Pre-Lithiation Strategies for Next-Generation Practical Lithium-Ion Batteries.

Next-generation Li-ion batteries (LIBs) with higher energy density adopt some novel anode materials, which generally have the potential to exhibit higher capacity, superior rate performance as well as better cycling durability than conventional graphite anode, while on the other hand always suffer from larger active lithium loss (ALL) in the first several cycles. During the last two decades, various pre-lithiation strategies are developed to mitigate the initial ALL by presetting the extra Li sources to effectively improve the first Coulombic efficiency and thus achieve higher energy density as well as better cyclability. In this progress report, the origin of the huge initial ALL of the anode and its effect on the performance of full cells are first illustrated in theory. Then, various pre-lithiation strategies to resolve these issues are summarized, classified, and compared in detail. Moreover, the research progress of pre-lithiation strategies for the representative electrochemical systems are carefully reviewed. Finally, the current challenges and future perspectives are particularly analyzed and outlooked. This progress report aims to bring up new insights to reassess the significance of pre-lithiation strategies and offer a guideline for the research directions tailored for different applications based on the proposed pre-lithiation strategies summaries and comparisons.

Revealing the Rate-Limiting Li-Ion Diffusion Pathway in Ultrathick Electrodes for Li-Ion Batteries.

Increasing the loading of active materials by thickening the battery electrode coating can enhance the energy density of a Li-ion cell, but the trade-off is the much reduced Li+ transport kinetics. To reach the optimum energy and power density for thick electrodes, the effective chemical diffusion coefficient of Li+ ( DLi) must be maximized. However, the diffusion of Li+ inside an electrode is a complex process involving both microscopic and macroscopic processes. Fundamental understandings are needed on the rate-limiting process that governs the diffusion kinetics of Li+ to minimize the negative impact of the large electrode thickness on their electrochemical performance. In this work, lithium Ni-Mn-Co oxide (NMC) cathodes of various thicknesses ranging from 100 to 300 µm were used as a model system to study the rate-limiting diffusion process during charge/discharge. The rate-limiting diffusion coefficient of Li+ was investigated and quantified, which was correlated to the electrochemical performance degradation of thick electrodes. It is revealed here that the under-utilization of the active material was caused by the limited diffusion of Li+ inside the porous electrode, leading to a critical electrode thickness, beyond which the specific capacity was significantly reduced.

Electrode Materials, Electrolytes, and Challenges in Nonaqueous Lithium-Ion Capacitors.

Among the various energy-storage systems, lithium-ion capacitors (LICs) are receiving intensive attention due to their high energy density, high power density, long lifetime, and good stability. As a hybrid of lithium-ion batteries and supercapacitors, LICs are composed of a battery-type electrode and a capacitor-type electrode and can potentially combine the advantages of the high energy density of batteries and the large power density of capacitors. Here, the working principle of LICs is discussed, and the recent advances in LIC electrode materials, particularly activated carbon and lithium titanate, as well as in electrolyte development are reviewed. The charge-storage mechanisms for intercalative pseudocapacitive behavior, battery behavior, and conventional pseudocapacitive behavior are classified and compared. Finally, the prospects and challenges associated with LICs are discussed. The overall aim is to provide deep insights into the LIC field for continuing research and development of second-generation energy-storage technologies.

Self-Healing Phenomenon Observed During Capacity-Control Cycling of Freestanding Si-Based Composite Paper Anodes for Li-Ion Batteries.

A different strategy of capacity-control cycling under fixed upper and lower capacity/voltage limits is used in an attempt to seek an extended cycle life for Si nanoparticle-multiwalled carbon nanotube (Si-MW) electrodes for Li-ion batteries. For cells using Si-MW 1:1 (w/w) electrodes in the electrolyte of ethylene carbonate-diethyl carbonate-fluoroethylene carbonate (EC-DEC-FEC, 45:45:10 w/w/w) tested at a current of 1 mA, stable 326 charge/discharge cycles at a designated capacity of 506 mA h g-1 are attained. The new cycling protocol allows for the observation of a self-healing phenomenon by studying the specific capacities and charge/discharge end voltages. Prolonged cycling under capacity control (500 mA h g-1) and the interesting pattern of variations in the discharge/charge end voltage are successfully reproduced under different electrode/electrolyte and current conditions: Si-MW 3:2 in the electrolyte of DEC-FEC (1:1 w/w) at 1 mA (490 cycles), Si-MW 3:2 in DEC-FEC at 0.5 mA (483 cycles), and Si-MW 1:1 in DEC-FEC at 0.5 mA (576 cycles), which can be explained by applying the proposed self-healing mechanism as well.

A hybrid electrochemical device based on a synergetic inner combination of Li ion battery and Li ion capacitor for energy storage.

Li ion battery (LIB) and electrochemical capacitor (EC) are considered as the most widely used energy storage systems (ESSs) because they can produce a high energy density or a high power density, but it is a huge challenge to achieve both the demands of a high energy density as well as a high power density on their own. A new hybrid Li ion capacitor (HyLIC), which combines the advantages of LIB and Li ion capacitor (LIC), is proposed. This device can successfully realize a potential match between LIB and LIC and can avoid the excessive depletion of electrolyte during the charge process. The galvanostatic charge-discharge cycling tests reveal that at low current, the HyLIC exhibits a high energy density, while at high current, it demonstrates a high power density. Ragone plot confirms that this device can make a synergetic balance between energy and power and achieve a highest energy density in the power density range of 80 to 300 W kg-1. The cycle life test proves that HyLIC exhibits a good cycle life and an excellent coulombic efficiency. The present study shows that HyLIC, which is capable of achieving a high energy density, a long cycle life and an excellent power density, has the potential to achieve the winning combination of a high energy and power density.

Modulation of electronic properties of tin oxide nanobelts via thermal control of surface oxygen defects.

Nanomaterials made from binary metal oxides are of increasing interest because of their versatility in applications from flexible electronics to portable chemical and biological sensors. Controlling the electrical properties of these materials is the first step in device implementation. Tin dioxide (SnO2) nanobelts (NB) synthesized by the vapor-liquid-solid mechanism have shown much promise in this regard. We explore the modification of devices prepared with single crystalline NBs by thermal annealing in vacuum and oxygen, resulting in a viable field-effect transistor (FET) for numerous applications at ambient temperature. An oxygen annealing step initially increases the device conductance by up to a factor of 105, likely through the modification of the surface defects of the NB, leading to Schottky barrier limited devices. A multi-step annealing procedure leads to further increase of the conductance by approximately 350% and optimization of the electronic properties. The effects of each step is investigated systematically on a single NB. The optimization of the electrical properties of the NBs makes possible the consistent production of channel-limited FETs and control of the device performance. Understanding these improvements on the electrical properties over the as-grown materials provides a pathway to enhance and tailor the functionalities of tin oxide nanostructures for a wide variety of optical, electronic, optoelectronic, and sensing applications that operate at room temperature.

Non-destructive monitoring of charge-discharge cycles on lithium ion batteries using 7Li stray-field imaging.

Magnetic resonance imaging provides a noninvasive method for in situ monitoring of electrochemical processes involved in charge/discharge cycling of batteries. Determining how the electrochemical processes become irreversible, ultimately resulting in degraded battery performance, will aid in developing new battery materials and designing better batteries. Here we introduce the use of an alternative in situ diagnostic tool to monitor the electrochemical processes. Utilizing a very large field-gradient in the fringe field of a magnet, stray-field-imaging (STRAFI) technique significantly improves the image resolution. These STRAFI images enable the real time monitoring of the electrodes at a micron level. It is demonstrated by two prototype half-cells, graphite∥Li and LiFePO4∥Li, that the high-resolution (7)Li STRAFI profiles allow one to visualize in situ Li-ions transfer between the electrodes during charge/discharge cyclings as well as the formation and changes of irreversible microstructures of the Li components, and particularly reveal a non-uniform Li-ion distribution in the graphite.

Vanadium oxide-carbon nanotube composite electrodes for energy storage by supercritical fluid deposition: experiment design and device performance.

Vanadium pentoxide (V2O5) deposited on porous multiwalled carbon nanotube (MWCNT) buckypaper using supercritical fluid CO2(scCO2) deposition shows excellent performance for electrochemical capacitors. However, the low weight loading of V2O5 is one of the main problems. In this paper, design of experiments and response surface methods were employed to explore strategies for improving the active material loading by increasing the organo-vanadium precursor adsorption. A second-order response surface model was fitted to the designed experiments to predict the loading of the vanadium precursors onto carbon nanotube buckypaper as a function of time, temperature and pressure of CO2, buckypaper functionalization, precursor type, initial precursor mass and stir speed. Operation conditions were identified by employing a model that led to a precursor loading of 19.33%, an increase of 72.28% over the initial screening design. CNTs-V2O5 composite electrodes fabricated from deposited samples using the optimized conditions demonstrated outstanding electrochemical performance (947.1 F g(-1) of V2O5 at a high scan rate 100 mV s(-1)). The model also predicted operation conditions under which light precursor aggregation took place. The V2O5 from aggregated precursor still possessed considerable specific capacitance (311 F g(-1) of V2O5 at a scan rate 100 mV s(-1)), and the significantly higher V2O5 loading (∼81%) contributed to an increase in overall electrode capacitance.