Search

Activated carbon is heat-treated in a H atmosphere at 600, 800, and 1000 °C for 1 h, respectively, to be used as electrode material for electrical double layer capacitors (EDLCs). After heat treatment, the surface morphology has no obvious change as compared with the raw material. The specific surface area and pore volume of sample treated at 600 °C have a slightly increase while those of samples treated at higher temperature decrease. XPS and elemental analysis indicate that oxygen containing functional groups on the sample are significantly reduced after treatment. The electrochemical performance of samples was evaluated using cyclic voltammetry and galvanostatic charge-discharge tests in 1 M TEABF/PC electrolyte. The sample treated at 600 °C shows the optimized electrochemical performance with increase capacitance, enhanced stability, and improved energy density. Its initial specific capacitance is near 127 F/g, and initial coulombic efficiency is about 52 %. At 3.0 V, its energy density reaches 32 Wh/kg and specific capacitance is about 70 F/g at 1 A/g even after 10,000 charge-discharge cycles. Thus, heat treatment at 600 °C under H atmosphere is an effective method to improve electrochemical properties of EDLCs based on activated carbon material. [ABSTRACT FROM AUTHOR]

The LiMnFePO/C nanocrystallites with a uniform size (∼50 nm) are successfully prepared by employing a facile solvothermal method at 180 °C. Amphiphilic carbonaceous material (ACM) is chosen as the carbon precursor and forms a homogeneous carbon layer covering the LiMnFePO particles. The as-prepared LiMnFePO/C sample, with a high Brunauer-Emmett-Teller (BET) surface area of 69.3 m g, is used as the cathode material for lithium-ion batteries (LIBs) and delivers a reversible capacity of 158 mAh g at 0.05 C. It shows remarkable rate capability with discharge capacities of 130 mAh g at 1 C, 105 mAh g at 10 C and 99 mAh g at 30 C. Meanwhile, the material maintains 121 mAh g at 1 C after 150 cycles with 93 % capacity retention. The kinetic behaviors of LiMnFePO/C sample are investigated by high throughout cyclic voltammetry. The small particle size, partial Fe substitution, and homogeneous carbon coating make a synergetic effect on enhancing the sample's electrochemical properties. [ABSTRACT FROM AUTHOR]

Lithium-rich layered oxide Li[LiNiMn]O, which also can be written as 0.6LiMnO·0.4LiNiMnO or 0.9Li[LiMn]O·0.4LiNiMnO, is synthesized using a solid-state reaction method. Its crystal structure and electrochemical behavior as the cathode material in lithium-ion batteries are studied. A reaction mechanism is proposed to interpret its unique electrochemical behavior shown in the first charge-discharge cycle. It includes four reactions: (1) LiNiMnO → Li + NiMnO + e, (2) Li[LiMn]O → Li + [LiMn]O + e, (3) [LiMn]O → 1/3 Li + 2/3 MnO + 2/3 O· + e, and (4) Li + NiMnO + e → LiNiMnO. The extraction of oxygen atoms (O·) in the reaction (3) results in the crystal structure rearrangement. Based on this hypothesis, it is found that the expected capacity of activated lithium-rich layered oxide xLiMnO·(1 − x)LiNiMnO (0 ≤ x ≤ 1) increases from 230 to 280 mAh g with increasing x value. Li[LiNiMn]O has an expected total first charge capacity of 396 mAh g, but its expected capacity is only 247 mAh g due to an initial capacity loss caused by the oxygen loss. Experimentally, within a charge-discharge voltage window from 2.0 to 4.8 V, Li[LiNiMn]O delivers a charge capacity of 310 mAh g and a discharge capacity of 215 mAh g, respectively, at 40 mA g during the first cycle. The electrochemical kinetic behavior of Li[LiNiMn]O is controlled by the charge-transfer process rather than by Li diffusion or blockage of solid-electrolyte interphase (SEI) layers at the end of Li extraction in the first charge. [ABSTRACT FROM AUTHOR]

To enhance the rate and cyclic performances of LiMnPO 4 cathode material for lithium-ion batteries, Mn is partially substituted with Fe, and LiMn 1− x Fe x PO 4 ( x = 0.2, 0.3, 0.4, 0.5) solid solutions are synthesized and investigated. Amphiphilic carbonaceous material (ACM) forms well carbon coating and connects the LiMn 1− x Fe x PO 4 crystallites by a three-dimensional (3D) carbon network. The synergetic Fe substitution and carbon connection obviously improve the samples’ rate capacities and cyclic stability. The optimized LiMn 0.6 Fe 0.4 PO 4 /C sample delivers discharge capacities of 160 mA h g −1 at 0.05 C, 148 mA h g −1 at 1 C, and 115 mA h g −1 at 20 C. All samples have well capacity retention (>92%) after 50 charge/discharge cycles at 1 C. The enhanced electrochemical properties are mainly attributed to the improvement of Li ion and electron transport in the LiMn 1− x Fe x PO 4 /C samples, respectively mainly resulting from their modified crystal structures caused by Fe substitution and the 3D carbon coating/connection originating from ACM carbonization. LiMn 1− x Fe x PO 4 materials exhibit two discharge plateaus at ∼4.0 and ∼3.5 V (vs. Li + /Li), whose heights respectively reflect the redox potentials of Mn 3+ /Mn 2+ and Fe 3+ /Fe 2+ couples. The plateaus’ lengths correspond to the Mn/Fe ratio in LiMn 1− x Fe x PO 4 and are affected by the kinetic behavior of samples. Though the ∼4.0 V plateau shrinks with increasing discharge rate, the ∼3.5 V plateau may slightly elongate. Moreover, the Fe substituted in the partial Mn sites could significantly improve the Li ion diffusion, thus enhance the kinetic behaviors of LiMn 1− x Fe x PO 4 . [ABSTRACT FROM AUTHOR]