Composite Positive Electrode Research Articles

All-Solid-State Batteries Using Rationally Designed Garnet Electrolyte Frameworks

Functioning bulk-type all-solid-state batteries in a practical form factor with composite positive electrodes, using Al-substituted Li7La3Zr2O12 (LLZO) as the solid electrolyte, have been demonstrated for the first time. The devices incorporate bilayers composed of dense LLZO membranes and porous LLZO scaffolds infiltrated with LiNi0.6Mn0.2Co0.2O2 and other components as positive electrodes, combined with lithium anodes. The porous scaffolds are prepared using an easily scaled freeze-tape-casting method. The unidirectional pores of the scaffold facilitate infiltration of cathode components and shorten lithium ion diffusion path lengths, while the addition of a soft ionically conductive solid to the scaffold ensures good contact among the components.

Reaction uniformity visualized by Raman imaging in the composite electrode layers of all-solid-state lithium batteries.

The reaction uniformity of LiCoO2 composite positive electrodes in all-solid-state cells was compared quantitatively by investigating the Raman band shifts corresponding to the state-of-charge (SOC) of LiCoO2. The quantitative SOC analysis was conducted using the Raman imaging data of composite electrodes with smaller or larger solid electrolytes. The electrodes exhibited different reaction uniformity although the cells showed similar initial charge capacities and average SOC. In the case of larger solid electrolytes, most LiCoO2 particles showed higher or lower SOC than the average SOC, and lower battery performance. The quantitative analysis of SOC in each LiCoO2 electrode demonstrated that a variable SOC outside the average SOC resulted in larger irreversible capacity and lower rate performance. The quantitative SOC analysis newly developed in the present study is a useful technique for designing composite electrodes showing higher battery performance.

Interlaced nickel phosphide nanoflakes wrapped orthorhombic niobium pentoxide nanowires array for sustainable aqueous asymmetric supercapacitor

To obtain simple and cost-effective functional energy storage system at a practical level, a novel core-shell porous nanocomposite based on orthorhombic niobium pentoxide (T-Nb2O5) nanowires and metallic phosphides nanoflakes is reasonably designed and synthesized by a highly efficient solvothermal and subsequently moderate phosphorization procedure. The T-Nb2O5 nanowires act as structure-directing agent to directionally grow hierarchical ultrathin porous nickel phosphide (Ni2P) nanoflakes, thus providing high electrical conductivity, large surface area and sufficient redox active sites. Owing to the distinctive nanostructure and synergistic effects of the T-Nb2O5@Ni2P core-shell heterostructure, its electrochemical properties have vastly improved. When the current density is 1 A g−1, a high specific capacity of 105 mAh g−1 can be obtained. In addition, an asymmetric supercapacitor assembled based on the T-Nb2O5@Ni2P composite positive electrode and activated carbon negative electrode exhibits a high energy density of 30.2 Wh kg−1 at a power density of 453 W kg−1 and impressive cycling stability (90%) after 5000 cycles. The well-defined oxide/phosphide core-shell materials can be expected to a new type of electrode materials for the high-performance supercapacitors.

Status and Targets for Polymer-Based Solid-State Batteries for Electric Vehicle Applications

As the market of electric vehicles (EVs) expands, numerous researchers and companies are focusing on the use of lithium metal anodes, which can enable higher energy densities required to extend the driving range of EVs and allow for lower cost [1]. However, the implementation of these metal anode batteries has several safety issues that emerge from dendrite growth in flammable liquid electrolytes. Solid polymer electrolyte has been studied as one of the promising candidates for lithium metal batteries because of the advantages of the non-flammability and relatively higher mechanical properties than liquid electrolytes [2, 3]. Compared to existing batteries based on liquid electrolytes, solid polymer electrolytes still have limitations, such as low ionic conductivity under room temperature conditions, which can significantly hinder its application in EVs. Hence, research on next-generation solid polymer electrolyte is being conducted to improve its transport properties. In this work, by using a model-based approach, the status of the present-day solid polymer electrolyte battery is examined, and compared with the traditional batteries with liquid electrolyte, with a focus on EVs. Using a mathematical model developed based on the macro-homogeneous approach pioneered by Newman and coworkers [4], we investigated Li-metal/LiFePO4 (LFP) batteries containing three different electrolytes, namely (i) liquid electrolyte (ii) polystyrene-b-poly(ethylene oxide) (SEO) block copolymer electrolyte and (iii) a single-ion conducting block copolymer electrolyte. Concentrated solution theory and modified Ohm’s law are used to calculate the mass transport and potential gradient inside the electrolyte. In addition, the mass balance of lithium, potential drop in the active material, and the charge-transfer reactions are considered for a composite positive electrode that consists of LFP active particles. More detailed information about the continuum model is available in the literatures [4, 5]. All the transport parameters for the solid polymers, which include SEO and single-ion block copolymer, are adopted from the previously-published experimental results [3, 6-8]. For validation, model predicted results of the three Li/LFP batteries are compared with experimental data. Next, we compared the performance of the three different electrolytes operating at the same specified electrode thickness and weight percent of electrolyte. Optimal design points of each cell that provides the highest specific energy and power are determined. Finally, the results demonstrate how the specific energy and power of a solid polymer cell should be improved to achieve similar performance as that of a liquid electrolyte cell. This work provides directions for improvements in present-day solid polymer electrolytes for usage in electric vehicle. References Gallagher, K.G., et al., Quantifying the promise of lithium-air batteries for electric vehicles. Energy & Environmental Science, 2014. 7(5): p. 1555-1563.Armand, M. and J.M. Tarascon, Building better batteries. Nature, 2008. 451(7179): p. 652-657.Bouchet, R., et al., Single-ion BAB triblock copolymers as highly efficient electrolytes for lithium-metal batteries. Nature Materials, 2013. 12(5): p. 452-457.Doyle, M., T.F. Fuller, and J. Newman, Modeling of Galvanostatic Charge and Discharge of the Lithium Polymer Insertion Cell. Journal of the Electrochemical Society, 1993. 140(6): p. 1526-1533.Fuller, T.F., M. Doyle, and J. Newman, Simulation and Optimization of the Dual Lithium Ion Insertion Cell. Journal of the Electrochemical Society, 1994. 141(1): p. 1-10.Bruce, P.G., et al., Li-O-2 and Li-S batteries with high energy storage. Nature Materials, 2012. 11(1): p. 19-29.Devaux, D., et al., Failure Mode of Lithium Metal Batteries with a Block Copolymer Electrolyte Analyzed by X-Ray Microtomography. Journal of the Electrochemical Society, 2015. 162(7): p. A1301-A1309.Pesko, D.M., et al., Comparing Two Electrochemical Approaches for Measuring Transference Numbers in Concentrated Electrolytes. Journal of the Electrochemical Society, 2018. 165(13): p. A3014-A3021.

Improvement of Inter-particle Contact in Positive Electrodes Using the Composite Deformable Solid Electrolyte in an Oxide-type All-solid-state Lithium Ion Battery

We evaluated the combined effect of a high-deformation property and the use of a high ionic conductivity solid electrolyte in a non-sintered positive composite electrode. The battery performance of...

Multiscale Interfacial Characterisation of Transport Properties in Composite Li-Ion NMC532 Electrodes

Good ionic and electronic conductivities are required in battery electrodes to facilitate redox reactions and thus efficient charge and discharge of the battery. However, limitations of charge transport properties in state of the art lithium-ion positive composite electrodes greatly restrain the electrode thickness. This is largely attributed to: (i) the multitude of interfaces/contacts that arise with the different materials, and at different scales inside the composite electrode; and (ii) to the degree of percolation and tortuosity of the different phases. The active material studied, LiNi0.5Mn0.3Co0.2 O2 (NMC532), consists of 1µm size grains (or particles) that agglomerate to form 4µm clusters. The binding polymer PVDF is used with Carbon Black (CB) to impart higher electronic conductivity. The aim of this project is to study the effect of the electrode morphology and the various interfaces on electronic and ionic limitations. By changing the morphology of the electrode (i.e. composition and porosity) the aim is to minimize the transport limitations in the electrode [1]. In doing so it is hoped to be possible to increase electrode thickness and thus minimize the non-electroactive part in the battery electrodes, reducing cost and increasing efficiency. This research may also help in developing a faster method to optimize new active materials and thus reduce its time to market. Broadband Dielectric Spectroscopy (BDS) apply AC perturbations, between 20Hz to 18GHz in our case, across a metal coated sample at the end of a coaxial waveguide. Complex permittivity and conductivity (resistivity) are then calculated from admittance (low-frequency) and the coefficient of reflection (high-frequency). The inferred electric polarisations (or susceptibilities) at each interface are additive in nature. It provides information about multiscale charge transfer phenomenon across interfaces. This allows for a multi-hierarchical study of the composite. The interfacial characterisation of resistive and capacitive responses is carried out using Nyquist plots. Measurements are undertaken using dry nitrogen, from room temperature to -100°C. The conductivity of NMC532 is thermally activated and activation energies are derived using Arrhenius plots. To characterise the NMC532 active material alone, samples with no carbon black were referenced. Electrode metal contact, clusters and particles of NMC532 were characterised for dry samples. Samples were then impregnated with solvent and electrolyte to study their effect on the active material. Both conductivity and interfacial capacitance were seen to increase for the impregnated electrodes. Industrially fabricated NMC532 composite electrodes with varying CB content and porosity, coated on aluminium, were also characterised. It was seen that the electrochemical performance was better for electrodes with a non-percolated CB-polymer network The results obtained bring to question the necessity to have percolated electrodes when using an active material with relatively high conductivity. This is investigated with the help of electrochemical impedance spectroscopy (1mHz-50kHz) characterising the charge-transfer resistance as well as operando BDS, allowing for interfacial characterisation of electrodes at different levels of charge/discharge. The research is part of the project ANR Pepite comprising researchers conducting further characterisations such as FIB-SEM X-Ray tomography and PF-NMR to allow for a complete characterisation of the composite electrode and a better understanding on the effect of the electrode architecture on its performance. Acknowledgement We are grateful to the ANR for the funding of the Pepite project (ANR-15-CE05-0001).

Na2S sacrificial cathodic material for high performance sodium-ion capacitors

Sodium sulfide (Na2S) has been used as sacrificial material for the presodiation of a Sn4P3 negative electrode in order to realize a high-performance sodium-ion capacitor (NIC). In two-electrode cells with Na counter/reference electrode and 1 mol L−1 NaClO4 electrolyte, sodium could be irreversibly extracted from Na2S at potential lower than 3.8 V vs. Na/Na+, with a capacity close to the theoretical value of 687 mAh g−1. In the realization of the composite positive electrode for a NIC, the relatively high capacity of Na2S allows to reduce its amount to 40 wt%, whereas the other materials are activated carbon (AC, 40 wt%), carbon soot (15 wt%) and binder (5 wt%). Once the pre-sodiation of Sn4P3 is completed, activated carbon being a part of the positive electrode stores charges in the electrical double-layer (EDL), while reversible sodium insertion occurs in the Sn4P3 negative electrode. The full NIC demonstrates stable performance in the voltage range 2.0 V–3.8 V with high specific energy ca. 48 Wh kg−1 at specific power of 1 kW kg−1 (per total mass of electrodes). Hence, Na2S is an excellent sacrificial material, which allows the NICs construction to be simplified and consequently the manufacturing costs to be reduced.

Cascade‐Type Prelithiation Approach for Li‐Ion Capacitors

AbstractAn industry‐relevant method for pre‐lithiation of lithium‐ion capacitors to balance the first charge irreversibility is demonstrated, which addresses the prime bottleneck for their market integration. Based on a composite positive electrode that integrates pyrene monomers and an insoluble lithiated base, Li3PO4, a “cascade‐type” process involving two consecutive irreversible reactions is proposed: i) oxidative electropolymerization of the pyrene moieties releases electrons and protons; ii) protons are captured by Li3PO4 and exchanged for a stoichiometric amount of Li+ into the electrolyte. (1H, 19F, and 31P) NMR spectroscopy, operando X‐ray diffraction, and Raman spectroscopy support this mechanism. By decoupling the irreversible source of lithium ions from electrons, the cascade‐type pre‐lithiation allows the simultaneous enhancement of the capacity of the positive electrode, thanks to p‐doping of the resulting polymer. Remarkably, the proton scavenging properties of Li3PO4 also boost the polymerization process, which enables a 16% increase in capacity without detrimental effect on power properties and cyclability. Full cells integrating a cheap carbon black based negative electrode, show much‐improved capacity of 17 mAh g‐1electrodes (44 F g‐1electrodes, 3–4.4 V) and excellent stability over 2200 cycles at 1 A g‐1. Thanks to its versatile chemistry and flexibility this approach in principle can be applied to any kind of ion‐battery.

The influence of electrode matching on capacity decaying of hybrid lithium ion capacitor

Hybrid lithium-ion capacitors (HyLICs), combining a battery-type negative electrode and a composite positive electrode, can display a more comprehensive performance than conventional lithium-ion capacitors (LICs), e.g. high energy density. However, owing to the coexisting of non-Faradaic and Faradaic energy storage mechanism in the composite positive electrode, the insufficient cycle stability of device becomes an urgent problem. In this paper, we discussed the influence of electrode matching on the cycling stability of HyLICs with a L30/HC@SLMP structure. The three L30/HC mass ratios were 0.8, 1.0 and 1.2, respectively. Three full cells with above ratios exhibit great difference in cycle stability when charging/discharging at a current density of 1 A g−1. Various methods, including galvanostatic charging/discharging, three-electrodes test and scanning electron microscopy, were used to analyze the performance of difference cells. It turns out that the poor rate performance of HC electrode is the main limited factor in HyLICs' cycling stability. Compared with the composite positive electrode, HC electrode is more sensitive to the current densities. Under high current density, solid electrolyte interface layers on the HC electrode surface accumulated fast, which leads to the increase in the impedance of HC electrode. From the Ragone plot, the best matching strategy of L30/HC electrode was achieved when the mass ratio of L30/HC was 1.0. Finally, the optimal HyLICs with L30/HC mass ratio of 1.0 were assembled and performed an excellent cycle stability of more than 80% retention after 62,000cycles in 40C, as well as better power and energy density compared with LICs.

Improvement of Cycling Capability of Li2S-FeS Composite Positive Electrode Materials by Surface Coating With Titanium Oxide

High-energy-density batteries are actively sought after for next-generation electric vehicle applications, triggering the search for sulfur-based positive electrode materials with high capacity for extended driving range. One of the primary issues of sulfur cathodes is the rapid capacity decay caused by the dissolution of sulfur in the electrolyte and the resulting parasitic reactions. This can be prevented by incorporating a suitable amount of carbon material into the sulfur electrodes, because of its inherent properties; however, introducing a large amount of low-density carbon materials will lead to a decrease in the energy density of the battery. Therefore, an alternative method to suppress sulfur dissolution is required for commercial applications. Recently, we developed a Fe-containing Li2S-based material (Li8FeS5) as the sulfur cathode; it has a relatively high initial discharge capacity (> 700 mAh∙g-1) and electric conductivity. However, rapid capacity degradation was observed during the initial several cycles, due to some unknown reactions between the electrolyte and Li8FeS5. In this study we demonstrate an effective method to improve the cycle performance of Li8FeS5 by coating its surface with a stabilizing material. We selected titanium oxide as the coating material, based on its high stability toward liquid electrolytes and its strong interaction with sulfur. Hence, we obtained TiOx-coated Li8FeS5 particles (Li8FeS5-TiOx) by a liquid-phase reaction. The Transmission electron microscopy observation revealed that the Li8FeS5 particles were coated with several tens of nanometers of TiOx layers. The Li8FeS5-TiOx cells exhibited improved cycling performance with a carbonate electrolyte. We also observed that the capacity originating from an iron redox was not affected by coating during the cycling test; in contrast, the degradation of the cell capacity corresponding to the sulfur redox was suppressed significantly after coating with the TiOx layer. Thus, the surface reaction of Li8FeS5 with the electrolyte, which could be the main reason for the cycling degradation, was effectively suppressed by coating with the TiOx layer.

Lithium Magnesium Tungstate Solid as an Additive into Li(Ni1/3Mn1/3Co1/3)O2 Electrodes for Li-Ion Batteries

Addition of 1 wt% Li4MgWO6 (LMW) solid in a positive electrode is effective to improve battery performance of Li(Ni1/3Mn1/3Co1/3)O2 (NMC)//graphite lithium-ion cell. The batteries with LMW additive show better capacity retention and lower resistance than those of the additive-free batteries. To elucidate the improvement mechanism, the NMC and graphite electrodes after long-term cycle are examined by means of electrochemical tests in Li half-cells, electron microscopy, synchrotron X-ray diffraction, and photoelectron spectroscopy. The results reveal that the addition of LMW suppresses imbalanced state of charge (SOC) between the positive and negative electrodes during cycles whereas evident imbalanced SOC causes the capacity degradation during long cycling under the LMW-free condition. The imbalanced SOC results in overcharge of NMC which is proved by formation of severe micro-cracks in NMC particles. Crystallinity of LMW added in the positive composite electrode is lost after 2000 cycles and tungsten is detected on the graphite electrode, which suggests that the additive is gradually dissolved in electrolyte and tungsten is deposited on the graphite. Because of the formation of surface layer containing tungsten ingredient on the graphite, irreversible side-reactions on the graphite electrode should be suppressed, which contributes to successfully suppress the SOC-unbalancing as well as capacity fading.

Numerical Prediction of Multiscale Electronic Conductivity of Lithium-Ion Battery Positive Electrodes

The electronic conductivity, at the multiscale, of lithium-ion positive composite electrodes based on LiNi1/3Mn1/3Co1/3O2 and/or carbon-coated LiFePO4, carbon black and poly(vinylidene fluoride) mixture is modeled. The electrode microstructures are acquired numerically in 3D by X-ray tomography and FIB/SEM nanotomography and numerically segmented to perform electrostatic simulations using Fast Fourier Transform (FFT) method. Such simulations are easy and quick to perform because they are directly computed on the grid represented by the voxels in the 3D volumes. Numerical results are compared with experimental measurements of the multiscale electronic conductivity by broadband dielectric spectroscopy (BDS). A good prediction is realized for the bulk conductivities of the C/LiFePO4 phase and the CB/PVdF mixture. The combination of X-ray and FIB/SEM tomography, FFT simulation method, and BDS is thus well adapted to understand the influence of the electrode composition and microstructure on its electronic conductivity.

Improvement of Cycle Capability of Fe-Substituted Li2S-Based Positive Electrode Materials by Doping with Lithium Iodide

For an attempt to improve the cycle capability of the Li8FeS5 cells, we have prepared LiI-doped Li8FeS5 composite positive electrode materials. The obtained Li8FeS5·xLiI sample cells showed the improved cycle capability, though the initial capacity value decreased with proportional to the mass of LiI. The improved cycle performance was attributable to the suppressed resistance rise of the cells, due probably to the suppression of high-resistive surface precipitates formed by the reaction between the active material and the electrolyte. The dopant LiI would stabilize the local structure against Li extraction/insertion reactions, as well as suppress the reaction with the electrolyte, leading to the improved cycle performance.

An elementary 1-dimensional model for a solid state lithium-ion battery with a single ion conductor electrolyte and a lithium metal negative electrode

An elementary 1-dimensional model is developed for a solid state lithium-ion battery having a single ion conductor electrolyte, a lithium metal negative electrode and a composite positive electrode. The battery topology is assumed to be of the layered variety, thereby justifying the 1-dimensional formulation. The governing equations for the electrochemical kinetics at the interface between the negative electrode and the electrolyte separator are stated, as are those for ion transport in the electrolyte. The positive electrode is assumed to be a particulate composite of storage material within a matrix of electrolyte. A mixture theory is developed for the positive electrode encompassing ion transport in the electrolyte matrix and storage and unstorage of lithium in the active material subject to electrochemical kinetics at the perimeter of the storage particles. Many simplifying assumptions are made with the advantage of them leading to closed-form or semi-closed-form solutions, including linearization of the equations governing the redox kinetics at interfaces in the battery between electrolyte and active material. An approximation is given for the concentration of lithium in the positive electrode of the battery during discharge, with the details depending on a length scale parameter that depends on the competition between the rate of lithium insertion into/extraction from the storage particles and the rate at which lithium ions are transported in the electrolyte. When the conductivity of the electrolyte is high and the redox reactions are relatively sluggish, this length scale parameter is comparable to the thickness of the positive electrode or larger than it. In that case lithium insertion into/extraction from storage particles occurs everywhere within an active zone of the positive electrode, but with the rates least at the current collector of the positive electrode. If the conductivity of the electrolyte is poor and the redox reactions rapid, the length scale for the solution is small compared to the thickness of the positive electrode and insertion into/extraction from storage particles occurs only in a narrow slice of the positive electrode. This slice moved along the positive electrode and separates a region of it that is completely filled/empty from a region of it that has not yet gained or lost any of its lithium. In all cases there will usually be a region of the positive electrode near the separator that is completely filled during discharge and completely empty during battery charging. Our results also give outcomes from which the internal resistance of the battery can be estimated.

Scanning electrochemical cell microscopy for visualization and local electrochemical activities of lithium‐ion (de) intercalation process in lithium‐ion batteries electrodes

Scanning electrochemical cell microscopy with a single barrel micro‐/nano‐pipette (SECCM) was applied to lithium iron phosphate (LiFePO4) composite positive electrodes and an isolated LiFePO4 secondary particle for lithium‐ion batteries. To analyze lithium‐ion (Li+) charge or discharge process on the electrodes using local probe, a pipette filled with LiCl electrolyte solution and Ag/AgCl quasi‐reference counter electrode (QRCE) was used. Both the local electrochemical activities of LiFePO4 on the composite electrodes and a single particle were revealed by SECCM as in‐situ direct measurement of current response‐related Li+ transport from mapping and cyclic voltammogram at confined area by the pipette. The mapping has visualized Li+ deintercalation process from LiFePO4 at +0.65 V (applied between the sample‐QRCE). We show that the SECCM system is a strong analytical tool for a characterization of local Li+ behavior of active electrode materials in lithium‐ion batteries.

A high-performance composite positive electrode based on graphene and Li (Ni1/3Co1/3Mn1/3)O2

The cathode electrodes in commercial Li-ion cells are usually coated on aluminum foils, while the anode part is coated on copper current collector. However, these metallic foils of the electrodes are relatively heavy counterparts when compared with the total cell weight. To overcome this issue, we comparatively studied LiNi1/3Mn1/3Co1/3O2 (NMC); NMC/graphene positive electrodes reinforced with graphene were produced in the form of freestanding electrodes by a facile sol-gel and vacuum filtration method. To confirm our results obtained with the half-cells, graphite@NMC@graphene full-cells were also produced and a specific capacity of 220 mAh g−1 after 250 cycles. Extraordinary electrochemical cycling, high conductivity, and enhanced rate properties are obtained by anchoring the NMC particles between the graphene layers. The results have also indicated that the freestanding graphene-based electrodes could be a useful tool for high-capacity lithium-ion batteries.

Biomass carbon/polyaniline composite and WO3 nanowire-based asymmetric supercapacitor with superior performance

The demand for advanced energy storage devices such as supercapacitors and lithium-ion batteries has been increasing to meet the application requirements of hybrid vehicles and renewable energy systems. Here, high energy density aqueous asymmetric supercapacitor (ASC) is assembled based on chestnut shell-based activated carbon (CAC)/PANI composite positive electrode and tungsten trioxide (WO3) nanowires negative electrode. The CAC/PANI composite and WO3 nanowires were synthesized through an interfacial polymerization method and a simple sodium sulfate assisted hydrothermal process, respectively. The CAC/PANI//WO3 ASC device operates with a voltage of 1.5 V in 1 M H2SO4 electrolyte and achieved a high energy density of 15.4 Wh kg−1 at a power density of 252 W kg−1. Furthermore, the device shows an excellent cycling performance with capacitance retention of 83% after 1500 cycles.

Grafting Nitrophenyl Groups on Carbon Surfaces by Diazonium Chemistry to Suppress Irreversible Reactions in High-Voltage LiNi0.5Mn1.5O4 Positive Electrodes

Nitrophenyl groups are grafted onto the surface of a conductive carbon using diazonium chemistry. The as-modified carbon is loaded as a conducting agent in a spinel-structured LiNi0.5Mn1.5O4 (LNMO) composite positive electrode to examine the beneficial effects given by the surface-grafted nitrophenyl groups. The Li/LNMO cell fabricated with unmodified carbon exhibits poor Coulombic efficiency due to parasitic reactions, such as the oxidative decomposition of the electrolyte and oxidation of the conductive carbon, prevailing at the working voltage of LNMO (4.6–4.9 V vs. Li/Li+). These reactions appear to occur mainly on the carbon surface on account of its ∼20 times larger area than that of LNMO. However, use of the nitrophenyl-modified carbon greatly improves the Coulombic efficiency of the Li/LNMO cell, indicating suppression of such parasitic reactions. Differential electrochemical mass spectrometry reveals that nitrophenyl grafting effectively suppresses carbon oxidation at high voltage. Although suppression of electrolyte oxidation is only moderate with nitrophenyl-modified carbon, CO2 generation by carbon oxidation is greatly suppressed. The latter feature can be attributed to the electron withdrawing ability of the nitro groups, which imparts the conductive carbon with high stability against electrochemical oxidation.

Analysis of structural and thermal stability in the positive electrode for sulfide-based all-solid-state lithium batteries

Sulfide-based all-solid-state batteries using a non-flammable inorganic solid electrolyte are promising candidates as a next-generation power source owing to their safety and excellent charge–discharge cycle characteristics. In this study, we thus focus on the positive electrode and investigated structural stabilities of the interface between the positive electrode active material LiNi1/3Mn1/3Co1/3O2 (NMC) and the 75Li2S·25P2S5 (LPS) glass electrolyte after charge–discharge cycles via transmission electron microscopy (TEM). To evaluate the thermal stability of the fabricated all-solid-state cell, in-situ TEM observations for the positive electrode during heating are conducted. As a result, structural and morphological changes are detected in the LPS glasses. Thus, exothermal reaction present in the NMC-LPS composite positive electrode after the initial charging is attributable to the crystallization of LPS glasses. On the basis of a comparison with crystallization behavior in single LPS glasses, the origin of exothermal reaction in the NMC-LPS composites is discussed.

Development of complex hydride-based all-solid-state lithium ion battery applying low melting point electrolyte

Abstract A thermally durable all-solid-state lithium ion battery composed of a complex hydride, oxide electrolytes, and LiNi1/3Mn1/3Co1/3O2 active material is developed. This battery exhibits a discharge capacity of 56 mAh g−1, and the tenth capacity retention ratio is 29% at 150 °C owing to the large contact resistance between the electrolyte layer and the composite positive electrode layer. This large contact resistance is reduced by introducing an adhesive layer comprised of a mixture of LiBH4 and LiNH2 that is easily melted by thermal treatment and fills the voids and pores at the interface between the two layers. As a result, repeated charge-discharge cycles are successfully demonstrated at 150 °C with a high discharge capacity and discharge capacity retention ratio. The first discharge capacity is enhanced to 114 mAh g−1 and the capacity retention ratio at the tenth cycle is improved to 71%. These results demonstrate that using an adhesive layer is an effective measure to reduce the contact resistance and thereby enhance the performance of the battery.