Yongyao Xia

Studies on an LiMnO spinel system (obtained by melt-impregnation) as a cathode for 4 V lithium batteries part 1. Synthesis and electrochemical behaviour of LixMn2O4

Abstract An Li x Mn 2 O 4 spinel phase is used as a cathode for 4 V lithium batteries and is prepared by the melt-impregnation method, in which melted LiOH or LiNO 3 is impregnated into MnO 2 pores, then reacted with MnO 2 at higher temperature. The effect of synthesis conditions on the electrochemical properties is investigated extensively. For optimum synthesis, the spinel Li x Mn 2 O 4 should be prepared in an N 2 atmosphere at a low temperature of less than 750 °C and for a short time of less than 48 h. The optimum LiMn 2 O 4 delivers an initial charge capacity of 135 mAh/g, and exhibits good rechargeability. The average specific capacity during first 50 cycles is about 120 mAh/g or more.

Enhancing the elevated temperature performance of Li/LiMn2O4 cells by reducing LiMn2O4 surface area

Abstract Lithium-rich spinels were obtained with the same structure but different surface area by two different synthesis routes, namely the “once-annealed” and the “twice-annealed” methods. The elevated temperature performance of Li/Li1+xMn2O4 cell is significantly improved using a spinel cathode with a small surface area: the cell at 50°C lost 5% of the initial capacity over the first 100 cycles based on a spinel cathode with the small surface area of 1.2 m2/g compared to 8% based on a large one of 6.2 m2/g. Also the mechanism responsible for the reaction of LiMn2O4 with LiOH to form lithium-rich spinel has been investigated.

Studies on Li-Mn-O spinel system (obtained from melt-impregnation method) as a cathode for 4 V lithium batteries Part IV. High and low temperature performance of LiMn2O4

Abstract The cycling profile of an Li/1 M LiPF 6 + ethylene carbonate/dimethyl carbonate (1:2 in volume)/LiMn 2 O 4 cell is examined at various operating temperatures (0, 25 and 50 °C). The capacity fades faster on cycling at a high operating temperature than that at a low one. It is found that the two-phase structure in a high-voltage region is very sensitive to the working temperature. This two-phase structure can be stably maintained for lithium-ion insertion/extraction at a low temperature in the high-voltage region, while it is effectively forced to transform to a more stable one-phase structure at high temperatures. The capacity loss at high operating temperature is due to the following factors: (i) an unstable two-phase structure co-exists in the high-voltage region for lithium-ion insertion/extraction; (ii) Mn slowly dissolves in the electrolyte solution, and (iii) the electrolyte solution decomposes on the electrode.

Studies on LiMnO spinel system (obtained from melt-impregnation method) as a cathode for 4 V lithium batteries Part II. Optimum spinel from γ-MnOOH

Abstract The mechanism for the formation of a spinel compound from the reaction of LiNO3 with a high density γ-MnOOH is extensively investigated in terms of pore-size distribution, X-ray diffraction, thermal and chemical analyses. The reaction of MnOOH with LiNO3 at about 300°C yields a product that consists of domains of lithiated MnO2 phase with an approximate composition of Li0.3MnO2. This product ultimately transforms to a disordered spinel phase of composition LiMn2O4 + y at 300–400°C. The disordered spinel is converted to a well-ordered as the temperature is raised above 550°C. The oxygen content depends on the final heating temperature and atmosphere. Compounds obtained at high temperature, or in N2, have a large capacity and unstable rechargeability. By contrast, some compounds prepared at low temperature, or with a slightly high lithium content in the starting materials, exhibit an improvement in rechargeability. The possible cause for the differences in electrochemical characteristics of both types of compound is proposed.

Differences in electrochemical behavior of LiMn2O4 and Li1+xMn2O4 as 4-V Li-cell cathodes

Abstract Stoichiometric spinel LiMn 2 O 4 delivers a large initial capacity of 140 mA hr/g and has unstable rechargeability. The capacity fading of the cell on cycling occurs only in the higher charge voltage range of x x Mn 2 O 4 . Nonstoichiometric spinel Li 1+ x Mn 2 O 4 has a slightly lower capacity of 110 mA hr/g and has an excellent cycling behavior.

Lithiated manganese dioxide, Li0.33MnO2, as a 3 V cathode for lithium batteries

Abstract The composition of the thermal synthesis products of the LiNO 3 –MnO 2 system has been determined by a continuous variation method in combination with thermal analysis and XRD technique. An optimal 3 V cathode material, a single-phase lithiated MnO 2 , Li 0.33 MnO 2 , was prepared by heating a mixture of LiNO 3 and MnO 2 with a Li/Mn molar ratio of 1/3 at 260°C, followed by heating at 320–350°C. The electrode material delivers an initial capacity of 210 mA h/g close to the theoretical capacity and shows a good rechargeability with a stable rechargeable capacity of 180 mA h/g at a current rate of 0.4 mA/cm 2 (C/3).

High Surface Area Electromigration Damage for 3 V Li‐Cell Cathodes

A new electrolytic manganese dioxide (EMD) suitable for use as a cathode for primary or secondary Li-cells has been developed. The heat-treated product is a high surface area EMD (HSA EMD), which was prepared by anodic deposition from an MnSO{sub 4}/H{sub 2}SO{sub 4} electrolyte at a high current density. It shows excellent electrochemical behavior as a cathode for Li cells. This electrode material discharges its theoretical capacity during the first discharge process and has satisfactory cycling behavior. Measurement indicates that HSA EMD has a quicker open-circuit-voltage (OCV) recovery than common EMD. This suggests that fast Li ion diffusion occurs in this material. A two or more phase reaction occurs during the first discharge in the Li/HSA EMD cell. In contrast, repeated discharge/charges are fundamentally a single phase reaction. The relationship between the OCV voltage plateau during the first discharge and structure of the HSA EMD is discussed.

Studies on the LiMnO spinel system (obtained from melt-impregnation method) as positive electrodes for 4 V lithium batteries. Part III. Characterization of capacity and rechargeability

Abstract A series of ‘lithium-rich’ or ‘oxygen-rich’ spinel compounds are prepared from the reaction of LiNO 3 with electrochemically prepared manganese dioxide (EMD) via the melt-impregnation method. The capacity and rechargeability of these compounds are semi-quantitatively discussed in terms of an LiMn 2 O 4 Li 4 Mn 5 O 12 Li 2 Mn 4 O 9 phase diagram. The capacity decreases as the lithium or oxygen content increases in the spinel matrix. By contrast, the rechargeability is improved.

Dependence of Battery Performance of Spinel Li 1+X Mn 2 O 4 on the Preparation Method

The formation process of the spinel phase by the melt-impregnation method was extensively investigated by the pore volume distribution measurement. Two kinds of the spinel structure compounds Li 1+x Mn 2 O 4 optimized in this method were examined as a 4V cathode in a lithium nonaqueous cell. The first one has an initial charge capacity of 135-147 mAh/g and an unstable rechargeability with the characteristic two-step process, and another one delivers a slightly lower capacity of 105-120 mAh/g and ideal rechargeability with the quasi-one step process. These compounds preserve a capacity of more than 110 mAh/g for the first 100 cycles. The capacity fading on cycling of the former spinel only occurs at the second charge plateau in the rage of x x Mn 2 O 4 , and is due to the unstable two-phase structure for lithium intercalation coexisting in this region. The excellent rechargeability for the later spinel results from a homogeneous reaction occurring over the entire intercalated region.