganese, in side reactions remains poorly understood. Our work aims to bridge this knowledge gap through fundamental electrochemistry. While the effects of manganese (Mn) dissolution from transition metal oxides have been documented, the fundamental physics of how manganese ions induce capacity fade are not yet determined. Delacourt et al. contend that Mn0 is deposited in the solid-electrolyte interphase (SEI) at the negative electrode creating an electronically conductive SEI, while Vissers et al. show that Mn2+ ion-exchanges with lithium-containing inorganic electrolyte reduction products1,2. Lastly, Wandt et al. demonstrated via operando X-ray absorption spectroscopy that Mn ions are always in the +2 oxidation state on lithiated and delithiated graphite, however Mn metal was detected on lithiated graphite in ex situ analysis3. This disagreement on the redox state of dissolved manganese impedes efforts to prevent SEI contamination by the transition metal ions. Even the standard reduction potential of Mn2+ to Mn0 in nonaqueous electrolyte is not agreed upon. Komaba et al. reported 1.87 V vs. Li/Li+, based on the standard redox potential vs. NHE in an aqueous environment4. Several studies have observed different reduction potentials for Mn2+ ions in both perchlorate- and carbonate-based electrolytes5,6. To better diagnose the redox state of manganese in a nonaqueous battery electrolyte, a fundamental understanding of the potential ranges for various oxidation states of manganese is necessary. In one part of this work, we determine the Mn2+/Mn0 redox potential in nonaqueous electrolyte via three-electrode cell electrochemistry, energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). To our knowledge, this is the first attempt to determine the precise redox potential for the Mn2+/Mn0 reaction in nonaqueous battery electrolytes. It is important to avoid electrolyte and lithium-ion reduction potentials obfuscating the reduction potential of Mn2+; therefore, a stable non-passivating, lithium-free electrolyte is used for this study (namely, 0.1 M tetrabutylammonium perchlorate in tetrahydrofuran). We vary the potential applied at a copper electrode in a manganese-doped electrolyte, correlate the current response to manganese reduction, and then confirm Mn0 deposition via EDS and XPS of the copper electrode. We have demonstrated that Mn0 can be electrochemically deposited on copper foil from a Mn-doped electrolyte and detected via EDS and XPS. In another part of this work, we determine how the presence of manganese affects the formation and characteristics of the SEI using redox mediators, such as the ferrocene-ferrocenium couple. SEI films are formed both in the presence and absence of manganese salts, by applying a constant potential for a specified amount of time. The SEI is probed by adding a redox mediator to the electrolyte, and measuring the current response around the mediator reaction potential. By comparing the cyclic voltammograms for SEIs formed in neat electrolyte versus Mn-contaminated electrolyte, the impact of manganese on through-film mass transport and reaction kinetics has been determined. Results show that the SEI formed in the presence of manganese has an effective through-film ferrocenium diffusivity eight times greater than the SEI formed in neat electrolyte, indicating that manganese may induce higher porosity or cracks. Future work will tune redox mediator size and chemistry to characterize pore or crack sizes in the SEI.