18 research outputs found
Towards High Capacity Li-ion Batteries Based on Silicon-Graphene Composite Anodes and Sub-micron V-doped LiFePO4 Cathodes
Lithium iron phosphate, LiFePO4 (LFP) has demonstrated promising performance as a cathode material in lithium ion batteries (LIBs), by overcoming the rate performance issues from limited electronic conductivity. Nano-sized vanadium-doped LFP (V-LFP) was synthesized using a continuous hydrothermal process using supercritical water as a reagent. The atomic % of dopant determined the particle shape. 5 at. % gave mixed plate and rod-like morphology, showing optimal electrochemical performance and good rate properties vs. Li. Specific capacities of >160 mAh g−1 were achieved. In order to increase the capacity of a full cell, V-LFP was cycled against an inexpensive micron-sized metallurgical grade Si-containing anode. This electrode was capable of reversible capacities of approximately 2000 mAh g−1 for over 150 cycles vs. Li, with improved performance resulting from the incorporation of few layer graphene (FLG) to enhance conductivity, tensile behaviour and thus, the composite stability. The cathode material synthesis and electrode formulation are scalable, inexpensive and are suitable for the fabrication of larger format cells suited to grid and transport applications
Role of disorder in limiting the true multi-electron redox in ε-LiVOPO4
Recent advances in materials syntheses have enabled ε-LiVOPO4 to deliver capacities approaching, and in some cases exceeding the theoretical value of 305 mA h g−1 for 2Li intercalation, despite its poor electronic and ionic conductivity. However, not all of the capacity corresponds to the true electrochemical intercalation/deintercalation reactions as evidenced upon systematic tracking of V valence through combined operando and rate-dependent ex situ X-ray absorption study presented herein. Structural disorder and defects introduced in the material by high-energy ball milling impede kinetics of the high-voltage V5+/V4+ redox more severely than the low-voltage V4+/V3+ redox, promoting significant side reaction contributions in the high-voltage region, irrespective of cycling conditions. The present work emphasizes the need for nanoengineering of active materials without compromising their bulk structural integrity in order to fully utilize high-energy density of multi-electron cathode materials
What Happens to LiMnPO<sub>4</sub> upon Chemical Delithiation?
Olivine MnPO<sub>4</sub> is the delithiated
phase of the lithium-ion-battery cathode (positive electrode) material
LiMnPO<sub>4</sub>, which is formed at the end of charge. This phase
is metastable under ambient conditions and can only be produced by
delithiation of LiMnPO<sub>4</sub>. We have revealed the manganese
dissolution phenomenon during chemical delithiation of LiMnPO<sub>4</sub>, which causes amorphization of olivine MnPO<sub>4</sub>.
The properties of crystalline MnPO<sub>4</sub> obtained from carbon-coated
LiMnPO<sub>4</sub> and of the amorphous product resulting from delithiation
of pure LiMnPO<sub>4</sub> were studied and compared. The phosphorus-rich
amorphous phases in the latter are considered to be MnHP<sub>2</sub>O<sub>7</sub> and MnH<sub>2</sub>P<sub>2</sub>O<sub>7</sub> from
NMR, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy
analysis. The thermal stability of MnPO<sub>4</sub> is significantly
higher under high vacuum than at ambient condition, which is shown
to be related to surface water removal
Electrochemical Performance of Nanosized Disordered LiVOPO<sub>4</sub>
ε-LiVOPO<sub>4</sub> is a promising multielectron cathode
material for Li-ion batteries that can accommodate two electrons per
vanadium, leading to higher energy densities. However, poor electronic
conductivity and low lithium ion diffusivity currently result in low
rate capability and poor cycle life. To enhance the electrochemical
performance of ε-LiVOPO<sub>4</sub>, in this work, we optimized
its solid-state synthesis route using in situ synchrotron X-ray diffraction
and applied a combination of high-energy ball-milling with electronically
and ionically conductive coatings aiming to improve bulk and surface
Li diffusion. We show that high-energy ball-milling, while reducing
the particle size also introduces structural disorder, as evidenced
by <sup>7</sup>Li and <sup>31</sup>P NMR and X-ray absorption spectroscopy.
We also show that a combination of electronically and ionically conductive
coatings helps to utilize close to theoretical capacity for ε-LiVOPO<sub>4</sub> at C/50 (1 C = 153 mA h g<sup>–1</sup>) and to enhance
rate performance and capacity retention. The optimized ε-LiVOPO<sub>4</sub>/Li<sub>3</sub>VO<sub>4</sub>/acetylene black composite yields
the high cycling capacity of 250 mA h g<sup>–1</sup> at C/5
for over 70 cycles