Capacity Decay Mechanism Research Articles

Instability of Zinc Hexacyanoferrate Electrode in an Aqueous Environment: Redox‐Induced Phase Transition, Compound Dissolution, and Inhibition

AbstractThe structural stability of electrode materials and their compatibility with electrolytes are the important properties for ion‐intercalative electrochemical energy‐storage devices. In the present work, we employed zinc hexacyanoferrates (ZnHCFs), which occurs as cubic or rhombic phases, as the probe to tailor the mechanism of capacity decay upon electrochemical cycling and the corresponding mitigating strategy. Capacity fading results from the loss of active materials, which is highly correlated to the phase states; this has been identified for both phases, where the cubic phase is demonstrated to be the dominant source of ZnHCF dissolution. In 1 m KNO3 electrolyte, rhombic ZnHCF behaves evidently more stable than the cubic phase for long‐term galvanostatic charge/discharge cycling. Even when simply immersed in an aqueous environment, the rhombic–cubic phase transition can spontaneously occur, which, in particular, can be accelerated considerably by electrochemical redox processes in the potential window of 0.8–1.1 V. Utilizing the common‐ion effect, specifically by incorporating ZnII into aqueous electrolytes, could considerably enhance the capacity retention of ZnHCF. Our results suggest that, if electrode materials are soluble at certain electrochemical stages, introducing electrochemically inert common ions into the electrolyte should be an efficient approach to improve the electrode–electrolyte compatibility for pursuing enhanced cycling performances.

Electrospun FeS2@Carbon Fiber Electrode as a High Energy Density Cathode for Rechargeable Lithium Batteries.

In this study, an FeS2@carbon fiber electrode is developed with FeS2 nanoparticles either embedded in or attached to carbon fibers by using an electrospinning method. By applying this binder-free, metal-current-collector-free FeS2@carbon fiber electrode, both the redox reaction and capacity decay mechanisms for the Li-FeS2 system are revealed by changing the electrolyte (conventional carbonate electrolyte and a "solvent-in-salt"-type Li-S battery electrolyte) and working voltage ranges (1.0-3.0 V and 1.5-3.0 V vs Li/Li(+)). The FeS2@carbon fiber electrode shows stable cycling performance in both the conventional carbonate electrolyte and the solvent-in-salt-type Li-S battery electrolyte in the voltage range of 1.5-3.0 V. Electrochemical tests in the solvent-in-salt-type Li-S battery electrolyte indicate that the Li-FeS2 system becomes a hybrid of the Li-S cell and Li-iron sulfide cell after the initial cycle. Based on the understanding on the capacity decay mechanisms, the cycling stability of the Li-FeS2 system in the voltage range of 1.0-3.0 V is then significantly enhanced by coating the FeS2@carbon fiber electrode with a thin layer of Al2O3. The Al2O3-coated electrode demonstrates excellent cycling performance with high discharge energy densities at both the material level (∼1300 Wh/kg-FeS2) and the electrode level (∼1000 Wh/kg-FeS2 electrode).

Enhancing lithium–sulphur battery performance by strongly binding the discharge products on amino-functionalized reduced graphene oxide

Lithium-sulphur batteries are one very appealing power source with high energy density. But their practical use is still hindered by several issues including short lifespan, low efficiency and safety concern from the lithium anode. Polysulphide dissolution and insulating nature of sulphur are generally considered responsible for the capacity degradation. However, the detachment of discharge products, that is, highly polar lithium sulphides, from nonpolar carbon matrix (for example, graphene) has been rarely studied as one critical factor. Here we report the strongly covalent stabilization of sulphur and its discharge products on amino-functionalized reduced graphene oxide that enables stable capacity retention of 80% for 350 cycles with high capacities and excellent high-rate response up to 4 C. The present study demonstrates a feasible and effective strategy to solve the long-term cycling difficulty for lithium-sulphur batteries and also helps to understand the capacity decay mechanism involved.

High-performance lithium/sulfur cells with a bi-functionally immobilized sulfur cathode

Lithium/sulfur (Li/S) cells have a theoretical specific energy five times higher than that of lithium-ion (Li-ion) cells (2600 vs. ~500Whkg−1). The conventional Li/S cells that use an organic liquid electrolyte are short-lived with low coulombic efficiency due to the polysulfide shuttle. We herein design carbon-coated NanoLi2S (NanoLi2S@carbon) composites, which consist of Li2S nanoparticles as the core and a carbon coating as the shell. The carbon shell prevents the NanoLi2S core from directly contacting the liquid electrolyte, which improves the performance of Li/S cells to provide longer cycle life and high sulfur utilization. The cyclability of Li/S cells is further enhanced by mixing the core–shell NanoLi2S@carbon composites with graphene oxide, which chemically immobilizes polysulfides in the cathode through their functional groups. The resulting Li/S cell shows an initial specific discharge capacity of 1263mAhg−1 (normalized to sulfur) at the C/10 rate and a capacity retention of 65.4% after 200 cycles. The capacity decay mechanism during cycling is also characterized in detail using near edge X-ray absorption fine structure (NEXAFS) spectra.

Capacity Decay Mechanism of Microporous Separator‐Based All‐Vanadium Redox Flow Batteries and its Recovery

The results of the investigation of the capacity decay mechanism of vanadium redox flow batteries with microporous separators as membranes are reported. The investigation focuses on the relationship between the electrochemical performance and electrolyte compositions at both the positive and negative half-cells. Although the concentration of total vanadium ions remains nearly constant at both sides over cycling, the net transfer of solution from one side to the other and thus the asymmetrical valance of vanadium ions caused by the subsequent disproportionate self-discharge reactions at both sides lead to capacity fading. Through in situ monitoring of the hydraulic pressure of the electrolyte during cycling at both sides, the convection was found to arise from differential hydraulic pressures at both sides of the separators and plays a dominant role in capacity decay. A capacity-stabilizing method is developed and was successfully demonstrated through the regulation of gas pressures in both electrolyte tanks.

Study of the intrinsic electrochemical properties of spinel LiNi0.5Mn1.5O4

In the past, the electrochemical properties of spinel LiNi0.5Mn1.5O4, i.e. rate capability at room temperature and cycle performance at elevated temperature, were not satisfactory. In this study, the influence of glycolic acid on the electrochemical properties of spinel LiNi0.5Mn1.5O4 was studied. With some amount of glycolic acid as an additional reactant, spinel LiNi0.5Mn1.5O4 was synthesized, and its electrochemical properties were improved. In addition, the mechanism of capacity decay at elevated temperature was also studied, which presents constructive view to further improve the electrochemical properties of spinel LiNi0.5Mn1.5O4.

Preparation and characterization of size-uniform Li[Li0.131Ni0.304Mn0.565]O2 particles as cathode materials for high energy lithium ion battery

Lithium-rich layered oxide, Li[Li0.131Ni0.304Mn0.565]O2 with uniform particle size, is prepared through co-precipitation of metal hydroxide with synergistic dispersion of polyvinylpyrrolidone and ethylene glycol and subsequent solid reaction with lithium hydroxide. The crystal structure and morphology of the prepared sample is characterized with X-ray diffraction and scanning electron microscope. It is found that the prepared sample is a solid solution of Li2MnO3 and LiNi0.5Mn0.5O2, with uniform particle size of about 230 nm. Charge/discharge tests indicate that the prepared sample exhibits improved elevated-temperature discharge capacity and cycling stability. In the region of 3.0–4.8 V, the prepared sample delivers initially a 0.1 C (1 C = 263 mAh g−1) capacity of 256 mAh g−1 at 50 °C and 205 mAh g−1 at 25 °C, and the 0.1 C capacity at 25 °C retains 178 mAh g−1 after 60 cycles. The capacity decay mechanism is understood in detail through dQ/dV analyses.

Vanadium oxides–reduced graphene oxide composite for lithium-ion batteries and supercapacitors with improved electrochemical performance

A facile approach to the surface reduced graphene oxide (rGO) modification of micro-nano structured vanadium oxides composites are developed as cathode materials of lithium ions batteries (LIBs) and supercapacitors (SCPs) for the first time. The as-prepared V2O5–rGO and VO2–rGO composites exhibit remarkably enhanced cycling performance when being used as cathode materials in LIBs and SCPs, respectively. The uniform coating of graphene around the surface of vanadium oxides ensures good close electrical contact, therefore higher specific capacity and enhanced cycling performance than pure VO2 and V2O5 electrodes. Meanwhile, the cycling performance enhancement and capacity decay mechanism are presented, which is not only important to design electrode materials in LIBs and SCPs, but also extendable to the design and fabrication of other functional materials for energy storage and transfer systems.

Surface Characterization on Lithium Insertion∕Deinsertion Process for Sputter-Deposited AgSn Thin-Film Electrodes by XPS

In this paper, the structural and surface changes of the sputter-deposited thin-film electrodes during Li insertion∕deinsertion have been investigated by exsitu X-ray diffraction and X-ray photoelectron spectroscopy (XPS). The study revealed a possible type of capacity decay mechanism for the Sn-based alloy electrodes. It was considered that the surface reactions of active material with electrolyte led to the formation of solid electrolyte interphase (SEI) layer in the first Li insertion process, which was damaged during the Li deinsertion process due to the strong shrinkage of the inner active material. Thus, in the Li deinsertion process, the electrolyte could readily permeate through the damaged SEI layer and reacted with the inner active material to form tin oxides. These tin oxides were conductively isolated, leading to the consuming of active material and capacity decay. It was found that the degree of the formation of tin oxides was strongly related to the identity of electrolyte. Much more SEI layer and tin oxides were formed in the propylene carbonate (PC)-based electrolyte than in the ethylene carbonate (EC)-based electrolyte, due probably to their different reactivity with the active material. This could be the reason why both the sputter-deposited thin film and the powder-pasted alloy electrodes showed rather poor cycling stability in the PC-based electrolyte.

Novel Composites Based on Ultrafine Silicon, Carbonaceous Matrix, and the Introduced Co-Milling Components as Anode Host Materials for Li-Ion Batteries

We report studies of the electrochemical behavior of the Si-M-C (M could be TiN, TiB 2 , graphite) composites, which were prepared from ballmilling silicon with the relatively inactive components and further followed a pyrolyzed poly(vinyl chloride) process. As lithium hosts, these composites showed large capacities of ca. 900 mAh g - 1 and good capacity retention. Research reveals that both the high-energy mechanical milling step and pyrolysis reaction play a key role in alleviating the morphology stress arising from silicon during cycling. A possible capacity decay mechanism with respect to such anode systems has also been investigated and discussed.