As advanced negative electrodes for powerful and useful high-voltage bipolar batteries, an intercalated metal-organic framework (iMOF), 2,6-naphthalene dicarboxylate dilithium, is described which has an organic-inorganic layered structure of π-stacked naphthalene and tetrahedral LiO4 units. The material shows a reversible two-electron-transfer Li intercalation at a flat potential of 0.8 V with a small polarization. Detailed crystal structure analysis during Li intercalation shows the layered framework to be maintained and its volume change is only 0.33%. The material possesses two-dimensional pathways for efficient electron and Li(+) transport formed by Li-doped naphthalene packing and tetrahedral LiO3C network. A cell with a high potential operating LiNi(0.5)Mn(1.5)O4 spinel positive and the proposed negative electrodes exhibited favorable cycle performance (96% capacity retention after 100 cycles), high specific energy (300 Wh kg(-1)), and high specific power (5 kW kg(-1)). An 8 V bipolar cell was also constructed by connecting only two cells in series.
Hybrid capacitors should ideally exhibit high volumetric energy density, favorable lowtemperature performance and safe operation. Here we describe a negative electrode comprising an intercalated metal-organic framework, 4,4′-biphenyl dicarboxylate dilithium [4,4′-Bph(COOLi) 2 ], which forms a repeating organic-inorganic layered structure of π-stacked biphenyl and tetrahedral LiO 4 units. The electrode shows a stepwise two-electron transfer and has a capacity of 190 mAh g −1 at 0.7 V vs. Li/Li + , which can suppress the lithium metal deposition reaction occurring an internal short circuit. A hybrid capacitor containing 4,4′-Bph(COOLi) 2 negative and activated carbon positive electrodes possesses high volumetric energy density of approximately 60 Wh L −1 and good high-rate performance, particularly at the low temperature of 0°C, because of low charge-transfer resistance along with low activation energy. Hopping mobility calculations suggest the observed low resistance properties are the result of high electron mobility arising from two electron-hopping pathways between adjacent molecules in the π-stacked biphenyl packing layer by lithium intercalation.
Stoichiometric lithium cobalt oxide LiCoO 2 is known to exhibit several structural phase transitions with x in Li x CoO 2 at ambient temperature (T); e.g., an initial rhombohedral (R3̅ m) phase transforms into a monoclinic (C2/m) phase at x ∼ 0.5. In contrast, lithium overstoichiometric (Li) 3b [Li δ Co 1−δ ] 3a O 2−δ with δ ≥ ∼0.02, where δ is the Li + ions at the 3a (Co) site, maintains the R3̅ m symmetry until x ∼ 0.5 in Li x (Li δ Co 1−δ )O 2−δ at ambient T, and this is the reason why such material has been widely used in commercial lithium ion batteries. We performed X-ray diffraction measurements in the T range between 100 and 300 K for the lithium overstoichiometric Li x (Li 0.02 Co 0.98 )O 1.98 samples with x = 1, 0.56, and 0.51 to understand the factors that govern the structural changes in Li x (Li δ Co 1−δ )O 2−δ with δ ≥ 0. Both x = 0.56 and 0.51 samples exhibit a structural phase transition from the high-T R3̅ m phase to the low-T C2/m phase at 250 K (=T s1 ). Furthermore, these samples indicate another structural phase transition at 170 K (=T s2 ); although their crystal structures still have the C2/m symmetry, the degree of monoclinic distortion starts to decrease below T s2 , associated with a magnetic anomaly and a freezing of the Li + ions at the 3b site. Because the two structural phase transitions of T s1 (=330 K) and T s2 (=150 K) are also observed for the stoichiometric Li x CoO 2 compound with x ∼ 0.5, the C2/m phase in Li x (Li δ Co 1−δ )O 2−δ is found to appear in the limited x and T ranges. The characteristics and possible origin of T s1 and T s2 for both stoichiometric Li x CoO 2 and lithium overstoichiometric Li x (Li 0.02 Co 0.98 )O 1.98 samples are discussed.
As advanced negative electrodes for powerful and useful high‐voltage bipolar batteries, an intercalated metal–organic framework (iMOF), 2,6‐naphthalene dicarboxylate dilithium, is described which has an organic‐inorganic layered structure of π‐stacked naphthalene and tetrahedral LiO4 units. The material shows a reversible two‐electron‐transfer Li intercalation at a flat potential of 0.8 V with a small polarization. Detailed crystal structure analysis during Li intercalation shows the layered framework to be maintained and its volume change is only 0.33 %. The material possesses two‐dimensional pathways for efficient electron and Li+ transport formed by Li‐doped naphthalene packing and tetrahedral LiO3C network. A cell with a high potential operating LiNi0.5Mn1.5O4 spinel positive and the proposed negative electrodes exhibited favorable cycle performance (96 % capacity retention after 100 cycles), high specific energy (300 Wh kg−1), and high specific power (5 kW kg−1). An 8 V bipolar cell was also constructed by connecting only two cells in series.
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