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Abstract: A three-electrode Li-ion cell with metallic lithium as the reference electrode was designed to study the charging process of Li-ion cells. The cell was connected to three independent testing channels, of which two channels shared the same lithium reference to measure the potentials of anode and cathode, respectively. A graphite/LiCoO2 cell with a C/A ratio, i.e., the reversible capacity ratio of the cathode to anode, of 0.985 was assembled and cycled using a normal constant-current/constant-voltage (CC/CV) charging procedure, during which the potentials of the anode and cathode were recorded. The results showed that lithium plating occurred under most of the charging conditions, especially at high currents and at low temperatures. Even in the region of CC charging, the potential of the graphite might drop below 0V versus Li+/Li. As a result, lithium plating and re-intercalating of the plated lithium into the graphite coexist, which resulted in a low charging capacity. When the current exceeded a certain level (0.4C in the present case), increasing the current could not shorten the charging time significantly, instead it aggravated lithium plating and prolonged the CV charging time. In addition, we found that lowering the battery temperature significantly aggravated lithium plating. At −20°C, for example, the CC charging became impossible and lithium plating accompanied the entire charging process. For an improved charging performance, an optimized C/A ratio of 0.85–0.90 is proposed for the graphite/LiCoO2 Li-ion cell. A high C/A ratio results in lithium plating onto the anode, while a low ratio results in overcharge of the cathode. [Copyright &y& Elsevier]

We evaluate poly(acrylamide-co-diallyldimethylammonium chloride) (AMAC) as a water-based binder for the graphite anode of Li-ion batteries. It is shown that AMAC has a similar bonding ability as the conventional poly(vinylidene fluoride) (PVDF) binder, and that the graphite electrodes bonded by AMAC and PVDF have nearly the same cyclability. Advantages of AMAC binder include: (1) it assists in forming a more conductive solid electrolyte interface (SEI) on the surface of graphite and (2) organic liquid electrolyte exhibits better penetration on the AMAC-bonded electrode. Impedance analysis shows that formation of the SEI on the surface of graphite includes two stages. The first stage takes place above 0.15 V and the second stage between 0.15 and 0.04 V. The SEI formed in the first stage is relatively resistive, while that formed in the second stage is highly conductive. For the first stage, the presence of AMAC may enhance the conductivity of the SEI. We performed a storage test on the AMAC-bonded graphite by monitoring the change of open-circuit voltage (OCV) of fully lithiated Li/graphite cells and by comparing their capacity change before and after storage. We observed that OCV of the cell increased gradually, and that capacity loss during the storage recovered in the subsequent lithiation process. Therefore, the OCV increase could be considered a self-delithiation process, which does not consume permanently Li+ ions. [Copyright &y& Elsevier]

We report an effect of surface modification on the performance of natural graphite in Li-ion battery. A graphite electrode was treated with a mixed solution of H3BO3 and H2C2O4 (2:3 molar ratio) in methanol, followed by condensation at 100–110 °C under vacuum. The above treatments result in formation of oxalatoborate coating on the graphite surface. It is shown that the resulting coating can effectively increase reversibility of the initial forming cycle of Li/graphite half-cell. More interestingly, such a coating significantly suppresses self-delithiation of the lithiated graphite, which hence increases the storage performance of Li-ion battery, especially at elevated temperatures. With progressive cycling, the coated graphite shows excellent capacity retention while the control one starts fast capacity fading from around 70th cycle. [Copyright &y& Elsevier]

Cycling performance of Li-ion cells was studied by using electrochemical impedance spectroscopy (EIS). Results showed that total resistance (Rcell) of the Li-ion cells is mainly composed of bulk resistance (Rb), solid-state interface resistance (Rsei) and charge-transfer resistance (Rct). During cycling, the Rb and Rsei remain unchanged while the Rct displays two minima in the same voltage regions where the major peaks of differential capacities are present. The Rct can be linked to kinetics of the cell electrochemical reaction. In response to the temperature change, the Rb and Rsei vary in a very similar manner, while the Rct shows significant difference. In the fully charged and discharged states as well as at the low temperatures (≤20 °C), the Rcell of the Li-ion cells is predominated by the Rct. Using the term of the Rct, we explained two low temperature phenomena of the Li-ion battery: (1) charging of a fully discharged cell is much more difficult than discharging of a fully charged cell, and (2) both the power (operating voltage) and energy (delivered capacity) are substantially reduced. [Copyright &y& Elsevier]

The effect of Li2CO3-coating on the performance of natural graphite in a Li-ion battery was studied. It is shown that Li2CO3-coating can effectively increase reversibility of the initial forming cycle of Li/graphite half-cell. More interestingly, the Li2CO3-coating significantly suppresses self-delithiation of the lithiated graphite, which enhances storage performance of the Li-ion battery. The Li2CO3–coated graphite also shows higher capacity retention after long-term cycling. [Copyright &y& Elsevier]

A symmetric cell was adopted to analyze low temperature performance of Li-ion battery. Results showed that impedances of both Li-ion and symmetric cells are mainly composed of bulk resistance (Rb), surface layer resistance (Rsl) and charge-transfer resistance (Rct). Among these three components, the Rct is most significantly increased and becomes predominant as the temperature falls to below −10 °C. Therefore, we may ascribe the poor low temperature performance of Li-ion battery to the substantially high Rct of the graphite and cathode. Comparing impedance spectra of the symmetric cells, we found that at −30 °C the delithiated graphite and lithiated cathode, both of which correspond to a discharged state in a Li-ion battery, have a much higher Rct than when charged. This means that the Li-ion battery in the discharged state suffers a higher polarization. This result explains the phenomenon that at low temperatures, charging of a discharged Li-ion battery is more difficult than discharging of a charged battery. [Copyright &y& Elsevier]

Poly(acrylonitrile-methyl methacrylate) (AMMA) plasticized with 1 m LiBF4 dissolved in γ-butyrolactone (GBL) liquid electrolyte was studied as a gel polymer electrolyte (GPE) of Li-ion cell. Typically, a GPE composed of 12 wt.% AMMA and 88 wt.% liquid electrolyte has an ionic conductivity of 3.9 mS/cm at 20 °C and 1.1 mS/cm at −30 °C. Results of cyclic voltammetry showed that such a GPE is electrochemically stable to withstand the normal operation of a Li-ion cell. With this GPE, Li/graphite and Li/cathode half-cells presented 89% and 80%, respectively, of Coulombic efficiency (CE), in the first cycle and approached to unity in the following cycles. For graphite/GPE/cathode Li-ion cell, its initial CE was 82%, being close to the lower one of either half-cell. Due partially to high ionic conductivity of the GPE, the Li-ion cell can be reversibly cycled in a wide temperature range. In particular, the Li-ion cell retained as high as 66% of discharge capacity even at −30 °C, as compared to that obtained at 20 °C, when it was cycled at 0.5 mA/cm2 (∼0.5 °C) between 2.5 and 4.2 V. [Copyright &y& Elsevier]

In this work, we used tris(2,2,2-trifluoroethyl) phosphite (TTFP), in which the oxidization number of phosphorus was three (III), to formulate nonflammable electrolytes of the Li-ion batteries. Using 1 m (mole solute per kilogram solvent) LiPF6 3:3:4 (w) propylene carbonate (PC)/ethylene carbonate (EC)/ethyl methyl carbonate (EMC) electrolyte as a baseline, the effect of TTFP on the flammability and conductivity of the electrolytes, as well as the cell performance was evaluated. It is observed that the addition of TTFP can substantially reduce flammability of the electrolytes at a small expense in the ionic conductivity. When the TTFP content reaches 15 wt.% versus the solvent, the electrolyte becomes nonflammable. In Li/graphite half-cell, TTFP not only suppresses PC decomposition and graphite exfoliation but also increases Coulombic efficiency (CE) of the lithiation and delithiation cycle. In Li/cathode (a lithium nickel-based mixed oxide cathode) half-cell, TTFP has negligible adverse impact on the cycling performance when the cells are cycled between 2.7 and 4.2 V. In graphite/cathode Li-ion cell using PC-based electrolytes, TTFP can improve cycling performance, especially at high temperature (60 °C), since its presence favors the formation of solid electrolyte interface (SEI) film on the graphite electrode and increases thermal stability of LiPF6-based electrolytes. [Copyright &y& Elsevier]

We report a new approach toward formulating an electrolyte for low temperature operation of Li-ion batteries. The core of this new approach is to use LiBF4 salt instead of LiPF6, which is the chosen solute in the state-of-the-art Li-ion electrolytes. We found that although LiBF4-based electrolyte has lower ionic conductivity than the LiPF6 analogue, it provides improved low temperature performance. In particular, at −30 °C, a Li-ion cell with 1 m (mol/kg solvent) LiBF4 dissolved in 1:1:3 (wt.) propylene carbonate (PC)/ethylene carbonate (EC)/ethylmethyl carbonate (EMC) mixed solvent delivers as high as 86% of capacity, in comparison to that obtained at 20 °C. Whereas the counterpart one, using LiPF6, only retains 72%. Furthermore, the cell with LiBF4-based electrolyte shows lower polarization at −30 °C. The above results suggest that the ionic conductivity of the electrolyte is not the only limitation to the low temperature operation of Li-ion batteries. Analysis of cell impedance reveals that the improved low temperature performance by LiBF4 arises from a reduced charge-transfer resistance. [Copyright &y& Elsevier]

Abstract: This paper describes the fabrication and performance of microporous gel electrolyte (MGE) Li-ion batteries. The MGE battery was prepared through three steps: (1) making microporous polymer membrane as battery separator by the phase-inversion method, (2) making the battery assembly and activating it with liquid electrolyte, and (3) forming MGE in situ by warming the battery. Depending on liquid electrolyte uptake and warming conditions of the microporous membrane, the resulting MGE may contain three phases: liquid electrolyte, gel electrolyte, and polymer matrix. Therefore, the MGE combines many advantages such as high ionic conductivity, good adhesion to the electrodes, and good mechanical strength. In this work, we used poly(acrylonitrile-methyl methacrylate) (AMMA, AN:MMA=94:6) as the polymer matrix, and a solution of 1.0 m LiBF4 dissolved in a 1:3 (wt.) mixture of ethylene carbonate (EC) and γ-butyrolactone (GBL) as the liquid electrolyte. Typically, an MGE gelled with 390 wt.% of liquid electrolyte vs. the dried membrane has an ionic conductivity of 2.2 mS/cm at 20 °C and the resulting Li-ion battery shows good cycling performance. [Copyright &y& Elsevier]